The effect of vanadium sources on the synthesis and catalytic activity of VMCM-41

Article abstract of DOI:10.1016/j.jcat.2004.10.005

Mesoporous VMCM-41 was synthesized hydrothermally using various vanadium sources, viz, tetravalent vanadium such as vanadyl sulfate and vanadyl acetylacetonate, as well as pentavalent vanadium like sodium vanadate and ammonium vanadate. The influence of different vanadium sources on the framework substitution of vanadium, as well as their catalytic activity for the oxidation of cyclohexane, was investigated. Among the different vanadium stocks, the tetravalent vanadium sources showed maximum vanadium incorporation in the silicate framework of MCM-41. As a consequence, these catalysts gave much higher substrate conversion and excellent product selectivity. On the other hand, the catalysts prepared from pentavalent vanadium sources showed lower activity owing to the smaller amounts of vanadium in the matrix. Although the activity of the catalyst slightly decreased after first recycle as due to leaching of small amounts of active vanadium species, it however remained nearly the same even after several recycles. This was further confirmed by washing experiments wherein non-framework vanadium ions were removed upon ammonium acetate treatment; the washed catalysts showed a similar activity to those of the recycled catalysts. Thus, recycled/washed VMCM-41 behaves truly as heterogeneous catalyst. Furthermore, the influence of pore size of the catalyst was tested for the oxidation of bulkier substrate, viz, cyclododecane.

Full text of DOI:10.1016/j.jcat.2004.10.005

Journal of Catalysis 229 (2005) 64–71
www.elsevier.com/locate/jcat
The effect of vanadium sources on the synthesis and catalytic activity
of VMCM-41
P. Selvam ∗, S.E. Dapurkar
Solid State and Catalysis Laboratory, Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400 076, India
Received 22 June 2004; revised 4 October 2004; accepted 6 October 2004
Available online 6 November 2004
Abstract
Mesoporous VMCM-41 was synthesized hydrothermally using various vanadium sources, viz, tetravalent vanadium such as vanadyl
sulfate and vanadyl acetylacetonate, as well as pentavalent vanadium like sodium vanadate and ammonium vanadate. The influence of
different vanadium sources on the framework substitution of vanadium, as well as their catalytic activity for the oxidation of cyclohexane,
was investigated. Among the different vanadium stocks, the tetravalent vanadium sources showed maximum vanadium incorporation in the
silicate framework of MCM-41. As a consequence, these catalysts gave much higher substrate conversion and excellent product selectivity.
On the other hand, the catalysts prepared from pentavalent vanadium sources showed lower activity owing to the smaller amounts of vanadium
in the matrix. Although the activity of the catalyst slightly decreased after first recycle as due to leaching of small amounts of active vanadium
species, it however remained nearly the same even after several recycles. This was further confirmed by washing experiments wherein non-
framework vanadium ions were removed upon ammonium acetate treatment; the washed catalysts showed a similar activity to those of the
recycled catalysts. Thus, recycled/washed VMCM-41 behaves truly as heterogeneous catalyst. Furthermore, the influence of pore size of the
catalyst was tested for the oxidation of bulkier substrate, viz, cyclododecane.
2004 Elsevier Inc. All rights reserved.
Keywords: Mesoporous VMCM-41; Vanadium sources; Selective oxidation; Cyclohexane; Cyclohexanol; Cyclododecane; Cyclododecanol
1. Introduction
of thermally stable hexagonal MCM-41 and cubic MCM-48
materials, overcomes many of these difficulties as these
materials possess a larger pore diameter (2–10 nm), high
surface area (700–1500 m2 g−1) and substantial amount of
silanol (defect sites) groups (30–40%) [9,10]. The large sur-
face area and high concentration of silanol groups in the
mesoporous materials stabilize the vanadium ions in a facile
manner to that of the corresponding microporous analogues
[11–14].
Several attempts have been made to incorporate vana-
dium in mesoporous MCM-41 type silicate framework, via
isomorphous substitution [15–18], impregnation [19,20],
ion-exchange [21], and chemical vapour deposition [22], as
well as hexagonal mesoporous silica (HMS) matrix [23,24].
Substitution of vanadium in hexagonal mesoporous alu-
minophosphate (HMA) network has also been reported [25].
Although several reports have appeared on the incorpora-
tion of vanadium in MCM-41, only a few of them address
Vanadium containing microporous molecular sieves were
found to be active in a number of oxidation reactions [1,2].
However, the activity of these catalysts depends strongly
on the preparation methods as well as the choice of vana-
dium sources for the synthesis [2–6]. Furthermore, vana-
dium leaching is one of the main problems in the liquid
phase oxidation reactions; thus the actual reaction is taking
place under homogeneous conditions rather than heteroge-
neous catalysis [1,2,6–8]. In addition, microporous-based
molecular sieves have yet another drawback of smaller pore
size, which restricts the accessibility of active sites to large
substrate molecules. However, the discovery of mesoporous
molecular sieves, designated as M41S family [9] consisting
*
Corresponding author. Fax: +91 22 2576 3480.
