national centre of excellence in physical chemistry...
TRANSCRIPT
i
CATALYTIC OXIDATION AND DEGRADATION OF ORGANIC COMPOUNDS USING SUPPORTED TRANSITION METALS/METAL
OXIDES.
A dissertation submitted to the University of Peshawar in partial fulfillment for
the degree of
DOCTOR OF PHILOSOPHY IN PHYSICAL CHEMISTRY
By
MOHSIN SIDDIQUE
(PhD Scholar)
NATIONAL CENTRE OF EXCELLENCE IN PHYSICAL
CHEMISTRY
UNIVERSITY OF PESHAWAR
(April 2014)
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Title
Page No.
i. Acknowledgment…………………………………………………….. vii
ii. Publications………………………………………………………….. viii
iii. List of Reviewers……………………………………………………. x
iv. Certificate……………………………………………………………… xii
v. Dedications…………………………………………………………………… xii
vi. Topic and Key words…………………………………………………… xiii
vii. Abstract……………………………………………………………….. xiv
Chapter 1-Introduction
1.1.Introduction………………………………………………………………….. 1
1.2.Aims and Objectives…………………………………………………………. 3
1.3.Heterogeneous Catalyst……………………………………………………… 4
1.4.Metals and Metal oxide as catalysts…………………………………………. 4
1.5.Cobalt Oxide…………………………………………………………………. 6
1.6.Palladium as Catalyst………………………………………………………… 7
1.7.Role of Molecular Oxygen in Oxidation Reactions…………………………. 8
1.8.Adsorption of Oxygen on Catalyst surfaces…………………………………. 10
1.9.Kinetics of the surface catalyzed oxidation reactions……………………….. 11
i. Mars and Van Krevelen Kinetics (MVK)……………………………. 11
ii. The Eley-Rideal Mechanism(ER)……………………………………. 12
iii. The Langmuir-Hinshelwood Mechanism(LH)………………………. 13
iv. Power rate law………………………………………………………... 17
1.10. Oxidation of Alcohols……………………………………………………. 18
1.11. Oxidative Degradation of Methylene Blue………………………………. 20
1.12. Catalytic degradation of Congo-Red Dye………………………………... 22
References Chapter 1…………………………………………………………… 23
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Chapter 2-Literature Review
2.1. Oxidation of Benzyl Alcohol Catalyzed by Mn Catalyst……………............ 30
2.2.Oxidation of Benzyl Alcohol Catalyzed by Palladium Catalyst……………... 33
2.3.Oxidation of Toluene by Palladium Catalyst…………………………............ 36
2.4.Catalytic degradation of Methylene Blue Dye………………………………... 38
2.5.Catalytic degradation of Congo-Red Dye……………………………………... 40
References Chapter 2…………………………………………………………….. 42
Chapter No.3 Experimental
3.1. Materials and Chemicals……………………………………………………..... 45
3.2. Synthesis of Catalyst ………………………………………………………….. 45
i) Manganese oxide………………………………………………………... 45
ii) Cobalt Oxide……………………………………………………………… 45
iii) Palladium Supported Zirconia……………………………………………. 45
3.2.1. Synthesis on Manganese Oxide Catalyst…………………….. 45
3.2.2. Synthesis of Cobalt Oxide…………………………………….. 47
3.2.3. Synthesis of Zirconia………………………………………….. 47
3.2.4. Synthesis of Palladium Supported Zirconia…………………... 47
3.3. Characterization of Catalyst……………………………………………………… 48
3.3.1. Surface Area Investigation…………………………………….. 48
3.3.2. Grain Size Distribution Analysis………………………………. 49
3.3.3. X-Ray Diffractometery (XRD) ………………………………… 49
3.3.4. Fourier Transform Infra Red Spectroscopy (FT-IR)................... 49
3.3.5. Scanning Electron Microscopy (SEM)………………………… 50
3.4. Oxidation Reactions……………………………………………………………… 50
3.5. Analysis of Reaction Mixture……………………................................................. 51
Chapter.No.4. Results and Discussions
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Chapter 4A (Solvent Free Oxidation of BzOH by Pd/ZrO2)……………………… 56
4A.1 Characterization of the Catalyst……………………………………………….. 56
4A.2 Nitrogen Adsorption…………………………………………………………… 56
4A.3 X-Ray Diffractometery (XRD)………………………………………………… 58
4A.4 FT-IR Analysis of Pd/ ZrO2 Catalyst………………………………………….. 58
4A.5 Scanning microscope analysis…………………………………………………. 58
4A.6 Grain Size Distribution………………………………………………………… 62
4A.7 Oxidation of Benzyl Alcohol in Solvent Free Conditions…………………….. 62
4A.8 Comparison of Activated and Fresh Catalyst………………………………….. 62
4A.9 Effect of Catalyst Loading……………………………………………………… 63
4A.10 Life Span of the Catalyst……………………………………………………….. 63
4A.11 Time Profile Investigation………………………………………………………. 63
4A.12 Comparison with Organic Solvents…...………………………………………… 67
4A.13 Effect of Mass Transfer………………………………………………………….. 70
4A.14 Kinetic Analysis………………………………………………………………….. 70
4A.15 Benzyl Alcohol Oxidation in Organic Solvents………………………………….. 77
4A.16 Conclusions …………………………………………………………………………
79
References Chapter No. 4A………………………………………………………………..80
Chapter 4B (Solvent Free Oxidation of BzOH by Pd/ZrO2)………………... 81
4B.1 Characterization…………………………………………………………. 81
4B.2 Catalyst Activity…………………………………………………………. 83
4B.3 Time Profile………………………………………………………………. 85
4B.4 Effect of Catalyst Loading………………………………………………... 85
4B.5 Effect of Oxygen Partial Pressure………………………………………… 90
4B.6 Conclusions………………………………………………………………… 92
References Chapter No. 4B………………………………………………………. 93
4C (Oxidation of BzOH by MnOx)…………………………..………………….. 94
4C.1 Characterization……………………………………………………………… 94
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4C.2 Nitrogen Adsorption…………………………………………………………. 94
4C.3 Particle Size Analysis………………………………………………………… 95
4C.4 X-Ray Diffractometery (XRD)……………………………………………….. 95
4C.5 Fourier Transform Infra Red Spectroscopy (FT-IR)…………………………. 96
4C.6 Scanning Electron Microscope (SEM) analysis ……………………………… 96
4C.7 Oxidation of Benzyl Alcohol………………………………………………….. 101
4C.8 Manganese oxide as Catalyst or Oxidant……………………………………… 103
4C.9 Catalyst Leaching……………………………………………………………… 103
4C.10 Catalyst Loading………………………………………………………………. 104
4C.11 Life Span of Catalyst………………………………………………………….. 104
4C.12 Time Profile Investigation…………………………………………………….. 104
4C.13 Effect of Oxygen Partial Pressure…………………………………………….. 107
4C.14 Comparison of Laboratory and Commercial Manganese Oxide……………… 107
4C.15 Comparison with other catalysts……………………………………………… 107
4C.16 Conclusions…………………………………………………………………… 113
References Chapter No. 4C………………………………………………………... 114
4D (Oxidative Degradation of Methylene Blue by Cobalt Oxide)……….. 116
4D.1 Characterization……………………………………………………………….. 116
4D.2 Surface Area……………………………………………………………………. 116
4D.3 X-Ray Diffractometery (XRD)………………………………………………… 118
4D.4 FT-IR Analysis………………………………………………………………… 118
4D.5 SEM…………………………………………………………………………… 121
4D.6 Particle Size……………………………………………………………………. 121
4D.7 Reaction Protocols…………………………………………………………….. 121
4D.8 Time Profile……………………………………………………………………. 123
4D.9 Detection of Products………………………………………………………….. 123
4D.10 Catalyst Loading……………………………………………………………….. 123
4D.11 Effect of Temperature………………………………………………………….. 123
4D.12 Effect of Oxygen Partial Pressure……………………………………………… 126
4D.13 Comparative Study of Oxygen, Nitrogen and Open to Atmosphere………….. 126
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4D.14 Life Span of the Catalyst………………………………………………………. 126
4D.15 Adsorption Studies……………………………………………………………... 130
4D.16 Conclusions………………………………………………………………………. 131
References Chapter No. 4C…………………………………………………………... 132
4E (Oxidative Degradation of Congo-Red Dye by Cobalt Oxide)…………... 133
4E.1 Reaction Protocols…………………………………………………………….... 133
4E.2 Characterization ………………………………………………………………... 134
4E.3 Time Profile……………………………………………………………………... 134
4E.4 Decolorization of Congo-Red with different Catalysts…………………………. 134
4E.5 Detection of Products……………………………………………………………. 134
4E.6 Effect of Catalyst Loading……………………………………………………….. 136
4E.7 Effect of Oxygen Partial Pressure………………………………………………... 136
4E.8 Life Span of the Catalyst…………………………………………………………. 136
4E.9 Conclusions………………………………………………………………………… 142
Conclusions and over view………………………………………………………………… 143
References Chapter 4E…………………………………………………………………… 149
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ACKNOWLEDGEMENT
First of all, I would like to express my gratitude to my supervisor Professor Dr.
Mohammed Ilyas for his continuous help, stimulating suggestions and encouragement to me in
all the time of research and writing of this thesis. I would also like to thank Professor Dr.
Christopher Hardacre and Dr. Haresh G. Manyar (Lecturer QUB) for providing me an
opportunity to work in the liquid phase heterogeneous catalysis group during my six months visit
to Queen’s University Belfast, United Kingdom under International Research Initiative
Program, supported by the Higher Education Commission of Pakistan.
I am thankful to the director NCE Physical Chemistry Prof Dr. Hasan M Khan and Ex-
director NCE Physical Chemistry Prof. Dr. S. Mustafa for providing me ample opportunity and
facilities for completing my Ph.D work.
I appreciate the help of Professor Dr. Ikram Ul Haq, Professor Dr.Mohammd Salim
Khan, Professor Dr. Abdul Naeem Khan and all the other technical and non technical staff
members of the NCEPC during my work.
I would also like to appreciate Dr.Muhammad Saeed, Dr. Mohammed Taufiq, Mr. Aziz
Ahmed, Mr. Nadir Khan and all the members of our research group, for their kind help and
encouragement during my candidature.
I am really grateful to Mr. Izhar Ahmed and his crew members for making some great
glass instruments during my experimental work
Special thanks must go to my parents and my brothers and sisters for their understanding,
encouragement and support.
Lastly, I am indebted to the Higher Education Commission of Pakistan for providing
me with a valuable research scholarship and funding under Indigenous 5000 Ph.D Scholarship
and IRSIP projects respectively.
Mohsin Siddique
October 2013
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Publications
The following papers have been published in ISI Indexed journals
I. Liquid Phase Aerobic oxidation of benzyl alcohol by mechanochemically synthesized
manganese oxide. Mohammed Ilyas, Mohsin Siddique and Muhammad Saeed. Chinese
Science Bulletin. Vol.58 No.19: 2354-2359. 2013.
II. Oxidative Degradation of Oxalic Acid in Aqueous Medium Using Manganese Oxide as
Catalyst at Ambient Temperature and Pressure, Arabian Journal of Science and
Engineering. M. Ilyas, M. Saeed, Mohsin Siddique and Aziz Ahmed. 2013. DOI
10.1007/s13369-013-0545-x
III. Catalytic Decolorization of Methylene Blue from aqueous solution using cobalt oxide.
Mohammed Ilyas and Mohsin Siddique, Journal of the Chemical Society of Pakistan.
Volume 34- No.5. 2012. 1197.
IV. Oxidative degradation of Phenol in aqueous medium catalyzed by Lab prepared Cobalt
oxide. M. Ilyas, M. Saeed and Mohsin Siddique. Journal of the Chemical Society of
Pakistan. Volume 34- No.3. 2012. 626.
The following papers have been presented in different national and International Conferences
I. 7th International and 19th National Chemistry, Conference, Organized by Kohat
University of Science and technology, Kohat (June 09-11,2009)
II. 8th International and 20th National Chemistry, Conference, Organized by Dept.of
Chemistry ,Quaid e Azam University, Islamabad , Pakistan ( Feb 15-17,2010)
III. 1st National Conference on Material Processing , Characterization, Properties and their
Economic Potential, Organized by University of Peshawar (1-3 December, 2010)
IV. 1st National Conference on Physical & Environmental Chemistry (PEC-2010) at Baragali
Summer Campus, National Centre of Excellence in Physical Chemistry, University of
Peshawar, Peshawar, Pakistan, September 26-30, 2010.
V. 9th International and 19th National Chemistry, Conference, Organized by Dept.of
Chemistry ,University of Karachi, Karachi , Pakistan ( Feb 15-17,2011)
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VI. 10th International and 20th National Chemistry, Conference, Dept.of Chemistry and Bio-
Chemistry, University of Faisalabad , Faisalabad , Pakistan ( Nov 21-23,2011)
VII. National Symposium on Kinetics and Catalysis (KC-2011), National Centre of
Excellence in Physical Chemistry, University of Peshawar, Pakistan (September 26-
28,2011)
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List of Reviewers
This thesis was reviewed and evaluated by following experts in the field of chemistry.
1. Prof. Dr. Ticheng An
Guangzhou institute of Chemistry
Chinese Academy of Sciences
Guangzhou 510640
P.R. China
2. Dr. Akhtar Saeed
Associate Professor
Toronto Institute of Pharmaceutical Technology
Canada.
3. Prof. Dr. Imtiaz Ahmad
Professor
Department of Chemistry,
Islamia College University Peshawar,
Pakistan.
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This is to certify that the dissertation submitted by Mr. Mohsin Siddique entitled
“Catalytic oxidation and degradation of organic compounds using supported transition
metals/metal oxides” is acceptable in its present form by the University of Peshawar, Peshawar,
Pakistan as satisfying the dissertation requirement for the degree of Doctor of Philosophy in
Physical Chemistry
________________________ ________________________
Professor Dr. Mohammad Ilyas Professor Dr. Hasan M. Khan (Research Supervisor) (Director)
Examination Satisfactory
Committee on Final Examination
___________________________
External Examiner
xii
Dedicated to My Parents
xiii
Topic
Catalysis, Heterogeneous Catalysis
Key Words
Palladium oxide, Zirconia, Cobalt oxide, Manganese Oxide, Toluene, Benzyl alcohol,
Benzaldehyde, Benzoic acid, Methylene Blue, Congo Red, Carbon dioxide, XRD, FTIR, SEM,
Langmuir-Hinshelwood.
xiv
Abstract
0.1 wt % Palladium/ZrO2, Manganese oxide and cobalt oxide were synthesized in our
laboratory. Zirconia was prepared in the laboratory by precipitation of zirconyl chloride with
ammonium hydroxide. Pd/ZrO2 was prepared by incipient wetness impregnation technique.
Cobalt oxide was prepared by solid state mixing of cobalt nitrate and ammonium bicarbonate in
an agate mortar at room temperature. Manganese oxide was also prepared by mechanochemical
addition of potassium permanganate and Ammonium bicarbonate in a molar ratio of 2:3
respectively.
The prepared catalysts were characterized by several physical/analytical methods that
include nitrogen adsorption studies (Surface area and Pore Size Analysis), X-Ray
Diffractometery, Fourier Transform Infra Red spectroscopy, particle size and Scanning Electron
Microscope analysis.
Pd/ZrO2 was tested for the solvent free oxidation of benzyl alcohol and Toluene
respectively. The reactions were carried out in liquid phase under mild conditions of temperature
and pressure. Molecular oxygen was used as oxidant. The oxidation of BzOH was > 70 %
selective towards benzaldehyde formation with a TOF>6000 per hour. Kinetic study showed that
Langmuir Hinshelwood (L-H) mechanism was found to be followed when the experimental data
was applied to the L-H equation.
The oxidation of toluene was more selective towards benzyl alcohol formation in lower
reaction temperature regime; however as the reaction temperature was increased the reaction
became more selective towards benzoic acid. The main oxidation products were benzyl alcohol,
benzaldehyde and benzoic acid however the main product was benzoic acid.
Manganese oxide was prepared by solid state mechanochemical addition of potassium
permanganate and Ammonium bicarbonate. The synthesized manganese oxide powder was
employed for the oxidation of benzyl alcohol in liquid phase using n-heptane as a solvent. The
reaction was found to very fast and 100% selective towards the formation of benzaldehyde at
xv
363K and atmospheric pressure of oxygen. The reactions were performed at very low reaction
temperatures i.e. 323-363K.
Cobalt oxide was prepared by was prepared by solid state mechanochemical mixing of
cobalt nitrate and ammonium bicarbonate. The synthesized catalyst was employed for the
catalytic degradation of two different dyes i.e. Methylene Blue and Congo red. The catalyst was
found to be extremely efficient towards the degradation of both these dyes. Both the organic dyes
were successfully destructed in a very quick reaction time i.e. 10 minutes. Reactions were carried
out in atmospheric conditions and room temperature. Various parameters affecting the
degradation performance of the dye were examined such as time, catalyst loading, temperature,
initial dye concentrations, speed of agitation and effect of partial pressure of oxygen. The
removal percentage of dyes increased with increasing mass of Cobalt oxide up to an optimum
mass but decreased with increasing initial concentrations. All the catalysts were heterogeneous in
nature, which could be separated easily by simple filtration.
1
Chapter 1
Introduction
1.1.Introduction
The catalytic oxidation of hydrocarbons using di-oxygen as an oxidant is widely used in
synthesis, and is a common strategy for rectification of contaminants from waste water and flue
gases [1]. A number of processes and oxidants have are reported in literature which includes
stoichiometric [2, 3] and catalytic methods [4, 5]. These stoichiometric oxidatants contains
harmful metals such as hexavalent chromium or explosive reagents, such as organic peroxides.
These oxidants have been censured with increasingly stringent environmental laws. Therefore,
from the environmental point of view, using green oxidants such as di-oxygen, air [4, 6, 7], H2O2
[5, 8] and ozone [9] qualify as good alternatives to the harmful stoichiometric oxidants, towards
green oxidations of organic compounds. An ideal oxidant for any large-scale oxidation reaction
should qualify the following qualities
It should be low cost
It should be pure
It should be environment friendly
And it should be available easily everywhere
Di-Oxygen is the one which best qualify theses qualifications [10]. Not only it is easily
available in air in a large quantity but it also produces negligible byproducts such as water and
carbon dioxide.
However, a number of drawbacks that are connected with the use of oxygen as oxidant
make it difficult for this cause i.e.
Though di-oxygen is highly potent oxidant, but it’s not very reactive toward many
organic molecules.
2
Such reactions which involve di-oxygen as an oxidant, generally involves radical
formations; these radicals produced are very reactive and hence react with
everything that comes in its way, Hence the selectivity of the reaction towards a
certain product is altered.
To take good advantage of di-oxygen as oxidant, an appropriate catalyst should be
employed, which has the ability to activate the molecules of oxygen for appropriate reaction.
Presently di- oxygen is used industrially for a number of commercial-scale oxidation reactions
that are catalyzed by different homogeneous and heterogeneous based catalysts; however, these
catalytic reactions involve higher temperature and pressure conditions. But, these oxidation
techniques are not adaptable for commercial scale production, where selectiveness and gentle
reaction conditions favor the production of good quality products. Hence, it is very important
that these oxidants should be displaced by such catalysts that have the capability to activate di-
oxygen under milder reaction conditions. An asservative elucidation to this problem lies in
homogeneous catalytic systems which show excellent selectivity towards catalytic oxidation
reactions under mild conditions of temperature and pressure using di-oxygen as the oxidant.
Although di-oxygen is potentially an admirable oxidation tool, the
thermodynamic and kinetic restrictions that are brought about by the spin preservation rule
makes it remarkably un-reactive towards organic compounds. Therefore, to make these oxygen
molecules react readily with these organic compounds, the electronic arrangement of the di-
oxygen need to be changed, so that it is able to conform to the electronic arrangement of the
reacting organic compounds. The change can be accomplished either by physical means for
example using light excitation or by chemical means i.e. by reacting the oxidant i.e. di-oxygen
with, for example, group “d” metals/metal oxides .This generally represents the procedures that
how the catalytic reactions take place in a homogenously catalyzed oxidation reaction. However,
with homogeneous catalytic systems, both the constituents (reactant and catalyst) remain in a
single/same phase, due to which it becomes difficult to separate the products from the catalyst,
which increases the number of steps [11]. Therefore, using heterogonous system having a solid
catalyst and liquid or gases as reactants would be advantageous as compared to the homogeneous
counterpart because in heterogeneous catalytic system the products and the catalysts are easily
separable and also the catalyst can be reused for further production or degradation.
