Insights in the aerobic oxidation of aldehydes

Article abstract of DOI:10.1039/c3ra42385a

The hydroformylation of olefins (oxo synthesis) is the most important process for the production of higher aldehydes (>C₄). The liquid phase oxidation of the latter to carboxylic acids by molecular oxygen or air has been known for more than 150 years and is an industrially important process. However, in the recent literature, several different oxidizing reagents and catalytic processes have been reported for this oxidation but most of them have limitations as they use environmentally unacceptable reagents or unnecessarily sophisticated conditions. Herein, we re-evaluated the air oxidation of aldehydes. We show that under mild conditions (air or oxygen and non-optimized stirring), reactions are transfer limited and thus catalyst has no effect on reaction rate. Using efficient stirring (self-suction turbine), uncatalysed air oxidation of 0.8 M aldehyde is possible in 50 min at room temperature whereas less than 10 min was necessary with 10 ppm Mn(ii). Thus, recommendations for avoiding common pitfalls that may rise during the evaluation of new catalysts are made. The Royal Society of Chemistry 2013.

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PAPER
†
Insights in the Aerobic Oxidation of Aldehydes
a
a,b*
a
a
a*
Laurent Vanoye, Alain Favre-Réguillon, Asma Aloui, Régis Philippe, and Claude de Bellefon
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
The hydroformylation of olefins (oxo synthesis) is the most important process for the production of
higher aldehydes (> C ). The liquid phase oxidation of the latter to carboxylic acids by molecular oxygen
4
or air has been known for more than 150 years and is an industrially important process. However, in the
recent literature, several different oxidizing reagents and catalytic processes have been reported for this
oxidation but most of them have limitations as they use environmentally unacceptable reagents or
unnecessarily sophisticated conditions. Herein, we reꢀevaluated the air oxidation of aldehydes. We show
that under mild conditions (air or oxygen and nonꢀoptimized stirring), reactions are transfer limited and
thus catalyst has no effect on reaction rate. Using efficient stirring (selfꢀsuction turbine), uncatalysed air
oxidation of 0.8 M aldehyde is possible in 50 min. at room temperature whereas less than 10 min. was
necessary with 10 ppm of Mn(II). Thus, recommendations for avoiding common pitfalls that may rise
during the evaluation of new catalysts are made.
1
0
1
5
subsequent condensation between the peracid and the aldehyde
1
. Introduction
25 produces the corresponding carboxylic acid via a tetrahedral
adduct similar to the Criegee intermediate observed in the Bayerꢀ
Villiger reaction. The migration of the hydride from the aldehyde
moiety to the nearest peroxide oxygen gives two carboxylic acids
The principal method for the commercial production of C ꢀC13
carboxylic acids is the oxidation in air (autoxidation) of the
corresponding aldehydes obtained by hydroformylation of
olefins (oxo synthesis). This reaction is catalysed on an
industrial scale by Co complexes. The autoxidation of aldehydes
in the liquid phase was shown by Bäckström to be a chain
reaction yielding, as the initial product, a peracid (Scheme 1). A
4
1
whereas the migration of the alkyl group from the aldehyde gives
2
2
0
5,6
30
an equimolar mixture of carboxylic acid and formate. The ratio
of hydride to alkyl migration was found to depend critically on
the structure of aldehydes, temperature, solvent, … .
1
3
,4
O
2
Gas phase
Liquid phase
O
O
Initiation
R
R
H
R
Rearrangement of the Criegee intermediate
O
O
+
O
2
OH
OH
O
+
R
O
O
R
O
O
O
R
Propagation
Termination
O
O
O
O
H
O
R
H
O
O
+
+
OH
O
H
R
O
R
2
H
O
R
O
R
R
O
R
H
R
OH
+
R
O
O
O
nonradical products
R
O
Free radical chain reaction
Scheme 1 Autoxidation of aldehydes to the corresponding acids
3
5
On the labꢀscale, the autoxidation of aldehydes has been 40 results difficult. Moreover, the pronounced effect of small
catalysed by different metallic ions in the form of salts or organic
amounts of inhibitors and initiators upon the rate has made
quantitative measurement and comparison of rates obtained by
4
,6ꢀ13
complexes of Mn, Co, Fe, Cu, Ni, Cr or Au.
