Metal nitrate-catalyzed one-pot oxidative esterification of benzaldehyde with hydrogen peroxide in alcoholic solutions at room temperature

Article abstract of DOI:10.1039/d0nj05671e

The activity of metal nitrate catalysts was investigated in the oxidative esterification reactions of benzaldehyde with hydrogen peroxide. Several types of metal nitrates (alkaline, alkaline earth, and transition metals) were evaluated as catalysts. Among the assessed salts, Fe(NO3)3 was the most efficient catalyst toward the formation of the target product (i.e., benzoic alkyl ester). In methyl alcohol, benzaldehyde was selectively oxidized to benzoic acid and then esterified to methyl benzoate. The efficiency of the catalyst was correlated with its higher Lewis acidity character, which was established through the pH measurements of methanolic solutions of the soluble metal nitrate salts. The influence of main variables of the reaction, such as catalyst load, temperature, and reactant stoichiometry, was investigated. The size of the carbon chain and steric hindrance played an essential role in the reaction selectivity. While methyl and ethyl alcohols selectively provided ester as the main product (ca. 70-75%) and acetal as the subproduct, the other alcohols gave ester, hemiacetal, and benzoic acid, which was formed in the least amount. The use of an inexpensive catalyst, a green oxidant, mild conditions, and short reaction times were the positive aspects of this one-pot process. The high TON (ca. 900) is evidence of the high catalytic activity of Fe(NO3)3. It is noteworthy that this methodology does not rely upon ligands and other additives.

Full text of DOI:10.1039/d0nj05671e

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Metal nitrate-catalyzed one-pot oxidative
esterification of benzaldehyde with hydrogen
peroxide in alcoholic solutions at
room temperature
Cite this: New J. Chem., 2021,
45, 3683
Marcio Jose da Silva * and Cesar Macedo de Oliveira
´
´
The activity of metal nitrate catalysts was investigated in the oxidative esterification reactions of
benzaldehyde with hydrogen peroxide. Several types of metal nitrates (alkaline, alkaline earth, and
transition metals) were evaluated as catalysts. Among the assessed salts, Fe(NO3)3 was the most efficient
catalyst toward the formation of the target product (i.e., benzoic alkyl ester). In methyl alcohol,
benzaldehyde was selectively oxidized to benzoic acid and then esterified to methyl benzoate. The
efficiency of the catalyst was correlated with its higher Lewis acidity character, which was established
through the pH measurements of methanolic solutions of the soluble metal nitrate salts. The influence
of main variables of the reaction, such as catalyst load, temperature, and reactant stoichiometry, was
investigated. The size of the carbon chain and steric hindrance played an essential role in the reaction
selectivity. While methyl and ethyl alcohols selectively provided ester as the main product (ca. 70–75%)
and acetal as the subproduct, the other alcohols gave ester, hemiacetal, and benzoic acid, which was
formed in the least amount. The use of an inexpensive catalyst, a green oxidant, mild conditions, and
short reaction times were the positive aspects of this one-pot process. The high TON (ca. 900) is
evidence of the high catalytic activity of Fe(NO3)3. It is noteworthy that this methodology does not rely
upon ligands and other additives.
Received 20th November 2020,
Accepted 20th January 2021
DOI: 10.1039/d0nj05671e
rsc.li/njc
agent, as well as the addition of pH controllers.8–11 Notwith-
standing, hydrogen peroxide requires an activation step, which is
Introduction
The production of benzoic esters has raised a lot of interest, performed by metal catalysts. Various organometallic com-
due to the extensive use of these products as a raw material in pounds have been successfully used as catalysts in the oxidative
the industrial production of polymers, cosmetics, perfumes, esterification of aldehydes.12–17 Nonetheless, their high cost, the
fibers, plasticizers, and dyes.1,2 In particular, the one-pot conver- difficult synthesis, and their thermal instability are negative
sion of aromatic aldehydes to esters has been demonstrated to aspects of their use.