3
Metals/Metal oxides in supported and unsupported form have been suggested as effective
heterogeneous catalysts for oxidation of organic compounds. Precious metals like platinum (Pt),
palladium (Pd) and ruthenium (Ru) have been shown good activity towards the oxidation of
theses organic compounds, but as these transition metals and their oxides are very expensive the
need of the day is to find such a catalyst which is inexpensive and is also environmental friendly
for this purpose the use non-precious transition metals like cobalt (Co), manganese (Mn) and
nickel (Ni) or the use of precious metals like Platinum, Gold or Palladium in supported form
qualify to be the ideal heterogeneous catalysts towards the catalytic oxidation of organic
compounds and molecules, using neat oxidants like as di-oxygen is of great value [12,13].
1.2.Aims and Objectives
The main aims of this project are to produce and develop environmental friendly methods for the
oxidation of biomass/petrochemicals hydrocarbon compounds such as Benzene, Toluene and
Phenols, Methylene Blue, Congo-Red and Cyclohexanol.
The use of metal/metal oxides in both supported and unsupported forms.
The use of di oxygen and air instead of stoichiometric reagents or oxidants like Hydrogen
peroxide, potassium permanganate and potassium dichromate.
The Synthesis of oxides of transition metals like Fe, Mn, Co and Pd
Applying different experimental conditions to investigate the catalytic activities of these
oxides for oxidation reactions of organic compounds like Industrial textile dyes
(Methylene Blue and Congo red), benzene, benzyl alcohol, toluene, phenol etc.
To characterize the catalysts under study and investigate the kinetics and the reaction
mechanisms of the reactions under different physical conditions.
4
1.3.Heterogeneous Catalyst
A catalyst is a material which accelerates chemical reactions without its self undergoing
any chemical change. Catalysts are found in many physical forms generally in the form complex
solid materials, containing sites on which the catalytic processes take place (heterogeneous
catalysis). Heterogeneous catalysts are classified broadly as supported or bulk catalysts.
Heterogeneous catalysts are used for the oxidation of organic molecules because of their
exceptional features as a catalyst. Heterogeneous catalysts are mostly inorganic compounds that
contain certain sites on its surface called as "active sites". It is actually these sites which are
responsible for the transformation of reactants into products by lowering their activation energy.
Heterogeneous catalysts are either uni-phase materials, e.g. zeolites or Raney nickel, or they may
be composed of an uncreative or inert supported material on which the active metal is thoroughly
distributed. By and large the active sites are transition metals, existing either in metallic or
cationic form, for instance 0.1% Pt dispersed on activated carbon. As the catalytic reactions are
taking place upon the surface of the active site therefore it is aimed to synthesize the catalyst in
such way that the grain size of the catalyst is maintained as small as possible, so that its surface
to bulk ratio is increased and the atoms of the reactants can approach the active sites with ease.
Commonly used support matter for heterogeneous catalyst is simple metals and metal oxides, i.e.
silica, alumina, Zirconia, Titania and ceria [14].
1.4.Metals and Metal Oxide as Catalysts
Keeping in view its potent role in catalysis, surface chemistry, and electronic technology,
colloidal materials (mono- and bimetallic) have, after a period of inactiveness, recently attracted
more attention. Now that more powerful analytical tools are available for the comprehensive
characterization of these materials, it has become possible to introduce ourselves the knowledge
of nanoscale metal colloids [15]. The number of potential applications of these colloidal
materials is gaining attraction for the reason that the metal nano particles have a unique
electronic structure, due to which not only metal particles are highly dispersed but also show and
extremely large surface to bulk ratio to the approaching reactant molecules[16]. Highly dispersed
monometallic and bimetallic can be used for a novel catalyst that is appropriate equally in both
homogeneous and heterogeneous catalysis fields [17]. Nano-particles that consist of one or two
dissimilar metal components are of immense technological importance [18, 19].
5
A metal oxide may consist of a single crystal that may be pure or defective, it may in the
form of powder which pertains to a larger number of crystals, it may be polycrystalline i.e. it
may contains crystals of various orientations and shapes or simply it is a thin metal oxide film. A
number of techniques have been discussed in literature for the preparation of metal oxides;
however the method of its selection vastly depends upon the desired results. Different types of
solid state materials are found in the literature among them are hydrides, halides and
chalcogenides [the binary compounds of oxygen (O) and sulfur(S)]. Oxides and Sulphides are
found widely applicable because of their excellent thermal and mechanical properties combined
with their promising behavior as a catalyst. In the early Era solid state materials especially metals
and metal oxides were synthesized by methods which demanded stringent conditions of
temperature and pressure and consumed a lot of time to synthesize them, but unfortunately these
materials usually possess very small surface area and hence were not very appropriate for a lot
of applications especially catalysis. Transition metals and metal oxides include oxides of
Ruthenium, Molybdenum, Platinum, Palladium and Vanadium etc. are of exceptional interest
and application in the field of catalysis especially heterogeneous catalysis. The shape and size of
metal and metal oxides greatly influence the catalytic properties for instance spherical nano
particles that have a mean diameter of 3nm, more than 50% of the reactant molecules can
approach the active sites (metal nano-particles) directly, which allows a stoichiometric chemical
reaction possible [19]. The ionic nature of some types of materials may incorporate a number of
defective sites that includes edges, corners or action/anion vacancies. Among other materials, the
materials synthesized through aero-gel routes possess good applications because of their low
densities and low thermal conductivities. Among their major contributors to its applications
includes radiation detectors, super insulators, solar concentrators, coatings, insecticides, glass
precursors, destructive adsorbents and catalysis. Large surface area of the nano scale particles
gives rise to a number of defective sites of the crystal structure. A number of studies were
conducted to clarify the types of defective sites that may exist in MgO nano powders [20, 21].
The catalytic properties as well as the electronic structure of these nano-materials can be readily
altered by changing the experimental conditions [22, 23]. The main use of metal nano particles is
in the field of catalysis, whereas metal oxides are applicable mainly in sensors, cosmetics,
ceramics, catalysis and petrochemical industries. Nearly 75-80% of chemical industry use
heterogeneous catalysis for their production of chemicals, which accounts for more than 30
6
present of the world gross production. It is obvious that the expenditure and effectiveness of the
catalyst employment in these production processes have great impact on the global market
directly. In this document we will try to elaborate a summary of the colloidal properties and
methods as well as the approach(s) those have been employed to link the philosophy of nano
science with catalysis. In the last decade or so the scientific literature regarding the preparation
of nano scale materials using a down-up approach has gained significant amount of attraction,
however in the recent past people are moving towards the synthesis of more ordered, highly
crystalline nano scaled metal oxide powders.
1.5.Cobalt Oxide
Cobalt oxide (Co3O4) is widely used in different industrial applications. It is used as
anodic material in lithium ion batteries, as sensors, as electro chromic material, as catalyst and in
other types optoelectronic devices [24-26]. Although single phase cobalt oxide particles were
synthesized some 30 years before using “forced hydrolysis” method at considerably low
temperatures [27]. Literature study has reported Cobalt oxide to be an effective heterogeneous
catalyst towards the oxidation of different organic materials [28-31]. As solid oxide, a cobalt
oxide catalyst represents p-type oxides, which has a great capability to adsorb oxygen on its
surface and produce electron-rich adsorbed species like O1- and O2-. Cobalt oxide has been tested
as a catalyst for the catalytic decay of O3 molecules in gas phase. Cobalt oxide has been widely
employed as a support material for other transition elements/metals. Cobalt oxide is a unique
oxide used in different catalytic processes e.g. catalytic oxidative degradation of VOCs,
oxidation and reduction of NOx, oxidation of Carbon mono-oxide using gold as promoter, vapor-
phase oxidation of organic molecules advanced physical applications (magnetic properties) [32-
37]. Different morphologies of cobalt oxide are reported in the literature that includes nano
spheres, nano cubes, nano rods, and meso porous structures [38-41]. The physical and chemical
properties of cobalt oxide mainly depend on their morphological feature such as crystallite size
and exposed crystal faces. Cobalt oxide catalysts are usually composed of poorly defined multi
crystal structures with a number of exposed crystal faces, due to which different types of active
sites are exposed to the reactants, these sites exhibit different reactivities towards the substrate
molecules which usually results in lowering of the catalytic reactivities [42, 43]. Substantial
research has been carried out to synthesize porous Co3O4 particles with controlled morphology
7
trough nano casting route, i.e. by crystallizing cobalt oxide inside the pores of both aperiodic and
periodic silica [44-46].
Beside all these things people are not reluctant to use cobalt oxides as catalysts in
oxidation processes of different chemical applications. As discussed earlier, surface area, particle
size distribution, surface and structural morphology, oxidation state and preparation condition of
the catalyst (cobalt oxide) plays are the key factors towards its catalytic efficiency. In literature
various synthetic routes for the preparation of cobalt oxide have been proposed, among them
includes; sol-gel route, pulsed laser deposition, gel hydrothermal oxidation reduction-oxidation
rout, staged oxidation process, homogenous precipitation, cobalt salt decomposition at high
temperature and solid state mechanochemical process . The mechanochemical process seems to
be a better choice for large-scale synthesis in which cobalt oxide can be synthesized through a
simple solid state displacement reaction at room temperature. Due to simplicity and less cost,
this process for synthesis suits large-scale production of cobalt oxide at room temperature [47-
51].
1.6.Palladium as Catalyst
Precious metals like Pt, Pd, Au, and Ru are valuable materials that possess many
important applications in the chemical industry. Palladium seems to be the most attractive
catalytic systems because of its promise as a metal that shows high catalytic efficiency. For
instance, supported palladium catalysts are used in: chemical industries for the hydrogenation of
different organic reactions, in the decomposition of ethanol and methanol[52], automotive
exhaust-gas [53,54], methane combustion [55], methane steam reforming [56,57], partial
oxidation of methane and methanol, oxidation of NOx, CO oxidation and reduction, oxidation of
alcohols to aldehydes, ketones and carboxylic acid, hydro-dechlorination of halogenated organic
compounds, aldehydes condensation, aniline synthesis from phenol etc, zeolite supported
palladium catalysts have been used extensively for the shape and enantioselective organic
reactions [58]. In most catalyzed reactions both active phase and supporting material are
responsible to bring about the chemical change in the substrate molecules. However, in other
case when support material only serves as a host material to keep the catalytically active species
intact, these active species must interact some way or other with the support. Due to these
interactions between the active material and support it is quite necessary that the active material
8
is dispersed to a maximum, the catalytic activity is also dependant on the morphology and the
electronic properties of the metal.
Probably the use of palladium in the field of fine chemicals is commercially the most
attractive and widely used catalyst up to date [58]. The surface morphology and reactivity of
supported palladium catalyst is entirely different from unsupported/ bulk palladium. The bulk
palladium cannot be used commercially because of its expensiveness, poor thermal and
mechanical stability leading to quick deactivation process of the catalyst. Common supports used
for palladium are alumina (Al2O3), Silica (SiO2), magnesium oxide (MgO), and Titania (TiO2)
[59, 60]. But some disadvantages are associated with these materials like, small surface area,
high acidic or basic surface charges, low volume activity, poor mechanical strength and phase
transition problems at higher temperature values, making them unsuitable for commercial
applications. The use of mixed oxides, larger surface area supports and inert supports can
overrule these drawbacks. Good choice of support is important in constructing a supported
catalyst for the desired activity and selectivity. The oxidation state and acid/base or electrophilic
/electrophobic properties of catalysts play an important role on the activity of the catalyst.
Support material benefits in the dispersion of catalytically active species but provides us with
economic benefits i.e. lower preparation costs and longer productive lifetime [61, 62].
1.7.Role of Molecular Oxygen in Oxidation Reactions
Oxygen plays substantial role in the field of heterogeneous catalysis on the on metal oxide
surfaces in a verity of ways. These can be abstracted:
1.1. Langmuir-Hinshelwood mechanism: Both oxygen and the reactant species get
adsorbed at the same time on catalyst surface and then in second step products are formed
and desorbed.
1.2. Eley-Rideal mechanism: First oxygen molecule interacts with the surface and in
2nd step the reactants attract with the catalyst surface and in 3rd step products are
desorbed off.
1. In a number of surface catalyzed reactions, the reactants and the catalysts reactant
together producing a change in the chemical composition of the catalyst. Transition
metals and metal oxides tend to be more reactive towards oxygen species, due to the
9
stoichiometry of the surface of metals and their oxides by reacting with oxygen. The
transformation in structure and composition of the catalyst into another generally affects
the catalytic activity; therefore it’s necessary to have knowledge about the state of the
catalyst during a catalytic chemical reaction.
2. Mars and Van Krevelen mechanism (MVK) says that the catalytic oxidation of organic
compounds catalyzed by solid heterogeneous catalysts undergoes the following
transformations/steps:
i. In the first step the lattice oxygen of the catalyst react with the reactants, to form
products and the catalyst becomes partially reduced.
ii. In the 2nd step he partially reduced catalyst is re oxidized by molecular oxygen
and hence the catalyst is regenerated into its original oxidation state [63].
This model certainly explains the fact that the oxidizing agent is actually the lattice
oxygen responsible for the product formation and the role of molecular oxygen is s merely to
fulfill the spaces that were left empty due to reaction of the lattice oxygen with the reactants.
Therefore it’s worth mentioning here that that the selectivity and activity of the reaction is
directly dependent upon the oxygen lying in the lattice of the catalyst (strength of metal oxygen
bond)
Generally, the metal oxides that are responsible for the oxidation of organic compounds contains
two distant lattice points i.e. the cations and anions, the anions are actually the oxygen anions, in
a lot of metal oxides these anion oxygen make co-ordinate covalent bonds with the metal ions
(cations) [64,65].
1.8. Adsorption of Oxygen on Catalyst Surfaces
Adsorbed surface oxygen on the metal and metal oxide catalyst exists in two different
forms i.e. physically adsorbed or physisorbed oxygen and chemically adsorbed or chemisorbed
oxygen.
The physisorbed oxygen does not dissociate and hence no significant charge migration
occurs from the adsorbent surface i.e. metal/metal oxide surface to adsorbed oxygen species,
hence the oxygen species remains unchanged the metal surface.
10
But the chemisorbed oxygen species are generally form through the transference of
electrons between the adsorbent i.e. metal oxide surface and the adsorbate i.e. oxygen, resulting
in the formation of some charged oxygen species that can hold the surface of metal oxide firmly.
Different form of adsorbed oxygen-surface linkages are possible that include molecular
oxygen-metal linkage, and negatively charged species (O21-, O1-, O2
2-)-metal linkages.
The chemisorption of oxygen at the metal oxide surface gives the idea of bond braking
and making in case of di-oxygen (O2) to produce anionic species O2-. According to molecular
orbital theory (MOT) the P orbitals (px, py and pz) of the two oxygen atoms interacts with three
bonding molecular orbitals and two antibonding molecular orbitals, giving rise to a bond order
equal to two i.e. a double covalent bond is present between the two oxygen atoms (one sigma –
one pi). The oxygen molecule gets attached to metal site by a coordinate covalent bond
chemically and hence a chemical bond is established between the oxygen molecule and metal
oxide surface i.e. electron transference between adsorbate and adsorbent takes place, when the
electron density is shifted towards oxygen, two electrons of the metal enters the two vacant
antibonding molecular orbitals of oxygen molecule and hence the linkage between the two
oxygen atoms gets weakened. The overall process can be represented as;
Step No.1. )(22 adsOO
Step No.2. )()( 12
12 adsOeadsO
Step No. 3. )()( 22
112 adsOeadsO
Step No.4. )(2)( 122 adsOadsO
Step No.5. )()( 211 latticeOSeadsO
Therefore, the possibility is there that oxy and peroxy anions may exist, these are the
intermediates for the final formation of O2- anion. But each individual transformation may be
slow or fast which mainly depends upon the properties and experimental conditions of the
reaction.
This all describes the main theme of the chemisorption of oxygen molecule on the metal oxide
surface of the oxidation process [66, 67].
11
1.9.Kinetics of the Surface Catalyzed Oxidation Reactions
Five types of kinetic rate laws generally explain the heterogeneous surface catalyzed
oxidation reactions.
i. Mars and Van Krevelen Kinetics (MVK)
In 1954 Mars and Van Krevelen published their data for the catalytic oxidation of different
compounds namely anthracene, naphthalene, toluene and benzene using vanadia based catalysts.
The investigation was based on the effect of the oxygen partial pressure on the conversion of the
target materials using a fluidized bed reactor.
The proposed mechanism was that first of all the substrate molecule abstracts lattice oxygen
from the surface of the catalyst and hence the catalyst became reduced. However the lattice
oxygen is then re oxidized by the gas phase molecular oxygen. The mechanism can be written as
follows
HC+ βOL P +β rred = k red PR………… (1)
+1/2 O2 OL r ox = k ox Pno2 (1- )…… (2)
Where β represents stoichiometric constant
P represents the reaction product
OL corresponds to the lattice oxygen anion
Corresponds to the representive vacancy
And R represents the substrate which is to be oxidized.
At equilibrium the oxidation rate will be equal to the reduction rate hence
Rred = k ox/β……………. (3)
Βk red PR= k ox Pno2 (1- )…. (4)
=koxPno2/βkredPR+ k ox Pno2…. (5)
Substituting equation (5) in equation (1) we get
12
1/red=1/kredPR+ β/ k ox Pno2…….. (6)
ii. The Eley-Rideal Mechanism (ER)
According to ER mechanism a reaction occurs between an adsorbed species and a second species
without absorbing.
The gas phase reactant gets physisorbed on the catalyst surface for not less than 10-13sec. In this
very small time the oxygen the gas phase reactant reacts with the appropriately charged oxygen.
As the mechanism is derived from Langmuir-Hinshelwood mechanism according to which the
absorption sites are described to be energetically and chemically uniform. I.e. all the adsorption
sites are exactly the same. The absorbing species must be must be in equilibrium with the species
that gets adsorbs from the gas phase reactants, which means that the oxygen and reacting species
in gas phase adsorbing and desorbing must be in dynamic equilibrium with each other. Figure 1
represents the dissociative adsorption of oxygen on a uniform surface having an equilibrium
constant K. The friction of the surface covered by atomic oxygen given by LH adsorption
mechanism can be given as
O2 (g) 2O (ads) )7...(..........1 5.0
22
5.022
PoKo
PoKoo
And then reaction of oxygen species with the hydrocarbons can be given as
HC+O (ads) P )8...(..........roPkRate
The reaction rate enhances with the increase in the surface coverage by the atomic oxygen and
carbon mono oxide pressure. A more precise version of equation (8) can be examined in table 1.
The simplest mechanism associated with ER mechanism when a gas phase hydrocarbon absorbs
on the catalyst surface is that the reaction is following a first order dependence on the
hydrocarbon partial pressures; First order when the partial pressure of oxygen low pressures
which progressively change to zero order when high partial pressures of oxygen are used.
KO2
13
iii. The Langmuir-Hinshelwood Mechanism (LH)
According to this system a catalyzed reaction is composed for five steps and the slowest or the
rate determining step involves the reaction between two adsorbed species on a uniform catalyst
surface. The adsorption of the species can be brought about on a single type of surface sites
(Figure 2) or each absorbing molecule can be adsorbed on their own type of surface sites. Each
absorbed molecule will be in a thermodynamic equilibrium with the corresponding gas phase
species. According to LH mechanism the overall reaction can be described by the following
equations.
O2 (g) 2O (ads) )9...(..........1 5.0
22
5.022
PoKo
PoKoo
HC (g) HC (ads) )10...(..........1 RR
RRCHC PK
PK
HC (ads) +O (ads) P )11...(..........roPkRate
The kinetic expressions predicts the reaction orders in both reactants varying from 1st order to
zero order as the partial pressure of oxygen is increased gradually. When this type of kinetics is
observed with one kind of adsorption sites, a characteristic feature is the appearance of
maximum rate where the partial pressure of one of the reactant is increased gradually and that of
the second reactant is held at a constant. This adds to the phenomena of inhibition of one reactant
when a high pressure of the second reactant is applied.
slow
k
KR
KO2
14
Figure 1: Eley- Rideal mechanism for the reaction of an oxygen species adsorbed on a
uniform surface, with CO reacting in the gas phase.
15
Figure 2: Langmuir- Hinshelwood mechanism for the reaction of oxygen and
Hydrocarbon species adsorbed on a single type of uniform surface sites.
