Organic Catalysis over Crystalline Aluminosilicates

Organic Catalysis over Crystalline Al um inosiIicates P. B . VENUTO

AND

P. S . LANDIS

Research Department. Applied Reaearch and Development Division and Central Ronearch Division, Mobil Reaearch and Develqpment Corporation. Paulaboro and Princeton. New Jeraey

I . Introduction and Scope ........................................ 269 A. Introduction ................................................ 269 B Scope ...................................................... 261 I1 Relation between Catalyst Structure and Catalytic Activity ......... 261 A . Factors Related to Accessibility of Sites ........................ 261 B Factors Related to Nature of Catalytically Active Sites . . . . . . . . . . 277 I11. Organic Reactions Catalyzed by Crystalline Aluminosilicates ......... 306 A. Olefin-Forming Eliminations and Related Reactions ............. 306 B Polymerization. Isomerization. and Related Reactions of Olefins . . 316 C. Electrophilic Aromatic Substitution and Related Reactions ....... 319 D Condensation and Cyclization Reactions ........................ 340 E Acetal and Ketal Formation .................................. 346 F . Beckmann Rearrangement of Ketoximes ....................... 348 G . Epoxide Transformations .................................... 361 H . Oxygen-Sulfur Exchange Reactions ............................ 362 I Olefin Carbonylation ......................................... 366 J . Amination Reactions ........................................ 367 K Hydrogenation. Dehydrogenation. and Related Reactions ........ 360 L . Miscellaneous Reactions ..................................... 366 References .................................................... 366

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I. Introduction and Scope A . INTRODUCTION

Z olit s (molecular sieves) are crystalline aluminosilicates composed of SiO4 and A104 tetrahedra arranged in various geometric patterns. The tetrahedra are linked together a t the corners by shared oxygen ions to form ordered lattices. which are often best visualized as threedimensional combinations of chains. layers. and polyhedra . At least

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30 naturally occurring zeolite minerals and three purely synthetic

varieties are known (1).Current workers in the zeolite field must further acknowledge a tremendous debt to the pioneering fundamental studies of Barrer I n typical syntheses, sodium zeolites result when reactive gelsformed from mixing aqueous solutions of sodium aluminate, sodium silicate, and sodium hydroxide-are allowed to crystallize a t temperatures of 25-100". Hydrated zeolites may be regarded as polyanionic frameworks surrounding an aqueous solution of cations. The cations are required to maintainelectrical neutrality since each A104 tetrahedron is associated with unit negative charge. Zeolites can undergo base exchange with a wide variety of cations, a procedure that substantially modifies their properties. Further, they exhibit electrical conductivity of an ionic type. The structure and properties of crystalline zeolites have recently been reviewed (1,2). The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. The striking affinity of dehydrated zeolites for adsorption of a wide variety of guest molecules in a highly selective manner was recognized a t an early date by Barrer and others (3-5). Such studies have ultimately led to the development of several important commercial separation, drying, and purification processes using zeolites. I n 1960, Weisz, Frilette, and co-workers first reported molecularshape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics ( 8 ) . I n 1961, Rabo and associates ( 9 )presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,

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the driving force being the ease of study of the crystallographically well-defined zeolite surface, with its potential for clean structure-reactivity correlations.

B. SCOPE

A broad spectrum of organic reactions, including both classical

acid-catalyzed transformations and those where strong acidity is by no means a requirement, is examined, and a considerable amount of previously unreported data is included. Where possible, mechanistic interpretations are given. Selected data on reactions with simpler molecules, more properly called “inorganic,” are also included. Cracking and related processes are not discussed in detail unless germane to structure-reactivity relations. Although reactions over a wide variety of crystalline aluminosilicates are covered, emphasis is placed on catalysis over X- and Y-type faujasites. Unusual opportunities for organic catalysis were found to exist when these were base exchanged to substantially eliminate their alkali metal content. It is not proposed to review the properties of zeolites in general. Structural aspects related to catalysis are, however, discussed in detail. Accessibility of sites and other related factors basic to an understanding of zeolite catalytic behavior are also examined. Considerable emphasis is given to the chemistry of activation processes, the characterization of acid zeolite catalysts, and the physicochemical interactions of adsorbed molecules with zeolite surfaces.

II. Relation between Catalyst Structure and Catalytic Activity A. FACTORS RELATEDTO ACCESSIBILITY OF SITES Let us assume that the chemical transformations in zeolite catalyst systems occur within the high surface area intracrystalline volumes. Then, for a reaction within a zeolite particle it is apparent that both the entry pores and the channel-cavity system must be open enough to allow transport of reactant molecules from the bulk phase to the active sites (and vice versa). Thus, any crystalline sieve that could sorb simple organic molecules such as n-hexane might conceivably have catalytic potential. Factors pertinent to these processes are discussed below.

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1. Qeometry of Internal Pore Systems

a . Sodalite Group. Members of this group have cubic symmetry and a framework based on simple combinations of truncated octahedra. Each of these octahedra (Fig. 1) (sodalite units) is composed of 24

FIG.1. Truncated octahedron or sodalite unit: vertices represent Si or A1 atoms; lines represent oxygens.

(Al, Si) ions (vertices) interconnected with 36 oxygen anions, and contains eight hexagonal and six square faces. The simplest member of the family is sodalite (Fig. 2), in which the truncated octahedra share

FIG.2. Line drawing of sodalite structure.

square and hexagonal faces. Sodalite is actually not a zeolite, and basic sodalite sorbs only small polar molecules such as water or ammonia through the 6-ring apertures. The next member of the series is zeolite type A (10).Here each sodalite unit is linked to its neighbor (by the square faces) by four bridging oxygen ions (Fig. 3). This configuration gives a roughly spherical internal cavity, 11.4 A in diameter (a-cage), that is entered through six circular orifices-formed by %rings of oxygen-with a diameter of 4.2 A. The

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FIG-.3. Line drawing of zeolite t y p e A structure.

sodalite units themselves enclose a second set of small cavities @-cage) of 6.6 11 internal diameter. The t9-cages interconnect with the large cavities by a distorted 6-ring of oxygen atoms 2.2 A in diameter. The unit cell composition of zeolite A (Linde 4A) is Na12. (A102)12*(Si02)12* 27H20. Eight of the sodium ions are situated in the center of the 6-rings in the hexagonal faces (type I), and four lie adjacent to the 8-ring. When 40% or more of the sodium cations within 4A molecular sieve are replaced by .:alcium ions [Linde 6A], the effective pore diameter becomes about 6 A, and n-paraffins, but not branched paraffins, may be adsorbed. the framework consists of a tetraI n faujasite-type zeolites (11,12), hedral arrangement of sodalite units linked by hexagonal faces with six bridge oxygen ions (Fig. 4). This results in a series of wide, nearly spherical cavities (supercages),each of which opens by common windows (distorted 12-rings of 8-9 A diameter) into four, identical, tetrahedrally distributed cavities. The overall view of the faujasite lattice is that of a tightly packed aggregate of oxygen atoms interlaced with large voidsa t least for a highly ionic model where the aluminum and silicon ions are small enough to occupy the center of the tetrahedron of four oxygen ions. The capacity of the approximately 12 A supercage is such that a t 25' i t can contain 2.8 isooctane or 5.4 benzene molecules (12).The small pore system, consisting of the interiors of the sodalite cages and the

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Fro. 4. Line drawing of faujasite structure.

hexagonal prisms, is inaccessible to most organic molecules. The unit H2O. The basic framecell of Linde 13X is Na~a.(AlO2)~a.(SiOz)los.264 work for zeolite Y is the same as that of zeolite X, but the Si/Al ratio is higher, generally ranging from 1.5 to 3.0.

b. Mordenite. In contrast to the sodalite zeolites with a threedimensional pore structure that contains cages, mordenite has only a two-dimensional, tubular pore system. A schematic diagram of a cross section of the mordenite structure is shown in Fig. 6. The crystal structure (13) consists of chains of 4- and 5-rings of Si and A1 tetrahedra n n n n

FIG.6. Cross section of mordenite struoture. From Moier (13).

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linked laterally so that a system of large, elliptical, parallel channels interconnected by smaller cross channels is created. These elliptical tubes, which are formed between the chains by rings of 12 tetrahedra, have a major and minor diameter of 6.95 and 5.81 A, respectively, and constitute the main sorption-diffusion system in mordenite. Some of the cations in mordenite are thought to be located in these channels. The large channels are perpendicularly intersected by smaller side pockets with a minimum free diameter of about 3.9 A. The unit cell of the mineral mordenite is Nas (Al0z)s (SiOz)40-24H20 (13).The great thermal stability and acid resistance of mordenite are thought to reflect the high Si/Al ratio and the presence of 4- and 5-rings. Mordenite also is sensitive to stacking faults (13), periodic lateral displacements of the lattice network, that can result in the reduction of the channel diameter from 6.6 A to about 4 A. Such faults may be responsible for the lack of capacity of mordenite for larger molecules. Recently, synthetic mordenites more nearly free of stacking faults have become available (14,15); these new compositions have considerable capacity for larger molecules such as benzene, cyclohexane, etc.

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c. Chabazite Group. Zeolites in the chabazite group ( 1 ) have aluminosilicate frameworks best represented in terms of sheets or layers of linked rings of tetrahedra; the shapes of their cavities differ considerably from the roughly spherical large cages of faujasite or Linde type A. Figure 6 depicts the large cavities in chabazite (16) (Car.

FIG.6. Line drawings of (a) chabazite structure and (b) erionite structure. From Barrer (1).

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(A~OZ)S-(S~OZ)IS26H20) and erionite (17) ([Ca, Mg, Na2, K214.6(A102)o.(SiO2)27*27H20), both of which are capable of sorbing nparaffins. The cavities in chabazite are roughly ellipsoidal, linked by distorted (elliptical) 8-rings with dimensions of about 4.4 x 3.1 A. Significantly, the framework of chabazite distorts upon dehydration, as the Cat+ ions move from the large cavities into the hexagonal prisms (2). The distorted 8-ring aperture of erionite, which is closely related to chabazite, has dimensions of about 3.6 x 5.2 A; the main cavities of erionite have free dimensions of about 6.3 A and 15.1 A. 2. Locus of Catalysis and Molecular-Shupe Selectivity From the previous discussion, it follows that the intracrystalline volume in zeolites is accessible only to those molecules whose size and shape permits sorption through the entry pores; thus, a highly selective form of catalysis, based on sieving effects, is possible. Weisz and coworkers (7) have conclusively established that the locus of catalytic activity is within the intracrystalline pores: when Linde 5A sieve (- 5 A pore diameter) was used, selective cracking of linear paraffins, but not branched paraffins, was observed. Furthermore, isoparaffin products were essentially absent. With the same catalyst, n-butanol, but not isobutanol, was smoothly dehydrated a t 230-260". A t very high temperatures, slight conversion of the excluded branched alcohol was observed, suggesting catalysis by a small number of active sites located a t the exterior surface. Similar selectivity between adsorption of n-paraffins and branched-chain or aromatic hydrocarbons is shown by chabazite and erionite (18). By making a comparison of the rates of dehydration of sec-butanol over Linde 1OX and 5A zeolites a t relatively high temperature and low conversion, Weisz (7) also found that the rate constant per unit volume of 5A was between two and three orders of magnitude smaller than that of 1OX. These relative magnitudes were consistent with the ratio of available surface areas (0.6-3.5 m2/gm for the external area of 1-5p sized crystals of shape-selective 5A and 500-700 m2/gm for lox, where the internal surface was accessible to the sorbate. The rapid adsorption kinetics and rectangular adsorption isotherms observed with faujasite-type zeolites, a t least with molecules whose dimensions are not close to those of the pores, attest to the strong driving force for occlusion within the intracrystalline volume. Further,

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with the relatively open (8-9 A pore diameter) faujasites, sieving effects based on molecular size or shape alone are not generally observed, except with large, bulky organic molecules. It is concluded that, for the most part, catalysis in the porous crystalline zeolites occurs within the intracrystalline voids. However, some role must be allocated to the external surface, especially a t higher temperatures, to explain the formation of isobutylene polymers over chabazite (19),the dehydrohalogenation of tert-butyl chloride over 5A (20), and the detection of small amounts of triphenylbenzene and triphenylphenanthrene in the condensation of acetophenone over hydrogen Y (HY) zeolite (21).Table I shows the external surface areas of TABLE I External Surface Area8 of Faujaaite Catalyeta External surface areaa

Average particle size (microns)

