Highly efficient one-step conversion of cyclohexane to adipic acid using single-site heterogeneous catalysts

Article abstract of DOI:10.1039/b513583d

A solid source of 'active' oxygen (acetylperoxyborate, APB), when dissolved in aqueous solution in the presence of a single-site microporous catalyst containing redox centres (Fe^IIIAlPO-31, Mn^IIIAlPO-5, Fe^IIIAlPO-5), converts cyclohexane with high efficiency (ca. 88%) and exceptionally high selectivity (ca. 81%) to adipic acid at 383 K; this procedure is also effective in converting styrene to styrene oxide and α-pinene and (+)-limonene to their corresponding epoxides. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513583d

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Highly efficient one-step conversion of cyclohexane to adipic acid using
single-site heterogeneous catalysts
Robert Raja,*a John Meurig Thomas,bc Mingcan Xu,d Kenneth D. M. Harris,d Michael Greenhill-Hoopere
and Kieran Quille
Received (in Cambridge, UK) 26th September 2005, Accepted 1st November 2005
First published as an Advance Article on the web 9th December 2005
DOI: 10.1039/b513583d
been developed, such as the one-step oxidation of cyclohexane
with either alkyl hydroperoxides or dioxygen.11–13 The formation
of styrene oxide from styrene has been used widely as a model
reaction for demonstrating catalytic epoxidation. It is estimated
that millions of lbs of styrene oxide are produced per annum in the
USA alone for the production of styrene glycol and phenylethyl
alcohol (perfumes), as a reactive diluent for epoxy resins and as a
chemical intermediate for cosmetics, surface coatings, fibres and
textiles. The solid redox catalysts that are eminently suitable for the
generation of active oxygen from peroxy precursors are transition-
metal-substituted microporous aluminophosphates4b,14 (e.g.
FeIIIAlPO-31, MnIIIAlPO-5 and FeIIIAlPO-5), which have been
described4b,14 (and used for other15 benign oxidations) by us
previously.
A solid source of ‘active’ oxygen (acetylperoxyborate, APB),
when dissolved in aqueous solution in the presence of a single-
site microporous catalyst containing redox centres (FeIIIAlPO-
31, MnIIIAlPO-5, FeIIIAlPO-5), converts cyclohexane with high
efficiency (ca. 88%) and exceptionally high selectivity (ca. 81%)
to adipic acid at 383 K; this procedure is also effective in
converting styrene to styrene oxide and a-pinene and (+)-
limonene to their corresponding epoxides.
Oxidation reactions are central to numerous processes that convert
bulk chemicals into useful, higher-value products.1 But most of the
current practices employed in industry suffer from several
disadvantages, such as the use of aggressive oxidants, low
selectivity, multiple-step reactions and the generation of excessive
amounts of waste. There is a pressing need for new oxidants,
especially if, by the parallel use of benign and inexpensive solid
catalysts, highly selective oxidations may be effected under
relatively mild conditions. It is acknowledged that the ideal
oxidant is air or oxygen2–4 and that the merits of hydrogen
peroxide,5 notwithstanding the practical problems associated with
its transport, storage and use, are considerable. Many striking
recent advances have been reported with this oxidant.6
On the laboratory scale, we have shown previously13 that,
with
a
carefully designed framework-containing FeIIIAlPO-
31 catalyst, aerial oxidation of cyclohexane may achieve some
65% selectivity towards adipic acid, but the conversion falls in
the range of 3 to 8% at temperatures of 373 to 403 K. Many
other heme and non-heme catalysts can facilitate the oxidation
of cyclohexane in H2O2 or in alkylhydroperoxides, but
the products are invariably a mixture of cyclohexanol and
cyclohexanone.
