规整化多级孔沸石基催化剂对烃类大分子吸附、扩散和反应性研究

The situation of worldwide reserves of crude oils is very clear: light crude oils production is declining; heavy and extra-heavy oils production is increasing. Heavy-oil resources are difficult to extract, transport and refine. A new technology can be used in the field to economically upgrade and significantly improve the properties of heavy oil by reducing viscosity, increasing gravity and removing contaminants. Fluid catalytic cracking units without any doubt belong to the most important refinery technologies. This process converts atmospheric and vacuum residues into fractions of motor fuels, mainly gasoline. The processing difficulty is that the molecules are multifunctional, containing many active sites or structural moieties, the reactions of which are all potentially significant in the conversions. In this respect, the present approach represents a step forward in the study of the adsorption and diffusion of reactants in zeolitic catalysts, ahead of traditional studies at lower temperature and with lower-reactivity zeolites. Adsorption and diffusion parameters are calculated under reaction conditions, and intrinsic kinetic parameters are evaluated independently. The modified riser simulator is employed to study heterogeneous kinetic model，the assessment of intrinsic kinetic constants, adsorption and diffusion parameters. Without doubt, it is very necessary to design and build effective catalysts with atom-by-atom precision and convert reactants to products with molecular precision. There is still great effort to look for more active, selective and stable materials as acid components of FCC catalysts to improve their desired performance. This advance will require not only a new level of understanding of reaction mechanisms and dynamics to specify the structure and properties of the catalyst. In the present project, a novel ordered hierarchical zeolitic catalyst is designed and prepared, in which the relatively large zeolitic mesopores (or macroporoes) with weak acid sites are used to in primarily cracking of heavy oil, the mesopores with medium-strong acid sites in secondarily cracking and the micropores with stronger acid sites in selective conversion. Generally, the philosophy of the FCC-catalyst preparation is: to have weak acid centers in macroporous part of catalyst particles to ensure pre-cracking the great molecules of residue to smaller molecules which could enter into the mesopores with stronger acidity. Products of cracking in mesopores could finally enter the zeolite micropores to be cracked over the strongest zeolite acid centers into smaller molecules. We will advance the field from catalyst discovery to catalyst design. Improvements in the catalytic processes across the chemicals and petroleum industries will increase resource and energy utilization efficiencies meanwhile reduce waste and overall environmental footprints.

针对重油的分子组成和结构，以芳香烃等烃类大分子为主要研究对象，制造一种适用于重油催化裂化的规整化多级孔沸石基催化剂。采用硅烷化、分步晶化、沸石化等技术和分级制备方法，构建以微孔沸石骨架为基本结构组成、中孔沸石为派生结构、大中/大孔沸石化材料为初级裂解单元的阶梯式孔催化剂。整个催化剂活性中心均来源于沸石的微孔骨架，规整化大中孔结构可提高胶质、沥青质分子的可接近性，有效地完成大分子的初始裂化并抑制副反应的发生；中孔沸石可提高预裂化产物的可接近性和合理转化；三级孔道同时具有对不同尺寸分子的择形功能。以分子模拟为先导，结合常规表征技术，认识重油分子的结构特点和反应特性，揭示催化裂化中正碳离子中间产物形成的基本条件和对酸基性能的依赖性，以及伴生热裂解中的自由基作用。采用Riser Simulator，研究FCC条件下典型重油分子的吸附、扩散和反应性，指导催化剂的制备，以期达到提高重油利用效率的目的。

随着原油越来越重质化，纯微孔沸石催化剂对大分子的利用效率越来越低。本着利用多级沸石增大的外表面来促进重油中大分子的预裂解、预裂解产物进一步在沸石微孔孔道中较强酸中心和限域空间作用下进行深度裂解、并最终转化成烯烃-汽油-柴油等目标产物的设计思路，构想将原油在沸石中的“一步裂解”设计为“多级裂解”，解决因重油重质化问题对原有催化剂提出的挑战。本项目从制备适宜重油大分子多级裂解的高效催化剂出发，以构筑大-中-微三级孔道入手，系统研究了纳米沸石合成及自组装、纳米沸石在核沸石外表面自组装构筑具有高外表面积的核壳沸石材料；利用软、硬模板制备了多种具有微-介-大孔结构的沸石催化剂；利用PS、PMMA微球模板合成了多种等规大孔硅铝材料，探索了在沸石化过程中等规孔道的影响和控制因素。通过介孔、大孔结构的引入，单晶核纳米多晶壳沸石的构筑，壳层生长对核相沸石外表面的亲疏水性及酸性的修饰调节等多重功效解决了微孔沸石活性位对大分子可接近性、扩散的限制以及传输方向等问题，实现了在全尺度上对沸石催化剂的多级孔结构和酸性位的有效调控。选用不同结构、不同空间尺度的反应分子在制备的大-中-微孔沸石催化剂上的催化评价来研究催化性能与表面酸性位点，酸强度的关系，揭示催化剂中有效活性位点，探索多级沸石催化剂在大分子反应中应用的可能性及其优越性，为适宜于大分子多级裂解催化剂的设计和制备奠定基础。