Calcium tungstate: A convenient recoverable catalyst for hydrogen peroxide oxidation

Article abstract of DOI:10.1039/c6gc00725b

Calcium tungstate was found to be an excellent catalyst for large scale "green" oxidations of organic substrates (amines, alkenes, alcohols, sulfides) with hydrogen peroxide. It displays the unusual dual characteristics of producing a soluble pertungstate species, allowing for homogeneous reaction conditions, but then precipitating, unchanged, at the end of the oxidation. These qualities allow for easy catalyst recovery and minimal waste stream generation for large scale application.

Full text of DOI:10.1039/c6gc00725b

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Calcium tungstate: a convenient recoverable
catalyst for hydrogen peroxide oxidation†
Cite this: DOI: 10.1039/c6gc00725b
Received 15th March 2016,
Accepted 11th August 2016
Caitlin M. Tressler, Peter Stonehouse and Keith S. Kyler*
DOI: 10.1039/c6gc00725b
www.rsc.org/greenchem
Calcium tungstate was found to be an excellent catalyst for large alcohols, alkenes and sulfides. Furthermore, the CaWO4 is
scale “green” oxidations of organic substrates (amines, alkenes, unchanged under even in the presence of stabilizing cofactors6
alcohols, sulfides) with hydrogen peroxide. It displays the unusual and can be recovered by simple filtration.
dual characteristics of producing a soluble pertungstate species,
allowing for homogeneous reaction conditions, but then precipi-
tating, unchanged, at the end of the oxidation. These qualities
Results and discussion
allow for easy catalyst recovery and minimal waste stream gen-
eration for large scale application.
When CaWO4 is suspended in 30% H2O2 at >45 °C, the color-
less solid slowly dissolves resulting in a homogeneous solution
whose yellow color is indicative of the formation of an active
pertungstate7 species. Also, this solution of CaWO4/H2O2 has
Introduction
an identical UV-Visible spectrum (1 mM, λmax = 335 nm) to
that of Na2WO4/H2O2. The CaWO4/H2O2 combination remains
homogeneous at high peroxide concentrations (1–35%), but
the CaWO4 readily precipitates when the peroxide level drops
below this level. In the absence of an oxidizable substrate,
these solutions exhibit slow, and mildly, exothermic decompo-
sition of hydrogen peroxide. Maintaining the temperature
≤65 °C leads to a loss of only 2.3% peroxide content after 8 h.
Above this temperature, the nonproductive decomposition sig-
nificantly increases, but can be minimized by the addition of
stabilizing agents such as 9,10-phenanthroline. The stabilizer
allows for oxidations to be carried out for periods of 24 h or
more. The phenanthroline also precipitates at room tempera-
ture along with the CaWO4 when the peroxide has been
exhausted to below 1%.
Oxidations using hydrogen peroxide/tungsten catalyst combi-
nations have attracted considerable attention because they offer
environmentally clean methods of oxidizing various organic
substrates.1 For large industrial scale application, catalyst recov-
ery and catalyst reuse become significant issues. These issues
have prompted a few reports of heterogeneous tungsten cata-
lysts such as mesoporous solid supported WO3-nanoparticles,2
insoluble polyoxytungstates,3 basic phosphotungstates,4 and
immobilized peroxotungstate.5 A drawback to their use is
related to mass transfer problems and mechanical mixing due
to the heterogeneous nature of the catalyst. This problem would
be exacerbated as the scale of such oxidations increases.
Ideally, for large scale batch reactions, the catalyst would be
homogeneous during the early stage of the reaction, then
become heterogeneous only at the end of the oxidation. We
now report herein that CaWO4 offers this dual behavior of
generating a homogeneous catalyst system at high hydrogen
peroxide levels, but re-precipitating at very low levels allowing
for easy recovery. We demonstrate the application of this
oxidation catalyst to a variety of substrates including, amines,
Table 1 shows that various substrates, including amines,8
alcohols,9 alkenes,10 and sulfides,11 are readily oxidized using
only 0.5–1.0 mol% of catalyst. Oxidations using the more con-
ventional Na2WO4 (Table 1; entries 2, 3, 5–7) gave no signifi-
cant improvement over CaWO4 with respect to time,
temperature and yield. Where appropriate, the catalyst may be
used with cosolvents such as t-butyl alcohol or methanol to
facilitate solubility of the substrate. At the end of each reac-
tion, the excess hydrogen peroxide was decomposed by
increasing the temperature to approximately 95–100 °C. When
the peroxide level drops below 1%, CaWO4 precipitates
unchanged from the mixture.
