Selective synthesis of vitamin K3 over mesoporous NbSBA-15 catalysts synthesized by an efficient hydrothermal method

Article abstract of DOI:10.1039/c2dt31096a

Well hexagonally ordered NbSBA-15 catalysts synthesized by an efficient hydrothermal method were used, for the first time, for the selective synthesis of vitamin K₃ by liquid-phase oxidation of 2-methyl-1-naphthol (2MN1-OH) under various reaction conditions. The recyclable NbSBA-15 catalysts were also reused to find their catalytic activities. To investigate the leaching of non-framework niobium species on the surface of silica networks, the results of original and recyclable NbSBA-15 catalysts were correlated and compared. To find an optimum condition for the selective synthesis of vitamin K₃, the washed NbSBA-15(2.2pH) was extensively used in this reaction with various reaction parameters such as temperature, time and ratios of reactant (2M1N-OH to H₂O₂), and the obtained results were also demonstrated. Additionally, the liquid-phase oxidation of 2M1N-OH was carried out with different solvents to find the best solvent with a good catalytic activity. Based on the all catalytic studies, the vitamin K₃ selectivity (97.3%) is higher in NbSBA-15(2.2pH) than that of other NbSBA-15 catalysts, and the NbSBA-15(2.2pH) is found to be a highly active and eco-friendly heterogeneous catalyst for the selective synthesis of vitamin K₃.

Full text of DOI:10.1039/c2dt31096a

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Selective synthesis of vitamin K over mesoporous NbSBA-15 catalysts
3
synthesized by an efficient hydrothermal method†
a
a
b
c
b
M. Selvaraj,* D.-W. Park, I. Kim, S. Kawi and C. S. Ha
Received 22nd May 2012, Accepted 27th June 2012
DOI: 10.1039/c2dt31096a
Well hexagonally ordered NbSBA-15 catalysts synthesized by an efficient hydrothermal method were
used, for the first time, for the selective synthesis of vitamin K by liquid-phase oxidation of 2-methyl-
3
1
-naphthol (2MN1-OH) under various reaction conditions. The recyclable NbSBA-15 catalysts were also
reused to find their catalytic activities. To investigate the leaching of non-framework niobium species on
the surface of silica networks, the results of original and recyclable NbSBA-15 catalysts were correlated
and compared. To find an optimum condition for the selective synthesis of vitamin K , the washed
3
NbSBA-15(2.2pH) was extensively used in this reaction with various reaction parameters such as
temperature, time and ratios of reactant (2M1N-OH to H O ), and the obtained results were also
2
2
demonstrated. Additionally, the liquid-phase oxidation of 2M1N-OH was carried out with different
solvents to find the best solvent with a good catalytic activity. Based on the all catalytic studies, the
vitamin K selectivity (97.3%) is higher in NbSBA-15(2.2pH) than that of other NbSBA-15 catalysts,
3
and the NbSBA-15(2.2pH) is found to be a highly active and eco-friendly heterogeneous catalyst for
the selective synthesis of vitamin K3.
with traces of hazardous transition metals, and are thus con-
1
. Introduction
2
,3,11
sidered as a “dirty” fine chemical industry process.
Since
Vitamin K3 (2-methyl-1,4-naphthoquinone or menadione) is
widely used as a blood coagulating agent and is an important
intermediate in the synthesis of other vitamins, such as vitamin
vitamin K is found to be more active than vitamin K and
3
1
vitamin K as an antihemorrhagic agent, the catalytic oxidation
2
1
of MN using different oxidizing agents over different catalysts is
2,12–15
K (phylloquinone), vitamin K (menaquinone) and vitamin K
1
2
4
extensively reported.
Recently, the new inorganic and
(acetomenaphthone). The catalytic oxidation of 2-methylnaptha-
organic modified mesoporous SBA-15 materials are used as
16,17
lene (MN) to vitamin K over CrO in sulfuric acid is firstly
reported.
