Insights about the effect of composition, branching and molecular weight on the slow pyrolysis of xylose-based polysaccharides

This work provides insights into the relationship between the pyrolysis products distribution and the structural features of xylose-based hemicelluloses. Three xylose-based hemicelluloses differing in composition, molecular weight, chain branching and origin were selected, structurally characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis and nuclear magnetic resonance and then used as feedstocks for slow pyrolysis tests up to 700 °C in nitrogen environment. Commercial beechwood xylan was selected as representative of the glucuronoxylan hemicellulose type, typical of hardwood species. Commercial corncob xylan was selected as it is composed by a mixture of low molecular weight xylo-oligosaccharides with a low acetylation degree. In the end, the xylan extracted from grape pruning, being an arabinoglucuronoxylan, was chosen as representative of hemicellulose structure common to most annual plants and agricultural wastes. Low-branched material (corncob xylan) produced the lowest yield of char and the highest yield of pyrolysis liquids, while highly-branched material (grape pruning xylan) exhibited an opposite trend. The liquid composition was qualitatively similar for all the considered xylans. However, the comparison of the quantifiable species showed different relative abundances. The pyrolysis products distribution indicated that the chain branching degree affected at a higher extent the feedstock pyrolytic behavior with respect to the ash content.