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A comparative F.t.i.r./g.c.-m.s. study of MTG conversion on a series of progressively dealuminated HZSM-5 and H-mordenite surfaces reveals the role of zeolite dealumination in modifying the zeolite pore void (i.e. catalytic volume) and therefore the shape-selectivity. This is illustrated by a different distribution of the primary alkenes formed under similar experimental conditions. G.c.-m.s. analysis of a reaction at the early stages of dimethylether conversion reveals that ethene, which forms in trace quantities on HZSM-5, is dominant on H-mordenite. The propene/butenes ratio is high (ca. 1.7) for H-mordenite, but is < 1.0 and decreases with increased dealumination for HZSM-5. Isobutene dominates for increasingly dealuminated HZSM-5. Methanol reformation proceeds concurrently with alkene formation, suggesting a route involving either propagation and/or decomposition to alkenes. The mechanistic implications of the interdependence of zeolite dealumination, modified pore void and identity of alkenes are analysed.

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