Catalyst

REX REY NH4Y-HY

2.3 1.2 2.6

m21gm

2.35 6.98 3.50

% of total BET surface area 0.49 1.23 0.52

~~~

Calculated on basis of cube with side equal to average particle size.

some modified faujasite catalysts discussed in detail later. 3. Factors in Sorption Rates and Diffusion a. General Factors.

It is well known in heterogeneous catalytic sys-

tems that the chemical transformation at the catalyst site may not be rate determining (22). Indeed, the adsorption of molecules colliding with a catalyst surface, the desorption of products from the exterior surface to the bulk fluid phase, the diffusion within the internal pore system, or even the diffusion through the bulk fluid from one catalyst particle to another may be rate determining or at least affect the overall rate. Frabetti (23) has made a study of the diffusion of CrCa hydrocarbons in Na+ mordenite a t temperatures from 25 to 140' and a t pressures ranging from 0.2 to 20 cm Hg. The experimental diffusion coefficients

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obtained by transient adsorption rate and desorption rate measurements under these conditions were of the order of lo-9-10-10 cmZ/sec. As tabulated by Frabetti, the experimental diffusion coefficients reported for zeolites with smaller pores, such as Linde A and members of the chabazite group, are seen to be extremely low (varying between 10-6 and 10-16 cm2/sec) when compared to values for other porous solids (24). Selected aspects of sorption kinetics and diffusion, as related to zeolite and substrate structure, are discussed below.

b. Physicochemical Interactions of Xorbates with Zeolites. Barrer ( 4 ) has defined the diversity of energy terms that contribute to the physical bond in zeolitic systems. The various interactions that, together with pore geometry, determine selectivity in adsorption are enumerated below : (1) dispersion energy

(4~);

(2) close range repulsion energy ( 4 ~ ) ; (3) polarization energy ( 4 ~ ) ; (4) field-dipole energy ( 4 ~ ~ ) ; ( 5 ) field gradient-quadrupole energy ( 4 i ~ ) ; (6) dipole-dipole energy (c$&~); (7) dipole-quadrupole energy ( 4 f l ~ ) ; (8) quadrupole-quadrupole energy ( ~ Q Q ) . Factors 1 and 2 are universally found in sorption systems, and the approximate additivity of dispersion energies for all atom pair interactions ensures the sorption, a t low temperatures, of large molecules, even n-paraffins. When a sorbent is composed of positive and negative ions (as in a zeolite), there exist local electrostatic fields, F, that polarize the sorbate of polarizability a. Thus $P = -&aFz, where the negative sign implies exothermal reaction. Such effects could be visualized in terms of distortion of electron clouds in certain p - or n-electron systems. The electrostatic factors 4 and 5 are a t least as important as the sum of dispersion, repulsion, and polarization [forces in zeolites. Thus a local field, F ,also interacts with molecules possessing permanent dipole moments, such as NHs or HzO: that is, 4~~= - F p cos 8, where p is a point dipole with its axis oriented a t angle 8 to the local field direction. Similarly, the field gradients, I”, associated with the anionic framework and associated cations will strongly interact with molecules possessing permanent quadrupole moments, i.e., Nz, CO, COz. Factors 7 and 8

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are normally small. Although most sorption studies involving zeolites have been conducted with rather simple molecules a t relatively low temperatures, it is probable that the interactions enumerated above play an important role in catalytic reactions a t higher temperatures. c. Factors Related to Molecular Sieving. In a zeolite-sorbate system, a tremendous number of kinetic collisions impinge upon the external pore openings on the crystal surface. The variety and proportions of sorbates admitted to the intracrystalline cavity system, however, are controlled by the free dimensions of the entry windows. Some of the factors related to the “molecular sieving effect” are discussed below.

(i) Zeolitic structure. ( a ) Ring size. Zeolites incorporating four, five, six, eight, ten, and twelve-membered oxygen rings into their lattice structure are known ( 1 ) .Even small molecules cannot diffuse through 4or 5-rings, and an appreciable energy of activitation is involved in the passage of HzO or NHBthrough the 6-ring of basic sodalite (25). Ease of penetration increases rapidly thereafter with ring size, notably with the 12-ring orifices in faujasite. ( b ) Ring shape. Owing to ring puckering, not all rings containing the same number of atoms are equivalent in size. Thus Linde A, ZK-5 (25a), chabazite, erionite, and some other members of the chabazite group all have 8-ring windows, but the free dimensions of their orifices vary with the degree of nonplanarity ( 1 ) .Linde A and ZK-5, purely synthetic aluminosilicates, are the only known zeolites possessing %ring planar windows which freely admit n-paraffins. Ring distortion considerably restricts the diffusion rate in erionite and some of the others. Structures with %rings can therefore exert a wide range of molecular sieving behavior based on ring distortion alone. (c) Eflect of cation. The number, size, valency, and location in the lattice of zeolitic cations have important effects on the size and shape of the entry pores. This is most dramatically demonstrated in zeolite A, where replacement of 30-40% of the Na+ ions by Ca++ results in an increase in effective pore diameter from about 4.0 A to about 5.0 A (2,lO).Marked effects of cation upon diffusion coefficients observed with various gases in chabazite, mordenite, and levynite have been reported by Barrer and Brook (26).Also, diffusion coefficients about 50% lower in hydrogen mordenite (H-mordenite)than in Naf-mordenite for simple

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low molecular weight hydrocarbons have been observed (23). Since cations tend to partially obstruct intracrystalline channels, Barrer and Baynham (27) have suggested that sorption rates might normally be expected to decrease with increasing cation density.

( d ) Silicon: aluminum ratio. Since Si-0 bonds are slightly shorter than A1-0 bonds, a given ring size may decrease slightly with higher silicon content (28).Also, since each lattice A104 unit is associated with unit negative charge and an associated cationic positive charge, an increased charge density will result from a decrease in Si/Al ratio. The overall result will be a decrease in the rate of sorption of polar organics such as carboxylic acids. ( e ) Effect of solvation. It is well known that the framework in crystalline aluminosilicates is not totally rigid (I),and that lattice distortions and changes in cation position accompany the hydration-dehydration processes (16,29).Thus, the time and temperature of activation will be variables influencing the sorption rates of certain substrates (30). Controlled addition of polar modifiers that are strongly adsorbed and relatively immobile a t the temperature of subsequent sorption can act as additional barriers in pore systems ( 1 ) .Thus Barrer and Rees (31,32) have shown that addition of small amounts of HzO, NH,, or CHsNH2 to zeolites such as chabazite or mordenite, results in the variation of diffusion coefficients of ethane or permanent gases by as much as a factor of i03-i04.

(ii)Sorbate structure. ( a )Effect of Molecular Dimensions. Whether or

not a sorbate molecule can enter a zeolite pore of given dimensions depends on its critical dimension (smallest projected diameter). Therefore, for a given cationic form of mordenite or chabazite, the experimental diffusion coefficients ( DE)for some simple gaseous molecules decrease with increasing sorbate diameter (32,32a).Sorption rates have generally been observed to vary inversely with the molecular weight (23,29)of the sorbate or with the number of carbon atoms in straight chain hydrocarbons. For the latter, entropy effects must play an increasing role as chain lengths increase. It will be recalled that molecules possess translational, rotational, and vibrational energy and are adaptable to conformational changes. In addition, atoms are really not rigid spheres, but soft and deformable:

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Breck (2)has observed that molecules with diameters about 0.5 b larger than the pore diameter can actually intrude into the internal zeolite cavities. Nevertheless, as the critical dimensions approach the pore diameter, interactions between sorbate and entry pore become increasingly important. Activation energies of diffusion may increase markedly (26), and minor changes in molecular structure may be reflected in decreased rates of diffusion. These difficulties in penetration would be expressed as an increased energy barrier, E, in the Arrhenius equation for the diffusion coefficient, D E = DOexp(-E/RT) (1).A more negative entropy of activation would also be expected, and the entropy term would appear in the preexponential coefficient DO.In the limiting caae of complete exclusion of one molecule-but not another-molecular shape-selective catalysis becomes possible. Frabetti (23) has observed an activated diffusion process in Na+-mordenite, where the activation energies for diffusion of methane and ethane in the temperature range 25-140" were 1.7 and 1.9 kcal/mole, respectively.

(b) Polarity of sorbate. However, steric and molecular weight considerations are not the only factors in molecular sieving. From the diversity of energy terms described above (Section 3,b), it follows that extremely strong interactions of molecules containing polar, i.e., -OH, ,C=O, -NH2, or polarizable, i.e., C-C, CsHs-, functional groups with zeolite surfaces can result. If such interactions are encountered with lattice structural elements (notably cations) a t the pore mouths on the external surface of the zeolite crystal, sieving effects may be observed. Goldstein (33) has observed an inhibition in sorption of simple aliphatic carboxylic acids, nitriles, and nitro compounds in zeolites containing 8-ring entry pores, where simple steric considerations were clearly not the determining factor. This inhibiting effect also increased as the silicon :aluminum ratio decreased (and cation density increased). (iii) Effect of temperature. Thermal vibrations of oxygen atoms surrounding the apertures and of the cations in various adjacent positions prevent total rigidity of the zeolite lattice. With temperatures from - 193 to 27", a variation in vibrational amplitude of 0.1-0.2 A is possible, and thermal vibration alone could cause a variation of 0.3 A in pore diameter (2).It also seems probable that even greater variation in pore size could occur at the higher temperatures of many catalytic reactions over zeolites. In addition, organic molecules have greater

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kinetic, rotational, and vibrational energy at higher temperatures, and consequent greater mobility, rate of change of conformation, and deformability. Accordingly, the sensitivity of molecular sieving in a zeolite-sorbate system can be greatly increased by lowering the temperature (1). d. Some Factors in Intracrystalline Diffusion. After the organic molecule has collided with the external pore openings on the zeolite crystal surface and passed through the pores, diffusion within the intracrystalline pore system of the zeolite can occur. From the studies of Barrer et al. (12)in near-faujasite systems, it seems likely that once it is occluded within the internal pores, the sorbate never leaves the influence of the pore wall, since no part of the channel is wide enough to escape the fields emanating from the lattice. Since the molecules must collide frequently with the pore wall, a Knudaen-like activated diffusion, similar to surface diffusion, must be involved (1).Similar tendencies would certainly exist in zeolites with smaller pores.

(i) Effects of lattice structure. Once within the intracrystalline pore system, mass transport could be retarded by various energy barriers located at intervals within the channel system. The source of these barriers would be channel constrictions due to small O-rings, cations of varying size and valence physically protruding into the pores, sorbed modifiers, and in short, all those variables discussed in Section 3,c,i. The gross geometry of the pore systems, i.e., three-dimensional, open in faujasite vs two-dimensional, tubular in mordenite, must also be considered.

(ii) Effect of sorbate structure. As described earlier (Section 3,c,ii,)the energy barriers can vary greatly with sorbate structure. Barrer and Brook (26) have reported a strong dependency on the polarity of the molecule in the observed diffusion coefficients and activation energies of diffusion for several molecules of similar size and shape (C3H8, CHZC12, CH3NH2) in zeolites with small pores. When mixtures of various low molecular weight gases are passed through a long column of a given cation-exchanged form of a zeolite, different retention volumes are often observed (34). These chromatographic effects are further evidence of the existence of diverse sorbate-lattice interactions. A fairly general observation has also been that the experimental diffusion coefficient is

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dependent on sorbate concentration in the zeolite, at least within certain ranges.

( i i i ) Temperature eflect on sorbate-lattice interaction. Kiselev (35)has made an interesting observation in this regard. In gas chromatographic studies with A- and X-type zeolites, the ratios of retention times of molecules with no T bonds or quadrupole moments (C2Hs/CsHa, Oz/CH4)did not change greatly with temperature. However, the ratios of retention times of species with T bonds and large quadrupole moments and those species lacking them (C2H&Hs, N2/CH4) decreased sharply as temperature was increased. This observation suggests that the contribution of interactions between zeolite cations and species with T bonds (+p) and quadrupole moments (+&) sharply decreases as temperature increases. Because it has been observed that diffusion in zeolites is an activated process (l,23),one might expect the experimental diffusion coefficient, DE, to increase correspondingly, owing to the increased thermal energy of the sorbate. Frabetti reported an apparent exception to this prediction (23): the observed relative insensitivity to temperature in values of DE obtained for propane and butane in Na+-mordenite within the temperature range 25-140". In this case, it was proposed that the effect of the additional thermal energy available to the diffusate at the higher temperatures was counterbalanced by increased chemical interaction between diffusate and zeolite. ( i v ) Effect of lattice defects. Many zeolitic lattice structures are probably not ideal, and may consist of polycrystalline aggregates, or be pervaded with disordered regions. These may profoundly influence the magnitude of any experimentally derived diffusion coefficients, since fluid phase or surface phase diffusion may occur through these domains. Thus stacking faults in mordenite (13),amorphous regions in mordenite (23) and chabazite (36), sintering in Linde A (37), and hydrolysis of surface skin in erionite (28) have been suggested as causes for irregulaities in diffusivities. The presence of impurities-such as trapped salts, alkali or silica, crystallites of other zeolites, or coke deposits-will all tend to lower diffusion rates. ( v ) The nature of the intrazeolitic phase. The narrow, rigid intracrytalline channels in zeolites exert a compelling tendency toward

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capillary condensation, strong adsorption, and a liquidlike state at low temperatures with hydrocarbons (12,38)and with small polar (25,39) substrates. The driving force for liquefaction is greatly amplified in the dehydrated zeolite because of the powerful Coulombic fields (40,41)and polarity operative within the pore system, no channel of which is wide enough to escape the influence of the lattice-derived fields (12). The influence of these forces is manifest in the type I adsorption isotherms commonly observed with zeolites. The curves rise very steeply initially, and approach their asymptotic equilibrium values at p / p o values that are much lower than for any other sorbent. From another standpoint, it may be said that the mobility of the guest species within the channels is considerably lower than that in the bulk fluid; that is, the occluded molecules have sustained significant decreases in translational, rotational, and vibrational degrees of freedom (entropy effects), compared with their vapors (12).Similar effects would be expected in relatively small pore ion-exchange gels, such as Dowex 60 or Amberlite I.R. 120, with aqueous or polar solvents systems (42). Thus, zeolitic water may be viewed as intermediate in mobility between the bulk liquid and ice ( 1 ) .Further, clusters of molecules may exist in zeolite cavities. In faujasite, due to its openness, these clusters are not merely isolated, but form continuous filaments of dense zeolitic fluid (12). For temperatures not far removed from 25O, the apparent saturation number of guest molecules varies with temperature approximately as the density of the bulk liquid. Thus the sorption value (at p/pO = 0.6) for toluene molecules occluded per unit cell in Na+-faujasite (12) decreases by only 3% as the temperature is increased from 40 to 80'. Similarly, a decrease of about 4% was observed for benzene over the temperature range 25-70'. Thus the intrazeolitic organic phase in catalytic reactions may clearly be visualized as liquid like in character a t temperatures below the boiling point of the reactant. Based on the previous discussion, it also seems probable that even at temperatures somewhat higher than the reactant boiling point, a rather dense intrazeolitic fluid exists, even if the bulk reactant phase contacting the zeolite crystal is gaseous.

( v i ) Catalyst aging. As a final consideration related to accessibility of sites, a brief general discussion of catalyst aging follows. Treatment of other specific aging pathways will be given in Section I11 under the appropriate reaction.

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In many of the continuous flow organic reactions catalyzed by crystalline ttluminosilicates, high initial conversions of reactants were obtained, with subsequent decreases in conversion with time. Such catalyst aging results from blocking or obliteration of active sites, generally by localized or strongly chemisorbed organic polymer of low mobility. In certain cases, the adsorbed material has become sufficiently hydrogen deficient to justify application of the term ‘(coke.” I n addition to decrease in overall conversion, more subtle variations in product distributions (selectivity) may arise from the changes in site density or modifications in the steric, electronic, dielectric, or other fundamental properties of the catalyst surface associated with aging. It is extremely difficult to obtain meaningful kinetic constants in such systems. An analysis of the aging process in the alkylation of benzene with ethylene over a rare earth-exchanged X zeolite (REX) (43) provides insight into the physicochemical phenomena operative. In this reaction, a catalyst lifetime of over 192 hours was observed at 204”, benzene: ethylene molar ratio of 5, and pressure of 500 psig. Under similar conditions, but a t atmospheric pressure, a drastic increase in the rate of catalyst aging was observed, with catalytic activity lasting only 2 to 3 hours. Subsequent mechanistic studies showed that most of the catalyst aging in this reaction could be attributed to complex, temperaturedependent side reactions of ethylene (44,44a). Within the intracrystalline pore system of the zeolite, ethylene underwent polymerization, followed by isomerization, cyclodehydrogenation, and intermolecular hydrogen transfer reactions. These processes resulted in low molecular weight paraffins being extruded into the gas phase and the formation of bulky, hydrogen-deficient aromatics within the pores (Fig. 7). The occluded aromatics encountered increasing steric hindrance of escape from the relatively large supercages through the narrower pore orifices, in effect, a kind of (‘reverse” molecular-shape selectivity. Further, some of these aromatic aging products were highly alkylated naphthalenes ( 4 4 ) , which showed a strong affinity for the zeolite surface. For maintenance of catalytic activity for extended periods, it is essential that aromatics of higher molecular weight eventually diffuse out of the catalyst pore system. Such diffusion appears possible in the benzene-ethylene system under 500 psig pressure, where the liquid phase in contact with the catalyst surface affords a sort of (‘solvation” for the desorbed molecule. In the atmospheric pressure system, however, an

276

P. B. VENUTO AND P. 9. LANDIS

C I

c-c-c, c-c-c-c,

C-c-C,

c-c-c-c,

ETC.

’

t HIGHER MW

t

CONDENSED POLYCYCLICS

FIG.7. Schematic visualization of intracrystalline hydrogen-transfer reactions of ethylene.

attenuated gas phase impinges on the catalyst surface, and there is a high energy barrier against desorption of the relatively high molecular weight aromatics. The aging products tend to concentrate within the pore system, and inevitably react further until such bulky and high molecular weight products are formed that egress from the zeolite cavity becomes impossible. Prior to the latter stage, aging products of moderately low diffusivity could conceivably be removed by solvent extraction or vacuum desorption; thereafter, the tendency is toward ultimate formation of three-dimensional carbonaceous residues and an aged catalyst that can be regenerated only by an oxidative process. Even in the relatively open three-dimensional faujasite structure (12-ring entry pores), serious aging problems can exist. More rigorous restraints limit catalysis with A-type zeolites, where narrower %ring pores are structural characteristics. In fact, mixtures of alkylaromatics and branched aliphatics have been isolated from within the pores of CaA zeolite after dodecane cracking (45). It would also be expected that aging would be extremely rapid in the two-dimensional, tunnel like mordenite channels, since once a given channel was blocked a t any point, no further diffusion could occur. Experimental results for benzene-ethylene alkylation over hydrogen mordenite support this contention (43).

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277

B. FACTORS RELATEDTO NATUREOF CATALYTICALLY ACTIVE SITES 1. Mono- and Divalent Cation-Exchanged Faujasites

It has been conclusively shown that catalytic activity in ion-exchanged

faujasites is influenced by cation type (including size and charge) (8,9,40,43,46,47), cation location in the lattice (40,48), zeolite Si/Al ratio (40,48),and the presence of proton donors (49-52).

a. Crystallographic Classijication of Cation Positions. In their original crystallographic study of the structure of (hydrated) synthetic Linde Na 13X, Broussard and Shoemaker (11)suggested that the 48 (out of 80) Na+ cations in the unit cell located by Fourier analyses were positioned near the center of the 6-rings at the mouths of the sodalite cages. The other 32 cations could not be precisely located. A more explicit picture of cation distribution has emerged from X-ray studies on a dehydrated, calcium-exchanged single crystal of natural faujasite (Y-type, Si/AlN 2.5) reported by Pickert, Rabo, and associates (40,48). Three cation sites, located at different positions in the lattice, (designated SI, SII, and SIII)were described by the Linde workers. In Fig. 8, these positions are schematically superimposed on a section of the line drawing of the faujasite structure shown earlier (Fig. 4). The SIsites (16per unit cell) are located in the interior of the hexagonal

Fm.8. Schematic drawing of cation positions in faujasite.

278

P. B. VENUTO AND P. 5. LANDIS

