Catalyst-free aerobic oxidation of aldehydes into acids in water under mild conditions

Article abstract of DOI:10.1039/c7gc02983g

The first example of catalyst-free aerobic oxidation of aldehydes in water under respective acidic, neutral and alkaline conditions was developed. The sole oxidant is molecular oxygen of 1 atmosphere and reactions can proceed under extremely mild conditions. This procedure covers a wide range of aldehydes, and operates easily. No additives and catalysts were required for this purpose, and most of the aldehydes can be converted to their corresponding carboxylic acids with good to excellent yields, in addition, no side-product formation could be observed during or after the reactions. To well illustrate why the oxidation rate becomes fast firstly and then slows with increased temperatures, five control reactions were carried out and a Fe³⁺/Fe²⁺ recycling system was introduced to facilitate the aldehyde oxidation rate. The generality of this method offers the potential for industrial aldehyde-containing waste water treatment.

Full text of DOI:10.1039/c7gc02983g

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Catalyst-free aerobic oxidation of aldehydes into acids in water
under mild conditions
Yue Zhang,a† Yujia Cheng,a† Huizhuo Cai,a Shaopo He,a Qiheng Shan,a Hongwei Zhao,b,c Yiping
Chen,a and Bo Wang*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The first example of catalyst-free aerobic oxidation of aldehydes in water under respective acidic, neutral and alkaline
conditions was developed. The sole oxidant is molecular oxygen of 1 atomosphric and reactions can proceed under
extremely mild conditions. This procedure covers a wide range of aldehydes, and operated easily. No additives and
catalyst were required for that purpose, and most of the aldehydes can be converted to their corresponding carboxylic
acids with good to excellent yields, in addition, no side-product formation could be observed during or after reactions. To
well illustrate why oxidation rate become fast firstly and then slows with increased temperatures, five control reactions
were carried out and a Fe3+/Fe2+ recycling system was introduced to facilitate aldehyde oxidation rate. The generality of
www.rsc.org/
this
method
offers
the
potential
for
industrial
aldehyde-containing
waste
water
treatment.
and usually considered not environmentally benign.10-12
Therefore, procedures with molecular oxygen as the terminal
oxidant and proceed in aqueous phase remains scare for clean
preparation of carboxylic acids from aldehydes.13
Introduction
Oxidation is one of the most fundamental reactions in nature,
and the oxidation of aldehydes into their corresponding
carboxylic acids is an important biological process for daily life
activities.1 One of the most representative examples is the
oxidation of acetaldehyde into acetic acid in liver cells at 37oC
in water using the sole oxidant oxygen and dehydrogenase as
the catalyst under neutral pH conditions as some aldehydes,
for instance, acetalhyde can cause damage to human DNA.2 As
P
revious works
hazarous oxidants
KMnO4/CrO4/KHSO3/KIO4, etc
a)
b)
[Ag]/[Cu] ligand containing P or NHC
1 atm O2, 1 equiv NaOH, H2O, 50o
C
FeIIIMO6
c)
O
O
O
Na2CO3, 1 atm O2, H2O, 50o
C
R
H
R
R
OH
OH
has been illustrated(Route
a,
Fig.1),3-6 most aldehydes are
This work
generally stable and inert towards autoxidation though being
prone to oxidation as most of them need strong oxidizer such
as KMnO4, CrO4, or KIO4 to assist their transformation to
carboxylic acids.7 In addition, most procedures for aldehyde
oxidation still rely on stoichiometric loadings of hazardous
oxidants such as dichromate/permanganate,8-9 oxone,5
periodate reagent,6 and etc promoted oxidation reactions,
which usually proceeded in harmful organic solvents, and
followed is the generation of large amounts of waste water
stream usually containing heavy metals, which are wasteful
O
no catalyst, no base
1 atm O2, H2O, 37oC
R
H
Fig.1 Representative examples and schematic profile of oxidation of
aldehydes into carboxylic acids
In addition, in some draining waste water from chemical
companies especially those from medicine chemical synthesis
corporations, ‘trace-less’ aldehydes usually exist, which are
difficult for natural biodegradation and may cause serious
environmental problems,13,14 for instance, diatoms can release
toxic polyunsaturated aldehydes.15 Therefore, before
discarded, aldehydes must be removed or converted to their
corresponding stable formats (carboxylic acids) that are not
harmful to living creatures, one important principle is the
removal process shouldn’t introduce new chemical(s) that
might be new pollution sources, and therefore base/acid or
oxidants for waste water treatment are unsuitable as further
downstream neutralization or reduction processes are
required, usually followed is salt stream requiring a second salt
removal treatment. Therefore, developing green and catalyst-
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free protocols using molecular oxygen as the sole oxidant and better conversions at 25oC in 8h, as when using H2O2 under the
proceeded smoothly at ambient temperatures covering a wide same conditions, the conversion was 47%(Entry 1, Table 1),
range of pH is highly desirable in both industry and academia, and a higher temperature when at 37oC, the yields declined
especially useful for aldehyde-containing waste water sharply to 8% (Entry 2, Table 1). Nevertheless when use
treatment processes.
