leader project final

8/6/2019 Leader Project Final

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Kinetics of Gas/Solid Catalytic Reactions: Hydrogenation

of Chloronitrobenzene as a Case Study

2010-2011

Bielser Jean-Marc &

Beswick Oliver

Assistants :Hunt Phil Morten

Fernando Crdenas-Lizana


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Kinetics of Gas/Solid Catalytic Reactions:

Hydrogenation of Chloronitrobenzene as a Case

Study Chemical engineering lab & project(2010-2011)

2

0 Table of contents

0 Table of contents ______________________________________________________________ 2

1 The Goals of the Project _______________________________________________________ 3

2 Introduction _________________________________________________________________ 5

3 Experimental _________________________________________________________________ 7

3.1 Characterization ________________________________________________________________ 7

3.1.1 Atomic Absorption Spectrometry (AAS) _____________________________________ 7

3.1.2 Temperature controlled reduction (TCR) _____________________________________ 7

3.1.3 X-ray diffraction (XRD) _________________________________________________ 7

3.1.4 Transmission electron microscopy (TEM) ____________________________________ 7

3.2 Manipulations __________________________________________________________________ 73.3 Blank tests and reproducibility ____________________________________________________ 11

3.4 Turnover Frequency Calculation ___________________________________________________ 11

3.5 Madon-Boudart correlation _______________________________________________________ 13

3.6 Activation energy ______________________________________________________________ 13

3.7 Contact time calculation _________________________________________________________ 14

3.8 Effect of partial pressure (H2 andp-CNB) ___________________________________________ 14

4 Results and Discussion ________________________________________________________ 16

4.1 Characterization _______________________________________________________________ 16

4.1.1 Atomic Absorption Spectrometry (AAS) ____________________________________ 16

4.1.2 Temperature controlled reduction (TCR) ____________________________________ 16

4.1.3 X-ray diffraction (XRD) ________________________________________________ 16

4.1.4 Transmission electron microscopy (TEM) ___________________________________ 17

4.2 Calibration ____________________________________________________________________ 18

4.3 Blank tests ____________________________________________________________________ 20

4.4 Reproducibility ________________________________________________________________ 20

4.5 Kinetic Regime ________________________________________________________________ 21

4.5.1 Madon-Boudart correlation ______________________________________________ 21

4.5.2 Apparent activation energy ______________________________________________ 22

4.6 Time-on -stream effect __________________________________________________________ 24

4.7 Deactivation __________________________________________________________________ 25

4.8 Contact time effect _____________________________________________________________ 27

4.9 Effect of partial pressure _________________________________________________________ 28

5 Conclusion __________________________________________________________________ 30

6 List of symbols ______________________________________________________________ 31

7 References __________________________________________________________________ 33


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1 The Goals of the Project

The aim of this laboratory project was to study in details the kinetics of a gas phase reaction over

(powder) supported catalyst in a fixed-bed (tubular) reactor. The model reaction selected,

hydrogenation ofpara-chloronitrobenzene (p-CNB) catalyzed by a gold-based catalyst, is induced to

producepara-chloroaniline (p-CAN). As the reactant used ispara-chloronitrobenzene, the cleavage

of the carbon-halogen bond is the unwanted step (see Figure 1). The desired product, para-

chloroaniline is a high valuated industrial intermediate product.

Figure 1: Main reaction pathways associated with the hydrogenation ofp-CNB. The targeted route top-CAN is

represented by the solid arrow. [3]

By examining in details different effects on the product distribution like change in contact time,

temperature and partial pressure of the reactants, this project could serve as a basis for a study which

would focus on the feasibility of the selective nitro group reduction as a viable way to produce aniline

compounds.

The first step was to demonstrate that the reaction is exclusively catalytic. Then, the reproducibility of

the system was analyzed. It was also essential to study the optimum reaction at which the intrinsic

kinetics can be approached in order to optimize the reaction rate. Obviously it was necessary to prove

that intrinsic kinetics controlled the reaction. Thus we wanted to study and understand the influence of

those parameters on the reaction kinetics.

Thep-CNB conversion will be calculated as well as the selectivity to p-CAN as the target product in

order to realize this present study.


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The catalyst activity was expressed in terms of turnover frequency (TOF) which is defined as being

the number of produced molecules per active site per time.


