Evaluation and Tailoring of Acid-Base Properties of Zeolites. Part 2

The most important application of acid catalysis by zeolites concerns the transformation of hydrocarbons. The reactant hydrocarbons, typically olefins or alkanes, must be encapsulated in the zeolite close to a catalytically active centre such as an acidic site, Broensted or Lewis, where reactive precursors can be generated. Although the detailed nature of the reactive precursors is unknown, several views are expressed concerning the mode of initial decomposition. In the case of alkanes these include direct protonation [42] radical/radical-ion formation [43] or hydride abstraction by Lewis acids. At higher conversion the reaction products, generally, tend to become mpre consistent with carbenium-ion chemistry. A general scheme (32) describes an overall view of kinetic paths which can explain most catalytic results (Scheme 4) Knowledge of the activated hydrocarbon-zeolite complex, the reaction precursor, is limited but we can presume that it is strongly sorbed within a reaction domain located, at least initially, near the catalytic site. The composition and structure of this domain will strongly influence the polarisation and configuration of the complex as will the nature of the active site (Broensted, Lewis or radical centre). The possibility of intermolecular rearrangements within this precursor complex, prior to the formation of carbenium ions, cannot be completely excluded. Such rearrangements may account for some of the unexpected products observed at low conversion of n-hexane [44].