“OSIRIS can uniquely probe the behaviour of lignin derivatives within porous zeolite catalysts," explains lead author and ISIS facility development student, Katie Morton. “By combining these results with molecular dynamics simulations, we can unravel the diffusive and adsorptive mechanisms to later optimise catalysts for the sustainable production of fuels and other important chemicals."

Key to the mission of reducing our reliance on crude oil is biomass, a renewable carbon-based feedstock. Lignin, a significant component of biomass, has historically been difficult to convert into useful products. Optimising acidic zeolites for lignin conversion can leverage existing infrastructure. But before optimisation, we must understand zeolite-lignin interactions, focusing on rate-limiting processes such as diffusion and adsorption.

The research team studied cresol isomers, model lignin derivatives, within the commercial zeolite catalysts H-Y and H-Beta. The Quasielastic Neutron Scattering (QENS) spectrometer OSIRIS at ISIS can uniquely probe the behaviour of lignin derivatives within porous zeolite catalysts. Their QENS experiments revealed rotational motions, with the proportion of rotating molecules depending on the size and shape of both the zeolite pores and the molecules.

​The more linear p-cresol molecule exhibited higher rotating fractions than m-cresol, especially within the larger pores of H-Y. This could be because, in the smaller channels of H-Beta, molecules could only rotate in the larger channel intersections. In the H-Y samples, where greater mobile fractions of rotating molecules were observed, the average rate of rotation was slower than in the H-Beta samples, which the group attributed to increased interactions between the cresol molecules.

Using Molecular Dynamics (MD) simulations, the team were able to validate their QENS results, reproducing molecular rotations and providing further insights into the motions occurring over different timescales. Their simulations revealed fast, rattling movements for seemingly static molecules bound to acid sites, and slower translational movements throughout the zeolite crystals. They attributed the unexpectedly rapid diffusion of p-cresol in H-Beta to its less effective adsorption on acid sites, which, despite the smaller pore size, facilitated increased diffusion. This underscores the complex interplay between the nature of adsorption and size of the pores on local and nanoscale mobility.

The trends in diffusion rate with sample type that the team calculated from their simulations at catalytic temperatures matched cresol conversion rates in previous catalytic studies. This suggests that initial cresol transformation may be limited by internal mass transport, highlighting the importance of this research. The excellent agreement between classical MD models and QENS paves the way for rapid screening of different model lignin derivatives and zeolites of varying structures and acidity, promising advancements in catalyst optimisation for lignin conversion in the future. 

Further information: the full paper can be found at DOI: 10.1039/D4CY00321G