UNIVERSITY OF CINCINNATI

Date:______

I, ______, hereby submit this work as part of the requirements for the degree of: in:

It is entitled:

This work and its defense approved by:

Chair: ______

Synthesis, Characterization and Kinetic Studies of Mixed Metal Mo-V-Nb-Te Oxide Catalysts for Propane

Ammoxidation to Acrylonitrile

A thesis submitted to the

Division of Research and Advanced Studies of the University of Cincinnati

in partial fulfillment of the requirements for the degree of

Master of Science

In the Department of Chemical and Materials Engineering of the College of Engineering

December 2005

by

Salil Bhatt

B.Tech., Indian Institute of Technology Bombay, India, 2002

Committee Members

Dr. Vadim V. Guliants (Chair) Dr. Peter Smirniotis Dr. Soon-Jai Khang

Abstract

The ample abundance and low cost of propane has recently spurred an interest in the

manufacture of acrylic acid and acrylonitrile from propane, both important intermediates in the manufacture of plastics and clothing. Commercially, both are currently

manufactured based on a process utilizing propylene as the feedstock, which is high in

demand and expensive. Many catalytic systems have been proposed and tested for the

oxidation and ammoxidation of propane to acrylic acid and acrylonitrile respectively. Of

these, the four component mixed metal oxide system Mo-V-Nb-Te has been found to be

the most promising for acrylic acid and acrylonitrile manufacture. The objective of this

work has been to study the effect of temperature, catalyst weight and flow rates on the

yield and selectivity to various product species and to formulate a reaction mechanism

that would assist in making further predictions on the reaction outcome for various

reaction parameters.

Acknowledgments

I would like to express my sincere gratitude to my advisor, Dr. Vadim V.

Guliants, for his ideas, support and guidance throughout the duration of the project. His

invaluable experience in the field of catalysts has been instrumental in gearing the progress and direction of this work. His dedication to research, science and technology

has been an ideal model to follow throughout my thesis work.

I am thankful to the Chemical Sciences, Geosciences and Biosciences Division,

Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under the

Grant No. DE-FG02-04ER15604, without which the funds needed to accomplish this

research would not be available. I would like to thank Dr. Junichi Ida, who has played a

vital role in assisting me during preparation of the experimental setup, testing and

calibration of the GC detectors and mass flow controllers. Beyond this, he has also

extended his invaluable experience in providing much needed suggestions and guidance

during the experimental stages of the project. I would also like to thank Mr. Rishabh

Bhandari for his assistance in guiding me through various phases of synthesizing the

catalyst, characterization and kinetic testing. I would also like to thank Mr.

Balasubramanian Swaminathan, Mrs. Li Yuan and Mr. Neelakandan Chandrasekaran not

only for their valuable suggestions and feedback but for their co-operation and moral

support. Last but not the least; I would thank all of my other group mates for their

valuable feedbacks and suggestions.

Table of Contents

List of Tables iii

List of Figures iv

Chapter 1 Introduction 1

Chapter 2 Current trends in acrylonitrile processing 5

2.1 Introduction 5

2.2 Structure of bulk mixed Mo-V-Nb-Te-Ox catalyst 10

2.3 Synthesis and Characterization of Mo-V-Nb-Te oxides 13

Chapter 3 Reaction Kinetics 19

3.1 Introduction 19

3.2 Kinetics as a function of propane conversion 22

3.3 Calculation of activation energy for propane conversion 26

3.4 Evaluation of reaction rate orders 29

3.4.1 Reaction rate order for oxygen 30

3.4.2 Reaction rate order for ammonia 31

3.4.3 Reaction rate order for propane 33

3.5 Reaction mechanism 35

Chapter 4 Design of a productive ammoxidation catalyst 46

4.1 Introduction 46

4.2 Effect on the purity level of nitrogen gas during calcination 47

4.3 Effect of catalyst composition 50

Appendix A Experimental Setup 52

A.1 Description of equipment 52 A.2 Schematic and design of the experimental setup 56

Appendix B Procedure for Chemical Analysis 59

B.1 Temperature ramp settings 59

B.2 Valve switching 62

Appendix C Calibration 63

C.1 Calibration of Mass Flow Controller 63

C.2 Calibration of the GC detectors 65

Appendix D Simulation code listing 66

References 70

ii List of Tables

1. Industrial processes under study or development for oxidative

transformation of light alkanes 01

2. Comparison of different catalysts for propylene ammoxidation 06

3. Comparison of different catalysts for propane ammoxidation 08

4. Comparison of surface areas of Mo-V-Nb-Te catalyst obtained using

different synthesis routes 17

5. Comparison of kinetic activity for catalysts synthesized using hydrothermal and

dry-up methods 18

6. Temperature vs. reaction rate using 0.073g of catalyst & 36.5 ml/min flow

rate 27

7. Reaction rate orders for individual products w.r.t. individual reactant

species 29

8. Reaction rate constants for propane ammoxidation pathway 37

9. Reaction rate constants for propane ammoxidation pathway with propylene to

acrylonitrile as an added path 38

10. Reaction rate constants considering propylene as an intermediate 40

11. Retention times of primary organic species (minutes) 61

12. Retention times of light and inorganic gases using just Porapak Q column 61

iii List of Figures

1. Electronegativity and catalytic behavior of metal oxides 07

2. Reaction pathway for conversion of propane to acrolein 09

3. Structure of the M1 phase 10

4. Structure of the M2 phase 11

5. XRD of catalyst prepared by (a) Dry-up and (b) hydrothermal methods 16

6. SEM image of catalyst prepared using dry-up synthesis 17

7. SEM image of catalyst prepared using hydrothermal synthesis 17

8. Effects of reaction temperature upon conversion, selectivities and yields 23

9. Variation of selectivity of various components at different propane conversions 24

10. Variation of the yield of various components at different propane conversions 24

11. Natural logarithm of reaction rate vs. inverse of temperature 28

12. Natural logarithm of rate of formation of product species vs. natural log of oxygen

concentration 30

13. Natural logarithm of rate of formation of product species vs. natural log of

ammonia concentration 32

14. Natural logarithm of rate of formation of product species vs. natural log of

propane concentration 34

15. Proposed propane ammoxidation reaction pathway 36

16. Proposed route for propane ammoxidation with intermediate propylene 39

17. Predicted vs. observed concentration data for propane 41

18. Predicted vs. observed concentration data for propylene 42

19. Predicted vs. observed concentration data for acrylonitrile 42

iv 20. Predicted vs. observed concentration data for acetonitrile 43

21. Predicted vs. observed concentration data for carbon oxides 43

22. Synthesis route followed for MoVNbTeOx metal oxides 46

23. Comparison of the X-Ray Diffraction pattern for industrial and pre-purified grade

nitrogen 48

24. Comparison of kinetic activity of catalysts calcined by industrial and pre-purified

grade nitrogen 49

25. Comparison of kinetic performance of catalysts with varying niobium contents 51

26. Diagram of the GC analysis system 56

27. Schematic of six-way valve used for injecting effluent samples 57

28. Schematic of six-way valve connecting Porapak Q and molecular sieve 57

29. Calibration curve for helium gas 64

30. Calibration curve for oxygen gas 64

31. Calibration curve for propane gas 65

v Chapter 1

Introduction

The industry today is predominantly reliant on the use of olefins as raw materials for the manufacture of a large number of chemical intermediates. However, the abundant natural abundance and lower cost of paraffin’s as compared to olefins has generated an interest in obtaining these intermediate products through direct catalytic conversion of paraffin’s by bypassing olefin manufacture as an intermediate step, as opposed to the current commercial method of using olefins. Various alternatives are now currently being sought in this regard. Different catalytic systems have been devised and investigated for the successful conversion of the corresponding alkane to the desired reaction product [1-

4]. So far, the one reaction that has been quite successfully implemented using an alkane is that of oxidation of n-butane to maleic anhydride, which is based on the now famous

Vanadium-Phosphorous metal oxide system [5-7].

Table 1.1: Industrial processes under study or development for oxidative

transformation of light alkanes (C1 – C6) [20]

Raw Material Product Stage of development

Methane Methanol Pilot plant

Methane Syngas Pilot plant

Methane Ethylene Pilot plant

Ethane 1,2-dichloroethane, vinyl chloride Pilot plant

Ethane Acetaldehyde Research

1 Ethane Acetic Acid Research

Ethane Ethylene Research

Propane Acrolein, Acrylic Acid Research

Propane Propyl Alcohol Research

Propane Acrylonitrile Demonstrative plant

Propane Propylene Research

n-Butane Acetic Acid Industrial

n-Butane Maleic Anhydride Industrial

n-Butane Butadiene Industrial, abandoned

Isobutane Methacrylic acid Pilot plant

Isobutane Isobutene Research

Isobutane t-Butyl Alcohol Research

n-Pentane Phthalic anhydride Research

Cyclohexane Cyclohexanol, cyclohexanone Industrial

Cyclohexane Cyclohexanol Research

The oxidation and ammoxidation of propylene is currently a very crucial reaction for the manufacture of acrylic acid and acrylonitrile, both of which have uses in textiles and polymer industry. Currently the industrial chemical acrylonitrile is being manufactured by the SOHIO process [8, 9], which is based on the ammoxidation of propylene. New alternatives to this process are being sought that produce acrylonitrile from the direct ammoxidation of propane [10-12, 4]. Various multi-component mixed metal oxide catalytic systems have been investigated for this purpose and the four

2 component metal oxide system composed of Molybdenum, Vanadium, Tellurium and

Niobium has been discovered to give the highest yields to acrylonitrile and overall optimum reaction performance. [13-14]

Of all the key reaction components, propane is the most stable molecule and requires the highest activation energy for oxidation, whereas the product species formed during the reaction are relatively unstable and are easily activated for further oxidation to yield carbon oxides. Based on the molecular orbital theory, this can be explained by the fact that the propylene and molecules of successive intermediates formed in the reaction such as acrolein have π-bonding molecular orbital as against propane which is solely composed of just σ-bonding molecular orbitals. The higher internal energy levels of the propylene and other product species make them more susceptible to further oxidation products under conditions of propane oxidation. Hence, the key challenge is to control the further oxidation of the desired unsaturated intermediates formed and limit the reaction to just the desired partial oxidation products.

