Mesoporous (Cr)MCM-41: A mild and efficient heterogeneous catalyst for selective oxidation of cyclohexane

Article abstract of DOI:10.1016/S0021-9517(02)93711-5

The oxidation products of cyclohexane (i.e., cyclohexanol and cyclohexanone) are important intermediates for producing adipic acid and caprolactam, which are used in the manufacture of nylon-6 and nylon-66 polymers. The liquid-phase oxidation of cyclohexane under moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieve was investigated. The catalysts showed high substrate conversion and excellent product selectivity. Cyclohexanol was the major product, with only small amounts of other products (i.e., cyclohexanone and cyclohexyl acetate). The catalytic activity decreased after the first recycle due to the leaching of nonframework Cr ions, but remained nearly the same in subsequent cycles. This observation was confirmed by washing experiments where the nonframework Cr ions were removed upon ammonium acetate treatment or under reaction conditions. The washed catalyst showed activity similar to that of the recycled catalyst. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide led to the formation of cyclohexane as the major product. The use of solvents such as methanol, dioxan, and acetone showed lower conversion. The use of initiators such as acetone, cyclohexanone, and acetaldehyde resulted in lower activity.

Full text of DOI:10.1016/S0021-9517(02)93711-5

Journal of Catalysis 211, 134–143 (2002)
doi:10.1006/jcat.2002.3711
Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst
for Selective Oxidation of Cyclohexane
A. Sakthivel and P. Selvam1
Department of Chemistry, Indian Institute of Technology—Bombay, Powai, Mumbai 400 076, India
Received February 25, 2002; revised May 27, 2002; accepted June 5, 2002
are produced on an industrial scale by the oxidation of cy-
clohexane or hydrogenation of phenol (1–5). In the early
1940s, Du Pont developed a process in which cyclohexane
was oxidized in the presence of air to cyclohexanol and
cyclohexanone using cobalt napthenate or cobalt acetate
as catalyst (1, 4). In this process, several by-products, viz.,
mono- and dicarboxylic acids, esters, aldehydes, ketones,
and other oxygenated materials, were generated. Later, in
the 1950s, Scientific Design (now Halcon International) de-
veloped a new process where anhydrous meta-boric acid
was added as a slurry in the oxidation vessel. This led to
the formation of borate ester of cyclohexanol (3, 5), which
prevents over oxidation (ring-opening products). The re-
sulting cyclohexyl ester is subsequently hydrolyzed to cy-
clohexanol. In comparison to the former, the latter process
showed good yield of cyclohexanol, which, however, re-
quires high investment and a high operating cost to recover
and recycle the boric acid.
On the other hand, for the laboratory-scale reaction ex-
tensive literature is available on the selective oxidation of
cyclohexane using a variety of transition metal compounds
in stoichiometric amounts or as homogeneous catalysts
(6–15). In the cases of the latter, the use of initiators, e.g., cy-
clohexanone (3), cyclohexylperoxide (3), methyl ethyl ke-
tone (10), and acetaldehyde (16), shortens the induction
period as well as enhances the catalytic activity. However,
owing to the limitations of these soluble (homogeneous)
catalysts, viz., catalyst separation from the product and the
disposal of solid/liquid wastes, which pose serious problems
to the environment, in recent years attention has been fo-
cused on the development of transition metal-based hetero-
geneous catalysts with oxygen or peroxides as nonpolluting
oxidants (6, 7, 17–25). However, in most cases, extreme re-
action conditions such as high pressure (2 MPa) and high
temperature (450 K) in conjunction with low activity make
the process less attractive. In addition, leaching of active
metal ions has often been observed under the reaction con-
ditions (20, 25, 26). Hence, the oxidation of cyclohexane
over heterogeneous catalysts under mild/moderate condi-
tions is a topic of great interest. In this regard, chromium-
containing mesoporous MCM-41 molecular sieves show
promise for certain oxidation reactions (21, 27–30). In
Liquid-phase oxidation of cyclohexane was carried out at mod-
erate reaction conditions over mesoporous (Cr)MCM-41 molecu-
lar sieves using acetic acid as solvent, hydrogen peroxide as oxi-
dant, and methyl ethyl ketone as initiator. Under these conditions,
the catalysts showed high substrate conversion and excellent prod-
uct selectivity. Although the activity of the catalyst decreased af-
ter the first recycle, owing to leaching of nonframework chromium
ions, it, however, remained nearly the same thereafter. This obser-
vation was further confirmed by washing experiments where the
nonframework chromium ions were removed upon ammonium ac-
etate treatment or under reaction conditions. Further, the washed
catalyst also showed a activity similar to that of the recycled cata-
lyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly
as a heterogeneous catalyst, and thus it forms the first example of
a nonleaching chromium-based catalyst reported so far. The use of
a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, re-
sulted in the formation of cyclohexanone as the major product. On
the other hand, the use of solvents like methanol, dioxan, and ace-
tone showed lower conversion. Likewise, the use of initiators such
as acetone, cyclohexanone, and acetaldehyde also resulted in lower
c
activity. ꢀ 2002 Elsevier Science (USA)
Key Words: cyclohexane; cyclohexanol; cyclohexanone;
(Cr)MCM-41; oxidation; mesoporous molecular sieves.