3
3
eco-friendly catalysts
materials.
and as light-emitting support
2,3
1
8,19
In this catalytic system the vitamin K3 yield is
reported at around 30–60%, ∼18 kg of inorganic waste is pro-
duced in the production of 1 kg of the target product, and there
is an essential treatment for the removal of inorganic waste from
Since the development of clean catalytic methods for the pro-
duction of fine chemicals has received increasing attention over
the last two decades, the development of cleaner, catalytic
2,3
the target product. The diverse catalysts are also applied in the
production of vitamin K by the oxidation of MN with hydrogen
methods for the synthesis of vitamin K is therefore a challen-
3
3
ging goal. Using different catalytic methods, the catalytic oxi-
dation of MN is carried out over diverse microporous and
4
–10
peroxide (H O ) or tert-butyl hydroperoxide (TBHP).
Even
2
2
6
–10,20–22
though the above catalytic systems produce a reasonably yield of
mesoporous solid catalysts.
The vitamin K selectivity
3
vitamin K by the catalytic oxidation of MN, some catalysts
3
is moderate due to the formation of numerous side-products; the
main one is an isomeric 6-methyl-1,4-naphthoquinone
become homogeneous with the leaching of the active species
from the solid catalysts during the oxidation process. These cata-
lytic systems cause the well-known problems with catalyst
separation and inevitably lead to contamination of the product
(6-MNQ). To avoid byproducts such as 6-MNQ and methyl
group oxidized products from MN, 2-methyl-1-naphthol
(
2M1N-OH) instead of MN is used as a substrate for the syn-
23,24
thesis of vitamin K with high selectivity.
Mo–V-phosphoric
3
Keggin heteropoly acids can be used as the reversibly acting oxi-
a
School of Chemical and Biomolecular Engineering, Pusan National
dants to convert 2M1N-OH into vitamin K with a yield around
3
University, Busan 609-735, Korea. E-mail: chems@pnu.edu;
Fax: +82-51-512-8563; Tel: +82-51-510-2397
25
8
0–85%. In addition to synthesis of vitamin K with a higher
3
b
Department of Polymer Science and Engineering, Pusan National
selectivity, the non-catalytic oxidation of 2M1N-OH with mo-
lecular oxygen is also carried out at elevated pressure (3 atm) at
University, Busan 609-735, Korea
c
Department of Chemical and Biomolecular Engineering, National
26
room temperature.
University of Singapore, Singapore 119260, Singapore
†
1
An atom-efficient, clean oxidizing agent as well as a very
active heterogeneous catalyst remains a great challenge in the
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2dt31096a
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development of a clean catalytic process for the oxidation of
M1N-OH. Recently, vitamin K3 has been synthesized by
the liquid-phase oxidation of 2M1N-OH over the mesoporous
2.3. Oxidation of 2-methyl-1-naphthol
2
Oxidation of 2M1N-OH to vitamin K was carried out under
3
2
3,27
vigorous stirring in a thermostated glass vessel reactor at differ-
ent temperatures. In a typical experimental procedure, 4 mg of
NbSBA-15(2.2pH) catalyst and 0.05 mmol of 2M1N-OH dis-
solved in 7 ml of acetonitrile (MeCN) were added to the reactor.
Thereafter, the reaction mixture was stirred under constant stir-
ring, and subsequently the reaction temperature was slowly
raised to 348 K, and 0.25 mmol of H O was carefully added
titanosilicate catalysts, TiMMM-2 and TiSBA-15.
However,
to the best of our knowledge, the oxidation of 2M1N-OH
using NbSBA-15 catalysts synthesized by pH-adjusting direct
hydrothermal (pH-aDH) method has not been clearly reported
with a higher vitamin K selectivity in the literature so far.
3
In this report, using the pH-aDH method, the two-dimensional
mesostructured NbSBA-15 catalysts synthesized with different
concentrations of niobium species have been used for the selec-
2
2
through the septum to the reactant mixture and refluxed for
5 min. After completion of the reaction, the NbSBA-15(2.2pH)
4
tive synthesis of vitamin K by the oxidation of 2M1N-OH with
3
catalyst was filtered, and the products were collected. The
oxidation of 2M1N-OH was also conducted with other
NbSBA-15 mesoporous catalysts.
