a study of molybdenum carbide catalyst for fischer-tropsch synthesis-mengsc process engineering...

A STUDY OF

MOLYBDENUM CARBIDE CATALYST

FOR

FISCHER-TROPSCH SYNTHESIS

by

Farhan Munir

A thesis submitted for the degree of Master of Engineering Science in Process

Engineering

School of Chemical Engineering and Industrial Chemistry

The University of New South Wales

November, 2001

ACKNOWLEDGMENTS

I would like to express my gratitude to those people who contributed a lot in many ways

during the course of this project.

Firstly, I am particularly indebted to my supervisor, Dr. A. A. Adesina, for his valuable

guidance and encouragement during the course of this project.

I wish to thank Professional officer, Mr. J. Starling for his technical assistance and

helpful advices during experimental work.

To all postgraduate students, I am very grateful for their friendly behaviour during this

study.

To all my family members, in Australia and Pakistan, I am extremely grateful for their

continued support in every sense. Their moral support, encouragement and their most

valuable guidance helped me to complete this report.

TABLE OF CONTENTS

CHAPTER #

1. INTRODUCTION 1

2. FISCHER-TROPSCH SYNTHESIS 3

Background 3

Reactions 4

Thermodynamics 7

Kinetics and Mechanisms 11

3. FISCHER-TROPSCH CATALYSTS 22

Active metals for FTS 22

Catalyst supports 23

Catalyst preparation and characterisation 25

4. Mo2C CATALYST 33

High surface transition metal carbides 33

Thermodynamic consideration in preparation of carbides 34

Preparative methods for carbides 34

Molybdenum carbide catalyst 36

5. OBJECTIVES 38

6. EXPERIMENTAL 39

Materials 39

6.1.1Chemicals 39

6.1.2Gases. 40

6.2Catalyst preparation 40

6.2.1Preparation of MoS2 catalyst by PFHS method 41

6.2.2Carburisation of MoS2 to Mo2C 43

6.3Experimental apparatus 46

6.3.1For MoS2 preparation 46

6.3.2For Mo2C preparation 47

6.4Catalyst characterisation 48

6.4.1Total surface area 48

7. RESULTS AND CONCLUSIONS 51

REFERENCES 55

APPENDIXES 59

1

Chapter 1

INTRODUCTION

The realisation of the fact that current petroleum and natural gas reserves are limited

and will not be available to supply all the energy requirements are the vital motivation

behind the researches on the hydrogenation of carbon monoxide to produce

hydrocarbons. Coal still comprises the majority of world fossil fuels resources compare

to petroleum. Coal gasification to CO and H2 followed by a CO hydrogenation step is

the one possible route to produce synthetic natural gas (SNG) via methanation reaction

or to manufacture longer chain hydrocarbons via Fischer-Tropsch synthesis (FTS)

reaction. It can be considered as an alternative to crude oil for the production of both

liquid fuels (gasoline and diesel) and chemicals like alkenes.

Fischer-Tropsch synthesis is a catalytic process and can occur on almost any group VIII

transition metals [20]. However, the product distribution differs greatly from one to

another. It is generally conceded that Fe, Co and Ru yield high molecular weight

compounds while Ni favours an almost exclusive production of CH4 [22]. Some of the

metals other than group VIII can also be used as FT catalysts, for instance Mo and W.

[5]. It is also believed that Mo is sulphur resistant and Molybdenum carbide (Mo2C) is

active for FTS [3] and has high olefin selectivity with the promotion of potassium [23].

According to Anderson [3], metallic Mo, Mo2C and MoN have the largest tendency to

produce higher hydrocarbons As mentioned earlier that Mo is sulphur resistant and that

2

is reason Mo is very useful component for catalysts, which needed to operate with

sulphur-containing or CO- rich feed.

Literature evidence shows that the Mo2C active for FTS and has high olefin selectivity.

In this project, an attempt was made to study the preparation method of Mo2C. Initially,

silica supported MoS2 was prepared from Precipitation from homogenous solution.

After getting silica supported MoS2 was than carburise to get he Mo2C with different

carburising conditions and surface area measurements were taken.

3

Chapter 2

FISCHER TROPSCH SYNTHESIS

2.1 Background

Fischer-Tropsch synthesis (FTS) look back to a history of about seventy years.

From early experiments of inventions in 1925 up to a 600,000 t per annum industrial

capacity in 1945[1], the main development took place in Germany, particularly in Franz

Fischer’s laboratories at Kaiser Wilhelm Institute for Coal Research (presently Max

Plank Institute) at Mulhein (Ruhr) in collaboration with Ruhrchemie Company for

commercialisation of FT processes [2].

The process is first commercialised in 1938 in which hydrogenation of carbon

monoxide was carried out at 400 – 450 oC and about 7 – 30 bar pressure over alkalised

iron turning catalyst. Earlier catalysts had consisted of cobalt and of nickel or

manganese oxides. Large-scale plants in Germany produced about 800 000 t per anum

of liquid fuels during World War II, using fixed bed reactors. After World War II, large

scale use of the Fischer-Tropsch synthesis was cantered in South Africa where, since

1955, Sasol has operated first fixed bed reactor plant with a capacity of 250 000 t per

annum and later, two fluid bed reactors plants with over 2 500 000 t per annum

capacity. Since 1993, shell Oil Company has operated a modified Fischer-Tropsch plant

4

in Malaysia, which uses syngas from natural gas to produce high-molecular weight

alkanes, which are then hydro cracked to diesel fuel.

