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formation of acetaldehyde. Acidic features of the catalyst or of the support
over which the active phase is deposited are a requisite to achieve high
selectivity to the acid, since desorption into the gas phase is facilitated, thus
saving the product from consecutive oxidative degradations.
d
n
4
r
4
n
g
|
3
8.3.3 Direct Ammoxidation, Oxychlorination
and Epoxidation of Ethanol
Ethanol has also been used as a reactant for the synthesis of chemicals
which are typically produced starting from other molecules, such as
(a) acetonitrile, by means of ethanol ammoxidation, whereas nitrile is typi-
cally obtained as a by-product of propylene ammoxidation, (b) 1,2-
dichloroethane, by means of ethanol oxychlorination, whereas it is typically
produced through ethylene oxychlorination, and (c) ethylene oxide, typically
being produced by olefin epoxidation. In this perspective, the mechanism
involved in each of the above reactions can be considered sequential, since
two in-series reactions occur over a single, but bifunctional, catalyst.
Ethanol ammoxidation can be carried out with the same reactor and
process by which propylene is ammoxidized to acrylonitrile;
97,98
in fact,
acetonitrile is a by-product of the acrylonitrile process and conditions aimed
at the ammoxidation of ethanol are not much different from those necessary
for propylene ammoxidation. However, some authors have investigated the
ammoxidation of primary alcohols to nitriles using dedicated catalysts and
conditions. Table 8.2 summarizes the results reported in the literature.
Outstanding results were obtained with crystalline silicon aluminium
phosphate (SAPO) and vanadium aluminium phosphate (VAPO).
In the aerobic ammoxidation of alcohols to nitriles, the activation of the
strong N-H bond in ammonia (107 kcal mol
1
) require severe reaction
conditions. One key reaction intermediate, as reported in the literature, is
the hemiaminal compound [1-aminoethanol, CH
3
CH(OH)NH
2
, in the case of
ethanol ammoxidation], obtained by addition of ammonia to the carbonyl
moiety in acetaldehyde; the hemiaminal may be the precursor of either
acetaldehyde imine (ethanimine, via dehydration) or acetamide (via
dehydrogenation or oxidative dehydrogenation). In this regard, it is claimed
.
Table 8.2
Summary of catalysts and conditions for the gas-phase ammoxidation of
ethanol to acetonitrile.
Ethanol conversion,
acetonitrile yield (%) By-products
Catalyst
T (1C) Ethanol : H
2
O:NH
3
:O
2
Ref.
84, 82
a
V/P/Sb/
O-Al
2
O
3
400
Ethanol : water 1 : 2;
NH
3
: air 2.1 : 1
Acetaldehyde 99
100, 99
b
SAPO
350
1 : 1 : 5 : air
-
100
100, 96.5
c
VAPO
350
1 : 1 : 5 : air
Acetaldehyde 101
a
The authors hypothesize a mechanism via acetic acid and acetamide.
b
The authors hypothesize a mechanism via either aldehyde and imine or ethylamine.
c
The authors hypothesize a mechanism via aldehyde and imine.
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