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Black plate (268,1) matter, free radical species may be spin-trapped from the gas- phase from a variety of different model catalytic systems, but how important they are in real catalytic reactions is unknown158. A co- catalytic affect was demonstrated when a mixture of methanol and toluene was injected into the reactor containing H-ZSM-5159,160. To account for this observation, the formation of resonance- stabilised carbenium ion intermediates in the zeolite by a side- chain alkylation mechanism was proposed159,160. Little further progress was made until the mid-1990s when Dahl and Kolboe advanced the ‘‘hydrocarbon pool’’ mechanism in which the function of methanol is to methylate hydrocarbon molecules of unknown structure adsorbed on the catalyst’s surface, and leads ultimately to olefins through parallel reaction pathways161,162. Haw’s group began to search for key intermediates in the hydro- carbon pool using experimental pulse-quench in situ NMR methods. Methanol conversions catalysed by both H-ZSM-5163,164 and H- SAPO-34165,166 were investigated. Among the species identified in H-ZSM-5 were the 1,3-dimethylcyclopentenyl carbenium ion in equilibrium with a related neutral cyclic diene, and methyl substi- tuted benzenium carbenium ions in equilibrium with neutral methylated benzene derivatives. On H-SAPO-34, the critical reac- tion centres appear to be methylated benzene derivatives. Haw has concluded that the initiation step in MTOyMTG catalysis is really a phantom, and that the first hydrocarbon pool species are formed by organic impurities. He contends that only an ‘‘indirect’’ hydro- carbon pool mechanism accounts for the observed outcome of MTOyMTG methanol conversion catalysis, and if any ‘‘direct’’ reaction happens at all it is much slower than those processes involving the hydrocarbon pool. Hence initiation of the MTOyMTG reaction is generally caused by organic impurities that form the first few reaction centres167. The impurities in the mixture undergo repeated methylation and subsequent olefin elimination with re- initialisation of the catalytic cycle. The impurities are larger organic molecules termed ‘‘organic reaction centres’’ which function as a scaffold onto which the reactants are tacked together and the products are removed therefrom. This circumstance avoids the need to invoke the high-energy intermediates required by all of the ‘‘direct’’ mechanisms. The rate of formation of the initial reaction centres and hence the duration of the induction period depend on the nature of the impurity species. Secondary reactions of primary olefin products are determined by the topology and acid strength of the porous catalysts. It is proposed that reaction centres form via some secondary routes that ‘‘age’’ into polycyclic aromatic 268 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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