三元多级孔结构对钴基费托合成选择性调控的研究

It has been the main problem for Fischer-Tropsch synthesis to improve the low selectivity of product. The project intends to utilize the regular pore channel of SiO2 support, the moderate interaction between ZrO2 and active species, the high hydrophobic property of carbon materials; and fully take advantage of the high hydrothermal stability of micropores, good diffusivity and selectivity of mesopores and macropores. The ternary hierarchical porous support, which used SiO2 as micropore,ZrO2 as mesopore and carbon as macropore, is firstly constructed through the method of casting and self-assembly of soft-hard template, along with the transfer crystallization process. Then the cobalt species are introduced and confined into the above support by the ultrasonic-assistant vacuum impregnation method. And by using the Fischer-Tropsch synthesis as probe-reaction, we systematically study the preparative chemistry of ternary hierarchical porous catalysts; the synergistic effect of multielement support; the interaction between support and active species; the influence of pore size and pore distribution on the selectivity of product and the adsorbtion-desorbtion diffusion behavior of reactant and product. By studying the relationship between texture of ternary hierarchical porous catalysts and product selectivity, the project aims to develop the basic idea of new type of structured catalyst design and reveal the basic discipline to regulate and control the selectivity of product for Fischer-Tropsch synthesis. This study would establish the foundation of theory and technology with which the project would obtain a category of structural catalyst with high efficiency and controllable selectivity for the Fischer-Tropsch synthesis.

费托合成产物选择性可调控性差是目前面临的难题。本项目拟利用SiO2载体的规整孔道结构、ZrO2与活性物种温和的相互作用以及炭载体高疏水性能特点，并且充分利用微孔结构水热稳定性高，介孔和大孔扩散性和选择性好的优势，将三种基元物质优势互补地通过软-硬模板进行浇铸自组装并加以转晶的方式构建"微孔SiO2-介孔ZrO2-大孔炭"结构的三元多级孔载体。结合超声辅助真空浸渍法将钴物种限域于该三元多级孔载体中，以费托合成为探针反应，系统研究三元多级孔催化剂的构建,考察多元载体协同效应及与活性物种间的相互作用、孔结构组成对产物选择性及反应物和产物吸脱附扩散行为的影响。本项目旨在通过对三元多级孔催化剂织构与产物选择性之间的关系研究，拓展新型结构催化剂设计的基本思路及揭示费托合成产物调控的基本规律，此研究将为获得具有高效可控选择性的费托合成结构催化剂奠定理论和技术基础。

1）制备了一系列第一个介孔孔径相同，第二个孔径逐渐增大的双介孔结构催化剂。双介孔结构对催化剂的结构、还原度以及费托合成性能有极大的影响。双介孔结构催化剂，第二个孔孔径越大，活性越高，甲烷选择性越低，同时，重质烃选择性也较低。具有2.5 nm和22 nm的双介孔结构催化剂，显示出较高的C5-18选择性。.2）我们通过考察不同颗粒尺寸的催化剂，发现在内扩散限制存在的条件下，大孔对改善内扩散限制有很好作用，大幅度改善了催化剂的费托反应性能。大孔存在条件下，大孔对于活性组分的分散也起到了很好的作用。.3）采用等体积浸渍法制备了双介孔催化剂，考察了不同还原温度对催化剂结构和费托合成性能的影响。经不同温度还原处理后的催化剂钴物种主要以单质钴形式存在，且主要以面心立方钴晶型出现。随着还原温度的提高，催化剂球形形貌被部分破坏，但是基本上较好地保持了活性物种的分散性，钴物种与载体间的相互作用增强，较难还原。随着还原温度的提高，催化剂活性位增加，活性增加，但增加到一定程度后活性降低，而甲烷选择性随着还原温度的提高逐渐增加。