However, the
4,8,14
wide variety of reaction conditions, catalyst characteristics and
methods for rate calculations make comparing the literature
different investigators very difficult.
The comparison is
further complicated because some reactions are carried out in a
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mass transfer limited regime where the rate limiting step is the 10 “green” oxidant and several procedures have been reported for
4
,7,8,11,14
rate of oxygen transfer from gas bubbles into the liquid.
Recently, the growing awareness of the environment has
created demand for efficient oxidation process with
environmentally friendly oxidants under mild conditions and has
the aerobic oxidation of aliphatic aldehydes to the corresponding
carboxylic acids using such green oxidant. In order to compare
the different systems described in the literature, air or oxygen
oxidation of aliphatic aldehydes at room temperature only will be
a
5
15
led to rich developments during recent decades. Highly 15 considered. The productivity of these reactions calculated as mM
efficient, selective and metalꢀfree oxidations of aldehydes have
been reported with hydrogen peroxide and oxone. However, diꢀ
oxygen and in particular diꢀoxygen from air is the ultimate
of aldehyde converted by volume of liquid phase by hour and for
1
6
0.2 bar of O (i.e. air) are listed in Table 1.
2
a
Table 1 Comparison of aldehyde oxidation by molecular oxygen at room temperature described in the literature.
b
Entry
Oxidant
bubbling
air bubbling
air
Catalyst
Substrate
2ꢀethylhexanal
nꢀheptanal
2ꢀethylhexanal
2ꢀethylhexanal
nꢀheptanal
nꢀoctanal
2ꢀethylhexanal
nꢀoctanal
Solvent
Time (h)
2.0
3.5
12
2.3
3.5
3.0
3.0
48.0
80.0
Conversion (%) Productivity (mM/L/h)
Ref.
5,6
1
2
3
4
5
6
7
8
9
O
2
_
_
_
_
CH
CH
water
water
3
CN
CNc
82ꢀ93
12
98ꢀ112
41
12
3
17
17
86
23
c
O
2
ballon
air bubbling
ballon
88
28
12
Au/CeOd
2
CH
3
CN
78
97
263
65
9
O
2
Ni (II)
dichloroethane
dichloroethane
d
10
18
19
O
2
Fe (III)
100
96
69
83
0.27
60
e
O
2
bubbling
H
2
TTPd
CH
water/ CH
3
CN
CN
air sat. solv.
Sb(II)
nꢀdecanal
3
a
b
Oxidation performed at room temperature and at atmospheric pressure in classical glassware using magnetic stirring. Productivity was calculated as
c
20
mM of aldehyde converted by volume of liquid phase by hour and for 0.2 bar of O
vide infra. soluble catalyst.
2
.
Aldehyde was insoluble in water; “on water” effect was discussed
d
e
H TTP : non metallated porphyrins and irradiation (λ>350 nm) was used.
2
limited to room temperature to decreased vapour pressure of
20
The productivity calculated for uncatalysed oxidation (Table 1,
entries 1ꢀ4) can vary by a factor of 4. These differences can be
explained by the different aldehyde reactivity (Table 1, entries 1
and 2) or by solvent effects (Table 1, entries 1 and 4). It should be
organic compounds.
.2 General
2ꢀethylhexanal (96%), nꢀoctanal (99%), cyclohexaneꢀ
2
2
3
3
4
4
5
0
5
0
5
mentioned that using the correction of the partial pressure of O2, 60 carboxaldehyde (97%) benzaldehyde (>99.5%), analytical grade
the same productivity for the same reaction performed with air or
solvents (>99%) (Aldrich) were used as obtained from the
supplier and stored at 4 °C under N with protection from light.
