Synthesis, Characterization and Catalyst Testing of Metal Modified Zeolites for

Isomerization of n-butane and Application in Fine Chemicals

Masters Thesis

SEELAM PREM KUMAR

Laboratory of Industrial Chemistry

Process Chemistry Centre

Faculty of Technology

Department of Chemical Engineering

bo Akademi University

Turku, Finland.

2007.

Preface

The present masters thesis was carried out at the Laboratory of Industrial

Chemistry, Process Chemistry Centre, Faculty of Technology, Department of

Chemical Engineering, bo Akademi University, Turku, Finland during the

academic year 2005-2006 as part of my graduation studies as an international

student from India. I would like to thank to bo Akademi University for giving me

the opportunity to study in Finland.

I wish to express my sincere thanks and gratitude to Prof. Tapio Salmi and Prof.

Dmitry Yu. Murzin, for giving me the opportunity to work in the top research

center in catalysis i.e. Laboratory of Industrial Chemistry. My special thanks to

my supervisor Docent Dr. Narendra Kumar who has been so helpful to me and

whose technical and moral guidance remained a source of research knowledge

in my life and I learnt so many things during interactions with N.Kumar. I would

like to express my gratitude to Laboratory of Inorganic Chemistry and Heat

Engineering Laboratory for giving me the opportunity to work during my studies.

I would like to thank the persons at the Laboratory of Industrial Chemistry, in

particular to Laboratory manager Dr. Kari Eranen for his help with the reactor

system. Special thanks go to Aton Tokarev for helping me in DCP

measurements, explanation of lactose oxidation results and also to Elena

Murzina for testing my catalysts in oxidation of lactose reaction. I am thankful to

Thomas Finnas for helping me in FTIR measurements and also to Jose Villegas

in catalyst testing. Finally, I am grateful to Ping Ping Lee for DCP and ICP

measurements at Laboratory of Analytical Chemistry.

I am very kind to my parents who made me a person with good character and

hard working; especially my elder brothers always encouraged me in the studies.

I wish to thank my friends for their valuable moral support in my life in

Hyderabad, India. And also special thanks to my friends in Denmark and Finland

for their valuable support in my life. Financial support from bo Akademi,

Laboratory of Industrial Chemistry is gratefully acknowledged.

i

ABSTRACT

Seelam Prem Kumar Synthesis, Characterization and Catalytic Testing of Metal Modified Zeolite Catalysts

For Application in Fine Chemicals and

Isomerization of n-Butane.

Masters Thesis Carried out under the supervision of Prof. Tapio Salmi, Prof. Dmitry Yu. Murzin and

Docent Narendra Kumar, Laboratory of

Industrial Chemistry, Process Chemistry

Centre, Department of Chemical

Engineering, Faculty of Technology,

bo Akademi University, 2005-2006.

Keywords Synthesis of zeolites, MCM-22, MCM-36, MCM-48, metal modification, Pd, Au,

ultrasound, n-butane isomerization,

lactose oxidation, deposition precipitation

method.

Iso-butane formed by isomerization of n-butane is an essential ingredient in

refinery production of alkylates and oxygenated compounds such as tertiary butyl

alcohol (TBA), methyl tertiary butyl ether (MTBE), isooctene, and polyisobutene

rubber. In recent years, the technology for the isomerization of normal butane (n-

C4) to isobutane (i-C4) has become increasingly important for motor fuel

applications. Isobutane is a primary feedstock for motor fuel alkylation processes,

which produces an excellent and environmentally superior gasoline-blending

component.

The modification of zeolites is of very great importance both from an industrial

and academic point of view because of the potential applications of metal and

ii

proton modified zeolite catalysts in the process of petroleum and in the chemical

industry.

The scope of the thesis was to investigate different catalysts preparation

methods. Proton forms of MCM-22 with varying acidities and palladium modified

MCM-22 catalysts with 28, 30, 50, and 70 (silica to alumina) ratios are

synthesized, characterized and tested at the laboratory. To investigate the

influence of acidity over isomerization of n-butane and lactose oxidation were

selected as test reactions. The effect of synthesis time of MCM-22-30 using

ultrasound irradiation method, and effect of palladium metal form of zeolites with

different catalyst preparation methods for applications in fine chemicals i.e.

oxidation of lactose had been investigated. Synthesis of MCM-36 with different

MCM-22 precursors and as well as preparation of MCM-48, which is difficult to

synthesize, and also its reproducibility are also investigated in this thesis work.

Acidity of MCM-22 catalytic materials which can be adjusted by silica to alumina

ratio was found to influence activity and yield of isobutane in n-butane

isomerization. H-MCM-22-30-R was found the most promising catalyst and more

active catalyst compared to Pd form catalysts in n-butane isomerization and in

the case of lactose oxidation, the Pd-impregnated catalysts i.e. MCM-22 exhibits

similar catalytic activity and Pd-H-MCM-22-30-IMP exhibits high active compared

to other preparation methods of Pd-H-MCM-22-30 in lactose oxidation.

The stability of the catalysts in the conversion of n-butane, yield and selectivity of

iso-butane were investigated with Pd-H-MCM-22-28-I.E catalyst with different

weight hourly space velocities and also effect of temperature.

iii

REFERAT

Seelam Prem Kumar Syntes, karakterisering och testning av metall- modifierade zeolitkatalysatorer fr

finkemika-lie applikation och isomerisering av

n-butan.

Diplomarbete Arbetet utfrdes under handledning av Prof. Tapio Salmi, Prof. Dmitry Yu. Murzin och

Docent Narendra Kumar, vid Laboratoriet fr

Teknisk Kemi, Processkemiska forskargrupp-

en, Kemisk-tekniska fakulteten, bo Akademi

2007.

Nyckelord Zeolitkatalysatorer, syntes, MCM-22, MCM-36, MCM-48, metallmodifiering, Pd, Au,

ultraljud, n-butan isomerisering, laktos

oxidering.

Isobutan, en isomeriseringsprodukt av n-butan, r en viktig frening fr

produktion av alkylat och syrehaltiga komponenter, s som tertirbutylalkohol

(TBA) och metyltertir butyleter (MTBE), samt iso-okten och polyisobutengummi.

Under de senaste ren har teknologin fr isomerisering av n-butan till isobutan

blivit alltmer viktig. Isobutan r en viktig rvara fr alkyleringsprocesser som

producerar utmrkta och miljvnliga bensinkomponenter. Modifiering av zeoliter

genom protonering eller metalltillsats r viktig ur srl ur industriell som

akademisk synvinkel.

Avsikten med diplomarbetet var att underska olika metoder fr syntes av

katalysatorer. Protonerade MCM-22 med olika surhets grad, kisel till aluminium

frhllande 28, 30, 50 och 70 syntetiserades och modifierades med palladium.

iv

Drefter karaketeriserades och testades katalysatorerena i laboratoriet. Till

testreaktioner fr att studera effekten av surhet valdes isomerisering av n-butan

och oxidering av laktos. Syntestidens effekt studerades med hjlp av

ultraljudsbehandling av MCM-22-30. Olika palladiumtillsatsmetoder p zeoliter

studerades med hjlp av laktosoxidation som exempel p finkemikalieapplikation.

Syntesen av MCM-36 utfrdes frn olika MCM-22 material. Den ytterst krvande

syntesen av MCM-48 samt reproducerbarheten av denna, ingr ven i detta

diplomarbete.

Olika surheter av MCM-22 katalysatormaterial pverkade aktiviteten och utbytet

av isobutan och isomeriseringen av n-butan. Den protonerade formen av H-

MCM-22-30-R hade hgre aktivitet n Pd-formen i isomeriseringen av n-butan.

Bda formerna uppvisade liknande aktiviteter i oxidationen av laktos.

Katalysatorstabiliteten, konversionen av n-butan, utbytet av och selektiviteten till

isobutan studerades med Pd-H-MCM-22-28-I.E under olika fldeshastigheter och

olika temperaturer.

v

PUBLICATION RELATED TO THE TOPIC 1. A.V.Tokarev, E.V. Murzina, Prem k. Seelam, A.J. Plomp, J.H.Bitter, N.Kumar, D.Yu.

Murzin, Influence of support nature on the catalytic activity of Pd catalysts in lactose

oxidation 2006 (submitted).

