CO2加氢制异构烷烃分子筛核壳催化剂的设计及其催化作用机制研究

Isoalkanes are ideal gasoline additives, and greenhouse gas CO2 hydrogenation to isoalkanes is certainly one of effective routes for recycling valuable carbon resource. As reported, Fe-Zn base oxides and zeolite mixed catalyst was usually used for synthesizing isoalkanes from CO2. Nevertheless, the main problems of the catalyst are, until now, that the synergistic effect of metal components and zeolite is poor, conversion of CO2 and selectivity of isoalkanes are not high as expected, and the catalytic mechanism still keeps unclear. With the aim of improving catalytic performance, a novel and multifunctional core-shell catalyst, which contains two main parts: Fe-Zn supported acidic M-SBA-15(M=Zr, Al) as a core, and metal modified M'ZSM-5(M'=H、Fe、Co、Ni、Cu、Zn and so on) zeolite as shell, is designed and prepared by a simple physical cladding or in-situ coating method in this project. It is expected that the catalyst is able to efficiently convert CO2 to isoalkanes, considering its enhanced synergistic interaction between metal components and zeolite. Wherein, the core-shell catalyst can be controllably synthesized by tuning preparation method, metal composition and optimizing crystallization conditions. Based on the systematic characterization of the catalyst, the origin of this catalyst supremacy as well as the structure-activity relationship between fundamental properties and catalytic activity are explored in detail, thereby proposing feasible catalytic mechanism of the isoalkanes synthesis over multifunctional core-shell catalyst. In brief, the project will offer a novel catalyst preparation technique, and lay a theoretical foundation for developing an efficient catalytic system and understanding the catalytic mechanism of isoalkanes synthesis from CO2 hydrogenation.

异构烷烃是理想的汽油添加组分，将温室气体CO2加氢转化为异构烷烃是碳资源循环利用的有效途径之一。CO2加氢制备异构烷烃通常将Fe-Zn基氧化物及分子筛机械混合制得复合催化剂，此时金属组分与分子筛间协同作用较差，导致CO2转化率及异构烷烃选择性低。为了提高复合催化剂的协同作用，本项目拟以酸性M-SBA-15(M=Zr、Al) 负载Fe-Zn的介孔材料为核，通过物理粘接法或原位包覆法将金属掺杂的M'ZSM-5(M'=H、Fe、Co、Ni、Cu、Zn等)分子筛均匀包覆于Fe-Zn/M-SBA-15表面，制备异构烷烃合成的核壳多功能复合催化剂。通过对制备方法、原料配比及晶化条件的调变与优化，实现核壳催化剂的可控合成；研究催化剂的物化性质与催化性能之间的构效关系，明确核壳催化剂上CO2加氢制异构烷烃的催化机制。本项目的实施，将为CO2加氢制异构烷烃高效催化剂的设计及其催化作用机理认识奠定基础。

异构烷烃(尤其是C5+异构烷烃)是理想的汽油添加组分，将温室气体CO2加氢转化为C5+异构烷烃是碳资源循环利用的有效途径之一。传统的金属氧化物及分子筛复合催化剂由于两组分间协同作用较差，导致异构烷烃选择性低及催化剂稳定性差等。本项目主要以Fe-Zn-Zr氧化物为核，HZSM-5分子筛为壳，采用物理粘接法制备出Fe-Zn-Zr@HZSM-5核壳催化剂。通过对HZSM-5硅铝比、核壳比例及两活性组分组装方式的考察及研究实现了不同酸性及壳层厚度核壳催化剂的可控合成。Fe-Zn-Zr@HZSM-5核壳催化剂具有优异的CO2加氢制汽油烃及C5+异构烷烃性能；催化剂的酸性、壳层厚度及核壳结构是影响C5+异构烷烃选择性的关键因素，恰当的酸性及壳层厚度对抑制烯烃加氢及芳构化等副反应作用显著。核壳结构不仅有利于汽油烃及C5+异构烷烃的形成，而且两组分间充分接触可进一步提高含氧化合物转化。此外，经四丙基溴化铵(TPABr)溶液高温水热处理获得的Fe-Zn-Zr-T能够显著促进含氧化合物生成，抑制有害物CO；水热处理不仅改变了Fe-Zn-Zr表面的元素组成，而且降低CO2的吸附。降低Fe-Zn-Zr中Fe元素含量，Fe-Zn-Zr-T(0.1:1:1)@HZSM-5催化剂上烃的选择性进一步提高，且反应120 h，催化剂稳定性良好，C5+烃中异构烷烃的选择性保持在93-95%之间。本项目制备的核壳催化剂有效克服了传统复合催化剂上C5+异构烷烃选择性低，芳烃选择性高及催化剂寿命短的问题，明确了核壳催化剂上影响C5+异构烷烃生成的关键因素，为CO2加氢制C5+异构烷烃反应路径的更深层次认识奠定良好基础。