H O . To prepare the highly selective synthesis of vitamin K
3
2
2
over washed NbSBA-15(2.2pH), the oxidation of 2M1N-OH has
been carried out with different reaction parameters and solvents.
For the investigation of catalytic stabilities, the recyclable
NbSBA-15 catalysts have also been reused. The catalytic results
of the mesoporous catalysts used for the oxidation of 2M1N-OH
have been correlated and compared for the selective synthesis of
vitamin K3.
The preliminary oxidation product was analyzed with an auth-
entic sample using gas chromatograph (GC) using a VF5-MS
capillary column coupled with a flame ionization detector (FID).
Additionally, the product was further confirmed using a HP-5MS
capillary column by gas chromatograph–mass spectroscopy
using the Hewlett-Packard model 6890 gas chromatograph com-
bined Hewlett-Packard 5973 mass spectroscopy (GC-MS), and
1
H nuclear magnetic resonance (NMR, Bruker 800). The coup-
ling/overoxidation byproducts were also confirmed by GC-MS
and H NMR.
2. Experimental
1
2
.1. Materials
Mesoporous NbSBA-15 catalysts viz., NbSBA-15(2.2pH) and
NbSBA-15(1.6pH) were reused in the oxidation of 2M1N-OH to
find their stabilities. In a typical experimental procedure, the
NbSBA-15(2.2pH) catalyst used in a catalytic run was separated
from the reaction mixture, washed with acetone several times
and dried at 393 K. Finally, the catalyst was calcined at 773 K
for 6 h in air to remove the adsorbed species. The treated catalyst
is defined as ‘recyclable NbSBA-15(2.2pH)’, which was again
used in further catalytic runs. A similar procedure was used
for recycling studies of NbSBA-15(1.6pH). The hot-catalysts
filtration experiments were examined twice after four runs, to
find the catalytic activity of NbSBA-15(2.2pH). After com-
pletion of the reaction, the catalyst was filtered and analyzed by
ICP-AES, EDS and XPS to find the percentage of niobium
For the synthesis of mesoporous NbSBA-15 catalysts, all chemi-
cals viz., triblock copolymer poly(ethylene glycol)-block-poly
(
propylene glycol)-block-poly(ethylene glycol) (Pluronic P123,
molecular weight = 5800, EO PO EO ), tetraethylorthosili-
2
0
70
20
cate (98%, TEOS), hydrochloric acid (37%, HCl) and niobium
chloride (>99.9%) were purchased from Sigma-Aldrich Chemi-
cal Inc. All the chemicals were used as received without further
purification.
For the synthesis of vitamin K , all chemicals viz., 2-methyl-
-naphthol (98%, 2M1N-OH), hydrogen peroxide (30% H O ),
2 2
3
1
acetonitrile (MeCN), acetone (Ac), acetic acid (AcOH), metha-
nol (MeOH) and ethanol (EtOH), were also purchased from
Sigma-Aldrich Chemical Inc. and used as received without
further purification.
content, and the 2M1N-OH conversion and vitamin K selectiv-
3
ity were calculated with standard formulae followed by analyz-
ing results of GC and GC-MS.
To remove the non-framework niobium ions, a high amount of
NbSBA-15(2.2pH) was washed with acetone several times and
dried at 393 K. The treated catalyst is named as ‘washed
NbSBA-15(2.2pH)’, which is used to find an optimal condition
with different reaction parameters such as time, temperature
and stoichiometric molar ratios of reactants (2M1N-OH to
H O ). For the identification of a better solvent over washed
NbSBA-15(2.2pH) catalyst, the oxidation of 2M1N-OH was
carried out with different solvents including MeCN, Ac, AcOH,
MeOH and EtOH.
