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SOME APPLICATIONS OF ZEOLITES IN ORGANIC CI-EMISTRY

Th.M. Wortel

Delft University Press

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o .^

HO UI

SCm APPLICATIONS OF ZEOLITES IN ORGANIC CHEMISTRY

BIBLIOTHEEK TU Delft P 1600 6094

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I - De door Jacobs, Tielen en Uytterhoeven gegeven kinetische afleidingen voor de omzetting van isopropylalcohol over X- en Y-zeolieten zijn voor een nadere beschouwing vatbaar.

P.A. Jacobs, M. Tielen en J.B. Uytterhoeven, J. Catal. 50, 98 (1977).

II - Bij de door Bergk en Wolf beschreven ontleding van cumeenhydroperoxide over X- en Y-zeolieten is geen rekening gehouden met de mogelijkheid van redox-ontleding.

K. Bergk en F. Wolf, Z. Chem. _1_5, 152 (1975).

III - Het gebruik van een "moving belt" voor de koppeling van een hoge-druk vloeistofchromatograaf en een massaspectrometer betekent slechts een gedeeltelijke oplossing.

IV - Voor een goede beoordeling van de door Tvigevman, Biron en Weiss uitge-voerde formose-reactie over NaX-zeoliet is een meer gedetailleerde productanalyse vereist.

Sh. Trigerman, E. Biron en A.H. Weiss, React. Kinet. Catal. Lett. 6(3), 269 (1977).

V - Enkele door Bveak gehanteerde aanduidingen voor "secondary building units" zijn misleidend.

D.W. Breek, "Zeolite Molecular Sieves", John Wiley, New York, (1974), p. 45.

VI - Voor zeoliet ZSM-5 is een betere classificatie mogelijk dan door

Kokotailo, Lawton, Olson en Meier wordt gegeven.

G.T. Kokotailo, S.L. Lawton, J.H. Olson en W.M. Meier, Nature 272, 437 (1978).

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voor de ringinversie van cis-decaline is te ver vereenvoudigd. F.D. Fessemier, M. Anteunis en D. Tavemier, Buil. Soc. Chim. Belg.

87 (3), 179 (1978).

VIII - De discussies over het al of niet gebruiken van remlichtsignaal-schakelaars en het al of niet voeren van meerdere mistlanpen zijn voorbij gegaan aan de echte problematiek: de standaardisering van de achterverlichting van motorvoertuigen.

IX - Bij de regel dat bij het tennisspel de scheidsrechter een beslissing van een lijnrechter kan herroepen of corrigeren is men er ten onrechte vanuit gegaan dat een tennisbal een puntmassa is.

X - Effectuering van een maximumsnelheid voor motorvoertuigen kan het best plaatsvinden door in te grijpen in het motorvermogen.

XI - Diverse technische beroepen zijn door de ontwikkelingen op het gebied van gereedschappen en van cosmetica zeer geschikt geworden voor vrouwen.

-XII - De invoering van een kattenbelasting kan een bijdrage leveren aan het binnen aanvaardbare omvang houden van het nationale kattenbestand.

XIII - Het ligt voor de hand om bij het tennisspel de telling binnen de games thans aan te passen aan de "tie break"-telling.

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Proefschrift t e r verkrijging van

de graad van doctor in de

technische wetenschappen

aan de Technische Hogeschool Delft,

op gezag van de rector magnificus

Prof.Dr.Ir. F.J. Kievits,

voor een commissie aangewezen

door het college van dekanen

te verdedigen op

donderdag 27 september 1979

te 14.00 uur door

Theodoras Maria Wortel

scheikundig ingenieur

geboren t e 's-Gravenhage

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Prof.Dr.Ir. H. van Bekkum

On the cover a line drawing of an a-cage of zeolite type X or Y

The investigation described in this thesis has been supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

Drawings: J.M. Dijksman

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CONIENIS SUNWARY 1. GENERAL INTRODUCTION History Z e o l i t e molecular s i e v e s S t r u c t u r e of t h e z e o l i t e s used Tieolite A Zeolites X and Y Zeolite L Zeolite mordenite Zeolite ZSM-5

Ion exchange capability

Hydroxyl groups and acidity of zeolites Adsorption by zeolites

References

2. SOffi APPLICATIONS OF ZEOLITES IN ORGANIC CHEMISTRY General introduction

Shape selectivity

Applications in industrial catalysis

Catalytic cracking Hydrocvaaking

Paraffin isomevization Methanol convevsion

Applications in (synthetic) organic chemistry

Shifting chemical equilibvia Reactions of alcohols

Reactions of alkenes

Reactions of aromatic compounds

Ring transfoïmiations of hetevooyclics

Scope of this thesis References

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3. SYNTHESIS OF ACETALS USING MOLECULAR SIEVES. THE USE OF SOLID

ACIDS AS CATALYSTS ^ 45

Introduction 45 Results and discussion 45

Sulfonated polystyrene resins 46

Silica-alumina cracking catalysts 47

Molecular sieve catalysts SI

Regeneration of catalyst and adsorbent . 5 3

Conclusion 56 Experimental 56

Reagents 56

Standard pvoaeduve for following acetal formation 57

Synthetic procedure 57

Regeneration 57

References 59

4. CYCLOHEXANONE, DIETHYLAMINE AND ETHANOL ADSORBED ON

SILICA--ALUMINA: A ^ \ NMR STUDY 61

Introduction 61 Experimental 62 Results and discussion 62

Conclusion 68 References 68

5. ZEOLITE CATALYZED LIQUID PHASE DEHYDRATION OF a-PHENYLETHANOLS 71

Introduction 71 Experimental 72

Standard procedure for dehydration 72

Results and discussion ' . 7 2

Adsorption 74

Effect of the activation temperature 74

Influence of the extent of cation exchange 75

Effect of the alcohol/zeolite ratio and solvent 77

Substituent effects 78

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6. SELECTIVE BROMINATION OF HALOBENZENES USING ZEOLITES 83

Introduction 83 Experimental 84

Reagents 84

Standard procedure for zeolite catalyzed avomatic bromination 85

Excess catalyst bromination 85

Solvent-free bromination 85

Adsorption 85

Results and discussion 85

Adsorption 86

Effect of the activation temperature 87

Influence of the extent of cation exchange 88

p/o-Selectivity and the action of HBr 91

Effect of partial poisoning of the zeolite surface 94

Solvent effect 96

Mechanistic considerations 98

References 99

7. A NOTE ON THE HYDROBRO^INATION OF 1-ALKENES IN THE PRESENCE OF

ZEOLITES 101 Introduction 101 Results and discussion 102

Conclusion 106 Experimental 107

Adsorption experiments 107

Hydrobromination experiments 107

References 108

8. REM3VAL OF PEROXIDE IMPURITIES BY ZEOLITES 109

Experimental 112

Techniques 112

Analysis 113

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9. CARBOHYDRATE SEPARATION BY X-ZEOLITES. CATION AND SOLVENT EFFECTS

Introduction

Results and discussion Conclusion Experimental Adsorption isotherms Column chromatography References 115 115 116 121 122 122 122 122 SAMENVATTING 125

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SUMMARY

This thesis describes some applications of zeolites in organic chemistry in detail. In addition to the application of zeolites as catalysts also the application as adsorbent in purification and separation procedures is

described. Generally in all reactions investigated a slurry technique has been used. In the purification and separation procedures a column technique was applied too. Partially the work described in this thesis has been publishedi"^ or is accepted for publication'*"^.

In Chapter 1 a. general introduction is given. The structures of the zeolites used in this thesis, their ion exchange capability, acidity and adsorptive properties are described.

In Chapter 2 the application of zeolites in organic chemistry is summarized. After a brief review of important industrial applications examples are given of the use of zeolites in (synthetic) organic chemistry. Especially the attention is focussed on reactions related with those described in the following Chapters.

