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

• Acids decreased the total amount of anhydrosaccharides from cellulose pyrolysis.
• Cellulose infused with alkalis had increased anhydrosaccharide yields.
• The β-elimination pathway was favoured by the presence of acidic species (e.g. H+ ions).
• Cellulose conversion extent via β-elimination depends on the amount and strength of acid infusion.
• The β-elimination route could be suppressed/controlled by the introduction of a neutralising species (OH− ions).

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 (H2SO4, H3PO4, H3BO3) and alkalis (Ca(OH)2, NH4OH) on the yields of levoglucosan and its derivatives (levoglucosenone, 1,4:3,6-dianhydro-α-d-glucopyranose, 1,6-anhydro-β-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 β-elimination pathway was catalysed by the presence of acidic species (e.g. H+ ions). The extent of cellulose conversion via the β-elimination pathway was dependant on the amount and strength of acid infused within the cellulose matrix. However, the β-elimination route could also be suppressed or controlled by the introduction of a neutralising species (e.g. OH− ions).