2
.2. Synthesis and characterization of mesoporous NbSBA-15
catalysts
In our synthesis conditions, no mineral acid was added to
the synthesis mixture. The mesostructured NbSBA-15 catalysts
2
2
(
nSi/nNb = 10) have been synthesized by the pH-aDH method
using the molar gel composition, 1 TEOS/0.1 Nb O /0.016
P123/127–210 H O according to our previously published pro-
2
5
2
28
cedure. The synthesized NbSBA-15 catalysts are named as
NbSBA-15(1.6pH), NbSBA-15(1.8pH), NbSBA-15(2.0pH) and
NbSBA-15(2.2pH), and the pH-adjusting values in the prep-
aration of samples were 1.6, 1.8, 2.0 and 2.2, accordingly.
All the synthesized catalytic samples were characterized using
the relevant instrumental techniques such as ICP-AES, XRD, N2
adsorption–desorption, UV-vis DRS, TEM, TEM-EDS, SEM
3
. Results and discussion
The mesoporous SBA-15 material is usually prepared using
Pluronic P123 triblock copolymer as the structuring agent under
strong acidic hydrothermal conditions. Under these conditions,
the introduction of more heteroatomic species into SBA-15 is
2
8
and XPS, according to the published procedure.
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2
8
very difficult because the formation of metal–oxo species in the
mesoporous materials is much reduced. To overcome this
problem, the pH-aDH method has been introduced and used for
the synthesis of MSBA-15 catalysts (M = Al, Cr, Mn, Sn, Ga,
Ce, Ti and V) with high metal content, and the synthesized
MSBA-15 catalysts have been used for certain catalytic
small amount of Nb O species is dispersed on the SBA-15.
2 5
Most of the Nb O5 species dispersed on the surface of
NbSBA-15(2.2pH) are completely removed by the washing treat-
2
28
ments. The results of TEM and FE-SEM correspondingly
show the uniform pore diameter and rope-like hexagonal meso-
porous structure of NbSBA-15 catalysts (Fig. 5S and 6S†). On
the basis of characteristic results, it is interestingly to note that
the well-ordered NbSBA-15 catalysts with a huge number of
29–39
reactions.
A high amount of niobium species are success-
fully substituted/incorporated into SBA-15 under the pH-aDH
2
4
5+
method without mineral acid. The mesoporous NbSBA-15
catalyst has more hydrothermal stability than NbMCM-41 due to
tetrahedral Nb species have been successfully synthesized
using the pH-aDH method.
2
8,40,41
having thicker silica pore walls.
The mesoporous NbSBA-15 catalysts have been used in the
liquid-phase oxidation of 2M1N-OH (Scheme 1) with the reac-
tion conditions noted in Table 2, and their catalytic activities
based on vitamin K3 selectivity are in the following order:
NbSBA-15(2.2pH) > NbSBA-15(2.0pH) > NbSBA-15(1.8pH) >
NbSBA-15(1.6pH). Among the all NbSBA-15 catalysts, the
mesoporous NbSBA-15(2.2pH) exhibits the best catalytic
activity and gives a higher 2M1N-OH conversion and vitamin
The NbSBA-15 catalysts synthesized by the pH-aDH method
have been characterized by ICP-AES, XRD, N2 adsorption–
desorption, UV-vis DRS, TEM, TEM-EDS, SEM and XPS
2
8
according to the published procedure.
ICP-AES and
TEM-EDS studies show that the rich Nb ions are highly incor-
porated on the silica pore walls. The elemental composition of
NbSBA-15 catalysts is listed in Table 1. XRD results confirm
that the calcined NbSBA-15 catalysts have well ordered 2D
hexagonal mesostructures with space group P6mm, and the unit
cell parameter in the calcined NbSBA-15 catalysts increases
with increasing Nb ion content, and with increasing pH from 1.6
to 2.2, as shown in Table 1 and Fig. 1S.† After washing treat-
ments, the washed NbSBA-15(2.2pH) has almost the same struc-
K selectivity than other NbSBA-15 catalysts (Table 2) due to
3
5+
the higher loading of tetrahedral Nb species on the surface of
inner silica pore walls. The higher catalytic activity of
NbSBA-15(2.2pH) is also tentatively ascribed to its two-dimen-
sional mesostructure that leads the higher number of accessible
active sites, and the large number of tetrahedral Nb species
incorporated in the framework of SBA-15 produces high
to enhance catalytic activity in
the oxidation of 2M1N-OH. Strukul et al. reported that the
vitamin K selectivity is 93% in the oxidation of 2M1N-OH
over Nb O –SiO catalyst. On the basis of catalytic activity,
5+
tural integrity. By increasing the Nb ion content, N adsorption
2
28,42
results reveal that the textural parameters of NbSBA-15(2.2pH)
increase as compared to other NbSBA-15 catalysts (Table 1).