In Chapter Z^ the synthesis of acetals from corresponding carbonyl compounds and alcohols in high yield is described. Commercial silica-alumina cracking catalysts, Y-type zeolites partially exchanged with multivalent cations, and sulfonated polystyrene resins are used as catalysts while zeolite A serves to shift the equilibrium by adsorbing the water formed. Proper activation of silica-alumina and Y-type zeolite is essential for obtaining a high catalytic activity. Procedures have been developed to regenerate the catalyst-adsorbent combination. Compared to earlier techniques the synthesis of acetals has been improved siilistantially.

In Chaptrv / "C l\^^^ chemical shifts of cyclohexanone, diethylamine and etlianol adsorbed on silica-alumina are reported. The chemical shifts are influenced by tlie surface coverage, the activation temperature of the

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silica--alumina, and the water content of the catalyst. The results are discussed in relation to the mechanism of silica-alumina catalyzed acetalization and enamine formation.

In Chaptered the dehydration of a-phenylethanol on Y-type zeolites is reported. The reaction rate and the selectivity for styrene formation is found to depend on the degree of cation exchange, the activation temperature, the alcohol/zeolite ratio, the solvent used, and the nature of the substituents. For obtaining a high selectivity to styrene a strongly competing solvent is required.

In Chapter s'^ the substitution of halobenzenes by bromine, catalyzed by

Y-type zeolites is described. Catalyst activity and para/ortho-ratio obtained depend upon the type of cation, the extent of cation exchange, the activation temperature, the solvent and the amount of catalyst used. In all cases the

para/ortho-ratio was substantially higher than with conventional procedures. Catalyst deactivation by hydrogen bromide, liberated during the reaction, was suppressed by adding sodium hydrogen carbonate and zeolite KA. The highest

para/ortho-ratios were obtained in a solvent-free procedure. The acidic Br^nsted sites of the zeolite are considered to be the catalytically active sites.

In Chapter 7^ the zeolite-catalyzed hydrobromination of 1-alkenes is described. Contrary to literature data normal (Markovnikov) addition of hydrogen bromide yielding 2-bromoalkanes is observed if peroxide impurities are removed thoroughly from the reactants. Substantial amounts of 3- and 4-bromoalkanes are formed due to zeolite-catalyzed isomerization of the alkene.

In Chapter 8^ X-zeolites and basic alumina are used for the removal of peroxide impurities from tetrahydrofuran, diisopropyl ether, cyclohexene, and 1-octene. The application of NaX or CoX 101, w/v in a slurry technique results in over 901 removal of peroxides from tetrahydrofuran and diisopropyl ether, whereas from cyclohexene and 1-octene the peroxides are nearly

completely removed. Also a column technique is applicable. CoX is an active catalyst for the decomposition of hydroperoxides.

In Chapter 9^ the adsorption of fructose and glucose on X-zeolites has been studied. The adsorption capacity and the selectivity of adsorption depend on the type of cation in the zeolite, the extent of cation exchange and the solvent used. Fructose and glucose can be conveniently separated by column chromatography using NaCaX as the stationary phase and eluting with aqueous methanol. This system also allows separation of other monosaccharides, of monosaccharides and disaccharides, and of monosaccharides and related alditols.

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References

^ Th.M. Wortel, W.H. Esser, G. van Minnen-Pathuis, R. Taal, D.P. Roelofsen, and H. van Bekkum, Reel. Trav. Chim. Pays-Bas 96, 44 (1977).

2 Th.M. Wortel and H. van Bekkum, Reel. Trav. Chim. Pays-Bas 97, 156 (1978). 5 Th.M. Wortel, W. van den Heuvel, N.A. de Munck, and H. van Bekkum, Acta

Physiaa et Chimiaa 24, 341 (1978).

"* Th.M. Wortel, D. OuïïTjn, C.J. Vleugel, D.P. Roelofsen, and H. van Bekkum, accepted for publication in J. Catal.

^ Th.M. Wortel, S. Rozendaal, and H. van Bekkum, accepted for publication in

Reel. Trav. Chim. Pays-Bas.

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GENERAL INTRODUCTION

History'-"^

Zeolites were first discovered by Cronstedt in 1756 as a new group of minerals consisting of hydrated aluminosilicates of the alkali and alkaline earths cations. As the minerals when heated, behaved like boiling stones, he introduced the name zeolite which comes from the Greek words zeo lithos,

meaning boiling stone. In 1840 Damour observed that the dehydration-hydration of zeolites is reversible and occurs without any change in the transparancy or morphology. Some years later the action of salt solutions on these

minerals was studied. In 1858 Eichhom reported that calcium and sodium could replace each other in the zeolites natrolite and chabazite.

The reversible dehydration-hydration of zeolites attracted many early investigators to study the possibility of adsorption of other molecules than water. Friedel proposed the idea that the structure of a dehydrated zeolite

resembles an open spongy framework. He observed that various liquids like alcohol, benzene and chloroform were adsorbed by a dehydrated zeolite and that this adsorption resulted in a change of the refractive indices of the minerals. The adsorption of gases like hydrogen sulfide, ammonia and air by chabazite was studied by Grandj'ean in 1909. In 1925 Weigel and Steinhoff found that chabazite rapidly adsorbs alcohol and water but that no substantial

adsorption occurred of benzene and acetone. McBain interpreted these results as a molecular sieving and concluded that the pore openings of the chabazite crystals had to be less than 0.5 nm.

Between 1926 and 1948 several papers on zeolite adsorption appeared in particular from the Barrer group in England which group developed the separation of normal- from iso-paraffins by chabazite. They discovered that variables such as pressure, temperature, particle size, variations in the

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chemical composition of the zeolite (e.g. by ion exchange) and the dehydration conditions influenced the rate of adsorption.

The attention had mostly been focussed on chabazite which was unfortunately a rare mineral only available in small quantities. Before 1949 numerous attempts to prepare analogues of zeolite minerals were without substantial success. In 1948 Milton and coworkers of the Union Carbide Coi-poration started a different synthesis prograrrme. Crystallization temperatures between 25 and 150 were employed in contrast to the earlier studies in which the tenperature ranged from 200-400 . In the following five years the zeolites A and X and several other new zeolites were synthesized. Some synthetic zeolites were structurally related to the zeolite minerals chabazite, mordenite and erionite. This successful development of methods for synthesizing zeolites has resulted in a wide scientific interest and in the development of numerous applications.

In 1954 the Linde molecular sieves of Union Carbide were released for general sale. The initial commercial application was in the drying of

refrigerants in household refrigerators. Several processes for the separation of straight chain from branched chain hydrocarbons with zeolite 5A were introduced in the late 1950's and the early 1960's. In 1959 Union Carbide evaluated a catalyst based on zeolite Y for the isomerization of pentane and hexane. In 1962 cracking catalysts containing zeolites, which were developed by Ntobil Oil Corporation, were used for the first time. Presently over 951 of the installed capacity in the USA employs zeolite catalysts.

The major applications in which zeolites are used today are the drying of process streams in industry as well as the drying of solvents and reagents

in laboratories, catalytic cracking, hydrocracking, purifications like the removal of sulfur and nitrogen cornpounds, air separation and bulk separations like normal- from iso-paraffins.

Recent promising applications are the use of zeolites as partial phosphate substitute in detergent formulations, and the use of zeolites to convert methanol to hydrocarbon mixtures rich in aromatics. . ,

-Zeolite molecular sieves

Zeolites, more commonly known as molecular sieves, are hydrous crystalline aluminosilicates. The framework structure consists of a three dimensional network of SiO. and AlO. tetrahedra linked together by coninon oxygen ions. The radii of the ions of the framework are silicon 0.039 nm, aluminum 0.057 nm, and oxygen 0.135 nm. Both silicon and aluminum ions (collectively denoted

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T-atoms) fit into the cavity formed by a dense tetrahedral arrangement of the four surrounding oxygen ions. The location of silicon and aluminum in the framework is governed by the rule of Loewenstein^ which states that two AlO. tetrahedra never share the same oxygen ion. The framework is schematically shown in Fig. 1.

S I Al S I S I Al Si

/ \ y\ / \ r^ y\ / \

0 0 0 0 0 0 0 0 G O 0 0

Fig. 1. Aluminosilicate framework of zeolites.

The T-sites of all natural zeolites are dominated by silicon and aluminum ions, but chemically related ions such as gallium, germanium, phosphorus and iron can be incorporated into synthetic zeolites.

