Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.
J Am Chem Soc. 2010 Jul 7;132(26):9129-37. doi: 10.1021/ja102778e.
We report here a general synthetic strategy to encapsulate metal clusters within zeolites during their hydrothermal crystallization. Precursors to metal clusters are stabilized against their premature colloidal precipitation as hydroxides during zeolite crystallization using bifunctional (3-mercaptopropyl)trimethoxysilane ligands. Mercapto (-SH) groups in these ligands interact with cationic metal centers, while alkoxysilane moieties form covalent Si-O-Si or Si-O-Al linkages that promote zeolite nucleation around ligated metal precursors. These protocols led to the successful encapsulation of Pt, Pd, Ir, Rh, and Ag clusters within the NaA zeolite, for which small channel apertures (0.41 nm) preclude postsynthesis deposition of metal clusters. Sequential treatments in O(2) and H(2) formed small (approximately 1 nm) clusters with uniform diameter. Titration of exposed atoms with H(2) or O(2) gave metal dispersions that agree well with mean cluster sizes measured from electron microscopy and X-ray absorption spectroscopy, consistent with accessible cluster surfaces free of mercaptosilane residues. NaA micropore apertures restrict access to encapsulated clusters by reactants based on their molecular size. The ratio of the rates of hydrogenation of ethene and isobutene is much higher on clusters encapsulated within NaA than those dispersed on SiO(2), as also found for the relative rates of methanol and isobutanol oxidation. These data confirm the high encapsulation selectivity achieved by these synthetic protocols and the ability of NaA micropores to sieve reactants based on molecular size. Containment within small micropores also protects clusters against thermal sintering and prevents poisoning of active sites by organosulfur species, thus allowing alkene hydrogenation to persist even in the presence of thiophene. The bifunctional nature and remarkable specificity of the mercapto and alkoxysilane functions for metal and zeolite precursors, respectively, render these protocols extendable to diverse metal-zeolite systems useful as shape-selective catalysts in demanding chemical environments.
我们在此报告一种在沸石水热结晶过程中封装金属团簇的通用合成策略。使用双功能（3-巯丙基）三甲氧基硅烷配体稳定金属团簇的前体，防止它们在沸石结晶过程中过早地胶态沉淀为氢氧化物。这些配体中的巯基（-SH）基团与阳离子金属中心相互作用，而烷氧基硅烷部分形成共价 Si-O-Si 或 Si-O-Al 键，促进连接的金属前体周围沸石成核。这些方案成功地将 Pt、Pd、Ir、Rh 和 Ag 团簇封装在 NaA 沸石中，由于其小的通道孔径（0.41nm），阻止了金属团簇的后合成沉积。在 O2 和 H2 中的顺序处理形成了具有均匀直径的小（约 1nm）团簇。用 H2 或 O2 滴定暴露的原子给出的金属分散度与从电子显微镜和 X 射线吸收光谱测量的平均团簇尺寸非常吻合，这与没有巯基硅烷残留物的可及团簇表面一致。NaA 微孔孔径限制了基于分子大小的反应物对封装团簇的进入。在 NaA 中封装的团簇上的乙烯和异丁烯加氢反应速率之比远高于分散在 SiO2 上的那些，甲醇和异丁醇氧化的相对速率也是如此。这些数据证实了这些合成方案实现的高封装选择性以及 NaA 微孔根据分子大小筛选反应物的能力。包含在小孔径内也可以保护团簇免受热烧结的影响，并防止活性位点被有机硫物种中毒，从而使烯烃加氢反应即使在噻吩存在下也能持续进行。双功能性质和硫醇和烷氧基硅烷功能对金属和沸石前体的显著特异性，使这些方案可扩展到多种作为苛刻化学环境中具有形状选择性的催化剂的金属-沸石体系。
