Efficient Synthesis of 2-Ethylhexanoic Acid via N-Hydroxyphthalimide Catalyzed Oxidation of 2-Ethylhexanal with Oxygen

Abstract

An efficient method for the synthesis of 2-ethylhexanoic acid has been reported. The method involves the 2-ethylhexanal oxidation using oxygen or air in the presence of N-hydroxyphthalimide in isobutanol as a solvent under mild conditions. A high selectivity of >99% for 2-ethylhexanoic acid was achieved. The influence of catalyst amount, solvent type and quantity, temperature, and reaction time on the product composition was studied. The developed method is in line with the global trends aimed at developing green oxidation processes as well as having potential for implementation in industry due to its high selectivity, cost-effective oxidizing agent, and mild reaction conditions. The use of isobutanol as a solvent is of crucial importance providing an opportunity for potential producers of 2-EHAL from butanal to employ the less valuable alcohol.

Full text

Citation: Czieszowic, Ł.; Orli´nska, B.;
Lisicki, D.; Pankalla, E. Efficient
Synthesis of 2-Ethylhexanoic Acid via
N-Hydroxyphthalimide Catalyzed
Oxidation of 2-Ethylhexanal with
Oxygen. Materials 2023,16, 5778.
https://doi.org/10.3390/ma16175778
Academic Editor: Barbara Pawelec
Received: 30 June 2023
Revised: 17 August 2023
Accepted: 19 August 2023
Published: 23 August 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
materials
Article
Efficient Synthesis of 2-Ethylhexanoic Acid via
N-Hydroxyphthalimide Catalyzed Oxidation of 2-Ethylhexanal
with Oxygen
Łukasz Czieszowic 1, 2, * , Beata Orli ´nska 2, * , Dawid Lisicki 2and Ewa Pankalla 1
1Grupa Azoty Zakłady Azotowe–K˛edzierzyn-S.A., Mostowa 30A, 47-220 K ˛edzierzyn-Ko´zle, Poland;
ewa.pankalla@grupaazoty.com
2Department of Chemical Organic Technology and Petrochemistry and PhD School,
Silesian University of Technology, Akademicka 2A, 44-100 Gliwice, Poland; dawid.lisicki@polsl.pl
*Correspondence: lukasz.czieszowic@grupaazoty.com or lukasz.czieszowic@polsl.pl (Ł.C.);
beata.orlinska@polsl.pl (B.O.)
Abstract:
An efficient method for the synthesis of 2-ethylhexanoic acid has been reported. The method
involves the 2-ethylhexanal oxidation using oxygen or air in the presence of N-hydroxyphthalimide
in isobutanol as a solvent under mild conditions. A high selectivity of >99% for 2-ethylhexanoic acid
was achieved. The influence of catalyst amount, solvent type and quantity, temperature, and reaction
time on the product composition was studied. The developed method is in line with the global
trends aimed at developing green oxidation processes as well as having potential for implementation
in industry due to its high selectivity, cost-effective oxidizing agent, and mild reaction conditions.
The use of isobutanol as a solvent is of crucial importance providing an opportunity for potential
producers of 2-EHAL from butanal to employ the less valuable alcohol.
Keywords: oxidation; 2-ethylhexanoic acid; N-hydroxyphthalimide; aldehyde
1. Introduction
2-Ethylhexanoic acid (2-EHA) and its derivatives are widely used in the chemical
industry, including production of alkyd resins, plasticizers, stabilizers of polyvinyl chloride,
lubricants, detergents, flotation agents, and corrosion inhibitors [1].
There are two primary industrial pathways for 2-EHA synthesis, both starting from
butanal, which is a product of propylene hydroformylation (Figure 1). In method (I),
2-ethylhexanol is obtained through butanal aldolization and subsequent hydrogenation.
This 2-ethylhexanol is then oxidized to 2-ethylhexanal (2-EHAL), which is further oxidized
to 2-EHA. In method (II), an unsaturated aldehyde, 2-ethylhex-2-enal (EPA), is obtained
through aldol condensation of butanal, followed by dehydration. Selective hydrogenation
of an EPA yields 2-EHAL, which is then oxidized to 2-EHA [
1
]. Notably, both methods
involve the oxidation of 2-EHAL.