The negative charge of the aluminosilicate structure, introduced by the isomorphic substitution of silicon by aluminum is neutralized by cations such as sodium, calcium, etc.

In the synthesis of zeolites at least four conponents are essential: a source of cations, usually a strong basic hydroxide, alimiinate, silicate and water. As alkali metals form soluble hydroxides, aluminates and silicates they are very suitable for the preparation of homogeneous mixtures. The simplest way to crystallize a zeolite is to produce supersaturated solutions of appropriate composition at temperatures between room tenperature and 175 . The time required for the crystallization varies from a few hours to several days. Under these conditions the actual product is determined by kinetic factors, and the truly stable situation may be completely irrelevant. Because of the absence of thermodynamic equilibrium variation of the reactant

formulation and the physical conditions can lead to the production of new zeolites or the modification of their chemical compositions.

Extensive empirical data on the relationship between composition,

temperature, types of reactants and other factors in the synthesis of zeolites have been accumulated. Typical reaction compositions required for the synthe-sis of type A, X and Y in the Na^O-A1^0,-SiO^ system are shown in Fig. 2^.

Generally the particle sizes of the individual zeolite crystals range from 1 to 10 pm. For most applications this very small particle size is unsuitable and the crystals must be pelletized in order to be packed in columns and beds

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SlOj

N Q J O AI2O3

Fig. 2. Reaction composition (mole %) diagram for zeolite synthesis. Areas identified by letters refer to compositions which yield the designated zeolite at 90-98% water content of the gels. The points marked with

(+) show the typical composition of the zeolite.

for use in adsorption or catalytic processes. The external surface of the crystals of the zeolite powder is small, about ^% of the total surface^. The

2

internal surface for zeolites A, X, Y is about 800 m /gi°''l.

The formula of zeolites can be represented by its unit cell composition:

M [ (AlOj) (SiO,) l.zHjO

The only part of a zeolite structure which can be specified precisely is the topology of the aluminosilicate framework. All other parts, such as the distribution of atoms on the crystallographically equivalent sites, are complex and uncertain. There are over 35 different framework topologies known and an infinite number is possible. Nearly 100 synthetic types have been reported.

At present seven groups of zeolites can be distinguished by structural considerations. Meier classified them as the analcime, natrolite, chabazite, phillipsite, heulandite, mordenite and faujasite group. Within each group a common sub-unit of structure is present which is a specific array of (Al,Si)0.

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tetrahedra. In the classification the Si-Al distribution is neglected. The sub-units, called secondary building units, as proposed by Meier^"^, are shown in Fig. 3. Of course the primary units are the (Al,Si)0. tetrahedra. In some cases the zeolite framework can be considered in terms of polyhedral units such as the sodalite unit of the types A, X, and Y.

• O O

S4R S6R S8R

D4R D6R

«-1 5-1 4-4-1

Fig. 3. The secondary building units in zeolite structures. Only the

positions of the tetrahedral T-atoms (Si,Al) are shown. Oxygen atoms lie near the connecting solid lines, which do not indicate bonds.

An excellent review of the synthesis, the structural and chemical data of a large number of zeolites was given by Breck^'^ srARobson'^'^. Generally zeolites are capable of reversible dehydration-hydration, ion exchange and sorption of molecules having a smaller critical diameter^^ than the tiniform pore openings of the structure.

Structure of the zeolites used

Information concerning the composition and structure of the zeolites used in the present work is summarized in Table I. Next the structure of these zeolites will be discussed in detail.

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type of

z e o l i t e A ZSM-5 X Y L Mordenite t y p i c a l u n i t c e l l composition Nai2Ali2Sii2°48-27H2° NaJVl Si 0 . 1 6 H 0 Na86^86Sil06O384-264H2O Na56Al56Sil36°384-264H20 Na3KgAl9Si270^2-22H20 N^8^8Si40O96-24H2O 0^ 8 10 12 12 12 12 b p o r e s (nm) 0.4 0.51 x 0. 0.74 0.74 0.71 0.67 X 0. 57 70 w a t e r a d s o r p t i o n c a p a c i t y (g/100 g) 29 10 33 33 15 IS pore volume (ml/g) 0.30 0.13 0.36 0.34 0.20 0.20 d e n s i t y (g/ml) 1.27 1.78 1.31 1.27 1.61 1.72

Number of oxygen ions forming the pore-opening. Crystallographic pore-opening.

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Zeolite A

The framework of the synthetic zeolite A is based upon the double 4-ring (D4R) building units. It can also be described by the sodalite unit, a truncated octahedron of 24 tetrahedra. The aluminosilicate framework is generated by placing a D4R unit in the center of each edge of a cube with edges of 1.23 nm. Each c o m e r of the cube is occupied by a sodalite unit enclosing a cavity (6-cage) with a free diameter of 0.66 nm. The center of the unit cell is a large cavity (a-cage) which has a diameter of 1.2 nm. A simplified model of tlie unit cell showing only silicon and aluminum ions in the vertices is shown in Fig. 4i'*)'5. xhe oxygen ions are situated near the middle of each line.

Fig. 4. (a) Skeletal diagram of the sodalite unit; (b) structure of the unit

cell of zeolite A, showing cation positions.

Zeolite A is synthesized in the sodium form. In the unit cell of NaA, eight out of the twelve sodium ions are located near the center of the 6-rings on the 3-fold axis inside the a-cage, called the S-, sites. The remaining four ions are less localized in the dehydrated zeolite. They are thought to be

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situated adjacent to the eight-mentered windows, called site S.^, controlling the access of the a-cages.

IVhen sodium in NaA (trade name 4A) is partly (45'„) replaced by potassium the effective pore diameter in KA (3A) is reduced to about 0.3 nm due to the larger ionic radius of potassium (0.133 nm v.s. 0.095 nm) which is situated at site II. When there is 68°6 ion exchange with calcium the eight site I positions are occupied by four calcium and four sodium ions in each unit cell and the site II positions are vacant in CaA (5A) resulting in a widening of the effective pore size to about 0.5 nm.

Zeolites X and Y

The framework structures of the synthetic zeolites X and Y and their mineral counterpart faujasite are characterized by the double 6-ring (D6R) as the secondary building unit. However, they also may be built from sodalite units. The structure^^''^ consists of a diamond like array of sodalite units which are joined tetrahedrally through the 6-rings (Fig. 5 ) . The linkage between adjoining sodalite units is a D6R unit or hexagonal prism (y-cage). The Si/Al-ratio varies from 1-1.5 for zeolite X and from 1.5 to about 3.0 for zeolite Y.

Fig. 5. Structure of faujasite type zeolites X and Y showing cation positions Sj-Sjj and four different types of oxygen ions 0 -0 .

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Structure analysis showed that the cation distribution is rather corrplex and that the hydrated cations move upon dehydration to positions near framework oxygens 1''. Site I is located in the centre of the D6R unit, site I' is on the inside of the g-cage adjacent to the D6R unit (Fig. 5 ) . Site II' is on the inside of the 6-cage adjacent to the single 6-ring. Site II approaches the single 6-ring on the outside of the e-cages and lies within the large cavity (a-cage) opposite site II'. Site III refers to positions close to the wall of the large cavity on the 4-fold axis in the large twelve-membered window. X-ray diffraction studies have shown that in dehydrated X and Y zeolites polyvalent cations occupy site I and to some degree site I' in preference to the other sites. However, the distribution of cations sometimes strongly depends on the residual water content^^.

The effective pore diameter may change somewhat when sodium is replaced by other cations. IVhen about 70°6 of the sodium ions in NaX (trade name 13X) is exchanged for calcium ions the new zeolite NaCaX (10X) possesses a somewhat reduced pore diameter. It has been suggested^ that if the population of tlie calcium ions is larger than 70°Ó and the Si/Al-ratio is less than 1.5, calcium ions in site II will slightly distort the framework of the zeolite. This has been confirmed by studies on the variation of the lattice constant and the adsorption of triethylamine with a critical diameter of roughly 0.8 nm. The latter compound was rejected by the dehydrated zeolite NaCaX (Si/Al < 1.5), with an effective pore diameter of nearly 0.8 nm and readily adsorbed at room temperature by NaCaY (Si/Al > 1.5) with an effective pore diameter of nearly 0.9 nm.