E-mail address: selvam@iitb.ac.in (P. Selvam).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.10.005
2004 Elsevier Inc. All rights reserved.

P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
65
the influence of vanadium sources on the extent of incorpo-
ration of vanadium in the framework vis-à-vis the catalytic
activity. Reddy et al. [23,24] have investigated the leaching
of active vanadium species from VHMS matrix and found
that the leaching depends, in addition to the experimental
conditions, on the nature of vanadium sources used in the
synthesis of catalysts, the type of solvent, substrate, oxidant,
etc. On the other hand, Deng et al. [26] have also docu-
mented the leaching of vanadium species from amorphous
vanadium containing porous silica, which also depends on
the nature of solvent, substrate, oxidant, etc. Therefore, in
this investigation, an attempt has been made to study system-
aticlly the effect of various vanadium sources, viz, vanadyl
sulfate, vanadyl acetylacetonate, sodium vanadate and am-
monium vanadate, on the synthesis and catalytic activity of
VMCM-41, as well as to evaluate the leaching behavior of
the catalyst. In this regard, we chose cyclohexane oxidation
reaction owing to the importance of the oxidation products,
viz, cyclohexanol and cyclohexanone, in the production of
adipic acid and caprolactam as they are the key intermediates
in the manufacture of nylon-6 and nylon-66 polymers [27].
Furthermore, the influence of the mesoporosity of the cata-
lysts was also tested for the oxidation of bulkier stubstrates
such as cyclododecane.
viz, vanadyl sulfate, vanadyl acetylacetonate, sodium vana-
date and ammonium vanadate. The as-synthesized sam-
ples are designated as VMCM-41(AVS), VMCM-41(AVA),
VMCM-41(ASV), and VMCM-41(AAV), respectively. All
as-synthesized VMCM-41 samples were calcined at 823 K
for 2 h in N2 with a flow rate of 50 ml min−1 and heating rate
of 1 K min−1 followed by 6 h in air. The calcined samples
were designated in a way similar to that of the as-synthesized
samples, i.e., VMCM-41(CVS), VMCM-41(CVA), VMCM-
41(CSV), and VMCM-41(CAV), respectively. Unless other-
wise stated, the VMCM-41 catalyst used in the present study
was with Si/V ratio of 50.
For a comparison, microporous vanadosilicate or vana-
dium silicalite-1 (VS-1) was synthesized as per literature
procedure using vanadyl sulphate as vanadium source [5].
The catalyst was prepared hydrothermally in a Teflon-lined
stainless steel autoclave with a typical molar gel composi-
tion of: SiO2:0.165(TPA)2O:22H2O:0.01V2O5. First, aque-
ous solution of vanadyl sulphate was added to tetraethyl-
orthosilicate and the mixture was stirred for 30 min. To this,
tetrapropyl ammonium hydroxide was added and the mix-
ture was stirred for 1 h to achieve homogenization of gel.
The resulting gel was transferred into Teflon-lined stainless-
steel autoclaves and kept in an air oven for crystallization
at 443 K for 48 h. The solid product obtained was washed,
filtered and dried at 383 K for 12 h. The resulting as-
synthesized samples were calcined at 823 K for 12 h under
oxygen. This sample is designated as VS-1(CVS). In ad-
dition, mesoporous siliceous MCM-41 was also prepared,
for both loading of vanadium oxide as well as for a blank
reaction, according to the procedure described earlier [29].
Vanadium oxide supported MCM-41 (V2O5/MCM-41) was
prepared by incipient wetness method. For this purpose, first,
an aqueous solution of vanadyl sulphate (5 × 10−3 M) was
added dropwise to 1 g preactivated calcined MCM-41 and
was kept under mild stirring for 3 h at room temperature.
The resulting sample was repeatedly washed, dried and cal-
cined in air for 8 h at 773 K. This sample is referred to as
V2O5/MCM-41 (CVS).
2. Experimental
2.1. Starting Materials
The following chemicals were employed for the prepa-
ration of VMCM-41 and for the oxidation of cyclohexane.
Fumed silica (SiO2, 99.8%, Aldrich), tetramethylammonium
hydroxide (TMAOH, 25 wt%, Aldrich), cetyltrimethylam-
monium bromide (CTAB, 99%, Aldrich), vanadium sulfate
hydrate trihydrate (VOSO4 · 3H2O, 99%, Aldrich), vanadyl
acetylacetonate (VO(acac)2, 97%, Lancaster), sodium vana-
date (NaVO3, 98%, Loba), ammonium vanadate (NH4VO3,
98%, Loba), sodium hydroxide (NaOH, 98%, Loba), sulfu-
ric acid (H2SO4, 98%, BDH), cyclohexane (99.5%, Merck),
hydrogen peroxide (H2O2, 30%, Qualigens), acetic acid
(99.5%, Fischer), methyl ethyl ketone (MEK, 99%, SD). All
the reagents used in this study were in as-received form.
