重油分子催化裂化中孔Y@ZSM-5沸石催化剂的结构设计、合成及性能研究

Processing heavy feedstocks containing bulky molecules offer new challenges to FCC catalyst. To meet the demands on the catalyst for processing heavier feedstocks, the focus of this project is to design and prepare a novel Y@ZSM-5 material with core/shell structure, which have microsphere cores consisting of the aggregates of zeolites Y nanocrystals with intracrystalline mesopores and shells consisting of the zeolites ZSM-5 nanocrystals with intracrystalline mesopores. The core/shell structure of zeolites is beneficial to the diffusion of product molecules to prevent side-reaction, compared with their mixture of Y and ZSM-5 zeolite. And both the reduction of particle size of zeolite and the generation of mesopore inner zeolite crystals will circumvent the limitation imposed by the small pore sizes of zeolites on the accessibility of the active sites and diffusion to larger molecules, and make them applicable for catalysis involving large molecules. Furthermore, the detailed and in-depth studies about how to control the mesoporous structure, core/shell structure and corresponding acidity etc. of catalysts in an extensive ranges through changing the synthesis condition, to accomplish the goal that the structure and performance of catalysts may be adjusted and elected according to the composition of raw feedstocks and expected product distribution. Moreover, the adsorption, diffusion and catalytic cracking of typical larger hydrocarbon molecules on various Y@ZSM-5 core/shell catalysts with different structure and properties will be investigated, and establish the relationship between the structure and composition of catalysts with their properties of adsorption, diffusion and catalytic cracking for different reaction molecules to clarify the promotion mechanism about the generation of mesopores inner zeolite crystals, reduction of particle size and core/shell structure on the accessibility of the active sites and diffusion of large molecules. Above studies and corresponding results will open a novel mode for designing and preparing the effective catalysts for catalytic cracking of heavy feedstocks, offer the basic data and academic foundation for the application of catalysts, and therefore facilitate the development of refining industry.

针对重油分子催化裂化反应对催化剂的特殊要求，本着功能性导向的设计思路，本项目欲设计和制备一种具有核壳结构的Y@ZSM-5双沸石催化剂，其中核是由具有晶内中孔的纳米Y沸石聚集而成的微球，壳由生长于微球表面的中孔ZSM-5纳米粒子组成，其目的在于通过沸石晶粒纳米化和晶内中孔双重功效解决微孔沸石活性位对大分子可接近性和扩散的限制问题，以及通过核壳化解决两沸石间的扩散问题；完成在全尺度上对沸石催化剂的中孔孔结构、核壳结构和酸性的调控，实现催化剂的结构与性能可根据反应原料组成以及对产品分布的要求进行调控；同时选择具有代表性的模型重油大分子考察其在催化剂中的扩散、吸附与裂化规律，并在构建其与催化剂孔结构和组成之间关系的基础上，揭示沸石晶内中孔、纳米化和双沸石核壳结构对反应分子的可接近性、扩散和催化裂化的促进作用机理，以此指导重油催化裂化催化剂的定向设计与制备，并为其应用提供完善的基础数据和理论依据。

本项目首次通过“蒸汽相法”实现了一种具有核壳结构的沸石催化材料Y@ZSM-5沸石的制备，其中核为具有晶内中孔的Y型沸石，其中孔结构起源于蒸汽处理过程的脱铝过程；壳层沸石由包裹生长于Y型沸石外表面的ZSM-5纳米粒子组成，纳米粒子间因晶粒的堆积形成晶粒间的“二次”介孔结构。选择正辛烷作为探针分子并考察了它在制备的核壳沸石中的扩散、吸附规律，结果表明正辛烷在多级核壳复合沸石上的扩散速率是在对应的由Y和ZSM-5组成的机械混合物上的扩散速率的80倍；研究表明通过沸石晶粒纳米化和晶内中孔双重功效可以解决微孔沸石活性位对大分子可接近性和扩散的限制问题；通过核壳化可以解决反应分子在两相沸石间的梯度扩散问题，揭示了沸石晶内中孔、纳米化和双沸石核壳结构形成对反应分子的可接近性、扩散的促进作用机理；实现了对沸石催化剂的中孔孔结构、核壳结构和酸性的初步调控。