Zeolite L

The main structural unit of zeolite L^^ is the D6R unit, however, additional oxygen bridges are required. Alternatively zeolite L may be built from e-cages (Fig. 6 a ) . The c-cages are symmetrically placed across the D6R unit

(Fig. 6b); cross-linking columns of e-D6R-e-units form wide channels in one direction parallel to the c-axis with an effective pore diameter of 0.8 nm

(Fig. 6c).

Fully hydrated the structure has four cation positions (Fig. 6b). The cations in site D appear to be the only exchangeable cations at room

temperature. During dehydration cations in site D probably withdraw from the channel walls to a fifth site E located between the A-sites.

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Fig. 6. (a) Skeletal diagram of the c-cage; (b) section of the framework of zeolite L showing the linkage of e-D6R-e units and the cation sites A (%), B (O), C (9), D (9) and E (O); (o) projection of the framework of zeolite L parallel to the a-axis showing the main channel.

Zeolite Mordenite

The framework of mordenite'^ may be built with the 5-1 secondary building unit or the characteristic configuration T„0., (Fig. 7 a ) . The structure consists of chains (Fig. 7b) cross-linked by the sharing of neighbouring oxygens. The high thermal stability of mordenite is probably due to the large number of 5-rings^^>^°.

Ji^

a

Fig. 7. (a) The characteristic TO unit found in mordenite; (h) skeletal

o JO

diagram showing the chain of S-membered rings in mordenite.

Tne dehydrated zeolite contains a channel system in two directions, but for the adsorption of molecules larger than methane the accessible channel system is one-dimensional. The elliptical channel walls are formed by twelve oxygen ions (Fig. 8a). The channel-like pores all run parallel and are interconnnect-ed by highly distortinterconnnect-ed eight-memberinterconnnect-ed rings with a free diameter of 0.23 nm

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(side pockets). The diffusion between the channels is impossible except perhaps for very small molecules like water.

Fig. 8. (a) Projection of the framework of mordenite along the axis of the main channels; (b) skeletal diagram showing the wall of a pore section and four side pockets only.

Natural mordenite appears to have an effective pore diameter of only 0.4 nm which is assumed to be due to the occurrence of stacking faults and the presence of amorphous parts in the pore system. The pores can be effectively widened by acid leaching. Mordenite may be synthesized as a large-port or a

small-port variety. The synthetic mordenite used in the present work was of the large-port type.

Zeolite ZSM-S

Zeolite ZSM-5 is a member of a new class of catalysts (ZSM) with Si/Al ratio's ranging from 25-100 developed by Mobil Oil Corporation. It is the most

siliceous zeolite known, however, recently a completely aluminum deficient form^' (silicalite) with the same structure has been synthesized.

The structure of ZSM-5 may be built from 5-1 secondary building units and consists of sheets which are formed by cross-linking chains (Fig. 9) across mirror planes thus generating the structure of ZSM-5.

ZSM-5 contains two intersecting channel systems, one sinusoidal running parallel to [OOl] and the other straight and parallel to [010] as indicated in Fig. 9c. The channels have an effective pore-diameter of roughly 0.6 nm. ^fodels give the impression of a circular geometry for the ten-memebered windows but the literature reported elliptical.

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Fig. 5^2. (a) Skeletal diagram showing a chain of characteristic

con-figurations and (b) the [lOO] face of the ZSM-5 unit cell. The ten--membered windows shown are the entrances to the sinusoidal channels which run parallel to \00l\; (c) the channel structure of ZSM-S.

It appears that changes in the alkyl groups of the ammonium hydroxide results in different ZSM-zeolites^^"^i. The use of tetrapropylammonium hydroxide results in the formation of ZSM-5.

Ion exchange capability

The cations, compensating the negative charge of the framework in zeolites, can be exchanged by other cations of different valency and nature which permits the introduction of catalytically important elements. A family of catalysts can be prepared with gradually changing properties like adsorption behaviour, stability, catalytic properties, etc. Since these properties depend upon controlled cation exchange, detailed information on cation exchange equilibria is important.

The extent of cation exchange of zeolites depend upon:

(i) the nature of the cation species, the cation size (anhydrous and hydrated), and the cation charge;

(ii) the concentration of the exchanging cation species in solution; (iii) the anion species associated with the cation in solution; (iv) the solvent, most exchanges are carried out in water, sometimes

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(v) the temperature; and

(vi) the structural characteristics of the zeolite including the cation to be exchanged.

The ion exchange process may be represented by an ion exchange isotherm, a plot of the equivalent fraction of the exchanging cation in the zeolite

(A ) versus the fraction in the solution (A ) at a given total concentration in the equilibrium solution and at a constant temperature. Different types of ion exchange isotherms^^ of cations in zeolites are shown in Fig. 10. In

Fig. 10. Types of ion exchange isotherms for exchange of cations in zeolites.

The equivalent fraction of the exchanging cation in the zeolite (A )

versus the fraction in the solution (A ) . (Of. text.)

curve A the zeolite exhibits a preference for the entering ion. In curve B the zeolite preferred the outgoing ion. In many cases the entering ion shows a selectivity reverse to the increasing fraction in the zeolite resulting in a sigmoidal isotherm as illustrated by curve C. In curve D, the isotherm terminates at a point where the degree of exchange,' X, is less than 1, due to an ion sieve effect. This effect is caused by one or more of the following phenomena:

(i) the solvated cation is too large to enter certain small cavities and channels;

(ii) sometimes the exchangeable cations are locked during synthesis and cannot be replaced at the temperature applied;

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for the cation.

Sometimes in the cases (i) and (ii) a higher degree of exchange is obtained by a sequence of repeated exchanges with thermal activation of the zeolite after each exchange. Such an activation induces the dehydrated cations to migrate to the former inaccessible sites.

The specific exchange capacities vary with the entering and exchangeable cation and the chemical composition of the zeolite; obviously zeolites with a low Si/Al-ratio have a higher exchange capacity. Table II summarizes the maximum degree of exchange and the type of ion exchange isotherm for several cations and zeolites used in the present work.

Table II Ion exchange properties of some sodium zeolites

z e o l i t e A A A A X X X X X Y

V

1 Y

j Ttordenite c a t i o n + K NH4'^ Ag^ Ca2" Ag^ NH4'^ Ca2+ Ce^^ La^" NH4 Ag^ Ca2^ La^" Ca2^ cone. (N) 0.1 0.2 0.1 0.1 0.1 0.05 0.1 0.5 0.3 0.1 0.1 0.1 0.3 0.1 t

(°C)

25 25 25 25 25 20 25 25 25 25 25 25 25 20 isotherm t y p e B C A • A A D A D D D A D D D X '^ max 1.0 1.0 1.0 1.0 1.0 0.62 1.0 0 . 8 0.85 0.68 1.0 0.68 0.69 0.6 T^ Entering cation. ^ Cf. Figure 10.

Max. degree of exchange achieved under conditions cited. •J, ^ equivalents of entering cation

max gram atoms of Al in zeolite

A limitation as to the nature of the cations that can be introduced by ion exchange is the acid stability of the zeolites. IVhen the exchange solution is

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strongly acidic in addition to a given cation, protons are also incorporated in the zeolite and a breakdown of the lattice may occur. Generally if the Si/Al-ratio is high, the zeolite will be more stable in acidic solutions. The zeolites A, X, and Y are stable within a pH range of 5-^2^^>^^. Zeolites with a higher Si/Al-ratio such as mordenite are also stable at lower pH-values

(Si/Al > 3, stable in 0.1 N HCl)^^. Zeolites which are decon^josed by treatment with acids can be divided into two groups:

(i) those with Si/Al > 1.5 precipitating insoluble silica without the formation of a gel and,

(ii) those with Si/Al < 1.5 which gelatinize.

