Liquid phase oxidation of 2-methylnaphthalene to 2-methyl-1,4- naphthoquinone over lanthanum doped MCM-41

Article abstract of DOI:10.1016/j.catcom.2014.01.027

Liquid phase oxidation of 2-methylnaphthalene was carried out under mild reaction conditions over lanthanum doped MCM-41 using aqueous hydrogen peroxide (30%) as oxidant and acetic acid as solvent without adding any initiator. The catalyst exhibited very high substrate conversion (95.8%) and reasonable product (2-methyl-1,4-naphthoquinone) selectivity (69.3%) under mild condition. Fast hot catalyst filtration experiment proved that the catalyst acted as a heterogeneous one and it can be reused two times without losing its activity to a greater extent. A possible mechanism was proposed.

Full text of DOI:10.1016/j.catcom.2014.01.027

Catalysis Communications 49 (2014) 10–14
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Liquid phase oxidation of 2-methylnaphthalene to
-methyl-1,4-naphthoquinone over lanthanum doped MCM-41
2
Guoli Zi, Daomei Chen, Bin Li, Zhiqiang Li, Xia Luo, Jinping Zhang, Lin Li, Jiaqiang Wang ⁎
The Universities' Center for Photocatalytic Treatment of Pollutants in Yunnan Province, School of Chemical Sciences & Technology, Yunnan University, Kunming 650091, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2013
Received in revised form 22 January 2014
Accepted 27 January 2014
Available online 31 January 2014
Liquid phase oxidation of 2-methylnaphthalene was carried out under mild reaction conditions over lanthanum
doped MCM-41 using aqueous hydrogen peroxide (30%) as oxidant and acetic acid as solvent without adding any
initiator. The catalyst exhibited very high substrate conversion (95.8%) and reasonable product (2-methyl-1,4-
naphthoquinone) selectivity (69.3%) under mild condition. Fast hot catalyst filtration experiment proved that
the catalyst acted as a heterogeneous one and it can be reused two times without losing its activity to a greater
extent. A possible mechanism was proposed.
Keywords:
©
2014 Elsevier B.V. All rights reserved.
Lanthanum doped MCM-41
Selective oxidation of 2-methylnaphthalene
2-Methyl-1,4-naphthoquinone
1
. Introduction
The well-ordered mesoporous materials, particularly, incorporated
with metal ions, have a good use in catalysis for their unusual properties:
monodispersed, single-pore distributed, huge pore volume and highly
ordered mesoporous structures [22,23]. For example, in our group a se-
ries of mesoporous materials have been used as heterogeneous catalysts
in selective oxidation of a number of substrates [23]. Interestingly, a few
microporous and mesoporous materials including TS-1, Ti-Beta, Fe-Beta,
and Ti-MCM-41, have been also used as catalysts in 2-MN oxidation.
However, both the conversion rate (10–28%) and the 2-MNQ selectivity
(35–63%) are low [24].
Lanthanum is one of the rare earth elements which is considered to be
pollution-free to environment [25]. Thus, lanthanum doped mesoporous
materials, such as La/MCM-48 [26], LaMn/MCM-41 [27], and CsLa/
MCM-41 [28], have been employed as catalysts in selective oxidations.
However, so far there is no report on using La-doped MCM-41 material
(La/MCM-41) as catalyst for catalytic oxidation of 2-methylnaphthalene
to 2-methyl-1, 4-naphthoquinone as we know.
It is well known that quinone derivatives play an important role in
biosystems and are useful intermediates and products of fine chemistry
1–3]. Vitamin K3 (2-methyl-1,4-naphthoquinone, 2-MNQ) is attracting
[
more attention because of its interesting pharmacological activities [4,5],
more anti-bleeding activity than Vitamin K1 and Vitamin K2 [6,7] and is
a key intermediate in the preparation of the other vitamins of group K,
and is of particular importance. Many methods have been described to
bring about the controlled oxidation of 2-methylnaphthalene using vari-
ous oxidizing agents in the presence of catalyst [6,8–14]. Traditionally,
2-MNQ is produced on industrial scale via stoichiometric oxidation of
2-methylnaphtalene (2-MN) by CrO in sulfuric acid with a yield of
40–60% [1,15]. However, this process is often cited as a typical example
3
of a ‘dirty’ fine chemical industry process leading to a huge amount of
waste [16,17]. Furthermore, several homogeneous catalytic methods in-
cluding methyltrioxorhenium [18–20] and metalloporphyrin catalysts
[
(
21] have been developed for 2MN oxidation to obtain vitamin K3
VK3). The CH ReO catalyzed oxidation of 2-methylnaphthalene,
using 85% H solution that gave a 46% yield of VK3 was reported
18,19]. However, this reaction is rather hazardous, as concentrated
In this investigation, La/MCM-41 was synthesized and used for the
oxidation of 2-methylnaphthalene under mild condition. The effects of
temperature, reaction time and solvents were also studied. The reusability
and fast hot catalyst filtration experiments were conducted.