The niobium content on the surface of the silica network also
increases when the pH is increased from 1.6 to 2.2 (Fig. 2S†).
The textural parameters of the washed NbSBA-15(2.2pH) also
remain constant with a good isotherm related to IUPAC classifi-
numbers of Lewis acid sites
3
43
2
5
2
the NbSBA-15(2.2pH) has superior catalytic activity in the syn-
thesis of vitamin K than other NbSBA-15 and Nb O –SiO
3
2
5
2
5
+
cation. The UV-vis DRS results prove that the Nb ions in the
calcined NbSBA-15 catalysts are tetrahedrally coordinated to
4
+
Si on the silica walls, and it can be also observed that
their intensities increase with increasing pH from 1.6 to 2.2
(
Fig. 3S†). However, the non-framework Nb O species dis-
2 5
persed on the surface of SBA-15 cannot be easily found by
UV-vis DRS analysis. The calcined NbSBA-15 catalysts and
washed NbSBA-15(2.2pH) have been further characterized by
5+
XPS (Fig. 4S†). The XPS results clearly confirm that the Nb
species can be tetrahedrally coordinated into SBA-15, whereas a
Scheme 1 2M1N-OH oxidation over NbSBA-15.
a
Table 1 Structural and textural parameters of mesoporous NbSBA-15 catalysts
n
Si/nNb ratio
Product
BET (m g−1)
2
d
(Å)
V
(cm g−1)
3
t
= a
− d
(Å)
Sample
pH in gel
Gel
ICP-AES
EDS
XPS
a
o
(Å)
A
p
p
w
o
p
NbSBA-15(1.6pH)
NbSBA-15(1.8pH)
NbSBA-15(2.0pH)
NbSBA-15(2.2pH)
NbSBA-15(2.2pH)
NbSBA-15(2.2pH)
NbSBA-15(1.6pH)
1.6
1.8
2.0
2.1
—
10
10
10
10
—
—
—
20.6
18.7
16.2
12.6
12.9
12.9
22.0
20.5
18.4
16.0
12.2
12.9
12.8
22.1
20.6
18.5
16.1
12.3
12.8
12.9
22.0
104.0
105.8
107.0
108.4
108.3
108.3
102.9
788
810
850
903
901
904
780
71.7
72.8
73.8
74.6
74.6
74.6
71.2
1.22
1.22
1.23
1.24
1.24
1.24
1.21
32.3
33.0
33.2
33.8
33.7
33.7
31.7
b
c
c
—
—
a
b
a
o
, unit cell parameter; ABET, specific surface area; V
p
, pore volume; d
p
, pore diameter, T
w
, wall thickness. The catalyst was washed four times using
c
acetone and dried at 393 K, prior to use in this reaction. The results were obtained for the reusable catalysts used for the 4th run in the oxidation of
M1N-OH.