Mn(II) 2ꢀethylhexanoate (40% in mineral spirit) was used as
obtained from Strem. Reconstituted air (O 21%wt and N
with pure O was obtained (Table 1, entries 3ꢀ4). More surprising
2
2
are the similar or lower productivities observed with the catalysed
reactions compared to the uncatalysed reaction, except for entry
2
2
5
, Table 1. Indeed, the liquidꢀphase oxidation of organic 65 79%wt) was supplied from Air Liquide.
compounds with air or oxygen as the oxidant involves a complex
interplay of intrinsic parameters (aldehyde reactivity, solvent, …)
and extrinsic parameters (concentration, oxygen partial pressure,
catalyst, initiators/inhibitors, …) as well as physical phenomenon
such as gas to liquid mass transfer. When oxygen transfer
becomes the rate limiting step, the rate of the overall process is
no longer controlled by the chemical mechanisms but rather by
physical transport phenomena (diffusion, convection). Under
these conditions, the chemical performances (productivity,
selectivity) can be severely falsified by oxygen starvation in the
2
.3 Procedure of the oxidation of aldehydes
The reactions were performed in a 135 mL thermoꢀregulated
stainless steel autoclave (120 mL of useful capacity) operating in
batch mode. Four baffles were mounted to the reactor walls. The
reaction medium was stirred by a Rushton turbine or a selfꢀ
suction turbine. A total volume of 40 mL of the desired mixture
of 2ꢀethylhexanal and solvent were placed in the reactor. The
temperature was set to 25 °C and the reactor was then pressurized
with lab bench compressed air (8 bar) in about 10 s. When the
pressure reading has stabilized (typically after about 10 s), the
aldehyde oxidation experiment was started by switching on the
agitator. The timeꢀcourse of the gas pressure decrease was
measured with a pressure transducer and recorded onꢀline using
Labview®. The reaction mixture was analysed at the end of each
experiment using a Shimadzu 2010 GCꢀMS system equipped
with a DBꢀ5 column. In all cases, the conversion calculated from
oxygen consumption and determined by GCꢀMS was within the
experimental error (5%). For higher pressure (24 to 40 bar), the
7
7
8
8
0
5
0
5
4,7,8,11,14
liquid.
Thus, one could suspect that the differences in
productivity observed in Table 1 could be explained by oxygen
mass transfer resistance. This prompted us to reꢀinvestigate the
labꢀscale oxidation of aldehyde with air at room temperature.
2
. Experimental section
2
.1 Safety
It is worth mentioning that in the case of oxidation reactions, fuel,
oxidant and energy are present at the same time in the reactor.
Thus, the safety of the process should receive explicit attention.
While oxidation reactions are run routinely and safely on large
scale following safety regulations and procedures, the possibility
of forming explosive vapour space on labꢀscale is greatly reduced
when pure oxygen is replaced by air and the temperature is
same procedure was used with reconstituted air (O 21%wt and
N2 79%wt). For catalytic reaction, catalyst was added to the
organic phase at room temperature before pressurization.
50
2
55
2
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3
. Results and Discussion
could be considered as a fairly reliable approximation of the
oxygen consumption rate (dnO /dt). From the P=f(t) plot, the
2
3
.1 Uncatalysed autoxidation of 2-ethylhexanal
45
dnO =f(t) plot is constructed as shown in Fig. 1.
2
Initial attempts to optimize the reaction conditions for the
autoxidation of aldehydes to the corresponding carboxylic acids
were done using 2ꢀethylhexanal, an industrially important
From the oxygen consumption rate (Fig. 1), three reaction
steps were distinguished: i) the first 100 s correspond to the
dissolution of gas in the liquid phase. No further pressure
decrease could be observed with pure heptane. ii) A concomitant
5
1
,21
aldehyde produced worldwide on a large scale.
For safety
reasons, the autoxidation of diluted aldehyde with air was 50 rapid increase of O consumption was observed which could be
2
studied. A blank experiment was first performed in a thermoꢀ
regulated stainless steel autoclave operating in batch mode with
0.8 M of 2ꢀethylhexanal in heptane. The reactor was pressurized
with lab bench compressed air (8 bar) at 25 °C. The reaction
related to aldehyde autoxidation through free radical chain
mechanism. The overall reaction rate increases to a maximum
after 400 s in this experiment. iii) Afterwards, the reaction rate
slows down to zero due to the decrease of the ratio of chain
1
0
medium was stirred at 1000 rpm by a Rushton turbine (RT) and 55 propagation over termination. This ratio drops because of the
the timeꢀcourse of the pressure decrease in the reactor was
recorded onꢀline (Fig. 1). Note that, under these conditions and
considering the volume of solution (40 mL) and the volume of
the reactor (120 mL), the molar ratio between the aldehyde and
the oxygen is 2.9 and oxygen is the conversion limiting reagent.
decrease of reagents concentrations (aldehyde, O …) in the liquid
2
phase. After 1400 s, the conversion of O was quantitative and
2
1
5
corresponds to the measured conversion of 2ꢀethylhexanal, i.e.