molecular oxygen as the oxidant at 37oC and the reaction time
Compared to commonly used organic solvents, the natural was prolonged to 24h, the yield was boosted to >99% (Entry 13,
abundance of water and its inherently greener characteristics Table 1), indicating that oxygen is more suitable for aldehyde
makes it extremely attractive for developing eco-friendly oxidative conversion compared to H2O2, and therefore in
reactions. Recently, aerobic oxidations of aldehydes using future evaluations, oxygen will be chosen as the oxidant and
molecular oxygen have received a plethora of attentions.7,16,17 reaction time is 24h. Reaction temperatures varied from 25oC
Li et al. developed the first example of a homogeneous to 50oC were also evaluated for suitable oxidation conditions,
catalyst silver-/copper- ligand promoted aerobic oxidation in when temperature was increased from 25oC to 37oC, and
water,7 an array of aldehydes was converted to acids with ideal finally to 50oC, the yield of product benzoic acid 2a was firstly
yields under mild conditions (50oC, Route
b, Fig.1). Wei et al. boosted from 78% to a quantitative yield of 99%, and then
reported the first example using a recyclable homogeneous declined to 94% at 50oC (Entry 6,9,13, Table 1), indicating that
catalyst iron(III)-catalysed aerobic oxidation of aldehydes in 37oC is more suitable among all tested temperatures for
water under mild conditions,17 a wide range of functionalized oxidation. Conventionally, those reactions proceed at a higher
aldehydes were transformed into carboxylic acids in good to temperature give faster reaction rates compared to those at a
excellent yields of up to 99% (50oC, Route
c, Fig.1). lower temperature, and therefore, the conversion of aldehyde
Nevertheless, in both methods, metal catalysts were involved, is supposed to increase before reaching a full conversion.
and strong alkalinity of reaction media is relied on, which are Table 1. Optimization of reaction conditionsa
incompatible with alkalinity sensitive substrates or substrate
with alkalinity sensitive group(s). And when reactions were
completed, an additional neutralization processes are required,
Temp.
(oC)
Time
(h)
8
leading to the formation of large amount of waste salt stream.
Herein, we present a catalyst-free aerobic oxidation process
for treatment of trace-aldehyde-containing waste water
through the way of converting aldehyde into carboxylic acids
at extremely mild conditions, noted that here we use mimic
waste water (dissolving aldehyde in water) instead of real-life
one aims at easy and accurate detection of aldehyde and their
related carboxylic acids, molecular oxygen of 1 atmospheric is
used as the sole oxidant. For testing the generality of the
protocol, acidic, neutral and alkaline conditions were assessed
to expect for satisfactory results (Table.2). A wide range of
aldehydes were evaluated and no additional additives (base or
acid) were required for that purpose. And 37oC was chosen as
the optimized temperature for reactions after optimization
reactions, five control reactions were carried out with/without
Fe3+ ions to investigate how and the effect of phase transfer on
aldehyde conversions with increased temperatures. Most of
the aldehydes can be converted to their corresponding
carboxylic acids with good to excellent yields of up to 99%, and
no side-product formation could be detected during or after
reactions.
Entry
C (mM)
Oxidant
Yield (%)
1
2
10
10
H2O2
H2O2
25
47
8
37
8
3
4
5
6
7
8
10
5
O2
O2
O2
O2
O2
O2
37
25
25
25
50
50
8
8
35
35
62
78
32
77
5
12
24
8
5
5
5
12
9
5
5
O2
O2
O2
O2
O2
O2
O2
50
37
37
37
37
37
37
24
8
94
37
10
11 b
12
13
14
15
5
8
99
5
12
24
24
24
81
5
>99
90
10
2
99
aConditions: all reactions were carried out at
a 5ml scale at varied
temperatures under pH = 5.0, after all substances were charged in closed
vessels. Yields were determined by HPLC analysis using a SHIMADZU LC-16
instrument equipped with a C18-WR column at the wavelength of 254 nm; b
Reaction was performed with conditions as in footnote a, except that the pH
of the reaction medium was made at 3.0 using Fe3+ of 0.5%mol.