The actual interest in FTS has grown up in consequence of environmental demands,

technological developments and changes in fossil energy reserves. Present areas with

high potential for early implementation of FT synthesis are the European North Sea, the

US State Alaska and countries around Arabian Gulf, particularly with its large natural

gas and its shrinking petroleum resources. The commercial FT synthesis on the basis of

low price coal in South Africa has now been directed towards more valuable olefins

instead of gasoline since the country has meanwhile access to the world oil market. [2]

2.2 Reactions involved in FTS

Fischer-Tropsch synthesis may be defined as the hydrogenation of carbon oxides to

produce longer-chain hydrocarbon fuels and chemicals. Generally it is regard as a

hydrogenation of CO and CO2 to produce a variety of hydrocarbons and / or alcohols

[3] and in most cases FTS refers to the hydrogenation of CO resulting in straight

paraffins, isomers, olefins, and certain oxygenated product like CO2 and water. This

reaction is normally carried out at about 453 K to 673 K and pressure is about 1 to 40

atmospheres over group metals.

5

The reaction involved in the FT synthesis can be schematically represented in the form

of general equations as [2.4]:

(2n+1) H2 + nCO = CnH2n+2 + nH2O (1)

2nH2 + nCO = CnH2n + nH2O (2.2)

2nH2 + nCO = CnH2n+1OH + (n-1)H2O (2.3)

Or

(n+1) H2 + 2nCO = CnH2n+2 + CO2 (2.4)

nH2 + 2nCO = CnH2n + nCO2 (2.5)

(n+1) H2 + (2n-1) CO = CnH2n+1OH + (n-1) CO2 (2.6)

The first three equation represents the case where water is produce as a predominant

oxygenated product while in the last three equations showing the case where CO2 .

These general equations can be used to represent various hydrocarbon reactions. In case

where n = 1, equation (1) will represents methanation reactions and if n > 1 results in

the production of Fischer-Tropsch hydrocarbons.

6

In addition to the above-mentioned reactions, there are also possibilities of some side

reactions depending on the reaction condition employed. These reactions includes:

Water gas shift reaction

CO + H2O = CO2 + H2 (2.7)

Boudouard reaction

2CO = C + CO2 (2.8)

Coke deposition

H2 + CO = C + H2O (2.9)

Carbide formation

XM + C = MxC (2.10)

Depending on the process conditions and the catalysts used, a different product

distribution can be obtained, such as low or high boiling compounds, and saturated or

unsaturated hydrocarbons. Thus, the kinetics of FT synthesis is complicated and

depends on the catalyst and operating conditions.

7

2.3 Thermodynamics of FTS

The FTS is an exothermic reaction system. Information on the heats of reactions is of

great practical importance as removal of the heat is one of the most difficult engineering

aspects in FTS. Excessive catalyst temperature usually leads to less desirable products,

carbon deposition, and catalyst disintegration. Figure 2.1 shows the behaviour of the

heats of reactions per carbon atom, ∆H/n, for reactions (2.1) and

(2.4) With temperatures. It is seen from the figure that:

1. For n-paraffins and 1- olefins, ∆H/n vary only slightly with temperature,

especially at very high temperature,

2. For reaction (2.1), which produces paraffins, ∆H/n increases (become less

negative) with increase in carbon number, that is, reaction heat decreases with

increase in product chain length,

3. For reaction (2.4), which produces olefins, ∆H/n decreases with increase in

carbon number, an opposite trend to that seen for paraffins

4. Reactions producing CO 2 give off heat than that producing H2O by the amount

of ∆H of water gas shift reaction (9 Kcal mol-1).

8

The relationship between the standard free Gibbs energy, ∆Fo of a chemical reactions

and its equilibrium constant is defined by the following equations:

- ∆Fo = RT ln Keq (2.11)

According to Anderson [4], the free energy characteristics of the FTS can be described

as follows:

9

1. Reactions producing CO2 [(2.4) and (2.5)] have more negative Gibbs free

energy change values ∆Fo (larger equilibrium constant) the corresponding

reactions producing water [(2.1) and (2.2)] as reactions (2.4) and (2.5) are

combination of reactions (2.1) and (2.2) with water gas shift reaction (2.7)

respectively, and the Gibbs free energy change of water gas shift (WGS)

reaction is negative.

2. The equilibrium constant of WGS reaction is large (>20) at FTS temperature

(453-623 K). Hence, if equilibrium were sustained for the WGS reaction, almost

all of the water produced by primary synthesis reaction should be converted to

CO2. But, in fact, nearly all of the oxygen appears as water in the synthesis on

Co and Ni and more than 25% in the synthesis on Fe. Thus, the kinetics rather

thermodynamics probably controls product distribution, since WGS reaction has

a relatively low rate of reaction within this temperature range.

3. Under usual FTS temperature, the standard free energy changes per carbon

atom, ∆Fo /n, for reaction producing CH4 and reactions producing carbon are

more negative the for corresponding reactions producing higher hydrocarbons.

Therefore, the production of higher hydrocarbons (rather than CH4 and carbon)

must depend on the nature of the catalyst.

4. The equilibrium conversion of synthesis increases with increasing pressure. For

a given conversion, the higher the operating pressure the higher the synthesis

temperature needed.

5. With increase in reaction temperature, ∆Fo for CH4 and other paraffins increases

(less negative) quicker than that for carbon formation, i.e., carbon formation will

be favoured under high temperature.

10

6. In this range of temperature and pressure, sizable yields of all hydrocarbons with

the exception of acetylene are thermodynamically possible. The equilibrium

constants of cyclic and aromatic hydrocarbons, which are formed for mono-

olefins of the same carbon number.

7. The ∆Fo of reactions resulting in hydrocarbons containing the same number of

the carbon atoms becomes more negative in the order of di-olefins, mono-

olefins, and paraffins.

8. Hydrogenation of carbon monoxide to hydrocarbons or alcohols, except for

acetylene and methanol is thermodynamically possible under most synthesis

conditions

9. Hydrogenation of olefins and dehydration of alcohols are thermodynamically

possible under usual synthesis conditions.

10. Reaction of any amount of ethylene or ethanol with H2 - CO mixtures is

thermodynamically possible at all FTS temperatures. Ethylene has a greater

thermodynamic tendency to incorporate than higher olefins.

