Chemistry Reference
In-Depth Information
The condensation of ethanol with methanol via the Guerbet reaction
also leads to isobutanol as the final product, but appears to go through
n-propanol as the intermediate.
32,93,94
That is, ethanol reacts with
methanol to give n-propanol, which further reacts with methanol to yield
isobutanol. This indicates that the reactivity of propanol is higher than
that of ethanol in the Guerbet condensation with methanol.
A variety of catalysts (homogeneous and heterogeneous) have been
studied for the Guerbet reaction of higher alcohol formation, however
heterogeneous catalysts such as Ni-Raney, Ni/Kieselguhr, Cu-Raney,
copper-chromite, Ru/C, Pd/C, MgO etc. were reported to show higher
activity.
26,32,42,43,91-103
An isobutanol selectivity of W90% has been re-
ported on some heterogeneous catalysts,
93-98
which shows great potential
for this reaction. A catalyst that is able to assist in rapid hydride transfer
between adsorbed alcohol and adsorbed carbonyl is found to be essential
for high yields and selectivities towards higher alcohol formation.
32,92
Since aldehydes are reaction intermediates of Guerbet reaction, it was
found that the yield could be further improved when an aldehyde
was added to the reactant mixture and the reaction was carried out a
lower temperature than that used in the non-catalyzed Guerbet
reaction.
92
In addition to the metal dehydrogenation catalysts which catalyze
hydrogenation-dehydrogenation steps of aldehydes and alcohols, a solid
base which catalyzes the condensation steps is also employed in Guerbet
reactions. The commonly used solid base reagents include magnesium
oxide, calcium oxide, potassium carbonate and sodium carbonate.
32,103
Other bases such as MeONa, EtONa, KOH, hydrotalcites and hydroxy-
apatites are also reported.
26,43,93-98
Koda et al. reported the effect of various bases on the iridium-catalyzed
Guerbet reaction of ethanol to n-butanol.
26
EtONa showed the highest
activity compared to other bases such as t-BuOK, EtOK, KOH and
Na
2
CO
3
. For the same reaction, Tsuchida et al. found that hydroxyapatite
exhibited the highest n-butanol yield, followed by hydrotalcite, Mg(OH)
2
,
and CaF
2
.
43
A homogeneous base like MeONa has been extensively studied, and
was found to give nearly complete isobutanol selectivity.
93-95,97
However
the main drawback of using MeONa was reported to be water poisoning,
due to the progressive hydrolysis of MeONa to MeOH and inactive NaOH
by reaction with the water produced in the condensation step.
93,96
Later,
a fully heterogeneous system was adopted and the MeONa base was
replaced by hydrotalcites which were found to be unaffected by
water poisoning.
96,98
Hence there remains a great level of improvement
possible in the field of catalyst development for successful Guerbet
reactions.
3.4 Homologation or cofeeding of lower alcohols
Homologation of lower alcohols is another route to produce higher al-
cohols from syngas.
27,28,77,104,105
The general reaction for higher alcohol
formation from CO and H
2
was shown in Eq (1). Addition of methanol
and/or ethanol
to syngas mixture seems to result
in a significant