INTRODUCTION
Selective oxidation reactions using heterogeneous cata-
lysts are of growing importance for the modern chemical
industry. The oxidation products of cyclohexane, viz., cyclo-
hexanol and cyclohexanone, are important intermediates
in the production of adipic acid and caprolactam, which
are used in the manufacture of nylon-6 and nylon-66 poly-
mers (1–4). In addition, they are also used as solvents for
lacquers, shellacs, and varnishes as well as stabilizers and
homogenizers for soaps and synthetic detergent emulsions.
Furthermore, cyclohexanol ester, viz., cyclohexyl phthalate,
iswidelyusedasaplasticizeraswellasinthesurface-coating
industry. Other uses of cyclohexanone are as starting mate-
rial in the synthesis of insecticides, herbicides, and pharma-
ceuticals. In general, both cyclohexanol and cyclohexanone
1 To whom correspondence should be addressed. Fax: +91-22-576-7152/
572-3480. E-mail: selvam@iitb.ac.in.
134
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

MESOPOROUS (Cr)MCM-41 IN SELECTIVE OXIDATION OF CYCLOHEXANE
135
particular, unlike many other systems based on micro- tematically by powder X-ray diffraction (XRD; Miniflex–
porous molecular sieves (26, 31, 32) and pillared clays Rigaku), Brunauer, Emmett, and Teller (BET) surface
(33, 34), the mesoporous molecular sieves stabilize chro- area (Smartsorp 90), simultaneous thermogravimetry–
mium ions in the matrix, designated (Cr)MCM-41 (29, 30); differential thermal analysis (TG–DTA; Shimadzu DT-30),
thus they can act as true heterogeneous catalysts. There- electron paramagnetic resonance (EPR; Varian E-112), and
fore, in this investigation, an attempt was made to explore diffuse reflectance ultraviolet–visible (DRUV–vis; UV-260
the catalytic activity of mesoporous (Cr)MCM-41 molecu- Shimadzu) spectroscopic techniques. The chromium con-
lar sieves for the titled reaction.
tent in the samples was determined by inductively cou-
pled plasma–atomic emission spectroscopy (ICP–AES) on
Labtam Plasma Lab 8440 equipment after the sample was
dissolved in HF/HNO3.
EXPERIMENTAL
Starting Materials
Reaction Procedure
The following chemicals were employed for the prepa-
ration of (Cr)MCM-41: tetraethyl orthosilicate (TEOS;
Aldrich, 98%), chromium nitrate nonahydrate (Cr(NO3)3) ·
9H2O; Alfa, 98.5%), tetradecyltrimethylammonium bro-
mide (TDTMAB; SISCO, 99%), and sodium hydroxide
(NaOH; Loba, 98%) as sources for silicon, chromium,
surfactant, and alkali, respectively. Cyclohexane (Merck,
99.5%), hydrogen peroxide (H2O2; Qualigens, 30%), ter-
tiary butylhydroperoxide (TBHP; Lancaster, 70%), acetic
acid (Fischer, 99.5%), methyl ethyl ketone (MEK; SD
Fine Chemicals, 99%), cyclohexanone (Merck, 99%), ace-
tone (SD Fine Chemicals, 99.5%), and acetaldehyde (SD
Fine Chemicals, 30%) were used for (liquid phase) cyclo-
hexane oxidation reactions. Authentic samples of cyclohex-
anol(SDFineChemicals, 98%)andcyclohexanone(Merck,
99%) were used for comparative analysis of the reaction
products.
