Single Event Microkinetics of (Cyclo)Alkane Cracking on Acid Catalysts

Home , Cycloalkane, Higher alkanes

Faculteit Ingenieurswetenschappen Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin

Single event microkinetics of (cyclo)alkane cracking on acid catalysts

Author: Pilar Díaz Ríos Promoters: Prof. Dr. Ir. G. B. Marin Prof. Dr. Lic. M.-F. Reyniers Coach: Ir. R. Van Borm

Thesis work submitted to obtain the degree of chemical engineer 2006-2007

FACULTEIT INGENIEURSWETENSCHAPPEN

Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin

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Single event microkinetics of (cyclo)alkane cracking on acid catalysts by Pilar Díaz Ríos

Thesis work submitted to obtain the degree of chemical engineer Academic year: 2006-2007

Universiteit Gent Faculteit Ingenieurswetenschappen

Promotors: Prof. Dr. Ir. G. B. Marin Prof. Dr. Lic. M.-F. Reyniers Coach: Ir. R. Van Borm

Overview

The study of pure hydrocarbons has been a continuous practice for obtaining a better insight in the chemistry of the FCC process. Single event microkinetic modelling is an advanced kinetic methodology in catalytic cracking. However, there are issues which are still to be addressed, such as coke formation and catalyst effects. The objective of this thesis is to obtain a better insight in the influence of the catalyst properties during catalytic cracking of (cyclo)alkanes in absence of coke formation.

In the first chapter, the aim and justification of this work is explained and a general introduction about the fluid catalytic cracking process and the use of zeolites in this area is given. In the second chapter, the set up in which the experiments are performed, is described. Next, the mechanism of hydrocarbon cracking is explained and more specific information about the catalyst used is given. In the fourth and fifth chapter, experimental results of the catalytic cracking of iso-octane and methylcyclohexane over different catalysts are presented. The reactor effluent is analysed by means of gas chromatography. Based on these results feed conversion and product selectivities can be calculated, as discussed in chapter 4 and 5. The estimation of acid dependent parameters for catalytic cracking of iso-octane according to the SEMK is developed in chapter 6, and general conclusions of the whole work performed are presented in chapter 7.

Keywords: catalytic cracking, single event microkinetic modelling, iso-octane, methylcyclohexane, acid sites, pore structure, Y zeolite, ZSM-5 zeolite, BIPOM zeolite

Single event microkinetics of (cyclo)alkane cracking on acid catalysts

Pilar Díaz Ríos

Supervisor: Rhona Van Borm

Abstract–The use of zeolites for hydrocarbon conversions is II. EXPERIMENTAL PROCEDURE based on their acid properties and their framework topology. This thesis aims at investigating the influence of these catalyst properties on the rates of the elementary reactions occurring A. Iso-octane cracking during catalytic cracking of alkanes and cycloalkanes. Cracking Catalytic cracking experiments have been performed in a experiments using iso-octane and methylcyclohexane as model recycle electrobalance reactor set up equipped with online gas components have been performed in a recycle electrobalance chromatographic effluent analysis. The reactant 2,2,4-triMe- reactor set up at a broad range of operating conditions. Based on pentane (iso-octane) is evaporated and diluted with nitrogen. the experimental data obtained for iso-octane cracking, acid Iso-octane cracking was performed over CBV 500 (Y zeolite), parameters can be estimated using the single event microkinetic modeling approach, for future optimizing the FCC process. CBV 3020 (ZSM-5 zeolite) and BIPOM3 (BIPOM material) and the activities and product selectivities obtained were Keywords–catalytic cracking, zeolite properties, Y zeolite, compared with previously available data. An overview of the ZSM-5 zeolite, BIPOM zeolite, single event microkinetic modelling. catalyst properties is given in table I. The catalyst pellet size varied from 0,5 to 0,71 mm for CBV 500 and BIPOM3. For CBV 3020 a larger pellet size (0,71-1 mm) was also used to study the absence of transport limitations. Temperature I. INTRODUCTION (475°C) and iso-octane partial pressure (7 kPa) were constant Fluid catalytic cracking is the most important process in a in all the experiments, but a broad range of space times was modern refinery. The growth in the use of USY zeolites in investigated. this process has improved the understanding of the nature and location of the catalytically actives sites in them. Elementary Table I. Properties of zeolites tested in iso-octane cracking. reactions such as isomerization, protonation or β-scission Active sites occur via carbenium ion intermediates. In the industrial FCC Catalyst Si/Al bulk Si/Al frame Structure (mol NH3/kg) process, an acid catalyst containing Y zeolite as main active CBV 500 2,6 3,9 1,50 FAU (NH4-USY) component is used [1]. CBV 3020 15 18,4 ? MFI The influence of the zeolite properties is studied by the (ZSM-5) Al-BIPOM1 50 47 ? MFI cracking of iso-octane and methylcyclohexane in a recycle (BIPOM) Al-BIPOM3 50 ? ? MFI electrobalance reactor over different zeolites. In the case of (BIPOM) the cracking of iso-octane, one Y zeolite is used to study the correct use of the set up, a type of ZSM-5 zeolite to study the absence of diffusion limitations and BIPOM samples to study B. Methylcyclohexane cracking the behavior of these new materials in catalytic cracking. For Experiments using methylcyclohexane as model component methylcyclohexane cracking, a set of Y zeolites differing in of cyclics were performed at similar operating conditions as acid characteristics have been tested at various operation with iso-octane (475°C, 7 kPa and a broad range of space conditions to study the influence of the catalyst properties on times) over four Y zeolites whose properties are shown in activity and selectivity. table II. The objective in this case is to compare activity and A single event kinetic model describing catalytic cracking product selectivities obtained on the catalysts. of hydrocarbons in absence of coke formation [2] has already been developed at the LPT but the kinetic parameters Table II. Properties of zeolites tested in methylcyclohexane cracking. determined still depend on the type of the catalyst used. A general application of this model in process optimization and Catalyst Si/Al bulk Si/Al frame Active sites structure (mol NH3/kg) catalyst development is consequently limited. Therefore, the LZY20 2,6 30 0,99 FAU kinetic model has been modified to be able to use it with other (H-USY) CBV 720 15 16 0,60 FAU catalysts. Currently, the model is extended to account for the (H-USY) influence of acid properties on alkane cracking. Acid CBV 760 30 100 0,23 FAU (H-USY) parameters were estimated based on available iso-octane CBV 500 2,6 3,9 1,50 FAU cracking data. (NH4-USY)

C. Experimental results of iso-octane cracking III. SINGLE EVENT MICROKINETIC MODEL It has been demonstrated based on the experiments performed that there is a good agreement between previously A. Iso-octane cracking model available and currently obtained data over CBV 500 during The rate coefficients in the current single event microkinetic the cracking of iso-octane. This confirms the proper operation model [3] depend on the acid properties and pore structure of and functioning of the set up. the acid zeolite catalyst. Therefore it is important to derive a When comparing activity and product selectivities obtained kinetic model that can accurately describe the influence of the over CBV 3020 no influence of the pellet size on the catalyst properties on the rates of the elementary reaction experimental data was observed. This indicates that transport steps. The elementary steps considered in the cracking of limitations inside the ZSM-5 pellets were absent under the acyclics are (de)protonation, hydride transfer, protolytic conditions investigated. scission, β-scission and PCP-isomerization. Kinetic Two newly developed zeolitic materials have been tested in parameters for these elementary reactions had been iso-octane cracking. At similar operating conditions, both determined previously for the cracking of iso-octane over the BIPOM1 and BIPOM3 showed a relatively low activity reference catalyst LZ-Y20 by means of regression of compared to the other zeolites tested. To compare the product experimental data [4]. distribution obtained, propylene selectivity over all catalysts The influence of acidity has been implemented in the tested in iso-octane cracking is represented in Figure 1. As kinetic model by introducing a limited number of catalyst can be seen, CBV 500 and CBV 3020 follow the same trend descriptors: the number of active sites C , the influence on the as Y and ZSM-5 zeolites [4], respectively. It has been t protonation enthalpy ∆(∆Hpr), and the influence on the observed that BIPOM1 resembles Y zeolite behavior, while activation energies of the elementary reaction families BIPOM3 shows an intermediate behavior between Y and considered ∆Ea,reac. A computer code has been developed to ZSM-5. estimate these acidity descriptors.

Propylene selectivity 120 B. Parameter estimation CBV 500 CBV 3020E 100 BIPOM1 Preliminary estimations for the acidity descriptors of CBV BIPOM3 Y-trendline 80 ZSM5-trendline 720 have been obtained based on unweighted regression to 60 experimental data. The best agreement between experimental 40 and calculated data was obtained with the following values: Selectivity (mol%) 20 ∆(∆Hpr) = 7,652±0.033 kJ/mol, ∆Ea,htf = 7.822±0.034 kJ/mol, 0 ∆E = 5.145±0.021 kJ/mol, ∆E = 7.699±0.033 kJ/mol, 0 1020304050607080 a,pr a,prsc Conversion (mol%) ∆Ea,beta = 5.194±0.024 kJ/mol and ∆Ea,PCP =10.43±0.004

kJ/mol. Figure 1. Propylene selectivity as a function of iso-octane conversion over the zeolites tested, at 475°C and 7 kPa. IV. FUTURE WORK D. Experimental results of methylcyclohexane cracking First of all, parameter estimations for the acidity descriptors over Y zeolites will have to be continued. Next, the influence For the four Y zeolites tested in methylcyclohexane of pore structure can be modeled. Based on the cracking cracking, it is demonstrated in Figure 2 that the acid results of methylcyclohexane kinetic parameters for cyclic properties do not influence the catalyst activity. species at free coke formation can be estimated. Finally,

cracking experiments will have to be performed with other + Conversion vs acid H /Fº model hydrocarbons (e.g., aromatics). 80

60 50 V. REFERENCES 40

30 [1] M. Stöcker. “Gas phase catalysis by zeolites”, Micropor. Mesopor.

Conversion (mol%) 20 LZY20 Mater., vol 82, no. 3, pp. 257-292, 2005. CBV 720 10 CBV 760 [2] M.-F. Reyniers, Y. Tang, and G. B. Marin, “Influence of coke CBV 500 0 formation on the conversion of hydrocarbons. II. i-Butene on HY 0 20 40 60 80 100 120 140 160 zeolites”, Appl. Catal. A.: Gen, vol. 202, no. 1, pp.65-80, 2000. H+/Fºmch (mol H+.s/mol mch fed) [3] Hans C. Beirnaert, J. R. Alleman, and G. B. Marin, “A fundamental kinetic model for the catalytic cracking of alkanes on a USY zeolite in Figure 2. Methylcyclohexane conversion as a function of acid sites the presence of coke formation”, Ind. Eng. Chem. Res., vol. 40, no. 5, available per feed flow rate over the Y zeolites tested, at 475°C and 7 kPa. pp. 1337-1347, 2001. [4] T.R.R. Pintelon. Master thesis, Department of Chemical Engineering, The most important reaction products were isobutane, Universiteit Gent, 2006. propane, n-butenes, propene, isopentane and toluene. The product distribution per hydrocarbon family globally evolved: alkanes >> alkenes > cycloalkanes ≈ aromatics. The product distribution was very similar for the four catalysts tested, which indicates that the acid properties do not influence the product selectivities for both feeds, iso-octane [4] and methylcyclohexane. i

TABLE OF CONTENTS

1. Introduction 1-22 1.1. Aim and justification 1 1.2. Working method 2 1.3. Fluid catalytic cracking process 3 1.3.1. An overview of the process 3 1.3.2. Typical process configuration of Fluid Catalytic Cracking 6 1.3.3. Industrial feeds and typical products for the process 10 1.4. Commercial catalysts 14 1.4.1. Introduction 14 1.4.2. Zeolites 18 1.4.2.1. Applications and General trends of development 18 1.4.2.2. General structure 19

2. Description of the set up 23-32 2.1. Introduction 23 2.2. Feed section 23 2.2.1. Liquid hydrocarbons 23 2.2.2. Gaseous hydrocarbons 24 2.2.3. Inert gasses 26 2.3. Reaction section 27 2.4. Sampling and analysis section 28 2.4.1. Introduction 28 2.4.2. Flame ionization detector 29 2.4.3. Thermal conductivity detector 30

3. Chemistry of catalytic cracking 23-58 3.1. Carbocations as intermediates os transition states 33 3.2. Influence of the catalyst on the kinetic of catalytic cracking 34 3.2.1. Acid properties 34 3.2.2. Pore structure 37

3.3. Elementary reaction steps in catalytic cracking process 42 3.3.1. Mechanism of catalytic cracking 42 3.3.2. Monomolecular cracking mechanism 46 3.3.3. Bimolecular cracking mechanism 46 3.3.4. Oligomeric cracking mechanism 48 3.4. Catalytic cracking of model compounds 48 3.4.1. Alkanes 48 3.4.1.1. General behavior 48 3.4.1.2. Iso-octane reaction pathways 48 3.4.2. Cycloalkanes 54 3.4.2.1. General behavior 54 3.4.2.2. Methylcyclohexane reaction pathways 55

4. Experimental results of iso-octane catalytic cracking 59-71 4.1. Introduction 59 4.2. Y zeolites: Correct operation of the set up 60 4.2.1. Activity 60 4.2.2. Product selectivities 62 4.3. ZSM-5 zeolites: Absence of diffusion limitations 63 4.3.1. Activity 63 4.3.2. Product selectivities 64 4.4. BIPOM catalysts 65 4.4.1. Activity 66 4.4.2. Product selectivities 67 4.5. Comparison between catalysts used on iso-octane cracking 68

5. Experimental results of methylcyclohexane catalytic cracking 72-82 5.1. Introduction 72 5.2. Influence of catalyst activity 73 5.3. Product selectivities 75 5.3.1. Alkanes 77 5.3.2. Alkenes 78 5.3.3. Cycloalkanes 79 5.3.4. Aromatics 80

iii

5.3.5. Comparison between catalysts used on methylcyclohexane cracking 81

6. Parameter estimation for the cracking of iso-octane at coke free conditions 83-94 6.1. Introduction 83 6.2. SEMK theorical fundamentals 84 6.3. Reaction network for iso-octane cracking over Y zeolites 86 6.4. Rate equations at molecular level 87 6.5 Definition of the rate coefficients 88 6.6. Acidity descriptors 90 6.7. Parameter estimation procedure 91

7. Conclusions 95-96

A. operation of the set up 97-102 A.1. Pretreatment 97 A.2. Feed 98 A.3. Perform analysis 99 A.3.1. Store sample 99 A.3.2. Direct injection in FID 99 A.3.3. Indirect injection in FID 99 A.3.4. Direct injection in TCD 99 A.3.5. Indirect injection in TCD 100 A.4. Shut down reactor 100 A.5. Load reactor 101

B.Calibration factors of the GC FID 103-104

C. Feed composition and calibration curve for methylcyclohexane flow rate 105-106

D. Physical properties 107-108

E. Product identification for the cracking of iso-octane 109-113

F. Product identification for the cracking of methylcyclohexane 114-120

G. Experimental data of iso-octane 121

H. Experimental data of methylcyclohexane 122-123

Bibliography 124-126

Abbreviations

BFB bubbling fluidized bed BIPOM bimodal porous material CCFI counter current feeder injector CFB circulating fluidized bed CSTR continuous stirred tank reactor CFD computational fluid dynamics CTAB cethyltrimethylammonium bromide DME dimethylether EFAL extraframework aluminium FCC fluid catalytic cracking FCCU fluid catalytic cracking unit FID flame ionisation detector GC gas chromatography HFAU acid exchanged faujasite zeolite LPG petroleum liquefied gas LPT laboratorium voor petrochemische techniek (Ghent, Belgium) MBCC moving bed catalytic cracking MFC mass flow controller PI pressure indicator PMA pulse mass analyser R2R two-stage regenerator process RTD riser termination device SEMK single event microkinetic modelling TCD thermal conductivity detector TEOS tetraethylorthosilicate TI temperature indicator TIC temperature indicator and controller TOS time on stream TPAOH tetrapropylammoniumhydroxil TPD temperature programmed desorption USY ultrastable Y zeolite

List of symbols

A preexponential factor 1/s Ã single event preexponential factor - aj group contribution factor g/mol CF calibration factor g/area + C k carbom atom number -

Ct molar concentration on the Brönsted acid centres mol/kgcat

C* number of free acid sites mol/gcat + CR1+ surface concentration of carbenium ion R1 mol/gcat

Ct total concentration of acid sites mol/gcat

Ea activation energy J/mol

FºiC8 molar flow of iso-octane μl/s

F°mch molar flow of methylcyclohexane μl/s h Planck’s constant J.s H enthalpy J/mol H+/F° site time mol H+.s/mol mch fed

ΔHr reaction-enthalpy J/mol ΔH°# standard activation enthalpy from the reactants to the transition state J/mol k elementary step rate coefficient - k single event rate coefficient - kB Boltzmann constant J/mol K≠ equilibrium coefficient - M molecular weight g/mol Mn+ compensating cation of a zeolite - Nr. number - ni,j number of contributions present in component i - n order of reaction - ne number of single events - nexp number of experiments - nresp number of responses - nj,i number of group contribution present in component i -

PA1 partial pressure of alkane A1 Pa

vii

pt total pressure Pa pºi partial pressure of component i Pa R universal gas constant J/(mol k) S entropy J/(mol k) ΔS°# standard activation entropy J/(mol k) T temperature °C Tc critical temperature °C w weight factor -

W mass of the catalyst Kgcat W/F ° space time kg.s/mol x molar fraction - y molar fraction - yji,calc calculated yield of product j in experiment i - yji,exp experimental yiel of product j in experiment I -

Greek symbols

κ thermal conductivity W/(m K) + θ B fraction of Brönsted acid sites covered by carbenium ions σ symmetry number of the species

σjk main diagonal elements of the covariance matrix reac σ gl global symmetry number of the reactant ≠ σ gl global symmetry number of the transition state

Φij parameter used to calculated dynamic viscosity of a gas mixture τ average residence time of the gas in the reactor

Subscripts c critical i component i j component j iC8 iso-octane

viii mch methylcyclohexane cat catalyst reac reactant gl global

Superscripts obs observated + positive charged

Chapter 1. Introduction 1

CHAPTER 1 INTRODUCTION

1.1 AIM AND JUSTIFICATION

This project focuses on two parts:

1. To determine the influence of acid properties on activity and selectivity during catalytic cracking of cyclic model components;

2. To develop an appropriate kinetic model to describe how the acidity of the catalysts affects the relative rates of the elementary reactions for the catalytic cracking of alkanes.

The use of USY zeolites in Fluid Catalytic Cracking has become a significant improvement to produce high quality gasolines. The growth in the use of these kinds of zeolites has improved the understanding of the nature and location of the catalytically active sites in them.

Acid-catalyzed hydrocarbon reactions, such as the zeolite-catalyzed processes taking place in petroleum refineries, occur via carbenium ion intermediates. Catalytic cracking of hydrocarbons proceeds through reactions as isomerisation, protonation or cracking via β- scission of carbenium ions causing the breaking of C-C bonds producing lighter and more valuable petroleum fractions.

A single event microkinetic model describing catalytic cracking of hydrocarbons in absence of coke formation has already been developed at the LPT but the kinetic parameters determined still depend on the type of catalyst used. A general application of this model in process optimization and catalyst development is consequently limited. Therefore, the kinetic model has to be modified to be able to use it with other catalysts.

Chapter 1. Introduction 2

1.2. WORKING METHOD

Catalytic cracking experiments have been performed in a recycle electrobalance reactor set up equipped with online gas chromatographic effluent analysis to register the fractions obtained after the catalytic cracking reactions, to study the influence of acid properties on the elementary reactions in the catalytic cracking of model components.

Iso-octane (2,2,4-trimethylpentane) cracking experiments were performed in the recycle electrobalance reactor. The experimental results available on iso-octane cracking will be used to develop a kinetic model for catalytic cracking of alkanes in FORTRAN, using the so-called single event approach developed at the LPT. This model can describe the influence of the acid properties on the rates of the elementary reactions.

The single event reaction network generation algorithm generates the complete network of elementary reaction steps for a given hydrocarbon feedstock. The introduction of the concept “single event rate coefficient” makes it possible to reduce the number of kinetic parameters. The single event rate coefficients are independent of the feedstock; they remain dependent on the catalyst, however. An advantage of this approach is that the kinetic parameters can be determined out of experiments with simple model components instead of real vacuum gas oil, allowing the analysis of all reaction products.

As the actual analysis techniques do not allow the characterisation of all individual components in heavy petroleum fractions, a certain degree of a posteriori relumping may be required in the modelling.

Moreover, experiments with methylcyclohexane, as cyclic model component, have been performed in the recycle electrobalance reactor to study the influence of acid properties. In the future, experimental data obtained, as activities and selectivities, will be used to develop a kinetic model for cyclics.

The most important catalyst properties that can influence activity and/or selectivity are the number of active sites, their strength (distribution) and the framework topology. Therefore, a series of Y and ZSM-5 zeolites differing in acid characteristics and in framework topology have been tested in the catalytic cracking of both reactants at various operating conditions.

Chapter 1. Introduction 3

To cut out the effect of coke formation occurring during the cracking experiments, all the data obtained have to be extrapolated to time zero.

1.3. FLUID CATALYTIC CRACKING PROCESS

1.3.1. An overview of the process

Catalytic cracking, developed by American engineers Warren K. Lewis and Edwin R. Gilliland, rapidly became one of the most important refining processes after it was first commercialized in 1936 by Eugène Houdry.

Since its introduction, catalytic cracking has become one of the most important of the petroleum refining processes. The reason for this durability and the most significant characteristic of the cracking process is its flexibility in treating the variety of feedstocks available from whatever crudes may need to be refined. This flexibility becomes increasingly important as refineries are obliged to resort to heavier crudes i.e., containing poisonous constituents, because of shortages and the high price of the more desirable feedstocks (Wojciechowski and Corma, 1986).

Nowadays, FCC (fluid catalytic cracking) units are present in most refineries with more than 300 units in operation. There are approximately 400 catalytic crackers operating world-wide, with a total processing capacity of over 2400 m3 gasoil per day (Sadeghbeigi, 1995).

The major objective of most FCC units is to maximize the conversion of gas oil to gasoline and Liquefied Petroleum Gas (LPG). The LPG produced in the catalytic cracker is rich in propylene and butylene. The process consists of breaking the hydrocarbon C-C bonds present in the feedstock in order to obtain light products such as gasoline. The gasoline is the most valuable product of the cracking unit and constitutes a significant fraction of the overall gasoline production. The objective is to achieve maximum gasoline yield with high octane number. The octane number of a gasoline is a quantitative measure of fuel performance in an internal combustion engine. The weight composition of a gasoline is about 5.7% of alkanes,

Chapter 1. Introduction 4

23.7% of isoalkanes, 34.4% of aromatics, 10% of cycloalkanes and 26.2% of alkenes, with an average molecular weight of 93 g/mol (Cotterman and Plumlee, 1989).