35%.
Under these conditions, a productivity of 140 mM/L/h for 0.2
60
bar of O (i.e. air) could be calculated. This value is comparable
2
to the best value obtained with pure oxygen (Table 1, entry 1) or
with catalysed reaction with air as oxidant (Table 1, entry 5) but
far exceeds the productivity for catalysed reactions listed in Table
1. The results could be explained by oxygen mass transfer
resistance obtained using classical glassware used in the literature
with poor agitation and without baffles. To emphasize such a
limitation, we studied the effect of the stirring rate and the turbine
type on the latter solution, i.e. 0.8 M.
9
8
7
-0.005
Measured pressure heptane solution
Measured pressure heptane/RCHO mixture
Calculated O2 pressure
dP/dt
-
-
-
-
0.004
0.003
0.002
0.001
6
7
7
5
0
5
6
2
3.2 Effect of stirring rate
1
0
Even under labꢀscale conditions (diluted solution, 25 °C), an
improvement of the oxidation rate was observed with increasing
the stirring rate (ESI). For these specific conditions, the reaction
controlled regime was only obtained using a selfꢀsuction turbine
(SST) for stirring rates above 1000 rpm (Fig. 2). Under these
conditions, the oxygen conversion was total in less than 600 s. It
should be emphasized that, as expected, the selectivity was not
influenced by the improvement of the oxidation rate.
0.000
1600
0
200
400
600
800
1000
1200
1400
Time (s)
Fig. 1 Reactor pressure, calculated oxygen partial pressure and oxygen
consumption rate as the function of time and composition of the liquid
phase. Liquid phase 40 mL, 80 mL of air at 8 bar, 25 °C, RT at 1000 rpm.
2
0
After stabilization at the desired pressure (8 bar) at room
temperature, the experiment was started by switching on the
agitator. A rapid and complex evolution of the reactor pressure
with time, in the absence of catalyst, could be noticed. After 1400
seconds, a constant pressure was obtained. GCꢀMS analysis of
the reaction mixture shows an aldehyde conversion of 35% and a
selectivity of 80% towards the carboxylic acid comparable to to
previously published catalysed autoxidation. The byꢀproducts
were identified, i.e. 3ꢀheptylformate, 3ꢀheptanol and are
compatible with a Criegee intermediate.
0
.0030
Self-Suction Turbine (SST)
Rushton Turbine (RT)
2
3
3
4
5
0
5
0
0.0025
0.0020
0.0015
5
,6
0
.0010
.0005
For given solvent and pressure conditions, the measured
reactor pressure includes the partial pressure of O , N , solvent
0
2
2
and solutes. For a given reaction, the partial pressures of N and
2
0.0000
250
500
750
1000
1250
1500
solvent are constant and the variation of the partial pressures of
solute could be neglected. Thus, from the reactor pressure, the
Stirring rate (rpm)
oxygen partial pressure (P O ) can be calculated quite directly
2
80
2
Fig. 2 Rate of pressure loss determined at P(O )=0.1 bar as the function of
stirring rate and turbine type. 2ꢀethylhexanal 0.8 M in heptane 40 mL, 80
mL of air at 8 bar, 25 °C.
and plotted as the function of time as shown in Fig. 1. Under
those conditions, we could estimate that the oxygen present in the
reactor was quantitatively reacted in less than 1400 s. This is in
agreement with the aldehyde conversion and the stoichiometry of
the reaction. Thus, the derivatives of the total pressure (dP/dt)
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3
.3 Substrate scope
selectivity was observed with the linear aliphatic aldehyde (Table
, entry 2), while the lowest selectivity was obtained with 2ꢀ
2
Under the same conditions, autoxidation of different aldehydes
was studied and the results are shown in Table 2. As expected
from literature data, a strong influence of aldehyde structure both
on selectivity and on productivity could be noticed. The highest
6
ethylheptanal (Table 2, entry 1). Aromatic aldehydes show the
2
2
lowest productivity due to higher termination rate constant.
