Active catalysts prepared using a vanadium-containing oligosilsesquioxane for selective photo-assisted oxidation of methane into methanal

Article abstract of DOI:10.1039/a707173f

Catalysts of characteristic pore structure derived from silica-supported vanadium-containing silsesquioxane show excellent activity towards the selective photo-assisted catalytic oxidation of methane into methanal (at 493 K, 139 μmol hr^-1, TON ca. 9h^-1, selectivity 81%), indicating its potent abilities as an excellent precursor for porous oxide catalysts.

Full text of DOI:10.1039/a707173f

View Article Online / Journal Homepage / Table of Contents for this issue
Active catalysts prepared using a vanadium-containing oligosilsesquioxane for
selective photo-assisted oxidation of methane into methanal
K. Wada, M. Nakashita, A. Yamamoto and T. Mitsudo*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01,
Japan
Catalysts of characteristic pore structure derived from silica-
supported vanadium-containing silsesquioxane show ex-
cellent activity towards the selective photo-assisted catalytic
homogeneous analog of silica-supported catalysts.7 Here, in
order to investigate the possibility of 1b as a precursor for
heterogeneous catalysts, we prepared supported 1b-based
catalysts, and found that thermal treatment of the catalysts
induces high activities towards the selective photo-assisted
catalytic oxidation of methane into methanal.
oxidation of methane into methanal (at 493 K, 139 mmol h21
,
TON ca. 9 h21, selectivity 81%), indicating its potent
abilities as an excellent precursor for porous oxide cata-
lysts.
Preparation of a vanadium-containing silsesquioxane 1b was
based on the procedure reported by Feher et al.5a The reaction
of vanadyl triisopropoxide with incompletely condensed sil-
sesquioxane 2b, which was prepared by the hydrolytic con-
densation of cyclopentyltrichlorosilane, gave a white powder of
dimer 3b in 44% yield.5a,7 The resultant product exists as a
monomer 1b in organic solutions at above room temperature. 1b
shows an absorption at ca. 252 nm in cyclohexane solution
(e = 4840 dm3 mol21 cm21). The silica-supported catalysts
were prepared by incipient wetness impregnation from a
pentane solution of 1b using silica (Alfa, evacuated at 423 K for
2 h, mass ratio 1b:silica = 1:3, V 1.9 mol%). The vacuum
dried catalyst was designated A. We also prepared catalysts B
and C by air-flow treatment (20 cm3 min21) for 2 h at 523 and
723 K, respectively. It should be noted that heat treatment of 3b
without silica support often produced non-porous glassy
materials.
The reaction was performed by using an upstream flow fixed-
bed reactor made of silica glass. The catalyst was mounted in a
flat cell (10 3 20 mm, inner thickness 1.5 mm), and covered by
another silica glass tube. UV irradiation was performed from the
outer side of the reactor using a 200 W high-pressure mercury
lamp with a water filter. Details of the reaction apparatus were
described elsewhere.2 Chemical actinometry using iron(iii)
ammonium oxalate revealed that the number of photons
irradiated into the catalyst bed is 6.631027 einstein s21
(250–500 nm, 1.631027 einstein s21 for 250–300 nm).
Liquid and gaseous products were collected in cold traps and
gas sampling bags, respectively, followed by analysis using gas
chromatography, GC–MS and 1H NMR spectroscopy.
Supported vanadium oxide catalysts are very important in the
chemical industry, especially for the selective oxidation of
hydrocarbons.1 We have reported the excellent activities of
silica-supported vanadium oxide catalysts towards the selective
photo-assisted catalytic oxidation of methane, ethane and
propane at relatively high temperature into the corresponding
aldehydes.2 Only highly dispersed, isolated tetrahedral VO4
species were considered to be active species, and the reactions
were quite sensitive to the state of the surface. However, strict
control of the surface structure is generally quite difficult.3 Thus
the development of novel methods for the preparation of
catalysts which have well ordered surface species is desirable.
On the other hand, oligometallasilsesquioxanes such as
compound 1a have attracted attention from the viewpoint of
R
R
O
O
OH
OH
V
Si
Si
O
O
O
O
O
O
O
O
O
OH
R
R
R
R
R
R
Si
Si
Si
Si
O
O
Si
Si
Si
Si
R
R
R
R
O
O
O
O
O
O
O
Si
Si
Si
O
Si
O
R
R
1a (R = c-C6H11
)
2b (R = c-C5H9)
b (R = c-C5H9)
R
R
R
Si
O
V
O
O
O
R
Si
O R
Si
Si
O
O
O
Silica-supported vanadium-containing silsesquioxane-based
catalysts showed fair to excellent activities for photooxidation
of methane into methanal [eqn. (1); see Table 1] and results are
shown in Table 1. Preliminary experiments showed that catalyst
O
O
O R
O
Si
Si
O
O
Si
R
Si
Si
O
O
O
R
O
Si
R
O
Si
O
O
R
O
V
O
O
Si
O
Si
O
R
Si
Table 1 Photo-assisted catalytic oxidation of methanea
R
R
R
3b (R = c-C5H9)
hn
CH4
+
O2
(1)
HCHO
+
H2O
493 K, [cat.]
well defined, homogeneous models for surface active sites of
supported catalysts or metal-containing zeolites.4 The catalytic
activities of these oligometallasilsesquioxanes, however, have
been mainly focused on alkene polymerization and metathesis.
Their activities for other reactions were not well investigated.5
It should be noted that the activity of a titanium-containing
silsesquioxane towards the epoxidation of alkenes was recently
reported.6
From the background described above, we have reported the
fair to excellent activity of a vanadium-containing silses-
quioxane 1b, a cyclopentyl derivative of 1a, for the photooxi-
dation of benzene and cyclohexane, indicating its ability as a
Yield/mmol h21
Run
Catalyst
HCHO
CO
CO2
1
2
3
4
None
B
C
0
7.2
139
61
0
9
0
18
6
26
13
b
V2O5/SiO2
7
a
Typical reaction conditions: amount of catalyst 50 mg, molar ratio
methane:O2 :He = 12:1: 11, total gas flow rate 20 cm3 min21, reaction
temperature 493 K, reaction time 1 h.
Chem. Commun., 1998
133