In industry, the exothermic process of 2-EHAL oxidation to 2-EHA (approx. 250 to
300 kJ/mol [
1
]), which occurs according to a radical chain mechanism [
2
], is typically
carried out in the liquid phase using air [
3
–
5
], or oxygen [
6
–
8
]. Various catalysts have
been employed, including transition metals, alkaline earth metals, hydroxides, or salts, and
combinations of both transition metals and alkaline metal salts [
9
–
12
]. Attempts have also
been made to carry out the process in gaseous phase; however, only a mixture of heptane,
3-heptanone, and 3-heptyl formate was obtained in this process [13].
The authors of the paper [
14
] studied the oxidation of 2-EHAL with air and observed
that the conversion of 2-EHAL increased as the temperature rose. The content of aldehyde
in the post-reaction mixture was 76% and 26% at 30
◦
C and 83
◦
C, respectively. However, the
acid 2-EHA content in the products was 18% and 50% at the above-mentioned temperatures.
Materials 2023,16, 5778. https://doi.org/10.3390/ma16175778 https://www.mdpi.com/journal/materials

Materials 2023,16, 5778 2 of 10
The main byproduct was the ester 3-heptyl formate (6% at 30
◦
C and 18% at 83
◦
C).
Increasing the molar ratio of oxygen to aldehyde from 0.625 to 10.0 resulted in a higher
yield of 2-EHA, increasing from 52% to 67% as well as an increase in ester yield from 19%
to 23% [14].
Materials 2023, 16, x FOR PEER REVIEW 2 of 10
Figure 1. Scheme of obtaining 2-EHA from n-butanal.
The authors of the paper [14] studied the oxidation of 2-EHAL with air and observed
that the conversion of 2-EHAL increased as the temperature rose. The content of aldehyde
in the post-reaction mixture was 76% and 26% at 30 °C and 83 °C, respectively. However,
the acid 2-EHA content in the products was 18% and 50% at the above-mentioned
temperatures. The main byproduct was the ester 3-heptyl formate (6% at 30 °C and 18%
at 83 °C). Increasing the molar ratio of oxygen to aldehyde from 0.625 to 10.0 resulted in a
higher yield of 2-EHA, increasing from 52% to 67% as well as an increase in ester yield
from 19% to 23% [14].
Gliński and Kijeński obtained an 80% yield of 2-EHA through oxidation with oxygen
at 40 °C, utilizing Mn(II) 2-ethylhexanoate as a catalyst [10]. Lehtinen et al. utilized Mn(II)
acetate as the catalyst and octanoic acid as a solvent for 2-EHAL oxidation with oxygen,
resulting in 83% yield of the acid [15]. By employing Fe(II), Ni(II), or Co(II) complexes as
catalysts, oxygen as the oxidizing agent, and dichloroethane as solvent at room
temperature, a 70% yield of 2-EHA was obtained [16]. Ko et al. achieved 84% yield of the
acid by oxidizing 2-EHAL with oxygen using KOH as a catalyst in a continuous stirred
tank reactor at 50 °C and under pressure of 0.8 MPa [17]. Furthermore, when EHAL was
oxidized using oxygen in the presence of Mn(II) 2-ethylhexanoate and sodium 2-
ethylhexanoate at room temperature and pressure of 0.5 or 0.75 MPa, a 97–98% yield of 2-
EHA was obtained [18,19].
The utilization of hydrogen peroxide as an oxidant in 2-EHAL oxidation has also
been reported [20]. The authors recommended the use of aqueous solutions of hydrogen
peroxide with a concentration of 3% to 30%. A yield of 65% of 2-EHA was achieved when
30% aqueous solution of hydrogen peroxide and a phase transfer catalyst, specifically a
quaternary ammonium salt [CH
3
(C
8
H
17
)
3
N]HSO
4
, were applied in a two-phase system at
90 °C for 2 h. However, the use of hydrogen peroxide on an industrial scale may be limited
due to its higher price compared to air or oxygen.
Figure 1. Scheme of obtaining 2-EHA from n-butanal.
