The effect of pore structure on the local and nanoscale mobility of anisole and guaiacol in commercial zeolite catalysts

Abstract

The dynamical behaviour of common lignin derivatives anisole and guaiacol within commercial acidic zeolite catalysts was investigated using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulations, to understand the diffusion mechanisms of simpler lignin-derived compounds in potential industrial catalysts for their conversion into value-added fuels and chemicals. QENS experiments probing timescales of $\sim$340 ps observed and quantified localised jump diffusion within the frameworks of industrial acidic H-Y and H-Beta samples (Si/Al = 15 and 12.5 respectively), and methyl rotations which differed in rate between systems. As the zeolite pore diameter increased from H-Beta to H-Y, and as molecular size decreased from guaiacol to anisole, an increase in the proportion of diffusing molecules was observed by a factor of 2–3 across the temperature range. Faster rates of diffusion, longer jump distances, and expanded regions of confined diffusion were observed for the smaller anisole molecule in both frameworks and for both molecules in H-Y over H-Beta, indicating that the ratio between catalyst pore diameter and molecular size significantly affects local diffusivity in these catalysts. QENS observables generated from the MD simulations over the experimental timescale reproduced this confined diffusion, along with the trends in mobility with molecular size and framework topology. Upon probing an extended nanosecond timescale with the MD, anisole still diffused more quickly than guaiacol in both zeolites, and guaiacol diffused more quickly in H-Y than in H-Beta as per the localised motions. However, in contrast with experimentally observed/modelled localised motions, nanoscale diffusion of anisole was faster in H-Beta than in H-Y due to the straight channels of H-Beta facilitating continuous diffusion over the nanoscale, whereas in H-Y the diffusion rate beyond the confining region was slower due to the barriers to jumping between supercages. In addition to its larger molecular size, guaiacol’s hydroxyl group allowed for stronger interactions with the zeolite Brønsted acid sites than the methoxy group which both molecules possess, hindering diffusion further. The study highlights the complex interplay between molecular shape, functionality, steric pore hindrance and acid site interactions on the local and nanoscale mobility of important derivatives of lignin in potential catalysts for their conversion to fuels and useful chemicals.