be an attractive route to synthesize esters.3
It is known that solid supported catalysts activate hydrogen
Stoichiometric oxidation processes commonly employs toxic peroxide in solution, and are an attractive option to oxidize
reactants such as N-bromosuccinimide/pyridine, diazonium alcohols and aldehydes.18–22 For instance, Thakur et al. assessed
salts, or chromium trioxide.4 Alternatively, Lewis acid metal the activity of titania-supported VO(acac)2 in the oxidation of
have been used as catalysts in oxidation reactions with environ- aromatic and aliphatic aldehydes with hydrogen peroxide to
mentally benign oxidants like hydrogen peroxide.5 In this sense, methyl esters.23 Although the immobilization of homogeneous
hydrogen peroxide has received considerable attention because it catalysts on a solid support is a widely explored field, there are
is an atom-efficient, non-flammable, and easy to handle oxidant, some drawbacks that motivate the search for a better under-
which generates only water as the by-product.6,7
standing of how these catalysts act in solution.24 Notwithstanding,
Various oxidative processes of organic compounds with the study of catalytic activity of the transition metal salts in solu-
aqueous hydrogen peroxide have been developed in the presence tion has been useful to optimize their use as heterogeneous
of an adequate solvent, avoiding the use of a phase transfer catalysts.25,26 In this sense, polyoxometalates such as transition
metal salts of heteropolyacids of Keggin type have been widely used
as catalysts in oxidation reactions with hydrogen peroxide.27–30
Chemistry Department, Federal University of Viçosa, Viçosa, Minas Gerais State,
In addition to transition metals, calcium and magnesium
chlorides were also found to be efficient catalysts in the oxidative
36590-000, Brazil. E-mail: silvamj2003@ufv.br; Fax: +55-31-3899-3065;
Tel: +55-31-3899-3210
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esterification of benzaldehydes; however, the reactions were slow acid (1.1), methyl benzoate (1.0), ethyl benzoate (1.0), propyl
(ca. 24–48 h).31 Among the series of metal salts assessed in benzoate (0.9), and butyl benzoate (0.8).
oxidative esterification of benzaldehyde in the CH3OH/H2O2
The acidity of the medium was used to verify the relation
system at room temperature, zinc bromide was the most active between the Lewis acidity of the catalyst salt and the decrease
and selective toward methyl esters; however, the reactions triggered by its addition to the reaction solution. For this, the
required a long time (ca. 16 h), besides a high catalyst load (ca. pH was measured before and after the addition of hydrogen
10 mol%), to achieve good yield.32 Iron salts (i.e., halides, acetate, peroxide to the reaction solutions, using a potentiometer (Bel
acetylacetonate, perchlorate, sulfate anions) were also investi- pH meter, model W3B) with a glass electrode. Before each
gated in similar reactions, but with ethyl alcohol, and Fe(ClO4)3 measurement, the pH meter was calibrated at pH 4.0 and 6.86,
was found to be the most active catalyst; however, a reaction time using potassium hydrogen phthalate and dihydrogen/hydrogen
of 20 h and a high catalyst load (10 mol%) were again required.33 phosphate NIST buffers, respectively.
In the present study, for the first time as far as we know,
Typically, the catalyst salt (ca. 10 mol% relative to benzaldehyde)
metal nitrate salts are used as catalysts in oxidative esterifica- was added to alcohol methyl solution, and magnetically stirred
tion reactions of benzaldehyde with hydrogen peroxide in until pH stabilization was reached. The effect of peroxide addition
alcoholic solutions. The focus was on correlating the activity on the pH of reaction solutions was also verified in the presence of
and selectivity attained in metal nitrate-catalyzed reactions with each catalyst salt.
the main reaction parameters (i.e., oxidant load, type and
concentration of metal nitrate catalyst, temperature, and nature
of alcohol). Simple experimental data such as pH measurements
and product selectivity allowed us to propose reaction pathways
that elucidated how the most active catalyst (i.e., Fe(NO3)3)
Results and discussion
Screening of the metal nitrate catalyst
contributed to converting benzaldehyde into alkyl ester.
In this work, our initial objective was to select the most active and
selective catalyst in the oxidative esterification of benzaldehyde
with hydrogen peroxide in metal nitrate-catalyzed reactions carried
out in methyl alcohol solution. The catalyst load (ca. 10 mol%) was
selected based on the literature.31–33
Experimental
It is important to highlight that the reaction conditions were
not optimized to achieve the maximum conversion, as our aim
was to select the most active catalyst (Fig. 1).