View Article Online
200
150
100
50
A afforded very small amounts of the products. The result
without catalyst shown in run 1 exhibits the absence of the
photo-chemical reaction of methane. Control experiments
without alkanes afforded none of the products, eliminating the
contribution by the oxidation of organic groups of the
silsesquioxane moiety. Reaction using silica support also did
not afford the products.
Silica
A
B
C
The reaction using catalyst C, calcined in air at 723 K,
produced methanal in a yield of 139 mmol h21 (0.56% of fed
methane) with the selectivity of > 80%. Turnover frequency for
the formation of methanal was ca. 9 h21. Only a trace amount
of methanal was formed by the reaction at ca. 300 K, indicating
the necessity of a reaction temperature as high as 493 K.
Reaction without UV irradiation did not proceed at all. For
comparison, we examined the performance of a silica-supported
vanadium oxide catalyst prepared by evaporation-to-dryness
impregnation using ammonium metavanadate, which showed
the highest activity in the previous study.2 However, it gave
only 61 mmol h21 of methanal under the same reaction
conditions, indicating the excellence of catalyst C. It should be
noted that catalyst B including organic substituents (vide infra)
also produced methanal, but the yield and the selectivity were
low. This probably indicates the difference of the activity
between the surface vanadium species on silica and that on the
silsesquioxane moiety.
V
0
1
5
Pore radius / nm
Fig. 1 Pore size distribution of silica and supported catalysts estimated by
the Dollimore–Heal method (desorption, integral data)
the possibility of oligometallasilsesquioxanes as excellent
precursors for porous oxide catalysts. We are now investigating
not only the detailed structure, but also the shape selectivity of
the supported catalysts.
Then we investigated various properties of the supported
catalysts. The XPS spectra of the catalysts A and B showed
almost the same ratio of C 1s, corresponding to organic
substituents of the silsesquioxane moiety, to Si 2p, indicating
the absence of oxidation of organic substituents by treatment at
523 K in air. On the other hand, catalyst C was found to consist
of oxides, since only trace amounts of carbonaceous materials
were detected on the surface of catalyst C. While the BET
surface area of original silica support was 259 m2 g21, those of
catalyst A (180 m2 g21) and B (171 m2 g21) were smaller. On
the other hand, catalyst C had a larger surface area (349 m2
g21). Fig. 1 shows the pore size distribution of the catalysts
estimated from the desorption isotherm of nitrogen at 77 K by
Dollimore–Heal method. It should be noted that the catalyst C
was more rich in mesopores around 30 Å than the parent silica
support. These characteristic features indicate the possibility of
oligometalla-silsesquioxanes as novel precursors for porous
oxide materials.
Footnote and References
* E-mail: mitsudo@scl.kyoto-u.ac.jp
1 G. I. Golodets, Heterogeneous Catalytic Reactions Involving Molecular
Oxygen, Elsevier, New York, 1983.
2 K. Wada and Y. Watanabe, in Methane and Alkane Conversion
Chemistry, ed. M. M. Bhasin and D. W. Slocum, Plenum, New York,
1995, p. 179 and references therein.
3 Preparation of Catalysts III, ed. G. Poncelet, P. Grange and P. A. Jacobs,
Elsevier, Amsterdam, 1983.
4 J. F. Brown, Jr. and L. H. Vogt, J. Am. Chem. Soc., 1965, 87, 4313;
F. J. Feher and T. A. Budzichowski, Polyhedron, 1995, 14, 3239;
R. Murugavel, A. Voigt, M. G. Walawalker and H. W. Roesky, Chem.
Rev., 1996, 26, 2205.
5 (a) F. J. Feher and J. F. Walker, Inorg. Chem., 1991, 30, 689; (b)
F. J. Feher, J. F. Walzer and R. L. Blanski, J. Am. Chem. Soc., 1991, 113,
3618; (c) W. A. Herrmann, R. Anwander, V. Dufaud and W. Scherer,
Angew. Chem., Int. Ed. Engl., 1994, 33, 1285.
6 H. Abbenhuis, S. Krijnen and R. van Santen, Chem. Commun., 1997,
331.
In conclusion, we have shown fair to excellent activities of
the heterogeneous catalysts prepared using a vanadium-
containing silsesquioxane 1b for the selective photo-assisted
oxidation of methane into methanal. The present results indicate
7 K. Wada, M. Nakashita, A. Yamamoto, H. Wada and T. Mitsudo, Chem.
Lett., in press.
Received in Cambridge, UK, 6th October 1997; 7/07173F
134
Chem. Commun., 1998