基于碱水电解制氢的电催化氢化解聚造纸黑液中木质素磺酸盐的研究

The straw of gramineous plants and grass as the main raw materials were used in paper industry of China. About 20 million tons of paper pulp and 200 million tons of black liquor containing 10%-15% Kraft lignin were produced annually. In view of the composition character of black liquor with alkali (NaOH) and Kraft lignin, this project will study the electrocatalytic hydrogenation and depolymerization of Kraft lignin in black liquor with the three-dimensional electrode reactor. For the electrocatalytic hydrogenation, in which hydrogen atom is generated by alkaline water electrolysis, chemisorbed hydrogen on the surface of the third electrode material could react with the Kraft lignin as well as the intermediate substrate. The hydrogen atom is a powerful reducing agent, although hydrogen atom was formed on the cathodes surface by reduction of water. By the one-electron redox of phenol group, the formation of hydroquinones (C6H4(OH)2) and benzoquinones (C6H4O2) in molecular chain of lignin adsorbed to the third electrode material surface may react with the chemisorbed hydrogen via a electrocatalytic reaction mechanism. As the hydrogen evolution electrodes will be prepared by nickel-based alloys materials, and the third electrode material will be composed with complex metal oxides, therefore a group of oxidation reduction potential corresponding to the active centers of catalyst for electrocatalytic reaction can be established on the surface of alloy electrode with different valence states. It is the new motive power for electrocatalytic hydrogenation of chemical bonds with different bond energy in the lignin molecule. It also supplies the activation energy for electrocatalytic hydrogenation of some special linking groups in the lignin molecule. Finally, the lignin will be decomposed into alkyl, aromatic hydrocarbon and phenolic compounds. These findings of this project will bring new academic significance to electrolysis reactions in organic synthesis. It has a good potential application in development and utilization of Kraft lignin in pulping black liquor.

我国造纸工业主要采用禾本科作物秸秆和草本原料，每年碱法制浆生产约2000万吨纸浆和2亿吨含10%-15%木质素磺酸盐的造纸黑液。本项目针对造纸黑液含有碱和木质素的特点，提出在三维电极反应器中，以较低的析氢电位电解碱水获取氢原子并吸附在第三电极材料表面，同时碱和电极表面得失电子促进木质素分子上的酚转变为醌式结构，最终让吸附的氢原子与木质素化合物在电极表面完成电催化氢化反应和解聚反应。采用镍基合金材料制备析氢工作电极，用复合金属氧化物制备第三电极材料，建立多个电催化活性中心以及用不同价态金属合金在电极表面构建出多组氧化还原电位，以此作为木质素分子上不同键能的化学键氢化反应的新动力，从而为木质素分子中特定连接基团进行氢化反应提供所需的活化能，把木质素解聚为小分子烷烃、芳烃和酚类化合物。本项目研究成果对电解有机合成反应赋予了新的学术意义，对造纸黑液木质素的开发利用具有良好的应用前景。

项目背景：选择了造纸工业的副产品松木木质素、玉米秸秆木质素、竹木质素、稻草木质素和麦草木质素为原料，用甲醇和乙醇两种强极性溶剂分别与水混合醇水溶液对它们进行分离提纯，得到纯度高、分子量分布窄松木木质素磺酸钠，将提纯后的木质素放入不同浓度NaOH溶液中，进行电催化氧化降解。.研究内容及关键技术：分别采用了5组电极催化降解不同的木质素：1）在碱性溶液中，以镍电极为阴极电极，Pb/PbO2电极为阳极电极，电催化还原降解玉米秸秆木质素为甲苯、邻二甲苯、间二甲苯等芳烃化合物。2）在碱性溶液中，以电沉积法制备的Pb/PbO2电极作为阳极，Cu电极作为阴极，对竹木质素进行电催化降解，电催化降解竹木质素得到24种产物。3）以Ti/SnO2-Sb2O3/α-PbO2/β-PbO2电极为阳极和钛基(Ti，Ti/Fe，Ti/Cu和Ti/Cu/Sn)电极分别为阴极，在氢氧化钠溶液中电催化降解稻草木质素，得到酚类、芳烃、芳香酯、芳香酸、芳香醛和芳香酮等16种化合物。4）以电沉积法制备的Cu/Ni-Mo-Co电极为阴极，阳极极化法制备的Pb/PbO2电极为阳极，在NaOH溶液中电催化降解玉米秸秆木质素。在实验过程中，氧化反应和还原反应同时发生，得到24种降解产物。5）麦草木质素为原料，以钢合金电极为阴极，自制的Pb/PbO2电极为阳极，在碱性溶液体系中通过电化学催化反应，将木质素解聚成小分子芳烃化合物。.科学意义：实现了用碱水电解制氢且吸附氢原子(chemisorbed hydrogen)与木质素完成氢化反应，对木质素进行氢化解聚反应属于木质素加氢提质的一种新机制；用电极电位驱动在催化剂表面完成木质素的氢化反应，木质素磺酸盐在碱溶液中完全溶解，促进木质素分子上的酚转变为醌，环己二烯共轭醌式结构才能够让吸附在电极表面的醌负离子接受氢原子，完成木质素的氢化反应和解聚反应；由于木质素原料的不同以及不同电极电位的电催化能不同，木质素实现了解聚在80%以上，获得降解产品6～24种不等，根据H/C元素比大于木质素原料，约50%的产品为还原产品，创新地实现了用碱水电解制氢获取氢原子(H)对木质素磺酸盐电催化氢化反应和解聚反应。