Is it possible to achieve highly selective oxidations in supercritical water? Aerobic oxidation of methylaromatic compounds

Article abstract of DOI:10.1002/adsc.200303218

We have demonstrated that different methylaromatic compounds [1,4-dimethylbenzene (p-xylene), 1,3-dimethylbenzene (m-xylene), 1,2-dimethylbenzene (o-xylene), 1,3,5-trimethylbenzene (mesitylene) and 1,2,4-trimethylbenzene (pseudocumene)] can be aerobically oxidized in supercritical water (scH₂O) using manganese(II) bromide as catalyst to give corresponding carboxylic acids in the continuous mode over a sustained period of time in good yield. No partially oxidized intermediates (i.e., toluic acid and benzaldehydes) were detected for the dimethylbenzenes and mesitylene reactions. By fine tuning pressure and temperature, ScH₂O becomes a solvent with physical properties suitable for single-phase oxidation since both organic substrate and oxygen are soluble in scH₂O. There is a strong structural similarity of metal/bromide coordination compounds in the active oxidation solvents (acetic acid and scH₂O) which does not exist in the much less active H₂O at lower temperatures. This may account for the successful catalysis of the reactions reported herein. Aromatic acids produced by the loss of one methyl group occurred in all of these reactions, i.e., 3-6percent benzoic acid formed during the oxidation of the dimethylbenzenes. Part of this loss is thought to be due to thermal decarboxylation. The thermal decarboxylation process is monitored via Raman spectroscopy.

Full text of DOI:10.1002/adsc.200303218

FULL PAPERS
Is it Possible to Achieve Highly Selective Oxidations in
Supercritical Water? Aerobic Oxidation of Methylaromatic
Compounds
a
a
b
b
Eduardo Garcia-Verdugo, Eleni Venardou, W. Barry Thomas, Keith Whiston,
c
a
a,
Walter Partenheimer, Paul A. Hamley, Martyn Poliakoff *
Clean Technology Group, School of Chemistry, University of Nottingham NG7 2RD, UK
a
Fax: ( ‡ 44)-115-951-3058, e-mail: Martyn.Poliakoff@nottingham.ac.uk
b
Invista Performance Technologies, Wilton, Cleveland, TS10 4XX, UK
c
Central Research and Development, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware,
DE 19880, USA
Received: November 27, 2003; Accepted: February 14, 2004
Abstract: We have demonstrated that different meth- similarity of metal/bromide coordination compounds
ylaromatic compounds [1,4-dimethylbenzene (p-xy- in the active oxidation solvents (acetic acid and
lene), 1,3-dimethylbenzene (m-xylene), 1,2-dimethyl- scH O) which does not exist in the much less active
2
benzene (o-xylene), 1,3,5-trimethylbenzene (mesity- H O at lower temperatures. This may account for the
2
lene) and 1,2,4-trimethylbenzene (pseudocumene)] successful catalysis of the reactions reported herein.
can be aerobically oxidized in supercritical water Aromatic acids produced by the loss of one methyl
(
scH O) using manganese(II) bromide as catalyst to group occurred in all of these reactions, i.e., 3 ± 6%
2
give corresponding carboxylic acids in the continuous benzoic acid formed during the oxidation of the
mode over a sustained period of time in good yield. dimethylbenzenes. Part of this loss is thought to be
No partially oxidized intermediates (i.e., toluic acid due to thermal decarboxylation. The thermal decar-
and benzaldehydes) were detected for the dimethyl- boxylation process is monitored via Raman spectro-
benzenes and mesitylene reactions. By fine tuning scopy.
pressure and temperature, scH O becomes a solvent
2
with physical properties suitable for single-phase Keywords: arenes; benzylic oxidation; CÀH activa-
oxidation since both organic substrate and oxygen tion; green chemistry; manganese; supercritical water
are soluble in scH O. There is a strong structural (scH O)
2
2
Introduction
used although many others have been evaluated. There
is a strong synergistic relationship of the metals with the
Processes involving the partial selective oxidation of bromide anion. The most common catalyst is a mixture
organic molecules (typically hydrocarbons) are diverse of cobalt (II) and manganese (II) acetates with hydro-
and widely used in almost all of the important commod- bromic acid at a metals/bromide ratio of 1:1 mol/mol.
[
1]
ity and fine chemical industries including the manufac- This area has been extensively reviewed. Nearly all
ture of pharmaceuticals, agrochemicals and polymer aerobic oxidations, such as the methylaromatic com-
intermediates. In an ideal partial oxidation process, only pounds considered here (Scheme 1), generate water as a
the organic substrate, air and (a small amount of) by-product. For this reason, water is a good candidate
catalyst should be introduced into the reactor. If the for use as an environmentally more friendly solvent for
catalyst can be recovered in some way (and subsequent- oxidation. Indeed, the use of water as solvent has
ly recycled via either a continuous mode of operation, or already been explored by different research groups.[
via a fresh charge of substrate), then the work-up of the Water, however, is a poor solvent for autoxidation, that
reaction mixture should only involve product recovery is, for oxidations in which molecular oxygen is activated
2]
and recycling of un-reacted substrate.
At least 73 different aromatic carboxylic compounds
have been prepared from methylaromatic compounds deactivate
via a free radical chain process.
