Weaker acid sites

The greater basicity of alcohols over olefins is responsible for the fact that dehydration can be performed by weaker acidic sites than are necessary for olefin isomerization. There are, however, also other factors, such as participation of neighboring groups, which may influence the rate of dehydration of alcohols. 

The results described above suggest that the methylation of naphthalene over MFI- metallosilicates occurs inside crystalline pores by a restricted transition-state mechanism, not with unordered sites at or near external sites. Weaker acid sites preferentially form 2,6-DMN and favor a decease in coke formation. 

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

The larger pore zeolites (H-Y) show rapid deactivation in contrast to the more restrictive HZSM-5 and the non-zeolitic catalysts (silica-alnmina, MCM-41), since coke deposits can accumulate inside the channel system of large pores. In contrast, coke deposits only on the outer surface of the zeolites having narrow pores. Fnrthermore the catalysts having weaker acid sites of lower density are better at tolerating coke deposition. For instance, the mild acidity of clays (and their pillared versions, e.g. pillared montmoriUonite) show good resistance to deactivation by coking. 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

Based on the above data, one may suggest that, CoPc molecules seem to lie flat on the surface of alumina and alumina-rich supports. This may arise most likely from the interaction of Co + with A13+ centers (with possibility of N(of CoPc)- weaker acid sites (of alumina) interaction [31,32]) resulting in a stoichiometry value-1. In this case, Co ion in molecular dispersed CoPc is accessible for O2 from one side. However, the higher stoichiometries obtained in the case of samples on silica and silica-rich support may reflect that CoPc molecules are oriented on the support surface. This implies that CoPc molecule is accessible for O2 from different sides (both sides ofcentral Co ion and exposed peripheral N s). Such increased accessibility is favored by inclined edge interactions mainly between nitrogens of the macro-cycle of Pc and the hydroxyl groups of the support surface (Br[Pg.413]

The dewaxing performance of ZSM-5 zeolite was shown to depend on the A1 content [168] and of crystallite size [168,169]. Moreover, isomorphous substitution of A1 by other trivalent cations (e.g. B, Fe, Ga) leading to weaker acid sites decreased the activity though it significantly increased the dewaxing selectivity by reducing the extent of secondary cracking [170],... 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

The selective methylation of meta-xylene to produce 1,2,4-trimethylbenzene (TMB) has been studied by Raj et al.16 The most effective catalysts were those based on the medium pore 10 ring MEL structure. They found that isomor-phous substitution of framework Al for Ga or Fe significantly enhanced the yield of 1,2,4-TMB. The reason for the higher yields was attributed to the weaker acid sites on the Ga and Fe substituted materials compared to the Al... 

In agreement with these results, SAPO-37 was found to be much less active than HY zeolite for the cracking of n-hepane (Table 1). This reaction requests the presence of strong acid sites. Nevertheless, the activity for SAPO-37 is significantly higher than that for NaY. For the cracking of 2,2,4-trimethyl pentane, a reaction that can be performed with much weaker acid sites, SAPO-37 presents a reasonable initial activity (for t = 5 min), although quite lower than that observed for HY. 

As coke is laid down on the (unpoisoned) HNa-ZSM-8 using methanol feed in the pulse reactor, the selectivity of substituted aromatics, i.e., xylenes and toluene, increases while that of benzene is somewhat decreased. Side-chain addition probably occurs preferentially over the weaker acid sites left uncoked. At these coke levels, steric hindrance is obviously not a problem since xylene selectivity is greater than that for toluene. 

Dealkylation and disproportionation are relatively unaffected by poisoning, since they require weaker acid sites. Increasing the particle size increases the selectivity of these products, probably because they are less affected by steric considerations. 

Recently, Tops e et al. have reconsidered the same system in a quite detailed study. Ammonia and pyridine adsorption have, in particular, been studied. The most active acidic site, characterized by the band at 3600 cm , are thought to be at the channel intersection. Weaker acidic sites, characterized by the band at 3720 cm" are, in contrast with previous assignments, related to terminal groups on the external surface of zeolite and possibly to non-zeolitic impurities. 

Analogues of aluminosilicate zeolites, in which the place of aluminium is taken by other Bivalent cations such as B, Ga or Fe, are readily prepared. These give solids with closely similar structure but with weaker acid sites that are also more susceptible to removal of the heteroatom from the framework, for example in the presence of water or steam. Such weakly acidic solids can be selective catalysts for molecular rearrangements that give unwanted by-products in the presence of stronger add catalysts. The Beckmann rearrangement is such a reaction (see Section 8.6.1). 

The acidic properties of cobalt and silicon-substituted AlPO-5, -11, and -44 have been characterized by JSnchen et al. [111,276] by adsorption calorimetry of acetonitrile at 303 K, after activation at 720 K. Adsorption calorimetric measurements indicated that the adsorption potential of the samples for acetonitrile was enhanced upon cobalt incorporation. The heat curves exhibited at least two steps indicating the existence of acid sites of different strengths. The heats of adsorption indicated the formation of strong acid sites, due to the cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P - OH groups. 

Wang et al. [89] defined the acidity and acid strength of H-ZSM-5 using the activity and selectivity in the conversion of various a-olefins and paraffins on H-ZSM-5 partially poisoned with pyridine. The conversion of olefins required weaker acid sites than the conversion of paraffins. Furthermore, sites with a higher acid strength favor the formation of aromatics, which indicates that dehydro cyclization requires very strong acid sites to proceed. 

Choudhary and Akolekar [90] studied the conversion of cumene, isooctane, and o-xylene over various low-silica and high-silica zeohtes. They defined strong acid sites as sites from which pyridine desorbs above 400 °C and found that isooctane cracking required essentially these very strong acid sites, while o-xylene isomerization and cumene cracking were also catalyzed by weaker acid sites. 

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