(66d) Production of Aromatics from Lignocellulosic Biomass: Vapor-Phase Hydrodeoxygenation of Propylguaiacol over a Bifunctional Molybdenum-Containing Zeolite

Lignin, accounting for 15-40 wt% of lignocellulosic biomass, is the most abundant renewable source of aromatic units in nature. Conversion of lignin-derived compounds into aromatic chemicals is a highly desirable target in the biorefinery field.¹ However, selective cleavage of the intrinsic C-C and C-O bonds within lignin remains a long-standing challenge.² Early studies have shown that supported Mo and Au catalysts exhibit high selectivity towards C-O bond cleavage in the hydrodeoxygenation (HDO) of lignin-derived monomer.^{3, 4} Meanwhile, dealkylation of monomeric lignin, which involves the C-C bond activation that requires higher dissociation energy compared to that of C-O bond, is usually achieved by acidic zeolite.^5,6 Accordingly, it is high desirable to develop efficient catalyst that enables simultaneous cleavage of C-O and C-C bonds for selective production of aromatics from lignocellulosic biomass.

This study investigates the production of aromatics (benzene, toluene and phenol) from lignin-derived propylguaiacol over a bifunctional Mo-containing zeolite. A synergistic effect was observed between the metallic sites and the Brønsted acid sites in promoting efficient C-C and C-O bonds activation to obtain fully deoxygenated aromatics. The close proximity of the metallic sites and the Brønsted acid sites was found to be the key factor for enabling high selectivity towards aromatic and for increasing on-stream stability. In addition, it was revealed a predominant contribution of the acid sites to the cleavage of the alkyl group. Further increase in the acidity of the zeolite support favors the formation of methylated products, resulting in more production of partially deoxygenated aromatics. The reaction network of propylguaiacol HDO was proposed by varying the conversion of propylguaiacol to determine the formation sequence of the primary products. The kinetics for the HDO of propylguaiacol and the intermediates propylphenol, propylbenzene were studied over Mo-containing zeolite. This work will be insightful for the development of bifunctional and bimetallic catalysts for upgrading complex dimeric/oligmeric lignin compounds in the future.

References

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