16
“Table 1: Summary of the Kinetics of Heterogeneous catalytic systems
Ser/
No
Mechanism Kinetic equation Linear form
1 Mars-Van
Krevelen
( Surface
oxidation
Reduction)
Rredn
ox
Rn
redox
PkPok
PPokkr
2
2 noxRred PokPkr
2
111
2 Eley-Rideal
(Reaction between
adsorbed O2 and
weakly adsorbed
R)
nnR
nn
PoKo
PPokKor
22
22
1 )
11(
11
22nn
r PoKokPr
3 Langmuir-
Hinshelwood
(Uniform surface
with one type of
sites)
222
22
)1( RRnn
Rn
Rn
PKPoKo
PPoKkKor
Liner transformation not possible
4 Langmuir-
Hinshelwood
(Uniform surface
with two type of
sites)
)1)(1( 22
22
RRnn
Rn
Rn
PKPoKo
PPoKkKor
)
1)(
1(
11
2
2
2 RRn
n
Rn P
KPo
KoKkKor
5 Power Law mR
nPkPor 2 Already straight line equation
ar is reaction rate; k is reaction rate constant; kox, kred are the rate constants of oxidation or reduction; n=1 or
0.5 for non dissociative and dissociative adsorption; R is the component to oxidize; Ko2 and KR are adsorption
coefficient of Ox or R: a is a stoichiometric coefficient.
17
iv. Power Rate Law: can also” be applied to catalyzed oxidation of organic compounds,
where two reactants are involved. The rates of heterogeneous catalytic reactions are complex,
because adsorption of the reacting species on the catalyst surface, reactions between the adsorbed
species and desorption of the products take place simultaneously. Essentially the catalytic
reactions can be associated with surface coverage concentrations. The surface concentration
effect upon the reaction rate can be eliminated by employing simple assumptions like quasi-
equilibria and quasi-steady state hypotheses. The overall rate law can be give as under:
The Power rate law can be applied to such reactions, which involves two reactants during the
process of catalytic oxidation. Heterogeneous reactions and their rates are very complex as the
reaction involves both adsorption and desorption at the same time, i.e. Adsorption of reactants
on the catalyst surface, then reaction of the adsorbed species to produce the products and finally
the desorption of the products from the catalyst surface-All these processes takes place
simultaneously- Generally the surface catalyzed reactions are described in the form of surface
saturations. The effect of catalyst surface saturation on the overall rate of the reaction can be
annihilated by applying simple assumptions like quasi-steady state hypotheses [68]. Therefore
the overall rate can be written as
])[(][
2 RpOKdt
Rdr (1.30)
If the pressure of oxygen is kept constant, equation (1.30) will be changed to equation (1.31)
][][
Rkdt
Rdr (1.31)
If we rearrange and then integrate equation (1.31) we get equation (1.32), i.e. first order rate
equation as.
18
tktR
Rr
O ][
][ln (1.32)
Where [R]o represents the initial molar concentration of the reactant and [R]t represents the molar
concentration of reactant R at any time t.
1.10. Oxidation of Alcohols
The selective oxidation of primary and secondary alcohols to its corresponding carbonyl
compounds is of immense importance for the production of industrially important fine chemicals
[69]. These carbonyl compounds are important fine chemicals used in perfumery, soap, pharma
and food processing industries. Traditionally inorganic stoichiometric reagents such as
chromium (IV) are used for the oxidation of alcohols. These oxidants are not only expansive, but
they also produce large amount of heavy metal waste [71]. Moreover, these reactions are
generally carried out in environmentally undesirable solvents such as halogenated hydrocarbons.
Concerning environmental as well as economical problems, the use of molecular oxygen as
source of oxidation in heterogeneous catalysis is developed as a preferred green procedure for
the selective oxidation of alcohols [72-75]. A number of transition metal catalysts are reported in
literature for the oxidation of alcohols, but palladium catalysts entertain particular attention
because of its high activity and selectivity [76-89]. Palladium immobilized on different supports,
e.g. Pd/Hydroxyapatite [76], Pd/Al2O3 [90-93], Pd/C [94] and Pd/MgO [95] have been
effectively used as catalysts for the selective oxidation of different alcohols using molecular
oxygen as oxidant.
The properties of the support material for the active species such as palladium influence
greatly the catalytic activity of the catalyst from different perspectives. On one hand, the support
may influence the physical characteristics of the catalyst, i.e. Particle size distribution on the
support as well as the electronic structure of the catalytically active material during the
interaction of active material and the support. On the other hand, the physio-chemical properties
of the support material will definitely influence the kinetics of catalytic reaction.
19
Lately zirconia has gained great interest as a catalyst and as its support due to its unique
properties. It has been tested in a number of reactions [96-100] due to its acid-base bi-functional
properties and its high thermal and mechanical properties. Due to these properties zirconia is
used on numerous occasions to improve the mechanical strength of classical alumina supports
[95].
In literature alcohol oxidation is carried out by the following methods;
i. Liquid phase catalytic oxidation of alcohols under solvent free conditions
ii. Liquid phase catalytic oxidation of alcohols using organic liquids as solvent
iii. Liquid phase catalytic oxidation of alcohols using water as a solvent
iv. Gas phase catalytic oxidation of alcohols
Liquid phase catalytic oxidation of alcohols towards aldehydes and ketones as products in
solvent free conditions is vastly reported in literature. Choudhary et al. [101] implemented gold
nano particles supported on different materials (U3O8, MgO, Al2O3, and ZrO2) for the oxidation
of benzyl alcohol towards benzaldehyde, he showed high conversion and selectivity towards
benzaldehyde and minimal amount of benzyl benzoate as a byproduct using 152 Kpa oxygen
pressure and 413 K temperature. He further reported that the oxidation of (Bz-OH) benzyl
alcohol catalyzed by Au/U3O8 was the most effective catalyst when the concentration of gold is
higher and the particle size is smaller. At higher temperature and longer reaction duration the
selectivity towards benzaldehyde was reduced
Chen et al.[102] reported 0.5 wt % Pd/MnOx as efficient catalyst for the solvent free
oxidation of benzyl alcohol showing 18 % conversion of the substrate and 99% selectivity
towards benzaldehyde, reaction conditions being as follows- Reaction temperature 160 oC,
oxygen pressure 1atm (20ml/min).
Enache et al. [103] used 2.5% Au–2.5% Pd/TiO2 for the selective oxidation of benzyl alcohol
to benzaldehyde, in 8 hours of reaction time they achieved 74% conversion and 91% selectivity
towards benzaldehyde using oxygen partial pressure equal to 0.1 M Pa and temperature of the
reaction was maintained at 363K.
20
The oxidation of alcohols is also reported vastly using different solvent systems; water is
regarded as best solvent for alcohol oxidation because it is greener and cleaner impact on the
environment. However, water as a solvent has been reported to enhance the oxidation of alcohols
towards carboxylic acids rather than aldehydes [70,104-107]. In order to refrain from this
problem dry conditions are necessary which can be achieved by using organic solvents. Toluene
is regularly used solvent for the liquid phase oxidation of alcohols [70, 75,104-106,108].
It has been reported that benzyl alcohol in waste water effluents is making a lot of
environmental problems, it has been estimated that waste water coming out of industries contains
benzyl alcohol in concentration range of 0.5-10 gm-L-1. Therefore, it’s very important to either
completely mineralize benzyl alcohol to water and carbon dioxide or convert it into other less
dangerous materials. Therefore considerable amount of effort has been put forward to eliminate
or convert this substrate either to carbon dioxide or aldehydes. In these circumstances catalytic
oxidation or catalytic degradation using a suitable heterogeneous catalyst seems to be a better
choice.
We have successfully explored in our work the selective oxidation of benzyl alcohol in liquid
phase in solvent and solvent free conditions. For solvent free oxidation of alcohols we have
successfully used 0.1% Pd/ZrO2 and used mechanochemically synthesized manganese oxide for
the oxidation of benzyl alcohol using n-heptane as solvent.
Benzyl alcohol, Glycerol and toluene were successfully oxidized to its corrissoponding
carbonyl compounds using different conditions of temperature and using oxygen and air as the
lone oxidant.
1.11. Oxidative Degradation of Methylene Blue Dye
Due to more rigorous environmental regulations and increasing public concern towards
harmful organic substances, there is a need to develop efficient technologies for the complete
removal of organic pollutants from wastewater effluents. Wastewater from different industries
generates a wide variety of contaminant in high concentration, including dyes, phenol and its
derivatives, hydrocarbons, sulphur, nitrogen and halogen containing organic compounds and
heavy metals [109,110]. Organic dyes, which are highly colored compound and resist chemical,
biochemical and photochemical degradation, are used by textile industries. Due to their
21
resistance to chemical, biochemical and photo degradation, these organic dyes have presented
significant environmental problems. These organic dyes are among the largest group of
pollutants released from different industries especially textile and paper industries These organic
dyes can be removed by adsorption on porous solids like activated carbon [111] and other
advanced oxidation process [112-115], but these techniques have the ability to produces toxic
metabolites which can affect the life of living organisms. Similarly it has been reported that azo
dyes are reduced readily in anaerobic environment and convert to potentially dangerous aromatic
amines [116]. The problem associated with removal of pollutants by adsorption is, how to treat
the saturated adsorbents. Similarly lack of selectivity is a problem associated with advance
oxidation processes. Therefore, wet oxidation is an alternative option. Consequently, appropriate
wet oxidation systems should be more selective towards target compounds.
The wet-air or thermal liquid-phase oxidation process is known to have a great potential for
the treatment of wastewater containing dyes. The efficient removal of pollutants via wet-air
oxidation process requires very high temperature and pressure, which leads to high installation
costs, and practical applications for these processes are limited. Therefore, the development of
catalytic wet oxidation (CWO) using various types of heterogeneous catalysts has been
attempted in order to reduce the severity of the oxidation conditions. The use of heterogeneous
catalysts makes the process more attractive by achieving high efficiency for oxidation of organic
wastes at considerably lower temperature and pressure [97,117-119]. A number of catalysts have
been reported for decolorizing MB in aqueous solution. For example cobalt oxide supported on
MgO has been used as a catalyst for the degradation of MB with Oxone as an oxidant [120].
Andre and co workers reported [121] recently the photo catalytic degradation/oxidation of MB
using Niobia as catalyst and hydrogen peroxide as the oxidant. They have reported that the
oxidative degradation of MB only occurs under the influence of UV. No decolorization has
occurred in the absence of UV [122]. Shijian et al. [123] explored the degradation of MB by
heterogeneous Fenton Reaction using titano magnetite as catalyst and H2O2 as oxidant. However,
the reaction was not very efficient as complete degradation of the dye was achieved in more than
45 hours at 30 ˚C (MB: 0.100 g/L, H2O2: 1.0 g/L).
22
Supported oxide of precious metal like Pt, Pd, Au, are very active towards the decolorization
of MB. However, in view of the economic factor the use of oxides of non-precious metals like
Co, Mn, Ni, etc. as catalysts have gained great attention [124-126].
In this work cobalt oxide is selected as a novel catalyst for oxidative
degradation/decolorization of MB in aqueous medium. Many synthetic routes can be applied for
the synthesis of cobalt oxide; however, the Mechano-chemical process in solid state is an
attractive one due to its simplicity. Mechano-chemical process is suitable for large-scale
synthesis [127].”
1.12. Catalytic Degradation of Congo-Red Dye:
The elimination of the non-biodegradable organic compounds is an important
environmental problem. Dyes are one of the important classes of these organic compounds that
are frequently used in the fabric industry. They are among common industrial pollutants which
need to be removed from the waste water.
Congo-Red dye one of those non biodegradable dyes which are not easy to remove from
the water streams. A number of conventional methods are used for the treatment of CR dye from
the fresh water streams which include coagulation techniques, adsorption technique and
advanced oxidation techniques but these techniques are generally very expansive or are not a
permanent resolve to the problem.
Therefore, we have taken a step to introduce a cheap and easy method of removal of this
dye from the waste water. In this study we have used cobalt oxide as heterogeneous catalyst in
presence of air/oxygen at room temperature and in the absence of any radiations for the complete
removal of this dye.
23
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30
Chapter 2
Literature Review
Organic substances catalyzed by the oxides of transition metal and metal oxides has been
practiced since long times. In literature transition metal oxides have been reported in both
supported and unsupported forms for the oxidation and degradation of organic compounds. The
oxidation of organic compounds has been reported vastly in both liquid and gas phase. There are
few reports in which metal oxides supported on different supports and in unsupported forms have
been used as catalyst. In the following pages a brief literature review related to oxidation of
organic compounds like benzyl alcohol, toluene and organic dyes catalyzed by metal and metal
oxides is presented here.
2.1. Oxidation of Benzyl Alcohol Catalyzed by Mn Catalyst
Benzyl alcohol oxidation is widely reported in literature using transition metal catalyst.
Precious metal catalysts in supported forms have generally been used. However, oxides of non
precious metals and in their metallic forms are also been discussed a lot. A few of them are
discussed below.
Su and co-workers [1] reported manganese oxide in different oxidation forms for the
catalytic oxidation of benzyl alcohol in absence of any solvent. He reported through different
experiments that among other oxides MnO2 was a better catalyst and showed good activity and
selectivity towards benzaldehyde i.e. 37.7% conversion and 99 % selectivity for benzaldehyde
(BzH) was achieved in 3 hours and 80oC and used microwave as an added energy source. The
oxidant used was molecular oxygen.
Makwana et al. [2] reported MnO-OMS (Octahedral molecular sieves) as heterogeneous
catalyst for the oxidation of benzyl alcohol using toluene as solvent and oxygen as oxidant. 100
% selectivity towards BzH with 97% conversion was achieved in 240 minutes. The MnO-OMS
was reported to be reusable after treating it with ethanol and calcinations at 300oC.
31
Yang et al. [3] reported Carbon Supported manganese oxide as catalyst for the selective
oxidation of benzyl alcohol temperature of the reaction was maintained at 80 ºC. The catalyst
was active and selective for the oxidation of benzyl alcohol to benzaldehyde. Furthermore, the
catalyst was easily filterable and recyclable for more than five times.
Makwana et al. [4] presented their work on the selective oxidation of benzyl alcohol using a
heterogeneous catalyst-Octahedral molecular sieve type manganese oxide using oxygen as an
oxidant. They applied their data to kinetic analysis and found that the Mars-van Krevelen (MVK)
model was found to be better fit. They had shown that two steps mechanism was involved in this
mechanism first involving exchange of gaseous oxygen and lattice oxygen of the catalyst. The
mechanism was further established by isotopic labeling technique of oxygen16 and oxygen 18.
Nair et al.[5] Used Co/Mn/Br– as catalyst under high aerial pressure (14-16 bars) and
temperature (100-120 oC) using water-dioxane solvent system. They were successful enough to
convert only 5% of benzyl alcohol to benzaldehyde in 180 minutes under these conditions.
Guo et al. [6] subjected Cu-Mn mixed oxides as heterogeneous catalyst for the oxidation of
benzyl alcohol using molecular oxygen at natural pH condition. Dichloromethane was used as
solvent, and the reaction temperature was maintained in range of 253-393 K in autoclave reactor.
Highest catalytic activity was observed when the mole ration of Cu to Mn was maintained at 1.
Benzyl alcohol oxidation was studied by Yang et al. [7] using manganese oxide catalyst
synthesized in the presence of 2, 2, 6, 6-tetramethyl-piperidyl-1-oxy (TEMPO). 23-26 %
conversion was achieved with 100% selectivity towards benzaldehyde in 360 minutes at 393K.
Tang. et al. [8] also reported the oxidation of benzyl alcohol using manganese oxide catalyst
stationed on active carbon as support.30% conversion of benzyl alcohol was achieved with 100%
selectivity towards benzaldehyde. The temperature of the reaction was maintained at 383 K. In a
typical reaction 2 milli moles of benzyl alcohol was taken in 10ml toluene and 200mg catalyst
was suspended in the reaction vessel. Molecular oxygen was used as oxidant. They described
through different characterization techniques that the manganese oxide was a mixture of Mn3O4
and MnO types of manganese oxide.
32
Yuanting et al. [9] reported Pd supported on MnCeOx as heterogeneous catalyst for the
aerobic oxidation of benzyl alcohol. The palladium nano clusters were well dispersed by simple
MARP technique successfully. The support material in mixed form, i.e. MnCeOx was found
remarkable in enhancing the activity of the catalyst in contrast when only MnOx or CeO2
supports alone. The highest TOF was found with Pd/7Mn3Ce-C with TOF=15,235 h-1. They
further explained that the catalytic activity was solely attributed to the palladium nano particles.
However, the enhanced activity of the catalyst was due to the synergic effect of Pd, MnOx and
CeO2. The catalyst was found to be reusable over five consecutive runs.
Yuanting and his group [10] reported manganese oxide and vanadium oxide catalysts
supported on activated carbon. 5 wt. % Mn/C and 4 wt. % V/C catalysts were prepared by wet
impregnation technique. The activated carbon was pre-oxidized before impregnating with the
active materials. The pre-treatment of the support material enhanced the catalytic activity of the
catalyst for the substrate. It was reported that due to pre-treatment the catalytic activity on Mn/C
increased from 31% to 67 % and for vandia catalyst it enhanced from 46% to 93 % in 3 hours of
reaction time. Oxygen was the lone oxidant used. In a typical run the initial concentration of
benzyl alcohol taken was 2.5 m.moles and temperature was maintained at 373 K and catalyst
weight was 0.2 grams.
Qinghu et al. [11] reported CuMn/Al2O3 catalyst as efficient catalyst for the aerobic
oxidation of benzyl alcohol to benzaldehyde. 91 % conversion of benzyl alcohol was achieved at
373K in 240 minutes. Mn1.5Cu1.5O4 microcrystalline phase was described to be the main reason
for the high catalytic activity of the catalyst. The calcinations temperature plays was found to
play a critical role towards the catalytic activity of the active catalyst. At higher calcination
temperatures the microcrystalline phase of Mn1.5Cu1.5O4 is transformed to Mn3O4 which results
in lower activity of the catalyst.
Roushown et al. [12] reported recently that Cu-Mn mixed oxide was very active and selective
towards the complete oxidation of benzyl alcohol to benzaldehyde at 373K. Oxygen being
employed as oxidant and the reaction was carried in toluene as solvent. They described that the
catalytic efficiency was dependent upon the calcination temperature. The optimum calcination
temperature for the catalyst to perform at its peak was 573K. The high catalytic performance of
33
the catalyst was attributed to the active MnO2 phase of manganese oxide rather than the inactive
phase -Mn2O3.
Cun et al. [13] reported the selective oxidation of benzyl alcohol using gold nano rods
supported on β-manganese oxide as heterogeneous catalyst. Molecular oxygen was used as
oxidant under solvent free conditions. 40 % conversion and 99% selectivity towards
benzaldehyde was achieved in 5 hours at 120oC and oxygen partial pressure was 0.3 MPa. Initial
concentration of benzyl alcohol was 200m.moles.
Choudhary et al. [14] prepared Au/MgO catalyst by depositing nano sized gold particles by
using homogeneous deposition precipitation technique. The catalyst was found to be highly
active towards the oxidation of benzyl alcohol and showed good selectivity toward benzaldehyde
formation. Nearly 100% conversion was achieved with 65% selectivity towards benzaldehyde
was achieved. The Au loading and the calcination temperature highly influenced the catalyst
performance.
Dong et al [15], effectively used manganese oxide supported on kieselguhr for the catalytic
oxidation of benzyl alcohol to benzaldehyde. Typically 2.4 grams of catalyst and 25ml
dichloromethane were placed in a flask and were stirred mechanically. To it 1 m.moles benzyl
alcohol in 5ml dichloromethane was added and refluxed it for 10 hours. Afterwards the solids
were filtered off and thoroughly washed with dichloromethane. The filtrate was evaporated to get
crude product which was purified by preparative TLC technique. This suggested that 90% of the
product was benzaldehyde.
Dong et al [16], synthesized manganese dioxide supported on graphite support. The same
procedure was applied as in reference no. (13) for the oxidation, identification and quantification
of the products. Benzaldehyde yield of >92 % was achieved in 10 hours.
2.2. Oxidation of Benzyl Alcohol by Palladium catalyst
Oxidation of benzyl alcohol using palladium catalysts is also reported widely in literature.
Benzyl alcohol oxidation by palladium catalysts is reported in solvent and solvent free
conditions. Palladium catalysts are versatile in the fact that this catalyst shows some very good
34
interaction with alcohols and shows excellent results towards oxidation of alcohols towards its
selective products. Some of the literature is cited in the below sentences.