Here we describe how a stable powder, acetylperoxyborate
(APB), which has hitherto been used mainly for bleaching
purposes,7 may be used,8 when dissolved in aqueous solution, to
generate high yields of (i) adipic acid (an extremely important
commodity chemical in the manufacture of urethanes and
polyamides such as nylon 6,6), and (ii) a variety of epoxides,
from their hydrocarbon precursors. Currently adipic acid is
manufactured by a two-step process involving nitric acid oxidation
of K-A oil, a mixture of cyclohexanol and cyclohexanone, the
latter being produced from cyclohexane using a homogeneous
cobalt-based catalyst.9,10 Alternative synthetic routes have also
We have carried out structural studies on APB both in the solid
state and on dissolution in water. Powder XRD shows that APB is
a crystalline material, but comprises a mixture of phases; the major
constituent(s) of this mixture are of unknown structure.{ Solid
state NMR spectroscopy also substantiates the complexity of the
solid material.{ However, when APB is dissolved in water, the
situation is substantially simpler, and it is clear that the different
phases present in the solid state give rise to the same solution state
species on dissolution in water. Thus, in aqueous solution, 13C and
1H NMR spectra reveal that acetic acid [CH3CO2H] and
peroxyacetic acid [CH3CO3H] are the only carbon-containing
species present (Fig. 1). The 11B NMR spectrum shows a single
peak characteristic of trigonal B(O)3 species (probably representing
the average over an equilibrium involving different species of this
type, including boric acid [B(OH)3]).
Integration of the 1H NMR signal intensities (CH3 groups)
shows that, after dissolution in water, the ratio of peroxyacetic acid
to acetic acid decreases relatively slowly as a function of time
(Fig. 2), as the system moves towards establishing the following
equilibrium:§ CH3CO2H + H2O2 = CH3CO3H + H2O. Clearly, a
decrease in the ratio [CH3CO3H]/[CH3CO2H] as the system
approaches equilibrium leads to an increase in the concentration
of hydrogen peroxide.
aDepartment of Chemistry, University of Cambridge, Lensfield Road,
Cambridge, UK CB2 1EW. E-mail: robert@ri.ac.uk;
Fax: +44-1223-336017; Tel: +44-1223-336335
bDepartment of Materials Science and Metallurgy, University of
Cambridge, Pembroke Street, Cambridge, UK CB2 3QZ.
E-mail: jmt2@cam.ac.uk; Fax: +44-1223-740360;
Tel: +44-1223-334467
cDavy Faraday Research Laboratory, Royal Institution of Great Britain,
Albemarle Street, London, UK W1S 4BS
dSchool of Chemistry, Cardiff University, Park Place, Cardiff, Wales
CF10 3AT. E-mail: harriskdm@cardiff.ac.uk; Fax: +44-29-2087-4030;
Tel: +44-29-2087-0133
eBorax Europe Limited, 1A Guildford Business Park, Guildford, UK
GU2 8XG. E-mail: mike.greenhill-hooper@borax.com;
Fax: +44-1483 242001; Tel: +44-1483 242059
448 | Chem. Commun., 2006, 448–450
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Table 1 Oxidation of cyclohexanea using APB and FeIIIAlPO-31
Product selectivity (mol%)
Time Conv.