Zeolites are readily destroyed by hydrogen fluoride in essentially the same manner as any porous siliceous material^^. Noteworthy is that adsorbed com-pounds are efficiently removed from the zeolite surface (Si/Al < 3) by acid

(2 N HCl) or alkaline (4 N KOH) treatment, resulting in a destruction of the zeolite structure. Sometimes the use of alkaline solutions results in a recrystallization of the zeolite phaseii>38_

Proton exchange also occurs when zeolites are brought into contact with distilled water^''^5,40_ P Q P example NaA and NaX slurried in distilled water produces respectively a pH of 10-10.5ii'"^i and 9-9.53^>'*^, due to a partial hydrolysis of the sodium ions and replacement by protons according to

N a * - z e o l i t e + HjO ^ ^ ^ H * - z e o l i t e + NaOH

Extensive washing of NaX resulted in a replacement of 20b of the sodium ions^5. An increase in the Si/Al-ratio or the presence of other cations seems to suppress this process. It appeared that after washing NaX and NaY with water less sodium ions were replaced in NaY than in NaX'*^. A water suspension

of a zeolite, containing residual sodium hydroxide or silicate from its synthesis, may have a pH as high as 122^.

Applications include the selective removal of radio-active cations from radioactive waste materials'*^ and the removal of ammonia, as ammonium ions from agricultural and industrial waste water'*'*'"^^ to prevent explosive growth of algae. Recently it has been shown that NaA with its high capacity for calcium and magnesium ions possesses significance as a phosphate substitute in detergents'*^"'*^.

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Hydroxyl groups and acidity of zeolites

Although the formal discription of the zeolite crystal structure does not include structural hydroxyl groups it is known that they can be generated within the zeolite by means of the following procedures:

(i) After thermal decomposition of ammonium or alkylammonium ions with liberation of the corresponding amines and the formation of protons onto the anionic lattice according to

A T / O , 0 . 0 , . 0 .

/ \ / \ / \ / \

0 0 0 0 0 0 0 0

I 1 1 I ' I 1 1 I

The decomposition^^ starts at 200 and is complete at about 400 . Generally tliis process is reversible with ammonia and primary alkyl-amines.

(ii) Upon water removal from zeolites, containing multivalent cations, which cannot satisfy the framework charge distribution, the cation--associated electrostatic field causes dissociation of the coordinated water molecules. © © e ® M.(H,0)n M . O H ^ H® / O . G / O x ^ / O s , ' > O s , AT /O,^:0/O^ / Ó . 9 / 0 ^ Al S I Al » Al S I Al + (H->0) 0 0 0 0 0 0 0 0 0 0 0 0

I I I 1 1 I I I I I I I

Alkaline-earth and rare-earth cation exchanged Y-zeolites obtain a maximal hydroxyl concentration5i"52 after activation at 200-300 . (iii) Acid stable zeolites allow the introduction of hydroxyl groups by the

direct exchange from acidic solutions (cf. the section Ion exchange capability). For zeolites which are not acid stable (Si/Al < 3) method

(i) is normally used, yielding H-zeolites which are, however, generally not stable during subsequent adsorption of water. If water is adsorbed below 100 H-faujasite (Si/Al 2.2) will collapse^^, \>hereas H-mordenite

is stable against treatment with water. Also in Na-zeolites washed with distilled water some hydroxyl groups are formed due to a partial

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hydrolysis of the sodium ions (Cf. the section Ion exchange capibility). In the OH-region of the infrared spectrum of X and Y zeolites three bands are observed, one at 3740 cm -attributed to non-acidic hydroxyl groups from the surface or from amorphous silica-, one at 3650 cm and one at 3540 cm . An X-ray study5'* has indicated indirectly that the latter two bands involve hydroxyl groups situated at the sites 0^ and 0_ (Fig. 5 ) . Later infrared spectra in the OH-region have been resolved in six components^5^ and the following proton distribution has been proposed 0^ (15.6), 0^ + 0. (16.7) and Oj (23.6) for zeolite HY.

A Lewis site is created upon dehydroxylation of two neighbouring hydroxyl groups. The formation of a trigonal coordinated aluminum and a positively charged silicon, is assumed^''.

® H 2 ,0 0 0,^ 0 _„ 0. 0 . 0 . .0. .0. ® .0. / \ / \ T^^ /••: / \ ^ / \ / \ 0 0 0 0 0 0 0 0 0 0 0 0

I M I I I I I t i l l

Generally this process becomes important at activation temperatures above 400-500 . The Lewis acidity can be increased by increasing the activation temperature.

The change of the degree of exchange of sodium for ammonium ions or multi-valent cations allows a systematic change in the hydroxyl concentration. Generally the number of hydroxyl groups increases with increasing Si/Al-ratio for X and Y zeolites containing polyvalent cations^6, probably due to an enlarged inter-aluminum distance.

The strength of the acidic hydroxyl groups seems also related with the Si/Al-ratio. An increase in the Si/Al-ratio results in an increased acidity of the hydroxyl groups^^.

A review of methods for determining the strength, density and nature of the acid sites is given by Jacobs^^. Important methods are infrared spectroscopy and titration with butylamine and indicators. With the application of tlie latter method, the amount of butylamine needed, to impart to the zeolite the colour of the basic form of the indicator adsorbed on the surface, represents the amount of acid sites whose strength is higher than the pK of the indicator titrated.

An infrared study of the adsorption of pyridine can be used to determine the concentration of Br^nsted and Lewis acid sites. The formation of a

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pyridinium ion results in an infrared absorption band at 1545 cm" which indicates Br^nsted sites. The coordination to Lewis sites is observed in the 1440-1450 cm" region. An excellent review on infrared studies of zeolite surfaces is given by Ward^'^.

Noteworthy is that for CaY titration with butylamine a maximum acidity was determined after activation at 500 6 0,6i_ ^ c^j. Q^Y no Lewis acidity was observed at this activation temperature^"^, it was suggested that tliis maximum indicated an optimum in the Brsiinsted acidity of the zeolite. However, with infrared spectroscopy a maximum of the hydroxyl concentration was observed after activation at 250 5i,62_ probably this discrepancy is caused by a coordination of some butylamine with the dehydrated calcium ions. Other investigations indicated an increase in the acidic hydroxyl concentration upon water adsorption on CaY activated at 400 ^ ^.

Adsorption by zeolites

Many data are available in the literature concerning the adsorption in zeolites from the gas phase. The amount of gas or vapour wliich is adsorbed by a dehydrated zeolite depends on the equilibrium pressure, on the

temperature, the nature of the gas or vapour and the nature of the micropores in the zeolite. Adsorption and desorption are in general completely reversible.

Since zeolites exhibit the Langmuir-type isotherm, the Langmuir equation has been applied with some success. Contrary to the usual interpretation of this

type of isotherm, the break in the isotherm corresponds to the filling of the pores and not to the completion of a monolayer. The pores are filled at a very low relative vapour pressure. From the agreement^"* between measured and calculated pore volumes, using normal liquid densities, it is concluded that the adsorbed intracrystalline phase resembles the liquid state. Of course this is only true for relatively small molecules. One must realize, however, that an increase in the molecular volume of the molecule results in a decrease of the number of adsorbed molecules per unit cell (Table III) and a less efficient packing.

In the section history the origin of the term molecular sieve is described which means that only molecules of a specific size and/or shape may diffuse

through the surface layers of the crystals into the crystalline voids, where repeatedly similar barriers have to be passed. Figure 11 gives an indication of the molecular sieve adsorption ability of various zeolites, especially in relation to compounds used in our investigations.

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TABLE III Number of molecules adsorbed in a large cavity of zeolites A and

y65, 66

adsorbate zeolite A zeolite X

water 32 36 iodine 7.5 methanol 12 propanol 5.4 dietliyl ether 3.7 5.1 pentane 4.5 hexane 2.7 3.8 heptane 3.5 benzene — 5.4 cyclohexane — 4.1 toluene — 4.6 isooctane — 2.8 1,3,5,-tri-ethylbenzene — 2.2

It must be realized that the effective pore diameter is influenced by the temperature and the cations involved in a given zeolite. The influence of the temperature on the effective pore diameter is illustrated by the adsorption of 1,3,5-tri-t-butylbenzene which is not adsorbed by NaX at

normal temperatures, while it is readily adsorbed up to 18 wt % at 180 upon heating an excess of 1,3,5-tri-t-butylbenzene together with NaX powder for three days^^.

