Insights into the reaction network and kinetics of xylose conversion over combined Lewis/Brønsted acid catalysts in a flow microreactor

The catalytic effect of Lewis acid on xylose conversion to furfural has been widely reported, while the underlying reaction network and kinetics are not fully elucidated. This work presents experimental and kinetic modelling studies on xylose conversion to furfural, using AlCl3/HCl as combined Lewis/Brønsted acid catalysts in monophasic water or a biphasic water–methyl isobutyl ketone system in a flow microreactor. The reaction network and kinetics were developed, where Al(OH)2+ was identified as the catalytically active species for isomerization between xylose, lyxose and xylulose, while both Al(OH)2+ and H+ catalyze the sugar dehydration to furfural and side reactions leading to humins (e.g., sugar condensation and furfural degradation). The promoting effect of AlCl3 is attributed to not only the tandem catalysis through xylulose, but also the parallel Al(OH)2+-catalyzed sugar dehydration. Within the studied range (120 to 180 °C, 0.05–0.4 M HCl, 0.04–0.12 M AlCl3), the furfural selectivity relied mainly on the concentration ratio of HCl to AlCl3 rather than their individual concentrations or the temperature. The volcano-like evolution of the maximum furfural yield with increasing HCl/AlCl3 ratio is related to (i) the lower reaction orders in Al(OH)2+ for reactions forming furfural than for side reactions; and (ii) a gradual shift of the dominant reaction pathway from the Al(OH)2+-catalyzed one towards H+-catalyzed one. An optimized furfural yield up to 90% was achieved with 40 mM AlCl3 and 100 mM HCl at 160 °C from 1 M xylose in a continuous slug flow microreactor. The catalytic aqueous phase could be recycled and reused four times without performance loss.