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ZEOLITE CATALYSTS

IN SOME ORGANIC REACTIONS

J.C. Oudejans

TR diss

1394

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ZEOLITE CATALYSTS

IN SOME ORGANIC REACTIONS

TR d i s s

1 3 9 4

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On the cover: Electromicrograph of two large ( dg d g e = 0. 7 m m ) single crystals of natural Faujasite, embedded in volcanic rock and surrounded by phillipsite. The specimen is mined by the author in pit no. I of the Limberg at Sasbach am Kaiserstuhl, Bavaria, FRG.

ZEOLITE CATALYSTS

IN SOME ORGANIC REACTIONS

Proefschrift

ter verkrijging van de graad van doctor in de technische wetenschappen aan de Technische Hogeschool Delft, op gezag van de Rector Magnificus,

prof. ir. B.P.Th. Veltman, in het openbaar verdedigen ten overstaan van het College van Dekanen op donderdag

10 mei 1984 te 16.00 door

Johannes Cornells Oudejans

scheikundig doctorandus, geboren te Velsen.

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Dit proefschrift is goedgekeurd door de promotor: prof.dr.ir. H. van Bekkum

To all who have contributed to the realization of this Thesis

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The investigation described in this thesis has b^en supported by

the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

Typing: Mrs. M.A.A. van der Kooij-van Leeuwen Drawings: Mr. W.J. Jongeleen

C O N T E N T S

1. GENERAL INTRODUCTION 1

The Role of Z e o l i t e s in C h e m i s t r y - I n t r o d u c t o r y Remarks 1 The S t r u c t u r e of the Z e o l i t e s used in t h i s Thesis 2

Z e o l i t e A. Z e o l i t e s X and Y Mordenite ZSM-5

New Developments in Structure Determination of Zeolites 8 Solid State Magic Angle Spinning NMR (MASNMR)

High Resolution Electron Microscopy

Ion Exchange in Zeolites 10

References 11

2. DEVELOPMENTS IN ZEOLITE CATALYSIS 15

Introduction 15 The Role of Zeolites in Catalysis 15

The Framework Composition

The Structure of the Zeolites; Pores and/or Channels The Acidity of Zeolites

Well defined Support for Metal Particles

Industrial Applications of Zeolite Catalysts 22 Petroleum Processing

Petrochemicals

The Use of Zeolites in C-l Technology New Perspectives in Zeolite Catalysis

The Use of Zeolites in Organic Laboratory Practice

-The Scope of this -Thesis 30

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3. SELECTIVE OXIDATION OF 2,6-DIALKYLPHENOLS BY t-BUTYL

HYDROPEROXIDE IN ZEOLITE CoX 38

Introduction 38 Experimental 39 Results 40

Decomposition of t-BuOOH on Zeolite CoX Selective Oxidation of 2,6-dialkylphenols

Discussion A3 Conclusions 46 References 47

4. THE DECOMPOSITION OF ETHYL DIAZOACETATE OVER ZEOLITE

NaCuX 48

Introduction 48 Experimental 48 Results and Discussion 50

Adsorption Experiments Decomposition of EDA

Influence of the Copper Exchange Level Influence of the Oxidation State of the Copper Influence of the Zeolite Activation Temperature Influence of the Type of Catalyst

Conclusions 58 References 59

5. CYCLOPROPANATION OF OLEFINS BY ETHYL DIAZOACETATE:

COPPER- AND COPPER-CONTAINING ZEOLITES AS CATALYSTS 61

Introduction 61 Experimental 62 Results and Discussion 64

The Cyclopropanation of Cyclohexene over Zeolite NaCuX

The Cyclopropanation of some other Olefins over Zeolite NaCuX

The Preparation and Characterization of Chiral Copper Complexes inside the Zeolite Cavity and the Attempted Use in Asymmetric Cyclopropanation

References

6. CONVERSION OF ETHANOL OVER ZEOLITE H-ZSM-5 IN THE PRESENCE

OF WATER 79

Introduction 79 Experimental 79 Results and Discussion 80

Influence of the Reaction Temperature

Influence of the Ethanol Space Velocity on the Formation of Aromatics

Influence of the Amount of Water in the Feed on the Formation of Aromatics

Concluding Remarks 86 References 87

7. AMMOXIDATION OF TOLUENE AND RELATED AROMATICS OVER ZEOLITE

ZSM-5 88

Introduction 88 Experimental 89 Results and Discussion 90

Influence of the Cation in Zeolite ZSM-5 Influence of the Reaction Temperature Influence of Individual Components In the Feed Ammoxidation Activity of some other Cu-containing Zeolites

Preliminary Experiments with Related Aromatics Spectroscopie Measurements

General Remarks 97 References 98

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A NOTE ON THE CONVERSION OF ETHANOL OVER ZEOLITE H-ZSM-5

IN THE PRESENCE OF AMMONIA AND OXYGEN 100

Introduction 100 Experimental 100 Results . 102

Effect of the Type of Catalyst Influence of the Reaction Temperature Effect of the Ethanol Space Velocity

Influence of the Presence of O2 in the Gas Feed Other Reactants

Discussion 104 References 106

9. A NOTE ON THE CONVERSION OF FURAN AND SOME FURAN

DERIVATIVES OVER ZEOLITE ZSM-5 108

Introduction 108 Experimental 109 Results and Discussion 110

Conversion of Furan and Derivatives to Hydrocarbons Conversion of Furan and Furan Derivatives in the Presence of NH3/O2

References 114

10. CHLORINATION OF PHENYL ACETATE AND ANIS0LE OVER ZEOLITE

CATALYSTS 115

Introduction 115 Experimental 117 Results and Discussion 118

Adsorption Experiments Chlorination of Phenyl Acetate Chlorinatlon of Anisole

Some Remarks on Halogen Activation on Zeolite Surfaces

References 131 SUMMARY

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CHAPTER 1

GENERAL INTRODUCTION

1. The Role of Zeolites in Chemistry - Introductory Remarks

More than 200 years ago, in 1756, Cronstedt discovered a new mineral species (stilbite) in rocks that were used as building stonel. This new species, a crystalline aluminosilicate, was called zeolite owing to the behaviour of this mineral upon heating. From that moment it took almost one century before the reversible dehydration-hydration character of zeolites was revealed by Damour in 1840^, This phenomenon has led to one of the most important uses of zeolites in chemistry both in the industry and in the laboratory, namely as drying agent for liquids and gases. Another quality of zeolites to be discovered was the ability of ion exchange in zeolites in which cations, neutralizing the negative charge of the lattice (see also 2), are replaced by others. The ion exchange feature has expanded the use of zeolites widely.

+

Based on this zeolites are used in the removal of NH4 ions from waste waters, in treating nuclear waste water and as detergent builder. Ion exchange enables to modify zeolite catalysts and adsorbents unlimitedly^.

Much of the pioneering work has been performed by Barrer and coworkers on another characteristic of natural zeolites: the adsorption of molecules in the zeolite pores . Zeolites proved to be high-capacity and selective adsorbents for two reasons:

1. They separate molecules based upon the size and configuration of these molecules relative to the zeolite's pore system dimensions.

2. Zeolites adsorb molecules containing e.g. a permanent dipole moment or other interaction effects, with a high selectivity.

However, the number of applicable natural zeolites and their available amounts were limited. This induced scientists to synthesize crystalline alumino-silicates. This resulted in the large scale synthesis of zeolites NaA and NaX by the Union Carbide Corporation6»7 in the 1950'0*

Synthetic zeolites have achieved a great success in the chemical industry as adsorbents both in molecular sieving and in selective adsorption. Examples of

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2

commercialized applications are a.o. separation of n-paraffins from branched chain paraffins on zeolite CaA, xylene separation, olefin separation, oxygen from air and separation of fructose from fruetose-dextrose-oligosaccharide mixtures.

The role of zeolites as catalysts started some 30 years ago. At the large industrial research centers in the United States the first observations were made of the acidic properties of modified (e.g. NH^-, H "* or Rare Earth-exchanged) zeolites NaX and NaY. The first industrial application of a zeolite catalyst was Introduced by the Union Carbide Corporation in 1959 with a zeolite Y isomerization^ catalyst".

However, the major Impetus for industrial zeolite catalysis came in 1962 with the introduction of a rare earth exchanged zeolite X in the catalytic cracking of heavy distillates9. Since then the developments in zeolite catalysis have been numerous both in catalysts and in processes.