The oxidation of cyclohexane (18 mmol) was carried out
in the presence of an initiator (5 mmol) under atmospheric
pressure at 373 K for 12 h using 50 mg of calcined or washed
(Cr)MCM-41 catalyst with 18 mmol of oxidant and 10 ml of
solvent. After the reaction, the catalyst was separated and
the products were extracted with ether and analyzed by gas
chromatography (Nucon 5700) with carbowax column. The
filtered catalysts were reactivated in oxygen atmosphere at
773 K for 6 h for recycling experiments. The reaction was
also carried out using solvents such as methanol, 1,4-dioxan,
and acetone under the same reaction conditions. Further-
more, the influence of different initiators, e.g., cyclohex-
anone, acetaldehyde, and acetone, as well as the effect of
various oxidants, viz., air, oxygen, H2O2, and TBHP, on the
reaction was also investigated.
RESULTS AND DISCUSSION
Synthesis
The (Cr)MCM-41 molecular sieves were hydrothermally
synthesized in a Teflon-lined stainless-steel autoclave at
373 K for 24 h with a typical molar gel composition of
1 SiO2 : 0.25 (TDTMA)2O : 0.25 Na2O : 45 H2O : xCr2O3
(where x = 0.0025–0.02 with a Si/Cr ratio of 25–200) ac-
cording to a procedure outlined previously (30). The as-
synthesized (Cr)MCM-41 was calcined at 823 K and 6 h
in oxygen atmosphere, referred to as calcined (Cr)MCM-
41. Since the calcined samples contain a certain amount of
nonframework chromium ions (29, 30), they were further
treated with ammonium acetate solution (1 M). The re-
sulting sample was designated washed (Cr)MCM-41. For a
comparison chromium-grafted MCM-41 (Si/Cr = 50), des-
ignated (Cr)MCM-41(grafted), and (siliceous) MCM-41 were
also synthesized and calcined as per the procedure de-
scribedearlier(35, 36). Further, themicroporousanalogues,
suchas(Cr)silicalite-1, (Cr)APO-5, and(Cr)zeolite-β, were
also prepared for comparison as per the literature proce-
dure (26, 32).
The XRD patterns of as-synthesized, calcined, washed,
and recycled (Cr)MCM-41 (Fig. 1) showed the reflections,
viz., 100, 110, 200, and 200, which are typical characteris-
tics of mesoporous (hexagonal) MCM-41 structure (37, 38).
Table 1 summarizes the average unit cell parameter, a0, of
(Cr)MCM-41 with different Si/Cr ratios. In general, the in-
˚
corporation of a larger cation, such as Cr(III) (0.76 A),
TABLE 1
XRD and ICP–AES Data of (Cr)MCM-41 with Different
Chromium Content
ICP–AESc
XRDb a0
(A)
Si/Cr
(molar ratio)
Chromium content
(wt%)
Samplea
˚
(Cr)MCM-41(200)
(Cr)MCM-41(100)
(Cr)MCM-41(50)
(Cr)MCM-41(25)
41.93
43.59
44.42
45.51
392
194
112
46
0.27
0.57
1.23
1.98
Characterization
a Numbers in parentheses indicate the nominal Si/Cr ratios.
b As-synthesized samples.
The as-synthesized, calcined, and washed (Cr)MCM-41
as well as the MCM-41 samples were characterized sys-
c Calcined samples.

136
SAKTHIVEL AND SELVAM
TABLE 2
XRD and ICP–AES Data of Various (Cr)MCM-41
ICP–AES
XRD a0
(A)
Si/Cr
(molar ratio)
Chromium content
(wt%)
˚
Sample
Calcined (Cr)MCM-41
Recycled (Cr)MCM-41
Washed (Cr)MCM-41
37.22
36.70
36.04
112
161
157
1.23
0.76
0.77
a consequence of the removal of nonframework chromium
ions from the mesopores (39, 40). However, the diffrac-
tion patterns (Fig. 1) remain nearly the same even after the
recycling experiment or washing treatment, indicating the
intactness of the structure.
