图1 利用可再生能源(风能、太阳能)的电化学过程可以将生物质转化为燃料及化学品

Fig.1 Renewable energy drives electrochemical transformation of biomass to fine chemicals and fuels

图2 作为平台分子的有机酸是从木质纤维素到生物燃料转化路径上的关键节点[34]

Fig.2 Organic acids, platform molecules derived from lignocellulose, stand at the crossroad of lignocellulose conversion route to advanced biofuels[34]

图3 糠醛 (fufural)和5-羟甲基糠醛 (5-hydroxylmethylfufural, HMF)，作为糖类衍生的代表性平台分子，可以被转化为合成可再生聚合物所需的单体；还原后也可以直接作为生物燃料[23]。糠醛也可以被转化为乙酰丙酸 (图中虚线箭头)，从而进入图2所示的转化路径

Fig.3 Fufural and 5-hydroxylmethylfufural (HMF) are representative platform molecules that can be (electrochemically) converted to monomers for renewable polymers (FA, fufuranic acid; FDCA, 2,5-furandicarboxylic acid) and biofuels (MF, 2-methylfuran; DMF, 2,5-dimethylfuran)[23]. Fufural can also be converted to levulinic acid (the dashed arrow), then undergoes another route of transformation as shown in Fig.2

图4 电解池内的电势曲线 (a), 其中CEM指阳离子交换膜，是一种用于分隔阳极和阴极电解液的典型隔膜[49]；阳极过电势 (ηa) 和阴极过电势 (ηc) 随电流的变化 (b)[50]

Fig.4 Potential profile in an electrolyzer for electrochemical conversion of biomass (a). CEM denotes cation exchange membrane, which is a representative separator between anolyte and catholyte[49]. Anodic overpotential (ηa) and cathodic overpential (ηc) as a function of cell current (b)[50]

图5 电化学反应器设计的一般性指导原则[55]

Fig.5 General guidlines for the electrochemical reactor design[55]

图6 用于木质纤维素转化的电化学反应器的设计路线

Fig.6 A fishbone design scheme of electrochemical reactors for lignocellulose conversion

图7 衣康酸电催化还原成甲基琥珀酸的反应，以及竞争性的析氢反应(a)； 以Ni为阴极时纯硫酸溶液(红色曲线)及衣康酸溶液的循环伏安曲线(黑色曲线)(b)； 以Pb为阴极时的循环伏安曲线(c)[62]

Fig.7 Electrocatalytic reduction of itaconic acid to methyl succinic acid, and competitive hydrogen evolution reaction(a); Cyclic voltagram of pure supporting electrolye (H2SO4, red curves) and itaconic acid solution (black curves) with Ni cathode(b), and with Pb cathode(c)[62]

图8 三种代表性的不同构型的电化学反应器示意图

Fig.8 3 representative types of electrochemical cells with different configurations

图9 电解液单次通过电化学反应器(a)； 电解液循环通过电化学反应器(b)(流程图中的分离装置以水力旋流器示意[58])

Fig.9 Single pass of electrolyte in electrochemical cell(a); multiple passes of electrolyte in cell, with a hydrocyclone as a representative separation unit(b)[58]

图10 乙酰丙酸在同一个电化学反应器里的还原及后续的氧化反应，最终产物是辛烷 (经授权修改自文献[40]; Copyright ? 2017, 英国皇家化学学会)

Fig.10 Consecutive reduction and oxidation of levulinic acid stream in a single electrochemical cell for the synthesis of octane

图11 用于木质素降解的板框式电化学反应器示意图(该反应器的部件都是商业化产品，其中的多孔阳极被凿出直径3 mm的孔(左下图)以便电解液能穿过阳极流动。经过流道改造的垫片(右下图)可以将电解液导入反应器中间的入口，使电解液能穿过阳极[84])

Fig.11 Schematic of a press-filter type electrochemical reactor for lignin depolymerization(The components of the reactor are all of commerical sources. The planar porous anode is drilled with holes of 3 mm diameter (lower left), allowing the electrolyte to flow through the anodes. Spacers with modified flow channels (lower right) could feed the electrolyte to the anodes in the interior of the reactor[84])

图12 用于木质素降解反应的电化学膜反应器(ECMR)示意图(a)(AEM指阴离子交换膜，NFM指纳滤膜)；反应器阳极室3D示意图(b)(其中镍阳极附近的静态混合器(static mixers)可以促进阳极电解液的混合，并促进阳极附近小分子产物通过陶瓷纳滤膜的原位分离)； ECMR里的纳滤膜原位过滤比反应之后的纳滤分离过程有更好的渗透性能(c); ECMR过程比普通的电化学降解及后续膜过滤过程有更高的小分子产物收率(d)[77] (1bar=105 Pa)

Fig.12 Flow scheme showing the electrochemical membrane reactor (ECMR) for lignin depolymerization(a)(AEM is anion exchange membrane, NFM is nanofiltration membrane); 3 D scheme of the anode compartment with static mixers right next to the Ni anodes to promote the liquid flow and in-situ product removal through the ceramic nanofiltration (NF) membrane(b); ECMR has better permeability through the NF membrane compared with post-reaction filtration of the reaction medium(c); Gel permeation chromotography shows that ECMR process has improvent in yield of small molecular components(d)[77]

图13 羟甲基糠醛转化为2,5-糠醛二酸的连续氧化电化学反应实验室装置图(2,5-糠醛二酸是一种合成生物塑料的重要单体[88])

Fig.13 Picture of the bench scale electrochemical cells for the continuous oxidation of HMF to FDCA, an important monomer for bioplastics[88]