Another major step in zeolite catalysis research was made in 1972 with the synthesis of zeolite ZSM-5 and the discovery of its extraordinary catalyst behaviour by the Mobil Oil Corporation* . From that moment much research has been devoted to obtain more insight in the possible applications of high silica zeolites.

Very recently the synthesis of a new class of molecular sieves was reported ; an aluminophosphate molecular sieve (Alpo) with the general composition AI2O3.1.0±0.2 P2O5. The aluminophosphate structures obtained include structural analog of the zeolites sodalite and erionite-offretite.

In conclusion it seems that zeolite chemistry and technology still is in a period of fast expansion.

2. The Structure of the Zeolites used in this Thesis

Zeolites are the crystalline members of the class of alurainosilicates. The smallest unit that can be discerned is a TO4 tetrahedron with T representing Si^+ or Al3+. The zeolite framework is built by linking these units together by a common oxygen ion. A schematic representation of the zeolite framework is given in Figure 1. Due to the charge of Si-, Al- and 0-ions the presence of Al provides a surplus of negative charge and the aluminosilicate structure has to be neutralized by cations (e.g. Na+, Ca2+( etc.). The TO, tetrahedra are linked together to form so-called secondary building units (SBU) that are shown in Figure 2. From these SBU's the three dimensional frameworks of the

Na Na St A! Si Si Al Si

A / \ A A A A

0 0 0 0 0 0 0 0 0 0 0 0

Fig. 1. The framework structure of zeolites.

5 4 R S6R

Fig. 2. Secondary Building Units (SBU's) in zeolite structures

zeolites are formed. Since a number of reviews concerning the structure of

used in the present work will be discussed in more detail. Data about the composition and structure of these zeolites are summarized in Table 1.

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4

Table 1. Comparison of the zeolites used in this thesis.

Type of zeolite Si/Al Typical unit cell corapositio

Mordenite ZSM-5 1 Na12(A102)i2(Si02)i2'27H2O 1. -1.5 Na86(AlO2)86(SiO2)l0ó-264H2O 1.5-3 Na56(AlO2)56CSiO2)l36.2 50H2O 5 Na8(AlO2>8(siO2)40-2ZlH2O > 15 Na4(A102)4(S102)92.16H20 D4R D6R 5-1 5-1 a See Fig. 2. i) Zeolite A

Zeolite A was the first synthetic zeolite to be prepared1^ and up to now no natural analogue has been discovered. It can be represented by the idealized formula Na12.Ali2.S±12-O^g.27H20 in the form in which zeolite A is synthesized. The framework of zeolite A is shown in Figure 3.

Fig. 3. The framework of zeolite A; a) the sodalite unit, b) zeolite A unit cell.

Tetrahedra of AlO^ and SiO^ are, by common oxygen atoms, arranged to truncated octahedra or sodalite units (see Figure 3a). These sodalite units are linked together by double 4-rings of oxygen atoms (Figure 3b). In this way a cavity is formed (the a-cage or supercage) with a free

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diameter of 0.114 nm and with a pore opening of 0.42 nm. The sodalite units enclose a small cavity called the 8-cage with a diameter of 0.66 nm and a pore opening of 0.22 nm. The pore opening to the a-cage can be altered by ion exchange. Replacing the sodium Ions for potassium decreases the pore opening from 0.42 nm to about 0.3 nm while exchange of the sodium ions for calcium increases the pore opening to 0.5 nm due to the reduced number of cations, leaving two of the six windows of the supercage free of cations.

Quite recently questions arose whether the currently accepted zeolite A structure was correct. Electron diffraction studies as well as 29gi_ and 27A1 MASNMR yielded results which implied that the Loewenstein rule16

(in which every Si^+-ion is surrounded by four Al^+ ions and each Al3+ by four Si Ions) should be abandoned and that the space group is R3 (rhombohedral) instead of Fm3c (cubic) '-J-0. However, recently new evidence In support of the Loewenstein rule for zeolite A has been published19. Vivid discussions are still going on on this subject

ii) Zeolites X and Y

Zeolites X. and Y are the synthetic analogues of the rare mineral zeolite faujasite. The Si/Al ratio in this type of zeolite might vary from 1-1.5 for zeolite X, and from 1.5-3 for zeolite Y and is around 2.4 for faujasite. Typical unit cell compositions are Nagg [(A102)86~

(Si02)io6l-264.H20 (for zeolite NaX) and Na5£ [(A102)56<S102)1361.250H2O (for zeolite Y ) . The framework can be built from the sodalite units by linking these tetrahedrally via double 6-rings in arrangement like carbon atoms in diamond (see Figure 4 ) . In each unit cell eight cavities (or supercages) are enclosed by these sodalite units with a pore opening of 0.74 nm and a diameter of 1.3 nm. As in zeolite A the pore opening can be changed somewhat by exchange of the sodium ions by e.g. calcium ~. In zeolite X and Y several crystallographic cation sites can be distinguished (see Figure 4 ) . The partition of the cations over these sites depend a.o. on the valency of the cation, the degree of dehydration of the zeolite and the presence of possible cation ligands1 • As in the case of zeolite A also zeolite X and Y were investigated with new techniques such as 29si_{^sfjH^21-22 # However, in contrast to zeolite A the Loewenstein rule was found to be valid for these type of zeolites with a Si/Al ratio in the range of 1.2-2.75.

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6

Fig. A. S t r u c t u r e of zeolite NaX and N a Y ( f a u j a s i t e t y p e ) ; S , , , a r e Cfce

cation positions; 0|-0/| are four types of oxygen i o n s .

iii) Mordenite

Natural mordenite has a nearly constant Si/Al ratio of 5 and is the most siliceous zeolite m i n e r a l1 2- The framework is built from 5-1 secundary building units and consists of chains, cross-linked by the sharing of neighbouring oxygens. Channel-like pores are thus formed. These channels are interconnected by "side pockets" with a free diameter of 0.28 nm (see Figure 5 ) . Therefore diffusion between the channels is not possible

Fig. 5. Projected framework representation of zeolite Mordenite along the channel a x i s .

7

(except for very small m o l e c u l e s ) . The mordenite framework can be seen as a pseudo unidimensional framework. Synthetic mordenite has an ideal unit cell composition of Nag [A102)8(S102)4ol «24H20 and can be made in a "large port" and a "small port" variety depending on the synthesis conditions. The "small port" mordenite exhibit an adsorption diameter of - 0.4 nm whereas the "large port" mordenite can adsorb molecules like benzene and cyclohexane.

iv) ZSM-5

Zeolite ZSM-5 is a representative member of a new class of high silica z e o l i t e s1^ seeming to have considerable significance as c a t a l y s t s ' . In contrast to the synthesis of the zeolites mentioned, organic template molecules e.g. tetrapropylammonium hydroxide are used in highly siliceous gels during the synthesis of zeolite ZSM-5. The framework of zeolite ZSM-5 contains a new configuration of linked tetrahedra shown in Figure 6a and consisting of 12 T a t o m s » • These units join to form chains (see Figure 6b) which on their turn can be connected to form sheets (Figure 6c and d ) . The linking of the sheets leads to a three dimensional framework In which two types of channels can be seen ("Figure 6 e ) . One channel system of sinussoidal channels runs parallel to [001]

(W k) (d) UJ

Fig. 6. S c h e m a t i c r e p r e s e n t a t i o n of z e o l i t e Z S M - 5 ; a ) e x t e n d e d 5-1 S E U , b ) c h a i n of extended S B U ' s , c ) and d ) sheets of ZSM-5 a l o n g [001] and [010] a x i s , r e s p e c t i v e l y e ) c h a n n e l s t r u c t u r e of Z S M - 5 .

and is i n t e r s e c t e d by a system of s t r a i g h t c h a n n e l s p a r a l l e l to [ 0 1 0 ] . The c r o s s - s e c t i o n of b o t h c h a n n e l s c o n s i s t s of 10 o x y g e n a t o m s . The s t r a i g h t c h a n n e l o p e n i n g h a s d i m e n s i o n s of 0.54 x 0.56 n m w h i l e the s i n u s o i d a l c h a n n e l o p e n i n g i s 0.51 x 0.55 n . Z e o l i t e Z S M - 5 h a s a S i / A l ratio v a r y i n g from 15 to > 4000 . In the u l t i m a t e c a s e (Si/Al ■* » ) an

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all silica molecular sieve (silicalite) is obtained with the ZSM-5 structure29>30# However, it is suggested that A l3 + ions present as impurities in the starting compounds are incorporated into the zeolite lattice31. Tt is this high Si/Al ratio that is held responsible for the high strength of the acid sites of the zeolite (as H-ZSM-5)3 1 and for a certain hydrophobicityJi- that is observed. The unique channel structure and its dimensions cause a strong shape selective actionJJ and a high

3. New Developments in Structure Determination of Zeolites

Recently two new and sophisticated techniques have been developed with which more insight can be obtained in the structure of zeolites. These techniques are:

1. Solid State Magic Angle Spinning NMR (MASNMR). 2. High Resolution Electron Microscopy.

These techniques can assist in solving a number of questions concerning zeolite structures and behaviour.