Gli´nski and Kije´nski obtained an 80% yield of 2-EHA through oxidation with oxygen
at 40
◦
C, utilizing Mn(II) 2-ethylhexanoate as a catalyst [
10
]. Lehtinen et al. utilized
Mn(II) acetate as the catalyst and octanoic acid as a solvent for 2-EHAL oxidation with
oxygen, resulting in 83% yield of the acid [
15
]. By employing Fe(II), Ni(II), or Co(II)
complexes as catalysts, oxygen as the oxidizing agent, and dichloroethane as solvent at
room temperature, a 70% yield of 2-EHA was obtained [
16
]. Ko et al. achieved 84% yield
of the acid by oxidizing 2-EHAL with oxygen using KOH as a catalyst in a continuous
stirred tank reactor at 50
◦
C and under pressure of 0.8 MPa [
17
]. Furthermore, when
EHAL was oxidized using oxygen in the presence of Mn(II) 2-ethylhexanoate and sodium
2-ethylhexanoate at room temperature and pressure of 0.5 or 0.75 MPa, a 97–98% yield of
2-EHA was obtained [18,19].
The utilization of hydrogen peroxide as an oxidant in 2-EHAL oxidation has also
been reported [
20
]. The authors recommended the use of aqueous solutions of hydrogen
peroxide with a concentration of 3% to 30%. A yield of 65% of 2-EHA was achieved when
30% aqueous solution of hydrogen peroxide and a phase transfer catalyst, specifically a
quaternary ammonium salt [CH
3
(C
8
H
17
)
3
N]HSO
4
, were applied in a two-phase system at
90
◦
C for 2 h. However, the use of hydrogen peroxide on an industrial scale may be limited
due to its higher price compared to air or oxygen.
In non-catalytic oxidation processes of 2-EHAL using air or oxygen at a temperature
of 82
◦
C, lower yields of the acid were obtained, respectively, 66 and 50% [
21
]. Shapiro et al.
achieved comparable yields of 2-EHA by oxidizing 2-EHAL in an aqueous suspension
using air and oxygen. They extended the reaction time with air by ten hours. A yield of
88% was obtained after 2 h of oxygen oxidation, and when air was used, the yield was
86% after 12 h of reaction [
22
]. By conducting the 2-EHAL oxidation process in octane as a

Materials 2023,16, 5778 3 of 10
solvent, with oxygen under pressure of 0.3 MPa and a temperature of 40
◦
C, 81% of 2-EHA
was obtained [23].
To enhance the economic and ecological aspects of oxidation processes with in-
dustrial importance, novel catalysts have been investigated. Recently, the use of N-
hydroxyphthalimide (NHPI) as an active organocatalyst for free radical processes [
24
],
including the oxidation reaction of various aldehydes to their corresponding carboxylic
acids, has been reported [
25
]. As a model reaction, the cyclohexanal oxidation was carried
out with oxygen at a low temperature of 30
◦
C for 3 h in various solvents such as acetoni-
trile, methanol, toluene, water, dioxane, and n-butanol. The authors observed that the
highest yields of acid were achieved when using acetonitrile, while cyclohexanal did not
undergo oxidation in both alcohols, methanol and n-butanol [
25
]. Under the optimized
conditions, a high yield of 90% for 2-EHA was obtained in MeCN as the solvent.
Patent authors [
26
,
27
] obtained 2-EHA by means of specific chemical reactions. Moi-
seevich et al. describe a method for obtaining acid from the waste from the production
of N-2-ethylhexyl-N’-phenyl-N-phenylenediamine. The authors of the patent alkylated
N-aminodiphenylamine with a solution of potassium 2-ethylhexanoate. Then, they added
water to the mixture and extracted several times with an aliphatic solvent. 2-Ethylhexanoic
acid was distilled from the organic layer under reduced pressure with a >99% yield. In
the patent [
27
], the authors used the reaction of 1-amino-2-bromo-3-ethylheptene with
dimethyl fumarate. In the process conducted for 3 h at 48
◦
C, 2-EHA was obtained with
96% yield. The acid was separated from the post-reaction mixture by seven-fold extraction
with a solution of 1,3-propanediamine and a nine-fold extraction with propylene glycol
methyl ether. The described method allows only limited amounts of 2-EHA to be obtained.
For industrial process, new solutions are being sought to improve the economic efficiency
while meeting very stringent environmental protection requirements.