In general, when the catalyst was active, benzaldehyde was
oxidized to benzoic acid and quickly esterified to methyl benzo-
ate; these two reactions occurred in a one-pot process, in the
presence of aqueous H2O2, and in methyl alcohol solutions
containing catalytic amounts of the metal nitrate. However,
depending on the cation present in the nitrate salt, a concur-
rent reaction occurred (i.e., condensation of benzaldehyde with
Materials and methods
All chemicals were commercially available and were utilized
without prior treatment. LiNO3 (99 wt%), Co(NO3)2Á6H2O (98
wt%), Mn(NO3)2Á4H2O (98 wt%), Cu(NO3)2Á3H2O (98 wt%),
Ni(NO3)2Á6H2O (98 wt%), Ba(NO3)2 (99 wt%), and Mg(NO3)2Á
2H2O (99 wt%) were obtained from Dinamica Ltd (Brazil).
Fe(NO3)3Á9H2O (98 wt%), FeCl3Á6H2O (98 wt%), FeSO4Á7H2O
(98 wt%), Fe2(SO4)3Á5H2O (98 wt%), Zn(NO3)2Á6H2O (98 wt%),
Sr(NO3)2 (99 wt%), Al(NO3)3Á6H2O (99 wt%), and AgNO3
(99 wt%) were acquired from Vetec (Brazil). Benzaldehyde
(ca. 99 wt%) and alkyl alcohols (i.e., methyl, ethyl, propyl, isopropyl,
butyl, sec-butyl and isobutyl; ca. 99.0–99.5 wt%) were obtained from
Sigma-Aldrich. Hydrogen peroxide was purchased from Vetec
(ca. 34 wt% aqueous solution).
Catalytic tests
Catalytic runs were performed under magnetic stirring in a
glass reactor (ca. 50 mL), fitted with a reflux condenser and
sampling septum, in a glycerine bath. Typically, benzaldehyde
(ca. 10 mmol) and metal nitrate catalyst (ca. 10 mol%) were
dissolved in CH3OH (ca. 10 mL solution), at room temperature
(ca. 298 K). After the slow addition of aqueous H2O2 solution
(ca. 30 mmol), the reaction was started.
The reaction progress was monitored by GC analysis of
samples periodically collected (GC 2010 Shimadzu, capillary
column, FID). The reaction products were identified by GC-MS
analysis (GC-MS 2010 ultra mass, i.e. 70 eV), and comparison
with chromatographic analytical standards. The main response
Fig. 1 Metal nitrate-catalyzed oxidative esterification of benzaldehyde
with H2O2 in CH3OH solutions. Reaction conditions: benzaldehyde
(1.0 mmol), H2O2 (2.0 mmol), catalyst (10.0 mol%), toluene (0.1 mL),
factors in relation to benzaldehyde were as follows: benzoic CH3OH (10.0 mL), 298 K.
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measurement of Lewis acidity requires sometimes a greater
effort.37 Nonetheless, in this case, as the reactions were carried
out in alcoholic solutions, we envisage that the same method
could be used to estimate the Lewis acidity. The Lewis acidity of
a metal cation is strictly associated with its ability to react with
a protic solvent releasing H+ ions, which are easily detected by a
glass electrode. Therefore, before the addition of oxidant, we
measured the pH of the solutions containing the metal nitrate
dissolved in methyl alcohol and correlated it with the reaction
conversion (Fig. 2). This approach was previously used by us in
other works.38–40 It is important to highlight that although this
procedure was not technically validated by any international
association such as ASTM or the like, it was useful to correlate
the activity of catalysts with pH measurements.
Although the expression ‘‘pH’’ should be preferentially used
for aqueous acid solutions (i.e., H3O+ ions), herein it was
successfully used to quantify the H+ ions produced by the
reaction of a metal cation and a protic solvent (i.e., methyl
alcohol).
Fig. 3 shows that the reactions with the most acidic solu-
tions (i.e., Fe(NO3)3, Cu(NO3)2, and Al(NO3)3) were those in
which higher benzaldehyde conversion and ester selectivity
were achieved. These results agree with the literature, which
describes that acidic catalysts are efficient to promote esterifi-
cation or acetalization reactions.41 However, it is essential to
understand how the metal cations activated oxidant/substrate
molecules to lead to the formation of the oxidized products.