3.1. MASNMR

MASNMR in zeolite research can be used in two ways:

a) to study the zeolite structure, by 29S i_ a nd/or by 2 .U-MASNMR, b) to investigate molecules adsorbed onto zeolites by i : iO and/or by

J-H-MASNMR.

Ad a. "SI—MASNMR studies on Na-A type of zeolite led Lipmaa et al. to the conclusion that each tetrahedrally coordinated Si ion is surrounded (via oxygen bridges) by 3 (and not 4) Al ions and one Si^+ ion while each Al3"*" ion is surrounded by 3 (and not 4) Si^+ ions and one Al ion • Hence, a new structural model was proposed1". The almost sacrosanct Loewenstein rule1" has been the subject of many discussions since then. In consequence of the MASNMR data a reassignment of space groups has been proposed3". However, in a Si-MASNMR study of zeolite ZK-4, the high silica analogue of zeolite A (Si/Al > 1.5) Thomas et al. returned on their earlier observations and statements concerning zeolite A and re-established the validity of the Loewenstein rule1". Zeolite Na-Y has also been the subject of a 29Si-MASNMR study2 1»2 2. Thermal treatment of this zeolite with S1C14 yielded an essentially Al-free

faujasite structure showing one single 2"Si-MASNMR signal characteristic of a S i ^+ ion surrounded by four other Si^+ ions. On the basis of advanced 29Si-MASNMR measurements at very high field (79.80 MHz) of ten

confirmed the fulfilment of the Loewenstein rule in these zeolites and came to a re-examination of the Si, Al ordening.

Recently structural information was obtained from ^'Si-MASNMR

measurements on zeolite ZSM-53 7>38 and silicalite37 Fyfe et al.3' showed that: 1) several crystallographic sites for Si atoms can be

distinguished by 29Si-MASNMR, 2) A l3 + ions can be detected in silicalite (the all silica analogue of ZSM-5) by 27A1-MASNMR, 3) this aluminium is present as AIO, tetrahedra even at very low Al concentrations and in two types of tetrahedral sites. They conclude that structurally zeolite ZSM-5 and silicalite are essentially indistinguishable. These

observations are of vital importance in view of the patents assigned to the Mobil Oil Corporation (zeolite ZSM-5) and the Union Carbide Corporation (silicalite) .

Nagy et a l .3 8 used the 29S1-MASNMR signal at -103 ppm to identify silanol groups present in zeolite ZSM-5, possibly associated to lattice defects.

Not only the surroundings of Si^+ ions in zeolites can be investigated using 29si-MASNMR but also the neighbouring T atoms of the A l3 + ions can be studied by 27A1-MASNMR. The combination of these two should be sufficient to reveal the Si, Al ordening in different type of zeolites. Moreover, MASNMR can be used to study isomorpheous substitution of e.g. A l3 + by B3 + in zeolites by applying ^ B NMR spectroscopy.

Ad b. The other possibility offered by Solid State MASNMR is to study the state of molecules adsorbed on the zeolite surface or reacting inside the zeolite. For this purpose *H- and 13C-MASNMR are frequently used. After the 5th International Conference on Zeolites a number of studies concerning adsorbed molecules on zeolites have been published. In most of them the state of hydrocarbons (paraffins, olefins and aromatlcs) adsorbed on zeolites of the faujasite type have been investigated " . ^C-MASNMR has been performed on ethylene adsorbed on zeolite

H-ZSM-54'-49 in order to contribute to the elucidation of the mechanism of the methanol to gasoline conversion (the "Mobil MTG Process").

H-MASNMR was used to study a.o. the mobility of H20 in zeolites , exchange kinetics of H2° Pr o t o n s * > the adsorbed state of formic acid

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in zeolite NH4-y and H-Y55'56.

3.2. High Resolution Electron Microscopy

High Resolution Electron Microscopy records the structure of both perfect and imperfect zeolites directly in real space. This technique can reveal information about the presence or lack of structural integrity in situations where X-ray studies are not helpful. Moreover, more insight might be obtained with respect to relationships between zeolite structure and catalytic activity.

Menter57 first observed images showing dm = 1.47 nm spacings in a synthetic faujasite in 1958 while it took two decades then for the first high resolution (~ 3 A) image was obtained from a Linde L zeolite by Sanders Bursill et al.5' were the first to demonstrate the potential of electron microscopy in obtaining valuable structural information from zeolites. The disadvantage of damaging the zeolite structure by the electron beam can be anticipated by dehydration of the zeolite in situ at high vacuum and for extended time. High resolution electron microscopy on zeolite Na-Y was performed after incorporating U0^+ ions Into the lattice to prolonge the lifetime of the zeolite framework"^.

Recently high resolution electron microscopy has been used to examine zeolites ZSM-5, ZSM-11 and NaY61.

Subtle differences were found between ZSM-5 and ZSM-11. Moreover, germs of zeolite ZSM-5 were observed within the amorphous precursors of this zeolite. Zeolite Na-Y was shown to retain its structure even after extensive

4 . Ion Exchange in Zeolites^ » °j

One of the most essential characteristics of zeolites is the ability to exchange its cations for other cations of different valency and chemical behaviour. This cation exchange behaviour of zeolites depends a.o. upon the nature of the cations, the concentration of the cation in solution, the solvent (mostly water) and the temperature.

Purposes of cation exchange In zeolites are:

i) To modify the pore opening to the cavity of the zeolite*2, In 2eolite A the pore opening can be varied by using monovalent cations of different size or by reducing the number of cations by exchange to higher valent

I 1

cations. In this way pore openings can be obtained of 0-3, 0.4 and 0.5 nm with IC1", Na4" and Ca2 + as cations, respectively. This modification of the pore opening has a large effect on the molecular sieving properties of zeolites.

practically all cases in which zeolites are used as catalysts the cations originally present after synthesis (Na+ in most cases) have been replaced by other cations that form the catalytically active sites or are precursors for them. Ion exchange thus offers the possibility of making "tailor made" catalysts or of heterogenizing homogeneous catalysts. Moreover, the metal cation can be reduced to form small metal particles (see also 2.4).

iii) To increase the stability of zeolites with regard to e.g. high temperatures or acidic environments*5-1. An example of this is the increased stability of zeolite H-Y which is obtained after ion exchange of Na+ for NH4 and consecutive calcination to expell NH3. Zeolite H-Y can endure higher temperatures and more acidic circumstances than zeolite NaY.

Some applications of cation exchange are already mentioned in 1. In addition it must be noted that of the cation exchange applications the use as detergent builder is - since 1975 - the most widely spread. In 1980 767, of the total amount of zeolites consumed In the USA was used in detergency whereas this percentage is expected to increase to 81% in 198566- Governmental rules and regulations to expell phosphates from detergents are held responsible for this increase.

1. A.F. Cronstedt, Akad. Handl. Stockholm, 17 (1756) 120. 2. A. Damour, Ann. Mines, _1_7_ (1840) 191.

3. H. Eichhorn, Poggendorf Ann. Phys. Chem., 105 (1958) 126.

4. E.M. Flanlgen in "Proceedings of the 5th International Conference on Zeolites", ed. L.V.C. Rees (Heyden, London, 1980), p. 760. 5. R.W. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular

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6. R.M. Milton, U.S. Patent 2,882,243 (1959). 7. R.M. Milton, U.S. Patent 2,882,244 (1959).