In a refinery, the FCC process is interconnected to several other refining processes receiving or suppling various hydrocarbons steams as it is shown in figure 1.1.

The original method for converting larger molecules into smaller molecules is thermal cracking. Above about 360°C, hydrocarbon molecules decompose into fragments. At that temperature, the random thermal energy in a hydrocarbon molecule is occasionally large enough to break that molecule into two pieces. After a short time as a free radical, each fragment rearranges into something that is chemically stable. Most of the time the new molecules are smaller than the old molecules. The higher the temperature, the more often such decompositions occur and the faster the petroleum cracks. The big molecules that are not suitable for gasoline generally decompose into smaller ones that are. Moreover, thermal cracking produces many alkene molecules that have higher octane numbers than the usual contents of crude oil. These alkenes are made when the free radical fragments of original hydrocarbon molecules rearrange internally to form double bonds. If the last carbon atom in a chain has only three neighbors, it can complete its electronic shell by forming a double bond with the carbon atom next to it. This rearrangement causes the neighboring carbon to abandon a hydrogen atom, which immediately becomes part of a hydrogen molecule. Therefore, thermal cracking creates many smaller molecules, with double bonds at their ends, and hydrogen molecules.

However, thermal cracking is difficult to control and also creates many large and useless molecules. As a rule, the higher the temperature in the cracking tank, the higher the octane number of the gasoline it produces but the smaller the yield. To make premium gasoline by thermal cracking, the refinery might have to waste all but 20% of the hydrocarbons it feeds to the cracking tank. Because this waste is intolerable, thermal cracking has been replaced almost completely by fluid catalytic cracking and reforming for the production of gasoline (Wojciechowski and Corma, 1986).

Chapter 1. Introduction 5

Figure 1.1- General refining scheme (www.USEnviromentalAgency.com).

Following this introduction, catalytic cracking has been the subject of many investigations designed to clarify the fundamental chemical processes involved and the kinetics of the overall reaction. After years of researches, Turkevich and Smith suggested that cracking takes place via “hydrogen exchange” in which the hydrocarbon adsorbs on specific locations in certain regions of a catalyst surface. In 1933, Gayer shows that a catalysts consisting of alumina supported on silica possesses acidic properties. This observation, confirmed later by others, led to speculation that these acid sites may in fact be the catalytically “active centers”. About the same time, Whitmore proposed acid sites as the active centers in his theory of carbenium ion reactions (Wojciechowski and Corma, 1986).

In 1947, Hansford described the mechanism in terms of carbenium (then called carbonium) and carbine ions, but later he modified the mechanism to include only carbenium ions. And in 1949, Thomas independently proposed a mechanism similar to that of Hansford. The carbenium ion postulate is now known to agree with the observed facts and to explain the following specific characteristics of catalytic cracking as the mechanism of the C-C bond

Chapter 1. Introduction 6

rupture, the higher cracking rates of C6 and higher alkanes, the preferential formation of three or more carbons from linear hydrocarbons, skeletal isomerisation or hydride transfer (Wojciechowski and Corma, 1986).

1.3.2. Typical process configuration of fluid catalytic cracking

The first developed technology proposed by Eugene Houdry was a fixed bed technology, including several fixed beds in parallel that could operate either in reaction or regeneration mode. In 1942, Esso (based on the work by Standard Oil of New Jersey) proposed the first fluid catalytic process utilizing small fluidized catalyst particles to enhance both the cracking reaction and the transport of the catalyst between the reaction and regeneration zones. This unit used upflow reactors. Both the catalyst and air flowed upwards through the reactor vessel and exited through the vessel overhead lines. External cyclones were used to collect the catalyst. In the first unit, the feed was vaporized before being introduced in the reactor. Heat was removed from the unit by catalyst coolers. Later, the liquid feed was vaporized by the hot catalyst coming from the regenerator. This reduced or eliminated the need for external heat removal (Gauthier et al, 2000). A moving bed technology (Moving Bed Catalytic Cracking, MBCC) and Fluid Catalytic Cracking (FCC) was later proposed. The MBCC operates with continuous movement of the catalyst between the reaction zone and the regeneration zone. In the reactor, the catalyst moved downward, against the fluid stream, in a very compact mass, into the regenerator. From the regenerator the catalyst was returned to the top of the reactor. Over the last fifty years, several evolutions came on the market to improve the catalytic cracking process and fluidization technologies. Developments and commercialization of both FCC and MBCC processes continued in parallel for some time. However, the FCC process proved to be more flexible and came to dominate the field (Froment and Bischoff, 1990).

In a FCC unit, heavy oil is cracked into lower molecular weight hydrocarbons, which are either blended to finished products or are routed to downstream units for further processing, gasoline is the major end product. A simplified flow sheet of the main parts of a Fluid Catalytic Cracking Unit (FCCU) is shown in figure 1.2. In industrial cracking, catalysts are composed of different amorphous and crystalline (matrix, binder) acid functions (zeolite), and

Chapter 1. Introduction 7 a series of additives. For the production of gasoline the main active component is Y-type zeolite, while ZSM-5 can be added to increase the yield of propene.

Figure 1.2 – FCC unit configuration (www.catalysis-ed.org).

The heat produced during regeneration is consumed in the endothermic cracking reactions, as it was mentioned in figure 1.2. FCC therefore became an integrated reaction-regeneration process where heat produced in the regeneration zone and transported by the catalyst is consumed to vaporize the liquid feed and to promote endothermic reactions. This very

Chapter 1. Introduction 8 specific characteristic implies that the catalyst flow rate in the reactor does not only depend upon the reaction requirements, but also upon the adiabatic requirements of the process. Therefore, a modification in the thermal balance of the unit can modify the catalyst circulation and the reaction zone performance (Froment and Bischoff, 1990).

In the following years, several evolutions concerning catalyst, fluidization technologies and reactor setup led to the improvement of the catalytic cracking process. For instance, feed preheating systems and flue gas systems were incorporated in the configuration.

In 1982, research performed by Total and IPF led to the development of a two-stage regenerator process (R2R). Computational Fluid Dynamics (CFD) was used to develop new reactor riser configurations so as to improve catalyst and feed contact. For instance, new counter current feeder injectors (CCFI) were installed; a new riser termination device (RTD) was incorporated to enable a quick and efficient separation of catalyst and products; the amount of injected steam and the catalyst flow profile were optimized by modifying the steam injection pattern prior to the feed injection (Patureaux, 2000). A scheme of the FCC unit is shown in the figure 1.3.

Figure 1.3-The R2R resid fluid catalytic cracking process (Patureaux et al, 2000).

The FCCU developed at that moment, was modified towards a short contact time system, referred to as MSCC process (Schnaith et al, 1998). Product and catalyst were separated by techniques based on the inertia of the catalyst-vapor mixture. In the configuration, the regenerator was elevated and the MSCC reactor was placed directly below the regenerator.

Chapter 1. Introduction 9

Regenerated catalyst flows straight down from the regenerator into the reactor. Reactions occur in a horizontal flow section of the reactor. The reaction products are directed through the external cyclones and spent catalyst falls into the stripper. The catalyst is lifted into the regenerator bed where coke burn-off is completed. With this configuration, the resid processing capability and the gasoline octane number were increased. The dehydrogenation by contaminant metals was reduced though metal levels increased. High gasoline yields are achieved for a big variety of crudes.

The withdrawal of older catalyst results in higher gasoline yield, reduced hydrogen production, lower coke selectivity and lower catalyst and additives consumption. Newly added catalyst particles are relatively fresh and active, while particles that have been in the unit for many months are catalytically dead. This lower activity is the result of repeated extended exposure to the hydrothermal deactivating environment of the regenerator, which reduces the zeolite surface area and crystallinity.

Moreover, risers were equipped with multiple vaporizing feed nozzles located around the riser circumference. Increased capacity of the unit and riser temperature limit was achieved. The flexibility of the original setup was limited by the not uniform cyclone temperatures in the regenerator. Experimental studies indicated that the primary cause of this uneven temperature profile was a bad catalyst distribution. A catalyst distributor was installed using an extension of the spent-catalyst standpipe to direct catalyst into air-fluidized nozzles which spreads the catalyst uniformly in the regenerator. The top of the catalyst distributor is located below the regenerator-bed level. This prevents volatile hydrocarbons from escaping directly into the dilute phase. The ignition of these volatile hydrocarbons causes high temperatures in the cyclones, which are an additional cause of the uneven temperature profile and cause also catalyst deactivation. The original regenerator design placed the regenerated catalyst standpipe at the level of the air distributor, resulting in poor air distribution in that area. Catalyst withdrawal was then moved below the air distributor to avoid this. The substantially higher catalyst activity obtained indicates uniform temperature profile due to a lower level of volatile hydrocarbons in the regenerator (Jonhson et al, 1998).

Dutta and Gualy (1999) show how a design tool can be customized to simulate accurately FCC technology. The FCC riser/reactor setup is represented by a tall circulating fluidized bed (CFB) section sandwiched between two short sections: a continuous stirred tank reactor

Chapter 1. Introduction 10

(CSTR) at the bottom and a bubbling fluidized bed (BFB) at the top. The CSTR represents a zone at the riser bottom where the regenerated catalyst is assumed to be intimately mixed with the vaporized feed-oil before the mixed stream ascends through the riser. The BFB section is introduced at the top to simulate the hydrodynamics of the dense bed stripper.

Further improvements in fluid catalytic cracking technology are sought in reactor control, better performance and flexibility for varying feedstocks and towards product specifications. Mostly, the improvements are related to solid flow, separation and circulation aspects of the complex process, etc., but not the basic design concept. This is due to the fact that many phenomena, like catalyst behavior, reaction chemistry or reaction/regeneration are closely coupled to each other. It is therefore not possible to independently make a major change in one without adversely affecting the other and the overall system performance. Several trends still drive the evolution of the process (catalyst and technology) and lead to new developments (Gauthier et al, 2000): o Heavier feeds tend to be processed. Conventional FCC treats vacuum distillates. Progressively, refiners tend to incorporate atmospheric residues in the FCC feed or heavy secondary cuts with no value from other refining processes. With resid processes such as the R2R, it is now possible to convert atmospheric or even hydrotreated vacuum residues; o Depending upon the markets, the refiner may be inclined to maximize the yields of gasoline, LPG (liquified petroleum gas), propylene or even middle distillates while minimizing heavy fuel oil cuts; o For a given unit, the refiner usually maximizes throughput or some products flow rates taking into account unit bottleneck constraints such as air blower capacity, regeneration temperature, wet gas compressor capacity or gas plant capacity; o Sulfur in liquid products, olefinicity of LPGs or gasoline, polyaromatics in middle

distillates or SOx (sulfur oxides), NOx (nitrogen oxides) and dust emissions in flue gas, have to be minimized.

1.3.3. Industrial feeds and typical products for the process

The typical petroleum fractions used as feeds for the FCC process are very complex mixtures of hydrocarbons. The feed quality is critical because of its effects on the product distributions and their quality, which ultimately defines the cracking severity. Traditionally the FCC

Chapter 1. Introduction 11 feedstock contains both heavy atmospheric and light/heavy vacuum gas oils with an average boiling point of 473-823 K and hydrocarbons with a carbon number from 14 to 44. Currently, apart from the atmospheric and vacuum gas oils, the FCC feedstock may contain atmospheric and vacuum residue, heavy straight run naphta, hydrotreated gas oils, deasphalted, coker gas oils, lube extracts, etc. The cracking of heavier hydrocarbons is particularly difficult because of their refractoriness, metal and sulfur content, and higher coke selectivity (Marcilly, 2001).

Each type of hydrocarbon reacts in a different way under cracking conditions owing to the different reaction that it can undergo, which ultimately results in certain typical reaction products. The major difference among these hydrocarbons of a particular type is in their crackability or extent of conversion for a given set of operating conditions.

Figure 1.4 - Typical feedstock in fluid catalytic cracking (www.oilcompany.com).

As principal families it is possible to have:

o Alkanes - The alkane compounds found in crude oil have the general formula CnH2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight chain alkane molecules are found in gases and alkane waxes. Examples of straight-chain molecules are methane, ethane, propane, and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms). The branched-chain (isomer) alkanes are usually found in heavier fractions of crude oil and have higher octane numbers than normal alkanes. These compounds are saturated hydrocarbons.

Chapter 1. Introduction 12

In the feed alkanes crack mostly to alkanes and alkenes (C3 and C4) and light gasoline

fraction (C5-C7), as a result of β-scission of the internal C-C bonds. The cracking of terminal C-C bonds occurs in a minor proportion yielding methane, ethane and ethylene, which composes the dry gas fraction. For a given carbon number, branched alkanes are fairly more reactive than normal alkanes as the latter requires a series of skeletal isomerizations before cracking.

o Alkenes - Alkenes are mono-alkenes with the general formula CnH2n and contain only one carbon-carbon double bond in the chain. Alkenes are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil.

They are very reactive hydrocarbons and they are practically absent in catalytic cracking feeds. However, their cracking patterns are important to be studied because of the relatively high proportion of alkenes in the cracked products and their participation in secondary reactions such as hydride transfer, alkylations and coke formation.

o Cycloalkanes - Cycloalkanes are saturated hydrocarbon groupings with the general

formula CnH2n, arranged in the form of closed rings (cyclic) and found in all fractions of crude oil except the very lightest. Single-ring cycloalkanes (monocycloalkanes) with five and six carbon atoms predominate, with two-ring cycloalkaness (dicycloalkanes) found in the heavier ends of naphtha.

The cycloalkanes crack more easily than normal alkanes and to a similar extent than isoalkanes. They typically undergo fast dealkylation, resulting in smaller cycloalkanes, and slow ring opening reactions, resulting in acyclic hydrocarbons, which rapidly are recracked to lighter products. Their cracked products are very similar to those obtained out of alkanes but with higher production of gasoline because the formed products are less susceptible to secondary cracking because of the ring stability. The gasoline formed from cycloalkanes feeds has improved quality (higher octane number), compared with that produced out of alkanes because of the relatively high content of aromatics encountered in the reaction products. Cycloalkanes are, indeed, aromatic precursors.

Chapter 1. Introduction 13

o Aromatics - Aromatics are unsaturated ring-type (cyclic) compounds which react readily because they have carbon atoms that are deficient in hydrogen. All aromatics have at least one benzene ring (a single-ring compound characterized by three double bonds alternating with three single bonds between six carbon atoms) as part of their molecular structure. Naphthalenes are fused double-ring aromatic compounds. The most complex aromatics, polynuclears (three or more fused aromatic rings), are found in heavier fractions of crude oil.

They have a different cracking behavior than that of alkanes and cycloalkanes. The benzene ring itself is not cracked and, therefore, condensed ring aromatics are not converted but highly transformed into coke via condensation reactions resulting in catalyst activity decay. Aromatics with small side chains crack extensively by shearing off the entire side chain. Alkylaromatics with long side chains have a similar behavior to that described for alkanes.

o Dienes and Alkynes - Dienes, also known as dio-alkenes, have two carbon-carbon double bonds. The alkynes, another class of unsaturated hydrocarbons, have a carbon- carbon triple bond within the molecule. Both these series of hydrocarbons have the

general formula CnH2n-2. Dialkenes such as 1,2-butadiene and 1,3-butadiene, and

alkynes such as acetylene, occur in C5 and lighter fractions from cracking. These compounds are more reactive than alkanes or cycloalkanes and readily combine with other elements such as hydrogen, chlorine, and bromine.

As non-hydrocarbons: sulfur compounds, oxygen, nitrogen compounds, trace metals, carbon dioxide, salts and cyclic acids.

As worldwide reserves evolve to higher molecular weight crude, several problems arise for oil companies as well as for design and engineering firms and catalyst manufactures. The FCC process is strongly affected because it has to undergo adaptations in operating conditions, hardware, etc.

Heavier oil fractions have higher density, higher boiling point, lower H/C ratio as well as higher hetero-atoms content of sulfur, nitrogen, oxygen, organo-metallics, etc. This will result in diffusion problems in the catalyst pores, accumulation of metals on the catalyst surface, which promotes dehydrogenation reactions and eventually destroys the zeolitic part of

Chapter 1. Introduction 14 catalyst, an increase of coke formation and finally, a decrease in the quality of the fuels because of the relative high content of sulfur and aromatics in the gasoline (Marcilly, 2001).

Since the composition of the feed is complex, the number of cracked products is also overwhelming and their individual quantification is practically impossible. In the refinery, there are some key product fractions that have been categorized by their boiling point, i.e. fuel gas, LPG, gasoline, LCO, HCO and coke. The yield of these fractions as well as the conversion level of the unit depends on the feedstock, catalyst and operating conditions used.

A typical yield pattern of an FCC unit is shown in table 1.1.

Product wt% on fresh feed

C3 & C4 15 Gasoline 40 - 50 Heavy Gas Oil 10 Coke 5

Table1.1- Typical catalytic cracking product distribution (www.oilrefineries.com)

1.4. COMMERCIAL CATALYSTS

1.4.1. Introduction

In the catalytic cracking process, hot hydrocarbon molecules are brought into contact with silica-alumina catalysts. Like all catalysts, these materials facilitate chemical reactions by reducing the activation energies needed to complete them. Therefore, an industrial catalyst needs to have enough activity to realize suitable conversion of the feedstock by increasing the rate of the chemical reaction.

When a hydrocarbon molecule attaches to the surface of the catalyst, the catalyst helps it to rearrange. The catalyst reduces the potential energy of the partially rearranged molecules so that less overall energy is needed to complete the rearrangement. Catalyzed rearrangements thus proceed at lower temperatures (Kazansky et al, 2003). An example is given in figure 1.5.

Chapter 1. Introduction 15

Figure 1.5 - A catalyst provides a special surface (a) to which a long unbranched paraffin molecule can attach (b). The catalyst helps the molecule crack into two parts (c). The final pair of carbon atoms in each of the new chains is joined by a double bond and a hydrogen molecule is released. Once the rearrangement has occurred, the new molecules leave and the catalyst is left unchanged (d) (Kazansky et al, 2003).

These catalysts also help to control the rearrangements, see figures 1.6. A particular catalyst will assist certain rearrangements more than others. Catalysts are particularly helpful in cracking larger molecules into smaller ones so that yields of gasoline molecules are much higher with catalysts than without.

Figure 1.6 - An unbranched paraffin molecule (a) attaches to the surface of an isomerizing catalyst (b). This catalyst helps a carbon atom and a hydrogen atom exchange places (c) by stabilizing the pieces during the exchange. The branched paraffin molecule leaves and the catalyst is left unchanged (d)(Kazansky et al, 2003).

For cyclics the behavior is very similar as it is shown in figure 1.7.

Chapter 1. Introduction 16

Figure 1.7 - A cycloparaffin (a) attached the surface of a reforming catalyst (b). This catalyst helps in the removal of hydrogen molecules from the ring (c) and creates an aromatic molecule. This molecule leaves the surface and the catalyst is left unchanged (d)(Kazanski et al, 2003).

Because all of the catalyst’s work is done by its surface, most commercial catalysts are designed to have lots of surface area. The silica-alumina catalysts used in fluid catalytic cracking are actually small particles of porous materials. These particles are only about 50 microns in diameter and they swirl around with the fluid they are cracking (Marcilly, 2001).

The reactions take only a few seconds to complete, after which the catalyst particles must be separated from the fluid. Unfortunately, the catalyst particles quickly accumulate a coating of very large molecules that don’t react and can’t be removed easily. Like most catalysts, they lose their catalytic activity when their surfaces become dirty. The only effective way to clean the surfaces of these particles is to burn the residue off them. This burning regenerates the catalyst particles and prepares them for their next trip through the fluid (Kazansky et al, 2003).

There are some very important properties to consider when choosing a catalyst. And due to the nature of the fluid catalytic cracking process, some extra aspects have to be taken into account (Van Camp, 2005): o Selectivity favors desired reactions and leads to the wanted products. Selectivity towards side products has to be minimized. o Thermal conductivity reduces the presence of hot spots.

Chapter 1. Introduction 17

o Attrition resistance is needed in FCC, due to the fluidized operation regime. o Thermal and hydrothermal stability is necessary for resisting the severe conditions during the regeneration. o Resistance to chemicals is important. FCC feedstocks contain metals and basic hetero- atoms which can poison or even destroy the zeolitic fraction of the catalyst. o Regeneration is necessary in order to restore the kinetic catalytic properties to an acceptable level. Deactivation of the catalyst due to coke formation causes a decrease in both selectivity and activity. And when deposits are important, such as in the coking of cracking catalysts, it should be possible to regenerate the catalyst frequently to obtain a reasonable catalyst life time. o Price is important for the commercial success of the heterogeneous by catalyzed process.

Many efforts have been focused on finding suitable cracking catalysts. Acidified natural clays were the first industrial cracking catalyst. They were soon replaced by synthetic catalysts which consisted of amorphous alumina-silica, developed in 1940. Their thermal stability and mechanical strength made them more appealing than acidified natural clays. The activity of the catalyst is connected with their acid properties.

It is known that an acid compound may exchange protons (H+) or electrons with a basic substrate. The acidity of pure silica is zero, but it is introduced by addition of Al. And as the amount of alumina increases, this acidity passes through a maximum. High alumina catalysts have a higher activity and increase the yield of gasoline for a given yield of coke. An increase in the amount of alumina also introduces a better stability and better mechanical properties. Other combinations of metallic oxides different from alumina-silica are also possible, e.g. silica- magnesia (Stöcker, 2005).

1.4.2. Zeolites

Chapter 1. Introduction 18

1.4.2.1. Applications and general trends of development

The present catalytic cracking catalyst contains Y zeolite as main active component. Amorphous catalysts in pellets form were initially utilized, whereas crystalline zeolitic catalysts with micro spherical shape are currently employed.

Historically, three generations of catalytic cracking catalysts are identified. The first one consisted in natural clays of the bentonite form in a granular form activated with acid leading to large pore and high surface area materials containing acid sites. They had good activity for cracking hydrocarbons, but they also were severely deactivated by the sulphur in the feeds. Natural clays were replaced by the second generation of catalytic cracking catalysts composed by synthetic mixtures of oxide catalysts, mainly SiO2-Al2O3 and SiO2-MgO, had more activity than the acidified clays and more resistance to contamination by sulphur. The synthetic amorphous catalysts whose thermal stability and mechanical strength were improved compared to acidified clays, also led to gasoline with higher octane number. The formulation of this type of catalysts underwent an important evolution and even a complete technology for its preparation was developed which was the basis for the preparation of zeolitic catalysts. A micro-spherical shape replaced the cylindrical pellets. One of the main drawbacks of the second-generation catalysts was their low stability which was improved by incorporating alumina but resulted in a bad quality of important products. The technology of catalytic cracking underwent a total change in 1962 with the incorporation of zeolites. This resulted in better kinetic properties, i.e. activity, selectivity and stability. Zeolitic catalysts also provided a significant increase in gasoline yield and a decrease in the formation of dry gas and coke, compared with the other catalysts (Van Camp, 2005).