2. N. Kumar, O. Russu, P. Seelam, T. Heikkil, V.-P. Lehto, H. Karhu, T. Salmi, D.Yu.

Murzin, Pt modified MCM-22, ZSM-5 and beta zeolite catalysts for n-butane

isomerization: influence of structure, acidity and Pt modification, Studies in Surface

Science and Catalysis (TOCAT-5) (accepted).

vi

CONTENTS Preface ........ i Abstract .......... ii Referat......................................................................................................................... iv Publication related to the topic................................................................................. vi Contents .................... vii 1. Introduction ...... 1

1.1 Isomerization of n-butane........1

1.2 Fine chemicals ........ 4

1.3 Oxidation of lactose ........... 5

2. Background and theory ........ 6 2.1. Introduction to zeolites ........... 6

2.1.1 Definition ........ 6

2.1.2 Framework and structure..... ....... 6

2.1.3 History ........... 8

2.1.4 Classification of zeolites ...... 9

2.1.5 Shape selectivity ............. 11

2.1.6 Acidity .... 14

2.1.7 Synthesis of zeolites: pre and post synthesis methods...... 15

2.1.8 Mesoporous molecular sieves .... 19

2.1.9 Zeolite pore structure and active sites... 22

2.1.10 Zeolites in industrial applications ....... 24

3. Catalyst preparation methods............. 26 3.1 Ion exchange method ...... 26

3.2 Impregnation method ....... 28

3.3 In-situ method ....... 30

4. Characterization methods ............ 31 4.1 Surface area measurement method...................................... 31

4.2 X-ray powder diffraction method .............. 32

4.3 Scanning electron microscopy.......... 33

4.4 Fourier transform infra red spectroscopy..........34

vii

4.5 Direct current plasma....... 36

5. Overview on MCM-22 and MCM-36 ............. 36 6. Overview on MCM-48 ............. 41 7. Overview on Ultra sound irradiation method .............. 42

7.1 Introduction ....... 42

7.2 Applications ...... 43

8. Experimental Procedure ........... 44 8.1 Catalysts ....... 44

8.2 Synthesis of catalysts ..... 47

8.3 Proton form catalysts........... 50

8.4 Metal modification .......... 51

8.5 Catalysts characterization . 54

8.5.1 Surface area measurement .... 54

8.5.2 X-ray powder diffraction... 55

8.5.3 Scanning electro micrograph...... 55

8.5.4 Direct current plasma... 55

8.5.5 Fourier transform infrared spectroscopy ...... 55

9. Catalytic Testing ........... 56 9.1 Isomerization of n-Butane ..... 56

9.2 Oxidation of lactose ........ 59

10. Results and Discussions ............ 60 10.1 Catalyst synthesis and characterization results ....... 60

10.2 Catalyst testing in n-butane isomerization and lactose oxidation....... 79

10.2.1 Influence of acidity in isomerization of n-butane .......... 79

10.2.2 Influence of palladium form of H-MCM-22

in isomerization of n-butane ....... 80

10.2.3 Effect of temperature......................................................................... 84

10.2.4 Effect of space velocity on isomerization of n-butane ... 87

10.2.5 Influence of catalysts preparation methods on lactose oxidation.. 90

10.2.6 Influence of SiO2/Al2O3 ratio on lactose oxidation.....93

viii

11. Conclusion ............... 96 12. References ............ 98

APPENDIX-I--------------------------------------------------------------------------------------------102

ix

REFERAT

Seelam Prem Kumar Syntes, karakterisering och testning av

metallmodifierade zeolitkatalysatorer fr

finkemikalieapplikation och isomerisering av

n-butan

Diplomarbete Arbetet utfrdes under handledning av Prof.

Tapio Salmi, Prof. Dmitry Yu. Murzin och

Docent Narendra Kumar, vid Laboratoriet fr

Teknisk Kemi, Processkemiska forskargruppen,

Kemisk-tekniska fakulteten, bo Akademi

2007.

Nyckelord Zeolitkatalysatorer, syntes, MCM-22, MCM-36,

MCM-48, metallmodifiering, Pd, Au, ultraljud,

n-butan isomerisering, laktos oxidering

Isobutan, en isomeriseringsprodukt av n-butan, r en viktig frening fr produktion av

alkylat och syrehaltiga komponenter, s som tertirbutylalkohol (TBA) och

metyltertirbutyleter (not sure) (MTBE), samt iso-okten (not sure) och

polyisobutengummi. Under de senaste ren har teknologin fr isomerisering av n-butan

till isobutan blivit alltmer viktig. Isobutan r en viktig rvara fr alkyleringsprocesser,

som producerar utmrkta och miljvnliga bensinkomponenter. Modifiering av zeoliter

genom protonering eller metalltillsats r viktig ur srl industriell som akademisk

synvinkel.

Avsikten med diplomarbetet var att underska olika metoder fr syntes av katalysatorer.

Protonerade MCM-22 med olika surhets grad, Si/Al (is Si/Al correct?) frhllande 28,

30, 50 och 70 syntetiserades och modifierades med palladium. Drefter karaketeriserades

och testades katalysatorerena i laboratoriet. Till testreaktioner fr att studera effekten av

surhet valdes isomerisering av n-butan och oxidering av laktos. Syntestidens effekt

studerades med hjlp av ultraljudsbehandling av MCM-22-30. Olika

palladiumtillsatsmetoder p zeoliter studerades med hjlp av laktosoxidation som

exempel p finkemikalieapplikation.

Syntesen av MCM-36 utfrdes utgende frn olika MCM-22 material. Den ytterst

krvande syntesen av MCM-48 samt reproducerbarheten av denna, ingr ven i detta

diplomarbete.

Olika surheter p MCM-22 katalysatormaterial pverkade aktiviteten och utbytet av

isobutan och isomeriseringen av n-butan. Den protonerade formen av H-MCM-22-30-R

hade hgre aktivitet n Pd-formen i isomeriseringen av n-butan. Bda formerna

uppvisade liknande aktiviteter i oxidationen av laktos.

Katalysatorstabiliteten, konversionen av n-butan, utbytet och selektiviteten till isobutan

studerades med Pd-H-MCM-22-28-I under olika fldeshastigheter och olika

temperaturer.

1

1. Introduction 1.1. Isomerization of n-Butane

Transformation of hydrocarbons is of prime economic importance in the

petrochemical industry. Generally, the transformation occurs under strong acidic

conditions. For example, liquid super acids such as SbF5/HF or SbF5/HSO3F can activate alkanes at temperatures below 273 K. However, the drawback of liquid

acids is that they are corrosive and difficult to recover and reuse. As a

replacement of liquid acids, solid acids exhibit a promising alternative because of

their environmental friendly characteristics (non-corrosiveness, ease of handling,

and easy to recover and reuse) [1]. Butane can be obtained from catalytic and

steam crackers of oil refineries and petrochemical plants. The isomerization of n-

butane to isobutane is an important reaction for the production of alkylates and

oxygenated compounds such as tertiary butyl alcohol (TBA), methyl tertiary butyl

ether (MTBE), isooctene, polyisobutene (PIB) and polyisobutene rubber.

In the petrochemical industry, halogenated alumina and comparable catalysts

containing precious metals, as well as organic chlorides have been used for this

reaction in order to reach the necessary acidic strength. Unfortunately, these are

non-environmental-friendly catalysts and, therefore, there is a great interest

among academic and industrial researchers to find and develop new active,

selective and resistant-to-deactivation catalysts. The main problem associated

with the use of strongly acid zeolites, e.g. H-Mordenite, as isomerization catalysts

is the fast deactivation [5, 6]. Zeolites such as ZSM-5, beta, and Mesoporous

molecular sieves as well a sulfated zirconia have been reported to be active

catalysts in the isomerization of n-butane. The stability and activity of the

catalysts have been improved by the introduction of metals combined with the

usage of hydrogen as a carrier [2]. Much discussion has been going on in the

literature whether the apparently simple isomerization of n-butane to isobutane

goes via a bi- or monomolecular mechanisms. The relative importance of these

2

mechanisms depends on the reaction temperature and the surface concentration

of reactants determined by temperature, reactant pressure and concentration of

acid sites. Skeletal isomerization of n-butane over zeolite catalysts is proposed

(see Figure 1) to proceed via a bimolecular dimerization-cracking route. The first step is the formation of the butylcarbenium ion from butane by: (i) protonation of

butane by a Brnsted acid site to form a penta-coordinated butylcarbocation eq.

(1) and subsequent abstraction of H2, eq. (2), (3) (ii) hydride abstraction by a

Lewis acid site eq. (3) and (iii) protonation of trace olefins formed eq, (4). The

butyl carbenium ion reacts with a olefin (butene) forming an octylcarbenium ion,

which yields n- and isobutane after isomerization and beta-scission. Byproducts,

mainly propane and pentanes, are obtained from the C8 intermediates after

disproportionation. The formation of olefin species is important for the skeletal

isomerization of n-butane, since they are needed in the dimerization step [3].

However, the possibility of the n-butane isomerization also via a monomolecular

mechanism has been proposed by Tran et al. [4]. The bimolecular mechanism

shown below is a simplified version of that used to describe n-butane

isomerization.

n-C4H10 C4H9+ + H .. (1)

C4H9+ C4H8 + H+. (2)

H+ + H H2 .. (3)

C4H8 + H+ C4H9+ (4)

C4H9+ + C4H8 C8H17+ i-C4H10 ..... (5)

Figure 1. Reaction mechanism of isomerization of n-butane (68).

3

The primary reactions (1.1-1.4) in butane conversion may involve cracking

of butane into propylene and methane or ethylene and ethane, disproportionation

of butane into pentane and propane, and isomerization of n-butane into iso-

butane. Butane disproportionation is more dependent on strong acid site density

of a catalyst than butane cracking (mono-molecular reaction) since two adjacent

acidic sites are needed to start disproportionation (bi-molecular reaction) [81].

Ethylene, propylene, propane and pentanes formed from the primary reactions

may undergo further reactions. The secondary reactions (1.5-1.12) may involve

oligomerization, isomerization, cracking, hydrogen transfer, dehydrocyclization,

aromatization, and coking (which leads to catalyst deactivation), etc. At low

conversions, the primary reactions will be predominant.

