2 research outputs found
Investigation of the Enhancing Effect of Solid Cocatalysts on Propene Formation in Ethene/<i>trans</i>-2-Butene Metathesis over MoO<sub><i>x</i></sub>/SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub>
The
metathesis of ethene and 2-butenes to propene over WO<sub><i>x</i></sub>/SiO<sub>2</sub> is an important industrial process
and has been intensely studied over alternative catalysts with supported
MoO<sub><i>x</i></sub> species. Such studies have, however,
not analyzed the effect of cocatalysts on propene production. Here,
we utilized CaO, Al<sub>2</sub>O<sub>3</sub>, or SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub> located upstream to MoO<sub><i>x</i></sub>/SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub> (0–100
wt % SiO<sub>2</sub>) to probe their influence on the rate of propene
formation and on stream stability. All three prebed materials did
not produce propene but significantly enhanced the metathesis activity
of supported MoO<sub><i>x</i></sub> species, with CaO having
resulted in the highest increase. The strength of the effect was established
to depend on the kind of MoO<sub><i>x</i></sub> species
and support material. From a mechanistic viewpoint, prebeds generate
a gas-phase promoter from <i>trans</i>-2-butene but not
from ethene, which further reacts with MoO<sub><i>x</i></sub>-containing catalysts and increases their activity. The promoter
is responsible for the stabilization and/or formation of catalytically
active Mo–carbene species but not for the reduction of MoO<sup>VI</sup> to MoO<sup>IV</sup>. The obtained results provide new insights
for the design of metathesis catalysts and open the possibility for
tuning their activity and on-stream stability through the operation
of prebed materials and metathesis catalysts in individual reactors
at different temperatures
Effect of Support and Promoter on Activity and Selectivity of Gold Nanoparticles in Propanol Synthesis from CO<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, and H<sub>2</sub>
Direct
propanol synthesis from CO<sub>2</sub>, H<sub>2</sub>, and
C<sub>2</sub>H<sub>4</sub> was investigated over TiO<sub>2</sub>-
and SiO<sub>2</sub>-based catalysts doped with K and possessing Au
nanoparticles (NPs). The catalysts were characterized by scanning
transmission electron microscopy and temperature-programmed reduction
of adsorbed CO<sub>2</sub>. Mechanistic aspects of CO<sub>2</sub> and
C<sub>2</sub>H<sub>4</sub> interaction with the catalysts were elucidated
by means of temporal analysis of products with microsecond time resolution.
CO<sub>2</sub>, which is activated on the support, is reduced to CO
by hydrogen surface species formed from gas-phase H<sub>2</sub> on
Au NPs. C<sub>2</sub>H<sub>4</sub> adsorption also occurs on these
sites. In comparison with TiO<sub>2</sub>-based catalysts, the promoter
in the K–Au/SiO<sub>2</sub> catalysts was found to increase
CO<sub>2</sub> conversion and propanol production, whereas Au-related
turnover frequency of C<sub>2</sub>H<sub>4</sub> hydrogenation to
C<sub>2</sub>H<sub>6</sub> decreased with rising K loading. The latter
reason was linked to the effect of the support on the ability of Au
NPs for activation of C<sub>2</sub>H<sub>4</sub> and H<sub>2</sub>. The positive effect of K on CO<sub>2</sub> conversion was explained
by partial dissolution of potassium in silica with formation of surface
potassium silicate layer thus inhibiting formation of potassium carbonate,
which binds CO<sub>2</sub> stronger and therefore hinders its reduction
to CO