Herein, an environmentally friendly technology of 2-EHAL oxidation to 2-EHA is
reported. The developed method uses oxygen as the oxidant and NHPI as the catalyst. It is
in line with the current global trends aimed at developing green oxidation processes that use
environmentally friendly oxidizing agents. We propose that this method holds potential
for implementation in the industry due to its high selectivity, cost-effective oxidizing
agent, and mild reaction conditions. Furthermore, the use of isobutanol as the solvent is
of crucial importance. The hydroformylation of propylene results in a mixture of both
valuable butanal and a smaller amount of less valuable isobutanal. Both aldehydes can be
subsequently hydrogenated to respective alcohols. Therefore, this presents an opportunity
for potential producers of 2-EHAL from butanal (Figure 1) to utilize the less valuable
isobutanol. The potential of employing isobutanol as a solvent in aldehyde’s oxidation
with oxygen in the presence of NHPI has been reported for the first time. Previous attempts
to use methanol or n-butanol in this reaction did not yield positive outcomes [25].
2. Materials and Methods
2.1. Materials
N-hydroxyphthalimide (Sigma-Aldrich, St. Louis, MO, USA, 97%), acetonitrile (Su-
pelco, Bellefonte, PA, USA, 99.9%), isobutanol (Grupa Azoty ZAK S.A., Kedzierzyn-Kozle,
Poland, 99.7%), n-butanol (Grupa Azoty ZAK S.A. 99.8%), 2-ethylhexanol (Grupa Azoty
ZAK S.A. 99.7%), heptane (Supelco 99.5%), decane (Sigma-Aldrich 99%), toluene (Chempur,
Piekary Slaskie, Poland, 99.5%), acetic acid (Chempur 99.5%), methanol (Chempur 99.8%),
and 2-ethylhexanal (Sigma-Aldrich 96%).
2.2. 2-EHAL Oxidation Reaction with Oxygen or Air
The oxidation processes of 2-EHAL with oxygen were carried out using a gasometric
apparatus, as depicted in Figure 2. The 2-EHAL, catalysts, and solvent were introduced
into a 10 cm
3
two-necked flask. The reaction flask was connected to a gas burette filled with
oxygen at atmospheric pressure. The oxygen uptake was monitored during the oxidation
process with an accuracy of 0.1 cm
3
. The conversion of 2-EHAL was calculated based on

Materials 2023,16, 5778 4 of 10
amount of 2-EHAL in the reaction mixture before and after reaction determined using
GC analysis. The selectivity of 2-EHA and byproducts were determined by means of GC
analysis.
Figure 2.
Gasometric apparatus: (A)—two-neck flask, (B)—oil bath, (C)—magnetic stirrer,
(D)—thermometer, (E)—water tank, (F)—connection to oxygen cylinder, (G)—measuring burette,
(H)—reflux condenser.
2.3. Analytical Methods
The composition of the reaction products was analyzed using an Agilent 8890 gas chro-
matograph equipped with a FID detector, DB-WAXetr column (50 m
×
0.32 mm
×
1
µ
m),
and automatic sample dispenser. The analysis was carried out using helium as a carrier
gas. Dispenser temperature: 250
◦
C; detector temperature: 270
◦
C; division: 20:1; injection
volume 1
µ
L; air 400 mL/min.; nitrogen 25 mL/min.; hydrogen 30 mL/min.; oven tempera-
ture program: 50
◦
C, 3
◦
C/min. to 65
◦
C, 5
◦
C/min to 120
◦
C, 15
◦
C/min to 200
◦
C, 200
◦
C

Materials 2023,16, 5778 5 of 10
for 15 min. Each sample was analyzed twice and the concentration of the substance was
calculated from pre-prepared calibration curves. Samples for analysis were taken directly
from the post-reaction mixture, no prior preparation was required prior to GC analysis.
The composition of the product was additionally confirmed by the gas chromatograph with
mass spectrometry (GC-MS) performed on the Agilent 8890 gas chromatograph equipped
with an automatic sample dispenser, DB-5ms column (30 m
×
0.25 mm
×
0.25
µ
m, helium
2 mL/min), coupled to the Agilent 5977B GC/MSD (EI 70 eV) mass spectrometer using the
NIST mass spectra library.
3. Results
In the drive to develop a more sustainable and environmentally friendly method of
2-EHA synthesis with industrial potential, NHPI was selected as the organocatalyst for the
oxidation of 2-EHAL using oxygen or air under atmospheric pressure. Due to the limited
solubility of NHPI in both the aldehyde and obtained acid, the use of a polar solvent was
needed to achieve a homogeneous reaction mixture. The influence of different solvent
types and quantities, catalyst amount, temperatures, and reaction time on the conversion
of 2-EHAL and the selectivity of 2-EHA were examined. The composition of the resulting
mixtures was determined by means of gas chromatography analysis. Apart from 2-EHA,
several byproducts were identified in the reaction mixture, including: heptane, 3-heptanone
(3H=O), 3-heptanol (3H-OL), and 3-heptyl formate (3HFE).