Wang et al [17]. reported recently synthesis of monometallic Pd and Rh catalyst supported on
N-doped mesoporous carbon (NMC), mesoporous carbon (MC) pristine and nitric acid treated
activated carbon (AC and ACN) catalysts. These catalysts were applied to the oxidation of
benzyl alcohol. Surface acidic and basic characteristics played a vital role in the catalyst
efficiency towards the oxidation reaction. Typically 2.5 gm of benzyl alcohol was taken in a two
necked flask and 20 mg of catalyst was added to it with 900 rpm stirring rate. The reaction
temperature was 160oC.Oxygen was bubbled into the reactor at a rate of 30 ml/min. Pd/NMC
and Rh/NMC showed better performance with 65% and 48% conversion respectively and more
than 90 % selectivity towards benzaldehyde was shown by both the catalysts.
Babak et al. [18] studied benzyl alcohol oxidation of recently using palladium nano particle
stabilized on meso-porous channels of SBA-15 in aerobic environment. The catalyst showed
99% yield in both oxygen and air environment. Benzaldehyde selectivity was found to be 83%,
and the catalyst was active for more than twelve cycles with total TON=3000. The catalyst was
also found to leach less in the reaction mixture and it was confirmed by the standard method of
hot filtration and by inductively coupled plasma technique.
Pd-Ag supported on pumice catalyst has also been reported by Liotta et al [19] for the
selective oxidation of benzyl alcohol to benzaldehyde. Kinetics of the reaction was performed at
333K using an autoclave reactor, at 2 bar oxygen pressure. Typically the oxidation reactions
were performed under flowing conditions of oxygen (160ml/min) at atmospheric pressure, at 75 oC, in a Pyrex glass reactor equipped with a magnetic stirrer. The reaction was vigorously stirred
at 1200rpm speed. The catalyst showed 100% selectivity towards benzaldehyde formation.
Liquid phase oxidation of benzyl alcohol to benzaldehyde using oxygen as oxidant by
modified palladium has been reported by Stuchinskaya and co workers [20]. Pd (II)-MO (MO=
CO (III), Fe (III), Mn (III) and Cu (II)) were used for the selective oxidation of benzyl alcohol at
373K. The addition of these cations to the palladium species greatly improved the catalytic
activity of the catalyst. Cobalt and Iron were found to be the most effective promoters. Cobalt
promoted palladium used for benzyl alcohol oxidation in toluene as solvent showed 53-95%
35
conversion and 85-100% selectivity towards aldehydes in a very short time of 15-60 minutes.
The catalyst can be reused for several times without the loss of activity or selectivity.
Miedziak group [21] reported supported gold palladium nano-particles for the selective
oxidation of benzyl alcohol in solvent free conditions under oxygen atmosphere. 1 % (Au-
Pd)/TiO2 showed the best performance with 29% conversion and 96% selectivity to
benzaldehyde. Reaction conditions employed were as follows: Benzyl alcohol to metal ratio was
equal to 58580. Temperature was 273K, pressure of oxygen was 10 bar, reaction time = 4hour,
stirring rate=1500rpm.
Dierk et al. [22] examined the catalytic activity of alumina supported palladium for the
oxidation of benzyl alcohol in critical carbon dioxide. The reaction mixture was purged by di-
oxygen 150 bar pressure. It was found through different characterization techniques that
palladium was mainly in metallic zero state. Typically benzyl alcohol oxidation was performed
in super critical carbon dioxide, using an in situ spectroscopic cell. It was found that the
oxidation of benzyl alcohol in super critical carbon dioxide showed better performance in either
liquid phase or gas phase oxidation. The reaction was found to go through a maximum when the
oxygen to alcohol ratio was increased, but if the oxygen concentration is increased from an
optimum value the reaction rate is decreased because the noble metal catalyst is in oxidized state.
Caravati et al. [23] reported the oxidation of benzyl alcohol to benzaldehyde with molecular
oxygen in “supercritical” carbon dioxide over 0.5% Pd/Al2O3 in a continuous fixed bed reactor.
The reaction conditions were optimized at 80oC and 150 bar oxygen pressure which resulted in a
high TOF (1585/hour) with a better selectivity to benzaldehyde i.e. 95 %. The study showed that
the catalyst activity was highly dependent on the concentration of oxygen and temperature.
Enache et al. [24] employed gold-palladium biocatalyst supported on titania support for the
oxidation of benzyl alcohol in solvent free conditions. Molecular oxygen was employed as
oxidant. A range of Au-Pd ratios were tested for the conversion and selectivity of benzyl alcohol
oxidation. All the catalysts were also compared over a temperature range, i.e. 100-160 oC. 2.5%
Au-2.5%Pd/TiO2 was found to be the best catalyst in terms of conversion (1.9%) while Au/TiO2
was the most selective to benzaldehyde formation( 100%).Reaction conditions: 100 oC, 0.1 g
catalyst, 40 ml benzyl alcohol, PO2 10 bar, Reaction time 30-60 minutes.
36
2.3. Oxidation of Toluene by Palladium Catalysts
The liquid phase selective oxidation of toluene to corresponding alcohol, aldehydes and ketones
acid using molecular oxygen is very important industrial application for the synthesis of many
fine chemicals. Toluene oxidation towards the production of benzoic acid is an important step for
the synthesis of phenol in the well known Dow-phenol process [25-29] and for the synthesis of ε-
caprolactum in the Snia-Viscosa process [30-32]. Toluene is also categorized among highly toxic
aromatic hydrocarbons by world health organization. Toluene oxidation to aldehydes, ketones
and carboxylic acids has attracted a good majority of scientists; some of the literature is cited as
under.
Rainone et al. [33] synthesized vanadia catalyst in the form of woven fibers by sol-gel as
well as by incipient wetness impregnation technique. The catalysts were subjected to
characterization techniques which show that the elemental fibers comprised of vanadia/titania
covered by silica core. When silica fiber material was replaced by fibrous alumina, the dispersion
of active vanadia/titania was increased dramatically. It was further found that the crystalline state
was not important factor for forming the active vanadia surface, rather the dehydration of Titania
(TiO2) layer without calcination lead to enhanced vanadia dispersion upon the support material.
The catalyst demonstrated excellent selectivity and activity for the production of benzaldehyde
and benzoic acid with the spherical/granulated texture with the same composition. The
conversion and selectivity were measured at 553K, 2 wt % toluene, 40 % v/v oxygen/argon,
catalyst loads corresponded to 9m2. Maximum 15 % conversion and 80% selectivity towards C7
compounds was achieved.
Ilyas et al. [28] investigated the liquid phase catalytic oxidation of toluene towards
benzoic acid employing platinum supported on zirconia as heterogeneous catalyst in solvent free
conditions. The oxidant used for oxidation of substrate molecule was Di-oxygen. The catalyst
(Pt/ZrO2) was found to be very active and selective towards the oxidative transformation of
toluene to benzoic acid. When the oxidation reaction was conducted for longer period time other
products i.e. benzyl benzoate, t-stilbene and meth-biphenyl carboxylic acid were detected.
Owing to reaction conditions at 363 K, 37 % conversion with 71% selectivity towards benzoic
acid was achieved in 3 hours and 0.2 grams of 1 % Pt/ZrO2 was used.
37
Okumura et al. [34] reported palladium supported on different supports (MgO, Al2O3,
SiO2, SnO2, Nb2O5, and WO3) for the catalytic combustion of toluene. Zirconia and silica
supports were the better of all showing complete combustion between 500-600 oC. Palladium
loaded on acidic and basic supports i.e. Tungsten oxide and Magnesium oxide were inactive in
the reaction as compared to the weakly acidic or weakly basic supports such as alumina, silica,
tin oxide and niobium oxide. The facile interaction Pd on these supports was the main reason
responsible for the extraordinary activity for the complete oxidation/degradation of toluene.
Li et al [35] investigated the catalytic behavior of Cu-Mn mixed oxide catalyst for the
selective oxidation of toluene, in liquid phase solvent free conditions. The oxidation reaction was
carried out in an auto stirred autoclave reactor. In a typical reaction 50ml toluene along with
100mgs of catalyst was subjected to the reaction vessel, pressure was adjusted to 1 M.Pa and
temperature was set to 463K. In 3 hours 17.2 % of substrate was converted and the following
selectivities were observed: benzyl alcohol 18.1 %, Benzaldehyde 11.7%, Benzoic acid 63.1%,
bezylbenzoate 3.1%, others 3.5%.
Li et al [32] demonstrated the catalytic activity of manganese oxides for the solvent free
oxidation of toluene using molecular oxygen as the sole oxidant. Four types of manganese oxides
namely MnO2, Mn2O3, Mn3O4 and MnO were synthesized by different techniques. All the
catalysts were effective for the selective oxidation of toluene but Mn3O4 was found to be the
most effective and recyclable. At 463K and 1M.Pa of oxygen pressure, 50ml substrate and 1
gram of catalyst 39% conversion with 93% selectivity towards benzoic acid was reported using
Mn3O4 as catalyst.
Wang et al. [36] reported the synthesis of manganese supported hexagonal mesoporous
silica and then modified the catalyst by immobilization of phenyl group on the catalyst surface.
Due to this modification the surface of the catalyst became more hydrophobic. This catalyst was
tasted for the oxidation of toluene in oxygen environment. Catalytic reactions were performed in
an autoclave reactor at 463K, 1MPa oxygen pressure, 15gram substrate and 0.1 gram catalyst
was loaded to the reactor. After 180 minutes 12.6 % conversion was achieved with 67%
selectivity towards benzoic acid.
38
Kesavan et al. [37] recently reported the selective solvent free oxidation of toluene by
Gold-Palladium alloy nano particle supported on carbon and titania support, under oxygen
atmosphere. Both the catalysts were found to be very active and selective towards the oxidation
of toluene to benzyl benzoate. At 433K and 10 bar oxygen pressure 50.8% of the substrate was
converted and 94% selectivity towards benzyl benzoate was achieved similarly under the same
conditions when Titania was used as support 24% conversion and 96% selectivity to benzyl
benzoate was achieved.
2.4. Catalytic Degradation of Methylene Blue Dye
Among organic dyes Methylene blue dye is one of the major water contaminant which
needs to be taken care. These organic dyes are dangerous because of the potentially carcinogenic
properties of these chemicals. Methylene Blue (MB) is a deeply colored chemical which is used
in coloring and printing industries and is a common waste water pollutant. A lot of literature is
cited for the catalytic and mostly photo-catalytic degradation of Methylene blue dye. Some of the
literature is cited in the coming lines.
Stoyanova et al. [38] recently investigated the oxidative degradation of Methylene blue
dye in aqueous medium using oxides of nickel and cobalt. The oxidative catalytic oxidation of
Methylene blue dye in aqueous medium using NaOCl as oxygen supplier was studied using
individual oxides of cobalt and nickel as well as in supported form upon iron oxide. The results
show that Methylene blue could be easily degraded using the oxides of cobalt and nickel at room
temperature. The cobalt oxide system was found to be the best catalytic system for the
degradation of Methylene blue dye. The degradation process was followed by first order kinetics.
Rauf et al. [39] reported 10 % Cr/Ti photo catalyst synthesized by sol gel method and
was used for the degradation of Methylene blue dye in the presence of UV- irradiation. The
catalyst was found to be active for the degradation of the dye. They were able to destroy 70 % of
the dye using this catalyst. Second order kinetics was followed by the degradation of the dye
molecule. The degradation products were analyzed by LC-UV/MS. It was found that the dye
undergoes the breaking of methyl groups which result in the formation of some intermediate
compounds. Most of the intermediate compounds were confirmed by a tandem mass
spectrometric technique.
39
Talebian and co. [40] compared the catalytic activity of SnO2, TiO2, ZnO, and In2O3 for
the degradation of Methylene blue dye. They observed that SnO2 was the best photo catalyst for
the degradation of Methylene blue, while indium oxide was found to be the least active.
Similarly at different pH conditions the same trend was observed. The catalytic performances
were attributed to micro structures of the applied catalysts.
Xiao and coworkers [41] used carbon doped-TiO2 for the degradation of Methylene blue
dye. They observed that when the calcination temperature was increased from 500oC-800oC the
surface area of the catalyst was decreased from 20 to15 m2/g. Due to this the active sites will not
be easily accessible by the reacting molecules. The decolorization of Methylene blue was
decreased from 49% to 39 % under the employed conditions and increasing the calcination
temperature. Hydroxyl radical production was found to closely related to the photo catalytic
activity. 600 oC was found to be an optimal degradation temperature for the decolorization and
production of OH radicals.
Lim et al. [42] also reported C doped Titania for the decolorization of Methylene blue
(MB) dye. They reported that with the increase in calcination temperature from 300 oC on wards
the BET surface area was decreased from 200 m2/g (200 oC) to 80 m2/g (400 oC). The decrease
in surface area with calcination temperature increased the crystallization and growth anatase.
When the calcination temperature was increased to 700 oC the BET surface area was less than 1
m2/g. with the increase in surface area the degradation of Methylene blue was observed to be
decreased.
Baiju et al. [43] also indicated the same trend when Ta2O5 was used as dopent for Titania.
It was observed that when the doping of Ta2O5 was increased from 1mol% to 10mol% the
crystallite size of Titania gradually decreased at a given temperature. It was also observed that all
those samples which were doped with Ta2O5 shown better activity than pure Titania. The
addition of Ta2O5 increased the stability of the anatase phase of Titania. The optimum calcination
temperature for the degradation of MB was found to be 500 oC.
Zhan et al. [44] reported the synthesis of various shapes and sizes of Mn3O4, and
successfully employed for the oxidative degradation of Methylene blue dye. To control the shape
and sizes of manganese oxide, the growth times, temperature of the reaction, the use of surfactant
40
and manganese source were varied. Octahedral geometries were obtained when CTAB and PVP
mixture were used, however when PVP or PEO and PPO were used as templates larges
agglomerates of manganese oxide were produced with the loss of octahedral geometries with the
production of crystallites having a quasi-spherical shapes. The nano scale octahedral manganese
oxide catalyst was found to be more effective than the others which showed 99.7 % removal of
the dye at reaction temperature of 80 oC and H2O2 as an oxidant.
Kuan et al. [45] recently reported the oxidative degradation of Methylene blue dye using
manganese oxide and micro waves as energy source. The study was performed to compare
manganese oxide (MnO2) in the presence of microwave (MW) and conventional heating (CH).
The degradation process and kinetics were investigated for sole MnO2, MnO2-CH and MnO2-
MW. It was found that all the three tested condition followed third order kinetic models.
However fast reaction conditions were observed when MW enhanced process was employed.
The results showed that 100 % degradation can be achieved under MW irradiation with a total
organic carbon removal of 92% at neutral pH in 10 minutes.
2.5. Catalytic Degradation of Congo-Red Dye
As like Methylene blue dye Congo-Red is also used vastly for dying purposed in textile
and paper industries. It is estimated that 25 % of the total dyes industry is drained into the fresh
water streams which produces a lot of problems to the living environments including plants and
animals. Some literature is cited here to discuss the degradation of Congo-Red dye.
Herrera [46] investigated the effectiveness of sonophotocatalytic system using nano scale
rutile and anatase phases of Titania with the Titania synthesized using an ultrasound system for
the degradation of Methylene blue dye. Under the sonocatalytic conditions efficiency of anatase
Titania was found to be superior to rutile Titania for the degradation of Congo-Red dye.
Furthermore the kinetic constant of the CR dye oxidation was found to be > 2 times more than
that obtained in the absence of ultrasound and UV light and P25 Titania.
Pouretedal et al. [47] investigated Congo-Red oxidative degradation using a cobalt doped
zinc sulphide nano particles. The size of the nano particles were determined to be in the range of
10-40 nm. 94 % dye was degraded using UV irradiation in 120 minutes. 98% degradation could
be achieved under sunlight in 12 hours. Pseudo first order kinetics was observed for both the
41
reactions i.e. under UV and sunlight reaction conditions. The rate constant for UV irradiated was
found to be 2.2 x 10 -2 per minute and 2.9x10-2 per minute for the reaction in the presence of
sunlight.
42
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45
Chapter No.3
Experimental
In this part the synthesis and characterization of different catalysts used in this work will be discussed in detail.
3.1. Materials and Chemicals
The following materials and chemicals –as illustrated in table 3.1-were used as supplied without further purification.
3.2. Synthesis of Catalysts
The following catalysts were synthesized and were tested for their catalytic activity
towards the oxidation and degradation of organic compounds.
1. Manganese oxide
2. Cobalt oxide
3. Palladium Supported zirconia.
3.2.1. Synthesis of Manganese oxide catalyst
Solid state chemical method was employed for the facile synthesis of manganese oxide at
room temperature.
Manganese oxide and potassium permanganate in solid state were mixed together in a fixed
molar ratio (2:3).The two solids were thoroughly mixed and grinded together using a mortar and
piston regularly for 30 minutes. After complete reaction, the brownish black solid of manganese
oxide was washed with plenty of water to remove any traces of the precursor material.
After complete washing, the paste was dehydrated at 110oC in an oven at 110oC. The catalyst
was kept at this for ~24 hours and afterwards the catalyst was subjected to calcination at 300oC
at a constant temperature rate of 5oC/ minute. The solid catalyst was kept at this temperature for
four hours. The catalyst was stored in an air tight glass jar and was used for further studies.
46
TABLE 3.1
Serial
Number
Materials
Purity/ Grade Supplier and Origin
1 Benzyl alcohol Synthesis grade Scharlau, Spain
2 Benzaldehyde Synthesis grade Scharlau, Spain
3 Benzoic acid Synthesis grade Scharlau, Spain
4 n-heptane 99 %+ Acros, USA
5 Sodium hydroxide Laboratory grade Merck, Germany
6 Ammonium bicarbonate Laboratory grade Merck , Germany
7 Manganese chloride 99.9 % Merck, Germany
8 Cobalt nitrate 99% Acros, USA
9 Palladium Chloride 99.9% Alfa Acer, UK
10 Zirconyl Chloride 98% min Acros organics
11 Ammonia Solution 32%, Reagent Grade Scharlau, Spain
12 Molecular Oxygen 99% BOC, Pakistan
13 Molecular Nitrogen 99% BOC, Pakistan
14 Oxygen Traps Fully activated C.R.S.Inc.202223
15 Moisture Traps Fully activated C.R.S.Inc.202268
16 Toluene 99.8% Sigma Aldrich
17 n-octane 99% Acros Organics
18 Methylene Blue 90% Sigma Aldrich
19 Congo red ------------------
20 Calcium hydroxide ------------------
21 Methyl benzoate Scharlau, Spain
47
3.2.2. Synthesis of Cobalt Oxide
Cobalt oxide was synthesized by solid state mechano-chemical mixing and milling of
Cobalt nitrate and ammonium carbonate using a mechanical mixer in a fixed molar ratio
(2:5).The mixing continued until the color of the mixture became homogeneous. The resultant
powder was dried at 273K and then washed with plenty of water to remove any residual material.
After wards the catalyst was dried and claimed using a programmable furnace at an ascending
black powder was assigned as cobalt oxide. The powder was grinded and stored for further
studies.
3.2.3. Synthesis of Zirconia
Monoclinic zirconia was synthesized using a dilute solution of zirconyl chloride by slowly
adding 35% ammonia solution with strong stirring at room temperature. The pH of the solution
was monitored with a pH meter. Throughout the addition of ammonia solution white and thick
precipitate of zirconium hydroxide was formed. Approximately at pH~10 the white precipitate
was zirconium hydroxide was collected and washed with plenty of water in a bucnal funnel, until
the chlorine test with silver nitrate solution became negative.
A drying oven was employed for the dehydration of the catalyst at 110 oC overnight. The
dried precipitate was then calcined with a programmable furnace at 950oC and kept at that
temperature for 240 minutes to get monoclinic zirconia. The solid was then grinded to fine
powder.
3.2.4. Synthesis of Palladium Supported Zirconia
Incipient wetness impregnation technique was employed for the synthesis of 0.1 wt % Pd
supported on monoclinic zirconia powder. Typically a known amount of palladium chloride
aqueous solution was added monoclinic zirconia powder so as to make 0.1 wt% Pt/ZrO2. Small
amount of TDW (triply distilled water) was added to make a paste. The paste was thoroughly
mixed and then dried it in a programmable oven at 383 K. After wards it was again calcined at
950oC and kept at that temperature for 240 minutes.
The prepared catalyst was then activated in hydrogen flow at 280 oC using an automatically
controlled tube furnace. The activated catalyst was kept in flow of hydrogen for 2 hours.
48
3.3. Characterization of Catalyst
Catalyst characterization plays a vital role in the field of heterogeneous catalysis and surface
science. Characterization of the catalysts helps us correlating the structural changes with the
catalytic activity and selectivity towards our desired and undesired products i.e. if we are able to
control certain characteristics of our catalyst we will be able to get our desired products and
selectivities.