The influence of the degree of cation exchange on the effective pore diameter is illustrated in Figure 12. IVhen exchanging in NaA, sodium ions for calcium ions, the pores suddenly widen at 30^ exchange, due to the removal of pore-blocking S^, cations, thus alloiving the adsorption of normal paraffins ^ ^.

The rate of adsorption is influenced by the temperature, the pressure or concentration of the sorbate and the character of the diluent or solvent. The critical diameter**2 and length of the sorbate, compared to the pore diameter of the zeolite, influence the rate of adsorjotion as well. For n-paraffins an increase in chain length results in a decrease of the diffusion rate into A zeolitcs^s. Attention has to be draivn to tliose cases where the critical

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> z >< o z —( o o o a t l D 'E 4> O -INI < O o

A 4

. A 4

^er glucose 1

& I

ecu cyclohexane < Ö > - B r — h CH3-CH-(CHj)„CH3 CHj = CH-(CH2)„-CH3, Br-(CHj)„CH3 1 ""'^„H2n.2 C2H5OH 1 < 0 z HBr.Brj HCl,Cl2 • < 3C HjO h [• e a 8 7 6 5 « zeolite effective pore size ( A )

Fig. 11. Chart showing a correlation between approximate effective pore size of various zeolites with the critical diameters^^ of various

molecules.

diameter approximates the effective pore diameter, as a long time may be required to reach the adsorption equilibrium.

Generally the separation of a mixture of compounds by zeolites may be divided in two classes: separations based on a difference in size and separations based on a difference in affinity. Probably the most common application of the first type is the adsorption of water; the drying of gas and liquid process streams in industry or the drying of solvents and gases in laboratory work. By chosing a type A zeolite which excludes the solvent, very low water contents (< 1 ppm) are possible. The residual water content is governed by the nature of the solvent^^"''^.

Another important industrial application of zeolites based on size

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0.20

7. exchanged 2 N a * Ca

Fig. 12. Effect of calcium exchange for sodium on the sieving properties of zeolite A. (1) nitrogen, 2 kPa, -196 ; (2) n-heptane, 6 kPa, 25 ;

(Z) propane, 33 kPa, 25 ; (4) isobutane, 53 kPa, 25 .

with branched chain paraffins, for the upgrading of gasoline and to purify n-paraffins used in the manufacture of biodegradable detergents. An excellent review of these processes together with an extensive bibliography of

literature on the use of zeolites for separation and purification is given by Kerr^^.

Sometimes, when all compounds are adsorbed at equilibrium, a partial separation may be achieved when a difference in the adsorption rates exists. For example, during the adsorption of a mixture of water and alcohol out of pentane solutions by zeolite CaA, a minimum in the ratio of adsorbed alcohol/ adsorbed water with time was observed''3.

If all compounds of a mixture penetrate the zeolite, and a difference in adsorption strength exists, a separation of the compounds will be possible. As an indication of the adsorption strength, the heat of adsorption may be used, which is strongly influenced by the degree of pore filling and the type of zeolite, including the presence of different cations. A valuable indication of the heat of adsorption may be obtained by calculation using the data

compiled by Kiselev and Lopatkin, shown in Table IV.

Generally, the more unsaturated and/or more polar compounds are preferred by the zeolite. Thus propene is selectively adsorbed from a mixture with propane^^ on type A zeolite.

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TABLE IV Increments of heats of adsorption of cevtain fragments of complex molecules on NaX zeolite''^, at 6 =0.1

T

molecule fragrent heat of adsorption, kJ/mole

-CH2- 10 -CH3 ^ ' 11.3 .

-H of alkanes -1.3 CH2=CH- 37.2 -0- of ethers, alcohols 46.0

-NO- of aliphatic compounds 72.4 -H of OH groups of water, alcohols • 13.8

-OH of aliphatic alcohols . " 61.1

-NH. of aliphatic amines 68.2

Contrary to this rule is the adsorption behaviour of highly siliceous zeolites. With aromatic-n-paraffin mixtures dealuminated hydrogen mordenite (Si/Al % 47) showed a preference for the n-paraffin in contrast to the

original mordenite (Si/Al -x, 6)''s. Similar results were obtained with ZS^-S^'^. Silicalite selectively adsorbs hydrocarbons from aqueous solutions''^^ Q ^ the other hand ZSM-5 adsorbs small alcohols like isopropyl alcohol from n-hexane solutions'7''. Many other unique uses of zeolites''2>79 have been developed like separation of oxygen from air, the removal of NO and SO from nitric and sulfuric effluent gas streams, and the separation of isomeric xylenes.

We have studied the adsorption of peroxide impurities from alkenes and ethereal solvents on NaX^". Especially with alkenes, a nearly complete removal of the peroxides was observed, which is probably due to the large difference in adsorption strength between the polar peroxide and the alkene.

We also investigated the adsorption of fructose and glucose on X-zeolites. By a proper choice of cation and mixed solvent a separation between these two important monosaccharides has been achieved^^.

References

^ R.M. Barrer, "Zeolites and Clay Minerals as Sovbents and Moleculav Sieves",

Academic Press, London (1978).

2 D.W. Breck, "Zeolite Moleculav Sieves", Wiley, New York (1974).

"Molecular Sieves", Society of the Qiemical Industry, London (1968).

3

"^ "Molecular Sieve Zeolites", Adv. Chem. Ser. 101 and'jü2 ( 1 9 7 1 ) . ^ "Molecular Sieves", Adv. Chem. Ser. 121 (1973

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6 "Molecular Sieves", ACS Symp. Ser. 40 (1977).

^ W. Loewenstein, Amer. Mineral. 39, 92 (1954).

s D.W. Breck and E.M. Flanigen, ref. 3, p. 47.

9 Ref. 2, p. 595.

10 J.W. Ward,

J. Catal.

J£, 34 (1968).

11 D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, and T.L. Thomas,

J. Am.

Chem. Soa. 78, 5963 (1956).

12 W.M. Meier, ref. 3, p. 10.

13 H. Robson,

Chem. Tech. (Leipzig)

176 (1978).

1'* T.B. Reed and D.W. Breck,

J. Am. Chem. Soc.

78, 5972 (1956).

15 L. Broussard and D.P. Shoemaker, J. Am. Chem. Soc. 82, 1041 (1960).

16 J.V. Smith, ref. 4, hr[, 171 (1971).

17 Ref. 2, p. 92.

18 R.M. Barrer and H. Villiger, Z. Kristallogr. J_28, 352 (1969).

19 W.M. Meier, Z. Kristallogr. U S , 439 (1961).

20 Ref. 2, p. 122.

2 1 E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton,

R.M. Kirchner, and J.V. Smith, Nature Tr\_, 512 (1978).

22 G.T. Kokotailo, S.L. Lawton, D.H. Olson, and W.M. Meier, Nature TJl, 437

(1978).

23 G.T. Kokotailo, P. Chu, and S.L. Lawton, Nature 2JS_, 119 (1978).

2'* U.S. Patent 3.950.496.

25 U.S. 4.086.186, C.A. 89, 77496 (1978).

25 Fr. Demande 2.228.371, C.A. 83, 134800 (1975).

27 Ger. Offen 2.625.340, C.A. 86, 794260 (1977).

28 U.S. 3.642.434, C.A. 76, 142994 (1972).

29 Ref. 2, p. 312.

30 U.S. 4.016.245, C.A. 86, 178180 (1977).

31 U.S. 3.832.449, C.A. 81, 177235 (1974).

32 Ref. 2, p . 531.

33 D.P. Roelofsen, Chemie en Techniek 7J_, 567 (1972).

3'* C.L. Angell and P.C. Schaffer, J. Phys. Chem. 69, 3463 (1965).

35 P.A. Jacobs, "Carboniogenic Activity of Zeolites", Elsevier, Amsterdam

(1977), p. 10.