2
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Table 2 Oxidation of 2M1N-OH over mesoporous NbSBA-15
a
catalysts
2
M1N-OH
Vitamin K
yield (%)
3
Vitamin K
3
selectivity (%)
Catalysts
conversion (%)
NbSBA-15(1.6pH)
NbSBA-15(1.8pH)
NbSBA-15(2.0pH)
NbSBA-15(2.2pH)
NbSBA-15(2.2pH)
NbSBA-15(2.2pH)
NbSBA-15(1.6pH)
91.4
93.2
95.5
100
96.5
96.6
90.4
3.5
81.2
84.0
91.7
97.3
91.2
91.5
70.1
<1
88.9
90.2
96.0
97.3
94.5
94.7
77.6
2.5
b
c
c
d
Filtrate solution
SBA-15
30.5
1.0
3.5
a
Reaction conditions: 4 mg of catalyst, 1 : 5 ratio of 2M1N-OH to H
2 2
O
(
4
0.05 mmol of 2M1N-OH and 0.25 mmol of H
2
O
2
), reaction time =
b
c
5 min, 7 ml of MeCN, temperature = 348 K. Washed catalyst. The
d
recyclable catalysts were used for the 4th run. The results obtained
from the hot-filtrate solution after the NbSBA-15(2.2pH) catalyst
removal.
Fig. 1 Synthesis of vitamin K
3
with different reaction temperatures.
Reaction conditions: 4 mg of washed NbSBA-15(2.2pH), 1 : 5 ratio of
M1N-OH to H (0.05 mmol of 2M1N-OH and 0.25 mmol of H ),
reaction time = 45 min, 7 ml of MeCN.
2
2
O
2
2 2
O
catalysts and is found to be a promising heterogeneous catalyst
in the liquid-phase oxidation of 2M1N-OH for the highly selec-
tive synthesis of vitamin K3.
44
For recycling studies, the two mesoporous catalysts viz.,
NbSBA-15(2.2pH) and NbSBA-15(1.6pH) synthesized using
different pH values in the gel under the pH-aDH method have
been examined to find their catalytic stabilities as follows.
Initially, the mesoporous catalysts used in the catalytic reaction
usually suffer from the loss of their catalytic activities, and hence
the catalysts need to be regenerated by calcination. The recycled
catalysts were washed four times with acetone and dried at
(2.2pH) and that this is a true heterogeneous process. This
reaction was also carried out using pure SBA-15 synthesized by
29,31
the pH-aDH method.
In this case, conversion of 2M1N-OH
(∼30%) forms with a trace amount of vitamin K selectivity
3
(Table 2), thus indicating that major activity is only due to
niobium ions incorporated in the framework of SBA-15.
Since the washed NbSBA-15(2.2pH) catalyst is a promising
catalyst in this catalytic oxidation reaction, it has been further
used in this reaction with different reaction parameters such as
reaction temperature, time, ratios of reactant (2M1N-OH to
H O ) and solvents, to find the best reaction conditions for the
3
6
2
93 K overnight. Finally the catalysts were calcined at 773 K for
h in air for complete removal of the organics and unreacted
M1N-OH molecules. The treated catalysts have been reused for
2
2
23
this reaction as shown in Table 2. In the first two runs
M1N-OH conversion decreases, and vitamin K yield as well
highly selective synthesis of vitamin K3.
2
The oxidation of 2M1N-OH over washed NbSBA-15(2.2pH)
catalyst has been carried out with different reaction temperatures
and times using the reaction conditions shown in Fig. 1 and 2,
3
as its selectivity also decreases (values not shown in Table 2).
On the basis of first two runs, it is observed that the extra frame-
work Nb O and their nanoparticles can be leached on the inner
2
5
for obtaining a high selective synthesis of vitamin K . When the
3
silica surface of pore walls, as shown in Table 1. But, after four
runs the 2M1N-OH conversion and vitamin K3 selectivity
remain constant, indicating that the niobium ions have not been
further leached on the mesoporous matrix, which is in good
agreement with the results of ICP-AES and EDS of filtrate solu-
tions where no niobium ions are detected, and the absence of
non-framework Nb O and their nanoparticles have been further
reaction temperature and time are decreased from 348 to 318 K
and 45 to 15 min, respectively, the rate of vitamin K formation
3
with 2M1N-OH conversion decreases due to the formation of
C–C coupling products such as dinaphthol and dinaphthaqui-
none due to lower catalytic activity on the surface of the catalyst
45
at a low reaction temperature or time. The 2M1N-OH conver-
2
5
sion and vitamin K yield as well as its selectivity are similar at
3
confirmed by the XPS results of 4th run of the catalysts
Table 1). Additionally, the washed NbSBA-15(2.2pH) was also
348 and 358 K. These results show that a higher temperature
(358 K) does not affect the catalytic activity of washed
NbSBA-15(2.2pH). After 45 min, the 2M1N-OH conversion
(
used in this reaction to find its catalytic activity. The effect of
this result is nearly the same as that of NbSBA-15(2.2pH) used
for the 4th run, as shown in Table 2. In a similar trend, the
slightly increases whereas the vitamin K yield as well as its
3
selectivity decreases due to the formation of C–C coupling pro-
5+
23,45
vitamin K yield also increases with increasing tetrahedral Nb
ducts with some amount of tars.