Primary reactions: n-C4H10 C3H6+CH4 (cracking) ................................. 1.1a

n-C4H10 C2H4+ C2H6 (cracking) ................................. 1.1b

2 n-C4H10C5H12+C3H8 (disproportionation) ............... 1.1c

n- C4H10i-C4H10 (isomerization) ................................. 1.1d

Secondary reactions: i- C5H12 C3H8 + C2H4 (cracking) ................................ 1.1e

C3H8 C2H4+CH4 (cracking) ........................................ 1.1f

2 C2H4n-C4H8 (oligomerization) ................................. 1.1g

C2H4+C3H6i-C5H10 (oligomerization) ...........................1.1h

i-C4H8 +i- C5H12i- C4H10+i-C5H12 (hydrogen transfer)...1.1i

n- C4H8i-C4H8 (isomerization).......................................1.1j

C6H12 C2H4+ C4H8 (cracking) .......................................1.1k

Olefins Aromatics + H2 (dehydrocyclization) ................1.1l

In the temperature range, except for cracking reactions (Reactions 1.1a,

1.1b, 1.1e, 1.1f, 1.1k) that are very endothermic; the other reactions are

exothermic or have small heats of reaction. Oligomerizations (Reactions 1.1g

4

and 1.1h) have large exothermic heats of reaction. The effect of temperature on

the equilibria of the reactions is different. For cracking reactions, a high

temperature is favored; while for oligomerization, a low temperature is favored.

For hydrogen transfer reactions and isomerization reactions, the effect of

temperature on reaction equilibrium is not as significant as on the other reactions

[81].

The zeolites to be used in skeletal isomerization of n-butane should have at least

10-membered-ring channels (0.45-0.6 nm), if the molecular sizes of reactant n-

butane (critical diameter 0.49 nm) and the product isobutane (critical diameter

0.56 nm) are taken into consideration. The channels of 8-membered-ring

zeolites (0.35-0.45 nm) are too small to allow branched molecules to diffuse with

any considerable rate, which results in a rapid deactivation of the catalyst.

1.2. Fine Chemicals Zeolites are important catalysts in a multitude of contemporary chemical

production processes. A continually increasing number of applications is

emerging in the production of organic intermediates and fine chemicals. In drug

manufacture, fine chemicals are pure, single chemical substances that are

produced by chemical reactions. Examples of fine chemicals are intermediates

for drug production and bulk active pharmaceutical ingredients ready to be

compounded with inert pigments, solvents and excipients and made into dosage

forms [7]. Most of the fine chemicals are used in the manufacturing of life saving

drugs; in order to prepare the fine chemicals we need catalysts with high activity

and selectivity towards desire product.

5

1.3. Oxidation of Lactose

Lactose oxidation is a consecutive reaction, resulting first in lactobionic acid, and

then in 2-keto-lactobionic acid (Figure 2). From industrial viewpoint this reaction is interesting because lactobionic acid possesses useful properties and can,

thus, be used as acidulant, complexing agent or antioxidant in food, pharmacy

and medicine. Lactose, an abundant disaccharide, is a by-product of dairy

industry available in big amounts [26-28].

Figure 2. Reaction of lactose oxidation.

Oxidation of sugars is very sensitive to several factors such as pH of reaction

media, since the formation of corresponding carbohydrate acids results into big pH

OO

O

OH

OH

OH

OH

OH

OH

OH

H

OH

OOCOONa

OH

OH

OH

OH

OH

OH

OH

OOCOONa

OH

OH

OH

OH

OH

OH

OH

OH

OOOH

OH

OH

OH

O

O

HO

OH

OH

Lactose Lactobionic acid (LBA), sodium salt

Lactulose 2-keto-Na-Lactobionate

IsomerizationOH-

O2

O2

OH

6

change. Another important parameter is the oxygen feed rate, since the reaction

rate can be retarded either due to the lack or due to oversupply of oxygen (i.e.

oxygen poisoning) at high oxygen feed rate.

2. Background and Theory

2.1 Introduction to zeolites

2.1.1. Definition

Zeolites are crystalline aluminosilicates characterized by a structure that

comprises a three-dimensional and regular framework formed by linked TO4

tetrahedral (T= Si, Al, etc), each oxygen being between two T-elements. This

tetrahedron is the fundamental building unit of all zeolites. These simple

tetrahedra are combined in a complex way to form secondary building units

(SBU), forming the building block of the different framework structures of zeolite

crystals [27].

Classical zeolite structures are composed from primary units of AlO4 and SiO4

and polyhedra with secondary units to form frame network with Si-O-Al atoms. At

the movement there are over 170 types of zeolites in which 50 are naturally

occurring zeolites. The aluminum atoms, particularly in high silica zeolites, may

not be uniformity distributed, often zoning effect is observed.

2.1.2. Framework and Structure

A defining feature of zeolites is that their frameworks are made up of 4-

connected networks of atoms (see Figure 3). One way of thinking about this is in terms of tetrahedra, with a silicon atom in the middle and oxygen atoms at the

7

corners. These tetrahedra can then link together by their corners (see illustration)

to from a rich variety of beautiful structures. The framework structure may contain

linked cages, cavities or channels, which are of the right size to allow small

molecules to enter - i.e. the limiting pore sizes are roughly between 3 and 10 in

diameter.

In all, over 130 different framework structures are now known. In addition to

having silicon or aluminum as the tetrahedral atom, other compositions have also

been synthesized, including the growing category of microporous

aluminophosphates, known as ALPOs [34].

Figure 3. Frame work of aluminosilicates in zeolites (78).

Figure 4. Zeolite structure (77) (ZSM-5). Figure 5. Zeolite cage (77).

The structure has channels and cavities with molecular sizes (Figures 4 and 5) that can host the charge-compensating cations, water or other molecules and

salts. A schematic representation of a chain of tetrahedral is shown in Fig. 3. In a real crystal, however, all four oxygen atoms are bridging atoms except where the

macromolecule terminates at the crystal faces, in which case a proton

8

coordinates to the non-bridging oxygen atom. Each tetrahedron containing

aluminum formally has one unit of negative charge because the aluminum atom

has a formal charge of +3 and each oxygen atom has a formal charge of -2.

There are enough metal cations, such as Na+, K+, Ca+2, Mg+2 or Sr+2, present in

the interstices of the aluminosilicate framework to make the crystal electrically

neutral. These cations are usually mobile and are responsible for the ion

exchange with another metal.

2.1.3. History of zeolites

The discovery of zeolite catalysts in the late 1950s stimulated the interest of

chemists, physicists, and engineers in the study of zeolite catalysts and reactions

catalyzed by them and before the discovery of zeolites in catalysis, there is much

information in the history (see Table 1) which tells about the application of zeolite materials.

Table 1. History of Zeolites (38).

Date Facts of zeolite History

1756 Zeolite mineral identified

1850 Ion-exchange properties

1858 Reversible adsorption/Desorption of water

1862 Synthetic zeolite

1926 Molecular sieves defined

1929 Modern concepts of zeolite structure identification

1948 Synthetic zeolite without natural occurrence

1953 Synthetic zeolite commercialized

1967 First International Conference on Zeolites, foundation of IZA

1972 High silica zeolites

1979 Pillared clays

1982 Aluminophosphates

1991 Mesoporous molecular sieves

9

2.1.4. Classification of zeolites

The natural or synthetic zeolites are hydrated micro porous molecular sieves

(

10

several purposes, such as oil dewaxing and ethylbenzene production. In 1980s

and 1990s, zeolites and molecular sieves were used for several petroleum-

refining applications particularly for hydrocarbon cracking and production of

octane-enhancement additives. The zeolites are also classified into different

groups depending upon the silica to alumina ratio as mentioned in Table 2.

Table 2. Zeolites classification (74) (depends on silica to alumina ratio).

Si/Al ratio Zeolite Properties

Low(1-1.5) A,X Relatively low stability of

framework, low stability in acids,

High stability in bases ,

High concentration of acid groups

with moderate acid strength

Hydrophilic

Intermediate(2-5) Erionite,

Chabazite,

Clinoptilolite,

Mordenite

High(10-infinite) ZSM-5, Erionite,

Mordenite,MCM-

41,MCM-22,MCM-

36, etc.

Relatively high stability of frame

work, high stability in acids and

low stability in bases.

The effect of silica to alumina ratio on physicochemical properties of the zeolites

as explained:

a. Increasing SiO2/Al2O3 ratio affects the following physical properties of the

zeolite:

11

Increases acid resistance Increases thermal stability Increases hydrophobicity Decreases affinity for polar adsorbents Decreases cation content.

b. Decreasing SiO2/Al2O3 ratio affects following physical properties of the

zeolite:

Increases hydrophilicity Increases cation exchange properties Decreases the pore size for same numbering ring, as Al has

lower atomic radius than Si.

2.1.5. Shape selectivity

The shape selectivity provided by the microporous crystalline structures of

zeolites is of crucial importance in hydrocarbon transformation. The rate of

deactivation by coke largely depends on the shape selectivity. The crystalline

structures of zeolites can be altered to a large extent achieving desired shape

selective properties. Three different types of shape selectivity are observed over

zeolites (shown in Figure 6) and summarized shortly here [32].

1. Reactant selectivity occurs when some of the molecules in the reactant

mixture are too large to diffuse through the catalyst pores.

2. Product selectivity occurs when some of the products formed within the

pores are too bulky to diffuse out. The bulky molecules are either converted to

less bulky molecules or coked, that eventually deactivates the catalyst.