8. R.M. Milton, in "Molecular Sieves", Soc. Chem. Ind., London, (1968), p. 199.

9. C.J. Plank, E.J. Rosinski, and W.P. Hawthorne, Ind. Eng. Chem., Prod. Res. Dev., 3_ (1964) 165.

10. K.J. Argauer and G.R. Landult, U.S. Patent 3,702,886 (1972).

11. S.T. Wilson, B.M. Lok, C A . Messina, T.R. Cannan, and E.M. Flanigen, J. Am. Chem. Soc., 104 (4) (1982) 1146.

12. D.W. Beck, Zeolite Molecular Sieves, (Wiley, New York, 1974). 13. W.M. Meier and D.H. Olson, "Atlas of Zeolite Structure Types". 14. L.B. Sand and F.A. Mumpton, "Natural Zeolites, occurrence, properties,

use" (Pergamon Press, Oxford, 1978).

15. J.V. Smith in "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS Monograph 171, 1976), p. 1.

16. W. Loewenstein, Amer. Mineral., J39_ ( 1954) 92.

17. G. Engelhardt, D. Zeigan, E. Lipmaa, and M. Maegi, Z. Anorg. Allg. Chemie, 468 (1980) 35.

18. J.M. Thomas, L.A. Bursill, E.A. Lodge, A.K. Cheetham, and C A . Fyfe, J. Chem. S o c , Chem. Commun., (1981) 276.

19. J.V. Smith and J.J. Pluth, Nature, 291 (1981) 265.

20. J.M. Thomas, C A . Fyfe, S. Ramdas, J. Klinowski, and G.C. Gobbi, J. Pbys. Chem., _86_ (1982) 3061.

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23. S.L. Meisel, J.P. McCullough, C M . Lechthaler, and P.B. Weisz, Chem. Tech., J_ (1976) 86.

24. C D . Chang and A.J. Silvestri, J. Catal., 47 (1977) 249.

25. G.T. Kokotailo and W.M. Meier, in "The Properties and Applications of Zeolites", ed. R.P. Townsend, (The Chemical Society, London, 1980), p. 133.

26. D.H. Olson, G.T. Kokotailo, S.L. Lawton, and W.M. Meier, J. Phys. Chem., 85 (1981) 2238.

27. D.H. Olson, W.0. Haag, and R.M. Lago, J. Catal., _6J_ (1980) 390. 28. E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.C Patton, R.M.

Kirchner, and J.V. Smith, Nature, 271 (1978) 512.

29. R.W. Grose and E.M. Flanigen, U.S. Patent 4,061,724 (1977).

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30. R.M. Dessau and R.B. LaPierre, J. Catal., _78_ ( 1982) 136.

31. D. Barthomeuf, in "Stud. Surf. Sci. Catal. 5, Catalysis by Zeolites", eds. B. Imelik et al., (Elsevier, Amsterdam, 1980), p. 55.

32. M.Y. Chen, U.S. Patent 3,732,326 (1973).

33. P.B. Weisz, in "Stud. Surf. Sci. Catal. 7, New Horizons in Catalysis", eds. T. Seyama and K. Tanabe, (Elsevier, Amsterdam, and Kodanska Ltd., Tokyo, 1981), p. 3.

34. L.D. Rollmann and D.E. Walsh, J. Catal., J36_ ( 1979) 139. 35. D.E. Walsh and L.D. Rollmann, J. Catal., 56 (1979) 195.

36. E.A. Lodge, L.A. Bursill, and J.M. Thomas, J. Chem. S o c , Chem. Commun., (1980) 875.

37. C.A. Fyfe, G.C. Gobbi, J. Klinowski, J.M. Thomas, and S. Ramdas, Nature, 296 (1982) 530.

38. J.B. Nagy, Z. Gabelica, and E.G. Derouane, Chem. Lett., (1982) 1105. 39. Nature, J100 (1982) 309.

40. NATO, Adv. Study Inst. Ser., Ser. C, _61_ (1980) (Magn. Reson. Colloid Interface Sci.).

41. K. Salzer, Z. Phys. Chem., (Leipzig) , _262_ (1981) 90. 42. J.B. Nagy, M. Guelton, and E.G. Derouane, in ref. 9, p. 583. 43. H. Kacirek, H. Lechert, W. Schweitzer, and K.P. Wittern, in "The

Properties and Applications of Zeolites", ed. R.P. Townsend, (Chem. Soc. Spec Publ. no. 33, 1980), p. 164.

44. M. Buelow, W. Schirmer, and J. Kaerger, in ref. 4, p. 58. 45. M.D. Sefcik, J. Am. Chem. S o c , 101 (1979) 2164. 46. H. Pfelfer and J. Kaerger, in ref. 9, p. 259.

47. V. Bolis, J.C. Vedrine, J.P. van den Berg, J.P. Wolthuizen, and E.G. Derouane, J. Chem. S o c , Faraday Trans. 1, 76 (1980) 1606.

48. E.G. Derouane, J.P. Gllson, and J.B. Nagy, J. Mol. Catal., 10 (1981) 331. 49. J.P. Wolthuizen, J.P. van den Berg, and J.H.C van Hooff, in "Stud. Surf. Sci. Catal. 5, Catalysis by Zeolites", eds. B. Imelik et al., (Elsevier, Amsterdam, 1980), p. 85.

50. W. Maiwald and W.D. Easier, in ref. 9, p. 629.

51. H. Pfelfer and W. Gruender, in "Magnetic Resonance and Related Phenomena", Proc. of the 20th Ampere Congress, 1978, eds. E. Kundla et al. (Springer, Berlin, 1979), p. 135.

52. W. Maiwald, W.D. Basler, and H.T. Lechert, in "Proc. of the 5th International Conf. on Zeolites", 1980, ed. L.v.C Rees, (Heyden, London, 1980), p. 562.

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53. W.D. Basler, in "The Properties and Applications of Zeolites", ed. R.P. Townsend, (Chem. Soc. Spec. Publ. no. 33, 1980), p. 167.

54. W.D. Basler and W. Maiwald, J. Phys. Chem., _83_(1979) 2148. 55. J.A. Reimer and R.W. Vaughan, J. Magn. Reson., 41 (1981) 483. 56. T.M. Duncan and R.W. Vaughan, J. Catal., _67_ (1931) 49. 57. J.W. Menter, Adv. Phys., _7_(1959) 299.

58. J.V. Sanders, in "Physics of Materials", eds. D. Borland et al., (University of Melbourne Press, 1978), p. 244.

59. L.A. Bursill, E.A. Lodge, and J.M. Thomas, Nature, _2R6_(1980) 111. 60. L.A. Bursill, J.M. Thomas, and K.J. Rao, Nature, 289 (1981) 157. 61. J.M. Thomas, G.R. Millward, S. Ramdas, L.A. Bursill, and M. Audier, in

"Faraday Discussions of the Chemical Society", _72_ (1981) 345.

62. H.K. Beyer and 1. Belenkaja, in "Stud. Surf. Sci. Catal. 5, Catalysis by Zeolites", eds. B. Imelik et al. (Elsevler, Amsterdam, 1980), p. 203. 63. J.D. Sherman, "Adsorption and Ion Exchange Separations", AICHE Symposium

Series, ]± (179) (1978) 98.

64. J.A. Rabo, "Zeolite Chemistry and Catalysis", ACS Monograph 171, (1976). 65. C.V. McDaniel and P.K. Maher, in ref. 64, p. 285.

66. P.L. Layman, Chemical & Engineering News, (Sept. 1982), 17.

15

CHAPTER 2

DEVELOPMENTS IN ZEOLITE CATALYSIS

1. Introduction

It was only 25 years ago that scientists discovered the use of zeolites as catalysts in chemical reactions. In the industrial research the acidic properties of zeolites X and Y and the shape selective properties of zeolite A were recognized and this soon resulted in the first major industrial application in 1962 of zeolite Y. Since then numerous workers in the field of heterogeneous catalysis have explored possible uses of zeolites as catalysts. This has led to a large amount of scientific publications and patents. Moreover, the use of zeolite catalysts has had an enormous impact on the processing of crudes in the petrochemical industry. However, the use of zeolites in catalytic reactions has not been limited to petrochemically processing.

Up to this moment the major part of zeolites catalysts is based on the acid character of these materials.

However, it is realized that zeolites offer good possibilities to heterogenize homogeneous catalysts and the development of new processes based on e.g. transition metals ion exchanged zeolites can be expected.

Moreover, the use of zeolites as carriers for metal catalysts is of interest since zeolites are able to stabilize very small metal particles in their framework. Metal containing zeolites are already applied in hydrocracking.

2. The Role of Zeolites in Catalysis

The role of zeolites in catalysis is based on a number of characteristic features of these materials which are summarized in Table 1. Some of these features are discussed in Chapter 1, others will be discussed below in more detail.

