What is active cellulose in pyrolysis? An approach based on reactivity of cellulose reducing end

•Elimination of the reducing ends prolonged the induction period.•Reducing ends activate cellulose at relatively low temperatures (<260 °C).•Thermal decomposition ↔ hydrolysis reactions expand disrupted regions between crystallites.•Hydrogen-bond rearrangement was proposed as the molecular mechanism for activation.•Other cellulose units in the paracrystalline regions are also potential activation sites.

The idea of active cellulose is based on the initial induction period that is observed during thermogravimetric analysis of cellulose. The role of the reducing ends of cellulose samples (Avicel PH-101 and Whatman No. 42) was studied in relation to active cellulose formation, because our previous studies suggested that these end groups have lower thermal stabilities. Elimination of the reducing ends from Avicel PH-101 by reduction with NaBH4 substantially prolonged the induction period. These features were only characteristic of Avicel PH-101, with a leveling-off degree of polymerization (DP) of DP¯n 218. Formation of water-soluble oligosaccharides was also lower (240 °C/60 min). For Whatman No. 42 cellulose with a higher DP (DP¯n 2170), similar elimination of the reducing ends had almost no effect on the weight-loss behavior. The reducing group analysis data showed that a large number of new reducing ends were formed in Whatman cellulose during DP reduction as the initial pyrolysis process. Based on these differences arising from the initial DP of cellulose, a mechanism for active cellulose was proposed: thermal decomposition of reducing end groups, which are originally present or are formed during pyrolysis in crystalline cellulose, activates the following pyrolysis reactions. The molecular mechanism for activation is also discussed; we propose that it includes hydrogen-bond rearrangement in the initiation of pyrolytic reactions such as depolymerization (transglycosylation) and dehydration, based on our previous proposal that proton donation (acting as an acid catalyst) through hydrogen bonding is a fundamental mechanism for these pyrolytic reactions.