3. Restricted transition-state selectivity occurs when certain reactions are

prevented because the corresponding transition-state would require more space

12

than the available in the cavities or in the pores. Neither the reactant nor the

products are prevented from diffusing through the pores.

The most important type of shape selectivity in the skeletal isomerization of n-

butane is the restricted transition-state selectivity (Figure 6). Concerning the restricted transition state-type selectivity, the lower transition state molecule is

easier to accommodate in the cavities than the upper one.

Figure 6. Schematic representation of the three types of shape-selectivity (70).

13

The Figure 7 gives the idea of the shape selectivity of the molecules entering into the zeolite pores which depends upon the size and shape selectivity of the

molecules. There are in principle as many different shapes, as shown in Figure 7,

and dimensions of intracrystalline cavity or channels, as there are zeolite

topologies. All molecular sieves are classified according to their dimensions (pore

diameter) into microporous (pore mouth less than 2 nm) and macroporous (pore

mouth more than 2 nm) materials. Zeolites are microporous molecular sieves and

are also divided in small, medium or large pores.

Figure 7. Shape-selective environments in different zeolite structure types (71): (a) large molecules have access to interrupted cavities and channel intersections

for pore mouth catalysis; (b) molecules are plugged into the pore aperture; (c)

molecules are converted in multiple pore mouths according to key-lock catalysis;

(d) molecules are converted in the intra-crystalline shape-selective environment.

14

2.1.6. Acidity

Zeolites and zeolite-type catalysts are usually acidic according to both definitions

of acidity e.g. Brnsted (proton donor) and Lewis (electron pair acceptor) acidity.

In hydrocarbon transformation, Brnsted acid sites (Figure 8) are considered to be more important. The proton form of a molecular sieve containing Brnsted

acid sites can easily be obtained from the ammonium form by calcination.

However, Brnsted acid sites can be present already in the as-synthesized

molecular sieves, as determined by the choice of the template [1]. The number of

acid sites is proportional to the concentration of the framework trivalent T-atoms.

There are, however, factors that affect the acid strength, like the nature of

trivalent T-atom and the Si/T-atom ratio in the framework. The acidity decreases

in the order: Al>Ga>Fe>>B. Brnsted acid sites in zeolites account for their

catalytic properties and can be investigated by NMR, FTIR or TPD

measurements.

Figure 8. Brnsted acid site as present in a zeolite (72).

15

2.1.7. Synthesis of zeolites

Zeolites are crystalline aluminosilicates with the general formula Mn/2.O.Al2O3

.ySiO2. By varying the "template" molecule added to an aqueous solution of

mineralized silica (and co metal) nearly 100 different zeolites have been

synthesized. Other synthetic variables including the source of the inorganic

precursors, the mineralizing agent (OH-, or F-) and the reactant concentrations

have also resulted in new crystalline materials. Various zeolites are synthesized

with varying the Si/Al ratio with minute value 1 to infinite.

a. Pre-synthesis Method

Aluminosilicates zeolites are formed by hydrothermal synthesis, typically under

mild conditions. Zeolites are synthesized by mixing a silica source (SiO2), an

alumina source (Al2O3), organic template, mineralizing agent (OH- anion, NaOH)

and water as solvent to form an aluminosilicate gel.

The nucleation and crystallization of the gel takes place in an autoclave (shown

in Figure 11). The zeolite crystals that are formed are filtered, washed and dried. Washing is required to reduce the alkalinity because of the addition of NaOH.

In order to remove the organic template, zeolites are typically calcined after

drying. Different calcinations techniques have an effect on the properties of the

synthesized zeolite. After calcinations the zeolite is in the so called sodium form,

denoted by Na at the beginning of the e.g. Na-ZSM-5 because it contains sodium

ions to balance the framework charge induced by Al as T-atoms.

Zeolite crystallization mainly depends on the sources of silica and alumina,

temperature, pressure, synthesis time, solution pH, composition of the gel and

the nature of the organic templates, pretreatment of the reactants, inclusion of

16

special additives, homogeneity or heterogeneity of the reactant mixture as well

as seeding effects.

The pH of the solution is an important parameter in the zeolite synthesis.

Increasing the alkalinity at constant temperature influences the kinetics of zeolite

crystallization in the same way as increasing the temperature at constant

alkalinity does. The induction time decreases strongly and the crystal growth is

accelerated with an increase in pH [14]. The decrease in the nucleation time and

enhanced rate of crystal growth with rising pH can be attributed to the much

greater concentrations of dissolved species. The induction time in zeolite

crystallization decreases rapidly with increasing temperature, up to a certain

point. Alkalinity also has effect on the Si:Al ratio of the zeolite [16]. Since OH-

ions serve as a mineralizing agent, the pH of the gel solution is of utmost

important. The OH ions bring the Si and Al oxides or hydroxides into the solution

at a particular rate. Crystallinity of zeolites increases with time and usually the as-

synthesized form of zeolite is not the most suitable for the catalytic purpose.

Template makes the stability, control the formation of a microporous framework

structure (see Figure 9) of the zeolite and then the template can be removed. Thereafter, the microporous voids channels and cavities are created by

calcination. A large number of organic molecules can be used as templates for

zeolites synthesis. Typical templates are alkyl ammonium cations R4N+, alkyl

phosphates (R4P+) and organic complexes, which act as structure directing

agents and help in the formation of the zeolite lattice. The templates contribute to

the stability by forming new bonds such as hydrogen and electrostatic bonds,

and assist in the formation of the particular structure through their form and size.

While choosing a template for the synthesis of a zeolite, important properties of

the template such as its solubility, stability during the synthesis, steric

compatibility, framework stabilization and removal of the template without

destroying the frame work structure of the zeolite must be taken into account.

The structure directing agents can be organic or inorganic compounds. They are

17

cations, ion pairs and neutral molecules. Of the three species, cations are the

most important because they not only acts as structure directing agents but also

affect the rate of zeolite crystallization. The mechanism of the zeolite synthesis is

illustrated in the Figure 11.

Figure 9. Zeolite pore structure formation (72).

Generally used silica, alumina and template sources for the synthesis of zeolites

as mentioned below.

1. Silica Sources: Tetra methyl orthosilicate (Si(OCH3)4), Tetra ethyl orthosilicate (Si(OC2H5)4), waterglass (Na2SiO3.9H2O), Na2O 11%, SiO2

29%, Ludox-AS-40 (colloidal silica), Fumed silica, Aerosil-200, etc.

2. Alumina Sources: sodium aluminate (NaAlO2) 54% Na2O, Al2O3,

Al (OH)3 aluminum hydroxide Gibbsite, etc.

18

3. Templates: Alkyl ammonium cations R4N+, alkyl phosphates (R4P+) and

organic complexes, and also inorganic cations like Na+, Li+, K+, etc.

b. Post synthesis method

The sodium form of zeolite is made to acidic form by ion-exchanging sodium

cations with NH4Cl or NH4NO3 to achieve an ammonium form (Figure 10). The ammonium form of zeolite is dried and calcined to obtain the proton form of

zeolite i.e. acidic form of zeolite.

Figure 10. Zeolite synthesis flow sheet (i.e. proton form).

Sodium form of zeolite formed (Na-zeolite form, parent zeolite)

Drying in order to remove water molecules at 100 oC and then calcination process to remove template or surfactant.

Nucleation and crystal growth takes place in autoclave at 150oC, 1-7 days

Sodium form to proton form with 1M NH4Cl sol. by ion exchange and then dried and calcined to remove ammonia molecules to get proton form.

Silica source + alumina source + NaOH (mineralizing agent) + Organic template ----------- leads to gel formation

19

Figure 11. Synthesis mechanism of zeolite (73).

c. Present day research on zeolites

Synthetic goals in research include the optimization of reported syntheses,

variation of the composition of zeolites with known topologies and understanding

the influence of synthetic conditions like synthesis time, silica to alumina ratio,

etc. that favor the formation of these low-symmetry crystalline phases and

moreover the high quality zeolites should be more feasible, high porosity, eco-

friendly, easy handling, easy recovery, less expensive, high thermal stable, high

activity, longer life and high selectivity.

2.1.8. Mesoporous molecular sieves

Mesoporous materials are those with pores in the range 2-50 nm in diameter.

They have huge surface areas, providing a vast number of sites where sorption

20

processes can occur. These materials have numerous applications in catalysis,

separation and many other fields. The synthesis of these materials is of

considerable interest and is constantly being developed to introduce different

properties. Molecular sieves are porous solids with pores of the size of molecular

dimensions, 0.3-2.0 nm in diameter or more. Examples include zeolites, carbons,

glasses and oxides. Some are crystalline with a uniform pore size delineated by

their crystal structure, e.g., zeolites, while others are amorphous carbon

molecular sieves. The most common commercial molecular sieves are zeolites. Mobile composite of matter (MCM) materials are mesoporous templated

molecular sieves (Figure 12). M41S are typically semi crystalline mesoporous silicates and aluminosilicates with pores 2-10 nm related to phyllosilicate

minerals, imogolite and alophane [19]. MCM-22, MCM-36, MCM-48 and MCM-50

are the members of mesoporous with high surface areas.

Figure 12. Ordered mesoporous materials

Amongst the current developments in the field of hierarchical pore structures, the

creation of mesopores in zeolite crystals is the most frequently employed way to

combine micropores with mesopores in one material. There are different

approaches to generate and characterize mesopores in zeolite crystals and

establish their impact on the catalytic action with better mass transport.