[3]
Water is generally known to both activate and
[
1,4]
metal-catalysed autoxidations. However,
by aerobic oxidation in yields from 75 ± 95% using with the amounts of water produced during most
around 30 different combinations of soluble metal/ reactions ( > 5 wt %) a large decrease in the rate of
[
1]
bromide catalysts. Acetic acid is the usual solvent reaction occurs. A direct comparison of water, i.e., non-
Adv. Synth. Catal. 2004, 346, 307 ± 316
DOI:10.1002/adsc.200303218
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
307

FULL PAPERS
Eduardo Garcia-Verdugo et al.
acid does not precipitate within the reactor, a factor in
reducing the amount of 4-carboxybenzylaldehyde in the
recovered product. Furthermore, the use of high tem-
perature water offers significant cost advantages
through enhanced energy recovery due to a higher
process temperature and the elimination of organic
solvents. The process totally eliminates the use of
organic solvents and has promise as a potentially cleaner
Scheme 1. Aerobic oxidation of xylene isomers in scH O.
2
and economical route to manufacture of terephthalic
scH O, as a solvent to acetic acid is available. For acid, as has been recently shown by Dunn and Savage.
[
5]
[12]
2
instance, in the Co/Mn/Br catalyzed autoxidation of p- In their workthe economic and environmental assess-
xylene to terephthalic acid, to obtain the same purity of ment of the scH O process is compared with the use of
2
terephthalic acid, 33 times more catalyst was required. acetic acid as solvent.
Furthermore, the use of water as a solvent required a
We chose to study the methylaromatic compounds o-,
two-stage continuous oxidation at 1858C and 2208C m- and p-xylene and 1,2,4-trimethylbenzene (pseudo-
rather than a single stage continuous oxidation at 2268C cumene), because the oxidation of these compounds
in acetic acid. Even then, there was no net decrease in gives products that are all large volume, industrial
[
13]
the carbon loss due to carbon dioxide and monoxide processes with a large variety of uses. p-Xylene, m-
formation and significant amounts of ring-brominated xylene and pseudocumene (1a, 1b, and 11) are usually
product were produced. Therefore non-scH O water as oxidised in the liquid phase while o-xylene (1c) is
2
a solvent is distinctly inferior and much less active than oxidised in the gas phase using vanadium/oxide hetero-
acetic acid/water as solvent.
geneous catalysis. The oxidations of p-xylene and m-
Supercritical water, scH O, has generated consider- xylene are continuous processes while pseudocumene
2
[
1]
able interest as a reaction medium. As water approaches oxidation requires a batch approach. The oxidation of
its critical point (T ˆ 3748C, P ˆ 22.1 MPa, 1 ˆ 1,3,5-trimethylbenzene (7) was chosen to compare with
c
c
c
À3
0
.317 g ¥ cm ), the physical and chemical properties of 1,2,4-trimethylbenzene (11) because it is known that
the fluid change. As the temperature is increased, at a methyl groups ortho to each other can deactivate the
fixed pressure of 20.0 MPa, the dielectric constant (e) oxidation system. Here, we show that despite signifi-
decreases from 80 (258C) to 36 (2008C), 21 (3008C), 14 cant reactivity differences of these reagents, high yields
3508C) and 2 (4008C). An additional change also of the expected aromatic acids can be easily achieved in
[6]
[
14]
[
7]
(
occurs as a result of the breakdown in hydrogen bonding continuous aerobic selective partial oxidation in scH O.
2
as the temperature is increased: the ionic product (Kw)
‡
increases with enhanced local concentrations of H O
3
À
and OH ions, which reach a maximum value at around Results
2
508C and 25 MPa and then decreases sharply at the
[8]
critical point. This property has allowed acid- and All the experiments were conducted using a continuous
base-catalysed reactions to be performed at this near- flow reactor, the configuration of which is shown in
critical temperature without adding further acid or Figure 1. The continuous oxidation was carried out in
[9]
base. scH O has been widely investigated as a medium scH O with MnBr as the catalyst, using the optimised
2
2
2
for the total oxidation of organic species, normally at conditions previously reported by us for the oxidation of
[
11]
temperatures of 550 ± 7508C. The advantage is that p-xylene to terephthalic acid. The expected products
scH O acts as a solvent for both O and organic and reactivity of methylaromatic compounds in the
2
2
substrates providing, in this way, a single phase oxida- metal/bromide-catalysed autoxidation of methylaro-
[
10]
tion which overcomes mass-transfer limitations.
matics in acetic acid have been detailed in the liter-
[
1]
We have recently reported the continuous partial ature. To a first approximation, the methyl groups are
selective oxidation of p-xylene to terephthalic acid using consecutively oxidised to their aromatic acids. The
manganese(II) bromide as the catalytic system.[11] Our intermediates in highest yield are benzylic alcohols,
process generates almost no 4-carboxybenzylaldehyde, acetates and benzaldehydes.
thought to be due to the faster reaction rates experi- In supercritical water oxidation, the same products
enced at higher temperature. 4-Carboxybenzaldehyde is have been seen for p-xylene.[
[
1,15]
11,16]
Similarly, in sub-
one of the most problematic impurities in current critical batch aerobic oxidation of toluene, m-xylene, o-
commercial processes for manufacturing terephthalic xylene, mesitylene (1,3,5-trimethylbenzene), and ethyl-
acid since it co-precipitates with the terephthalic acid benzene, the same intermediates and products have
and requires an additional plant to hydrogenate it back been reported which have also been seen in metal/
[
1,17]
to p-toluic acid, which is then separated by fractional bromide-catalysed autoxidation in acetic acid.