3
3
2 2
O
[
hydrogen peroxide is potentially explosive. And these homogeneous
media have several disadvantages owing to the typical problems of sep-
aration of products from the catalyst and inevitably leading to contam-
ination of the product with traces of hazardous transition metals.
Therefore, the development of cleaner catalytic methods for the synthesis
of Vitamin K3 is a challenging goal. Especially, it is of great practical
interest to develop a more efficient, easily separable, reusable, and
environment-friendly mesoporous catalyst for the oxidation process.
2. Experimental
2.1. Synthesis of catalysts
La/MCM-41 with Si/La ratio of 50, synthesized by a modified process
as described in the literatures is [23] as follows: 5.0 g cetyltrimeth-
ylammonium bromide (CTAB) was added to 66 mL solution containing
1
.0 g sodium hydroxide. Then, 20 mL tetraethyl orthosilicate (TEOS)
was added to it slowly until the solution became a clear gel. Subsequently
the aqueous solution of Lanthanum nitrate hydrous was introduced
⁎
Corresponding author. Tel./fax: +86 871 65031567.
E-mail address: jqwang@ynu.edu.cn (J. Wang).
1566-7367/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2014.01.027

G. Zi et al. / Catalysis Communications 49 (2014) 10–14
11
dropwise. The final mixture was stirred for 2 h and then transferred into a
Teflon bottle and treated under autogenous pressure without stirring at
wavenumber decreases with incorporation of lanthanum in the structure.
−
1
The wavenumber of this band decreases from 1090 cm
(for the
−
1
363 K for 7 days, filted, washed, dried and calcined at 823 K in air for 24 h.
MCM-41 sample) to 1076 cm (for La/MCM-41 sample). In general,
this shift toward the lower wavenumber is considered an indication of
the incorporation of La into the framework of MCM-41 [32]. In the
2
.2. Oxidation of 2-methylnaphthalene
−
1
hydroxyl region (3000–4000 cm ), the broad band is observed at ca.
−
1
−1
The oxidation reactions were carried out at the atmospheric pressure
3421 cm
for La/MCM-41. The band shift from 3433 to 3421 cm
as follows: the catalyst (40 mg), 2-methylnaphthalene (500 mg) and
5 mL of solvent (acetic acid, methanol, acetic ether, acetone, acetonitrile)
were used as received without further purification and added succes-
sively into a temperature controlled, round bottom, three-necked-
flask having a reflux condenser. 6 mL the mixture of 30% aqueous
for La/MCM-41 compared with MCM-41 is probably attributed to the
presence of more defect sites (SiOH group). This band was assigned to
silanol group vibrations situated inside the channels of MCM-41,
resulting from silanol groups interacting via hydrogen bonding.
1
−
1
Furthermore, the broad bands are observed at 1630 cm
MCM-41 and La-MCM-41.
for both
2 2
H O and acetic acid (1:1, V/V) was added dropwise after the reaction
mixture heated to the set temperature. Reaction mixture was filtered
under reduced pressure after the set time. The products were filtered
and analyzed by high-performance liquid chromatography (HPLC)
3.2. Catalytic performance
(
2
Agilent 1100 series) with a packed column (Kromasil ODS C18
50 × 4.6 mm 5 μm) and an ultraviolet detector (diode array detector).
