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The organization of aluminium atoms in zeolites affects their catalytic properties. Here we demonstrate that the aluminium distribution is a key parameter controlling the reaction pathway of acid catalysed reactions over ZSM-5 zeolites. We study ethanol transformation over two ZSM-5 samples with similar Si/Al ratios of~15, and with aluminium atoms located mainly at the channel intersections but differently distributed in the framework. One of the samples contains mostly isolated aluminium atoms while the other has a large fraction of two aluminium atoms located in one ring. The FT-IR time-resolved operando study, supported by catalytic results, reveals that the reaction pathway in ethanol transformation over ZSM-5 is controlled by the proximity of aluminium atoms in the framework. ZSM-5 containing mostly isolated Al atoms transforms ethanol in the associative pathway, and conversely ZSM-5 containing a dominating fraction of two aluminium atoms in one ring transforms ethanol in the dissociative pathway.

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This study attempts to offer an insight into o-xylene isomerization process in terms of steric constraints of 10-ring zeolites. The zeolites under investigation, i.e. ZSM-5, IM-5 and TNU-9, have purely microporous structures and crystals with the same shape and offer comparable protonic functionality, both in manner of amount and strength of Si(OH)Al groups. Under these conditions, micropore topology is recognised to deliver differentiated catalytic performance of each zeolitic structure. Moreover, 10-ring zeolites of reduced dimensionality, i.e. TNU-10 and ZSM-22, are chosen as reference materials. The 2D COS analysis of IR spectra reveal that interaction and catalytic activity in o-xylene isomerization process are strongly influenced by diffusion of reagents in rigid microporous environment and the formation of intermediate species is characterised by IR bands at 1485 and 1455 cm. With a prolonged time of the reaction (15 min), the formation of the latter species is observed for zeolite IM-5 characterised by limited 3-dimensionality. Rapid scan experiments show that in a shorter reaction time (4 min), these intermediate moieties are present for all the structure studied. The intermediate species are believed to originate from arenium ions such as methylbenzenium CH-CH-CH and/or methyl-substituted cycloheptatrienyl ions CH-CH. Their amount is strictly related to the rigidity of the microporous system: the most spacious environment for o-xylene transformation, e.g. TNU-9, is characterised by the lowest population of intermediates, whereas structures offering sufficiently tight geometry, e.g. TNU-10, can accommodate intermediate species in higher quantities, finally leading to the production of p-xylene with high selectivity.