Silica-Alumina-Supported,Tungsten-Based Heterogeneous Alkane Metathesis Catalyst: Is it Closer to a Silica- or an Alumina-Supported System?

摘要

Grafting of [W(ident to C-t-Bu)(CH_2-t-Bu)_3] (1) on silica-alumina partially dehydroxylated at 500 deg C generates a surface complex [(ident to SiO)W(ident to C-t-Bu)-(CH_2-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,[(ident to 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 C_(n+1)>C_(n+2),which are fully consistent with olefin metathesis being the key homologation process.