pH (h) (mol%) Cy-olc Cy-oned acid
Adipic Other
acidsb
Oxidant
APB
5.2
3
16
3
49.0
88.6
61.2
97.0
42.0
82.6
48.0
89.8
11.2 71.5
11.3
6.5 48.3
17.1
81.2
21.8
33.1
22.5
72.5
17.5
51.2
—
7.5
23.5
56e
2.7
6.5
—
PAA
1.65
16
3
16
3
—
—
17.3 57.5
3.3 17.7
13.5 49.7
3.5 16.8
PAA + Neobor1 5.1
PAA + NaOAc 5.1
19.2
24.3f
16
a
Cyclohexane ; 2.5 g; FeAlPO-31 ; 0.25 g; T ; 383 K; solid APB
3.49 (shown by titration studies to liberate 0.701 of
;
g
g
peroxyacetic acid and 0.045 g of H2O2 immediately on dissolution);
20.5 g of double-distilled (d-d) water; PAA ; peroxyacetic acid (25%
peroxyacetic acid solution in acetic acid) ; 4.2 g + 20.5 g of d-d
water; PAA + Neobor1 ; 4.2 g (25% peroxyacetic acid solution in
acetic acid) + 1 g Neobor1 + 1 g NaOH + 20.5 g of d-d water;
PAA + NaOAc ; 4.2 g (25% peroxyacetic acid solution in acetic
acid) + 0.934 g sodium acetate trihydrate + 1 g NaOH + 20.5 g of
1
Fig. 1 (A) Solution state H NMR spectrum of APB dissolved in D2O
[1.76 ppm: CH3 of acetic acid (averaged over the equilibrium CH3CO2H
= CH3CO22 + H+); 1.91 ppm: CH3 of peroxyacetic acid (averaged over
2
the equilibrium CH3CO3H = CH3CO3 + H+); 4.70 ppm: 1H in D2O
b
c
d-d water. Other acids ; glutaric, succinic and valeric acids.
;
(residual 1H), B(OH)3, H2O2, and the CO2H groups of acetic acid and
peroxyacetic acid, all of which undergo rapid exchange in the acidic
solution]. (B) Solution state 13C NMR spectrum of APB dissolved in D2O
[22.3 ppm: CH3 of acetic acid; 180.0 ppm: C(O)O of acetic acid; 16.9 ppm:
CH3 of peroxyacetic acid; 172.7 ppm: C(O)OO of peroxyacetic acid; the
13C signals are averaged over the equilibria shown above].
d
e
f
Cyclohexanol. ; Cyclohexanone. ; 10.5% CO2. ; 4.1% CO2.
(MIII ions) are located. The use of the solid oxidant (APB) in
conjunction with the FeIIIAlPO-31 catalyst improves the overall
conversion by an order of magnitude ($ 13 fold), but more
interestingly the selectivity for adipic acid is also significantly
enhanced (from 65% to 81%), compared to our earlier work13
when air was used in combination with this catalyst.
For the purpose of comparison, the oxidation of cyclohexane
was also carried out using neat peroxyacetic acid (PAA) rather
than APB as the oxidant. Although high conversions are observed,
the selectivity for adipic acid is reduced drastically. Fragmentation
products such as succinic, glutaric and valeric acids, tar and CO2
seem to predominate with PAA. In order to rule out the possibility
that the decreased adipic acid selectivities observed with neat PAA
are due to changes of pH, additional experiments were carried out
by adjusting the pH to be the same as that in the experiments with
APB. In one set of experiments, a combination of peroxyacetic
acid, sodium acetate trihydrate and NaOH was used to bring the
pH close to that for APB, while in another set, Neobor1 borax
pentahydrate (Na2B4O7?5H2O; ex Borax Europe Ltd), an
important component which is present in APB, was used. The
addition of Neobor1 significantly enhances the selectivity for
adipic acid, whereas in the absence of Neobor1, fragmentation
products and CO2 are observed (in agreement with the result for
neat PAA). In the experiments in which Neobor1 is added to
PAA, the conversions (activity) and selectivities for adipic acid are
not as high as those observed with solid APB. Although Neobor1
is a major component in the preparation of APB, we emphasize
that other components (sodium perborate monohydrate and
H2O2) are also involved in the preparation and may influence
the activity and selectivity. However, this series of experiments
shows that the boron (borate) component, contained in Neobor1,
is critical for the high activities and selectivities observed when
APB is used as the oxidant. Thus, while APB serves as a source of
‘‘active’’ oxygen for these catalytic oxidation processes, the role of
key boron-containing species that arise from dissolution of APB is
also implicated. Clearly, identification of these species and
understanding their role is an important issue for future research.