2.3. Characterization
All the samples were systematically characterized
by various analytical and spectroscopic techniques such
as low angle powder X-ray diffraction (XRD, Rigaku),
thermogravimetery-differential thermal analysis (TG-DTA,
Dupont 9900/2100), transmission electron microscopy
(TEM, Philips CM 200 operated at 160 kV), nitrogen sorp-
tion (Sorptomatic 1990), Fourier transform-infrared spec-
troscopy (FT-IR, Nicolet Impact-400), 29Si magic angle
spinning nuclear magnetic resonance (29Si MAS-NMR,
Varian 300X), diffuse reflectance ultraviolet-visible spec-
troscopy (DRUV–vis, Shimazdu UV-260), electron para-
magnetic resonance spectroscopy (EPR, Varian E-112), and
inductively coupled plasma-atomic emission spectroscopy
(ICP-AES, Labtam Plasma Lab 8440).
2.2. Synthesis of VMCM-41
The VMCM-41 samples were synthesized hydrother-
mally using various vanadium sources with different Si/V
(molar) ratio as per the procedure described elsewhere
[18,28] with a typical molar gel composition of: SiO2:
0.135 (CTA)2O:0.13 Na2O:0.075 (TMA)2O:68 H2O:(0.02–
0.0025) V2O5. The pH of the gel was adjusted to 11.5 and
it was subjected to hydrothermal treatment at 373 K for
3 d. The final solid product, designated as as-synthesized
VMCM-41, obtained was filtrated and dried for overnight.
The samples were prepared from various vanadium sources,

66
P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
2.4. Reaction procedure
with acetone followed by drying at 353 K. Then, in order
to remove the adsorbed molecules, the catalyst was acti-
vated at 823 K for 6 h in air. The reaction was then car-
ried out on the recycled catalyst. The following catalysts,
viz, VMCM-41(CVS), VMCM-41(CSV), V2O5/MCM-41
(CVS), and VS-1(CVS), were used for the recycling pur-
pose. The recycled catalysts were designated as VMCM-
41(RVS), VMCM-41(RSV), V2O5/MCM-41 (RVS), and
VS-1(RVS), respectively.
The oxidation of cyclohexane (18 mmol) was carried out
using 50 mg of the catalysts with 30% H2O2 (18 mmol)
as oxidant and acetic acid as solvent medium (10 ml). The
reaction was performed both in the presence and in the ab-
sence of methyl ethyl ketone (MEK, 5 mmol) as initiator
at 373 K for 12 h. Oxidation of cyclododecane (12 mmol)
was carried out under similar conditions as mentioned above
for cyclohexane reaction with the only change of cyclodode-
cane mixed solvents (5 ml acetic acid + 5 ml CH2Cl2) being
utilized to make a homogeneous reaction mixture. After the
reaction, the catalyst was separated and the products were
extracted with diethyl ether and analyzed by gas chromatog-
raphy (GC, Nucon 5700) with carbowax column. Further
confirmation of oxidation products was made by GC-MS
(Hewlett Packard G1800A) equipped with a HP-5 capillary
column.
3. Results and discussion
All as-synthesized and calcined VMCM-41 samples, ir-
respective of the vanadium sources and vanadium content,
were white. However, the color of the calcined samples upon
exposure to air at room temperature turns slowly into faint
yellow, which further intensifies with increase in vanadium
content. This could be attributed to the additional coordina-
tion of water molecules to form hexacoordinated vanadium
ions in the mesoporous matrix [16]. Furthermore, the white
color of the as-synthesized samples and the yellow color of
the calcined samples suggest the pentavalent nature of vana-
dium in the mesoporous network. Powder XRD patterns (not
reproduced here) of the various as-synthesized and calcined
VMCM-41 obtained from different vanadium sources show
characteristic diffraction patterns [28], which are typical of
hexagonal mesoporous MCM-41 structure [9,10]. Table 1
summarizes the XRD, ICP-AES and nitrogen sorption data
of different VMCM-41 samples having different Si/V (mo-
lar) ratios. As expected, for all the samples, an increase in
the d-spacing (or a0 values) was observed as compared to
the siliceous analogue, MCM-41. The pragmatic expansion
in the unit cell dimension with increase in Si/V ratio could
possibly be attributed to a larger (crystal) radius of V5+
(0.495 Å) than that of Si4+ (0.40 Å) [31], and/or the longer
V–O bond distance (1.8 Å) as compared to Si–O distance
(1.6 Å) [15]. This clearly indicates a possible substitution
of pentavalent vanadium ions in the framework structure of
MCM-41. A similar observation was made earlier of vana-
2.5. Washing studies
In order to check the stability of vanadium ions in the
mesoporous matrix, we treated the calcined samples, e.g.,
VMCM-41(CVS), with 1 M ammonium acetate solution.