The industrial chemical acrylonitrile is related in a significant way in the manufacture of the various objects that we interact with in our day to day life. Right from polymers and plastics of which are made day to day use utensils to the clothes that we wear almost every day directly or indirectly possess some relation to acrylonitrile during their manufacturing process. For example, poly-acrylonitrile is an important chemical that is used in the manufacture of garments. Currently more than 10 billion pounds of acrylonitrile is being manufactured across the globe. [11]

Research is ongoing in this field striving to develop new synthesis methods and conditions to enhance the yield of acrylonitrile and reduce the costs involved in the

3 process. This has led us to investigate the use of propane, being much cheaper than propylene, as a use for the raw material of the ammoxidation reaction to yield acrylonitrile. Various catalysts have been investigated for support of this reaction and achieve the same levels of efficiency and productivity currently exhibited by the propylene ammoxidation process being used commercially. Amongst all the candidate catalysts investigated to date, the four component mixed metal Mo-V-Te-Nb oxide catalysts have delivered the most promising results.

4 Chapter 2

Current trends in acrylonitrile processing

2.1 Introduction

The current industrial process for the manufacture of acrylonitrile is based on the

SOHIO process [8, 9], which involves the ammoxidation of propylene to acrylonitrile.

The overall reaction for propylene ammoxidation to acrylonitrile is as follows:

CH2 = CH – CH3 + 1.5O2 + NH3 → CH2 = CH – CN + 3H2O

Currently multi-component mixed metal oxides comprising primarily of Bi-

Mo(W)-O phases modified with Fe, Co and Ni metals are employed for this reaction. So far, 87-98% conversion and 90-97% selectivity to acrylonitrile have been achieved. The temperature used is in the range of 250-450°C [14].

The formation of acrylonitrile from propylene is a three step process: (i) The insertion of an oxygen atom into a C-H bond to transform propylene to an π-allylic alcohol (ii) conversion of π-allylic alcohol to σ-allylic alcohol with N-addition (iii) dehydrogenation of the allylic alcohol to form acrylonitrile.

The results obtained for the conversion of propylene to acrolein/acrylonitrile are tabulated below.

5 Table 2.1.1 Comparison of different catalysts for propylene ammoxidation [14]

Conversion Selectivity Oxides Temperature (°C) (mole %) (mole %) Mo-Bi-Fe-Co-Ni-P-Mg 350 98 95

Mo(W)Bi Fe, Co, Ni, Si 250-450 96 90-93

Mo, Bi, Fe, Co, Ni, Sn, K 300 87 97

The catalytic oxidation of lower alkanes is composed of two primary steps: (i) the

dehydrogenation step leading to olefin conversion and (ii) oxygen addition step to the

unsaturated carbon atom in the reactant or intermediate [21]. Every metal oxide species

has oxygen ions with varying levels of electronegativity. Three types of oxygen ionic

- - 2- species form on the surface of the catalyst, O , O2 and in rare cases O . Of these, all three are nucleophilic species. Of the more abundant oxygen ionic species, O- is far more

- nucleophilic than O2 species and capable of reacting with almost any hydrocarbon. In essence, the dehydrogenation step depends on the nucleophilic ability of the oxygen ion as opposed to the addition step which depends on the electrophilic nature of the oxygen ion at the catalyst surface. Hence the selectivity to the final products is dependent on the nature of the oxygen ions being generated at the surface of the catalyst. Species having a high electronegativity could be detrimental by leading to the production of a large amount of carbon oxides by olefin addition. At the same time, the oxygen species should be sufficiently nucleophilic in nature to initiate the reaction by hydrogen abstraction to form the corresponding olefin.

6

Fig 2.1.2: Electronegativity and catalytic behavior of metal oxides [21]

As can be seen from the above figure, the lone oxygen atom is the most electronegative and responsible for additions at unsaturated carbon bonds in olefins.

Oxides of Ru and Os are highly electronegative as well and result in complete oxidation of reaction products to carbon monoxide and dioxide. Oxides of sodium, potassium etc. would be effective only for the dehydrogenation step and would fail to produce partial oxidation products. It is evident that oxides possessing an intermediate nature are most selective towards partial oxidation products such as oxides of vanadium, molybdenum,

antimony and tin.

7 Good catalytic performance coupled with the lower temperature required for the reaction has initiated interest in further optimization of the catalyst development process and fine tuning of the reaction so as to achieve yields exceeding 40%. Several mixed multi-component metal oxide systems have been tested for the same and their performance is summarized as below.

Table 2.1.3 Comparison of different catalysts for propane ammoxidation [14]

Catalyst Temperature Conversion Acrylonitrile Selectivity

VSbSnCu 470 14% 61%

BiMoFeCrWMg 470 15% 54%

BiMoP 529 44% 54%

VSbWP 500 85% 37%

MoVNbTe 420 55% 53%

BiVSbMo 500 09% 49%

The postulated reaction pathway from propane to acrolein (or acrylonitrile with

NH insertion) is depicted below

8 Propane

CH3CH2CH3 -117.1 -174.1 -150.4 CH2=CHCH3 CH3CH(OH)CH3

CH3CH3CH2OH -104.5 -186.3 CH2=CHCH2OH -172.3

CH COCH -195.5 CH3CH2CHO 3 3

-134.4 -615.4 CH2=CHCHO Acrolein -850

CH3CHO + CO2

CH3COOH + CO2

Fig. 2.1.4: Reaction pathway for conversion of propane to acrolein [14]

The numbers in the above figure denote the enthalpy change for each reaction pathway in KJ/mol. As can be interpreted from the above figure, the enthalpy change for the conversion of propane to 1-propanol and 2-propanol is definitely higher as compared to that for propylene conversion. The latter path is hence favored at higher temperatures.

The path towards acetone and acetic acid through 2-propanol cannot be totally eliminated though. The path through 1-propanol may lead to the formation of acetaldehyde depending on the active sites and surface properties of the catalyst. However, the conversion of either propylene or 1-propanol to acetonitrile is a process with a much larger enthalpy change as compared to the formation of either 1-propanol or propylene, in

9 accordance with the lower reactivity of a saturated alkane to an unsaturated olefin.

Hence, the first step requires a higher temperature as compared to the second step.

2.2 Structure of bulk mixed Mo-V-Nb-Te-Ox catalyst

The four component molybdenum, vanadium, tellurium and niobium mixed metal

oxide system were first developed by Mitsubishi, Japan [22]. The catalyst operates at

temperatures ranging from 340°C to 480°C with 420°C being reported as the optimum

temperature achieving the highest yield towards acrylonitrile.

The four component Mo-V-Nb-Te mixed metal oxide systems are found to

comprise of three different phases: (i) The M1 phase having a composition of

Mo7.8V1.2NbTe0.94O28.9 which has an orthorhombic structure; (ii) The M2 phase having a composition of Mo4.67V1.33Te1.82O19.82 which has a hexagonal structure; (iii) trace amounts of monoclinic TeMo5O16 [15-16].

Fig.2.2.1 Structure of the M1 phase [17]

As can be seen from the above figure, the M1 phase is comprised of twelve

distinct crystallographic lattice positions having a space group of Pba2. The M1 phase is

10 found to contain vanadium sites with an oxidation state of +5, which is absent in the M2

phase. This site is mainly found to be responsible for the abstraction of a hydrogen atom

from propane for the dehydrogenation reaction to propylene, through redox reactions

involving a change in the oxidation state of vanadium from +4 to +5 and vice-versa.

Hence, the M1 phase plays a critical role in initiating the propane dehydrogenation

reaction to propylene, based on which further reactions proceed to yield the final product,

acrylonitrile. It can be concluded that the M1 phase is the most vital for the propane

ammoxidation reaction to acrylonitrile. Also, niobium present in the M1 phase plays two

very important roles: (i) site isolation, i.e. spatially separates active catalytic centers from

each other and imparts the structure its high selectivity to acrylonitrile (ii) acts as a

stabilizing agent for the entire structure [17]

Fig. 2.2.2 Structure of the M2 phase [17]

The structure of the M2 phase contains five distinct crystallographic lattice positions. The main difference between the M2 phases from the M1 is the lack of V5+

11 sites which are crucial for propane activation due to which the M2 phase cannot initiate

the reaction. Another difference is the lack of niobium in the structure of the M2 phase.

Vanadium plays a vital role for the activation of propane to form propylene. The

chemisorbed propylene then interacts with the two Te4+ atoms undergoing an α-H

abstraction to form an π-allylic alcohol intermediate bonded to a Mo6+ of the active

center. The ammonia molecules are easily accessible through this molybdenum site

which acts as an NH inserter converting the allylic intermediate to acrylonitrile. The Nb5+ atoms serve the purpose of site isolation by keeping the Mo and Te sites apart, which is essential to achieve optimum selectivity.

The M1 phase is quite efficient at low conversions and the complete process of conversion from propane to acrylonitrile occurs within the M1 sites of the catalyst only.

However, as the conversion levels increase, the rate of NH insertion and acrylonitrile formation cannot keep up with the rate of abstraction α-hydrogen from vanadium sites on

the M1 phase. This is when the M2 phase comes into play, and with a rich abundance of

Tellurium and molybdenum activation sites, is able to successfully convert all of the

remaining un-reacted propylene from the M1 phase into acrylonitrile, which would

otherwise be attacked again by the vanadium sites of the M1 phase and be converted to

partial/complete oxidation products such as carbon oxides. For optimal yield of

acrylonitrile, it has been suggested that a composition of 60%/40% of M1/M2 phases

respectively be utilized [17]

Based on the assumption that the presence of V5+ atom leads to propane activation and two V5+ atoms leads to further combustion into waste carbon oxides, the structure of the M1 phase has been analyzed and being found to contain a rough composition of

12 46%/44%/10% of active/inactive/waste sites respectively. Based on this, it can be calculated that the maximum attainable selectivity to acrylonitrile is 81%. However, the experimental maximum being obtained so far is just 72%. This can be accounted on the basis of waste sites present which are more selective towards complete oxidation products as against desired intermediates.