This result shows that the
3
species on the surface of inner silica pore walls of SBA-15.
After finding the results of four runs, hot-catalyst filtration
experiments were also carried out twice at 348 K during oxi-
dation of 2M1N-OH over NbSBA-15(2.2pH). In this case, the
filtrate solution gives a very low 2M1N-OH conversion (3.5%)
vitamin K can be affected at a longer reaction time. One can
3
conclude from the corresponding catalytic results that the suit-
able parameters like 348 K and 45 min promote the decompo-
sition of an intermediate, 2-methyl-1-naphthol peroxo-niobium
complex, which further reacts with H O for the selective for-
2
2
with a trace amount of vitamin K yield due to leaching of non-
mation of vitamin K with an excellent selectivity.
3
3
framework Nb O on the surface of catalyst, indicating that the
To find the best ratio of 2M1N-OH to H O over washed
2
5
2 2
oxidation of 2M1N-OH takes place on the surface of NbSBA-15
NbSBA-15(2.2pH) catalyst for the selective synthesis of vitamin
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a
Table 3 Oxidation of 2M1N-OH with different solvents
Solvent
7 ml)
2M1N-OH
conversion (%)
Vitamin K
yield (%)
3
Vitamin K
3
selectivity (%)
(
MeCN
Ac
AcOH
MeOH
EtOH
96.5
96.4
95.4
97.4
81.0
85.0
97.2
95.5
67.0
91.2
87.1
39.6
18.0
14.9
53.8
52.5
73.8
7.1
94.5
90.4
41.5
25.6
18.5
63.3
54.0
77.3
10.6
b
c
MeCN
MeCN
d
e
Ac
Solvent free
a
Reaction conditions: 4 mg of washed NbSBA-15(2.2pH), 1 : 5 ratio of
M1N-OH to H (0.05 mmol of 2M1N-OH and 0.25 mmol of H ),
reaction time = 45 min, temperature = 348 K; 328 K. 2 ml of solvent.
2
2
O
2
2 2
O
b
c
d
e
5
mmol of water. 318 K.
Fig. 2 Synthesis of vitamin K
conditions: 4 mg of washed NbSBA-15(2.2pH), 1 : 5 ratio of 2M1N-OH
to H (0.05 mmol of 2M1N-OH and 0.25 mmol of H ), reaction
temperature = 348 K, 7 ml of MeCN.
3
with different reaction times. Reaction
4
5
due to the formation of its cleavage products. The C–C dimer
2
O
2
2 2
O
products can be formed when the reaction is conducted with
23,24,27
uneven ratios of 2M1N-OH to H O .
In addition, the
2
2
somewhat active surfaces on the catalyst may be deactivated by
the C–C dimer products and unreacted substrates. On the basis
of the ratio results, it is manifestly found that 1 : 5 mmol ratio of
2
M1N-OH to H O is an optimum ratio for the highly selective
2 2
synthesis of vitamin K3.