The
crystallisation in water. In our process, the first example response to increases in catalyst concentration, temper-
of continuous selective oxidation in scH O, terephthalic ature, and reaction time reported in scH O are similar to
2
2
308
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Adv. Synth. Catal. 2004, 346, 307 ± 316

Highly Selective Oxidations in Supercritical Water
FULL PAPERS
the corresponding dicarboxylic acid, 66% of isophthalic
acid (5b, Entry 2, Table 1) and 46% of o-phthalic acid
(
5c, Entry 1, Table 1). In all cases a high selectivity was
achieved for the production of the corresponding
dicarboxylic acid derivative 5a ± c and with levels of
the incompletely oxidised intermediate products such as
tolualdehydes 2a ± c, toluic acids 3a ± c and carboxyben-
zaldehydes 4a ± c below the detection limits. The only
side product detected was benzoic acid 6, which is likely
to be produced by decarboxylation of either the
dicarboxylic acid or other intermediates (3a ± c or 4a ±
c), since such a process is favoured by longer residence
[
16]
times at high temperature. The lower yield observed
for m-xylene oxidation (66%) as compared to p-xylene
(
90%) may be expected since there is no resonance
stabilisation of the radical intermediate at the meta
position as seen at the ortho and para position.
[
17]
Figure 1. Configuration of the continuous aerobic oxidation
reactor. D-01: water feed vessel, D-02: catalyst feed vessel, D-
The mesitylene oxidation (7) was also successful with
a 78% yield and a selectivity of 93%, see Table 2. The
only by-product detected was phthalic acid (5c, 7%). In
this case, benzoic acid was not detected. The reaction of
03: organic feed vessel, D-04: NaOH feed vessel, P-01: feed
water pump (HPLC pump, Gilson model 305 Q-rate ˆ
1
0 mL/min, DP ˆ 60 MPa), P-02: feed catalyst pump
max
1,2,4-trimethylbenzene (10, pseudocumene) gave tri-
(
HPLC pump, Gilson model 305 Q-rate ˆ 10 mL/min,
DPmax ˆ 343 bar), P-03: feed organic pump (HPLC pump, mellitic acid (11) with 80% selectivity and 50% yield, see
Gilson model 306 Q-rate ˆ 5 mL/min, DP ˆ 343 bar), P-04: Table 2 and Figure 2. The major by-product was iso-
max
feed NaOH pump (HPLC pump, Gilson model 303 Q-rate ˆ
phthalic acid (5b), the concentration of which was
around twice that of o-phthalic (5c) and terephthalic
acids (5a), 10, 5 and 5% respectively.
5
mL/min, DP ˆ 343 bar), G-01: pressure gauge (Bourdon
max
gauge), PM-01: pressure transducer and pressure monitor
(
water line), PM-02: manometric module, Gilson 802C,
Thus even given the significant differences in reac-
maximum pressure: 60 MPa (catalyst line), PM-03: pressure
transducer and pressure monitor (organic line), PM-03:
manometric module, Gilson 802C, maximum pressure:
[
1]
tivities of these substrates, we have demonstrated that
all of the methyl groups on a given methylaromatic
compound can be selectively oxidised to the carboxylic
acid in good yield.
60 MPa (NaOH line), T-01, T-02, T-03 and T-04: thermocou-
ples, E-01: pre-heater (1/4 in AISI 316 tubing with 300 cm
heat supplied by a cartridge heater (Watlow, 240 V, 200 W,
FIREROD0247 NFRG2E164A) and a band heater (Watlow,
2
40 V, 200 W, NTB25X6UA3, 47 02 DM)), R-01: tubular
reactor (1/4 in Hastelloy C276 pipe, 30 cm), E-02: cooler, F-
1:0.5 mm filter, BPR-01: Tescom, 26-1722-24-043 back
0
pressure regulator max. P: 6000 psi.
those in acetic acid-based reactions.[1,11] For instance,
increasing the catalyst concentration will result in
increasing activity with the expected changes in p-toluic
and terephthalic acid yields. These results suggest a
similar operating mechanism in both acetic acid and
scH O.Therefore it seems reasonable to assume that the
2
reactivity of the methylaromatic compounds reported
here will be similar to that in acetic acid, which has been
[
1]
detailed in the literature.
The results obtained in the oxidation of the xylene
Figure 2. Plot showing the variation in selectivity of the
aerobic oxidation reaction of 1,2,4-trimethylbenzene (10) in
isomers in scH O are summarised in Table 1. In the case
2
of p-xylene (1a, Entry 3, Table 1) the previously opti-
mised conditions gave around a 90% yield of tereph-
thalic acid (5a).[ Under the same experimental con-
ditions (residence time, catalyst, and oxygen concen-
scH O observed during a continuous run. Samples were
2
collected at 15 min intervals after the reactor had reached
stable conditions. Catalyst strength: Mn 1719 ppm/Br
11]
5
000 ppm, flow rates: H O (P-01): 8.19 mL/min, catalyst
2 2
tration, see Experimental Section) the oxidation of m- (P-02): 4.023 mL/min, organic (P-03): 0.061 mL/min, NaOH
xylene (1b) and o-xylene (1c) led to reasonable yields of (P-04): 3.4 mL/min, temperature 3808C and pressure 230 bar.
Adv. Synth. Catal. 2004, 346, 307 ± 316
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Eduardo Garcia-Verdugo et al.
Table 1. Yields and selectivities observed in the continuous Mn/Br-catalysed oxidation of xylene isomers in scH O.