36 J.A. Rabo, "Zeolite Chemistry and Catalysis", ACS Monograph Vn_ (1976).

37 C.V. McDaniel and P.K. Maher, ref. 36, p. 307.

38 Ref. 2, p. 504.

39 A.P. Bolton, J. Catal. 22, 9 (1971).

••o E.A. Lombardo, G.A. Sill, and W.H. Hall, ref. 4, 102, 346 (1971).

'*i L.F.A.M. van Klooster, Th.M. Wortel, and H. van Bekkum, unpublished results.

'*2 A. Maes and A. Cremers, ref. 5, p. 230.

'*3 L.A. Bray and H.T. FuUam, ref. 4, m_, 450 (1971).

'*'* Ref. 2, p. 588.

'*5 F.A. ^'tumpton and L.B. Sand, "Natural Zeolites, Occurrence, Properties and

Use", Pergamon Press, London (1978), p. 3, p. 441.

'*5 M.J. Schwuger and H.G. Smolka, Colloid Polymer Sci. 2SA_, 1062 (1976).

'*7 M.J. Schwuger, H.G. Smolka, and C.P. Kürzendorfer, Tenside Deterg. V5,

305 (1976).

'*8 P. Berth,

Tenside Deterg.

15, 176 (1978)

'*9 W.K. Fischer, P. Gerike, anJ G. Kurzyla, Tenside Deterg. }5_, 60 (1978).

50 J.W. Ward, J. Catal. 9, 225 (1967).

51 K.H. Steinberg, H. Bremer, F. Hoffman, Ch.M. MLnachev, R.V. Dimitriev, and

A.N. Detjuk, Z. Anorg. Allg. Chem. 404, 129, 142 (1974).

52 K.H. Steinberg, H. Bremer, and F. HöTEiian, Z. Anorg. Allg. Chem. 407, 162,

173 (1974).

53 H.G. Karge, Z. Phya. Chem. N.F. 95, 241 (1975).

5'* D.H. Olson a n d E . Dempsey, J. Catal. U, 221 (1969).

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55 P.A. Jacobs and J.B. Uytterhoeven, J. Chem. Soa. Faraday Trans. I 69, 359 (1973).

56 J.W. Ward, J. Catal. ]]_, 355 (1970).

57 D. Barthomeuf, Acta Physica et Chimica 2A_, 71 (1978). 58 Ref. 35, p. 35.

59 J.W. Ward, ref. 36, p. 118.

60 M. Ikemoto, K. Tsutsumi, and T. Takahashi, Bull. Chem. Soc. Jap. 45, 1330 (1972).

61 W. Kladnig, J. Phys. Chem. 80, 262 (1976). 6 2 Ref. 35, p. 132.

63 J.W. Ward, J. Catal. y\_, 238 (1968). 6'* Ref. 2, p. 425.

65 Ref. 1, p. 121.

66 D.P. Roelofsen and H.W. Saeys, Chemie en Techniek 28, 76 (1973).

67 D.P. Roelofsen, "Molecular Sieve Zeolites, Properties and Applications in Organic Synthesis", Thesis Delft (1972), p. 24.

68 Ref. 2, p. 641.

69 D.R. Burfield, K.H. Lee, and R.H. Smithers, J. Org. Chem. A2, 3060 (1977). 70 D.R. Burfield, G.II. Gan, and R.H. Smithers, J. Appl. Chem. Bioteahnol. 28^,

23 (1978).

71 D.R. Burfield and R.H. Smithers, J. Org. Chem. 43, 3966 (1978). 72 G.T. Kerr, Sepn. and Purif. Methods 2, 283 (1973).

73 D.P. Roelofsen, E.R.J. Wils, and H. van Bekkum, Real. Trav. Chim. Pays-Bas

90, 1141 (1971).

7"* A.V. Kiselev and A.A. Lopatkin, ref. 3, p. 252.

75 A. Grossman and W. Schirmer, Chem. Tech. (Berlin) 20, 34 (1968). 76 p.E. Eberly, ref. 36, p. 392.

77 T. Huizinga, Th.M. Wortel, and H. van Bekkum, unpublished results. 78 U.S. 4.061.724.

79 R.A. Anderson, ref. 6, p. 637. 80 Chapter 8.

81 Chapter 9.

82 The critical diameter of a sorbate is defined as the diameter of the narrowest imaginary cylinder which can still accomodate the molecule; we estimated it by inspection of CPK space-filling molecule models.

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S»1E APPLICATIONS OF ZEOLITES IN ORGANIC CHEMISTRY

General introduction

In addition to the use of zeolites as adsorbants in large scale separations ((?.ƒ. Chapter 1 ) , based on their molecular sieve effects, the high acidity of these materials together with special field effects and ingress-egress selectivity has resulted in their use as catalysts. The large scale

applications of zeolite catalysts are found in the petroleum and petrochemical industry. Commercial catalysts are of the faujasite type (X and Y) the

mordenites and the (natural) erionite type zeolites. The well-established industrial processes that utilize zeolite-based catalysts are catalytic crackingi"'* today by far the most important application, hydrocracking'*"6 and paraffin isomerization5. Other processes that use zeolite catalysts are

aromatic alkylation, disproportionation and iscmerization of aromatic hydro-carbons 7. Recently a process of zeolite-catalyzed methanol conversion to gasoline has been described8>9. in synthetic organic chemistry zeolites are so far just incidentally used as catalysts.

In the following section a short description will be given of the most important uses in industry, followed by a review of some applications of zeolites in organic chemistry. First attention will be given to so-called shape selective catalysis exerted by zeolites which is due to their uniform pore size.

Shape selectivity

Generally the molecular dimension of organic reactants constitute an important factor when zeolites are applied as catalysts. Only molecules with a critical diameter less than the effective pore diameter can enter the pore system and react inside the zeolite. Furthermore, only products which can desorb from

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the zeolite will appear in the final product. Bulkier molecules will react or will be formed only on the outer surface of the zeolite. This phenomenon of shape selectivity was firstly described by Weisz et al. 10>11. These authors observed a selective dehydration of 1-butanol in the presence of isobutanol on CaA zeolite and only cracking of n-paraffins in the presence of iso-paraff ins. In the latter case only straight chain products were formed.

Three types of shape selectivity may be considered, in general6>i2. (i) Reactant selectivity, only a part of the reactants can pass through

the zeolite pores.

(ii) Product selectivity, only a part of the products formed in the zeolite are desorbed. The formation of bulkier products inside the zeolite results in pore-blocking, unless they are in equilibrium with smaller products, which are capable to desorb from the inner void.

(iii) Reaction selectivity, which occurs when inside the zeolite cavities certain reactions out of a set of two or more parallel reactions are stimulated or suppressed. The latter for instance because the corre-sponding transition state requires more space than available in the cavities or the pore mouths.

It is noteworthy that although the external surface is rouglily M of the total surface, the existence of intracrystalline diffusion could make the interior sites less effective and as a result catalysis on the aselective

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outer surface becomes more important than corresponds with its surface area. Shape selectivity may be inproved by either selective removal of exterior catalytic sites or by irreversibly poisoning them with strongly adsorbing molecules which have a critical diameter larger than the effective pore size of the zeolite.

Examples of shape selectivity have been reported for small pore zeolite A (0.3-0.5 nm), erionite (0.4 nm), zeolite T (0.4 n m ) , offretite (0.6 n m ) , and Z9>1-5 (0.6 n m ) . With faujasites (0.7 nm) shape selectivity occurs only when relatively large molecules are involved. Examples of shape selective cracking, hydrocracking and transalkylation are given by Csiasery^.

Applications in industrial catalysis

Catalytic cracking

At present, all commercial cracking catalysts contain zeolites (5-401), generally rare-earth exchanged Y-zeolites, (REY and REHY) dispersed in a matrix of silica-alumina or clay. At the same conversion level of gasoil, the use of zeolite-promoted cracking catalysts results in increased yields of light cycle-oil, increased yield and octane number of the gasoline product fraction and a decreased production of coke. Over zeolites a mixture is obtained, which is ridier in paraffins and aromatics than would be the case with the conventional silica-alumina catalysts.