Results and discussion
Initial investigations were carried out using benzaldehyde as
the model substrate under aqueous conditions, in order to
meet some principles of green chemistry, H2O2 and molecular
oxygen were selected as candidates for oxidation of aldehydes
into carboxylic acids as the resulted sole by-product is water
after reaction. Oxygen was simply flushed into a balloon and
sealed along with the flask after substrate aldehyde and water
as the solution were charged. Compared with those employing
oxidant H2O2, reactions using molecular oxygen could offer
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
Fe(II)
Fe(III)
Aldehyde
O2
O2
O2
O2
O2
Carboxylic
acid
O2
Fig.2 Oxygen on/below the surface of reaction medium and Fe3+/Fe2+
recycling system
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With increased temperatures from 25oC to 50oC,
a
transformation, and also similar results were provided by Li et
promotion in the reaction rate was observed, however, the al. and Wei et al. as in whose reported procedures,
solubility of molecular oxygen in water shows a reverse NaOH/Na2CO3 was essential for such transformation. By
tendency, reaction rate slowed. The decline in the yield (94%) prolonging the reaction time to 24h, the results made basically
of product 2a at 50oC may possibly attribute to mass transfer no difference in both reactions as both could provide yields of
resistance of oxygen from oxygen gas on liquid surface down up to 99%. Replacing ion Fe3+ in reaction
to liquid solution. Larger mass transfer resistance of oxygen reaction , similar results can still be obtained, indicating that
makes the concentration of oxygen lower, and as
when with no assistance of ion Fe3+ after a prolonged time, the
consequence, fewer benzaldehyde molecular will be conversions can still approach theoretical conversions.
E and compared with
D
a
O
O
successfully transformed to benzoic acids though oxidation
reaction rate maybe accelerated at a higher temperature.
To well address increased mass transfer resistance of oxygen
with increased temperatures and whose effects on the
oxidation of aldehydes in water, five control reactions A, B, C,
Ph
H
Ph
OH
Fe(III)
Fe(II)
D, E (Fig. 3) under different conditions were carried out and
O2
evaluated. A Fe3+/Fe2+ recycling system was introduced and
functioned as a shuttle moving up and down from the surface
of solution down to inside it (Fig. 2), Fe3+ ion can oxidize
benzaldehyde into benzoic acid easily and very fast, which was
then converted to a reduced format Fe2+(Scheme A) after
oxidizing aldehyde into acid, and when ion Fe2+ moves to the
surface of reaction medium, it was converted back to Fe3+ the
moment when meeting with oxygen, and again back to
solution ready for oxidation in a following round. In reaction A,
ion Fe3+ of 0.5mol% was dissolved in water, and the solution
was set at pH3.0, after 8h, an almost full conversion of >99%
1atm
O2
O2
O2
N2
O2
O
O
O2, balloon
Fe(III), 0.5 mol%
pH = 3.0
A
B
was achieved. However, in reaction B
, with no Fe3+ ions, in the
Ph
Ph
Ph
H
H
H
Ph
OH
Yield, >99%, Reaction time, 8h
same time 8h, only a poorer conversion of 38% was obtained.
Based on these results, it is indicated that the oxygen transfer
from on the solution surface down to inside the solution was
greatly accelerated. By prolonging the reaction time to 24h, we
found that a quantitative yield could also be afforded,
indicating that under pH 3.0, ion Fe3+ could greatly enhance
oxidation rate and reduce reaction time. In addition, the
results also indicated that with enough oxygen gradually
dissolving in solution, the conversion can also be very close to
that using a Fe3+/Fe2+ recycling system, a conclusion can thus
be drawn that with increasing temperatures, lower
conversions were associated before full conversions, a longer
reaction time was necessary for theoretically ideal conversions.
O
O
O2, balloon
no Fe(III)
pH = 3.0
Ph
OH
Yield, 38%, Reaction time, 8h
Yield, 99%, Reaction time, 24h
N2, balloon
O
O
Fe(III)
0.5, 50, or 100 mol%
pH = 3.0
C
Ph
OH
Yield, 0.5%, Reaction time, 8h
Fe(III) 0.5mol%
Yield, 51%, Reaction time, 8h
Fe(III) 50mol%
Yield, >99%, Reaction time, 8h
Fe(III) 100mol%
In reaction C
, we used N2 instead of O2 and ion Fe3+ of 0.5mol%
O
O
O
O2, balloon
to test the oxidation capacity of Fe3+, and as expected, about
0.5% of aldehyde was successfully converted to benzoic acid,
even when reaction was prolonged to 24h. After reaction, the
volume of N2 after reaction remains the same as before,
proved that the airtightness of reaction vessel was quite good.