2
Entry
Compound
Yield [%][a] of
Selectivity [%]
dicarboxylic acid
5
[%][b]
4 [%][c]
(carboxylic
benzalde-
hyde)
3 [%][d]0
(toluic
acid)
2 [%][e]
(tolualde-
hyde)
6 [%][f]
(benzoic
acid)
(dicarboxy-
lic acid)
1
2
3
4
5
o-xylene
m-xylene
p-xylene
o-xylene[
m-xylene[
46
66
90
±
94
97
95
±
<0.1
<0.1
<0.1
1
<0.1
<0.1
<0.1
30
<0.1
<0.1
<0.1
±
6
3
5
g]
69
g]
16
80
±
20
±
[
[
[
[
[
[
[
a]
b]
c]
d]
e]
f]
All calculated by HPLC. Yield [%] refers to the corresponding diacids phthalic, isophthalic and terephthalic acid
respectively.
Selectivity for ortho-, meta-, para-dicarboxylic acid calculated as % 5 ˆ (molar conc. 5/molar conc. 5 ‡ molar conc. 2 ‡ molar
conc. 3 ‡ molar conc. 4 ‡ molar conc. 6)  100.
Selectivity for ortho-, meta-, para-carboxylic benzylaldehyde as % 4 ˆ (molar conc. 4/molar conc. 2 ‡ molar conc. 3 ‡ molar
conc. 4 ‡ molar conc. 5 ‡ molar conc. 6)  100.
Selectivity for ortho-, meta-, para-toluenecarboxylic acid calculated as % 3 ˆ (molar conc. 3/molar conc. 5 ‡ molar conc. 2 ‡
molar conc.3 ‡ molar conc. 4 ‡ molar conc. 6)  100.
Selectivity for ortho-, meta-, para-tolualdehyde calculated as % 2 ˆ (molar conc. 2/molar conc. 5 ‡ molar conc. 2 ‡ molar
conc.3 ‡ molar conc. 4‡molar conc. 6)  100.
Selectivity for carboxylic acid calculated as % 6 ˆ (molar conc. 6/molar conc. 5 ‡ molar conc. 2 ‡ molar conc. 3 ‡ molar
conc. 4 ‡ molar conc. 6)  100.
g]
[17]
Results from Holliday et al.
Table 2. Yields and selectivities observed in the continuous Mn/Br-catalysed oxidation of trimethylbenzene isomers in scH O.
2
Entry
Compound
Yield [%][a] of
Selectivity [%]
tricarboxylic acid
8
(
[%][b]
trimesic
11 [%][c]
(trimellitic
acid)
5a [%][d]
(terephtha-
lic acid)
5b [%][e]
(isophtha-
lic acid)
5c [%][f]
(phthalic
acid)
6 [%][g]
(benzoic
acid)
acid)
1
2
mesitylene
pseudocumene
78
50
93
±
±
80
±
5
±
10
7
5
<0.1
<0.1
[
[
[
a]
b]
c]
All calculated by HPLC. Yield [%] refers to trimesic and trimelitic acid respectively.
Selectivity for trimesic acid calculated as % 8 ˆ (molar conc. 8/molar conc. 8 ‡ molar conc. 5b ‡ molar conc. 6)  100.
Selectivity for trimelitic acid calculated as % 11 ˆ (molar conc. 11/molar conc. 11 ‡ molar conc. 5a ‡ molar conc. 5b ‡ molar
conc. 5c ‡ molar conc. 6)  100.
[
[
[
[
d]
e]
f]
Selectivity for terephthalic acid calculated as % 5a ˆ (molar conc. 5a/molar conc. 11 ‡ molar conc. 5a ‡ molar conc. 5b ‡
molar conc. 5c ‡ molar conc. 6)  100.
Selectivity for isophthalic acid calculated as % 5b ˆ (molar conc. 5b/molar conc. 11 ‡ molar conc. 5a ‡ molar conc. 5b ‡
molar conc. 5c‡molar conc. 6)  100.
Selectivity for phthalic acid calculated as % 5c ˆ (molar conc. 5c/molar conc. 11 ‡ molar conc. 5a ‡ molar conc. 5b ‡ molar
conc. 5c ‡ molar conc. 6)  100.
g]
Selectivity for carboxylic acid calculated as % 6 ˆ (molar conc. 6/molar conc. 5 ‡ molar conc. 2 ‡ molar conc. 3 ‡ molar
conc. 4 ‡ molar conc. 6)  100.
Discussion
Comparison of Results to the Literature
The 90% yield of terephthalic acid obtained in this work
via continuous oxidation can be compared to the
maximum yield of 57% obtained by Dunn, Urquhart
and Savage via batch oxidation in supercritical water[
16]
Scheme 2. Aerobic oxidation of trimethylbenzene in scH O.
and to 64% terephthalic acid yield from batch oxidation
2
310
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Highly Selective Oxidations in Supercritical Water
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in sub-critical water by Holliday et al.,[17] see Table 1. reaction conditions for each particular substrate should
Comparable yields of terephthalic acid can be obtained lead to the corresponding aromatic acid with higher
in hot water and in acetic acid ( > 90%) but the latter yields and selectivities in scH O.
2
requires a two-stage continuous process with a Co/Mn/
Br catalyst. The first stage has an 80 min residence time
[
18]
and the second stage a 60 min residence time. This is Methyl Carbon Atom Loss:Evidence for
in contrast to that reported here with much lower Decarboxylation in scH O
2
catalyst concentration and very fast reactions (residence
time < 20 seconds), with comparable yields, and no It has been reported that the terephthalic acid yields in
detectable 4-carboxybenzaldehyde in the product.