11. Reactions of water plus graphite to give hydrocarbons have positive values of

∆Fo and heat change. These reactions, which approximate the desired overall

equation for the production of synthetic fuels from coal, are thermodynamically

impossible under all practical conditions.

11

2.4 Kinetics and Mechanisms of Fischer-Tropsch Synthesis

2.4.1 Mechanism of the Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis converts two of the simplest compounds, H2 and CO, into a

complex array of products, containing predominantly of alkenes and alkanes but also a

variety of minor compounds including a range of oxygenate compounds. The

motivation behind the study of mechanism is to understand how catalyst composition

and reaction conditions govern product distribution in CO hydrogenation. Currently, the

mechanisms available are more and due to the complexity of FTS, no single mechanism

are capable of explaining all the various observations made during the synthesis

reaction. Generally, the synthesis mechanism is divided into the following steps [5]:

1. Adsorption of reactants,

2. Chain initiation,

3. Chain growth,

4. Chain termination,

5. Desorption of products, and

6. Reabsorption and further reaction.

More specifically, the synthesis gas mechanism starts with the chemisorption of CO and

H2 on the catalyst surface with the formation of a primary complex that lead to

weakening of a C-O bond and formation of a C-H bond followed by chain growth by

12

reaction of the primary complex with synthesis gas or with the products already formed

and adsorbed with chain termination for example by hydrogenation or by reaction of the

growing chain with synthesis products followed by desorption from the surface. The

pertinent feature for FTS is given in the figure 2.2.

Most of the mechanistic studies focus on steps 2 to 4. Various mechanisms reported can

be grouped into four types. [5,13,14]

1. Surface Carbide Mechanism,

2. Enolic mechanism,

3. CO insertion Mechanism, and

4. Alkoxy Mechanism.

13

2.4.1.1 Surface carbide mechanism

This mechanism is related CO hydrogenation and the authors hypothesized that the CO

reacts with metal of the catalyst to form bulk carbide, which subsequently undergoes

hydrogenation to form methylene groups [15]. The methylene species were assumed to

polymerise to form hydrocarbon chains that then desorb from the surface as saturated

and/or unsaturated hydrocarbons. Figure 2.3 shows the schematic of carbide

mechanism.

14

Schematic diagrams of the above mentioned mechanism is given in Appendix 2.

15

2.4.1.2 Enolic Mechanism

This mechanism is based on the work of Anderson [3] and Storch [16]. In this

mechanism, it is believed that enol, formed from simultaneous chemisorption of CO and

H2 on the catalyst surface, is the intermediate in the synthesis. The reaction between

two primary enolic complexes leads to the formation of C-C bond and water with the

concurrent release of a carbon atom from the surface by hydrogenation. Through this

stepwise condensation and hydrogenation, chain growth continues by the addition of

one carbon at a time.

16

2.4.1.3 CO insertion mechanism

This mechanism involves the insertion of a carbonyl group into a meral alkyl bond.

The resulting alkyl intermediate can then undergo a variety of reactions to form

acids, aldehydes, alcohols, and hydrocarbons. In addition branched hydrocarbons

can also be formed [17,18].

17

2.4.1.4 Alkoxy mechanism

This mechanism is based on the idea that CO chemisorbs on metal surface and reacts

with hydrogen to yield oxygen rather than a carbon coordinated species. Thus the bond

strength of metal oxides should correspond directly to specific activity of machination

but should be related inversely to methane selectivity.

18

More recent studies on the mechanism are done by Davis, B.H [43]. He studies the

mechanism for generation of hydrocarbon and oxygenated products from synthesis gas

using FTS, as describe earlier. The data indicate from the study shows that there are

similarities between iron and cobalt catalytic synthesis mechanisms, the details differ. It

also appears from his study that the surface carbide mechanism is a better choice. The

detailed schematic diagrams of these mechanisms are given in Appendix2.

19

2.4.2 KINECTICS OF FTS

It is very obvious from the available literature that the kinetic expressions were

influenced by the type of catalyst and the operating conditions employed by the various

investigators [5]. Nevertheless, it is generally conceded that FT reactions are about first

order in H2 partial pressure and zero order in CO as long as the H2/CO ratio is between

1 and 3 [6,7,8].

Vannice [9] has summarized almost all known rate expressions for the FTS reported

before 1974.Recently Wojciechowski [10] and Sarup and Wojciechowski [11] presented

a rather comprehensive kinetic study of FTS over a Co catalyst. For Fe catalysts,

Anderson [3] found that the first order equation

r = k PH2 (2.12)

fit the data well up to synthesis gas conversion of 60%. Considering that inhibition by

water may occur at conversion larger than 60%, Anderson [3] proposed a rate equation

including water inhibition. Thus

r = ko PCO PH2 (2.13)

PCO + a PH2O

20

Which can be derived from the enolic mechanism by assuming that the hydrogenation

of chemisorbed CO is the rate-determining step [12]. To handle the observed H2

dependence, Huff and Satterfield [13] derived an alternate rate form of

r = ko PCO P2H2 (2.14)

PCO + a PH2O

Using two different mechanism, the carbide mechanism taking the hydrogenation of

surface carbon as the rate determining step, and an enol / carbide mechanism with

hydrogenation of surface enol as the rate limiting step. Ledakowicz [14] develop the

following relationship in order to accommodate situations with high water gas activity

and /or low H2/CO ratios as

r = ko PCO PH2 (2.15)

PCO + PCO2

Based on the enolic mechanism with hydrogenation of surface CO as the rate

determining step assuming that CO and CO2 are only gaseous species which adsorb

significantly on the catalyst surface. They also presented the following rate equation

21

r = ko PCO PH2 (2.16)

PCO + a PH2O + cPCO2

To account for inhibition by both water and CO2. This generalised rate expression may

be used for catalysts with low WGS activity, where water concentration are high, as

well as for catalysts with high shift activity which shows inhibition by CO2.The

activation energy reported by the authors for Fe Catalysts is about 80 to 103 kJ mol-1,

regardless of catalyst type.