prisms, positioned between two puckered 0 6 rings in 6-fold coordination to oxygen (Ca-0 = 2.42 A), They are thereby effectively hidden from the surface of the large pore system. Ca++ions strongly prefer SI over SII. The SII sites (32 per unit cell) are found next t o the 06 rings (in the hexagonal faces) a t the mouths of the sodalite cages on the 3-fold cubic axes (Ca-0 = 2.34 A). These cations have 3-fold oxygen ion coordination. Similarly, the SIII sites are situated next t o the 0 4 rings on the surface of the large pore system on the 4-fold inversion axes. These sites are probably populated only in the univalent cation form of the zeolite. From these considerations, the Linde workers have inferred some major differences between the charge distributions around SI and SII and between X- and Y-type zeolites (vide infra). Useful information has also been obtained about the environments of cations in nickel- (as),manganese- ( 5 3 ) ,and copper- ( 5 4 )exchanged zeolites by ESR techniques, and in zeolites containing sodium and copper cations by NMR studies ( 5 4 ) . b. Effect of Silicon: Aluminum Ratio. From the discussion above, (Subsection a,), it follows that a cation lying on the surface of a large cavity in an X- or Y-type zeolite, i.e., near an SII site, is in relatively low (3-fold) coordination with the lattice oxygen. Hence, it is not well shielded electrically: the fields are accessible and extend into the main cavities. The effects of these fields, however (say, for an SIIsite), vary greatly with the Si/Al ratio (cation density) of the zeolite (S),and may easily be visualized structurally for a divalent cation-exchanged zeolite. Although an increase of the Si/Al ratio from 1 to 2 in the Naf form increases the distance between cation sites, it does not significantly alter the bonding or location of the monovalent cation with respect to the A104 tetrahedra. A similar increase in the Si/Al ratio, however, will shift a divalent cation position from a location essentially equidistant between two A104 sites (Fig. 9a), to an asymmetric position close to one of the A104 units (Fig. 9b). As Fig. 9b suggests, the bond energy and bond length of one of the Ca-0 bonds will approximate that of a monovalent cation; the other bond, however, will be long and weak, Thus, a polarization exists between the unscreened cationic charge and the unsatisfied lattice A104 negative charge. Using a simplified fully ionic model, Pickert and associates ( 4 0 )have calculated an electrostatic field magnitude of 6.3 volts/A for points 2 A

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

279

(b)

Fro. 9. Effect of Si/Al ratio on position of Cat+ ions in faujasite.

toward the center of a 10 A pore starting from an occupied SII site in a CaY zeolite with Si/Al = 2. Similar calculations for a CaX type zeolite with Si/Al = 1 gave a field magnitude of 5.6 volts& a value 0.7 volts/A less than that obtained for the corresponding Y -zeolite. Those workers (40)experimentally found that, for a given cation, the catalytic activity for a Y-type catalyst was much greater than for an X-type, i.e., MgY had greater n-hexane isomerization activity than did MgX. It was suggested that the calculated differences in electrostatic field strengths could account for the observed differences in catalytic activity. c. Eflect of Cation Position. The existence of several crystallographically distinct cation locations provides the basis for a potential heterogeneity of sites in catalytic reactions. The Linde workers (40,48) made calculations (on the same basis as described in Subsection b ) for the electrostatic field strengths of surface cations near SII and SIII in an X-type zeolite of Si/Al ratio 1.0 and in a Y-type zeolite of Si/Al ratio 2.0. I n both cases, the fields were significantly larger near SIIIthan SII (by about 0.9 volt/A) at a distance of 2 A from the center of the cation for the univalent cation-exchanged systems, provided both sites were occupied. I n theory, at least, the catalytic activity induced a t locations S I I and SIIIcould be significantly different. Further evidence supporting site heterogeneity is provided by the observation of Pickert et al. (40) that in a Y-type faujasite with Si/Al = 2.5, the relative proportions of silicon and aluminum are such that it is impossible to distribute the aluminum so as to render all cation sites equivalent.

280

P. B. VENUTO AND P. 9. LANDIS

d . Effect of Cation Type. Fields associated with bivalent cations have been calculated to be greater than those arising with univalent cations (40,48). It could further be predicted that the Coulombic fields (for a given cationic charge) would be greater as cationic radius decreased; that is, the electrostatic field strength would become larger with increasing cationic charge density. Trends consistent with these expectations were found experimentally by Pickert et al. (40) for the n-hexane isomerization activity of a series of cationic zeolite catalysts, where the order of reactivity was NaY < BaY < SrY < CaY < MgY. Similar trends were reported by Rabo et al. (48) in a comparison of the cumene dealkylation activity of NaY and Cay faujasites.

e. Concept of Carboniogenesis. Based on the considerations advanced above, the Linde group has proposed a concept of catalytic activity where the cations themselves are carboniogenic centers (40,48). The calculations strongly suggest that the actual magnitudes of the electrostatic fields in the cavities at distances as far as several angstroms from the bivalent cation (where these fields would be accessible to reactant molecule) are as great as 1 volt/A. A field of this strength may cause substantial shifts of bonding electrons in adsorbed reactant molecules. Further, the stronger the field,the greater would be the polarization of the adsorbate, resulting in shifts approaching the structure C@Hevisualized in Fig. 10, where A@is a lattice-associated cation and

Fro. 10. Visualization of the polarization of a paraffin substrate induced by cation fields.

R1 and Rz could be alkyl groups. The electron-deficient carbon atom of a C-H bond so polarized could then serve as the active center for reactions of the carbonium ion type; complete cleavage of the C-H bond is not required. The Linde workers were unable to correlate zeolite hydrogen content with hexane isomerization activity (40); nor did they attribute the great rise in cumene cracking activity (48) obtained by replacing univalent cations with bivalent cations in zeolite Y as arising

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

281

from the 0.1 to 1.0 wt% of structural OH groups still remaining after activation.

f. Extent of Cation Exchange. The extent of ion exchange is an important variable in determining catalytic activity of zeolites, as was clearly shown by Rabo et al. ( 9 ) . There, the hexane isomerization activity of Y-type faujasites (loaded with 0.5 wt% Pt) was greatly dependent on the degree of replacement of Na+ by Ca++. Similarly, Venuto et al. (43)observed maximum alkylation activity in rare earth-exchanged faujasites (vide infra) only after a very high degree of replacement by rare earth of Na+ cations. Since SI sites have a strong affinity for bivalent cations (i.e., Ca++) (40) and these sites are hidden and relatively inaccessible to organic reactants, it follows that a large fraction of SI sites must fist be filled with bivalent cations before sodium ions at the potentially catalytically active SII sites can be replaced. For example, with a CaY of Si/A1 ratio 2.5, 58% of the bivalent cations should occupy SI. Thus, even at 65% ion exchange, only a fraction of the SII positions would carry Ca++ions. As a consequence, catalytic activity could increase more than linearly with increasing degree of exchange, even at high levels, since most of the incoming cations would enter the SII sites, where lower coordination with oxygen and potentially more powerful unscreened electrostatic fields can exist. g. Effect of Proton Donors. It has been demonstrated that water plays an important role in the catalytic behavior of both alkali- (49,50)and alkaline earth- (51,52)metal exchanged faujasites. Basset and Habgood (49)have demonstrated that water promotes the isomerization of cyclopropane over NaX. Gourisetti et ul. (51,52) have observed a similar effect in the dehydration of tert-butanol over CaX. There, maximum activity was observed when the number of water molecules equaled the number of zeolitic Ca++ions. A promoting role for water has been observed in numerous reactions over silica-alumina (55). It is evident that the presence of water (or perhaps of other proton donors) introduces ambiguities into the interpretation of catalytic results based on a series of cation-exchanged zeolites with different calculated (40,48) electrostatic field strengths. Although the water source could be an added promoter ( 4 9 - 5 4 ,it could also arise via elimination in alcohol dehydration, a reaction which is known (6,7,51,52,56)to proceed smoothly over mono- and divalent cation-exchanged faujasites.

282

P. B. VENUTO AND P. 9. LANDIS

Certainly water, with its high dielectric constant, can effectively screen the cation fields (Le., by solvation), and thus modify the magnitudes of the fields accessible to reactants. In an extreme case, a catalytically inactive, fully hydrated zeolite would result. It will also be recalled that reactant diffusivities (31,32) and cation positions (2) in certain zeolites are dependent on the degree of hydration. Evaluation of thermochemical and spectroscopic observations furnishes some clues as to one possible role of water. Barrer and Bratt (41) reported a high isosteric heat of adsorption, approaching 30 kcal/mole at low coverage, for water in certain mono- and divalent cationexchanged faujasites. This high value reflects the magnitudes (4O,48) of the electrostatic fields (i.e,, 4 ~ available ~ ) for interaction with sorbates within the rigid zeolite. It is known from infrared spectroscopic studies (57,58) that faujasites containing alkali and alkaline earth cations retain OH groups on their surface, even after evacuation a t 450'. Bertsch and Habgood (57) have detected infrared OH-stretching vibrations in NaX-Hz0 systems that suggest that isolated water molecules are adsorbed simultaneously by an ion-dipole interaction (i.e., +F,,) with the exchangeable cation and by hydrogen bonding of one of the hydrogens to lattice oxygen (Fig. 11).Extending this reasoning, Hirschler (59) suggested that the polarizing action of the cation

0

\o

'

At

0

,"; ' 0

FIO.11. Interactions of water in NaX faujasite.

fields in Na or Ca-X zeolites might render acidic the proton of a hydroxyl group attached to an adjacent lattice atom or of a water molecule solvating the cation itself. If then, the cationic fields are powerful enough to polarize parafin C-H bonds (Fig. lo), where the energy requirements approach those for hydride abstractions, we would expect the facile labilization of the

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

283

considerably weaker OH bonds in water (or a related proton donor) with the generation of a pool of protonic species, which could give rise to the observed (49-52) promoting effects. Equations (1-3) illustrate

this reasoning for a bivalent cation(M++)-exchangedzeolite. It is assumed that all positive charges are balanced by an equivalent anionic charge in the lattice, and that all steps are fast and reversible. Such a mechanism would also only be operative a t low surface coverages of water, probably at HzO/lattice A104-unit ratios of about one or less, as has been found experimentally (50,51). In fact, deactivation might reasonably be expected a t only moderately higher HzO/AlOd-unit ratios. Equation (1) essentially represents a simple solvation reaction (iondipole interaction); the events in Eq. (2), which probably occur at the higher temperatures required for catalytic reactions depict the labilization (polarization) of one of the hydrogens (Hid(+)) of the solvating water molecule. The process could proceed to the extent of complete dissociation, Eq. (3), and formation of a proton, H,.H1 could thus participate in a catalytic reaction, i.e., alcohol dehydration, and be subsequently returned to the surface hydrogen pool. It is almost certain that the populations of such entities as the right-hand members of Eqs. (2) and (3) are extremely small and that cation (M)-OH vibrations might be undetectable by infrared. Finally, one might predict that rigorously controlled experiments could reveal a trend in catalytic activity in the presence of proton donors correlating with cation charge density, similar to that reported by the Linde group (40,48),wherein the effect of the cation would be mediated through the protons. This effect is analogous to that of various

284

P. B. VENUTO AND P. 8 . LANDIS

electron-withdrawing groups (transmitted via the inductive effect through chains of atoms) on the dissociation constant, K g , of organic carboxylic acids, RC02H, where K , decreases according to the order R =CFs > CCls > CHs. 2. Rare Earth-Exchanged Faujasitee

Since a large number of organic reactions were found to be catalyzed by rare earth-exchanged zeolites (Section 111) a discussion of their properties is included. a. Pertinent Properties of Rare Earth Cations (60).In Table I1 there is listed the relative proportions of the cations (together with their atomic numbers and crystal ionic radii) present in the lanthanide mixture used in preparation (43,44) of mixed rare earth-exchanged faujasites (designated REX and REY). Since the selectivities for ion exchange of the rare earths vary, it is to be understood that the relative proportions of cations finally exchanged into the zeolite lattice may differ somewhat from those in the original exchanging solution (TableII). TABLE I1 Proportions and Propertiee of Rare Earth Mixture

Rare earth (RE) La Ce Pr Nd Sm Qd Otherb a

Gm atom

(%)

Atomic number

Trivalent orystal radius (A)

26.0 47.2 6.9 19.3 1.9 0.7

67 68 69 60 62 64

1.061 1.034" 1.013 0.996 0.964 0.938

Trace

-

-

T

I

AT = -0.123 A

In Ce4+, T = 0.92 A. Small amounts of Ca++ also present in exohange mixture.

Although the electronic configuration of the lanthanides imparts unusual properties, it nevertheless allows many similarities to more common elements such as the alkaline earths (i.e., trivalent members form oxides M209 that resemble those of Group 11-A). Since the shielding of the 4f electrons is sufficient to render them sterically inaccessible

ORQANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

285

for extensive bond formation, the generation of complex species, although known, is far less characteristic of the lanthanides than of the d-type transition metals. It has thus been generally concluded that most ligands are held by electrostatic (ionic) interactions. In terms of size, the tripositive lanthanide ions are all significantly larger than other tripositive ions, e.g., the cationic radii of Al3+, Fe3+, and Cr3+ are 0.57, 0.67, and 0.65 A, respectively. Hence they would not be expected to have as great acidity as the latter. Nevertheless, the lanthanide ions possess fairly high charge density. For comparison, it should be mentioned that in Ca++,r = 0.99 A and in Ba++,r = 1.35 A. Since within the lanthanide series, the trivalent ionic radius decreases as atomic number increases from 57 through 71 (partial listing in Table 11),a corresponding increase in charge density and a tendency to attract negative ions (acidity) will be observed. However, because of their similar electronic configurations and size (Ar = -0.2 A in going from La through Lii), the trivalent rare earth ions show great similarities in their patterns of chemical behavior. Finally, it should be observed that all lanthanides form very insoluble hydroxides, M(OH)3 (i.e., KspLa(OH)3= 1.01 x lO-lO), whose basicity decreases with increasing atomic number. Further, the trivalent rare earth cations (RE3+)in aqueous solution are slightly hydrolyzed:

-

+ Ha0

[RE(HaO)n]3+

+

$ [RE(HsO)~-I(OH)]++ HsOf

The smaller Ce4+ cation, with its higher charge density, undergoes hydrolysis far more extensively than do any of the trivalent series. b. Chemistry of Activation in Rare Earth X - and Y-Type Faujasites (REX and RE Y ) .I n any comparisons of catalytic activity, it is evident that a detailed knowledge of catalyst analyses, physicochemical properties, and preparative history-including activation procedures-is essential. It is particularly critical for modified faujasite catalysts, where variables in the activation process-such as temperature, time, atmosphere, pressure, and sample geometry-are of great importance (43). The chemistry of activation as related to a specific topic in acid catalysis is discussed below. ( i ) Ethylation activity us temperature of activation. A study of the relation between catalytic activity and temperature of activation for the benzene-ethylene alkylation reaction was made by Venuto et al. (43) for three modified faujasite catalysts. This alkylation is relatively difficult,

286

P. B. VENUTO AND P. S, LANDIS

since the benzene ring is not substituted with an electron-donating group and a primary carbonium ion-like electrophile is involved; hence, strongly acidic catalysts are required. The results of this study are shown in Fig. 12. Although the maximum alkylation activities for these three CONDITIONS :

-

177 'C LHSV 5 . 4 COHO:C,H,

= 12

I ATMOSPHERE $ m 7 5 1

2

rr

REX

25

0

0

-L I00

CALCINATION TEMPERATURE, OC (Tc)

FIQ.12. Profiles of ethylene-benzene alkylation activity for modified faujaaites calcined for 3 hours in oxygen at various temperatures ( 4 3 ) .

catalyst samples happen to be relatively close, the activation temperato attain these maxima-400" (REX), 280' (REY), and 550tures (Tc) 600" (HY derived from thermal decomposition of NH4Y)-differ strikingly. Although different activation patterns may occur with other catalyst compositions and reactant combinations, the above activation procedures were applied successfully throughout most of the catalytic operations employing REX, REY, and HY, reported in Section 111.

( i i ) Physicochemical characterization of activation process. Calcination renders REX and REY active for alkylation. No major lattice changes as high as 800". were observed even a t calcination temperatures (Tc) In the following paragraphs, using the REX profile in Fig. 12 as a frame of reference, the experimental details supporting these conclusions are documented. Spectroscopic, physical, and chemical coordinates for parent

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

287

(uncalcined), high ethylation-activity, and low ethylation-activity catalyst compositions are compared for the REX sample in Table 111. TABLE I11

Unit Cell Compoaitionfor REX __________~

A102 units SiOa units RE3+ x 3" Ca++ x 2 Na+ NH~+

~

86 106 80.7 2.16 3.91 0

Calculated on basis of 140 for average atomic weight of RE cation (La= 139; Ce = 140; Nd = 144).

( a )Stability of lattice. Several approaches clearly demonstrate that the structure of the lattice in REX was maintained essentially intact throughout the range of Tc.Figure 13 shows that sorptive capacity,

0

200 400 600 800 TEMP. OF CALCINATION, OC

FIG. 13. Cyclohexane adsorption vs temperature of calcination for modified faujaaite catalysts.

as evidenced by cyclohexane adsorption, was maintained through T c = 800".Similarly, X-ray diffraction studies showed high crystallinity throughout the same range. The differential thermal analysis (DTA) profile of the parent REX (Fig. 14, profile A) showed reorganization of

288

P. B. VENUTO AND P. 9. LANDIS CONDITIONS : 0.2 MV, FULL SCALE 10 *C/M INUTE 0, ATMOSPHERE 150 MESH VYCOR REF.

0

i

W

I

I

200

I I 1 400 600 800 TEMPERATURE. 'C

I

10 3

FIG.14. Differential thermal analysis profiles for rare earth-exchanged X catalysts.

the lattice only at a very high temperature (sharp exotherm at 930"). This exotherm was also present in samples calcined at temperatures as high as 800". IR studies of the major Si-0 stretching vibrations showed only a slight shift to higher frequencies in going from the parent ( Y S I - o = 972 cm-1, Fig. 15,profile A) to the sample at Tc=400" ( v s ~ - - o= 986 cm-1, Fig. 15, profile B) with high ethylation activity. These shifts were accompanied by minor changes in profile. Finally, the sample with low ethylation activity (Tc = 600", Fig. 15, profile C) differed from the sample at Tc = 400" only in exhibiting a slightly flatter I R profile.

( b ) Desorption of water. Figure 13 shows that at Tc = 300" about 94% of the potential sorptive capacity of REX has become available for admission of hydrocarbons, clearly demonstrating the expulsion of physically adsorbed water. This conclusion has been confirmed by thermogravimetric analysis (TGA), IR studies of the OH region, and by DTA. Examination of Fig. 14, profile A, shows the presence of several clearly defined minima between 100" and 350" in the parent REX. These endotherms are probably associated with release of water molecules with increasingly high energy barriers to desorption. Similar DTA profiles in the region 75-400" have been reported for various zeolites by Barrer and Bratt (39) and Barrer and Denney (61).Also associated with the loss of adsorbed water (at increasing Tc) were X-ray changes

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILZCATES

289

t

z

P n ?! f

n

fd k-

FREQUENCY, CM"

FIQ.16. Infrared profiles in Si-0 region for (a)parent; (b) high ethylation activity; and (c) low ethylation activity REX catalysts. (KBr disks; catalysts calcined 3 hours in oxygen.)

that reflected stabilization of the cations, i.e., the process of changing from a less ordered system of mobile, aquated cations (similar to a concentrated aqueous solution) in the parent, to one of greater order and lower potential energy.

( c ) Some properties of REX with high ethylation activity. The composition with high ethylation activity (Tc= 400") showed maximum sorptive capacity (Fig. 13) and virtually no endotherm (100-300") in its DTA profile (Fig. 14, profile B), as expected if almost all of the physically adsorbed water had been desorbed. The change in color from pale yellow or white to deep yellow, observed after calcination of REX and REY in 0 2 at 400-450" or higher, probably represents the transition -

Ces+

2~ e 4 +

(no color change observed in Nz calcinations). This change did not appear to significantly affect ethylation activity in REX or REY catalysts. The valence state of Ce, however, could conceivably influence certain other reactions.

290

P. B. VENUTO AND P. 9. LANDIS

( d ) Differences with R E Y . Characterization of the activation process in REY was generally similar to that described above for REX, except for a trend to water removal and development of activity at somewhat lower temperatures (Figs. 12 and 13), which perhaps reflects its higher Si/Al ratio and lower charge density. In REY, there was virtually no shift to higher frequencies for the major Si-0 skeletal I R band (1030 cm-1) upon calcination at higher temperatures, although the profile tended to flatten slightly. (e) Visualization of the development of alkylation activity in R E X and R E Y . From our earlier discussion of the powerful accessible electro-

static fields near certain alkali and alkaline earth metal cation sites in near-faujasites, it is apparent that such t,endencies would be even more intensified in the rare earth cations, because of their higher charge density. On the basis of the foregoing considerations, one may visualize the sites active for alkylation in REX and REY faujasites as arising via a mechanism similar to that proposed by Plank (62) and by Venuto et al. (44). This mechanism is consistent with the observation that the addition of proton donors to a REX catalyst previously calcined a t 400' significantly increases its activity for benzene-ethylene alkylation (44). Further confirmation was provided by deuterium tracer studies with olefinic organic substrates passed over DzO-treated rare earth zeolites (44) (vide infra). It also seems probable that the concept of carboniogenesis does not hold for rare earth or other trivalent cationexchanged zeolites, particularly in light of the tendency that rare earth cations have toward hydrolysis. 3. Hydrogen Zeolites and Related Compositions a, General Properties. Some years ago, Pauling (63) suggested an analogy between an aluminum tetrahedron with corners shared by silicon tetrahedra and the perchlorate ion. He further predicted that the acid obtained by replacing the potassium cation of an aluminosilicate (mica) with a proton would be very strong. Later, Benesi (64) quantitatively related the acidity of silica-alumina to that of 90% HzS04. The rapidly accumulating body of knowledge about hydrogen zeolites (zeolites containing a protonic species instead of a metallic cation associated with the negatively charged lattice A104 unit) testifies to the accuracy of his prediction. Most important, certain protonic forms have broad catalytic application.

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291