Scheme 1 M(NO3)3-Catalyzed oxidative esterification of benzaldehyde
with hydrogen peroxide in methyl alcohol solutions.
methyl alcohol led to the formation of benzaldehyde acetal)
(Scheme 1).
Nitrate salts of cations belonging to the earlier transition
metal series and to the main group of the periodic table (i.e., I,
II, and XIII) were evaluated. Among the first group, Fe(III) and
Cu(II) nitrates were the most efficient catalysts, while Al(III)
nitrate was the most efficient in the second group. Alkaline
and alkaline earth metal cations were less efficient in the
benzaldehyde oxidation reaction. Although Mg(NO3)2 achieved
a conversion near 40%, no oxidation product was detected, only
the acetal.
When we focused on the conversion and reaction selectivity,
we verified that apparently a high Lewis acidity is a key aspect
for both oxidation and esterification reactions (Fig. 2). The
reactions that achieved higher conversion and that provided
greater ester selectivity were those carried out in the presence of
typical Lewis acid metal cations (i.e., Fe(III), Cu(II) and Al(III)
cations).34
Discussions on the reaction mechanism involving metal
catalysts and hydrogen peroxide
Using pH measurements as a useful tool to investigate the
effects of Lewis and Brønsted acidity of metal nitrate catalysts
The metal-catalyzed oxidation reactions with hydrogen peroxide
can be distinguished by two mechanisms: those that may
involve peroxometal or oxometal intermediates.42 In the first case
(i.e., peroxometal pathway), the oxidation number of the metal
cation remains unaltered along the catalytic cycle. Moreover, no
stoichiometric oxidation of the substrate by the metal ion occurs.
There are several ways to estimate the Lewis acidity, for
instance, by using the traditional techniques of 31P NMR or
infrared spectroscopy with probe molecules such as triphenyl-
phosphine oxide or pyridine, respectively,35,36 or new methods
where fluorescence spectroscopy is the main tool.36
Different from Brønsted acidity, which is easily measured in
solution considering the amount of H+ ions present, the
Fig. 2 Conversion and selectivity of metal nitrate-catalyzed oxidative
esterification of benzaldehyde with H2O2 in CH3OH solutions. Reaction
conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), catalyst (10.0 mol%),
toluene (0.1 mL), CH3OH (10.0 mL), 298 K.
Fig. 3 Conversion and pH measurements of metal nitrate-catalyzed
oxidative esterification of benzaldehyde with H2O2 in CH3OH solutions.
Reaction conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), catalyst
(10.0 mol%), toluene (0.1 mL), CH3OH (10.0 mL), 298 K.
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It is very frequent when transition metals with d0 configuration
orbitals are the catalysts, which are weak oxidants (i.e., Mo6+, W6+,
Re7+, Ti4+ ions). Conversely, in the oxometal pathway, there is
variation in the oxidation number of metal catalysts, which are
normally strong oxidants (i.e., Cr6+, Mn7+, Os8+) and may oxidize
the substrate even in the absence of peroxide.43
In the present study, the cations of the metal catalysts have a
low oxidation state, and the reaction may proceed through the
other route; hydrogen peroxide can react with a metal cation,
leading to the peroxometal intermediate. It can occur when a
Mn+/Mn+1 redox pair is involved.44 The metal cation can
enhance the oxidizing character of the peroxo-species acting
as a Lewis acid due to the electron-withdrawing effect and
promote the transfer of the oxygen atom to the substrate more
efficiently.
Considering the literature and the experimental data, we
have proposed a reaction pathway where both Lewis and
Brønsted acids are found to be effective in converting benzal-
dehyde to benzoic acid. We suppose that M+ ions may have
reacted with hydrogen peroxide in the presence of alcohol,
generating H+ ions and MOOH (eqn (1), Scheme 2).
The alcohol plays an essential role in this step, providing the
H+ ion. No metal alkoxide is formed because a proton of hydrogen
peroxide regenerates the alcohol. Although it could be possible for
the metal nitrate catalyst to react with the aqueous hydrogen
peroxide solution and release H+ ions, apparently, under these
reaction conditions, this does not happen. To support this
hypothesis, we carried out a comparison of pH measurements
before and after the addition of the oxidant (Fig. 4).37
In most of the solutions, the addition of aqueous hydrogen
peroxide triggered an increase of the pH value. It may be
attributed to the reaction of metal catalyst with hydrogen per-
oxide which leads to the hydroxyl species.45 Nonetheless, in the
case of the more active metal nitrate catalysts, this effect was
absent (i.e., Fe3+ and Cu2+ ions) or minimum (i.e., Al3+ ions).
