Characterization of Silica-Supported Tungsten Bis- and Tris-hydrides by Advanced Solid-State NMR

Abstract

Tungsten-hydrides supported on oxide supports are unique catalysts regarding the direct transformation of ethylene to propylene, alkane metathesis, and the low-temperature hydrogenolysis of waxes to lower molecular paraffins. The number of hydrides coordinated to the tungsten center and their structure on the siliceous support with very high surface silica (KCC-1) is unknown. KCC-1(700) silica of extremely high surface area allows for a high tungsten metal loading of 14 wt %. We show here the full characterization of supported tungsten bis- and tris-hydrides, which, after reaction with N2O gas, yield well-defined tungsten bis- and tris-hydroxide species on KCC-1(700). The obtained tungsten-hydroxide species are perfectly suitable for a detailed NMR study. The obtained tungsten hydroxo complexes are proven to be a tungsten bis-hydroxo and tungsten tris-hydroxo species. This analysis supports the conclusion that supported tungsten-hydride complexes coexist on the support as bis-hydride and tris-hydride species. They are, respectively, in close proximity to the silicon bis-hydride and the silicon mono-hydride. This proximity is explained by the mechanism of the formation of tungsten-hydride on the silica surface.