HETEROGENEITY OF THE ACID SITES

In many cases multiple probe molecules are used in order to deduce the existence of acid strength distributions. For example, the heterogeneity of the acid sites in H-ZSM-5 was studied by monitoring the adsorption of a series of weak electron donor molecules, such as hexane, fluorobenzene, chlorobenzene, benzene, toluene, p-xylene, and ethylene, to the acid sites. The frequency shift of the acidic OH group was taken as a... 

The El mechanism has, as the rate-determining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group X. 

The goal of our work was to recognise if the cyclohexanol conversion can be used as a test method in the studies of heterogeneity of Bronsted acid sites in zeolites. 

In the field of heterogeneous catalysis, pyridine is frequently employed as a molecule probe of the surface acid sites of zeolites and other oxide catalysts, with which the amount and the strength of the acid sites are determined, for instance by infrared measurements of the intensity and the peah position of pyridine adsorbed on zeolite surfaces [9]. Despite its importance in this respect, details of the structure and the molecule orientation within the adsorbed pyridine layer on these nonconductive substrates are not known, due to the lack of techniques to yield such data before the advent of AFM, The series of our present work constitutes the first report on the determination, by means of. A.FM. of the array structure of the pyridine base adlayers on zeolite surfaces, and the estimation of the molecular orientation within the adsorbed layer. 

The breadth of the resonances for the a and g carbons may arise from several factors. An important consideration is the presence of N dipolar coupling. The fact that we do not observe "well resolved" doublets for these carbons may be due in part to the diffusion of the molecule and/or the strength of the applied magnetic field relative to the N quadrupolar coupling constant(24). Perhaps of equal importance to the linewidth is a heterogeneity oT the acid sites on the surface of the alumina. The presence of a distribution of site acidities would lead to a corresponding distribution of chemical shifts, and hence appear as a line broadening mechanism. 

The rate of reactions occurring through heterogeneous acid catalysis is obviously determined by the characteristics of the acid sites (i.e. their number, nature, strength and density). For skeletal transformations of hydrocarbons (and cokipg is obviously of this type) the rate depends essentially on the protonic acidity of the catalysts [10]. The Lewis acid sites alone do not seem to be active. However, when located in the vicinity of protonic sites they can increase their strength and consequently their activity [11]. Therefore for skeletal transformations of hydrocarbons the C/P ratio must depend only on the strength and on the density of the protonic sites. 

In another study investigating a series of dealuminated samples [146], greater initial differential heats compared to the parent sample (Si/Al = 15) were observed at a low dealumination level but not at a higher one. The increase of the acid site strength was accompanied by a heterogeneous strength distribution, as a result of dealiunination [146]. 

Mixing Nafion solutions and a silica sol is anotlier way to prepare nanocomposites. While Nafion is a strong acid heterogeneous catalyst by itself, it has low surface area, which limits the number of available acid sites. Mixing Nafion solutions and a silica sol increases the accessibility of the acid sites. 

The incorporation of Al in the framework of SBA-15 increased the acidity of the samples, depending on the amount of Al introduced. The acidic sites are heterogeneous and samples comprise both Bronsted and Lewis. These mesoporous solids demonstrated good adsorption capacities towards pollutant molecules. 

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