2.3 Synthesis and Characterization of Mo-V-Nb-Te oxides

Two different synthesis methods have been successful in producing Mo-V-Nb-Te mixed metal oxides as valuable catalysts for propane ammoxidation. These are :-

(i) Hydrothermal method and (ii) The dry-up method [18].

1. Hydrothermal method: - 5.35g of ammonium paramolybdate and 1.12g of telluric

acid are taken and mixed with 20ml of de-ionized water at 80°C and stirred to

form a homogenous solution. In parallel, two more homogenous solutions are

prepared; one constituting the element vanadium is prepared by mixing 2.22g of

hydrated vanadium sulfate and 10 ml of de-ionized water at room temperature;

another constituting the element niobium is prepared by mixing 1.44g of hydrated

niobium oxalate in 10ml of de-ionized water heated to 80°C. After all the three

solutions prepared in parallel have turned homogenous, the solution containing

vanadium is added drop wise to the solution constituting of molybdenum and

tellurium and stirred until completely mixed and homogenous. Then, the solution

containing niobium is added into the remaining mixture and stirred until

homogeneity is achieved. This solution is then transferred to an acid digestion

13 bomb, closed and sealed for any leaks and heated to 175°C for 48 hours. After

this, the solution is taken out and filtered for about 6 hours and then dried again at

80°C for another 6 hours or so. Upon drying, the resulting black solid obtained is

calcined in a stream of pure nitrogen at 600°C for 2 hours. Then the four

component Mo-V-Nb-Te oxide sample is obtained which can be used as the

catalyst for the propane oxidation/ammoxidation reaction. The formula of the

mixed metal oxide obtained was Mo1.0V0.3Te0.17Nb0.12.

2. Dry-up method: - 4.21g of ammonium metavanadate is mixed with 117ml of de-

ionized water until the salt is dissolved. 4.13g of telluric acid and 15.89g of

ammonium paramolybdate are added step-wise until the solution turns

homogenous. In parallel, another homogenous solution is made with mixing 3.89g

of ammonium niobium oxalate with 17.9ml of de-ionized water. This solution is

then added into the larger solution now comprising of the other three elements

Mo, V and Te. The resulting solution is stirred again for some more time until

homogeneity is obtained. After this, the solution is dried at 150°C for about 5

hours until a solid residue is obtained. This residue is then calcined in a fashion

similar to the hydrothermal catalyst in a pure nitrogen environment at a

temperature of 600°C for 2 hours. The formula of the mixed metal oxide obtained

was Mo1.0V0.3Te0.2Nb0.1.

Of the two methods, the slurry method is more efficient in preparing the catalyst due to two reasons: (i) little loss of the metal oxide components during the drying phase as compared to the loss suffered in the hydrothermal method during filtration; (ii) the time involved in making the catalyst is of the order of one day at the maximum whereas

14 the time involved in making the catalyst based on the hydrothermal method takes time of the order of 4 or more days. Hence, very large amounts of the catalyst can be obtained in a short period of time by using the dry-up method. However, the atmospheric conditions

(pressure) during the dry-up method is harder to control since the catalyst solution is left open during evaporation whereas the catalyst is sealed in an air tight acid digestion bomb in the hydrothermal method wherein the pressure within would be just a function of the vapor pressure of water. Hence, the hydrothermal method is more reproducible and utilized for laboratory level testing.

Characterization of the obtained Mo-V-Nb-Te mixed metal oxide catalyst was done by X-Ray Diffraction studies, Transmission Electron Microscopy (TEM) and

Scanning Electron Microscopy (SEM). TEM and SEM imaging were used to visually see the structure of the metal oxides and unit cells. XRD was used to identify the various peaks present and judge the extent of crystallization and identify the different phases

(M1/M2) present in the catalyst, and cross-check the possibility of oxidation of individual metal oxide species within the catalyst which would otherwise impede the performance of the catalyst.

A comparison of the XRD results obtained by synthesizing the catalyst using the three different methods outlined above is shown under.

15

Fig. 2.3.1: XRD of catalyst prepared by (a) Dry-up and

(b) hydrothermal methods [18]

It is evident that the difference in the X-Ray diffraction patterns of the four

component Mo-V-Nb-Te catalyst prepared by the hydrothermal method is very similar to

the one prepared by the dry-up method. Consequently, they can be deduced to have

similar phases in similar compositions and be expected to show similar conversion levels

and selectivity to acrylonitrile for a given weight, flow rate and temperature conditions.

The M1 and M2 phases both exhibit a strong characteristic peak at about 22.1°.

The other major peaks that characterize the M1 phase occur at 7.7°, 9.0°, 27.3°, 29.2° and

35.4°. Those characterizing the M2 phase occur at 28.2°, 36.2°, 44.7° and 50.0° [13]. In

addition, a third rutile phase having a composition of TeMo5O16 that exists in minority exhibits a peak at 54.2°. The strongest line exhibited by the M1 phase occurs at about

16 22.1° and that for the M2 phase occurs at 28.2° which can be used to characterize these

phases [23].

Fig. 2.3.2: SEM image of catalyst prepared Fig. 2.3.3: SEM image of catalyst prepared

using dry-up synthesis [18] using hydrothermal synthesis [18]

The catalyst prepared using the hydrothermal method has almost twice the surface area as compared to the catalyst prepared by the dry-up method. However, similar morphologies of the catalyst prepared using both methods relate to similar selectivities to acrylonitrile observed in the reaction.

Table 2.3.4: Comparison of surface areas of Mo-V-Nb-Te catalyst obtained using

different synthesis routes [18]

Synthesis procedure Surface area (m2g-1)

Hydrothermal 19.1 Dry-up 9.9

As can be predicted from the higher surface area of the hydrothermal catalyst, the conversion and yield to acrylonitrile obtained are higher as compared to the catalyst synthesized using the dry-up procedure.

17 Table 2.3.5: Comparison of kinetic activity for catalysts synthesized using hydrothermal and dry-up methods. Conditions: GSHV = 7900h-1, amount of catalyst = 50mg, reaction

temperature = 683K, propane / ammonia / air = 1.0 / 1.2 / 15.0 [18]

Synthesis procedure Propane conversion ACN selectivity Rate of reaction (mole %) (mole %) (mg ACN/m2 cat./h) Hydrothermal 27 70.9 14.1 Dry-up 18.5 66.5 17.2

It can be concluded that four component mixed metal oxides prepared using the hydrothermal method are more effective and achieve the highest throughput to acrylonitrile. It is essential therefore, to study the reaction mechanism of the propane ammoxidation reaction in presence of the four component catalyst discussed to fully understand the different parameters affecting the overall rate of formation of acrylonitrile and realize the optimal combination of parameters for the reaction. These studies would form the kernel of the optimal sequence based on which the industrial process for acrylonitrile manufacture shall be implemented.

18 Chapter 3

Reaction Kinetics

3.1 Introduction

A utilitarian analysis of the reaction kinetics for the ammoxidation of propane shall involve determining all of the parameters in the following standard rate model:-

γ1 γ 2 γ 3 − rA = k[C3 H 8 ] [O2 ] [NH 3 ] , where the terms in the square brackets denote the concentrations of the gaseous species represented by the formulae and γ1, γ2 and γ3 are the rate orders of the reaction w.r.t. to propane, oxygen and ammonia respectively.

The rate constant, k is related to the activation energy of the reaction through the following equation:-

−E / RT k = Ae , where A is the Arrhenius constant, E the activation energy of the reaction in presence of the catalyst, and T is the temperature at which the reaction is carried out.

The utility of the above equation lies in its simplicity. For most purposes of analysis, the estimated parameters can be used to successfully predict the influence of the reaction conditions on the rates of formation of various product species.

The four constants present in the above two equations can be determined by performing a series of experiments where in the parameters pertaining to one of the constants is changed, keeping the others constant. For the determination of the reaction rate orders, this would imply that the concentration of any individual gaseous species be

19 varied, keeping the temperature and concentrations of the other gaseous species a

constant. The determination of the activation energy is done by keeping the

concentrations of each individual species a constant and varying the temperature of the

reaction.

It may be noted that the accurate estimation of all of the above constants demand

that the reactant concentrations be kept as low as possible to keep the rates of the

reversible reactions from the product species to the reactants, negligible.

As evident from the literature reports discussed in the previous section, the hydrothermal four component Mo-V-Nb-Te oxide catalyst is the most effective for the propane ammoxidation reaction. The following studies have been made on the catalyst:

(i) Comparison of the conversion and selectivity to acrylonitrile obtained keeping temperature and weight/flow rate ratios a constant; (ii) Determination of the activation energy of the reaction using the hydrothermal catalyst and subsequent comparison with the activation energies reported for the catalyst prepared by the dry-up method; (iii)

Comparison of the reaction rate orders for propane, ammonia and oxygen for the catalysts synthesized by the two different methods.

The following definitions were used in analysis of the reaction kinetics

1. Conversion: - Defined as the number of moles of propane reacted divided by the

number moles of propane initially detected in the feed.

2. Selectivity: - Defined as the number of moles of carbon obtained for a particular

product species upon the number of moles of carbon of propane reacted as a result

of propane conversion

20 3. Yield: - Defined as the product of the conversion and selectivity for a particular

component

4. Reaction rate: - Calculated as moles of propane converted upon the weight of the

catalyst multiplied with the flow rate and the surface area per unit weight of the

catalyst. The reaction rate is obtained in units of µmolm-2s-1.

21 3.2 Kinetics as a function of propane conversion

The rate expression for a plug flow reactor having a bed of catalyst is as follows:

dX F = (−r ) , A0 dW A where FA0 represents the molar flow rate of the reactant gas, X the conversion, W the weight of the catalyst loaded and rA the reaction rate. It is evident that the conversion is a function of the ratio of the weight of the catalyst loaded to the inlet molar flow rate of the reactants.

The effect of the propane conversion w.r.t. to the selectivity and yields of the various product species were explored. It was found that the conversion increased up to a threshold value of about 60 – 65% at which the selectivity and yield to acrylonitrile dropped significantly.