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2 Introduction

Functionalized anilines are important intermediates in chemical industry processes. Often used in themanufacture of agrochemicals (herbicides), pharmaceuticals, dyes and also pigments, they are

obtained via hydrogenation of the correspondent functionalized nitrobenzenes. Nevertheless, the

hydrogenation must be highly selective to avoid competitive hydrogenation of a double bond, triple

bond or the cleavage of a carbon-halogen bond [2][3].

Amines in industry are still produced by the so-called Bechamps reaction and some other non-

catalytic reactions [1]. The process based on Bechamp synthesis route consists in using iron filings to

reduce aromatic nitro compounds into their corresponding aniline in an acidic medium. Because of

lower selectivities than catalytic reactions those processes produce large amounts of waste which

sometimes may be toxic or dangerous as iron sludge. The use of catalysts offers a cleaner alternative.

A good catalyst must be highly selective, active and stable. Indeed catalysts undergo deactivation and

this phenomenon should not be limiting for industrial uses. For industries using catalytic reactions the

catalyst regeneration must be easy to realize. Its price should not be limiting for process purposes. A

commercially available catalyst should be used since the preparation of a heterogeneous catalyst is

often tedious [2][3].

Many research groups, at industrial and academic level, have studied the hydrogenation of

halonitrobenzenes. The key challenge in this group of reactions is the exclusive nitro-group reduction,

i.e. high selectivity. In order to achieve this goal a number of parameters have been modified such as

the addition of organic or inorganic modifiers, tailoring the particle size of the metal or the support. Pt

catalysts modified with Pb or H3PO2 (promoted with FeCl2 and V complexes respectively) are

examples of modified catalysts [2].

Pt, Pd and Ni based catalysts have already shown good results in terms of selectivity, conversion and

thus yield in the liquid phase hydrogenation process of chloronitrobenzenes. For example selectivity of

98-99.9% and conversion of 100% were obtained with a bimetallic Pt-Fe/TiO2. Good selectivity has

been obtained even in presence of carbon-carbon double or triple bonds [2]. Nevertheless no catalyst

previously mentioned gave a 100% selectivity response.

In recent literature 100% selectivity was obtained over supported silver and/or gold catalysts on

different oxide supports, e.g. TiO2 and Al2O3 [2][3]. This is a result that confirms the potential of this

metal in terms of selectivity.

Despite on the number of publications in the subject, the selective hydrogenation of halonitrobenzenes

continues gaining attention, the main parameters that have been demonstrated to impact on catalytic

response can be summarized as follow [13]:

Metal catalyst (Pt/Pd/Ru): Noble metals have an excellent activity for the hydrogenation ofhalonitrocompounds. The activity or the hydrodehalogenation are factors that change

depending on the metal used [13].

Metal support (Al2O3/TiO2/Fe2O3): Many studies made with different metal particles.Differences in both activity and selectivity were observed by changing a support. For example,

A tenfold increase in TOF over Pt/TiO2compared to Pt/-Al2O3 was observed [13].


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Metal particle size: Generally the selectivity increases when the particle size is reduced. Onthe other hand the highest selectivity was reached with a large Pt particle (12.3nm). The

dechlorination was suppressed to some extent because p-CAN easily desorbs from large

particle surfaces. It was also shown that the specific activity could increase (15 times withRu/-Al2O3 catalyst with a dispersion decrease from 1 to 2.510

-4[H/Ru]) [13].

Reaction parameters (reaction temperature, hydrogen pressure, concentration of substrate,reaction medium etc.): Interesting studies have been made, for example it has been shown that

the reduction rate of nitro group was higher in the protic solvents than in the non-protic

solvents, following the order: ethanol > methanol > cyclohexane > methylacetate [13].

Regardless on the number of studies on hydrogenation ofp-chloronitrobenzene, no study was found

about the kinetics of this reaction in gas phase over gold supported catalyst. This is the reason why this

particular reaction was studied on gold based catalyst. In gas phase the reaction is run at atmospheric

pressure and often at lower temperatures than in liquid phase. This implies energy savings which arenot negligible for large industrial processes. Furthermore the gas phase allows to synthesis more

complex aromatic compounds than liquid phase. Therefore this study could serve as a basis for other

catalytic synthesis.

Some characterization of the used catalyst will be done for this project. Certain information about the

catalysts size for example must be measured for the calculations.


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3 Experimental

3.1 Characterization

The focus of this project is the kinetics of the studied reaction. Nevertheless some characterization

experiments were made to define clearly the catalyst used. Some parameters must be known in order

to perform the calculation made in this project. Therefore the techniques used will be briefly

explained.