The world zeolite market has developed strongly over the last decades and at present represents some 1.6 Mt per year with about 290.000 t/year for natural zeolites (approximately 18%). The manufacture of zeolite catalysts for petroleum refining is a $700 million per year business worldwide ($350 million per year in the U.S.) that is presently growing at approximately 5% per year. The fluidized catalytic cracking application is the largest as well as the oldest application of zeolite catalysts (Marcilly, 2001).

Chapter 1. Introduction 19

The Y-type zeolite present in the FCC catalysts alone accounts for almost 95% of the world consumption of zeolitic catalysts. The production of other zeolites remains marginal (Marcilly, 2001).

Zeolites have been present in refining since the beginning of the sixties (cracking on FAU) and in petrochemicals of first generation intermediates since the seventies (transformation of aromatics on MOR).

Considering the specifications imposed relating to petroleum products in general and fuels in particular, the refining industry and that of petrochemicals has, and will always have a need for new or improved adsorbents and catalysts. From that point of view, zeolites will continue to hold a strong position to offer innovative solutions. The opportunity concerning the progress that they can provide must be sought first of all in existing processes or new processes aiming at producing the following hydrocarbons (Marcilly, 2001):

o light alkenes from C3 to C5 as raw materials for petrochemicals or for the production of clean, good quality fuels;

o highly branched alkanes from C5 to C12 for the gasoline pool or longer and slightly branched paraffins for the kerosene and gas oil pools; o specific molecules as first and/or second generation intermediates in petrochemicals: alkylmonoaromatics in particular. The development of processes of inter- transformation of these aromatics can certainly be expected.

The growth in using zeolites in this industry should be sustained by the research for new crystalline structures and chemical compositions, the ability to incorporate elements in addition to Si and Al into the framework of known zeolites, and the improvement in the understanding of the nature and location of the catalytically active sites in zeolites.

1.4.2.2. General structure

Zeolites are crystalline materials with different properties that make them attractive as heterogeneous catalysts: well-defined crystalline structure, high internal surface areas (>600 m2/g), uniform pores with one or more discrete sizes, good thermal stability, ability to sorb

Chapter 1. Introduction 20 and concentrate hydrocarbons and highly acidic sites when ion exchanged with protons (Chen and Degnan, 1988).

The crystal structure of a zeolite is defined by the specific order in which a network of tetrahedral units is linked together. Zeolites consist of a three-dimensional network of metal- oxygen tetrahedra (in few cases also octahedral) which provide the periodically sized microporous structure, in which the active sites are part of the structure. Their acidity derives from the protons that are required to maintain electrical neutrality in the structure. The general structure formula therefore is:

Mx/n * [(AlO2)x * (SiO2)y] * wH2O

With Mn+ the compensating cation that makes the structure electrically neutral. The introduction of cations generates active Brönsted acid sites, as it is shown in figures 1.8 and 1.9, which enhance the cracking activity of the catalyst.

Figure 1.8 - Brönsted and Lewis acid sites (Stöcker, 2005).

Chapter 1. Introduction 21

Figure 1.9 - Formation of Lewis acid sites in zeolites (Stöcker, 2005).

Acid sites result from the imbalance of the metal and the oxygen formal charge in the primary building unit. This can easily recognized in the case of zeolites, which consist of a three- dimensional network of Si-O tetrahedra. A lattice comprising of only Si-O tetrahedra is neutral (the 4+ charge at the silicon is balanced by four oxygen atoms with each 2- charge, however, belonging to two tetrahedra). Replacing one Si4+ atom by Al3+ causes a formal charge on the tetrahedron of 1-. This negative charge is then balanced by a proton or metal cation forming an acid site. The bare, negatively charged tetrahedron is then the corresponding base. This “ion exchange” character adds versatility to the properties of zeolites, in addition to adding a valuable property that can be used in this own right in several process applications. It is therefore interesting to know precisely where in the zeolite structure these acid sites will be located. These acid (and base) properties are not just a function of the chemical composition, since other factors, like the framework density, the type of cation or the local strain have an influence as well (Stöcker, 2005).

The size of the zeolite pore openings is determined by the number of tetrahedral units (or alternatively, oxygen atoms, required to form the pore) and the nature of the cations that are present in or at the mouth of the pore (Chen and Degnan, 1988). Also, zeolites with more than one pore system are classified according to their largest accessible pore, see table 1.2.

Chapter 1. Introduction 22

Pore size Number of tetrahedra Max. Free diameter (Å) Small 6, 8 4,3 Medium 10 6,3 Large 12 7,5

Table 1.2- Zeolite sizes (Chen and Degnan, 1988).

The number and strength of the acid sites are complex functions of the nature and concentration of the tetrahedral groups, their location and concentration of exchangeable cations that are present. Generally, the higher the silica to alumina ratio, the more thermally stable the zeolite is. Zeolites with silica to alumina (SiO2/Al2O3) ratios greater than 10 are typically classified as high silica materials.

Several industrial applications of the zeolites are based upon technology adapted from the silica/alumina catalysts originally developed for the catalytic cracking reaction. This means, that the activity requested is based on the formation of Brönsted acid sites arising from the creation of “bridging hydroxyl groups” within the pore structure of the zeolites. The “bridging hydroxyl groups”, which are products associated with negatively charged framework oxygen linked into alumina tetrahedra, are the Brönsted acid sites (Stöcker, 2005).

Figure 1.11- “Bridging hydroxyl groups” in zeolites (Stöcker, 2005).

For zeolites, it can be stated that the concentration of aluminium in the lattice is directly proportional to the concentration of acid sites. The catalytic reactions must take place at active sites on the internal surface. An extended description of zeolites is given in Chapter 3.

Chapter 2. Description of the set up 23

CHAPTER 2 DESCRIPTION OF THE SET UP

2.1. INTRODUCTION

The recycle electrobalance reactor is a suitable set up to perform catalytic cracking experiments. In such reactor the catalyst mass is measured continuously and registered by means of an electrobalance. The use of a recycle reactor allows to work at gradientless conditions and at high conversion. Under the conditions prevailing the rate of coke formation can be calculated directly from the increase in the catalyst mass, because then the rate of coke formation is uniform over the whole catalyst mass. On-line gas chromatographic analysis of the reactor effluent allows to determine activity and selectivity of the catalyst. A schematic overview of the set-up is given in figure 2.1.

2.2. FEED SECTION

2.2.1. Liquid hydrocarbons

Liquid hydrocarbon is fed out of a Pyrex reservoir (1). Parallel to the reservoir a measuring buret (2) (5,0 ml, subdivided into 0,02 ml) is installed. The reservoir and the buret are connected and operate as communicating vessels. The buret is used to measure the hydrocarbon flow rate streaming out using a chronometer (close valve V-1 to measure the flow correctly). The desired flow rate is set by putting pressure above the reservoir and buret using helium. The overpressure with respect to atmospheric pressure is read on a manometer of 0-1 bar. The helium pressure is adjusted with a DRD Mess- und Regeltechnik needle valve (NV-1).

The feed flows through a capillary tube (7m x 0,25mm) located in a thermostatically controlled Lauda bath (3). The temperature of this bath is set at 40°C. Using a capillary tube

Chapter 2. Description of the set up 24 the feed flow rate of liquid hydrocarbon can be adjusted by means of pressurizing the reservoir with helium. Since the resistance of the feed tube is nearly completely in this capillary tube, it is supposed to be constant. At sufficiently high helium pressure the differences in static height due to differences in level in the reservoir can be neglected, so that the feed flow rate only depends on the helium pressure set.

Then, the liquid feed is passed to the evaporator (4) via a needle. Around this needle flows inert dilution gas (N2). The flow rate of this dilution stream is adjusted using a Brooks 5850S mass flow controller (MFC-2, 0-100 n-ml/min N2). For all mass flow controllers adjustment and reading the flow rate is carried out on a Brooks 5878 read out unit. A check valve (CV-2) prevents liquid to flow into this tube. The needle is located in a small tube filled with glass pearls which buffers small irregularities during evaporation. The whole is heated with electrical heating elements. Experimentally it turned out that a smooth evaporation of both iso-octane and methylcyclohexane requires a temperature of 180°C.

The gas leaving the evaporator is kept warm by heating the tube using Habia resistance wire (approximately 120°C). Two coupled three-way valves allow to send the gaseous reaction mixture either to the reactor (during the experiment) or to the vent (during the pretreatment of the catalyst). A bubble vessel in the outlet tube to the vent allows to check the regularity of the gas flow rate visually.

2.2.2. Gaseous hydrocarbons

This option is currently not in use and is not represented on figure 2.1. The set-up is provided with a Brooks 5850E mass flow controller (0-200 n-l/h air) to adjust the flow rate of a gaseous hydrocarbon.

Chapter 2. Description of the set up 25

FI Figure 2.1 – Schematic overview of the experimental set up. – Figure 2.1

Chapter 2. Description of the set up 26

2.2.3. Inert gasses

Currently only N2 is used as inert gas. It is used as diluent, as pretreatment gas for the catalyst and as protection of the electrobalance. The first application of N2 is already discussed in section 2.2.1.

During the pretreatment the catalyst is heated under a flow of nitrogen at a rate of +5°C/min up to 500°C. The catalyst is maintained at this temperature during 1 hour and then cooled down to the desired reaction temperature. A Fischer pressure regulator (0 to 7 bar) combined with a Hoke needle valve (NV-2) is used to set a given nitrogen flow rate which can be read on a Brooks rotameter. A three-way valve allows to send the gas either to the reactor through the coupled three-way valves (during the pretreatment) or to the vent (during the experiment). After the experiment the catalyst is brought back under pretreatment conditions to be able to determine the total amount of coke deposited. A good measurement of the amount of coke requires a fast stabilization in the reactor at the beginning of the experiment. Therefore it is useful to set the nitrogen flow rate equal to the feed flow rate and to switch the coupled three- way valves slowly to cause as few oscillations of the catalyst basket as possible. Experimentally it was found that transient phenomena have disappeared after 5τ, where τ represents the average residence time of the gas in the reactor.

Nitrogen is utilized also in the balance chamber. Experimentally it was found that the presence of O2 influences the product distribution markedly. Therefore all oxygen is dispelled from the balance chamber before the start of an experiment by flushing with a sufficiently high nitrogen flow. This is called “flushing” the reactor. The nitrogen is fed via a Kytola rotameter (0-14 l/min air), from which the pressure is adjusted using a Martonair pressure regulator (0 to 1 bar). However, this rotameter does not permit to set the nitrogen flow rate accurately. Therefore, when starting the pretreatment N2 is fed using a Brooks 5850E mass flow controller (MFC-1, 0-25 n-ml/min N2). The balance chamber is kept constantly at a higher pressure than the reactor to prevent warm gaseous hydrocarbons to ascend to the balance chamber and damage the electronic part. This pressure difference is read on a

Yamamoto Manostar pressure difference meter (-1 to 4 mm H2O). The balance chamber is provided with an overpressure safety device. When the pressure in the balance amounts to more than 1 psi (6,89 kPa) above atmospheric pressure, a safety valve (PSV) is opened and N2 is removed via a bubble vessel.

Chapter 2. Description of the set up 27

2.3. REACTION SECTION

The reaction section consists of the reactor (5), a balance (6) and a ventilator driven by an electric motor (8).

The reactor is constructed with AISI 316 steel and consists of two legs. The right leg is 177 mm long and has a diameter of 14 mm. The feed gas is led in here through a narrow tube with internal diameter 1 mm which ends at the bottom of the reactor. The effluent exits the reactor at the top through a tube with internal diameter 2,2 mm positioned concentrically around the inlet. In- and outlet are positioned sufficiently far away from each other to avoid short circuit (i.e. feed flows directly to the outlet). The left leg is 234 mm long and has a diameter of 22,4 mm. Centrally in this leg, approximately 130 mm from the bottom, hangs the catalyst basket with diameter 17 mm. To reduce the bypass around the basket accessories are placed which centralize the gas flow and lead it preferentially through the basket. The catalyst basket is attached to the weight arm of the balance with an annealed Kanthal wire with diameter 0,2 mm. A counterweight is attached to the other arm of the balance.

The balance (6) is a high pressure Sartorius balance (type 4436 MP8-1) coupled to a Sartorius D/A Converter (type 7287). The balance continuously measures the weight difference between the weight arm (wire, catalyst basket, catalyst and coke) and the counterweight. This is registered by a recorder (Kipp & Zonen, type BD 112). The theoretical resolution is 0,001 mg. However, the experimental accuracy is limited to 0,01 mg because of the vibrations caused by the motor and by the oscillations of the basket due to the flow inside the reactor. Currently, the balance is programmed such that from a weight value ab, cde mg the numbers b, c and d (decade 3) are passed on automatically to the recorder via de converter.

Since the left leg of the reactor and the balance chamber are connected through a narrow joint, nitrogen is supplied to the balance chamber to protect the electronic part (see paragraph 2.2.3). Besides, the temperature of the balance chamber must keep between 10 and 30°C to assure an accurate measurement. Therefore, a cooling water jacket (K1) is placed around the joint as additional protection.

The reactor is heated using four IR radiation elements. These are controlled two by two (top and bottom) by two coupled Omron EC5N temperature controllers. With these the

Chapter 2. Description of the set up 28 temperature can be controlled accurately up to 1°C and a temperature programme can be imposed. An Omron E5C2 temperature limiter prohibits the temperature to increase above 520°C. The temperature is measured at different positions inside the reactor using NiCr-Ni thermocouple (see figure 2.1). The read out is performed with a Jumo temperature indicator, using a Jumo selector switch.

The pressure in the reactor is a little bit higher than atmospheric pressure and is controlled via a Hoke needle valve (NV-3) which determines the effluent flow rate through the sampling valve of the analysis section. The pressure in the reactor is measured (compared to atmospheric pressure) with a Jumo pressure converter (type 4 AP-30) and read on a Jumo pressure indicator (0-250 mbar).

A ventilator causes perfect mixing of the gases in the reactor. The ventilator is driven by a DC motor (8). The motor (Dunkermotoren, type BG 43X50) is equipped with rotational speed control and has a theoretical maximum of 12000 rotations per minute. To limit oscillations to a minimum and to avoid blowing out the catalyst basket, it was chosen to use rotational speeds between 7000 and 10000 rpm. The seal of the ventilator axis cannot withstand temperatures above 200°C. Therefore, a cooling water jacket (K2) is placed around the axis.

The flow rate of coolant (tap water) is adjusted using MMA 42 Dwyer rotameters, both for cooling the axis and for protecting the balance chamber.

2.4. SAMPLING AND ANALYSIS SECTION

2.4.1. Introduction

After the reactor the effluent is immediately mixed with the internal standard, dimethyl ether, in a cyclone (9). It was selected because dimethyl ether can be detected both by flame ionization and based on thermal conductivity. The flow rate of dimethyl ether is controlled by a Brooks 5850E mass flow controller (MFC-3, 0-55 n-ml/min N2). To avoid condensation of dimethyl ether the tubes are heated using Habia resistance wire (approximately 120°C).

Chapter 2. Description of the set up 29

Part of the gas mixture is sent to the valve system (10), the rest is removed to the vent via a bubble vessel. This fraction is adjusted using a Hoke needle valve (NV-3). This split system determines the resistance of the effluent tube, which imposes a slight overpressure in the reactor.

The valve system consists of three valves and is maintained at 180°C to make sure that all components of the effluent mixture remain gaseous. A six-way valve allows to choose between both gas chromatographs. A six-loop valve enables to store samples. There are five loops with content 1 ml and one with content 0,5 ml. An eight-way valve determines whether a sample is stored or injected. Some important valve positions are represented in figure 2.2.

2.4.1. Flame ionization detector

A Chrompack CP-9003 gas chromatograph with flame ionization detector is used to detect the individual hydrocarbons in the effluent. This detector (FID: Flame Ionisation Detector) relies on the formation of ions during the combustion of compounds containing hydrocarbons (C-C, C-H) in a hydrogen rich flame. The combustion of hydrogen in air produces very few ions and electrons, so that in the presence of pure carrier gas (helium) a constant background signal is obtained. However, when compounds containing hydrocarbons are present in the column, considerable ionization occurs. The organic molecules are broken and the collection of positive ions and electrons formed at the cathode and the anode respectively causes a sharp electric signal (mV) which is registered with XChrom.

The components are separated on a capillary column (CP-Sil PONA CB for ASTM D 5134- 90 fused silica). The stationary phase is located at the inside under the form of a homogeneous film. A capillary column requires a flow splitter to avoid overloading the column. The gas chromatograph has an electronic pressure controller which maintains the pressure in the column at 100 kPa. For a good separation of the components a temperature programme is used. The initial temperature of -50°C is attained by cooling the oven with liquid nitrogen. If necessary gaseous N2 can be fed via a pressure controller to start the cooling. Table 2.1 represents the characteristics of the column and the set values of the GC-FID.

Chapter 2. Description of the set up 30

Table 2.1 – Characteristics of the column used and set values of the GC-FID.

2.4.2. Thermal conductivity detector

A Chrompack CP-9000 gas chromatograph with thermal conductivity detector is used to detect N2 in the effluent. This detector (TCD: Thermal Conductivity Detector) is based on the difference in thermal conductivity κ between the carrier gas (helium) and the component to be detected. The thermal conductivity is measured using four resistances connected as a Wheatstone bridge circuit. One resistance is located in the gas flow ahead of the sample injection so only the carrier gas flows through (reference cel). A second resistance is located immediately after the column so both carrier gas and effluent flow through (analysis cel). When these gasses flowing through differ in thermal conductivity the temperatures of the filaments in the reference and analysis cel will differ. Consequently, the resistance changes (in the analysis cel). The resulting electric charge is registered with XChrom.

The components are separated on a packed column (Alltech SS Porapak N). Also on this gas chromatograph a temperature programme is used for a good separation of the components. After the elution of dimethyl ether the flow direction of the carrier gas in the column is reversed (“backflush”) by switching a Valco valve. This causes the heavier components to be removed from the column. Table 2.2 represents the characteristics of the column and the set values of the GC-TCD.

Chapter 2. Description of the set up 31

Table 2.2 – Characteristics of the column used and set values of the GC-TCD.

Five chromatograms from the FID and one from the TCD are obtained after each experiment. The equations of the peaks (calculated by means of integration with the XChrom program) of the five chomatograms of the FID are used to identify each component separated. Also for the

TCD, N2 and DME are identified and their areas are calculated by the program using an internal method. With these values it is possible to calculate in a worksheet in Excel the initial conversion of the reactant and the product selectivities.

Chapter 3. Chemistry of catalytic cracking 32

Figure 2.2- Schematic representation of the most important valve positions (6-loop valve, 8-way valve and 6-way valve).

Chapter 3. Chemistry of catalytic cracking 33

CHAPTER 3 CHEMISTRY OF CATALYTIC CRACKING

3.1. CARBOCATIONS AS INTERMEDIATES OR TRANSITION STATES

It is generally accepted that the reaction mechanisms of hydrocarbon conversion and formation on acid zeolites and related catalysts involve the formation of carbocations. However, whether these carbocations act as transition states or as intermediates is still under discussion, and is, in addition, depending on the type of hydrocarbon. The behaviour of carbocations and their reaction pathways in zeolites and related microporous materials are strongly depending on the shape-selective effect due to the confinement of the reacting molecules in the microstructure of the catalysts.

Concerning the type of carbocations related to the conversion or formation of hydrocarbons, one has to distinguish between alkylcarbenium ions (containing a tri-coordinated positively charged C-atom with three substituents being either alkyl groups or hydrogens) and alkylcarbonium ions (consisting of a penta-coordinated positively charged C-atom with the same type of substituents).

The stability of alkylcarbenium ions depends on the inductive effect of the substituents on the positively charged C-atoms, with the tertiary alkylcarbenium ions as the most stable ones. However, this effect is less pronounced for the alkylcarbonium ions. Due to the charge delocalization, there is an energy order for the carbenium ions concerning the number of alkyl groups bonded to charge bearing carbon atoms, i.e. primary, secondary and tertiary. The former ions are energetically less favored compared with the latter.

It has been mentioned in literature that carbocations involved in zeolite hydrocarbon catalyzed processes do not occur as stable intermediates but are (part of) transition states stabilized by the interaction with the acidic and basic parts of the zeolite lattice. In has been stated that

Chapter 3. Chemistry of catalytic cracking 34 carbenium ions are, in reality, activated complexes with a maximum in the potential energy surface, whereas the real surface intermediates are covalently bonded alkoxides, which are formed from the surface though the carbenium-ion like transition state (Quintana, 2006).

Carbocations are formed by reactions that can be grouped in four main classifications (Wojciechowski and Corma, 1986): o The addition of a cation to an unsaturated molecule; o The addition of a proton to a saturated molecule; o The removal of an electron from an electrically neutral species; o Heterolytic fission of a molecule.

Different types of carbenium ions can be identified depending on the hydrocarbon type from which they originate. These carbenium ions can be conveniently grouped in saturated and unsaturated ones. For saturated carbenium ions, only the number of alkyl substituents bonded to the carbon atom bearing the charge mainly determines their nature and, hence, their stability, i.e., primary, secondary or tertiary. The size of the alkyl substituents affects the energy only slightly. For unsaturated carbenium ions, two different subgroups of carbenium ions can be distinguished, viz., those where the charge is conjugated with the double bond referred to as allylic carbenium ions, and those where there is no conjugation. The properties of the latter subgroup are assumed to be the same as saturated alkylcarbenium ions, whereas those of the allylic carbenium ions have to be defined in a different way because of the occurrence of resonance between the charge and the double bond.

3.2. INFLUENCE OF THE CATALYST ON THE KINETICS OF CATALYTIC CRACKING

3.2.1. Acid properties

A conventional FCC catalyst consists of micro-spheres with an average diameter of 75 µm. It consists of a zeolitic component, i.e. Y zeolite from 20 to 50 wt%, and non-zeolitic components, a silica-alumina matrix from 5 to 20 wt%, and a low surface area inert, normally natural clay (filler), which provides mechanical resistance to the catalyst. In industrial operation, the catalyst is subjected to very severe conditions during the reaction, stripping and

Chapter 3. Chemistry of catalytic cracking 35 especially regeneration. The catalyst undergoes deactivation not only due to the thermal and hydrothermal effects but also to mechanical attrition. The latter results in a decrease in the particle size and ultimately, in catalyst loss.

The micro-spheres are composed by zeolite particles in the range 5-30 µm. The zeolite particles are dispersed in a matrix consisting of silica-alumina or alumina. Apart from binding the zeolite crystals in a microspherical catalyst particle with sufficient mechanical resistance, the matrix has other functions. It serves as diffusion medium to and from the zeolite particle due to its large number of meso/macropores which “precrack” large hydrocarbon molecules into smaller ones, acts as energy carrier and as sink for sodium and other contaminant metals and protects the zeolite from structural damage due to thermal and hydrothermal effects (Quintana, 2006).

