Fig xA Response surface and contour plot of
For the β-1,4-glycosidic bonds in the four model compounds, the C4O″ bonds are consistently stronger than the C1O′ bonds, indicating that during pyrolysis the depolymerization of hemicellulose would be expected to occur through the symmetrical breakage of C1O′. Zhang et al. (2011a) also found C1O′ homolysis to be the most favorable reaction pathway in Agarose GPG/LE pyrolysis. Compared with the xylose residue without an O-acetyl group, it was found that an O-acetyl group further strengthened C4O″ in the β-1,4-glycosidic linkage, which may be ascribed to the accumulation of bonding electrons induced by the O-acetyl groups. Besides, C5O, corresponding to ring-opening of the monosaccharide residue, also showed the lowest thermal stability (the values for both C1O′ and C5O were between 0.7 and 1.0). Thus, these two bonds are very easily cleaved, and the corresponding depolymerization and ring-opening may occur simultaneously at the onset of thermal degradation. In a previous study, Huang et al. (2013) investigated the decomposition pathways of a xylosyl oligomer by a molecular dynamics method, and also concluded that glycosidic bond cleavage and ring scission could occur simultaneously within a narrow temperature range. As both C1O′ and C5O were broken, the original C1O would be converted into a CO bond, and might be preserved in the final pyrolytic products. Evidence for this is provided by the detection of furfural as a typical compound containing CO at the C1 position in the Py-GC/MS analysis. The bond order values of X-OAc and X-hydroxyl at the C2 and C3 positions showed that these two bond types have similar stabilities, especially for those at the C2 position (0.99–1.17). Thus, breakage of these bonds, leading to deacetylation and dehydration, respectively, may occur in a similar temperature range. This is in accordance with the results of the above TG-FTIR analysis; the temperatures corresponding to the strongest release intensities for acids and H2O were almost the same. Since the formations of CO2 and CO were related to profound degradation involving rupture of the carbon chain, their strongest release intensities were attained at higher temperatures.