As is known, ion Fe3+ will precipitate when pH of the solution
turns to be >3.2. And in order to test whether oxidation
reaction can still proceed well when pH of the reaction
medium was made at 9.0 (in such case, the majority portion of
Fe3+ will become precipitate, leading to a very small portion of
metal ion dissolved and remained in the reaction system, and
the left Fe3+ ions were supposed to loss their capability for
oxidation of aldehyde). After 8h, the conversion was measured
by HPLC to be 40%, a slight increase compared to that
proceeded under pH 3.0, it was proposed that stronger
alkaline can make positive contributions to oxidative
Fe(III), 0.5 mol%
pH = 9.0
D
E
Ph
Ph
H
H
Ph
OH
Yield, 40%, Reaction time, 8h
Yield, 99%, Reaction time, 24h
O
O2, balloon
no Fe(III)
pH = 9.0
Ph
OH
Yield, 39%, Reaction time, 8h
Yield, 99%, Reaction time, 24h
+ PhCOOH + H+
a)
Fe(III)
Fe(II)
+ PhCHO + H2O
+ O2 + H+
Fe(II)
Fe(III)
b)
+ H2O
Fig. 3 Control experiments for exploring possible effects on aldehyde
oxidation caused by mass transfer resistance of oxygen
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The concentration of benzaldehyde can also have some full conversion (99%) was achieved (Entry 13, Table 1),
effects on conversion of benzaldehyde during oxidation indicating that either by increasing the concentration of
process, as has been addressed that the bottleneck in this molecular oxygen or lowering aldehyde’s concentration, the
reaction is continuous dissolving oxidant oxygen in reaction yields of product 2a can be theoretically obtained to be
solution. When concentration of benzaldehyde 1a was made quantitative, however it is quite tough to increase the
at 10mM, after reacted for 24h, the yield of product 2a was 90% concentration of oxygen at a higher temperature with 1
(Entry 14, Table 1), nevertheless, when substrate 1a was set at atmospheric oxygen. Therefore,
a
lower aldehyde
a lower concentration 5mM under the same conditions, almost concentration of 5.0mM will be chosen in future investigations.
Table 2 Results of oxidation under acidic, neutral and alkaline conditions
Entr
y
C
Time
(h)
Yield
(%)
C
Time
(h)
Yield
(%)
C
Time
(h)
Yield
(%)
Aldehyde
Product
pH
Entry
16
pH
Entry
pH
9.0
(mM)
(mM)
(mM)
1
10
5.0
24
90
10
7.0
24
92 31
10
24
96
1a
2
3b
5
5
5.0
3.0
24
8
96
99
17
18d
5
5
7.0
7.0
24
8
99 32
39 33f
5
5
9.0
9.0
24
8
>99
41
1b
1c
4
5
5.0
24
74
19
5
7.0
24
83 34
5
9.0
24
90
5
5
5.0
24
84
20
5
7.0
24
84 35
5
9.0
24
86
1d
1e
1f
6
7
8
5
5
5
5.0
5.0
5.0
24
24
24
87
98
99
21
22
23
5
5
5
7.0
7.0
7.0
24
24
24
97 36
>99 37
96 38
5
5
5
9.0
9.0
9.0
24
24
24
93
99
99
1g
1h
1i
9
5
5
5
5
5.0
5.0
5.0
5.0
24
12
24
24
99
95
63
99
24
25
26
27
5
5
5
5
7.0
7.0
7.0
7.0
24
24
24
24
>99 39
95 40g
65 41
>99 42
5
5
5
5
9.0
9.0
9.0
9.0
24
36
24
24
89
99
85
99
10
11
12
1j
1k
13
5
5.0
24
75
28
5
7.0
24
78 43
5
9.0
24
89
14c
15
5
5
3.0
5.0
8
99
99
29e
30
5
5
7.0
7.0
8
47 44h
99 45
5
5
9.0
9.0
8
51
1l
24
24
24
>99
aConditions: Unless otherwise stated, all reactions were carried out at a 5ml scale at 37oC under varied pH conditions with oxygen of 1 atmospheric, after all
substances were charged in closed vessels and oxygen was flushed in a balloon and sealed, which was stirred in a shaker incubator for 24 h. Yields were
determined by HPLC analysis using a SHIMADZU LC-16 instrument equipped with a C18-WR column (4.6 mm×250 mm,5μm) at the wavelength of 254 nm;
b,cReaction was performed with conditions as in footnote a, except that the pH of the reaction medium was made at 3.0 when using Fe3+ of 0.5%mol, and
when at pH = 5.0, and 9.0, no Fe3+ ions were employed herein, reaction time, 8h;d,e,f,hReaction time, 8h; gReaction time, 36h.