Nowadays most petroleum companies have developed modifications of the fluid catalytic cracking process based on the principle of risercracking. The relationship between catalyst properties, feedstock composition, reactor operating conditions and results obtained are very complex and have been reviewed in several articlesi"3.

Hydrocracking

Catalytic cracking in the presence of hydrogen and a dual function catalyst possessing both cracking and hydrogenation-deliydrogenation activity, is at present the second largest application of zeolite catalysts5. The purpose of hydrocracking is to convert high boiling feedstocks into lower boiling products. Polycyclic aromatics must first be partially hydrogenated before the cracking of the ring system can take place. The sulfur and nitrogen atoms, present as simple sulfides and more complex heterocyclics, are con-verted into hydrogen sulfide and ammonia. An additional and probably more important role of the hydrogenation component is to hydrogenate the coke precursors rapidly and to prevent their conversion to coke residue on the

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catalyst.

Generally such catalysts consist of an acidic, hydrothermally stable large pore zeolite (rare-earth exchanged X, Y) loaded with a small amount of a noble metal (Pd or Pt). Advantages of these catalysts are (i) the high activity at relatively low temperature and hydrogen pressure, and (ii) the ability to operate in the presence of significant amounts of hydrogen sulfide, ammonia and other nitrogen-bases, in contrast with silica-alumina catalysts.

Two hydrocracking processes in which the shape selectivity of zeolites is exploited are in current use. The Selectoforming process of Mobil Oil employs a metal loaded zeolite of the offretite-erionite type for the selective hydrocracking of normal paraffins of a catalytic reformate5>6. The second process is catalytic dewaxing, which employs a metal loaded tubular-pore

zeolite of the.mordenite type6.

Paraffin isomerization

Here, the catalyst is also a dual functional one5. In hydroisomerization low octane number paraffins like pentane and hexane are converted into iso-paraff ins. However, since hydrocracking results in a direct reduction of the gasoline yield, the process operating conditions are more stringently defined. By adjusting the lowest possible reactor temperature and hydrogen pressure consistent with economical production rates and run lengths the yields of the highly branched isomers are maximized and hydrocracking is minimized. It is interesting to note that the hydroisomerization process, Hysomer, of Shell Oil Company can be operated in conjunction with the Union Carbide Corporation's

Isosiv-process for the separation of normal from isoparaffins, thus enabling complete isomerization of a C5/C6 stream. The combined process' trade-name is Total Isomerization Process5. . - '

Methanol conversion

Recently Mobil Oil described8>9 a methanol-to-gasoline process using an acidic

ISA-S catalyst. This process offers an alternative to the well-established Fischer-Tropsch process for converting hydrogen and carbon monoxide into hydrocarbons. Tlie Mobil-process produces aromatic rich gasoline, which

contains no aromatic compounds with more than ten carbon atoms. No diesel fuel or residua of any kind are produced. Ihe only quality problem that has appeared so far is the presence of small amounts of durene (1,2,4,5-tetramethylbenzene). Durene has a very high octane number, however, it also freezes at 79° and probably could plug carburators. Any such problems can be eliminated by keeping durene concentrations below M. Although zeolite-catalyzed conversion of

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methanol is an interesting route from coal to motor fuel, it is not yet of commercial interest because the processing costs are about 2-3 times too h i ^ .

Applications in (synthetic) organic chemistry

Most attention in this field has been given to the application of zeolites in gas phase reactions, relatively few data on the application in liquid phase procedures have been presented in the literature. The most extensive source of information concerning organic reactions catalyzed by zeolites at relatively high temperatures is the review of Venuto and Landis'^^. A recent review given by Jaaobs^^ is mainly concerned with acid catalyzed reactions over zeolites and bifunctional metal loaded zeolites carried out in the period 1970-1975. A review of Csiosery^ concerning shape selective catalysis mainly describes reactions in petrochemistry. Poutsma^^ describes transfor-mations of hydrocarbons and reactions of molecules containing hetero atoms. Catalysis over faujasite zeolites is described by Rudham and Stoakwell^^. Venuto'^'' describes the mechanistic aspects of electrophilic aromatic

substitution and reactions of alkylaromatics over zeolites.

The following section is not exhaustive and treats mainly work related with our investigations. Also some attention is paid to reactions which received little or no attention in the reviews mentioned above.

Shifting chemical equilibria

The use of zeolites as selective adsorbents and as catalysts may be success-fully combined in shifting chemical equilibria e.g. as has been shoivn by work from this laboratory. A selective removal of component D by adsorption

in a zeolite will shift the equilibrium to the right. The following procedures can be applied.

...

r^

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Often the equilibration is homogeneously catalyzed and the zeolite only serves to adsorb selectively component D (I). However, catalysis b y wide-pore zeolites is sometimes possible (II). Here the co-adsorption of A, B and C is a disadvantage, resulting in a partial conversion. Method (II) may be improved by using a combination of a large pore zeolite to catalyze the reaction and a small pore zeolite for a selective adsorption of component D (III).

Important variables in shifting chemical equilibria are the amount and type of zeolite (adsorbent), the solvent used, the temperature, the amount and type of catalyst (homogeneous or heterogeneous) and the experimental technique. Generally three experimental techniques are applicable.

(i) Slurry technique. The zeolite is suspended in the liquid reaction mixture. A fast removal of D is thus possible. However, unwanted

interactions of the catalyst or one of the reactants with the zeolite may occur (,e.g. ion exchange).

(ii) Extraction technique. The condensed vapor of the boiling reaction mixture flows through a compartment filled with zeolite (Soxhlet apparatus). After (partial) adsorption of D by the zeolite the solvent returns to the reaction mixture.

(iii) Flow technique. The reaction mixture flows through a fixed bed of zeolite. Here similar advantages and disadvantages occur as in the slurry technique. When using a solid catalyst a two compartment technique as applied for the first time in this investigation, where zeolite adsorbent and catalyst are placed in separate compartments, may be applied. In this way a separate regeneration of catalyst and adsorbent is possible.

In the present study the slurry and two compartment flow technicjues have been explored [cf. Chapter 3 ) .

It must be realized that very low concentrations of D are necessary to obtain a complete conversion, as illustrated in Table I.

Table I Concentration lowering of D needed to shift the equilibrium A + B ::^ C + D to 95, 99 and 99.9% convevsion i n i t i a l conversion (l)^ 25 75 K 0,11 9.0 c o n c e n t r a t i o n 95"6 3.10-4 23.10"^ (M) of D a t 99°» I.IO"^ -4 9.10 conversion of 99.9°i, I.IO"^ 9.10-'^

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In several well-known chemical equilibria like ester, acetal and enamine formation the equilibrium is shifted by water adsorption. For example, the data in Table II show that the required low water concentrations can be obtained by adsorption in zeolite A especially in apolar solvents.

Table II Residual water content of several solvents after treatment with 5% w/v zeolite NaA at 26-30 °ci8

solvent residual water content (ppm) (M) toluene 0.01 4.8 lO"''

benzene 0.03 , 1.5 10"^ dichloromethane 0.07 5.2 10"^ diethyl ether 2 7.9 lO"^

1,4-dioxane 13 7.4 lO"*^ tetrahydrofuran 28 1.4 10

A short description follows of some reactions studied in this laboratory. Water adsorption by zeolite A during the enamine formation, starting from a carbonyl compound (e.g. cyclohexanone, camphor, acetophenone) and a secondary amine (e.g. diethylamine), resulted in a nearly complete conversion into the corresponding enaminesi9.

\ , ^ ° \^/NR,

I + H N R ! :;:= II + HjO

Some experiments have been performed with the zeolites H-mordenite and SK-500 (commercial REY) as catalysts, however, the amorphous silica-alumina cracking catalysts appeared to be the most active. This is noteworthy because adsorption of cmiine on the acidic surface would poison the catalyst and deactivate the amine for reaction witli the carbonyl compound. In Chapter 4 attention is paid to this phenomenon.

IVlien using A zeolites for water adsorption and using homogeneous acid catalysis, acetals are prepared in high yields20.2i i^y reaction between a carbonyl compound (e.g. acetone, benzaldehyde) and primary or secondary alcohols (e.g. methanol, propanol, cyclohexanol).

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