The following techniques have been used to correlate the structural changes of the catalyst
with the catalytic activity and selectivity towards certain products.
1. Surface area and pore size analysis by nitrogen adsorption
2. Grain size distribution
3. Powder X-ray diffraction (XRD)
4. Drift Spectra (FTIR)
5. Scanning electron microscopy (SEM)
3.3.1. Surface Area Investigation
The pore size and surface area of the prepared catalyst was determined using N2
adsorption at -195.6oC. Quanta Chrome Nova 1200e, USA instrument was used for the analysis
of surface area and pore size distribution. Before adsorption the catalyst under examination was
degassed at 423 K for a period of 120 minutes, in molecular nitrogen. The BET (Brumer-Emmit-
Tailor) surface area of the catalyst samples was determined by nitrogen adsorption in the range
of P/P 0.05 – 0.15. The pore size distribution was calculated by desorption portion of nitrogen
by BJH method. The constant cross-sectional area of the nitrogen was 16.2 oA2/mol.
3.3.2. Grain Size Distribution Analysis
Analysette 22 Compact, Fritsch, Germany, particle size analyzer was used for the
determination of particle size distribution of the prepared catalyst. Wet method of analysis was
used for analyses.
49
3.3.3. X-ray Diffractometery (XRD)
The XRD patterns of the as prepared catalyst were recorded using JEOL (JDX-3532)
Japan, using Cu-K radiation at a tube voltage of 40 KV and 20 mA current. The patterns were
recorded over a range of 2 angles from 0 to 70. X-Ray diffractograms for all samples were
recorded under same scales and conditions.
Scherer’s equation was used for the determination of the crystallite size of the synthesized
catalyst.
CosD
9.0 (3.2)
Where
D represents the average crystallite size
is1.54 ºA (Wave length of X-Rays)
β represents peak broadening in radian
is angle at which the peak under study is taken in to consideration.
3.3.4. Fourier Transform Infrared Spectroscopy (FTIR)
IR-Prestige 21, Shimadzu, Japan instrument was used to record the DRIFT spectra of all
the catalysts before and after reaction indifferent environments. The IR spectra were recorded in
KBr medium in the range of 400-4500 cm-1.
3.3.5. Scanning Electron Microscopic (SEM) Analysis
JEOL JSM-5910, Japan, Japan was used to record the micro images of the used and fresh
catalyst. The catalyst samples were stabilized over sample stubs and then coated with a thin layer
of gold. An automatic software programmed computer system was used to analyze the samples
at desired resolutions.
50
3.4. Oxidation Reaction
Oxidation reactions were conducted in a flat bottom three necked flask as batch reactor.
The volume capacity of the reactor was 50ml. The reactor was fitted with a reflux condenser and
a digital temperature controller for accurate measurement of the reaction temperature. Reaction
temperature was maintained using a hot plate (or oil bath stationed on a hot plate). Typically
predetermined quantity of the substrate material was taken in the glass reactor and the reactor
was saturated with molecular oxygen by passing it through the reaction mixture at a flow rate of
40 or 60 ml/min. The flow of the gases was maintained at a regular flow using the conventional
bubble flow meter. After the attainment of the desired temperature and gas flows the reactor was
injected with the required amount of catalyst. At regular intervals of time, small aliquots from
the reaction mixture were drawn from the reactor and the catalyst was separated from it and the
reaction products were analyzed using UV-Visible spectrophotometer and Gas Chromatography.
To carry out the reaction at different partial pressures of oxygen nitrogen was mixed with
molecular oxygen in different ratios. Following formula was used to calculate the partial pressure
of oxygen.
kPaFF
Fp
NO
OO 2.101
22
2
2
(3.3)
Where pO2 represents the partial pressure of oxygen and FO2 and FN2 represents the
oxygen flow rate and nitrogen flow respectively.
3.5. Analysis of Reaction Mixture
The reaction mixture was analyzed using either Gas Chromatograph and/or UV-Visible
spectrophotometer
GC-FID was effective instrument for the analysis of solvent free oxidation reactions and for the
reactions carried out in organic solvents such as n-heptane and toluene. Clarus 500 Gas
51
Chromatograph (Perkin Elmer, USA) equipped with FID was used for the analysis of reaction
mixture. Excellent separation of the components of reaction mixture was achieved under the
conditions of GC presented in table 3.1
UV-Visible double beam spectrophotometer (UV-160 A, Shimadzu, Japan) was used for
the analysis of the reaction mixture processed in aqueous medium. Silica quartz Cuvettes of 1 cm
path length were used for the determination of absorbance of various component of the reaction
mixture. Previously prepared calibration plots were used for the determination of concentration
of various components of the reaction mixture.
The degradation of the organic dyes was analyzed using UV-Visible spectrophotometer
(UV-160 A, Shimadzu, Japan). Silica Quartz Cuvettes of 1 cm path length were used for the
absorbance of the organic dyes at regular intervals of time. Calibration curves were prepared
using standard concentrations.
52
Figure 3.2 Experimental Setup (A) Oxygen or Nitrogen Supply (B) Trap for moisture and impurities (C) Saturator (D) Three necked Pyrex Glass Reactor (E) Reaction mixture (F) Thermometer (G) stirrer bar (H) Magnetic Stirrer/Hot plate and Oil Bath (I) Condenser (J) Soap bubble meter
Figure 3.1: Three necked Batch reactor
53
Table 3.1. Operating Conditions of GC.
Injector
Sample volume
Temperature
3 µL
483 K
Detector
Detector
Temperature
Air
Hydrogen
Auto zero
FID
453 K
450 mL/min
45 mL/min
On
Oven
Temperature
Ramping
473 K
373 K for zero min, 10 °C/min to 200 °C, hold for 10 minutes.
Channel Parameter
Column
Sampling rate
Run time
Attenuation
Offset
Carrier gas
Split flow
Capillary, Elite-5, 30 m, 0.25 mmID, Cat. No. N9316076
12.5 pts/sec
20 minutes
-3.0
5.0 mV
Nitrogen, 14 psi
20 mL/min
54
References
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55
56
Chapter 4A
Results and Discussion
Reactant: Benzyl Alcohol
Catalyst: 0.1% Pd/ ZrO2
Oxidation of benzyl alcohol in solvent free conditions by palladium supported zirconia
catalyst:
4A.1. Characterization of the Catalyst:
Catalyst characterization is of a significant importance in the field of heterogeneous
catalysis. It not only helps us to determine the structural and compositional features of the
active sites of the catalyst but it’s also helpful in giving a deep knowledge of the reaction rates,
activity and selectivity of certain chemical reactions.
4A.2. Nitrogen Adsorption
The nitrogen adsorption studies for the as synthesized catalyst were measured at -196 oC
using a still volumetric machine (Quanta-Chrome Nova 1200e surface area and pore size
analyzer). Prior to adsorption, the catalyst sample were freed from any impurities by degassing
the samples at 200 oC for 120 minutes under evacuated environment. The adsorption desorption
isothermal plots of the prepared catalyst are shown in figure 1. According to classification of
IUPAC, the isotherms corresponds to type II, the hysteresis loop is of type H3. This type
hysteresis loop is characteristic of such solid surfaces which consists of irregular shaped particles
with no or very small pores [40].The specific surface area was calculated using Brunauer-
Emmett-Teller (BET) method [1] which was found to be equal to 11.0 m2g-1 and 12.4 m2g-1 for
zirconia (and 0.1 wt %Palladium/Zirconia (Pd-ZrO2) respectively. Barrett Joyner Halenda (BJH)
method was for the determination of the pore size distribution. The desorption branch of the
isotherm was used to find the pore size distribution of the solid catalyst [2]. The adsorption
desorption isotherm and pore size distribution of Pd/ZrO2 are demonstrated in figure 1 and 2
respectively.
57
0.0 0.2 0.4 0.6 0.8 1.0
2
4
6
8
10
12
14
16
18
20
0.05 0.10 0.15 0.20 0.25 0.3010
20
30
40
50
60
70
80
90
1/[
W((
Po
/P)-
1)]
Relative Pressure (P/Po)
y = 278.35x+ 2.72R² = 0.99
Vol
ume
(cm
3 /g)
Relative pressure ( P/Po)
Adsorption Desorption
(a)-
(b)
Figure 1 (a) Nitrogen adsorption desorption Isotherm (b) BET surface area analysis.
10 20 30 40 50 60 70 80 90
0.000
0.005
0.010
0.015
0.020
0.025
Po
re V
olu
me
(c
m3/g
)
Pore Radius (A)
Figure 2 Pore size distribution of Pd/ZrO2
58
4A.3. X-Ray Diffractometery
The X-ray diffraction patterns reported in figure 3 , shows clearly that zirconia is present
mainly in the monoclinic phase. No tetragonal peaks were found in the zirconia structure. The
main peaks appearing at 2θ=28.18 and 31.38 degrees represents the monoclinic phase of zirconia
[3-5], the absence of peaks at 2θ = 30.94o indicates that there is no tetragonal phase and zirconia
is purely present in monoclinic phase. The peaks for palladium should occur at F2θ = 40.4° and
46.9°, but these peaks seems to be suppressed, may be because of the very low loading of
palladium on monoclinic zirconia. Furthermore there is no difference between fresh (a) and the
used catalyst, which suggest that the composition of catalyst is retained in both oxygen (b) and
nitrogen (c) atmosphere.
4A.4. FT-IR Analysis of Palladium/ Zirconia Catalyst
The DRIFT spectra of Pd/ZrO2 are demonstrated in figure 4. [(a) Fresh, (b) used in
oxygen atmosphere and (c) used in nitrogen atmosphere].The peaks at 3380 cm-1 and 1565 cm-1
corresponds to the vibrational stretching and twisting of the Oxygen-Hydrogen covalent bond.
It may be due to the presence of water molecules in the crystal lattice of the catalyst. The IR
spectra contain another absorption band at 466 cm-1. This band corresponds to the vibrational
stretching of the Zr-O bond in ZrO2 [6]. Furthermore, there seems no significant difference
between the used and unused catalyst, which may suggest that all the substrate and the products
are well removed from the surface of the catalyst.
4A.5. Scanning Microscope Analysis
The SEM images are shown in figure 5 for (a) Fresh, (b) Used and nitrogen used catalysts. The
images reveal that the catalyst particles are bunched together, moreover there seems no
difference between the morphologies of the fresh and used catalysts which suggests that the
morphology of the catalyst remains constant during and after the course of reaction. It can be
also seen from the micrograph that the palladium particles are evenly dispersed on the support
surface i.e. zirconia.
59
10 15 20 25 30 35 40 45 50 55 60 65
Angle (2)
a-Fresh
Inte
nsi
ty (
CP
S)
b-Used O2
C-Used N2 m
mm = monoclinic phase
Figure 3: XRDs of (a) Fresh, (b) Used in oxygen, (c) Used in nitrogen atmosphere
60
4000 3600 3200 2800 2400 2000 1600 1200 800 400
5
10
15
20
25
30
35
40
45
% T
ran
sm
itte
nce
wave number)cm-1
(a)
(b)
(c)
Figure 4: DRIFT Spectra of (a)Fresh , (b)Used in oxygen atmosphere for benzyl alcohol
oxidation and (c)Used in nitrogen atmosphere for benzyl alcohol oxidation.
61
Figure 5: SEM Images of (a) Fresh, (b) Used in oxygen
Figure 6: The particle size distribution of as synthesized catalyst.
62
4A.6. Grain Size Distribution
The grain size distribution of the as synthesized catalyst is shown in figure 6. The figure
suggests that more than 35% of the particles were in the range of 2-5 micron meter.
4A.7. Oxidation of Benzyl Alcohol in Solvent Free Conditions
The oxidation of BzOH (benzyl alcohol) was performed in three necked Pyrex glass
batch reactor. The three necked flask of 50ml capacity was provided with a cooling condenser
and a closed bulb space-immersed in reaction mixture- for recording the reaction temperature,
using a digital thermometer. The reaction mixture was maintained using hot plate and oil bath. In
a typical reaction 97.6 m.moles of benzyl alcohol (10ml benzyl alcohol) was taken in the glass
reactor. The system was well saturated with molecular oxygen by bubbling it (oxygen) in the
reaction mixture at 60 ml-min-1. The moisture if any present in the lines was removed using
standard traps. When the reaction temperature is achieved, calculated amount of catalyst (usually
0.2 g) was injected into the reaction mixture. The oxygen flow and the stirring rate were kept
constant at 60 ml/min and 950 rpm respectively, throughout the reaction using a bubble flow
meter.
The reaction mixture was analyzed using a GC-Clarus 500-Perkin Elmer, USA
instrument equipped with a flame ionizing detector. Standard calibration plots were prepared for
quantifying the reaction products.
4A.8. Comparison Of Activated (in H2) And Fresh Catalyst:
First of all the catalyst was tested without reducing the catalyst in hydrogen flow, which
revealed that the catalyst was not reactive towards the oxidative transformation of benzyl alcohol
but when the catalyst was activated in reducing environment of hydrogen flow at 280oC, the
catalyst became reactive towards the oxidative transformation of benzyl alcohol towards its
reaction products. This may be due to the fact that before oxidation the palladium surface was in
oxidized state i.e. PdO. But after treatment in the hydrogen flow the palladium oxide was
converted to it metallic or zero valent form. In literature the metallic form of palladium is
reported to be active towards the oxidation reactions.
63
4A.9. Effect of Catalyst Loading
The effect of catalyst load on the progress of benzyl alcohol oxidation was monitored in
the range 0.03-0.3 g, keeping all other parameters as constant. The conversion of BzOH
increased linearly with increasing the catalyst loading, which becomes almost constant after
0.2 grams of the catalyst loading which may suggest that gas liquid mass transport resistance
might not be significant under these circumstances. Figure 7 represents the effect of the
catalyst loading upon the conversion of benzyl alcohol.
4A.10. Life Span of the Catalyst
The catalyst re-usability is an important factor which needs to be taken care. A good
heterogeneous catalyst is one which can be used for longer period of times, hence minimizing the
cost of reaction and eliminating the risk of environmental disarray. For this purpose the used
Pd/ZrO2 catalyst after recovery from the reaction mixture was washed with double distilled water
and pure ethanol.
The catalyst was then dried at 100 oC overnight. It was observed that the catalyst efficiency
remained the same as for fresh catalyst.
In another experiment the used catalyst was reused without washing, it was observed that
the catalyst was still able to catalyze benzyl alcohol but the efficiency of the catalyst was
reduced up to 5 percent. The results are demonstrated in figure 8 and 9.
4A.11 Time Profile Investigation
Palladium oxide supported on zirconia as a catalyst was found to be an efficient catalyst
in solvent free conditions towards benzyl alcohol oxidation. The oxidative transformation of
benzyl alcohol as a function of temperature was monitored periodically. The time profile
investigation for the oxidation of benzyl alcohol was carried out at 100oC (373K).
Benzaldehyde and benzoic acid were detected as the reaction products in the reaction course;
however, the selectivity towards benzaldehyde was more than 70 % while only small amount
of benzoic acid was formed. This shows that although a parallel reaction is possible
(consecutive reaction is also possible- as benzaldehyde may react with the water formed during
64
the reaction which results in the formation of benzoic acid in the presence of active oxygen)
but the selectivity towards benzoic acid is small.
Figure 10 and figure 11 demonstrates the time profile study for the solvent free oxidation
of benzyl alcohol at various temperatures ranging from 70-100 oC
4A.12. Comparison with organic solvents:
To know the effect of solvents with the solvent free reaction of benzyl alcohol, oxidation
reactions for benzyl alcohol were conducted in two different solvents using the same catalyst in
the same conditions. The reaction was conducted in two organic solvents i.e. n-heptane and n-
octane. Interesting thing was that in organic solvents the reaction was ~ 100 % selective towards
benzaldehyde formation. The reason could be due to complete removal of H2O molecules during
the reaction in the presence of the organic solvents.
We calculated that at an oxygen flow of 60 ml/min and at 100 % conversion a total of
0.002 m.moles of water will be formed in the case of the organic solvent and ~97 m.moles of
water molecules will be produced in case of solvent. At condenser temperature (25 oC) 23.3
m.moles of water can be vaporized in 300 minutes that is the reason we suggest that all of the
water formed in case of solvent will be evaporated while there is still some water in the solvent
free reaction which may react with the benzaldehyde to form benzoic acid in the company of
molecular oxygen. The results are summarized in Table 2.
65
0 50 100 150 200 250 300
5
10
15
20
25
30
35
Co
nve
rsio
n (
%)
Catalyst loading (mg)
Figure 7 Effect of Catalyst loading on the conversion of benzyl alcohol
Reaction Conditions: BzOH 97.6 m moles, Temperature 100 °C (373 K),
Pressure of oxygen 101 kPa, Agitation 950 rpm, Time 240 minutes
66
Figure 8. Catalyst life Span
Reaction conditions :BzOH= 97.6 , Catalyst. 200 mg, Temp. 100 °C (373 K), Pressure of
oxygen 101 kPa, Agitation. 950 rpm, Time 300 minutes.
Figure 9: Catalyst Reusibity without washing for successive reactions
Reaction conditions BzOH= 97.6 , Catalyst. 200 mg, Temp. 100 °C (373 K), Pressure of
oxygen 101 kPa, Agitation. 950 rpm, Time 300 minutes.
31
32
33
34
35
36
37
Fresh Once Twice
Conversion %
Catalyst Used
0
5
10
15
20
25
30
35
40
Fresh UsedOnce
UsedTwice
UsedTrice
Conversion % 36 32 27 23
% C
on
vers
ion
67
Figure 10 Conversion of benzyl alcohol as function of time at 100 ºC
Figure 11 Conversion of benzyl alcohol as function of time at different
temperatures
Reaction conditions: BzOH 97.6 m mols, Catalyst amount 200 mg, Agitation. 950 rpm,
Pressure of oxygen 1atm.
68
Table 2: Oxidation of Benzyl alcohol
Temperature (oC)
Selectivity ( % ) Conversion (%) TOF*/Hour
BzOH Bz-OOH
Solvent Free Conditions
100 71.7 29.7 36.2 6178
90 73.5 25.9 26.4 4506
80 75.8 24.0 13.9 2372
70 74.8 24.9 10.3 1758
Solvent 80 oC
n-heptane 100 ---- 40.8 36
n-octane 100 ---- 98.3 87
*Turnover frequency (TOF): molar fraction of BzOH converted to the palladium content in the
catalyst per hour.
69
4A.13 Effect of mass transfer
The catalytic benzyl alcohol oxidation in liquid phase at a certain agitation speed is
typical slurry based reaction. The reaction consists of a liquid reactant i.e. benzyl alcohol a
gaseous reactant i.e. molecular oxygen and a solid catalyst i.e. Pd/ZrO2. To investigate the true
kinetics of a multi phase reaction, the mass transfer effect must be investigated.
The mass transfer variation upon the reaction rate can be examined by investigation of
the agitation speed upon the reaction rate. If the rate of the reaction is varying with agitation
speed then we say that the mass transfer exists in a certain reaction, however if the reaction
conversion is independent while the agitation speed is changing, than the reaction will be free
from mass transfer limitation and this will be our area of interest when we are going to calculate
the oxidation kinetics.
The agitation effect upon the conversion of reaction was investigated in the range of 150-
1000 rpm at 100 oC for one hour each. The agitation speed vs. conversion is shown in figure 12.
The conversion of benzyl alcohol increases in initial stages with increasing agitation speed which
denotes the mass transfer regime from 150-650 rpm, however above this point the conversion of
the substrate becomes constant, which indicates that the oxidation of benzyl alcohol is free from
any mass transfer limitations i.e. the catalytic reaction is now taking place in the kinetically
restricted region. This is the area of significance and all the following reactions were carried out
at 950 rpm.
4A.14. Kinetic Analysis
From the knowledge of the stirring effect upon the benzyl alcohol conversion and the
activation energy (to be discussed later) it can be concluded that oxidation reaction is most
probably taking place in the kinetically controlled region.
The time profile data at various temperatures and at 1 atm partial pressure of di-oxygen
was applied to the kinetic models. Method of non linear least square regression method of
analysis was used. For this purpose Curve Expert 1.4 software was employed.
Langmuir-Hinshelwood model was applied to describe benzyl alcohol oxidation catalyzed by
palladium supported on zirconia support. It must be assumed that the catalytic reaction between
the active oxygen and benzyl alcohol is taking place at surface of palladium particles. According
70
to Langmuir-Hinshelwood (L-H) mechanism, in the first step the molecular oxygen in gaseous
phase and the substrate molecule gets adsorbed at the catalyst surface, in the second step the
absorbed gaseous species and the substrate molecule react together to give the respective
products. In the last step the products formed gets desorbed from the catalyst surface [7,8].
