Standard and rapid scan infrared spectroscopic studies of o-xylene transformations in terms of pore arrangement of 10-ring zeolites – 2D COS analysis

Abstract

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⁻¹. 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 CH₃–C₆H₅⁺–CH₃ and/or methyl-substituted cycloheptatrienyl ions CH₃–C₇H₇⁺. 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.