Slow pyrolysis of xylan as pentose model compound for hardwood hemicellulose: A study of the catalytic effect of Na ions

In this study, the effect of sodium (Na) ions on the slow pyrolysis of hemicellulose is assessed using xylan as a representative of hardwood hemicellulose. Thermogravimetric analysis and pyrolysis tests were conducted at slow heating rates (5 K/min) up to 973 K on commercial xylan, demineralized xylan and Na+ doped xylan samples. Commercial xylan was demineralized by using a cation-exchange resin to reduce the presence of inherent inorganics (mainly alkali or alkaline-earth metals). Subsequently, the demineralized xylan was doped with controlled amounts of Na ions (0.4, 0.5 and 1.1 wt.%) by using cation-exchange resin in Na+ form. The pyrolysis products yields (gas, liquid and char) and the composition of the gas mixture were quantified for each sample. Gas chromatography/mass spectrometry (GC/MS) analysis were performed to quantify the main species in the liquid products. The results, discussed considering of the available hemicellulose pyrolysis models, demonstrate that the presence of Na+ greatly affects the pyrolytic behavior of the samples, whereas only slight quantitative effects are observed on the solid residues yields, which increase with Na+ concentration. Furthermore, Na+ favors the ring opening reactions (increasing the production of CO2, CO and hydroxy ketones) and the rearrangement of the xylose ring to form furan derivatives. The reaction pathways competing with 2- furaldehyde production mechanism could explain the non-monotonous trend of this compound as function of Na+ content.