It was observed that the selectivity to acrylonitrile passed through a maximum and then decreased again. This can be explained on the basis that the reaction to form propylene is an intermediate in the pathway to form acrylonitrile, and a low conversion would imply a low rate of reaction in which the gases do not have sufficient time to stay in contact with the catalyst bed to convert the propylene formed into acrylonitrile. The selectivity to acrylonitrile reaches an optimal amount since beyond a certain critical rate or contact time with the catalyst; the reaction of the formed acrylonitrile to carbon oxides would begin to be more predominant as compared to the rate of formation to acrylonitrile. Propylene would be converted to other intermediates at higher reaction rates and hence is expected to decrease with increasing conversion.

22 70 Acrylonitrile

) 60

% 100 50 Selectivity ole 80 Yield m 40 on (

% 60 i 30 s le r o e 40 20 m b) onv

C 10 a) 20 0 0 320 340 360 380 400 420 440 320 340 360 380 400 420 440 Temperature (°C) Temperature (°C)

Carbon Dioxide Propylene

15.0 40 Selectivity Selectivity 12.0 Yield Yield 30 9.0 % % c) le le 20 o 6.0 mo d) m 10 3.0

0 0.0 320 340 360 380 400 420 440 320 340 360 380 400 420 440 Temperature (°C) Temperature (°C)

Acetonitrile

12.0 Selectivity 10.0 Yield 8.0 %

le 6.0 e)

mo 4.0 2.0 0.0 320 340 360 380 400 420 440 Temperature (°C)

Fig 3.2.1 Effects of reaction temperature upon (a) net conversion; selectivities and yields

of (b) acrylonitrile; (c) carbon dioxide; (d) propylene and (e) acetonitrile

23 The above data was collected using a catalyst weight of 200mg and a flow rate of

8ml/min over a 20 hour interval, taking 2 readings for each data point.

90 80 70 %) e l 60 Acrylonitrile o

m 50 Propylene ( y

it 40 CO2 iv t

c 30 Acetonitrile le

e 20 S 10 0 0 102030405060 Conversion (mole %)

Fig 3.2.2 Variation of selectivity of various components at different propane conversions

35

30

) 25

% Acrylonitrile e

ol 20 Propylene m 15 CO2 d ( l e

i Acetonitrile

Y 10 5

0 0 102030405060 Conversion (mole %)

Fig 3.2.3 Variation of the yield of various components at different propane conversions

The operating conditions used for the above was a reactor temperature of 360°C and a flow composition of 6% propane, 7% ammonia, 17% oxygen and 70% helium as used and reported by Asakura et. al. in the Journal of Catalysis [13]. Four different batches of calcined catalysts were used for each data point, each being tested for 4 hours.

24 The selectivity to acrylonitrile is found to peak at a specific level of conversion.

At low conversion, the selectivity to propylene is higher owing to low contact times due to which propylene doesn’t undergo further oxidation. At high conversions, the further oxidation of acrylonitrile takes place giving carbon oxides and acetonitrile. This behavior was in sharp contrast with the reported behavior for the catalyst prepared by the dry-up synthesis procedure, in which the selectivity to acrylonitrile is found to peak at lowest propane conversion levels and decreases abruptly only around a conversion level of 80%.

This however agrees with the behavior reported for the four component Mo-V-Nb-Te oxide catalyst prepared by the hydrothermal synthesis procedure for propane [19].

The conversion of any reaction taking place in a continuous flow packed bed reactor is a function of the ratio of the weight of the catalyst to the flow rate (W/F).

Higher W / F ratios imply a higher conversion for the same catalyst, feed compositions and temperature. This is so since the reactants interact with the catalyst bed for longer time duration to give a greater conversion. This increased conversion level plays an important role in affecting the selectivity of any product species.

25 3.3 Calculation of activation energy for propane conversion

This set of experiments was conducted at very low conversion levels, not exceeding 15%. The W/F ratio was fixed to be 0.002 and data points were obtained at temperatures ranging from 320°C to 380°C. The catalyst was not unloaded at any point in the reaction. Since the reaction was carried out at very low conversion levels, the influence of the reversible reactions from the product species to propane can be essentially neglected. The reaction rate was obtained as function of moles of propane converted per unit time per unit gram of the catalyst. The rate of reaction is a function of two independent variables, the rate constant which is a direct function of the reaction temperature, and a function of the flow characteristics within the reactor and the weight of the catalyst (W/F ratio). Since the W/F ratio is a constant, the change in the former can be directly associated with the change in the reaction rate upon temperature. The equation employed for relating the rate constant to the rate of the reaction and the temperature is:

ln k = C − E / RT

Where C is any constant that is obtained through an arithmetic combination of the

Arrhenius constant and the dissociation constant obtained as the ratio of the stoichiometric coefficients of the reaction products to the reactants; in essence independent of the temperature. E represents the activation energy of the reaction and T the temperature of the reaction.

For the purposes of this experimental series, we substitute the use of the reaction rate constant with the actual reaction rate as follows:-

γ1 γ 2 γ 3 − rA = k[C3 H 8 ] [NH 3 ] [O2 ]

26 When the concentrations of all the reactants, propane, ammonia and oxygen are kept at a constant, the logarithm of the reaction rate can be expressed as:-

ln(−rA ) = ln k + (γ 1 ln[C3 H 8 ] + γ 2 ln[NH 3 ] + γ 3 ln[O2 ]) = ln k + C

Hence a plot of the logarithm of the reaction rate takes the same form as that of the logarithm of the reaction rate constant vs. the temperature. The slope of the line would still yield the negative of the activation energy divided by the ideal gas constant.

The results obtained from the series of experiments are tabulated below. For each data point, two individual readings were taken, with the total testing time being 16 hours.

Table 3.3.1 Temperature vs. reaction rate using 0.073g of catalyst & 36.5 ml/min flow

rate

Temperature Conversion Reaction rate (°C) (mole %) (µmolg-1s-1) 320 0.93 1.94 340 2.3 4.71 360 6.2 12.82 380 12 24.94

27 ) 3.5 -1 s -1 g

l 3 mo µ 2.5 te ( a 2 tion r

ac 1.5

1 y = -16797x + 28.985 R2 = 0.9964 l log of Re

a 0.5 tur

Na 0 0.0015 0.00156 0.00162 0.00168 0.00174 1 / Temperature (K-1)

Fig. 3.3.1 Natural logarithm of reaction rate vs. inverse of temperature.

The slope of the above graph is the activation energy divided by the ideal gas constant. From this, the activation energy turns out to be (33 ± 3) kcalmol-1K-1, which is within 5% of the reported value of (32 ± 1) kcalmol-1K-1 [13].

28 3.4 Evaluation of reaction rate orders

The evaluation of reaction rate orders also needs to be done at low conversion levels so as to minimize the rate of the backward reactions that may originate due to the formation of product species. This would give a more accurate estimate of the reaction rate orders since the conversion and reaction rates would depend solely on the concentration of the reactant species. It may be recalled that the reaction rates are powers of the concentrations in the reaction rate expression.

The reaction rate orders were evaluated individually for each of the three reactants: oxygen, propane and ammonia. For each of the three cases, a freshly calcined catalyst (calcination done at 600°C for 2hrs in UHP nitrogen for all 3 cases) of weight

100 mg was loaded. The total flow rate was kept a constant at 50 ml min-1. The flow rates of the individual reactant species and the helium gas were correspondingly adjusted. The rate orders for ammonia and oxygen were determined using the same batch of catalyst, with a total of 9 data points and a testing time of 36 hours. Determining the rate orders of propane consisted of collecting 4 data points and a run time of 16 hours.

The table below lists the results obtained. The reaction in reality being much more complex in nature, shows a poor correlation for many of the constants, which are omitted.

Table 3.4.1 Reaction rate orders for individual products w.r.t. individual reactant species

Reaction rate Overall Acrylonitrile Acetonitrile Propylene Carbon Dioxide Propane 1.1 ± 0.4 1.02 ± 0.05 2 ± 1 0.7 ± 0.3 - Ammonia 0.69 ± 0.05 0.6 ± 0.3 - 0.5 ± 0.4 - Oxygen - -1 ± 1 -2.3 ± 0.5 0 2 ± 1

29 3.4.1 Reaction rate order for oxygen

The typical concentration for oxygen in the reaction is about 17 mol %. The concentration of oxygen was varied from 11% to 23% in increments of 3% (all mol %) each and the respective conversion and reaction rates for individual product species evaluated.

Oxygen was found to have a profound influence upon the rate of formation of acetonitrile and a weaker correlation was found to be present for the rate of formation of acrylonitrile and carbon dioxide. The rate of formation of both acetonitrile and acrylonitrile were found to decrease with increase in the oxygen concentration which could be attributed to the fact that the presence of oxygen favors propane oxidation reaction over the ammoxidation reaction. Correspondingly, carbon dioxide formation was found to increase with increase in oxygen concentration.

The charts below are a plot of the logarithm of oxygen concentration vs. logarithm of the rate of production for specific reaction products.

6 4 a) 5 3 4 y = 1.555x + 16.768

r 2 R = 0.8875 3 r y = -2.3029x - 15.558 2 ln ln 2 2 R = 0.9908 1 1 b)

0 0 -8.4 -8.2 -8 -7.8 -7.6 -7.4 -8.4 -8.2 -8 -7.8 -7.6 -7.4 ln C ln C

Fig 3.4.1 Natural logarithm of rate of formation of (a) CO2 (b) acetonitrile vs. natural log

of oxygen concentration

30

6 5

5.6 4.75 5.2 y = -0.0122x + 4.2461 r r R2 = 0.5436 4.5 ln 4.8 ln y = -1.3333x - 5.3525 R2 = 0.895 4.25 c) 4.4 d) 4 4 -8.4 -8.2 -8 -7.8 -7.6 -7.4 -8.5 -8.25 -8 -7.75 -7.5 ln C ln C

7 e) 6.6

6.2 r ln 5.8 y = -0.4232x + 1.163 2 R = 0.7547 5.4

5 -8.5 -8.25 -8 -7.75 -7.5 ln C

Fig 3.4.1 Natural logarithm of the rate of formation of (c) acrylonitrile; (d) propylene and

(e) overall reaction rate vs. natural log of concentration of oxygen

3.4.2 Reaction rate order for ammonia

Increase in the concentration of ammonia was found to have a positive effect upon the overall rate of reaction and the rate of formation of acrylonitrile, propylene and acetonitrile. For the formation of carbon dioxide, the rate of formation was found to pass through a maximum and then decrease again with increasing concentration. This could be attributed to the combined effect of the increase in the rate of formation of acetonitrile

31 and acrylonitrile, both of which lead to carbon dioxide as final oxidation products and the increased presence of ammonia which reduces the rate of the oxidation reaction as compared to ammoxidation.

