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

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. This article is part of a discussion meeting issue ‘Science to enable the circular economy’.

The effect of molecular shape and pore structure on local and nanoscale cresol behaviour in commercial zeolite catalysts

“…The same simulations were ran at 653 K to match the temperature of the experiments carried out by Imbert and colleagues 33,33,34 . Furthermore, the third catalytic system -H-ZSM5 -was included, which was not measured experimentally due to the low probability of observing dynamics relating to catalysis over the instrumental timescale of OSIRIS, as observed with 2,5-dimethylhexane loaded into H-ZSM5 83 . H-ZSM5 has frequently been tested for its ability to convert biomass feedstocks 10,15,16,21,31 .…”

“…33,34 Furthermore, a third catalytic system -H-ZSM5was Catalysis Science & Technology Paper included, which was not measured experimentally due to the low probability of observing dynamics relating to catalysis over the instrumental timescale of OSIRIS, as observed with 2,5-dimethylhexane loaded into H-ZSM5. 83 H-ZSM5 has frequently been tested for its ability to convert biomass feedstocks. 10,15,16,21,31 As with the other systems, the diffusion of 10% wt of p-and m-cresol throughout the zeolite framework was simulated over 5 ns, with the MSD plots for the simulations ran at 653 K shown in Fig.…”

Diffusion of confined fluids in microporous zeolites and clay materials

“…Omojola et al [327] showed the importance of this factor while studying methanol and dimethyl ether in HZSM-5 zeolite. O'Malley et al also studied the rotational dynamics of octane and 2, 5-dimethylhexane in HZSM-5 as a function of its Si/Al ratio [328]. They showed that the rotational motions of both hydrocarbons were much more hindered when additional Brønsted sites (a lower Si/Al ratio) were present in the system, implying the importance of the composition of zeolite in the dynamic behavior of hydrocarbons in a zeolite.…”

Silver Jubilee for the OSIRIS spectrometer: Achievements and Outlook

“…More recently the quasielastic response to pressure of barocaloric materials was resolved [19]. QENS was also applied studying the diffusion of small molecules in framework materials for catalytic applications and attracted a growing user community [20,21]. Within softmatter and bioscience the relaxations and mobility of large molecules and the water environment were investigated [22][23][24].…”