According to Langmuir-Hinshelwood kinetic theory, the catalytic rate of reaction is directly
related to the fraction of the surface coverage by benzyl alcohol , θ,
2OBzOHrkRate (1)
Where rk , BzOH and 2O corresponds to the rate constant, fraction of the surface covered
by benzyl alcohol and surface covered by molecular oxygen respectively.
In the solvent free conditions due to the negligible change in concentration of benzyl alcohol,
BzOH (at all stages of conversion in the present case) can be taken equal to 1 and therefore,
equation (1) is changed to
2
'OkRate (2)
In these conditions at a constant oxygen pressure equation 2, will become
"kRate (3)
71
Figure 12: Benzyl alcohol conversion as a function of agitation speed.
Reaction conditions: BzOH: 97.6 mmol, Catalyst: 200 mg, Temperature: 100 °C (363 K),
Pressure of oxygen: 1atm, Time: 60 minutes.
3
5
7
9
11
13
150 350 550 750 950
Conversion (%)
Agitation speed ( rpm )
72
It is a pseudo zero order reaction and integration will change it to
tkBzOH t" (4)
Where (BzOH)t is the amount of benzyl alcohol left at time “t”. It is also evident that in these
conditions the value of "k depends upon oxygen partial pressure (equation 2 and 3).
Equation 4 was applied to time profile data for benzyl alcohol oxidation at various partial
pressures of oxygen which resulted in straight lines (figure 13).
The slopes of these lines (giving the values of "k ) depend upon the partial pressure of
oxygen as listed in Table 3.
Considering that 2
'" Okk (equation 2 and 3), and if the Langmuir adsorption isotherm is
applicable for the adsorption of oxygen then:
22
22
2 1 OO
OOO pK
pK
(5)
Whereas 2OK and
2Op are the adsorption equilibrium constant for oxygen and partial pressure of
oxygen respectively. Insertion of Equation (5) for 2O :
73
22
22
1
'"
OO
OO
pK
pKkk
(6)
Application of equation (6) to the data using non-linear least square using the software curve
expert 3.1 resulted in a very good agreement between experimental and theoretical values of "k
as shown in figure 14. The values of rate coefficient 'k and adsorption equilibrium “K” obtained
by non-linear least square fit are 0.236 m.mol /min. and 0.0144/kPa respectively, with
correlation coefficient (R2) of 0.9932. It can be summarized that the rate of oxidation of benzyl
alcohol in the liquid phase solvent free conditions is:
As BzOH in solvent free conditions is constant or equal to 1 and thus this equation is reduced to:
4A.15. Benzyl Alcohol oxidation in organic solvents
In the solvent free conditions it is not possible to determine the reliance of the reaction
rate on oxidation of benzyl alcohol concentration therefore experiments were performed using n-
heptane as solvent. Figure 15 represents the comparison of the experimental and calculated
amount of benzyl alcohol at various time intervals. These calculations were made by using
Temkin isotherm.
)7(1
22
22
OO
OOBzOH pK
pKkRate
)8(1
22
22
OO
OO
pK
pKkRate
74
Table 3: Apparent rate Constant “k’ ” at various oxygen partial pressures
pO2
(kPa)
k’’
(m.mol/min.)R2
101.1 1.38E-01 0.9501
84.1 1.30E-01 0.9541
67.3 1.20E-01 0.9541
50.5 9.86E-02 0.9833
33.6 7.06E-02 0.9615
16.8 5.07E-02 0.9038
Figure 13: Benzyl alcohol oxidation as a function of Time: Effect of Oxygen Partial Pressure
75
.
Figure 14: Comparison of apparent rate coefficients, experimental “k'(exp)” and calculated
“k’(calc)” at various oxygen partial pressures. Calculations were made according to Langmuir
Adsorption isotherm, using "k = 0.236 m.mol/min and adsorption equilibrium constant for
oxygen= 0.0144/kPa.
76
By using krK1 = 4.31 exp -03 m.mol/min and K2 = 8.71 L/m.mol values for figure 15 were
obtained.
This combined with the dependence of the rate on oxygen partial pressure in solvent free
conditions 10:
This shows that both oxygen and benzyl alcohol react in the adsorbed states. It also reveals the
fact that benzoic acid is formed only by the reaction of benzaldehyde with water produced in the
reaction. No benzoic acid formation was observed in conditions where water is completely
removed from the reaction mixture immediately after it is produced.
)9])([ln( 21 BzOHKKkRate r
)10)(1
(][ln(22
22
21OO
OOr pK
pKBzOHKKkRate
77
Figure 15: Comparison of experimental and calculated values according to Temkin Isotherm
78
4A.16. Conclusion
Palladium zirconia was found to be an efficient catalyst for the conversion of benzyl
alcohol to benzaldehyde at a comparably faster rate. The TOF was found to be among the highest
as compared to previously reported results in the literature. Langmuir Hinshelwood model was
found to be applicable to benzyl alcohol oxidation in solvent free conditions in liquid phase
when the partial pressure data was subjected to kinetic analysis. However when a solvent such as
n heptane was used Temkin isotherm was found to be applicable. The activation energy was
found to be 49.1 KJ/mol.
79
References
1. Haas-Santo, K.; Fichtner, M.; Schubert, K. Applied Catalysis A: General. 2001, 220, 79-
92.
2. Chen, Y.; Zheng, H.; Guo, Z.; Zhou, C.; Wang, C.; Borgna, A.; Yang, Y. J. Catal. 2011,
283, 34-44.
3. Benyagoub, A. Acta Mater. 2012, 60, 5662-5669.
4. Trubelja, M. P.; Potter, D.; Helble, J. J. Journal of Materials Science. 2010, 45, 4480-
4489.
5. Sato, K.; Abe, H.; Ohara, S. J. Am. Chem. Soc. 2010, 132, 2538-2539.
6. Santos, V.; Zeni, M.; Bergmann, C.; Hohemberger, J. Rev Adv Mater Sci. 2008, 17, 62-
70.
7. Önal, Y.; Schimpf, S.; Claus, P. J. Catal. 2004, 223, 122-133.
8. Vincent, M. J.; Gonzalez, R. D. Applied Catalysis A: General. 2001, 217, 143-156.
80
Chapter 4B
Results and Discussion
Reactant: Toluene
Catalyst: Pd/ZrO2
Oxidation of toluene in solvent free conditions by Palladium supported zirconia.
4B.1 Characterization
The BET surface area of 11.00 and 12.4 m2/g was found for Zirconia (ZrO2) and Pd/ZrO2
respectively however the BET surface area of the non-calcined sample was found to in the range
of 87 m2 /g. It may be because with the increase in calcination temperature the crystallite size of
the catalyst increases hence the pores of the as synthesized catalyst are decreased due to which
the surface area of the catalyst tends to decrease with increase in calcination temperature.
Figure 16 illustrates the scanning electron micrograph of the synthesized (fresh) catalyst
which was used for the oxidation of toluene in the solvent free conditions. The SEM image
revels that the palladium particles are well dispersed on the surface of zirconia, however the
particle size of the catalyst cannot be found from the SEM image because the particles of the
catalyst seem to cling together.
Figure 17 represents the diffraction patterns of the catalyst calcined at 950 oC. The
diffraction patterns show that the supported palladium catalyst is mainly dominated by
monoclinic phase of zirconia. The monoclinic peaks of zirconia can be found at a 2 angle of
28.18o and 31.38o [9, 10]. The diffraction pattern for Palladium could not be fully resolved, may
be due to the very small loading of palladium on zirconia ( 0.1 wt %). We found no difference
between the used and unused catalyst, after recovery from the reaction mixture and also before
and after reduction in hydrogen atmosphere at high temperatures.
4B.2 Catalytic Activity
Batch oxidation of toluene using 0.1 % Pd/ZrO2 as catalyst and molecular oxygen as
oxidant was carried out in solvent free condition at five different temperatures (333-373 K). For
typical run 0.0936 moles (10 mL) toluene was taken in three necked batch reactor. The
temperature of the reactor was kept at the desired temperature using a hot plate. Molecular
oxygen was constantly purged through the reaction mixture at oxygen flow rate of 60 mL min-1.
81
Figure 16 – SEM image of Palladium Zirconia Catalyst
10 20 30 40 50 60
Inte
nsity
(C
PS
)
2 (angle)
(a)-Fresh
(b)- O2 Used
(c)- N2 Used
Figure 17 XRD Patterns of the Catalyst before and after use in the reactions.
82
A small saturator containing pure toluene was placed in the way of the reactor at the
reactor temperature, so that if there is any loss of toluene during the reaction with the flow of
oxygen the saturator will fill in the deficiency. After attaining the desired conditions of
temperature, flow of oxygen and agitation speed, the reaction was stopped and the sample was
immediately analyzed by GC to determine any chemical change in toluene during heating period.
Afterwards after repeating the above steps and after the attainment of desired conditions of
temperature and pressure the reaction mixture was added with the desired amount of catalyst
(mostly 0.2g). After regular intervals of time samples were drawn using a glass syringe, the
samples were immediately filtered and subjected to GC and UV spectrophotometer for the
qualitative and quantitative analysis of the reaction mixture. The known products of the
oxidation of toluene in present case were benzyl alcohol, benzaldehyde and benzoic acid. Table
4B.1 shows the results. Conversion of toluene rises from 3.6% to 28.1% with temperature from
333 K to 373 K. The yield of benzyl alcohol gradually decreases with the increase in reaction
temperature. At lower temperature, reaction is more selective for benzyl alcohol. As the
temperature increases, the reaction becomes more selective for benzoic acid. In present
investigation higher conversion and TOF were obtained as compared to other solvent free
oxidations.
4B.3. Time Profile
The catalytic oxidation of the substrate molecule i.e. toluene with respect to time at
varying temperatures is been shown in figure 18. A linear trend of conversion was seen with
time. Figure 19 and Figure 20, shows conversion/yield and selectivity of different products at
various temperatures. At lesser conversions, the reaction is more selective for benzyl alcohol. An
increase in selectivity towards benzoic acid was observed with increase in conversion of the
substrate molecules. Similarly with increase in temperature, selectivity of oxidation reaction for
benzyl alcohol decreases with selectivity for benzoic acid increases as given in Figure 18. The
present catalytic system, Pd/ZrO2, is more efficient as compared to Pt/ZrO2 which has been
reported from this laboratory [11], as in present investigation there is no induction period.
Benzaldehyde, which is the product of oxidation of benzyl alcohol, appears in the beginning of
reaction. In case of Pt/ZrO2 only benzyl alcohol was detected as the reaction product in
beginning.
83
Temperature (K) Conversion (%) Selectivity (%) TOF*
BzOH BzH BzAc
333 3.6 75.8 15.8 8.3 1793
343 8.6 60.6 17.2 22.1 4274
353 14.4 59.2 23.5 17.4 7158
363 19.7 53.7 24.3 21.9 9799
373 28.1 53.7 24.3 21.9 13999
Turn over frequency (TOF*): Molar ration of toluene to Pd content per hour.
Reaction Constants : Catalyst 0.2 g, Pressure of oxygen 101 kPa, Time 1 hour, agitation
950 rpm, Toluene 0.0936 moles
Table 4B.1: Conversion and selectivities of Toluene at different temperatures
84
4B.4. Effect of Catalyst loading
The effect of the loading of the catalyst was studied in the range of 50-300mg at 373 K and 101
kPa partial pressure of oxygen. The agitation speed of the reaction mixture was maintained at
950 rpm. The results are demonstrated in Figure 21. The conversion of toluene enhanced with
increase in weight of the catalyst from 0.05-0.35 g; however the % conversion per minute, per
gram of catalyst was observed to be decreased with the early increase in catalyst weight i.e. from
0.05 to 0.08 g, however, when the catalyst weight is increased further i.e.0.08-0.2 g the
conversion of toluene per minute, per gram of the catalyst becomes almost constant. The
characteristics of toluene conversion at lower catalyst loading i.e. below 0.1 g suggest transport
limitations of toluene to the metal surface. When the catalyst weight was increased above 0.1
grams, the rate of the reaction per gram of the catalyst becomes constant, which suggests the
existence of kinetic control region. Thus a catalyst weight of 0.2 grams was selected as optimum
catalyst loading.
4B.5. Effect of oxygen partial pressure
The effect of oxygen partial pressure was investigated for the oxidation of toluene in the
range of 16-101 kPa oxygen partial pressure. The experiments were performed using Pd/ZrO2
catalyst at 373 K with 0.0936 moles of toluene and 0.2 g of the catalyst loading. Figure 22
indicates that the conversion of toluene increases with the increasing oxygen partial pressure.
Oxygen partial pressure was calculated by mixing oxygen with nitrogen in different ratios
keeping the total flow of the gases equal to 60 mL min-1. The partial pressure of oxygen was
calculated by applying equation below.
kPaFF
Fp
NO
OO 2.101
22
2
2
85
Figure 18 Time profile for the conversion of toluene in solvent free conditions.
Reaction Conditions: Toluene 10 ml (0.093 moles), Oxygen 1 atm (60 ml/min), Catalyst wt 0.2
grams, Stirring speed 950 rpm
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
0.00 50.00 100.00 150.00 200.00
Con
vers
ion
(%
)
Time (mins)
60 C 70 C 80 C 90 C 100 C
86
Figure 19 Time profile for the conversion of toluene with respect to yield at different temperatures in solvent free conditions.
Reaction Conditions Toluene 10 ml (0.093 moles) Oxygen 1 atm (60 ml/min) Catalyst wt 0.2 grams Stirring speed 950 rpm
87
Figure 20 Time profile for the conversion of toluene with respect to Selectivities at different
temperatures in solvent free conditions.
Reaction Conditions Toluene 10 ml (0.093 moles) Oxygen 1 atm (60 ml/min) Catalyst wt 0.2
grams stirring speed 950 rpm
88
Figure 21 Effect of catalyst loading (grams) against the conversion of toluene. Reaction Conditions Toluene 10 ml (0.093 moles)Oxygen 1 atm (60 ml/min) Catalyst wt
0.2 grams Stirring speed 950 rpm
89
Figure 22. Effect of Partial pressure of oxygen against the conversion of toluene
Reaction Conditions: Toluene 10 ml (0.093 moles) Oxygen 16.8Kpa-101Kpa (60 ml/min)
Catalyst wt 0.2 grams stirring speed 950 rpm
90
4B.6. Conclusion
Palladium zirconia was found to be an efficient catalyst for the oxidation of Toluene to
industrially important products such as benzyl alcohol benzaldehyde and benzoic acid; however
it was found that the catalyst was found to be more selective towards benzyl alcohol at lower
temperatures but at higher temperatures it was found to be more selective towards benzoic acid.
The TOF was found to be among the highest as compared to previously reported results in the
literature.
91
References
1. Haas-Santo, K.; Fichtner, M.; Schubert, K. Appl. Catal. A. Gen. 2001, 220, 79-92.
2. Chen, Y.; Zheng, H.; Guo, Z.; Zhou, C.; Wang, C.; Borgna, A.; Yang, Y. J. Catal. 2011,
283, 34-44.
3. Benyagoub, A. Acta Mater. 2012, 60, 5662-5669.
4. Trubelja, M. P.; Potter, D.; Helble, J. J. J. Mater. Sci. 2010, 45, 4480-4489.
5. Sato, K.; Abe, H.; Ohara, S. J. Am. Chem. Soc. 2010, 132, 2538-2539.
6. Santos, V.; Zeni, M.; Bergmann, C.; Hohemberger, J. Rev Adv Mater Sci. 2008, 17, 62-
70.
7. Önal, Y.; Schimpf, S.; Claus, P. J. Catal. 2004, 223, 122-133.
8. Vincent, M. J.; Gonzalez, R. D. Appl. Catal. A. Gen. 2001, 217, 143-156.
9. D’Souza, L.; Suchopar, A.; Zhu, K.; Balyozova, D.; Devadas, M.; Richards, R. M.
Microporous Mesoporous Mater. 2006, 88, 22-30.
10. Ferino, I.; Casula, M. F.; Corrias, A.; Cutrufello, M. G.; Monaci, R.; Paschina, G. PCCP.
2000, 2, 1847-1854.
11. Ilyas, M.; Sadiq, M. Catal. Lett. 2009,128, 337-342.
92
Chapter 4C
Results and Discussion
Reactant: Benzyl Alcohol
Catalyst: Manganese Oxide (MnOx)
Oxidation of Benzyl alcohol in liquid phase by manganese oxide catalyst:
4C.1. Characterization
Characterization of the catalyst is an imperative step in the field of heterogeneous
catalysis. Material and surface science provides us the facility to investigate the composition,
nature and the structure of the catalytically active sites of the material and the correlation of this
information with the catalytic reactions, their rates, activities and selectivities.
4C.2. Nitrogen Adsorption
Nitrogen adsorption studies were conducted by first removing all the physically adsorbed
species from the surface of the catalyst. This was made possible by degassing process, in which
the solid sample was heated at elevated temperature in full vacuum environment. The adsorption
isotherms were deduced by plotting the volume of the total nitrogen adsorbed at -195.6 oC
against the relative pressure i.e. P/ P°, shown in 1 (a). The Isotherm can be classified as type II
according to Brunauer, Deming, Deming and Teller (B D, D T) classification of isotherms [1-5].
This type of isotherm represents the unrestricted monolayer- multilayer adsorption. Its hysteresis
loop can be classified as an H3 type according to IUPAC classification of hysteresis loop [5]. It
is believed that this type of isotherm occurs in such types of materials which has aggregates of
plate like particles having a mesoporous structure that are very poorly defined[6,7].
BET equation (equation no.1) was employed for the determination of specific surface
area of manganese oxide. Figure 23 (b) shows the physically adsorbed nitrogen over the catalyst
surface. Simplified form of the BET equation can be given as follows;
omm
o PPCWCCWPPW /)/1(/1]1)/[(/1 (4.1)
93
Where
W = Amount of Nitrogen adsorbed at Relative Pressure (P/P0)
Wm=mono layer capacity for nitrogen adsorption
C=constant
The surface area and pore size analysis are shown in table 4.1. The average pore volume
and pore radius were determined using the BJH method taking into consideration the desorption
branch of the isotherm as shown in figure 24.
4C.3. Particle Size Analysis
The particle size distribution of the as prepared catalyst was found to be in the range of 3-
10 micro meters using wet method.
4C.4. X-ray Diffractometery (XRD)
The XRD patterns of different manganese oxides catalysts are shown in Figure 25. In the
XRD pattern of the prepared catalyst, peaks at 2θ = 12.7°, 25.2°, 35.1°, 40.7°, and 58.85°are
apparent and show the crystalline nature of the catalyst. Among the peaks, those at 2θ = 12.7°
and 25.2° are assigned to -MnO2. These peaks are very weak, suggesting that this phase is not
present in our catalyst in significant quantity. The peaks at 2θ = 35.1°, 40.7°, and 58.9° are
assigned to MnO [6]. Lia and coworkers [7] reported peak at 2 = 25.2 to Mn2O3. Wang and his
group [8] assigned the peaks at 2, 36.2 and 44.4 to Mn3O4. In comparing the XRD data of the
prepared catalyst with the date reported in literature, it was argued that the manganese oxide
catalyst is a mixture of -MnO2, Mn2O3, and MnO in nature. Based on the XRD data, it was
concluded that the catalytic activity of manganese oxide is due to the presence of hydrated MnO.
It was also found that the catalytic activity of the commercially available catalyst was found to
be poor towards the oxidation of benzyl alcohol. The crystal size of the catalyst used for
oxidation of benzyl alcohol was calculated using Scherer’s equation [9] and was found to be 3.84
nm. The XRD spectra of fresh(a) and used (b) for the oxidation of benzyl alcohol show no
difference, which clearly suggests that he nature of the catalyst remains the same even after the
oxidation reaction [10-12].
94
4C.5. Fourier Transform Infrared Spectroscopy (FTIR)
Figure 26 shows the FT-IR spectra of different catalysts. Absorption bands are observed
in the ranges 3300–3500, 1500–1700, 1000–1100, and 500–600 cm−1. The 3300–3500 cm−1
bands are assigned to O−H stretching vibrations; the bands observed between 1100 and 1600
cm−1 are most usually assigned to O−H bending vibrations for groups joined to manganese.