5 5.5

a) 4.6 b) 5.25 4.2 y = 0.5221x + 8.8534 r r 2 5 ln R = 0.9471 ln 3.8 y = 0.5795x + 10.107 R2 = 0.9825 4.75 3.4

3 4.5 -9 -8.8 -8.6 -8.4 -8.2 -9 -8.8 -8.6 -8.4 -8.2 ln C ln C

4 4

3.6 3.6 y = 0.9848x + 14.777 y = 1.4195x + 20.689 3.2 R2 = 0.7672 3.2 r r R2 = 0.723 ln ln 2.8 2.8

2.4 c) 2.4 d) 2 2 -9 -8.8 -8.6 -8.4 -8.2 -9 -8.8 -8.6 -8.4 -8.2 ln C ln C

Fig 3.4.2.1 Natural logarithm of reaction rate for (a) propylene; (b) acrylonitrile; (c)

carbon dioxide and (d) acetonitrile vs. natural log of ammonia concentration

32

6

y = 0.6897x + 11.647 5.75 R2 = 0.9987

r 5.5 ln

5.25 e) 5 -9 -8.8 -8.6 -8.4 -8.2 ln C

Fig 3.4.2.1 Natural logarithm of total reaction rate vs. natural log of ammonia

concentration

3.4.3 Reaction rate order for propane

Increasing the concentration of propane was found to have a strong positive correlation to the rate of formation of acrylonitrile formation. The contribution of propane to the rate of formation of acetonitrile, propylene and net conversion was also found to be positive in spite of being more weakly correlated. This can be explained on the basis that acetonitrile and propylene being intermediate products, convert to other end products such as acrylonitrile and carbon dioxide whereas acrylonitrile and carbon dioxide tend to be more stable products given the reaction temperature of 380°C. At temperatures exceeding 400°C a similar drop in the yield of acrylonitrile can be observed followed by a corresponding increase in the yield of carbon dioxide which takes place due to further oxidation of acrylonitrile.

33 6 6 b) 5.5 5.6 5 5.2 r r 4.5 ln ln y = 1.0766x + 15.002 4.8 R2 = 0.969 4 y = 1.0169x + 14.99 2 R = 0.9955 4.4 a) 3.5 3 4 -10.5 -10 -9.5 -9 -10.5 -10 -9.5 -9 ln C ln C

4 5.6

3.5 5.2 3 r r 2.5 4.8 ln ln y = 1.031x + 12.733 y = 0.9767x + 17.739 2 2 R = 0.917 2 R = 0.9054 4.4 c) 1.5 d) 1 4 -10.5 -10 -9.5 -9 -10.5 -10 -9.5 -9 ln C ln C

6.6

e) 6.2

r 5.8 ln y = 1.093x + 16.554 R2 = 0.9189 5.4

5 -10.5 -10 -9.5 -9 ln C

Fig 3.4.3 Natural logarithm of rate of formation of (a) propylene; (b) acrylonitrile; (c) acetonitrile; (d) carbon dioxide and (e) total rate vs. natural log of propane concentration

34 As can be seen, with the exception of acrylonitrile which is the main reaction product, the reaction rates for acetonitrile, propylene and carbon oxides do not show any reliably strong correlations to the change in concentrations of propane. For a more through analysis, a reaction network modeling simulation was accomplished by simultaneously computing concentrations of individual components through each reaction path and obtaining the best fit of the rate constants that match with the experimental data. A detailed study of the simulations is described in the next section for propane.

3.5 Reaction mechanism

It would have been observed that the simplistic rate orders evaluated for each individual reaction species do not show any power-law correlation for the vast majority of cases. This is because in reality, the entire reaction chain leading to a particular product can be broken down into a series of first-order elementary reaction steps. The combination of these first-order reactions may make the overall reaction rate expression of a form different than the power-law expression. Hence, a detailed study of the elementary reaction pathways involved is needed in order to fully understand the dependence of each constituting product species on the reactants. The simplistic rate orders can be however used as an approximation when the conversion is low.

Several different permutations of possible reaction paths were tested based on earlier findings for propane oxidation and ammoxidation reactions for various catalysts

[24-28]. The changes in product concentrations were modeled as a function of the first- order linear rate constants and input propane concentration. The best fit of parameters was that which minimized the relative error in the experimental vs. the theoretically

35 predicted concentrations of products at the exit of the reactor bed. The root mean square method was used for purposes of optimization.

The optimal mechanism discovered suggested the direct formation of acrylonitrile as the dominating route as opposed to the intermediate conversion of propylene. This can be attributed to the dominance of the M1 phase during the catalytic oxidation process, in which the product propylene is not released as an intermediate but rather instantly converted to further oxidation products. Consequently, the rate of reaction for the formation of propylene would differ from the rate of formation of acrylonitrile. This however does not entirely rule out the role of propylene in the generation of acrylonitrile.

Similarly, the formation of acrylonitrile is claimed to pass through the intermediate formation of acrolein, which is hardly ever detected in actual analysis. However, experiments conducted for propylene ammoxidation on similar reaction conditions show significant concentrations of acrolein produced as a by-product. [24]

Propylene 1

2 Acrylonitrile

Propane 3 Acetonitrile 4 6

5

COx

Fig. 3.5.1 Proposed propane ammoxidation reaction pathway

The rate expressions for the individual reaction pathways are as follows:-

dC1 C1 (k1 + k2 + k3 + k4 ) − F0 = dW (1+ k7C1 )

36 dC2 k1C1 F0 = − k5C2 dW (1+ k7C1 )

dC3 k2C1 F0 = − k6C3 dW (1+ k7C1 )

dC4 k3C1 F0 = dW (1+ k7C1 )

dC5 k4C1 F0 = + k5C2 + k6C3 dW (1+ k7C1 ) where C1, C2, C3, C4 and C5 represent the concentrations of propane, propylene, acrylonitrile, acetonitrile and carbon dioxide respectively; k1-6 represent the rate constants of the reaction pathways shown in Fig 3.5.1; and k7 is the adsorption rate constant for propane discussed below.

The following equations take into account the adsorption/desorption effects of gas species on the catalytic surface according to the Langmuir-Hinshelwood model, which is represented by the rate constant k7. The model is given by:

kaCa θ a = 1+ kaCa

Where θa represents the adsorption concentration of any species ‘a’, and k and C represent the adsorption constant and concentration of species ‘a’ in the gas phase.

The respective rate constants obtained by minimizing the error for each reaction data point is as follows:-

Table 3.5.1 Reaction rate constants for propane ammoxidation pathway

k1 k2 k3 k4 k5 k6 k7 Mean (µ) 0.42 0.72 0.093 1.2 2.3 1.3 1.3 Std. Dev. (σ) 0.0389 0.0685 0.0113 0.7129 0.8325 0.3998 0.4775 95% C.I. ±0.03 ±0.05 ±0.008 ±0.5 ±0.6 ±0.3 ±0.4

37

The bottom row indicates the width of the deviation of the 95% confidence intervals for each individual value from the mean based on the standard deviation observed for all the data points.

It was observed that adding more reaction paths produced results with larger deviations of k-values from the mean and broader confidence intervals those above.

Statistically, every reaction path would be theoretically feasible. The likelihood of each would depend upon how well the observed data fits the simulated results. Since the above reaction network and combination of coefficients produced results with the least observed error, it can be said to be the most probable path for the reaction. It may be noted that this proposed network is similar in many ways to the one for V-Sb-Al mixed metal oxide catalysts [24]. The direct reaction from propane to acrylonitrile is found to contribute the maximum to the rate of generation of acrylonitrile, which can be attributed to the M1 phase of the catalyst. It may be noted that simulation results for an additional propylene to acrylonitrile pathway yield a reaction rate co-efficient of propylene larger than that for propane (with larger error estimates), in concord with the higher activity of propylene as compared to propane. However, the low concentration of propylene makes the direct conversion of propane crucial to be optimized for obtaining the maximum yield.

Table 3.5.2 Reaction rate constants for propane ammoxidation pathway with propylene to

acrylonitrile as an added path

k1 k2 k3 k4 k5 k6 k7 k8 Mean (µ) 0.44 0.67 0.093 1.2 0.7 1.2 1.2 2.2 Std. Dev. (σ) 0.0442 0.0790 0.0113 0.7054 0.7820 0.3099 0.4884 0.4704 95% C.I. ±0.03 ±0.06 ±0.008 ±0.5 ±0.6 ±0.2 ±0.4 ±0.3

38 The modified equations for C2 and C3 corresponding to the above table is thus:

dC2 k1C1 F0 = − k5C2 − k8C2 dW (1+ k7C1 )

dC3 k2C1 F0 = + k8C2 − k6C3 dW (1+ k7C1 )

As can be seen, the mean value of k8 is larger than that of either k1 or k2.

However, the low concentration of propylene makes the net reaction rate of formation of acrylonitrile through propane much more profound than that from propylene.

