Coupling biomass pretreatment for enzymatic hydrolysis and direct biomass-to-electricity conversion with molybdovanadophosphoric heteropolyacids as anode electron transfer carriers

Abstract Owing to their acidity, oxidizing ability and redox reversibility, molybdovanadophosphoric heteropolyacids (Hn+3PMo12-nVnO40, abbreviated as PMo12-nVn) were employed as electron transfer carriers for coupling biomass pretreatment for enzymatic hydrolysis and direct biomass-to-electricity conversion. In this novel coupled process, PMo12-nVn pretreatment that causes deconstruction of cell wall structure with PMo12-nVn being simultaneously reduced can be considered as the “charging” process. The reduced PMo12-nVn are further re-oxidized with release of electrons in a liquid flow fuel cell (LFFC) to generate electricity is the “discharging” process. Several Keggin-type PMo12-nVn with different degree of vanadium substitution (DSV, namely n) were prepared. Compared to Keggin-type phosphomolybdic acid (PMo12), PMo12-nVn (n=1–6) showed higher oxidizing ability but poorer redox reversibility. The cellulose enzymatic digestibility of PMo12-nVn pretreated wheat straw generally decreased with increase in DSV, but xylan enzymatic digestibility generally increased with DSV. PMo12 pretreatment of wheat straw at 120 °C obtained the highest enzymatic glucan conversion (EGC) reaching 95%, followed by PMo11V1 pretreatment (85%). Discharging of the reduced heteropolyacids in LFFC showed that vanadium substitution could improve the maximum output power density (Pmax). The highest Pmax was obtained by PMo9V3 (44.7 mW/cm2) when FeCl3 was used as a cathode electron carrier, while PMo12 achieved the lowest Pmax (27.4 mW/cm2). All the heteropolyacids showed good electrode Faraday efficiency (>95%) and cell discharging efficiency (>93%). The energy efficiency of the coupled process based on the heat values of the products and generated electric energy was in the range of 18%–25% depending on DSV. PMo12 and PMo11V1 seems to be the most suitable heteropolyacids to mediate the coupled process.