Knocking on Wood: Base Metal Complexes as Catalysts for Selective Oxidation of Lignin Models and Extracts

Abstract

This work began as part of a biomass conversion catalysis project with UC Santa Barbara funded by the first NSF Chemical Bonding Center, CATSB. Recognizing that catalytic aerobic oxidation of diol C-C bonds could potentially be used to break down lignocellulose, we began to synthesize oxovanadium complexes and explore their fundamental reactivity. Of course there were theories regarding the oxidation mechanism, but our mechanistic studies soon revealed a number of surprises of the type that keep all chemists coming back to the bench! We realized that these reactions were also exciting in that they actually used the oxygen-on-every-carbon property of biomass-derived molecules to control the selectivity of the oxidation. When we found that these oxovanadium complexes tended to convert sugars predominantly to formic acid and carbon dioxide, we replaced one of the OH groups with an ether and entered the dark world of lignin chemistry. In this Account, we summarize results from our collaboration and from our individual labs. In particular, we show that oxidation selectivity (C-C vs C-O bond cleavage) of lignin models using air and vanadium complexes depends on the ancillary ligands, the reaction solvent, and the substrate structure (i.e., phenolic vs non-phenolic). Selected vanadium complexes in the presence of added base serve as effective alcohol oxidation catalysts via a novel base-assisted dehydrogenation pathway. In contrast, copper catalysts effect direct C-C bond cleavage of these lignin models, presumably through a radical pathway. The most active vanadium catalyst exhibits unique activity for the depolymerization of organosolv lignin. After Weckhuysen's excellent 2010 review on lignin valorization, the number of catalysis studies and approaches on both lignin models and extracts has expanded rapidly. Today we are seeing new start-ups and lignin production facilities sprouting up across the globe as we all work to prove wrong the old pulp and paper chemist's adage: you can make anything from lignin except money!

Catalytic Scissoring of Lignin into Aryl Monomers

“…The importance of lignin as the biggest sustainable reservoir for aromatic rings, as not only chemical feedstocks but also material precursors, has been acknowledged broadly. Particularly, for chemical feedstocks, versatile methods have been developed, and these included reduction, oxidation, and solvolysis, and pyrolysis . The utilization of the aromatic nature to prepare aromatics from lignin is recognized as a high‐value valorization.…”

Ethanolysis of Kraft lignin to platform chemicals on a MoC1-x/Cu-MgAlOz catalyst

“…Lignin has a macromolecular structure composed of phenyl-propane units linked by C-O and C-C bonds, with β-O-4 as the most common linkage [2][3][4]. Hence, lignin has been shown promising to be used as a resource for the production of aromatic chemicals [5][6][7]. For centuries, Kraft lignin has been produced in a great amount as the byproduct of pulping process in which cellulosic fiber is separated and utilized to produce paper, while lignin is left in the black liquor as a pollution waste [8,9].…”

Complete lignocellulose conversion with integrated catalyst recycling yielding valuable aromatics and fuels

“…Moreover, under these processing conditions the lignin component is structurally modified, rendering its further catalytic valorization very challenging [4][5][6][7] . This remains true despite impressive advances in the selective conversion of lignin model compounds 8,9 and depolymerization of organosolv lignin [10][11][12][13][14] . Recently, elegant research has focused on lignocellulose fractionation in the presence of a catalyst 4,[15][16][17] .…”