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Table 1. Characteristic features of zeolites in catalysis.

1. The framework composition

2. The structure of the zeolite: channels and/or pores 3. The ion exchange ability

4. The acidity of zeolites - Surface hydroxyl groups - Hydrogen as cation 5. The well defined support for small metal particles

2.1. The Framework Composition (1)

The composition of the zeolite framework is of major importance in zeolite catalysis. The framework composition controls the framework charge and influences the stability (both thermal and acid) of the zeolite considerably.

Three types of zeolites can be distinguished in this respect: low silica zeolites, intermediate silica zeolites and high silica zeolites.

Table 2 summarizes the most important zeolites, both natural and synthetic, used for catalytic purposes, subdivided in these three classes.

Table 2. Catalytic important zeolites according to Si/Al ratio.

Si/Al class Type of zeolite

Low silica zeolites A, X Si/Al - 1-1.5

Intermediate silica zeolites Y, L, Mordenite, Erionite, Si/Al - 2-5 Chabasite, Clinoptilolite, Omega

High silica zeolites Dealuminated Y and Mordenite Si/Al = 10-4000 (large p o r t ) , ZSH-5

All silica "zeolite" Silicalite Si/Al =

-Increasing the Si/Al ratio in zeolites has several effects:

a ) The stability of the zeolite towards high temperatures and reactive

17

environments e.g. acids increases'>'. The low silica zeolites are stable up to temperatures of 800-900 K while zeolite R-ZSM-5 is stable up to 1300 K. Low silica zeolites are relatively readily destroyed at pH < 4 while the high silica zeolites are stable in - even boiling - concentrated acids. However, a decreased base stability is observed for high silica zeolites in contrast to the low silica ones.

b) The electrostatic field inside the zeolite changes and consequently the adsorptive interaction with guest-molecules. Low silica zeolites are very hydrophilic while high silica zeolites are more hydrophobic (and o r g a n o p h i l i c )1. The all silica molecular sieve Silicalite can be used to adsorb organic compounds from aqueous solutions due to its extreme hydrophobicity^ «^.

c) The acid strength of the Br^nsted acid sites present in the zeolite increases with increasing Si/Al ratio. This can be very well illustrated by comparing the acidity of the zeolites H-Y, H-mordenite and H-ZSM-5 with a Si/Al ratio of 2.4, 5 and 25, r e s p e c t i v e l y5'6. The increase in acid strength is thought to be caused by more isolated positions of -Al-O-H groups at higher Si/Al ratios.

d) The concentration of cations decreases evidently since this is a function of the Al content of the zeolite. This will have an effect on cation specific interactions in adsorption, ion exchange and catalysis. The cation exchange selectivity is most strongly affected. The affinity to adsorb water (hydration of cations) is also Influenced by a decrease in cation concentration (see also b ) ) .

e) With Increasing Si/Al ratio a changing structure of the zeolite secundary building units is observed from 4, 6 and 8 rings for zeolites A, X and Y to a more stable 5 ring for zeolites mordenite and ZSM-5, respectively.

2.2. The Structure of the Zeolites; Pores and/or Channels

In general the internal voids system of the zeolites consists of: a) channels, unldirectinal or interconnected, and/or

b) cavities, interconnected by apertures of oxygen rings.

The structures of the zeolites used in the present work, are discussed in Chapter 1.

The role of the zeolite structure in catalysis is very important. It offers the possllibity of performing catalytic reactions according to "Shape Selectivity R u l e s " '- 9. Shape-selective catalysis was first reported by Weisz and Frilette more than 20 years a g o1 0.

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The main principal of shape selectivity is found in the zeolite structure and pore dimensions. This will have a large effect on the (adsorptlve) interaction with host (guest) molecules as well as on the diffusivities of the molecules adsorbed.

Three kinds of shape selectivity can be distinguished (see Figure 1 ) :

reactant selectivity

product selectivity

transition state selectivity

Fig. 1. Types of shape selectivity in zeolite catalysis.

1* Reactant selectivity: this occurs when only a fraction of the reactants has access to the active sites within the zeolite pores or channels. Both geometric factors and large differences in diffusivities play a role. In this respect the zeolite outer surface plays a minor role (< 5% of the total surface)•

2' Product selectivity: this takes place when only part of the products formed at the active site can diffuse out of the zeolite lattice. Products that are too large to leave the zeolite lead to deactivation of the catalyst or can react further to yield smaller-sized molecules that can diffuse out of the zeolite- Moreover, large differences In diffusion rates of alkylation, isomerlzation or disproportlonation products of alkylbenzenes will have a shape selective action, e.g. high para-xylene selectivity in xylene

19

isomerlzation over zeolite H-ZSM-5 (a-c).

3, Transition state selectivity: when the transition state required for a given reaction cannot be reached in the zeolite due to steric and space restrictions.

Another important item is the relation between the zeolite structure and the occurrence of catalyst deactivation by coke formation. Many catalytic processes suffer from carbonaceous deposits formed out of reacting species which polymerize and/or condense Into heavy polynuclear aromatics. Walsh et al.1^'1^ suggested that alkylation of aromatics could be the first step In coke formation.

In wide pore zeolites such as zeolite X and Y. formation of coke or coke-precursors is possible. Indeed it Is found that zeolite Y, used in fluid catalytic cracking, deactivates rapidly by coke formation and has to be regenerated continuously10. Zeolite mordenite has a pseudo unidimensional channel structure in which large polyaromatics cannot be formed. However, the pore structure makes this zeolite very sensitive to deactivation by pore blocking by small coke residues17. In contrast to the zeolites mentioned, zeolite H-ZSM-5 is much less sensitive for deactivation by coke formation. The differrence probably lies in the size of the pores (10-ring for ZSM-5, 12-ring for Y and mordenite) and the spatial constraints which prevents the alkyl-aromatlcs, once formed, to react further to produce coke (by cyclization, hydrogen transfer, repeated alkylation, etc.) in the smaller pores of zeolite ZSM-52.

2.Z. The Acidity of Zeolites

One of the most important features of zeolites in catalytic reactions is the behaviour as solid acids due to the presence of acid sites, both of the BrfJnsted and Lewis type. The ratio of these two depends a.o. on the zeolite activation procedure and on the reaction conditions.

Several reviews have been published concerning correlations between the acidic and catalytic properties i.e. between the proton number or acid strength and the occurrence of carbenium ion reactions

Brfinsted acid sites In zeolites can originate from several sources (see Figure 2 ) :

1. Upon thermal treatment of ammonium exchanged zeolites, (see Fig. 2a) 2. After dehydration of multivalent cation exchanged zeolites during which

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20

surface, (see Fig. 2b)

3. By treatment of acid stable zeolites with acidic solutions, thus directly exchanging the cation for protons. This method cannot be used in case of zeolite A, X and Y because of their lattice instability towards aqueous acids, but is quite suitable to convert zeolite Na-ZSM-5 to its proton form H-ZSM-524.

Lewis acid sites are in general obtained by dehydroxylation of two neighbouring hydroxyl groups by heat treatment (T > 750 K) (see Fig. 2c).

A A

o o o o

© © e 8

M. (H,0) M OH H

' ° \ ? ' \ / ° ^ A AT / O

x

! e , o

x

ó e

x

o

x Al Si Al > Al Si Al

/ \ A A / \ A A

o o o o o o o o o o o o

I I I I I I I I I I I I

o eo o

H

,o e o o o e o

/ \ A T^~ A A A / \

o o o o o o o o o o o o

Fig. 2. Types of acid (Br^nsted and Lewis) sites in zeolites.

Especially since the introduction of the high silica zeolites like zeolite ZSM-5 more insight is obtained in the relations between the Si/Al ratio of the zeolite, the resulting Br^nsted acidity and the catalytic activity of the corresponding zeolites ' • It has been found that the more siliceous the

21

zeolites are, the stronger are their Brénsted acid sites.

Frequently used methods to determine the strength and number of Brfins ted acid sites are"^:

- Spectroscopie methods

e.g. IR spectroscopy of hydroxyl groups or of molecules adsorbed on the zeolite, like NH3, amines, pyrldine and piperidine.

- Temperature Programmed Adsorption/Desorption (TPA, TPD)

e.g. TPD measurements of NH3 adsorbed on zeolite H-ZSM-5 revealed a number of Bronsted sites with different acid strength and their relative concentrations. A unique type of Br^nsted site showed to be present, with a very high acid strength (AHads (NH3) ■ 150 kj/mole). This acid site is supposed to play an important role in e.g. the Mobil MTG Process.

