Thermal Activation of Molecular Tungsten Halide Clusters with the Retention of an Octahedral Metal Framework and the Catalytic Dehydration of Alcohols to Olefins as a Solid Acid Catalyst

Abstract

When the molecular tungsten halide cluster (H3O)2[(W6Cl8)Cl6]·6H2O, with an octahedral metal framework, is heated to 50 and 150 °C in flowing helium gas, it changes into (H3O)2[(W6Cl8)Cl6] and [(W6Cl8)Cl4(H2O)2], respectively. Activation at 250 °C yields a poorly crystallized solid state cluster, [W6Cl8]Cl2Cl4/2, which exhibits catalytic activity for the dehydration of ethanol to yield ethylene and a small amount of ethyl ether and acetal. The activity is attributed to the Bronsted acidity of the hydroxo ligand that is produced by elimination of hydrogen chloride from the chloro and aqua ligands. The catalytic activity increases with increasing temperature, and reaches a maximum at 300 °C. The catalytic activity then disappears above 350 °C, at which temperature the crystallinity of the cluster improves and the active sites are included in the crystal. In the case of primary alcohols, the reactivity decreases with increasing length of the carbon chain, and secondary alcohols are more reactive than the corresponding primary alcohols. Halide clusters of niobium, molybdenum, and tantalum having the same metal framework are also active catalysts for these reactions.