3.1.1 Atomic Absorption Spectrometry (AAS)

The percentage of gold in the catalyst is measured using AAS. The concentration of gold is given in

ppm. The light absorbed by the detector measures the atomic density in the solution. It is then possible

to convert the result in weight percent.

3.1.2 Temperature controlled reduction (TCR)

A known amount of catalyst is contacted to a hydrogen flow. The experiment starts at room

temperature and is heated linearly. A detector will measure the amount of hydrogen and detect either

hydrogen consumption (negative peak) or hydrogen release (positive peak). These peaks indicate for

example the temperature at which the catalyst precursor reduces to metallic gold.

3.1.3 X-ray diffraction (XRD)

X-ray diffraction is used to characterize the crystalline structure of the solid compound of interest or to

determine the chemical composition of the crystalline structures. Libraries of profile are available in

order to relate the results to different compounds. This technique will not be discussed in more details

as it is beyond the scope of this project but it will nevertheless be interesting to comment the profile

obtained.

3.1.4 Transmission electron microscopy (TEM)

An electron beam is passed through a thin layer of catalyst. A special camera will take the picture of

the interactions between the electrons and the catalyst. High resolution pictures could be obtained.

This technique was used to determine the particle size of the catalyst before and after the reaction.

3.2 Manipulations


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The reactions were conducted at atmospheric pressure in gas phase over Au/Al 2O3 in a fixed-bed

(tubular) reactor over the following temperature range: 120C T 170C. This is a vertical tubular

glass reactor containing the fixed catalytic bed. An oven is used to heat up the column. The reaction is

conducted under isothermal conditions, thus the temperature in the catalyst bed is continuouslymonitored using a thermocouple inserted in a thermowell. A co-current flow of hydrogen and organic

compound passes through it. The organic compound is dropping at a constant rate (using an infusion

pump) and the drops will evaporate in the heated column. Different layers are disposed on top of a

fixed bed (see Figure 2). From top to bottom these are:

- Glass beads: They ensure the mixing of the hydrogen with the vaporized organic reactant.- Glass wool: As the glass beads they ensure a good mixing between the reactants.

Moreover it provides a physical separation between the catalyst and the glass beads to

ensure that the catalyst bed remains homogeneously distributed forming a thin layer.

- Catalyst mixed with ground glass (75m): Ground glass is used to maintain the volume ofthe bed constant. To control the activity of the catalyst, the amount of catalyst will be

modified. Therefore, by adjusting the volume of ground glass, and if the volumetric flow

of gases is constant too, the contact time between the reactant and the catalyst is

maintained constant. This combination also ensures the isothermal (1K) condition.

- Glass wool: Physical separation between the catalytic bed and the porous glass frit.


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Figure 2: Schematic diagram of the continuous gas phase reactor [3]


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The effluent is condensed by using a cold trap (liquid nitrogen), collected every 30 minutes and taken

for GC analysis. The temperature is monitored during the whole reaction to ensure isothermal

conditions. After each sampling the gas flow rate will also be monitored using a continuous digital

flowmeter.

The catalyst precursor used was HAuCl4. The catalyst has been previously activated (to formed

metallic gold, active form for the catalytic reaction) and passivated, before each run the passivating

oxygen layer on the catalyst will be removed by leaving it for 30 min in the reactor with 60 cm3min

-1

H2 at the reaction temperature since we have demonstrated elsewhere the removal of this passivation

layer at T > 120C.

The conversion and the selectivity were then calculated using the gas chromatography (GC) results.

As it will be shown in the calibration part, the area is proportional to the amount of moles in the

analyzed sample. Therefore the conversion (Equation 1) and the selectivity (Equation 2) were

calculated using the following equations:

(1)100

%Area100

CNB-p

CNBpCNBpX

CNBoutp

in

outin

CNB-p

(2)%Area100

%Area

CNBpCNBp

CANpS

CNBoutp

CANoutp

outin

out

CAN-p


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How the calculations have been realized is shown in details hereinafter.

In order to calculate total number of gold atoms at the surface NS, Au, the specific surface area of gold

SSAAu (Equation 3) and the dispersion D need to be determined:

(3))g(md

6SSA 12

AuAu

Au AuAu

Where Au is the gold density and dAu is the diameter of the gold particles. The diameter of the

particles is found using transmission electron microscopy (TEM). The dispersion is the ratio of

number of atoms of metal at the surface over the total number of metal atoms (Equation 4).

