The effect of uronic acid moieties on xylan pyrolysis

Abstract

The 4-O-methyl-D-glucuronic acid groups attached to xylose chains in wood xylan can form uronate salts with alkali and alkaline earth metal cations. These uronic acid and metal uronate moieties may act as acid and base catalysts, respectively, during xylan pyrolysis. In this study, commercial beechwood xylan (in which most uronic acids were present as sodium salts) and a demineralized version of the same material containing free uronic acid groups were pyrolyzed at 220–340 °C under N2 and the decomposition behaviors of the uronic acid and xylose units were determined. In the case of the original xylan, differential thermogravimetric (DTG) peaks were assigned to the degradation of sodium uronate and some xylose units (at lower temperatures) and to the decomposition of the remaining xylose units (at higher temperatures). In contrast, the demineralized xylan generated only one DTG peak in the intermediate temperature range, corresponding to the simultaneous decomposition of both uronic acid and xylose units. Thus, the effect of sodium uronate on the pyrolysis of the original xylan was only regional, whereas the free uronic acid affected the demineralized xylan in a more homogeneous manner. Additional data from pyrolysis-gas chromatography-mass spectrometry revealed that the compositions of the pyrolysis products were also different for these two xylan samples. The results of this work provide an improved understanding of wood pyrolysis mechanisms.