Similarly, the bands in region 500–900 cm−1 are assigned to Mn-O vibrations. The IR spectrum
suggests the existence of some interstitial bound water in the catalyst. The spectrum of the fresh
catalyst suggests that it is mainly composed of MnO, having characteristic bands at 833, 948, and
816 cm−1. The broad peak at 3200–3500 cm−1 was assigned to the stretching vibration of H2O
and the hydroxyl group in the lattice. The peak near 1600 cm−1 in the fresh catalyst (Figure
26(a)) was assigned to the bending vibration of water and OH, which suggests that hydroxyl
group is present in the prepared catalyst. The spectra of used catalysts (Figure 26 (b) and (c))
show bands at 1592 cm−1, which may be due to MnH2 with H derived from benzyl alcohol [13-
20].
4C.6. Scanning Electron Microscopic (SEM) Analysis
A scanning electron micrograph (SEM) gives valuable knowledge about the surface structure and
morphology of the prepared catalyst and informs somewhat about the particle distribution. SEM
of the catalyst is shown in Figure 27. Similar morphology has been observed for plate-like
layered manganese oxide catalysts [21-23]: however, the larger surface area in these studies was
probably caused by the presence of pillared material (e.g., alumina, lithium). The presence of
pillared material creates a larger interlayer distance and results in pores having the form of
parallel plates.
Figure 26 suggests that the average grain size of the manganese oxide catalyst is in the order of
microns, while XRD data suggests particle size of the order of a nanometer. The average
crystallite size is calculated by Scherrer equation based on XRD data, while the SEM image
shows the size of particles that are aggregates of several crystallites.
95
0.0 0.2 0.4 0.6 0.8 1.00
5
10
15
20
25
Vo
lum
e (c
c/g
ram
)
P/Po
Adsorption Desorption
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26
4
6
8
10
12
1/[W
((P
o/P
) -
1)]
P/Po
(b)
y = 47.661x + 0.429R² = 1
(a)
Figure 23: Nitrogen adsorption investigation: a) Type II isotherm, b) BET
isotherm.
96
0 50 100 150 200 250 300 350 400
0.00
0.05
0.10
0.15
0.20
0.25
0.30
P
ore
Vo
lum
e (c
c/g
ram
)
Radius (A0)
Pore Volume
Figure 24: Pore size distributions according to BJH method.
Table 4.1 Surface area and porosity of the catalyst.
Parameters
MnO
BET surface area (m2g-1)
Pore volume (cm3g-1)
Pore radius (Å)
Pore surface area (m2g-1)
72.50
0.27
45.0
5.50
97
5 10 15 20 25 30 35 40 45 50 55 60 650
500
1000
1500
0
500
1000
1500
Co
un
ts p
er
Se
con
d
2 Angle)
(b) Used
(a) Fresh
Figure 25 XRD of Manganese oxide fresh (a) and used (b) catalysts.
98
Figure 26 FT-IR spectra of fresh (a) and once-used (b) and twice-used (c) catalysts.
99
4C.7 Oxidation of Benzyl Alcohol
The oxidation of benzyl alcohol in liquid phase was conducted in a three necked round
bottom flask as batch reactor, of 50ml capacity, supplied with a reflux condenser and a digitally
controlled thermometer. The thermometer was used for the measuring the temperature of the
reaction. Reaction temperature was kept constant using a hot plate combined with an oil bath
(stationed on the hot plate).
Typically a predetermined quantity of the substrate material (2 mmol of benzyl alcohol)
and 10ml of n-heptane as solvent was taken in the glass reactor and the reactor was saturated
with molecular oxygen by passing it through the reaction mixture at a flow rate of 60 ml/min.
Moisture from the gases was removed using standard traps. Drying of oxygen is an important
factor because it has been reported elsewhere [24] that the presence of water in reaction mixture
tends to change the selectivity of benzyl alcohol from aldehydes to ketones. The flow of the
gases was maintained at a regular flow using the conventional bubble flow meter.
After the attainment of the desired temperature (70 0C) and gas flow the reactor was injected 100
milli grams of the catalyst. The gas flow was maintained and regularly checked at 60 ml/min,
while stirring the reaction mixture at a constant rate of 950 rpm. A solvent saturator was also
installed in the way of the gases at the condenser temperature, so as to minimize the solvent loss
during the course of the reaction. At regular intervals of time, small aliquots from the reaction
mixture were drawn from the reactor and the catalyst was separated from it and the reaction
mixture was analyzed using UV-Visible spectrophotometer and/or Gas Chromatography. GC
(Clarus 500, Perkin Elmer, USA) equipped with Flame ionizing detector (FID) and UV-Visible
spectrophotometer model number -Shimadzu UV-160A, Japan- were used for the analysis of the
reaction mixtures. Previously prepared standard curve were used for the quantification of the
reactants and the products. Products were identified by comparison with standard chemicals
bought from the market.
4C.8. Manganese Oxide as Catalyst or Oxidant
Manganese and their oxides are extensively used as stoichiometric oxidants for oxidation
reactions. It is because of the oxygen that is contained in the lattice of manganese oxide.
100
Figure 27 SEM image of prepared catalyst (Manganese Oxide).
101
Therefore manganese oxide acts only as oxidant and not as heterogeneous catalyst for the benzyl
alcohol oxidation.
Therefore it is very important to distinct weather our catalyst acts as a stoichiometric
oxidant or as heterogeneous catalyst. For this very reason 0.002 mole of benzyl alcohol in n-
heptane as solvent was carried in nitrogen atmosphere. Under nitrogen flow only 10 %
conversion of benzyl alcohol was possible as compared to 96 % conversion in the flow of
molecular oxygen in 90 minutes at 343 K. In another experiment the substrate amount was
doubled and the time of the reaction was also doubled which revealed about the same conversion
(91%). These results revealed that our prepared catalyst is only effective if oxygen atmosphere is
provided and is not effective in absence of oxygen; hence we can confidently say that lattice
oxygen of manganese oxide does not play role as oxidant rather it acts as heterogeneous catalyst
for the activation of molecular oxygen for the oxidation of benzyl alcohol.
Partial pressure of oxygen also proved evidence for the catalytic role of manganese oxide
for the oxidation of benzyl alcohol. The % conversion of (BzOH) was reduced from 96 to 20%
when the oxygen partial pressure was reduced from 101.6 KPa to 33 KPa. These results
confirmed that our catalyst acts as heterogeneous catalyst rather than stoichiometric oxidant.
4C.9. Catalyst leaching
The catalyst leaching is an issue which is an obstacle in heterogeneous based reactions,
because if catalyst leaches into the reaction mixture it cannot be regarded as a heterogeneous
reaction. To test whether the catalyst particles leaches into the reaction mixture we employed the
classical hot filtration method. We performed a simple experiment to determine whether
catalyst leaching occurred in our system. N-heptane was stirred with catalyst in a flask under the
same experimental conditions (70°C for 2 h, constant oxygen flow). After two hours, the catalyst
was removed from the reaction mixture and the benzyl alcohol substrate was added. The reaction
mixture was stirred for a further 2 h, after which no oxidation products were detected by GC
(FID) or UV-visible spectrophotometry. This experiment confirmed that the catalyst is not
leaching into the reaction mixture
102
4C.10. Catalyst loading
The influence of catalyst load upon the conversion of benzyl alcohol oxidation was
monitored over the catalyst weight range of 0.03–0.12 g, keeping all other parameters of
temperature and pressure constant. Figure 28 demonstrates that the conversion of benzyl alcohol
enhances with catalyst loading, although the increase is nonlinear. Full conversion (100%) was
achieved at loadings of 0.15 g and higher.
4C.11. Life Span of Catalyst
The reusability of a catalyst plays important role towards the economics and the greener
environmental protocols. Therefore it was intended to know whether catalyst which was used
for the oxidation of benzyl alcohol was able to repeat its activity and selectivity towards the
oxidation of benzyl alcohol towards benzaldehyde or not.
To investigate catalyst reuse, the used catalyst was collected from the reaction and was
washed several times in an ethanol-water mixture. The washed catalyst was then reused. We
found that used MnO had the same catalytic performance as fresh catalyst, as shown in Figure
29. These results confirm the high effectiveness of the current process for easy and convenient
oxidation of benzyl alcohol by an easily removed and reusable manganese catalyst.
4C.12. Time Profile Investigation
The use of manganese oxide as catalyst resulted in gradual conversion of benzyl
alcohol to benzaldehyde over time. The time profile investigation of benzyl alcohol oxidation
was monitored continuously. The time profile study was carried out at 343 K. In the early part of
the reaction, only benzaldehyde was detected as the reaction product. As the conversion of
benzyl alcohol to benzaldehyde neared completion (~95%), formation of benzoic acid was
detected. This shows the consecutive nature of the reaction. Figure 30 shows the time profile of
the reaction. Formation of benzoic acid was only detected at the beginning of the oxidation
process when oxygen was not dried by passing it through a silica moisture trap. We concluded
that dry conditions are necessary to control the selectivity of the product distribution.
103
0.02 0.04 0.06 0.08 0.10 0.1250
60
70
80
90
100
Co
nv
ersi
on
( %
)
Catalyst Loading ( Grams)
Conversion
Figure 28: Effect of catalyst loading on oxidation of benzyl alcohol. Reaction conditions:
Initial concentration of benzyl alcohol: 0.2 mol/L in n- heptane, volume: 10 mL,
Temperature 343 K, oxygen pressure 1 atm (1 atm=1.013×105), 950 r/min, 90 min.
104
Fresh Etanol Washed Unwashed0
20
40
60
80
100
Co
nver
sion
(%
)
Catalyst
Figure 29 Life span of catalyst for the oxidation of benzyl alcohol.
Reaction conditions: Initial concentration of benzyl alcohol: 0.2 mol/L in n- heptane, volume:
10 mL, Temperature 343 K, oxygen pressure 1 atm (1 atm=1.013×105), 950 r/min, 90 min.
105
Figure 31 shows the time profile for benzyl alcohol oxidation at various temperatures in range
313–343 K, with all other experiment conditions remaining unchanged.
4C.13. Effect of Oxygen partial pressure
The effect of oxygen partial pressure on benzyl alcohol oxidation was investigated over
the range 33–101 KPa with a constant initial concentration of 0.2 mol/L benzyl alcohol at 70°C.
The results in Figure 32 show that the conversion of benzyl alcohol increases with increasing
oxygen partial pressures.
The conversion of benzyl alcohol was reduced from 96% to 23% when the partial
pressure of oxygen was reduced from 101 to 33 KPa. The low conversion in the presence of
nitrogen could be due to oxygen from the lattice taking part in the reaction. This apparent
reduction of manganese oxide could be limited to a few surface layers because no difference in
the XRD patterns of the unused and used catalyst was observed (Figure 24).
4C.14. Comparison of laboratory-prepared and commercial manganese oxide
The activity of our laboratory-prepared catalyst was compared with commercially available
manganese oxide catalyst. The results show that our laboratory-prepared catalyst was superior to
the commercial catalyst (Figure 33).
4C.15. Comparison with other catalysts
Comparison of the activity of the present catalyst with other manganese-containing
catalysts (Table 4C.2) shows that the present catalyst is highly active and superior for benzyl
alcohol conversion to benzaldehyde.
4C.16. Batch analysis
To determine the reproducibility of our catalyst preparation, three batches of catalyst
were prepared and tested for activity in benzyl alcohol oxidation. The results shown in Table
4C.3 show no significant difference between batches.
106
Figure 30- Time profile for conversion of benzyl alcohol (BzOH) to benzaldehyde (BzH) and benzoic acid
(BzOOH). Reaction conditions: Initial concentration of benzyl alcohol: 0.2 mol/L in n- heptane, volume: 10
mL, Temperature 343 K, oxygen pressure 1 atm (1 atm=1.013×105), 950 r/min, 90 min.
107
Figure 31: Effect of temperature on the Conversion benzyl alcohol.
Reaction Conditions: Initial concentration of benzyl alcohol: 0.2 mol/L in n- heptane, volume:
10 mL, Temperature 343 K, oxygen pressure 1 atm (1 atm=1.013×105), 950 r/min, 90 min.
108
Figure 32 Effect of partial pressure of oxygen on the reaction of benzyl alcohol at
343 K. Reaction conditions Initial concentration of benzyl alcohol: 0.2
mol/L in n- heptane, volume: 10 mL, Temperature 343 K, oxygen
pressure 33 Kpa-101 Kpa, 950 r/min, 90 min.
109
Figure 33 Comparison of Laboratory-prepared and commercial catalyst.
Reaction conditions: Initial concentration of benzyl alcohol: 0.2 mol/L in n- heptane,
volume: 10 mL, Temperature 343 K, oxygen pressure 1 atm (1 atm=1.013×105), 950
r/min, 90 min.
110
Table 4C.2 Comparison of catalysts: Liquid-phase oxidation of benzyl alcohol using molecular
oxygen as oxidant catalyzed by manganese-containing catalysts
Catalyst Amount
(mg)
BzOH
(mmol)
Solvent
(volume, mL)
Temperature
(K)
Conversion
(%) References
MnOx (110) 50 1 Toluene (10) 383 72 [15]
K(2)Mn(1)/C 200 2 Toluene (10) 373 92 [25]
K(1)Mn(1)/γAl2O3 200 2 Toluene (10) 373 42 [26]
MnxOy 100 2 n-heptane (10) 343 100 Present
work
Table 4.3 Comparison of catalyst activity between batches
Batch Number Conversion (%) Time (min)
1 96 90
2 97 90
3 96 90
111
4C.17. Conclusions
Manganese oxide was prepared by a simple mechanochemical process in the solid state
and was used as catalyst for the oxidation of benzyl alcohol. The synthesized manganese oxide
was characterized as having an octahedral layer structure. The catalyzed oxidation of benzyl
alcohol was carried out in n-heptane solvent over a temperature range 313–343 K and with the
partial pressure of oxygen ranging between 33 and 101.6 KPa. The prepared catalyst showed
remarkable catalytic activity for oxidation of benzyl alcohol under relatively mild conditions.
The prepared manganese oxide was found to be an effective, reusable, and easily separated
heterogeneous catalyst for oxidation of benzyl alcohol.
112
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114
Chapter 4D
Results and Discussions
Reactant: Methylene Blue
Catalyst: Cobalt Oxide
Oxidation/ Degradation of Methylene Blue (MB) using mechanochemically synthesized Cobalt Oxide.
4D.1. Characterization
Cobalt oxide synthesized by mechanochemical route. Characterization of the catalyst was done by surface area and pore size analyzer, XRD, SEM and FT-IR Techniques.
4D.2. Surface Area
The nitrogen adsorption-desorption isotherm of Cobalt Oxide was obtained by plotting
the volume of nitrogen adsorbed against relative pressure, P/Po as given in Figure 34. This
isotherm may be classified as type II according to the Brunauer, Deming, Deming, and Teller
(BDDT) classification [1,2]. This type of isotherm represents unrestricted monolayer-multilayer
adsorption. Its hysteresis loop can be classified as type A, which is a characteristic of cylindrical
shaped capillaries open at both ends [3]. BJH method was used to calculate the average pore
radius, the desorption branch of the isotherm was used for calculation [4]. The average pore
radius was ~ 4.50 nm. Figure 35 shows that 88% of the pores are < 4.5nm where as there is
negligible contribution from pores having radius of < 3 nm. The BET surface area is ~72.48
m2/g.
4D.3. X-Ray Diffractometery
In the XRD pattern of the prepared catalyst various peaks can be observed (Figure. 36 a)
which shows the crystalline nature of the catalyst. Peaks at 2θ = 18.8, 31.2, 36.8, 45.5 and 59.1
are assigned to spinal Co3O4 crystal lattice. Furthermore, as there seems no splitting of the peaks
which conforms that the material is in single phase [5-9]. It can be seen from the XRDs of (36.a)
115
0.0 0.2 0.4 0.6 0.8 1.00
20
40
60
80
100
120
140
160
180
Adsorption Desorption
Vo
lum
e A
ds
(c
c/g
ram
)
Relative Pressure (P/Po)
Figure 34 Nitrogen Adsorption Isotherm for cobalt oxide
Figure 35 Pore size distribution of Cobalt Oxide Catalyst.
116
unused (36.b) used in oxygen flow and (36.c) used in nitrogen flow that there is no difference in
the unused (Figure 36.a) and used (Figure 36.b) catalyst. But the catalyst used in the Nitrogen
atmosphere (figure 36 c) shows that the XRD pattern is distorted. Similar XRD pattern has been
reported for MB intercalation with layered manganese oxide [10]. Considering the reports of MB
intercalation with various inorganic host lattices [11,12], it is concluded that, it is a typical
intercalation reaction, as it occurs near room temperature (30°C). It is reversible. MB can be
extracted back in ethyl alcohol from the catalyst, used in N2 atmosphere, at room temperature. It
is a topochemical process. The structural integrity of the host lattice (cobalt oxide) is conserved
in the course of forward and reverse reaction. The intercalated MB has been widely used as a
second generation biosensor employing an electron transfer mediator. It facilitates electron
communication between the redox center of an enzyme and the electrode surface, as well as it
also overcomes the influence of oxygen variation and interference of electro active species [13].
4D.4. FT-IR Analysis
FT-IR spectra are shown in figure 37. There is no difference in the spectra of fresh
(unused) and the one used in oxygenated atmosphere (figure. 37. a and b). It again confirms our
earlier conclusion from XRD results that MB oxidative degradation occurs on the surface and the
surface is restored to its original configuration due to the presence of oxygen. Figure.37 c shows
the spectrum of the catalyst used in nitrogen atmosphere while 37 d and 37 e are spectra of a
physical mixture of cobalt oxide with MB and of MB respectively. Similarity of (37 c) and (37 d)
again confirms our earlier conclusions (section B 1.2) that MB is intercalated in cobalt oxide by
adsorption and that it is a reversible phenomenon. The absorption bands shown at 3500-3200 cm-
1 and 1600 cm-1 in the entire samples are attributed to bending and stretching vibrations of
adsorbed water. The bands observed at 570 and 660 cm-1 are assigned to spinal Co3O4 [13, 14].
4D.5. SEM
Surface morphology of the unused catalyst, and the one used in nitrogen atmosphere shows that
there is no marked difference in the fresh and nitrogen used catalysts (Figure. 38,a and c). It
could be due to the fact that in the presence of nitrogen MB is only adsorbed and intercalated
between the layers of host lattice as shown by XRD.
117
5 10 15 20 25 30 35 40 45 50 55 60 65
450
900
1350
1800
1530
2040
2550
3060
3300
3450
3600
3750
2 Theta angle
Fresh (a)
Inte
nsi
ty (
CS
P) Oxygen Used(b)
Used In Nitrogen (C)
Figure 36 XRD of cobalt oxide, (a) fresh, (b) under O2 and (c) Under N2 atmosphere.
Change all other captions accordingly.
118
Figure 37 DRIFT Spectra of MB and cobalt oxide (a) fresh, (b) used in O2, (c) used in N2
atmosphere, (d) cobalt oxide mixed with MB and (e) MB.
119
No degradation reaction occurs on the surface of cobalt oxide. As MB is intercalated in
the layered cobalt oxide, therefore, morphology of the surface is unchanged. Figure 38 b shows
that catalyst used in oxygen atmosphere has its surface scuffed up. This could be due to the fact
that in this case degradation of MB occurs by using some of the lattice oxygen thus reducing
cobalt oxide. Re-oxidation of the surface due to the presence of oxygen could occur resulting in
restoration of the same structure (as shown by XRD) with slightly different morphological
features. The difference in the surface morphology could also be due to the presence of carbon,
resulted from the burnout of MB on the surface.
4D.6. Particle size
Particle size of the prepared cobalt oxide catalyst was in the range of 3-10 micron using
the wet method of analysis.
4D.7 Reaction Protocols
Catalytic oxidation reaction was performed in a three necked magnetically stirred Pyrex
glass batch reactor outfitted with a condenser and a digitally controlled thermometer. The
reaction temperature was kept constant at 313 K using an oil bath and a hot plate.
Characteristically 40 ml of 40 ppm MB Solution was taken in the reactor and was saturated with
molecular oxygen, passing molecular oxygen gas at 60 mL/min for 30 minutes. After getting the
required conditions of temperature and pressure 60 mg of the catalyst was transferred to the
batch reactor and temperature and flow of oxygen were kept constant at desired value. The
reaction was agitated at a stirring speed of 900 rpm. Aliquots of the reaction mixture were taken
after the desired intervals of time. Catalyst was removed from reaction sample using Whatman
Glass microfiber filter No. 1825055 and a glass syringe.
The reaction sample was analyzed by using a UV-Visible spectrophotometer model
Shimadzu UV-160A, Japan.
4D.8. Time Profile
The time study for the oxidative degradation of Methylene blue was monitored from time
to time (figure 39). This analysis was of the reaction was carried out at 313 K for 120 minute.
120
Figure38. SEM Images of cobalt oxide, (a) fresh (b) used in N2 (c) used in O2.