(4)(-)NA

MSSAD

AAu

AuAu

Where MAu is the atomic weight of gold, AAu is the surface of one atom of gold and NA is the

Avogadro number.

(5)surface)at theatomsgoldof(numberDM

NloadingmN

Au

Acat

surfaceAu,

Where mcat is the total mass of catalyst (including the support) and the loading is the ratio of the mass

of metal in the catalyst over the total mass of catalyst.

The next step is to calculate the number of molecules converted per time which requires the

conversion (Equation 6).

(6)time)ofunitperconvertedCNBof(moleculesNM

QXN

ACNB-p

CNB-pCNB-pCNB

Where Qp-CNB, p-CNB, and Mp-CNB are the volumetric flow rate, the density and the atomic weight ofp-

CNB.

Finally the TOF (Equation 7) is obtained by dividing the reactant converted by the metal atoms on the

surface:

(7)surface)at theatomsAuofnumberperandtimeofunitperconvertedCNB-pof(moleculesN

NTOF

surfaceAu,

CNB


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3.5 Madon-Boudart correlation

To ensure that neither external nor internal mass transfer occurs, a Madon-Boudart correlation was

applied [7]. The measurements were made at two different temperatures changing the gold loading but

keeping the same dispersion. The ln(rate) vs ln(concentration of active sites) was plotted. If a slope of

one is observed it means the reaction is not limited by mass transfer.

The number of gold particles at the surface per gram of catalyst was calculated using Equation 8.

(8)DM

1

100

weight%m(mol)n

Au

AusurfaceAu,

This value was then divided by the total mass of catalyst (0.5g).

The reaction rate was calculated in terms of moles converted per second per gram of catalyst using

Equation 9.

(9)m

1X

3600

Frate

catalyst

CNBp

in

3.6 Activation energy

Another way to know if there is mass transfer limitation is to calculate the apparent activation energy.

If this last is greater than 15 kJ/mol the reaction is not limited by mass transfer limitations. Theapparent activation energy will be calculated using an Arrhenius plot. Therefore it is necessary to

calculate the kinetic rate constant of the reaction (at different temperatures). At this point it is only

possible if one of the reactants (hydrogen) is set in excess (the concentration is assumed to be

constant) and a first order is assumed. From the balance of a plug flow reactor Equation 10 is

obtained:

(10)dwr)()dF(FF AAAA

AAAAA Ck'r:sassumptionthefromanddCQdFandCQF

(11)Ck'dw

dCQ A

A

A

(12)dXCCandX)(1CCC

CCX AAA0A

A0

AA0

(13)dwQ

k'

X)(1

dXA


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14

(14)Q

wk')

X1

1ln(X)ln(1 A

k is then expressed in moles of reactant per unit of time per moles of gold. Different values of k willbe calculated by changing the amount of catalyst. Equation 14 was expressed with different units

(refer to the list symbols). The slope of the plot of Equation 15 is k.

(15)F

nk')

X1

1ln(

CNB-p

The activation energy was then calculated using a plot of the Arrhenius equation (Equation 16).

(16)T

1

R

Eln(A))ln(k' A

3.7 Contact time calculation

The contact has a similar definition than the residence time in the sense that a volume has to be

divided by a volumetric flow rate to obtain it. In this case the volume of the catalytic bed and the

volumetric flow rate of hydrogen are considered.

The volume of the catalytic bed has been calculated by assuming its height being constant. Therefore

the same value has always been used.

(17)4

hdV reactor

2

reactor

bedcat.

Where dreactor and hreactor are the diameter and the height respectively of the reactor.

(18)Q

VmeContact t i

2H

bedcat.

3.8 Effect of partial pressure (H2 andp-CNB)

The effect of partial pressure was measured to determine the partial reaction orders for both hydrogen

and p-CNB.

To measure the effect of the H2 partial pressure, the reactant was diluted and the concentration fixed.

The hydrogen flowrate was then modified from 1 to 1 stochiometry to far excess of H 2 (up to 129

times more hydrogen than necessary).

Same experiments were made to measure the effect of the partial pressure ofp-CNBusing a constant

hydrogen flow and varying thep-CNB concentration.


