苯酚类分子选择性直接脱氧的双金属催化剂构建与反应过程机理研究

Hydrodeoxygenation of lignin derived phenolics is a key step to conversion of lignin to fuels and chemicals. This reaction follows two reaction paths on industrial hydrotreating catalysts at high temperatures and high pressures, i.e., direct deoxygenation path (DDO) and prehydrogenation of the phenolic ring followed by deoxygenation path (HYD). The typical products are mixtures stemmed from deoxygenation, phenolic ring hydrogenation as well as C-C hydrogenolysis small molecules. The selectivity of this process is low while the hydrogen consumption is high. In corresponding to these weaknesses, in this project, we propose combination of a hydrogenation metal (such as Ni, Pd, Pt) and an oxophilic metal (such as Re, Mo, Fe) to form bimetallic catalyst with controllable surface structures for selective direct deoxygenation of phenolics into aromatics under mild conditions (250-300ºC, atmospheric pressure, and vapor phase). In this project, through tailoring the bimetallic catalyst composition and preparation, we investigate its fine structure, understand the reaction pathway and reaction kinetics of methylphenol on bimetallic catalyst, combine experimental and theoretical studies to investigate the adsorption structure and energy of methylphenol on bimetallic catalyst. With comparison to monometallic catalyst, we correlate the fine structure of bimetallic catalyst, methylphenol adsorption, and reaction kinetics, to reveal the reaction mechanism of phenolics selective direct deoxygenation (C-O cleavage) to aromatics on bimetallic catalyst. Through the investigation of this project, it could provide a general rule on improving direct deoxygenation while suppressing phenolic ring hydrogenation and C-C hydrogenolysis during phenolics conversion. As a result, the project has both scientific and potential application values.

木质素衍生苯酚类分子加氢脱氧是木质素转化为燃料和化学品的关键。该反应在工业加氢处理催化剂上高温高压下遵循两条路径(直接脱氧和先加氢后脱氧)，产品为脱氧、苯环加氢及C-C氢解产物的混合物，选择性低且氢耗高。针对这些不足，本项目提出结合加氢金属(如Ni，Pd，Pt)和亲氧金属(如Re，Mo，Fe)组成表面结构可控的双金属催化剂，在温和条件下(250-300ºC，常压，气相)催化酚类分子选择性直接脱氧制备芳烃。本项目拟调变双金属催化剂的组成和制备，研究其精细结构，理解甲基苯酚在双金属催化剂上的反应路径、反应动力学，实验和理论计算探究甲基苯酚在双金属催化剂上的吸附结构和吸附能。对比单金属催化剂，关联催化剂表面结构-分子吸附-反应动力学，研究双金属催化剂在酚类分子选择性直接脱氧制备芳烃中的作用机理。通过本项目研究得出提高直接脱氧而抑制苯环加氢和C-C氢解的一般规律，具有一定科学意义和应用价值。

将可再生的非粮食木质纤维素生物质转化为液体燃料和化学品，可以降低对化石能源的依赖，减少二氧化碳排放，有利于实现国家2060碳中和目标。作为生物质的重要组成部分(可高达30%)，木质素解聚后生成酚类分子，是自然界中重要的芳烃结构来源，将其选择性转化为芳烃具有重要意义。本项目系统研究了酚类分子在过渡金属催化剂上的反应路径与机理，构建了加氢金属-亲氧金属的双金属催化剂，实验与理论计算结合揭示了双金属催化剂在选择性直接脱氧中的作用机制。.理论计算表明苯酚在Pt(111)上加氢脱氧遵循部分加氢机理，造成脱氧困难的主要原因是表面吸附的苯酚的O远离表面，促进O吸附有利于直接脱氧。Ni粒径从22减到2 nm，反应TOF提高3倍，甲苯生成TOF提高6倍，而C-C氢解生成甲烷TOF减少3/4。理论计算表明C-O断裂能垒在Ni(111)和缺陷位Ni(211)分别为175.6和120.5 kJ/mol。这表明缺陷位有利于O在过渡态的吸附，进而促进C-O键断裂。.亲氧Re、Mo和W与Ni形成双金属催化剂，由于性质(亲氧性、亲碳性、加氢能力)不同，最优结构不同。NiRe形成表面合金；NiMo形成MoOx修饰的Ni核；NiW形成Ni核，Ni1W1合金和WOx。由于电子效应和几何效应，双金属催化剂增加对O的吸附而弱化苯环吸附，促进C-O直接断裂反应而抑制C-C氢解反应。理论计算表明亲氧金属有利于吸附O，促进C-O断裂，如NiRe表面能垒降低到94.6 kJ/mol。最优结构的NiRe室温可以化学吸附CO，而NiW和NiMo不能吸附CO，表明前者仍然具有较强苯环吸附能力。双金属都能提高间甲酚加氢脱氧的活性，提高直接脱氧生成甲苯的选择性，降低甲烷选择性，但程度不同。NiMo和NiW催化剂在250-350°C能够完全抑制C-C氢解和加氢反应，高选择性生成甲苯；NiRe催化剂不能完全抑制加氢(Re具有较强加氢能力)和C-C氢解反应(仍有一定苯环吸附能力)。.通过本项目研究得出的提高酚类分子直接脱氧而抑制苯环加氢和C-C氢解的一般规律，为木质素利用提供了候选催化剂，具有一定科学意义和应用价值。