With all optimized reaction conditions in hand, the scope of withdrawing electron to the aldehyde moiety, making the
substrates and functional-group compatibility of this aldehyde moiety less inactive for oxidation compared with
procedure were further evaluated (Table 2) under varied pH conventional aromatic rings with no substituent(s). In most
conditions (pH = 3.0, 5.0, 7.0, and 9.0). An array of aldehydes conditions, reactions proceeded under pH 9.0 could provide
was then conducted and some of them bear various functional better yields than those under pH 5.0, indicating that better
groups (electron- donating/withdrawing group(s)), all reactions conversions can benefit from the addition of base (Entry 1-
were proceeded smoothly and the results were summarized in 3,16-18,31-33, Table
2), which correlated well with the
Table 2. To our satisfied, most of the reactions could afford reported procedures by Li et al. and Wei et al. However, using
good to excellent yields of up to 99% except aldehyde 1i, the base for assisting oxidation process by molecular oxygen might
yields were 65%, 63%, and 85% respectively when carried out be unpractical especially for aldehydes bearing alkaline
under pH 5.0, 7.0 and 9.0 (Entry 11, 26, 41, Table 2). It was sensitive functional groups, and also for consideration of green
supposed that in the molecular structure of 1i, the aromatic and sustainable development for industrial purpose as in the
heterocyclic ring as
a functional group functioned as following downstream treatment process, large amounts of
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9
L.T. Sandborn, Org. Synth. 1921,
10 M. Yoshida, Y. Katagiri, W.-B. Zhu, K. Shishido, Org. Biomol.
Chem. 2009, , 4062.
1, 340.
waste water were usually generated. As to the aliphatic
aldehyde n-butyl aldehyde, this aerobic oxidation seems to be
not compatible well as poor conversion of 32% was obtained.
Further optimisation of the conditions for aliphatic aldehydes
so that these substrates can be utilized is underway.
7
11 L. Han, P. Xing, B. Jiang, Org. Lett. 2014, 16, 3428.
12 P. de Frémont, N. M. Scott, E.D. Stevens, T. Ramnial, O.C.
Lightbody, C.L.B. Macdonald, J. Clyburne, A.C.C.D. Aber-
nethy, S.P. Nolan, Organometallics 2005, 24, 6301.
13 S.S. Stahl, Science 2005, 309, 1824.
14 I. Arends, U. Hanefeld, Green Chemistry and Catalysis, ed. R.
A. Sheldon, Wiley-VCH, Weinheim, 2007.
15 M. Pepi, H. J. Heipieper, C. Balestra, M. Borra, E. Biffali, R.
Casotti, Chemosphere, 2017, 177, 258.
In conclusion, we have, for the first time developed a
catalyst-free aerobic oxidation of an array of aldehydes
proceeding in aqueous solution. During oxidation reactions,
ultra-green molecular oxygen is used as the sole oxidant, the
generality of the procedure under acidic, neutral and alkaline
conditions were evaluated and most of them gave satisfactory
conversions. To illustrate why the conversions became lower
at a higher temperature 50oC, five control reactions were
carried out, and some were introduced with a Fe3+/Fe2+
recycling system to facilitate fast oxidation. It was found that
the solubility of molecular oxygen plays an essential role in
increased mass transfer resistance of oxygen. A wide array of
aldehydes was subject and most of the reactions offer the
corresponding carboxylic acids with good to excellent yields of
up to 99%, no side-product formation reactions were observed.
Offering potentials for sustainable and clean synthesis of
carboxylic acids from aldehydes, especially for ‘trace-less'
aldehyde waste processing in chemical industry.
16 M. Liu, H. Wang, H. Zeng, C.-J. Li, Sci. Adv. 2015, 1, e1500020
17 H. Yu, S. Ru, G. Dai, Y. Zhai, H. Lin, S. Han, Y. Wei, Angew.
Chem. Int. Ed. 2017, 56, 3867.
Acknowledgements
Financial support from the National Natural Science
Foundation of China (21502036), Innovative Research Team
Project of Hainan Natural Science Foundation (2016CXTD006),
Hainan Provincial Natural Science Foundation (217041) and
Hainan University (kyqd1408) are gratefully acknowledged.
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