Lignin Hydrogenolysis:\nPhenolic Monomers from Lignin\nand Associated Phenolates across Plant Clades

Abstract

The chemical complexity of lignin remains a major challenge\nfor\nlignin valorization into commodity and fine chemicals. A knowledge\nof the lignin features that favor its valorization and which plants\nproduce such lignins can be used in plant selection or to engineer\nthem to produce lignins that are more ideally suited for conversion.\nSixteen biomass samples were compositionally surveyed by NMR and analytical\ndegradative methods, and the yields of phenolic monomers following\nhydrogenolytic depolymerization were assessed to elucidate the key determinants controlling the\ndepolymerization. Hardwoods, including those incorporating monolignol <i>p</i>-hydroxybenzoates into their syringyl/guaiacyl copolymeric\nlignins, produced high monomer yields by hydrogenolysis, whereas grasses\nincorporating monolignol <i>p</i>-coumarates and ferulates\ngave lower yields, on a lignin basis. Softwoods, with their more condensed\nguaiacyl lignins, gave the lowest yields. Lignins with a high syringyl\nunit content released elevated monomer levels, with a high-syringyl\npolar transgenic being particularly striking. Herein, we distinguish\nphenolic monomers resulting from the core lignin vs those from pendent\nphenolate esters associated with the biomass cell wall, acylating\neither polysaccharides or lignins. The basis for these observations\nis rationalized as a means to select or engineer biomass for optimal\nconversion to worthy phenolic monomers.