Activation of zeolites

P. A. Jacobs, Carboniogenic Activity of Zeolites, Elsevier Science Publishing Co., Inc., New York, 1977. 

Activation of zeolites is a dehydration process aceomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specifie pore sizes that will increase their seleetive nature due to the size and orientation of the molecules to be adsorbed. Moleeules above a specific size could not enter the pores and therefore would not be adsorbed. 

Study of the relation between the acidity and activity of zeolites in the conversion of hexane... 

The activity of the Pt-exchanged catalyst for n-C f, transformation increases when the crystallites size increases, which was totally unexpected. External diffusional limitations cannot be invoked since the size of the grains of catalyst is the same. Moreover, this would lead to the opposite result. Other experiments showed that the activity of zeolite-... 

Zicovich-Wilson, C.M., Corma, A. and Viruela, P. (1994). Electronic confinement of molecules in microscopic pores. A new concept which contributes to explain the catalytic activity of zeolites. J. Phys. Chem. 98, 10863-10870... 

Two points are emphasized (i) zeolites can be successfully operated at the same or higher severities (with respect to P/O (feed alkane/alkene) ratio and OS V (alkene space velocity)) than the liquid acids (ii) the productivities of zeolite catalysts (i.e., the total amount of alkylate produced per mass of catalyst) are roughly the same as of that of sulfuric acid. If the intrinsic activities of zeolites (which have 0.5-3 mmol of acid sites per gram) are compared with that of sulfuric acid (which has 20 mmol of acid sites per gram), zeolites outperform sulfuric acid. Nevertheless, the price of a zeolite catalyst and the high costs of... 

The activity of zeolite catalyst for the cracking of cumene was measured at several onstream periods at cumene flow rates of u = 0.01 mol/s and 0.32 mol/s (IECPDD 22 609, 1983). The results are tabulated. Taking the relation to be... 

Vaudry, F., Di Renzo, F., Fajula, F., and Schulz, P. (1998) Origin of the optimum in catalytic activity of zeolite beta. JCS Faraday Trans., 94, 617-621. 

Jacobs, P.A. (1977) Carhoniogenic Activity of Zeolites, Elsevier, Amsterdam. 

The catalytic activity of zeolites has its origin in the fact that some of the silicon atoms in the crystalline framework of the solids are replaced by an aluminum atom. Since aluminum is trivalent, the replacement of the tetravalent silicon results in the introduction of a negative charge into the zeolite lattice. This negative charge has to be compensated by cations and particularly by protons, the latter resulting in the so-called Brpnsted acidity (Figure 13.2) that plays an important role in the catalytic activity of zeolites. 

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]. 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

Catalytic activities of zeolites were also studied for the dehydration of the azeotropic mixture of water and dimethylvinylcarbinol with formation of isoprene (30), for the hydrogenation of ethylene (31), the crotonic condensation of n-butyl aldehyde (32), and many other reactions (33-41)-... 

Influence of Various Substances on the Catalytic Activity of Zeolites... 

One of the most promising methods for controlling the intensity and selectivity of processes is the introduction of various substances into the reaction mixture. Venuto et al. (58) attained a highly selective dehydrogenation of hydrocarbons over cation exchanged zeolite X by conducting the reaction in the presence of NH3. It is also well known that the addition of small amounts of water increases the activity of zeolites for carbo-nium-ion type reactions cracking (59), alkylation (58), isomerization (56,60), disproportionation (60,61,62) and others (56). 

Thus, the addition of compounds to reaction mixtures can influence the catalytic activity of zeolites. The effect may be increased conversion or a shorter reaction time. The effects may be caused by surface modification or by variation in adsorption-desorption in the system reagent-product-zeolite. Sometimes the properties of the zeolite change so radically that it is possible to talk about the action of new catalytic systems. 

Figure 2. Dependence of catalytic activity of zeolites type A and X on size of nickel crystals in benzene hydrogenation = type A O = type X...

O ynthetic zeolites have been used as catalysts for many reactions. Their catalytic activity depends strongly on the nature of exchangeable metal cations. Pickert and co-workers (1) proposed that the high catalytic activity of zeolites for carboniogenic reactions was caused by the strong electrostatic field near surface cations, resulting in polarization of reactant molecules. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

Intraparticle diffusion can affect catalyst selectivity and activity. Similarly, intracrystalline diffusion can affect the selectivity and activity of zeolitic catalysts where intraparticle diffusion is negligible. Therefore, one... 

The results in Table 2 show that the pyridine is less active than any of the X zeolites and Ge faujasite except the lithium form which shows slightly lower activity, whereas all Y zeolites show lower activity than pyridine. Piperidine, however, is more active than any of the zeolite samples studied here. From this comparison, it appears that, most of the basic sites of the zeolites must have pK<10.3. However, the fact that zeolites are also active for catalyzing the condensation of benzaldehyde with ethyl malonate, indicate that these samples have some basic sites with pK< 13.3. On a quantitative bases, and comparing the activity of zeolites for condensation with ethyl cyanoacetate, ethyl acetoacetate and ethyl malonate (Fig. 2), we can conclude that most of the basic sites of the zeolite have pK<9.0 with a sensible amount with 9.0basic strength of different solid base catalysts. 

A graphical representation of the dependence of the activity per acidic site of the zeolites on their Si/Al ratio is given in Fig. 2. It is known that zeolite acidity is enhanced by incrasing the Si/Al ratio (ref. 17,18). Therefore, the obtained linear correlation strongly suggests that the activity of zeolites in hydration of alkynes is a direct and single function of the acid strength and is not dependent of the zeolite framework. Such a result is consistent with the correlation obtained in olefin hydration (ref. 19). 

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. 

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