FeMn@HZSM-5核壳结构催化剂的设计及其对合成气直接制低碳烯烃的调控机制研究

Light olefins are basic building blocks that are widely used in the chemical industry. It is of great strategic significance to develop the technology for direct synthesis of light olefins from syngas. The Fischer-Tropsch to olefins (FTO) process is a prospect way of producing light olefins directly from syngas. But the hydrocarbons obtained with the FTO process typically follow the so-called Anderson-Schulz-Flory distribution, which result in wide product distribution and low light olefins selectivity. Accordance with the existing problems, we design a new concept of FeMn@HZSM-5 core-shell catalyst. This catalyst consists of a FeMn core catalyst and a HZSM-5 zeolite shell. The confined reaction space afforded by zeolite shell will facilitate the conversion of syngas to light olefins, depressing heavy hydrocarbons formation, as well as enhancing the light olefins selectivity. The influences of core-shell, the thickness and acidity of HZSM-5 zeolite shell on the light olefins selectivity will be investigated. The adsorption and desorption behavior of reactants, products and reaction intermediates and their conversion mechanism will be characterized by operando spectroscopy. The relationship between the structure and activity of the core-shell catalysts will be revealed. This project plans to clarify the regulation mechanism that how controls distribution of product over the core-shell catalysts. It will provide theoretical basis for the design and preparation of FTO catalysts with high efficiency.

低碳烯烃是重要的化工原料，开发合成气经费托合成直接制低碳烯烃技术具有重要的战略意义。然而费托合成制烯烃的产物分布服从ASF分布，存在产物分布较宽、低碳烯烃选择性低的问题。针对这一问题，本课题提出了构建FeMn@HZSM-5核壳催化剂的设想，该催化剂的内核为FeMn费托合成催化剂，在其表面包裹一层HZSM-5分子筛膜，形成核壳结构。在合成气直接制低碳烯烃反应中，这种结构使得内核表面生成的长链烷烃在分子筛壳层的空间限域效应和酸性作用下裂解为低碳烯烃和烷烃，抑制长链烷烃的产生，促进低碳烯烃的高效直接合成。研究核壳结构合成条件、分子筛膜厚度以及酸性对低碳烯烃选择性的影响规律，对反应物、产物和反应中间体的吸脱附行为及其转化机理进行原位、动态的表征，揭示催化剂的构效关系。本课题将阐明核壳催化剂对低碳烯烃的选择性调控机制，为高效FTO催化剂的设计和研制提供理论依据。

低碳烯烃是重要的基本有机化工原料，其传统生产技术依赖于石油资源，而我国石油资源严重短缺，制约了其发展。因此，开发合成气经费托合成直接制低碳烯烃技术具有重要的战略意义。本项目采用在FeMn费托合成催化剂表面包裹一层HZSM-5分子筛膜的方法制备了具有核壳结构的FeMn@HZSM-5核壳催化剂。该核壳结构催化剂与FeMn催化剂相比具有较高的低碳烯烃选择性。通过调变核与壳两组分的比例、选用不同硅铝比的HZSM-5分子筛实现了不同的壳层厚度和酸性的核壳催化剂可控制备，对比反应结果，随着壳层的增加，低碳烯烃选择性逐渐增加。降低分子筛硅铝比可以增加壳层的酸性，提高分子筛壳层的裂解作用，可以将长链烷烃裂解为低碳烯烃，从而提高了低碳烯烃的选择性。研究了催化剂的核壳结构对合成气合成低碳烯烃的产物分布的调控机制。FeMn@HZSM-5核壳催化剂性能的提高可归因于其特殊的核壳结构以及封闭的反应环境。这种特殊的核壳结构阻碍了反应生成的水向核内的扩散，有效抑制了水煤气反应，降低了二氧化碳的生成。另外，核壳催化剂的壳层分子筛的对内核表面生成的长链烷烃的具有裂解作用，从而进一步提高了低碳烯烃的选择性。通过本项目的研究，对设计和研制高效定向合成低碳烯烃催化剂具有重要的指导意义。