ReOx/AC-Catalyzed Cleavage of C-O Bonds in Lignin Model Compounds and Alkaline Lignins

An effective method for the cleavage of the C-O bonds in lignin alpha-O-4 dimers and various alkaline lignins including the lignin from pulp industry was developed over a heterogeneous ReOx/AC catalyst. High yield of aromatic monomers (up to 98.0%) from alpha-O-4 model compounds and up to 80.1% lignin oil with 24.5% yield of monomers from realistic lignin were obtained under optimized conditions. Alkaline poplar lignin provided syringylpropane and 4-propenylsyringol with the highest yield of 6.6% and 7.4%, respectively. The active sites of ReOx/AC could be assigned to ReIV-VI according to XPS and EXAFS characterization. The lignin structures of different botanical species (hardwood, softwood and herbaceous crop) were compared and the activity of ReOx/AC on these lignins was investigated in detail. GPC and 2D HSQC NMR analysis of isolated lignin and lignin oil showed that hardwood lignin was more liable to be deconstructed than softwood and grass lignins due to the less amounts of beta-5 linkages, lower molecular weight and richer syringyl (S) units in hardwood lignin.