Cobalt Clusters in NaY Zeolite Cages:  Synthesis and Characterization

Abstract

The adsorption of Co 2 (CO) 8 onto dehydrated NaY powder under a N 2 atmosphere and into a tetrahydrofuran slurry of NaY under a CO/H 2 atmosphere predominately yielded supported Co 4 (CO) 12 and supported Co 6 -(CO) 16 , respectively. The molecular cobalt-carbonyl clusters and their decarbonylated products have been structurally characterized by in situ infrared, extended X-ray absorption fine structure (EXAFS), and diffuse reflectance spectroscopies and NO chemisorption. The infrared spectrum of the species assigned to Co 4 -(CO) 12 /NaY and Co 6 (CO) 16 /NaY is shifted significantly from that observed for externally supported species on NaY and that observed for clusters, in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. The carbonyl activity of Co 6 (CO) 16 in isotopic exchange with 13 CO(g) is lower than that of Co 4 (CO) 12 . EXAFS coordination numbers showed that the successively decarbonylated sample maintained its small cluster size, which depended on the temperatureprogrammed oxidation of the precursor. The results of the research provide further indications of the potential of zeolite cages as microreactors for the preparation of metal clusters with controlled nuclearites. IntroductionNanostructures dispersed in the molecular-scale cages of zeolites include metal, 1,2 metal carbonyl clusters [Fe, 3 Rh, 4 Pd, 5 Os, 6 Ir, 7 and Pt 8 ], and metal oxides. 9,10 The metal-carbonyl clusters have been prepared in the zeolites cages, often by the use of "ship-in-a-bottle" synthesis technology. 4,5,8 The preparation of uniform, encapsulated nanoclusters, especially in the respect of a higher metal loading process is often difficult, as mixtures often form both inside and outside the cages. The activation of these clusters for catalytic reactions usually requires their decarbonylation to give nearly structurally uniform materials that offer the prospect of being size/shape selective catalysts. 11-13 However, during the decarbonylation, the metal often migrates out the zeolite pores to form crystallites on the external surface. 14 Preparation of metal carbonyl clusters with uniform nuclearities in zeolites has not been well demonstrated. The decarbonylated metal clusters are not yet well characterized. Chini et al. 15 demonstrated that the synthesis of Co 4 (CO) 12 and [Co 6 (CO) 15 ] 2-could be accomplished by boiling Co 2 (CO) 8 with 2-propanol in vacuum at 323 K and by boiling Co 2 (CO) 8 with ethanol in vacuum at 333 K, respectively. On the basis of the synthesis of Rh 6 (CO) 16 4 and [Pt 12 (CO) 24 ] 2-8 inside NaY cages, we infer the intrazeolite enviroment acts as a "solidsolvent". This suggests that the intrazeolite Co 4 (CO) 12 /Co 6 -(CO) 16 could be analogously formed with homogeneous preparation. The goals of the research are characterization of the cobalt carbonyl species and their decarbonylated products by infrared, extended X-ray absorption fine structure (EXAFS), and UV/vis spectroscopies and NO chemisorpt...

RETRACTED: Effect of pretreatment temperature on catalytic performance of the catalysts derived from cobalt carbonyl cluster in Fischer-Tropsch Synthesis

“…Among them, the heterogeneous catalysts prepared by metallic carbonyl cluster precursors loaded on solid supports exhibited distinct characteristics and activities in heterogeneous catalyst system [5,6]. The published studies indicated that the carbonyl precursors, such as Co 2 (CO) 8 , Co 4 (CO) 12 and Os 3 (CO) 12 [7][8][9], are excellent precursors due to their small metallic particles, which possibly emerge from the bonds between metal atoms in their zero oxidation state in these complexes. Moreover, these complexes were assumed to mimic small metallic particles covered with CO ligands [10].…”

Study on Modification of NaX Zeolites: The Cobalt (II)-Exchange Kinetics and Surface Property Changes under Thermal Treatment

“…Many ways are used to prepare the transition metal containing zeolites: (1) by ion exchange, either from aqueous solution [2][3][4][5] or by solid-state reaction [8], (2) by hydrothermal synthesis [9], and (3) by adsorption and decomposition of volatile organometallic compounds [10].…”

Diffuse Reflectance Infrared Spectroscopy Study of Co2(CO)8 Supported on Alumina

“…The traditional precursor for a supported CoMo HDS catalyst is cobalt nitrate. 2a-c Recently, however, Okamoto et al2d have obtained promising results with use of Co 2 (CO) 8 as precursor. Many studies by IR spectroscopy have been made of the interactions between Co 2 (CO) 8 and Co 4 (CO) 12 and oxidic supports. − Silica is the simplest of these supports. Since “Co 2 (CO) 8 (ads)” and Co 4 (CO) 12 (ads) on silica give similar IR spectra, it has been suggested that dicobalt dimerizes to tetracobalt species. , The surface of alumina is much more heterogeneous than silica, and the surface reactions are different; various surface species, including Co 2 (CO) 8 , Co 4 (CO) 12 , Co 6 (CO) 16 - x , and Co(CO) 4 - and also some unidentified α and β species, have been observed by IR spectroscopy. − Similar surface species have been found on magnesia, , which is more basic than alumina, and also on zeolites, although the species formed on zeolites depend on the type of zeolite and the exchanged cation. − The identification of these surface species is not always unambiguous although clear identifications are essential if we are to understand reaction mechanisms.…”