All the as-synthesized (Cr)MCM-41 samples were green
in color and changed to yellow upon calcination. The for-
mer is due to the presence of trivalent chromium ions in
octahedral geometry and the latter is due to higher va-
lent chromium ions, viz., chromate and/or polychromate
ions, in the tetrahedral environment. The BET surface area
(1066 m2 g−1) of the calcined sample and TG (45 wt%
loss) of the as-synthesized sample further support the meso-
porous nature of (Cr)MCM-41. TG of calcined (Cr)MCM-
41 (41) showed a relatively small weight loss (10 wt%)
compared to siliceous MCM-41 (18 wt%), which suggests
that part of the silanol groups in the siliceous matrix
may possibly be consumed for stabilization of chromium
ions (Scheme 1) in a manner similar to that reported for
chromium oxide supported on a silica matrix (42). Further
29Si MAS–NMR studies showed distinct signals at −111
and −100 ppm (43), which are assigned to Q4 and Q3 (sili-
FIG. 1. XRD patterns of (Cr)MCM-41: (a) as-synthesized, (b) cal-
cined, (c) washed, and (d) recycled.
≡
con) sites, respectively (44). However, the Q3 site ( Si–
˚
OH) is considerably reduced for (Cr)MCM-41 compared
to siliceous MCM-41, which clearly supports the stabiliza-
tion of chromium ions via silanol groups (defect sites).
The estimated (ICP–AES) chromium content in various
(Cr)MCM-41 samples is shown in Table 1. It is clear
from this table that the samples show a considerable de-
crease in chromium content compared to that of the initial
gel composition. The loss in chromium could however be
in tetrahedral geometry for Si(IV) (0.40 A) is expected to
increase to a significant extent the lattice parameter. How-
ever, as can be seen from Table 1, only a small increase in
the lattice constant is noticed. This indicates, that there is
no appreciable incorporation of chromium into the meso-
porous matrix. However, the observed slight expansion in
a0 values and shifting of the reflections to the lower 2θ val-
ues (higher d values) can be explained by filling the pores
with chromium species, which slightly deform in the ma-
trix. This effect has also been observed for a mesoporous
matrix filled with InP (39) and CdS (40) particles. Unless
otherwise stated, the catalyst used in the present study was
(Cr)MCM-41 with Si/Cr = 50. Table 2 lists a0 values of var-
ious forms of a (Cr)MCM-41 sample synthesized with a
Si/Cr (molar) ratio of 50. The decrease in a0 values for the
calcined samples compared to the as-synthesized samples
could be attributed to the shrinkage of the lattice as a con-
sequence of the removal of template molecules. However, a
slight decrease of the a0 values for the washed and recycled
samples could be due to reversal of the deformed matrix as
SCHEME 1

MESOPOROUS (Cr)MCM-41 IN SELECTIVE OXIDATION OF CYCLOHEXANE
137
accounted for by the presence of soluble chromium hydrox-
ide in the mother liquor of the as-synthesized samples. On
the other hand, the washed sample (see Table 2) shows a
furtherlossofchromiumowingtotheremovalofnonframe-
work chromium ions from the matrix. Interestingly, unlike
many other chromium-based microporous molecular sieves
(26, 31, 32), the chromium content in the (Cr)MCM-41 re-
mains nearly the same even after recycling experiments (cf.,
Table 2), which suggests that there is no further leaching of
chromium under the reaction conditions.
The DRUV–vis spectra of as-synthesized (Cr)MCM-
41 showed typical absorption bands (29, 41) at ∼642,
4
4
445, and 290 nm, corresponding to A2g(F) → T2g(F),
4A2g(F) → T1g(F), and 4A2g(F) → T1g(P) transitions, re-
spectively, typical of trivalent chromium in octahedral coor-
dination (45, 46). On the other hand, the calcined, washed,
and recycled samples (Fig. 2) show strong charge-transfer
(oxygen to chromium) bands at 375 nm (1t1 → 2e sym-
metry allowed) along with a weak shoulder at 445 nm
(1t1 → 2e symmetry forbidden) and a very weak shoulder at
322 nm (1t1 → 2e symmetry forbidden) (see Fig. 2 inset) as-
signed to chromate, polychromate, and dichromate species,
respectively (45, 46). The EPR spectra of as-synthesized
(Cr)MCM-41 showed a broad spectrum with geff ∼ 1.98
(41), indicating the presence of trivalent chromium in octa-
hedral coordination, while for the calcined samples a sharp
signal at geff = 1.97 was observed (Fig. 3), which is charac-
teristic of pentavalent chromium in tetrahedral coordina-
tion (see also Scheme 1). The results are in good agreement
with literature, where chromium is present as both Cr(V)
and Cr(VI) species (45, 46). On the other hand, the EPR
signal for washed and recycled catalysts samples is about
two to two and a half times higher than the calcined sam-
ples owing to change in coordination of chromium species
upon various treatments (47).
