镍基催化剂上木质素磺酸钠催化转化制备酚类化合物的研究

Catalytic conversion of lignin to phenols remains one of the heated but challenging task in biomass conversion. Lignosulfonate constitutes basic lignin structure but is more soluble than lignin in water, which thus enables lignosulfoante a suitable candidate for phenol production via a catalytic process. However, due to its high sulfur content and complex structure, it is difficult for utilizing common catalysts for this conversion. Previously we reported the first process of catalytic hydrogenolysis of lignosulfonate to phenols over Ni-based catalysts. However, it is still unclear of catalytic mechanism and reaction details. Herein, we propose to achieve the process of lignosulfonate converting to phenols over nickel-based catalyst using alcohols as solvent and hydrogen source under mild conditions. We will specifically focus on the modification of activated carbon support to achieve ideal hybrid atom content. We will study the carbothermal reduction method to achieve suitable particle size, reduction degree, stability and charge environmental in the catalyst preparation. We will synthesize a series of model compounds with typical lignosulfonate structure and investigate their conversion. This could provide the detailed mechanism of desulfurization and C-O bond cleavage over nickel- based catalyst, establishing the relationship between the structure and activity of catalyst. We will further establish the methods of in situ characterization of nickel phase evolution and the methods of analyzing complex product mixture. We believe this study will pave a new avenue for lignin conversion to value-added products.

木质素催化转化制备酚类化合物是目前生物质转化的热点和难点之一。木质素磺酸钠具有来源稳定、溶解性较好等特点，适合用作原料制备酚类化合物。但是木质素磺酸钠较高的含硫量和复杂的结构增加了其转化难度，研究报道很少。基于前期的研究基础，本项目以醇类试剂为溶剂和氢源，在温和条件下，采用镍基催化剂，催化转化木质素磺酸钠制备酚类化合物。主要研究活性炭载体杂原子修饰处理方法；研究碳热还原方法，对催化剂颗粒大小、还原程度、稳定性以及电荷环境的影响，开发出适合木质素磺酸钠转化的高效催化体系；研究合成具有木质素磺酸钠典型结构的模型化合物，研究木质素磺酸钠及其模型化合物在镍基催化剂上发生碳硫键和醚氧键断裂的机制，阐明催化剂结构与性能之间的关系；建立原位表征镍活性相的变化和复杂产物分析的方法，可为木质素的高值化利用提供重要的技术参考。

围绕降低目标化学键尤其是醚氧键的键能展开实验，研究了高效转化木质素到芳香化学品的策略，取得了一些创新性的研究成果：提出了促进木质素解聚同时抑制再聚的氧化-加氢策略以及催化加氢断裂β-O-4链接结构的脱羟-加氢策略。针对氧化-加氢转化策略，第一步采用O2/NaNO2/DDQ/NHPI催化体系，氧化木质素β-O-4结构中的Cα–OH到Cα=O，降低了Cβ–OPh键的键能。第二步采用具有温和加氢能力的NiMo硫化物作为催化剂，实现了对预氧化模型化合物中Cβ–OPh键的加氢裂解，转化率高达98%。该策略可用于桦木粉转化，单体酚类产物的收率可达30%。对NiMo硫化物催化转化木质素模型化合物的进一步研究，发现该催化剂可以实现β-O-4-A的Cβ–OPh键的一步加氢断裂。β-O-4-A首先在催化剂表面的酸中心上活化，生成碳正离子（PhCHδ+CH2OPh），该碳正离后被还原为苄基自由基物种（PhCH•CH2OPh）。该自由基物种中Cβ–OPh的键解离能远低于β-O-4-A分子中Cβ–OPh的键解离能，使得Cβ–OPh键更易活化断裂。断裂产生的中间体被加氢，生成酚类、醚类及芳香烃类产物。在β-O-4-A直接加氢转化过程中，NiMo硫化物催化剂的酸中心、氧化还原中心与加氢中心之间的协同催化起到了关键的作用。氧化-加氢两步策略以及脱羟-加氢一步策略为木质素的高效解聚提供了新的思路，为木质素转化的催化剂的筛选提供借鉴。