11926
J. Am. Chem. Soc. 1999, 121, 11926-11927
Powerful Redox Molecular Sieve Catalysts for the
Selective Oxidation of Cyclohexane in Air
Robert Raja, Gopinathan Sankar, and John Meurig Thomas*
The Royal Institution of Great Britain
DaVy Faraday Research Laboratory
21 Albemarle Street, London, U.K. W1X 4BS
ReceiVed September 29, 1999
Apart from its intrinsic importance in the chemistry of C-H
activation,1-4 the selective oxidation of cyclohexane to yield
cyclohexanol and cyclohexanone is the center-piece of the
commercial production of Nylon.5 We have recently explored
framework-substituted aluminum phosphate (AlPO) molecular
sieves as catalysts for the aerobic selective oxidation of linear
and cyclic alkanes, alkenes, and ketones,6-10 and have found that
AlPO-36 (pore aperture 6.5 × 7.5 Å) in which a few percent of
the AlIII ions have been replaced by CoIII (or MnIII) is a good
catalyst for cyclohexane oxidation, and that CoIII (or MnIII)
substituted AlPO-18 (pore aperture 3.8 Å) for the regioselective,
terminal oxidation of linear alkanes predominantly to yield the
corresponding n-alkanoic acids.7
A key factor7 in achieving the highest catalytic performance
of a MAlPO sieve is for the transition metal ions (M) to be in a
high oxidation state while still remaining an integral part of the
AlPO framework. It so happens that the extent to which M ions
originally present in the II oxidation state can be raised to their
III state is dependent, for a given ion, on the structure of the
AlPO in which it is incorporated substitutionally. This fraction
of MIII ions is evaluated7,11 from X-ray absorption spectroscopy.
In the MAlPO-18 structure all the CoII ions, but less than 20%
of the CoII ions in MAlPO-5 (up to ca. 4 atom % of the AlIII
which they replace), may be raised to the III state upon calcination
Figure 1. (a) Fe K-edge XANES of calcined FeAlPO-5 (tetraethylam-
monium hydroxide was used as the templating agent). Note that the edge
position and in particular the pre-edge feature is similar to that of
framework-substituted FeZSM-5 catalyst {in part b}. Fourier transforms
{in part c} of the Fe K-edge EXAFS of calcined FeAlPO-5. The solid
line is the experimental and dashed curve the computed data generated
using EXCURV98. Structural data for the oxygen shell yields an average
(1) (a) Barton, D. H. R.; Gastinger, M. J.; Motherwell, W. B. J. Chem.
Soc., Chem. Commun. 1983, 731. (b) Ito, T.; Lunsford, J. H. Nature 1985,
314, 721.
15
Fe-O distance of 1.86 ( 0.02 Å, which is similar to that of FePO4
and FeIII containing ZSM-516 wherein the FeIII is present in tetrahedral
coordination.
(2) MacFaul, P. A.; Wayner, D. D. M.; Ingold, K. U. Acc. Chem. Res.
1998, 31, 159.
(3) Hill, C. L., Ed. ActiVation and Functionalization of Alkanes; Wiley-
Chichester: Chichester, 1989; Chapters 6-8.
(4) Thomas, J. M. Nature 1985, 314, 669.
(5) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.;
Wiley-Interscience: New York, 1992.
(6) Raja, R.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1998, 1841.
(7) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G. Nature 1999, 398,
227.
(8) Sankar, G.; Raja, R.; Thomas, J. M. Catal. Lett. 1998, 55, 15.
(9) Raja, R.; Sankar, G.; Thomas, J. M. J. Chem. Soc., Chem. Commun.
1999, 829.
(10) Raja, R.; Thomas, J. M.; Sankar, G. J. Chem. Soc., Chem. Commun.
1999, 525.
(11) (a) Thomas, J. M.; Greaves, G. N. Science 1994, 265, 1675. (b) Barrett,
P. A.; Sankar, G.; Catlow, C. R. A.; Thomas, J. M. J. Phys. Chem. 1996,
100, 8977.
(12) We observed cyclohexyl hydroperoxide (cHHP) in the reaction mixture
when CoAlPO-5 was used as the catalyst, and in the early stages of the reaction
(3 h) with MnAlPO-5. FeAlPO-5 was more reactive than CoAlPO-5 and
MnAlPO-5, and hence cHHP was not detected in the reaction mixture.
(13) Sheldon, R. A.; Wallau, M.; Arends, I. C. W. E.; Schushardt, U. Acc.
Chem. Res. 1998, 31, 485 and references therein.
(14) To illustrate this point we took an equimolar mixture of n-hexane and
cyclohexane and subjected it to a catalytic test over MnAlPO-18 (or CoAlPO-
18). There was, unsurprisingly, absolutely no conversion of the cyclohexane
molecule, as it is too large to access the active, framework tetrahedral sites in
the AlPO-18 structure. The conversion of n-hexane, however, was as expected
quite substantial, there being good selectivity (66%) for terminally function-
alized7 products. However, when the same reactant mixture was dissolved in
acetic acid, the MnAlPO-18 catalyst now gives rise to considerable oxidation
of both cyclohexane (conversion ) 8.7%) and n-hexane (conversion ) 5.2%),
C2 and C3 alcohols and ketones being main products (selectivity ) 91%).
Clearly acetic acid as a solvent results in homogeneous as well as heteroge-
neous catalysis.
Figure 2. Comparison of the fraction of M(III) ions present (estimated
from EXAFS) in the calcined FeAlPO-5, MnAlPO-5, and CoAlPO-5 and
their catalytic activity (TON) for the oxidation of cyclohexane after 24
h at 403 K. Individual product distributions are also shown.
in O2 or air at ca. 550 °C. However, the pore dimension of the
MAlPO-18 is too small to permit ingress of cyclohexane to the
10.1021/ja9935052 CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/04/1999
Raja, Robert
