分子筛催化反应中客体分子扩散机制的脉冲梯度场核磁共振研究

As a typical microporous crystalline material, zeolite has been widely applied to a large number of heterogeneous catalytic reactions. Molecular transport inside these micropores plays an important role in the catalytic performance of zeolitic catalysts. Despite its importance, only a few reports have been focused on the diffusion dynamics of guest molecules in zeolitic catalysts under reaction conditions. Pulsed field gradient NMR has proven to be a powerful tool for studying the molecular transport in porous materials. In this proposal, we will employ PFG NMR technique to study the diffusion mechanism of guest molecules in zeolitic catalysts under reaction conditions, in which methane aromatization was chosen as a probe reaction, and Mo/HZSM-5 catalyst was chosen as a research object. The methods of PFG NMR studies in zeolitic catalysis system will be established to investigate the diffusion mechanism and diffusion dynamics of guest molecules in the channels of zeolitic catalysts and to explore the diffusion properties of probe molecules at different reaction time intervals. Moreover, by combining PFG NMR results with the catalytic reaction performance, the physical and chemical properties of the catalysts, and other NMR characterization, the molecular transport properties of zeolitic catalysts in the reaction, the location of active species, the location and the blocking degree of carbon deposits in the zeolite channels as well as the removal efficiency of coke by regeneration of catalysts will be clarified in this study, which would offer important reference for developing high-performance zeolitic catalysts and for optimizing the relevant reaction process.

分子筛作为一种典型的微孔晶体材料，在多相催化反应中具有广泛、重要的应用。分子筛催化剂的催化性能与其分子传质性能密切相关，然而，目前国内外对于反应过程中分子筛催化体系的扩散动力学研究少有报道。PFG NMR技术是表征孔材料中分子扩散的一种重要实验方法，本项目以分子筛催化反应中客体分子扩散机制的PFG NMR研究为出发点，以甲烷芳构化为探针反应，以Mo/HZSM-5分子筛催化剂为研究对象，建立分子筛催化体系的PFG NMR扩散研究方法，揭示客体分子在分子筛催化剂孔道内的扩散机制和扩散动力学行为，并原位探测不同反应时间段内分子在催化剂孔道内的扩散性质，同时结合反应活性数据、催化剂的理化性质表征和其他NMR方法，阐明催化剂在反应过程中的传质性能、活性物种的落位、积碳的沉积位置和堵孔程度、以及催化剂再生除积碳效果等问题，从而为开发、设计高效分子筛催化剂和优化反应过程提供重要的实验依据和理论指导。

分子筛是一种重要的微孔多相催化剂，反应物和产物分子在分子筛孔道中的扩散性质对其催化性能具有重要影响。脉冲梯度场核磁共振（PFG NMR）通过测定探针分子的扩散系数，能够直观、真实地反映多孔材料孔道内的传质情况。在本项目中，我们以甲烷芳构化为探针反应，以Mo/HZSM-5分子筛催化剂为研究对象，通过优化材料合成方法和反应性能，利用PFG NMR技术，探索了甲烷分子在HZSM-5分子筛载体和载钼的6Mo/HZSM-5催化剂孔道的晶内自扩散，进而获得分子筛孔道内部传质性能、钼物种落位和积碳形成过程等信息。研究发现，在不同吸附量下，甲烷在6Mo/HZSM-5的自扩散都要比HZSM-5慢，说明钼物种很可能落位于分子筛孔道内，粒径很小并且高度分散，129Xe NMR的结果也支持这一观点；进一步原位探测了反应过程中分子筛的传质变化，考察了甲烷在不同反应阶段的6Mo/HZSM-5的自扩散行为，结果表明，在反应3 h后自扩散系数显著降低了两个数量级，积碳已大量生成，严重堵塞了分子筛孔道，而在反应6 h后，甲烷的扩散系数下降不明显，积碳的生成速率放缓。该工作可为设计高效分子筛催化剂和优化反应过程提供一些实验参考。