3.1. Effect of Solvent Type
The limited solubility of NHPI in 2-EHAL was demonstrated through solubility tests.
Hence, research was conducted to evaluate the effect of different solvents on the studied
reaction. Solvents commonly available in the plant producing 2-EHAL, namely iso-butanol
(i-BuOH), n-butanol, and 2-ethylhexanol, were used. Additionally, reactions were carried
out in acetonitrile (MeCN), heptane, decane, toluene (PhCH
3
), acetic acid (AcOH), and
methanol (MeOH) for comparison purposes. The conversion of the raw material, selectivity
towards the main product 2-EHA, and byproducts are shown in Table 1.
Table 1. The influence of the type of solvent on the oxidation of 2-EHAL.
Entry Solvent
Conv.
2-EHAL
[%]
Sel.
2-EHA
[%]
Sel.
Heptane
[%]
Sel.
3H=O
[%]
Sel.
3HFE
[%]
Sel.
3H-OL
[%]
1 AcOH 99.9 61.9 1.6 nd 8.5 6.1
2 MeCN 99.8 47.1 nd 1.5 1.9 8.5
3 PhCH399.5 60.7 0.7 0.5 21.7 0.5
4 Heptane 98.9 68.7 ** 0.4 14.4 2.2
5 Decane 98.8 71.5 0.8 0.2 16.0 1.0
6 2-ethylhexanol 55.2 93.0 1.6 2.1 0.8 6.3
7 i-BuOH 47.8 92.6 nd 0.9 0.8 1.0
8 n-butanol 42.7 97.0 nd * 0.7 4.1
9 MeOH 0.0 nd nd nd nd nd
2-EHAL 2 mmol, solvent 8 cm
3
, NHPI 5% mol, 30
◦
C, 3 h, 800 RPM, oxygen—0.1 MPa, “nd” not detected, - * the
solvent has the same retention time as 3H=O, ** solvent.
It was observed that the use of both polar solvents, such as MeCN or AcOH, and
non-polar solvents, such as heptane, decane, and toluene, resulted in high conversions of
2-EHAL (
≥
99%). However, the 2-EHA selectivity was found to be unsatisfactory, ranging
between 47 and 69%. Additionally, it was noted that the share of 3HFE reaction formation
was significantly higher in non-polar systems. This suggests that the oxidation mechanism
(Figure 3) in non-polar systems favors the migration of the alkyl group (pathway B) rather
than hydrogen (pathway A). As a result, the adduct decomposes to the carboxylic acid
and formate [
15
,
18
,
28
,
29
]. The formation of 3HFE in non-polar systems in the amount of
several to several dozen percent (Table 1, entry 3–5), aligns with the results obtained by

Materials 2023,16, 5778 6 of 10
Lehtinen, Nevalainen, and Brunow [
29
,
30
]. The oxidation of 2-EHAL did not occur when
methanol was used as a solvent, which aligns with the results obtained by Dai, Qu, and
Kang in their pioneering use of NHPI for this reaction [19].
Materials 2023, 16, x FOR PEER REVIEW 6 of 10
Figure 3. Oxidation of aldehyde to the corresponding acid via reactions of adduct.
Surprisingly, encouraging results were obtained when using the so-called OXO alco-
hols, namely i-BuOH, n-butanol, and 2-ethylhexanol, as solvents. Although the conversion
of 2-EHAL was lower compared to other solvents used (43–55%), the selectivities towards
2-EHA were remarkably high (>90%). Moreover, from a technological standpoint, it is eas-
ier to recover and reuse the unreacted raw material 2-EHAL, than to remove various im-
purities from the final product. Furthermore, it was confirmed that the esterification reac-
tion between 2-EHA and i-BuOH is limited under elevated temperature of studied reac-
tion. Therefore, i-BuOH was chosen for further investigation.
3.2. Effect of the Amount of i-BuOH
The effect of the amount of i-BuOH on the oxidation of 2-EHAL in the presence of
NHPI with oxygen was determined. The resulting conversion of 2-EHAL, as well as selec-
tivity of the acid and byproducts are depicted in Table 2.