That is, about 100 mg of the calcined sample was stirred with
30 ml of the ammonium acetate solution for 12 h at room
temperature and then it was filtered and washed repeatedly
with distilled water; the solid residue was dried at 373 K.
Further, the dried sample was calcined at 723 K for 6 h in air
so as to decompose the residual organics; such samples were
designated as washed catalysts, VMCM-41(WVS).
2.6. Recycling studies
To check the stability and recycling ability as well as
leaching of vanadium ions from the mesoporous matrix un-
der reaction conditions, several recycling experiments were
carried out for all the catalysts as per the procedure out-
lined elsewhere [30]. After the reaction was over, the cat-
alyst was separated from the reaction mixture and washed
Table 1
XRD, ICP-AES, and N sorption data of VMCM-41 synthesized using vanadyl sulfate as vanadium source
2
VMCM-41(AVS)
VMCM-41(CVS)
b
b
d
Si/V
a
a
0
Si/V
Vanadium
(wt%)
Pore volume
Pore diameter
(Å)
Surface area
(m g
0
a
c
−1
2
−1
)
ratio
(Å)
(Å)
ratio
(ml g
)
25
50
49.67
49.05
48.26
47.34
46.64
48.02
46.08
43.62
41.30
40.84
35
70
1.23
0.76
0.32
0.21
–
0.69
0.85
0.81
0.86
0.91
24
29
27
26
27
892
919
980
1012
1040
100
200
130
240
e
f
∞
∞
a
Nominal Si/V ratios in the synthesis gel.
b
c
d
e
f
2
2
2
2
2
2
XRD data; average unit cell parameter calculated using the relation: 1/d = 4/3[(h + hk + k )/a ] + [l /c ].
Actual Si/V ratio in the catalyst.
ICP-AES data.
Vanadium-free as-synthesized sample, MCM-41(A).
Vanadium-free calcined sample, MCM-41(C).

P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
67
Table 2
a
XRD, ICP-AES, and N sorption data results of various VMCM-41(50) prepared using various vanadium sources
2
b
b
c
−1
2
−1
Sample
a
0
(Å) Sample
a
0
(Å) Vanadium content (wt%) Pore volume (ml g
)
Pore diameter (Å) Surface area (m g )
VMCM-41(AVS) 49.05
VMCM-41(AVA) 48.76
VMCM-41(ASV) 46.97
VMCM-41(AAV) 46.80
VMCM-41(CVS) 46.08
VMCM-41(CVA) 45.50
VMCM-41(CSV) 43.89
VMCM-41(CAV) 43.82
0.76
0.66
0.46
0.35
0.85
0.75
0.78
0.76
29
27
28
28
919
945
960
930
a
Number in parenthesis is nominal Si/V molar ratio in synthesized gel.
b
c
2
2
2
2
2
2
XRD data; average unit cell parameter calculated using the relation: 1/d = 4/3[(h + hk + k )/a ] + [l /c ].
ICP-AES data.
dium incorporated zeolite-β [32] and HMS [24]. Table 2
presents XRD, ICP-AES and nitrogen sorption data of var-
ious VMCM-41 samples synthesized, with a Si/V (molar)
ratio of 50, using different vanadium sources. However, it is
interesting to note that a considerable variation in the diffrac-
tion data is observed for the same initial (gel) composition,
which could be accounted for the very different amounts of
vanadium incorporated in the framework. This is well sup-
ported by the ICP-AES results wherein distinct vanadium
content is noticed for all these samples under consideration.
TEM studies (not reproduced here) of VMCM-41(CVS)
having Si/V ratio of 50 showed a regular hexagonal array
of uniform channels [28] typical of MCM-41 structure. ED
investigation (not reproduced here) performed on the same
sample also confirms the periodicity and high crystallinity
[28] of VMCM-41. Both the TEM and ED results are consis-
tent with XRD data, thus validating the hexagonal structure
of the samples [10].
cropore filling at low P/Po. The pore volume, surface area
and pore diameter deduced from N2 sorption isotherms for
all the VMCM-41 samples are compiled in Tables 1 and 2.
All the results corroborate the mesoporous nature of cat-
alysts [9,10]. Table 2 also presents the vanadium analysis
data. It is clear from this table that relatively low vanadium
incorporation was observed in the case of pentavalent vana-
dium as the starting sources. On the contrary, tetravalent
vanadium sources favor high vanadium incorporation in the
matrix. The very different influence of these two sources on
the vanadium incorporation in the MCM-41 network may
be explained as follows: Under the same synthesis con-
ditions and initial vanadium concentration, the tetravalent
3−
vanadium sources form monomeric VO4
species while
the pentavalent vanadium sources produce both monomeric
3−
2−
VO4 and dimeric V2O7 species [34]. It seems likely
that the dimeric species hinder the incorporation of vana-
dium in the matrix, and hence part of the vanadium ions
remains in solution.