121
As the reaction time increase, degradation of MB increases and ~ 90 % of MB is degraded within
10 minutes of the reaction time, showing the excellent efficiency of our catalyst.
4D.9. Detection of Products
Carbon dioxide and water were the only reaction products detected. Lime water test was
employed to confirm the evolution of carbon dioxide. The gas flow passing through the reaction
mixture was passed through limewater, which turned milky, confirming the presence of carbon
dioxide in the reactor effluents.
4D.10. Catalyst Loading
The effect of catalyst weight as a function of oxidative degradation of Methylene blue
dye was investigated in the range of 40– 120 milligrams. All other parameters of temperature and
oxygen pressure as well as agitation speed were kept constant. Figure 40 shows the dependence
of degradation/ oxidation of MB on catalyst loading, suggesting that with the increase of catalyst.
4D.11. Effect of Temperature
Experiments were carried out to study the effect of temperature on the degradation of
MB. The catalytic degradation of MB was investigated as a function of temperature (Figure. 41).
Degradation of MB reaches 100% at 40 °C and above. Activation energy (Figure . 42) of the
reaction is 13.4 kJ mol-1. This low energy of activation shows that the reaction could be due the
diffusion controlled nature of the reaction.
4D.12. Effect of Oxygen Partial Pressure
For reaction at various oxygen partial pressures oxygen and nitrogen were mixed together
in a definite ratio keeping the total flow of the mixed or unmixed gases equal to 60 mL/min.
Equation No.4D.1 was employed to calculate the partial pressure of oxygen
kPaFF
Fp
NO
OO 2.101
22
2
2
(4.D1)
122
Figure 39 Time Profile Studies of MB aqueous solution with cobalt oxide:
Reaction conditions Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow rate: 60 mL/min. Time: 90 minutes, Stirring rate:
900 rpm.
Figure 40 Effect of catalyst loading at 30 ˚C Reaction conditions Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow
rate: 60 mL/min. Time: 90 minutes, Stirring rate: 900 rpm.
123
Figure 41 Temperature profile studies: Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow rate: 60 mL/min. Time: 90
minutes, Stirring rate: 900 rpm.
Figure 42 Arrhenius plots for energy of activation (only four values at the lower temperatures from figure 4.18 were used).
124
Where FO2 represents the flow rate per minute of oxygen
FN2 represent the flow rate per minute of nitrogen.
The effect of oxygen partial pressure on the oxidative degradation of MB was
investigated in the range ~ 0-1 atmosphere. All other parameters of temperature agitation speed
and concentration of MB dye was kept constant (Fig. 42). It is clear from the figure that by
increasing the partial pressure of oxygen the degradation of dye increases.
4D.13. Comparative Study of Oxygen, Nitrogen Flow and Open to Atmosphere
The behavior of the degradation process of the dye studied in various atmospheres.
Figure 43 shows a comparative study of the degradation of MB when oxygen and nitrogen were
bubbled into the reaction mixture or the reaction was kept open to atmosphere. It is evident that
the dye is easily degraded in the oxygen flow as compared to air.
Decolorization of the dye also occurs in nitrogen atmosphere; however, it is a slow
process.
4D.14. Life Span of the Catalyst
There is some reduction in the activity of the catalyst when it is reused. In this case the
catalyst was only dried and reused. It shows a 20 % reduction in activity when the catalyst was
used for the 3rd time. Regeneration of the catalyst in air at elevated temperature is expected to
reinstate the same activity level as shown for the first time. The comparison is shown in figure
44.
4D.15. Adsorption Studies
It has been shown that in the presence of nitrogen MB is adsorbed by cobalt oxide.
Therefore, the adsorption capacity of the catalyst was investigated as a function of MB
concentration (Figure 44). It is interesting to see that the amount adsorbed (mg of MB/g of
Cobalt Oxide) is directly proportional to the initial concentration (Ci) of MB.
125
Figure 42 Effect of oxygen partial pressure at 30 ˚C:
Reaction conditions Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow rate: 60 mL/min. Time: 90 minutes, Stirring rate:
900 rpm.
126
Figure 43 Effect of oxygen, nitrogen and air atmosphere on decolorization of MB
Reaction conditions Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow rate: 60 mL/min. Time: 90 minutes, Stirring rate:
900 rpm. Temperature 30 ˚C
127
Figure 44: Life Span of Catalyst: Temperature.
Reaction conditions Catalyst weight: 60 mg, Solution volume: 40 mL MB concentration: 40 ppm, pO2: one (1) atmosphere, Oxygen Flow rate: 60 mL/min. Time: 90 minutes, Stirring rate:
900 rpm. Temperature 30 ˚C
128
Figure 44 Amount of MB (mg) adsorbed per gram of Cobalt Oxide at 30 ˚C, initial
concentration of MB: 10-50 ppm.
129
4D.16. Conclusion
From the results of the present study we can conclude that Co3O4 can be used as an
effective catalyst for oxidative degradation of MB in liquid phase. The solid catalyst can be
easily separated from the reaction mixture by simple filtration. The reaction is 100% efficient for
complete mineralization of MB and maximum (more than 90 %) of the dye is degraded within
the first ten minutes. Moreover, the efficiency of the catalyst is good enough to be used for
longer periods of time and the degradation of the dye decreases with the decrease of oxygen
contents in the reaction mixture. Furthermore, in the absence of nitrogen MB is adsorbed and can
be reclaimed by simply washing with ethyl alcohol. The simple procedure of intercalation of MB
with cobalt oxide is a very important finding of this work, which could be exploited further to be
used as biosensor material.
130
Chapter 4E
Results and Discussions
Reactant: Congo-Red Dye
Catalyst: Co3O4
Oxidation/ Degradation of Congo-Red (Cr) By Cobalt Oxide Dye.
4E.1 Reaction Protocols
Experiments on the oxidation of Congo-Red at pH 6-7 in the presence of the cobalt oxide
catalyst were carried out having an initial concentrations of Congo-Red = 50 ppm, at three
different temperatures (30,35, and 40 °C). For a typical run, 50 mL of CR solution with a
specified concentration was taken in reactor and the temperature of the reactor was kept constant
at a desired value by the help of heating plate. After reaching the desired temperature, 1 mL
sample was taken to determine any change in the CR concentration during the heating period.
Then catalyst (30 mg) and molecular oxygen (60 mL/min) were introduced immediately into the
reactor, while a magnetic agitator continuously stirred (950 rpm) the solution. The prepared
cobalt oxide catalyst was found to be very active towards the degradation/ decolorization of
Congo-Red dye at room temperature. At appropriate time intervals of time small aliquots were
taken from the reaction mixture and catalyst was separated from reaction mixture with Whatman
Glass microfiber filter No. 1825055 using glass syringe. Analysis of the reaction mixture was
carried out by UV-Visible spectrophotometer (Shimadzu UV-160A, Japan) The percent
degradation of Congo red, was calculated from equation (4.11)
100)()(
)()((%)
o
to
dyeC
dyeCdyeCtionDecoluriza (4E.1)
Where C (dye)0 is the initial concentration and C (dye)t is the concentration of Congo-Red dye
at specified reaction time.
131
4E.2. Characterization
The BET surface area of the catalyst was found to in the region of 73 m2/g. The average
pore radius of the as synthesized catalyst was found to be around 4.5 nm. The diffraction patterns
were found to have dominated by Co3O4 phase and the catalyst is crystalline in nature. The FT-IR
analysis reveals that the catalyst is mainly composed of spinal form of Co3O4.
4E.3. Time Profile
Reaction time on the removal of dye is displayed in figure 45. The result shows that
cobalt oxide is a very active catalyst for the degradation of Congo-Red dye under the flow of
molecular oxygen. More than 80 % conversion/ decolorization, was achieved in the first 30
minutes at room temperature and normal pH, where the initial concentration of the dye was 50
ml, 50ppm. 96% dye was degraded in 90 minutes reaction time; however 98-99% decolorization
can be achieved at higher temperatures. Hence we can attribute that cobalt oxide synthesized by
mechanochemical process can be effectively used for the decolorization of Congo-Red dye.
4E.4. Decolorization of Congo-Red with different catalysts
A number of catalysts were tested for the decolorization of Congo-Red dye, among them
manganese oxide, nickel oxide, iron oxide and cobalt oxide were tested for this purpose. Table
4E.2 represents the decolorization efficiency of these catalysts in 90 minutes reaction time.
Among the catalysts tested cobalt oxide was found to be the most efficient at room temperature.
4E.5. Detection of Products
Carbon dioxide and water were the only reaction products detected. The formation of
carbon dioxide was confirmed by limewater test. The gas flow passing through the reaction
mixture was passed through limewater, which turned milky, confirming the presence of carbon
dioxide in the reactor effluents.
4E.6 Effect of Catalyst Loading
The effect of catalyst loading on the progress of oxidation of CR was studied in the range
of 10–100 mg, while all other parameters were kept constant. Figure 46 shows the dependence of
degradation/ oxidation of CR on catalyst loading, suggesting that with the increase of catalyst
132
Figure 45 The color removal of Congo-Red dye over time, pH 7, Atmospheric pressure. Reaction Conditions: 50 ml, 50 ppm, Reaction temperature 30 oC – 40 oC , Agitation speed 950
rpm
Table 4E.2: Decolorization of Congo-Red with different catalysts
75
80
85
90
95
100
0 20 40 60 80 100
% Decolorization
Time (minutes)
30 C 35 C 40 C
S/No Catalyst Name Decolorization% Reaction Time(mines) Reaction temperature( oC)
1 Nickel oxide 63 90 30
2 Iron Oxide 52 90 30
3 Manganese oxide 79 90 30
4 Cobalt Oxide 96 90 30
133
load under the same conditions the degradation process of the dye increases in linearly at the
start and then becomes constant for loading above 70 mg loading. Complete dye removal is
achieved n 30 minutes with loading above 70 mg. It also shows (Figure 46) that for loading
below 60 mg the degradation efficiency is lower.
4E.7 Effect of Temperature
Temperature effect on the degradation of Congo-Red dye was investigated at different
temperatures was investigated. Complete removal of dye is achieved at temperature above 30oC.
Figure 47 represents the dependence of temperature on the dye removal. Activation energy of the
reaction is 11.1 kJ mol-1 (Figure 48). The low activation energy suggests the reaction is occurring
in the diffusion controlled region.
4E.8. Effect of Oxygen Partial Pressure
Oxygen partial pressure was for the reaction was investigated in the range of 17 – 101
Kpa. For this purpose oxygen was mixed with nitrogen gas keeping the total flow equal to
60ml/min. The partial pressure of oxygen was calculated according to equation 4.1.
The effect of oxygen partial pressure on the progress of oxidation/degradation of CR was
studied in the range ~ 0-1 atmospheres while all other parameters were kept constant, (Fig. 49).
It is evident that with the increase of partial pressure of oxygen the rate of degradation of the dye
increases, reaching 99% activity at 101 Kpa in 90 minutes.
4E.9. Life Span of the Catalyst
The reusibity of the catalyst was found to be very good, and the catalyst was found to
work good for 5 consecutive times; however its activity decreased to some extent with each use.
It was observed that when the catalyst was washed in water followed by washing in ethanol and
then it was calcined at 300 oC, the activity of the catalyst was regenerated. The comparison is
shown in the following figure (Figure 50)
134
Figure 46 Effect of catalyst loading on the decolorization of Congo-Red dye
Figure 47 Temperature profile studies, 50 ml.50 ppm solution, and reaction temperature 30 oC, Oxygen 60ml/min (1 atm)
66
71
76
81
86
91
96
101
0 20 40 60 80 100
Deg
rad
atio
n (
%)
Catalyst Loading (mg)
80
85
90
95
100
105
15 20 25 30 35 40 45 50
Decolorization ( %)
Temprature ( Degree Centigrade)
135
Figure 48 Arrhenius Activation energy (Ea) plot for Congo-Red degradation- Calculated at four temperatures only
136
20 40 60 80
40
50
60
70
80
90
100
% D
eg
rad
atio
n
Time (mins)
101 kpa 84.1 kpa 67.3 kpa 50.5 Kpa 33.66 Kpa 16.83 kpa
Figure 49: Effect of oxygen partial pressure at 30 ˚C: Same conditions as employed for figure 47.
137
Fresh 2nd Use 3rd Use 4th Use 5th Use Ethanol Treated0
20
40
60
80
100
Deg
rada
tion
(%)
Figure 50: Life Span of Catalyst: Temperature 30˚C: other conditions same as employed for figure 47.
138
4E.10. Conclusion
From the results of the present study we can conclude that Co3O4 can be used as an
effective catalyst for oxidative degradation of CR from aqueous solution. The solid catalyst can
be easily separated from the reaction mixture by simple filtration. The reaction is 100% efficient
for complete mineralization of CR and maximum (more than 90 %) of the dye is degraded within
the first 30 minutes. Moreover, the efficiency of the catalyst is good enough to be used for longer
periods of time and the degradation of the dye decreases with the decrease of oxygen contents in
the reaction mixture.
This catalyst is very active for the decolorization of these organic and persistent dyes, as
we have shown for the decolorization of Methylene Blue and Congo-Red and hence we suggest
that it will be effective for other organic dyes, we are planning to use it for other dyes in the
future.
139
Conclusions and Overview
The general conclusions that can be deducted from this work are as follows.
1) Benzyl Alcohol Oxidation under solvent free conditions
Catalyst: 0.1 wt % Palladium/Zirconia
Oxidant: Oxygen
Palladium supported zirconia was synthesized by impregnating palladium oxide on
monoclinic zirconia. After calcination and reducing/ activation under hydrogen flow, it was
employed as heterogeneous catalyst for the oxidation of benzyl alcohol in solvent free
conditions. Monoclinic zirconia was found to be inactive for the alcohol oxidation however;
monoclinic zirconia was very active towards the oxidation of benzyl alcohol. The reaction
was carried out under mild condition of temperature (343-373K). The reaction was carried
out under atmospheric pressure. A green, cheap and easily available oxidatant such as
oxygen was used as oxidant. The catalyst was found to be very active in oxidizing benzyl
alcohol. Only two reaction products were detected when analyzed by GC i.e. Benzaldehyde
and benzoic acid. However the reaction under these conditions were highly selective towards
benzaldehyde formation and the TOF was found to be more than 6000/hour. This TOF is
among the highest reported in the literature so far. The catalyst is easily recovered from the
reaction mixture by simple filtration. The catalyst was checked for a number of parameters
like catalyst loading, time profile, life span of the catalyst, effect of mass transfer limitations,
and Kinetic Analysis. The time profile data at various partial pressure of oxygen was applied
to kinetic models. Langmuir Hinshelwood model was found to be applicable to benzyl
alcohol oxidation in liquid phase solvent free conditions. When as solvent such as n-heptane
was used, Temkin equation was found to be applicable to the benzyl alcohol oxidation
reaction. The activation energy was found to be 49.1 KJ/ mole. The summary of the results
is shown in table 4.4.
140
Table 4.4 Oxidation of Benzyl Alcohol
Temperature (oC)
Selectivity ( % ) Conversion (%) TOF*/Hour
BzOH Bz-OOH
Solvent Free Conditions
100 71.7 29.7 36.2 6178
90 73.5 25.9 26.4 4506
80 75.8 24.0 13.9 2372
70 74.8 24.9 10.3 1758
Solvent 80 oC
n-heptane 100 ---- 40.8 36
n-octane 100 ---- 98.3 87
*Turnover frequency (TOF): molar fraction of BzOH converted to the palladium content in the
catalyst per hour.
141
2) Toluene Oxidation under solvent free conditions
Catalyst: 0.1 wt % Palladium/Zirconia
Oxidant: Oxygen
Palladium/zirconia (0.1 wt% and monoclinic phase of zirconia) was checked for the oxidation of
toluene in batch reactor conditions. Molecular oxygen was used as oxidant. The reactions were
run at five different temperatures (333-373K). 0.0936 moles of toluene was used in the reactor.
The samples were analyzed for qualitative and quantitative analysis by GC and UV-visible
spectrophotometer. The known products of the oxidation were benzyl alcohol, benzaldehyde and
benzoic acid. At lower reaction temperature the reaction was found to be more selective to
benzyl alcohol formation but as the temperature of the reaction was increased gradually the
selectivity of the product was changed to benzoic acid formation. The TOF was found to be
14000/hour. The summary of the above discussion is represented in table 4.5. Different
parameters like catalyst loading, time profile, life span of the catalyst, effect of mass transfer
limitations were also considered in this study.
142
Table 4.4: Conversion and selectivities at different temperatures
Temperature (K) Conversion (%) Selectivity (%) TOF*
BzOH BzH BzAc
333 3.6 75.8 15.8 8.3 1793
343 8.6 60.6 17.2 22.1 4274
353 14.4 59.2 23.5 17.4 7158
363 19.7 53.7 24.3 21.9 9799
373 28.1 53.7 24.3 21.9 13999
Turn over frequency (TOF*): Molar ration of toluene to Pd content per hour.
Reaction Constants : Catalyst 0.2 g, Pressure of oxygen 101 kPa, Time 1 hour, agitation
950 rpm, Toluene 0.0936 moles
143
3) Benzyl Alcohol Oxidation in liquid phase conditions
Catalyst: Manganese Oxide
Oxidant: Oxygen
Solvent: n-heptane
Benzyl Alcohol oxidation was carried out in batch reactor conditions. n-heptane was used as
solvent and oxygen was employed as oxidatant. Laboratory prepared manganese oxide was used
as heterogeneous catalyst for the oxidation of benzyl alcohol. Manganese oxide was prepared by
solid phase mechanochemical method. The catalyst was characterized by several analytical
methods before and after the reaction. The following analytical/physical methods were used to
characterize the catalyst. (i) Surface area and pore size analysis, (ii) Particle size analysis, (iii)
XRD, (iv) FT-IR, (v) and SEM analysis. Based on the XRD data it was concluded that MnO
phase was responsible for the activity of benzyl alcohol oxidation. Manganese oxide catalyst was
found to 100% selective towards the formation of benzaldehyde. When all the benzyl alcohol is
converted to benzaldehyde than benzoic acid formation starts which suggests the consecutive
nature of the reaction. The catalyst was found to be easily recoverable and can be used efficiently
for a number of times again with no change in selectivity. However the activity decreases to
some extent with each consecutive use. However if the catalyst after regeneration is washed
thoroughly with plenty of water-Ethanol mixture and then dried, its activity is regenerated. This
method is suitable for the large scale production of benzaldehyde because manganese oxide can
be very easily synthesized by mechanochemical method.
4) Methylene Blue Oxidative degradadation in aqueous medium
Catalyst: Cobalt Oxide
Oxidant: Oxygen
Cobalt oxide was synthesized by solid state mechanochemical method and was characterized by
the techniques mentioned above. The cobalt oxide synthesized was determined to be in Co3O4
phase. The catalyst was found to be extremely efficient in removing the dye from the water. In
the first 10 minutes > 90 % of the dye was examined to be removed. The solid catalyst can be
easily separated from the reaction mixture by simple filtration. The reaction is 100% efficient for
the complete mineralization of MB dye. Moreover the efficiency of the catalyst is good enough
to be used for longer periods of time and the degradation of the dye decreases with the decrease
144
of oxygen contents in the reaction mixture. Furthermore in the absence of oxygen MB is
adsorbed and can be reclaimed by simple washing with ethanol. The simple procedure of
intercalation of MB with cobalt oxide is very important finding of this work, which could be
exploited further to used as biosensor material.
5) Congo Red Dye Oxidative degradadation in aqueous medium
Catalyst: Cobalt Oxide
Oxidant: Oxygen
From the results of the present study we can conclude that cobalt oxide can be used and efficient
catalyst for the oxidative degradation of CR from aqueous solution. The solid catalyst can be
easily recovered from the reaction mixture by filtration. The reaction is 100% efficient in the
removal of the dye. The catalyst can be used for longer times.
The summary of the results can be shown in the table below.
Catalyst Reactants Conversion
Pd/ZrO2
Benzyl alcohol and
Oxygen 36 % conversion of 10 mL, 0.097 moles at 100 °C in 180 minutes.
Toluene 28% of 10 mL, 0.0936 moles at 90 °C in 180 minutes.
Cobalt oxide
Methyleneblue and
oxygen Full conversion of 40 mL, 40 ppm at 30 °C in 90 minutes.
Congo red and
oxygen Full degradation 50 mL, 50 ppm at 30 °C in 90 minutes.
Manganese oxide Benzyl alcohol and
oxygen
About 97% conversion of 10 mL, 0.2 M at 70 °C n-n-heptane as
solvent,180 minutes.
145
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