Our results for the continuous oxidation of m-xylene due to −decarboxylation×.
supercritical water decrease with longer reaction times
[
11,16]
Here we find that o-, m-
and o-xylene both gave better selectivity and yields than and p- xylene produce benzoic acid, mesitylene produ-
those obtained by Holliday et al. using batch reactors ces isophthalic acid and pseudocumene produces three
[
17]
and sub-critical temperatures in water. The continu- different diacids. The appearance of the by-product
ous oxidation of o-xylene gave an up to 46% yield of containing one less methyl carbon atom is not unique to
phthalic acid, while none was detected in the batch supercritical water oxidation but occurs in all metal- and
process, see Table 1. The long reaction time used in the metal/bromide-catalysed homogeneous autoxida-
[
1]
batch experiments ( ꢀ 60 min for batch experiment tions.
against < 20 seconds in a continuous reactor) favours Benzoic acid formed during dimethylbenzene oxida-
the reaction of o-xylene to phthalic acid followed by the tion can be formed in at least three different ways: 1) via
thermal decarboxylation at the o-position. Indeed, the decarbonylation (rxn 2, Scheme 3), 2) via thermal
major product produced by a batch process was benzoic decarboxylation (rxn 3, Scheme 3) and 3) metal-cata-
acid, instead of phthalic acid (Entry 4, Table 1). Holli- lysed decarboxylation (rxn 4, Scheme 3). For the metal-
day obtained a 16% of isophthalic acid from m-xylene catalysed pathway, Mn(III,IV) and Co(III) are known to
[
17]
[1]
(
Entry 5, Table 1) in contrast to this workwith 66%.
The results by Holliday are likely to have been that they can decarboxylate aromatic acids.
incomplete oxidations as evidenced by the presence of decarbonylation mechanism is important in aliphatic
be present in autoxidation and it is well-established
[
20,21]
The
[
22]
4
% m-toluic acid and the poor mass accountability aldehydes but not aromatic ones. To confirm this we
(
20%).
aerobically oxidised heptaldehyde and p-tolualdehyde
Except for p-xylene, the yields reported here for the with a Co/Mn/Br catalyst in acetic acid at 958C and
other methylaromatic compounds are less than those in atmospheric pressure. The vent carbon monoxide level
a conventional metal/bromide acetic acid solvent proc- during the oxidation of heptaldehyde was 5 vol % but
ess.[ Thus reported yields in acetic acid for o-xylene to only 0.03% for p-tolualdehyde.
1]
[39]
Thus most of the
o-phthalic acid of 82 ± 83% compare to 46% reported observed benzoic acid is from a decarboxylation mech-
here. Similarly for m-xylene to isophthalic acid the anism and not from decarbonylation.
reported yields are between 61 ± 93% compared to 66%
Previously we provided experimental evidence for the
in this work. For mesitylene to trimesic acid the reported metal-catalysed decarboxylation mechanism using a Co/
[
14]
yields are 71 ± 88% compared to 78% found here. The Mn/Br catalyst in acetic acid.
During the aerobic
yields of trimellitic acid from pseudocumene are 90 ± oxidation of 1,2,4-trimethylbenzene, the addition of
1
3
9
2% as compared to 50% found here However, these benzoic acid which has a C-labelled carboxyl group,
1
3
are perhaps unfair comparisons since the reported yields resulted in the observation of CO in the vent gases.
2
in acetic acid are optimised yields obtained by variation The temperature of the reaction was 958C where
of the type of catalyst, the catalyst concentration, the thermal decarboxylation should be very minimal. The
reagent concentration, the temperature and the pres- increase in carbon dioxide formation is directly propor-
sure. These variables are adjusted for each reaction to tional to the concentration of benzoic acid added. The
take into account the initial reactivity of the methylar-
omatic compound and the decreasing reactivity of the
reagent as the successive methyl groups have been
reacted to carboxylic acid groups. This has been
[
1,19]
discussed in detail.
In this work, the optimised
conditions for the oxidation of p-xylene to terephthalic
acid in scH O were used for all the othermethylaromatic
2
compounds. For instance, if pseudocumene was oxidised
in an identical manner to the commercial process where
p-xylene is converted to terephthalic acid using a Co/
Mn/Br catalyst in acetic acid, much lower yields would Scheme 3. Pathways leading to loss of methyl carbon atoms
be obtained. Therefore, a further optimisation of the during autoxidation.
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Eduardo Garcia-Verdugo et al.
addition of o-phthalic acid, rather than benzoic acid,
results in more than twice the rate of formation of
carbon dioxide, on a mole basis, consistent with ortho
acid groups being more effectively decarboxylated than
[
14]
para or meta-ones.
The oxidation of the phenyl radical to phenol and its
subsequent complete combustion to carbon oxides,
rxn 6, Scheme 3, is probably important since, except
for p-xylene, the mass accountabilities are < 100.
The significantly lower yield of o-xylene to o-phthalic
acid as compared to the para- and meta-isomers and its
higher benzoic acid yield suggest that the decarboxyla-
tion of the ortho diacid grouping is more important than
for the carboxylic acids in the para- and meta-positions.
This is particularly striking during the oxidation of 1,2,4-
trimethylbenzene where the yield of terephthalic acid is
twice as high as that of o-phthalic acid. The results are
consistent with the reported thermal decarboxylation of
Figure 3. Raman spectra of an aqueous solution of 0.08 M
phthalic acid and 0.04 M sodium hydroxide, heated isobari-
cally at 300 bar stepwise from 25 to 3508C. An NIR GaAlAs
diode laser (785 nm) was used to excite the samples with ꢀ
3
00 mW output power using an Almega (Thermo Nicolet)
Raman spectrometer, exploiting 1808 (back-scattered) geom-
terephthalic and isophthalic acid which gave yields of etry of scattering.