Therefore, we suggest that the reaction of metal cations
(i.e., Fe3+, Cu2+, and Al3+ ions) with methyl alcohol provides H+
ions, which are essential for both oxidation and esterification
reactions. In eqn (2) (Scheme 2), we suppose that MOOH
species may promote the transfer of an oxygen atom to
Fig. 4 Impact of hydrogen peroxide addition on the pH of the methanolic
solutions containing benzaldehyde and the metal nitrate catalyst.a Reaction
conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), catalyst (10.0 mol%),
toluene (0.1 mL), CH3OH (10.0 mL), 298 K.
benzaldehyde resulting in benzoic acid. It has been described
that Fe(III) cations can react with hydrogen peroxide forming
intermediates such as MOOH. The literature has reported that
iron and copper nanoparticle catalysts are efficient in the oxida-
tion of para-substituted-benzaldehydes.46 The presence of an
active metal cation catalyst is the key in this step. Herein, in
the absence of an efficient metal catalyst, the oxidation reaction
practically did not occur, as demonstrated by the uncatalyzed
reactions. Moreover, the presence of H+ ions provides the correct
stoichiometry so that this reaction occurs releasing water.
Nonetheless, it is important to highlight that it is not the only
reaction pathway possible; for instance, Thakur et al. proposed
that the methyl ester of aromatic aldehydes can be directly
obtained in the presence of titania-supported vanadium acetate
without the acid formation.23 However, as will be demonstrated in
the next sections benzoic acid is also a product formed herein.
An important point is that when we added the metal nitrate to
the ‘‘pure’’ hydrogen peroxide aqueous solution (i.e., in the
absence of alcohol), a violent decomposition reaction was imme-
diately noticed, due to the collapse of hydrogen peroxide, which
led to a quick evolution of molecular oxygen. Nonetheless, it is
noteworthy to highlight that in the reaction solutions (i.e., ben-
zaldehyde/CH3OH/metal nitrate) no gas release was observed.
After the formation of benzoic acid, it was almost instanta-
neously esterified to methyl benzoate. In GC analyses, only
traces of benzoic acid were detected. Typically, esterification
reactions are catalyzed by Lewis or Brønsted acids.47–49 In
eqn (3) (Scheme 2), we have described that the M+ cation in
the presence of alcohol may provide H+ ions, which can
protonate the carbonyl group of benzoic acid, facilitating its
nucleophilic attack by alcohol, generating ester and water.50
Assessment of the nature of the iron salt catalyst
Other iron salts were investigated in benzaldehyde oxidation
reaction (Fig. 5). An equal load of iron was used in all the runs.
Except for ferric sulfate, all were soluble in methyl alcohol.
Possibly, this was the reason for the lowest efficiency of the
Fe2(SO4)3 catalyst.
Scheme 2 Probable route for Lewis and Brønsted acid-catalyzed oxida-
tive esterification of benzaldehyde.
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Fig. 7 Effect of the catalyst load on the conversion of benzaldehyde in
Fe(NO3)3-catalyzed reactions with hydrogen peroxide. Reaction condi-
tions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), CH3OH (10 mL),
temperature (298 K), reaction volume (10 mL).
Fig. 5 Effects of the type of anion and oxidation number of the cation on
iron-catalyzed oxidative esterification of benzaldehyde with H2O2 in
CH3OH solutions. Reaction conditions: benzaldehyde (1.0 mmol), H2O2
(2.0 mmol), catalyst (2.5 mol% of Fe2+ or Fe3+ ions), toluene (0.1 mL),
CH3OH (10.0 mL), 298 K.
interval of 10.0 to 1.25 mol%, regardless of the catalyst load,
all the reactions had a maximum conversion after a 3 h reac-
tion. Below this range, a decrease in catalyst load resulted in a
concomitant decrease in both rate and reaction conversion.
The maximum TON achieved was 900, with 54% of conversion
using 0.06 mol% of catalyst, indicating a high activity at room
temperature.
