A novel, zeolite-encapsulated m3-oxo Co/Mn cluster catalyst for
oxidation of para-xylene to terephthalic acid
Suhas A. Chavan, D. Srinivas and Paul Ratnasamy*
National Chemical Laboratory, Pune 411 008, India. E-mail: prs@ems.ncl.nes.in
Received (in Cambridge, UK) 2nd March 2001, Accepted 10th May 2001
First published as an Advance Article on the web 31st May 2001
Trinuclear, m3-oxo mixed metal acetato complexes,
[CoMn2(m3-O)(MeCO2)6(py)3] (py = pyridine) encapsulated
in zeolite HY, exhibit high catalytic efficiency in the selective
aerial oxidation of para-xylene to terephthalic acid; inter-
estingly, the formation of 4-carboxybenzaldehyde, the worri-
some impurity in the conventional process, is suppressed
significantly over these solid catalysts.
2924, 1624, 1458, 1340, 1221, 680 and 623 cm21. The FT-IR
bands due to pyridine were observed at around 1545, 1489 and
790 cm21. A shift in the position of the bands due to
encapsulation was observed. Representative FT-IR spectra of
encapsulated clusters, Mn3(O)-Y and CoMn2(O)-Y, are shown
in Fig. 1. The ‘neat’ cluster complexes in MeCO2H–H2O–NaBr
medium showed a characteristic band of ligand origin and a
charge transfer band (O ? Mn/Co) in the UV–VIS spectra, the
positions of which were sensitive to the metal ions [Co3(O):
250, 355 nm; Mn3(O): 254, 320 nm and CoMn2(O): 254, 345
nm). The encapsulated clusters in zeolite-Y exhibited a marked
shift in the band positions (Fig. 2). The diffuse reflectance UV-
VIS spectra (Fig. 2) are dominated by the band of ligand origin;
the weak charge transfer band could be seen only in Co3(O)-
Y.
EPR spectra provided evidence for the formation and stability
of cluster complexes in zeolite-Y. Mn3(O) and Co3(O) com-
plexes showed a broad EPR signal at g = 2.008 (peak-to peak
line width = 500 G) and 2.259 (peak-to-peak linewidth of 1050
G), respectively. The intensity of these signals decreased with
temperature to 77 K corresponding to an antiferromagnetic
behaviour of the complexes. For Co3(O), the signal arises from
the low lying excited states while Co2Mn(O) was EPR silent.
Mn3(O) in frozen solutions of 38 ml HOAc–5.6 ml H2O–86.5
mg NaBr [Fig. 3(a)] at 82 K, showed an EPR signal with
partially resolved Mn hyperfine features. These hyperfine
features could not be seen in the solid complexes due to
intermolecular interactions. Encapsulated Mn3(O) clusters
showed EPR signals (g = 2.012) similar to that of frozen
solutions [see Fig. 3(a) and (b)] and indicate the formation and
isolation of cluster molecules in zeolite-Y. CoMn2(O)-Y
exhibited signals at g = 2.026. Co3(O)-Y showed a broad signal
Terephthalic acid, one of the largest volume commodity
chemicals, is commercially manufactured by dioxygen oxida-
tion of para-xylene using cobalt and manganese salts, at
473–500 K, in acetic acid (MeCO2H) solvent and bromide ion
as promoter.1,2 High yields and selectivity for terephthalic acid
are obtained. Replacement of the homogeneous catalyst with a
solid catalyst is a desirable alternative which will eliminate
toxic metal ions from the waste effluents in the process. One
such method of preparing solid, heterogeneous catalysts is
encapsulation of active metal complexes inside the pores of
zeolites or zeolitic materials.3–5 We had reported earlier the
presence of reactive m3-oxo Co/Mn mixed cluster complexes in
homogeneous reaction medium.6 In this communication we
report, for the first time, the encapsulation of these complexes,
[CoMn2(m3-O)(MeCO2)6(py)3]n (n = +1 or 0) inside the cages
of zeolite HY [hereafter referred to as CoMn2(O)-Y]. The
hydrothermal stability and catalytic activity of these solid
cluster catalysts in the heterogeneous dioxygen oxidation of
para-xylene are also reported. It is found that these heteroge-
neous catalysts were highly active and selective for the
oxidation of para-xylene to terephthalic acid. A comparative
study indicates that the zeolite-Y-encapsulated heteronuclear,
cluster complex, CoMn2(O)-Y is more efficient than the
corresponding homonuclear cobalt and manganese cluster
complexes, [Co3(m3-O)(MeCO2)6(py)3] and [Mn3(m3-O)(Me-
CO2)6(py)3], respectively, in HY [hereafter referred to as
Co3(O)-Y and Mn3(O)-Y, respectively].
Co(II), Mn(II) and mixed Co(II)/Mn(II) exchanged HY
zeolites were prepared by the ion exchange method, wherein
zeolite HY was contacted with aqueous solutions of Co(Me-
CO2)2·4H2O and Mn(MeCO2)2·4H2O in requisite proportions at
338 K with stirring for 4 h. The ion-exchanged zeolites (Co-Y,
Mn-Y, Co,Mn-Y) were washed with distilled water several
times and dried at 373 K. In a typical preparation of the
encapsulated metal cluster complex, the corresponding ion
exchanged zeolite Y sample (1.5 g) was suspended in 15 ml
glacial MeCO2H. To this slurry was added pyridine (3 ml),
NaBr (0.5 g), aq. H2O2 (50%, 10 ml) and distilled water (5 ml).
The mixture was stirred, while bubbling air through the
solution, for 2 h at 298 K. The solid product [Co3(O)-Y, pink;
Mn3(O)-Y, pale brown; CoMn2(O)-Y, purple] was filtered off,
washed with glacial MeCO2H and dried under vacuum. ‘Neat’
cluster complexes of the composition [Co3(m3-O)(MeCO2)6-
(py)3]ClO4 [referred to as Co3(O)], [Mn3(m3-O)(MeCO2)6-
(py)3]ClO4 [referred to as Mn3(O)] and [CoMn2(m3-O)-
(MeCO2)6(py)3] [referred to as CoMn2(O)] were prepared, for
comparative studies, by known procedures.7–9 The formation
and purity of the complexes was confirmed by elemental
analysis, FT-IR, UV–VIS and EPR spectroscopies.
The FT-IR spectra of the encapsulated complexes showed
characteristic bands corresponding to acetate groups at ca.
Fig. 1 FT-IR spectra (Nujol mull) of cluster complexes Mn3(O) (A) and
CoMn2(O) (B) encapsulated in zeolite HY.
1124
Chem. Commun., 2001, 1124–1125
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102016c