317 / 2018-07-25 17:54:05

A novel route for bioethanol production with co-generation of electricity from lignocellulosic biomass mediated by polyoxometalates under mild conditions

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As the most abundant renewable materials in natural world, lignocellulosic biomass has attracted great attention for producing biofuels, materials, chemicals and electricity. The polysaccharides of lignocellulose can be converted to fermentable sugars for bioethanol production, while lignin which contains structure rich in electrons can be used as an electron donor for electricity generation by a fuel cell technology. To achieve an integrated utilization of lignocellulosic biomass for energy production, a novel technical route involving conversion of cellulose to ethanol and lignin to electricity has been developed based on the acidic and redox properties of polyoxometalates (POMs). In this process, the pretreatment of biomass using POMs to oxidize lignin with the POMs being reduced simultaneously can be considered as a “charging” process, while the re-oxidation of POMs in a liquid flow fuel cell with generation of electricity is a “discharging” process. By this process, lignocellulosic biomass can be not only well pretreated and further converted to bioethanol, but also directly converted to electricity under mild conditions. Several Keggin-type POMs at concentration of 0.25 M including phosphomolybdic acid (H3PMo12O40, PMo12), phosphotungstic acid (H3PW12O40, PW12) and phosphomolybdovanadic acid (H4PMo11VO40, PMo11V) were compared for pretreating wheat straw. It was found that PMo12 and PW12 could obtain a high cellulose digestibility with 80% conversion within 24-h hydrolysis when pretreatment was conducted at 95oC. The PMo12-pretreated substrates could be well converted to bioethanol by a simultaneous saccharification and fermentation (SSF) process. Ethanol concentrations were 9.92, 18.3 and 22.9 g/L at initial solid loadings of 5, 10 and 15%, respectively, which corresponded to ethanol yields of 84.0, 77.5 and 77.6%, respectively. The reduced POMs could be re-oxidized in a liquid flow fuel cell with generation of electricity. Fe3+ was screened as a cheap and effective liquid mediator to transfer electrons to air, the final electron acceptor in the “discharging” process. The highest output power densities of 10.8 mW/cm2 and 12.4 mW/cm2 were obtained for discharging of reduced H3PMo12O40 and H4PMo11VO40, respectively. This power density was 5000-6000 times higher than that of phenol-fueled MFCs, and 10 times higher than that of recently reported direct biomass fuel cell with an air-cathode covered by Pt catalyst. Mass balance indicates that by this technical route, 11 gram of ethanol can be produced with generation of 10 mW electricity from 100 gram of wheat straw, but the process efficiency can be further improved by process optimization. However, this work provides a new idea for energy generation from biomass under mild condition. Moreover, this work also provides a conceptual combination of direct biomass fuel cell and redox flow cell, which can be flexibly switched between electricity generation from biomass and stationary energy storage.