The reaction rate followed the trend Fe(NO3)3 4 FeCl3
4
FeSO4 4 Fe2(SO4)3 (Fig. 5). Even when using the same iron
load, the reactions presented different conversions and selec-
tivity. Once again, we may suppose that there is a correlation
between the ester selectivity and the pH of solutions after the
addition of iron salts (Fig. 6).
On the other hand, the selectivity of reactions was more
sensitive to variation of the catalyst load (Fig. 8). The catalyst
concentration was crucial for ester formation; it was the major
product only until 1.25 mol%, with a high conversion (ca. 95%).
Below this load, benzaldehyde acetal became the main product.
Indeed, at the lowest concentration (ca. 0.06 mol%), despite
the conversion of 50%, no ester was detected. This suggests
that under these conditions, condensation of benzaldehyde
with methyl alcohol, which is also an acid-catalyzed competitive
reaction, occurs more favorably than esterification and oxida-
tion reactions.
The reactions carried out in more acidic solutions were
those that achieved higher conversion and ester selectivity.
The oxidation state was also important; among the totally
soluble catalysts, ferrous sulfate was the lowest effective.
When the catalyst was less efficient, acetal was the major
product; conversely, ester was the most selective product when
the iron catalyst was acidic enough to lead the pH value to a
level lower than 2.0.
Impact of Fe(NO3)3 catalyst load on benzaldehyde oxidation by
H2O2 in CH3OH
Scheme 2 describes the possible participation of both Lewis
(i.e., M+ ions) and Brønsted (i.e., H+ ions) catalysts in the
oxidative esterification reaction. We can note that the oxidation
depends on the presence of active species MOOH. Therefore,
The effect of catalyst load was assessed in a wide range of values
(ca. 0.06 to 10.0 mol%, Fig. 7). Within the concentration
Fig. 6 Effects of the type of anion and oxidation number of cations on the
conversion, selectivity, and pH value of oxidative esterification reactions of
benzaldehyde with H2O2 in CH3OH solutions. Reaction conditions: benzal-
dehyde (1.0 mmol), H2O2 (2.0 mmol), catalyst (2.5 mol% of Fe2+ or Fe3+ ions),
toluene (0.1 mL), CH3OH (10.0 mL), 298 K.
Fig. 8 Influence of the catalyst load on the conversion and selectivity of
metal nitrate-catalyzed oxidative esterification of benzaldehyde with H2O2
in CH3OH solutions. Reaction conditions: benzaldehyde (1.0 mmol), H2O2
(2.0 mmol), catalyst (variable), CH3OH (10.0 mL), 298 K.
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the decrease in the selectivity of oxidation products is a con-
sequence of the decrease of metal catalyst load. On the other
hand, the acetalization reaction which is the concurrent reac-
tion was less impacted by the reduction of the catalyst load.
Possibly, the significant excess of alcohol in solution was
advantageous toward the acetal formation reaction.
Different from the selectivity, the catalyst load has no effect
on the reaction equilibrium and therefore it should not affect
the conversions. Nonetheless, since they did not reach equilibrium,
we verified that the reaction conversions were also impacted by the
catalyst load. Within the period studied, the conversions were
lower when the catalyst load was lower than 1.25 mol%. It can
be assigned to the lower H+ amount generated in the solution,
which depends on the metal catalyst load.
Fig. 10 Effect of oxidant load on the conversion and selectivity of
Fe(NO3)3-catalyzed oxidative esterification reactions of benzaldehyde with
H2O2. Reaction conditions: benzaldehyde (10 mmol), Fe(NO3)3 catalyst
(0.625 mol%), CH3OH (163 mmol), temperature (298 K), reaction volume
(10 mL).
Effects of molar ratio of the oxidant to substrate
The oxidant load played a key role in Fe(NO3)3-catalyzed benzal-
dehyde oxidation with H2O2 in CH3OH solutions. Fig. 9 and 10
display the main data of conversion and selectivity obtained in
reactions with different amounts of oxidant.
benzaldehyde, and consequently, the lower the ester selectivity
(Scheme 3).
An excess of oxidant was detrimental for both rate and
reaction conversion. This can be assigned to the reversible
character of the esterification reaction, which gives water as a
co-product. Nonetheless, the ester selectivity was higher when a
higher oxidant load was used (Fig. 10).