1
0 ± 15% at 3508C for 30 min but for trimellitic anhy-
[
23]
dride, complete degradation was observed. Similarly
it has been shown that o-phthalic acid thermally
decarboxylates much easier than terephthalic and iso-
phthalic acids. However, the relative importance of tions have discussed the structure of the Co/Mn/Br
thermal and metal-catalysed decarboxylation, remains catalyst in acetic acid and acetic acid/water mixtures,[25]
uncertain.
the nature of the metal bromide species that activates
A better understanding of methyl carbon atom loss the methylaromatic compound via benzylic radical
[
26]
processes in the autoxidation process in scH O is formation, and the activity and selectivity enhance-
2
[
27]
important in order to minimise the extent of this side ments due to the presence of cobalt and zirconium in
reaction. The stability of the aromatic carboxylic acid in the catalysts. The keyquestion here is why H O lowers
[
28]
2
[
23]
ncH O and scH O has received previous attention.
the activity of MnBr in CH CO H, but does not appear
2
2
2 3 2
We have started to study such reactions using high to affect its activity under supercritical conditions. We
temperature, high pressure Raman spectroscopy. We suggest that this is largely the consequence of the low
have demonstrated that phthalic acid 2a decarboxylates dielectric constants of CH CO H and scH O, both of
3
2
2
À
to benzoic acid 6 (Figure 3) at high temperature and in which promote ion-pairing. The role of Br is discussed
the presence of NaOH. The decarboxylation takes place in detail in separate paper in this issue.[38]
above 3008C and can be monitored using Raman
Current evidence suggests that MnBr2 (Mn/Br
spectroscopy.[ The first clear observation is that a 1:2 mol/mol) produces higher yields of terephthalic
24]
new band, which corresponds to the mono-substituted acid in batch autoxidation in sub- and supercritical water
À1
[9,10]
benzene ring, appears at 1000 cm , whilst the existing than CoBr2.
In acetic acid, the Mn/Br catalyst is
peakcorresponding to the disubstituted benzene ring at
much less active than the Co/Br catalyst. Also, increas-
À1
ꢀ
1049 cm , decreases significantly in intensity. Fur- ing the bromide/metal ratio above 1/1 mol/mol reduces
[
1,28]
ther investigations based on this technique are in the activity.
This may suggest that MnBr might be a
2
progress in order to obtain kinetic data relevant to this negative catalyst in scH O rather than a positive one, i.e.,
2
process, which will allow us to minimise its effect.
decreases the rate of reaction rather than enhances it.
With a system that produces high yields in 20 sec rather
than 0.5 ± 2.0 h, the necessity for catalyst control of the
Mechanism of Autoxidation in Acetic acid and scH O desirable reaction pathways may be much more impor-
2
tant. For example, we have found in the highly active
The mechanism of metal- and metal/bromide-catalysed non-catalysed autoxidation of benzaldehyde in acetic
aerobic oxidation of hydrocarbons is thought to be a acid that addition of Co and Co/Mn/Br catalysts reduces
metal-modified free radical chain mechanism. Benzylic the rate of autoxidation and increases the selectivity as
.
[39]
radicals (PhCH ) are initially formed which react well.
2
rapidly with dioxygen to form benzyl peroxy radicals
In metal/bromide autoxidations in acetic acid, the
.
(
PhCH OO ). Benzyl hydroperoxides (PhCH OOH) bromine is known to enhance selectivity by reducing the
2
2
are formed via a subsequent propagation step with a steady state concentration of the higher oxidation states
second molecule of the hydrocarbon. Recent publica- of the metals via the reaction in Eq. (1).
312
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Highly Selective Oxidations in Supercritical Water
FULL PAPERS
Table 3. Selected properties of H O, scH O, acetic acid, and acetic acid/water mixtures and self dissociation constant (log K) of
2
2
Co(II), Mn(II), Ni(II) bromide mixtures.
Solvent
T [ 8C]
e
Self-dissocia-
tion [log K]
Geometry of
Co(II) coordi-
nation compound
Metal bromide stability constants, K , K
1 2
Co(II)
Mn(II)
Ni(II)
H O
HOAc
25
25
25
78
À 14
oct.
oct., tet.
oct.
0.74, 0.40
851, 295
small
1.9, 0.55
0.76, -
not available
not available
2
[
34]
[34]
[15]
6.2
À 13.9
large
Small
17[
34]
À 8.1
[34]
[25]
[15]
10%
HOAc-H O
2
scH O
427
25
4
1.35
À 20
oct., tet
oct. tet
not available
not available
not available
large[31]
not available
2
acetone
±
K  K ˆ
1
2
9
[34]
1
.6 Â 10
The structures of these compounds, as drawn by Fulton,
are reproduced in Figure 4 and compared to those in
anhydrous and hydrous acetic acid. Tomiyasu and co-
workers have described a similar transition for CoCl2
using UV-visible spectroscopy in water from 50 to
ꢁ
1†
Since these are homogeneously catalysed autoxidations,
the speed of this critical reaction is obviously related to
the metal/bromide coordinationcompounds that exist in
solution. We would suggest that the metal/bromide
coordination compounds in the active acetic acid, water/
acetic acid, and scH O solvents are similar to each other
but dissimilar to the much less active non-scH O water.
The structural change of Ni(II)/Br mixtures from non-
scH O water to scH O has been studied by Fulton and
Using XAFS, near IR, and molecular
dynamics, they followed the change from the simple
[
35]
4
008C.
In the following discussion we shall assume that the
coordination chemistry for Ni(II) is the same as that for
Co(II) and Mn(II). This assumption is reasonable since
these are first row transition metals in the same
oxidation state. Some of the similarities are shown in
various different solvents in Table 3.