The preparation of hydrogen zeolites has generally been attempted either by direct ion exchange with mineral acids or by thermal decomposition of the NH4+ form of the zeolite. Hydronium ion-exchanged zeolites of low Si/Al ratio (prepared by contact with weak mineral acids) may be stable when hydrated, but may lose much or all of their crystallinity upon dehydration a t higher temperatures (9,65). Nevertheless, substantially complete replacement of the Naf cations in Na+-mordenite was reported by Keough and Sand (14). The resulting hydrogen form (H-mordenite)was strikingly acid stable in spite of the absence of metallic counter ions, and had high catalytic activity for cracking (14,47,66)and other reactions. From DTA and TGA studies, Barrer and Peterson (65) have suggested that the water of hydration of the proton is lost a t a relatively low temperature. As early as 1949, Barrer (67) reported generation of the hydrogen forms of mordenite and chabazite from thermal decomposition of the ammonium forms. I n attempts t o prepare hydrogen X (HX) by this method, Rabo et al. ( 9 )observed collapse of the crystalline lattice structure. This breakdown was believed to be promoted by the low Si/Al ratio and consequent close proximity of aluminum atoms, which facilitated rearrangement of the aluminosilicate framework. I n 1960, Rabo et al. ( 9 )observed that calcination of an NH4f-type faujasite was associated with the appearance of a protonic form showing an I R OH stretching vibration a t 3570 cm-1. Since that time, knowledge of the underlying processes operative in the chemistry of ammonium, hydrogen, and related forms of zeolites has greatly expanded (68,69).Considerable impetus in this direction has been provided by refined infrared spectroscopic techniques (57,58,70-72) A possible analogy between hydrogen zeolites and certain heteropoly acids may exist. A heteropoly electrolyte is the free acid or salt of a large anion that contains a t least two different kinds of atoms in positive oxidation states. An example is the 12-tungstophosphate series, P04W1203s3-. Baker (73,74)has recently reported the preparation of a number of new anions in this series, where an octahedrally coordinated metal ion such as Co++or Gas+ replaces one of the 12 tungsten atoms of the conventional Keggin structure. I n some of these anions, the central cavity is occupied by Hz++instead of a metal cation; still other hydrogens appear to be firmly linked to the exterior oxygen atoms of the complex, probably to those surrounding the metal.

292

P. B. VENUTO AND P. 9. LANDIS

b. Decationated Y-type Faujasites. ( i ) Properties and thermal decomposition products. The refined IR studies of Hall and co-workers (70,72) and of Angel1 and Schaffer (71) have significantly contributed to an understanding of decationated Y-type zeolites. Such zeolites are formed by replacing Na+ ions with NH4+ by cation exchange, with subsequent thermal decomposition of the NH4+ form. Upon release of NHs, the point-charge protons remaining to screen the negative charge bonds on the A104 tetrahedra react with lattice oxygen in Si-0-Al to form a 3-coordinate aluminum that is adjacent to an SiOH group (IIb): H@

NHp

The position of the major IR OH stretching vibrations (Fig. 16, 3660 and 3570 cm-1) in a deaminated, 75% decationated form (70,72) indicates that the released proton has become attached to a particular I - EVALUATED AT 411 t

2- ADOVE

i ISpnok

NH,

3- I + 4 S p d NH,

3700

3500 3300 3100 FREQUENCY, CM”

2900

Fro. 16. Spectral changes on readsorption of NH3 on a decationated Y-type zeolite after evacuation at 415’: H-stretching region. From Uytterhoeven et al. (70).

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

293

oxygen (IIb). A classical Bronsted form (IIa) is proposed as being in equilibrium with the silanol form, and proton donation can occur upon the approach of a strong base (NH3);with weaker bases, the magnitude of K E is critical, and is probably a function of temperature (70). Structure (11)will be designated as hydrogen Y (HY) in succeeding sections. On the basis of the preceding discussion, it seems quite improbable that the concept of carboniogenesis (40) could be applicable to hydrogen zeolites. Structure (11)reacts as a Bronsted acid with the strong base, NH3, causing the disappearance of both the 3660 and 3570 cm-1 I R bands, but not a weak silanol-type band at 3745 cm-1 (70). In fact, the 3660 and 3570 cm-1 bands may be viewed as silanol-type bands whose frequency has been lowered from 3745 cm-1 by a Lewis acid-base interaction between the electron pairs of the OH group with adjacent 3coordinate A1 atom (70-72). Thus, it is this interaction and the resulting ability of the A104 tetrahedra to reform that contributes t o the acidity of this group. Uytterhoeven et al. (70) have demonstrated from NH3 adsorption experiments (Fig. 16) that the transformations (I)+(11) above are essentially reversible. Venuto et al. (75) confirmed this conclusion by gross scale reconstitution studies on a 90% decationated Y sample:

(111)

(IV)

(VI

The reconstituted sample (V) was rigorously shown to be virtually identical with that of the parent NH4Y (111). A second process, dehydroxylation, is important in decationated zeolites. At higher temperatures, the protonic form exhibits instability and decomposes in a process involving the loss of one mole of water per pair of A104 tetrahedra to form a defect structure (9,68-72): OH ' 0 0'

.O

0

'0

0 '

.

.o

'0"

The latter (VI) is viewed as consisting of one reformed A104 tetrahedron bearing a negative charge, and a %coordinate aluminum ion adjacent to a 3-coordinate silicon ion, the latter bearing a positive charge.

294

P. B . VENUTO AND P. S . LANDIS

(ii) Physicochemical characterization of decationated Y -type active in catalysis of benzene ethylation. ( a ) Ethylation activity us temperature of calcination. The study of benzene-ethylene alkylation activity versus temperature of calcination ( 4 3 )also included a 90(yodecationatcd Y-type catalyst, the aiialysis of which is shown in Table 1V. I n Fig. 12, it is seen TABLE IV Unit Cell Composition for NH4Y A102 units Si02 units

Naf

NH4+

51 141 5.1 45.9

that ethylation activity rapidly becomes maximum near Tc = 550-600", but decreases sharply thereafter. The higher temperatures necessary to obtain maximum catalytic activity with the NH4-t zeolite reflect the higher energy required for decomposition of the NH4 groups, the detailed chemistry of which was discussed in the preceding section. In contrast to the studies of Hall and associates (70,72),in which deamination was accomplished by heating and evacuation within an infrared cell, thermal decompositions in 0 2 were accompanied by an oxidative process (76)in which appreciablc amounts of Nz gas, in addition to NH3, were detected in the reactor effluent. Similar oxidations were reported earlier by Barrer (67) in the thermal deamination of the NH4+ forms of mordenite and chabazite. Ethylation activity for samples activated in a flow of inert gas a t a given temperature paralleled that for samples calcined in 0 2 a t a temperature about 50" lower. This parallel reflects the residual nitrogen content in the sample; for the presence of the 0 2 carrier is associated with more removal of zeolitic NH4+ groups at a given temperature (via the oxidative process) than was removed in the presence of the inert gas, Thus a catalyst calcined for 3 hours in He a t 550" (wt yo N = 0.27) would have ethylation activity roughly equal to that of a similar sample calcined in 0 2 for 3 hours a t 500".

( b ) Physicochemical properties of decationated Y species with high and low ethylation activity. In Table V, various physical and chemical properties of an uncalcined NH4Y faujasite (Table IV) are compared with those of compositions with high and low ethylation activity

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

295

TABLE V

Properties of Decationated Y -Type Faujasitefl

Property

Activation temp. ("C), 3 hours in He yo Nitrogen (Kjeldahl) Infrared bands (om-') OH-stretching (F1uorolube)b Major Si-0 skeletal (KBr) Pulsed NMR (50 MHz) Spin-spin relaxation time (T2a)'

H, total, meqlgmd Acidic H, CaDs exchange a t 177"(meq/gm)d DTA profile (Fig. 18) 200-300" (endo) 270 and 510' (exo) 980" (exo) X-Ray diffraction Relative crystallinity (yo) Position of peak near 28 = 58'

High ethylationactivity catalyst

Low ethylationactivity catalyst

Uncalcined 4.28

550" 0.27

700" 0

-

3575 ( 8 ) 3650 (sh, m) 3726 (w)

3575 (w) 3625 (sh, w) 3730 (w)

1010

1048

1058

3.3 -

0.18 4.08 f 0.14 2.74 & 0.13

0.43 2.17 f 0.13 1.26

Strong Very strong Strong

Absent Extremely weak Strong

Absent Absent Strong

l0OC

75

-

58.08

58.20

58.20

Parent NH4Y

-

Data largely from Venuto at al. ( 7 5 ) . Intensities in parontheses; sh = shoulder peak. e 02-calcined samples. d Run a t least in triplicate. e No amorphous species or other zeolites present. a

b

produced from its thermal decomposition in He. The infrared spectrum and DTA profile of the parent NH4Y are shown in Figs. 17 and 18, respectively. The endotherm a t 200-300" in Fig. 18 represents the desorption of physically adsorbed water, a process with considerably lower energy requirements than denitrogenation. The exotherms a t 370" and 510" are associated with the decomposition of zeolitic NH4+ groups (76)) and the sharp exotherm a t 980" reflects a profound lattice rearrangement and loss of crystallinity.

296

P. B. VENUTO AND P. 8. LANDIS

FREQUENCY, CM"

Fro. 17. Infrared spectrum in the region 1600-600 cm-1 for parent NHIY catalyst (KBr disk).

200

400 600 WO TEMPERATURE, *C

1000

FIG.18. Differential thermal analysis profile for parent NH4Y catalyst. (Conditions: 0.2 mv, full scale; lO"/minute;0 2 atmosphere; 160 mesh Vycor reference.)

High ethylation-activity H Y : Although small amounts (i.e., 0.27 yo)of residual nitrogen may be tolerated (Table V), substantial denitrogenation was generally a requirement for significant ethylation activity. The IR profile for the HY with high ethylation activity was similar to that reported by Uytterhoeven et al. ( 7 0 ) ;it differed mainly by showing a greater intensity of the 3575 cm-1 band. It will be recalled that the hydrogens in the zeolitic OH bonds undergo isotopic exchange with Dz at 500' (70)or when exposed to the full vapor pressure of DzO with subsequent activation at 500" (71).A shift to higher frequencies in the position of the major Si-0 stretching vibration (relative to the parent NH4Y) was observed with the active species. Both total hydrogen (NMR) and acidic hydrogen (CsDs exchange at 177') values were of the same order of magnitude as the number of wt

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

297

decationated sites (4.07 meq H per gram of catalyst would be expected on the basis of Na+ free A102 units). Values for the acidic hydrogen were also confirmed by reconstitution studies with NH3 (75). Although difficult to interpret accurately in terms of structure, the pulsed NMR Tzavalues may reflect different environments for hydrogen. The X-ray data are thought to reflect a loss of order, i.e., introduction of defects (dehydroxylation), but not lattice collapse or formation of amorphous regions. Thus, all the data define an ethylation active catalyst containing protons at 67% of Na+ free Al02 units, with about 28% of the protonic sites having undergone dehydroxylation, and about 5% of the original NH4+ groups undecomposed. In the catalyst with low ethylation activity ( T c = 700°),infrared OH bands with very much weaker intensities, lower total (NMR) and acidic (CsDs exchange) hydrogen, a shift to still higher frequencies in the skeletal Si-0 band, and the X-ray data all suggest that this catalyst is extensively dehydroxylated. Its sorptive properties (Fig. 13), DTA profile, and other characteristics argue for the persistence of an intact aluminosilicatelattice, however.Thesedata support the mechanism of decationation proposed earlier for NH4Y decompositions in vmuo within an IR cell (70). They further integrate the earlier data into a larger coordinate system that includes gross scale catalytic reactions.

( i i i ) Catalytic proton transfer reactions with hydrogen zeolites. ( a )Olefin substrates. Liengme and Hall (72)have studied the isotopic exchange reactions of low molecular weight gaseous olefins over an exhaustively deuterated (D2, 500") decationated Y-type faujasite. At low temperatures (within an IR cell), no exchange occurred between the hydrogen of CzH4 and the catalyst OD groups. At temperatures greater than 150", however, exchange between C2D4 and catalyst OH residues occurred, which suggests the intervention of a primary carbonium ion-like species i.e., C D B H - C D ~With ~ . CH3-CH=CH2, however, isotopic exchange and polymerization occurred even at room temperature. This observation suggests that, under these conditions, CHs-CH=CHz is a stronger base, one that readily forms a (relatively stable) secondary carbonium ion as shown: Q

CHs-CH=CHa

+ DQO-Zeol + CHs-CH-CHaD Q

+ CHs-CH=CHD +e

Q

0-Zeol

H@

0-Zeol

298

P. B. VENUTO AND P. 8. LANDIS

Venuto et al. (75) prepared a synthetic partially deuterated decationated Y catalyst by thermal decomposition (in He) of a deuteroammonium Y sample prepared by a reconstitution procedure. This catalyst (designated nonstoichiometrically as DHY) showed V O H / V O ; ) ratios of 1.356-1.365, a range consistent with the ratios of corresponding groups in other decationated zeolites ( 7 4 , Si02 (77) and H2S04 (78). When this catalyst was contacted with 2,3-dimethylbutene-1 or 1-hexene a t 0-25" in the liquid phase, the transfer of small amounts of deuterium to organic reactant accompanied the isomerization and polymerization reactions (79). ( b ) Aromatic substrates. The partially deuterated decationated Y catalyst (DHY) was also employed in an extensive study of the isotopic exchange reactions of simple aromatics (75). Thus, when toluene was passed over fixed beds of DHY catalyst at temperatures ranging from 25 to 150" in continuous-flow systems, deuterium originally located on catalyst oxygen (IRVOD = 2635 cm-1) exchanged with the hydrogen on the aromatic ring, forming C-D bonds. The disappearance of catalyst OD-stretching bands with standing at 25" is shown for a mull of a DHY-toluene system in Fig. 19. No deuteration of the a-methyl group

-

3500

3000

2500

FREQUENCY, CM"

FIG.19. Comparative infrared spectra [Fluorolube mull] of DHY catalyst: Curve 1 , immodiately after discharge from exchange with toluene at 36"; curve 2, after standing for 6 days at room temperature (75).

occurred, even in exchanges a t temperatures up to 150", where exchange rates were extremely rapid. Ring deuteration occurred predominantly in the ortho and para positions a t temperatures up to 75" (Table VI); in comparative experiments a t 75" (in separate runs), the rate of exchange of deuterium into

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

299

TABLE VI Repreaentative Ring Orientation Data in Toluene-& Samplea from Exchanges at Variow, Temperatures Time Relative rates Exchange on stream (min) para/meta ortho/meta ortho/para temp. ("C) 24 36 76 1008 160"

106 76 64 47 31

12.2 4.0 1.67 1.04 1.13

7.8 3.4 1.22 1.14 1.17

0.64 0.86 0.78 1.10 1.04

a Near-statisticaldistributionat higher temperaturesreflects the decreasing role of activation energy differences for the different ring positions, compared to the overall energy of reaction.

toluene was 1.69 times faster than with benzene. These patterns of reactivity and deuterium distribution, where the catalyst deuterium species seeks out sites of greater electron density (i-e., ortho, para vs meta position, toluene vs benzene), are characteristic of electrophilic substitution reactions. Similar isotopic exchange reactions are observed in exchanges of toluene with HzS04 (84, HClOd (84, andCF3COOH (82). In the present case, the exchangeable catalyst deuterium is the electrophile, i.e., a deuteron (D+)or a deuteron-like species associated with the polyanionic aluminosilicate lattice. Equimolar mixtures of toluene and benzene were passed over beds of DHY a t low temperatures (25-60') in experiments where the two aromatics of different reactivity competed for the electrophilic deuterium (75).The distribution of deuterium between toluene and benzene (apparent kC&,CH3/kCaH6) and among the ring positions of the toluenedl samples was determined. A plot of logpf vs selectivity factor (&) for these data from the competitive experiments a t 25-60' (Fig. 20, black circles) falls on the line obtained from a study of 47 electrophilic substitution reactions by H. C. Brown and associates (83).The partial rate factors pf and mi give the rate of substitution of the para position and one of the meta positions in toluene, relative to the rate of substitution of one of the six equivalent ring positions of benzene. Points a, b, c, d, and e fall quite close to the line, which represents a linear free energy relationship in both positional and substrate selectivity.

300

P. B. VENUTO AND P. 8 . LANDIS

Fro. 20. Plot of log pr and selectivity factor, Sr: 0,data from Competitive hydrogen deuterium exohange reeations of toluene and benzene over DHY (75); 0,data from competitive ethylation reeatiom of toluene and benzene over REX (128).

These competitive experiments were generated in a heterogeneous system, a rigid polar zeolite with entry pore size 8-9 A. Since reasonable adherence to the Brown selectivity relationship was observed, any sorption or diffusion effects were considered to be negligible or to cancel out. These data then furnish a link between a catalytic system involving a three-dimensional, porous, acidic aluminosilicate and more conventional, homogeneous catalyst systems. Finally, the existence of reactivity at low temperatures, and the relatively low ring-positional and substrate selectivity, suggest that the catalyst hydrogen species is very strongly acidic in nature. (iv) Evidence for site heterogeneity in decationated Y-type faujasites. Within the past few years a considerable amount of information has been amassed that suggests that site heterogeneity exists in decationated Y -type aluminosilicates. Pickert et al. (40) reported crystallographic evidence that supported the concept of different cation sites in Y-type zeolites. In studies of a series of Y-type aluminosilicates with varying degrees of decationation, Turkevich et al. (84) observed that cumene dealkylation activity increased at a considerably greater than linear rate as the degree of decationation became extensive. Although this

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

301

phenomenon may be a site-density effect, the possibility exists that at higher degrees of decationation, acidic sites with greater catalytic activity may be generated upon release of strongly bound NH3. Hall and associates (70,72) have proposed that the 3570 cm-1 IR band observed in addition to the major OH band at 3660 cm-1 in the thermal deamination of decationated Y samples arises from an A104site in a different environment. This observation means that there are two separate, noninteracting species at different oxygen locations, i.e., near oxygens forming the cubooctahedra (sodalite cages) and near those bridging them together (hexaganol prisms). The possibility also exists that the lower frequency band may result from interaction of two neighboring lattice OH groups, presumably through hydrogen bonding (71).Also, differential thermal analysis (76) suggests the existence of at least two major protonic sites holding NH3 in decationated Y sieves. Finally, evidence for site heterogeneity in various cationexchanged faujasites has been provided by CO (85) and CzH4 (86) sorption studies. 4. Spectroswpic Observations of Adsorbed Molecules

The application of spectroscopic techniques to the study of substrates adsorbed on zeolites has revealed a whole gradation of interactions, ranging from mild perturbations of the symmetry of electron distribution by electrostatic fields through electron transfer from a substrate to an acceptor site in the lattice (Tables VII-IX). Although most of these observations have been made near room temperature, it is not difficult, in many cases, to visualize the operation of similar phenomena in catalytic chemical reactions at higher temperatures.

a. Metal Cation-Induced Perturbations Detectable by I R Spectroscopy. The nondissociative interactions described in Table VII generally involve polarization by cation electrostatic fields of electronic charge density in C-C n bonds (86), aromatic n-electron systems (87,88), p-type orbitals on oxygen or nitrogen (72,88,89), or the carbon lone pair orbital on CO (85). Energy terms ( 4 ) contributing to these physical bonds probably include polarization (+p), field-dipole (+F~),and field ~) That the specific orbital interactions gradient-quadrupole ( + i energies. may be considerably more complex, however, was shown by Carter et al.

302

P. B. VENUTO AND P. 5. LANDIS

(86) for ethylene strongly adsorbed on AgX zeolite. The spatial arrangement shown in Fig. 21, which includes not only Ag 5s,p-hybrid orbital overlap with the olefin rr electrons, but also back donation of Ag 4d electrons into the olefin r * orbital, was suggested as being consistent with the observed absence of free rotation in the adsorbate. Field strength-dependent infrared shifts, similar to those observed with CO adsorbed on various zeolites (85), have also been observed for COz adsorbed on alkaline earth cation-exchanged faujasites (90,91). I n the study of Carter et al. (86), the heats of adsorption of the ethylene (18.1-8.3 kcal/mole) were related to changes in frequency of the double bond vibration of the adsorbed molecules. TABLE VII Infrared Spectroacopic Detection of Interactions between Organic Subatrates and Zeolite Cations

Zeolite

Substrate

LiX, NaX, KX, BaX, CaX

CHa=CHz

CdX

CHa=CH2

AgX NaX, CaX

NaX NaX Dehydroxylated Y NaX, CaX, SrX Alkaline earth transition metal

x, y

CH30H CH3OD

co

Diagnostic IR vibrations Position of va C=C str., v3 and Y ~ CHa Z def., CH str.

Probable nature of interaction

Cation-dependent physical adsorption; free rotation; cationwelectron bond Same as above Similar to above but stronger Very strongly held Same as above Enhanced intensity of Cation-weleetron Y 1 3 C C band at 1486 cm-1; appearance of forbidden modes Y C H displaced to Cation-.rr-olectron higher frequencies V O H and V N H shifted Cation-dipole to lower frequencies Strong band at 1446 Cation-pyridine N om-' Cation-dipole; anionShifts in VOH, Y O D a t low (0.3) 0 dipole ( ? ) Shifts in Y C 0~band Cation-carbon lone pair orbital near 2200 om-1

Ref.

ORQANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

303

TABLE VIII Hydrogen Bonding Intermtiow of Zeolite Acidic Hydroxyl Grmps with n-Electron System of Olefiw and Aromatics

Decationated Y zeolite

OH frequency (om-1) Substrate

CaH4 CaD4 C3H6 C3D6 CeHe C6Hab CeHsCHab

HY HY NiY,MgY HY HY

Unper- Hydrogenturbed bonded=

Ref.

3660

3300

(72)

3660

3200

( 72)

3640 3670 3670

3310 3340 3260

(71) ( 75) ( 75)

Bands are all broadened. C band near 1490cm-1 greatlyenhanced; see Abramov et al. (87). a

* Intensity of Y 1 3 C

TABLE IX Ionic, Radical, and Related Species Detected on Zeolite Surfacea by Spectroecopic Meana Zeolite

Substrate

Diagnostic feature

HY

C6HsN

CaX

(CeHs).&OH

Dehydroxylated Y

CsH5N

Dehydroxylated Y

(CeH&C=CHa (CeHs)aN CsHsNHa Perylene (CaoHla) CsHe ESR (C&)&eH 2-CsHlo ESR I-CsHle ESR NaF4

Cay, REY, CeX REY 6A, NaX, CeX

Species

Strong IR bands at 1660 Pyridinium ion and 1638 om-1 UV bands, Amax at 410 Trityl and 440 mp oarbonium ion Strong IR band at 1466 Lewis-bound om-1 pyridine ESR Radical cations (Art)

Ref. (72) (59) (72) (69)

Radical (94) cations (Art) (94) Radical species NFa radical (96)

304

P. B. VENUTO AND P. 8. LANDIS

FIG.21. Schematic diagram showing the spatial arrangement of the orbitals involved in the bonding between ethylene and AgX. From Carter et at?. (86).

b. Hydrogen Bonding Interactions With T Bases. Table VIII compares the shifts to lower frequency of the OH vibrations near 3600 cm-1 in several decationated Y zeolites when the aluminosilicates were contacted with simple olefinic and aromatic bases. The interactions are probably of the charge transfer type, with the adsorbate molecules acting as electron donors, and the polarized hydroxyl groups acting as acceptors. The shifts to lower frequency and broadening (hydrogen bonding) presumably result from the increased positive charge on the proton of the Si-OH group due to its proximity to the adjacent 3-coordinate aluminum. The larger shifts to lower frequency for propylene and toluene, relative to ethylene and benzene, respectively, are in accord with the more basic nature of the former substrates. I n the ethylene-HY system (72), rotation appeared to be quenched in the adsorbed species, although the adsorption was physical and reversible a t low temperatures. With propylene, however, the adsorption was stronger and not entirely reversible a t room temperature. Although the detection of such rr-type complexes on zeolites does not imply their intervention as intermediates in chemical reactions, it is not difficult to imagine that they may collapse to form reactive carbonium or benzenonium ion-type intermediates. c. Ionic, Radical and Related Surface Species. Other adsorbed species detected by spectroscopic means on zeolite surfaces are shown in Table IX. The formation of most of these involves a net transfer of negative charge from an organic substrate to an aluminosilicate framework. IR spectroscopy has proven useful in demonstrating the existence of pyridinium ions (72), from transfer of a lattice proton to pyridine. Hirschler (59) has successfully used ultraviolet (UV) spectroscopy to detect a

