Faujasite catalytic activity

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

Zeolites are used in various applications such as household detergents, desiccants and as catalysts. In the mid-1960s, Rabo and coworkers at Union Carbide and Plank and coworkers at Mobil demonstrated that faujasitic zeolites were very interesting solid acid catalysts. Since then, a wealth of zeolite-catalyzed reactions of hydrocarbons has been discovered. Eor fundamental catalysis they offer the advantage that the crystal structure is known, and that the catalytically active sites are thus well defined. The fact that zeolites posses well-defined pore systems in which the catalytically active sites are embedded in a defined way gives them some similarity to enzymes. 

Fatty acid, MRNi hydrogenation, 32 243-245 Faujasites, 34 160-183 acidic sites, 27 151-154 alkaline and rare earth forms, 27 160-165 amine titration, 27 163 infrared smdies, 27 160-163 surface acidity and catalytic activity, 27 163-165... 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

The active component for olefin oxidation is Pd2+, while Cu2+ acts as a promoter for the reoxidation of Pd. The sequence of ion exchange of Pd + and Cu2+ on the faujasite zeolite influences the catalytic performance. Best results seem to be obtained when Pd + is introduced in the second step of the ion exchange as it will then be located mainly at the more easily accessible cation sites II and/or III [23], The amount of exchanged Pd + determines the catalytic activity of Pd +Cu +Y, provided that Cu2+ is present in sufficient amounts to assure fast regeneration of Pd2+. A Pd/Cu atomic ratio of four is required here. Increasing acidity in Pd +Cu +NaY results in a decrease of both the activity and selectivity in the olefin oxidation [26]. 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

Recent work by Rabo et al. (57) opens new possibilities for controlling the activity and selectivity of zeolite catalysts. Occlusion of various guest molecules into the sodalite cavities of Y zeolites can significantly change the catalytic properties of the zeolites for carbonium-type reactions. Anions of occluded salts are located close to the center of the sodalite cavity and strongly influence the arrangement of cations in the faujasite lattice and hence the catalytic activity. 

The ability of water molecules to promote a reaction depends on many factors. In most cases, zeolites with monovalent cations have low activity. However, the addition of water molecules to X and Y zeolites with monovalent ions increased the isomerization of cyclopropane (63). De-cationized zeolites can be promoted readily with water, and the process is reversible (2, 60, 64). It was shown (2) that the promoting ability of water molecules in faujasites is less when the Si02/Al203 increases. Dealu-minated faujasites are even more difficult to promote. For erionite and mordenite the maximum effect of water was observed only after treatment with liquid water and subsequent heating (2). The effect of water on zeolites saturated with polyvalent cations is less pronounced (65, 66, 67). However, the presence of multivalent cations stabilizes the catalytic activity. Water and alcohols were reported to promote ion exchanged zeolites for n-pentane isomerization (68) and n-hexadecane hydrocracking (69). 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

Recently, we have shown that acid faujasites oan catalyze reactions involving carbonyl groups in the liquid phase and at moderate temperatures, and have observed that physicochemical modifications on the Y zeolite influence their catalytic activity (refs. 1-4). In this context, little information is currently available concerning the use of other acidic large pore zeolites, especially with Beta (p) struoture, as acid catalysts in... 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

Surface acidity and catalytic activity. Faujasitic zeolites exchanged with multivalent ions demonstrate significant catalytic activity for reactions involving carbonium ion mechanisms, in contrast to the inactivity of the alkali metal ion-exchanged forms. Several possible sources of the observed activity were proposed initially. Rabo et al. (202, 214) suggested that electrostatic fields associated with the multivalent ions were responsible for the catalytic activity. Lewis acid sites were proposed as the seat of catalytic activity by Turkevich et al. (50) and by Boreskovaet al. (222). Br0nsted acid sites formed by hydrolysis of the multivalent metal ions were proposed as the catalytic centers by Venuto et al. (219) and by Plank (220). 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

Claridge, R. P., Lancaster, N. L., Millar, R. W., Moodie, R. B. and Sandall, J. P. B. Faujasite catalysis of aromatic nitrations with dinitrogen pentoxide. The effect of aluminum content on catalytic activity and regioselectivity. The nitration of pyrazole, J. Chem. Soc., Perkin Trans. 2, 2001, 197-200. 

The often uncontrollable hydrolysis chemistry of Mn in aqueous solution, is attenuated by the geometry of the super- and hypercages in faujasite zeolites. This way, not only specific species are stabilized, but also new catalytically active complexes are formed. 