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The power rate law relates the reaction rate to the partial pressures of the reactant and can be written:

(19)PPkr m CNBpn

H2

Where n and m are the partial reaction orders of, respectively, H2 andp-CNB.

Nevertheless if one parameter is fixed then the power rate law is dependent on only one parameter

(e.g. if the partial pressure of hydrogen is fixed it will only depend on the partial pressure ofp-CNB).

As the studied reaction is catalytic the reaction rate was always expressed in terms of turnover

frequency. The turnover frequency is proportional to the reaction rate.

(20)PPkTOF m CNBpn

H2


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4 Results and Discussion

4.1 Characterization

4.1.1 Atomic Absorption Spectrometry (AAS)

AAS has confirmed 1.05% wt. Au content in the Au/Al2O3 catalyst.

4.1.2 Temperature controlled reduction (TCR)

The precursor (HAuCl4) put on the alumina support was used for this analysis. Hydrogen consumption

is observed at around 175C where Au3+

is reduced to Au0 (Figure 3). This is the reason why the

catalyst was activated to 333C because the metallic gold is the active form of the catalyst.

Figure 3: Temperature controlled reduction

4.1.3 X-ray diffraction (XRD)

Figure 4 is the profile obtained by x-ray diffraction over the catalyst used in this project. The profileobserved is characteristic of delta Al2O3 (i.e. main peak at 67o, a secondary reflexion at 46o and a


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broad peak over the range 33-41o). No characteristic peak for gold was observed for two possible

reasons. Either not enough gold is present in the sample or the gold particle size is smaller than the

detection limit (~


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Figure 5: Transmission electron microscopy (TEM) was used to take this picture. The gold particle (dark spherical

form) has a diameter of about 7nm

4.2 Calibration

The samples are analyzed by GC (gas chromatography), therefore the first step is to identify the

correlation factor between the number of moles and the area percentage. A calibration curve is

therefore calculated experimentally.

Figure 6 shows the correlation between the mole fraction and the area fraction of p-CAN in a solution

containing both p-CNB and p-CAN. The solvent of this solution is ethanol. Each point corresponds to

a different solution with known concentrations of both compounds.

Figure 6: Calibration curve ofp-CAN.


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The point (0; 0) is obvious and the line was therefore forced through this point. The linear correlation

between the number of moles and the area is hereby proven to be one.

The same method is applied for all the possible product distribution combinations and the two

following figures (Figure 7 and Figure 8) show the results obtained.

Figure 7: Calibration curve of nitrobenzene (NB).

Figure 8: Calibration curve of aniline (AN).


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Graphically the similarity of both results seems obvious. The conversion decreases in the same way

with time and the selectivity is found to be 100% any time of the experiment.

The selectivity top-CAN was observed to be 100% in each reaction made. This property was already

discovered in other studies in liquid phase [2].

4.5 Kinetic Regime

4.5.1 Madon-Boudart correlation

To ensure that no mass transfer limitation occurs and thus that the reaction is under kinetic control, a

Madon-Boudart diagram was used (see Figure 10). The plot is ln(rate) vs. ln(concentration of activesites) and if no mass limitation occurs (nor external nor internal), a slope of one is obtained for two

different temperatures [7]. Figure 10 confirms that the reaction is under kinetic control.

Figure 10: Madon-Boudart diagram


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The reaction is the slowest step of the overall reaction if no mass transport occurs. It means that the

reaction kinetics can be studied in more details to characterize the intrinsic kinetics of this selective

hydrogenation reaction.

4.5.2 Apparent activation energy

As an example the calculation ofk (as explained in 3.6 Activation energy) is shown for a reaction

temperature of 120C. The range of catalyst used is 0.0200.045g.

Figure 11: Treaction=120C and catalyst amount range = 0.020 - 0.045g

The assumption that the reaction follows a first order kinetic is verified in Figure 11 as the

experimental data fits the model derived in the experimental part. Therefore from Figure 11 the rate

constant could be extracted using the slope. The relation used was derived in the experimental part and

the equation obtained was:

(15)F

nk')

X1

1ln(

CNB-p

The Arrhenius equation was then used to plot the three k obtained at different temperatures (range:

120 170C). The activation energy was then calculated using the slop of the line obtained (Figure

12).

CNBpCNBp

Amol

kJ85mol

J84561.710171)(8.314slopeRE


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Figure 12: ln(k') versus 1/T, the activation energy was extracted from the slope

Again the result proves that over the temperature range tested the reaction is under kinetic control as

the apparent activation energy is greater than 15kJ/mol.

