(260b) Measurement of Relative Recalcitrance of Biomass Substrates Using Hot Water/ CO2 Reactive Pretreatment

Biomass extracts, particularly sugars from biomass hydrolysis are important products for further conversion in the production of bioethanol. This research is concerned with investigating a more environmentally-benign method for conducting biomass hydrolysis with the aid of subcritical water and supercritical carbon dioxide (SC-CO2). Current methods of degrading biomass utilize acidic or basic medium which needs to be neutralized after the conclusion of the reactive pretreatment step. Carbon dioxide at elevated pressures can acidify the aqueous medium allowing for the depolymerization of the constituent carbohydrate polymers (cellulose and hemicellulose) to monomeric sugars, glucose and xylose from the biomass substrate.

An orthogonal experimental design L9(34) was used to determine the most effective reaction conditions for select biomass substrates, such as corn cob, corn stover and switch grass. The typical range of parameters investigated consisted of temperatures 160-180C, CO2 pressures from 15-45 MPa, 30-90 minutes reaction time, and various sieved particles of each biomass substrate. Small samples of each target biomass were placed in a tubular reaction vessel filled with neat water and rapidly brought to one of the above temperatures, to serve as a baseline for assessing the effect of CO2 on the depolymerization process. Likewise, similar experiments were performed with the aid of an Applied Separations Spe-ed unit to pump SC-CO2 into the tubular reaction vessel contained the biomass substrate and water. Using identical conditions as those employed for depolymerization in neat water, carbonated hot water hydrolysis was performed on each biomass substrate in triplicate.

The sugar yields were calculated based on the size exclusion chromatographic (SEC) determination of the monomeric sugar content of the resultant hydrolyzate. SEC was also used to access the relative recalcitrance of the above biomass substrates to depolymerization under hot carbonated water reactive pretreatment. Calibration of the biomass depolymerization profiles obtained during SEC was done relative to pullulan and sugar standards. Substantial shifts were observed for each type of biomass, including pine tree residues, various rice substrates, and assorted corn cob products in addition to the previously mentioned substrates. Some of the above biomass substrates showed a greater resistance to depolymerization under identical conditions relative to the other substrates which were highly converted into lower oligomeric and monomeric sugar ?containing hydrolyzates. Results compared with traditional dilute sulfuric acid ? based hydrolysis will be reported as well as in terms of the recalcitrance of the targeted biomass to hydrolysis using subcritical water - SC-CO2 mixtures.