22

Chapter 3

FISCHER-TROPSCH CATALYSTS

3.1 Active Metals for FTS

The discovery of the gasoline synthesis by Fischer and Tropsch is based on iron and

cobalt as catalysts, both the metals remaining until today the only ones for industrial

applications [21]. Essentially, Fischer-Tropsch synthesis can occur on almost any group

VIII transition metals [20]. However, the product distribution differs greatly from one to

another. It is generally conceded that Fe, Co and Ru yield high molecular weight

compounds while Ni favours an almost exclusive production of CH4 [22]. Some of the

metals other than group VIII can also be used as FT catalysts, for instance Mo and W.

[5]. It is also believed that Mo is sulphur resistant and Molybdenum carbide (Mo2C) is

active for FTS [3] and has high olefin selectivity with the promotion of potassium [23].

According to Anderson [3], metallic Mo, Mo2C and MoN have the largest tendency to

produce higher hydrocarbons. As mentioned earlier that Mo is sulphur resistant and that

is reason Mo is very useful component for catalysts, which needed to operate with

sulphur-containing or CO- rich feed.

23

3.2 Catalyst Support

The major drawbacks with bulk metal catalyst are catalyst efficiency and they are

costly. These types of catalysts also shows low thermal stability and surface area losses

occurs during sintering. To avoid these problems the active metals component is very

often supported on high surface area carriers. Chen [5] summarize the major purposes

for the employment of support in a catalysts as:

1. To stabilize the catalyst against agglomeration and coalescing, usually referred

to as a thermal stabiliser,

2. To introduce resistance to poisons or resistance to by-product formation,

3. To decrease the density of the catalyst and also to dilute the costly ingredient by

less costly materials,

4. To prepare the catalyst in such a form that its resistance to breakage and

minimization of pressure drop is accomplished.

Snel [33] also pointed out the influence of support on catalytic behaviour of a

supported catalyst by support basicity effect, support dispersion effect, electronic

modification effect, and strong metal-support interaction effect.

There are various materials, which can be utilized as catalyst support. Table 3.1

shows the general classification.

24

Table 3.1 General classification of Catalyst Support [Chen [5]]

Class Supports

Inert Support SiO2

Catalytically active

supports

Al2O3, SiO2-Al2O3 and

Zeolites etc

Those influence active

component by strong

interaction

TiO2, Nb2O5, V2O5 etc

Structural supports Monoliths

The most widely used supports in FT catalysts are Al2O3 and SiO2 .According to

Ishihara [34] others metal oxides are used as FT catalysts for improved activity and

selectivity.

Silica

Silica is a refractory oxide, which is widely used as a catalyst support after alumina. It

occurs in many polymorphs depending on the temperature and pressure. Silica has

characteristics that make it useful in many cases in which alumina is inapplicable. Silica

is also primarily much more resistant to acid media and as a consequence is more

satisfactory than alumina in this type of environments. However, alkali environments

adversely affect both silica and alumina and neither is suitable for use in basic system.

25

At 1 atm and below 846 K, silica exists in low-quartz form. It may also react with

catalytic compounds and cause the incipient formation of silicates when the temperature

exceeds 723-773 K. [35]. The thermal stability range and the figure of colloidal silica

are mentioned in Appendix 3.

3.3 Catalyst Preparations and Characterisation

3.3.1 Catalyst Preparation

The production of supported catalysts can be divided into two main groups [36]:

1. Application of the active precursor onto a separately produced carrier, and

2. Selective removal of one or more components from solids of an initially small

specific surface area.

The first group of preparation methods has the advantage that a number of properties of

the support can be adapted to the requirements of a catalytic process. Especially when

shaped pellets are utilised, the pore-size distribution and mechanical strength of the

pellets may be adjusted. The active precursor can be applied onto the support by the

following methods:

1. Adsorption

2. Impregnation and drying

3. Precipitation

26

Adsorption of active metals is mostly done from liquids. While impregnation and

subsequent drying is utilised to obtain higher loadings or to apply active precursors that

do not markedly adsorb onto the support. The first two methods are important but are

not relevant to the objectives of current studies.

Precipitation

Precipitation of an active precursor in the presence of suspended support is also utilised

to produce supported catalysts. After completion of the precipitation the solids are

filtered, dried, processed to pellets and thermally treated. Since precipitation can be

carried out more rapidly than drying, this procedure has some advantages. However, the

distribution of the active material throughout the support is even worse than with

impregnation and drying.

On addition of a precipitating agent to a suspension of the support in a solution of the

precursor, the precipitant initially contacts the dissolved precursor outside the pore

system of the support. To get very small precipitated particles of the active components,

nucleation of the precipitate must proceed rapidly. Consequently, small precipitated

particles of the precursor will develop outside the pores of the carrier. Provided the

precipitated particles are attracted by support, they will be attached to the carrier.

However, the diffusivity of colloidal particles rapidly drops with the particle size. As a

result, the porosity of the carrier must be kept small to limit transport problems as much

as possible. Transport problems are completely prevented by coprecipitation with the

support, if the active precursor and the support nucleate simultaneously [36].

27

Using a compound that slowly reacts to a precipitant can separate addition and reaction

of a precipitating agent. The slowly reacting compound can be added and a

homogeneous solution can be established before the precipitating agent has attained

marked concentration. Though nucleation of the active precursor can occur as rapidly as

require to generate very small particles, homogenizing the suspension can be completed

before the precursor starts to precipitate. Working at different temperatures can extend

the time available to homogenize the suspension considerably. The solution can be

mixed and homogenized at a temperature where no marked formation of the precipitant

takes place after which the temperature is raised and the precipitant develops rapidly.