5
0
1
Table 2 Influence of aldehyde on selectivity, productivity and maximum oxygen consumption. Aldehyde 0.8 M in heptane 40 mL, 80 mL of air at 8 bar,
5 °C, SST 1500 rpm
2
c
d
e
Conversion (%)
RCHO
Time
(s)
Selectivity
(%)
Productivity
(mM/L/h)
Entry
1
Aldehyde
a
b
O
2
[
[
[
[
b]
35
35
35
35
100
100
100
100
300
2000
120
85
99
95
99
672
101
1680
67
CHO
b]
b]
b]
2
3
CHO
CHO
4
CHO
3000
a
b
c
d
GC/MS analysis. an aldehyde / oxygen ratio of 2.9 is used.e time required to obtain a constant reactor pressure. Selectivity towards the corresponding
carboxylic acid, determined by GC/MS analysis of the crude. Productivity calculated as mM of aldehyde converted by volume of liquid phase by hour
2
and for 0.2 bar of O .
35
variation of the oxygen consumption rate as the function of time
are noticed (Fig. 3). For a concentration of 0.8 M, maximum
overall reaction rate was obtained after 400 s until a decrease of
the ratio of chain propagation over termination was noticed.
15
3.4 Effect of aldehyde concentration
The influence of the aldehyde concentrations in heptane from
0
.16 M to 6.4 M (100% w/w) was studied using 2ꢀethylheptanal.
Results are reported in Fig. 3.
3
.5 Solvent effect
4
0
Solvent effect for air autoxidation of 2ꢀethylꢀhexanal was then
studied with an aldehyde concentration of 0.8 M (Fig. 4). With all
the solvents tested (except for DBE and iPrOH where the reaction
was stopped before completion), total conversion of oxygen was
found (i.e. an aldehyde conversion of 35% was determined by
GC/MS) and the determined selectivity for the corresponding
carboxylic acids was only influenced by the solvent to a small
0
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
.00
6.4 M (100 vol%)
3.2 M (50 vol%)
45
6
extent. For comparison purposes, one experiment was performed
17
under “on water condition” (Table 1, entries 3ꢀ4). Using water,
0/1 volume ratio, reaction rate comparable to those obtained
1
.6 M (25 vol%)
3
0
.8 M
0.16 M
0
.4 M
50 with organic solvents using an autoclave stirred with a selfꢀ
suction turbine. Thus, the “on water effect” observed with
hydrophobic aldehyde may be tentatively interpreted by a
concentration effect (Fig. 4) and by aldehyde spreading on the air
0
200
400
600
800
Time (s)
23
bubble surface obtained by magnetic stirring. Such spreading
increases the oxygen transfer usually obtained in classical
glassware as used by A. Vigalok et al. and thus increase the
20
Fig. 3 Oxygen consumption rate as the function of time and concentration
of 2ꢀethylhexanal in heptane. 40 mL of solution, 80 mL of air at 8 bar,
5
6
6
5
0
5
2
5 °C, SST at 1500 rpm.
17
reaction rate.
An influence of the solvent on the reaction rate of aldehydes
Under these conditions, total conversion of oxygen was
observed (except for 0.16 M where O was in large excess but the
6,24
2
autoxidation has been noticed.
In particular, under our
25
reaction was stopped before completion) and the selectivity
determined by GCꢀMS was comparable within experimental
errors. As expected from free radical chain reaction mechanism,
the oxygen consumption rate was strongly dependent on the
aldehyde concentration as it is an important factor for radical
chain reaction. For concentrations above 0.8 M (i.e. 12.5 vol%)
maximum overall reaction rate was observed in less than 100 s.
After, the reaction rates are limited by oxygen concentrations in
the reactor. For lower aldehyde concentration, i.e. 0.16 and 0.4
M, the aldehyde autoxidation proceeds slowly and very little
conditions, low reaction rates were obtained using iPrOH and
dibutylether (DBE) (Fig. 4). The latter, and especially DBE, can
form peroxide via an autoxidation process that could compete
with the free radical chain reaction and thus inhibit aldehyde
autoxidation. Surprisingly, the reaction rate observed with
hexanoic acid, acetonitrile and heptane was inversely
proportional to the calculated oxygen solubility in the solvent
30
(
ESI). The highest O consumption was obtained in heptane after
2
4
00 s (Fig. 4), whereas the lower were obtained for acetonitrile
4
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and particularly for hexanoic acid (decreased by a factor of 7).
