Probing adsorption interactions of lignin derivatives in industrial zeolite catalysts through combining vibrational spectroscopy and ab initio calculations

Abstract

Understanding the adsorption interactions of lignin derivatives within established industrial zeolite catalysts is crucial for optimising the catalytic conversion of more sustainable, non-fossil, lignin-based feedstocks into value-added fuels and chemicals using established infrastructure. The adsorption of key lignin pyrolysis oil derivatives (p-cresol, m-cresol, o-cresol, anisole and guaiacol) within commercial samples of zeolite catalysts relevant to lignin conversion (H-Y, H-Beta, H-ZSM5) was investigated using a combination of inelastic neutron spectroscopy (INS) and density functional theory (DFT) calculations. A wide range of adsorption energies were calculated (-97 to -157 kJ mol−1), with stronger adsorption observed in the smaller pored zeolites, correlated to the increased stabilising interactions with the pore walls. The observed variations in adsorption strengths among lignin derivatives was attributed to the specific adsorption geometries influenced by their functional groups, and the molecular shape relative to the pore topography at the location of the acid site, with most derivatives, except anisole, favouring a configuration involving two hydrogen bonds via a five-membered ring structure. Using DFT phonon calculations to directly reproduce the INS spectra, we assigned vibrational modes for each structure within the 200-1200 cm−1 range. Changes in peak positions, intensities, and widths between the INS spectra of the pure compounds and those dosed in the zeolite catalysts were explained through comparisons with the phonon calculated spectra. Specific peaks attributed to H-bonding group vibrations showed the most significant shifts in H-ZSM-5, followed by H-Beta and HY, correlated with a breaking of adsorbate-adsorbate interactions in the smaller-pored zeolites due to strong interactions with the acidic sites and pore walls. In contrast, the larger H-Y supercages facilitated increased adsorbate-adsorbate interactions, resulting in spectra more similar to those of the pure crystalline compounds. Where the pure compound contained no hydrogen bonding groups, in the case of anisole, the vibrations attributed to the methoxy group bend decreased in intensity upon adsorption due to H-bonding to acid sites, this effect was amplified in the smaller pores of H-ZSM5. The study brings significant insights into factors governing the interactions fundamental to the conversion of biomass based feedstocks with established catalytic technologies.