ePubs

The open archive for STFC research publications

Full Record Details

Persistent URL http://purl.org/net/epubs/work/67298934
Record Status Checked
Record Id 67298934
Title Probing the behaviour of lignin derived monomers in catalytic zeolite systems
Contributors
Abstract The fundamental dynamical behaviour of lignin pyrolysis derivatives containing aromatic, methyl, hydroxyl, and methoxy functionalities has been investigated, both in their pure forms and adsorbed within three commercial Brønsted acidic zeolite structures. Quasielastic neutron scattering (QENS) experiments combined with Molecular Dynamics (MD) simulations and inelastic neutron scattering (INS) with density functional theory (DFT) calculations were employed to analyse diffusion and adsorption processes, respectively. The study aims to understand how adsorbate-acid site and adsorbate-adsorbate interactions and steric pore hindrance affect local/nanoscale mobility and the nature and strength of such adsorption, to optimise zeolites for the conversion of lignocellulosic biomass feedstocks and for developing accurate computational models to characterise such processes. Chapter 3 explores the dynamics of the model lignin derivative p-cresol using QENS and MD simulations, which revealed isotropic rotation and jump-diffusion. Two MD force fields were evaluated for their ability to replicate experimental observations, with the OPLS2005 force field providing a better match due to reduced molecular polarity, but still overrepresented hydrogen bonding. Chapter 4 extends this analysis to a broader range of lignin derivatives, showing that rotation and jump-diffusion rates decrease in the order: anisole > guaiacol ≈ o-cresol > p-cresol ≈ mcresol, primarily controlled by hydrogen bonding interactions. Simulations accurately reproduced QENS observables for anisole and guaiacol where no/less hydrogen bonding is present. However, simulations of the cresol isomers showed slower dynamics due to increased hydrogen bonding. By mapping the MD output onto the experimental space, we highlight crucial issues in extracting accurate coefficients for both diffusional and rotational dynamics from commonly applied analytical models of QENS data. However, experimentally verified MD models can calculate the true self and rotational diffusion coefficients. Chapter 5 focuses on the behaviour of p- and m-cresol in H-Y and H-Beta. Only isotropic rotation was observed on the timescale of the QENS instrument (∼54 ps) with confinement to the zeolite micropores and adsorption to acid sites slowing diffusion. QENS observed a larger population of rotationally mobile p-cresol in each catalyst due to its more linear shape, and the larger pores of H-Y allowed for greater mobility of both isomers. An increased rotationally mobile cresol population from H-Beta to H-Y correlated with a decreased rotational rate due to an increase in adsorbate-adsorbate interactions. The MD simulations reproduced the motions and gave further insight into a rapid rattling motion that occurred when bonded to acid sites. Accessing longer timescales in simulations showed extremely restricted diffusion and higher activation energies, but the same trends were observed with pore topology and molecular shape. Relatively fast diffusion was observed for p-cresol in H-Beta due to the longer axis of the molecule inhibiting favourable 180° angled hydrogen bonding to zeolite acid sites. Due to the agreement between experiment and simulation, we modelled dynamics at a higher, catalytically relevant temperature. The study included H-ZSM5 in which diffusion was significantly reduced in its smaller channels. This is attributed to a lower zeolite pore size-to-molecular size ratio rather than increased hydrogen bonding interactions. The relative diffusion rates within the simulations directly correlate with cresol conversion rates before coke formation, suggesting that diffusion is a limiting factor.
Organisation ISIS , ISIS-IRIS , ISIS-OSIRIS , ISIS-TOSCA , ISIS-LET , STFC
Keywords
Funding Information
Related Research Object(s):
Licence Information:
Language English (EN)
Type Details URI(s) Local file(s) Year
Thesis PhD, University of Bath, 2025. https://researchp…rs-in-catalytic-zeo/ 2025