The matrix is also diluent of the FCC catalyst as it moderates its activity in order to avoid overcracking of products formed. The incorporation of alumina to the matrix, originally only composed by inactive silica, provided acidity and eventually controlled porosity to improve the bottom selectivity and metals tolerance. At chemical level, the silica-alumina basic structural element is a tetrahedron with one oxygen at each corner while the center is occupied by silicon or negatively charged aluminium. This ultimately results in the formation of Brönsted acid sites. Lewis acid sites also exist as a result of the presence trivalent aluminium having an electron deficiency (Quintana, 2006).

The zeolite component provides most of the activity and selectivity of the catalytic cracking catalyst. Y faujasite is the currently utilized zeolite in the formulation of the FCC catalyst. The simplest structural building block of Y zeolite is a sodalite cage (see figure 3.1.), which is a cube-octahedron. These sodalite cages are linked via their hexagonal faces to form the cubic faujasite zeolite structure or supercages. The association of supercages produces complex crystalline structures with high surface area and a high standard porosity. In the case of Y faujasite, the supercage is in the order of 1.3 nm while the diameter of the aperture giving access to the cavities is about 0.74 nm. These dimensions are in the range of the kinetic diameter of several hydrocarbons, which confers to these materials peculiar properties for catalysis (Quintana, 2006).

Chapter 3. Chemistry of catalytic cracking 36

Figure 3.1- Detail structure of the sodalite cage and supercage of a Y zeolite (Quintana, 2006).

In the synthesized form, no more than 50% of the tetrahedron is aluminium. Y zeolite is normally synthesized at a Si/Al ratio of 5/1, which cannot be utilized as such in catalytic cracking. The presence of larger amounts of aluminium destabilizes the zeolite and provides an excessive activity to the catalyst leading to overcracking of the formed products and to very high coke formation. A dealumination stage of the Y zeolite is typically performed to increase its stability and to control its activity. Dealuminated zeolites can be further exchanged with bivalent cations, e.g., Ca, Mg, Mn, etc., or better with trivalent ions, i.e. rare earths (cerium or lanthanum), so as to produce high-activity zeolites because of the generation of additional acid sites. These ions also increase the stability of the crystalline network (Quintana, 2006).

The formation of light products in the cracking of n-alkanes is related to the distribution of charges in the space surrounding the molecules in the pores. An acid site cannot be considered independently from the structure it belongs to, i.e., from the topology and distribution of Al and Si atoms in the tetrahedral (Barthomeuf, 1993).

The parameters that can be used to describe Lewis and Bronsted acidity are the number and density of sites, their location, strength and also an efficiency coefficient comparable to activity coefficients in solution. The density of sites is a number of acid sites per unit volume or unit mass. The acid number depends only on the zeolite chemical composition. For Lewis

Chapter 3. Chemistry of catalytic cracking 37 and Brönsted acid sites the acid strength can be defined in general terms as the energy of a bond between the acid center and a base. In addition for protonic acidity the strength can be expressed as the energy of the O-H bond between the framework oxygen and the attached proton. The structure and topology of the zeolite should also be considered in studying those properties (Barthomeuf, 1993).

3.2.2. Pore structure

Building-in catalytically active sites within the intracrystalline cavities and pores of zeolites is the basis for molecular shape-selective catalysts. Shape-selective catalysis may be achieved by virtue of geometric factors and diffusional effects.

Diffusion plays a role of paramount importance. The molecules with high diffusivity will react preferentially and selectively while molecules which are excluded from the zeolite interior (their diffusivity is hence zero) will only react on the external non-selective surface of the zeolite. Products with high diffusivity will be preferentially desorbed while the bulkier molecules will be converted and equilibrated to smaller molecules which will diffuse out, or eventually to larger species which will block the pores. The latter will lead to a progressive deactivation of the catalyst by coking (Gates et al, 1979).

The Y-type zeolite present in the FCC catalysts accounts for almost 95% of the total world consumption of zeolites used within catalyst. Recently, attention has turned to the use of high silica faujasite zeolites that have a higher Si/Al ratio and lower acid site density. The net result is that this zeolite produces more high-octane alkenes (Stöcker, 2005).

In these kinds of zeolites, the sodalite cage, contains 8 hexagonal faces, 6 square faces, 24 vertices and 36 edges. Four sodalite cages are arranged in a tetrahedral configuration around a fifth sodalite cage. The five units are joined by hexagonal prisms (see figure 3.1). The joining of many such building blocks into a regular array results in a crystalline material whose structure contains pores (see figure 3.2). These pores are accessible to a variety of molecules of the type present in petroleum (Stöcker, 2005).

It has been observed that the acidity of the Y zeolite for gas oil cracking has a maximum for a Si/Al ratio of 5-8, which clearly indicates that this process requires the presence of strong

Chapter 3. Chemistry of catalytic cracking 38

Brönsted acid sites. Unfortunately, it has not been possible to prepare Y zeolite with a framework Si/Al ratio above 4 by direct synthesis; therefore, these kind of zeolites have to be prepared by dealumination of commercially prepared Y zeolite samples with Si/Al ratios in the range of 2.6, and finally they have a much better hydrothermal stability which is a pre- requisite for the application as FCC catalyst (Stöcker, 2005).

Figure 3.2- Y zeolite (Roy et al, 1999).

Also, it is known that higher octane numbers can be obtained by adding small amounts of ZSM-5 either as a separate additive or admixed with high silica Y (due to the pore architecture, this zeolite increases the octane number of the gasoline by selectively upgrading low octane gasoline components into lower molecular weight compounds with higher octane number). The ZSM-5 component selectively converts the low-octane alkanes and alkenes in the heavier fraction of the gasoline to higher-octane components and light olefins (such as propene, n-butene and isobutene). ZSM-5 turned out to be excellent in this respect, especially for the enhancement of propene. The relatively nonselective processing of large crude oil molecules is best done with the largest-pore zeolites, like faujasite, but the most selective petrochemical reactions are optimized over intermediate-pore materials like ZSM-5 (Stöcker, 2005).

As it can be seen in figure 3.4, the ZSM-5 framework contains two types of intersecting channels: one type is straight, has elliptical (0,51-0,58 nm) openings, while the other has near- circular (0,54-0,56 nm) openings. ZSM-5 has a channel (or pore) opening consisting of 10- membered oxygen rings with channel openings of 5,1 to 5,6 Å (see figure 3.3). These openings are smaller than the 7-8 Å cage openings for a commercial fluid catalytic cracking Y

Chapter 3. Chemistry of catalytic cracking 39 zeolite (Derouane, 1980). Hence, access of molecules larger than monomethyl aliphatics into ZSM-5 is restricted, and reaction of molecules with critical diameters larger than 6 Å is severely diffusion limited. Besides controlling accessibility of hydrocarbons, ZSM-5 contains strong Brönsted acid sites necessary for cracking hydrocarbons, and does not catalyze coke formation as readily as a Y zeolite (Stöcker, 2005). Different properties of Y and ZSM-5 zeolites are in table 3.1.

Figure 3.3 - ZSM-5 zeolite (Roy et al, 1999).

The properties of ZSM-5 stated above allow this material to be used as an FCC additive for boosting gasoline octane number. ZSM-5 also increases C3 and C4 LPG selectivity with a concurrent decrease in gasoline selectivity. It should be noted that the increased LPG fraction consists predominantly of alkenes. There is, however, a significant change in gasoline composition. The effect of ZSM-5 on gasoline structure may be summarized as follows:

o C7+ straight chain and branched alkenes and alkanes decrease,

o C5 hydrocarbons increase and the molecular weight of gasoline decreases, o the concentration of aromatics and cycloalkanes increases.

Since the rate of alkene cracking is more than two orders of magnitude higher than alkane cracking (Wojciechowski and Corma, 1986), the decrease of C7 alkenes is due to the latter being cracked to give mainly C3-C5 olefins. In the presence of such reactive alkenes, alkane cracking is substantially reduced. There are several pathways for gasoline alkane formation during catalytic cracking. One secondary pathway involves hydrogenation of primary alkenes. By cracking these primary alkenes ZSM-5 reduces the reactant alkenes needed to produce alkanes. Thus, low octane number alkanes in gasoline are decreased as a direct result of olefin cracking.

Chapter 3. Chemistry of catalytic cracking 40

The decrease in gasoline molecular weight and increase in C5 hydrocarbons are important reasons for the increase in gasoline octane number. There is essentially no incremental formation of aromatics and cyloalkanes due to the presence of ZSM-5. However, since the total volume of gasoline is reduced due to the decrease in C7 aliphatics, the concentration of aromatics and cycloalkanes in the gasoline is increased. This again helps in enhancing gasoline octane numbers.

Zeolite Pore diameter (Å) Cavity (Å)

Large pore: Y faujasite 3 6,6 - 11,4

Medium pore: ZSM-5 5.1-5.4 interconnected channels

Table 3.1- Porosity of Y and ZSM-5 zeolites (Chen and Degnan, 1988).

Figure 3.4 – Comparison of Y faujasite and ZSM-5 zeolites(www.catalystscience.com). A current handicap in zeolite technology is the zeolite particle size of typically around one micrometer, this is the case per Al-zeogrid zeolites (see figure 3.5). Shortcomings can in principle, be overcome by generating mesopores in the zeolite crystals, as found in the ultrastabilization of Y zeolite, or development of nanozeolites or of zeolite films and membranes. With respect to improved mass transfer, much is to be expected from hierarchical

Chapter 3. Chemistry of catalytic cracking 41 materials that have structural order at the meso-and/or macroscale in addition to the microscale (Kirschhock, 2005). Therfore, new materials heve been synthesized at KU Leuven.

Aluminium containing clear solutions of nanoslabs were prepared by dissolving aluminium powder in aqueous TPAOH (tetrapropylammoniumhydroxil) follow by TEOS hydrolysis (tetraethylorthosilicate). The aluminium containing clear solutions of nanoslabs (Si/Al = 50) were subjected to heating for one day at 120°C. A suspension of colloidal particles was obtained. These colloidal particles are known to be ZSM-5 crystallites with dimensions around 200 nm. The colloidal particles in the suspension were precipitated with ethanolic CTAB (cetyltrimethylammonium bromide). Calcination is performed to remove the TPA and CTAB and create the final Al-zeogrid.

By XRD techniques, it has been found that these zeolites present high surface areas (around 1000 m2/g) and super-micropores. The samples were characterized by FT-IR spectroscopy, and it supports the presence of MFI zeolite structure. SEM analysis revealed the morphology of Zeogrid type materials (spheres with diameter of 2-10 µm), and TEM and EDX techniques revealed that they consist of ZSM-5 particles and amorphous material that were aggregated disorderedly (Aerts, 2007).

Super-micropore, 1-2 nm (CTAB)

Ultra-micropore = MFI pore, 0,55 nm (TPA)

Figure 3.5 – Strucure of Al-zeogrid zeolite (Aerts, 2007).

Al-BIPOM3 zeolites are synthesized with organo-trialkoxy-silanes in order to create extra porosity. As standard zeolite synhesis, the MFI procedure based on the “clear solution” systems was used with a general mixture of Si:TPAOH:H2O:EtOH = 25:9:400:100 with addition of Al. The high crystallinity was demonstrated by FT-IR, XRD as well as TEM measurements. The TEM measurements show that the crystal phase is that of MFI, and it also

Chapter 3. Chemistry of catalytic cracking 42 present a high number of defects. By incorporating organosilane, the zeolite framework achieves additional pores. The new pore system is larger in size than the MFI framework but below the mesoporous range, 2 nm (Gagea, 2007).

Figure 3.6 – Mesoporous zeolite crystal synthesized with Q3 silica sources (Ph-, ClPr- and NH2Pr- trimethoxysilane) (Gagea, 2007).

3.3. ELEMENTARY REACTION STEPS IN CATALYTIC CRACKING PROCESS

3.3.1. Mechanism of catalytic cracking

This part discusses the cracking mechanism and not the reactivity of the model hydrocarbons. It is known that the reactivity of the model hydrocarbons is structure dependent, i.e. polynuclear aromatics > aromatics > alkenes > cycloalkenes ~ branched alkanes > normal alkanes (Quintana, 2006).

The catalytic cracking of hydrocarbons is assumed to proceed through the carbenium ion mechanism. This is a chain mechanism which comprises three global stages: initiation, propagation and termination.

Chapter 3. Chemistry of catalytic cracking 43

The initiation corresponds to the attack of a Brönsted acid proton on a hydrocarbon, e.g., alkane, alkene, etc., to produce a reactive intermediate. The two most common reaction mechanisms in the initiation steps are shown in figure 3.8. The propagation stage is a charge transfer process where a hydride ion is transferred from a saturated hydrocarbon, e.g., an alkane, to an absorbed carbenium ion resulting in the formation of a new carbenium ion. Traditionally, chain propagation occurs when a carbenium ion abstracts a hydride ion from an alkane. Nevertheless, a β-scission reaction can be also considered responsible for the propagation as a carbenium ion is formed. When the C-C bond is broken, a carbenium ion is left on the surface. This can either desorb, giving an alkene or an alkane when the desorption occurs through deprotonation or hydride transfer respectively, or can interact with one molecule of the reactant. The product distribution is highly dependent on the chain propagation mechanism, and more specifically on the number of propagation events per initiation step. This parameter is controlled by the reaction conditions and the nature of the catalyst. Finally, the termination step is given by carbenium ion desorption leading to an alkene and the return of the proton to the solid (and the Bronsted acid site is regenerated) (Quintana, 2006).

As the initiation is the most important step in the mechanism, it is explained as follows: The initiation stage, namely, how the first carbenium ion is formed, has been one of the main drawbacks of the classical catalytic cracking mechanism when pure alkane hydrocarbons are cracked. Alkanes are much less proton acceptors than alkenes, therefore, the formation of carbenium ions is not straightforward. Several possibilities have been proposed to account for the initiation step leading to the formation of the first carbenium ions: o It was first thought that the presence of small amounts of alkenes was necessary to initiate the cracking of an alkane through formation of a carbenium ion, i.e. resulting from the protonation of alkenes. A carbenium ion then abstracts a hydride from an alkane, forming another carbenium ion, which then can either crack by β-scission or undergo hydride transfer or isomerization. This classical catalytic cracking mechanism is also known as the bimolecular or hydride transfer catalytic cracking mechanism. For alkenes, it consists of the attack of a Bronsted acid site from the catalyst to the double bond of the reactant alkene to form a carbenium ion. This carbocation can then either rearrange without increasing the branching of the chain or rearrange to generate a branched carbocation (Wojciechowski and Corma, 1986).

Chapter 3. Chemistry of catalytic cracking 44

o Impurities in the feed, e.g. alkenes, that can rapidly be protonated. These impurities are produced through a free-radical mechanism at high temperatures. In the industrial process, these are formed when the hot catalyst abstract a hydride from the alkane (Quintana, 2006). o Lewis acid sites on the catalyst abstract a hydride from the alkane. The Lewis acid sites or electron acceptor sites (which for zeolites are associated with extra-framework Al or EFAL) can enhance the dehydrogenation of alkanes and form alkenes that can initiate the cracking reaction (Quintana, 2006). o Protonation of alkanes to form non-classical carbonium ions, which ultimately collapse to carbenium ions and alkanes or dihydrogen (Quintana, 2006). o Kissin has defined the cracking initiation mechanism based on the formation of transient hydroxyloxoniumions. The ions undergo the scission of the C-C bond in their alkyl groups in the β-position, with the formation of alkene molecules and smaller hydroxyloxoniumions (Kissin, 1996). This mechanism is shown in figure 3.7.

Figure 3.7 - Mechanism of transient hydroxyloxoniumions (Kissin, 1996).

However, when pure alkanes are cracked the first carbenium ion is formed via a different route. The Haag-Dessau or monomolecular mechanism (or protolytic scission mechanism) accounts for that fact. This states that a zeolite Brönsted acid site protonates an alkane to give a non conventional penta-coordinated carbonium ion transition state. This carbonium ion rapidly collapses to form a shorter alkane (or hydrogen) and a carbenium ion.

Chapter 3. Chemistry of catalytic cracking 45

Nowadays, the protolytic scission mechanism to form non-classical carbonium ions, which ultimately collapse to carbenium ions and alkanes or dihydrogen, is widely accepted. It has to be recalled that when the catalyst is not a pure zeolite, it is very complex to determine the reaction mechanism.

Figure 3.8 - Comparison between classical and non-classical mechanism for alkane molecules (Stöcker,2005).

Once the problem of the initiation of the cracking reactions starting from pure alkanes (or saturated hydrocarbons) is consistently explained, in practice two different reaction mechanisms can be clearly identified for the catalytic cracking of alkanes on Brönsted acid sites: monomolecular and bimolecular. Recently, a third mechanism, oligomerization, has been proposed to account for the formation of hydrocarbons heavier than the feed (Quintana, 2006).

These three mechanisms have been delimited based on the observed reaction products, proposed transition states, relative reaction rates and deactivation behavior. A description of each one of them is given below.

Chapter 3. Chemistry of catalytic cracking 46

3.3.2. Monomolecular cracking mechanism

It is demonstrated that zeolites can protonate alkanes to give carbonium ions, which are transition states in cracking. The carbonium ions collapse to give the cracking products: a smaller alkane (C-C bond is attacked) or H2 (C-H bond is attacked) and a carbenium ion (see figure 3.7). Haag-Dessau cracking dominates at low conversions, high reaction temperatures, low reactant pressures and with small and medium pore zeolites having a low concentration of Brönsted acid sites (Stöcker, 2005). All these conditions favour a low reactant concentration in the pores and impede hydride transfer. The decay of the carbonium ion into an alkane and a smaller carbenium ion is the main step in Haag-Dessau cracking mechanism (see figure 3.9).

The intrinsic activation energy of this mechanism was reported to be independent of the alkane reactant or the zeolite studied; moreover, the position in the molecule where protolytic attack occurs is practically irrespective to the stability of the carbenium ion. However, it has been demonstrated that the protonic attack occurs on the most nucleophilic centers of the hydrocarbon, i.e. on the C-C and C-H bonds (Quintana, 2006).

Figure 3.9 - Monomolecular cracking mechanism (Reyniers et al, 2000a).

3.3.3. Bimolecular cracking mechanism

The classical cracking mechanism is based on the fact that a carbenium ion abstracts a hydride from an alkane forming another carbenium ion, which cracks by β-scission (cleavage of the C-C bond located in position β to the trivalent positively charged carbon atom), forming an alkene. This the classical chain process involving hydride transfer between a reactant alkane and an adsorbed carbenium ion. The latter is assumed to be formed via protonation of an alkene on a zeolitic Bronsted acid site. The resulting carbenium ion isomerizes and then

Chapter 3. Chemistry of catalytic cracking 47 cracks in β-position leading to an alkene and a new carbenium, which either propagates the reaction via hydride transfer or terminates it by forming an alkene via deprotonation. It is well known that extensive skeletal isomerization and shifting of the charge position may precede the bond scission to avoid the formation of not thermodynamically favored carbenium ions such as primary ones (Quintana, 2006).

The overall process is governed by the stability of the carbenium ions in the different states of the reaction. In addition, the reaction rate decreases in the sequence tertiary > secondary > primary carbenium ions formed. Furthermore, the activation energy usually increases with increasing energy level of the final state. Therefore, the rate for reactions starting from a tertiary carbenium ion and ending with a tertiary carbenium ion is faster than the reaction starting from and ending with a secondary carbenium ion. The whole mechanism is represented in figure 3.10.

Figure 3.10 - Bimolecular cracking mechanism (Reyniers et al, 2000a).

The mechanism is favored at lower temperatures, high alkenes partial pressure and high conversions. Also the rate of this pathway could depend on the external surface area of the zeolite particle, which is higher for steamed particles containing mesopores. The incidence of this mechanism is not favored when using small or medium pore zeolites as the bulky transition state involved in hydride transfer is sterically hindered. Concerning the product distribution, the ratio between branched and normal products largely exceeds one for this mechanism. This is an intrinsic characteristic of the bimolecular catalytic cracking when using large pore zeolites, e.g., USY zeolites, extensively used in FCC, rather than an incidental occurrence. Large pore zeolites like Y zeolite show usually a greater tendency to crack the hydrocarbons according to the classical mechanism. However, the small and medium pore

Chapter 3. Chemistry of catalytic cracking 48 zeolites like ZSM-5 favour the non-classical Haag-Dessau mechanism which allow monomolecular reactions while restricting the bi-molecular (hydride transfer) reactions due to steric limitations in the pores (Quintana, 2006).

Skeletal isomerization of small carbenium ions is rather difficult to occur. For instance, as the formation of primary carbenium ions is not energetically favored, the cracking of a monobranched alkane proceeds once di-branched or tri-branched carbenium ions are formed.

3.3.4. Oligomeric cracking mechanism

At higher reactant partial pressure the classical cracking mechanism is gradually replaced by oligomerization cracking. It is described by the same chemistry as the bimolecular mechanism but with the presence of an alkylation (or oligomerization) reaction, which is the reverse reaction of β-scission. This alkylation occurs between an adsorbed carbenium ion and an alkene leading to a larger carbenium ion. The latter either can undergo further isomerization and β-scission as in the bimolecular mechanism, or it can remain adsorbed on the surface because of its high molecular weight and bulky structure or it can undergo another alkylation. The operating conditions where oligomeric cracking is favored are high alkenes partial pressure, low reaction temperature and high surface carbenium ions concentration (Quintana, 2006).

3.4. CATALYTIC CRACKING OF MODEL COMPOUNDS

3.4.1. Alkanes

3.4.1.1. General behavior

A general overview of the mechanism of alkane cracking is given in figure 3.11.

3.4.1.2. Iso-octane reaction pathways

Interaction of 2,2,4-trimethylpentane with a Brönsted acid site results, via protolytic scission, in the formation of hydrogen, methane, ethane, propane, butane and the corresponding

Chapter 3. Chemistry of catalytic cracking 49 carbenium ions. Involvement of 2,2,4-trimethylpentane in hydride transfer reaction with a + surface carbenium ion then produces the tertiary i-octyl carbenium ion (i-C8 ) and the corresponding alkane. The i-octyl carbenium ion formed during conversion of 2,2,4- trimethylpentane can undergo (t,t)-β-scission thereby producing i-butene and a i-butyl carbenium ion.

Figure 3.11 – Elementary steps in the catalytic cracking of alkanes (Feng et al, 1993).

Chapter 3. Chemistry of catalytic cracking 50

The latter gives rise to i-butane by hydride transfer. C3 and C5-hydrocarbons can be formed via two possible routes: chain isomerisation of the i-octyl carbenium ion followed by β- scission or alkylation of the i-octyl carbenium ion with a product alkene followed by β- scission. Cyclization and alkylation of surface carbenium ions with a higher carbon atom + number (Ck ) lead to aromatics and coke. This is illustrated in figure 3.12.

Figure 3.12 - Schematic representation of the catalytic reaction network of iso-octane (Reyniers et al, 2000a).