Furthermore, the maximum O2 consumption rate is shifted in
Aldehydes autoxidation involves different types of radicals and
●
among then the acylperoxy radical “RCO ” (Scheme 1).
3
time, 800 and 1300 s for acetonitrile and hexanoic acid 10 Radicalꢀradical recombination reactions require so little
respectively. activation energy that they are usually diffusion controlled in
25
solvents of normal viscosity. Thus, depending of the viscosity
of the solvent, the diffusion of the radical species out of the
0
0
0
0
0
0
.025
.020
.015
.010
.005
.000
26
heptane
solvent cage could be decrease and favour the recombination
(termination step). The latter decrease the reaction rate. To further
probe such a “cage effect”, air oxidation of 2ꢀethylhexanal was
studied under the same conditions with alkanes of various
viscosity, i.e. pentane (nꢀC5), heptane (nꢀC7), nonane (nꢀC9),
decane (nꢀC10) and tetradecane (nꢀC14) (Table 3).
1
5
20
"
on water" condition
CH CN
3
DBE
iPrOH
nC CO H
6
2
0
600
1200
1800
2400
3000
3600
Time (s)
5
Fig. 4. Oxygen consumption rate as the function of time and solvent. 2ꢀ
ethylhexanal 0.8 M 40 mL, 80 mL of air 8 bar, 25 °C, SST at 1500 rpm.
2
Table 3 Viscosity, O solubility and Maximum oxygen consumption rate as the function of solvent
[
a]
ꢀ1
[b]
ꢀ1
ꢀ1
[c]
ꢀ1
Entry
Solvent
pentane
heptane
nonane
decane
Viscosity (mPa.s )
O
2
solubility (mmol.L .PO
2
(bar) ) Maximum oxygen consumption rate (mmol.s )
1
2
3
4
5
0.225
0.393
0.648
0.92
2
12.29
7.15
5.41
4.56
3.57
0.033
0.023
0.012
0.008
0.003
Tetradecane
a
27 b
23
c
ref. . ref.
.
determined with 2ꢀethylhexanal 0.8 M 40 mL, 80 mL of air at 8 bar, 25 °C, SST at 1500 rpm.
2
3
3
4
5
0
5
0
However, even for alkanes, the viscosity and the solubility of
oxygen are coupled. The viscosity of the solvent increase while
0
.014
2
4 bar
2
7
chain length of the alkanes is increasing but the solubility of
0.012
4
0 bar
23
oxygen decreases with increasing chain length of the alkanes.
0.010
Therefore, determination of kinetic parameters, especially
reaction orders, is required for further discussion.
In all previous reactions, aldehyde was present in large molar
excess (aldehyde / oxygen ratio of 2.9). In order to determine the
apparent reaction order, the reaction had to be performed using
0.008
0.006
30 bar
0.004
excess of oxygen. Thus, pressurized reconstituted air (O 21 %wt
2
0
.002
.000
and N 79 %wt) was used. Under our conditions, with a reactor
2
pressure of 24 bar, oxygen is in stoichiometric amount and with
0
0
500
1000
1500
2000
2500
3000
3500
4000
3
0 and 40 bar of reconstituted air, an excess of oxygen is
Time (s)
obtained. The reactor was pressurized with reconstituted air at
25 °C to the desired pressure (24 to 40 bar). When the pressure
reading has become constant, the experiment was started by
switching on the agitator.
Fig. 5 Oxygen consumption rate as the function of time and pressure. 2ꢀ
ethylhexanal 0.8 M in heptane 40 mL, 80 mL of O /N (21/79 wt%),
5 °C, SST at 1500 rpm.
