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

Abstract

The behaviour of model lignin derivatives m- and p-cresol within commercial acidic zeolite catalysts was investigated using a combined quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation approach, to understand the diffusion mechanisms in industrial catalysts of molecules relevant to the conversion of lignocellulosic biomass into value-added fuels and chemicals, and to link such behaviours to catalytic characteristics. QENS experiments observed isotropic rotation of both isomers in H–Y and H-beta. The more linear p-cresol isomer exhibited more mobility in each catalyst, while the larger pores of H–Y allowed for greater mobile populations of both isomers over H-beta. Notably, decreased rotational rates were observed with increasing mobile populations due to increasing adsorbate–adsorbate interactions in the catalyst micropores. QENS observables calculated from MD simulations reproduced the experimental trends in mobile populations of rotating cresols with zeolite topology. Exploring cresol dynamics within the MD simulations over longer timescales saw extremely restricted diffusion and high activation energies (21–32 kJ mol⁻¹) for all systems, with the same trends in diffusivity with pore topology and molecular shape observed as for the mobile populations observed in the experiment. Diffusivity was lower for m-cresol than p-cresol by a factor of 3.3 when confined within H-beta channels due to its propensity to form favourable 180° H-bonds with zeolite Brønsted acid sites, whereas the longer axis of p-cresol inhibits this favourable orientation, increasing its likelihood of unhindered diffusion at an orientation parallel to the zeolite channel. The agreement between the QENS experiments and simulations allowed for reliable modelling at a catalytically relevant temperature of 653 K, and we include simulation of the extensively catalytically tested H-ZSM5, revealing that the rate of reactant diffusion directly correlates with cresol conversion rates before the formation of coke, observed previously in catalytic studies. The interplay between the nature of adsorption onto acid sites, steric pore hindrance, local/nanoscale mobility, and their influence on catalytic properties is highlighted and explained for important derivatives and model monomers in the zeolite catalysed conversion of lignocellulosic biomass feedstocks.