Fig. 1 which depicts the nitrogen adsorption–desorption
isotherms of VMCM-41(CVS), shows reversible type IV
adsorption isotherms [33]. A sharp inflection between rel-
ative pressure P/Po ∼ 0.3 corresponds to capillary conden-
sation within the mesopores (see the inset of Fig. 1). The
sharpness of this step reflects the uniformity of the pores.
Further, the isotherms also confirm the absence of any mi-
TG of as-synthesized VMCM-41 samples (not repro-
duced here) showed > 50 wt% weight loss [28] attributed
to the removal of adsorbed water and surfactant molecules.
On the other hand, the calcined sample showed a relatively
large weight loss (not reproduced here) of > 20 wt% [28]
suggesting that part of the silanol groups in the matrix may
possibly be consumed for stabilization of vanadium ions
in a similar manner to that reported for chromium incor-
porated MCM-41 [35]. DTA traces (also not reproduced
here) showed the corresponding endothermic/exothermic
transitions characteristic of mesoporous MCM-41 mate-
rials. The above conjecture is well supported by 29Si
MAS-NMR studies (not reproduced here) of as-synthesized
MCM-41(∞) and VMCM-41(AVS) with a Si/V ratios
of 25–100, wherein the spectra showed three signals at
−110.0 (Q4), −100.0 (Q3), and −90.1 (Q2) ppm corre-
sponding to Si(OSi)4, Si(SiO)3(OH), and Si(OSi)2(OH)2,
respectively [28]. The spectra also confirmed the presence
of large number of internal silanol groups (defect sites
≡Si–OH; Q3 sites) in the case of siliceous MCM-41 [36].
However, as the vanadium content in the sample increases,
the intensity of the Q3 sites decreases, or the intensity ra-
tio of Q4/Q3 sites increases. This implies that there is an
interaction between vanadium ions and hydroxyl groups of
the silicate MCM-41 matrix. That is, during the synthesis,
in addition to the isomorphous substitution, vanadyl species
Fig. 1. N sorption isotherms of VMCM-41(CVS). The inset shows pore
2
size distribution.

68
P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
(VO22+) may also bind to the surface termination groups
(≡Si–OH) in the MCM-41 matrix via condensation to form
(SiO)3V=O type units [13,14,18].
structure of the tetrahedral vanadium species can be de-
tected. On the other hand, DRUV–vis studies of the vari-
ous calcined samples (Fig. 2e–h) prepared from different
vanadium sources showed considerable broadening of the
spectra towards lower energy (in the region 400–500 nm).
Such broadening could be attributed to the additional water
coordination of the distorted tetrahedral vanadium species,
which results in the formation of hexacooridnated vanadium
species in a way similar to the reported for vanadium sup-
ported silica [39,40] and vanadium incorporated HMS [24].
This implies that a small part of V5+ is present on the wall
surface as distorted tertrahedral arrangement, while the ma-
jority of V5+ is located mainly in the regular tetrahedral
environment in the framework of VMCM-41. Photolumi-
nescence studies [41] have shown evidence for these two
different kinds of (regular and distorted) tetrahedral vana-
dium species in the framework sites, which are accessible
and inaccessible to water molecules, respectively.
Table 3 summarizes the cyclohexane oxidation results
over various vanadium containing catalysts. It can be seen
from this table that the catalysts VMCM-41(CVS) and
VMCM-41(CVA) show excellent activity as compared to
VMCM-41(CSV) and VMCM-41(CAV). The modest ac-
tivity of the latter catalysts is accounted for the low vana-
dium content in the samples. It is however interesting to
note that, irrespective of the nature of catalysts, the washed
and recycled catalysts show only a small loss in activity as
compared to their calcined counterparts due to leaching of
non-framework vanadium ions from the mesoporous matrix,
which is consistent with vanadium analyses in the samples
(cf. Table 3). The observation of loss of vanadium content
upon recycling/washing is well supported by DRUV–vis
studies [28] where a slight decrease in absorption band inten-
sity was noted for the recycled/washed catalysts as compared
to the calcined catalyst (not reproduced here). However, the
spectra remain nearly the same even after the treatment for
the washed and recycled samples. At this juncture, it is also
interesting to note that the reaction carried out in the ab-
sence of catalyst or initiator or with vanadium-free MCM-41
showed much lower conversion (Table 3), thus confirming
the role of vanadium ions in the reaction. In addition, fil-
trate and quenching studies, carried out on washed catalyst,
show no catalytic conversion of cyclohexane. Thus, the ob-
served high activity of the mesoporous VMCM-41 catalysts
is due to the stabilization of vanadium ions in the silicate
framework. Moreover, unlike in most cases when extreme
reaction conditions such as high pressure and high tempera-
ture are adopted, in the present investigation where the use
of methyl ethyl ketone as initiator facilitate the reaction un-
der standard experimental conditions.
