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Octane isomer dynamics in H-ZSM-5 as a function of Si/Al ratio: a quasi-elastic neutron scattering study

O'Malley, Alexander J., García Sakai, Victoria, Dimitratos, Nikolaos, Jones, Wilm, Catlow, C. Richard A. and Parker, Stewart F. 2020. Octane isomer dynamics in H-ZSM-5 as a function of Si/Al ratio: a quasi-elastic neutron scattering study. Philosophical Transactions A: Mathematical, Physical and Engineering Sciences 378 (2176) , 20200063. 10.1098/rsta.2020.0063

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Abstract

Dynamical behaviour of n-octane and 2,5-dimethylhexane in H-ZSM-5 zeolite catalysts of differing Si/Al ratios (15 and 140) was probed using quasi-elastic neutron scattering, to understand molecular shape and Brønsted acid site density effects on the behaviour of common species in the fluid catalytic cracking (FCC) process, where H-ZSM-5 is an additive catalyst. Between 300 and 400 K, n-octane displayed uniaxial rotation around its long axis. However, the population of mobile molecules was larger in H-ZSM-5(140), suggesting that the lower acid site concentration allows for more molecules to undergo rotation. The rotational diffusion coefficients were higher in H-ZSM-5(140), reflecting this increase in freedom. 2,5-dimethylhexane showed qualitative differences in behaviour to n-octane, with no full molecule rotation, probably due to steric hindrance in the constrictive channels. However, methyl group rotation in the static 2,5-dimethylhexane molecules was observed, with lower mobile fractions in H-ZSM-5(15), suggesting that this rotation is less hindered when fewer Brønsted sites are present. This was further illustrated by the lower activation barrier calculated for methyl rotation in H-ZSM-5(140). We highlight the significant immobilizing effect of isomeric branching in this important industrial catalyst and show how compositional changes of the zeolite can affect a range of dynamical behaviours of common FCC species upon adsorption.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Cardiff Catalysis Institute (CCI)
Publisher: Royal Society, The
ISSN: 1364-503X
Date of First Compliant Deposit: 15 July 2020
Date of Acceptance: 20 April 2020
Last Modified: 16 Jul 2020 09:45
URI: http://orca.cf.ac.uk/id/eprint/133446

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