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the latter mechanism involves a cycle of electron exchange, first between the
substrate and the metal cation and then between the latter and O
2
.
The nature of the mechanism is a function of the catalyst type and
reaction conditions used; for instance, in the case of the Veba-Chemie
process, which makes use of an Ag catalyst in the form of wire gauze or
crystals, the mechanism is a dehydrogenation combined with hydrogen
combustion. Depending on the reaction temperature (which can range from
500 to 650 1C, depending on the alcohol : air inlet ratio used and the type of
reactant used, either air or oxygen), the per pass conversion can be between
30 and 70%, with a selectivity between 90 and 99%. The main by-product is
acetic acid, but minor amounts of formic acid, ethyl acetate, CO and CO
2
may also form. Other catalysts claimed are based on Cu oxide.
In recent years, because of the increasing interest in the development of
a more sustainable chemical industry, the transformation of bioethanol
into chemicals has been attracting considerable interest.
22,23
Indeed, it has
to be recognized that the value-adding chain based on biomass-based
ethanol is not new, since many processes for the transformation of this
platform molecule into chemicals were developed and extensively used
since the mid-twentieth century, but were later abandoned because of the
more favorable costs when the same chemicals started to be produced from
oil. Even nowadays, the economics for the production of base chemicals
from bioethanol are fairly uncertain and, in most cases, an economic
assessment would discourage the use of this molecule for the synthesis
of chemicals, at least in Europe. The case is different in regions such as
South America, India or even North America, where the margins of
profitability are greater and plants have already been started up to produce
ethylene through ethanol dehydration, the olefin being then used for
polyethylene or PVC production. Recently, Abengoa claimed a new
process for the Guerbet condensation of ethanol into 1-butanol, a fuel
additive.
24,25
Nevertheless, over the last 15 years, the scientific community has shown
increasing interest in reactions aimed at the valorization of ethanol.
26-31
Here, some important achievements in this field are outlined, with a specific
focus on gas-phase oxidations, such as oxidation to acetaldehyde (and acetic
acid), direct oxidation to ethene oxide, oxychlorination to 1,2-dichloroethane
and ammoxidation to acetonitrile.
d
n
4
r
4
n
g
|
3
.
8.3.1 Oxidation of Ethanol to Acetaldehyde and Acetic Acid:
Metallic Catalysts
Catalysts for the gas-phase oxidation of ethanol to acetaldehyde and acetic
acid include noble metals, which usually catalyze the oxidation to acetic acid
(with the exception of Au, the selectivity of which is, however, remarkably
affected by the reaction conditions used) and other transition metal oxides
(or mixed metal oxides), which are more selective towards acetaldehyde,
such as supported V oxides, Mo oxides and polyoxometallates.
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