2.4- Well defined Support for Metal Particlee^S^ZB

Metal containing zeolites are used in the chemical industry in several commercial processes such as (hydro)cracking, the Hysomer process and selective forming.

In many cases noble metals like Pt, Pd or ru are involved. Important factors in the application of metal supported catalysts are:

i) the dispersion of the metal on the support ii) the stability of the metal particles on the support

iii) the catalytic activity of the support (dual function catalysts) iv) the stability of the support.

ad i) In order to obtain a maximal activity with a minimal metal content one aims to have a high metal dispersion as possible.

In principle zeolites offer the possibility of creating an atomic dispersion of metals. Metals can be Introduced by ion exchange into the zeolite as isolated ions. Reduction of these metal ions can yield Isolated metal atoms. In practice metal atoms will agglomerate. However, the reduction procedure applied will determine the size of the metal particles and the siting of them in the zeolite lattice (whether in the supercage or in the sodalite unit) • ad ii) The zeolite lattice is able to stabilize metal particles inside the

zeolite pores. If metal particles are formed in zeolites X and Y larger than the supercage (d > 0.13 nm) a local breakdown of the zeolite lattice is necessary. It has been shown for different metals in zeolites (Pt, Pd, N1) that the size of the metal particles depends

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22

a.o. on the reduction temperature, the pre-calcination conditions and the reducing agent**"*, Most studies have been performed on zeolites X and Y. Quite recently other types of zeolites were investigated, in particular zeolite ZSM-528.

ad iii) An important feature of zeolites as support for metal particles is the ability to design bifunctional catalysts with a metal function and an acid function (Br(insted sites). Acid sites can be present in the zeolite or can be introduced upon reduction of metal ions with hydrogen according to:

Me+n„zeol,-n + n/2 H2 > Me0 + nH*-zeol."« (22)

ad iv) Zeolites are stable support materials under general rection conditions. Zeolites X and Y are thermally stable up to 750 K while the high silica zeolites like mordenite and ZSM-5 can endure temperatures up to 1200 K. Also the acid stability is increased with increased Si/Al ratio. Moreover, zeolites possess a high internal surface (up to 800 m2/g).

Depending on a.o. the kind of metal, the kind of zeolite, the metal reduction procedure, the Si/Al ratio of the zeolite (and consequently the number and strength of acid sites) it will become possible to create "tailor made" catalysts.

In view of the variation in the kind of zeolite, the Si/Al ratio (and consequently the number and strength of the acid sites), the kind of metal and the metal reduction procedure it will become more and more possible to design "tailor made" catalysts.

3. Industrial Applications of Zeolite Catalysts

Zeolite catalysts are being applied to the following general areas of industrial processes™ >3^;

1. Petroleum processing 2. Petrochemicals

3. The Use of Zeolites in 0-1 Technology

Up to this moment the first area (Petroleum processing) has grown to the most important and mature application of zeolite catalysts. The other fields are still in their industrial Infancy. However, looking at the progress that has been made in the last decade, the future for zeolite catalysts In many

industrial processes looks very promising.

3.1. Petroleum Processing

The major application of zeolites as catalysts Is found in the petroleum industry and is related to two important zeolites properties: i) the high acidity of these materials and li) the molecular sieve effect.

Some details of the most important uses of zeolite catalysts in petroleu processing are given below:

The first major application of zeolites as catalysts in catalytic cracking of vacuum distillates took place in 1962. The increased catalytic activity and the improved yields to gasoline compared to the amorphous silica-alumina catalysts caused a revolution in catalytic cracking. At this moment all of the catalytic cracking in the United States and Canada is done with zeolites, while worldwide in about 95% of all cracking units zeolites are used33. Zeolite based fluid cracking catalysts (FCC) all contain a faujasite type of zeolite (synthetic type X or Y zeolite) immersed in a matrix of silica-alumina or clay. The zeolites are modified by ion exchange with protons and rare earth cations and have a low sodium content to increase their stability. Some functions of the matrix material are » • i) a role as heat sink in the regenerating section and protecting the heat sensitive zeolite, il) the synerglstic effect on activity and steam stability, iii) the role as sink for sodium ions, extracted from the zeolite.

All modern cracking units are moving-bed units (Fluid Catalytic Cracking, FCC, risercracklng33»36-38), in which pretreated oil is contacted with FCC at temperatures between 743 and 793 K. During cracking coke is formed on the catalysts (1-2 wt % ) . The catalyst Is regenerated continuously at temperatures between 863 and 973 K. The regenerated catalyst flows into the Incoming oil and the cyclo is completed.

Hydrocracking3 ^

The second major industrial application of zeolite catalysts is found in the hydrocracking process in which Inferior crude fractions are converted to more valuable products in catalytic cracking with addition of hydrogen.

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the large pore zeolites (already met in catalytic cracking) is combined with a hydrogenation-dehydrogenation function provided by combination of Ni, Co, Mo, W, Pt or Pd (usually Ni-W, Co-Mo or Pt-Pd). The advantages of zeolite hydrocracking catalysts can be found in a) a high activity and therefore a lower reactin temperature and hydrogen pressure without increase of deactivation rate, b) the processing of heavier feed stock or cracking at higher space velocities, c) resistance to the presence of S- and N-containing compounds.

The hydrocracking process (e.g. the Unicracking-JHC Process-'" >^) is operated at 525-700 K and hydrogen pressures from 1,36 MPa up to 13,6 MPa. Hydrocracking processing in which a shape selective zeolite is used, is found in the Mobil Oil Selectoforming Process , in this process C5 to C9 normal paraffins from reformer product streams are adsorbed selectively and hydrocracked. Nickel exchanged or impregnated erionite or erionite­ el inoptilo lit e combinations are used as catalysts.

Another shape selective hydrocracking process is hydrodewaxing using a "large port" hydrogen exchanged mordenite containing Pt * . A disadvantage of this zeolite is its pseudo-unidimensional channel structure (see Chapter 1) which causes rapid deactivation even at small coke levels by blocking the pores. The new high silica zeolite ZSM-5 with its three dimensional narrow channel structure seems to offer better perspectives 4»^-).

Hydro-Isomerization

One way to increase the octane number of gasoline is to replace the low-octane paraffins (11-C5 and n-C^) by aromatics and isoparaffins. A hydro-isomerization process based on a zeolite bifunctional catalyst and operating at 523 K has been developed (the Shell Hysomer Process^>*'). The catalyst in this process consists of a noble metal (Pt), highly dispersed on a high acidic mordenite with a low sodium content. In the hydroisomerization process the metallic function dehydrogenates n-paraffins to olefins which at the zeolite acid sites are converted to iso-olefins via the well known carbenium ions. These iso-olefins are hydrogenated at the noble roets.1 to give lso-paraffins^°* Usually, the Hysomer process is integrated with Union Carbide's Isosiv process in which branched-chain and cyclic hydrocarbons are separated from the straight-chain paraffins by zeolite CaA which on their turn are isomerized to isoparaffins. The total process is called TIP (Total Isomerization Process).

25

3.2. Petrochemicals

Although zeolite catalysis has been commercially applied in the petroleum industry for converting mixtures of hydrocarbons for 20 years now, chemical companies have recently recognized the value of zeolite catalysts in converting hydrocarbon compounds into higher value chemicals. An increasing number of patents reflects this interest. However, at this moment only two processes in the field of aromatics technology are commercialized:

Benzene Alkylation

The alkylatIon of benzene with ethylene to yield ethylbenzene is a major intermediate process step in the production of styrene. Recently, the Mobil-Badger Process has been introduced In which a zeolite of the ZSM-5 type is usediy>->u. This zeolite seems to offer major advantages over the traditional aluminum chloride catalyst with respect to corrosion problems and product contamination.

Alkylation of benzene with propylene to yield cumene (intermediate in the production of phenol and acetone), promoted by zeolites has been reported ^. However, this process is not yet applied on large scale.

Xylene Isomerization

Xylenes are primarly obtained from the Cg aromatic cut of the product stream from the catalytic reforming of petroleum naphthas. From the xylene isomers p-xylene is the most valuable, Its main use being the starting material for terephthalic acid. Rare earth exchanged Y zeolites were used as Isomerization catalysts * "* as an alternative Friedel-Crafts catalyst but only on a small scale due to a.o. the deactivation characteristics exhibited by these zeolites.