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

305

relatively low concentration of a n adsorbed carbonium ion. Dehydroxylation is believed to lead to t,he formation of cation-anion vacancy pair sites (defect sites), which may act as strong Lewis acids (69,70,72). The observation of IR bands characteristic of coordinately held (Lewisbound) pyridine is consistent with this view. The nondissociative interactions of cations with aromatic r electrons were also reflected by intense heats of adsorption, i.e., 15.5 and 25 kcal/mole for benzene adsorption on NaX (92) and Na-mordenite (93), respectively. With dehydroxylated Y-type faujasites, Stamires and Turkevich (69) demonstrated single electron transfer from aromatics with low ionization potential, such as 1,l-diphenylethylene, to the lattice electron acceptor sites (probably the 3-coordinate aluminum defect sites). The aromatic radical-cations so generated were identified through their ESR spectra. Radical species have also recently been reported from aromatics with higher ionization potential (such as benzene) and olefins adsorbed on calcium and rare earth zeolites (94). Although the exact nature of the electron acceptors appears uncertain, the possibility exists that Ce4+ operates as an electron sink in the rare earth systems. In fact, Richardson (95),in recent ESR studies of polycyclic aromatics adsorbed on alkali, alkaline earth, and transition metal ion-exchanged faujasites, has demonstrated that electron transfer (oxidation) occurs at the cation with the formation of organic radical ions. Elucidation of the possible role of such radical ions as intermediates in catalytic reactions would be of considerable interest. Zeolites have also been found to function as a convenient medium for storage and isolation of free radical species such as NFz (96)or Noz(97).Dollish and Hall (97a)have recently demonstrated that the amount of available oxygen-either gas phase or tenaciously chemisorbed-significantly affects the concentration of radical ions generated from aromatics adsorbed on synthetic zeolites.

III. Organic Reactions Catalyzed by Crystalline A1u m inosil icates A. OLEFIN-FORMING ELIMINATIONS A N D RELATED REACTIONS 1. Dehydration of Alcohols and Associated Processes

a. /3-Eliminations and Etherijkation. Conditions for representative dehydration reactions of simple CI-CS alcohols over alkali- and alkaline

306

P. B. VENUTO AND P. 8. LANDIS

earth metal-exchanged zeolites and hydrogen zeolites are tabulated in Table X. Decomposition was mainly to olefins (a)although an associated TABLE X Repreeentative Dehydrationa of Simple Alcohols over Zeolite Catalyeta

Reactant CHsOH CaHaOH CaHaOH CaHaOH CaHsOH iso-, n-CsH70H iso-CsH7OH iso-CsH7OH n-CrHgOH n-,iso-CIHBOH n-C4HgOH

0

b c

Catalyst

Temp. for appreciable activity ("C)

NaX H-Mordenite Zeolite A seriesc Mg > C a > Na > Li Mordenite seriesc Mg > Ca > Na > Li MgX, CaX, NaX, LiX Cay H X > NaX > CaAc NaX-COa CaA CaX, C a y Zeolite X seriesc Li > Na > K > R b CaX H-Mordenite CaX, C a y NaX CaY

Ref.

260a 260b 310 310 270-360 260-276 210 236 220-260 220-276 360 109-130 100-200 168-228 276-326 160-170

98.4% conversion to (CHs)aO. Similar conversions over active AlaOa a t 340'. Order of reactivity under identical conditions a t indicated temperature.

bimolecular process to form ethers (b) was observed in some cases. CnHantiOH

-+

2CnHan+iOH -+

+ Ha0 (CnHan+i)zO+ Ha0

CnHan

(a)

(b)

Similar dehydration-etherification processes accompanied the use of

C2-Clo alcohols as alkylating agents in the presence of rare earth-X (REX) and -Y (REY) and HY catalysts (43).

The relative rates of olefin production and etherification have been shown to vary with reaction conditions (99-101),including temperature and contact time, and with the geometry of the catalyst pore system (99). Bryant (99) has demonstrated kinetic effects in ethanol and nbutanol dehydration that greatly favor olefin formation using a series

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

307

of ion-exchanged zeolite A and mordenite catalysts; etherification is favored in the corresponding zeolite X series. Perhaps the operation of a Rideal-type etherification mechanism is not favored in the narrower zeolite A and mordenite pore systems: CH, -CHI

-0

/H

Rideal-type attack

Few detailed studies of olefinic dehydration products have been reported. Rapoport et al. (105) found cis- and trans-2-pentene (in equilibrium amounts) and 1-pentene as major products from n-pentanol dehydration over NaX ; only traces of 2-methyl- l-butene and 2-methyl2-butene were reported. Thus, it is evident that double bond isomerization has accompanied or followed the dehydration reaction. Several authors (99,101,106)have suggested that diffusion processes may be rate controlling, or at least of some significance, in zeolite-catalyzed dehydration reactions. Molecular-shape selective alcohol dehydration (7) was discussed earlier. Several studies on a series of cation-exchanged zeolites A, X, and mordenite (Table X) (99,103) have demonstrated that activity for n-alcohol dehydration is dependent on cation size and valency. Specifically, the catalytic activity was higher in zeolite systems possessing greater cationic charge density and higher calculated (40) electrostatic field strengths. Similarly, the position of the cation in the lattice (probably as related to the effective field strength) has been suggested as important in determining dehydration activity (52). Using surface modifying treatments and ion-exchange techniques, the dehydration activity of H-mordenite has been related to the protonic acid sites (104). Since zeolites have a tremendous affinity for water, even a t very low concentrations, it ,is evident that the eliminated water (aside from any reversibility of the dehydration reaction) will play some role in the behavior of the catalyst system. Thus, changes in diffusivity (31,32) may occur, or “self-promotion” by the proton-generating hydrolytic

308

P. B. VENUTO AND P. 9.LANDIS

mechanisms discussed earlier. It has been experimentally observed (51,52) that pretreatment of CaX with small amounts of water is associated with an increase in activity for tert-butanol dehydration. Frilette and Munns (102) observed that catalytic activity for the dehydration of isopropanol at 236" was reversibly induced in NaX by the presence of C02. Kinetic and sorption studies suggested that the catalytic sites were formed by chemisorption of C 0 2 molecules; impurity cations or defect centers, rather than Na+ ions, were thought to be responsible for the sorption. Since Frilette and Munns observed no isopropanol dehydration activity at temperatures below 300" over carefully prepared NaX samples, the possibility was suggested that Tsitsishvili et al. (101) were actually working with contaminated or hydrolyzed zeolite NaX. The above data suggeh a possible role for polar or ionic intermediates in many of the dehydration reactions. For many catalyst systems, the generally observed order of decreasing temperature for appreciable reaction (RCH20H > R2CHOH > RsCOH) parallels the increasing stability of the corresponding carbonium ions from loss of hydroxyl ion. The common denominator in these zeolite-catalyzed dehydrations seems to be the presence of an extensive, rigid, ordered polar surface, a major function of which is to give assistance to ionization or heterolytic bond scission in the adsorbed alcohol. The relatively basic hydroxyl function is thereby rendered a better leaving group by interaction with

protons in hydrogen zeolites, metal cation fields (either unscreened or mediated by a proton donor), lattice defect sites, or other electron-deficient sites. Carbon-oxygen bond cleavage is thus facilitated. b. Methanol Decomposition. Methanol cannot dehydrate to form an olefin unless recombination of one-carbon fragments takes place. Mattox (98) has reported a 98.4% conversion of methanol to dimethyl ether over NaX at 260"; closer scrutiny of the data, however, reveals that the remaining 1.6 mole % of product is a mixture of CZ to C g olefins, with butene the predominant product. This product distribution suggests that an a-elimination process is occurring in the methanol

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

r\ H CI&-OH

L

n :C&

-

I&O

+

309

:C&

(Cbh

n = 2, 3, 4, 5

adsorbed on the zeolite surface. The divalent carbenoid species so formed can then polymerize to form simple, low molecular weight olefins. Analogous results, but with simple olefins comprising as much as 34-43y0 of the gaseous products, have been observed by Schwartz and Ciric (107) for methanol decomposition over REX and ZnX catalysts a t higher temperatures (Table XI). Although decomposition by radical TABLE XI Melhanol Decomposition over Metal Cation-Exchanged Faujasiteaa

Temp. ("C) yo Conversion of CH30H to gaseous products Gaseous product

330-380

360-390

61

30

Tr

10.3 43.0 17.0 2.9 9.2

22.6 24.6 2.7 10.8 4.4 8.0 13.9 6.2 4.3 2.7 100.0

-

7.8 4.0 2.0 3.8 100.0

4 Continuous-flow; 1 atm; 1.6 LHSV; samples analyzed at 2-3 hours on stream. b 9.46 wt % Zn. c Liquid product mainly unreacted CHsOH, HzO, and (CH3)zO.

pathways probably produced the methane, a carbene-type mechanism may also explain the formation of the monoolefins. The large quantities

310

P. B. VENUTO AND P. S. LANDIS

of paraffins other than methane observed in the reaction over R E X probably arose from secondary hydrogen-transfer reactions of ethylene (44). Coke formation was extremely rapid in this reaction.

c. Displacement Reactions of Ethanol. Zeolites also show catalytic activity for the gas phase conversion of ethanol and hydrogen chloride to ethyl chloride and water a t temperatures ranging from 150' to 270" (108).At 170' and C2HSOH/HCl molar ratio of 1.82, the following order CaH& H

+ HCI + C&C1+

Ha0

of activity was observed: HY, REX, REY > CaX, NaX > Linde 6A. Under similar conditions, neutral alumina was about as active as Linde 6A. I n these continuous-flow runs, steady state conversion t o ethyl chloride (over 3 to 6 hours in many cases) without apparent catalyst deactivation was observed for all of the above catalysts. Maximum activity for HY, REX, and REY was obtained a t temperatures of 180-200". Considerably higher temperatures were required to achieve comparable conversions with the other zeolites or alumina. With REX, REY, and HY catalysts the increased rate of elimination of HCl to form ethylene a t temperatures above 200"-and the associated catalystpoisoning side reactions of ethylene-resulted in decreased ethyl chloride formation and increased coke formation. I n homogeneous systems, halogen acids convert alcohols to alkyl halides by nucleophilic displacement on carbon by halide ion. Since hydroxide ion is a poor leaving group, the reaction is assisted by protonation of the alcohol or by complex formation with a Lewis acid. I n the zeolite system, it was experimentally shown that HCl adds to ethylene at 190" (hydrohalogenation) to form ethyl chloride; large amounts of coke were also formed. Three times as much ethyl chloride and about one-fourth the amount of coke were formed in the analogous ethanolHC1 reaction. This observation suggests that ethylene is not necessarily an intermediate in this reaction, and that replacement may occur without prior elimination, as shown here: H

LO/

CI

,/'

CH,-CH,---?

q/H I

;1I u

TfiTfi

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

311

2. Dehydrohalogenation and Related Reactions

In 1953, Barrer and Brook (19) observed that tert-butyl chloride adsorbed upon the external surface of chabazite crystals reacted smoothly at room temperature to give HCl and oily polymers derived from isobutylene. Decomposition of isopropyl chloride over chabazite did not occur until 160°, however. Milton and Breck (20) demonstrated smooth dehydrochlorination of tert-butyl chloride over clay-bonded CaA at 100-150". These 13-eliminationsmay be assumed to have occurred on the external surface of the small pore zeolites. Recently a study was made of the intracrystalline catalytic dehydrohalogenation patterns of two-carbon haloalkanes over zeolite catalysts in continuous flow systems (108).At 260°, REX, NaX, Nix, AgX, HY, metal-hydrogen Y, and Linde 5A zeolites all converted substantial amounts of ethyl chloride t o ethylene in gas phase reactions, while amorphous silica-alumina was considerably less reactive. Catalysts such aa y-Al203 or CaCl2 require temperatures of 360-420" for comparable conversions (109).With REX, conversion increased rapidly from about 6% at 150" to nearly 100% at 316" and higher. Moderate aging was observed in all ethyl chloride runs, with about two-thirds to one-half initial activity remaining after 2 hours on stream. Typical aging curves are shown in Fig. 22. The profile of the AgX shows a sharp, initial peak followed by a very rapid loss of ell activity. ~ H a C lFLOW RATE (2S.r) MOLL /pm CATALYST/IOUR

z r o t RUNS 177 Y RUNS

0.0410 0.0205

80

20 0

-

REX, 177% -0

-01 0

200 300 100 TIME ON STREAM, MINUTES

FIQ. 22. Ethyl chloride dehydrohalogenation activity AgX (108).

VB

time for REX, NaX, and

312

P. B. VENUTO AND P. 9. LANDIS

Crystalline AgC1, but only small amounts of coke, were detected in the discharged catalyst. Since this catalyst showed slight activity at 6 6 O , a silver ion-assisted elimination seems probable :

X-Ray analysis of NaX catalysts discharged after dehydrochlorination runs showed an extensive loss of lattice crystallinity and the presence of NaCl. Since only small amounts of coke were formed, it appears that the rapid aging observed with NaX Catalysts is related to lattice collapse of HX formed from the reaction of NaX with HC1. Barrer and Brook (19) reported a similar breakdown in the chabazite lattice associated with H F liberated from the dehydrofluorination of occluded fluoromethanes HCFs and HCF2C1. a-Eliminations, by carbenoid intermediates, appear to be operative here, as in the case of methanol. Carbene intermediates may also be involved in the formation of C02 observed by Cannon (110) in the decomposition of HCFzCl over Linde 4A and 6A sieves. Modified faujasites REX, HY, and metal-hydrogen Y, after reaction with ethyl chloride for several hours, contained large amounts of coke, but showed negligible loss of lattice crystallinity. Complex intracrystalline hydrogen-transfer reactions of product ethylene (44) were the major source of catalyst aging in the modified faujasites. Elimination reactions of other two-carbon haloalkanes were studied using REX catalyst. Ethyl bromide and ethyl iodide formed ethylene and hydrogen halide in high yield at 66'. The reactions shown here also 163"

CHs-CClS-CHsdCls 240'

CHS--CHCls-CHadHCl+ (malor)

2880

CHaCl- CHaCl-CHdHCl+ (malor)

+ HCI CaH&l+ HCI (trace)

CaH&l+ CHs-CHCla + hydrocarbons + HCl (all minor)

proceeded in good yield at the temperatures indicated. Further reactions of product vinyl chloride were shown to be responsible for the side produats with 1,2-dichloroethane (Fig. 23). This reaction scheme involves an intermolecular hydride ion shift.

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

CH,=

CHCI

-HCI

[CH,POLYMER

/

COKE

$

CH,-

POLYMER

+

C3HB ,C,H,,

-

CH,

H-C

CH

?

- CI]

1

-

313

C-H

C1°

CH,

- CHCI,

He CH,CI

1

-HCI

= CH,

,ETC.

FIQ.23. Reactions of vinyl chloride over REX catalyst at 260" (108).

It is generally agreed that liquid phase /I-eliminations proceed by ionic mechanisms (111).There is also a class of gas phase eliminations that is essentially heterolytic in character (UZ),with the stability of the carbonium ion formed from the alkyl halide being the major factor controlling the rate. Additional evidence supporting ionic mechanisms has been compiled for HCl elimination on solid catalysts such as CaC12, CaO, and A1203 (113).The patterns of substrate structure vs elimination temperature and side reactions observed in the zeolite-catalyzed dehydrohalogenations also tend to support the proposition that ionic intermediates intervene. Elimination activity in REX catalyst was inhibited by nitrogeneous bases, yet strong acidity, such as that required for alkylation of benzene with ethylene, is not necessary, as shown by the data for NaX and Linde 5A. Thus the zeolite surface appears to assist in the heterolysis of the carbon-halide bond in a variety of ways, as described earlier under dehydration of alcohols. It is also possible that the anionic lattice oxygens may assist in this catalysis by polarizing C-H bonds. Zeolites can also function as reservoirs for selective halogenation of hydrocarbons (114) and as convenient media for addition of chlorine to olefins, with subsequent dehydrohalogenation to form reaction mixtures rich in highly halogenated alkenes (108).Similarly, highly chlorinated benzenes have been prepared in low yield by passing cyclohexane over NaY in the presence of NOCl and HCl, NO and Clz, or Cla alone a t 149"-260" (115).

314

P. B. VENUTO AND P. 5. LANDIS

3. Dehydrosulfurization ,

Zeolite NaX functions as an efficient dehydrosulfurization catalyst for mercaptans at temperatures of 200-600' (116).I n a continuous flow system a t 500" (LHSV = 2) ethanethiol gave 100% conversion to gaseous products consisting primarily of ethylene and H2S. Under similar conditions, methanethiol gave 97 yo conversion with predominant formation of methane (47%)and H2S (47.9%).A small, but reproducible quantity of ethylene (1.8%) was also observed, which suggests a dual path for the decomposition. Radical-type decomposition would account for the methane and HzS, and the presence of carbene intermediates would lead t o ethylene formation. trans-4-tert-Butyl-cis-2-methylcyclohexanethiol underwent H2S elimination a t 200' (LHSV = 0.5) over the same catalyst. Five isomeric olefins were produced in the ratio of 1.2 : 1.1 : 1.0 : 0.08 : 0.02, as shown. tort

-0 u SH

teri-0u

tort-0u

tort-0u

I

I

ioril-0u

tert -0u

I

I

H,.

Pd/C

tert -0u

Since hydrogenation of each of the five olefins produced 3-tert-butyl-lmethylcyclohexane, i t is apparent that elimination was associated with double bond, but not skeletal, isomerization. I n a somewhat related reaction, Landis (117)has demonstrated the formation of good yields of trans-stilbene from condensation of 2 moles of benzyl mercaptan over NaX. Ck-SH

NaX

200-300'

truns-C,&CH=CHC,&

+

2

YS

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

316

B. POLYMERIZATION, ISOMERIZATION, AND RELATED REACTIONS OF OLEFINS Table XI1 summarizes pertinent polymerization, isomerization, isotopic exchange, and related reactions of C2-C6 olefins over various faujasite-type catalysts. It is probable that a diversity of mechanistic pathways is represented in these reactions. Generally, double bond isomerization was observed, and reactions over HY, dehydroxylated Y, and REX occurred at lower temperatures than with the other catalysts. In a study of polymerization over several zeolite catalysts, Norton (118) observed that the order of substrate TABLE XI1 Representative Polymerization and Related Reactions of 0lejh.n wer Faujaaite Catalysts Reactant

Catalyst HY Dehydroxylated Y REX Nix CaX > NaX Nix HY NaX Nix Dehydroxylated Y NaX HY REX HY

Reaction Reaction temp. ("C) reporteda Ref.

2 160 100 92-2 13 260-360 200-300 260-360 2 26 263-379 226 0 232 - 8 0 to 20 64

26

Key: (1) Isotopic exchange of reactant hydrogens with catalyst deuterium or vice-versa; (2) polymerization; (3) isomerization.

reactivity was (CH&C----CH2 > C H s C H d H 2 > CH2=CH2. He further noted that the structures of the liquid branched polymers (mainly c6, Ce, (212) obtained from propylene polymerization over CaX were of the conjunct type reported for conventional acid catalysts such as H3P04 supported on Kieselguhr. Catalysis by zeolite acidic sites was proposed for these polymerizations.

316

P. B. VENUTO AND P. 9. LANDIS

The gas phase isotopic exchange and proton transfer reactions of ethylene and propylene over HY (72)were discussed under decationated Y faujasites; the complex intracrystalline polymerization-hydrogen transfer reactions of ethylene over REX were described under catalyst aging. Similar hydrogen-transfer reactions have been observed for CzH4 and C3H6 over HY (44,72)and for C3H6 and l-C4H* over REX (44). The isomerization of cyclopropane to propylene (as),a waterpromoted reaction, is also included in Table XII. Eberly (118a) has recently discussed similar hydrogen transfer reactions of C3-Cs olefinsleading to aromatic formation--over HY zeolite. In small pore zeolite systems, the polymerization of propylene (118), isobutylene (124,vinyl chloride (121),and styrene (121)over Linde SA, the polymerization of propylene and isobutylene over chabazite (19),and the double bond isomerization of 2-methyl-1-pentene over Linde SA (122)have been reported. Since most of these reactants and the products derived from them cannot pass through the 4-5 A entry pores, it is assumed that these reactions occurred on the external surface of the zeolite. Crystalline X-type catalysts, prepared by partial exchange of the Na+ cations of clay-bonded NaX with Nit+ ions (Nix, Table XII), actively catalyzed the polymerization of low molecular weight olefins to products that were predominantly dimers. In a typical example, a N i x catalyst containing 1.76-wt yo Ni produced 45% and 70% maximum conversion of ethylene to butenes (at 270" and 320", respectively) in continuous flow systems (GHSV = 0.29 L/hr/ml catalyst). Maximum conversions generally were obtained between 1 and 6 hours reaction time. Similar results were obtained with a catalyst containing 3.4 wt. % Ni; X-type catalysts containing chromium or cobalt cations were less effective. Equilibrium mixtures of 1-butene and cis- and trans-2-butene were generally obtained. Although initial activities were generally high, conversion markedly decreased at longer times on stream. Deactivation of the N i x catalyst was shown to be due to the intracrystalline formation of a viscous oily polymer. Activity could be restored upon removal of the polymer by solvent extraction or by heating in vacuo at 370". In some cases, "induction periods," during which no butenes appeared in the gaseous effluent,were observed. In one run, the reaction was stopped after the induction period and the entrapped organic matter extracted and analyzed. The product consisted of C12-C35 aliphatic polymers, with major

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

317

carbon numbers at c 2 1 - C ~ ~ The . polymers contained C=C double bonds. The "induction periods," then, involved the active intracrystalline polymerization of ethylene to an aliphatic species of fairly high molecular weight. Allene and methylacetylene also polymerized over Nix at 200" to form mesitylene and pseudocumene. Evidence for a carbonium ion type of isomerization-polymerization reaction involving proton transfer was shown for the reaction of 1hexene over a deuterated REX catalyst (44).The catalyst was prepared by adding DzO (1.4 eq of D per gram atom of Al) to a REX sample that had been precalcined at 500", and showed a broad envelope at 2597-2326 cm-1 (0-D stretching vibrations) in its IR spectrum. The origin of protonic acidity in such REX catalysts was discussed earlier. 1-Hexene and catalyst (0.54 wt : wt ratio) were placed in a sealed tube and heated for 1.75 hour at 64". Analysis of the liquid product (mole %) gave 1-hexene ( 19.0yo),trans-hexene-2 and -3 (74.5y0),hexene dimers (5.5'3"), and hexene trimers (1.0%). Evidence for extensive transfer of deuterium from catalyst OD groups to olefin as C-D bonds is given in Table XIII. TABLE XI11

Diatribution of Deuterium among Liquid Monoolefink Reaction Products of 1-Hexene over Deuterated R E X Catalyst Degree of deuteration

Isomer (mole C6

yo)"

ClZ

CIS

23.0 23.9 18.8 14.8 9.8 6.9 2.9 1 .o

-

6.6 9.8 13.0 12.9 12.9 12.9 9.6 6.4 9.6 3.2 3.2

100.0

99.9

-

-

-

a Mass spectroscopic analysis, 7 eV; weak IR band present at 2166 cm-1 characteristic of C D stretch in -CHaD or -CHD-.

318

P. V. VENUTO AND P. 9. LANDIS

The following typical carbonium ion type of mechanism is consistent with the above data: (1) Initial adsorption on proton-donating surface, e

CHa=CH-CdHg

+ D"O-Zeol+ CH~TCH-C~H~

(2) Deuteron transfer and carbonium ion equilibration, I

+ CHzD-CH-CHaCsH7 d Q

+ CHaD-CHa-CH-CH2-CaHs B e

+ Other

carbonium ions

