Chemistry Reference
In-Depth Information
alloy, toluene was not formed and the main products were benzaldehyde and
molecules deriving from aldehyde.
85
As alternatives to noble metals, low-cost metals and metal oxides
are currently being explored as catalysts for the reaction of alcohol
oxidation.
68,86
The use of a manganese-doped cobalt mixed oxide catalyst prepared by a
solvothermal method was proposed in the oxidation of vanillyl alcohol to
vanillin without using a base.
87
Three different types of metal oxides were
observed in the prepared catalysts, which could be identified as Co
3
O
4
,
Mn
3
O
4
and CoMn
2
O
4
. Among these, the tetragonal phase of CoMn
2
O
4
was
found to be the most active and selective for vanillyl alcohol oxidation. The
successful recycling of the catalyst was also achieved in this oxidation
reaction.
A recent study by Zhu et al.,
68
using various metal oxides supported on
activated carbon (AC), indicated that cobalt and nickel oxides are the most
promising catalysts for the aerobic oxidation of benzyl alcohol, whereas
manganese, iron and copper oxides showed lower conversion and selectivity.
The Co
3
O
4
/AC catalyst showed high activity under mild conditions in the
absence of an added base and the activity was ascribed to the synergistic
effect of Co
3
O
4
and AC. The proposed reaction mechanism indicated that
Co
3
O
4
is responsible for the alcohol dehydrogenation step, whereas carbon
provides sites for molecular oxygen activation. However, a high load of co-
balt oxide over the support was necessary to obtain significant activity and
the catalyst could be reused only after thermal treatment at high tempera-
ture (350 1C).
d
n
4
r
4
n
g
|
1
.
1.3.2 Selective Oxidation of Alcohols Derived from Biomass
The selective oxidation of oxygenate compounds derived from renewable
feedstocks has been extensively explored over the past 10 years.
65,66,88
The
use of metals, mainly Au-, Pd- and Pt-supported catalysts, for the liquid-
phase oxidation of a variety of molecules of interest in biomass conversion
(polyols, carbohydrates, glycerol and furans) has attracted a great deal of
attention from researchers. The main example of this class of reactions is
glycerol oxidation.
88-91
Glycerol is a major by-product of biodiesel synthesis
and its oxidation to high-value chemicals - such as tartronic acid, glyceric
acid and dihydroxyacetone (Scheme 1.18) - can help biodiesel economics
become more competitive.
The use of gold catalysts for the selective oxidation of glycerol has in-
creased significantly over the past decade.
92,93
Davis and co-workers dem-
onstrated that Au was more active than Pd and Pt with the use of high base
concentration, whereas the activity of bimetallic Pd/Au systems was signifi-
cantly lower than that of monometallic Au but led to greater formation of
glyceric acid.
94
It was also demonstrated that hydrogen peroxide was formed
during glycerol oxidation and its formation was associated with an increased
presence of base in solution;
95
the H
2
O
2
formed can cause carbon-carbon
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