DRUV–vis spectra of various as-synthesized vanadium
samples (Fig. 2a–d) prepared from different vanadium
sources show two intense absorption bands at 275 nm and
340 nm. These bands are attributed to charge transfer tran-
sitions associated with O2− to V5+ [37]. Further, the ab-
sence of d–d transitions in the region 600–800 nm, and of
a tetravalent vanadium EPR signal (not reproduced here)
confirms the above assignment [16]. While the charge trans-
fer bands at 275 nm are consistent with V5+ in tetrahedral
environment, the other charge transfer band at 340 nm is
attributed to V5+ with V=O double bond and V–O single
bonds [24,38]. Indeed, these bands closely resemble those
of NH4VO3 (Fig. 2i) indicating isolated tetrahedral environ-
ment of V5+ in the mesoporous VMCM-41 matrix, which is
in good agreement with literature [38]. On the other hand,
the bands at 275 nm and 340 nm may also be assigned to
distorted tetrahedral arrangement of V5+ (as a result of an-
choring/grafting on the pore walls of MCM-41) in a way
similar to that reported for various vanadium-loaded/grafted
amorphous/mesoporous silica materials [30,38]. Hence, it
is highly difficult to distinguish between these two types
of species and DRUV–vis, and therefore only an averaged
In order to check the vanadium leaching as well as the
repeated use of the catalysts, several recycling runs were
performed over VMCM-41(CSV). Fig. 3 illustrates the re-
cycling studies. It can be seen from this figure that during
the first recycling experiment (or second run), a decrease
in conversion is noticed, and thereafter a minimal or no
Fig. 2. DRUV–vis spectra of various mesoporous vanadosilcates:
(a) VMCM-41(AVS), (b) VMCM-41(AVA), (c) VMCM-41(ASV),
(d) VMCM-41(AAV), (e) VMCM-41(C), (f) VMCM-41(CVA),
(g) VMCM-41(CSV), (h) VMCM-41(CAV), and (i) NH VO .
4
3

P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
69
Table 3
a
b
Oxidation of cyclohexane over vanadium containing catalysts (Si/V = 50) prepared using various vanadium sources
Catalyst
c
Vanadium content
Conversion
Selectivity (wt%)
(wt%)
(wt%)
d
Cyclohexanol
Cyclohexanone
Others
VMCM-41(CVS)
VMCM-41(CVS)
VMCM-41(RVS)
VMCM-41(WVS)
VMCM-41(CVA)
VMCM-41(CSV)
0.76
0.76
0.63
0.65
0.66
0.46
0.37
0.35
0.59
0.32
0.54
0.30
–
99.0
10.9
93.2
93.4
94.0
72.0
65.3
58.0
97.3
75.8
36.8
24.2
11.6
9.0
94.5
90.5
96.1
90.0
93.2
92.5
88.6
92.8
86.3
99.6
93.9
89.7
96.4
78.1
1.9
1.2
3.6
5.5
4.0
4.6
8.9
5.6
7.6
–
3.6
8.3
0.3
5.5
2.8
2.9
2.5
1.6
6.1
0.4
2.4
1.8
3.6
21.9
e
f
g
f
VMCM-41(RSV)
VMCM-41(CAV)
V O /MCM-41 (CVS)
2
5
f
V O /MCM-41 (RVS)
2
5
VS-1(CVS)
2.7
8.5
–
f
VS-1(RVS)
MCM-41(C)
No catalyst
h
–
–
a
Number in parenthesis is nominal Si/V molar ratio in synthesized gel.
Reaction conditions: substrate-to-oxidant (molar) ratio = 1; acetic acid (solvent) = 10 ml; time = 12 h; temperature = 373 K; initiator = MEK (0.5 ml);
b
catalyst amount = 3.3 wt%.
c
ICP-AES data.
Mainly cyclohexyl acetate.
Without initiator.
Recycled catalyst: first cycle (or second run).
Washed catalyst.
Vanadium-free calcined catalyst.
d
e
f
g
h
Table 4
a
Oxidation of cyclododecane over mesoporous and microporous vanadosilicates with a Si/V ratio of 50
b
Catalysts
Selectivity (wt%)
Cyclododecanol
Conversion
(wt%)
Vanadium content
(wt%)
Cyclododecanone
Others
VMCM-41(CVS)
VMCM-41(RVS)
VS-1(CVS)
0.76
0.63
0.54
93.8
81.6
18.0
80.0
72.8
65.5
12.0
20.5
17.5
8.0
6.7
17.0
c
a
Reaction conditions: 373 K; 12 h; substrate:oxidant (H O ) = 1:1; catalyst = 50 mg (2.48 wt%); solvent = (acetic acid 5 ml + dichloromethane 5ml);
2
2
MEK = 5 mmol.
b
ICP-AES data.
Third cycle (or fourth run).
c
loss in activity is observed. The decrease in the conver-
sion during the second run is ascribed to the leaching of
small amounts of non-framework vanadium under reaction
conditions, which is consistent with ICP-AES results (Ta-
ble 3) as well as DRUV–vis studies (not reproduced here)
of the recycled catalysts, viz, VMCM-41(RSV). In this re-
gard, it is noteworthy here that under a similar reaction
conditions, the chromium containing mesoporous molecular
sieves leach much higher amounts of chromium ions [35].
