Zhu Kake, Hu Jianzhi, She Xiaoyan, Liu Jun, Nie Zimin, Wang Yong, Peden Charles H F, Kwak Ja Hun
Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Am Chem Soc. 2009 Jul 22;131(28):9715-21. doi: 10.1021/ja901317r.
Dispersion and quantitative characterization of supported catalysts is a grand challenge in catalytic science. In this paper, heteropoly acid H(3)PW(12)O(40) (HPA) is dispersed on mesoporous zeolite silicalite-1 derived from hydrothermal synthesis using carbon black nanoparticle templates, and the catalytic activity is studied for 1-butene isomerization. The HPAs supported on conventional zeolite and on mesoporous zeolite exhibit very different activities and thus provide good model systems to investigate the structure dependence of the catalytic properties. The HPA on mesoporous silicalite-1 shows enhanced catalytic activity for 1-butene isomerization, while HPA on conventional silicalite-1 exhibits low activity. To elucidate the structural difference, supported HPA catalysts are characterized using a variety of techniques, including (31)P magic angle spinning nuclear magnetic resonance, and are shown to contain a range of species on both mesoporous and conventional zeolites. However, contrary to studies reported in the literature, conventional NMR techniques and chemical shifts alone do not provide sufficient information to distinguish the dispersed and aggregated surface species. The dispersed phase and the nondispersed phase can only be unambiguously and quantitatively characterized using spin-lattice relaxation NMR techniques. The HPA supported on mesoporous zeolite contains a fast relaxation component related to the dispersed catalyst, giving a much higher activity, while the HPA supported on conventional zeolite has essentially only the slow relaxation component with very low activity. The results obtained from this work demonstrate that the combination of spinning sideband fitting and spin-lattice relaxation techniques can provide detailed structural information on not only the Keggin structure for HPA but also the degree of dispersion on the support.
负载型催化剂的分散及定量表征是催化科学中的一项重大挑战。本文中，杂多酸H(3)PW(12)O(40)（HPA）负载于使用炭黑纳米颗粒模板通过水热合成法制备的介孔沸石硅沸石-1上，并对其催化1-丁烯异构化的活性进行了研究。负载于传统沸石和介孔沸石上的HPA表现出截然不同的活性，从而为研究催化性能的结构依赖性提供了良好的模型体系。负载于介孔硅沸石-1上的HPA对1-丁烯异构化表现出增强的催化活性，而负载于传统硅沸石-1上的HPA活性较低。为阐明结构差异，采用多种技术对负载型HPA催化剂进行了表征，包括(31)P魔角旋转核磁共振，结果表明在介孔沸石和传统沸石上均存在一系列物种。然而，与文献报道的研究相反，仅传统的核磁共振技术和化学位移并不能提供足够的信息来区分分散和聚集的表面物种。只有使用自旋晶格弛豫核磁共振技术才能明确且定量地表征分散相和非分散相。负载于介孔沸石上的HPA含有与分散催化剂相关的快速弛豫成分，活性更高，而负载于传统沸石上的HPA基本上只有活性很低的慢速弛豫成分。这项工作所得结果表明，自旋边带拟合和自旋晶格弛豫技术的结合不仅可以提供关于HPA的Keggin结构的详细结构信息，还可以提供关于其在载体上的分散程度的信息。
