Mo-containing SBA-1 mesoporous molecular sieves as catalysts for partial oxidation of methane

Article abstract of DOI:10.1246/cl.2000.794

Mo-Containing SBA-1 mesoporous molecular sieves synthesized under strongly acidic conditions exhibited catalytic performance, superior to that of Mo-impregnated pure silica SBA-1 or silica gel, for the partial oxidation of methane with oxygen as an oxidant.

Full text of DOI:10.1246/cl.2000.794

794
Chemistry Letters 2000
Mo-Containing SBA-1 Mesoporous Molecular Sieves as Catalysts
for Partial Oxidation of Methane
Lian-Xin Dai,* Yong-Hong Teng, Kenji Tabata, Eiji Suzuki, and Takashi Tatsumi†
Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292
†Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai,
Hodogaya-ku, Yokohama 240-8501
(Received April 14, 2000; CL-000358)
Mo-Containing SBA-1 mesoporous molecular sieves synthe-
sized under strongly acidic conditions exhibited catalytic per-
formance, superior to that of Mo-impregnated pure silica SBA-1
or silica gel, for the partial oxidation of methane with oxygen as
an oxidant.
Since the discovery of ordered mesoporous molecular sieves
with uniform pore sizes larger than 20 Å, much research work
has been devoted to this new class of mesoporous materials,
denoted M41S.1,2 Recently, Huo et al. reported the synthesis of
a novel mesoporous molecular sieve denoted SBA-1 (Pm3n
cubic) under strongly acidic synthesis conditions.3,4 From the
point of view of catalytic application, the synthesis of three-
dimensional mesoporous materials such as SBA-1 containing
transition-metal has been collecting much attention.
tion method by using (NH4)6Mo7O24·4H2O as the Mo source,
then dried at 393 K overnight and calcined at 903 K for 4 h. The
partial oxidation of methane was carried out using a fixed-bed
quartz reactor. The products were analyzed by an on-line gas
chromatograph connected to a methanator.
The direct partial oxidation of methane to oxygenates has
been focused in connection with the chemical utilization of natu-
ral gas. Among the oxidation catalysts, silica-supported MoO3
catalyst has been found to be the most active and selective for
oxygenates production.5 It is conceivable that the large surface
(> 1000 m2 g–1) of cubic mesoporous materials makes it possible
to create highly dispersed, i.e., much more active catalytic sites
by incorporating transition-metal ion Mo into its silica-based
frameworks. The large pore (> 20 Å) with a 3-D channel system
makes it easy to discharge the produced oxygenates to the outside
of pore, preventing the deep oxidation. In the present study Mo-
containing SBA-1 with cubic structure is synthesized and used as
a catalyst for the partial oxidation of methane.
Mo-Containing mesoporous materials, Mo-SBA-1, were
synthesized under acidic conditions using cetyltriethylammonium
bromide (CTEABr) as a surfactant, TEOS as a silica source, and
(NH4)6Mo7O24·4H2O as a Mo source in an aqueous solution of
HCl. The surfactant CTEABr was synthesized by the reaction of
cetyl bromide with an equimolar amount of triethylamine in ace-
tone solution under reflux conditions. A typical synthesis was
conducted as follows: CTEABr, HCl (35%) and distilled water
were combined to obtain a homogeneous solution, which was
stirred for 30 min and cooled at 273 K. TEOS and Mo precur-
sor solution, which were precooled to 273 K, were added to the
above mixture under vigorous stirring. After stirred for 5 min,
the mixture was allowed to react at 273 K under static condition
for 6 h. The molar composition of the gel was as follows: 1
TEOS/0.13 CTEABr/(5–10) HCl/125 H2O/0.2 Mo. The precipi-
tate was filtered, dried (without washing) at 393 K overnight and
then calcined in air at 903 K for 4 h. Siliceous SBA-1 was syn-
thesized with the same procedure (HCl/TEOS = 10) except that
no Mo was added. Supported Mo catalysts on SBA-1 (Mo/SBA-
1) and amorphous silica (Mo/SiO2) were prepared by impregna-
Figure 1 illustrates the XRD patterns of various samples pre-
pared by different methods. Table 1 summarizes the preparation
conditions and physical properties of the Mo-containing SBA-1
mesoporous materials, together with the data of Mo/SiO2 for
comparison. As displayed in Figure 1, all the samples showed
the typical characteristic XRD patterns of the SBA-1 cubic phase
which can be indexed to the Pm3n space group.3,4 Morey et al.6
reported the incorporation of Mo into SBA-1 using MoCl5 (aq) as
the metal atom source with the Si/Mo ratio of 16 in the starting
gel. They found that only 0.4 at.% Mo was incorporated due to
the high solubility of Mo species under strongly acidic condi-
tions. In our study, however, a significantly large amount of Mo
was successfully substituted into the SBA-1 by adding
(NH4)6Mo7O24·4H2O to the gel with the following mole ratios: 1
TEOS/0.13 CTEABr/(5–10) HCl/125 H2O/0.2 Mo. Interestingly,
although the Mo species would be easily soluble in the concen-
