(362b) Progress in Thermochemical Production of Cellulosic Sugars

Robert C. Brown

Iowa State University, Ames, IA 50011, U.S.A

rcbrown3@iastate.edu

AIChE Annual Meeting

Boston, MA

November 7-9, 2021

The conventional approach for production of cellulosic sugars from lignocellulosic biomass involves pretreatment to open up the cell wall structure followed by acid or enzymatic hydrolysis to convert polysaccharides into fermentable sugars. Enzymatic has been favored in recent years because of its selectivity toward sugars. Despite advances over the last several decades, commercialization of biofuels via enzymatic hydrolysis remains elusive. Among the major challenges has been the high cost of pretreatments to separate polysaccharides from lignin, production of cost-effective enzyme cocktails, and effective use or disposal of process wastes.

Research at Iowa State University is advancing thermal depolymerization of biomass, using heat and catalysts instead of enzymes to break glycosidic bonds in plant polysaccharides, as an alternative pathway to cellulosic sugars. This pathway has been relatively little explored, likely because the low selectivity and unfavorable thermodynamics of high temperature thermal processes would suggest poor prospects for thermal depolymerization. Indeed, pyrolysis of most lignocellulosic biomass yields very little sugar, with yields that are at best only a few percent of the theoretical yields of hexose and pentose from cellulose and hemicellulose. We have found that alkali and alkaline earth metals (AAEM) normally chelated into the polymeric structure of biomass can be ion exchanged with sulfuric acid or ferrous sulfate to form AAEM salts that are thermally stable at pyrolysis temperatures. The large anions in these salts shield the smaller AAEM cations from interacting with the biomass, thus effectively passivating their catalytic activity, and dramatically increasing sugar yields from the pyrolysis of biomass. We have learned how to recover these sugars from other products of pyrolysis and purify them for use in fermentations or chemical synthesis for production of various products. This paper examples the pretreatments, processing conditions, separations, and purifications required to produce pure monosaccharides via thermochemical processing.