Table 2. The influence of the amount of solvent on the oxidation of 2-EHAL.
Entry
Volume of
Solvent
(cm
3
)
Conc. of
2-EHAL
(mol/dm
3
)
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 8 4 47.8 92.6 nd 0.9 0.8 1.0
2 6 3 59.1 95.5 nd 1.0 1.0 0.7
3 4 2 61.3 94.9 nd 0.7 1.1 0.1
4 2 1 59.0 99.4 nd 0.6 1.4 0.2
2-EHAL 2 mmol, solvent i-BuOH, NHPI 5% mol, 30 °C, 3 h, 800 RPM, oxygen—0.1 MPa, “nd” not
detected.
The study revealed that conducting the process using i-BuOH in the amount of 2 cm
3
,
corresponding to a concentration of 1 mol/dm
3
, resulted in the highest selectivity towards
the acid. Although further reducing the amount of i-BuOH would have been economically
advantageous, it might have been insufficient for solubilizing NHPI.
3.3. Effect of the Amount of Catalyst
The effect of the amount of NHPI on the oxidation reaction of 2-EHAL at 30 °C using
oxygen in i-BuOH was investigated. The results are depicted in Table 3.
Table 3. The influence of the amount of NHPI on the oxidation of 2-EHAL.
Entry
NHPI
(mol%)
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1
8
61.2
96.5
nd
0.2
1.5
nd
2 6 62.1 95.2 nd 0.3 1.5 nd
Figure 3. Oxidation of aldehyde to the corresponding acid via reactions of adduct.
Surprisingly, encouraging results were obtained when using the so-called OXO alco-
hols, namely i-BuOH, n-butanol, and 2-ethylhexanol, as solvents. Although the conversion
of 2-EHAL was lower compared to other solvents used (43–55%), the selectivities towards
2-EHA were remarkably high (>90%). Moreover, from a technological standpoint, it is
easier to recover and reuse the unreacted raw material 2-EHAL, than to remove various
impurities from the final product. Furthermore, it was confirmed that the esterification
reaction between 2-EHA and i-BuOH is limited under elevated temperature of studied
reaction. Therefore, i-BuOH was chosen for further investigation.
3.2. Effect of the Amount of i-BuOH
The effect of the amount of i-BuOH on the oxidation of 2-EHAL in the presence
of NHPI with oxygen was determined. The resulting conversion of 2-EHAL, as well as
selectivity of the acid and byproducts are depicted in Table 2.
Table 2. The influence of the amount of solvent on the oxidation of 2-EHAL.
Entry
Volume of
Solvent
(cm3)
Conc. of
2-EHAL
(mol/dm3)
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 8 4 47.8 92.6 nd 0.9 0.8 1.0
2 6 3 59.1 95.5 nd 1.0 1.0 0.7
3 4 2 61.3 94.9 nd 0.7 1.1 0.1
4 2 1 59.0 99.4 nd 0.6 1.4 0.2
2-EHAL 2 mmol, solvent i-BuOH, NHPI 5% mol, 30 ◦C, 3 h, 800 RPM, oxygen—0.1 MPa, “nd” not detected.
The study revealed that conducting the process using i-BuOH in the amount of 2 cm
3
,
corresponding to a concentration of 1 mol/dm
3
, resulted in the highest selectivity towards
the acid. Although further reducing the amount of i-BuOH would have been economically
advantageous, it might have been insufficient for solubilizing NHPI.
3.3. Effect of the Amount of Catalyst
The effect of the amount of NHPI on the oxidation reaction of 2-EHAL at 30
◦
C using
oxygen in i-BuOH was investigated. The results are depicted in Table 3.

Materials 2023,16, 5778 7 of 10
Table 3. The influence of the amount of NHPI on the oxidation of 2-EHAL.
Entry NHPI
(mol%)
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 8 61.2 96.5 nd 0.2 1.5 nd
2 6 62.1 95.2 nd 0.3 1.5 nd
3 5 59.0 99.4 nd 0.6 1.4 0.2
4 4 55.8 94.4 nd 0.2 1.4 nd
5 2 47.7 96.8 nd 0.3 1.5 nd
6 - 22.9 23.4 nd nd 0.2 nd
2-EHAL 2 mmol, solvent 2 cm3i-BuOH, 30 ◦C, 3 h, 800 RPM, oxygen—0.1 MPa, “nd” not detected.
