Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

Abstract

Abstract:Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal-metal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150-250 °C, catalytically active sites develop, and the cluster framework is retained up to 350-450 °C. One of the active sites is a Brønsted acid resulting from a hydroxo ligand that is produced by the elimination of hydrogen halide from the halogen and aqua ligands. The other active site is a coordinatively unsaturated metal, which can be isoelectronic with the platinum group metals by taking two or more electrons from the halogen ligands. In the case of the rhenium chloride cluster Re 3 Cl 9 , the cluster framework is stable at least up to 300 °C under inert atmosphere; however, it is reduced to metallic rhenium at 250-300 °C under hydrogen. The activated clusters are characterized by X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure analysis, thermogravimetry-differential thermal analysis, infrared spectrometry, acid titration with Hammett indicators, and elemental analyses. OPEN ACCESSMetals 2014, 4 85 Keywords: niobium and tantalum; molybdenum and tungsten; rhenium; chloride and bromide; inorganic cluster complex; Brønsted acid; coordinatively unsaturated site; bi-functional catalyst

Catalytic Reactions over Halide Cluster Complexes of Group 5–7 Metals

“…Under these conditions, Re 3 Cl 9 is reduced to metallic Re by hydrogen [1]. The same degradation has been reported over Pt, Ni, and Rh metal on SiO 2 and Al 2 O 3 under hydrogen streams.…”

“…These similar reactivities of Mo atom in the halide cluster (H 3 O) 2 [(Mo 6 Cl 8 )Cl 6 ]· 6H 2 O to those of the platinum metals can be interpreted in terms of the isoelectronic status of the Mo atom in the cluster and the platinum metals [1].…”

“…Under a hydrogen stream, the clusters of niobium and molybdenum catalyzed the decomposition of phenyl acetate similarly. However, hydrogenolysis to benzene proceeded in around 20% selectivity, which could be attributable to the platinum-like property of the cluster catalysts [1]. Figure 10 shows the catalytic activity of crushed and screened (150-200 mesh) …”

“…The reactions proceeded over halide cluster catalysts are listed in Table 1. The retention of the cluster framework and the active sites developed on the clusters by heat treatment are described in a previous paper [1], which can be summarized as follows. When molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing a hexanuclear octahedral metal framework were treated with progressively increasing temperatures in a hydrogen or helium stream, they lost crystallization water at 100-200 °C.…”

“…Consequently, the halide clusters of Group 6 metals, Mo and W, exhibited similar catalysis to the platinum metals showing variable selectivity in helium and hydrogen streams. This catalytic behavior of Group 6 metal clusters is interpreted in terms of the isoelectronic state with the platinum group metals [1].…”