Similarly, when alcohols with a carbon chain size greater than
that of ethyl alcohol are used, the approximation of carbonyl
carbon becomes less favorable. The following tendency was
observed in terms of conversion of alcohols; CH3OH 4 C2H5OH
4 C3H7OH 4 C4H9OH 4 iso-C4H9OH 4 iso-C3H7OH 4 sec-
C4H7OH 4 tert-C4H9OH. Truly, the reaction selectivity was
strongly impacted by alcohol’s nature. An increase in carbon
chain size affected the selectivity of esters (Scheme 3). For linear
primary alcohols, it was verified. The same occurred for secondary
alcohols. Particularly, for C4-alcohols, the ester selectivity was
n-butyl 4 iso-butyl 4 sec-butyl 4 tert-butyl (Scheme 3). Moreover,
acetalization reactions were also impacted, mainly by the size of
the carbon chain of alcohol. Acetal and hemiacetal were the
expected products; nonetheless, depending on the type of alcohol,
only one of them was selectively formed (Scheme 4).
Effect of alcohol on the Fe(NO3)3-catalyzed oxidative
esterification of benzaldehyde
When the hydroxyl groups are sterically hindered the reactivity
of alcohols is affected. It may be a key aspect for both esteri-
fication and acetalization reactions. Therefore, to assess this
effect various alcohols were evaluated in reactions with hydro-
gen peroxide and benzaldehyde in the presence of Fe(NO3)3 as a
catalyst (Fig. 11).
Alcohols with a greater carbon chain size and with a high
steric hindrance were less reactive (i.e., primary 4 secondary 4
tertiary) (Fig. 11). The more hindered the hydroxyl group of the
alcohol, the more difficult the attack of the carbonyl carbon of
We have found that only alcohols with shorter carbon chain size
efficiently acetalized benzaldehyde (i.e., CH3OH and C2H5OH);
Fig. 9 Impact of oxidant load on the conversion of benzaldehyde in
Fe(NO3)3-catalyzed reactions with hydrogen peroxide. Reaction condi-
tions: benzaldehyde (10 mmol), Fe(NO3)3 catalyst (0.625 mol%), CH3OH
(10 mL), temperature (298 K).
Fig. 11 Effect of the alcohol on the conversion of benzaldehyde in
Fe(NO3)3-catalyzed reactions with hydrogen peroxide. Reaction condi-
tions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), catalyst (2.5 mol%),
temperature (298 K), reaction volume (10 mL).
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Scheme 3 Ester selectivity of Fe(NO3)3-catalyzed oxidative esterification
reactions of benzaldehyde with hydrogen peroxide in alcoholic solutions.
Reaction conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), Fe(NO3)3
catalyst (2.5 mol %), temperature (298 K), reaction volume (10 mL).
Scheme 4 Acetal and hemiacetal selectivity of Fe(NO3)3-catalyzed benzalde-
hyde reactions with alkyl alcohol in the presence of hydrogen peroxide.
Reaction conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), Fe(NO3)3
catalyst (2.5 mol%), temperature (298 K), reaction volume (10 mL).
in the reactions with other alcohols, hemiacetal was the con-
densation product formed. This can be assigned to the steric
hindrance of the hydroxyl group of secondary and tertiary
alcohols as well as a longer carbon chain.
Another important change occurred when the reactivity of
the alcohols in esterification and acetalization reactions was
diminished; benzoic acid began to be formed in detectable
amounts. Only in the reaction with methyl alcohol, it was not
verified. For better visualization of the product distribution, as
well as the final conversion of benzaldehyde, these data are
summarized in Fig. 12.
In all the reactions, except for tert-butyl alcohol, ester was the
most selective product. However, the formation of hemiacetal or
acid depended on the type of alcohol.
The values of pH were also measured in the reactions with
different alcohols. The following values were obtained: methyl
alcohol (1.1); ethyl alcohol (1.5); propyl alcohol (1.6); isopropyl
alcohol (1.7); butyl alcohol (1.7); isobutyl alcohol (1.7). In
addition to the effects of the size of the carbon chain and steric
hindrance on the hydroxyl group, we suppose that the pH of the
alcoholic solution was also important to explain the decrease in
the conversion of reactions; with an increase in pH values, the
reaction became less favorable.