The oxidation of 2M1N-OH over washed NbSBA-15(2.2pH)
catalyst has been carried out with different solvents like MeCN,
Ac, AcOH, MeOH and EtOH, using the reaction conditions
noted in Table 3. MeCN is a dipolar solvent that produces a
higher 2M1N-OH conversion as well as vitamin K selectivity
3
because of the formation of reactant complexes on the catalytic
surface for a long time in the liquid-phase catalytic oxidation. A
similar effect was obtained for Ac. However, the Ac solvent
involves in the formation acetone peroxide and produces a large
amount of undesired side products at a low temperature (Table 3)
43
with a decrease in vitamin K selectivity. Although MeOH,
3
EtOH and AcOH are green solvents, they give the low vitamin
K selectivity because of leading low catalytic activity on the
3
Fig. 3 Synthesis of vitamin K
3
with different ratios of 2M1N-OH to
surface of catalysts. When this reaction is carried out with 2 ml
of MeCN under similar reaction conditions, the 2M1N-OH con-
2 2
H O . Reaction conditions: 4 mg of washed NbSBA-15(2.2pH), reaction
temperature = 348 K, reaction time = 45 min, 7 ml of MeCN.
version and vitamin K selectivity decrease because the high
3
quantity of 2M1N-OH may not be homogeneously dissolved
and react completely with H O (Table 3). It is clearly found that
2
2
K , the oxidation of 2M1N-OH has been carried out with differ-
3
vitamin K selectivity dropped only with maximum 2M1N-OH
3
ent ratios of n2M1N-OH/n
using the reaction conditions noted
H O
2
2
conversion when the catalytic reaction is conducted with
addition of more quantity of water under similar condition
in Fig. 3. The 2M1N-OH conversion (96.5%) and vitamin K3
yield (91.2%) as well as its selectivity (94.5%) can be observed
(Table 3). This may be due to the formation of C–C coupling
in the ratio of 1 : 5. The vitamin K selectivity decreases in other
3
products. The vitamin K3 forms with ∼10.6% of selectivity
when the catalytic reaction was carried out without solvent under
similar reaction conditions. Overall, from the catalytic activity
compared with different solvents, it is obviously found that MeCN
is the best solvent for the selective synthesis of vitamin K3.
ratios in the following order 1 : 4 < 1 : 3 < 2 : 5 < 3 : 5. In
addition, the 2M1N-OH conversion and vitamin K selectivity
3
gradually decrease in the order of ratios, 1 : 5 < 1 : 4 < 1 : 3. But,
in the ratios of 2 : 5 and 3 : 5 the 2M1N-OH conversion only
increases whereas the vitamin K selectivity decreases. A poss-
3
ible reason is that the formation of C–C coupling products such
as dinaphthol and dinaphthoquinone as the byproducts can be
observed when the concentration of 2M1N-OH is increased. But,
the formation of menadione epoxide can be only observed when
the concentration of H O is increased. However, a trace amount
4
. Conclusions
The mesostructured NbSBA-15 catalysts have been successfully
used in the liquid-phase oxidation of 2M1N-OH with H O for
2
2
2 2
of menadione epoxide is observed with a very low selectivity
the synthesis of vitamin K . From the recycling studies, it is
3
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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found that NbSBA-15(2.2pH) has a higher catalytic activity in
the liquid-phase oxidation of 2M1N-OH as compared to other
NbSBA-15 catalysts. From the optimized reaction conditions
over washed NbSBA-15(2.2pH), it is obviously found that a
higher vitamin K3 selectivity can be obtained at 348 K for
17 S. K. Das, M. K. Bhunia and A. Bhaumik, J. Solid State Chem., 2010,
183, 1326.
1
1
8 Y.-J. Li, B. Yan and L. Wang, Dalton Trans., 2011, 40, 6722.
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4
5 min using 1 : 5 ratio of 2M1N-OH to H O with 7 ml of
2 2
2
MeCN. From the studies of different solvents, it is found that
MeCN is the optimal solvent. On the basis of all catalytic
studies, it can be clearly observed that the NbSBA-15(2.2pH)
catalyst is a highly active, recyclable and promising hetero-
geneous catalyst for the selective synthesis of vitamin K3.
6
8
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Acknowledgements
This work was supported by a 2-year Research Grant of Pusan
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