β-Zeolite Assisted Lignin-First Fractionation in a Flow-Through Reactor

Lignin is one of the main constituents of lignocellulosic biomass, whose valorization is essential for an economically feasible biorefinery process scheme. In the present work, a hydrogen-free one step catalytic fractionation of woody biomass using commercial b-zeolite as catalyst in a flow-through reactor was carried out, leading to a maximum aromatic monomer yield of 20.5 wt.%. Birch, spruce and walnut shells were used and compared as lignocellulosic feedstocks. Relevant insights in the reaction mechanism were obtained through 2D HSQC NMR analysis, revealing that b-O-4 cleavage is catalyzed by the zeolite. To optimize system operation, a rate limiting step analysis was performed by using different reactor configurations. It was found that the system operated in a mixed regime where the rates of both solvolytic delignification and zeolite-based depolymerization/dehydration affect the net rate of aromatic monomer production. Oxalic acid addition was found to enhance monomer production at moderate concentrations by improving solvolysis; however, it caused structural changes to the zeolite leading to lower monomer yields at higher concentrations. Zeolite stability was assessed through catalyst recycling and characterization using NH₃-TPD, XRD, N₂ physisorption and TGA. Main catalyst deactivation mechanisms were found to be coking and leaching, respectively leading to larger pore size and lower concentration of acid sites.