Chemistry Reference
In-Depth Information
so-called M1 phase, active and selective in propane oxidation and ammox-
idation to acrylic acid and acrylonitrile, respectively), has a profound effect
on catalytic performance. As reported by Sobolev and Koltunov,
85,91
the
Mo-V-Nb-Te four-component catalyst proved to be very ecient in the
synthesis of acetic acid, with no need for co-fed water; an acetic acid yield
higher than 95% was observed at 250 1C, whereas at lower temperatures and
partial ethanol conversion the prevailing product was acetaldehyde. The
ethanol content in the feed could be increased up to 20% and the prod-
uctivity of acetic acid and ethyl acetate remained above 3 g g
cat
1
h
1
. The
ratio between acetic acid and ethyl acetate yields was greatly affected by the
spatial velocity used; under certain conditions, at 200 1C, similar yields to
ethyl acetate and acetic acid (
d
n
4
r
4
n
g
|
3
42% each) were found. The three-component
mixed oxide, containing Mo, V and Nb, prepared by precipitation in the
presence of colloidal TiO
2
, was studied by Li and Iglesia;
84
a yield of acetic
acid as high as 95% was achieved, where titania favored the dispersion of the
active component without negatively affecting the catalytic behavior. In this
case, water was shown to increase the selectivity by inhibiting acetaldehyde
synthesis more than its conversion to acetic acid, thus minimizing the
undesired combustion of acetaldehyde.
Other catalysts based on molybdenum oxide were studied (less recently) by
Appel and colleagues.
92-96
In SnO
2
-supported MoO
3
catalysts, the active site
was the four-coordinate isolated molybdate species, formed by a reaction
with basic hydroxyl species and probably also with Lewis acid sites. The
adsorption of ethanol to the molybdenyl unit generated an alkoxide species,
which was then selectively oxidized to acetic acid. Under the best reaction
conditions, an acetic acid yield close to 45% was achieved at 275 1C; at lower
temperatures, acetaldehyde was the prevailing product. The addition of
water to the feed gas decreased the ethanol conversion and significantly
increased the selectivity to acetic acid; in fact, water promoted the desorption
of ethanol and carboxylates, present as bridging and monodentate species,
because of active site blocking by hydroxyl groups. Doping with Ce increased
both activity and selectivity to acetic acid and acetaldehyde.
Another class of catalysts showing excellent performance in the direct
oxidation of ethanol to acetic acid is based on titania-supported V
2
O
5
.
82,83,91
For example, using 50 wt% aqueous ethanol, acetaldehyde was the prevailing
product at 175-200 1C and 2.7 bar pressure, with yields above 90%. In
contrast, acetic acid was the predominant product at low space velocity; a
selectivity above 80%, even at temperatures as low as 165 1C, could be
reached.
83
In general, however, for fixed reaction conditions, acetaldehyde
was the preferred compound at low temperatures, whereas acetic acid
formed more readily at high temperatures; a yield of the latter compound as
high as 75%, at 200 1C, was achieved even in the absence of co-fed water.
91
In conclusion, the results reported in the literature highlight that catalysts
based either on molybdenum or vanadium oxide show the greatest select-
ivity to acetic acid, in the presence of co-fed steam, especially if temperatures
exceed 200-250 1C. Conversely, lower temperatures lead to the preferential
B
.
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