合成气制芳烃新型纳米核壳催化剂的研制与性能研究

Production of aromatic from coal-derived syngas is becoming a highly effective pathway with the diminishment of petroleum resources.The conversion of syngas to aromatics (STA) is usually catalyzed by a bifunctional catalyst,comprising a metal active stie for CO reduction and a shape-selective zeolite acid site.The consecutive-reaction nature of the STA process demands a sequential distribution of the metal sites and zeolite acid sites. Such distribution of the two active sites can be hardly achieved on the reported bifunctional catalysts,since these catalysts are generally prepared by the impregnation or mechanically mixing method.To acquire such desired active site distribution, a novel core-shell bifunctional nanocatalyst is designed in this work.Specifically,the core of the catalyst is a nano-sized syngas-to-methanol or a syngas-to-olefin metal catalyst, while the shell consists of a zeolite which selectively transforms methanols or olefins into aromatics.To fully understand the relationship between the catalyst structure and its catalytic performances,and prepare an effective core-shell nanocatalyst for STA process,systematic investigations will be carried out to elucidate the effect of preparation on the morphology control of the core nanoparticles,the influence of the surface properties of the core on the growth of the zeolite shell,and the effect of the preparation and reduction procedures on the distribution of the active sites in both the core and the zeolite shell.The results of this work will be useful to explore routes for the synthesis of efficient core-shell nanocatalysts.

煤基合成气制芳烃具有重要的理论意义和实用价值。目前，常用于合成气制芳烃的催化剂是具有金属活性位和沸石酸性位的双功能催化剂。这些双功能催化剂通常由浸渍法或机械混合法制备，催化剂上双活性位点协同催化效率低。针对上述问题，本项目拟设计和研制一种新型的纳米核壳双功能催化剂，其中核催化剂为金属催化剂，壳为分子筛催化剂，合成气在核上生成甲醇或烯烃，再在壳上转化为芳烃。为了有效耦合核壳催化剂上的两步反应，实现合成气高效转化制芳烃，本项目拟通过多种表征和固定床反应评价，对催化剂制备和反应过程中的科学问题进行研究，具体包括：（1）核催化剂形貌尺寸的调控机制；（2）核催化剂的表面化学性质对壳分子筛生长的作用机制；（3）制备、还原条件对核、壳催化剂的活性位分布的影响规律。本项目的成功实施将为高性能纳米核壳催化剂的设计和研制提供理论和实践借鉴。

C1分子的催化转化是目前成为研究热点和焦点之一。本项目以合成气（CO+H2）加氢制低碳烯烃和芳烃、CO2加氢制二甲醚及CH3OH脱氢制甲酸甲酯等经典C1反应为探针反应，通过设计和制备具有核壳结构的纳米金属或金属-酸性分子筛双功能催化剂体系，结合原位表征、验证试验和理论计算，深入研究和明确了两个基本的科学问题：核壳催化剂中壳的“限域”作用本质，Cu0和Cu+在C1催化中的催化机制和反应机理，具体结果包括：（1）壳的“限域”作用不会影响核催化剂活性位对反应物的吸附，只是会大幅度提高了纳米核的化学和热稳定性；（2）在甲醇脱氢制甲酸甲酯反应中，甲醇在Cu0位点上吸附并断裂O-H键生成CH3O，进一步断裂C-H键生成HCHO，HCHO与 CH3O*反应生成H2COOCH3，该中间体在Cu0位点上脱氢生成HCOOCH3，整个过程的速控步骤为CH3O脱氢到HCHO。Cu+位点没有断裂O-H键和C-H键的能力，Cu0上生成的HCHO脱附扩散到Cu+位点上，会发生分解反应，生成副产物CO和CO2。本项目的成功实施，不仅为纳米核壳结构的催化剂的设计和制备提供了指导，也为C1分子催化转化提供了借鉴。