Chemistry Reference
In-Depth Information
Fig. 11 Total optimized energy change at 0 K for reaction of acrolein and hydrogen on a
Mo
3
O
9
cluster model of the MoO
3
surface, TS
=
transition state. The inset gives overall
D
r
G
for the three reactions, with (a) reactants at 0 K, (b) products at 0 K, (c) products at 323 K,
and (d) experimental values of the products at 323 K (Adapted from Moberg et al.
89
).
This proposed reaction path is similar to that over the sulphide cata-
lysts (Fig. 5). The oxygen vacancies are formed through dehydroxylation
of surface terminal sites to form water. The resulting coordinatively un-
saturated molybdenum sites selectively chemisorb acrolein, which is fa-
vorable due to the Lewis acid-base interaction between the coordinatively
unsaturated metal ion and the oxygen lone pair. After adsorption of the
reactant on the Lewis acid site, proton donation occurs. The proton is
present in the hydroxyl groups on the surface of the oxide. To enhance
the proton donating capability, hydroxyls of high Brønsted acid strength
must be present on the catalyst surface. The relative Brønsted hydroxyl
acidity of different oxides decreases in the order:
89
WO
3
WMoO
3
WV
2
O
5
WNb
2
O
5
MoO
3
and WO
3
function as catalyst, because they contain strong Lewis
and Brønsted acid sites, indicating that this kind of oxide is a potential
bio-oil hydrotreating catalyst. For example, unsupported low-surface-area
MoO
3
and MoO
2
catalysts were tested for hydrotreating of 4-methyl
phenol and the activity was compared to that of a commercial MoS
2
catalyst.
90
The catalysts are stable under the reaction conditions (598-684 K
and 2.41-4.83 MPa of H
2
), with the exception of MoO
3
, which underwent
reduction to a mixed oxide containing Mo
4
O
11
,MoO
2
, and metallic mo-
lybdenum. This partially reduced molybdenum oxide is found to have
high activity for the decomposition of 4-methylphenol because of the
formation of anionic vacancies and Brønsted surface acidity. Both MoO
2
and MoS
2
display similar conversion, selectivity, and activation energy.
Oxide nickel-tungsten catalysts supported on activated carbon were also
tested by Echeandia et al. in the hydrodeoxygenation of phenol at 423 to