Using this method the active material to be applied on to the support has to be present

within the pores of the carrier before formation of the precipitant set in. Consequently

the volume of the dissolved active precursor together with the inchoate precipitant can

be at most equal to the pore volume of the support. When a highly loaded support is to

be produced, the support must be impregnated by a concentrated solution. With

concentrated solutions the precipitated particles of the active precursor are likely to

cluster, subsequent thermal treatment causes the cluster to sinter, which leads to

relatively large active particles [36].

29

To avoid clustering of active particles especially at high loadings of the support would

be favourable. The concentration of a saturated solution is given as a function of the

temperature (solubility curve) is given in figure 3.1 and 3.2. The figure on the top is

showing the difference in free energy between a solution with a solid particle and

homogeneous solution of equal overall composition is represented as a function of

particle size. When the concentration of the solution is below that of the solubility

curve, the free energy grows on formation of a solid particle. Since the free energy of

larger particles increases linearly with the volume of the particles, the increase is

proportional to the third power of the particle size. At concentrations above that of the

solubility curve, the free energy of a solid particle and a saturated solution is lower that

that of the homogeneous solution. With large particles the decrease in free energy is

proportional to the third power of the particle size. The decrease in free energy per unit

volume grows with the difference between the concentration of the homogeneous

solution and that of the solubility curve the difference in the free energy is zero.

Considering the above figure in which case finely divided carrier suspended in a

solution of the active precursor. It is assumed that the ions of the active species

chemically interact with the surface of the carrier. We also assumed in this case that the

solubility curve has been shifted to higher concentrations. The top of the figure shows

the difference in free energy for the same concentration.

Simple addition of a precipitating agent to a suspension of the carrier in a solution of the

precursor does not lead to the homogeneous increase in concentration required to get

30

deposition precipitation. When the solution of the precipitant is poured into the

suspension of the support, the concentration can locally raise oboe the super solubility

of the bulk compound.

Local concentration differences in the suspension of the support can be minimized by

the following two methods. The first procedure separated addition and reaction of a

precipitating agent. As an example we can consider the increase in hydroxyl ion

concentration by hydrolysis of urea. Since the hydrolysis occurs at a marked rate only

about 60oC, the solution can be homogenized at a lower temperature and subsequently

brought t at a temperature where the reaction rapidly proceeds. In the second method, a

solution of the precipitant is injected into the suspension of the support below the level

of the liquid.

Precipitation according to the first method is known as precipitation from a

homogeneous solution used in gravimetric analysis to prepare well-crystallized,

relatively large crystallites that are easy to filter. Deposition precipitation on the other

hand, can provide extremely small particles. Besides the well describe utilisation of urea

a number of other methods has been developed, which are summarize in Figure 3.3.

Raising the pH- level of a solution of the active component can precipitate many active

precursors.

31

Cyanate is utilized when precipitation has to be done at lower temperatures than about

70 o C, the temperature at which urea hydrolyses rapidly. To avoid formation of soluble

amine complexes, nitrite can be favourably used.

32

3.3.2 Catalyst Characterisation

Generally, the two principal objectives in the application of physical techniques in the

study of catalysis are:

1. The characterisation of the catalysts; and

2. The acquisition of the information relevant to understand the catalytic

phenomenon.

The first objective consists of establishing the identity for the catalyst, to indicate its

structure, morphology and other physiochemical data. The second objective concerns

the catalytic process.

For physical properties of catalysts, physisorption of gases and mercury porosimetry are

the most common techniques to determine total surface areas and pore structure [45].

Selective chemisorption is a classic method for the measurement of the number of

surface metal atoms, metal surface areas and average particle size [46]. There are also

numerous methods employed for the characterisation of catalysts, from X-rays to

Infrared spectroscopy, to transmission electron microscopy. All these techniques are

employed or can be employed for catalyst characterisation, depending properties we

intend to find out. Schematic representation of physical techniques principal along with

the comparative physical characteristics for many physical techniques are given in the

Appendix 3 in the form of tables and charts.

33

Chapter 4

MOLYBDENUM CARBIDE (Mo2C) CATALYST

4.1 High Surface Transition Metal Carbides

The alloying of main group elements such as C, N or O, with early transition metals

produces a class of materials known as carbides, or oxycarbide [24-26]. The materials

have high melting points (>3300K), hardness (>2000 kg mm-2) and strength (> 3 * 105

Mpa).

The monometallic carbides often adopt simple crystal structure with the metal atoms

arrange in cubic close-packed (ccp), hexagonal close-packed (hcp) or simple hexagonal

(hex) arrays. The non-metallic elements C, N, and O, occupy interstitial spaces between

metal atoms, and for this reason the materials are also known as interstitial alloys.

The crystal structure adopted by the binary carbides is similar to those found in the

noble metals. The resemblance is not coincidental, and has been explained using Engel-

Brewer valence bond theory. The crystal structure and composition of carbides and

nitrides are given in Figure 1, Appendix 4.

34

The carbides have been found to be exceptional hydrogenation catalysts [27]. They have

activity close to or surpassing those of group VIII noble metals.

4.2 Thermodynamic considerations in the Preparation of

Carbides

Strategies for preparing are numerous and involve widely differing starting metallic

compounds, as well as different carbon sources. Carbide formation from elemental

carbon and transition metals show a number of trends (Table 1 – 4 Appendix). First, the

free energy of formation is strongly negative for the early transition metals, and

becomes less favourable in going to the group 8 metals. As temperature is raised,

carbide stability decreases slightly among the early transition metals, but increases

markedly for the late metals. In general, trends in free energy are mirrored by values of

the heats of reaction but, towards the right in the periodic table, entropic effects are

important in stabilizing the compounds.

4.3 Preparative Methods for Carbides

Oyama [29] surveyed many types of preparation methods for carbides and nitrides.

Because of the interest in transition metal carbides, this section of the thesis discusses

some important preparation methods for carbides.