Electrophiles (11) and (111) must be closely associated as ion pairs with anionic sites in the aluminosilicate lattice, Elimination of a proton from (11) or (111) results in the formation of the 2- and 3-hexene isomers. Attack of free liexene on the adsorbed CS electrophile t o form a CIZ carbonium ion, with subsequent proton elimination, results in dimer formation. The inorganic catalyst skeleton of the discharged REX catalyst was dissolved after termination of the 1-hexene isomerization run. Analysis of the brown, oily organic extract (mole yo) showed hexene dimers (67.1%), trimers (22.8%), tetramers (7.4%), and other higher polymers (2.7%) based on pentamer. The IR spectrum of the oil showed an intense C-D stretch near 2143 cm-1, characteristic of -CHzD or -CHD-. No aromatics were present, as shown by ultraviolet analysis. A selective retention of the less mobile polymer fractions of higher molecular weight has thus occurred. This "reverse" molecular-shape selectivity, observable in an uncomplicated way a t low temperatures, probably precedes the complex aging and coke-forming reactions observed in olefin-zeolite systems a t higher temperatures. As discussed earlier, transfer of deuterium to organic reactant was observed when 2,3-dimethylbutene-1 was contacted with a synthetic partially deuterated decationated Y zeolite (DHY). From Table XIV, it is seen that such HY catalysts have appreciable catalytic activity for isomerization even a t extremely low temperatures. Transfer of deuterium to olefinic reactant was associated with a decrease in the intensity of the catalyst 0-D band near 2620 cm-1. Similar patterns of deuterium transfer accompanied the isomerization of 1-hexene over DHY a t 25".

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

319

TABLE XIV Reaction of 2,3-Dimethylbutene-l over HY at Low Temperature8 Temp. ("C)

20a 00 -70 to -80

Initial ratio of reactant to catalyst (wt:wt) 9.2 6.9 6.7

Contact time (hr) 21 6 6

Liquid product composition, wt% 2,3-DMB-l

2,3-DMB-2

Other

20.1 13.9 60.7

77.6 86.6 36.6

2.3 0.6 2.7

a Small amounts of deuterium (as di only) transferred in reactions over deuterated catalyst (DHY); mass speotroscopic analysis at 7 eV.

Loading of zeolites with alkali metals provides catalysts that have double bond isomerization activity and little or no accompanying skeletal isomerization. A NaX zeolite loaded with 10.4 w t % sodium showed 94% conversion of 1-pentene to 2-pentene at 2 5 O , and 38% conversion of 1,3-cyclohexadiene at 100" to a mixture of 44% 1,4-cyclohexadiene and 56% benzene. Only a narrow range of sodium loadings proved effective, however.

C. ELECTROPHILIC AROMATIC SUBSTITUTION AND RELATED REACTIONS 1. Aromatic Alkylations a. Scope and Reaction Conditions. A considerable body of literature

exists concerning the Friedel-Crafts alkylation using conventional protonic acids, proton donor-promoted Lewis acids, and many acidic oxides and mixed oxides as catalysts (123).A recent study demonstrated that a number of crystalline aluminosilicates are versatile catalysts for a wide variety of alkylation reactions (43,44). Modified faujasites REX, R E Y , and HY have shown the broadest application, although CaX, NaX, and H-mordenite were useful in some cases. The tabulation in Table XV indicates that a wide variety of simple monocyclic aromatic nuclei-such as benzene, phenol, thiophene-can be alkylated with a wide variety of alkylating agents, including c 2 - C ~ olefins, alcohols and haloalkanes, ethers, and paraffins. Minachev et al. (12&127) have recently published a series of papers on the use of bivalent cationexchanged zeolites (mainly Cay) in alkylations.

320

P. B. VENUTO AND P. 9. LANDIS

Operation in the liquid phase is generally essential for efficient use of modified faujasites such as HY or REX, because catalyst aging and many side reactions are extremely rapid in the vapor phase. Either continuous flow or stirred, batch-type reactors systems may be employed. With low molecular weight species such as ethylene, high molar ratios of aromatic to alkylating agents are necessary to minimize the unfavorable coke-forming reactions of the alkylating agents and to favor monoalkylation. With higher molecular weight (C6-CI6) alkylating agents, lower ratios may be employed. TABLE XV Repreaentative Aromatic Alkylationa over Zeolite Catalysts

Aromatio

Alkylating agent

Catalyst CaYb REX REX REX CaY MgY CdY > SrY > BaYb REX REX REX REX CaYb HY HY REX REX REX REX HY, REX REX REX NaX NaX HY CaX > REY > HY > REX H-mordenite

- -

Temp. for highest mtivity ("C)

Conditione"

400 204 218 204 260-300

1 2 1 1 1

160 200 160 371 376 160 160 200 316 218 204 182 182 210 371 330 182 204

1 1 2 1 1 3 3 1 1 2 1 3 3 3 1 2 3

200

3

1

0 Key: (1) Continuous flow apparatus at 1 atm; (2) continuous flow apparatus under pressure; (3) stirred, liquid-phase reaator. b Work of Minachev et al. (124-127).

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

321

b. General Reaction Characteristics and Product Distributions. I n typical alkylations of substituted benzenes, ortho/para orientation generally predominated. Meta isomers did not appear in significant quantities except at higher temperatures or when reactants were subjected to prolonged exposure to the catalyst. Similarly, 2,4-dialkylphenols were identified as the major polyalkylation products in phenol alkylations; 2-substitution was favored in thiophene systems. Likewise, in competitive systems, selectivity for attack on the more reactive aromatic substrate was shown. Thus, kCaHsOH/kCeHa was 8.5 in a competition for ethylene over REX catalyst at 205'. More quantitatively, obedience to the Brown selectivity relationship has been observed for the competitive ethylation of benzene and toluene over REX catalyst (128). These data (points h, i, j from Table XVI) are shown TABLE XVI Compeditive Ethylationa Over REX Calalyata

Reaction

h

i

j

Temp. Conversionb ("C) 126 160 180

2.83 6.87 9.20

Isomer distribution in ethyltoluenes

Ortho

Meta

Para

30.8 22.6 17.7

26.6 30.6 26.6

43.6 46.8 66.7

Reactivity ratio (ktoluene/kbenzene) 1.92 1.84 1.60

a Continuous flow system; LHSV = 2.72; total aromatic/ethylene (molar)ratio = 6.9; C&CHa/CsHe= 1; analyses on samples accumulated between 20 and 60 min on Stream. * Conversion of aromatic to monoalkylaromatic; only trace amounts of dialkylate formed at 160' and 180".

on the same plot (Fig. 20) as those from the electrophilic hydrogendeuterium exchange experiments discussed earlier. With alkylating agents of three or more carbon atoms, the monoakylate generally contained a mixture of isomeric alkylaromatics. Structurally, these isomers consisted of unbranched paraffin chains with the aryl residue attached at various secondary carbon atoms along the chain as in (A) and (B). With primary alcohols and haloalkanes, however, variable amounts of n-alkylaromatics (C) were formed. These patterns

322

P. B. VENUTO AND P. €4, LANDIS

are consistent with acid-catalyzed double bond isomerization of olefin

reactants, which must be rapid compared to the alkylation step. In a few cases, products with skeletal rearrangement in the side chain were observed, but these were attributed to side chain isomerization of product arylalkanes. Generally, for a given aromatic, the temperature for appreciable reaction with different alkylating agents decreased, or, for a given temperature, the conversion of alkylating agent increased, in accord with the stability of the expected carbonium ion intermediate. This is visualized as taking place by double bond protonation in olefins, and heterolytic dissociation of carbon-oxygen or carbon-halogen bonds in alcohols or haloalkanes. In summary, then, analysis of the structures of product alkylaromatics, patterns of substrate reactivity, and side reaction pathways in modified faujasite-catalyzed alkylations reveals great similarity to the corresponding features commonly reported for electrophilic aromatic substitutions in the presence of strong protonic acids or promoted Lewis acids. For similar reactants, the modified faujasites, with their ordered, rigid structure, consistently catalyzed alkylation a t lower temperatures than did amorphous silica-alumina-type catalysts. Many of the undesirable sidereactions encountered at the higher temperatures were thereby eliminated. This observation is consistent with the fact that the modified crystalline faujasites are considerably more acidic than are the amorphous catalysts (46,47).On the other hand, promoted Lewis acids such as AlC13-HCl or very strong protonic acids such as 98% HzS04 or liquid HF generally showed significant activity at lower temperatures than did the modified faujasites. If it is assumed that the modified faujasites are very strong acids, it seems reasonable to relate the higher reaction temperatures to the additional energy barriers and entropy requirements imposed by sorption-desorption, intracrystalline diffusion, etc., as discussed earlier in relation to site accessibility. However, it is also possible that, under the conditions of alkylation, the zeolite acid

ORGANIC CATALYSIS OVER CRYSTALLINE ALTJMINOSILICATES

323

sites may not be as strong as those of 98% HzS04 or AlCls-HCl. The observed temperature dependence probably reflects both of these factors.

c. Specijk Reaction Systems. (i) Alkylation of aromatic hydrocarbons. ( a ) Catalyst comparisons in benzene alkylation with ethylene or propylene. Although vapor phase alkylation of benzene over modified faujasites REX, REY, and HY was useful as a testing procedure for comparing catalyst activities (Fig. 12), it was nevertheless associated with rapid catalyst aging as shown in Fig. 24. The side reactions of ethylene,

I-

,-. I

- I

I I

-

CONDITIONS :

2 18

OC

@ .

'\

= 19 can, : 5 M 4 LHSV I

I2

ATMOSPMERE

I I

P

Y -O o:q;

-4 -0-

FIO.24. Rapid aging in benzene-ethylene alkylations over REX crttalyat (43).

which lead to this aging, were discussed earlier. Under the conditions shown in Fig. 12, CaX showed no reaction at or below 218'. Minachev et al. (126) reported significant benzene-ethylene alkylation activity at 400' with Cay. With the rare earth cation-exchanged faujasites, low sodium levels were critical for high activity, and NsX was completely inactive, even at 316". H-Mordenite showed moderate initial activity at 1 7 7 O , but had a strikingly rapid aging profile. The immensely greater activity of hydrogen zeolites and trivalent cation (rare earth) exchanged zeolites, as compared to the less active bivalent and monovalent cation forms is a salient feature of the benzene-ethylene reaction. Optimum conversion of benzene and ethylene to ethylbenzene over

324

P. B. VENUTO AND P. 8. LANDIS

REX catalyst was realized under liquid phase, continuous-flow conditions. Within the bivalent cation-exchanged Y series, the order Ca, Mg, Cd > Sr > Ba has been observed (125,127) in benzene propylation at 260-300". Activity of the CaY for the propylation reaction reached a sharp peak in activity when 60% of the Na+ ions had been replaced by Ca++ (125). This increase in activity corresponds with the increasing occupation by Ca++ of the SII sites in zeolite Y at higher degrees of Ca++exchange, as predicted by Pickert et al. (40). ( b )Characteristics of other alkylations. REX catalyzed the atmospheric pressure alkylation of benzene with low molecular weight primary alcohols and alkyl halides at temperatures of 200-218", and with isopropanol at 160". Initial yields of the expected alkylbenzenes were good, with the exception of the methanol-benzene system, where a competing coke-forming reaction was prominent. Catalyst aging was severe in all cases except with isopropanol, where an apparent "steady state" formation of isopropylbenzenes was observed for the duration of the %hour run. In continuous-flow, liquid phase runs, high conversions of 1-decene to mixtures of decylbenzene isomers occurred in reaction with benzene over REX catalyst (Fig. 26). I n analogous benzene alkylation attempts with 1-decene or 1-decanol at atmospheric pressure, side reactions of CONDITIONS I

100 -

L H S V 9 3.1 C,M, I I-CK)Mzo

400 Pate

-

5

80

40

0

-

I

0

10

1

I

I

20 30 40 50 TIME ON STREAM, HOURS

00

FIG.26. Aotivity of REX oatalyst vs time for the elkylation of benzene with I-decene in the liquid phaw (43).

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

325

the alkylating agents were predominant. When a sufficiently rapid reaction rate could be maintained at temperatures near their boiling point, other aromatics such as m-xylene underwent smooth alkylation when stirred with catalyst and Clo-alkylatingagent at 150'. In reactions of m-xylene over HY catalyst, large amounts of n-alkylate were observed with 1-chlorodecane as alkylating agent. Isomers attached at secondary carbon atoms were almost exclusively formed with 1-decene. Several alkylation reactions with extremely high energy requirements have also been observed over zeolites. Small amounts of toluene and ethylbenzene, together with light paraffin gases, were observed in the alkylation of benzene with isobutane over REX catalyst a t 371". In a reaction that involved fragmentation of the benzene ring, Frilette and Rubin (129) observed the formation of toluene and ethylbenzene when benzene alone was passed over H-mordenite at 400".

(ii) Alkylation of phenol and other polar aromatics. In phenol alkylations, the product distributions and side reactions were entirely analogous to those observed in the other systems described earlier. With phenol reaction systems, only C-alkylation products were detected. (In contrast, the alkylation of thiophenol produced mainly S-alkylation.) However, some important differences in reactivity patterns, arising largely from the presence of the polar OH group and consequent strong adsorption of phenol, were observed (130). These differences included the higher temperature required for ethylation of phenol, relative to that required for alkylation of benzene, and the still higher temperature necessary to ethylate benzene in the presence of phenol. At a fairly high temperature (371') NaX served as a catalyst for the disproportion of anisole to a mixture of phenol, methylphenols, and methylanisoles. Aromatic amines were also alkylated at high temperatures over zeolite catalysts . (iii) Alkylation of thiophene and other heterocyclics.Crystalline zeolites

also showed catdytic activity for the alkylation of simple heterocyclics such as thiophene with alkylating agents capable of forming stable carbonium ions. Catalyst aging, however, was generally severe. Attempts to alkylate thiophene with ethylene, using REX or Nix, were unsuccessful. As indicated in Table XV, the less acidic CaX was a more suitable catalyst for thiophene alkylation than REX or HY; even NaX, at sufficiently high temperatures (288"), showed catalytic activity.

326

P. B. VENUTO AND P. 9. LANDIS

d . Mechanistic Considerations. ( i )Reaction pathways in alkylation with olefins. Using the same deuterated REX catalyst described earlier in reference to 1-hexene isomerization, the reaction of 1-hexene and benzene to form mono- and dihexylbenzenes and hexene polymer was studied (Table XVII). The Cia-alkylaromatics were almost exclusively TABLE XVII

Distribution of Deuterium i n Liquid Cia and CIS Fraction8 or Reaction Product8 of Hexene- 1 and Benzene over Deuterated REX Catalyst' Mole Degree of deuteration

yo isomer"

c11

CIS

Monoolefin Alkylbenzene

Monoolefin

Alkylbenzene 29.6 33.2 21.6 9.2 3.6 2.9

31.4 38.4 18.6 11.6

66.9 31.9 9.0 3.2

27.6 24.1 24.1 24.1

-

-

-

-

99.9

100.0

100.0

100.0

-

-

0 Equimolar mixture of CeHe and l-CeHls heated at 80°C for 1 hour in sealed tube with 1.4 times its weight of deuterated REX. * Mass spectroscopic analysis, 7 ev; IR spectrum (neat) of product mixture showed IR band at 2166 cm-1 (C-D in -CHDor -CHaD but not on aromatic ring).

2- and 3-phenylhexane. As shown in Table XVII substantial amounts of deuterium were transferred from catalyst OD groups to aliphatic carbon as C-D bonds in hexene polymer and hexyl residues attached to benzene. As visualized below, benzene attack on the adsorbed electrophile derived from 1-hexene (Eq. (2) in Section II1,B) in a Rideal-like

ORQANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

327

mechanism will explain the alkylation step. Since several arylalkanes were formed, olefin isomerization appears to be more rapid than alkylation under these conditions. The electrophile is a carbonium ion, or carbonium ion-like species, associated with the rigid polyanionic lattice. The reaction patterns observed for ethylene-benzene alkylations over modified faujasite catalysts (vide supra)also may be explained by such a mechanism. It is known that aromatics are adsorbed on certain zeolite sites (75,87,88,92,93)and may even compete with olefins for them. However, those aromatics that are reactants in alkylation and attack adsorbed electrophiles are almost certainly only weakly adsorbed and behave more like fluids in the intrazeolitic environment. Most of the reported interactions of aromatics with zeolites discussed earlier appear to involve, at some stage, a net transfer of negative charge from substrate to catalyst surface. Thus, an aromatic adsorbed strongly enough to have donated significant ring n-electron density to an electron-withdrawing site, either as n-charge drift or actual electron transfer, would be expected to have some degree of positive charge. As such, it would not be expected to attack positively charged electrophiles, since significant Coulombic repulsive forces would be encountered. Further, substrate selectivity in competitive ethylations of benzene and toluene (128) should not be expected if the aromatic reactants were strongly adsorbed. Hence, a Rideal-like pattern is proposed.

(ii) Alkylations with Alcohols and Haloalkanes. Alkylations with alcohols and haloalkanes proceed analogously to those with olefina, by generation of electrophiles from heterolysis of carbon-oxygen or carbon-halide bonds. The larger amounts of n-alkylate (B) often observed with primary alcohols (and halides) probably arise by S~2-like Rideal processes (A). In alkylations involving alcohols and chloralkanes over catalysts such as REX, the role of the eliminated HzO or HC1 is not

H

..I

6 (+)

-S(t) 0 --- CH, - R '\

(A)

-

@CH,R

(B)

+

H~O

+

H 0-ZEOL

@@

328

P. B. VENUTO AND P. 9. LANDIS

clear. It is evident, however, that HgO could modify the dielectric properties of the system, be involved in a “self-promotion” effect, or modify substrate diffusivities. Minachev et al. (126) noted that the presence of HaO, both as an impurity or as a byproduct in aromatic alkylations with alcohols, did not appear to affect the catalytic activity of bivalent cation-exchanged Y-type faujasites.

(iii)Explanation of some anomalies in phenol alkylation. The operation of a Rideal-type mechanism in phenol alkylation was complicated by the strong adsorption of phenol on REX catalyst at moderately low (93-149’) temperatures. Recent experiments (7‘9)suggest that phenol is specifically adsorbed at (or near) sites active for alkylation, thus hindering adsorption of alkylating agent at these same sites and preventing the generation of the electrophile required for alkylation of the aromatic ring. Thus, it was observed that phenol alkylation with ethylene occurred at considerably higher temperatures than might be expected considering its greater nucleophilicity (relative to benzene). The explanation for this difference is that reaction via a Rideal-type mechanism does not occur until temperatures high enough to desorb phenol from the active sites are attained. At these elevated temperatures ethylene can then compete for adsorption. Any other factors that enable the ethylating fragment to compete more favorably with phenol for adsorption-imposition of pressure in the case of ethylene, or use of ethanol, which is a more polar substrate-would facilitate the alkylation process. (iw) Other reaction pathways. Pickert et al. (131) recently reported benzene-propylene alkylation reactions over certain crystalline zeolites in which alkylation activity was enhanced as the temperature of calcination was increased. It was observed that the alkylation activity was maximum when all residual OH groups associated with catalytically active sites were removed. These data were cited as additional support for the idea that carbonium ion-like species are formed through polarization of reactant hydrocarbons by cation (A@)fields (40).With an olefin reactant, an implicit portion of this mechanism is the transfer of a proton from its original position on the aromatic ring to the aliphatic portion of the arylalkane product:

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

329

Such a transfer is necessary in order to maintain stoichiometry. Notwithstanding the zeolites' strong tendencies to adsorb small traces of potentially cocatalytic HzO, and their known smooth catalysis of alkylations with alcohols as reactants (43,126),there is a n alternative mechanism, which involves the possible role of residue sites as proton donors (74,132).Hall and associates (132) and Ozaki and Kimura (133) have observed that the deuterated organic residues formed on silicaalumina when slugs of propylene-d6 or butene-de were contacted with fresh catalyst surface were capable of furnishing deuterons for carbonium ion formation when subsequent slugs of nondeuterated olefin were introduced. That is, in the absence of a more favorable cocatalyst, substrate molecules may act as cocatalysts in reactions of the FriedelCrafts type (72). It further seems probable that reactant structures, as well as catalyst types, must be considered, when correlating maximuin alkylation activity with activation conditions. 2. Isomerixation and Transalkylation of Alkylaromatics I n Table XVIII there are listed conditions for various transformations of alkylaromatics over crystalline aluminosilicate catalysts, many of which occurred under alkylation conditions. Toluene disproportionation, mainly to benzenes and xylenes over REX (43),occurred in low yield a t 264" and 400 psig in the liquid phase; severe aging occurred in this reaction, even under hydrogen pressure. o-Xylene isomerized to mixtures of m- and p-xylenes in liquid phase continuous flow reactions over H-mordenite at 200-600" (134) and over REX a t 177-204' (43). Liquid phase transalkylation of polyethylbenzenes with benzene yields ethylbenzene (43). For R E X catalyst, optimum conditions of 232", 800 psig, LHSV = 2, and a Cdh/(CzH&C6H4 molar ratio of 9

330

P. B. VENUTO AND P. 9. LANDIS TABLE XVIII Ieomerization and Transalkylation Reactions of Alkyhromatice over Zeolite Ca?.ulyata

Aromatic CHsCeHs o-(CHa)aCeH4 o-(C&)zCeHr (CaHs)zceH~-CeHs (CzHs)aC& aec-C4HgCeH6

Catalyst

REX

H-mordenite REX REX 40% Ce3+ - 60% Decationated Y REX

Reaction temp. ("C)

Reaction reporteda

Ref.

284

1 2 192 1 1,2

(43) (134) (43) (43) (135)

13

(44)

200-600

117-204 232 170 200

0. Key: ( 1 ) Transalkylation;(2)positional isomerization;(3)side-chain isomerization; 10% of aec-butyl groups in total product had isomerized to isobutyl groups.

afforded 100yo conversion of diethylbenzene. Under these conditions, little or no decline in activity was observed in 776 continuous hours on stream. At atmospheric pressure, there was lower conversion and more rapid catalyst aging. Side chain isomerization was observed in the sec-butylbenzene-REX system (44). The most detailed study in the diethylbenzene system is that of Bolton et al. (135),where partially cerium-exchanged, partially decationated zeolite Y catalysts were employed. All reactions were run a t 170' in the liquid phase, essentially under alkylation conditions. Their findings are summarized in the reversible reaction scheme below. Starting with any one of the three diethylbenzene isomers, the same equilibrium product distribution was obtained: about 2 1 mole % ethylbenzene, Et

Et

Et

61 mole yodiethylbenzene, and 28 mole yotriethylbenzene. The diethylbenzene fraction consisted of 6% ortho, 62% meta and 32% para, and the triethyl portion, of 32% 1,2,4- and 68% 1,3,6 isomers.

ORQANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

33 1

The primary feature of this system is that isomerization does not occur in the absence of (intermolecular) transalkylation of the diethylbenzenes to 1,2,4-triethylbenzene and ethylbenzene. The disubstituted isomers are all derived from the same source, the 1,2,4 isomer. 1,lDiphenylethanes, and not ethyl carbonium ions, were postulated as intermediates. 3. Dealkylation

True reversibility of the Friedel-Crafts alkylation-which implies dissociation of an alkylbenzene into benzene and olefin-does not occur ArCRH-CH2R’

+ ArH + C H R S H R ’

