Chemistry Reference
In-Depth Information
Recently, a two-bed configuration was suggested for ethanol conversion to
1,2-dichloroethane: the first bed consists of a ZSM5 zeolite and the second
bed of a conventional oxychlorination catalyst made of Na-doped CuCl
2
-g-
Al
2
O
3
catalyst.
105
Under the optimized reaction conditions, at 300 1C and
with a feed containing 8% ethanol, 18% HCl and 31% air, an 82% yield of
dichloroethane was obtained. However, the mechanism proposed by
the authors was not one that would be readily envisaged; in fact, it was
demonstrated that the main product generated in the zeolitic bed was
ethyl chloride, produced by ethanol hydrochlorination, followed by the
disproportionation of two ethyl chloride molecules into ethylene and
dichloroethane. Conversely, ethanol dehydration followed by ethylene
oxychlorination played a minor role in the temperature range studied.
The direct transformation of ethanol into ethylene oxide was studied by
Lippits and Nieuwenhuys, using Al
2
O
3
-supported catalysts based on metallic
Au, Ag or Cu doped with Li
2
O.
106,107
Lithium affected the selectivity by
suppressing the formation of diethyl ether and ethylene, two compounds
formed mainly over the acidic support. A selectivity to ethylene oxide as high
as 50%, with ethanol conversion close to 80% at 300 1C, was obtained with
the Au-Li
2
O/Al
2
O
3
catalyst in the absence of oxygen; the product, however,
was observed only during the first heating cycle, because the deposition of
coke poisoned the active sites responsible for ethylene oxide formation. In
the presence of co-fed oxygen, with an equimolar content in the feed of
ethanol and O
2
, the same catalyst showed an almost 90% selectivity to
ethylene oxide at 200 1C, with 80% ethanol conversion. Higher temperatures
favored the formation of ethylene, diethyl ether and CO
2
; oxygen also
prevented the formation of coke on the catalysts. Similar behavior was
shown by Li
2
O-doped Ag NPs, supported over alumina and also by supported
Cu NPs; in this case, however, the presence of Li did not affect the catalytic
behavior. The authors did not discuss the possible mechanism for ethylene
oxide formation; however, the results shown were not in favor of a sequential
mechanism with ethylene as the reaction intermediate for epoxide for-
mation, but rather the direct transformation of ethanol into the epoxide.
d
n
4
r
4
n
g
|
3
.
8.4 Gas-Phase Oxidation and Ammoxidation
of Glycerol
The growing trend of biodiesel production shown over recent decades in
Europe has led to the availability of large volumes of glycerol, produced via
the transesterification of triglycerides, in addition to the traditional direct
hydrolysis to produce fatty acids. Since the late 1990s, this abundance of
glycerol has significantly affected the glycerin market, resulting in lower
prices, while making glycerol become a desirable, low-cost raw material for
conversion into different chemicals and biofuel additives. On the other
hand, from 2007, the glycerin market has faced very volatile conditions,
recent price increases and more recently a supply shortage of refined
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