Acid Catalysis with Reactants that Contain Heteroatoms

1 Acid Catalysis with Reactants that Contain Heteroatoms 

For the reasons outlined above, some typical acid-catalysed reactions, such as hydration and etherification, may be better performed over non-microporous acid catalysts, but microporous acids have found uses in this area. Asahi, for example, have established the zeolite-catalysed hydration of cyclohexene as a commercial process, where in a two-phase reaction mixture (aqueous and non-aqueous layers) the H-ZSM-5 catalyst stays in the aqueous phase but adsorbs enough cyclohexene, because of its relative hydrophobicity, that the reaction proceeds in the zeolite pores. This has the advantage over the previously used cyclohexene/sulfuric acid system that the aqueous layer is not acidic and corrosive. Furthermore, the medium-pore structure impedes etherification to dicyclohexyl ether and the highly siliceous zeolite has long-term stability in boiling water. 

The conversion of methanol to hydrocarbons is the most studied reaction of oxygenates over microporous solids, for both commercial and academic reasons. Methanol can be generated from syngas over copper- and zinc-based catalysts using the ICI process, and syngas can be prepared from methane, which is a readily available resource. Under the correct economic conditions, methanol conversion reactions can provide an important route to higher 

The nature of the reactions in the induction period are of interest, since they must take place on exposure of the freshly prepared acid form of the zeotype catalyst to the reactants, and give rise to the active hydrocarbon pool. Although direct reactions have long been envisaged to be responsible for this first C-C bond forming event. Haw et al. consider that the observed features might also be explained by the presence of small amounts of impurities (such as ethanol) in the methanol feed. In any case, the acid sites are responsible for the initial generation of the hydrocarbon pool. 

The demonstration that a reactive hydrocarbon pool is of importance in the MTO and MTG reactions bears striking resemblance to the observed reactivity 

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