Department of Chemistry, Indiana University of Pennsylvania, Indiana,
Pennsylvania, 15705, USA. E-mail: Keith.kyler@iup.edu
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6gc00725b
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12
Table 1 Oxidation of compounds with 30% H2O2 and CaWO4
30% H2O2
(mole equiv.)
CaWO
(mole equiv.)
Na2WO4
(mole equiv.)
Substratea
Product
Temp (°C)
50°
Time (h)
Yieldf (%)
95
1
2
4.0
0.005
0.5
4.0b
0.01
0.01
50°
50°
50°
50°
50°
50°
50°
50°
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
93
92
92
92
92
92
82
85
0.01
0.01
4.0
3
4
4.0c
4.0
4.0
4.0
4.0
0.01
0.01
0.01
0.01
0.01
70°
70°
70°
70°
70°
6
6
6
6
6
98b
98
99
98
99
5
6
3.0d
0.01
50°
50°
4
4
98
98
0.01
0.01
4.0c,e
75°
75°
75°
75°
75°
75°
16
91
92
92
92
91
91
0.01
0.01
0.01
0.01
0.01
20
20
20
20
20
7
8
75°
65°
65°
20
8
8
92
76
78
5.0
5.0
0.01
0.01
0.01
65°
4
78
9
2.0
2.0
0.01
0.01
25°
0.5
97
10
25°
0.5
96
a All reactions were run on a 0.1 mole scale unless otherwise indicated. b Run on 2.3 mol scale. c An equal volume of methanol cosolvent was
used. d An equal volume t-butanol cosolvent was used. e 9,10-Phenanthroline (1 mol%) stabilizer was added. f Isolated yields.
The catalyst is easily recovered by filtration, and we have tely heterogeneous throughout the reaction, and their ten-
demonstrated that the recovered catalyst shows no loss in dency to rapidly, nonproductively decompose hydrogen
activity after five cycles of oxidation (e.g. Table 1; 2, 4, 6). peroxide solutions makes them synthetically useless.
Additionally, no difference in appearance or IR spectrum of
The oxidation of amines12 proceeded rapidly at ambient
the catalyst was noted after these repeated cycles.
temperature to afford high yields of the corresponding nitro
By contrast to the CaWO4/H2O2 system, MgWO4 displayed products. The nitro product could be separated as an insoluble
substantially higher water solubility making it unattractive for organic phase, and the catalyst recovered from the aqueous
efficient recovery. We also observed that BaWO4, ZnWO4 and residue by filtration. As an example of the benefit of CaWO4
Zr(WO4)2 catalyzed various oxidations, but these were comple- for large scale batch oxidations, we applied this method to the
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oxidation of ∼2 lbs (1 kg) of p-toluidine (entry 2) and obtained
a 92% yield of p-nitro-toluene and with a >99% recovery of the
Conclusion
catalyst. The course of the amine oxidations proceeds via both As “green” oxidation methods gain popularity for commercial
the hydroxylamino and nitroso intermediates which could be production, the need for homogeneous, but easily recovered,
easily detected by thin-layer chromatography. Therefore, slow catalysts becomes paramount. Our current work demonstrates
addition of the amine to the homogeneous H2O2/CaWO4 the value of using CaWO4 as the catalyst of choice for large
mixture was important in suppressing the formation of azo- scale, batch mode oxidations. It is sufficiently broad in scope
and/or azoxy by-products which often accompany these oxi- to oxidize various classes of organic substrates. This material
dations. By carefully monitoring the reaction progress, we is readily available, inexpensive and provides technical simpli-
could consistently obtain high yields of the desired nitro city for recovery thus minimizing waste stream generation and
compounds.
