Cellulose hydrothermal conversion promoted by heterogeneous Bronsted and Lewis acids: Remarkable efficiency of solid Lewis acids to produce lactic acid

摘要

Crystalline cellulose treated in hydrothermal conditions (190°C, 24 h) is partially solubilised, 30%, into water soluble oligosaccharides/polymers with the formation of small amounts of glucose and 5-HMF. In the presence of solid Brensted catalysts such as Cs2HPW_(12)O_(40) and HY zeolite, the extent of the cellulose depolymerisation was not changed when no leaching occurred. However, a quite different products distribution was obtained, in favour of further transformations of glucose and HMF in levulinic and formic acids. On the opposite, solid Lewis acids such as tungstated zirconia (ZrW) and tungstated alumina (A1W) exhibited a remarkable promoting effect on the cellulose depolymerisation which was raised up to 45% while an unexpected decrease of the proportion of water soluble oligosaccharides/polymers was observed. Yields of 27mol% and 18.5 mol% in lactic acid were achieved on A1W and ZrW, respectively. Moreover, these tungsten based Lewis acids exhibited a good stability and recyclability. The efficiency of the solid Lewis acids ZrW and A1W to produce lactic acid directly from crystalline cellulose was explained by a positive synergy between water autoprotolysis responsible of the cellulose depolymerisation into soluble intermediates which are further converted on the solid Lewis catalyst surface.