Fig. 12 Effect of the alcohol on the conversion and selectivity of benzal-
dehyde in Fe(NO3)3-catalyzed reactions with hydrogen peroxide. Reaction
conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol), Fe(NO3)3 catalyst
(2.5 mol%), temperature (298 K), reaction volume (10 mL).
Effect of temperature on the Fe(NO3)3-catalyzed oxidative
esterification of benzaldehyde
An increase in reaction temperature resulted in a higher con-
The impact of temperature on conversion and selectivity was version of benzaldehyde.
also investigated (Fig. 13). It is an expected effect, since an
Nonetheless, herein the reactions were less sensitive to the
increase in temperature means a higher amount of energy is increase in temperature; the difference between the conver-
provided to the reactant molecules and consequently an sions achieved at the highest and lowest levels of temperature
increase in the number of effective collisions.
was small. Even though the catalyst load was also varied, the
Ethyl alcohol was selected aiming at a greater temperature gap between the conversions was not large enough in that the
interval reaction, and consequently, the number of effective reactions carried out at the intermediate temperatures between
collisions was also increased, improving the reaction rate. these two levels could not be efficiently measured (Fig. 13).
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oxidant (i.e., aqueous H2O2) using an inexpensive commercial
Lewis acid catalyst (i.e., Fe(NO3)3) in alcoholic solutions was
developed. Among the several metal nitrate salts assessed,
Fe(NO3)3 was the most active catalyst. The main aspects that drive
the reaction selectivity were studied. Benzoic acid and alkyl ester
were obtained in a one-pot process. Methyl and ethyl alcohols were
efficiently converted to esters (ca. 72–75% selectivity), in 1 h of
reaction at 298 K with a low oxidant excess (ca. 1:2). In this case,
acetal was a secondary product. When the reaction scope was
extended to the other alcohols, the formation of two side-products
was noticed: benzoic acid and hemiacetal. Depending on the
reaction conditions (i.e., catalyst and oxidant loads), alkyl benzoate
or hemiacetal(acetal) benzaldehyde were the main products. Sec-
ondary alcohols with a carbon chain size greater than that of ethyl
alcohol provided hemiacetal as the co-product and lower ester
selectivity. The high activity of the Fe(NO3)3 catalyst may be
assigned to its higher Lewis acidity strength. After performing
the pH measurements before and after adding the oxidant to the
reaction medium, we concluded that the most efficient catalysts
were those that led to a higher decrease in pH values (Fe(NO3)3 4
Cu(NO3)2Al(NO3)3). Noticeably, we demonstrated that these metal
nitrate catalysts promoted the oxidation reactions reacting with
both solvent and oxidant. It is noteworthy that this methodology
does not rely upon ligands and other additives.
Fig. 13 Effects of temperature and catalyst load on the conversion of
Fe(NO3)3-catalyzed oxidative esterification reactions of benzaldehyde by
H2O2. Reaction conditions: benzaldehyde (1.0 mmol), H2O2 (2.0 mmol),
Fe(NO3)3 catalyst (variable), temperature (variable), ethyl alcohol volume (10 mL).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the Federal University of Viçosa, the PPGMQ-
MG, and the development agencies CNPq and FAPEMIG. This
Fig. 14 Effect of the catalyst load on the conversion and selectivity of
Fe(NO3)3-catalyzed oxidative esterification reactions of benzaldehyde with
hydrogen peroxide in ethyl alcohol solutions. Reaction conditions: ben-
zaldehyde (1.0 mmol), H2O2 (2.0 mmol), Fe(NO3)3 catalyst (variable),
temperature (333 K), reaction volume (10 mL).
˜
study was financed in part by the Coordenaçao de Aperfeiçoa-
´
mento de Pessoal de Nıvel Superior – Brasil (CAPES) – Finance
Code 001.
To circumvent this obstacle, the reactions should be carried out
using a lower alcohol excess; nonetheless, it is out of the scope
of this study, because the main reaction (i.e., oxidative esteri-
fication) will be compromised.
Under these reaction conditions, no significant difference
was found in the reactions carried out at the lowest (ca. 298 K)
and highest temperature (ca. 333 K). Thus, only data obtained
at 333 K were presented (Fig. 14).
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