2
2
At concentrations normally used in autoxidations
(0.01 ± 0.02 M), these three metals are essentially octa-
2
2
[
31]
co-workers.
2‡
hedral ions of the structure [M(II)(H O) ] in non-
2
6
2
‡
scH O. Very little bromide is coordinated to the metals
[
Ni(II)(H O) ] at room temperature, to an equilibri-
2
2
6 oct.
as evidenced by their small measured stability constants,
see Table 3. Little ion-pairing occurs between the
um mixture of octahedral and tetrahedral species at
258C to a series of tetrahedral, ion-paired, NiÀBr
species at 4258C, Eq. (2).
3
[
32]
hydrated metals and the bromide.
By contrast, in acetic acid most of the bromide is
coordinated to the metals (see Table 2) and virtually all
of the non-coordinated bromide exists as ion pairs.[
Furthermore, the solution contains both octahedral and
25]
[
25,33]
tetrahedral Co/Br species.
Additionally, there is
evidence for both mononuclear and polynuclear metal
ꢁ
2†
Figure 4. Predominant metal/bromide coordination compounds in active (scH O, HOAc, H O/HOAc) and inactive (non sc-
2
2
water) solvents A): in water M ˆ Ni(II), Mn(II), Co(II). B): In supercritical water. C): In anhydrous acetic acid. D): In hydrous
acetic acid. Adapted from Refs.[
31,35]
Adv. Synth. Catal. 2004, 346, 307 ± 316
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
313

FULL PAPERS
Eduardo Garcia-Verdugo et al.
species.[25] The change in coordination from water to the catalytic system. However, to quote V. Krukonis
anhydrous acetic acid can be approximately illustrated ™There is no point in doing something in a supercritical
as shown in Eq. (3).
fluid just because it×s neat. Using the fluids must have
[
36]
some real advantage∫. We believe that continuous
selective partial oxidation is an excellent example of the
use of supercritical fluids to develop processes which are
not only environmentally more friendly but also have
additional technical and commercial advantages.
Certainly, a remarkable advantage is the absence of
detectable intermediate alcohols and aldehydes during
ꢁ
3†
Addition of water to acetic acid, results in most of the the continuous scH O aerobic oxidation of p-, m- and o-
2
bromide being no longer bound to the Co and Mn and xylenes. This is important because it is not enough to
the tetrahedral species no longer exists. But, because the produce a product in high yield but also it must be of
dielectric constant only increases modestly (6.2 to 17 sufficient purity. This is commercially important, be-
when changing from 0 to 10 wt % water), virtually all of cause the purification process can be as expensive as the
[
34]
the anions are ion-paired with the metals.
The reaction producing the product. In terephthalic acid
oxidation activity of the coordination compounds in manufacture, for example, the oxidation reaction pro-
this mixture is attributed to the presence of the easily duces solid terephthalic acid with a purity of about
accessible ion-paired bromide to the metallic oxidants 99.6%. However, the purification of these solids to
[
typically Co(III), Mn(III), Mn(IV)] and the labile, remove 4-carboxybenzaldehyde requires a separate
[
25]
weakacetic acid ligand in the coordination sphere.
plant to be built which hydrogenates the 4-carboxyben-
The decreased activity in the presence of the water is due zaldehyde backto p-toluic acid which can be subse-
to many factors including the replacement of the acetic quently separated by fractional crystallization from
acid ligands by water and the reduced reaction rate via water. In the case of o-xylene oxidation, the selectivity
the metal-catalysed pathways. The transition of cobalt/ achieved here in scH O is better than that reported for
2
bromide mixtures from anhydrous acetic acid to hy- the equivalent transformation of o-xylene in the gas
drous acetic acid can be written as in Eq. (4).
phase (78% based on o-xylene). The by-products in the
gas phase process include o-toluic acid, phthalide,
benzoic acid, and maleic acid. Two-stage distillations
are required to reach a purity of at least 99.8%. In our
case, simple selective crystallisation may give the
product with the required degree of purity and would
eliminate the need for expensive distillations. The
replacement of the current gas-phase process by the
ꢁ
4†
Thus, in the transitions from non-scH O to either acetic liquid phase scH O oxidation would result in about a 10-
2
2
[
3]
acid or scH O, the types of coordination compounds fold increase in the space-time yield.
2
formed are similar. Transitions occur from simple
Additionally, the elimination of the acetic acid solvent
hexaaquometal(II) ions to dimeric species and to by use of scH O would constitute a major process
2
mixtures of octahedral and tetrahedral species, the improvement because the metal-catalysed and free
bromide being either predominately directly bonded or radical-catalysed decomposition of acetic acid is one of
ion-paired to the metal. The probable reason for these the major variable costs in the current manufacture of
structural changes is understandable from the changes terephthalic acid. Indeed, one of the major uses of acetic
in dielectric constant in Table 3. Non-scH O water has a acid is to replace the acetic acid losses in terephthalic
2
[
16]
value of 78 at room temperature which changes to 4 in acid and dimethyl terephthalate manufacture.
scH O. The dielectric constant in scH O is similar to that Summarising, the use of supercritical water for this
2
2
of acetic acid. Acetone, another solvent of low dielectric process provides: i) chemical advantages (high selectivity
constant, also has an equilibrium between octahedral by fine-tuning of solvent properties), ii) the environ-
and tetrahedral bromo compounds with large equili- mental advantage of avoiding the use of acetic acid as
brium constants.[
26b]
solvent and its subsequent waste by unwanted oxidation
and production of CO , iii) economic advantages result-
2
ing from more selective and faster reaction, plus oper-
ation at high temperature allowing for more efficient
energy recovery from the exothermic reactions. and iv)
Conclusion
By tuning the pressure and temperature, scH O be- processintensification:fromalargereactorvolumewitha
2
comes a solvent with physical properties which provide ca. 40 minute residence time, to a reactor with a residence
for single-phase oxidation (organic and oxygen are time of the order of a few minutes. The new technology
soluble in scH O) and an environment which activates offers a cleaner route to aromatic carboxylic acids.
