Mercaptosilane-Assisted Synthesis of Metal Clusters within Zeolites and Catalytic Consequences of Encapsulation

Abstract

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.

Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures

“…The negligible catalytic activities also reveal that the catalytic contribution of hydrogen spillover is not visible in these materials. Our result is consistent with earlier reports showing that Pt/NaA is not active for the hydrogenation of organic molecules, and hence there is no indication of the catalytic contribution of hydrogen spillover 6,7 . In contrast, the amorphitized samples (Pt/NaHA-n and Pt/HA-n) showed remarkably larger catalytic activities than the Pt/NaA-n samples in both hydrogenation and dehydrogenation (Fig.…”

“…Highly selective Pt encapsulation in NaA was attained by using a mercaptosilaneassisted metal encapsulation method 6 . To control the zeolite crystallite size of the Pt/NaA, varied amounts of polyethyleneglycol (PEG) were added into the zeolite synthesis gel as a crystal growth inhibitor 17 .…”

“…Pt-encapsulated NaA zeolites were prepared by crystallization of the Pt-containing aluminosilicate gel in the presence of (3-mercaptopropyl)-trimethoxysilane at 353 K 6 . In a typical synthesis, 0.…”

“…1b). Particularly, interesting examples are Pt in LTA zeolites (NaA and KA) that have very small pore apertures (0.3-0.4 nm) and hence selectively allow the diffusion of H 2 (0.29 nm) over various organic molecules [6][7][8][9][10] . Earlier catalytic studies using these materials, however, have not led to generally agreeable conclusion.…”

“…Earlier catalytic studies using these materials, however, have not led to generally agreeable conclusion. Pt/NaA was reported to show negligible hydrogenation activities for isobutylene 6 and ethylbenzene 7 , indicating inappreciable catalytic function of hydrogen spillover. It was also reported that the isobutylene hydrogenation activity of Pt/KA is negligible but significantly increases when it is mixed with NaY and H-ZSM-5 (ref.…”