Photocatalysis of EtOH oxidation by tetrachloroferrate
Table 4. Comparison data for the catalytic oxidation of alcohols under mild conditions
Reference
[3]
[4]
This work
Catalyst
V-doped TiO2
Ethanol
Neat
FeCl3/TEMPOa/silica
FeCl4ꢀ/Dowex 2-X8
Alcohol
1-Heptanol
Ethanol
Concentration
Solvent
1 M
1 M
—
Toluene
Toluene
Sample volume (ml)
Energy source
O2 pressure (atm)
Reaction time (h)
Yield (%)
0.002
5
1
300 W Xe lamp, 350–450 nm
Heat, 80 °C
100 W Hg lamp, > 345 nm
12
1
5
24
11
1
1
100
15
a2,2,6,6-Tetramethylpyridine N-oxide.
approximately unity.[35] In ethanol, decomposition of small to
medium concentrations of 1,1-dihydroxyethanol to acetaldehyde
should therefore be nearly complete. The equilibrium constant for
the formation of the acetal, 1,1-diethoxyethane, from acetaldehyde
in ethanol has been determined to be 34,[36] consistent with the large
ratio of acetal to acetaldehyde in our photolysates. Neither 1,1-
dihydroxyethanol nor the hemiacetal, 1-ethoxyethanol, are detected
in any of our samples.
concentration predicts that the percentage yield should de-
crease as the ethanol concentration increases, which is again
what is observed.
Conclusions
A comparison can be made with other potentially green methods
to oxidize alcohols, as is shown in Table 4. Conditions are so dissim-
ilar among the methods compared that it is impossible to establish
any of these as superior.
Dependence of Yield on Ethanol Concentration
The particular advantages of FeCl4ꢀ/Dowex include its simplicity
and low cost, the possibility of using sunlight as the sole energy
source and the ability to work at room temperature and ordinary
pressures. If sunlight is to be employed, the yield can be greatly
improved over that of the experiments reported here by focusing
the incident solar radiation on the sample volume, by stirring the
suspension of catalyst and solution and by using oxygen in place
of air. It would, of course, be highly advantageous if photooxidation
were to stop at the aldehyde, but this does not happen.
A simplified picture of the mechanism outlined in Eqs (2)-(8) is
obtained by noting that, despite the fact that it is a chain with
multiple entries, each molecule of 1-hydroxyethylhydroperoxide
formed is responsible for the production of two molecules of
acetaldehyde (or its acetal), one produced simultaneously with
the hydroperoxide, Eq. (5), and the other during the re-oxidation
of Fe2+, Eq. (8). At the risk of oversimplification, the steps that
regenerate the 1-hydroxyethyl radical, Eqs (5) and (8), may be
regarded, once the photostationary state is established, as
multiplying the rate of hydroperoxide (and thus acetaldehyde)
formation by a factor that is constrained by the rate of self-
termination of 1-hydroxyethyl radicals (forming acetaldehyde
and ethanol in the process[37]). The rate is also constrained by
the recombination of chlorine atoms with the iron(II) species, i.
e. the reverse of Eq. (2):
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FeCl3 þ Cl → FeCl4
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In other words, the rate of acetaldehyde production should be
an asymptotic function of the ethanol concentration. This
function was fitted to the yield data in Fig. 2, generating an R2
value of 0.96. Equation (10) assumes that the rate of photodisso-
ciation in Eq. (2) is constant and also assumes that the concen-
tration of the iron(II) species, FeCl3ꢀ, can be treated as a
constant. Note that while the experimental yield increases to-
wards an asymptotic limit with increasing ethanol concentra-
tion, as predicted by Eq. (10), dividing Eq. (10) by the ethanol
Appl. Organometal. Chem. 2014, 28, 874–878
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