2
314
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Highly Selective Oxidations in Supercritical Water
FULL PAPERS
Experimental Section
Thermal Stability Study by Raman Spectroscopy
The cell and the experimental set-up has already been
described in detail in a recent publication.[ The cell was
constructed from the heat-resistant EI437B Ni-Cr alloy and
was designed to withstand pressures of up to 1500 bar at
temperatures up to 5508C. The cell was installed in the sample
compartment of the Almega (Thermo Nicolet) Raman spec-
trometer, exploiting 1808 (back-scattered) geometry of scat-
tering to collect spectra. An NIR GaAlAs diode laser (785 nm)
was used to excite the samples with ꢀ 300 mW output power.
The motorised stage of the Almega made itpossible to align the
cell by using the xyz coordinates via the Omnic 6.0a Software
24]
Oxidation of Polymethylbenzenes
All the experiments were conducted using a continuous flow
reactor, the configuration of which is shown in Figure 1. The
continuous oxidation was carried out in scH O with MnBr as
the catalyst, using the optimised conditions previously report-
ed by us for the oxidation of p-xylene to terephthalic acid.[
The exotherm was minimised by using relatively dilute
solutions (<5% organic w/w). An aqueous solution of H O
of 2% vol. was used as the source of O , supplied via a
reciprocating pump which allows for accurate control of the
2
2
11]
2
2
2
(
¹ 1992 ± 2000 ThermoNicolet Corp.). A 100 mm slit provided
O :substrate ratio; a critical parameter which is difficult to
2
sufficient resolution. Each spectrum was collected over four
scans of which each had an exposure time of five seconds. For
each experiment, spectra were collected at regular time
intervals, typically during a 30 min period.
achieve reproducibly using a gas booster pump to compress
oxygen gas. The H O solution was pumped into the system and
2
2
decomposed in the pre-heater under the required experimen-
tal conditions to generate a homogeneous mixture of O and
2
Reagent grade samples of the acids were purchased from
Aldrich and were used as received. NaOH was obtained in
pellet form. After ensuring the alignment was complete and
that the highest S/N ratio was achieved for the spectrum of the
solution under question, the cell was heated up to the desired
temperature and pressure. After thermal equilibrium was
reached, a set of spectra was obtained with a GaAlAs diode
laser (excitation at 785 nm), with an output of ca. 300 mW.
Each trace was the sum of 16 scans, at 10 seconds each, using a
100 mm slit. At the end of each experiment, the heater was
switched off and the cell was allowed to cool down to ambient
temperature. The system was then depressurised and the
contents were collected for high performance liquid chroma-
tography (HPLC) analysis.
scH O. Tester and co-workers have shown that there are no
2
significant differences in the oxidation of MeOH in scH O
2
[
37]
using either O or decomposed H O as the oxidant. The
2
2
2
residence time in the pre-heater was long enough to guarantee
total decomposition of H O to H O and O . The residence
2
2
2
2
time calculated from the total reactor volume divided by the
volumetric flow rate, was around 5 seconds. The total volume
was taken as the sum of the volume of the tubular reactor,
1
constructed from a 30-cm length of ³
4
in o.d. Hastelloy C276
pipework. The volumetric flow rate was based on the physical
properties of H O at the reaction conditions (Tˆ 3808C and
2
230 bar), as published in International Steam Tables and by the
US National Institute. The volumetric flow rate was
1
2.184 mL/min, being: flow rates: H O (P-01): 8.19 mL/min,
2
2
catalyst (P-02): 4.023 mL/min, organic (P-03): 0.061 mL/min,
NaOH (P-04): 3.4 mL/min. All the chemicals were purchased
from Aldrich Ltd and used without further purification.
CAUTION: This type of oxidation is potentially extremely
hazardous, and must be approached with care and a thorough
safety assessment must be made. Before each run, the
apparatus was hydrostatically pressure tested when cold, and
was then heated with a flow of pure water (5 ± 10 mL/min).
Once the operating temperature had been reached, the pumps
for organic, H O solution, catalyst solution MnBr (catalyst
Acknowledgements
Support from the Royal Society, the EPRSC (Grant GR/
N06892), the Marie Curie Research Programme are greatly
appreciated. We thank Dr. G. R. Aird and ProfYu. E. Gorbaty
for their help and Invista Performance Technologies for
financial support.
2
2
2
strength: Mn 1719 ppm/Br 5000 ppm) and NaOH (0.5 mol/L)
were started. Typically, an experiment was run for 1 ± 4 h. The
products were collected for sequential periods of 15 min and
analysed. The yield was calculated by HPLC as a percentage of
the stoichiometric amount expected from the measured
amount of organic pumped into the apparatus. The selectivity
was calculated by HPLC as a percentage based on the molar
amount of a particular component in the solution recovered
from the reactor. AWaters Xterra reverse phase C18 column,
maintained at 408C, was used (flow rate 0.7 mL/min, run time
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3
3
2
3
2
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4
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3
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[
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Adv. Synth. Catal. 2004, 346, 307 ± 316
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¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
315

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¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 307 ± 316