Xylan Slow Pyrolysis: what have we learned so far?

A detailed investigation on the slow pyrolysis of xylose-based materials was conducted since many years in our group to shed light on the pyrolytic behavior of hemicellulose and identify and circumscribe the role of structural characteristics and alkali and alkaline earth metals on hemicellulose decomposition mechanisms. Studies on three xylose-based hemicelluloses differing in composition, molecular weight (MW), chain branching, monomers composition and origin, indicated that: i) the production of char and gaseous species is promoted by higher MW, branching and composition with the ashes playing a prominent role; ii) the production of anhydrosugars, furfural, formic acid and acetic acid is suppressed by the presence of ashes, while the chain branching seems to favor the production of linear ketones and to inhibit the production of cyclic and hydroxylated ketones; iii) the production of water seems to be favored by the increase of MW and branching and suppressed by the presence of ashes. Studies on demineralized and metal doped xylan samples led to the acquisition of details regarding the catalytic effects of alkali and alkaline earth metals on hemicellulose pyrolysis behavior. The comparison of the results of pyrolysis tests on a demineralized xylan (DX) sample and a raw commercial xylan (X) indicated that metal ions in X were responsible of: i) a slight anticipation of the initial decomposition temperature and of the presence of a second important event (peaked at 550 K) in the devolatilization curve that is only slightly visible at higher temperature in the DX decomposition curve; ii) a higher amount of solid residue compared to the demineralized sample. The results of pyrolysis tests on samples obtained by doping a demineralized xylan through cationic exchange with controlled amounts of K+ (from 0.3 to 1.2 wt.%) or Na+ (from 0.4 to 1.1 wt.%) allowed to define the specific catalytic effects of K+ and Na+. The experimental results showed that K+ and Na+ have similar catalytic effects: both Na+ and K+ catalyze ring opening reactions (increasing the production of CO2, CO and hydroxy-ketones) and the rearrangement of the xylose ring to form furan derivatives. In the end, the comparison of above reported results with those of pyrolysis tests on a set of doped xylan samples obtained introducing controlled amounts of KCl or NaCl on a demineralized xylan sample through a conventional wet impregnation approach demonstrated how the catalytic effect of alkali metals on xylan pyrolysis is also affected by the adopted doping approach. The results showed that the introduction of K+ by wet impregnation using a chloride salt negligibly affected the pyrolytic behavior of the demineralized sample and indicated that the doping approach based on wet impregnation using chloride salts is not appropriate for the study of the effect of alkali metals on the pyrolysis of polysaccharides bearing acidic functional groups as xylan.