A substantial Improvement was achieved with the introduction of zeolite ZSH-550.54-59^ T h e c u a t m e] _ p o r e system of zeolite ZSM-5 causes a shape selective action in favour of the para Isomer. The high acidity of zeolite H-ZSM-5 leads to a high isomerization activity. Due to the zeolite structure only monomolecular isomerization occurs while the bimolecular

dlsproportionatlon is suppressed (transition state selectivity). A continuous shift of the thermodynamic equilibrium mixture to p-xylene is caused by a substantial higher diffusion rate of p-xylene out of the zeolite compared to the other two isomers. (The dlffusivity of p-xylene is believed to be at least

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26

three orders of magnitude higher than that of o- and m-xylene.) Modification of zeolite ZSM-5 by a.o. Mg6 2, P63, B6i( or Sb65 resulted in very high p-xylene selectivities (> 95%) probably due to both the adjustment of the zeolite acid character and the reduction of the channel dimensions.

For a number of promising zeolite-catalyzed chemical transformations which might reach the commercial state in the forthcoming decade the reader is referred to Section 2.3.5.

3.3. The Use of Zeolites in C-l Technology

Solid fossil fuels, in particular coal, are far more abundant long-term resources than oil and natural gas. Coal can be transformed into fuels or into chemical feedstocks by several processes o.a. gasification, liquefaction and pyrolysis. Gasification of coal with steam, yielding syn (synthesis) gas (a mixture of CO and H2) has substantial advantages over the other methods • Syn gas can be processed in a number of ways of which especially the conversion to oxygenates is of growing interest.

The use of zeolite catalysts (especially the high siliceous zeolites like H-ZSM-5) is expected to become of importance with respect to an improved activity and selectivity of the syn gas conversion or of combined processes (using e.g. dual-purpose catalysts). Two of such processes will be discussed in some more detail:

1. The Mobil Methanol to Gasoline Process (the MTG process). 2. Improved Fischer-Tropsch Synthesis.

1. The Mobil Methanol to Gasoline Process

A new and simple catalytic process for the conversion of methanol to hydrocarbons and water was announced some years ago " « The MTG process is based on a new class of zeolites, developed by the Mobil Oil Corporation. The high siliceous zeolite ZSM-5 (used in its H-exchanged form) is the prominent member of this class of pentasil zeolites (see also Chapter 1.2).

The high performance of zeolite H-ZSM-5 in the methanol conversion is attributed to the high strength of its acid sites and its shape selective channel structure which also is believed to have a retarding effect on the deactivation of the zeolite.

The mixture of hydrocarbons, obtained from the MTG process, was found to be predominantly in the gasoline boiling range (C4 to C^Q) with a high research octane number (RON -- 95). A typical composition of such a mixture is given In

27

Table 3.

It should be noted that as no hydrogen is formed - the overall H/C ratio of the product mixture Is equal to that of the (dehydrated) starting material. The largest aromatic compound observed is durene (1,2,4,5-tetramethylbenzene). The presence of water in the methanol (up to 30%) was found not

Table 3. Typical composition of synthetic gasoline from the MTG process7^.

Components wt %

Paraffins 56 Olefins 7

Naphthenes k

Aromatics 33

to be disadvantageous. To the contrary, water In the feed appeared to increase the amount of aromatics formed and to lower the rate of deactivation when e.g. ethanol-water mixtures were used as the feedstock™.

The discovery of zeolite ZSM-5 and its activity In the MTG process led to a considerable shift In scientific zeolite research. Substantial experimental work yielded a general scheme for the MTG process (see Scheme 2) '

~H20 -H20

methanol > dimethyl ether > light olefins > higher olefins

-> aromatics + (cyclo-)paraffins + C5 olefins (gasoline)

In this Scheme the formation of the first C-C bond has been a matter of much dispute'^-' . Several mechanisms are proposed such as a carbene-like mechanism*",72^ a carbenium ion mechanism10 or a mechanism involving a Stevens-type rearrangement of an 0-ylide (to an ethoxy group)''» . Recent Investigations on the conversion of methanol over H-ZSM-5 indicated that, once sufficient C3 olefins are formed during a (short) initiation phase, only the reaction of methanol with these olefins is responsible for Its conversion. Ethene is then formed by cracking of higher olefins'^.

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28

gasoline processing projects is taking place. The MTG process completes the sequence: coal (or natural gas) - syn gas (CO + H2) + methanol -> gasoline. In fact, the first MTG process unit is expected to come into operation within three years in New Zealand, where natural gas will be used as the feedstock79-80.

2. Improved FIscher-Tropsch Synthesis

The conversion of syn gas to hydrocarbons according to the well known, low selective Fischer-Tropsch (FT) chemistry yields a broad mixture of

hydrocarbons (up to > C3 0) , either consisting of mainly linear paraffins or of olefins depending on the technology used. At this moment these technologies are applied on a large scale in the three South African SASOL

Manufacturies81"8^ The use of zeolites (especially zeolite H-ZSM-5) in this process offers the possibility to Improve the selectivity of the reaction to a hydrocarbon mixture of C5 to Cu compounds8A~89. The primary synthesis products from the FT catalyst can be converted on zeolites of the pentasil type in a single stage or in a two stage process. In such a process the FT catalyst should yield light products (olefins or oxygenates). In the single stage process both FT catalyst and zeolite function under the same conditions in one reactor. However, serious problems can be expected since conventional FT catalysts and ZSM-zeolites have different optimum working temperatures (500-600 K and 650-700 K, respectively). In the two stage process the product stream from the FT synthesis Is led to a second reactor In which it Is converted to gasoline by zeolite H-ZSM-5. Both reactors can operate at optimum conditions while both catalysts can be regenerated independently.

A new approach will be the combination of a FT catalyst and the zeolite catalyst. For example, incorporation of the FT metal function (e.g. Fe, Mn) into zeolite H-ZSM-5 can yield a catalyst that will convert syn gas directly to gasoline"'-'.

3.4. New Perspectives in Zeolite Catalysis

Ever since the discovery of the applicability of zeolites as catalysts numerous research workers have tested zeolite catalysts in a wide variety of organic (and inorganic) reactions. Examples can be found in a,o. the Proceedings of the International Conferences on Zeolites9*- , while a number of excellent reviews summarize this work sufficiently ' ^ .

Reactions with hydrocarbons have been studied next to conversion reactions

29

of compounds containing hetero atoms. Acid catalyzed reactions and non acid catalyzed reactions have been reviewed, illustrating the multiple use of zeolites as catalysts. Both gas phase reactions and reactions In the liquid phase have been mentioned with the emphasis on the former. Heterogenizing homogeneous catalysts was a major objective in many studies,

The major trends in future commercial applications will probably comprise substantial growth in most of the presently existing catalytic processes as well as the development of new applications. From the beginning of the zeolite catalysis era in 1962 up to now relatively few (although on large scale) Industrial applications of zeolite catalysts are commercialized. However, the zeolite research performed both in the industry and on the universities has revealed the versatility of these materials. Especially since the discovery of the high silica zeolites, with Its prominent member zeolite ZSM-5, a new impetus is given to improve existing and develope new technologies based on zeolite catalysts.

Some projected industrial applications of zeolite catalysts are summarized In Table 4. In this Table not only future applications are mentioned but also some existing processes on which major improvements are made recently or can expected to be made in the near future.

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Table 4. Projected and improvable applications of zeolites in catalysis.

Application Potential catalysts

Methanol to gasoline Ethanol-water to gasoline CO + H2 to hydrocarbons

- via modified FT synthesis - via methanol ~" MTG combination Hydrocarbon processing - alkylation - isomerization - disproportionation - selective p-substitution Methanation Methanol carbonylation Hydroformylation Selective oxidation/ammoxidation Water splitting Hydrogenation/dehydrogenation Oligomerization Claus reaction (H2S + SC2 - 2H2O + 3/n Sn) N0X reduction by NH3 Heteroaromatics from simple compounds Toluene + methanol ■* styrene Diels-Alder reactions H-ZSM-5 H-ZSM-5 ZSM-5 + Fe, Ru, Zn, RuNaY ZSM-5, H-Mordenite, Zn/ Cu de-alum. H-Y 66-68 70 87,101 102-104 9,31,105

RuNaY, NiNaY, PdNaY ZSM-5 + Ru, NI, Pd RhNaY, RhNaX

RhNaX, RhNaY, CoNaX, CoNaY ZSM-5, NaX, NaY with trans.metals AgNaY, CrNaY, InNaY

Metal containing zeolites Metal containing zeolites

NaX, NaY H-Mordenite, H-ZSM-5 H-ZSM-5 H-ZSM-5 Cu(I)-zeolites 109-114 115 100,116-117 118-119 199,22,29,98 100

4. The Use of Zeolites in Organic Laboratory Practice - The Scope of this Thesis

In this laboratory applications of zeolites in (synthetic) organic chemistry are under investigation since 1 9 6 71 2 3 - 1 2 4. Initially, Roelofsen123 applied zeolites to shift organic chemical equilibria (e.g. in acetalization, ester interchange) by means of selective adsorption of one of the reaction products onto the zeolite.

