费托合成直接制低碳烯烃用疏水性胶囊催化剂的设计和调控机制研究

Light olefins are the key building blocks in chemical industry. Developing alternative routes to synthesize light olefins from non-oil-based resources, instead of the general petroleum-based process, has been paid more attentions in past decades. Although some alternative routes, such as methanol to olefins (MTO), dimethyl ether to olefins (DMTO), can realize the production of light olefins from syngas, these routes are indirect conversion ways with methanol or dimethyl ether as intermediates products. Fischer-Tropsch to olefins (FTO) is an interesting and more promising option to directly convert syngas to light olefins compared to MTO and DMTO. Iron-based FTS catalysts are prevailing for FTO. However, the faced challenges, such as carbon deposition, residual heavy hydrocarbons, unsatisfied light olefins selectivity and higher CO2 selectivity derived from water-gas-shift (WGS) reaction severely hinder the further development of iron-based catalyst. In this project, we present a new concept of hydrophobic zeolite capsule catalyst. This novel catalyst consists of an iron core catalyst and a hydrophobic zeolite shell. The confined reaction space afforded by zeolite shell will facilitate the conversion of syngas to light olefins, depressing the carbon deposition and heavy hydrocarbons formation, as well as enhancing the catalyst lifetime. On the other hand, the hydrophobic shell can effectively inhibit the diffusion of the produced H2O back to the iron core catalyst for WGS reaction, whereby to depress the production of CO2. This project plans to clarify the real mechanism and rules that how the hydrophobic zeolite capsule catalysts control the direct synthesis of light olefins from syngas and depress the formation of CO2 at the same time.

低碳烯烃是重要的基础化工原料。开发以非石油基路线合成低烯烃的工艺具有重要意义。虽然甲醇制烯烃（MTO）、二甲醚制烯烃（DMTO）等可实现合成气制低碳烯烃, 但这些路线均是间接合成，需要甲醇、二甲醚作中间产品。费托合成制烯烃（FTO）是更具有前途的直接从合成气制低碳烯烃的路线，然而铁基催化剂面临的挑战（积碳、重质产物残留、高二氧化碳选择性等）阻碍了FTO的进一步发展。本课题提出全新的具有Core-Shell结构的疏水性分子筛胶囊催化剂的概念。该催化剂包含铁基Core催化剂和疏水性Shell催化剂。分子筛Shell提供封闭反应空间和裂解功能以抑制积碳和重质烷烃生成，从而促进低碳烯烃的高效直接合成。此外，Shell的疏水性功能又会显著抑制水扩散返回到铁基催化剂上，以此来抑制二氧化碳的产生。本课题计划阐明疏水性胶囊催化剂对低碳烯烃合成的调控机制和对CO2抑制机制，揭示合成气直接制低碳烯烃的转化规律

低碳烯烃是重要的基础化工原料。由合成气直接定向合成低碳烯烃具有重要的意义。针对Fe基费托合成催化剂存在产物服从Anderson-Schulz-Flory（ASF）分布，低碳烯烃选择性低、副产物高的缺点。本课题以FeMn为核心催化剂，HZSM-5为壳层催化剂，通过物理粘结法成功制备了具有核壳结构的FeMn@HZSM-5胶囊催化剂。该胶囊催化剂在合成气直接制低碳烯烃反应中，表现出对于低碳烯烃的定向合成能力，同时表现出对产物中CO2生成的抑制作用。本课题执行期间详细考察了胶囊催化剂的壳层厚度、HZSM-5硅铝比对合成气直接制低碳烯烃的影响等。研究结果表明，适当的分子筛壳层厚度有利于降低CO2的生成。增大分子筛层厚度会阻碍H2O向核心扩散，从而降低CO2生成。由高Si/Al比的HZSM-5制得胶囊催化剂可以有效地抑制WGSR反应，可获得更高的烃类选择性和较低的CO2选择性。此外，本课题研究提出了胶囊催化剂定向合成低碳烯烃的反应机制，在FTO反应过程中，合成气首先进入到Core催化剂上，发生传统的费托合成反应，产物扩散通过Shell离开催化剂。依靠Shell提供的反应空间限域效应和酸点，可有效的促进低碳烯烃合成，同时裂解产物中的多余重质产物为低碳烯烃或烷烃。阐明了核壳催化剂区别于物理混合催化剂抑制WGSR反应的作用机理。通过HZSM-5构建核壳结构的胶囊催化剂可以有效地抑制水煤气反应的发生，降低二氧化碳的生成，从而定向高效合成低碳烯烃，为合成气定向催化转化研究提供了新的研究思路和科学基础。