Abstract:

The HPLC and LC-MS, GC-MS and other analytical methods were used to qualitatively and quantitatively analyze the intermediates and final products of Ni/W₂C catalyzed glucose hydrotreating under different process conditions. Ni-W₂C was recently reported as a promising catalyst for the hydrogenolysis of cellulose or glucose to EG with the highest yield of 75%, yet this was achieved at the substrate concentration less than 1% because the concentrated substrate will lead to coking. This meant low productivity and high cost for commercial production, thus hinder this promising process from industry application. Therefore, insights into the reaction network to elucidate the coking mechanism and to optimize the process are needed. In this work, the mechanistic study on the glucose conversion, especially with the concentrated glucose substrate, over 2%Ni-30%W₂C/AC was systematically carried out. The reaction parameters including temperature, initial glucose concentration and H₂ pressure have been investigated and the results, in combination with the intermediate analysis by LC, LCMS and GCMS, showed that three parallel reaction pathways including retro-aldol reaction, isomerization and hydrogenation were involved in the reaction. C₂ product (EG) was originated from glucose hydrogenolysis while the C₃ products (1,2-PG and glycerol) were originated from fructose hydrogenolysis, and the dehydration of fructose led to 5-HMF and finally to coke in concentrated glucose conversion. Based on these understanding, the ratio of C₃ products to C₂, on the one hand, has been manipulated by tuning of isomerization of glucose into fructose with base additive and on the other hand, coking has been avoided by accelerating its competing reactions even at the glucose concentration of 10%（mass）. More interestingly, it was indicated that glucose of low concentration favors retro-aldol reaction while the one of high concentration tends to hydrogenate.

Key words: biomass, Ni-W₂C catalyst, hydrogenation, glycols, mechanism

摘要：

采用HPLC、LC-MS、GC-MS等分析手段对不同工艺条件下Ni/W₂C催化葡萄糖加氢转化的中间产物及终产物进行定性定量分析，研究了葡萄糖加氢转化过程的反应机制和历程。研究发现：葡萄糖加氢转化过程中同时平行发生了加氢、异构和逆向羟醛缩合（氢解）三类反应；葡萄糖发生加氢反应能够得到六元醇且其不会再进一步转化，发生逆向羟醛缩合反应主要生成乙二醇（C₂产物），发生异构反应则可生成果糖，其进行逆向羟醛缩合的产物则为1,2-丙二醇和甘油（C₃产物）；高浓度葡萄糖条件下，其异构产物果糖发生脱水反应生成的5-HMF浓度过高发生聚合，进而导致结焦。根据葡萄糖加氢转化的反应网络，提出了调控反应过程中C₂产物和C₃产物分布的策略，并通过增加催化剂用量来加快果糖脱水的竞争反应速率(加氢、氢解)，进而实现了高浓度（10%，质量分数）葡萄糖的加氢转化。此外，葡萄糖加氢转化过程中存在明显的浓度效应，反应物浓度越低，越有利于发生逆向羟醛缩合反应，反之则有利于发生加氢反应。

关键词: 生物质, Ni-W₂C催化剂, 加氢, 低碳二元醇, 反应机理