Considering a stand-alone pathway from propane to propylene which acts as an intermediate for all subsequent steps is impractical as can be seen in the results tabulated

Table 3.5.3. Acrylonitrile

5 2 1 Propane Propylene COx 4

3

Acetonitrile

Fig. 3.5.2 Proposed route for propane ammoxidation with intermediate propylene

The following are the equations corresponding to the above reaction network.

dC1 k1C1 F0 = − dW 1+ k6C1

dC2 k1C1 F0 = − k2C2 − k3C2 − k4C2 dW 1+ k6C1

dC F 3 = k C − k C 0 dW 2 2 5 3

39 dC F 4 = k C 0 dW 3 2

dC F 5 = k C + k C 0 dW 5 3 4 2

Where C1, C2, C3, C4 and C5 represent the concentrations of propane, propylene, acrylonitrile, acetonitrile and carbon oxides respectively and k1-5 are the rate constants for the reaction pathways depicted in Fig. 3.5.2.

Table 3.5.3 Reaction rate constants considering propylene as an intermediate

k1 k2 k3 k4 k5 k6 Mean (µ) 2.4 20 2.8 30 -10 -20 Std. Dev. (σ) 0.6942 20.5954 0.8965 18.1539 28.2803 41.7999 95% C.I. ±0.5 ±20 ±0.7 ±10 ±20 ±30

Here k6 represents the adsorption rate constant.

The rate constants obtained fail to show any strong correlation to support the rate mechanism proposed. The values obtained do not converge to any specific numerical value and show huge deviations from the mean for every data set.

The conversion of propane to acrylonitrile is the fastest and the slowest being the conversion to acetonitrile, which agrees with the observed low concentrations of acetonitrile. The fact that most of the acrylonitrile is produced from direct conversion to propane and does not rely on a route passing through propylene suggests the active dominance of the M1 phase in the reaction as opposed to the M2 phase. It may be noted that the conversion of acetonitrile to carbon oxides have negligible weight as opposed to the contribution from propylene and acrylonitrile, thus denying any accurate rate constant estimates for the same. The rate coefficients for the formation of carbon oxides from

40 propylene and acrylonitrile are higher as compared to those for propane, owing to the higher activity of the former owing to the presence of unsaturated double and triple bonded carbon atoms in their structure.

Below is a comparison of the concentration predicted for each species using the model with the optimized coefficients with the observed concentrations for each.

Fig 3.5.3 Predicted vs. observed concentration data for propane

41

Fig 3.5.4 Predicted vs. observed concentration data for propylene

Fig 3.5.5 Predicted vs. observed concentration for acrylonitrile

42

Fig 3.5.6 Predicted vs. observed concentration data for acetonitrile

Fig 3.5.7 Predicted vs. observed concentration data for carbon oxides

43 A reasonable fit between the simulated and experimental results is observed for all the species except carbon oxides. This could be partly due to the poorer correlations observed for the rate constants leading to carbon oxide formation from propane, propylene and acrylonitrile. The formation of carbon oxide from acetonitrile has not been considered since the very low concentrations of acetonitrile will have a negligible contribution to carbon oxides formation as compared to the contribution of the other species. Another factor contributing to the ineffectiveness of the model is the inaccuracy of data obtained due to limitations of the instrument in successfully estimating the amount of carbon monoxide for most cases, due to the low concentrations of CO relative to oxygen, which it follows in the elution sequence.

No reaction network or mechanism has been reported for propane ammoxidation on Mo-V-Nb-Te oxide catalysts. A model for propane ammoxidation on V-Sb-Al catalysts was developed by Centi G. and Grasselli R.K. [24] assuming propylene as an intermediate for all further oxidation products including acrylonitrile. However, the rate constants obtained using the model exhibit large uncertainties and confidence intervals, akin to the deviations in rate constants observed in this work when a similar intermediate path through propylene was considered [Table 3.5.3]. The propane oxidation reaction for

Mo-V-Nb-Te metal oxide system has been developed and reported by Lintz H-G. [26]

This model shows that the acrylic acid is formed mainly through the intermediate path through propylene as against directly from propane, which was successful in explaining the observed data. It has been proposed that vanadium (V5+) sites are primarily responsible for propane activation [19]. Molybdenum (Mo6+) sites are responsible for further oxidation of propylene to acrolein and tellurium (Te4+) sites convert the acrolein

44 formed to acrylic acid. It may be inferred that the in the presence of ammonia, the intermediate propylene formed is not desorbed but reacts with ammonia to give acrylonitrile and other ammoxidation products; whereas in the absence of ammonia, propylene is desorbed upon formation and then adsorbed on Mo6+ sites for further oxidation to acrolein and ultimately acrylic acid.

45 Chapter 4

Design of a productive ammoxidation catalyst

4.1 Introduction

Design of an effective, productive and economical catalyst is the first most

essential step on the road to obtaining a good yield of acrylonitrile. Every step in a

catalyst preparation/synthesis procedure plays some contribution on the catalyst’s morphology and structure which in turn influences a catalyst’s kinetic behavior. 5.35g ammonium 1.12g telluric 2.22g hydrated 1.44g of paramolybdate acid vanadyl sulfate hydrated niobium oxalate

Mixed with Mixed with 10ml Mixed with 10ml water water at 80°C 20ml water at 80°C

Mixed drop by drop, stirred for 5min.

Stirred for 10min.

Filtered and Heated in hydrothermal Calcined at 600°C bomb at 175°C for 2 days dried at 80°C for 2 hr in N2

As-synthesized Calcined samples samples

Fig. 4.1.1 Synthesis route followed for MoVNbTeOx metal oxides

46 As can be seen, there are several steps involved starting from the mixing of ‘raw materials’ to obtaining the final product. Understanding these vital parameters that play a lead role in affecting the performance of the catalyst as a whole would go a long way in the synthesis of catalysts capable of delivering a performance both desirable and economical on a large scale. Here we analyze our findings with respect to the variation of two critical synthesis parameters.

4.2 Effect on the purity level of nitrogen gas during calcination

The atmosphere during calcination plays a significant role in producing the phases in the catalyst selective for propane oxidation and ammoxidation. The phases produced in a completely inert atmosphere (nitrogen or helium) as opposed to those in air are entirely different. The catalyst calcined in air shows significant amounts of molybdenum trioxide,

MoO3, as opposed to that calcined in nitrogen or helium [29]. The presence of such phases makes the catalyst unselective to propane oxidation in general since they lack the symbiosis offered by the M1 phases, wherein each element has a specific role in the steps towards the desired product. In addition, the surface area obtained from catalysts synthesized in an inert atmosphere is much higher than that obtained from calcination in air [29]. As a result, conversions obtained are much lower and the products obtained are more in resemblance to an oxidative dehydrogenation reaction of propane to propylene which occurs naturally at high temperatures. It has been observed that oxygen concentrations of the order of 1000ppm or more significantly affect the active phases of the catalyst and lower the conversion obtained for propane oxidation/ammoxidation reactions [13].

47 Three different purity levels (grades) of nitrogen were tried out and compared for kinetic performance: industrial grade, pre-purified nitrogen grade and ultra high purity nitrogen. Pre-purified and ultra high purity grades of nitrogen contain 99.998% and

99.999% of nitrogen respectively whereas the industrial grade is delivered bereft of any quality control. It is obtained as a residual of the product of oxygen purification from air.

X-Ray diffraction studies indicate the peaks corresponding to the M1 and M2 phases in the catalyst calcined using industrial grade nitrogen are much lower in intensity as compared to the peaks obtained when the catalyst is calcined using pre-purified or ultra high purity grade nitrogen. This difference in the peak intensities correlate to a lower order of crystallinity for the catalyst calcined using industrial grade nitrogen.

Pre-purified

Industrial

5 1525354555 Angle (2Θ)

Fig. 4.2.1 Comparison of the X-Ray Diffraction pattern for industrial and pre-purified

grade nitrogen

48 Both of the catalysts calcined with different purity grades of nitrogen were subjected to a propane ammoxidation reaction test. The selectivity to acrylonitrile for both the catalysts match reasonably well but the conversion and the yield to acrylonitrile obtained by the pre-purified grade catalyst is drastically more than that compared to the industrial grade catalyst, supporting the hypothesis that oxygen contamination during calcination decreases the available active sites for catalysis during the reaction.

70

60

50

40

% Industrial e l 30 Pre-purified mo

20

10

0 Acrylonitrile Propylene COx Acetonitrile Acetic Acid

Conversion Selectivity

Fig. 4.2.2 Comparison of kinetic activity of catalysts calcined by industrial and pre-

purified grade nitrogen respectively at W/F = 0.05 and temperature of 380°C with feed

composition of C3H8/NH3/O2/He = 6/7/17/70

49 4.3 Effect of catalyst composition

Niobium has been a passive contributor to the performance of the four-component mixed metal oxide system, Mo-V-Nb-Te. Where vanadium, molybdenum and tellurium play a direct role in converting the propane feed into acrylonitrile, niobium does not interact with the reactants or any of the intermediate reaction produced radicals. Hence, investigating the performance of niobium has been an issue the justification of which would be necessary to decide if more of the former three components could be added for higher conversions and selectivity.

Two catalysts were tried and tested with two different levels of niobium content, a composition of Mo1.0V0.30Nb0.12Te0.16 was tried and a composition of

Mo1.0V0.30Nb0.17Te0.16. The kinetic behavior of the catalysts for the propane ammoxidation reaction was studied at different conversions and temperatures. It was observed that the catalyst having higher niobium content showed a better selectivity and yield to acrylonitrile as compared to the catalyst with low niobium content.

50 80 70 60 50

% Nb/Mo=0.12

le 40 Nb/Mo=0.17 mo 30 20 10 0 y y n n e e t t y y i i o o il il t t i i i i r r v v t t 2 2 i i s s v v i i n n er er ct ct cti cti e e lo CO lo CO l l e e nv nv y y l l r r o o se se c c se se C C A A at 360°C at 400°C

Fig 4.3.1 Comparison of kinetic performance of catalysts with varying niobium contents

Mo-V-Te mixed metal oxides containing niobium have been reported to be more selective to acrylic acid in propane oxidation reactions as compared to those not containing niobium [30]. It has been reported that the difference in the yield of acrylic acid obtained increases in significance as the conversion is increased. Hence, it is conferred that the niobium plays a greater role in stabilizing the final product and preventing further oxidation reactions as against participating in its formation. For propane ammoxidation, at higher temperatures, the activity of the catalyst and selectivity towards acrylonitrile were both found to be lower as compared to the catalyst containing higher niobium content. The selectivity to carbon dioxide is found to be much higher at

400°C in the case of the sample containing a lower niobium content which suggests the possibility of abundant further oxidation of acrylonitrile. However, the contribution of niobium towards activating propane and being an essential helper in the generation of acrylonitrile cannot be ruled out for the propane ammoxidation reaction.

