Lignin Hydrogenolysis: Phenolic Monomers from Lignin and Associated Phenolates across Plant Clades

The chemical complexity of lignin remains a major challengeforlignin valorization into commodity and fine chemicals. A knowledgeof the lignin features that favor its valorization and which plantsproduce such lignins can be used in plant selection or to engineerthem to produce lignins that are more ideally suited for conversion.Sixteen biomass samples were compositionally surveyed by NMR and analyticaldegradative methods, and the yields of phenolic monomers followinghydrogenolytic depolymerization were assessed to elucidate the key determinants controlling thedepolymerization. Hardwoods, including those incorporating monolignol p-hydroxybenzoates into their syringyl/guaiacyl copolymericlignins, produced high monomer yields by hydrogenolysis, whereas grassesincorporating monolignol p-coumarates and ferulatesgave lower yields, on a lignin basis. Softwoods, with their more condensedguaiacyl lignins, gave the lowest yields. Lignins with a high syringylunit content released elevated monomer levels, with a high-syringylpolar transgenic being particularly striking. Herein, we distinguishphenolic monomers resulting from the core lignin vs those from pendentphenolate esters associated with the biomass cell wall, acylatingeither polysaccharides or lignins. The basis for these observationsis rationalized as a means to select or engineer biomass for optimalconversion to worthy phenolic monomers. Hydrogenolyticdepolymerization of lignin from 16 biomasssamples shows the significant impact of lignin composition on phenolicmonomer yields.