Fig. 2 Plot of the ratio R = [CH3CO3H]/[CH3CO2H] (determined from
solution state 1H NMR) as a function of time after dissolution of APB in
water at (a) 25 uC and (b) 65 uC.
The key feature of the use of the novel solid oxidant APB is that
the peroxyacetic acid, formed on dissolving APB, releases its active
oxygen in the presence of the single-site16 solid redox catalysts
FeIIIAlPO-31, MnIIIAlPO-5 or FeIIIAlPO-5 and dissolved boron-
containing species (see below). The active oxygen so liberated
oxidizes cyclohexane and epoxidizes any alkene introduced to the
sphere of reaction with high yields and high selectivities. Typical
results for the oxidation of cyclohexane are presented in Table 1.
The FeIIIAlPO-31 microporous catalyst has a one-dimensional
˚
channel system with a pore aperture of 5.4 A, which is significantly
III
smaller than that of Fe AlPO-5 (7.3 A). The more puckered inner
˚
walls therefore provide a constrained environment for the
oxidation of cyclohexane and thus modify the selectivity of the
reaction, as we have described earlier.13 What we capitalize upon is
‘‘product shape selectivity’’ in a highly localized manner within the
restricted environment, where the isolated catalytically active sites
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Chem. Commun., 2006, 448–450 | 449

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Table 2 Liquid-phase epoxidationa of olefins and terpenes using APB
Notes and references
Product selectivity
(mol%)
{ The powder XRD pattern contains subsets of peaks that differ
significantly in linewidth, suggesting that the material is a mixture of
phases of differing crystallinity. The main intense peaks are comparatively
broad. So far, successful indexing of the powder XRD pattern has not been
achieved. Other narrower peaks of lower intensity are assigned as impurity
phases, one of which is identified as NaCH3COO(CH3CO2H)2 (A. Perotti
and V. Tazzoli, J. Chem. Soc., Dalton Trans., 1981, 1768). The 2A phase of
boric acid (W. H. Zachariasen, Acta Crystallogr., 1954, 7, 305) is probably
also present as a minor impurity.
TimeConv.
Substrate Catalyst Oxidant pH (h) (mol%)Eb DiolBc Others
Styrene MnAlPO-5APB
Styrene FeAlPO-5 APB
Styrene MnAlPO-5PAA
Styrene MnAlPO-5PAA + 5.1 1
Neobor1
Styrene MnAlPO-5PAA + 5.1 1
5.2 1
4
5.2 1
4
1.651
95.2 100
98.8 86.513.5 —
84.1 100
—
—
—
—
—
—
—
—
99.1
97.6
73.0
79.6
78.0
92.7
87.312.5 —
{ The high resolution solid state 13C NMR spectrum of APB indicates at
least three different CH3 environments and at least two different CLO
environments, suggesting that there are several structurally distinguishable
acetyl/acetate [CH3C(O)OZ/CH3C(O)O2] and/or peroxyacetyl/peroxyace-
tate [CH3C(O)OOZ/CH3C(O)OO2] groups in the material (where Z is
probably a boron-containing moiety). Solid state 11B NMR spectroscopy
shows at least two different boron environments, and solid state 23Na
NMR spectroscopy confirms the presence of sodium in the material (the
spectrum is broad due to quadrupolar interaction).