Likewise, the washed catalyst, VMCM-41(WVS), was also
tested for vanadium leaching and recycling ability. Fig. 4 de-
picts the results of such a study. It is clear from this figure
that the catalytic activity is nearly the same as in the first
and subsequent cycles. That is, nearly no change or only a
very slight change in the activity is noticed. This is ably sup-
ported by ICP-AES analyses, which yielded no evidence for
leached vanadium in filtrate solution. Furthermore, the XRD
patterns [28] of the catalysts remain nearly the same (not re-
produced here) even after recycling or washing experiment,
indicating the intactness of the MCM-41 structure. Thus, the
samples behave truly as heterogeneous catalysts.
The catalytic activity of VMCM-41(CVS) was also com-
pared with its microporous analogue, VS-1(CVS) (Table 3).
It is clear from this table that the activity of the latter is
much lower, which could mainly be attributed to the low
diffusivity, and hence little accessibility, of active sites of
cyclohexane (kinetic diameter = 6 Å) in the micropore chan-
nels of MFI structure [6]. This conjecture is consistent with
the results obtained for the bulkier substrates, wherein the
microporous catalyst VS-1(CVS) showed meager activity
as compared with mesoporous VMCM-41(CVS) catalyst
(Table 4). At this juncture, it is also important to men-
tion that the catalyst VS-1(CVS), as well as V2O5/MCM-
41 (CVS), showed a continuous decrease in activity upon
cycling, which is attributed to repeated leaching of active
vanadium ions from the matrix. Table 4 presents the results
of the oxidation of a bulkier cyclododecane over microp-
orous and mesoporous vanadosilicate catalysts. It is inter-

70
P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
4. Conclusion
In summary, it is clear from this study that the use of
different vanadium sources for the synthesis of VMCM-41
significantly influences the extent of vanadium incorpora-
tion in the mesoporous matrix and therefore the catalytic
activity. Under similar synthesis conditions, the tetravalent
vanadium sources incorporate more vanadium due to the for-
3−
mation of monomeric VO4 species, while the use of pen-
tavalent vanadium sources results in the formation of both
3−
2−
monomeric VO4 and dimeric V2O7 species. Further,
the recycling and washing studies also showed very little
or no loss in activity; thus the vanadium containing meso-
porous MCM-41 behaves as truly heterogeneous catalysts
irrespective of vanadium sources employed for the prepara-
tion. On the other hand, the yield is directly related to the
vanadium content. In this regard, the tetravalent sources are
preferred for the synthesis as they facilitate vanadium incor-
poration much more efficiently than the pentavalent vana-
dium. Finally, this investigation also demonstrates that the
VMCM-41 catalysts are a promising alternative for the oxi-
dation of bulkier molecules.
Fig. 3. Recycling studies of VMCM-41(CSV) (reaction conditions: sub-
strate:oxidant = 1; T = 373 K; t = 12 h; catalyst amount = 3.3 wt% of
substrate).
Acknowledgments
The authors thank Dr A. Sakthivel and Mr S.K. Moha-
patra for the experimental assistance, and SAIF/RSIC, IIT-
Bombay for TEM and ED, 29Si MAS-NMR, EPR and ICP-
AES measurements.
References
[1] B.I. Whitting, J.R. Anderson, J. Phys. Chem. 97 (1993) 1032.
[2] G. Bellussi, M.S. Rigutto, Stud. Surf. Sci. Catal. 35 (1994) 177.
[3] J. Kornatowski, M. Sychev, S. Kuzenkov, K. Strnadova, W. Pilz, D.
Kassner, G. Pieper, W.H. Bauer, J. Chem. Soc., Faraday Trans. 91
(1995) 2217.
[4] J. Kornatowski, B. Wichterlova, J. Jirkovsky, E. Loffler, W. Pilz,
J. Chem. Soc., Faraday Trans. 92 (1996) 1067.
[5] T. Sen, P.R. Rajamohanan, S. Ganapathy, S. Sivasanker, J. Catal. 163
(1996) 354.
[6] P.R.H.P. Rao, A.A. Belhekar, S.G. Hegde, A.V. Ramaswamy, P. Rat-
nasamy, J. Catal. 141 (1993) 595.
[7] A.P. Singh, T. Selvam, Appl. Catal. A 143 (1996) 111.
[8] M.J. Haanepen, A.M. Elemans-Mehring, J.H.C. van Hooff, Appl.
Catal. A 152 (1997) 203.
[9] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, K.D. Schmidtt,
C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Hig-
gins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834.
[10] P. Selvam, S.K. Bhatia, C. Sonwane, Ind. Eng. Chem. Res. 40 (2001)
3237.
[11] P.R.H.P. Rao, A.V. Ramaswamy, P. Ratnasamy, J. Catal. 137 (1992)
225.
