The chemical complexity of lignin remains a major challenge
for
lignin valorization into commodity and fine chemicals. A knowledge
of the lignin features that favor its valorization and which plants
produce such lignins can be used in plant selection or to engineer
them to produce lignins that are more ideally suited for conversion.
Sixteen biomass samples were compositionally surveyed by NMR and analytical
degradative methods, and the yields of phenolic monomers following
hydrogenolytic depolymerization were assessed to elucidate the key determinants controlling the
depolymerization. Hardwoods, including those incorporating monolignolp-hydroxybenzoates into their syringyl/guaiacyl copolymeric
lignins, produced high monomer yields by hydrogenolysis, whereas grasses
incorporating monolignolp-coumarates and ferulates
gave lower yields, on a lignin basis. Softwoods, with their more condensed
guaiacyl lignins, gave the lowest yields. Lignins with a high syringyl
unit content released elevated monomer levels, with a high-syringyl
polar transgenic being particularly striking. Herein, we distinguish
phenolic monomers resulting from the core lignin vs those from pendent
phenolate esters associated with the biomass cell wall, acylating
either polysaccharides or lignins. The basis for these observations
is rationalized as a means to select or engineer biomass for optimal
conversion to worthy phenolic monomers.