Similar values for selective hydrogenation in liquid phase were already obtained in different studies.

One study investigated the hydrogenation of para-nitrophenol to para-aminophenol in a laboratory

scale batch slurry reactor. The apparent activation energy obtained was 61kJ/mol [11]. A second study

investigated the selective hydrogenation of ortho-chloronitrobenzene over Pd/Al2O3 in a downflow

microreactor (liquid phase) at atmospheric pressure and calculated an activation energy of 41kJ/mol

[12].

The values found in this project and in different studies were in the same order of magnitude.

Obviously these values were always found with a kinetic controlled reaction. The fact that these valuesare comparable independently of the phase shows that the interaction between the catalyst and the

molecules is a dominant fact in the kinetics of a reaction. This is also the reason why apparent

activation energies were used. The real activation energy is part of it but there is also the heat of

adsorption that must be considered.

adsorptionrealA,app HEE


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4.6 Time-on -stream effect

The time-on-stream theory has been developed in the purpose of understanding the ageing of catalysts

by relating their activity to the length of time they are used [8]. In the following Figure 13, the

conversion which is related to the activity of the catalyst has been plotted versus the time. Three

different experiments have been realized at same conditions varying the temperature. Indeed three

temperatures have been investigated in order to see its influence on the time-on-stream effect:

Figure 13: Conversion versus time, observation of the effect of time-on-stream for three temperatures

For all three temperatures deactivation is observed and stabilization occurs after around 2 hours. This

confirms what has already been said during the deactivation section. The deactivation rate is more

important for higher temperatures as the conversion at which it stabilizes. As mentioned the

deactivation is linked to the initial conversion, but in order to compare the deactivation between the

different experiments the percentage of deactivation were calculated (Equation 21)

(21)X

XXonDeactivati%

0

1500

Where X150 is the conversion at 150 min (stabilization has occurred) and X0 is the initial conversion

which has been calculated with a polynomial function.


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Table 2: Percentage of deactivation measured for T=120C, 150C, 170C

Temperature X0 X150 % Deactivation

170C 0.183 0.0496 0.729

150C 0.111 0.0295 0.733

120C 0.041 0.0092 0.776

No tendency can be emphasized for the % of deactivation with respect to temperature. The mean value

for the deactivation is 0.746 0.03 (Table 2). The causes of deactivation are discussed in the following

section (4.7).

4.7 Deactivation

Deactivation is an inevitable phenomenon for all catalysts. It seems obvious that catalysts with such

compositions i.e. supported small metal particles undergo thermal, chemical, or mechanical effects [6].

One quality of a catalyst that industries are looking for is stability. Indeed the catalyst should stabilize

after a certain time. This has been observed for the catalyst of interest. Also it could be interesting to

determine the causes of deactivation in order to partly avoid the effects of a part of them. The often

observed reasons of deactivations are thermal sintering (ageing), poisoning, loss of catalyst to gas

phase and deposition.

Some effects are easier to investigate than others. Deactivation due to temperature has already been

reported during hydrogenation ofmeta-chloronitrobenzene into meta-chloroanaline [9]. A manner to

notice thermal sintering is to use transmission electronic microscope (TEM) in order to see if the

particles became bigger. The two following pictures show the catalyst before a reaction and after a

mimicked reaction:


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Figure 14: TEM pictures of the catalyst before and after reaction

The two pictures (Figure 14) show that before the reaction the particles have a size of 7nm but that

after a simulated reaction the particles have increased to 10-50nm particles. Therefore the surface-to-

volume ratio is decreased causing deactivation.

It has been reported that Al2O3-based catalyst, which was the case for this project, are prone to

deactivation [12]. This might be one of the causes of decrease in conversion during the reactions. The

difficulty into proving the deactivation effect of alumina support is that it has a positive effect on the

activity of the catalyst. Therefore substituting it by another support would give completely different

results in terms of conversion versus time. On the other hand adding more alumina instead of a part of

the glass power in the catalyst bed could be a way to observe the effect of this compound on the

catalyst activity.

Poisoning is another source of loss of activity. It occurs when impurities chemisorbs on the active site.