51 Appendix A

Experimental Setup

A.1 Description of equipment

The setup used for carrying out the reaction and due detection and analysis of the components consisted of the following independent units: -

ƒ Four Mass Flow Controllers: - The mass flow controllers used were

independently calibrated for a specific reactant species. Care was taken to ensure

that the range of the mass flow controller matched with that to be used with the

reactant species. For the gas helium, the mass flow controller with the highest

range was used, since helium is the most concentrated gas in the reaction mixture,

having a concentration of about 70%. For the other gases, controllers with a lower

range were used.

ƒ Mass Flow Controller Readout: - The four mass flow controllers were

independently connected to one common controller readout in which the flow rate

and the gauge correction factors were set. It included a four channel display

ƒ 12” Tubular Lindberg Blue Reactor: - A tubular reactor was employed for

actually carrying out this reaction. The reactor included a programmable PID

controller with a 16 segment display adjustable. The gap within the reactor was 2”

in diameter and the tube used within the reactor had an O.D. of 9 mm from the

reactants end and an O.D. of 6 mm at the products end.

52 ƒ Gas Chromatograph: - This single instrument was used for the separation and

subsequent detection of all the reaction products coming out from a tube

connected directly at the reactor exit to the inlet of the sample loop present in the

Gas Chromatograph. The various sub-units of the Gas Chromatograph are: -

o Porapak Q column: - This column, 5” in length and 1/8” in O.D., was used

to accomplish separation of the organic species present in the reaction, in

particular propane, propylene, acetonitrile, acrylonitrile and acetic acid in

the order mentioned above. The only exception to the above case was the

elution of ammonia which passed through the column relatively

unadsorbed and eluted much before propane. Small amounts of

acrolein/acetone were also observed to elute between the elution of

acetonitrile and acrylonitrile, but these were negligible and generally not

reported in the actual kinetics data. Acetic acid formed a major reaction

product for some reactions only, in many other cases it was not detected or

reported as with the case of acrolein or acetone mentioned above.

o Carbosphere column: - The Carbosphere column consisted of a molecular

sieve and was 4” in length. This column was capable of separating all the

major inorganic gases present in the reaction products, in particular

oxygen and carbon dioxide. Carbon monoxide was a minority component

and found not detected or detected in very minute concentrations for

almost all reactions. The ammonia gas being detrimental to this column

was not passed through and instead eluted through the first column as

described above.

53 o Thermal Conductivity Detector (TCD): - The thermal conductivity

detector is based on the principle of detecting gases through the difference

in the conductivities of a standard reference/carrier gas (in this case

helium) and the eluting gas. This is framed onto a Wheatstone bridge

setup, in which each resistor is separated from a hot filament by the

passing gas which is either the reference or the eluting gas. The change in

the conductivity of the passing gas would change the heat transferred onto

the resistor, which would then change the current passing and create an

imbalance which would be detected as an electrical current coming out

from the circuit. This current is then recorded and the area of the signal

computed by integration. This area is then calibrated with different species

for different concentrations and the calibration employed to obtain the

actual concentration of the eluting gas in the reaction effluent. o Flame Ionization Detector (FID): - The flame ionization detector is based

on the principle of combusting gases using a mixture of hydrogen and air

at a high temperature and passing a current through this reacting mixture.

At these temperatures and kinetic conditions, several species of the

effluent gas are bound to be in ionic form and can easily conduct current.

This current is then recorded and the area obtained in a plot of current vs.

time employed to obtain the actual gas concentration in the effluent. The

area has to be calibrated by injecting a few solutions of a known

concentration first.

54 o Two Six-way valves: - One of these valves is used to control the flow of

the reactor effluent into the GC columns and the second valve serves as a

separator between the Porapak Q and the Carbosphere column, of which

the former is always passed through. These are used to control the flow of

the gases through the columns and based on an approximate idea of the

elution time of certain gaseous species; their positions are changed over

time.

ƒ Mass Spectrometer: - In this instrument, gases are passed through an ionization

chamber and then through a setup of two magnets sitting opposite to each other.

Depending on the ionization of gas and the resulting mass to charge ratio (Z), the

ions are deflected between the magnets and collected onto a plate. The Z ratio is

determined by the deflection experienced by the gas ions. This technique can be

used to identify almost any gas by comparing the spectrum obtained with the

standard gas spectrum for any gas in the literature. The spectrum is a function of

Z ratio vs. the intensity of the signal. This unit was used to verify the

compositions of the gases eluting from the column at any particular time.

ƒ Integrator: - The integrator collects the signal output from both the TCD and FID

detectors and is used to obtain a numerical analysis by detecting and processing

different peaks as they appear in the chromatogram and computing the areas for

each peak. It is used to produce a report of the different peaks detected, the time at

which the peak is reached (maximum signal value), the area of the peak and the

relative concentration of the species in the sample. To obtain the concentration of

the species, different weight factors need to be programmed into the integrator

55 which is obtained by an analysis of the signal sensitivity of each species for the

FID/TCD detectors.

A.2 Schematic and design of the experimental setup Injection port

Effluent Inlet For organics 6-way valve Porapak Q

Molecular Sieve 6-way Sample valve loop For Inorganic gases

FID TCD Restrictor

Fig A.2.1 Diagram of the GC analysis system

The first six way valve is concerned with controlling the flow of the effluent in and out of the system. This is implemented as two possible sets of connectors between six different ports, hence the term six-way valve. One set of connectors would lead the effluent directly to the exit route and the other set into the system. A detailed schematic of the valves is shown below.

56 Effluent in/out

1 2

6 3 He carrier gas Sample loop in/out 5 4 Into GC system

Fig A.2.2 Schematic of six-way valve used for injecting effluent samples

The points 1, 5 and 6, 2 will be connected in the above figure when the reaction effluent has to be collected into the sample loop. The procedure for injecting the reaction effluent is to maintain this valve position for a fixed duration of time and then change positions and connect sample loop to the helium carrier gas inlet and send the effluent mixture into the analysis system for separation and quantitative measurements. The points 6, 4 and 3, 5 would now be connected.

From Porapak Q To TCD, FID 1 2

6 3 Molecular Sieve Restrictor 5 4

Fig A.2.3 Schematic of six-way valve connecting Porapak Q and molecular sieve

The second six way valve working on a similar principle is concerned with regulating the flow between the Porapak Q and the Carbosphere columns. The light reaction gases are initialed passed through both the columns hereafter which the

57 connectors change their positions within the valve and the flow passes through just the

Porapak Q column before being routed towards the exit. This is implemented as follows.

The points 1, 5 and 6, 2 in the Fig 3.2.3 are initially connected in the valve and thus the gases flow through both the columns. To restrict the gases to flow through just one column, points 1, 4 and 3, 2 are now connected and the gases routed through a restrictor.

The main function of the restrictor is to act as a buffer for any pressure pulses that occur in the system typically as a result of valve switching. Two restrictors were installed in two different locations, one between the TCD and FID detectors as shown above for regulating the pressure whenever the position of the injection valve changes. The second restrictor was installed in between the exit of the column and the entry to the TCD. This served the purpose of regulating the pressure when the injection valve was used to inject the effluent from the reactor onto the system and back into the injection mode.

58 Appendix B

Procedure for Chemical Analysis

The procedure followed for quantitative analysis and identification of components is based wholly on the peak signals recorded and the observed elution time of the components. It is essential that the peaks of different components observed be as far apart from each other in time as possible and have a minimal of overlap for the most accurate results. The elution time of any component depends on two factors: (i) Nature of the component and the extent of interaction with the column, which is described by the dissociation constant ĸ and (ii) The flow rate of the inert carrier gas passed through the

GC columns.

B.1 Temperature ramp settings u The mean flow rate of the eluting component is given byv = , where u is the κ +1 flow rate of the carrier gas through the column.

If the carrier gas takes t0 seconds to elute out of the GC column, then the time taken by any component is to(ĸ+1). It may be noted that the dissociation constant, ĸ is a function of temperature and in general decreases with increase in temperature. Thus species which are strongly bonded onto the column can be removed by heating the column to higher temperatures once the elution of low bonding components is completed.

This has been accomplished for this reaction system, in which the light gases such as oxygen, carbon dioxide, ammonia, propane and propylene get eluted at a temperature of

100°C within 25 minutes, preserving a minimum time difference between successive peaks of about a minute. However, the elution of the heavier reaction products is

59 impractical at this temperature. It was estimated from simulation that the time taken for acrylonitrile to elute would be close to 3 hours! Hence, the column temperature has to be raised after the elution of propane in order to hasten the elution of the remaining components all of which are equally strongly bonded at 100°C. From determining the actual elution times of different organic species at various temperatures, the temperature of 170°C was chosen as the optimum temperature for fast elution of the organic species while preserving sufficient time lag between the different organic species within the elution sequence.

A formula for estimating the time dependence of the dissociation co-efficient was computed from the Langmuir rates of adsorption and desorption respectively

f (n)P R = exp(−E ads / RT ) ads 2πmkT a

' des Rdes = v. f (n)exp(−Ea / RT)

Where f(n) and f’(n) represent the surface coverage of empty and occupied sites respectively.

One of the assumptions of the Langmuir isotherm is that adsorption takes place only at specific sites at the surface and complete occupancy of these sites leads to complete adsorption. Thus, we can say that the above functions f (n) and f’ (n) depend only upon the concentration of adsorbate in the mobile phase and stationary phase respectively. The dissociation constant, κ (which is a ratio of the concentrations of adsorbate in stationary and mobile phases) can hence be concluded to be directly

f (n) proportional to . Also, at equilibrium, the rates of adsorption and desorption are f ' (n)

60 equal. Hence, taking the ratio of Rads and Rdes and clubbing all the constant terms onto one gives us the following:

k = A T exp(B /T )

For a temperature regime in which the species is strongly adsorbed, the dissociation factor is strongly dependent on the exponential term hence the adsorbance of the species decreases with increase in temperature. The retention times for the main organic species at different temperatures are given below.