The detector wavelength λ = 265 nm and temperature of column was
13.15 K. The mobile phase was CH OH (chromatographically pure):
O = 70:30 (V/V) and the flow rate was 1 mL min . Reference sub-
stances were used for the identification of the products. The efficiency
of reacted H for the production of 2-methyl-1,4-naphthoquinone
2 2
The oxidation of 2-methylnaphthalene with the aqueous H O
over different catalysts is summarized in Table 1. The catalysts are
compared under similar conditions and it is found that 2-methyl-1,
4-naphthoquinone is obtained as the main products, and only small
amount of other products, such as, 2-naphthoic acid and 6-methyl-1,
4-naphthoquinone, are detected for the oxidation of 2-MN. For compar-
ison, we have also studied the oxidation of 2-methylnaphthalene using
MCM-41, Co/MCM-41, Cu/MCM-41, and Ce/MCM-41 as a catalyst under
the same conditions with those used for La/MCM-41. It is apparent that
La/MCM-41 exhibits highest conversion and TOF. This indicates that the
active component was lanthanum. However the factors such as the dif-
ference between the dopant ions, number of active sites, can also mod-
ify the efficiency. Therefore, the advantage of La/MCM-41 over those
metal ions doped mesoporous materials is obvious. Due to the good
performance of La/MCM-41, in the following we concentrate on the
study of the influence of various parameters on the 2-MN conversion
and selectivity of 2-MNQ over La/MCM-41. Besides, as shown in
Table 1, the trend of the conversion of 2-MN does not always agree
with selectivity of 2-MNQ. Therefore, both conversion of 2-MN and
selectivity of 2-MNQ should be taken into account to evaluate the opti-
mum yield.
3
H
3
−
1
2
2 2
O
was detected by iodometric titration method at the optimum reaction
condition [29].
2
.3. Characterizations
Experimental details for the X-ray diffraction (XRD), surface area
measurements, high-resolution transmission electron microscopy
HRTEM), and Fourier transforms infrared (FI-IR) spectroscopy were
described in detail in Supplementary data.
(
3
. Results and discussion
3
.1. The characterization of La/MCM-41
Fig. S1 in Supplementary data shows the N
2
adsorption/desorption
isotherm and BJH pore-size distribution plots (inset) of La/MCM-41.
Typical IV adsorption/desorption isotherm is observed which indicates
the mesoporous structure of prepared material. La/MCM-41 is white in
color with BET surface area of 1047.4 m /g, pore volume of 0.85 cm /g
and average pore diameter of 2.9 nm. The BJH pore-size distribution of
La/MCM-41 indicates that the catalysts have regular mesoporous
channels.
3.3. Effect of solvents
Table 2 depictes the effects of various solvents on the catalytic oxida-
tion. It is known that the nature of solvents has a major influence on
reaction kinetics and product selectivity in the oxidation of 2-MN. It
shows that a significantly lower activity is obtained in the case of
methanol, acetic ether, acetone and acetonitrile, respectively. It
seems that acetic acid with the strongest polarity has highest yield
though acetonitrile also with strong polarity has very weak yield.
2
3
From Fig. S2 it is seen that the low-angle powder X-ray diffraction
(
XRD) patterns of the sample La/MCM-41 at 1–8° shows several diffrac-
tion lines in the low 2θ region. It is clearly indicating that the sample has
a regular mesoporous structure which is in good agreement with
reported literatures [30]. This is supported by the high BET surface
This is due to possible partial decomposition of H
2 2
O because it was
reported that the decomposition of H was faster in these solvents
2 2
O
than in acetic acid [33]. Moreover, it has been reported that acetic
acid does not only act as a solvent, but also serve as a good oxidizing
agent because of the formation of the framework titanium–peracetic
2
areas (1047 m /g). Interestingly, compared with the XRD patterns of
La
lanthanum oxides are observed in the wide-angle X-ray diffractograms
Fig. S2, inset). This indicates that our lanthanum species are well incor-
2 3
O [31], the sample is amorphous and no peaks corresponding to
(
Table 1
porated into the MCM-41 framework.
a
Comparison of catalytic activities of different catalysts for the oxidation of 2-MN.
High resolution transmission electron microscope image of La/
MCM-41 is shown in Fig. S3. It shows that many residual pores are dis-
tributed between layer and layer, indicating the presence of an ex-
tremely complex hierarchical pore network in La/MCM-41 and it can
be direct evidence for the existence of mesopores channels, which is
in agreement with the mesoporous structure of prepared material char-
Catalyst
Conversion of
-MN (wt.%)
Selectivity of
2-MNQ (wt.%)
TOF (h−1)
2
MCM-41
89.4
95.8
84.2
75.9
80.3
59.3
69.3
50.4
46.0
35.1
57.0
66.1
56.7
52.4
54.0
La-MCM-41
Cu-MCM-41
Ce-MCM-41
Co-MCM-41
2
acterized by low-angle XRD and typical IV N adsorption/desorption
isotherm.
a
Note: Reaction condition: substrate, 500 mg 2-methylnaphthalene; reaction time,
h; reaction temperature, 338 K; catalyst, 40 mg; solvent, acetic acid; TOF, turn
The FT-IR spectra of La/MCM-41 are recorded between 500 and
4
−
1
4
000 cm in transmission mode and shown in Fig. S4. The vibration
over number (millimole of oxidized products per millimole of metal in the catalyst per
hour (h )).
