(164b) Metal and Oxide Clusters Encapsulated Within Zeolites: Synthesis, Shape Selective Properties and Protection From Organosulfur Poisons

Encapsulation of metal and metal oxide clusters within zeolites or other microporous solids can protect such clusters against sintering and prevent their contact with toxic impurities, while selectively allowing reactants to access active sites based on molecular size [1, 2]. The confinement of such clusters within small-pore and medium-pore zeolites cannot be achieved via post-synthesis exchange from aqueous or vapor media because cationic or anionic precursors together with their charge-balancing double layer or gaseous complexes cannot diffuse through the windows or channels of these zeolites [3, 4]. We have developed a general synthesis strategy for the encapsulation of noble metals (Pt, Pd, Ru and Rh) and their oxides within SOD (0.28 nm x 0.28 nm), LTA (0.41 nm x 0.41 nm) and GIS (0.45 nm x 0.31 nm) zeolites via direct hydrothermal synthesis using amine and ethylene diamine containing metal precursors. These ligands stabilize the metal precursors and prevent their precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (< 380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during the self-assembly of microporous frameworks [4]. Metal precursor stabilization during synthesis proved infeasible for ANA (0.42 nm x 0.16 nm) and MFI (0.53 nm x 0.56 nm) because of high crystallization temperatures (> 413 K). Encapsulation required, in this case, interzeolite transformations using precursor zeolites within which metal clusters had been previously encapsulated via either direct hydrothermal synthesis using ligand-stabilized metal precursors or post-synthesis exchange.  GIS and BEA zeolites containing reduced metal clusters (Pt and Ru) were converted to ANA and MFI, respectively, via local recrystallization processes while retaining metal clusters within zeolite frameworks during the transformation. The latter approach represents, to the best of our knowledge, the first example of metal clusters encapsulation within microporous solids through interzeolite transformation.  

The presence of small metal clusters was verified using X-ray diffraction, transmission electron microscopy (TEM), and chemisorption measurements. X-ray diffractograms for all metal-containing zeolites showed the features expected for the crystalline forms of the intended structures without detectable lines for metal/oxide phases.  TEM images confirmed that metal clusters were small (1.1 -1.9 nm) and uniformly distributed throughout the samples and TEM-derived size distributions agreed well with the mean cluster sizes obtained from H₂ chemisorptive titrations of exposed metal atoms (1.2-1.8 nm).

Zeolite apertures of molecular dimensions select reactants and products based on molecular size. The preferential encapsulation of metal clusters within zeolite voids was confirmed by measuring the ratio of oxidative dehydrogenation (ODH) and hydrogenation rates for small (methanol (kinetic diameter 0.37 nm), ethene (0.39 nm), toluene (0.59 nm)) and large reactants (isobutanol (0.55 nm), isobutene (0.45 nm) and 1,3,5-triisopropyl benzene (TIPB, 0.85 nm)) on unconstrained clusters dispersed on SiO₂ (Χ_SiO2 = r_{small reactant}/r_{large reactant}) and on clusters of the same metal in zeolites (Χ_zeolite). The ratio of these relative reactivities denotes the encapsulation selectivity parameter (Φ = Χ_zeolite/Χ_SiO2), indicative of the ratio of active surfaces within zeolite crystals to total exposed metal surface area. This encapsulation selectivity parameter would approach unity for clusters with unimpeded access to reactants, such as those at external zeolite surfaces or supported on SiO₂. The encapsulation selectivity (Φ) values of 8-83 for ODH of methanol and isobutanol and 7-83 for hydrogenation of ethene and isobutene for Pt, Pd, Ru and Rh clusters dispersed in LTA, GIS and ANA indicate that the active sites are predominantly present within zeolite structures. Similarly, encapsulation selectivity (Φ) values of 4-20 for hydrogenation of toluene and TIPB for Pt and Ru clusters dispersed in MFI is consistent with encapsulation of these metal clusters.

Hydrogenation of ethene with and without added thiophene as a strong titrant of surface atoms were also used to confirm the encapsulation of metal clusters in LTA, GIS and ANA. The addition of thiophene to the feed decreased ethene hydrogenation rates by a factor of 2 in Pt/LTA, 7 in Pt/ANA, 13 in Pt/GIS and 530 in Pt/SiO_2. After stopping thiophene addition, rate recovered to 0.73 of its initial rate in Pt/LTA, 0.85 in Pt/GIS and 0.70 in Pt/ANA as a result of the immediate removal of physisorbed species that inhibit ethene diffusion, but did not restore detectable rates on Pt/SiO₂. Similarly, H₂-D₂ exchange in the presence and absence of H₂S (0.36 nm) were used to confirm encapsulation of metal clusters within SOD.

Metal clusters encapsulated within LTA, GIS, ANA and MFI zeolites provide practical catalysts for the selective hydrogenation–dehydrogenation of linear hydrocarbons and oxygenates over branched or cyclic analogues as confirmed by selective oxidative dehydrogenation of methanol and isobutanol, selective hydrogenation of ethene and isobutene and toluene and TIPB. These catalysts can also effectively prevent inhibition by larger and more strongly binding species, such as arenes and organosulfur compounds, as confirmed by the protection of active sites in LTA, GIS and ANA from thiophene poisoning in ethene hydrogenation and in SOD from H₂S poisoning in H₂-D₂ exchange reaction.

1. Weisz, P.; Frilette, V.; Maatman, R.; Mower, E., J. Catal. 1962, 1 (4), 307.

2. Zhan, B.-Z.; Iglesia, E., Angew. Chem. Int. Ed. 2007, 46, 3697.

3. Choi, M.; Wu, Z.; Iglesia, E., J. Am. Chem. Soc. 2010, 132 (26), 9129. 

4. Goel, S.; Wu, Z.; Zones, S. I.; Iglesia, E., J. Am. Chem. Soc. 2012, 134 (42), 17688. 

The authors gratefully acknowledge financial support for these studies from Chevron Energy Technology Company.