Hydrolysis of Bio-Oil Anhydrosugars Derived from Autothermal Pyrolysis of Cellulosic Biomass

Anhydrosugars are the major carbohydrate products of autothermal pyrolysis of lignocellulosic biomass that can be upgraded into biofuels and bio-based chemicals. Our research group has developed a fast pyrolysis process, namely autothermal pyrolysis which can produce high yields of bio-oil by thermal depolymerization of pretreated biomass. A significant fraction of the heavy ends of bio-oil contains sugars, mainly levoglucosan, but also xylose as the major carbohydrate products. While some of the anhydrosugars can be directly fermented, most yeasts prefer glucose, which can produce renewable fuels such as ethanol, butanol, and value-added platform chemicals such as organic acids and 5-hydroxymethylfurfural. The anhydroglucans can be acid-catalyzed to glucose; however, the currently available bio-oil hydrolysis processes could be potentially cost-intensive for scaling up the hydrolysis reactor and integrated biorefinery scenario.

The requirement of pressurized vessel at hydrolysis temperature (~2.7 bar at 135°C), complex handling of high concentrations of sulfuric, and difficulty of handling water-insoluble solids associated with the use of some common neutralization agents (e.g. calcium hydroxide) could increase the overall cost of production of biofuel and biochemicals from biomass. Furthermore, a high sulfuric acid concentration could be detrimental to the downstream fermentation unit if it ends up as a neutralized salt surpassing the admissible toxicity level for the fermenting microorganisms to remain reactive for glucose conversion.

We have a developed a process for hydrolyzing pyrolytic bio-oil at relatively mild conditions, with temperatures as low as 95-105°C and sulfuric acid concentrations of only 50-150 mM. A response surface method was used to optimize the hydrolysis reaction conditions for levoglucosan. Overall, a 95% conversion of levoglucosan could be achieved and a high concentration of glucose (160.8 g/L) could be produced at 115°C using only 150 mM sulfuric acid at 3 h of reaction time. Modeling of bio-oil hydrolysis shows a good overall fit with first order conversion of levoglucosan to glucose with possibility of other C₆ anhydrosugars conversion to excess glucose.