−
1
−1
band at ca. 1090 cm is assigned to νas(Si–O–Si) of MCM-41 and its

12
G. Zi et al. / Catalysis Communications 49 (2014) 10–14
1
00
80
60
70
Table 2
Effects of solvent on the oxidation of 2-methylnaphthalene.
a
6
5
4
3
2
0
0
0
0
0
Solvent
Conversion of
-MN (wt.%)
Selectivity of
2-MNQ (wt.%)
TOF (h−1)
2
acetic acid
methanol
acetic ether
acetone
95.8
75.4
45.0
51.2
25.6
69.3
59.1
47.3
12.7
15.9
66.1
52.0
31.0
35.3
17.7
acetonitrile
4
0
a
Note: Reaction condition: substrate, 500 mg g 2-methylnaphthalene; reaction time,
h; reaction temperature, 338 K; catalyst, 40 mg; solvent, 15 ml; TOF, turn over
Conversion of 2-MN
Selectivity of 2-MNQ
4
number (millimole of oxidized products per millimole of metal in the catalyst per hour
20
0
−
1
(h
)).
10
0
0
2
4
6
8
10
2 2
acid complex when it reacts with H O in the presence of titanium-
Time (h)
containing zeolites (TS-1) [34]. Similarly, if in our system a complex
with peracetic acid was possibly formed in the pores of La/MCM-41
then it would be relatively more stable and hydrophobic, as com-
pared with hydrogen peroxide. In general, it is considered that acetic
acid does not only facilitate homogeneity of the liquid phase, but also
be responsible for the increase in overall oxidation activity. In short,
the synergistic effects among doped lanthanum, mesoporous frame-
Fig. 2. Effect of reaction time on the conversion and selectivity over La-MCM-41. Reaction
condition: reaction temperature, 338 K; catalyst, 40 mg; solvent, acetic acid.
resulted in slightly decrease in the conversion of the 2-MN and selectivity
of 2-MNQ. Considering the effect of time on both conversion of the 2-MN
and selectivity of 2-MNQ, the yield of 2-MNQ reached the maximum at
4 h. Therefore, the optimum conversion and selectivity could be achieved
at about 4 h.
work of MCM-41, acetic acid and H
2 2
O make La/MCM-41 an effective
catalyst for the oxidation of 2-MN under mild conditions.
3
.4. Effect of reaction temperature
3.6. Oxidation of 2-MN with different amount of catalysts
As shown in Fig. 1, the catalytic performances are affected by reac-
tion temperature. It is clear that both conversion of 2-MN and selectivity
of 2-MNQ increase as the temperature rose and passed through a max-
imum at 338 K. The optimum temperature is at about 338 K, at lower
temperatures, the reaction is too slow, at higher temperatures, a series
of side reactions starts to dominate and resulted in significant decrease
in the selectivity of 2-MNQ. This was probably owing to a quicker decom-
position of H O at higher temperature which resulted in a relatively low
2 2
activity. Similar phenomenon has also been reported [35]. Considering
the effect of temperature on both conversion of 2-MN and selectivity of
Fig. 3 presents the effect of catalyst concentration on the reaction
over La/MCM-41. An initial steep increase in the conversion of 2-MN
and selectivity of 2-MNQ was observed when the amount of the catalyst
was increased up to 40 mg. Beyond this amount, the selectivity of
2-MNQ increased a little initially while decreased finally and the
conversion of 2-MN decreased, these results indicate that in the
system catalyzed by La/MCM-41 only small amount of catalyst is active
in the oxidation of 2-MN. This could be attributed to the competent
interaction of metal oxo-species in La/MCM-41 matrix with 2-
methylnaphthalene [36], thus inhibiting the catalytic reaction. And it
is also probable that large catalyst dosages provide excess active sites
in the system that accelerate side reactions. Therefore, 40 mg was
selected as the suitable amount of the catalyst for oxidizing 2-MN.
2
-MNQ, 338 K is chosen as the suitable temperature for the oxidation
of 2-MN.