4
4
FIG. 3. EPR spectra of (Cr)MCM-41: (a) calcined, (b) first recycled,
and (c) washed.
Figure 4 depicts the effect of reaction time over calcined
(Cr)MCM-41. It can be seen from the figure that the (cyclo-
hexane) conversion increases with time, while the (cyclo-
hexanol) selectivity decreases. At the initial stages, a small
amount of cyclohexyl acetate was observed along with cy-
clohexanol. However, as the time proceeds a slight decrease
in cyclohexanol selectivity is noticed, with a correspond-
ing increase in both cyclohexyl acetate and cyclohexanone
selectivity. The formation of the latter, however, could be
attributed to secondary oxidation of cyclohexanol. The for-
mation of cyclohexyl acetate at a higher reaction time could
be due to a possible termination reaction between unre-
acted cyclohexyl and acetoxy radicals. Figure 5 shows the
influence of reaction temperature, and it is clear from the
figure that both the substrate conversion and the product
selectivity increase with reaction temperature. At higher
reaction temperature (383 K), the activity decreases, owing
to a possible decomposition of H2O2 (6) as well as to the
formation of cyclohexyl acetate by interaction of a cyclo-
hexyl radical and a acetoxy radical. At lower temperature,
the cyclohexanol selectivity decreases due to incomplete
hydrolysis of cyclohexyl acetate. Figure 6 shows the effect
of chromium content on the reaction. It can be seen from
FIG. 2. DRUV–vis spectra of (Cr)MCM-41: (a) calcined, (b) recycled,
(c) washed, and (d) washed and recycled.

138
SAKTHIVEL AND SELVAM
FIG. 6. Effect of Si/Cr ratio on the conversion and selectivity over
(Cr)MCM-41.
FIG. 4. Effect of reaction time on the conversion and selectivity over
(Cr)MCM-41.
Table 3 presents the result of oxidation of cyclohexane
over various (Cr)MCM-41 catalysts under optimized ex-
perimental conditions. In all cases, cyclohexanol was ob-
tained as the major product, and only a small amount of
other products, viz., cyclohexanone and cyclohexyl acetate,
were identified. It can also be seen from this table that
the calcined catalyst showed high activity while a consider-
able decrease in activity was noticed, the first time, for the
recycled samples, owing to leaching of active (nonframe-
work) chromium ions under the reaction conditions. On
the other hand, the activity remained nearly the same in
the subsequent cycles. The lower conversion during recy-
cling experiments could, however, be attributed to lower
chromium content (0.76 wt%) as well as to the presence of
more pentavalent chromium ions in the matrix (see Fig. 3).
It is interesting that the washed catalyst (0.77 wt%) also
showed a result similar to that of the recycled (Cr)MCM-41.
Further, recycling of both cycled and washed catalysts does
not affect the reaction any further; thus the catalyst be-
haves truly as heterogeneous. This observation of the loss
of chromium content upon recycling/washing treatments
is well supported by DRUV–vis studies, where the initial
decrease in the absorption band intensity was noted for
the calcined (Cr)MCM-41 (Fig. 2a). However, spectra re-
main nearly the same for the washed and recycled samples
(Figs. 2b–2d). Further, the results are corroborated by the
filtrate as well as by the quenching studies carried out on
washed catalysts (see Table 3). In addition, the ICP–AES
data (see Table 2) are in line with the above findings. The cy-
clohexane reaction was also carried out on catalyst contain-
ing Si/Cr = 25 (1.98 wt% chromium), which showed very
high conversion (Table 3). The activity remained high even
after the cycling and/or washing treatments (1.07 wt%).
the figure that cyclohexane conversion increases with an in-
crease in chromium content and remains constant at a Si/Cr
of 50. However, the cyclohexanol selectivity decreases for
Si/Cr = 25 due to the presence of more chromium, which
favors the formation of ring oxidized products such as
hexanoic acid and heptanal. This is further supported by
reaction over a homogeneous catalyst (CrO3) where the
increase in catalyst amount (from 3 to 5 wt%) leads to con-
siderable amount of ring-oxidized products (from 0.7 to
4.4 wt%).
FIG. 5. Effect of reaction temperature on the conversion and selec-
tivity over (Cr)MCM-41.