Mesopores can be created (see Figure 13) via several routes from which steaming and acid leaching are the most frequently applied. Novel approaches

using secondary carbon templates that are removed after syntheses have

recently been launched. For the characterization of mesopores, nitrogen

physisorption and electron microscopy are commonly used. More recently, it was

shown that electron tomography, a form of three-dimensional transmission

21

electron microscopy, is able to reveal the three-dimensional shape, size and

connectivity of the mesopores. The effect of the presence of mesopores for

catalysis is demonstrated for several industrially applied processes that make

use of zeolite catalysts in the cracking of heavy oil fractions and synthesis of fine

chemicals over zeolite Y, and the production of cumene and hydroisomerization

of alkanes over mordenite. For these processes, the mesopores ensure an

optimal accessibility and transport of reactants and products, while the zeolite

micropores induce the preferred shape-selective properties [38-39].

Figure 13. Schematic representation of the assembly of zeolite nanocrystals to a mesoporous structure (31).

The fundamental reagents used in the synthesis of mesoporous templated

molecular sieves are

1. The long chain quaternary ammonium ions used as surfactants form micelles or

liquid crystals in aqueous solutions.

2. The silicate or aluminosilicate species, reflecting the micellar array condense and

polymerize around the hydrophilic parts of the surfactant.

3. Condensed inorganic species aggregates to form the walls of the porous solid.

4. Calcination of the materials removes the organic templates. This procedure

improves the stability of the structure.

22

2.1.9. Zeolite pore structure and active sites

The adsorption and catalytic process is over zeolites involve diffusion of

molecules in the zeolites pores, only those with a minimum of 8 tetrahedral

atoms apertures allowing this diffusion are generally considered for the particular

reactions [12], for e.g.:

i. Small pore zeolite with eight membered ring pore aperture having free

diameters 0.3-0.45 nm.

ii. Medium pore zeolites with ten membered ring apertures 0.45-0.60 nm in free

diameter.

iii. Large pore zeolites with 12 membered ring apertures 0.6-0.8 nm as

represented in Figure 14.

Figure 14. Pore openings of common zeolites with 12, 10 and 8 member ring structures (74).

Active sites in zeolite pores act as an acid, acid base, or bifunctional catalysis.

For example in fluid catalytic cracking, a catalyst containing acid sites e.g., FAU

zeolite is applicable. Also methanol conversion to olefins reactions, acetylation

reactions etc, need acid sites. The hydrocarbon reactions as well as many

transformations functionalized compounds are catalyzed by protonic sites only.

23

The maximum number of protonic sites is equal to the number of framework

aluminum atoms. The number of protonic sites can be adjusted either during the

synthesis or during post synthesis pretreatment of the zeolite: dealumination, ion-

exchange, etc. The parameters determining the acid strength of the zeolite

protonic sites are important in catalytic applications. The first feature of zeolites is

their stronger acidity compared to amorphous aluminosilicates. A relation exists

between the T-O-T bond angles and the acid strength of the associated proton in

the zeolites. Hence, the greater the angle, the stronger the active sites in

zeolites. The protonic sites (bond angles) of the zeolites are influenced by the

basicity of the reactants and the temperature will also play a role. Some of the

zeolites catalysts acidity measurements using pyridine adsorption are presented

in Table 3. For example, an acid site giving surface hydroxyl groups (-OH groups) i.e. the Brnsted acid site, which are more selective property for a zeolite

in acid catalysis. The higher Al atoms content in zeolite framework is proportional

to higher Brnsted acid sites (maximum acidity at Si/Al = 9-12).

Table 3. Brnsted and Lewis acid sites of the different zeolite catalysts (74).

S.No Catalyst Brnsted acid

sites (mol/g.cat)

Lewis acid sites

(mol/g.cat)

1 H-Beta-11 183 128

2 Mordenite 294 109

3 H -Y 291 165

4 H-MCM-41 89 168

5 H-MCM-22 187 175

6 Silica 0 7

7 Alumina 7 156

24

2.1.10. Zeolites in Industrial Applications

Applications of zeolites are in three main areas [13]:

i. Ion exchange: It is one of the important properties of the zeolites. The most

important end application of synthetic zeolites is in detergents. Zeolites replaced

phosphates builders that are caused algae overgrowth and consequently oxygen

deficit in water resources. Zeolites in sewage waters accumulate and immobilize

phosphates etc.

ii. Adsorption: zeolites are important industrial adsorbents for the separation of

both gases and liquids. They possess excellent capacity to remove volatile

organic chemicals from air streams and to separate isomers and mixtures of

gases.

iii. Catalysis: with the use of X and Y zeolites in isomerization and cracking,

hydrocarbon transformation, etc. Many industrial applications are presented in

the Table 4.

25

Table 4. Application of zeolites in various industries (10).

Zeolite/microporous material

Process or application technology

LTA (A-type zeolites) Detergent builder, separation, desiccation

FAU(X-and Y-type

zeolites)

Catalytic cracking, hydrocracking, separation,

Purification and desiccation, aromatic alkylation.

BEA (Beta zeolite) FCC additive, cumene and ethylbenzene production.

MOR (Mordenite) Hydrocracking, hydroisomerisation, dewaxing, NOx

reduction, adsorption, cumene synthesis, transalkylation of

aromatics.

MWW (MCM-22) Ethyl benzene and cumene production, Isomerization etc.

MFI (ZSM-5) Dewaxing, hydrocracking, ethyl benzene (Mobil-Badger) and

styrene production, xylene isomerisation, methanol to

gasoline (MTG), benzene alkylation, adsorption, catalytic

aromatization, FCC additive, toluene disproportionation

ERI (Erionite) Selectoforming, hydrocracking

LTL (KL-type zeolites) Catalytic aromatization

CHA (SAPO-34) Methanol to olefins (MTO)

FER (Ferrierite) n-Butene skeletal isomerisation

TON (Theta-1, ZSM-22) Long-chain paraffin isomerisation

AEL (SAPO-11) Long-chain paraffin isomerisation

26

3. Catalyst preparation methods Methods of metal modification of zeolites and mesoporous sieves are ion

exchange, impregnation, and in-situ. New methods of metal modification are

direct metal introduction (in-situ), isomorphous substitution of metal in framework

of zeolite and chemical anchoring.

3.1. Ion-exchange method

After post synthesis method, an acidic form i.e. proton form zeolite catalysts is

obtained. To this metal precursor solution is added in order to get metal modified

form under continuous stirring for 24 h and measure the pH value then filtration,

drying, and step calcination are performed.

Ion-exchange method consists of replacing an ion in an electrostatic interaction

with the surface of a support by another ion species. The support containing ions

A is plunged into an excess volume (compared to the pore volume) of a solution

containing ions B. Ions B gradually penetrate into the pore space of the support,

while ions A pass into the solution, until equilibrium is established corresponding

to a given distribution of the two ions between the solid and the solution. For

example, using a proper salt solution at ca. 100 C (to increase the exchange

rate), it is possible to prepare the acid form of zeolite by exchanging NH4+ for Na+

and successive calcination.

Natural exchangers are composed of a framework bearing electric charges

neutral by ions of opposite sign. For zeolites, for example, these charges are

negative and are due to the particular environment of aluminum. Aluminum, just

like silicon, is effectively situated in the center of a tetrahedron of four oxygen

atoms, which provides it with four negative charges, whereas the aluminum itself

has only three positive charges. The tetrahedron (AlO4) is thus an overall bearer

of a negative charge distributed over the oxygen atoms, and this charge is

27

neutralized by the presence of various cations Na+, K+ etc. These cations are not

definitely linked to the framework but may be replaced by the other cations during

an ion exchange operation. Whatever the exchanger conditions, and in particular

the pH, zeolites are cation exchangers and have a constant number of exchange

sites, which is equal to the number of aluminum atoms in their framework.

There are natural ion exchangers other than zeolites. Clays and silicates are

cation exchangers, whereas hydrotalcites are anion exchangers. As in the case

of zeolites, the number of exchange sites is not pH dependent.

Oxide surfaces contacted with water are generally covered with hydroxyl groups

which can be schematically represented as S-OH, where S stands for Al, Si, Ti,

Fe, etc. Some of these groups may behave as Brnsted acids, whereas other

hydroxy groups may behave as Brnsted bases, giving rise to the following

equation:

S-OH = S-O- + H+ . (1)

S-OH + H+ = S-OH+2 (2)

The resulting surface charge which arises from an excess of one type of charged

site over the other, is a function of the solution pH. A given value of pH exists for

which the particle is not charged overall. This value is characteristic of the oxide

and is called the pristine point of zero charge (PPZC or ZPC). Alumina is

amphoteric and may adsorb cations as well as anions and it ranges between 7

and 9. Silica ZPC values range between 1.5 and 3 and silica may adsorb cations.

The general procedure to introduce cations into the zeolite framework consists of

exchanging the Na+ cations, which balance the negative charge born by AlO4

tetrahedra with a solution of metal salt of the ion-exchange method. It has been

found that the equilibrium of the exchange reaction depends on temperature and

28

the concentration of the exchanging metal salt solution [12]. Cation exchange

into the hydrogen form of the zeolite is more difficult than into the Na-form

because of the strong bonding of the protons with the lattice oxygen. The proton

exchange limitation can be overcome by transforming the H-form into the NH4-

form, which has an ion-exchange property similar to that of Na+-form zeolite.