Wortel124 has investigated the use of zeolites as selective catalysts in some synthetic organic reactions (dehydration, halogenation, HBr addition) as well as some applications of zeolites as selective adsorbents. Practically all reactions were carried out in the liquid phase at mild temperatures using a slurry technique in which the zeolite is suspended in the liquid reaction mixture. During the work described in this thesis, the slurry technique was applied initially to study zeolite catalyzed conversions of relatively low volatile reactants and products. However, new developments (a.o. new applications of zeolite ZSM-5) required higher reaction temperatures and at this moment gas phase experiments at relatively high temperatures (up to 725 K) play an important role in the zeolite research program of this laboratory.

Wortel studied the use of X and Y zeolites in the removal of peroxide impurities from solvents. Zeolite NaCoX was found to adsorb t-butyl hydroperoxlde (t-BuOOH) and to catalyze its decomposition into t-butanol and dioxygen , As a follow-up of this work an application of the Co-catalyzed decomposition of t-BuOOH in zeolite CoNaX is described in Chapter 3. The t-butylperoxy radicals formed upon decomposition of t-BuOOH were used to oxidize 2,6-dialkylphenols inside the zeolite to the corresponding benzoquinones and diphenoquinones. The influence of the restricted space inside the zeolite pores on the selective oxidation to benzoquinones was investigated.

Transition metal ion exchanged zeolites were found to be active catalysts for a variety of reactions. Especially copper-exchanged zeolites have been investigated in selective oxidation of hydrocarbons, (de)-hydrogenations, cyclization etc. In our laboratory it was found that copper exchanged zeolites were also able to decompose ethyl diazoacetate132. This reaction was

investigated further (Chapter 4) and an application was found in the cyclo-propanation of olefins (Chapter 5). It was observed that copper complexes could be formed Inside the zeolite cavity. Attempts were made to construct chiral copper complexes Inside zeolites and to use them in the asymmetric cyclopropanation of di-oleflns as a reaction step in the synthesis of pyrethrines.

As part of a joint research project of the Departments of Biotechnology, Chemical Technology and Organic Chemistry of this university the conversion of ethanol in the presence of water into a mixture of hydrocarbons over zeolite H-ZSM-5 was Investigated (Chapter 6 ) . This conversion could be used as a process step in the continuous conversion of glucose Into hydrocarbons133. Relatively few reactions with compounds containing hetero atoms have been

(24)

32

studied1-^-1371 ^ possible ammoxidation activity of zeolite ZSM-5 was found in the conversion of acetaldehyde in the presence of ammonia1^ .

The ammoxidation activity of zeolite ZSM-5 (in its acid form or partially exchange e.g. with Cu(Il) ions) was investigated. The conversion of toluene to benzonitrile was used as a model reaction (Chapter 7). Zeolite ZSM-5 was found to be an effective catalyst for the amination of ethanol to ethylamines in a non oxidative atmosphere. We studied the conversion of ethanol and ammonia in the presence of oxygen. Pyridine and raethylpyridine formation was found (Chapter 8 ) .

In extension to our findings as described in Chapters 6 and 8 some orienting experimental work has been done on the conversion of furan and furan derivatives (e.g. from an acid-catalyzed conversion of hexoses and pentoses) over zeolite ZSM-5 both in the absence and in the presence of ammonia and oxygen (Chapter 9).

Halogenatlon of aromatics catalyzed by zeolites was reported in a few publications1^6"1^1. Chlorination of phenyl acetate over zeolite catalysts has not been reported. The use of zeolites in the chlorination of phenyl acetate and anisole is described in Chapter 10. Zeolites might influence - due to their restricted pore system - the selectivity of the chlorination in favour of the 2,A ,5-trichlorophenyl acetate which can be used as a powerful herbicide precursor.

References

1. E.M. Flanigen, in "Proceedings of the 5th Int. Conf. on Zeolites", ed. L.V.C. Rees, (Heyden, London, 1980), p. 760.

2. C.V. McDaniel and P.K. Maher, in "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS Monograph 171, 1976), p. 285.

3. E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, and J.V. Smith, Nature, 271 (1978) 512.

4. E.M. Flanigen and R.C. Patton, U.S. Patent, 4,073,865 (1978). 5. D. Barthomeuf, in "Stud. Surf. Sci. Catal. 5, Catalysis by Zeolites",

eds. B. Imelik et al., (Elsevier, Amsterdam, 1980), p. 55. 6. D. Barthomeuf, J. Phys. Chem., _83_ (1979) 249.

7. S.M. Csicsery, In "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS Monograph 171, 1976), p. 680.

8. E.G. Derouane, In "Stud. Surf. Sci. Catal. 5, Catalysis by Zeolites", eds. B. Imelik et al., (Elsevier, Amsterdam, 1980), p. 5.

33

9. P.B. Weisz, in "Stud. Surf. Sci. Catal. 7, New Horizons In Catalysis", eds. T. Seyama and K. Tanabe, (Elsevier, Amsterdam and Kodanska Ltd., Tokyo, 1981), p. 3.

10. P-B. Weisz and V.J. Frilette, J. Phys. Chem. , _64_ ( 1960) 382. 11. W.W. Kaeding, C. Chu, L.B. Young, and S.A. Butter, J. Catal., 69 (1981)

392.

12. W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, and S.A. Butter, J. Catal. _6_7_ (1981) 159.

13. W.W. Kaeding and F.G. Dwyer, J. Am. Chem. Soc., 101 (1979) 6783. 14. D.E. Walsh and L.D. Rollraann, J. Catal., _49_ ( 1977) 369. 15. D.E. Walsh and L.D. Rollmann, J. Catal., _56_ (1979) 195.

16. P.B. Venuto and E.T. Habib, Jr., "Fluid Catalytic Cracking with Zeolite Catalysts". (Marcel Dekker Inc., New York, 1979).

17. M.L. Poutsma, in "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS Monograph 171, 1976), p. 437.

18. J.W. Ward, Adv. Chem. Ser., 101 (1971) 380.

19. J.A. Rabo and M.L. Poutsma, Adv. Chem. Ser., 102 (1971) 284. 20. J.W. Ward, in "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS

Monograph 171, 1976), p. 118.

21. D. Barthomeuf, ACS Symposium Series, _40_(1977) 453.

22. P. A. Jacobs, "Carbon!ogenic Activity of Zeolites", (Elsevier, Amsterdam, 1977).

23. H.W. Haynes, Jr., Catal. Rev. Sci. Eng. , _17_ ( 1978) 273. 24. This Thesis, Chapters

25. A. Auroux, P.C. Gravelle, J.C. Vedrine, and M. Rekas, Proc. of the 5th Int. Conf. on Zeolites, ed. L.V.C. Rees, (Heyden, London, 1980), p. 433. 26. K.M. Minachev and Y.A. Isakov, in "Zeolite Chemistry and Catalysis", ed.

J.A. Rabo, (ACS Monograph 171, 1976), p. 552.

27. C. Naccache and Y. Ben Taarit, in "Proceedings of the 5th Int. Conf. on Zeolites", ed. L.V.C. Rees, (Heyden, London, 1980), p. 592.

28. "Stud. Surf. Sci. Catal. 12, Metal Microstructures in Zeolites", eds. P.A. Jacobs et al., (Elsevier, Amsterdam, 1982).

29. P. Gallezot, Catal. Rev. Sci. Eng., j?0_ (1979) 121.

30. D.E.W. Vaughan, In "The Properties and Applications of Zeolites", ed. R.P. Townsend, (The Chemical Society, London, 1980), p. 294. 31. A.P. Bolton, In "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS

Monograph 171, 1976), p. 714.

32. J.S. Magee and J.J. Blazek, in "Zeolite Chemistry and Catalysis", ed. J.A. Rabo, (ACS Monograph 171, 1976), p. 615.

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