MESOPOROUS (Cr)MCM-41 IN SELECTIVE OXIDATION OF CYCLOHEXANE
139
TABLE 3
Oxidation of Cyclohexane over (Cr)MCM-41a
Selectivity (%)
Conversion
Catalyst
(wt%)
Cyclohexanone
Cyclohexanol
Cyclohexyl acetate
Othersb
Calcined (Cr)MCM-41
First recycle
Second recycle
Washed (Cr)MCM-41
First recycle
Second recycle
98.9
72.1
74.5
72.2
74.6
71.1
5.4
0.3
0.6
0.8
1.4
—
0.5
—
—
1.0
1.7
1.3
0.7
—
92.9
95.3
95.3
90.7
93.3
98.5
80.1
83.3
88.7
91.7
91.9
85.6
96.4
78.1
6.8
4.1
3.9
7.9
6.7
—
—
—
—
—
—
—
—
7.2
2.9
2.5
2.8
—
—
1.0
Filtrate solution
Quenched solution
Calcined (Cr)MCM-41c
First recyclec
19.9
16.7
3.1
3.7
4.3
10.9
3.6
21.9
6.3
99.0
90.1
89.7
11.3
11.6
9.0
Washed (Cr)MCM-41c
Calcined (Cr)MCM-41d
MCM-41
No catalyst
—
a Reaction conditions: substrate : oxidant (H2O2) = 1 : 1; catalyst = 3.3 wt%; temperature = 373 K; and
time = 12 h.
b Hexanoic acid, cyclohexene, and heptanal.
c Catalyst with Si/Cr = 25.
d Without initiator.
Further, the reaction was carried out in the absence of cata-
lystorinitiatororwithchromium-freeMCM-41. Inallcases,
a low conversion (see Table 2) was noted. Thus, it is clear
from all these studies that the amount of chromium ions
present in the mesoporous matrix plays an important role
in the reaction.
The reaction was also carried out (48) over (Cr)MCM-
41(grafted), (Cr)zeolite-β, (Cr)silicalite-1, and (Cr)APO-5,
FIG. 7. Oxidation of cyclohexane over various chromium-containing
molecular sieves.
FIG. 8. DRUV–vis spectra of (a) (Cr)MCM-41, (b) (Cr)MCM-
41(grafted), (c) (Cr)APO-5, (d) (Cr)zeolite-β, and (e) (Cr)silicalite-1.

140
SAKTHIVEL AND SELVAM
SCHEME 2
and the results are shown in Fig. 7. As can be seen from this
Scheme 2 depicts the possible reaction pathway over the
figure, the activity of (Cr)MCM-41(grafted) catalyst showed a (Cr)MCM-41 catalyst. At first, acetic acid interacts with
lower activity than the corresponding (Cr)MCM-41, and H2O2, leading to the formation of peroxy acetic acid 2a,
the microporous analogues show much lower activity. Thus, which further reacts with chromate species 2b to produce
it is clear that (Cr)MCM-41 shows much better activity than chromium peroxospecies 2c, which is in accordance with
the other catalysts under investigation. It can also be no- Muzart (50). The latter then interacts with ketone (initia-
ticed from this figure that the microporous analogues, e.g., tor) to produce a chelate complex of type 2d followed by
(Cr)zeolite-β and (Cr)silicalite-1, show a drastic decrease reaction with cyclohexane, which leads to the cyclohexyl
in conversion upon recycling, owing to continuous leaching radical 2e. The cyclohexyl radical in turn reacts with
of active chromium species under the reaction conditions. peroxyacetic acid 2a and H2O2 through a chain transfer
However, in the case of mircoporous (Cr)APO-5, after the free radical reaction (51) to produce the desired product,
initialloss, noleachingwasobserved, whichisingoodagree- viz., cyclohexanol (see Scheme 3). Table 4 presents the
ment with literature (26). This could account for the forma- results of the influence of various initiators on the reaction.
tionofCr2O3 clustersuponcalcinationoftheas-synthesized It is, however, worth mentioning here that the use of MEK
samples, which is clearly evidenced form the DRUV–vis resulted in a significant increase in conversion. This could
studies (Fig. 8c), where an additional broad band around be attributed to the reduction in the induction period of the
630 nm was attributed to such species (45, 49). Such an reaction (4, 10, 16). That is, the use of unsymmetric ketone
effect can also be seen in the case of (Cr)MCM-41(grafted) (MEK) resulted in the formation of chelate complex
(Fig. 8b) and (Cr)silicalite-1 (Fig. 8e), but to a lesser (2d), which may easily be cleaved into a free radical capa-
extent.