Metal-modified zeolites can be obtained either by means of a multistage process

of ion-exchange with intermediate zeolite heating or by a single-stage process of

ion-exchange at high temperature in autoclaving conditions. The pH of the

exchange solution is important, firstly because it is known that acid solutions can

partially hydrolyze and dealuminate zeolite and secondly, because metal

solutions of different pH values can readily precipitate basic salts within a zeolite

matrix to various degrees. The theoretical ion-exchange capacity depends on the

chemical composition of the zeolite: the lower the Si/Al ratio in the lattice is, the

higher the ion-exchange capacity is [13].

Ion exchange is composed of a frame work bearing electric charges neutralized

by ions of opposite sign. For example, [PtCl6]2- (hexachloro planitinic acid-II) ion-

exchanged with H-ZSM-5 (proton form of ZSM-5).

3.2. Impregnation Method The impregnation method is used to obtain higher loadings and to apply active

precursors that do not adsorb on support easily. The solution of the precursor is

broken up into small discontinuous elements presented in the pore of the support

by gradually evaporating the solvent. Impregnation is the easiest method of

making a catalyst.

A carrier, usually porous, is contacted with a solution of one or more suitable

metallic compounds. The carrier is then dried, and the catalyst is activated as in

the case of precipitated catalyst. The active agent is never introduced into a

porous support in its final form but by the intermediary precursor, the choice of

29

which is very important for the quality of the final deposit. The size and the shape

of the catalyst particles are that of the carrier. The impregnation mechanism

which is shown in the Figure 15 gives the general idea about the process.

Two types of impregnation can be considered, depending on

i. If an interaction exits between the support and the precursors at the moment of

wetting, and

ii. If there is no wetting.

a. Impregnation with no interaction between support and catalyst

If the support does not have its own catalytic activity then it gives the fine catalyst

by adding the precursor solution in order to have contact with the support i.e.

metal-support non-bonding. This is a relatively rapid operation because the pores

are filled with the solution and forms air bubbles and then released after 10

minutes. The maximum amount of metal precursor that can be introduced will be

depending on the solubility of the precursor salt in the solvent and also the pore

volume of the support. The impregnation technique required less equipment

since the filtering and drying steps are eliminated and washing may not be

needed.

b. Impregnation with interaction between support and catalyst

Impregnation with interaction occurs when the solute deposited and establishes a

bond with the surface of the support at the time of wetting. Such interaction

results in a near atomic dispersion of the precursor's active phase. The

interaction can be as ion-exchange, adsorption, or a chemical reaction.

Impregnation is the preferred process in preparing supported noble metal

catalysts for which it is usually economically desirable to spread out the metal in

finely divided form. The noble metal is usually present in the order of 1 wt% or

30

less or 5 wt% of the total catalyst. This makes maximum use of a very expensive

ingredient, in contrast, to precipitated catalysts where some of the active

ingredient will usually be enclosed by other material present and thus unavailable

for reaction. This work mainly deals with incipient wetness impregnation, an

amount of solution is added to the support just enough to fill the pore volume.

The disadvantage of this method is broad particle size distribution.

Figure 15. Mechanism of impregnation method (74).

3.3. In-situ Method

Metal solution is added after preparing the gel solution of zeolites i.e. during pre-

synthesis, In-situ method is used to get higher metal loadings and highly metal

species dispersion into the zeolite support.

31

4. Characterization Methods For application of zeolites as shape selective property in catalysis and in order to

correlate the effect of the synthesis parameters with the catalytic and physico-

chemical properties, synthesized materials need to be characterized properly.

The important parameters, which were investigated in this thesis work, are the

specific surface area, phase purity, crystallinity, structure, morphology, the

presence and the amount of different acid sites, the amount of an introduced

metal in the ion exchanged and impregnation catalysts (i.e. metal loadings or

metal content). A brief description of the each of the methods used is given

below.

1. Surface area measurements (N2 adsorption)

2. X-ray powder diffraction (XRD)

3. Scanning electron microscopy (SEM)

4. Fourier Transform Infra Red spectroscopy (FTIR)

5. Direct Current Plasma (DCP)

4.1. Surface area measurements The surface area of zeolites catalysts were measured by nitrogen adsorption.

First, catalysts were dried over night at 373 K and out gassed in a burette at 473

K for three hours.

The most common method of measuring of surface area is N2 adsorption using

BET (Brunauer, Emmett and Teller) isotherm method for mesoporous materials.

00

)1(1)( CPV

PCCVPPV

Pmm

+= (1) where

32

V= Volume of the adsorbed gas at pressure P

Vm= Volume of the gas adsorbed in the monolayer

P0 = Saturation pressure of adsorbate gas at the experimental temperature

P= Experimental pressure

C= A constant related exponentially to the heat of adsorption and heat of

liquefaction of the gas

RTHH condadeC /)( = (2)

where,

Had= Heat of adsorption on the first layer

Hcond= Heat of liquefaction of adsorbed gas on all other layers

A graph from the nitrogen adsorption between P/V (P-P0) vs. P/ P0 gives a

straight line whose slope and intercept can be used to evaluate volume of the

gas adsorbed in the mono layer.

iSVm +=

1 Where, S is the slope and i is the intercept.

The execution of the experiment was carried out with help of surface area

measurement instrument i.e. Carlo Erba sorptomatic 1900 instrument using BET

for mesoporous and Dubinin isotherm for microporous materials.

4.1 . X-ray powder diffraction (XRD) The XRD or XRPD method gives elemental composition, catalyst structure

(phase purity) and particle size of the materials. Material should be sufficiently

crystalline to diffract X-rays (3-5 nm) and being present in desired amounts for In-

situ measurements.

33

For crystal size determination the following equation is used.

T=K / cos . (1)

T= is the thickness of crystal perpendicular to diffraction plane.

K= is a constant that depends on instrument

B = is the full width at half maximum (FWHM) of the diffraction peak

= angle of reflection

The purpose of XRD is to determine the unit cell parameters and thus unit cell

volume when the zeolitic structure is known, then one can determine if an

element has been introduced into the lattice framework position.

XRD method is also a measure of the purity of a compound, compared with

reference spectra. If there is no evidence of crystalline or amorphous

contaminants present, then one must compare the intensity of the reference with

the authentic sample to check for the same composition and crystal size.

The principle working of diffractometer is the divergence of the primary X-rays

beam which is limited by an automatic divergence slit (ADS) and a 15 mm mask.

The irradiated sample length was set to a fixed 12 mm length. On the diffracted

side, a 0.2 mm receiving slit and a 1 mm anti scatter slit is present. The diffracted

X-rays beam was filtered with a Ni Ka filter. The measured diffractograms were

analyzed using X pert high score software and the powder diffraction file (PDF)

database. The PDF database was used to identify the sample peaks and the

corresponding phases. All the samples were first screened through an angular

range 0.5-0.9(2theta) using 0.02 steps and 1s measuring time for each step.

34

4.3. Scanning electron microscopy (SEM analysis)

The morphology of zeolites catalysts was investigated by SEM analysis and SEM

of the samples were obtained by back scattering electrons. The purpose of using

back scattering electron was to observe relatively large crystalline materials. For

small crystalline catalysts SEM is not useful but high resolution transmission

electron microscope was used.

The pore shape, size, channels and small crystalline mesoporous molecular

sieves were investigated by high resolution transmission electron microscope

(TEM).

4.4 . Fourier transform infrared spectroscopy Infrared spectroscopy reveals information about molecular vibrations that cause

a change in the dipole moment of molecules. It offers a fingerprint of the

chemical bonds present within materials. FTIR (see Figure 16) is a very powerful analytical tool for examining both inorganic and organic materials. A

beam of radiation from the source is focused on a beam splitter, where half the

beam is reflected to a fixed mirror and the other half of the beam is transmitted to

a moving mirror which reflects the beam back to the beam splitter from where it

travels, recombined with the original half beam, to the detector. The IR intensity

variation with optical path difference (interferogram) is the Fourier transform of

the (broadband) incident radiation. The IR absorption spectrum can be obtained

by measuring an interferogram with and without a sample in the beam and

transforming the interferograms into spectra [21]. For example, the formation of

OH groups on the external and internal surface of Y-type zeolites as studied the

adsorption of small molecules such as ammonia and ethylene and later

employed pyridine as a probe molecule to discriminate Brnsted and Lewis acid

sites.

35

Figure 16. FITR spectrometer (75). Pyridine has extensively been used to probe Bronsted and Lewis acidity in

catalytic materials. Specific IR vibrations could be identified, depending on the

type of interaction of the molecule with surface sites. As an example, the

vibration mode ascribed to combined C-C stretching and N-H bending modes

was found at 1439 cm-1 for the gas phase, 1438 cm-1 for pyridine physically

adsorbed on silica alumina, 1450 cm-1 and 1545 cm-1 for pyridine chemically

sorbed respectively on Lewis and Brnsted acid sites. When adsorbed in HY

zeolite pores, pyridine was shown to form pyridinium ions (characterized by an

intense band at 1540 cm-1) by interaction with the so-called HF hydroxyls, siting

in the supercages and vibrating at 3640 cm-1. The HY zeolite structure presents a

second type of hydroxyls, the so-called LF hydroxyls, vibrating at 3550 cm-1 and

located in sterically less accessible cavities (sodalite cages and hexagonal

prisms). Although not accessible to pyridine because of too small cage opening,

LF sites have been shown to interact with pyridine, which suggested the mobility

of interacting LF sites toward positions accessible to pyridine, i.e. supercages.

