Insight into molecular behaviour in microporous catalysis using quasielastic neutron scattering

Abstract

The potential of quasielastic neutron scattering (QENS) to give a unique and detailed insight into the behaviour of active species in both established and emerging microporous catalytic systems is illustrated in 3 case studies, studying the methanol-to-gasoline (MTG) and ammonia selective catalytic reduction (NH -SCR) processes, and the zeotype catalysed Beckmann rearrangement.
We analyse the dynamics of methanol in ZSM-5 catalysts both with and without the hydrocarbon pool, resulting from the MTG reaction taking place at 623K and 673 K, to determine the effects of catalyst use on molecular mobility. Using QENS, we observe that methanol is immobile on the instrumental time scale in the fresh catalyst (ZSM-5-F) and in the sample used to convert methanol for 3 days at 623 K (ZSM-5-623). However, in zeolite ZSM-5-673 (MTG at 673 K for 3 days) we observe isotropic methanol rotation. The observed differences between the zeolites in methanol dynamics are attributed to the development of mesoporosity in ZSM-5-673 due to the high reaction temperature of 673 K, leading to dislodgement of lattice Al as is evident from NMR data.

In collaboration with Johnson Matthey, QENS and MD were combined and applied to the Cu-CHA zeolite catalysed NH3-SCR process. The effect of counterion presence on ammonia mobility was studied through comparing the diffusivity in a Cu-CHA sample and a counterion free sample. Our studies of the effect of Cu
presence in CHA found little impact of the counterion on NH diffusivity, with the NH performing jumps through the 8-rings between cages at the same rate between both samples. The finding was rationalized by MD simulations which observed that coordination shells formed around the Cu , shielding other NH molecules which were mobile, allowing the unimpeded intercage jump diffusion.
In the context of the zeotype catalysed Beckmann rearrangement, we study the mobility of cyclohexanone oxime in a microporous catalyst in comparison with a hierarchical micro/mesoporous analogue.

Couterintuitively, we find that the introduction or larger mesopores into the catayst structure lowers the overall mobility of the oxime due to the presence of a larger immobile fraction. The increase in the immobile fraction is explained by the walls of the mesopore being terminated by strongly adsorbing terminal silanol groups, immobilising the oxime at the low loadings studied, the interaction of which has been observed by neutron vibrational spectroscopy.

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Identifiers

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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