This is an irreversible phenomenon. Inhibition is different because due to physisorption and therefore

is a reversible phenomenon which allows in some cases the regeneration of the catalyst [6]. In the

particular reaction of hydrogenation ofp-CNB chloride is present in the molecule and could be apoisoning compound. Based on Figure 13 where deactivation occurs during about 2 hours it is more

likely that physisorption occurs. If the phenomenon of inhibition truly happens the physisorbed

molecules are in equilibrium at the active sites when stabilization is reached. Nevertheless

chemisorption cannot be excluded as its effects could be slow on the catalyst activity.

It has been proved that presence of chloride increases severely the sintering for Cu-based synthesis [4].

As gold belongs to the same elements group (I B) an assumption could be made that the catalyst used

during the project was partly prone to this type of deactivation. To investigate this negative effect on

the catalyst activity experiments should be made where additional chloride is put in contact to the

catalytic bed. If no further deactivation is observed this would mean that chloride has no inhibition

effect.


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4.8 Contact time effect

The TOF was calculated for several reactions during which two parameters were varied. First the

volumetric flow rate of hydrogen was varied at a fixed temperature in the range of 1.2 l/h to 5.4 l/h.

The temperature range was 120 to 170C. The results of TOF versus the contact time were plotted in

Figure 15.

Figure 15: Turnover frequency (TOF) versus contact time at T=120C, 150C, 170C

The TOF is increasing with the contact time independently of the temperature (see Figure 15). Indeed

bigger is the contact time bigger is the chance that a higher number of molecules of H2 which come in

contact with active sites of the catalyst bed will react with the organic reagent. It can be noticed that

the decrease of TOF tends to diminish when contact time is lowered. Indeed even if the contact time is

decreased by increasing the H2 volumetric flow rate it is slightly compensated by the fact that the

number of H2 molecules that pass through the catalyst per unit of time is more important.

Increasing the temperature leads to obtain a better TOF at a fixed contact time meaning that the rate

follows the Arrhenius law.

The tendency mentioned above is more obvious for higher temperatures where the energy of the

system is more important and therefore the chance of reaction between two reagent molecules is

higher.


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4.9 Effect of partial pressure

The partial orders were calculated for the reaction at 150C. Figure 16 and Figure 17 show the effect

on turnover frequency of the changes of partial pressure of respectively the hydrogen and thep-CNB.

The partial order of hydrogen is 0.18 and the partial order for p-CNB is 0.49 at 150C. Small partial

orders result in the fact that small changes of partial pressure of the reactants do not have a high

influence on the reaction rate. It could be advantageous to have this property because then the reaction

design is not too sensitive. Pressure fluctuations in a system easily occur and if the reaction rate is

highly influenced by this last it might be difficult to design a reproducible and reliable process.

Figure 16: Partial pressure of hydrogen modified with a fixed partial pressure forp-CNB

Figure 17: Partial pressure ofp-CNBmodified with a fixed partial pressure of hydrogen


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Further experiments at different temperature would allow calculating more accurate rate constants

with the real partial orders. The apparent activation energy could then also be calculated more

accurately. This was not done for this project because of lack of time.

A Langmuir-Hinshelwood model could be tested with the different kinetic parameters calculated. The

rate limiting step (either adsorption, reaction or desorption) could be identified.


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5 Conclusion

100% selectivity is one of the major points of the investigated catalytic reaction. This advantage over

the mentioned studies is crucial for further process development. It avoids the formation of unwanted

by-products which could involve more expensive separation steps in the process.

The gold metal catalyst used during the experiment was characterized as being a spherical (diameter =

7nm) particles using transmission electronic microscope (TEM) and X-ray diffraction (XRD). The

active form was determined to be Au0

using temperature controlled reduction (TCR).

In the range of pressure and temperature used for the experiments the reaction was shown to be under

kinetic control by the Madon-Boudart correlation and the calculated apparent activation energy. This

requirement is essential for a further optimization of the Au/Al2O3 catalyst.

Deactivation is observed until stabilization which occurs after around 2 hours of reaction. It was

demonstrated by TEM pictures that the decrease in activity of the catalyst was due to thermal

sintering. Also it is likely that chloride was responsible of a part of the deactivation. The stabilization

of the catalyst activity is really encouraging for further studies which could lead to the process

development based on this particular reaction.

The turnover frequency can be varied by adjusting the contact-time and temperature. Indeed the TOF

increased with higher contact-time and temperature (Arrhenius control).

At the tested temperature of 150C the TOF slightly increased with higher partial pressure of both

reagents (H2,p-CNB).