It was observed that reducing the NHPI amount below 6 mol% adversely affected the
aldehyde conversion, while increasing the catalyst amount to 8 mol% had minimal impact
on the conversion decrease. The selectivity remained consistently high (>94%) in all studied
reactions using from 2 to 8 mol% of NHPI. In contrast, the selectivity of the non-catalytic
reaction was significantly lower. This indicates the key role of the NHPI in the 2-EHA
formation. Probably, NHPI participates not only in the formation of the peracid from the
aldehyde, as proposed by the authors of [
25
], but also promotes the peracid decomposition,
generating phthalimido-N-oxyl radical (PINO) and 2-ethylhexanoyloxyl radical, which is
subsequently transformed into 2-EHA. This aligns with the paper regarding the NHPI-
catalyzed oxidation of benzaldehyde to benzoic acid [31].
Fine particles of undissolved NHPI were observed in the pre-oxidation mixture for
all systems. Additionally, systems containing 6 and 8 mol% of NHPI showed the presence
of sediment of undissolved NHPI in the post-reaction mixture. Considering the potential
problem of sediment formation in large-volume installations, it was decided to proceed
with further research using the addition of NHPI in the amount of 5 mol%.
For comparison, a non-catalytic oxidation reaction of 2-EHAL with oxygen at 30
◦
C was
performed for three hours using MeCN as a solvent. These conditions yielded 95% aldehyde
conversion and 38% selectivity towards acid. Subsequently, the oxidation process was
conducted in the presence of 5 mol% of NHPI in MeCN, under the same, aforementioned
conditions. As a result, the 2-EHAL conversion increased to 99.5% and 2-EHA selectivity
to 47%.
3.4. Influence of Temperature and Reaction Time
Table 4presents the results of the studies on the effect of temperature and reaction
time on the oxidation of 2-EHAL with oxygen.
Table 4. The influence of temperature and time on the oxidation of 2-EHAL.
Entry Temp. (◦C) Time
(h)
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 30 3 59.0 99.4 nd 0.6 1.4 0.2
2 35 3 70.3 75.0 nd 0.1 1.2 0.0
3 40 3 73.9 85.2 nd 0.4 1.6 0.1
4 50 3 76.0 71.4 nd 0.6 1.9 0.4
5 60 3 99.9 59.3 0.1 0.3 1.8 0.8
6 35 0.5 59.7 76.2 nd 0.2 1.1 0.2
7 35 1 62.8 63.0 nd 0.1 0.6 0.0
8 35 2 69.2 71.0 nd 0.1 1.1 0.0
9 35 3 70.3 75.0 nd 0.1 1.2 0.0
2-EHAL 2 mmol, solvent 2 cm3i-BuOH, NHPI 5% mol, 800 RPM, “nd” not detected.

Materials 2023,16, 5778 8 of 10
It was observed that increasing the temperature from 30 to 60
◦
C resulted in an increase
in 2-EHAL conversion from 59.0 to >99%. However, this temperature increase also led to a
decrease in selectivity to 2-EHA, which was unfavorable.
Extending the reaction time beyond 3 h is not justified as the rate of the 2-EHAL
oxidation reaction is low, as demonstrated in Figure 4which shows the oxygen consumption
over time of reaction.
Materials 2023, 16, x FOR PEER REVIEW 8 of 10
Extending the reaction time beyond 3 h is not justified as the rate of the 2-EHAL ox-
idation reaction is low, as demonstrated in Figure 4 which shows the oxygen consumption
over time of reaction.
Figure 4. Oxygen consumption versus reaction time.
3.5. Effect of Oxidizing Agent
When implementing a process on an industrial scale, it is necessary to use a cost-
effective oxidizing agent. In the context of industrial oxidation processes, air may be a
more suitable option compared to oxygen. Thus, the oxidation processes of 2-EHAL with
oxygen and air were compared. The results are depicted in Table 5.
Table 5. Effect of the oxidizing agent and temperature on 2-EHAL oxidation.