The different reactions involved are:

o Protolytic scission. This is the direct cracking of a alkane via protonation by an acid site. It may be involved in cracking at higher temperatures and in the initiation of alkane cracking (see figures 3.13 and 3.14).

Figure 3.13 - Example of 4-methylnonane C-C protolytic scission (Pintelon, 2006).

Chapter 3. Chemistry of catalytic cracking 51

Figure 3.14 - Example of pentane C-H protolytic scission (Pintelon, 2006).

o Protonation The molecule catches a hydrogen atom and forms a charged positively species (see figure 3.15).

Figure 3.15 - Example of 2-methyl-2-pentene protonation (Pintelon, 2006).

o Hydride transfer Hydride transfer in which a hydride ion is transferred from an incoming alkane to an carbocation, transforming this one into a alkane and the initial paraffin into another carbocation. This will be involved in bringing a new alkane into cracking reactions in the absence of a hydrogenation function and a possible equilibrium between alkenes and carbocations (see figure 3.16).

Figure 3.16 - Example of 4-methylnonane hydride transfer (Pintelon, 2006).

o Isomerizations The process of skeletal isomerization leads to the formation of tertiary carbon atoms, which in turn leads to the enhancement of the cracking rate of the isomerized products. The rate of isomerization is governed not only by the formation of the carbocation, but also by the subsequent rearrangement of the ion on the surface (see figures 3.17, 3.18 and 3.19).

Chapter 3. Chemistry of catalytic cracking 52

Figure 3.17- Example of 1,2-hydrideshift (Pintelon, 2006).

Figure 3.18 - Example of 1,2-alkylshift (Pintelon, 2006).

Figure 3.19 - Example of rearrangement of PCP-isomerization (Pintelon, 2006).

o β-scission In figure 3.20 an example of β-scission is shown.

Chapter 3. Chemistry of catalytic cracking 53

Figure 3.20 - Example of 4-methyl-2-pentylcarbenium ion β-scission (Pintelon, 2006).

o Alkylation This is the reverse reaction of β-scission.

Figure 3.21 - Example of alkylation (Pintelon, 2006).

o Deprotonation When the protonation occurs, a double bond is formed and the zeolite acid site is restored, as in figure 3.22.

Figure 3.22 - Example of deprotonation (Pintelon, 2006).

Chapter 3. Chemistry of catalytic cracking 54

3.4.2. Cycloalkanes

3.4.2.1. General behavior

The primary product of the cracking of cycloalkanes is the corresponding alkene chain isomer. On zeolite catalysts, the conversion of cycloalkanes gives a large amount of isomerisation, and significant amounts of aromatic products. It should be noticed that the amount of H2 found in the products of such cracking is greater than in the case of the corresponding alkanes.

The carbocation of a cycloalkane can rapidly be converted into a mixture of isomeric ions, which may lead to cracking, dehydrogenation, or ring contraction (see figure 3.23). The observed hydrogen-donating ability of cycloalkanes in cracking is well illustrated by the high alkane-to-alkene ratio obtained in the products when a mixture of cycloalkanes and alkenes or alkanes is cracked.

Figure 3.23 - General reactions for cyclics (Feng et al, 1993).

Chapter 3. Chemistry of catalytic cracking 55

3.4.2.2. Methylcyclohexane reaction pathways

The general chain mechanism is represented in figure 3.24.

Figure 3.24 –Methylcyclohexane chain mechanism (Cerqueira et al, 2001).

The cracking of methylcyclohexane gives a mixture of hydrocarbons belonging to C1-C7 alkanes, C2-C7 alkenes, C5-C7 cycloalkanes, C5-C7 cycloalkenes and C6-C8 aromatics.

Greensfelder and Voge (Wojciechowski and Corma, 1986) found 27% hydrogen plus C1 to C4 products in the cracking of methylcyclohexane.This is a surprising conclusion in view of the fact that in all these cases the catalysts are purely acid catalysts and do not contain any metal hydrogenation-dehydrogenation functions.

The percentage of C7 in the cracking products is small and indicates that the reaction of figure 3.25 is fast.

Figure 3.25 – Protonation followed by (t,t) β-scission (Cerqueira et al, 2001).

The isopropyl carbenium ion formed can desorb into propene or can undergo various reactions with the other products. Likewise, isobutene can be protonated, with the resulting tert-butyl carbenium ion undergoing various intermolecular reactions such as hydride transfer (figure 3.26) and the alkylation (figure 3.27).

Chapter 3. Chemistry of catalytic cracking 56

Figure 3.26- Hydride transfer (Cerqueira et al, 2001).

Figure 3.27- Alkylation followed by isomerization and β-scission (Cerqueira et al, 2001).

The reaction of figure 3.27, explains the formation of the C5 and C6 products found in large amounts with all of the zeolites and particularly with large pore zeolites.

Hydride transfer reactions are responsible not only for alkane formation but also for aromatic formation. Toluene results most likely from the steps shown in figure 3.28.

Figure 3.28- Toluene formation (Cerqueira et al, 2001).

Benzene and xylenes can result from toluene disproportionation.

Figure 3.29- Benzene and xylenes formation (Cerqueira et al, 2001).

Chapter 3. Chemistry of catalytic cracking 57

Reactions such as protolytic C-H and C-C scission (figure 3.30), respectively, occur via the pentacoordinated carbonium ion transition state mechanism, analogously to alkanes.

Figure 3.30- Protolytic C-H and C-C scission (Cerqueira et al, 2001).

Carbenium ions with methylcyclohexane skeleton undergo isomerization into carbenium ions with a dimethyl or ethylpentane skeleton.

Figure 3.31- Carbenium ion PCP- isomerization (Cerqueira et al, 2001).

Carbenium ions with methylcyclohexane and dimethylcyclopentane skeleton undergo β- scission reactions leading to C7 alkenylcarbenium ions. For the reactions in figure 3.32 the second one is slower than the first one because it involves a tertiary carbenium ions to be converted instead of a secondary.

Chapter 3. Chemistry of catalytic cracking 58

Figure 3.32- β-scission reactions (Cerqueira et al, 2001).

The alkenyl carbenium ions, which are very reactive, undergo rapid intramolecular reactions such as branching or intermolecular reactions such as hydride transfer, with the formation of a new carbenium I, which can undergo a new cycle of reactions (chain mechanism, figure 3.24).

Figure 3.33- Hydride transfer of olefinic carbenium ions (Cerqueira et al, 2001).

Chapter 4. Experimental results of iso-octane catalytic cracking 59

CHAPTER 4 EXPERIMENTAL RESULTS OF ISO-OCTANE CATALYTIC CRACKING

4.1. INTRODUCTION

Iso-octane cracking experiments have been carried out in the recycle electrobalance reactor at 475°C and 7 kPa partial pressure, varying the space times in a range which depends on the catalyst studied. Therefore, the amount of catalyst used for each experiment changes. The pellet size varied from 0,5 to 0,71 mm and from 0,71 to 1 mm.

The feed used is pure 2,2,4-trimethylpentane (iso-octane, with 99,6 wt%). As pointed out in Chapter 3, initial carbenium ions are formed through the rather difficult step of protolytic scission. Once the initial carbenium ions are formed, the reactions of the catalytic cracking cycles take place (isomerisation, β-scission, hydride transfer, deprotonation, alkylation or cyclisation).

A typical chromatogram of the reactior effluent and the product distribution of the catalytic cracking with retention times and calibration factors are given in Appendix E. With the peak areas obtained from the chromatograms, it is possible to analyse the effect of the operating conditions on conversion and product distribution. A general overview of the experimental conditions applied during the cracking of iso-octane is given in a table in Appendix G.

The properties of the catalysts used in the experiments are given in Table 4.1. The asterisk in some of the cells means that these values have not yet been determined. Firstly, three experiments with CBV 500 have been performed to apprehend the work methodology. The results obtained are compared with already available results of conversion and selectivities of iso-octane cracking under similar reaction conditions of temperature, partial pressure and space time. Secondly, the absence of diffusion limitations over ZSM-5

Chapter 4. Experimental results of iso-octane catalytic cracking 60 zeolites in iso-octane cracking at 475°C and 7 kPa, has been studied experimentally with CBV 3020. Thirdly, experiments with BIPOM3 have been carried out operating in iso-ocatne cracking under similar reaction conditions as with the other catalysts used. This experimental catalyst was synthesized at KU Leuven and has been tested to analyse activity and product selectivities.

To eliminate the effect of coke formation occurring during the cracking experiments, the conversion and the product selectivities obtained have been extrapolated towards initial conditions (time zero).

Si/Al Active sites Catalyst Si/Al bulk EFAL Structure frame (mmol NH3/g)

CBV 500 NH4-USY 2,6 3,9 yes 1,50 FAU CBV 3020 ZSM-5 15 18,4 yes ?* MFI Al-BIPOM1 BIPOM ±50 47 yes ?* MFI Al-BIPOM3 BIPOM ±50 ?* ?* ?* MFI

Table 4.1 - Properties of tested zeolites.

The consequences on acid properties can be particularly drastic since extraframework species deposited in the cavities may in some cases generate acidity. Therefore, an acid site cannot be considered independently from the structure it belongs to, i.e. from the topology and distribution of Al and Si atoms in the tetrahedra (Barthomeuf, 1993).

4.2. Y ZEOLITES: CORRECT OPERATION OF THE SETUP

4.2.1. Activity

To check the proper opertation of the electrobalance reactor set up, CBV 500 was used as mentioned. Table 4.2 shows the experimental conditions for the three experiments performed during the cracking of iso-octane.

Chapter 4. Experimental results of iso-octane catalytic cracking 61

Nr. T (°C) p° (kPa) W/F°(kg.s/mol) F°iC8 (μl/s) Conversión (mol%)

1 475 7,83 13,75 0,50 57,80 2 475 7,16 15,07 0,46 58,11 3 475 7,89 15,56 0,44 58,64

Table 4.2- Reaction conditions applied and conversions obtained for iso-octane cracking over CBV 500.

The conversions obtained are compared with a set of available results (Pintelon, 2006) for the same experimental conditions (Table 4.3). The previously available data correspons with the set of results called “CBV 500(1)” and the currently performed experiments correspond with “CBV 500(2)”. Figure 4.1 shows the initial iso-octane conversion as a function of space time at 475°C and 7 kPa.

Zeolite T (°C) p° (kPa) W/F°(kg.s/mol) CBV 500(1) 475 7 18,7 – 143,7 CBV 500(2) 475 7 49,5 – 56,02

Table 4.3- Experimental conditions for iso-octane cracking over CBV 500. The previously available data (Pintelon, 2006) correspons with the set of results called “CBV 500(1)” and the currently performed experiments correspond with “CBV 500(2)”.

Conversion vs space time

20 Conversion (mol%)

10 CBV 500(1) CBV 500(2) 0 0 20 40 60 80 100 120 140 160

W/Fo (kg.s/mol)

Figure 4.1- Conversion of iso-octane as a function of the space time during the cracking over CBV 500 at 475° C and 7 kPa partial pressure.

Chapter 4. Experimental results of iso-octane catalytic cracking 62

Taking into account that the conversion was extrapolated towards initial conditions, the agreement between the current data (“CBV 500(2)”) and available data (“CBV 500(1)”) is very good.

4.2.2. Product selectivities

The most important reaction products of iso-octane cracking over the Y zeolite studied are isobutane, isobutene, isopentane and propylene. Isobutane and isobutene are formed via direct β-scission of the 2,2,4-trimethylpentyl carbenium ion, while for propylene it has to isomerize first before it cracks. In Figure 4.2 the product selectivities of the main products separated are plotted separately as a function of iso-octane conversion.

Propylene Selectivity Isobutane Selectivity

25 140 120 20 100 15 80 10 60 40 Selectivity (m ol%) Selectivity (m ol%) 5 CBV 500(1) 20 CBV 500(1) CBV 500(2) CBV 500(2) 0 0 0 1020304050607080 0 1020304050607080 Conversion (mol%) Conversion (mol%)

Isobutene Selectivity Isopentane Selectivity

12 18 16 10 14 8 12 10 6 8 4 6 Selectivity (m ol%) Selectivity (m ol%) 4 2 CBV 500(1) CBV 500(1) CBV 500(2) Figure 4.2- Propylene selectivity as a function2 CBV 500(2) of iso-octane conversion. 0 0 0 1020304050607080 0 1020304050607080 Conversion (mol%) Conversion (mol%)

Figure 4.2 - Selectivities of the main products as a function of iso-octane conversion over CBV 500, at 475°C and 7 kPa.

It can be said for this specific range of conversion and these products, that selectivities decreases with increasing of iso-octane conversion, or therefore with increasing activity of the zeolite. Again, a good agreement between the current results (“CBV 500(2)”) and previously

Chapter 4. Experimental results of iso-octane catalytic cracking 63 obtained data (“CBV 500(1)”) is found. Based on figures 4.1 and 4.2 it can be concluded that the electrobalance reactor set up is properly operated.

4.3. ZSM-5 ZEOLITES: ABSENCE OF DIFFUSION LIMITATIONS

Working with CBV 3020 catalyst with different pellet size but in the same experimental conditions of available data, it could be possible to study the diffusion limitations based on activities and product selectivities obtained in iso-octane cracking.

The absence of diffusion limitations at pellet level was verified experimentally. Iso-octane cracking over CBV 3020 has been carried out over catalyst pellets varying size. At equal operating conditions, the presence of diffusion limitations will lead to decreased activity in case of larger pellets.

4.3.1. Activity

The operating conditions of the catalytic cracking experiments performed over CBV 3020 are given in Table 4.4. The data used (Pintelon, 2006) to study the variation of activity as a function of space time (Figure 4.3) are given in Table 4.5. The previously available data correspond with the set of results called “CBV 3020(1)” and the currently performed experiments correspond with “CBV 3020(2)”.

Conversión Nr. T (°C) p° (kPa) W/F°(kg.s/mol) F° (μl/s) iC8 (mol%) 1 475 6,94 48,456 0,34 7,564 2 475 6,94 63,108 0,37 6,226 3 475 7,01 78,876 0,37 9,962 4 475 7,96 106,308 0,39 1,829

Table 4.4 - Reaction conditions applied and conversions obtained for iso-octane cracking over CBV 3020.

Chapter 4. Experimental results of iso-octane catalytic cracking 64

Zeolite T (°C) p° (kPa) W/F°(kg.s/mol) Pellet size (mm) CBV 3020(1) 475 7 29,8 – 121,7 0,5 – 0,71 CBV 3020(2) 475 7 49,5 – 56,02 0,71 - 1

Table 4.5 - Experimental conditions for iso-octane cracking over CBV 3020. The previously available data (Pintelon, 2006) correspond with the set of results called “CBV 3020(1)” and the currently performed experiments correspond with “CBV 3020(2)”.

Conversion vs space time

16 14 12 10 8 6 4 Conversion (mol%) CBV3020(1) 2 CBV3020(2) 0 0 20406080100120140 W/Fo (kg.s/mol)

Figure 4.3- Conversion of iso-octane as a function of the space time during the cracking over ZSM-5 zeoliteswith different pellet size at 475° C and 7 kPa partial pressure.

From figure 4.3 it can be seen that equal activities are obtained for both CBV 3020 pellet sizes.

4.3.2. Product selectivities

It is known that there is a shift in the product distribution towards C1-C3 hydrocarbons, compared to Y-zeolites (C4) (Lungstein et al, 1999). This points to a change in the reaction mechanism of iso-octane cracking and might indicate that the conversion of iso-octane only occurred at the external surface or at the pore mouths (Barthomeuf, 1993).

Also for the product selectivities (see figure 4.4), a good agreement is found between the two sets of experimental data. Therefore, it can be concluded that diffusion limitations at pellet

Chapter 4. Experimental results of iso-octane catalytic cracking 65 level are absent during catalytic cracking of iso-octane over ZSM-5 zeolites at the experimental conditions investigated.

Propylene selectivity Isobutane selectivity 90 30 80 25 70 60 20 50 15 40 30 10 Selectivity (mol%)

20 Selectivity (mol%) CBV 3020(1) 5 10 CBV 3020(1) CBV 3020(2) CBV 3020(2) 0 0 0 5 10 15 051015 Conversion (mol%) Conversion (mol%) Isobutene selectivity Isopentane selectivity 12 12 CBV 3020(1) CBV 3020(2) 10 10

8 8

6 6

4 4 Selectivity (mol%) Selectivity (mol%)

2 CBV 3020(1) 2 CBV 3020(2) 0 0 0 5 10 15 0 5 10 15 Conversion (mol%) Conversion (mol%) Figure 4.4 - Selectivities of four typical products as a function of iso-octane conversion over CBV 3020 at 475°C, 7 kPa and different pellet size.

4.4. BIPOM CATALYSTS

Two BIPOM materials have been tested to study their behavior in iso-octane cracking. BIPOM stands for Bimodal Porous Material. This means that these catalysts have two different pore structures. Both catalysts tested have the MFI structure (same as ZSM-5) as basis combined with a larger pore system, the size of the latter depending on its synthesis method. BIPOM1 corresponds to Al-zeogrid and was synthesized by A.Aerts at KU Leuven. BIPOM3 is a third generation material and was synthesized by B.Gagea also at KU Leuven. More information about them is explained in Chapter 3.

Chapter 4. Experimental results of iso-octane catalytic cracking 66

4.4.1. Activity

The reaction conditions investigated and the conversion obtained of the BIPOM3 experiments that have been performed are given in Table 4.6. The reaction conditions applied to comparise the two types of BIPOM catalysts tested are displayed in Table 4.7. The previously available data correspond with the set of results called “BIPOM1” and the currently performed experiments correspond with “BIPOM3”.

Conversion Nr. T (°C) p° (kPa) W/F°(kg.s/mol) F° (μl/s) iC8 (mol%) 1 475 7,150 111,78 0,37 1,413 2 475 7,010 68,94 0,36 0,860 3 475 8,31 163,764 0,3 0,975

Table 4.6 - Reaction conditions applied and conversions obtained for iso-octane cracking over BIPOM3 catalyst.

Zeolite T (°C) p° (kPa) W/F°(kg.s/mol) BIPOM1 475 7 40.5 - 148.1 BIPOM3 475 7 68.94 – 163.76

Table 4.7 - Experimental conditions for iso-octane cracking over two BIPOM catalysts. The previously available data correspond with the set of results called “BIPOM1” and the currently performed experiments correspond with “BIPOM3”.

Figure 4.5 represents the iso-octane conversion over both BIPOM catalysts as a function of space time. Over both catalysts, a low cracking activity is obtained which could be explained by a collapse of porous structure due to the pelletization of the powder or due to the reaction temperature. Subsequent analysis based on N2 sorption (see figure 4.6) demonstrated that the low activity was not related to a porosity change. Another reason could be that these kind of zeolites are not very active for alkane cracking due to the low Al content compared to the Y and ZSM-5 zeolites tested.

Chapter 4. Experimental results of iso-octane catalytic cracking 67

Conversion vs space time

8 BIPOM1 BIPOM3 6

Conversion (mol%) 2

0 0 20 40 60 80 100 120 140 160 180 W/Fo (kg.s/mol)

Figure 4.5 - Conversion of iso-octane as a function of the space time during the cracking over two BIPOM catalysts at 475° C and 7 kPa partial pressure.

Figure 4.6 – Analysis based on N2 sorption of treated samples for different times.

4.4.2. Product selectivities

Based on the four plots of figure 4.7, it can be seen that the product selectivities of propylene and isobutane are clearly different over the two zeolites. The behavior of these two BIPOM materials is different in iso-octane cracking.

Chapter 4. Experimental results of iso-octane catalytic cracking 68

Propylene selectivity Isobutane selectivity 100 100

80 80

60 60

40 40 Selectivity (mol%) Selectivity (mol%) 20 20 BIPOM3 BIPOM3 BIPOM1 BIPOM1 0 0 0 0,5 1 1,5 2 2,5 00,5 11,5 22,5 Conversion (mol%) Conversion (mol%)

Isobutene selectivity Isopentane selectivity 100 14

12 80 10 60 8

40 6 4 Selectivity (mol%) Selectivity (mol%) 20 BIPOM3 2 BIPOM3 BIPOM1 BIPOM1 0 0 00,511,522,5 0 0,5 1 1,5 2 2,5 Conversion (mol%) Conversion (mol%)

Figure 4.7 – Selectivities of the main products as a function of iso-octane conversion over two BIPOM catalysts tested at 475°C and 7 kPa.

4.5. COMPARISON BETWEEN CATALYSTS USED IN ISO-OCTANE CRACKING

The activity of CBV 500 is higher than the activity of CBV 3020. On weight basis, the activity of the zeolites tested increases in the following order:

BIPOM3 ≈ BIPOM1 < CBV 3020 < CBV 500

This is shown by plotting conversion as a function of space time for the four of them (see figure 4.8).

Chapter 4. Experimental results of iso-octane catalytic cracking 69

Conversion vs space time 80 CBV 500 70 CBV 3020 60 BIPOM1 BIPOM3 50 40 30

Conversion (mol%) 20 10 0 0 20 40 60 80 100 120 140 160 180 W/Fo (kg.s/mol)

Figure 4.8 – Comparison of the activity of the four catalysts used in iso-octane cracking at 475°C and 7 kPa.

At comparing the activity of the four catalysts, the influence of the acid properties cannot be charged, since the intrinsic active sites of CBV 3020, BIPOM1 and BIPOM3 is not known. Only the influence of pore size can be studied. Therefore, the activity of CBV 500 is higher than the activity of CBV 3020, BIPOM1 and BIPOM3. This means that a larger pore size leads to a higher activity during the catalytic reactions, in the assumption that the intrinsic acid sites of the four catalysts have no significant influence on it.

The cracking of iso-octane mainly leads to the formation of alkanes and alkenes. However, small amounts of cycloalkanes and aromatics were consistently observed as will be pointed out below.

C1 and C2 hydrocarbons which are typical products of protolytic scission are formed in very low quantities over Y zeolites, but in large amounts over ZSM-5.

The formation of light products (

Chapter 4. Experimental results of iso-octane catalytic cracking 70

Namely, when large pore zeolites are used, the hydride transfer mechanism (as initiation reaction step) can be favored because the formation of the bulky intermediate molecules is not hampered as it is, for instance, with low medium pore zeolites as ZSM-5. Hence, the adsorbed carbenium ions can undergo skeletal isomerizarion before cracking (Stockër, 2005). The high yields of C4 should be explained in terms of the prevailing cracking mechanism on large pore zeolites. On Y zeolites, the highest yields were found for C4-hydrocarbons, mostly isobutane, formed from high branched carbenium ions. These kind of species, along with C3, can be formed both primary and secondary cracking.

Hydrocarbons with more than 6 carbons are not observed in high amount as they undergo secondary reactions. Cracked products with carbon numbers of 8 and higher were detected in very small proportions. This confirmed the hypothesis that over Y zeolites the iso-octane is preferentially cracked into molecules with a carbon number of at least 3. C1 and C2 hydrocarbons are therefore the result of protolytic scission.

