Heterogeneous catalysts for the valorisation of renewable feedstocks by depolymerisation of lignins

Abstract

Lignin is a plant derived, amorphous, aromatic, polymer and a potential source of carbon feedstock for generation of fuels and bulk chemicals. Nonetheless, for so long the commercial use of lignin has not been favorably brought to the market due to its difficult processing as a raw material. Like the variety of depolymerization methods opted in the literature, this thesis contributes to the catalytic hydrodeoxygenation of lignin model molecules to obtain functional aromatics. The work is an attempted contribution to the improvement of quality of lignin obtained by organosolv pulping. We synthesized lignin model molecules in close resemblance to the native lignin in terms of structure and chemical reactivity. The hydrodeoxygenation of the model compound was carried out under mild conditions in the presence of mixed oxides catalysts obtained by the thermal decomposition of layered double hydroxide. The conversion and selectivity variation was manipulated by the detailed characterization (XRD, TG, TPR, SEM, EDX and N2 physiosorption) of the solid catalysts. We observed that MeOH, as an efficient H-donor solvent can successfully cleave α-O-4 linkage, causing Meerwein–Ponndorf–Verley reduction and selective hydrodeoxygenation of the lignin model under mild condition, consequently generating products of solvolysis and successive hydrogenation. The depolymerized lignin products were characterized by GC (MS, FID). Our original contribution to the subject through this thesis is the conversion of α-O-4 linkage of lignin model (4-(benzyloxy)-3-methoxybenzaldehyde) to the hydrogenated products (alcohols, phenols) under mild conditions (160-200°C) using mixed oxides from Ni-Cu-Fe and related LDH systems. The goal of the project was a heterogeneous catalyst-assisted step ahead in development of solvolysis of lignocellulosic biomass. To the best of our knowledge the mixed oxides obtained from this set of LDH cations, for this reaction and conditions with above mentioned model have never been reported in the literature.