The nature of the bonding between pyridine and LF hydroxyls was recently

clarified by Parker et al. a partial protonation of pyridine was suggested [17].

36

4.5 . Direct Current Plasma Direct Current Plasma Spectrometer (DCP) is used for efficient determination of

major and minor elements in sample solutions. A direct-current plasma (DCP) is

created by an electrical discharge between two electrodes. A plasma support gas

is necessary, and Ar is most common one. Samples can be deposited on one of

the electrodes, or conducting can make up one electrode. Insulating solid

samples are placed near the discharge in a way that ionized gas atoms sputter

the sample into the gas phase where the analyte atoms are excited. This

sputtering process is often referred to as glow-discharge excitation [23].

5. Overview on MCM-22 and MCM-36 zeolites

MCM-22 possesses a unique crystal structure and belongs to MWW group of

zeolites (Figure 17) containing two independent non-interconnected pore systems [51-53]. One of the channel systems contains two-dimensional

sinusoidal 10-MR (member ring) channels 0.55 nm 0.4 nm, while the other

system consists of large supercages (12-member ring) with dimensions 0.71

nm 0.71 nm 1.81 nm. The super cages stack one above another through

double prismatic six-member rings and are accessed by slightly distorted

elliptical 10-MR connecting channels. In general, the synthesized MCM-22

zeolites crystallized as very thin plates with an extremely large external surface

area, on which distributed the 12-member ring pockets. The protonic form of

MCM-22 is an active catalyst for many reactions requiring acidic sites such as

catalytic cracking, olefin isomerization, and conversion of paraffins to olefins and

aromatics, and alkylation of paraffins with light olefins. Indubitably, the acidity

(number, location, and strength of the acid sites) plays an important role in the

catalysis [54-56]. Many studies have revealed that MCM-22 behaves like both

10- and 12-member ring zeolites [15].

37

6 10-ring viewed normal to [001] 10-ring viewed normal to [001]

between layers within layers

7H 8H

10-ring viewed normal to [001] 10-ring viewed normal to [001] Within layers between layers

MCM-36 is a unique porous zeolitic material comprising microporosity inside its crystalline layers and slit-like mesoporosity in the interlayer space. The

mesopores are created by intercalation of the zeolitic layers with polymeric silica

species mentioned in [24] and formation of a structure similar to that of pillared

clays. MCM-36 can be prepared from an as-synthesized layered precursor of

zeolite MCM-22 by applying first a swelling treatment to expand the interlayer

distances and then a pillaring (intercalation) procedure to stabilize the expanded

structure with flat-shaped mesopores between the layers [57]. Note that the

consideration of the mesopores as flat-shaped slits can easily be an improper

simplification as we do not know the distribution or density of the pillars and also

their thickness. Besides the dual porosity, the material shows another dual

nature: the crystalline zeolitic layers form together an amorphous structure

resulting from irregular arrangement of the pillars, i.e., the Figure 18 gives the idea of distribution, dimensions, and internal ordering. Since the zeolitic layers

can be intercalated with various metal oxides, not only SiO2, MCM-36 has

38

become a family of materials with adjustable properties. The introduction of

various metal oxides as single or mixed composites is an effective tool for

adjusting the MCM-36 properties, being important for potential catalytic

applications, by: (a) tailoring mesoporosity, i.e., distance between the layers

mentioned in ref [24], (b) creating acidic and basic centres of various strength

due to the choice of proper metal and its amount and; (c) influencing the acid

base character of the materials via ion-exchange (d) changing mechanical and/or

thermal stability via formation of more stable oxide species (e) affecting

adsorption properties in a broad sense [24].

Figure 17. MWW group of materials and its structures (76).

Pillared layered structures which are shown in the Figure 18 (MCM-36) are built

of inorganic layers with inorganic or organic pillars appended on both sides of the

sheets. These materials are potentially most attractive for catalysis, because they

combine high specific surface areas and good accessibility for larger molecules

to a large number of catalytic sites. Traditionally the focus was on pillared clays,

but pillaring of other layered phases such as zirconium phosphates, silicas and

metal oxides has also been explored. The combination of their specific pore

structure and catalytic properties has been exploited in many commercial

applications. Most notably, however, these materials contain moderately strong

to weak acid sites, much weaker than the strong Brnsted acid sites (bridging

hydroxyl groups) in zeolites [25].

39

Figure 18. A schematic representation of the MCM-36 structure.

In the MCM-36 phase, the polymeric silica as pillars is formed during the

hydrolysis and condensation of silicates from tetraethylorthosilicate. The

hydrolysis reaction replaces an alkoxy group (OR) with a hydroxyl (OH) group.

Subsequent condensation reactions involving the silanol groups produce

siloxane bonds (SiOSi) and alcohol or water as byproducts, leading initially to

oligomeric and polymeric structures. Depending on the conditions, the final

structures of the polymeric SiO2 can be formed as nearly linear polymeric

structures or three-dimensional branched structures. Based on the MCM-22

precursor with different Si/Al ratio, MCM-36 was produced by swelling and

pillaring techniques. The resulting MCM-36 contains a mesoporous region

between the microporous layers and has the properties of a medium-pore zeolite.

With silica as a pillaring material, the surface area of MCM-36 is about 2.5 times

higher than that of MCM-22. Silica pillaring increased the concentration of

terminal SiOH groups compared with MCM-22, the number of Brnsted acid

40

sites, however, was lower in MCM-36 than in MCM-22. This is attributed to the

dealumination induced by the swelling and pillaring processes. Alkane sorption

reveals that the 10-membered ring channel system of MCM-22 persists

unperturbed in MCM-36. The major fraction of the acid sites, and thus the

favored sorption sites, is located in the 10-membered ring channel of these

layers. Only about 10% acid sites exist on the outer surface and are accessible

through the mesopores. Consequently, alkane sorption in MCM-22 and MCM-36

at low equilibrium pressures is dominated by the 10-membered ring channels in

the layers. Catalytic tests indicate that MCM-36 is an active, selective and stable

solid acid catalyst for alkylation of isobutane with 2-butene, demonstrating that

the open mesoporous structure is utilized successfully for sorbing and converting

larger molecules [25].

MCM-36 contains more mesoporous region and less microporous region

compared to MCM-22 and MCM-36 exhibits higher BET surface area than MCM-

22. The MCM-36 has more terminal Si-OH groups compared to MCM-22 due to

silica pillars. The Brnsted acid sites are less in MCM-36 compared to MCM-22.

The typical XRD patterns of the MWW group materials are shown in the Figure 19.

41

Figure 19. XRD patterns of MCM-22(P) and its derivatives MCM-36 [57].

6. Overview on MCM-48 (mesoporous molecular sieve)

MCM-48 is a member of M41S Mesoporous silica's. It has recently attracted

much attention for its three-dimensional mesoporous channel systems, which

can be used as a prospective catalyst, an adsorbent, and even a template for the

synthesis of nanostructures. A lot of work has been done in the synthesis of pure

silica MCM-48 [29] and the incorporation of its framework with various

heteroatoms. MCM-48 was usually synthesized using cationic surfactants as

supramolecular template materials. Cationic-neutral or cationic-anionic

surfactants were also used as its structure-directing agent. With almost no

exception, MCM-48 was prepared in basic solution, and its yields were usually

low. The product yield was only ca. 50% using cetyltrimethylammonium bromide

(CTAB) as a single surfactant. One can gain a yield of ca. 80% using cationic-

neutral surfactant mixture by adjusting the pH of the reaction mixture to 10 during

42

synthesis. It is important to identify procedures suitable for a cost-efficient

synthesis of MCM-48 with high yields. Herein, by adjusting the pH=5 of the

solution in a synthesis process, a high product yield of 98% was gained using

CTAB as a surfactant. Some of the comparison between MCM- 48 and MCM-41

[30] is given below.

1. Porosity of MCM-48 is similar to that of MCM-41.

2. Particles of MCM-48 are much better organized than MCM-41.

3. Three dimensional channel system in MCM-48 as opposed to a one

dimensional channel system in MCM-41.

4. Both have similar thermal stability.

5. MCM-48 is more difficult to synthesize than MCM-41.

6. MCM-48 is cubic and MCM-41 hexagonal structure.

7. Ultra Sound Irradiation Method for Catalyst Preparation 7.1. Introduction

Ultrasound is simply sound pitched above the frequency bond of human hearing.

It is a part of sonic spectrum that ranges from 20 KHz to 10 MHz and

corresponds to the wavelengths from 10 to 103 cm. The application of

ultrasound, in connection to chemical reactions, is called sonochemistry. The

range from 20 KHz to around 1 MHz is used in sonochemistry, since acoustic

cavitation in liquids can be efficiently generated within this frequency range [45].

The origin of sonochemical effects in liquids is the phenomenon of acoustic

cavitation. Ultrasound is transmitted through a medium via pressure waves by

inducing vibrational motions of the molecules, which alternately compress and

stretch the molecular structure of the medium due to a time-varying pressure [41,

42, and 43]. Molecules start to oscillate around their mean position and if the

strength of the acoustic field is sufficiently intense, cavities are created in liquids.

43

This will happen if the negative pressure exceeds the local tensile strength of the

liquid.