Also it is important to point out that the acid properties of Y-zeolites do not influence the product selectivities in iso-octane cracking and the same holds for ZSM-5 zeolites tested (Pintelon, 2006), as mentioned before.

Figure 4.9 shows the comparison between the four catalysts tested. In the same plots, the trend of the available data of Y and ZSM-5 zeolites (Pintelon, 2006) and for BIPOM1, is represented by discontinuous lines. In this way it is possible to compare the behavior of the four catalysts, concluding that CBV 500 and CBV 3020 follow the same trend as Y and ZSM- 5 zeolites, respectively, while BIPOM3 has an intermediate behavior between BIPOM1 and ZSM-5.

Chapter 4. Experimental results of iso-octane catalytic cracking 71

Propylene selectivity Isobutane selectivity 120 140 CBV 500 CBV 3020E 120 100 BIPOM1 BIPOM3 Y-trendline 100 80 ZSM5-trendline 80 60 60 CBV 500 CBV 3020E 40 40 BIPOM1 Selectivity (mol%) Selectivity (mol%) BIPOM3 20 20 Y-trendline ZSM5-trendline 0 0 0 1020304050607080 0 1020304050607080 Conversion (mol%) Conversion (mol%)

Isobutene selectivity Isopentane selectivity 120 18 CBV 500 CBV 3020E 16 100 BIPOM1 14 BIPOM3 Y-trendline ol%) 80 12 ZSM5-trendline 10 60 tivity ( 8 CBV 500 lec40 m 6 CBV 3020E Se Selectivity (mol%) 4 BIPOM1 20 BIPOM3 2 Y-trendline ZSM5-trendline 0 0 0 1020304050607080 0 1020304050607080 Conversion (mol%) Conversion (mol%)

Figure 4.9 - Selectivities of the main products as a function of iso-octane conversion over all the catalysts tested at 475°C and 7 kPa.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 72

CHAPTER 5 EXPERIMENTAL RESULTS OF METHYLCYCLOHEXANE CATALYTIC CRACKING

5.1. INTRODUCTION

Experiments have been performed in the electrobalance reactor in order to study the cracking behavior of cycloalkanes. The experiments, as detailed further, were carried out at 475°C, 7 kPa methylcyclohexane partial pressure and varying space times. An overview of the experimental conditions applied during the cracking of methylcyclohexane is given in

Appendix H.

During the cracking of methylcyclohexane more than 100 different compounds (peaks) could be identified. A complete list of the observed hydrocarbons and a typical chromatogram are presented in Appendix F, respectively. For all the GC analyses, less than 3% of the total area corresponded to unidentified peaks. This has a minor impact on the final product yields.

The reaction products detected are formed out of any of the following reaction families: isomerizations (methylcyclohexane isomers), endocyclic-beta scissions (alkanes and alkenes

C3-C6), protolytic scission (C1-C2), aromatizations (aromatics) and disproportonations

(aromatics) as explained in Chapter 3.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 73

C4, C3, C5 and C7 are dominant carbon atoms number of the formed reaction products, whereas C9, C10, C2 and C1 are obtained in the lowest proportions. C3 are mainly constituted by propylene. Isobutane and non-branched alkenes are important constituents of C4, whereas isopentane is the highest contributor to C5. C6 are largely constituted by isoparaffins, e.g., 2- methylpentane. C7 are basically methylcyclohexane isomers, i.e., dimethylcyclopentanes in the highest proportion and ethylcyclopentane, whereas only traces of acyclic C7 hydrocarbons were observed. C8 are constituted by aromatics (xylenes), cycloalkanes and unsaturated C8 isomers. C9 and C10 are essentially aromatic hydrocarbons, e.g., trimethylbenzenes. Within C5 to C7 hydrocarbons, cyclic alkenes, i.e., methylcyclopentenes and 1-methylcyclohexene were systematically detected but in low proportions.

5.2. INFLUENCE OF CATALYST ACTIVITY

The available experimental data allow us to discuss the effect of acidity on methylcyclohexane cracking. This transformation was carried out at various space times to obtain a broad range of conversions. The physical properties of the four Y zeolites tested are represented in table 5.1. As shown in figure 5.1, conversion increases with increasing space time. Higher space time indicate a higher amount of catalyst available for reaction, and a higher methylcyclohexane conversion for each catalyst.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 74

Conversion vs W/Fº 80 70 ) 60 50 40 30

20 LZY20 Conversion (mol% Conversion CBV 720 10 CBV 760 CBV 500 0 0 20 40 60 80 100 120 W/Fº (kg.s/mol)

Figure 5.1 - Conversion of methylcyclohexane as a function of space time for the four

Y zeolites tested, at 475°C and 7 kPa.

To study the influence of acid properties on activity, the conversion can be represented as a

+ function of the site time H /F°mch (this ratio can be calculated multiplying W/F°mch with the number of acid centers, as determined by NH3-TPD). In this way the activity of four Y zeolites is correlated with the number of Brönsted acid sites which are effectively present on the zeolite grains for each molecule of methylcyclohexane fed. The intrinsic activity of tested zeolites is:

CBV 760 ≈ LZY20 ≈ CBV 720 ≈ CBV 500

With the four types of Y zeolites tested, the increase in Si/Al ratio (table 5.1) does not influence the activity; as it can be shown in figure 5.2, they look equally active, but space time has a positive effect on the methylcyclohexane conversion.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 75

active sites Catalyst Si/Al bulk Si/Al frame structure (mol NH3/kg)

LZY20 H-USY 2,6 30 0,99 FAU CBV 720 H-USY 15 16 0,60 FAU CBV 760 H-USY 30 100 0,23 FAU

CBV 500 NH4-USY 2,6 3,9 1,50 FAU

Table 5.1- Properties of tested zeolites.

Conversion vs acid H+/Fº

Conversion (mol%) 20 LZY20 CBV 720 10 CBV 760 CBV 500 0 0 20406080100120140160

H+/Fºmch (mol H+.s/mol mch fed)

Figure 5.2 - Methylcyclohexane conversion versus site time, at 475°C and 7 kPa

for the four Y zeolites tested.

5.3. PRODUCT SELECTIVITIES

At our operating conditions, the product yields evolved globally as follows: alkanes > alkenes > cycloalkanes and aromatics. For the Y zeolites, more or less the same amount of cycloalkanes and aromatics is formed, except for CBV 500 more aromatics than cycloalkanes are formed, as shown in figure 5.3.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 76

LZY20 CBV 720 120 120 Alkanes Alkanes Alkenes Alkenes 100 Cycloalka(e)nes Aromatics 100 Cycloalka(e)nes Aromatics 80 80

60 60

40 40 Selectivity (mol%) Selectivity (mol%)

20 20

0 0 0 20406080100 0 20406080100 W/Fº (kg.s/mol) W/Fº (kg.s/mol)

CBV 760 CBV 500

120 140 Alkanes Alkanes Alkenes Alkenes 100 Cycloalka(e)nes 120 Cycloalka(e)nes Aromatics Aromatics 100 80

80 60 60 40 Selectivity (mol%) Selectivity (mol%) 40 20 20

0 0 0 20406080100 0 20406080100 W/Fº (kg.s/mol) W/Fº (kg.s/mol)

Figure 5.3 - Initial methylcyclohexane selectivity per hydrocarbon family (alkanes, alkenes,

cycloalkanes and aromatics) as a function of space time at 475°C and 7 kPa, for the four Y zeolites

studied.

The product distribution was very similar for the four catalysts tested. Product distributions can be accounted for by means of carbocation chemistry and they allow to correlate the sample ability to catalyze particular reactions like cracking, isomerization and hydride transfer as a function of its properties. Cracking and isomerization depend on the total amount of acid sites, while hydride transfer is sensitive to the zeolitic properties (Cerqueira et al,

2001).

It has been written in literature that when acid sites more actively participating in the reaction propagation are no longer available, conversion decreases. In general, it is been reported that

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 77 alkenes can be converted even on acid centers with very low strength, whereas the cracking of saturated species, e.g., cycloalkanes, in which hydride transfer (or eventually protolytic scission) is involved, demands relatively high acid strength (Quintana, 2006).

5.3.1. Alkanes

The selectivity towards isobutane (as main alkane product), which is represented in figure 5.4, increases with increasing conversion. Taking into account that both conversion and selectivity have been extrapolated to initial conditions, it can be concluded that the isobutene selectivity is not affected by the acid properties of the Y zeolites tested.

Isobutane selectivity 60

Selectivity (mol%) LZY20 CBV 720 10 CBV 760 CBV 500 0 020406080 Conversion (mol%)

Figure 5.4 - Isobutane selectivity as a function of methylcyclohexane conversion

over Y zeolites tested, at 475°C and 7kPa.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 78

5.3.3. Alkenes

Alkenes show a characteristic behavior owing to their susceptibility to undergo secondary reactions, for the C3-C5 fraction. These are primary but unstable reaction products where β- scission (cracking) and desorption of carbenium ions via deprotonation are favored over hydride transfer typically responsible for their saturation (Quintana, 2006). Propylene is the main alkene obtained, and as can be observed in figure 5.5, selectivity decreases with increasing methylcyclohexane conversion. Also for propylene, no influence of acid properties of Y zeolites on selectivity is observed.

Propylene selectivity

20 18 16 14 12 10 8 LZY20 6

Selectivity (mol%) CBV 720 4 CBV 760 2 CBV 500 0 0 1020304050607080 Conversion (mol%)

Figure 5.5 - Propylene selectivity as a function of methylcyclohexane conversion

over Y zeolites tested, at 475°C and 7kPa.

At industrial level, when evaluating the effect of catalyst and operating conditions in a FCC unit, the alkane-to-alkene ratio on light alkenes and gasoline provides precious information about hydride transfer of the system including both operating conditions and catalyst properties. In figure 5.6, a higher conversion favors alkane-to-alkene ratio, independently of space time. High conversion results in high catalyst carbenium ion coverage which enhances hydride transfer reactions that yield alkanes in detriment of alkenes (Quintana, 2006). Also,

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 79 for the four catalysts, the alkane yield was higher than the alkene yield, indicating that bimolecular hydride transfer reactions are very important.

Alkane-to-alkene ratio vs Conversion

12 LZY20 CBV 720 10

o CBV 760 CBV 500 8

Alkane-to-alkene rati 2

0 0 1020304050607080 Conversion (mol%)

Figure 5.6 - Alkane-to-alkene ratio as a function of methylcyclohexane conversion for

the four Y zeolites tested at 475°C and 7 kPa.

5.3.4. Cycloalkanes

Cycloalkanes are primary products of methylcyclohexane cracking. Cycloalkanes with six carbon atoms are formed via direct protolytic scission of methylcyclohexane, whereas cycloalkanes with seven carbon atoms are formed via pcp-isomerization (ring contraction). In figure 5.7, 1-methylcyclopentene selectivity is plotted as an example of cycloalkanes selectivity over Y zeolites, its selectivity increases with increasing conversion.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 80

1-methylcyclopentene

5 LZY20 CBV 720 4 CBV 760 CBV 500 3

Selectivity (mol%) 1

0 0 1020304050607080 Conversion (mol%)

Figure 5.7- 1-Methylcyclopentene selectivity as a function of methylcyclohexane conversion over

Y zeolites tested, at 475°C and 7kPa.

5.3.4. Aromatics

Aromatics are expected to be hydrocarbons with an analogous behavior to alkanes in the sense that their formation is enhanced at high hydride transfer conditions, i.e., high conversions and low reaction temperature (Quintana, 2006). Aromatics from C6 to C8 appeared as primary products. This indicates that despite the number of elementary reactions involved in their formation out of the feed, i.e., hydride transfer/deprotonations for toluene, and disproportionations for xylenes, they are rapidly formed. Toluene selectivity as a function of methylcyclohexane conversion is represented in figure 5.8. This aromatic is formed in the largest quantity over the Y zeolites at the conditions tested. Toluene selectivity is found to increase with conversion.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 81

Toluene selectivity 20 LZY20 CBV 720 CBV 760 15 CBV 500

10 Selectivity (mol%) 5

0 0 1020304050607080 Conversion (mol%)

Figure 5.8- Toluene selectivity as a function of methylcyclohexane conversion over Y zeolites tested,

at 475°C and 7kPa.

5.3.5. Comparison between catalysts used on methylcyclohexane cracking

A general overview of the behavior of alkanes, alkenes, cycloalkanes and aromatic hydrocarbons is given in figure 5.9. The same general results are obtained as with the study of activity, the product yields evolved globally as follows: alkanes > alkenes > cycloalkanes and aromatics.

The acid properties of the Y zeolites tested do not influence the product selectivities obtained in methylcyclohexane cracking, as it was the case for iso-octane.

Chapter 5. Experimental results of methylcyclohexane catalytic cracking 82

LZY20 CBV 720

120 120 Alkanes Alkanes Alkenes Alkenes 100 Cycloalka(e)nes 100 Cycloalka(e)nes Aromatics Aromatics 80 80

60 60

40 40 Selectivity (mol%) Selectivity (mol%) 20 20

0 0 0 102030405060 0 102030405060 Conversion (mol%) Conversion (mol%)

CBV 760 CBV 500 100 140 Alkanes Alkanes 90 Alkenes 120 Alkenes 80 Cycloalka(e)nes Cycloalka(e)nes 70 Aromatics 100 Aromatics %) 60 80 50 40 60 30

Selectivity (mol 40 Selectivity (mol%) 20 20 10

0 0 0 1020304050 0 1020304050607080 Conversion (mol%) Conversion (mol%)

Figure 5.9 -Initial methylcyclohexane selectivity per hydrocarbon group (alkanes, alkenes,

cycloalkanes and aromatics) as a function conversion at 475°C and 7 kPa, for the four Y zeolites

studied.

Currently results of selectivities of the main products of each hydrocarbon family on methylcylohexane cracking can be compared with available dat for the mixture methylcyclohexane/1-octene (Quintana, 2006). The selectivities of the four components are obtained in the same reange for a given conversion.

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 83

CHAPTER 6 PARAMETER ESTIMATION FOR THE CRACKING OF ISO-OCTANE AT COKE FREE CONDITIONS

6.1. INTRODUCTION

The single event microkinetics (SEMK) modelling approach is one of the most advanced methodologies for describing catalytic cracking. It is based on a complete network of elementary steps of carbenium ion reaction intermediates. Elementary steps on Brönsted sites are taken into account.

The single event kinetic model was originally developed for hydrocracking. Then it was extended to the catalytic cracking of hydrocarbons on Y zeolite (Feng et al, 1993) at non- deactivating conditions. This fundamental modelling approach takes the detailed carbenium ion chemistry occurring on the active sites of the zeolite surface into account. It is based on the elementary reaction steps and leads to rate coefficients that are independent of the feedstock.

The application of the current catalytic cracking model is limited because the rate coefficients are specific for a given catalyst. No fundamental parameter(s) have been introduced to describe in an effective way the effect of variations in the acid strength distribution of the acid sites on the rate of the elementary steps. Therefore, the kinetic model will be extended to enable the description of the influence of the acid properties of the catalyst. This will allow to assess the effects of changes of the latter and, hence, to provide guidelines for catalyst optimization.

The program used in this case is developed to estimate acid dependent parameters (catalyst descriptors) for cracking of iso-octane in a mixed flow reactor (CSTR) according to the single event microkinetic modelling approach. The program used can be seen in Appendix I.

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 84

6.2. SEMK THEORICAL FUNDAMENTALS

According to the transition state theory, the rate coefficient of an elementary step can be expressed in terms of the equilibrium coefficient (equation 6.1).

kT* k= B * K≠ (6.1) h with h = Plank constant [J*s] k = elementary step rate coefficient

kB = Boltzmann constant [J/K] K ≠ = equilibrium coefficient T = temperature

The equilibrium coefficient can be expressed in terms of the standard activation enthalpy and the standard activation entropy for the formation of the transition state from the reactant, as it is written in equations 6.2, 6.3 and 6.4.

ΔΔH °≠ S°≠ kT* −− kee= B **RT* R (6.2) h

°≠ °≠ ° Δ=HHH −reac (6.3)

°≠ °≠ ° Δ=SSS −reac (6.4) and R = universal gas constant [J/mol*K] ΔH °≠ = standard activation enthalpy from the reactants to the transition state [J/mol] ΔS °≠ = standard activation entropy [J/mol*K] The standard entropy of a hydrocarbon species consists of a translation, a rotation and a vibration contribution. The rotation contribution of the symmetry to the standard entropy is given by equation 6.5.

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 85

° SRsym =− *ln(σ ) (6.5) with σ = symmetry number of the species

If this species contain chiral centra (e.g. n centra), a global symmetry number is defined by equation 6.6. The global symmetry contribution contains the effect of symmetry of the reactant as well as factors for optical isomerism.

σ σ = (6.6) gl 2n

In this way, the standard activation entropy (equation 6.7)can be written as the summation of an intrinsic contribution and a global symmetry contribution.

°° SS=−int r R*ln(σ gl ) (6.7) and so, the entropy change can be expressed as equation 6.8.

≠ °≠ °≠ σ gl Δ=SSint r − R*ln(reac ) (6.8) σ gl

reac with σ gl = global symmetry number of the reactant

≠ σ gl = global symmetry number of the transition state

This results in equation 6.9 for the elementary rate coefficient k .

reac ΔH °≠ ΔS°≠ σ −−int r gl kTB * RT* R ke= ≠ ***e (6.9) σ gl h

The number of single events of an elementary reaction is defined with equation 6.10.

reac σ gl ne = ≠ (6.10) σ gl

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 86 And so, the rate coefficient of an elementary step (equation 6.11) can be expressed in terms of the single event rate coefficient (Baltanas et al, 1989).

k = ne* k̃ (6.11) with k̃ = single event rate coefficient

ne = number of single events

The number of single events incorporates the differences in structure between the reactant and the activated complex. This results in single event rate coefficients which are entirely independent of the feedstock composition. Nevertheless, the rate coefficients are still catalyst dependent.

The calculation of the number of single events is implemented in the network generation algorithm by calculating the global symmetry number for both reactant and transition state of each elementary step.

The number of rate coefficients is extremely large and therefore, a series of assumptions has been derived for reducing that number. For instance, methyl-and primary carbenium ions are disregarded because they are much less stable than secondary and tertiary ions.

6.3. REACTION NETWORK FOR ISO-OCTANE CRACKING OVER Y ZEOLITES

The modelling of iso-octane cracking starts with the generation of the entire reaction network. A network generation algorithm is used to identify all species and all elementary reactions in the catalytic cracking of iso-octane. In this algorithm, hydrocarbon species are represented by a Boolean relation matrix and a corresponding vector. Reactions are implemented as simple matrix operations. More information about the representation of hydrocarbon species and the generation of the reaction network can be found in literature (Quintana, 2006 and Pintelon, 2006). The reaction network generated for iso-octane is represented in table 6.1. Only alkanes, alkenes and alkylcarbenium ions are considered, with a maximum of 8 carbon atoms. No more than 5 primary carbon atoms were allowed in a hydrocarbon and no branches larger than

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 87 methyl groups. The reaction network consists of 235 species and contains over 800 elementary reactions.

Hydrocarbon species Elementary reactions 35 alkanes 179 protolytic scissions 78 hydride abstractions 94 alkenes 137 protonations 106 carbenium ions 88 hydride shifts 36 methyl shifts 193 PCP-isomerizations 21 β-scissions 6 alkylations 78 hydride donations

Table 6.1 - Reaction network generated for catalytic cracking of iso-octane over Y zeolites.

6.4. RATE EQUATIONS AT MOLECULAR LEVEL

Based on the reaction network generated, elementary step rate equations can be derived. As an example, the rate equation corresponding to the protolytic scission of an alkane A1 on a + free acid site * producing a carbenium ion R1 and a second alkane A2 is written.

prsc )m(k + 1 *A ⎯+ ⎯→ +⎯⎯ AR 21

Assuming that the carbenium ion formed is of type m, the single event rate equation is given by equation 6.12. ~ = *nr C*P*)m(k (6.12) prsc e prsc 1 *A

with C* = number of free acid sites [mol/gcat]

P = partial pressure of alkane A1 [Pa] A1

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 88

+ + For a PCP-isomerization of carbenium ion R1 into a second carbenium ion R2 equation 6.13 is obtained, assuming the first is of type m and the latter of type n.

+ PCP )n;m(k + R 1 ⎯⎯→⎯⎯ R 2 ~ PCP = e *nr PCP C*)n;m(k + (6.13) R1

+ with C + = surface concentration of carbenium ion R1 [mol/gcat] R1

The number of free acid sites is calculated based on the surface concentrations of carbenium ions, which are calculated applying the pseudo-steady state approximation. This leads to equation 6.14 for the site balance.

*t += CCC + (6.14) ∑ Ri i with Ct = total concentration of acid sites [mol/gcat]

The net rate of formation of species i is obtained by summing all elementary reaction rates converting and producing this species. For alkylcarbenium ions it is assumed that this net rate of formation is zero.

6.5. DEFINITION OF THE RATE COEFFICIENTS

A full network starting from individual compounds for all hydrocarbon families involves a huge number of reaction species and reactions. However, the reactions can be grouped in a limited number of reaction families, e.g. protonation, β-scission, protolytic scission, etc. Within a reaction family the rate coefficients depend, as discussed earlier, on the type of carbenium ions involved. An overview of the reaction rate coefficients per reaction involved is given in table 6.2.

Reaction family Alkylcarbenium ions involved Number of coefficients protolytic scission (p),(s),(t) 3 hydride transfer (s),(t) 2 protonation (s),(t) 2 deprotonation (p),(s),(t) 3

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 89

PCP-isomerization (s,s),(s,t),(t,s),(t,t) 3 β-scission (s,s),(s,t),(t,s),(t,t) 4 alkylation (s,s),(s,t),(t,s),(t,t) 4

Table 6.2 – Single event rate coefficient per reaction family involved in the model.

When performing the estimation of rate coefficients without accounting for coke formation, the coefficients can be separately estimated with a reasonable accuracy for each type of hydrocarbon. For instance, if the estimation of the rate coefficients for the cracking of acyclic species is performed in advance, those values can be fixed during the cracking of cyclic hydrocarbon.

Certain activation energies and pre-exponential factors are associated with each reactions family. The simultaneous estimation of both activation energies and pre-exponential factors however, would lead to a significant increase in the number of parameters to be estimated. Hence, the number of parameters could be reduced if the pre-exponential factors are calculated in advance.

The calculation of the pre-exponential factors using transition-state theory and statistical thermodynamics was proposed several years ago (Yaluris et al, 1995). The pre-exponential factors can be defined as follows:

ΔΔH °≠ S°≠ kT* −− kee= B **RT* R (6.15) h

If the Arrhenius expresión is used for the rate constant

E − a kAe= * RT* (6.16) with A = pre-exponential factor [l/s]

Ea = activation energy [J/mol]

°≠ and with the approximation of ΔH = Ea , the pre-exponential factor can be calculated from equation 6.17.