with AlCls or similar catalysts at the reaction temperatures operative for liquid phase alkylation (123).Since the thermodynamic equilibrium is so far on the side of alkylation a t moderate temperatures, alkylation may be considered reversible only in a kinetic sense. At higher temperatures, in vapor phase processes, there is some indication of a true equilibrium. Table XIX lists representative zeolite-catalyzed dealkylations, largely with cumene, that have been reported in the literature. It is evident that temperatures considerably higher than those required for alkylation are necessary. Bivalent cation-exchanged forms of X- (8) and Y- (40,48)type faujasites have shown greater cumene dealkylation activity than do the corresponding monovalent forms. Pickert, Rabo et al. (40) showed that, within a bivalent cation series (Table XIX), dealkylation activity TABLE XIX Dealkylation Reactwna over Synthetic Zeolite8

Reactant Cumene Cumene Cumene Cumene Cumene Cumene tert-Butylbenzene Cumene

Reaction temp. (“C)

Catalyst NaX LiX > NaX > K X > RbX CaX BeY MgY Cay SrY > BaY > Nay CeY Dehylroxylated Y REX Silica-alumina N

N

N

610

600 410 460 460

326-360 260 460-660

Ref.

332

P. B. VENUTO AND P. 9. LANDIS

decreased as cation radius increased. Since these workers were unable to make any correlation between hydrogen content and activity for dealkylation (40,48), they concluded that catalytic activity was due directly to the cations themselves, (carboniogenesis), and not to the 0.1-1 .O% of structural OH groups remaining after activation. Rabo et al. (as),however, suggested that the OH residues might be proton donors in trivalent cation-exchanged zeolites such as CeY. Galich et al. (136) showed that, within a n alkali metal-exchanged X series (Table XIX), as cationic radius increased, cumene conversion decreased. Also, the products contained larger amounts of l-methyl-3ethylbenzene and less toluene, ethylbenzene, and propenylbenzene. The dealkylation of tert-butylbenzene (43) occurred a t a significantly lower temperature (260') over R E X catalyst than did other related dealkylations. The major liquid product was benzene, with small amounts of toluene, ethylbenzene, and cumene. Isobutane was the major gaseous product, and no olefins were observed. Cumene dealkylation over dehydroxylated Y-type zeolites was studied by Turkevich, et al. (84).Activity for cumene cracking a t 328" was decreased by progressive poisoning with quinoline. It was estimated that 1.2-1.6 x 1021 sites per gram of catalyst were capable of dealkylating cumene; similar patterns of activity were observed for ethylene polymerization and n-butene isomerization. Further, the activity per site was shown not to be uniform, with the greatest activity occurring in samples with the largest number of decationated sites. They concluded that the sites active in dealkylation were probably not protonic, but rather a small population of 3-coordinate aluminum defect sites produced by dehydroxylation. Such sites have been shown to produce radical cations by trapping electrons from adsorbed molecules (69) or to accommodate electrons produced from y-irradiation of Y-zeolites (138). Similar conclusions for the mechanism of cumene dealkylation were proposed by Topchieva and associates (139). It is difficult, however, to write a mechanism for cumene dealkylation that does not involve the addition of a proton t o cumene as shown.

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

333

Although it is possible that several pathways for cumene dealkylation exist, complete rejection of a protonic mechanism (40,84)seems unjustified at this time. Very recently, Ward has, in fact, demonstrated correlations between Bronsted acidity, hydroxyl group concentration, and cumene dealkylation activity in HY ( 1 3 9 ~and ) alkaline earth cationexchange Y zeolites (139b). 4. Chloromethylation

Using the bifunctional chloromethyl ether as an alkylating agent, three products were isolated in the zeolite-catalyzed chloromethylation reaction of benzene. Results are shown in Table XX. The products TABLE XX Chloromethylation of Benzene over Cryatulline Zeolitean

Catalyst0 HY H-Mordenite ZnClad

Time at 70°C (hr) 1.1 3.2 20.0 0.6 2.25 1.3

Observed Product distribution, mole Yo mole yo conversion of BenzylBenzyl Diphenylbenzenec methyl ether chloride methane 3.1 7.1 9.5 0.9 4.6 9.6

60.0 44.0 46.4 18.2

Trace 7.7

7.1 11.7 16.3 81.8 83.4 61.6

42.9 44.3 38.3

Trace 16.6 30.8

a Stirred reaction, CeH&H30CH2Cl (molar) ratio = 4; reactantlcatalyst (weightlweight) = 15-16. b No reaction when glass beads or NaX stirred with reactants at 70' for 20 hours. c To all other products. d Promoted with traces of H2O.

included benzylmethyl ether (I), benzyl chloride (11), and diphenylmethane (111).Product (111) arose from further reaction of benzene with (I)or (11). Hydrogen zeolites showed greatest efficiency in this reaction.

334

P. B. VENUTO AND P. 8 . LANDIS

Zinc chloride provided higher conversions than the zeolitic catalysts, but benzylmethyl ether was a minor product. I n an analogous reaction, where anisole (0.5 mole), chloromethyl ether (0.1 mole), and REX catalyst (2 gm) were stirred for 14 hours a t room temperature, a 28% yield of a mixture of 2,2'-dimethoxydiphenylmethane, 2,4'-dimethoxydiphenylmethane, and 4,4'-dimethoxydiphenylmethane in the ratio of 1 : 5.7 : 5.4 was obtained. No 1 : 1 adduct (chloromethyl- or methoxymethylanisole) was observed. The lower reaction temperatures required with anisole reflect the activating effect of an electron-donating substituent in electrophilic aromatic substitution. 5 . Condensation of Carbonyl Compounds with Aromatics

Crystalline aluminosilicates show high catalytic activity for condensations of carbonyl compounds with aromatics (21). Hydrogen Y zeolite derived from thermal deamination of NH4Y was most efficient in this respect, with an observed order of activity for condensation of phenol and formaldehyde a t 182" of HY > REY > REX > CaX 9 NaX. Silica-alumina and a sulfonated polystyrene ion exchange resin (Amberlyst 15)also catalyze this reaction. An overall reaction scheme, including reactant combinations employed, major reaction paths, and side reactions, is shown in Fig. 26. Mixtures of isomeric bisarylalkanes (IV) with

&

Rl t

\ c-0 /

- I&

\b

RZ

II

I REACTANT COMB INATIONS

I R

a. OH

II

-Rl

RI

n

n

rn

I : I ADDUCT CONDENSATION

/

SIDE REACTI ON S

CONDENSATION

R

R,

R

2: I ADDUCT Ip

FIG.28. Reaction scheme for condeneation of carbonyl compounds with aromatics over HY catalyst at 182".

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

336

ortho/para orientation were generally formed. However, ortho-substituted carbinol (IIIf)was the exclusive product from reaction of phenol and hexafluoroacetone. In a typical synthesis, a mixture of C13H1202 bisphenols was prepared in 80% yield by slow addition of a solution of trioxane (0.036 mole) in benzene (over 1.75 hour) to a stirred, liquid phase suspension of phenol (0.64 mole) and HY zeolite (5 gm) at 182'. The ratio of the 2,2', 2,4', and 4,4' isomers was 1.3 : 1.8 : 1.0. This technique, which afforded very high instantaneous ratios of phenol to aldehyde, prevented rapid catalyst aging. Generally, high yields were observed for carbonyl reactants with no a-hydrogens, since competitive intracrystalline aldol condensation reactions were eliminated. Evidence for the presence of organic cations was provided by bright red or purple colors observed immediately upon addition of the carbony1 compounds to the catalyst-aromatic mixtures, and by isolation of side products derived from hydride shifts to intermediate carbonium ions. Mechanistically, these reactions are visualized as proceeding by initial Rideal-like attack of aromatic on the adsorbed conjugate acid derived from the carbonyl compound, with the formation of an intermediate tert-benzylic carbinol:

Y//o-z EOL'&~

R2

'c-0 /

- ;Ia*

R, 00: H 0 ZEOL

0-C" &*

Ar ,?2

PI

+

I

R,-C-R,

I

+

tDQ H 0-ZEOL

OH

Ar H

The reactive carbinol is then rapidly converted to an electrophile, which is in turn attacked by a second mole of aromatic to form a bisarylalkane. Consistent with this proposal, salicyl alcohol rapidly reacted with phenol to form the expected mixture of bisphenols:

336

P. B. VENUTO AND P. 9. LANDIS

Similarly, no tert-benzylic carbinols were detected, with the single exoeption of the product from the reaction of phenol and hexafluoroaaetone (A); the failure of (A) to condense with another

mole of phenol at 182' is probably related to the destabilizing influence of the two trifluoromethyl groups on (B), the intervening electrophile in this process. Since formation of (A) was shown to be a direct, kinetic, acid-catalyzed process, the exclusive ortho orientation was thought to arise through intervention of the hemiketal-type O-alkylation product

(C). 6. Fries Rearrangement

When m-xylene and acetic anhydride were stirred with REX catalyst for 4 hours at 144', about 1% of ketone was obtained as a direct acylation product:

Almost all other direct acylation attempts failed, largely owing to coke formation following the strong selective adsorption of acyl halide or acid anhydride within the catalyst pores. Somewhat better results were obtained in the Fries rearrangement, which may be visualized as an intramolecular acylation. At a pressure of 400 psig and 204', about 5% rearrangement of phenyl acetate to a

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

337

mixture of phenolic ketones (I)and (11)was observed in a continuousflow run using REX catalyst: 0

&

II 0-c-CH,

OH __c

&cmHs (I 1

+

HO-@COCHs

(n)

The ratio of (11)to (I)was 3.36 at 204'; at 93', (11)was the only product. In the reaction of phenyl acetate over ACl3 (140),the para isomer is favored at lower temperatures (25'), and the ortho isomer, at higher ones (165'). Catalyst aging, largely from degradation of acyl residues to coke and water, was rapid. CaX was a less active catalyst for this reaction. The major product in all runs was phenol, which presumably arose from hydrolysis of the starting acetate. 7 . Electrophilic Hydrogen-Deuterium Exchange Reactions

a. General Characteristics. Hydrogen-deuterium exchange reactions in With benzene, such liquid phasesystemshavebeen reviewed by Gold (141). exchanges occur with measurable rates at temperatures of 25-70' when and CFsCOOH-Dz0 strong deuteron acids such as HzS04-DzO (142), (143)are employed; somewhat lower rates are observed with liquid hydrogen halides such as HBr-DBr (144).In such exchanges, the role of steric effects is minimal, and secondary isotope effects are usually negligible in isotopic exchange of the nucleus. Further, the laws of electrophilic substitution are generally followed. Likewise, in the isotopic exchange reactions between partially deuterated decationated Y catalyst (DHY) and the simple aromatics described in Section II,B, the patterns of reactivity were characteristic of electrophilic substitution.

b. Isotopic Exchange Reactions of Benzene over Decationated Y - Type Zeolites. ( i ) Exchanges over DHY. The exchange reactions of the aromatic ring hydrogens of benzene with DHY were also examined in some detail (75).Typical curves for the rate of appearance of deuterium in benzene vs time on stream in continuous-flow reactions at various

338

P. B. VENUTO AND P. 9. LANDIS

temperatures are shown in Fig. 27. Slow isotopic exchange with benzene also occurred at 28". Similar profiles for the appearance of hydrogen were generated in benzene-& exchanges of analogous HY

L/H1 20

I

40

I

1

I

I

1

60 80 100 I20 140 T I M E ON STREAM, MINUTES

I

1

160

I80

FIG.27. Rates of isotopic exchange at several temperature8 as a continuous flow of benzene "sweeps out" the deuterium from a fixed bed (1.76 gm) of DHY catalyst (75). The curve8 are extrapolated to intersect the abscissa at the times of initial appearance of benzene in the reactor effluent.

samples. Table XXI shows typical deuterium distributions for the first benzene samples removed (Fig. 27, fist points) in exchange of DHY at TABLE XXI Degree of Deuteratwn in Firat Sample8 Removed in Benzene Exchange8 of DH Y at varww, Temperaturea Exchange temp. ("c)

do

di

da

d3

75

0.9031

0.0899

0.0067

100 125 177

0.8667 0.6768 0.6728

0.1168 0.2323 0.2063

0.0167 0.0694 0.0789

d4

da

d6

0.0003

-

-

-

0.0017 0.0177 0.0301

0.0001 0.0034 0.0096

-

-

0.0004 0.0021

0.0002

-

Total D in sample (meq) 0.425 0.870 1.74 2.39

ORGANIC CATALYSIS OVER CRYSTlLLLINE ALOMINOSILICATES

339

Table XXII shows the deuterium distribution vs time for a representative run at 125". 76-177'.

TABLE XXII Degree of Deuteration v8 Time on Stream in Benzene Exchange of DHY at 126' Time on stream (min)

Total deuterated isomerso (mole %)

9 16 20 27 36 124

32.2 6.7 1.4 0.7 0.6 0.2

Deuterium distribution in benzene, dl-ds (%) d1

ds

ds

d4

da

de

71.8 84.6 89.0 96.6 98.1 100.0

21.6 12.6 8.0 4.3 1.9 0.0

6.6 2.6 2.2 0.0 0.0 0.0

1.0 0.4 0.7 0.0 0.0 0.0

0.1 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

Remainder is do.

These product distributions are to be expected for runs where a continuous stream of fresh aromatic sweeps out deuterium in a fixed bed of catalyst. The decreasing concentrations of dz-ds in the effluent with reaction time (Table XXII) reflect the rapid depletion of catalyst deuterium and the consequent decreased probability of polydeuteration. Benzene-& is the major deuterium carrier in most samples, especially at lower temperatures. (ii)Exchanges over DzO-treated H Y . A sample of HY was treated with 1.822 equivalent of deuterium (from DzO) per gram atom of aluminum and then exposed to benzene at 177" in a continuous-flow experiment (75). The patterns of deuterium depletion and distribution of deuterium in benzene were similar to those observed in exchanges of benzene over DHY. A total of 96.1% of maximum possible deuterium had been incorporated into benzene after 90 minutes' reaction time. (iii)Mechanistic considerations.In all of the aromatic isotopicexchange reactions over HY or DHY, about 2-3 wt yo of strongly chemisorbed organic matter (not removable by heating at 100-144" for 6 hours at 0.6 torr) was observed in the pale yellow discharged catalysts. There was essentially no change in catalyst crystallinity after the exchange reactions.

340

P. B. VENUTO AND P.

a. LANDIS

Based on the considerations advanced in this section, and in Section II,B, a schematic visualization of the chemical events occurring in isotopic exchange of benzene over DHY is shown here.

cm, From the present data i t is impossible to determine whether T - (I) or 0-complex (11)formation, or some other process, is rate determining, or to define the stereochemistry when going from (11) to (111). It is certain, however, that a strongly chemisorbed aromatic is involved at some stage. It is improbable that the defect sites produced by dehydroxylation (69,70) are directly involved in the electrophilic hydrogendeuterium exchange process. If one includes the possibility that hydronium ion-like species may be involved, the mechanistic discussion above may be applied to the HY-DaO system.

D. CONDENSATION AND CYCLIZATION REACTIONS 1. Aldol Condensation

The crystalline aluminosilicate-catalyzed aldol condensation of acetophenone to form dypnone has been reported (21). As shown in Table XXIII, hydrogen zeolites were the most effective catalysts for this conversion. Operation at low temperatures in the liquid phase is critical for this reaction, to avoid both coke formation and condensation with aromatic solvents. Catalyst aging was rapid, however. Only transient conversions of acetone to mesitylene were obtained over REX or H-mordenite at 316' owing to rapid intracrystalline self-condensation and coke formation.

ORGANIC CATALYSIS OVER CRYSTAUINE ALUMINOSILICATES

341

TABLE XXIII Aldol Condenaation of Acetophenone over Crystalline Aluminosilicatea

Catalyst" H-Mordenite H-Mordenite HY HY

Total reactant/ catalyst ratio (Wwt) 12.6 12.P 12.6 20.6b

Stir time Temp. (hr) ("C) 1 4 1 4

200 166 206 166

Conversion of Selectivity acetophenone (%) for dypnone 14.6 4.4 32.3 24.4

99 99 92 98

a With REX, Nix, Nay, NaX, or COs-promotedNaX, trace amounts only of dypnone were formed. b 60 wt yo solution in m-xylene.

The mechanism shown here seems reasonably to explain the observed

results; similarly, further condensation of dypnone with additional molecules of acetophenone would explain the formation of small amounts of triphenylbenzene and triphenylphenanthrene observed in the gas phase reaction of acetophenone over HY at 260-300". 2. Cannizzaro Reaction

The Cannizzaro reaction involves the self-condensation of aldehydes that have no hydrogen atoms on the carbon adjacent to the carbonyl group. It is generally accepted that an intermolecular hydride ion transfer occurs in this process. Cannizzaro-type reactions, in which products arising from hydride transfer were isolated, have also been observed in

342

P. B. VENUTO AND P. 9. LANDIS

the presence of zeolite catalysts. I n the reaction of benzaldehyde over NaX at 300", the isolation of benzyl alcohol and benzoic acid-and products from their further reaction or decomposition-are typical:

Similarly, formaldehyde yielded dimethyl ether, traces of methanol, and formic acid decomposition products: 4CHpO -+ C H 3 - 0 4 H s

+ 2CO + Ha0

In a typical reaction, HCHO-from vaporization of paraformaldehyde into a Nt-carrier stream-was passed through a fixed bed of NaX catalyst ( 5 gm) at 303" in a continuous-flow system. Analysis of a typical reactor effluent at about 1 hour on stream showed CHsOCH3 (5.5%), CHsOH (0.6%), and CO (3.2%). Linde 4A and 5A, and N a y were also effective catalysts for this reaction; HY and REX gave only extremely low yields. 3. Condensation of Aldehydes With Esters The condensation, using heterogeneous catalysts, of methyl esters of acetic or propionic acids with formaldehyde to form methyl acrylate or methyl methacrylate has been the subject of a number of recent patents. Thus, lead acetate on silica (l45),manganese dioxide on silica (l46),and Ba-, Ca-, Sr-, and Mg- Decalso derivatives (147')have been effective catalysts for this reaction.

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

343

TABLE XXIV Condenadion of Formaldehyde and Melhyl A&E ovcr H Y Catalyata

Temp. ("C)

300 360 376

Time on stream (hr)

Conversion of HCHO to methyl acrylate (yo)

2 2 1

22.6 60.0 60.0

Continuous-flow system: LHSV = 0.8; CsHaOs/ HCHO (molar)ratio = 26.8;Na carrier.

As shown in Table XXIV, crystalline aluminosilicates, notably, hydrogen zeolites, also catalyze the conversion of formaldehyde and methyl acetate to methyl acrylate:

Above 400°, pyrolytic and hydrolytic reactions producing acetic acidand its fragmentation products-become important. 4.

Prim Reaction

In the presence of strong acids such as aqueous HzS04, carbonyl compounds may react with olefins to form unsaturated alcohols and other products, depending on the reaction conditions. Using H-mordenite as catalyst in a continuous-flow system, 10% conversion of formaldehyde to isoprene was observed at 300" using an isobutylene-to-HCHO (molar) ratio of 3.7. A carbonium ion-type reaction scheme, involving a Prins reaction (1,2) and a subsequent dehydration-rearrangement step (3), explains the formation of isoprene. The useof REX as catalyst gave only traces of isoprene.

344

P. B. VENUTO AND P. 8. LANDIS

-

0 ZEOL Q

H

\C -OH

H'"t CH,

-

__c

CH20H

- CH, - C

-t.p

CH,OH

- CH

8 @'CH, 0-ZEOL

/CH3 C \

,CH3

7

3

C

\

(2)

cn3

CH3

6 . Reaction of Aldehyde8 with Ammonia

Alkylpyridines may be synthesized by passing gaseous acetaldehyde and ammonia over crystalline aluminosilicates.NaX and H-mordenite, both well-authenticateddehydration catalysts, accelerate the formation of methylpyridines at 300-400" (Table XXV).Initial conversions were high, but catalyst deactivation by coking was relatively rapid.

F + NI&-

CfZCHO

TABLE XXV Methyl Pyridine Syntheeia Over Zeolite Catdyata" Reaction temp. ("C)

Conversion to CHsCsH4N ( % ) b

NaX

300 400

46

H-Mordenite

400

6.4 19

Catalyst

420

72

a Continuous-flow system: NHs/CHsCHO (molar) ratio = 1-3.7; LHSV = 1.2-2.6. b Samples analyzed after 0.6 hours.

ORGlANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

345

The major products were a-(I)and y- (11) picolines; traces of pyridine and small amounts of ethylpyridines and higher molecular weight bases were also formed. Similar products have been observed when the same reactants were passed over silica-alumina (148).Zeolite catalysts, notably silver-exchanged X-type faujasites, have also proven effective in the synthesis of methylpyridines from acetylene and NH3, and methylacetylene and NH3, at temperatures ranging from 100" to 300" (149). 6. Condensation of Carbonyl Sdjide and Aniline

When carbonyl sulfide and aniline in 10 : 1 molar ratio were passed over a fixed bed of NaX at 310" at atmospheric pressure and = 0.36, about 10% of the aniline was converted to a LHSVC,H,NH, 1 : 1 mixture of sym-diphenylurea (I) and sym-diphenylthiourea (11), aa shown:

Since it is known that carbonyl sulfide readily disproportionates to COZ and CSZ over NaX under these conditions (116), the origin of the Nsubstituted ureas (I)and (11)can probably be attributed to the reactions involving isocyanate intermediates shown below:

7. Fischer Indole Cyclization

A number of ketone phenylhydrazones have been found to undergo cyclization to indole derivatives (Fischer synthesis) in the presence of

346

P. B. VENUTO AND P. 9. LANDIS

crystalline aluminosilicates:

Fairly good yields of 2-methylindole and tetrahydrocarbazole were formed from the cyclization of the phenylhydrazones of acetone and cyclohexanone, respectively, over CaX or REX catalysts as shown in Table XXVI. TABLE XXVI

Fiachet Indole S y n t M over Zeolite Calalyataa

Phenylhydrazoneb Catalyst Cyclohexanone Cyclohexanone Acetone Acetone

Cyclization product yield (wt yo)

REX CaX REX CaX

71.3 72.6 68.6

46.6

~~

a Continuous-flow system: 160'; LHSV carrier. 26 wt yo phenylhydrazone in toluene.

= 0.6-1.2;

Ng

*

E. ACETALAND KETALFORMATION H-Mordenite catalyzes the smooth conversion of simple aldehydes and alcohols to form acetals at 30' in the liquid phase. From the examples in Table XXVII, it is apparent that in these heterogeneous catalytic systems, acetal formation is dependent on the structures of both the aldehyde and the alcohol involved. Thus, for a given aldehyde, yields of acetal decreased in the order primary > secondary B tertiary; that is, branching at the u-carbon of the alcohol reduced the equilibrium conversion to acetal. In the isobutyraldehyde reactions, an extremely sharp drop in conversion was observed upon changing from isopropanol to tert-butanol as reactant. This observation suggests that, in addition to the increased steric interactions between organicreactants encountered in the tert alcohol system, molecular sieving-type interactions within the narrow mordenite pore system are operative.

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

347

TABLE XXVII Acetal Fvrmdwn from V a h Aldehyde8 and Alcohola Ueing H-Mwdenile C d y a t a

Aldehyde n-Butyraldehyde

Isobutyraldehyde

a b

Alcohol

Alcohol/aldehyde molar ratio

Methanol n-Propanol n-Butanol Isopropanol Ethanol Isopropanol tert-Butanol

6 10 10 10 10 10 10

Conversion to metal ( % ) b

66 64 62 17 43 16 0.2

Mixture (0.6 g m catalyst/0.6 mole reaotants) stirred 3 days at 30". Acetals trapped by preparative gas chromatography and identified by IR and NMR.

The reaction undoubtedly proceeds through formation of the hemiacetal, followed by acid-catalyzed etherification of the hemiacetal by excess alcohol. A possible mechanism is illustrated here.

R RCHO t R'OH

&

OH \ / C

H'

OH^

$8

H 0-ZEOL

\c/

/ \ H OR'

\OR0

-

6-ZEOL

-H,O

R'

All of the steps in the acetal formation are visualized as reversible, since acetals may be hydrolyzed by aqueous acids and by solid acidic zeolites in aqueous solution. Both aldehydes and ketones react with 1,2-glycolsin the presence of zeolite catalysts, preferably with continuous removal of water, to form cyclic acetals on ketals. For example, cyclohexanone reacts with

348

P. B. VENUTO AND P. 9. LANDIS

ethylene glycol in the presence of H-mordenite, HY, or REY (Table XXVIII) forming 1,4-dioxaspiro(4.6)decane: TABLE XXVIII

Rurction of Cyclohxamne with Ethylene Qlycol

Catalyst

NeX

Reflux time (hr)a

CeX Linde 6A H-Mordenite

7 7 7 18

REY

18 18

HY

a

Conversion of ketone to ketal (yo) <1 <1

3.1 83.6 87.6 86.2

Ketone (0.32 mole), glycol (0.32 mole), and catelyst

(0.6 gm)refluxed in 76 ml n-hexane; Ha0 removed as

ezeotrope.

This reaction is a well-known acid-catalyzed process (150), and it is apparent from the examples in Table XXVIII that the acidic faujasites are far superior to the alkali- or alkaline earth-cation forms as catalysts for this reaction. Fused alumina is also effective.

F. BECKMANN REARRANGEMENT OF KETOXIMES The oximes of acetone, acetophenone, and cyclohexanone undergo rearrangement to the corresponding amides when they are passed over acidic crystalline aluminosilicates a t 300-600" in continuous flow EYEtems (151).As shown in Table XXIX, high conversions of oximes were generally observed, but seleotivity to rearranged amides was variable. HY catalyst was most effective for this transformation. N-Methyl acetamide was the only product observed in the reaction of acetone oxime at 325"; at higher temperatures, acetamide decomposition was observed. Acetophenone oxime yielded acetanilide (A) and N-methylbenzamide (B). The high ratio of A to B is in accord with the