35

A. Direct Reaction of Metals and Non-metals

M + C � MC

This method of preparation is carried out by contacting metallic powders and solid

carbons, sometimes in the presence of gaseous hydrocarbons, at 1500-2300 K.

Thermodynamics indicate that carbide formation from the elements is favourable at

lower temperatures, but high temperatures are used to counter solid-state diffusion

limitations [28].

B.Reaction of Metal Oxides in the Presence of Solid Carbons

MO + 2C � MC + CO

This transformation is carried out by intimately mixing metal oxides, powders with

carbon, again as with pure metals, at temperature 1500 – 2300K.

C. Reactions of Metals or Metals Oxides with Gas- Phase Reagents

M + 2CO � MC + CO2

36

MO + HxCy � MC + H2O + CO

Carburisation with gaseous carbon sources such as methane, higher hydrocarbons, and

carbon monoxide was initially carried out mainly with metal wires. For catalytic

applications metals have been carburised with methane and ethane [30], propane [31],

and carbon monoxide [32].

4.4 Molybdenum Carbide (Mo2C) Catalyst

Molybdenum carbide, Mo2C, has been shown to have excellent catalytic activity for

hydrogen transfer reactions and has been suggested as a possible substitute for noble

metals [26,37].

Saito and Anderson [38] compared the performance of unsupported molybdenum metal,

carbide, nitride, oxide, and sulphide for CO methanation, and found that the Mo2C had

the highest activity.

Considerable attention has been focused in the recent years on the chemical and

physical properties of transition metal carbides and nitrides. The utility of these

materials ranges from wear-resistant coatings, to superconductors, to heterogeneous

catalysts [27]. The carbides of transition metals are catalytically active for number of

reactions including hydrogenation [39].

37

Miyao and co-workers [40] studied the preparation and characterisation of alumina

supported Mo2 C. Mo2C was prepared in this study by nitridation of 12.5 wt%

MoO3/Al2O3 in a flow of NH3 at 700Oc, followed by carburisation in a flow of 20 %

CH4/H2 also at 700Oc for 3 hours. The sample was compared to an unsupported

materials prepared from MoO3 in the same manner. The results suggested that Mo2C

was formed on the alumina supported by the carburisation treatments at 700oC, in the

same manner as the unsupported reference sample. Prenitridation before carburisation

resulted in the formation of carbide with a larger surface area and less free carbon,

compared to the carbide formed by direct carburisation.

38

Chapter 5

OBJECTIVES

The main objectives of this study were:

1. To study the preparation methods for Mo2C catalysts

2. To study the effects of carburisation temperature on the surface area of the

Mo2C Catalyst.

3. To investigate the effect of C/H ratio of carburising gas on the total surface area

of the catalyst.

4. To study the effect of flow rate of carburising gas on the surface area of the

Mo2C catalyst.

39

Chapter 6

EXPERIMENTAL

6.1 Materials

6.1.1Chemicals

All chemicals utilize for the catalyst preparation in this study are listed in Table 6.1.

Deionized water was used for all solution preparations.

Table 6.1 Chemicals Employed for the Catalyst Preparation

Chemicals Formula Grade Morphology Manufacturer Mol. Wt

Molybdenum

Trioxide

MoO3 AR AJAX 143.94

Silica SiO2 Pure Precipitated AJAX 60.09

Thioacetamide CH3CSNH2 AR --- AJAX 75.13

Urea NH2CONH2 AR --- AJAX

Nitric Acid HNO3 AR --- AJAX 63.01

40

6.1.2 Gases

All gases used in this study were supplied by ‘ Reaction Engineering and Technology

Group’, School of Chemical Engineering and Industrial Chemistry, UNSW Sydney

Australia. The source of these gases to reaction group is BOC Gases. All gases utilize

during the course of this study are listed in Table 6.2.

Table 6.2 Gases employed in this study with specification and Applications

Gas Specification Application

H2 Ultra High (99.999%) Reactant

N2 Inert

C3H8 Reactant

6.2 Catalyst Preparation

The main objective of this study was to prepare Mo2C catalyst. The preparation of

Mo2C was achieved in two steps. Firstly, the preparation of silica supported MoS2

catalyst via precipitation from homogeneous solution (PFHS). The next step is to

carburise MoS2 catalyst to get finally Silica supported Molybdenum Carbide Mo2C

41

catalysts. These two steps for catalyst preparation are discussed in detail in the coming

sections of this thesis.

6.2.1 Preparation of MoS2 catalyst by PFHS Method.

A weighed sample of silica was suspended in an aqueous solution of 100 ml in a 250 ml

conical flask containing 10 ml MoO3 (0.1 M) solution, 1 g urea, 1 ml of 0.75 M

concentrated nitric acid and 30 ml of thioacetamide (0.133 M). The flask content was

kept at 90oC in a water-bath for 3 h with intermittent shaking. The precipitated obtained

was then filtered, washed and dried at 120oC.In order to prepare 2 % of Mo/SiO2

catalyst, 4.7 g of silica were used in the suspension. The recipe for the preparation of the

catalyst is given in Table 6.3

Table 6.3

Chemicals Formula Concentration Amount

Molybdenum

Trioxide

MoO3 0.1 M 10 ml

Silica SiO2 -- 4.7 g

Thioacetamide CH3CSNH2 0.133 M 30 ml

Urea NH2CONH2 -- 1 g

Nitric Acid HNO3 0.75 M 1 ml

42

The above recipe is employed in order to prepare Silica supported MoS2 catalyst of 2 %

of Mo/SiO2 catalysts.

Precipitation

The suspensions containing all the above-mentioned ingredients were heated at 90oC in

a water bath. For shaking of the suspension, the shaker was equipped in a water bath.

The flask content was kept for 3 h wit intermittent shaking, which is provided by the

shaker. The next step was the filtration of precipitates.