handling. We are presently using this catalytic system for
In the case of alkene oxidations13 (entries 4 & 5), methanol oxidizing multi-pound quantities of heterocyclic amines
or t-butanol was employed as a cosolvent to facilitate solubili- toward commercial production of heterocyclic nitro analogues.
zation of the substrate. The epoxidation of norbornene (entry
5) gave >99% of the expected exo-epoxide, as was determined
by H & 13C spectra.14 Additionally, we did not observe the for-
1
Acknowledgements
mation of norborneol, or norbornanone which has been
observed with other hydrogen peroxide/catalyst systems.10c,15
This work was supported by an Innovation Award Grant spon-
As previously mentioned, temperature plays a significant sored by Indiana University of Pennsylvania and a supporting
role in determining the final yield of product due to the com- grant provided by Aerojet Corporation.
peting nonproductive decomposition of H2O2 by this catalyst.
This is particularly important for substrates, such as alcohols,
that are oxidized slowly by hydrogen peroxide systems. Alcohol
oxidations were conducted between 65–70 °C to avoid increas-
Notes and references
ing the rate of nonproductive decomposition of hydrogen per-
oxide which occurs at higher temperatures. However, it is
possible to employ reaction temperatures as high as 85–90 °C
by adding stabilizing agents such as 9,10-phen-anthroline. The
stabilizing 9,10-phenanthroline, being water insoluble at room
temperature, was recovered along with the catalyst. We did not
observe any change in the recovered CaWO4 when used in
combination with this stabilizer. We used this combination
for the oxidation of cyclohexanol due to the sluggish rate of
conversion to cyclohexanone when compared with other types
of substrates.
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to afford 3-pentanone in good yield. The oxidation of geraniol
(entry 8) gave mostly the 2,3-epoxide in 78% yield, but was
complicated by approximately 20% hydrolysis of the epoxide to
afford the corresponding 1,2,3-triol.16 Notwithstanding, oxi-
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yields of the corresponding sulfones. Interestingly, the use of
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ides.16 Under these conditions, we obtained ∼45–50% yields of
sulfones and only small amounts (<5%) of sulfoxides, the
remaining being unreacted sulfide. Attempts to suppress the
over-oxidation to obtain only sulfoxide by employing slow
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spectra were consistent with reported data. The aqueous
phase was filtered and the CaWO4 (3.4 g, >99%+) was
recovered.
13 General procedure for the oxidation of alkenes. To 9.0 mL of
30% H2O2 (80 mmol) and 9 mL of t-butanol was added
0.057 g (0.2 mmol) of CaWO4. The mixture was warmed to
45 °C and the solid dissolved to produce a yellow solution.
Then was added 1.88 g (20 mmol) of norbornene and the
mixture was stirred vigorously for 4 h at 65 °C. The mixture
was then warmed to 95 °C for approximately 10 min at which
point the yellow color of the aqueous phase disappeared and
the colorless CaWO4 precipitated. The mixture was cooled to
room temperature, and organic product was extracted with
ethyl acetate. The organic phase was dried over anhydrous
MgSO4, and evaporated to afford 2.15 g (98%) of exo-2,3-
epoxynorbornane. 1H-NMR (CDCl3) δ 0.92 (2H, d), 1.32
(4H, d), 2.20 (2H, br s) and 3.18 (2H, br s). Decoupled
13C-NMR (CDCl3) δ 24.4, 25.5, 34.2, 51.0. The aqueous phase
was filtered and the CaWO4 (3.4 g, 99%+) was recovered.
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12 General procedure for the oxidation of amines. To 2.2 L of
30% H2O2 (12.4 mol) was added 3.4 g (0.12 mol) of CaWO4.
The mixture was warmed to 45 °C and the solid dissolved
to produce a yellow solution. Then was added dropwise
over a 30 min period, 1 kg (2.3 mol) of p-toluidine while 14 S. Berger, S. Braun and H.-O. Kalinowski, 13-
maintaining the temperature below 50 °C. After complete
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