trated aqueous solution of HCl, Mo incorporation increased
monotonously with the concentration of HCl in the starting gel
(HCl/TEOS = 5 → 10) (Table 1), whereas no significant change
in the structural regularity of Mo-SBA-1 was observed (Figure
1b–d). For HCl/TEOS = 10, the Mo-SBA-1 containing up to 8.8
wt% Mo (Si/Mo = 16) was formed while maintaining a fairly
well-ordered cubic structure (Figure 1d). Therefore, the differ-
6−
ent Mo precursors (Mo5+ cation/Mo7O24 anion) resulted in a
remarkable difference in the amount of Mo incorporation; the
anionic Mo species favored the incorporation of Mo into the
SBA-1 framework according to the S+ X− I+ synthesis route.3,4
It can be seen from Table 1, both siliceous SBA-1 and
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
795
to HCHO on the former could be related to their highly dis-
persed Mo on the large surface of SBA-1 as described above.
For the Mo-SBA-1 series catalysts synthesized under different
acidic conditions (HCl/TEOS = 5 → 10), the selectivity and
yield for the formation of HCHO linearly increased with
increasing Mo content on the catalyst compared at the similar
CH4 conversion. These results are consistent with the observa-
tion by Faraldos et al.,8 who reported that CH4 conversion and
HCHO selectivity are dependent on the metal loading and the
dispersion of the surface species.
Mo-SBA-1 series samples possess characteristics of a meso-
porous material, i.e., a BET surface area over 1000 m2 g–1 and a
uniform pore size of ca. 20 Å. A considerable decrease in the
surface area for the impregnated sample Mo/SBA-1 should be
attributed to the loss of crystallinity seen in the XRD pattern
(Figure 1e), indicating that the cubic structure of SBA-1 was
partly destroyed during the impregnation of SBA-1 with an
aqueous solution of (NH4)6Mo7O24·4H2O. Mo/SiO2 sample
only had a considerably small BET surface area below 100 m2
g–1 and a small pore volume of 0.12 cm3 g–1.
Figure 2 shows the Raman spectra of crystalline MoO3,
Mo/SiO2, Mo/SBA-1 and Mo-SBA-1 samples. The absence of
sharp peaks at ca. 996, 820, 665 cm–1 and several others below
400 cm–1 (seen in MoO3 and Mo/SiO2) corresponding to bulk
It is also clear from Table 2 that the Mo-SBA-1 prepared
by direct synthesis showed a slightly higher CH4 conversion
and HCHO selectivity in comparison with the Mo/SBA-1 sam-
ple prepared by the impregnation method. Higher dispersion
and isolation of the active sites Mo incorporated into the SBA-1
phase can be attributed for the enhanced catalytic performance
in the partial oxidation of methane.
The present study demonstrates that a considerably large
amount of Mo can be effectively incorporated into the cubic
SBA-1 mesophase under strongly acidic conditions. The Mo-
SBA-1 synthesized by direct synthesis method exhibits higher
activity and selectivity for HCHO formation than the Mo-
impregnated siliceous SBA-1 (Mo/SBA-1) and amorphous sili-
ca (Mo/SiO2) catalysts, due to the presence of high dispersion
and isolation of Mo in the SBA-1 framework.
7
MoO3 in the spectrum of Mo-SBA-1 sample (Figure 2d) indi-
cates that no free MoO3 phase was presented and the Mo was
highly dispersed in the silica-based SBA-1 catalyst. For the Mo-
SBA-1 sample, the Mo species might consist of an octahedral
polymolybdate (Oh-Mo) at 952 cm–1 and a tetrahedral molybdate
(Td-Mo) at 880 cm–1.7 In the case of Mo/SBA-1 prepared by
impregnation method, besides of Oh-Mo (952 cm–1) and Td-Mo
(shifted to 850 cm–1) species, a trace amount of MoO3 (996 and
820 cm–1) was also detected. Thus, the degree of Mo dispersion
on the catalysts could be arranged in the order Mo-SBA-1 >
Mo/SBA-1 >> Mo/SiO2. This order is in agreement with the
expectation by the BET results as shown in Table 1.
We acknowledge financial support of New Energy and
Industrial Technology Development Organization (NEDO). L.-X.
Dai and Y.-H. Teng were supported by a fellowship from NEDO.
References and Notes
1
2
3
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S.
Beck, Nature, 359, 710 (1992).
K. A. Koyano and T. Tatsumi, J. Chem. Soc., Chem. Commun.,
1996, 145.
Q. Huo, D. I. Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E.
Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth, and G. D.
Stucky, Chem. Mater., 6, 1176 (1994).
4
5
6
7
8
Q. Huo, D. I. Margolese, and G. D. Stucky, Chem. Mater., 8, 1147
(1996).
R. G. Herman, Q. Sun, C. Shi, K. Klier, C.-B. Wang, H. Hu, I. E.
Wachs, and M. M. Bhasin, Catal. Today, 37, 1 (1997).
M. S. Morey, A. Davidson, and G. D. Stucky, J. Porous Mater., 5,
195 (1998).
C. C. Williams and J. G. Ekerdt, J. Phys. Chem., 95, 8781 (1991)
and references cited therein.
M. Faraldos, M. A. Banares, J. A. Anderson, H. Hangchun, I. E.
Wachs, and J. L. G. Fierro, J. Catal., 160, 214 (1996).
As shown in Table 2, both Mo-SBA-1 and Mo/SBA-1 cat-
alysts exhibited much higher CH4 conversion and selectivity to
HCHO than those of amorphous Mo/SiO2. The high selectivity