In order to investigate the formation and destruction of xylo-oligomers further, experi­ments were conducted using xylan. Both Birchwood xylan and Beechwood xylan were used for these studies. These materials were supplied as fine powders from Sigma and used as received. Because this biopolymer is not soluble, stock solutions of the samples in acid could not be prepared. Instead, approximately 20 mg of sample was added to 2 mL of a 1.2% w/w aqueous solution of sulfuric acid and the measured products were normalized to the actual amount of xylan used. In the absence of heating, HPLC measurements of the liquid portion of these slurries detected no xylose or furfural, while these species were observed when the samples were heated in the microwave reactor. Figure 9.10 shows plots of xylose and furfural

concentrations (normalized to 10 mg/mL of added xylan) as a function of residence time of the xylan slurry in the reactor. Figure 9.10a shows the plot of xylose formation at 115° C, where it is apparent that increasing the residence time results in increased formation of xylose. This result suggests that, at this temperature, xylan has not been fully converted to xylose. At 125, 135, and 145°C, the amount of xylose formed remains constant at all measured residence times, which suggests that the xylose residues in the xylan have been completely released. We note that approximately 73% of the mass in the xylan has been converted to xylose in these experiments. This value is close to the known concentration of xylose in this xylan [Johnson, D. K. (2006) unpublished work] (69%). At higher temperatures, the concentration of xylose decreases with increasing residence time. This observation suggests that at these tempera­tures, the xylose is being dehydrated to form furfural, which is also confirmed by the furfural plot in Figure 9.10b. Note that at temperatures below 145°C, independent measurements with pure xylose in acid show little formation of furfural, whereas at these temperatures large amounts of xylose are formed from xylan. Xylobiose was only observed at 115° C and only at a normalized level of 0.002 mg mL-1. These observations again show that hydrolysis reactions of xylo-oligomers and xylan are much faster than dehydration reactions of xylose.