ble of initiating chain transfer by hydrogen abstraction with

MESOPOROUS (Cr)MCM-41 IN SELECTIVE OXIDATION OF CYCLOHEXANE
141
TABLE 4
Influence of Initiators on the Oxidation of Cyclohexanea
Selectivity (%)
Conversion
Initiator
MEK
Cyclohexanone
Acetone
(wt%)
Cyclohexanone Cyclohexanol Cyclohexyl acetate Othersb
98.9
67.0
64.6
45.9
0.3
31.3
32.1
9.7
92.9
62.5
64.1
88.5
6.8
1.5
2.8
1.4
—
4.7
1.0
0.4
Acetaldehyde
a Reaction conditions: substrate : oxidant (H2O2) = 1 : 1; catalyst = 3.3 wt%; temperature = 373 K;
and time = 12 h.
b Hexanoic acid and cyclohexene.
TABLE 5
Oxidation of Cyclohexane over (Cr)MCM-41 with Different Solventsa
Selectivity (%)
Conversion
Solvents
(wt%)
Cyclohexanone Cyclohexanol Cyclohexyl acetate Othersb
Acetic acid
Methanol
1,4-Dioxan
Acetone
98.9
36.7
36.6
27.6
0.3
—
2.4
92.9
98.3
95.9
73.7
6.8
0.4
0.3
0.1
—
1.3
1.4
0.9
25.3
a Reaction conditions: substrate : oxidant (H2O2) = 1 : 1; catalyst = 3.3 wt%; temperature =
373 K; and time = 12 h.
b Hexanoic acid and cyclohexene.
SCHEME 3

142
SAKTHIVEL AND SELVAM
TABLE 6
Oxidation of Cyclohexane over (Cr)MCM-41 with Different Oxidantsa
Selectivity (%)
Conversion
Oxidant
(wt%)
Cyclohexanone
Cyclohexanol
Cyclohexyl acetate
Othersb
H2O2
O2
Air
98.9
86.5
52.4
60.5
0.3
—
—
92.9
97.2
99.0
—
6.8
2.8
1.0
8.3
—
—
—
TBHP
80.8
10.9
a Reaction conditions: substrate : oxidant (H2O2) = 1 : 1; catalyst = 50 mg (3.3 wt%); tem-
perature = 373 K; and time = 12 h.
b Hexanoic acid and cyclohexene.
cyclohexane (52). On the other hand, the use of symmet-
ric ketones such as cyclohexanone and acetone may pro-
duce a stable chelate complex, which may be difficult to
cleavage, and therefore a decrease in conversion. In the
case of acetaldehyde, the lower conversion is attributed to
a possible oxidation of the aldehyde to the corresponding
acids.
ions which come out from the matrix. However, unlike
the other chromium-based systems, the chromium ions in
mesoporous material remain intact, i.e., a certain amount
is stabilized in the matrix which do not leach out even after
several recycling/washing treatments. Hence, (Cr)MCM-41
behaves truly as a heterogeneous catalyst, thus forming the
first example of chromium-based non-leaching solid mate-
rials.
Table 5 summarizes the influence of various solvents on
the reaction. It can be seen from the table that a relatively
lower conversion was obtained in the case of methanol,
acetone, and 1,4-dioxan due to possible partial decompo-
sition of H2O2 under the reaction conditions. However, in
the case of acetic acid, the activity can be attributed to the
stabilization of H2O2 as peroxy acetic acid species 2a (6).
The reaction was also carried out with different oxidants,
e.g., 70% TBHP, molecular O2, and air, and the results are
listed in Table 6. It can be seen from the table that the use of
molecularO2 andairleadstoadecreaseinsubstrateconver-
sion due to a milder oxidizing nature as well as to difficulty
in activation (6). Although the use of TBHP decreases the
conversion, itproducescyclohexanoneasthemajorproduct
with considerable amounts of cyclohexyl acetate and other
ring oxidized products. The observed higher cyclohexanone
selectivity may possibly be due to stronger oxidation effi-
ciency of TBHP (24), thereby leading to a secondary re-
action, viz., oxidation of cyclohexanol to cyclohexanone,
and other ring oxidation reactions, e.g., hexanoic acid
(24).
ACKNOWLEDGMENT
We thank RSIC, IIT—Bombay, for EPR and GC–MS measurements.
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