Silica-alumina-supported, tungsten-based heterogeneous alkane metathesis catalyst: Is it closer to a silica- or an alumina-supported system?

Grafting of [W( C-t-Bu)(CH2-t-Bu)(3)] (1) on silica-alumina partially dehydroxylated at 500 degrees C generates a surface complex [( SiO)W( C-t-Bu)(CH2-t-Bu)(3)] (2) as evidenced by mass balance analysis. IR, and NMR spectroscopy. Upon treatment of this species under H-2, a tungsten hydride derivative, [( SiO)(EO)W(H)(x)] (3), (E=Si or Al), is formed. Both of these complexes are active as alkane metathesis catalysts. The activity of this hydride is similar to that observed for the tungsten hydride-supported on alumina and much greater than that obtained on silica. Noteworthy are the selectivities in higher alkane homologues, that is Cn+1 > Cn+2, which are fully consistent with olefin metathesis being the key homologation process.