Selective oxidation of cycloalkanes over iron-substituted hexagonal mesoporous aluminophosphate molecular sieves

Article abstract of DOI:10.1246/cl.2004.198

Iron-substituted hexagonal mesoposours aluminiphosphate (FeHMA) molecular sieves oxidize cycloalkanes into cycloalkanols and cycloalkanones with excellent yield in the presence of molecular oxygen or air under mild reaction conditions. The catalyst showed

Full text of DOI:10.1246/cl.2004.198

198
Chemistry Letters Vol.33, No.2 (2004)
Selective Oxidation of Cycloalkanes over Iron-substituted
Hexagonal Mesoporous Aluminophosphate Molecular Sieves
Susanta K. Mohapatra and Parasuraman SelvamÃ
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400076, India
(Received November 22, 2003; CL-031136)
Iron-substituted hexagonal mesoposours aluminiphosphate
were carried out under similar reaction conditions of cyclohex-
(FeHMA) molecular sieves oxidize cycloalkanes into cycloalka-
nols and cycloalkanones with excellent yield in the presence of
molecular oxygen or air under mild reaction conditions. The cat-
alyst showed higher activity as compared to mesoporous
FeMCM-41 as well as microporous FeS-1 and FeAPO-5 molec-
ular sieves, and that FeHMA can be recycled without much loss
in activity.
ane except that mixed solvents (5 mL acetic acid + 5 mL di-
chloromethane) were ued to make a homogeneous reaction mix-
ture. After the reaction, the catalyst was separated and the
products were extracted with ether and analyzed by means of
gas chromatography (GC; Nucon) with Carbowax column. Fur-
ther, the oxidation products were confirmed by GC–MS (Hewlett
Packard G 1800A) equipped with HP-5 capillary column. While
the separated catalyst was washed with distilled water for three-
times, dried in an air-oven and then activated at 773 K for 6 h.
The regenerated catalyst was then used for the subsequent recy-
cling studies.
Table 1 summarizes the results of cycloalkanes oxidation
over FeHMA. It can be seen from this table that the mesoporous
catalyst successfully transforms the substrate molecules into de-
sired products with good conversion under mild reaction condi-
tions. Further, the reactions predominantly give cycloalkanols
and cycloalkanones as the major products along with a small
amount of secondary reaction product, viz., cycloalkyl acetates.
It can also be seen from this table that the catalyst exhibits high
TON, which clearly reflects the efficiency of FeHMA. At this
juncture, it is noteworthy that in the presence of iron-free
HMA catalyst as well as without the use of catalyst (blank reac-
tion), the reaction showed meager activity (<5% conversion).
Thus, the high activity of FeHMA for the chosen reactions could
be accounted primarily to the isolated trivalent iron present in
the framework structure of HMA.
The development of environmentally benign catalytic oxi-
dation process with eco-friendly oxidants, e.g., hydrogen perox-
ide (H2O2), molecular oxygen (O2) and air, under mild reaction
conditions is of immense industrial importance.1 In recent years,
several groups have put forward a number of promising hetero-
geneous catalysts for the oxidation of cyclohexane.2 However,
till date, no such advancement has been made for the oxidation
of bulky cycloalkanes such as cyclooctane and cyclododecane.
On the other hand, a number of homogeneous catalysts have
been reported for the oxidation of these bulky cycloalkanes.
However, low yields, longer reaction periods, deactivation of
the catalysts by oxidative degradation of ligands, separation dif-
ficulties, production of hazardous by-products, etc. have made
these routes extremely unattractive.3 In order to overcome many
of these problems, heterogeneous catalysts as well as few other
attractive processes have been proposed but the obtained yield is
significantly low (ꢀ40%).4,5 Hence, in this investigation, we em-
ployed trivalent iron-substituted hexagonal mesoposours alumi-
niphosphate (FeHMA) molecular sieves for the selective oxida-
tion of cycloalkanes in the presence of O2 and air under mild
reaction conditions (Scheme 1).
The synthesis and characterization of the FeHMA catalyst
having 2.3 wt % iron was carried out as per the procedure report-
ed elsewhere.6 For a comparison, FeMCM-41 (2.10 wt % Fe),
FeAPO-5 (1.95 wt % Fe) and FeS-1 (1.90 wt % Fe) were also
prepared according to the literature procedures.7 The oxidation
of cyclohexane (18 mmol) was carried out using 10 mL of sol-
vent (acetic acid), 5 mmol of initiator (methyl ethyl ketone;
MEK), 50 mg of the catalyst with O2 or air (40 mL/min flow)
as oxidant at 373 K for 12 h under atmospheric pressure. The ox-
idation of cyclooctane (18 mmol) and cyclododecane (12 mmol)
It is also important to mention here that the use of air as ox-