Table B.1: Retention times of primary organic species (minutes)

Sample 120°C 150°C 170°C 200°C

Acetic Acid 100.97 38.65 22.01 12.13 Acetonitrile 37.47 19.74 12.96 8.83 Acrylonitrile 60.41 26.85 18.38 11.56 It can be observed that at 200°C, the retention times of acetic acid and acrylonitrile are too close to each other to assure a sufficient separation. Hence, a temperature of 170°C was chosen for the separation of highly adsorptive organic reaction species.

Table B.2: Retention times of light and inorganic gases using just Porapak Q column

Sample 50°C 100°C 120°C 150°C Propane 58.4 15.02 10.35 6.99 Propylene 50.37 13.57 9.48 6.6 Air 1.57 1.63 1.66 1.77 Ammonia 7.54 4.34 - 4.34 Carbon 4.47 2.7 2.59 2.57 Dioxide

61 As can be seen in the table above, the retention times of propane and propylene have an appreciable difference of about 1.5 minutes which is just sufficient for complete peak separation at 100°C, which was selected as the initial oven temperature for the analysis. The GC oven would be kept at this temperature until the final component, propane was eluted. After this, it would be heated up to 170°C at a rate of 10°C/min for the separation of the remaining organic components.

The elution time of air can be approximated to the elution time of helium since oxygen and nitrogen do not bond at all to the co-polymer network of the Porapak Q column.

B.2 Valve switching

The reaction effluent was initially passed through two columns, a Porapak Q and a Molecular Sieve column in order to accomplish the separation of the light gases such as oxygen and carbon oxides. For the remaining gases and organic components, the Porapak

Q column was sufficient to provide an effective separation. The flow between these two columns was regulated with the means of a six-way valve. Hence, a valve switch was implemented at a point after the elution of carbon dioxide but before ammonia to direct gases eluting from the first (Porapak Q) column into the detectors instead of routing them through the second column. Upon completing the elution of all the organic species after the temperature ramp (the last being acetic acid), the temperature in the oven was once again reduced to 100°C and the columns connected in series again by means of a valve switch. Upon this, the elution of light gases followed such as oxygen and carbon dioxide which cannot be accurately separated otherwise by the use of just the first Porapak Q column.

62 Appendix C

Calibration

The calibration involved two major components; calibration of the mass flow controller for controlling the flow rates of different gases passing through the reactor tube and calibration of the TCD and FID detectors in the GC analysis system to determine the concentration of the individual gases present in the reaction effluent mixture.

C.1 Calibration of Mass Flow Controller

The calibration of the mass flow controller involved the calibration of four gases which were fed into the reaction mixture; propane, ammonia, oxygen and helium. For each of these gases, a mass flow controller capable of providing an appropriate flow rate was implemented. For example, since helium would be present in large quantities as a dilutant gas, the mass flow controller used was in the range of 0 – 50 sccm (cm3 / min).

Whereas for propane and ammonia which were present in the lowest concentration, the range of the mass flow controllers used was 0 – 10 sccm. The one used for oxygen was of the range of 0 – 20 sccm. The calibration of these flow controllers was accomplished through the adjustment of the scaling correction factor, which is unique for a particular gas and a particular mass flow controller. Alternatively, this could be manually adjusted by matching a known flow rate of the gas being passed to the readout indicator by changing the scaling factor set until the value in the readout matches the known flow rate of the passing gas, which is typically measured by using a soap bubble meter. This was done for the butane mass flow controller which was being used for the propane gas since the correction factors were inapplicable. However, for the other gases, correction factors

63 were identified and set on the knobs on the controller readout. Additionally, a correlation was manually established between the flow rate actually measured and the flow rate set for each of the four gases. The correction factors were calculated based on the correlation:

Scaling Control Factor = Gauge Factor × Gas Correction Factor

The gas correction factor is unique for every gas and is based on the formula: -

s GCF = 0.3106* dC p

Where d represents the density of the gas in grams/liter at STP (0°C, 1 atm), Cp its specific heat in calg-1°C-1 and s represents the molecular structure correction factor which equals

ƒ 1.030 for mono-atomic gases

ƒ 1.000 for di-atomic gases

ƒ 0.941 for tri-atomic gases

ƒ 0.880 for poly-atomic gases

They can also be found typically tabulated in any mass flow controller handbook and directly pulled in from those.

45 25 y = 0.805x y = 1.1529x 2 R2 = 0.9975 20 R = 0.9985 te

u 30 15 r min 10 15 ml pe ml per minute 5

0 0 0 1020304050 0 5 10 15 20 sccm sccm

Fig C. 1: Calibration curve for helium gas Fig C. 2: Calibration curve for oxygen gas

64 16 y = 0.9763x )

n 2 i 12 R = 0.9994 m / l (m

e 8 t a r w

o 4 fl

0 0 5 10 15 20 sccm

Fig C. 3: Calibration curve for propane gas

C.2 Calibration of the GC detectors

The calibration curves were made for both the FID and the TCD detectors. The correlation obtained for the FID detector was used for computing the concentrations of propane, propylene, acetonitrile, acrylonitrile and acetic acid; in the order of elution. The

TCD calibrations were used for oxygen, carbon dioxide and ammonia respectively. The calibration data is available for both the detectors in the case of the organic species but only for the TCD detector for the latter gases. It was found that the FID detector had sensitivity about 10 times higher than that of the TCD for the organic species.

The calibration of gases was typically done by making a mixture of the gas with helium and varying the concentration of the gas by varying the individual flow rates of the gas to be calibrated and helium, keeping the total flow rate a constant. The moles corresponding to an individual gas mixture was computed and correlated with the actual area reports obtained from the TCD and FID detectors.

65 Appendix D Simulation code listing

The following is a listing of the code used to run the computations in MATLAB. rate_orders is the main function in the program followed by est_error, which estimates the sum of the squares of the relative differences between the predicted concentration of each species and the experimental data (stored in datatbl). The input of the function is a

1-dimensional matrix of rate constants, K. This return value of this function is optimized by using the inbuilt MATLAB library function fminsearch(), yielding the best fit for K, the co-efficients in the rate equation. The function compute is called upon to calculate the exit concentrations of all products from the effluent stream, given the rate constant matrix

K and the weight of the catalyst, which is 0.1g for all reaction rate order experiments.

This function calls upon the function dcdw (which estimates differential changes in concentrations) through the ordinary differential equation solver (ode45) and is integrated in the range of the entire weight of catalyst. The rate equations are compiled in the function dcdw.

66 List D.1 Reaction rate model parameter estimation program function rate_orders

Ko = [1; 1; 1; 1; 1; 1; 1];

K = [0; 0; 0; 0; 0; 0; 0];

K_set = zeros(7, 7);

% Data Table Format

% Propane Initial, Propane Final, Propylene, Acrylonitrile, Acetonitrile, COx datatbl = [[4.944E-04, 4.418E-04, 1.441E-05, 3.046E-05, 4.840E-06, 1.353E-05]

[4.944E-04, 4.391E-04, 1.520E-05, 3.199E-05, 5.060E-06, 1.415E-05]

[3.662E-04, 3.155E-04, 1.269E-05, 1.874E-05, 2.719E-06, 5.214E-05]

[3.662E-04, 3.137E-04, 1.279E-05, 2.319E-05, 2.748E-06, 4.383E-05]

[7.886E-04, 6.547E-04, 2.795E-05, 5.283E-05, 5.667E-06, 1.480E-04]

[7.886E-04, 6.769E-04, 2.590E-05, 4.308E-05, 5.690E-06, 1.166E-04]

[1.223E-03, 1.054E-03, 4.598E-05, 7.331E-05, 1.048E-05, 1.574E-04]]; h = waitbar(0, 'Computing co-efficients, please wait ...'); for i = 1:7 %7 data points

iter_count = 0;

%Minimize the relative error for each product concentration for K

K = fminsearch(@(K) est_error(datatbl(i, 1), datatbl(i, 2), datatbl(i, 3), datatbl(i,

4), datatbl(i, 5), datatbl(i, 6), K), Ko);

waitbar(i / 7, h);

for j = 1:7

K_set(i, j) = K(j);

67 end end

%The mean of the rate constants for all data points mean (K_set, 1)

%The standard deviation of the rate constants for all data points std (K_set, 0, 1) close(h);

function rms_diff = est_error(c0, c1, c2, c3, c4, c5, K) conc = zeros(5);

WC = compute(c0, 0.1, K); count = size(WC); conc(1) = (WC(count(1), 2) - c1) / c1; conc(2) = (WC(count(1), 3) - c2) / c2; conc(3) = (WC(count(1), 4) - c3) / c3; conc(4) = (WC(count(1), 5) - c4) / c4; conc(5) = (WC(count(1), 6) - c5) / c5; rms_diff = sqrt((conc(1)^2)+(conc(2)^2)+(conc(3)^2)+(conc(4)^2)+(conc(5)^2));

function WC = compute(sc_p, weight, K)

[W C] = ode45(@(w, c) dcdw(w, c, sc_p, K), [0 weight], [sc_p 0 0 0 0]);

WC = [W C];

68

function dc = dcdw(w, c, sc_p, K) dc = zeros(5,1); dc(1) = -(c(1) * (K(1) + K(2) + K(3) + K(4))) / (1 + (K(7) * c(1))); dc(2) = (((c(1) * K(1)) / (1 + (K(7) * c(1)))) - (c(2) * K(5))); dc(3) = (((c(1) * K(2)) / (1 + (K(7) * c(1)))) - (c(3) * K(6))); dc(4) = (c(1) * K(3)) / (1 + (K(7) * c(1))); dc(5) = (((c(1) * K(4)) / (1 + (K(7) * c(1)))) + (c(2) * K(5)) + (c(3) * K(6)));

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