高分散亚纳米Pt金属簇的构筑及丙烷催化脱氢反应性能研究

Catalytic dehydrogenation of propane is an important industrial route of producing propylene, which is filling the gap of propylene from oil resources. Al2O3-supported platinum catalyst exhibit the superior catalytic performance in propane dehydrogenation. However, carbon deposition leads to the rapid deactivation of catalyst, and thus needs the frequent regeneration of catalyst. In propane dehydrogenation based on supported platinum catalyst, the reaction intermediates are apt to multiple-site adsorb on the flat surface of platinum nanoparticle, which causes the deep dehydrogenation, dimerization, aromatization, and subsequently carbon deposition. In dehydrogenation process, these sites also catalyze the hydrogenolysis and cracking reaction of propane, accelerating the formation of coke deposits. In this project, we aim to prepare the highly-dispersed platinum cluster with subnanometre size and metal character for minimizing the flat site of platinum, and as a sequence, to a large extent prohibit the generation of coke deposits and improve the catalytic performance in propane dehydrogenation. Initially, we will prepare the alumina supported platinum cluster with subnanometre size and metal character by tuning the bonding of hydroxyl group and Lewis acidic-basic pair on the surface of Al2O3 and the binary platinum-tin cluster. Similarly, boron nitride and metal nitride supported platinum clusters with subnanometre size and metal character will be prepared using the monodisperse platinum cluster. Such well-defined catalysts are used to study the influence of size and construction of platinum cluster on the dehydrogenation reaction and coking behavior at a same scale as the kinetic diameters of reactants, identifying the curial factors of further improving the catalytic performance. In conjunction with the kinetic study for dehydrogenation, hydrogenolysis, cracking, and other side reactions involving in coking formation, we will set up the real correlation between both geometric and electronic structure of platinum and the elementary reaction steps about dehydrogenation and coking, revealing the acting mechanism of platinum site in the propane dehydrogenation and the coking process. This study may provide scientific basis for the design and development of novel Pt-based catalysts with superior catalytic performance.

丙烷催化脱氢是工业上增产丙烯的重要途径，可有效缓解我国丙烯原料对石油资源的过度依赖。氧化铝负载的Pt系催化剂活性高，但工况条件下易积炭失活，需频繁再生。Pt催化剂上引发和加速积炭过程的副反应多发生在配位饱和的Pt纳米晶平台位。因此，本项目提出构建与反应物分子动力学直径处于同一量级的亚纳米Pt金属簇，尽可能消除积炭反应发生的位点，提升催化效率。从单分散Pt金属簇出发，选择氧化铝、氮化硼、金属氮化物作为载体，调配其与载体表面的相互作用机制，构筑高分散、亚纳米Pt金属簇催化剂；亚纳米尺度研究Pt金属簇尺寸、构型对脱氢、积炭反应的调变，建立Pt金属簇几何结构与反应行为之间的本征关联，明确抑制积炭、提升催化效率的关键；利用三类载体对Pt金属簇化学态的调控效应，对比研究获得Pt电子结构与脱氢、积炭行为之间的关联规律，揭示Pt中心调控反应行为的电子效应机制，为进一步设计高效Pt脱氢催化剂提供理论基础。

负载型Pt催化剂上丙烷脱氢是工业上增产丙烯的重要途径，但是Pt催化剂配位饱和平台位引发的积炭失活问题是该体系面临的科学难题。本项目从构建亚纳米Pt金属簇、消除积炭反应位的角度出发，开发出了三类丙烷脱氢活性和反应稳定性兼具的高效Pt基脱氢催化剂。基于SnCl2对Pt-Cl键的插入反应，成功制备了基本结构单元为[NH(CH2CH3)3]3[Pt(SnCl3)5]的双金属单分子簇单晶，以其为前驱制备的Pt@Sn/Al2O3催化剂在临氢条件下的丙烷脱氢反应中表现出远超传统氯化物前驱制备催化剂的活性、稳定性和可再生性能。将此前驱进一步嫁接到Beta分子筛羟基窝制备的K-Pt@Sn/Beta催化剂，在低温、非临氢条件下丙烷脱氢反应获得了接近平衡转化率的丙烷转化活性和高丙烯选择性。球差电镜和同步辐射等结构表征证实活性组分大部分以粒径0.8-1.0nm左右的亚纳米团簇形式存在，Pt近邻配位元素主要为Sn。这些研究结果很好地践行了原子精度构建催化剂、分子尺度调控催化反应的研究范式。以边缘富含位错缺陷的h-BN纳米片为载体成功实现了Pt/Cu纳米簇的高度分散，在丙烷脱氢反应中表现出了优越的反应性和催化效率。例如，仅含有100 ppm Pt、520度时，该催化剂就展现出了与现有文献所报道的1000 ppm Pt系催化剂相当的反应性能。进一步开发出多孔硅表面涂敷湿化学改性方法，在显著提升Pt/Cu催化剂丙烷转化活性和丙烯选择性的同时，将Pt/Cu催化剂体系的使用温区从520度拓展到了工况条件下的600度，为推动Pt/Cu合金簇催化剂走向实用提供了解决方案。选用纳米氮化铝（n-AlN）为载体，优选Zn助剂，利用自刻蚀效应实现了Pt物种的高分散和稳定固载，制备的Pt-Zn纳米簇催化剂展示出了优异的丙烷脱氢反应性能，揭示了Zn助剂在n-AlN负载Pt催化体系中独特的促进效应。采用尿素辅助球磨氮气热解法制得了高比表面积MoVZrCrNbNx熵增氮化物，揭示了其独特丙烷脱氢反应活性和选择性，在常压、590°C、4.7 gC3H8·gcat-1·h-1、N2稀释的反应条件下，丙烷转化率可达12.6%，丙烯选择性可达96.8%，无明显失活发生。动力学数据证实丙烷在该催化剂上的反应级数和活化能均与铂催化剂相当，其催化活性源于金属氮化物类贵金属特性。