The Enhancing Effect of Bronsted Acidity of Supported MoOx Species on their Activity and Selectivity in Ethylene/trans-2-Butene Metathesis

Supported catalysts with a nominal Mo surface density of 0.15 and 1.5 Mo atoms nm(-2) were synthesized by impregnation of alumina, silica, and alumina-silica supports with silica content between 1 and 70 wt%. They were tested for their activity and selectivity in the metathesis of ethylene and trans-2-butene to propene between 343 and 603 K at 125 kPa. The catalysts were characterized by UV/Vis, Raman, and IR spectroscopy, XRD and H-2 temperature-programmed reduction for elucidating the distribution, degree of polymerization, reducibility, and acidity of MoOx species. We established that Bronsted acidity of highly dispersed tetrahedral and polymerized octahedral MoOx species is required to ensure high metathesis activity. The acidic character of these species is influenced by their structure and support. Tetrahedral MoOx species with Bronsted acidic character are only formed on supports possessing such acidity, whereas Bronsted acidic octahedral MoOx is also created on supports without such acidic sites.