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

3 9

TABLE XXIX Repreaentalive Beckmann Rearrangement Reactions over Zeolite Catalyata Highest conversion of oximec Oxime reactant''

Acetone (20%)

Catalyst

Temp. ("C)

HY

Total conversion

Selectivity for amide (yo)

31 66 93 86

100 31 96 76 68 61 62 66

326 460

Acetophenone (20%) Cyclohexanone (30%) Cyclohexanone (23%) Cyclohexanone (23%) Cyclohexanone (20%) Cyclohexanone (26%) Cyclohexanone (6%) a

HY HY REX

cox

ZnX NIX H-Mordenite

300 380 336 330 326 300 326

40 91 69 44 61

41

All reactions at 1 atm; Na carrier; LHSV = 0.9-1.6.

* As solutions by weight in benzene.

At time on stream of maximum activity (usually 2 or 3 hours).

anti position of the phenyl ring in the original oxime. The small

amount of benzamide may arise from isomerization of the oxime prior to rearrangement.

PHENYL M ICRATION/

0 H

CH,-C-

NH

a

NH

- CH,

95 Y

5 %

0

High initial conversions of cyclohexanone oxime (I) were observed with HY at 380-390' (Table XXIX). The major product was caprolactam (11),together with moderate amounts of 5-cyanopentene-1 (111) and small amounts of cyclohexanone and cyclohexanol. The ketone aroae from hydrolysis of the oxime, and the alcohol was produced by an

360

P. B. VENUTO AND P. 9. LANDIS

intermolecular hydride transfer reaction. Catalyst aging was rapid,

particularly at elevated temperatures. The deactivator was identified as a viscous, oily polymer that could be removed by benzene extraction. As shown in Table XXX, the nature of the solvent has an important influence on the rearrangement of cyclohexanone oxime over REX. The observed solvent effect-decreased conversion with increasing solvent polarity-was opposite to that observed in many homogeneous reactions, where rates are enhanced by solvents of high dielectric constant. This unusual result probably stems from competitive adsorption of the polar solvent molecules at catalyst acidic sites, which effectively blocks the reactant from these sites. The rearrangement reaction probably involves initial adsorption of the ketoxime (I) at a catalyst acid site, forming the conjugate acid (Ia). The conjugate acid can undergo intramolecular alkyl migration in a sequence that ultimately yields the lactam (11), or it can undergo /3-scission and loss of water to form the cyanopentene (111).

66 0-Zeol

0 0 O-Zeol S C = N ' " (1)

$&?zeil

-yo

$ -= O / H.

(Ia)

~o

OH

__c

I

00

H 0-Zeol

O ~ Q ~ L N I CCATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

351

TABLE X X X E f f a of Solvent on Converewn of Cyclohexamne Oxime over REXa

Solvent

Highest total conversion of oxime

Cyclohexane Benzene Methanol Acetic acid

30 40 38 2

a b

Selectivity for caprolactem (yo) 60 46

26 0

Reactions at 1 atm; Na carrier; LHSV = 1.0-1.1. 20 wt yo solutions.

G. EPOXIDETRANSFORMATIONS 1. Ismerimtion

Epoxides are isomerized to the corresponding aldehydes, ketones, or alcohols using a variety of crystalline aluminosilicates as catalysts. At 10&200" NaX and REX are effective catalysts for the conversions shown here. At higher temperatures further condensation reactions of 0

/ \

C H a 4 H a -+ CHsCHO 0

/ \

CHa-CH-CHa

0 +CHsCHaCHO +CH&Ha

+ CHadH-CHaOH

the aldehyde and ketone products occur, which ultimately produce aromatic hydrocarbons. In a typical example, ethylene oxide is passed over NaX at 200" (LHSV = 1.0) to produce 51% acetaldehyde, the remainder of the product consisting of dioxane and crotonaldehyde. Dioxane can be optimized by using longer contact times and higher temperatures. 2. Hydration and Ammonolysis of Ethylene Oxide

High activity of both Linde A- and faujasite-type aluminosilicates has been reported in the hydration of ethylene oxide to ethylene glycol (152). Representative data are shown in Table XXXI. Reactions in

352

P. B. VENUTO AND P. 9. LANDIS

TABLE XXXI

Hydration of Ethylene Oxide over Zeolite Catalyataa CaHaO/HaO molar ratio

Temp. ("C)

Linde 4A

0.1

Linde 6A

0.1

NsX CaX

1 .o 0.1

93 238 96 238 93 17 96

Catalyst

a

Glycol in product (wt

yo)

36.6 26.2 19.1 23.4 16.7 29.6 23.8

Continuous-flow system; LHSVH,~= 0.86.

both the gas and liquid phase have been demonstrated, although greatest selectivity against aldehyde formation is evident in liquid phase reactions at low temperatures. NHs and H2S also undergo addition to ethylene oxide in the presence of NaX, but the initial addition products rapidly condense to produce heterocyclic molecules. At 340°, ethylene oxide and NHs produced piperazine, morpholine, and p-dioxane. Byproducts included ethylene and acetaldehyde, the latter arising from an isomerization reaction. HzS reacts with ethylene oxide at 200" to produce p-dithiane, p-thioxane, and p-dioxane, along with byproduct acetaldehyde. These transformations are summarized in the accompanying diagram. 0

CH,

H

H.

/-\ - CH,

H,O

.

CH,OH I

H

OXYGEN-SULFUR

EXCHANGE REACTIONS

Conversion of Furans to Thiophenes The reaction of furan and its derivatives with hydrogen sulfide to produce thiophene and its homologs is smoothly catalyzed by crystalline zeolites. Representative reaction conditions using NaX are shown 1.

ORGANIC CATALYSIS OVER CRYSTALLINE ALUWNOSILICATES

363

in Table XXXII.Generally faujasites containing alkali metal cations were more effective than 'those containing alkaline earth or transition metal cations. Small pore zeolites were less effective. TABLE XXXII Typical Reactions of Oxygen Heterocycles A h Hydrogen SuljW over NaXa

Reactant Furan 2-Methylfurad Benzofuran

Temp. ("C)

LHSV

Pressure

340 300 600

0.26 1.0 0.7

1 atm 200 psig 1 atm

Selectivity Conversion of for sulfur reactant (yo) derivative (yo) 99

100

86

94 88

24

4 Continuous-flow system, 6/1 molar ratio of HaS to organic substrate; samples analyzed after 0.6 hours on stream. b Similar results were obtained with 2-phenylfuran.

The furan-thiophene conversion is optimum in the temperature range

of 300-500", with a fivefold excess of hydrogen sulfide. Reversal of the reaction occurs to only a very minor extent. The thermodynamics of the forward reaction (AF,298"= -26 kcal/mole) preclude this possibility at ordinary temperatures. At 600°, minor reaction of thiophene with water is observed, which produces hydogen sulfide and butadiene, with a trace of furan. Benzofuran yields benzothiophene, but higher temperatures (500-600") are required to produce reasonable yields. A t these temperatures, however, the poor hydrolytic stability of many of the crystalline aluminosilicates curtails their use. With furfural, the oxygen of the aldehyde group, as well as that of the furan ring, was replaced by sulfur. In addition, condensation and cleavage reactions led to a complex mixture of products. NPX CHO

LHSV = 1.1 80% conversion

12%

11%

3%

364

P. B. VENUTO AND P. 9. LANDIS

2 . Conversion of Butyrolactone to Thiobutyrolactone

As shown in Table XXXIII, a number of crystalline aluminosilicates are very effective catalysts for the conversion of y-butyrolactone t o y-thiobutyrolactone. Alkali metal cation-containing X-type faujasites TABLE

XXXIII

Sulfur-Oxygen Interchange Reaction of y- Butyrolactone over Varioua Catalysts

-

Catalyst

NaX csx REX CaX HY Ne-Mordenite H-Mordenite

Conversion t o y-thiobutyrolactone at 300" (yo)"

97 90 41 30 11 14 12

0. Cont,inuouu-flowsyst,em,HzS/C4H60a molar ratio = 5, LHSV = 0.5; samples analyzed after 1 hour on st,ream.

were more effective than more acidic faujasites or smaller pore zeolites. A temperature of 300" was optinium for these transformations. Moderately rapid aging was observed, howevcr: after 3 hours on stream over NaX a t 300", conversion t o y-thiobutyrolactone had decreased from 97% to 48%. 3. Other Related Conversions

The oxygen-sulfur interchange waction was extended to an acyclic anhydride system. High conversioii of r,cet,ic anhydride t o a mixture of thioacetic acid and acetic acid was obtained employing alkali metal

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

-

CH,-C<

P

cb-cjo + rqs crq-c*o

+

CH,-C<

365

/o SH

P

+

otherproducts

OH

cation exchanged X-type faujasites (Table XXXIV). The reaction proceeded smoothly at 200-300"; at higher temperatures (400") with NaX, selectivity for thioacetic acid was sharply decreased. TABLE XXXIV Reaction of Acetic Anhydride with Hydrogen Suljdea

NaX RbX csx

300 260 260

91 98 91

38.6 42.0 46.2

67.9 66.1 64

3.4 1.8 0.6

a Continuous-flowsystem, H2S/C4HeO3 molar ratio = 6, LHSV = 0.6; sample analyzed after 1 hour on stream.

In addition, the reciprocal conversions illustrated below are possible.

H

Using continuous flow systems, about 93% conversion of furan to pyrrole, and about 20% conversion of pyrrole to thiophene were observed with NaX zeolite. In all of the oxygen-sulfur interchange reactions, optimum catalysis is achieved with aluminosilicatesof low or negligible acidity. It is probable that the exchange proceeds via a multistep catalytic process. In

366

P. B. VENUTO AND P. S, LANDIS

addition to catalyzing the H-SH addition to a double bond, the zeolite catalysts have well-authentioated dehydration activity (7) which would be important in the final step of the depicted reaction pathway.

I. OLEPIN CARBONYLATION Garwood (153) has reported the use of a number of crystalline aluminosilicates as catalysts for the carbonylation of olefins to form aldehydes. I n a typical run, H-mordenite (60 gm), propylene (1.6 mole), CO (1.1 mole), and Hz (1.1 mole) were charged (in the order stated) to a glass-lined stainless steel autoclave and shaken for 21-24 hours a t the conditions shown: CH,-CCH=CH,

+

25-150’

CO -CH,=C 70-400 atm.

H,

+

plyolefin

‘cH0

Low yields of methacrolein and mixtures of polymer-mostly propylene dimer and trimer-were observed. The order of catalyst activity for the carbonylation reaction was H-mordenite > REX > HY B NaX; under comparable conditions, silica-alumina did not catalyze carbonylation. Yields of methacrolein were higher in the presence of H2. I n runs utilizing H-mordenite, polymerization could be inhibited by increasing the pressure of the CO-H2 gas mixture or through the addition of small amounts of methacrolein to the reaction mixture. Attempts a t paraffin carbonylation under conditions similar to those shown above produced only traces of oxygencontaining products. It is possible that the initial step in the carbonylation reactions may involve formation of a surface complex of CO with the acid zeolite, which is subsequently attacked by propylene. Indeed, it is known that CO is weakly and reversibly adsorbed on faujasite zeolites containing a variety of cations (48,85). However, the reaction scheme shown in

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

367

Fig. 28, involving a sequence of Rideal-like attack by unadsorbed CO on isopropyl carbonium ion, hydride transfer, and p-elimination, seems more likely in this case.

CH, -CH

9

CH,

0-ZEOL

HQ~-zEOL

CH,-CH

e

(D

-CH,

J

4

0-ZEOL

PROPY LE NE POLYMER

CH,

\ C

- CHO

FIG.28. Propylene carbonylation-polymerizationscheme.

In similar rocking autoclave experiments, isobutane was alkylated by ethylene at 40-150' using REX catalyst (154). The products at low temperatures were chiefly hexanes, with 2,3-dimethylbutane predominating. A t higher temperatures, large amounts of pentanes and moderate amounts of heptanes and octanes were also formed.

J. AMINATIONREACTIONS 1. Reaction of Ammonia with Alkylaromatic Hydrocarbons

Jones and Landis (155) have shown that aromatic nitriles can be produced in good yield by reacting methylaromatic hydrocarbons with NH3 in the presence of certain heavy metal-containing crystalline aluminosilicates.Generally,large pore zeolites containing transition metal cations capable of forming coordination complexes were required for ArCH3

+ NH3 -+ArCN + 3Ha

368

P. B. VENUTO AND P. 9. LANDIS

high conversions; transition metal-exchanged A-type zeolites showed only slight activity. I n Table XXXV, data for the formation of benzonitrile from reaction of toluene and NH3 a t 640" over a variety of transition metal-exchanged faujasites is presented. Conversions of toluene to benzonitrile as high TABLE XXXV Conversion of Toluene and NHa to Benzonitrile over Zeolite Cataly8ta at 540'5 Catalyst NaX CdX CrX cox CUX FeX HgX PtX Nix A@ ZnX

Maximum Naf exchanged Molar ratio, conversion to by heavy metal ( % ) b NHs/CaH&Hs benzonitrile per pass

7 70 36 76 41 40

72 4 63 96 43

10 6 100 90 60 10 10 100

20 13

0 14 8 14 42 34 9 3 19 46 73

Continuous-flow apparatus, LHSV = 0.26. All catalysts showed high (X-ray) crystallinity and sorptive capacity. 5

as 73% per pass were achieved with a 43% Zn-exchanged X-type zeolite. Ag-, Cu-, and Fe-containing zeolites also showed high activity. The use of a large excess of NH3-perhaps to partially poison catalyst acid sites and thereby inhibit coke formation-was necessary to maintain catalyst activity. Nevertheless, relatively rapid catalyst aging was observed at these high temperatures. With the ZnX catalyst, conversion dropped from a maximum of 73% a t 6 hours to 13% after 7 hours. The transition metal-containing zeolites also catalyze NH3 decomposition to some extent under these conditions. Since relatively low coke levels were observed in the discharged catalysts, it is possible that the deactivation may in part, arise from reduction of cations to the elemental metal by Hz. A proposed mechanism for the reaction of toluene with NH3 over a transition metal(M++)-exchangedzeolite is shown in Eqs. (1-6),where

ORGANIC CATALYSIS OVEX CRYSTALLINE ALUMINOSILICATES

359

it is assumed that each cationic positive charge is balanced by a corresponding lattice anionic change. M++

+ zNH3 + [M(NHa),]++

+

[M(NHa)zl+++ [M(NHdZ-i(NHa)l+ H+

+

+

CeHsCH3 + CeH&Ha@ He

CsH&Ha@ [M(NH3)z-i(NHz)lf + [M(NH3)z-i(NHzCHzCeH5)lt+

[M(NH3)z-1(NHzCHzCeHs)]++

+ NH3

+ CeH5CHzNHa

$ [M(NHs)z]++

CeHsCHaNHz -+ CeH&=N

+ 2Hz

(1) (2)

(3) (4)

(6) (6)

The reaction scheme involves initial formation of an ammine complex that can provide amide ions for reaction with benzyl carbonium ions formed by hydride ion abstraction from toluene. The resulting coordinated benzylamine is displaced from the complex by NH3, and subsequently rapidly dehydrogenated to benzonitrile. In support of this mechanism, i t has been demonstrated that the events depicted in Eq. (6) occur very rapidly over ZnX catalyst a t 500". In addition, the use of an olefin to provide ca,rbonium ions for abstraction of hydride ions from toluene enhances the overall reactmionrate. Using a COXcatalyst under similar conditions (570°,N H S / C ~ H ~ C ~ H ~ molar ratio = lo), ethylbenzene was converted to benzonitrile (17%) and styrene ( 10%). p-Xylene and mesitylene yielded only p-tolunitrile and 3,5-dimethylbenzonitrile, respectively, under reaction conditions similar to those for toluene. Apparently, the electronegative cyano group deactivates the second methyl group, thus effectively preventing hydride ion abstraction. The electron-donating amino group facilitates conversion of the p-methyl group in p-toluidine, which provides good yields of p-aminobenzonitrile. In somewhat related reactions in aliphatic systems, acetonitrile was prepared in 31% yield from the reaction of NH3 (3.4moles) and acetic acid (1 mole) over H-mordenite a t 300' in a continuous-flow system (LHSV = 1.3). Contacting mixtures of NH3 and methane or ethane with Nix, COX,or CrX at 760" gave only traces of HCN. 2. Ammonolysis of Halobenzenes As shown in Table XXXVI, transition metal-exchanged faujasites have catalytic activity for the conversion of halobenzenes to anilines in the presence of NH3 (155a).The use of CuX was associated with higher

360

P. B. VENUTO AND P. S. LANDIS TABLE XXXVI Halobenzene Ammonolyeie Reactiona over Zeolite Catalyetsa

Catalyst* ZnX CUX

b

Temp. Total conversion ("C) of CeHsCI (mole yo) 638 660 327 371 460

4.8 9.9 2.3 20.0 69.7

Product distribution (mole CeHe 32.6 68.2 67.2 48.4 93.6

CeH4Cla 20.9 4.4 0 0 0

0/6)

CeHsNHa 46.6 27.4 42.8 61.6 6.6

Continuous-flowsystems, NHs/CaHsCI (molar ratio) = 8, LHSV = 1.3. All catalysts showed high (X-ray) crystallinity and sorptive capacity.

overall yields of aniline at lower temperatures. With ZnX, transchlorination was a minor side reaction. With both catalysts, particularly at higher temperatures, competing cleavage reactions to form benzene were observed. The production of aniline in this reaction is consistent with an intermediate step involving the formation of amide ion from reaction of NH3 with the metal zeolite.

K. HYDROQENATION, DEHYDROGENATION, AND RELATED REACTIONS 1. Hydrogenation Reactions

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,156-16O), hydroisomerization (161-165), . hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. Generally, preparation of metal-loaded zeolite catalysts involvesinitial introduction of the metal component by impregnation, cation exchange, or-occasionally-physical adsorption of a volatile inorganic (such as Ni(C0)4), followed by an i n situ thermal decomposition or reduction step. Thus, a Pt-containing zeolite catalyst was prepared by Rabo et al.

OEGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

361

(162)by impregnating c&Y zeolite with an aqueous solution of HzPtCla. Far better dispersion of Pt, however, was obtained when cation exchange with Pt(NH&++ salts was effected. Similarly, exchange of a Na+-zeolite with Ni++ salts followed by reduction of Ni++cations produced a dispersion of Ni atoms within the zeolite cavities (2). It is evident that protons were dso produced, dong with Ni" atoms, during the reduction process: e

e

+ 20-Zeol+ Ha +Ni" + 2H@+ 20-Zeol

Ni++

Although the reduction step has commonly been effected with Hz, the use of alkali metal (Na") vapors has been employed by Rabo (48) for reduction of Ni++Yto Ni+Y. Generally, the stronger the electrostatic field of the intrazeolitic cation, the greater the tendency toward reduction (48). Metals such as Hg, Cd, and Zn, formed within the faujasite structure by Hz-reduction of the corresponding bivalent cations, can also be removed from the zeolite by heating at high temperatures in a stream of Hz (170). A molecular-shape selective catalyst with hydrogenation activity imparted from Pt impregnation has been reported (7). Using an equimolar mixture of isobutylene and propylene with a Linde 5A sieve containing 0.31 w t % Pt, it was possible to hydrogenate propylene at 343" without affecting the isobutylene. The size-selective principle has also been demonstrated (160) in the hydrogenation of benzene and triethylbenzene using Pt-impregnated NaX and CaX aluminosilicates. The discriminatory action of the faujasite systems is demonstrated by the results in Table XXXVII. TABLE XXXVII Molecular-Shape Selective Hydrogenations in Faujaaite Catalyst Systems

Moles of aromatic hydrogenated per hr per gm catalysta Catalyst

Benzene

Triethylbenzene

Pt/SiOa Pt/NaX Pt/CaX

21.4 4.8 4.0

17.4

a

Ha pressure, 30 psig.

1 .o

0.2

362

P. B. VENUTO AND P. 9. LANDIS

2. Dehydrogenation

Galich et al. (171,172) have explored the use of zeolites as catalysts for the dehydrogenation of n-alkanes. Unmodified Na and Ca forms of zeolites A and X showed little inherent activity for dehydrogenation of n-hexane a t 475-528"; cracking was the major process, When Ni-, Co-, Pe-, or Cr-containing A- or X-type catalysts were prepared-by ion exchange or hydride suspension methods-moderate dehydrogenation activity was observed. Nickel-modified A and X zeolites gave olefins and small amounts of aromatics. A maximum liquid yield of 7.7% olefins was observed a t 500" over CaX containing 16% Crz03. The products from paraffin dehydrogenation-and accompanying side react,ions-were not basically different from those observed in analogous, nonzeolite catalyst systems. The ZnX and COX catalysts described in Section J,1 function as dehydrogenation catalysts for olefins, alkylaromatics, and aralkylamines. Activity was enhanced by the addition of NH3 to the reactants. As shown in Table XXXVIII. small amounts of butadiene were formed TABLE XXXVIII Butene Dehydrogenation over Z n X Catalyet

Temp. ("C)

Molar ratio, NHa/l-CdHs

638 677 682

1.0 3.0 10.0

Product distribution (wt

yo)

CSHB

I-C4Hs

2-CdHs

C4H6

Other

2 6 9

39 48 66

40

6 10 11

7 13 18

23 7

when 1-butene was passed over ZnX in the presence of NH3. Similarly, ZnX and COX effect dehydrogenation of ethylbenzene to styrene, and curriene t o a-niethylstyrene in the presence of a 10-fold excess of NH3 (Table XXXIX). I n the absence of NHa, extensive dealkylation was observed. Passage of benzylamine alone over ZnX a t 540" yielded benzonitrile (52%) and toluene (43%); under similar conditions, but, in the presence of a 2-fold excess of NH3, the yield of benzonitrile was SS%, and toluene, 310/,. Finally, as shown in Table XL, oxidative

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

363

dehydrogenation of ethylbenzene to styrene and selective oxidation of benzyl alcohol to benzaldehyde have been observed in continuous-flow reactions over MnY zeolite. TABLE XXXIX Alkylaromatic Dehydrogenation over Zeolite Catalysts" Conversion to Temp. ("C) alkenylaromatic (yo)

Catalyst

Reactant

COX

CzH5CeHs

640

COX

i-CsH7CeH5

ZnX

i-CsH7CoHs

440 4406 640 640

a

b

1.6 10.0 14.0 0.6 24.0 19.0

600

LHSV = 0.26-0.36; NHs/aromatic (molar) ratio Tenfold excess of NHs replaced by Nz.

= 10.

TABLE XL Oxidative Dehydrogenations over MnY Catalyst" Reactant

Temp. ("C)

Product

Yield (Yo)

CeHsCHzOH

260

CzH5CeHs

300 330 370

CeHsCHO CeHSCHO CeHsCHO CaH&H=CHz

9.4 16.6 16.2

a

8.3

LHSV = 6.76; air flow = 8.6 liters/hour.

3. Reduction of Nitro Compounds

The reduction of the nitro group by H2S in the presence of crystalline aluminosilicates yields amines, sulfur, and HzO (173).Thus nitrobenzene provides high selectivity for aniline upon reaction with a 3-fold molar excess of H2S at 200-300' using NaX or NaY catalyst. About 16% conversion of CsHsNOz to CsH5NH2 was observed a t 143 minutes on stream in a continuous flow run (LHSV = 0.5) a t 300' over NaX; only small amounts of low molecular weight products (excluding sulfur and HZO) were formed. The stoichiometry of the reactants and the temperature of this reduction must be carefully controlled. At low ratios of

364

P. B. VENUTO AND P. 9. LANDIS

HzS to nitrobenzene, and a t low reaction temperatures, measurable amounts of hydrazobenzene and azobenzene are detected. A t high

temperatures and high conversions, appreciable reaction of product aniline with sulfur is observed. These observations point to a sequential reduction :

Aliphatic nitro compounds are similarly reduced to amines on contact with HzS and Naf zeolites. 4. Dehydrocyclizationof o-Ethylphenol Using Zeolites and Carbonyl Suljide

When o-ethylphenol is vaporized over certain zeolites a t 250"-550", the major products are phenol and p-ethylphenol, which arise from dealkylation and isomerization reactions. When carbonyl sulfide is introduced into the reaction, dehydrocyclization to produce benzofuran OH

OH

ORGANIC CATALYSIS OVER CRYSTALLINE ALUMINOSILICATES

365

is the major reaction pathway (174).Conversions of 30-70% o-ethylphenol to benzofuran have been observed in the temperature range 550-600' over NaX zeolite. Lower conversions-with catalysis presumably occurring on the external surface-have been observed under similar conditions with 4A and 5A molecular sieves. Acidic zeolites such as REX are less useful, since they promote extensive dealkylation, although benzofuran formation is still the predominant reaction at 600". L. MISCELLANEOUSREACTIONS

A variety of other reactions, mostly inorganic, have been reported to be catalyzed by crystalline aluminosilicates. These reactions are summarized in Table XLI. TABLE XLI Inorganic and Miacellaneow, Zeolite-Catalyzed Reactions

Reactants

Products

Catalyst

Ref.)

4A; 5A; NaX; natural chabazite 4A; Na-, Ca-X; Na-, Ca-chabazite 4A; Na-, Fe-. Co-, Zn-X; Na-, H-mordenite NaX NaX Pt/5A Nio-zeolite X NaX Na-, Ca-X NaX NaX 4A, CaX NaX NaX

(175) (176) (177) (116) (116) (178) (2) (179) (116) (180) (184 (182) (116) (116)

NaX Dehydroxylated Y 5A

(180)

Isotopic exchange with lattice oxygen

(84)

(183)

ACKNOWLEDQMENTS The authors are deeply appreciative of the efforts of many of their colleagues who were helpful in supplying experimental details for this manuscript. Unpublished work and contributions from the following persons are herein recognized: R. D. Offenhauer, D. G. Jones, L. A. Hamilton, D. E. Boswell, 0. Frangatos, J. N. Miale, and V. J. Frilette.

366

P. B. VENUTO AND P. S. LANDIS

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