(267d) Pyrolysis of Cellulose with the Use of Ionic Liquid for Efficient Production of Levoglucosenone

Pyrolysis is a most practical method to directly convert cellulose into condensable organic product, which is in general termed tar, with high yield. The tar consists of a variety of organic compounds, such as anhydrosugars and furan derivatives, and this is due to progress of various reactions in parallel and in series. For selective production of valuable chemicals, thus, the pyrolysis is currently less suitable compared to indirect methods via saccharification. A compound, levoglucosenone, can be produced by means of pyrolysis more easily than by general chemical methods, but it is difficult for any methods reported so far to convert cellulose to this particular compounds with either high selectivity or yield. This study proposes the pyrolysis of cellulose with the use of a type of ionic liquid (IL). ILs have widely been used as solvents for catalytic conversion of cellulose due to their characteristic properties such as solvent powers for cellulose, low vapor pressures and thermal stabilities. The proposed pyrolysis employs a type of IL, 1-butyl-2,3-dimethyl imidazolium trifluoromethanesulfonate ([BMMIM][OTf]), with an expectation of its high thermal stability occurring from alkyl substituent to 2-carbon. TG/DTA analyses in fact demonstrated its perfect stability at temperature up to 350 °C, where the most of the ILs underwent self-decomposition. The anion of [BMMIM][OTf], i.e., [OTf], is the conjugated base of a superacid, trifluoromethanesulfonic acid (pK_a < -14), and it was therefore expected catalysis of this IL in the pyrolysis of cellulose. Mixed with [BMMIM][OTf] at a mass ratio of 1 and temperature of 60 °C, cellulose was swollen. The swelling was maintained even after the mixture was cooled down to temperature below the melting point of [BMMIM][OTf], 41 °C. A TG analysis showed that the pyrolysis of cellulose in the mixture started at 160 °C, caused rapid and slow mass releases at 200–275 °C and at higher temperature up to 350 °C. Neither decomposition nor evaporation of [BMMIM][OTf], if any, was detected in the course of heating. The products from the pyrolysis at 300 °C were 32% char, 30% tar, 33% water, and 3–5% gas on a cellulose mass basis. Most importantly, levoglucosenone accounted for more than 70% and 27 mol-% on tar mass and cellulose monomer bases, respectively. Formation of levoglucosenone during the pyrolysis of a phosphoric-acid-treated cellulose was reported previously, but its yield was less than half of that from the cellulose-IL mixture. Such a high yield of levoglucosenone seems to be attributed to a catalytic role of catalysis of the [OTf] anion. Repeated use of [BMMIM][OTf] was also examined. The IL was separated from the char by washing the mixture with ethanol, and then recovered by means of evaporative removal of ethanol. The IL recovery was complete within the margin of experimental error. TG analysis of the used IL and also its repeated use in the subsequent pyrolysis of cellulose confirmed little or no change in the property of the IL in the processes.