3
.5. Influence of reaction time
As the reaction time was increased, the conversion of the 2-MN and
3.7. Oxidation of 2-MN with different amount of oxidant
selectivity of 2-MNQ increased, respectively (Fig. 2). It is seen that the
conversion of the 2-MN increased with time up to 6 h while the selectivity
passed through a maximum at 4 h, a further increase in the reaction time
The effect of the volume of oxidant on the reaction was also investi-
gated (Fig. 4). It is clear that the conversion of 2-MN and selectivity of 2-
1
00
80
60
40
20
0
100
100
80
60
40
20
0
80
60
40
20
0
7
5
2
5
0
5
0
Conversion of 2-MN
Selectivity of 2-MNQ
Conversion of 2-MN
Selectivity of 2-MNQ
308
318
328
338
348
358
368
10
20
30
40
50
60
Temperature (K)
Catalyst (mg)
Fig. 1. Effect of temperature on the conversion and selectivities over La/MCM-41. Reaction
Fig. 3. Effect of catalyst concentration on the conversion and selectivity over La-MCM-41.
conditions: reaction time, 4 h; catalyst, 40 mg; solvent, acetic acid.
Reaction condition: reaction temperature, 338 K; reaction time, 4 h; solvent, acetic acid.

G. Zi et al. / Catalysis Communications 49 (2014) 10–14
13
1
00
90
condition. Meanwhile, the catalytic activity over MCM-41 with different
amounts of La was evaluated. It indicates that the suitable ratio of Si/La
is 50 (Fig. S5) though the La loading has a weak effect on the catalytic
activity.
7
6
4
3
1
0
5
0
5
0
5
80
60
40
20
0
3.8. The reusability and fast hot catalyst filtration experiment
The stability and recyclability of the catalyst were studied by running
the reaction successively (under the optimum reaction conditions) over
two recycles as shown in Table 3. The typical recycling procedure was as
follows: after the initial reaction, the catalyst was separated from the re-
action mixture and washed with acetone and dried at 363 K, followed
by the activation at 723 K for 2 h. The catalyst La/MCM-41 showed
excellent reusability in the oxidation reactions. In three successive trials
the conversion did not show any significant change. The present study
indicates that the catalyst can be recycled a number of times with
almost the same activity. All the observations suggest that La/MCM-41
is a stable and highly effective catalyst for the oxidation of 2-MN.
In order to prove whether the catalyst is a heterogeneous one, ex-
periments with fast hot catalyst filtration and studying the reactivity
of the filtrate had been done by a modified process as Ref. [37]. We
found that after hot filtration the filtrate reacted little further comparing
that observed when the catalyst was not filtered. This observation was
also well supported by ICP-AES analysis of the filtrates obtained from
the fast hot catalyst filtration where negligible amount of leaching of
active lanthanum species (≤0.003 wt.%) was detected. These indicated
that the active component (lanthanum) did not leach to the solution
and the La/MCM-41 did act as a heterogeneous catalyst.
Conversion of 2-MN
Selectivity of 2-MNQ
2
4
6
8
10
12
Concentration of oxidant (mL)
Fig. 4. Effect of oxidant concentration on the conversion and selectivity over La-MCM-41.
Reaction condition: reaction temperature, 338 K; reaction time, 4 h; solvent, acetic acid.
Table 3
Recycled experiment of catalytic efficiency.
TOF (h−1)
Catalyst
Conversion of
-MN (wt.%)
Selectivity of
2-MNQ (wt.%)
2
La-MCM-41
First recycled
Second recycled
95.8
94.6
92.5
69.3
65.4
59.3
66.1
65.3
63.8
Note: Reaction condition: reaction time, 4 h; reaction temperature, 338 K; catalyst, 40 mg;
solvent, acetic acid.
Based on the above experimental results, a possible reaction pathway
for the oxidation of 2-MN over La/MCM-41 catalyst is depicted in
Scheme 1. The first step involves the rapid reaction between the acetic
acid and H O yielding the formation of peroxy acetic acid a, which
2 2
further reacts with lanthanum species b to produce lanthanum
peroxospecies c and d. The latter interacts with 2-MN to produce e and
MNQ were found to increase with increase in volume of oxidant and
passed through a maximum at 6 mL. Therefore, 6 mL oxidant was selected
as the suitable amount of the oxidant for oxidizing 2-MN. The efficiency of
2 2
H O for the production of 2-MNQ was 20.6% under optimum reaction
Scheme 1. A possible reaction pathway for the oxidation of 2-MN over La/MCM-41 catalyst.

14
G. Zi et al. / Catalysis Communications 49 (2014) 10–14
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Appendix A. Supplementary data
1
23–128.
Supplementary data to this article can be found online at http://dx.
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