以单分散包覆型多钼酸簇为前驱体构建合成气制乙醇高效负载型催化剂及其构效关系研究

Low yield has become a bottleneck on industrial applications of direct synthesis of ethanol from synthesis gas. It is currently challengeable to design the active site on atomic scale in turning the size and morphology for high reaction performance. This proposal focuses on a novel strategy to prepare alumina-supported Mo and Me-Mo catalysts with turnable size and structure of active sites by hydrothermal deposit method using Mo-based inorganic-organic surfactant-encapsulated Mo-clusters (Mo-SECs) as a superior precursor instead of conventional Mo-anions precursor. By utilizing electrostatic self-assembly between long-chain ammonium cations (such as cetyltrimethylammonium cations, dodecyltrimethylammonium cations, et al.) and Polyoxomolybdate anions (Mo-POMs), mono-dispersed Mo-SECs in water phase with a uniform size will be synthesized. Particle size and structure of Mo-SECs sol are finely turned by changing composition and preparation conditions in order to improve hydrothermal stability. Under hydrothermal conditions, mono-dispersed nano-Mo-SECs will be deposited into the pores of γ-Al2O3, which provides an effective route to prepare Mo-supported catalysts with controllable dispersion, adjustable stacking height and improved promoting effect of active phase. The influence of the relevant parameters (hydrothermal temperature, hydrothermal time and pH, etc.) in hydrothermal deposit method is systematically investigated and optimized. To understand the influence of the Mo-SECs-derived method on the promotion effect, the bimetallic Me-Mo/Al2O3 catalyst (Me=Co, Ni, Cu et al.) was prepared by the initial wet impregnation method, successive impregnation method and vacuum impregnation method using Mo-SECs-derived Mo/Al2O3 as the “secondary support”. The preparation method and conditions of bimetallic Me-Mo/Al2O3 catalyst will be are optimized. The synergistic effect between promoter and Mo will be demonstrated through transient method, in-situ FT-IR and material characterization, in order to get insight into the mechanism for the improvement of catalytic performance and the suppression of methane products. The “structure-activity” relationships will be discovered according to the relations between composition, active phase structure (including dispersion, MoS2 slab length and stacking layer number) and the performance (including CO conversion, hydrocarbon selectivity and ethanol selectivity). Thereby, a novel precursor for preparing Mo-based catalysts will be successfully developed and the roles of the Mo-SECs-derived preparation method in turning the size and morphology of supported metal sulfide nanoparticles for direct synthesis of ethanol from synthesis gas are demonstrated, shedding a light on the relational design and controllable fabrication of supported metal sulfide catalysts. We also expand the application of the novel catalyst to the production of hydrogen.

本申请旨在通过多钼酸阴离子和阳离子表面活性剂的静电组装，合成具有单分散特性的表面活性剂包覆多钼酸簇（Mo-SECs）活性前体，代替传统多钼酸前体，探索在纳米尺度上调变负载型催化剂活性位结构的新方法，并研究其催化特性。通过优选表面活性剂种类和优化制备条件，实现Mo-SECs溶胶粒子颗粒尺寸、组成和结构的精细调变，提高Mo-SECs的水热稳定性；以Mo-SECs溶胶粒子为活性前体，水热沉积至Al2O3孔道中，在有机壳层保护下实现Mo分散度及活性组分-载体间作用强度的调控，利用“网阱”效应避免活性组分再分布，并以焙烧后的MoO3/Al2O3为“第二载体”，探索助剂的最佳引入方式和助剂效应；针对合成气制乙醇反应，研究催化剂组成、活性位结构（MoS2片晶长度、堆垛层数等）对反应活性、乙醇选择性的影响规律，探寻抑制烃类生成的途径，解析催化剂的“构-效”关系，为负载型催化剂设计和制备提供新的思路，并拓展其在能源催化领域的应用。

乙醇作为重要的化工原料和潜在的液体燃料资源，其生产方法主要有农产品发酵法和乙烯水合法，从合成气直接合成乙醇（混合醇）在经济上具有较强的竞争力，但将碳链增长控制在碳二面临巨大挑战，提高催化剂活性和选择性以提高生产过程的效率，成为制约乙醇合成工业化的技术瓶颈。本项目按照计划执行，完成设定的研究目标。主要包括：① 通过以CTAB、TEAB等表面活性剂包覆Mo-基前驱体、结合水热法实现了催化剂活性相前驱体尺寸和结构的调控，经硫化后可有效调控硫化钼片晶的尺寸和结构（层数、长度），发展了一系列合成气制备高级醇的催化剂；② 研究了酸性质对钼基催化剂的合成气转化和产物分布的影响规律，利用碱刻蚀、金属掺杂等手段强化活性中心的协同作用，进而调节了金属-载体间的相互作用，提高钼的硫化度和边缘位的暴露程度；③ 提出了合成气制高级醇的反应网络，建立了活性位结构（层数、长度）与反应性能的构效关系，提出了NiSx和 Ni-KMoSx双活性中心的协同作用效应；④ 以所提出的反应网络为出发点，研究了耦合催化剂对反应网络的调节作用，利用Zn-Cr-Al具有活化CO生成中间物种（CH3O*）的能力，通过耦合MoS2基催化剂实现了中间物种的有效传递，实现了CO转化率和C2+含氧化合物选择性的提升。⑤ 项目共发表标注论文30篇、申请相关专利1件；参加了254th American Chemical Society National Meeting (Washington)、第十八届全国催化学术会议（天津）、第十一届全国环境催化与环境科学会议（沈阳）、第十届全国催化剂制备科学与技术研讨会（成都）、第十九届全国催化学术会议（重庆）等。⑥ 项目执行期间，培养研究生7名。其中4名研究生已获得学位毕业，3人获得国家奖学金，1人评为北京市优秀毕业生、2人校级优秀毕业生，1人获得2016年中国石油大学（北京）优秀硕士毕业论文。