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Abstract:
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.