≠ kT* ΔS A ≈ B *e R (6.17) h

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 90

This calculation requires the absolute entropies of all species participating in the various reaction steps, including the transition states. More details of this calculation can be found elsewhere Yaluris et al. (1995) and Martens (2001).

6.6. ACIDITY DESCRIPTORS

The influence of acidity on activation energies is illustrated in figure 6.2. This figure represents the energy diagram of an alkane being protonated, forming carbenium ion 1, which subsequently undergoes an isomerization reaction. The corresponding activation energies

Earef, and the protonation enthalpy ΔH pr, ref are displayed (green curve). When the same reactions take place over a catalyst with higher acid strength (blue curve), the corresponding protonation enthalpy ΔH pr will increase with a factor Δ(ΔH pr ), reflecting the increased stability of the carbenium ions. It is assumed that the latter is independent of the type of carbenium, p, s, or t. The increased acidity can also cause a change in the activation energies by a factor ΔEa :

EEa,,,, reac= a reac ref−Δ E a reac (6.18)

Figure 6.1 – Elementary steps considered in the catalytic cracking network.

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 91

Five different ΔEa values are introduced, corresponding to the five reaction families displayed in figure 6.1, leading to seven catalyst descriptors in the model: the total concentration of acid sites Ct, reflecting the influence of the number of acid sites, and six parameters reflecting the influence of the acid strength on the activation energies.

∆(∆Hpr) - ∆Ea,pcp

E refpcp,a, Energy

E pcpa,

E pra, + HHC +

ΔH pr ΔH refpr,

∆(∆Hpr) ∆(∆Hpr)

1ion 1ion 2ion 2ion

Reaction Coordinate

Figure 6.2 – Energy diagram of alkene protonation, followed by isomerization over two catalysts differing in acid strength. The grey lines correspond to the catalyst with the lowest acid strength which is chosen as a reference.

6.7. PARAMETER ESTIMATION PROCEDURE

In this section, parameter estimations (see table 6.3) for acyclic species using information of iso-octane cracking at coke free conditions are outlined.

Kinetic parameter Elementary reaction

kprsc(p),kprsc(s),kprst(t) protolytic scission

kht(s),kht(t) hydride transfer

kprot(s),kprot(t) protonation

kdep(s),kdep(t) deprotonation

kpcp(s,s),kpcp(s,t),kpcp(t,s),kpcp(t,t) PCP-isomerization

kβ(s,s), kβ(s,t), kβ(t,s),kβ(t,t) β-scission

kalk(s,s), kalk(s,t), kalk(t,s),kalk(t,t) alkylation

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 92

Table 6.3 – Kinetic parameters for cracking of acyclic species.

The incorporation of calculated pre-exponential factors requires an additional parameter related to the catalyst: the total concentration of acid sites on the catalyst Ct. This is an adjustable parameter. In the case of catalytic cracking, this value is strongly dependent on the type of zeolite used, i.e., the presence of rare earths, the use of USY zeolite, etc. (Quintana, 2006).

The required input data for the parameter estimation algorithm used in the FORTRAN code are:

o An initial set of acidity descriptors for the elementary reactions. o Single event pre-exponential factors as calculated based on transition state theory and statistical thermodynamics, as discussed in the previous section (see table 6.4). o Reference activation energies which have been obtained on the reference catalyst LZ- Y20 (shown in table 6.5).

o The experimental conditions: T, pt, N2 flow rate, HC flow rate, W/F°iC8 and the

number of acid sites Ct. o The experimentally measured product conversions. o The reaction network, the number of single events of each elementary reaction. o Weight factors. Unweighted regression is performed in this work to get preliminary results for future continuation with weighted regression.

Reaction Family A (1/s kPa) or (1/s) protolytic scission 1,96 106 protonation 1,98 106 deprotonation 2,71 106 β-scission 2,71 106 hydride transfer 1,55 106 PCP isomerization 1,51 106

Table 6.4 - Single event pre-exponential factors as calculated based on transition state theory and statistical thermodynamics.

Reaction Ea (KJ/mol) htf (s) 108,6 htf (t) 84,0 pr (s) 70,2

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 93

pr (t) 57,4 dep (p) 90,8 dep (s) 111,7 dep (t) 175,4 proto (p) 222,7 proto (s) 180,0 proto (t) 153,8 PCP (s,s) 116,6 PCP (s,t) 81,2 PCP (t,s) 179,7 PCP (t,t) 157,4 β (s,s) 187,9 β (s,t) 180,4 β (t,s) 274,6 β (t,t) 246,3

Table 6.5 - Reference activation energies which have been obtained on the reference catalyst LZ-Y20.

Parameter estimation is performed by minimizing the objective function:

nnresp resp nexp S =−∑∑σ jk ∑()yyyy ji,, calc jiexp,,( ki calc− ki exp) (6.19) jk i=1

with nexp = number of experiments

nresp = number of responses

y jicalc, = calculated product conversion of product j in experiment i

y ji,exp = experimental product conversion of product j in experiment i

σ jk = weight factors (in this work weight factors were equal to 1)

The product conversion of hydrocarbon j in experiment i, y jicalc, , is calculated by solving the continuity equation for a CSTR in steady-state regime (equation 6.20).

Wi y calc,ji − 0 ji = 0R* (6.20) F 8iC,i

The net rate of formation Rji of product j in experiment i is calculated by summing over all elementary reactions involving species j, as discussed above.

The ODRPack subroutine was used for performing the minimization of the objective function based on the Marquardt algorithm. Explicit orthogonal distance regression is performed, using forward finite differences. This subroutine also generates statistical data such as confidence intervals, correlation matrix, significance of the regression, etc.

Chapter 6. Parameter estimation for the cracking of iso-octane at coke free conditions 94

In figure 6.3 it can be seen the parity between experimental and calculated conversions.

isobutane isobutene

0.4 0.01

0.3

0.2 0.005 cal cal 0.1

0 0 00.20.40.6 0 0.02 0.04 0.06 0.08 exp exp

isopentane iso-octane

1 0.06

0.04 0.5 cal cal 0.02

0 0 0 0.02 0.04 0.06 0.08 0.1 0 0.2 0.4 0.6 0.8 1 exp exp

Figure 6.3 – Parity between experimental and calculated data.

The final correlation coefficient has a value of 2,912 and the F-value for significance regression is 6,457 102 (that is usually compared with a tabulated value of 3,09).

Chapter 7. Conclusions 95

CHAPTER 7 CONCLUSIONS

This work has been performed as a part of a PhD project in which the catalytic cracking of hydrocarbons is studied and modelled on acid catalysts. Fundamental models such as single- event microkinetics (SEMK) represent an important tool for the prediction of variations in the operating conditions and the optimization of industrial units. The extension of the SEMK for modelling catalytic cracking reactions in free coke formation requires a series of tasks in which the generation of reliable experimental data from the cracking of representative hydrocarbons is a key point. This information will then be used for the estimation of SEMK rate coefficients.

The catalytic cracking of a representative alkane, iso-octane, and a typical cycloalkane, methylcyclohexane, has been performed in a recycle electrobalance reactor at 475°C, 7 kPa and a broad range of space times, using different catalysts. In the cracking experiments that have been performed to develop this master thesis, the Y faujasite has been the most used zeolite to test the influence of acid properties on iso-octane and methylcyclohexane elementary reactions. Experiments have been performed with Y, ZSM-5 and BIPOM zeolites to study respectively, the correct operation of the setup, the absence of diffusion limitations inside the pellet and the behaviour of newly synthesized zeolites.

The cracking of iso-octane mainly leads to the formation of alkanes and alkenes. However, small amounts of cycloalkanes and aromatics were consistently observed. First, the correct operation and functioning of the recycle electrobalance reactor set up has been demonstrated by reproducing previously available data. Next, the absence of diffusion limitations in the ZSM-5 pellet at the conditions applied was verified experimentally. Then, two newly developed BIPOM materials have been studied to investigate their activity and product selectivities in iso-octane cracking. Under the conditions tested a maximum conversion of only 2 mol% was obtained over these materials, indicating a rather low activity compared to Y and ZSM-5. Comparing the product distribution obtained over these materials, it can be concluded that BIPOM1 (Al-zeogrid) shows a similar behaviour to Y zeolites, while BIPOM3 has an intermediate behaviour between Y and ZSM-5.

Chapter 7. Conclusions 96

An experimental study of methylcyclohexane cracking was carried out, at similar conditions as with iso-octane, testing four different Y zeolites. It was demonstrated that the acid properties of the Y zeolites do not influence the product selectivities obtained in methylcyclohexane cracking, as was the case for iso-octane cracking. At the operating conditions applied, the product distribution was very similar for the four catalysts tested and the global trend was: alkanes >> alkenes > cycloalkanes and aromatics. Also, it was probed for the experimental conditions used, that the four of them seem equally active, indicating that the acid properties do not influence activity in methylcyclohexane cracking, contrary to what was found in iso-octane cracking. At the operating conditions used, higher reactivity was observed for methylcyclohexane than for iso-octane.

A single event microkinetic model has been developed which takes the influence of the acid properties on rates of the elementary reactions in alkane cracking into account. A limited number of catalyst descriptors have been incorporated in the model: the number of active sites, the influence on the protonation enthalpy and the influence on the activation energies of the elementary reactions. A set of activation energies on a reference catalyst (LZ-Y20) had been obtained before. Based on these reference activation energies, the catalyst descriptors can be estimated by means of regression to experimental data on iso-octane cracking. Parameter estimation has been performed by means of the ODRPack subroutine. Preliminary estimations for the acidity descriptors of CBV 720 have been obtained based on unweighted regression. The best results so far have been obtained with the following values: Δ(ΔHpr) =

10,43±0.004 kJ/mol, ΔEa,htf = 7,652±0,033 kJ/mol, ΔEa,pr = 7,822±0.034 kJ/mol, ΔEa,prsc =

5,194±0.024 kJ/mol, ΔEa,beta = 7,699±0.033 kJ/mol and ΔEa,PCP = 5,145±0.021 kJ/mol.

In the future, parameter estimations for the acidity descriptors over Y zeolites will have to be continued. Next, the influence of pore structure can be modelled. Based on the cracking results of methylcyclohexane kinetic parameters for cyclic species (at coke free conditions) can be estimated. Then, other cracking experiments (e.g. with aromatics) will be carried out for elaborating the complete kinetic model for catalytic cracking of model hydrocarbons at non-deactivating conditions. Finally, the single event kinetic model will also have to be elaborated for the incorporation of coke formation.

Appendix A. Operation of the set up 97

APPENDIX A OPERATION OF THE SET UP

The experimental procedure is described in day-to-day order, that is, from morning to evening.

A.1. PRETREATMENT

1. Check the gas bottles and replace them when needed. Check the liquid nitrogen vessel. 2. Check the bubble vessels and refill them when needed. 3. Feed nitrogen to the balance using MFC-1: open valve (V-6, vertically) mass flow controller, adjust set value on Brooks read out unit, close feed via panel (close grey valve (V-7, horizontally), close black valve (V-8, vertically)). 4. Turn on cooling: open grey valve (V-9, vertically) on panel. 5. Switch on motor: push orange button (light is on), set the rotational speed desired. It takes at least 20 min for the motor to run stable after start-up, so check rotational speed regularly.

6. Feed N2 for pretreatment catalyst: open black valve (V-5, vertically), set flow rate using needle valve (NV-2), switch grey three-way valve in position “reactor” (vertically), coupled three-way valves in bottom position (“vent”). 7. Set split valve at desired value (fully open). 8. Start temperature programme: switch on temperature controllers (orange button, setpoint: 488 °C, switch in position “voorbehandeling”. 9. Start chronometer. 10. Turn on heating evaporator (red button, 0,1 A). 11. Turn on heating oven (green button, 0,45 A). 12. Turn on heating feed and effluent tubes (white buttons, 2,15 A). 13. Measure room temperature and look up atmospheric pressure on website KMI: http://www.meteo.be/nederlands/index.php?menu=Menu1_3. 14. Make XChrom files for analyses on TCD and FID.

Appendix A. Operation of the set up 98

15. After 2 h 30 min set reaction temperature: switch temperature controllers in position “reactie”.

A.2. FEED

1. Set flow rate N2 (pretreatment) at the desired value for the experiment (choose equal to total molar feed flow rate to cause as few pressure fluctuations as possible when switching to the feed). 2. Set split valve at the desired value (desired overpressure in the reactor). 3. Switch on evacuation system (black button). 4. Switch on Lauda bath (position 1, 40 °C), check water level and refill when needed. 5. Feed diluent N2: open valve (V-4) MFC-2 (vertically), adjust set value on read out unit. 6. Feed DME: open valve (V-10) MFC-3 (vertically) and adjust set value on read out unit. 7. When evaporator temperature is 180 °C or more, feeding of liquid hydrocarbon can start: open valve V-1 (vertically). 8. Feed helium: open valve (V-3, vertically) and set pressure above fluid reservoir using needle valve. 9. Measure flow rate liquid hydrocarbon: open valve V-2 (vertically), valve V-1 should be closed (horizontally) to measure the flow rate correctly. 10. Turn on balance recorder: on/off button, record on, pen down, range: 20V Var, rate 5 mm/min. 11. After stabilization at reaction temperature note the observed weight difference. 12. After 3 h 30 min the feed is sent to the reactor by switching the coupled three-way valves (position “reactor”). 13. Restart chronometer. 14. Perform first analysis as soon as the transient phenomena have disappeared (reactor in steady state). 15. Measure and check regularly flow rate of liquid hydrocarbon, overpressure in reactor, pressure difference between balance and reactor and temperatures of evaporator, DME-tube, catalyst basket, bottom reactor, effluent tube and oven.

Appendix A. Operation of the set up 99

A.3. PERFORM ANALYSIS

A.3.1. Store sample

1. Switch the eight-way valve in position “opslaan”. 2. Put the six-loop valve in the desired position to store the reactor effluent. 3. Let the gas flow through the desired loop for at least 1 minute and then switch the six- way valve to the next position to store the sample.

A.3.2. Direct injection in FID

1. Wait until the GC is ready (continuous green light). 2. Put the six-way valve in position “FID”. 3. Follow the instructions to store a sample. 4. After the gas has flown through the loop for at least 1 minute, start GC (red button) first and then XChrom (start). 5. Switch the eight-way valve in position “injectie” to pass the sample to the FID.

A.3.3. Indirect injection in FID

1. Wait until the GC is ready (continuous green light). 2. Put the six-way valve in position “FID”. 3. Put the six-way valve in the position ahead of the loop which contains the sample to be analysed. 4. Start the GC and XChrom. 5. Switch the eight-way valve in position “injectie”. 6. Immediately switch the six-loop valve one loop further.

A.3.4. Direct injection in TCD

Appendix A. Operation of the set up 100

2. Wait until the GC is ready (continuous red light). 3. Put the six-way valve in position “TCD”. 4. Follow the instructions to store a sample. 5. After the gas has flown through the loop for at least 1 minute, start GC (red button) first and then XChrom (start). 6. Switch the eight-way valve in position “injectie” to pass the sample to the TCD.

A.3.5. Indirect injection in TCD

1. Switch the Valco valve in the correct position, so that the carrier gas flows through the column in the correct direction. It takes about five minutes before the carrier gas flow rate is stabilized. 2. Wait until the GC is ready (continuous red light). 3. Put the six-way valve in position “TCD”. 4. Put the six-way valve in the position ahead of the loop which contains the sample to be analysed. 5. Start the GC and XChrom. 6. Switch the eight-way valve in position “injectie”. 7. Immediately switch the six-loop valve one loop further.

A.4. SHUT DOWN REACTOR

1. Close hydrocarbon feed by switching the coupled three-way valves (position “vent”). 2. After stabilization note the observed weight difference. 3. Close liquid hydrocarbon feed: close valves V-1 and V-2 (horizontally). 4. Close He feed: close valve V-3 (horizontally). 5. Turn off heating evaporator (red button) and feed tubes (white button). 6. Turn off heating Lauda bath (position 0). 7. Switch off temperature controllers reactor (orange button). 8. Shut off DME supply: close valve V-10 (horizontally).

9. Close diluent N2: close valve V-4 (horizontally). 10. Open isolation box (with gloves) to accelerate the cooling down. 11. Switch off heating effluent tubes and oven after the last GC injection.

Appendix A. Operation of the set up 101

12. When the reactor temperature has dropped under 200 °C, turn off water cooling (valve V-9 horizontally). 13. Switch off motor: set rotational speed at 0 and press orange button.

14. When the reactor temperature has dropped under 30 °C, shut off protective gas N2 for the balance (valve V-6 horizontally). 15. Switch off evacuation system (red button).

A.5. LOAD REACTOR

1. Switch off balance (on/off button). 2. Open reactor, open balance chamber, detach Kanthal wire and catalyst basket from the balance arm and slowly let it down using a thin copper wire. 3. Fill the basket with the desired amount of catalyst. 4. Check Kanthal wire (straight, hooks all right). 5. Attach basket with catalyst to the Kanthal wire, pull it up with the copper wire in the reactor and attach back to the balance arm. 6. Check with Maglite whether the catalyst basket hangs centrally in the reactor leg. If needed move the motor block (loosen 2x4 butterfly nuts to shift L/R and U/D). 7. Switch on balance (on/off button), equilibrate and adjust counterweight if needed. Possible displays: o H: Counterweight is too heavy. o L: Counterweight is too light. o - -: Balance is nearly in equilibrium. o Flashing number: arm where -- appears is that many mg heavier. o 0: Balance is in equilibrium. The number in the top display indicates the difference in weight between both balance arms. 8. Close balance chamber, if necessary rub the seals with grease. 9. Check graphite ring, if necessary rub bolts with Neverseez. Close reactor. 10. Start flushing balance chamber: open grey valve on feed panel (V-7, vertically), open black valve behind panel (V-8, horizontally). The pressure difference meter should

indicate at least 1 mm H2O overpressure. This overpressure is determined by the flow

Appendix A. Operation of the set up 102

rate of N2 through the Kytola rotameter and by the position of the split valve (NV-3). Flushing takes all night. 11. Open split valve. 12. Close isolation box using 4 clips.

Appendix B. Calibration factors of the GC fid 103

APPENDIX B CALIBRATION FACTORS OF THE GC FID

It is known that the peak surface area for a certain component i, obtained during the analysis of a mixture in a flame ionization detector, is proportional to the weight fraction of that component in the mixture, A calibration factor is the value by which the peak surface area has to be multiplied in order to obtaine the weight fraction of that component.

The calibration factor for the component i can be calculated by means of the “group contribution method” (Dierickx et al, 1986). The correlation used (T. Van Camp, 2005) for representing the calibration factors is shown in equation B.1.

CF()* i M () i= ∑ nj,i * a j (B.1) j

With a j = group contribution factor

CF = calibration factor

M = molecular weight

nij, = number of group contributions present in component i

The difference between the experimental and the calculated weight fractions is to be minimized. There are ten different group contribution factors (table B.1).

a1 CH3 Methyl group in an aliphatic chain

a2 CH2 Methylene group in an aliphatic chain

a3 CH Tertiary carbon in an aliphatic chain

a4 C Quaternary carbon in an aliphatic chain

a5 (5) Additional contribution for a 5-membered naphthenic ring

a6 (6) Additional contribution for a 6-membered naphthenic ring

a7 Caro-H Aromatic carbon atom, connected to a hydrogen atom

a8 Caro-C Aromatic carbon atom, connected to another carbon atom

Appendix B. Calibration factors of the GC fid 104

a9 C=H2 Olefinic carbon atom, connected to 2 hydrogen atoms

a10 C=CH Olefinic carbon atom, connected to a hydrogen and a carbon atom

Table B.1 – Group contribution method (Dierickx et al, 1986).

A complete list of the calibration factors for the different reaction products identified during our experiments with methylcyclohexane is available in appendix F.

Appendix C. Feed composition and calibration curve for methylcyclohexane flow rate 105

APPENDIX C FEED COMPOSITION AND CALIBRATION CURVE FOR METHYLCYCLOHEXANE FLOW RATE

By manually injecting pure feed into the FID and identifying the peaks obtained in the chromatogram, the methylcyclohexane composition can be determined, as shown in table C.1.

Component wt% benzene 0,001289 cyclohexane 0,013646 cyclohexene 0,004476 1,2-trans-dimethylcyclopentane 0,010285 n-heptane 0,032241 methylcyclohexane 99,73772 toluene 0,004113 1t,4-dimethylcyclohexane 0,012878 1,3t-ethylmethylcyclopentane 0,021364 i-propylcyclopentane 0,026567 1-octene 0,002776 1c,2-dimethylcyclohexane 0,013008 1c,3c,5-trimethylcyclohexane 0,01325 ethylbenzene 0,007099 n-nonane 0,027219 1-methyl-3-ethylbenzene 0,017864 1,2,3-trimethylbenzene 0,045234 1,4-dimethyl-2-ethylbenzene 0,008976

Table C.1 – Feed composition of methylcyclohexane.

A new calibration curve relating methylcyclohexane flow rate to the total pressure above the liquid (see figure C.1) has been constructed.

Appendix C. Feed composition and calibration curve for methylcyclohexane flow rate 106

Calibration methylcyclohexane 2,5

1,5 l/sec) μ

1 Flow rate ( Flow rate

0,5

y = 3,0837x2 - 4,7582x + 1,8309 0 1 1,11,21,31,41,51,61,7 Pressure (bar)

Figure C.1 – Calibration curve for methylcyclohexane flow rate as a function of the total pressure above the liquid.

Appendix D. Physical properties 107

APPENDIX D PHYSICAL PROPERTIES

Viscosity

For prediction of the vapour viscosity of pure hydrocarbons at low pressure (Tr < 0,6) the method of Siel and Thodos is the most accurate (Perry et al, 1998) as it can be shown in equation D.1. It can only be used for methylcyclohexane, for the other components it was used the “Graph for determining absolute viscosity of a gas as a function of temperature near ambient pressure” (Perry et al, 1998).

1/2 2/3 −4 NM** Pc μv = 4,60*10 * 1/6 (D.1) Tc

with μv = vapour viscosity NT= 0.00034* r0.94 M = molecular weight P = pressure of the component used

Tc = critical temperature of the component used

For the gaseous mixtures of hydrocarbon and non-hydrocarbon gases at low pressures the method of Wilke is recommended (Reid et al, 1988). It is based on the application of the equation D.2.

n ynii* ηm = ∑ n (D.2) i=1 ∑ y j *Φij j=1

Appendix D. Physical properties 108

With ηm = viscosity of the mixture

ηij = pure component viscosity

yij = mole fraction for pure component

To apply the equation D.2 is necessary apply the equation D.3 and D.4.

1/2 1/4 2 [1+ (ηηij / ) * (MM j / i ) ] Φ=ij 1/2 (D.3) [8*(1+ MMij / )]

η j M i Φ ji =**Φij (D.4) ηijM

The results of the viscosities are shown in table D.1, taking into account that methylcyclohexane is component number 1, propylene is 2, isobutene is 3 and nitrogen is 4.