Filtration

The precipitates were filtered using vacuum filtration unit. The contents of the flask

were poured into the funnel of the filtration unit and vacuum was applied. The

precipitates obtained from filtration were then dried. Washing is also done in this

section.

Drying

The washed precipitates obtained after the filtration and washing was then dried at

120oC for about 14 h.

43

6.2.2 Carburisation of MoS2 (Prepared via PFHS Method) to Mo2C

Catalyst.

This is the second and most important part of Mo2C Catalyst preparation. The catalyst

design for carburisation is obtained with the help of statistical method known as

Fractional Factorial Design (FFD). FFD is a statistical method, which enables

experimenters to get necessary information on a multi factor system with minimum

experiments [41].

For carburisation of MoS2, mixture of propane( C3H8) and H2 is utilize in the presence

of N2.

Catalyst Design

The catalyst designs for carburisation of MoS2 are based on FFD. Three factors namely,

C/H ratio, temperature and time at two levels were used for the catalyst design. The

general design of the carburisation step is given in table 6.4

44

Table 6.4 Designs of Carburisation Experiments

Factor Level 1 Level 2

C3H8/H2 Ratio 1 6

Temperature

(oC)

400 600

Time

(Hours)

1 5

On the basis of the above design, the outline of experiments for carburisation We can

observe from the above table that each factors are at two levels and there are three

factors in total. The total number of experiments was 8 ( 23 = 3 factors and 2 levels)

The flow rates of the carburising gas mixture are calculated by using the calculations

given in Table 6.5. Total gas flow rate of carburising mixture were kept at about 100-

ml/ min. After calculation of C3H8 /H2 Ratio, the amount of inert gas, which was N2

was utilize in the carburising mixture.

Table 6.5: C3H8 /H2 Ratio Calculations

C3H8 /H2 Ratio

C3H8 H2

45

1: 1 1/3 1

6: 1 2 1

The outline of the experiments along with the experimental details are given in Table

6.6

Table 6.6 Experimental Details for Carburisation

Catalyst

Sample

C3H8/H2

Ratio

Temperature

(oC)

Time

(Hours)

C3H8

Ml/min

H2

Ml/min

N2

Ml/min

C1 1 400 1 20 60 20

C2 1 600 1 20 60 20

C3 6 400 1 60 30 10

C4 6 600 1 60 30 10

C5 1 400 5 20 60 20

C6 1 600 5 20 60 20

C7 6 400 5 60 30 10

C8 6 600 5 60 30 10

46

6.3 Experimental Apparatus

6.3.1 Apparatus Employed for MoS2 Preparation

The apparatus employed in this section of catalyst preparation consist of the following

items:

• Conical Flask. 100,250 and 500ml

• Measuring Cylinder 100 ml

• Volumetric Pipette

• Beakers

• Filter Papers ( Whatman’s 90 mm)

• Vacuum Filtration Unit

47

• Dryer

• Sample Bottles

6.3.2 Apparatus Employed for Mo2C Preparation

The schematic diagram of experimental rig employed for the carburisation of MoS2 is

illustrated in Fig. 6.1. The whole experimental rig was placed in a fume cupboard in

order to avoid any effluent gas escapes. The system consists of a reactor, furnace,

temperature controller, flow controllers, and mixing vessel.

Reactor

The reactor is 10 mm ID, 40 cm long quartz cylinder. The reactor is fabricated with

quartz so that it can with stand high temperature for the reaction of carburisation. The

nature of the material also ensures the inert behaviour, which is also feasible to the

carburisation. The bed of catalyst consisted of 1 g of MoS2 catalyst. The catalyst was

carefully supported by glass wool on both sides.

Temperature Controller

48

Temperature controllers are employed in order to ensure the correct temperature inside

the reactor.

Mass Flow Controllers

A Brook Instrument 3- channel mass flow controllers were utilizes to monitor the flow

rate of C3H8, H2, and N2 . In order to achieve accurate flow rate to maintain correct

C3H8/ H2 ratio, these mass flow controllers were calibrated. After calibration of these

mass flow controllers, correct values are calculated from the calibration curves. The

details of the calibration of these mass flow controllers are given in the appendix 6. The

flow was stable throughout the experiment.

Mixing Vessel

The mixing vessel is a steel cylinder about 5 cm ID and 7 cm height. The large volume

of this vessel ensures the good mixing of the coming feed gases

6.4 Catalyst Characterisation

Catalyst characterisation plays a vital role in providing important information related to

physical and chemical properties of catalysts. As catalysis is a surface phenomenon.

49

Catalytic rates and selectivities depend on the available active surface area and their

accessibility in a catalyst, the intrinsic activity of the active sites on the surface and the

process conditions. Hence catalyst characterisation studies provide a basis for the

understanding the interrelationship between the activity and selectivity of a catalyst.

6.4.1 Total Surface Area

The method utilizes to calculate the ‘total surface area’ of the catalyst is known as BET.

Brunauer, Emmett and Teller jointly developed this method and is the most frequently

used for the measurement of total surface area of the catalyst. A schematic of the BET

equipment is show in Fig 6.3 (Appendix 6). A 30% N2 in He was used as measuring gas

and He was employed as flushing gas. The adsorption and desorption of N2 from the

measuring gas were used to determine the total BET surface area of the catalyst [42]. A

mixture of 30% N2 in the He has been suggested to give the best agreement with multi-

point BET methods.

The sample was first dried and degassed at 393 K for 1 hour and then cooled to room

temperature. A flow of the measuring gas was switched to pass the sample at a

temperature of liquid nitrogen (77 K). After the adsorption equilibrium had been

established, the temperature of the sample was raised to the ambient level and the

amount of N2 desorbed was measured. The formula used for the calculation BET total

surface area is as follows:

50

ABET = 4.35 V (273/ T ) ( 1 – X (P/Ps ) ( 760 / Patm )

Ws

Where ABET is the BET surface area. m2/ g

V is the volume of N 2 adsorbed/desorbed, ml

T is the room temperature, K

X is the mole fraction of N 2 in the measuring gas,

P is sample pressure, mm Hg,

Ps is saturation pressure of N 2, mm Hg,

Patm is the atmospheric pressure, mm Hg,

Ws is the mass of the sample, g.