Entry
Temp. (°C)
Oxidizing
Agent
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 30 oxygen 59.0 99.4 nd 0.6 1.4 0.2
2 30 air 48.0 86.6 nd 1.1 1.3 0.5
3 40 oxygen 73.9 85.2 nd 0.4 1.6 0.1
4
40
air
58.1
63.4
0.1
0.4
1.0
0.5
2-EHAL 2 mmol, solvent 2 cm
3
i-BuOH, NHPI 5% mol, 30 °C, 3 h, 800 RPM, “nd” not detected.
Replacing oxygen with air led to a significant reduction in 2-EHAL conversion and
the 2-EHA selectivity (entry 1 and 2). Additionally, increasing the reaction temperature
when using air did not yield the desired effect of (entry 3 and 4). The obtained data indi-
cate that utilizing air would require higher pressure.
4. Conclusions
(1) The study demonstrated the feasibility of the oxidation process of 2-EHAL to acid
under mild conditions with oxygen in the presence of NHPI as a catalyst in i-BuOH
as a solvent.
(2) 2-EHA was obtained with high selectivity of 99.4% and a conversion of 59.0% (30 °C,
3 h, 0.1 MPa, 5 mol% NHPI, i-BuOH)
(3) The developed method holds potential for implementation in industry due to its high
selectivity, cost-effective oxidizing agent, and mild reaction conditions.
Figure 4. Oxygen consumption versus reaction time.
3.5. Effect of Oxidizing Agent
When implementing a process on an industrial scale, it is necessary to use a cost-
effective oxidizing agent. In the context of industrial oxidation processes, air may be a
more suitable option compared to oxygen. Thus, the oxidation processes of 2-EHAL with
oxygen and air were compared. The results are depicted in Table 5.
Table 5. Effect of the oxidizing agent and temperature on 2-EHAL oxidation.
Entry Temp. (◦C) Oxidizing
Agent
Conv.
2-EHAL
(%)
Sel.
2-EHA
(%)
Sel.
Heptane
(%)
Sel.
3H=O
(%)
Sel.
3HFE
(%)
Sel.
3H-OL
(%)
1 30 oxygen 59.0 99.4 nd 0.6 1.4 0.2
2 30 air 48.0 86.6 nd 1.1 1.3 0.5
3 40 oxygen 73.9 85.2 nd 0.4 1.6 0.1
4 40 air 58.1 63.4 0.1 0.4 1.0 0.5
2-EHAL 2 mmol, solvent 2 cm3i-BuOH, NHPI 5% mol, 30 ◦C, 3 h, 800 RPM, “nd” not detected.
Replacing oxygen with air led to a significant reduction in 2-EHAL conversion and the
2-EHA selectivity (entry 1 and 2). Additionally, increasing the reaction temperature when
using air did not yield the desired effect of (entry 3 and 4). The obtained data indicate that
utilizing air would require higher pressure.
4. Conclusions
(1)
The study demonstrated the feasibility of the oxidation process of 2-EHAL to acid
under mild conditions with oxygen in the presence of NHPI as a catalyst in i-BuOH
as a solvent.
(2)
2-EHA was obtained with high selectivity of 99.4% and a conversion of 59.0% (30
◦
C,
3 h, 0.1 MPa, 5 mol% NHPI, i-BuOH)

Materials 2023,16, 5778 9 of 10
(3)
The developed method holds potential for implementation in industry due to its high
selectivity, cost-effective oxidizing agent, and mild reaction conditions.
(4)
i-BuOH enables the dissolution of NHPI in the reaction mixture, does not undergo
esterification under the reaction conditions, and facilitates heat exchange. Addition-
ally, the use of i-BuOH as a solvent provides an opportunity for potential producers
of 2-EHAL from butanal to utilize this less valuable alcohol.
(5)
It was observed that the use of air is feasible, however, it would require higher pressure.
Author Contributions:
Conceptualization, B.O. and E.P.; methodology, B.O., Ł.C. and D.L.; GC
analysis, Ł.C.; investigation, Ł.C.; writing—original draft preparation, Ł.C. and D.L.; supervision,
B.O. All authors have read and agreed to the published version of the manuscript.
Funding:
The research was co-funded under the MEiN (Ministry of Education and n) program
“Implementation Doctorate” No. 32/014/SDW/001-08 and by Rector of the Silesian University of
Technology grant No. 04/050/RGJ22/0143. The publication cost was co-financed by Grupa Azoty
Zakłady Azotowe K˛edzierzyn S.A.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
All data are available in the manuscript or upon request to the corre-
sponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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