Influence of selected mineral acids and alkalis on cellulose pyrolysis pathways and anhydrosaccharide formation

摘要

The primary/initial thermal degradation pathway is via intermolecular transglycosylation reactions within the glucose monomers of cellulose. Formation of liquid tar from cellulose can also occur via β-elimination. Under this mechanism, volatile acids (e.g. carboxylic acids) formed from the initial cellulose decomposition are able to attack the remaining cellulose as Bronsted acids thus catalysing heterolytic (ring-opening) reactions. Here, we have studied the effects of acids (H_2SO_4, H_3PO_4, H_3BO_3) and alkalis (Ca(OH)_2, NH_4OH) on the yields of levoglucosan and its derivatives (levoglucosenone, l,4:3,6-dianhydro-α-D-glucopyranose, 1,6-anhydro-P-D-glucofuranose). It was found that the acids generally decreased the total amount of anhydrosaccharides whilst cellulose infused with the alkalis had increased anhydrosaccharide yields. The results indicate that the P-elimination pathway was catalysed by the presence of acidic species (e.g. H~+ ions). The extent of cellulose conversion via the P-elimination pathway was dependant on the amount and strength of acid infused within the cellulose matrix. However, the P-elimination route could also be suppressed or controlled by the introduction of a neutralising species (e.g. OH~- ions).