45
2
2
2
The reaction profiles were almost identical as a function of the
initial reactor pressure. After 2000s, a stable pressure was
obtained (ESI). For all pressures, GC/MS analysis of the crude
reaction mixture shows a complete conversion of the 2ꢀ
ethylhexanal with a selectivity of 85% towards the corresponding
carboxylic acid product. The oxygen consumption rate profiles
50
(Fig. 5) are identical for the three pressures within experimental
errors clearly indicate that the apparent reaction order in oxygen,
under those conditions, is zero. Thus, the variation of the
55
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maximum reaction rate observed for alkanes (Table 3) could not
oxygen consumption rate using catalytic amount of Mn(II) is
be explain by the oxygen solubility in the solvent but by the 30 obtained by significantly decreasing the initiation time (Fig. 7
viscosity of the solvent (Fig. 6). Aldehyde autoxidation shows a
complex solvent effect and among others, viscosity of the solvent
was clearly an important factor.
insert). The maximum oxygen consumption rate reaction rate is
achieved in 10 and 500 s for catalysed and uncatalysed reactions
respectively.
5
Conclusions
0
0
0
0
0
0
0
.012
.010
.008
.006
.004
.002
.000
n-C5
35 The liquid phase autoxidation of aldehyde by air has been known
4,8
for a long time and the reaction mechanism and kinetics of the
oxidation have been widely studied. Recently, several new
catalysts have been reported for this oxidation. However, in most
case, we can suspect that the reactions are carried out in a mass
transfer limited regime, i.e. the reaction rate is limited by the
oxygen mass transfer. Such phenomenon could lead to much
confusion in the interpretation of the results.
We have shown that uncatalysed oxidation of aldehydes using
oxygen from air at room temperature is an effective method and a
safe process to produce carboxylic acids. A strong influence of
stirring rate, solvent type, solvents viscosity on the chemical rate
was fully demonstrated. These classical effects have been
neglected in most recent papers on liquid phase autoxidation of
aldehyde by air and may have a strong impact on catalyst
screening interpretations.
n-C7
n-C9
4
4
5
0
5
0
n-C10
n-C14
0.0
0.5
1.0
1.5
2.0
-1
Viscosity (mPa.s )
Fig. 6 Maximun reaction rate as the function of viscosity of the solvent
see ). 2ꢀethylhexanal 0.8 M 40 mL, 80 mL of air at 8 bar, 25 °C, SST at
2
7
(
1
500 rpm.
10
3.6 Catalytic oxidation of 2-ethylhexanal
Acknowledgements
All the previous oxidation reactions were performed without
added catalyst. The promoting effect of stainless steal originated
from autoclave walls has already been demonstrated for oxidation
This work received funding from Project PROCIP (ANRꢀ2010ꢀ
CD2Iꢀ013ꢀ01) from the French National Research Agency under
the CD2I Program.
2
8
reaction and cannot be neglected
autoxidation in metal free reactor will be discussed in a
forthcoming publication. However, manganese(II) 2ꢀ
ethylhexanoate was evaluated as catalyst for the air oxidation of
ꢀethylhexanal and a comparison with uncatalysed reaction is
presented in Fig. 7.
a
priori.
Aldehyde
15
55
Notes and references
1
1
a
Laboratoire de Génie des Procédés Catalytiques (LGPC), UMR 5285
2
CNRS/CPE Lyon, Université de Lyon, 43 boulevard du 11 novembre
1918, 69616 Villeurbanne cedex, France. E-mail: cdb@lgpc.cpe.fr
b
Conservatoire National des Arts et Métiers, Ecole SITI, Département
6
6
0
5
CASER, Equipe CGP, 2 rue Conté, 75003 Paris, France. E-mail :
favrereg@cnam.fr
Electronic Supplementary Information (ESI) available: Figures of the
evolution of the reactor pressure as the function of the parameters studied,
rate of pressure loss against oxygen residual pressure determined as the
function of the parameters studied, oxygen consumption rate as a function
2
8.0
uncatalysed
30
29
28
27
26
25
24
23
22
27.8
2+
†
2
2
2
2
2
2
7.6
7.4
7.2
7.0
6.8
6.6
10 ppm Mn
2
of the parameters studied, Calculated solubility of O in solvent. See
26.4
6.2
26.0
2
DOI: 10.1039/b000000x/
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TOC
For catalysed or uncatalysed air oxidation of aldehyde, efficient mixing is critical because the
reaction rate is limited by oxygen mass-transfer.