The chemical transformation of Ru-complexes in faujasite-type zeolites in the presence of water and of carbon monoxide-water mixtures is reviewed and further investigated by IR, UV-VIS spectroscopic and volumetric techniques. The catalytic activity of these materials in the watergasshift reaction was followed in a parallel way. The major observations could be rationalized in terms of a catalytic cycle involving Ru(I)bis and triscarbonyl intermediates stabilized in the supercages of the faujasite-type zeolite. The turnover frequency of this cycle is found to be determined by the nature, number and position of the charge compensating cations, as well as by the nature of the ligands present in the Ru-coordination sphere. 

Discrimination between surface and intrazeolite sites is often difficult. The chemical reactivity of zeolite-bound complexes to reagents of different sizes is helpful in determining the location of an irmnobilized complex. For example, in rhodium faujasites prepared from Rh(aUyl)3 and H-Na-X (i.e. the faujasite Na X in which a fraction of the sodium ions have been replaced with protons), catalytic activity for hydrogenation can be limited to the intrazeolite sites only when P(n-Bu)3 is used to poison the surface rhodium sites. Tri-n-butylphosphine is too large to penetrate the zeolite pores, thus the rhodium complexes within the zeolite remain catalytically active. 

The existence of surface" bivalent cations at lattice positions with 3-fold coordination in the large cavities (Site 2) exposed to adsorbed molecules was shown from a structure analysis of a Ca-exchanged fauja-site single crystal (12, 30). The assignment of Ca ions to various lattice positions in mineral faujasite helped to explain by comparison the observation that with Y zeolite the catalytic activity emerges at approximately 50% Ca exchange, because of the preference of Ca ions for the fully... 

Air-Calcined Faujasites. Since calcination conditions were shown to have a strong effect on catalytic activity, acidity measurements were... 

The early use and success of molecular sieve catalysis was spurred by the dramatic improvement in activity selectivity for catalytic cracking of vacuum gas oil achieved by using the faujasite based catalysts in comparison to the previously used amorphous SiOj/AUOj. These catalysts had a factor of about 10 -10 higher catalytic activity than the amorphous SiOj/AfrOj catalysts [42]. Paraffin, C4 to C8 isomerization [43] was one of the first successful non-petroleum processing applications using zeolite catalysts. The complexity of tailoring zeolite catalysts, however, is well illustrated by the fact that is only four years back that Shell has developed the first zeolite based process for isomerization of n-butene to isobutene [44]. 

The thermal stabilities of hydrogen faujasites and mordenites with different Si/Al ratios are reported. The temperature fields are outlined which characterize the thermal resistance of the lattice, framework Al, hydroxyl coverage, and the active sites. By choosing the proper conditions for activation of hydrogen zeolites, it is possible to induce the release of Al from the framework and in this way to promote the formation of strong add sites and enhance the catalytic activity. 

Reports on the thermal stabilities of faujasites and mordenites are largely confined to their resistance to collapse at elevated temperatures. There is, however, a need to extend these works to the investigations of reactions which occur during the thermal treatment of hydrogen zeolites. These include aluminum migration, dehydroxylation and formation of new active sites. The present study is concerned with the effect of calcination temperature on the crystallinity, the extent of thermal dealumination, concentration of hydroxyl groups and catalytic activity of hydrogen faujasites and mordenites with different Si/Al framework ratios. 

The catalytic activity and stability of aluminosilicate zeolites is strongly influenced by removal of Al from the framework sites into extraframework positions. This can be accomplished by dehydroxylation or calcination followed by steam treatment, the latter process producing ultrastable zeolites. Early Al NMR studies indicated the appearance of the 30 ppm resonance attributed to Al when faujasite and other zeolites are steamed (Gilson et al. 1987). Double rotation Al NMR studies at two magnetic fields (Ray and Samoson 1993) indicated that the nature of the Al species depends on the method of dealumination the 30 ppm resonance in zeolite-Y samples treated... 

Source of Activity in Zeolites.—Most of the experimental work designed to elucidate the catalytically active sites in zeolites has used faujasitic zeolites. This has been reviewed recently in detail.Much is broadly applicable to non-faujasitic zeolites, but in this section three factors influencing catalytic activity are emphasized silica/alumina ratio crystal structure modification of the zeolite by thermal treatment, cation exchange, etc. 

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