2-43: Identification of xylodisaccharides in xylose reversion products

Pretreatment is an essential step in the conversion of biomass to renewable fuels, but the loss of fermentable sugars often limits its economic viability. Xylose yield is reduced by dehydration reactions, which form furfural, as well as acid catalyzed reversion reactions which result primarily in the formation of xylodisaccharides. Thus far, there has been limited research to identify the precise structure of the products formed from these reversion reactions. In order to identify the structure of xylose reversion products, mildly acidic solutions of xylose (300 g L^-1 in 1.2 wt% H₂SO₄) were heated to 140 ^oC for 3 min and then fractionated by HPLC. Reaction products in each fraction were characterized by GC/MS, anion-exchange chromatography and NMR. Linkage types in the xylodisaccharides were assigned by permethylation / GC/MS analysis. The isomeric structure of each xylodisaccharide was identified by comparing the anomeric proton signal patterns in ¹H-NMR spectra with the patterns from authentic glucodisaccharide isomers of known structure. Comparison of the anomeric carbon signal patterns in ¹³C-NMR spectra supported the results from ¹H-NMR spectral analysis. 2D-NMR analyses (COSY, TOCSY and HSQC) were also used to assign the ¹H and ¹³C signals for each of the xylodisaccharides. Using this approach, 8 of the 9 possible a or b isomers of the 1→1, 1→2, 1→3 or 1→4-xylodisacharides have been identified. This work will ultimately assist in finding the optimal pretreatment conditions to reduce the loss of fermentable sugars.