Polyoxometalate‐Mediated Lignin Oxidation for Efficient Enzymatic Production of Sugars and Generation of Electricity from Lignocellulosic Biomass

Abstract

Conversion of lignocellulosic biomass to various renewable fuels, chemicals, materials, and electricity has attracted more and more interest in recent decades to decrease the dependence on fossil resources and reduce net CO2 emissions. Herein, we have developed an integrated process of biomass pretreatment for enzymatic hydrolysis of cellulose coupled with electricity generation with polyoxometalates (POMs) and ferric ion as electron mediators and proton carriers. The pretreatment is a “charging” process, and the re‐oxidation of the POMs is effectively a “discharging” process. The pretreated substrate showed a good enzymatic digestibility (≈80 % cellulose conversion) within a short incubation period (<24 h). Fe3+ was screened as a cheap, effective liquid mediator to transfer electrons to air, which is the terminal electron acceptor in the “discharging” process. The highest output power densities of 10.8 and 12.4 mW cm−2 were obtained for discharging of reduced H3PMo12O40 and H4PMo11VO40, respectively. This power density is 5000–6000 times higher than that of phenol‐fueled microbial fuel cells, and 10 times higher than that of a recently reported direct biomass fuel cell with an air cathode covered with Pt catalyst. Moreover, this work also provides a conceptual combination of a direct biomass fuel cell and a redox flow cell, which can be flexibly switched between electricity generation from biomass and stationary energy storage.

Keggin‐type Heteropolyacids‐Catalyzed Selective Hydrothermal Oxidation of Microalgae for Low Nitrogen Biofuel Production

“…Among these different POMs, the Keggin-type heteropolyacids (HPAs) are recognized as the most effective homogeneous catalysts for acid catalyzed reactions such as the transesterification, hydrolysis, due to their high Brønsted acidities. [29] Besides their acidities, HPAs have reversible redox properties, which enable the selective oxidation of cellulosic biomass with remarkable reusability of the light transition metals in oxygen atmosphere. [30,31] Accordingly, HPAs play a dual rule in HTL of microalgae both as catalysts of the hydrolysis of cellular compounds into monomers such as fatty acids, monosaccharide, and amino acids, and oxidizers of the monosaccharide into small molecular organic acids.…”

“…POMs, with various topology structures disclosed, are well‐defined metal‐oxyanions composed of oxygen bridges of light transition metals at their highest oxidation, such as vanadium, molybdenum, or tungsten. Among these different POMs, the Keggin‐type heteropolyacids (HPAs) are recognized as the most effective homogeneous catalysts for acid catalyzed reactions such as the transesterification, hydrolysis, due to their high Brønsted acidities [29] . Besides their acidities, HPAs have reversible redox properties, which enable the selective oxidation of cellulosic biomass with remarkable reusability of the light transition metals in oxygen atmosphere [30,31] .…”

Lignocellulose, Cellulose and Lignin as Renewable Alternative Fuels for Direct Biomass Fuel Cells

“…Thus research was addressed to identify an inexpensive mediator to quickly transfer the electrons released from the reduced POMs of the anode to a low‐cost oxidant such as air. Fe 3+ turned out to be an effective liquid catalyst (mediator) to transfer electrons to air, as the final electron acceptor [97] . Moreover, Fe 3+ is much cheaper than POMs, and Fe 3+ can be easily regenerated by oxidation of Fe 2+ by air at low temperatures.…”

“…Fe 3 + turned out to be an effective liquid catalyst (mediator) to transfer electrons to air, as the final electron acceptor. [97] Moreover, Fe 3 + is much cheaper than POMs, and Fe 3 + can be easily regenerated by oxidation of Fe 2 + by air at low temperatures. The cathode halfcell reaction was:…”

“…PMo12 (electron mediator at the anode side) and ferric iron (electron mediator at the cathode side) were successfully used in DBRFFCs for both power generation and sugar [97] or bioethanol [98] production. Some Fe 3 + salts, namely FeCl 3 , Fe 2 (SO 4 ) 3 and Fe(NO 3 ) 3 , were compared as cathode electron mediator in DBRFFCs at an Fe 3 + concentration of 0.8 M. [97] Using Fe(NO 3 ) 3 the highest power density (MPD = 7.29 mW cm À 2 at 80°C) was attained followed by FeCl 3 (MPD = 7.04 mW cm À 2 ) and Fe 2 (SO 4 ) 3 (MPD = 4.86 mW cm À 2 ). It has to be remarked, however, that Fe(NO 3 ) 3 is not a suitable electron mediator for discharging, because NO 3…”