Intrazeolite anchoring of ruthenium carbonyl clusters: Synthesis, characterization, and their catalytic performances

This paper focuses attention on the intrazeolite anchoring of ruthenium carbonyl clusters and their catalytic performances. The synthesis involves the adsorption of metal carbonyl species or metal ion exchange into zeolite cages followed by reductive carbonylation under a mixed CO and H-2 atmosphere. The characterization of the structure and properties of these samples was based on a multianalytical approach, including FT-IR, UV-vis, EXAFS spectroscopies, CO/H-2 gas chemisorption, and (CO)-C-13 isotopic exchange. Methane homologation was carried out on the zeolitic ruthenium clusters using a two-step process. The research encompassed several key points as follows. (i) [Ru-3(CO)(12)] guests in Na56Y zeolite were thermally activated in a hydrogen atmosphere, generating intrazeolitic [H4Ru4(CO)(12)]. (ii) Hexammineruthenium(III) complexes in Na56X zeolite were thermally activated progressively in a mixed CO and H-2 atmosphere. The generation process was considered to occur through conversion of the intermediate [Ru(NH3)(5)(CO)](2+) and Ru-I(CO)(3) to [Ru-6(CO)(18)](2-). (iii) Internal and external confinements of ruthenium carbonyl clusters were compared. (iv) A rapid (CO)-C-13/(CO)-C-12 isotopic exchange was found to reversibly occur for [Ru-6(CO)(18)](2-)/Na56X under H-2 coexistence. (v) Oxidation fragmentation under an O-2 atmosphere and reductive regeneration under a mixed CO and H-2 atmosphere were found to reversibly occur for the intrazeolite anchoring of [Ru-6(CO)(18)](2-). (vi) Comparison of the orbitally degenerate ground state for free Ru carbonyl clusters versus the intrazeolitic anchoring site provides a theoretical indication of the symmetry distortion. (vii) Surface carbonaceous fragments were generated by CH4 dissociation on the activated ruthenium clusters based on the analysis of TPD, FT-IR, and mass spectroscopies. These fragments are the precursors for hydrocarbon formation. (viii) The catalytic properties of the intrazeolitic ruthenium clusters showed cluster size dependence. Basic Na56X is superior to Na56Y in enchancing methane conversion and C2+ hydrocarbons selectivity