15.535.2 39.7 9.5
87.312.5 —
81.318.5 —
—
—
4
63.327.5 9.2—
43.529.7 11.314.2
NaOAc
a-Pinene MnAlPO-5APB
4
5.2 1
4
77.8 100
88.6 68.7—
78.0 100
93.7 61.3—
—
—
—
—
—
—
31.2
—
(+)-
Limonene
MnAlPO-5APB
5.2 1
4
—
38.5
a
Styrene ; 2.8 g; a-pinene, (+)-limonene ; 3.7 g; MeAlPO-5 ;
0.25 g; T ; 338 K; solid APB ; 3.49 g (shown by titration studies
to liberate 0.701 g of peroxyacetic acid and 0.045 g of H2O2
immediately on dissolution); 20.5 g of double-distilled (d-d) water;
PAA ; (25% peroxyacetic acid solution in acetic acid) ; 4.2 g +
20.5 g of d-d water; PAA + Neobor ; 4.2 g (25% peroxyacetic acid
solution in acetic acid) + 1 g Neobor1 + 1 g NaOH + 20.5 g of d-d
water; PAA + NaOAc ; 4.2 g (25% peroxyacetic acid solution in
acetic acid) + 0.934 g sodium acetate trihydrate + 1 g NaOH +
§ The reported equilibrium constant for this reaction at 25 uC (defined as
K = [CH3CO3H][H2O]/[CH3CO2H][H2O2]) is ca. 3.28 (see: K. Murai,
G. Akazome and Y. Murakami, Kogyo Kagaku Zasshi, 1960, 63, 1233). As
[H2O] >> [H2O2] in aqueous solution, then at equilibrium, the ratio
[CH3CO3H]/[CH3CO2H] (= K[H2O2]/[H2O]) must be ,, 1.
1 (a) K. Weissermel and H.-J. Arpe, in Industrial Organic Chemistry,
Wiley-VCH, Weinheim, 4th edn, 2003; (b) Modern Oxidation Methods,
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2 (a) C. L. Hill, in Advances in Oxygenated Processes, ed. A. L.
Baumstark, JAI, London, 1988, vol. 1, pp. 1–30; (b) J. M. Thomas,
R. Raja, G. Sankar, B. F. G. Johnson and D. W. Lewis, Chem.–Eur. J.,
2001, 7, 2973; (c) J. M. Thomas, R. Raja, G. Sankar, R. G. Bell and
D. W. Lewis, Pure Appl. Chem., 2001, 73, 1087.
b
c
20.5 g of d-d water. E ; epoxide. B ; benzaldehyde.
The liquid-phase epoxidations of styrene, a-pinene and (+)-
limonene with MnIII- and FeIIIAlPO-5 are summarized in Table 2.
For styrene, both catalysts, at 65 uC and 1 h contact time, give very
high conversions and close to 100% selectivity towards styrene
oxide. Longer contact time, not surprisingly, yields some diol
products and a corresponding diminution in selectivity towards the
desirable styrene oxide. Blank experiments (in the absence of the
solid catalysts) afford low conversions and virtually no styrene
oxide is produced (only diols and polymeric products are
observed), and a Mukaiyama style epoxidation17 (entailing the
use of sacrificial benzaldehyde for the in situ production of
perbenzoic acid in the presence of the redox catalyst) is much less
effective than the APB procedure.
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Analogous experiments were performed with styrene to exclude
the effect of pH and to investigate the importance of the boron
(borate) component. As in the case of cyclohexane, the addition of
Neobor1 significantly improves the selectivity for the epoxide,
whereas in the absence of Neobor1, cleavage, polymeric and other
degradation products are observed (in good agreement with the
result for neat PAA).
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While previous uses of borates in the oxidation of organic
compounds have been reported,18 the use of the solid oxidant APB
together with single-site framework-substituted metal aluminopho-
sphate catalysts, reported for the first time in the present paper,
represents a highly promising new opportunity for effecting a
range of oxidation processes. Thus, the high activities and high
selectivities observed using APB in the oxidation of cyclohexane to
adipic acid and in the epoxidations of olefins and terpenes (Tables
1 and 2), coupled with the obvious advantages that this solid
oxidant offers in handling, transport and storage, augur well for
the future use of APB in a range of industrially significant
reactions and processes.
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We thank Fang Guo for help with powder XRD and Borax
Europe for financial assistance
450 | Chem. Commun., 2006, 448–450
This journal is ß The Royal Society of Chemistry 2006