52

Chapter 7

RESULTS AND CONCLUSIONS

7.1 Results

The results obtained from the BET total surface area measurement are shown in table

7.1.

Table 7.1 BET Total Surface Area for Catalyst Samples

Catalyst Sample

C3H8/H2 Ratio Temperature(oC) Time (Hours) ABET

m2/g

C1 1 400 1 140

C2 1 600 1 134

C3 6 400 1 155.48

C4 6 600 1 288.08

C5 1 400 5 288.27

C6 1 600 5 210.48

C7 6 400 5 167.4

C8 6 600 5 173.18

53

BET Total Surface Area of Mo2C Catalysts

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8

Catalyst Samples

BET Total Surface Area, m2/g

Effect of C3H8/H2 Ratio on BET Total Surface Area w.r.t Carburisation Time

100

120

140

160

180

200

220

240

260

280

300

0 1 2 3 4 5 6 7

C3H8/H2 Ratio

BET Total Surface Area, m2/g

Carburisation Time 1 Hour

"Carburisation Time 5 Hour"

54

7.2 CONCLUSIONS

The BET Total surface area was measured for all samples. The Mo2C catalyst was

prepared at different conditions. Three parameters were used for carburisation namely

C3H8/H2 ratio, time and time . The following conclusions were made from the result

obtained. Table 7.1 showing the details of the experiments conducted for carburisation

of MoS2 catalysts to obtained Mo2S. The surface area for Mo2S is found to be 166.15

m2/g.

Effect of C3H8 /H2 Ratio on BET Total Surface Area:

The C3H8/H2 ratio utilizes for carburisation of the Mo2S catalysts were 1:1 and 6:1.

Catalyst samples C-4 and C-5 have found to be highest BET surface area. We can easily

see that these two catalysts attained the highest surface area regardless of different

conditions for carburisation. The same surface area is achieved when we carburise the

catalyst at 6:1 and 1:1 C3H8/H2 ratio, at 600 and 400 oC but for 1 and 5 hours time were

used respectively.

We can also see in the case of sample C-1, when 1:1 C3H8/H2 ratio were used at 400oC

for 1 hour, the surface area was substantially low.

55

Effect of Temperature on BET Total Surface Area:

There have been no major changes in surface area of the catalyst at two different

temperature levels. Considering the catalyst sample C-1 and C-2 (Table 7.1), we can see

there is s decrease in surface area of the two samples dur to the increase in temperature

by 200oC. In this comparison, all other parameters are the same for the two catalysts in

question. The similar situations appear to be with catalyst sample C-7 and C-8.

Effect of Time on BET Total Surface Area:

On examination of the results obtained from BET measurements for the catalyst

samples, it is quite clear that the time has also an important role on the out comes of the

BET surface area. Considering the catalyst samples, C-7 and C-8 for instance, the

values obtained for the above samples implies that the increasing carburising time with

higher C3H8/H2 ratio has an adverse affect on the surface area of these catalysts samples.

56

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59

APPENDICES

60

Appendix 1

63

Appendix 2

64

Fischer-Tropsch Synthesis: Current

Mechanisms

[Davis, B.H., 2001]

70

Appendix 3

77

Appendix 4

87

Appendix 6

88

CALIBRATION CALCULATIONS AND GRAPH OF MASS FLOW METERS.

CALIBRATION OF MASS FLOW METERS

FOR H2 CHANNEL # 1

MAIN READING FLOWRATES (ml/min)

3 26.96

5 43.44

8 68.8

89

FOR N2 CHANNEL # 3

MAIN FLOWMETER

0.26 19.1

0.3 23.1

0.4 32.1

FOR C3H8 CHANNEL # 2

MAIN FLOWMETER

1 34

0.7 21.81

2 74.07

CALIBRATION GRAPH FOR MASS FLOW METERS FOR FOR H2

y = 8.3747x + 1.7347

R2 = 1

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9

MAIN READING

FLOWRATES

90

CALIBRATION GRAPH FOR MASS FLOW METERS FOR FOR N2

y = 92.308x - 4.7718

R2 = 0.9994

0

5

10

15

20

25

30

35

0.2 0.25 0.3 0.35 0.4 0.45

MAIN READING

FLOWRATE

Series1

Linear (Series1)

CALIBRATION GRAPH FOR MASS FLOW METERS FOR C3H8

y = 40.167x - 6.2463

R2 = 1

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5

MAIN READING

FLOWRATE (ml/min)

Series1

Linear (Series1)

91

BET Surface Area Measurements Calculations

92

Sample: C-1

Parameters:

Area Volume (cc)

Calibration Small Loop 335016 0.50 Atm. Press 760 mm Hg

Calibration Large Loop 3031314 5.00 LN2 Sat P 775 mm Hg

Sample Desorption 1816535 2.97 Sample P 0 mm H2O

Sample Duplicate 1855143 3.04 Room Temp 25 'C

N2 Conc. 0.302

Sample Wt 0.0602 g

BET Surface area 8.34 m2 138.5 m2/g

BET duplicate 8.52 m2 141.5 m2/g

Average 140

Sample: C-2

93

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0644 g



Average 134.44

Sample: C-3

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0506 g



Average 155.48

94

Sample: C-4

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0469 g



Average 228.08

Sample: C-5

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0506 g



Average 288.27

Sample: C-6

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0552 g



Average 210.48

95

Sample: C-7

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0537 g



Average 167.40

Sample: C-8

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0524 g



Average 173.18

Sample: C-MoS2

Parameters:

Area Volume (cc)





N2 Conc. 0.302

Sample Wt 0.0515 g



Average 166.15