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31

6 List of symbols

A0 pre-exponential factor [mol1-n

Ln-1

s-1

] for n order

Ai area taken at the surface by one atom i [m2]

Ci concentration of compound i [molm-3

]

di diameter [m]

D dispersion [-]

Ea activation energy [kJmol-1

]

F molar flow rate [molh-1

]

k rate constant [cm3g

-1cats

-1]

k rate constant [molp-CNBmolAu-1

h-1

]

m mass [g]

M atomic weight [gmol-1

]

Ns number of sites [-]

Ni, Surface number of atoms i at the surface [-]

i molecules of reagent converted per time [-]

Q volumetric flowrate [cm3

h-1

]

density [kgm-3

]

r rate of reaction [molg-1

cats-1

]

Si selectivity with respect to compound i [-]

SSA specific surface area [m2gcat.]

T temperature [K]

TOF turnover frequency [h-1

or s-1

]

xi molar fraction [-]

X conversion [-]

i stoichiometric factor of compound i [-]

Abbreviations

Au gold

cat. catalyst

p-CNB para-chloronitrobenzene

p-CAN para-chloroaniline


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32

Constants

Gas constant R=8.314 [Jmol-1

K-1

]

Avogadro number NA=61023

[atoms or moleculesmol-1

]


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7 References

[1]Bechamp reduction . (n.d.). Retrieved october 2010, from wikipedia:http://en.wikipedia.org/wiki/Bechamp_reduction

[2] Blaser, H.-U., Steiner, H., & Studer, M. (2009, septembre 10). Selective Catalytic Hydrogenation ofFunctionalized Nitroarenes: An Update. ChemCatChem , pp. 210-221.

[3] Crdenas-Lizana, F. (2010). Kinetics of gas/solid catalytic reaction: "Hydrogenation ofChloronitrobenzene as case study". Travaux Pratiques (2010-2011) .

[4] Forzatti, P., & Lietti, L. (1999, September 14). Catalyst deactivation. Catalysis Today , pp. 165-181.[5] Kratky, V., Kralik, M., Mecarova, M., Stolcova, M., Zalibera, L., & Hronec, M. (2002, April 23). Effect

of catalyst and substituents on the hydrogenation.Applied Catalysis A: general , pp. 225231.

[6] Lassi, U. (2003).Deactivation Correlations of Pd/Rh Three-way Catalysts Designed for Euro IVEmission Limits: Effect of Ageing Atmosphere, Temperature and Time . Retrieved December 18, 2010,

from herkules.oulu.fi: http://herkules.oulu.fi/isbn9514269543/html/c476.html

[7] Madon, R. J., & Boudart, M. (1982, November). Experimental Criterion for the Absence of Artifacts inthe Measurement of Rates of Heterogeneous Catalytic Reactions.Industrial Engineering Chemical

Fundamentals , pp. 438-447.

[8] Pachousky, R. A., Best, D. A., & Wojciechowski, B. W. (1973, July). Applications of the Time-on-Stream Theory of catalyst Decay.Ind. Eng. Chem. Process Des. Dev. , pp. 254-261.

[9] Rode, C., Vaidyaa, M., Jaganathana, R., & Chaudharia, R. (2001, March 7). Hydrogenation of m-Nitrochlorobenzene to m-Chloroaniline: Reaction Kinetics and Modeling of a Non-Isothermal Slurry

Reactor. Chemical Engineering Science , pp. 1645-1653.

[10]Tijani, A., Coq, B., & Figuera, F. (1991, April 12). Hydrogenation of para-chloronitrobenzene oversupported ruthenium-based catalysts.EIsevier Science Publishers B.V. , pp. 255-266.

[11]Vaidya, M., Kulkarni, S. M., & Chaudhar, R. V. (2003, January 24). Synthesis of p-Aminophenol byCatalytic Hydrogenation of p-Nitrophenol. Organic process research & developement, pp. 202-208.

[12]Vishwanathan, V., Jayasri, V., & Mahaboob Basha, B. (2007, May 11). Gas phase hydrogenation ofortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) over unpromoted and alkali metal

promoted-alumina supported palladium catalysts.Akadmiai Kiad, Springer, pp. 291-298.

[13] Wang, X., Liang, M., Zhang, J., & Wang, Y. (2007, december 25). Selective Hydrogenation ofAromatic Chloronitro Compounds. Current Organic Chemistry , pp. 299-314.

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