Layered Double-Hydroxide Catalysts for Lignin Depolymerization

A critical step in the conversion of lignin to renewable value-added aromatic chemicals is selective depolymerization of the biopolymer to its constituent monomers.  For optimal process economics, this depolymerization should occur over heterogeneous catalysts in the absence of high-pressure hydrogen, but few catalysts exist that perform well and meet these criteria.  One promising class of catalyst is layered double-hydroxide materials, such as hydrotalcite.  Our group has recently shown that nickel-loaded hydrotalcite selectively cleaves the β-O-4 bond in a lignin model dimer, and reduces the molecular weight of two types of whole lignin.  Here we expand on this finding by varying the many tunable parameters of hydrotalcite catalysts, including the type and location of metal and the identity of the charge-compensating anion(s), as well as the reaction solvent.  Application of these catalysts to a variety of lignin model dimers and real lignin streams shows that these catalysts do not rely only on basic or metal sites for activity, and that the solvent has an impact on catalyst activity and product spectrum.