Φ12 = 0,5548 Φ31 = 1,3981

Φ13 = 0,7025 Φ32 = 0,7918

Φ14 = 0,3725 Φ34 = 0,5273

Φ21 = 1,7664 Φ41 = 2,8413

Φ23 = 1,2667 Φ42 = 1,5754

Φ24 = 0,6576 Φ43 = 2,021

Table D.1 – Coefficients to calculate the viscosity of the mixture.

With the values of viscosities calculated it can be modified the methylcyclohexane Excel file.

Appendix E. Product identification for the cracking of iso-octane 109

APPENDIX E

PRODUCT IDENTIFICATION FOR THE CRACKING OF ISO-OCTANE

A typical peak distribution and a chromatogram for the cracking of iso-octane is given in this appendix, as it can be shown below.

Product name Calibration factor Retention time (min) methane 0.976351477 5.38 ethylene 0.865499797 5.67 ethane 0.900247542 5.90 propene 0.872512682 7.67 propane 0.879883013 7.87 DME 2.876628463 9.57 isobutane 0.929507444 10.72 isobutene 0.818478926 11.96 1-butene 0.95418395 12.05 1,3-butadiene 0.833056359 12.20 n-butane 0.91609351 12.46 trans-2-butene 0.902551234 13.05 cis-2-butene 0.870761751 13.80 3-methyl-1-butene 1.217816261 15.49 isopentane 1.199689147 16.42 1-pentene 1.186971533 17.23 2-methyl-1-butene 1.204892145 17.59 n-pentane 1.251443192 17.81 isoprene 1.142138419 18.04 trans-2-pentene 1.129912219 18.24 cis-2-pentene 1.129912219 18.62 2-methyl-2-butene 1.14613953 18.86 2,2-dimethylbutane 1.253135934 19.60 cyclopentene 1.07886732 20.47 4-methyl-1-pentene 1.145733466 20.60

Appendix E. Product identification for the cracking of iso-octane 110

3-methyl-1-pentene 1.145733466 20.70 trans-1,3-pentadiene 1.0726677 20.77 cyclopentane 1.19434781 21.01 2,3-dimethylbutane 1.129527034 21.13 cis-4-methyl-2-pentene 1.10101003 21.22 2-methylpentane 1.196521701 21.35 trans-4-methyl-2-pentene 1.10101003 21.50 3-methylpentane 1.196521701 22.04 2-methyl-1-pentene 1.200755303 22.28 1-hexene 1.18588634 22.35 n-hexane 1.239318753 22.86 cis&trans-3-hexene 1.138038714 22.93 trans-2-hexene 1.138038714 23.09 2-methyl-2-pentene 1.151725106 23.20 cis-3-methyl-2-pentene 1.151725106 23.35 cis-2-hexene 1.138038714 23.52 trans-3-methyl-2-pentene 1.151725106 23.81 Methylcyclopentane 1.151453715 24.15 2,4-dimethylpentane 1.159870073 24.30 3,4-dimethyl-1-pentene 1.117989554 24.58 2,2,3-trimethylbutane 1.229045618 24.72 2,4-dimethyl-1-pentene 1.16254467 24.85 1-methylcyclopentene 1.107941468 25.06 trans-3-methyl-1,3-pentadiene 1.102515026 25.16 Benzene 1 25.27 cis-2-methyl-3-hexene 1.111702696 25.42 Cyclohexane 1.261730886 25.67 trans-2-methyl-3-hexene 1.111702696 25.75 4-methyl-t&c-2-hexene 1.111702696 26.01 2-methylhexane 1.194251372 26.13 2,3-dimethylpentane 1.159870073 26.22 Cyclohexene 1.155024717 26.35 3-methylhexane 1.194251372 26.42 cis-3,4-dimethyl-2-pentene 1.122875355 26.60 cis-1,3-dimethylcyclopentane 1.122654233 26.71 trans-1,3-dimethylcyclopentane 1.122654233 26.82 trans-1,2-dimethylcyclopentane 1.122654233 26.96 2,2,4-trimethylpentane 1.222869229 27.28

Appendix E. Product identification for the cracking of iso-octane 111 trans-2-heptene 1.14391529 27.39 n-heptane 1.230733221 27.47 cis-3-heptene 1.14391529 28.03 cis-1,2-dimethylcyclopentane 1.122654233 28.45 Methylcyclohexane 1.211116386 28.54 2,2-dimethylhexane 1.256319436 28.62 2,4,4-trimethyl-2-pentene 1.187922688 28.85 ethylcyclopentane 1.155513994 28.96 4-methylcyclohexene 1.124960931 29.03 2,4-dimethylhexane 1.162363199 29.32 1,2,4-trimethylcyclopentane 1.101982628 29.40 3,3-dimethylhexane 1.256319436 29.56 2,3,4-trimethylpentene 1.139852894 29.71 Toluene 1.081014379 29.95 2,3,3-trimethylpentane 1.222869229 30.08 1-methylcyclohexene 1.171069251 30.26 3,4-dimethylhexane 1.162363199 30.59 Ethylbenzene 1.093185138 33.78 p-&m-xylene 1.149534802 34.10 3,4-dimethylheptane 1.164318475 34.77 o-xylene 1.149534802 35.02 1-methyl-3-ethylbenzene 1.153063395 37.50 1-methyl-4-ethylbenzene 1.153063395 37.62 1,3,5-trimethylbenzene 1.20824457 37.76 1-methyl-2-ethylbenzene 1.153063395 38.20 1,2,4-trimethylbenzene 1.20824457 38.71 1,2,3-trimethylbenzene 1.20824457 39.75

Appendix E. Product identification for the cracking of iso-octane

112

28.22 28.09

28 27.49

Method: fid

27.10 Time(minutes) 26.97

Project: defproj

26.61

26.31 26.20

26 25.45

24.48

24.33 23.98

24 23.70

23.52

23.36

23.25 23.03

22.44 22.20

21.51

21.28 21.17

Standard 1

20.62

19.75 20

19.14

19.01

18.77 19

18.38

18.18

17.95

17.74 18 17.37

17 16.57

16 15.63 Response(mV) 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 Acquisition Time: Apr19 at 2006 12:29.33 Instrument:channel1 Analysis:6041901,1,1

Appendix E. Product identification for the cracking of iso-octane 113

42 41.05 41 Method: fid Time(minutes)

Project: defproj 40.01

38.97

38.77 39

38.45

38.01 37.75 38 37

36 35.25

35 34.33

Standard 1 34.01 34

33 32.48

30.79 31

30.46

30.16 29.92

29.17

29.05 28.76 29 Response(mV) 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 Acquisition Time: Apr19 at 2006 12:29.33 Analysis:6041901,1,1 Instrument:channel1

Appendix F. Product identification for the cracking of methylcyclohexane 114

APPENDIX F

PRODUCT IDENTIFICATION FOR THE CRACKING OF METHYLCYCLOHEXANE

A typical peak distribution and a chromatogram for the cracking of methylcyclohexane is given in this appendix, as it can be shown below.

Product name Calibration factor Retention time (min) methane 1 5.33 ethylene 0.886463346 5.62 ethane 0.922052727 5.85 propylene 0.893646092 7.62 propane 0.901194942 7.82 DME 2.876628463 9.55 isobutane 0.952021343 11.25 isobutene 0.838303567 12.52 1-butene 0.977295547 12.57 1,3-butadiene 0.853234085 12.74 n-butane 0.938282506 13 trans-2-butene 0.924412217 13.58 cis-2-butene 0.891852752 14.32 3-methyl-1-butene 1.058394909 15.6 isopentane 1.042640771 16.93 1-pentene 1.031587989 17.7 2-methyl-1-butene 1.047162657 18.07 n-pentane 1.087619821 18.27 trans-2-pentene 0.981998171 18.71 cis-2-pentene 0.981998171 19.08 2-methyl-2-butene 0.996101204 19.31 2,2-dimethylbutane 1.08909097 20.02 cyclopentene 0.937635434 20.82 4-methyl-1-pentene 0.995748297 21.14 cyclopentane 1.074341021 21.34 2,3-dimethylbutane 0.818052841 21.5

Appendix F. Product identification for the cracking of methylcyclohexane 115 cis-4-methyl-2-pentene 0.956879496 21.59 2-methylpentane 1.01556292 21.75 trans-4-methyl-2-pentene 0.979798943 21.83 3-methylpentane 1.039950718 22.41 1-hexene 1.030644856 22.67 n-hexane 1.114793332 23.24 trans-&-cis-hexene-3 1.012751109 23.35 trans-2-hexene 1.023689803 23.47 2-methyl-pentene-2 1.036001001 23.61 cis-3-methyl-2-pentene 1.036001001 23.69 3-methyl-cyclopentene 0.953299386 23.77 cis-2-hexene 1.023689803 23.9 trans-3-methyl-pentene-2 1.036001001 24.18 methylcyclopentane 1.035756879 24.44 2,4-dimethylpentane 1.043327571 24.65 cis-pentene 1.314823987 24.87 1-methylcyclopentene 0.996616696 25.22 benzene 0.899521071 25.5 3-methyl-1-hexene 1.035011958 25.55 2-methyl-3-cis-hexene 1 25.64 3,3-dimethylpentane 1.14032483 25.76 cyclohexane 1.134953518 25.93 2-methyl-trans-3-hexene 1 26.01 4-methyl-1-hexene 1.035011958 26.12 4-methyl-trans/cis-2-hexene 1 26.16 2-methylhexane 1.074254273 26.33 2,3-dimethylpentane 1.043327571 26.41 cyclohexene 1.03896907 26.54 3-methylhexane 1.074254273 26.73 cis-3,4-dimethyl-2-pentene 1.010050042 26.88 1,3-cis-dimethylcyclopentane 1.009851138 27.03 1,3-trans-dimethylcyclopentane 1.009851138 27.15 1,2-trans-dimethylcyclopentane 1.009851138 27.27 2,2,4-trimethylpentane 1.099996638 27.52 trans-2-heptene 1.028975907 27.59 n-heptane 1.107070465 27.79 cis-2-heptene 1.25632731 28.05 1,2-dimethylcyclopentane 1.009851138 28.23

Appendix F. Product identification for the cracking of methylcyclohexane 116 methylcyclohexane 1.089424709 28.87 ethyl-cyclopentane 1.039409186 29.18 2,4-dimethylhexane 1.04557019 29.31 1,2,4-trimethylcyclopentane 0.991256594 29.5 3,3-dimethylhexane 1.130085805 29.8 2,3,4-trimethylpentene 1.025321696 29.92 toluene 0.972395212 30.13 2,3,3-trimethylpentane 1.099996638 30.29 1-methylcyclohexene 1.053401467 30.53 3,4-dimethylhexane 1.04557019 30.8 1cis,2trans,3-trimethylcyclopentane 3.17920613 31.02 1trans,4-dimethylcyclohexane 5.79557012 31.09 trans-1,3-ethylmethylcyclopentane 3.040319651 31.41 trans-1,2-ethylmethylcyclopentane 3.040319651 31.5 1,1-methylethylcyclopentane 3.040319651 31.55 1cis,4-dimethylcyclohexane 5.79557012 31.67 iso-propylcyclopentane 2.238984175 32.08 1cis,2-dimethylcyclohexane 5.79557012 32.73 1cis,3cis,5-trimethylcyclohexane 3.995750603 33.19 ethylbenzene 0.983393083 33.81 1cis,2trans,2trans-trimethylcyclohexane 3.995750603 33.96 p-&m-xylene 1.034030777 34.14 3,4-dimethylheptane 1.047329002 34.87 o-xylene 1.034030777 35 i-butylcyclopentane 1.782815443 35.13 n-nonane 1.096908091 35.51 1-methyl-3-ethylbenzene 1.03720482 37.43 1-methyl-4-ethylbenzene 1.03720482 37.52 1,3,5-trimethylbenzene 1.086841449 37.69 1-methyl-2-ethylbenzene 1.03720482 38.08 1,2,4-trimethylbenzene 1.086841449 38.6 iso-butylbenzene 1.119574735 38.83 n-decane 1.03136797 38.93 1,2,3-trimethylbenzene 1.086841449 39.56 n-butylcyclohexane 2.201776053 40.35 1-methyl-3-n-propylbenzene 1.086614775 40.62 1,4-dimethyl-2-ethylbenzene 1.08419697 41.28 1,3-dimethyl-4-ethylbenzene 1.08419697 41.35

Appendix F. Product identification for the cracking of methylcyclohexane 117

1,2-dimethyl-4-ethylbenzene 1.08419697 41.54 1,3-dimethyl-2-ethylbenzene 1.08419697 41.67 1,2,3,4-tetra-methylbenzene 1.132616167 52.48 1,2,3,5-tetra-methylbenzene 1.132616167 42.7

Appendix F. Product identification for the cracking of methylcyclohexane

118

18.94

18.70

18.32 17.89

18 17.67 Method: fid2 Time(minutes)

Project: defproj 16.53

13.86

13.11

12.53 12.03

12 10.78

Standard 1

9.69 10 7.93

8 7.75 5.99

6 5.77 5.47 0 Response(mV) 180 160 140 120 100 80 60 40 20 Instrument:channel1 Acquisition Time: Apr27 at 2007 12:19.31 Analysis:7042701,1,1

Appendix F. Product identification for the cracking of methylcyclohexane

119 33.95 34 Method: fid2 Time(minutes) Project: defproj

31.06

30.55

30.17 29.86

29.46

29.14

28.81 28.15

27.16

27.04

26.91

26.54

26.43

26.25

26.13

25.83

26 25.39

Standard 1

24.41 24.28

22.97 22.14

21.45

21.22

21.09

20.55 19.70 20 0 Response(mV) 180 160 140 120 100 80 60 40 20 Instrument:channel1 Acquisition Time: Apr27 at 2007 12:19.31 Analysis:7042701,1,1

Appendix F. Product identification for the cracking of methylcyclohexane 120 48 Method: fid2 Time(minutes) Project: defproj 46

43.22

43.10 41.96

42 40.98

Standard 1 39.95

38.93

38.38 37.96

38 37.69

35.22 34.33 0 Response(mV) 180 160 140 120 100 80 60 40 20

Analysis:7042701,1,1 Instrument:channel1 Acquisition Time: Apr27 at 2007 12:19.31

Appendix G. Experimental data of iso-octane 121

APPENDIX G EXPERIMENTAL DATA OF ISO-OCTANE

The experiments performed in the recycle electrobalance reactor for iso-octane cracking are given in table G.1.

pºiC8 pt F°iC8 W/FºiC8 Conversion Experiment T (ºC) (103 Pa) (103 Pa) (10-6 mol/s) (kg.s/mol) (mol%)

O_CBV500_35 475 7,16 101,16 0,458 54,252 58,11

O_CBV500_36 475 7,83 101,96 0,5 49,5 57,804

O_CBV500_37 475 7,89 101,15 0,44 56,016 57,637

O_CBV3020_16 475 6,53 101,96 0,358 48,456 6,226

O_CBV3020_17 475 6,94 100,83 0,35 63,108 6,226

O_CBV3020_18 475 7,01 101,75 0,35 78,876 9,962

O_CBV3020_20 475 7,02 102,97 0,389 106,308 12,829

O_CBV3020_21 475 7,01 102,4 0,339 114,876 11,600

O_BIPOM3_01 475 7,150 99,89 0,36 111,78 1,4126

O_BIPOM3_02 475 7,010 101,7 0,36 68,94 0,86

O_BIPOM3_03 475 7,31 102,59 0,25 163,764 0,975

Table G.1 – Experimental conditions investigated for iso-octane cracking.

Appendix H. Experimental data of methylcyclohexane 122

APPENDIX H EXPERIMENTAL DATA OF METHYLCYCLOHEXANE

In table I.1, an overview is given of the conditions applied in the experiments with methylcyclohexane.

pºmch pt F°mch W/Fºmch Conversion Experiment T (ºC) (103 Pa) (103 Pa) (10-6 mol/s) (kg.s/mol) (mol%)

M_LZY20_03 475 6,85 101,88 0,3 63,756 53,571

M_LZY20_04 475 6,88 100,99 0,3 63,756 51,098

M_LZY20_05 475 7,18 102,5 0,36 71,64 52,701

M_LZY20_06 475 7,130 102,37 0,31 81,54 51,903

M_LZY20_07 475 6,530 102,72 0,28 56,412 46,105

M_LZY20_08 475 6,81 102,33 0,3 42,984 47,608

M_LZY20_09 475 7,36 102,89 0,35 27,396 45,133

M_LZY20_10 475 7,18 103,2 0,31 30,744 49,249

M_LZY20_11 475 7,11 101,75 0,34 14,976 33,057

M_CBV720_01 475 7,16 99,92 0,32 30,204 42,489

M_CBV720_02 475 7,34 100,9 0,33 38,844 48,786

M_CBV720_03 475 6,84 100,96 0,3 53,460 51,825

M_CBV720_04 475 7,19 100,59 0,32 60,552 52,685

M_CBV720_05 475 6,86 100,85 0,3 63,864 47,109

M_CBV720_06 475 7,02 101,51 0,31 83,700 62,235

M_CBV760_03 475 7,04 101,77 0,32 79,452 41,952

M_CBV760_04 475 6,97 101,65 0,3 56,608 32,180

M_CBV760_06 475 6,85 101 0,304 37,870 30,612

M_CBV760_07 475 6,99 102,34 0,31 42,696 39,891

Appendix H. Experimental data of methylcyclohexane 123

M_CBV760_08 475 6,99 102,61 0,28 114,340 43,178

M_CBV500_01 475 6,96 102,26 0,28 91,476 56,527

M_CBV500_02 475 6,91 102,14 0,26 98,350 67,463

M_CBV500_03 475 7,05 101,94 0,3 73,692 67,469

M_CBV500_04 475 6,91 101,48 0,3 64,188 67,603

M_CBV500_05 475 6,92 101,73 0,3 49,248 72,316

M_CBV500_06 475 6,7 102,07 0,289 33,370 51,179

Table I.1 – Experimental conditions for methylcyclohexane cracking.

124

BIBLIOGRAPHY

Aerts A. (2006). Presentation SBO-project BIPOM. KU Leuven.

Barthomeuf D. (1993). Zeolite Acidity Dependence on Structure and Chemical Environment. Correlations with Catalysis. Mat. Chem. Phys., 17:49-71.

Cerqueira H.S., Mihindou-Koumba P.C., Magnoux P. and Guisnet M. (2001). Methylcyclohexane Transformation over HFAU, HBEA, and HMFI Zeolites: I. Reaction Scheme and Mechanism. Ind. Eng. Chem. Res., 40:1032-1041.

Chen N.Y. and Degnan T.F. (1988). The implementation of definitions of quantities. Oil Gas J., 78(21):75.

Cotterman R.L. and Plumlee K.W. (1989). Effects of gasoline composition on octane number. Symposium on the Hydrocarbon Chemistry of FCC Naptha Formation, 34(4):756.

Derouane E.G. (1980). New Aspects of Molecular Shape-Selectivity. Catalysis by zeolite ZSM-5. Stud. Surf. Sci. Catal. Catalysis by Zeolites. Imelik et al.

Dierickx J.L., Plehiers P.M. and Froment G.F. (1986). Online gas chromatographic analysis on hydrocarbon effluents. Calibration factors and their correlation, J. Chrom., 362(2):155- 174.

Dutta S. and Gualy R. (1999). Modernize process reactors. Hydrocarb. Process., 78(11):91.

Feng W., Vynckier E. and Froment G.F. (1993). Single-event Kinetics of catalytic cracking. Ind. Eng. Chem. Res., 32(12):2997-3005.

Froment G. and Bischoff K. (1990). Chemical Reactor Analysis and Design. Wiley and Sons, New York.

Bibliography 125

Gagea B. (2006). Presentation SBO-project BIPOM. KU Leuven.

Gates B.C., Katzer J.R. and Schuit G.C.A. (1979). Chemistry of catalytic processes. McGraw- Hill, New York.

Gauthier T., Bayle J. and Leroy P. (2000). FCC: Fluidization phenomena and technologies. Oil Gas Sci. Technol., 55(2):187-207.

Johnson T.E., Goolsby T.L., Silverman M.A., Kowalczyck D.C. and Moore H.F. (1998). Catalyst separation technology improves FCC gasoline yields. Oil Gas J., 96(24):65.

Kazansky V. (1999). Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons. Catal. Today, 51(3):419-434.

Kirschhock C.E.A., Kremer P.B., Vermant J., Van Tendeloo G., Jacobs P.A., Martens J.A. (2005). Design and Synthesis of Hierarchical Materials from Ordered Zeolitic Building Units. Chem.-Eur. J., 11:4306-4313.

Kissin Y.V. (1996). Chemical mechanism of hydrocarbon cracking over solid acidic catalysts. J. Catal., 163:50-62.

Marcilly C. (2001). Evolution of Refining and Petrochemicals: What is the place of zeolites. Oil Gas Sci. Technol., 56(5):499-514.

Patureaux T and Barthod D. (2000). Use of CFD modelling for improving an FCC riser operation. Oil Gas Sci. Technol., 55(2):219-225.

Perry R.H., Green D.W. and Maloney J.O. (1984). Perry’s Chemical Engineers’ Handbook. McGraw-Hill Book Company, New York.

Pintelon T.R.R. (2006). De invloed van zuursterkte en zeolietporiënstructuur op de kinetiek van het katalytisch kraken van iso-octaan. Master Thesis, Universiteit Gent.

Bibliography 126

Quintana R. (2006). Single-Event Microkinetics for Coking in Catalytic Cracking: Development Application, PhD thesis, Universiteit Gent.

Reid, Prausttnitz and Poling. (1987). The properties of Gases and Liquids. McGraw-Hill Book Company, New York.

Reyniers M.F., Beirnaert H.C. and Marin G.B. (2000a). Influence of coke formation on the conversion of hydrocarbons. I. Alkanes on a USY-zeolite. Appl. Catal. A: Gen., 202(1):49-63.

Roy A.H., Broudy R.R., Averbach S.M. and Vining W.J. (1999). Teaching Materials that Matter: An Interactive, Multi-media Module on Zeolites in General Chemistry. Chem. Educ., 4(3):14.

Sadeghbeigi R. (1995). Fluid Catalytic Cracking Handbook: Design, Operation and Troubleshooting of FCC Facilities. Gulf Publishing Company, Houston, Texas.

Schnait M.W., Sexson P.A., True D.R., Bartolic D.B., Lee Y.K., Yoo I. and Kang S. (1998). New cracking processes have a good reliability. Oil Gas J., 96(25):53.

Stöcker M. (2005). Gas phase catalysis by zeolites. Micropor. Mesopor. Mater., 82(3):257- 292.

Van Camp T. (2005). Study of catalytic cracking of n-decane and methylcyclohexane in the presence of coking reactions in an oscillating microbalance reactor. Master thesis, Universiteit Gent.

Wojciechowski B.W. and Corma. A. (1986). Catalytic Cracking: Catalyst, Chemistry and Kinetics. Dekker. Inc., Basel, New York.