Sunday, May 4, 2008
3-18

Kinetics of xylose reversion reactions during acid pretreatment

Heidi M. Pilath1, Mark R. Nimlos1, Michael E. Himmel2, and David K. Johnson2. (1) National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, (2) Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

Acid pretreatment of cellulosic biomass solubilizes hemicellulose (i.e. xylan), allowing subsequent enzymatic hydrolysis of the cellulose.  Conversion of cellulose and xylan into constituent sugars in high yields is essential for the economic viability of bio-ethanol. Thus, acid-catalyzed reactions that result in the loss of sugars are of particular interest.  A loss reaction that has received little attention is the reversion reaction of sugars that result in the formation of oligomers, which cannot be fermented. The economics of pretreatment dictate that high biomass loadings are necessary and under these conditions reversion reactions may be important.  In this study, the reversion reactions of xylose (and for comparison, glucose) under dilute acid conditions were examined.  Experiments were conducted in highly agitated microwave-irradiated reactor vessels.  This reactor set-up allowed for rapid heating of the sample and limited the effects of mass transport, permitting the measurement of intrinsic kinetics. At high sugar loadings, xylose reversion reactions were found to result in conversion of up to 10% of the xylose into oligomers that are most likely to be dimeric.  Under similar conditions the reversion reactions of glucose produced readily identifiable 1,x- linked disaccharides. Using kinetic modeling of the experimental results we obtained the kinetics for formation and hydrolysis of reversion products, in addition to the equilibrium constants.  From these measurements, we were able to calculate activation energies. The kinetic measurements from this study are critical for designing and operating pretreatment processes to optimize xylose yield.

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