idant shows lower TON as compared to molecular O2. The reac-
tion may proceed through radical path, as evidenced for the ox-
idation of cyclohexane over FeAPO-5,2a and that direct attack of
O2 on cycloalkanes is endothermic process, and hence the use of
carbonyls as reaction initiator is inevitable. Further, the oxida-
tion of cyclooctane showed high TON as compared to the inert
cyclohexane and the highly steric cyclododecane. On the other
hand, the conversion of cyclooctane and cyclododecane can be
Table 1. Oxidation of cycloalkanes over FeHMA
Selectivity/wt %
Oxi- Conv.
Substrate
dant
TONa -ol -one Othersb
/wt %
62.7
46.0
74.6
52.5
82.5
55.8
Cyclohexane
O2
Air
O2
564 62.2 34.0
414 67.8 31.2
671 52.5 46.0
472 55.0 43.2
492 50.0 47.3
335 55.6 43.0
3.8
1.0
3.5
1.8
2.7
1.4
OH
O
Cyclooctane
[O]
Air
Cyclododecane O2
Air
+
n
FeHMA
n
n
n = 1, 3, 7
aTurn over number = number of moles of cycloalkanes convert-
ed per mole of iron. Mainly cycloalkyl acetate is obtained as
chain-terminating product.
Yield = 60−80%
b
Scheme 1. Oxidation of cycloalkanes (C6,C8, and C12).
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.2 (2004)
199
enhanced to 92 and 95%, respectively using H2O2 (30%) as ox-
idant, however, with the formation of excess quantities
(ꢀ8{10%) of secondary chain-terminating products such as cy-
cloalkyl acetates. While the use of TBHP (70%) gave 72 and
68% conversion of cyclooctane and cyclododecane, respective-
ly. The lower activity could be attributed to the deactivation of
the catalyst as a result of the formation of t-butanol as a conse-
quence of TBHP decomposition under the reaction conditions.
For a comparison, the cyclododecane reaction was carried
out over mesoporous FeMCM-41 as well as microporous Fe-
APO-5 and FeS-1 catalysts, and the results are presented along
with FeHMA in Figure 1. It can be seen from this figure that both
the microporous catalysts show much lower activity as com-
pared to their mesoporous analogues, viz., FeHMA and
FeMCM-41. The lower activity of both FeAPO-5 and FeS-1
could largely be attributed to the small pore openings and there-
by hinder the diffusion of bulky molecules, viz., cyclododecane,
inside the pore aperture. On the other hand, the lower activity of
FeMCM-41 as compared to FeHMA could be explained on the
basis of higher pore wall thickness of the former than the latter
(see Table 2), which suggests that low amount of active species
exposed for the substrate molecules.8
ported by the ICP–AES analysis where no change in iron content
was noticed for the reused catalysts. In addition, it may also be
noted here that the mesoporous catalysts keep their structure in-
tact and porosity intact even after three cycling procedures. It is,
however, important to note that, unlike FeHMA, the activity of
FeMCM-41 decreases upon recycling owing to leaching of ac-
tive iron (12%) under reactions conditions. This could be attrib-
uted to the lower stability of trivalent iron in the silicate ma-
trix.6,7a
In conclusion, in the present investigation, it was demon-
strated that FeHMA is an efficient and novel heterogeneous cat-
alyst for the selective oxidation of cycloalkanes with molecular
oxygen or air as oxidant. The catalyst showed better activity than
many other catalyst systems reported so far, and thus opens up a
new possibility as a potential catalyst for the synthesis of fine
chemicals.
The authors thank Sophisticated Analytical Instrumentation
Facility, IIT-Bombay for ICP–AES and GC–MS analyses.
References
1
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In order to check the reusability of the various catalysts, re-
cycling experiments were executed for the oxidation of cyclodo-
decane with O2, and the results are illustrated in Figure 1. It is
interesting to note from this figure that the activity remains near-
ly intact even after three recyclings (4th run). This is well sup-
2
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Table 2. Structural data of calcined FeHMA and FeMCM-41
SBET
/m2gÀ1
Pore volume
/ml gÀ1
H-K pore FWT
ꢀ
ꢀ a
diameter/A /A
Catalyst
3
4
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FeHMA
FeHMAb
FeMCM-41
923
890
637
0.48
0.46
0.50
28
28
33
8.8
9.2
18.2
a Framework wall thickness (FWT) = ao – H-K pore diameter
b
Used catalyst (cyclododecane oxidation) – after 3rd recy-
cling (or 4th run).
100
5
6
1st run 4th run
75
50
25
7
0
FeHMA FeMCM-41 FeAPO-5 FeS-1
8
a) W. Zhang, J. Wang, P. T. Tanev, and T. J. Pinnavaia,
Chem. Commun., 1996, 979. b) S. K. Mohapatra, F. Hussain,
and P. Selvam, Catal. Commun., 4, 57 (2003).
Figure 1. Recycling studies of cyclododecane oxidation over
various iron-containing molecular sieves.
Published on the web (Advance View) January 25, 2004; DOI 10.1246/cl.2004.198