M.L. de Araújo et al.
Molecular Catalysis 500 (2021) 111307
been scarcely investigated. Some recent studies include the production
of lactic acid over a Sn-Beta supported gold catalyst [21] and the
oxidation of acetol as intermediate during the conversion of glycerol to
formic acid and acetic acid by Lewis acid sites on ZrP- supported va-
nadium [22] and metal-free silica [23] catalysts.
2.2. Determination of H2O2 concentration
The H2O2 concentration was determined by iodometric titration. The
standard solution consisted of a 0.01 M Na2S2O3 solution standardized
with I2 as primary standard. The sample was prepared by adding 1 mL of
One obstacle to the development of catalytic routes for chemical
reactions is that many metallic catalysts are based on noble metals [24];
for example, palladium, rhodium, ruthenium, and iridium, which are
expensive. To avoid the use of noble, rare, and expensive metals, various
strategies for heterogeneous and homogeneous catalysis have been
developed by using metals belonging to the first transition series. Among
these metals, iron is particularly relevant because it is one of the most
abundant metals on the Earth’s crust, not to mention that it is inex-
pensive, relatively non-toxic, and versatile, as shown by the wide range
of biological and man-made reactions mediated by iron [25–38].
Recently, we have reported that acetol oxidation by FeCl3-H2O2 in
acetonitrile, under mild conditions, yields formic and acetic acids as the
main products [20]. By using 2-butanol as model, we have also reported
that the same catalytic system works well for the oxidation of
short-chain secondary alcohols in water [39].
ethanol, 100 μL of reaction solution, 1 mL of a 0.5 M H2SO4 solution,
0.12 g of KI, and three drops of a 3% m/v (NH4)6Mo7O24 catalyst solu-
tion into a 25-mL Erlenmeyer flask. The flask was then closed and left in
the dark for 10 min to allow I2 to form. The sample was then titrated
with the Na2S2O3 solution.
3. Results and discussion
3.1. Ethanol oxidation by Fe(ClO4)3-HClO4-H2O2 system
Ethanol was oxidized by the Fe(ClO4)3-HClO4-H2O2 system (Fig. 1).
The main products were formic acid (HFO) and acetic acid (HAC). At the
beginning of the reaction, acetaldehyde also emerged at low concen-
trations, but it was rapidly consumed. Under the studied conditions, the
reaction system oxidized the substrate to full conversion within 30 min.
The maximum HFO accumulation was 0.058 M at 30 min of reaction,
which was equivalent to a turnover number (TON) of 58, whereas the
maximum HAC accumulation was 0.085 M at 1 h of reaction
(TON = 85). After these reaction times, strong over-oxidation occurred
until 2 h of reaction, when almost all the H2O2 had been consumed
(Fig. 2). After 2 h, the concentration of products slightly decreases due to
the formation of peroxide oxidation products, which slowly decompose,
inducing the oxidation of HFO and HAC.
Oxidation reactions that use the decomposition of H2O2 with a
metallic catalyst are well known [40–45], and the use of H2O2 as a green
oxidant (to generate H2O as byproduct) has been extensively studied for
the oxidation of alkanes [34,46–50], alkenes [51–55], and alcohols [37,
44,56–58]. Additionally, water is the most attractive solvent for green
oxidations [59–63]. Therefore, the use of H2O2 as oxidant combined
with low-cost and non-toxic iron as catalyst and water as solvent gen-
erates a greener oxidation system for alcohol oxidation. Moreover, the
oxidation of organic compounds catalyzed by metal complexes can be
significantly improved in the presence of additives, such as organic and
inorganic acids and compounds containing nitrogen heterocycles [33,
34,48,56,64–66].
We calculated the turnover frequency (TOF) of this reaction for the
time interval in which the conversion was close to 50 % (in this case,
2 min of reaction). The TOF was 656 and 372 hꢀ 1 for HFO and HAC,
respectively.
Here, we report the oxidation of renewable ethanol and acetol (a
potential biodiesel waste) by Fe(ClO4)3-HClO4-H2O2 in aqueous me-
dium, under mild conditions, and we examine the kinetics and mecha-
nism of these reactions, which are still poorly explored in the literature
and, in the case of acetol, have not been explored at all.
3.2. Kinetics and mechanism of ethanol oxidation reaction
To understand the reaction mechanism better, we measured the
initial reaction rate (W0) when the initial concentration of the catalyst,
substrate, and oxidant agent and the temperature were varied (Fig. 3)
(more details can be found at the Supplementary Material). From the
Arrhenius plot, we calculated the activation energy as 24.1 kcal/mol.
On the basis of the data in Fig. 1, ethanol disappeared before the
2. Material and experimental methods
2.1. Reactions and analyses
The reactions were conducted in a jacketed reactor at 60 ◦C, under
air, in water. The following concentrations were used: 0.1 M ethanol or
acetol, 0.5 M of H2O2, 0.03 M HClO4, and 0.001 M Fe(ClO4)3. The vol-
ume of the reaction solution was 5 mL. For the gas chromatography (GC)
analyses, the samples were prepared by adding 800
μ
L of acetonitrile,
100 μL of reaction solution, 100 μL of a solution of nitromethane (in-
ternal standard), and a small amount of triphenylphosphine (to
decompose H2O2 and stop the reaction) to a GC vial. The GC analyses
were carried out using a Shimadzu GCMS-QP2010 chromatograph
equipped with a SOLGEL-WAX column (30 m x0.25 mm x0.25
simple quadrupole MS detector.
μm) and a
The reactions that were performed to determine the initial rates
involved changing the initial concentration of one of the reaction
components or the temperature, while the other parameters were kept
constant. The parameters range for the reactions that involved ethanol
as substrate were: [Fe(ClO4)3]0 within 0.3–5.0 M, [Ethanol]0 within
0.05 – 0.5 M, [H2O2]0 within 0.1–2.0 M and T within 50–70 ◦C. When
acetol was the substrate, the parameters range were: [Fe(ClO4)3]0 within
0.3–1.5 M, [Acetol]0 within 0.1 – 0.6 M, [H2O2]0 within 0.1–1.5 M and T
within 50–80 ◦C. The samples were collected within the first 10 min of
reaction and analyzed using a Shimadzu GC-2010 chromatograph
equipped with an SGE BP-20 column (30 m x0.25 mm x0.25
FID detector.
μm) and an
Fig. 1. Concentration of ethanol (curve 1), acetic acid (curve2), and formic
acid (curve 3) during ethanol oxidation by Fe(ClO4)3-HClO4-H2O2. Conditions:
0.1 M ethanol, 0.001 M Fe(ClO4)3, 0.03 M HClO4, and 0.5 M H2O2; T =60 ◦C.
2