The cavities are also called cavitation bubbles and the process itself is referred to

as cavitation. Two types of cavitation are known: stable and transient. Stable

cavities are bubbles, which form and oscillate around their equilibrium position

over several rarefaction/compression cycles, before collapsing or never

collapsing at all.

7.2 . Applications

Ultrasound has proved extremely useful in the synthesis of a wide range of

nanostructured materials, including high surface area transition metals, alloys,

carbides, oxides and colloids [41, 42]. Sonochemical decomposition of volatile

organometallic precursors in high boiling solvents produces nanostructured

materials in various forms with high catalytic activities. Nanometer colloids,

nanoporous high surface area aggregates, and nanostructured oxide supported

catalysts can all be prepared by the general route. The mechanism of the rate

enhancements in reactions of metals has been unveiled by monitoring the effect

of ultrasonic irradiation on the kinetics of the chemical reactivity of the solids,

examining the effects of irradiation on surface structure and size distributions of

powders and solids, and, determining depth profiles of the surface elemental

composition.

Ultrasonic irradiation of liquid-powder suspensions produces another effect: high

velocity inter-particle collisions [41-44]. Cavitation and the shockwaves it creates

in slurry can accelerate solid particles to high velocities. The resultant collisions

are capable of inducing dramatic changes in surface morphology, composition,

and reactivity. Heterogeneous catalysts often require rare and expensive metals.

The use of ultrasound offers some hope of activating less reactive, but also less

costly, metals. For example, the effects of ultrasound on hydrogenated catalyst

44

i.e. Ni powder, with the chemical consequence of enormously increasing the

catalytic rates of hydrogenation by Ni powder (>105-fold) [44].

8. Experimental Procedure 8.1. Catalysts

This work mainly deals with three mesoporous materials i.e. MCM-22, MCM-36,

and MCM-48 which were prepared using different methods as explained in the

sections 2.1.8, 3.1, 3.2 and 3.3. The main focus was on MCM-22 materials with

different silica to alumina ratios which were prepared as explained below,

characterized and tested in catalytic reactions i.e. isomerization of n-butane and

oxidation of lactose. Synthesis of MCM-36 with MCM-22 (with different silica to

alumina ratios) as precursors and synthesis of MCM-48 mesoporous materials

were also prepared in the lab.

All sodium forms MCM-22, MCM-36 and MCM-48 are characterized by nitrogen

adsorption method, X-ray powder diffraction method, and SEM which is

explained in detail in the theory part 4.

Catalysts nomenclature which was used during the work and the list of prepared

catalysts are given below.

Na-MCM-22-30-R means sodium form of MCM-22 with silica to alumina ratio equal to 30 prepared by the rotation mode. The rotation mode was used for all

MCM-22 based catalysts.

Na-MCM-36-22-30-R means sodium form of MCM-36, as MCM-22 precursor (no-

calcined) with silica to alumina ratio equal to 30 prepared by the rotation mode.

H-MCM-22-30-R means a proton form of MCM-22 with silica to alumina ratio equal to 30.

45

Pd-H-MCM-22-30-R-IE stands for palladium modified proton form of MCM-22

with silica to alumina ratio equal to 30 prepared by ion-exchange method.

Pd-H-MCM-22-30-R-IMP stands for palladium modified proton form of MCM-22

with silica to alumina ratio equal to 30 prepared by impregnation method.

Pd-H-MCM-22-30-R-IS stands for palladium modified proton form of MCM-22 with silica to alumina ratio equal to 30 prepared by in-situ method.

Pd-H-MCM-22-30-R-SSIE stands for palladium modified proton form of MCM-22 with silica to alumina ratio equal to 30 prepared by solid state ion-exchange

method.

Au-H-MCM-22-30-R-DP stands for gold modified proton form of MCM-22 with silica to alumina ratio equal to 30 prepared by Deposition Precipitation Method.

The catalysts which are synthesized, characterized and tested are listed below.

Proton forms

1. H-MCM-22-30-R

2. H-MCM-22-50-R

3. H-MCM-22-70-R

4. H-MCM-22-28-R

Ion-exchange catalysts

5. Pd-H-MCM-22-30-R-IE

6. Pd-H-MCM-22-28-R-IE

7. Pd-H-MCM-22-50-R-IE

8. Pd-H-MCM-22-70-R-IE

Impregnation catalysts

9. Pd-H-MCM-22-30-R-IMP

46

10. Pd-H-MCM-22-50-R-IMP

11. Pd-H-MCM-22-70-R-IMP

12. Pd-H-MCM-22-28-R-IMP

In-situ catalyst

13. Pd-H-MCM-22-30-R-IS

Solid state ion-exchange catalyst

14. Pd-H-MCM-22-30-R-SSIE

Deposition precipitation catalyst

15. Au-H-MCM-22-30-R-DP

The Na-form of catalysts which are synthesized and characterized in

laboratory is mentioned in the list below.

1. Na-MCM-22-28-static

2. Na-MCM-22-28-96 h-R

3. Na-MCM-22-30-R

4. Na-MCM-22-28-R

5. Na-MCM-22-50-R

6. Na-MCM-22-70-R

7. Na-MCM-36-22-30-R

8. Na-MCM-36-22-28-R

9. Na-MCM-36-22-50-R

10. Na-MCM-22-30-30 h-USI

11. Na-MCM-22-30-40 h-USI

12. Na-MCM-22-30-48 h-USI

13. Na-MCM-22-30-30 h-without USI

14. Na-MCM-22-30-40 h-without USI

15. Na-MCM-48-A and Na-MCM-48-B

47

Figure 20. Catalysts in powder form (sieved 150 m-250 m).

Proton forms of MCM-22 with different silica to alumina ratios i.e. 30, 50, 70 are

characterized by infrared spectroscopy for acidity measurement which is an

important method in order to determine active sites in the sample.

Palladium forms of catalysts were characterized by Direct Current Plasma for

metal loading i.e. Pd wt% in the zeolites and four different preparation methods

were employed during this thesis work i.e. ion-exchange method, in-situ and

SSIE method with nominal loadings 2 to 3 wt% Pd (using palladium nitrate

solution) and 5 wt% Pd as nominal loading by impregnation method (finally the

catalysts are crushed and sieved to 150 m-250 m pellets, see Figure 20).

8.2. Synthesis of Catalysts

Na-MCM-22-30-R (Si/Al = 30)

The Na-MCM-22-30-R was synthesized as mentioned in article [30] with some modifications. 3.02 g of sodium aluminate was added to 345.73 g of water and

thereafter 4.16 g of sodium hydroxide was added. The mixture was stirred for 10

minutes and pH was measured. To this mixture 24.07 g of hexamethyleneimine

(HMI) and subsequently 28.82 g of fumed silica was added. The mixture was

stirred for 20 minutes and the pH measured. The gel was transferred to teflon

cups and then inserted to autoclaves. The synthesis is carried out in rotation

48

mode for seven days at 150oC. After the completion of synthesis, the material is

filtered, washed (pH neutralization) with distilled water, dried at 100oC for 24 h

and calcined at 550oC for 8 hours.

Na-MCM-2250-R (Si/Al = 50)

The Na-MCM-22-50-R was synthesized as mentioned in article [30] with some modifications. Sodium aluminate (1.64 g) was added to 351.60 g of water and

then 2.26 g of sodium hydroxide was added. The mixture was stirred for 10

minutes and pH measured. To this mixture 21.77 g of hexamethyleneimine (HMI)

and then 26.12 g of fumed silica was added. The mixture was stirred for 20

minutes and pH measured. The gel was transferred to teflon cups and then

inserted to autoclaves. The synthesis is carried in rotation motion at 150oC for 7

days.

After completion of synthesis material is filtered, washed (pH neutralization) with

distilled water, dried at 100oC and calcined at 550oC for 8 hours.

Na-MCM-2270-R (Si/Al = 70)

The Na-MCM-22-70-R was synthesized as mentioned in article [30] with some

modifications. Sodium aluminate (1.14 g) was added to 345.73 g of water and

then 4.16 g of sodium hydroxide was added. The mixture was stirred for 10

minutes and pH was measured. To this mixture 24.07 g of hexamethyleneimine

(HMI) was added and then 28.82 g of fumed silica. The mixture was stirred for 20

minutes and pH was measured. The gel was transferred to Teflon cups and then

inserted to autoclaves. The synthesis is carried in rotation motion and was

carried out at 150oC for 7 days.

After completion of synthesis material is filtered, washed (pH neutralization) with

distilled water, dried at 100oC and calcined at 550oC for 8 hours.

49

Na-MCM-36-22-30-R with MCM-22-30-R as precursor

The Na-MCM-36-22-30-R was synthesized as mentioned in article [28] with some modifications. First, CTMACL (25% sol) and TPAOH (20% sol) was added

to MCM-22 precursor (non-calcined MCM-22 dried) in the ratio 1:4:1.2 as

mentioned in [28] and pH was measured. The reaction mixture is kept under

heating up to 98oC to 102 oC for 68 h under stirring. This process is known as

swelling process. The swollen MCM-22 is added to tetraethyl orthosilicate

(TEOS) in the ratio 1:5 heated up to 78 oC under nitrogen pressure for 25 h. This

process is known as pillaring process. Finally, the swollen and pillared MCM-36

is hydrolyzed in the ratio 1:10 and the pH is adjusted to 8. Then the material is

heated up to 40 oC, filtered,