[12] I. Moudrakovski, A. Sayari, C.I. Ratcliffe, J.A. Ripmeester, K.F. Pre-
ston, J. Phys. Chem. 98 (1994) 10895.
Fig. 4. Recycling studies of VMCM-41(WVS) catalyst (reaction conditions:
substrate:oxidant = 1; T = 373 K; t = 12 h; catalyst amount = 3.3 wt% of
substrate).
esting to note that VMCM-41(CVS) showed good activity
for the cycloalkane oxidation, while VS-1(CVS) catalyst
showed fairly small activity owing to their pore size re-
striction. Furthermore, it is noteworthy that the mesoporous
catalyst showed much higher activity as compared to many
other catalysts reported for the oxidation of cyclododecane
[15,42].
[13] D.H. Park, C.-F. Cheng, J. Klinowski, Bull. Korean Chem. Soc. 18
(1997) 70.
[14] D.H. Park, C.-F. Cheng, J. Klinowski, J. Mater. Chem. 7 (1997) 159.

P. Selvam, S.E. Dapurkar / Journal of Catalysis 229 (2005) 64–71
71
[15] K.M. Reddy, I. Moudrakovski, A. Sayari, J. Chem. Soc., Chem. Com-
mun. (1994) 1059.
[16] Z. Luan, J. Xu, H. He, J. Klinowski, L. Keven, J. Phys. Chem. 100
(1996) 19595.
[17] D. Wei, W.-T. Chueh, G.L. Haller, Catal. Today 51 (1999) 501.
[18] A. Sakthivel, S.E. Dapurkar, P. Selvam, in: T. White, D. Sun (Eds.),
Advances in Environmental Materials, Pollution Control Materials,
vol. 1, Materials Research Society, Singapore, 2001, pp. 67–72.
[19] H. Berndt, A. Martin, A. Brückner, E. Schreier, D. Müller, H. Kosslick,
G.-U. Wolf, B. Lücke, J. Catal. 191 (2000) 384.
[28] S.E. Dapurkar, PhD thesis, IIT-Bombay (2004).
[29] S.E. Dapurkar, S.K. Badamali, P. Selvam, Catal. Today 68 (2001) 63.
[30] A. Sakthivel, S.E. Dapurkar, P. Selvam, Catal. Lett. 77 (2001) 155.
[31] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 25 (1969) 925.
[32] S. Dzwigaj, P. Massiani, A. Davidson, M. Che, J. Mol. Catal. A 155
(2000) 169.
[33] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Mosenu, R.A. Pierotti, J.
Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
[34] S.E. O’Donnell, M.T. Pope, J. Chem. Soc., Dalton Trans. (1976) 2290.
[35] A. Sakthivel, P. Selvam, J. Catal. 211 (2002) 134.
[36] H. Landmesser, H. Kosslick, W. Storek, R. Fricke, Solid State Ion-
ics 101–103 (1997) 271.
[20] B. Solsona, T. Blasco, J.M. Lopez Nieto, M.L. Pena, F. Rey, A. Vidal-
Moya, J. Catal. 203 (2001) 443.
[37] G. Catana, R.R. Rao, B.M. Weckhuysen, P. Van Der Voort, E. Vansant,
R.A. Schoonheydt, J. Phys. Chem. 102 (1998) 8005.
[21] Q. Zhang, Y. Wang, Y. Ohishi, T. Shishido, K. Takehira, J. Catal. 202
(2001) 215.
[38] G. Centi, S. Perathoner, F. Trifiro, A. Aboukais, F. Aissi, Mguelton,
J. Phys. Chem. 96 (1992) 2617.
[22] G. Grubert, J. Rathousky, G. Schulz-Ekloff, M. Wark, A. Zukal, Mi-
cropor. Mesopor. Mater. 22 (1998) 225.
[39] A.E. Stiegman, H. Eckert, G. Plett, S.S. Kim, M. Anderson, A.
Yavrouian, Chem. Mater. 5 (1993) 1591.
[40] N. Das, H. Eckert, H. Hu, I.E. Wachs, J.F. Walzer, F.J. Feber, J. Phys.
Chem. 97 (1993) 8240.
[41] S. Dzwigaj, P. Massiani, M. Anpo, M. Che, J. Phys. Chem. B 104
(2000) 6012.
[42] D.H.R. Barton, E. Csuhai, N. Ozbalik, Tetrahedron 46 (1990) 3743.
[23] J.S. Reddy, A. Sayari, J. Chem. Soc., Chem. Commun. (1995) 2231.
[24] J.S. Reddy, P. Liu, A. Sayari, Appl. Catal. A 148 (1996) 7.
[25] S.K. Mohapatra, P. Selvam, Catal. Lett. 93 (2004) 47.
[26] Y. Deng, C. Lettmann, W.F. Maier, Appl. Catal. A 214 (2001) 31.
[27] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. da Cruz, M.C.
Guerreiro, D. Mandelli, E.V. Spimace, E.L. Pires, Appl. Catal. A 211
(2001) 1.