Chemistry Reference
In-Depth Information
Later, Burcham et al. made a slight change to Beretta et al.'s double
bed configuration, keeping the same 3% Cs-promoted Cu/ZnO/Cr
2
O
3
catalyst in both beds, but with different temperatures.
90
The first bed was
at 598 K, similar to that of Beretta et al., but the second bed was at a
relatively lower temperature (613 K, as compared to 678 K in Beretta
et al.). This was based on the idea that the copper-based catalyst in the
second bed, instead of a copper-free catalyst, would be more active and
hence lower temperatures could be used. They found that this modified
configuration resulted in a greater isobutanol yield than achieved by
Beretta et al. Isobutanol was preferentially formed in both Beretta's and
Burcham et al.'s work; this could be due to the steric hindrance and the
lack of two a-hydrogens in isobutanol, as a-hydrogens are required
for the chain growth processes.
34
This is also consistent with the
thermodynamic analysis of Mawson et al. (Fig. 4), which shows that
the isobutanol structure was the dominant structure at
reaction
temperatures of 550-700 K.
64
3.3 Guerbet reaction (Condensation of alcohols)
The Guerbet reaction has been identified as a promising method for the
direct conversion of lower alcohols to higher alcohols (Fig. 6). In this
reaction, a primary or secondary alcohol reacts with itself or another
alcohol to produce a higher alcohol by means of solid base catalysis.
91
This reaction has been extensively studied and several types of
homogeneous and heterogeneous catalysts have been reported for the
synthesis of higher alcohols.
32,91,92
The mechanism of the Guerbet reaction is based on a cross-conden-
sation between alcohols in the presence of a bifunctional catalyst that
acts both as a base as well as a hydrogenating/dehydrogenating metal
species.
92-94
The mechanism was proposed by Veibel and Nielsen,
92
and
consisted of a chain growth process with three essential steps - de-
hydrogenation of alcohols to corresponding aldehydes, followed by the
aldol condensation of the resulting aldehydes, and then hydrogenation of
the condensation products
to higher alcohols
through different
intermediates.
Isobutanol can be obtained directly from methanol and n-propanol
mixtures via the Guerbet reaction. It is the main product due to its
high steric hindrance and the presence of only one hydrogen in the
a-position.
34
Of the three mechanistic steps described above, Carlini et al.
showed that the key to high i-BuOH selectivity from methanol and
n-propanol mixtures is the rapid reaction of aldehyde intermediates to
give the cross-condensation alcohol, thus reducing self-condensation
reactions.
94
Fig. 6 Examples of Guerbet reactions for higher alcohol synthesis.
