Monday, April 30, 2007

Xylan and lignin removal during flow through pretreatment of corn stover

Sridhar Viamajala1, Melvin P. Tucker2, Michael J. Selig2, Todd Vinzant2, Stephen R. Decker2, and Richard T. Elander2. (1) Biological and Irrigation Engineering, Utah State University, 4105 Old Main Hill, Logan, UT 84322-4105, (2) National Renewable Energy Laboratory, 1617 Cole. Blvd MS 3511, Golden, CO 80401

In this study, we report on xylan and lignin removal during hot water flow-through (FT) experiments with corn stover. During all FT pretreatments, insoluble dark precipitates were observed in the effluent. Nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies confirmed the precipitates as lignin. The lignin released could be partially re-deposited on filter paper placed downstream of the reactor containing corn stover. Increased re-deposition on filter paper was observed during higher temperature experiments. These results suggest that lignin released from corn stover retained its affinity for cellulose and the greater amounts of deposition at higher temperature suggest a hydrophobic affinity of lignin for cellulose. 

Oligomeric and monomeric xylose concentrations were measured in the effluent during all the FT experiments, but at temperatures beyond 200 °C, significant degradation of xylan to unknown products was observed.  However, in all cases, total xylan removed was proportional to lignin and acetate release although increasing the reaction temperatures from 200 to 230 °C did not significantly enhance the kinetics of either xylan, lignin or acetate removal. These results suggest that cleavage of ester bonds, de-lignification and removal of xylan occur simultaneously. Since xylan is covalently linked to lignin via ferulic ester linkages, it seems likely that under hot water FT conditions, cleavage of this ester bond occurs and facilitates xylan release. Melting and mobilization of lignin also likely contribute to the process of xylan release. Through this ongoing work, we are developing a better mechanistic understanding of lignin-xylan interactions at high temperatures.