Synthesis and Catalytic Activity of Niobium-Containing Hexagonal Mesoporous Silica

Article abstract of DOI:10.1246/cl.2003.992

Niobium-containing hexagonal mesoporous silica has been synthesized using n-dodecylamine as a template; the novel material is very active as a catalyst in the epoxidation of propylene with hydrogen peroxide and alkyl peroxides as oxidants.

Full text of DOI:10.1246/cl.2003.992

992
Chemistry Letters Vol.32, No.11 (2003)
Synthesis and Catalytic Activity of Niobium-Containing Hexagonal
Mesoporous Silica
Yanyong Liu,ꢀ Kazuhisa Murata, and Megumu Inaba
Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology,
AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
(Received August 13, 2003;CL-030751)
Niobium-containing hexagonal mesoporous silica has been
distilled water, air-dried at room temperature, and finally cal-
cined in air at 923 K for 4 h. Ti-MCM-41, Ti-HMS, Nb-MCM-
41, and TS-1 were prepared by the methods reported in the liter-
atures.1,4,13,14
synthesized using n-dodecylamine as a template;the novel ma-
terial is very active as a catalyst in the epoxidation of propylene
with hydrogen peroxide and alkyl peroxides as oxidants.
X-ray diffraction (XRD) patterns were recorded using a
MAC Science MX-Labo diffractometer with Cu radiation oper-
ated at 40 kV and 50 mA. Infrared spectra were recorded on a
JASCO FT/IR-680 spectrophotometer using a KBr pellet tech-
nique. Epoxidation of propylene was carried out in a 50-mL
stainless-steel reactor. A portion of 0.2 g of catalyst, 6.5 mmol
of 30% H2O2 or 70% TBHP, and 10 mL of methanol were added
in the autoclave reactor, and then the reactor was charged with
propylene at a constant pressure (0.8 MPa) at 298 K. After heat-
ing, the mixture reacted at 323 K under magnetic agitation for
4 h. The products were detected by gas chromatography and
the residual H2O2 and TBHP were determined by iodometric ti-
tration.
The low-angle powder XRD patterns of the various samples
are shown in Figure 1. Each sample exhibits an intense reflection
corresponding to the (100) plane at 2–3 degrees and a broad
shoulder near 5 degrees. These patterns are typical wormhole
structures of HMS materials assembled from long alkyl chain
neutral amines as surfactants.10 The d100 spacings were 3.71
and 3.54 nm for HMS before and after calcination, and the
d100 spacings were 3.86 and 3.69 nm for Nb-HMS before and af-
ter calcination. The d100 spacing shifted to lower diffraction an-
gle by the Nb modification. Both HMS and Nb-HMS possess
high surface areas above 1000 m2gꢁ1 measured by the N2 ad-
The synthesis of silica-based mesoporous materials has at-
tracted great interest because it extends the range of molec-
ular-sieve materials into the very large pore regime.1 Incorpora-
tion of transition metal ions in these mesoporous structures is a
very interesting and promising field of research. Hexagonal mes-
oporous silica (HMS) has several advantages among the meso-
porous materials: 1) HMS can be easily formed by sol-gel reac-
tion in the presence of primary alkylamine as template at room
temperature;2 2) transition metal ions, such as Ti4þ, Al3þ
,
Zr4þ, V5þ, Cu2þ, Cr6þ, and Mn2þ, can be incorporated into
the HMS framework uniformly with high contents;2–7 3) HMS
possesses thicker framework walls, small crystallite size of pri-
mary particles, and complementary textural porosity,8 which
causes that transition metal-modified HMS usually shows high
activity in the organic reactions.2,9,10 Niobium compounds and
materials have shown great promise in heterogeneous catalysis
within the past decade.11 Recently, Nb-MCM-41 has been recog-
nized as a very attractive catalyst in the oxidation of organic
compounds with hydrogen peroxide.12,13
Propylene oxide (PO) is one of the most important chemical
feedstocks for producing resins such as polyurethane. It is pro-
duced commercially by etherification of chlorohydrin or organic
hydroperoxide processes. The latter (Halcon process) has more
ecological and economical benefits than the former and accounts
for more than one million tons annual production of propylene
oxide worldwide. Titanium silicate (TS-1) has been used to cat-
alyze the epoxidation of propylene with hydrogen peroxide un-
der mild conditions.14 However, TS-1 cannot catalyze the epox-
idation of propylene with alkyl hydroperoxide, such as tert-butyl
hydroperoxide (TBHP), owing to the limited size of the silicalite
pore.15 In the present study, we report on the synthesis of niobi-
um-containing mesoporous silica (Nb-HMS) at the first time.
The novel material is very active as a catalyst in the epoxidation
of propylene with both hydrogen peroxide and TBHP as oxi-
dants.
2000
D
C
B
A
Nb-HMS samples were synthesized at room temperature us-
ing n-dodecylamine as surfactant molecule. A solution which
containing 0.1 mol of tetraethyl orthosilicate (Si(OC2H5)4) and
the required amount of niobium penta-ethoxide (Nb(OC2H5)5)
in 0.7 mol ethanol was added to a solution of n-dodecylamine
(0.027 mol) and HCl (2 mmol) in water (3.63 mol). The resulting
gel was stirred for 5 min and aged for 18 h at room temperature.
The solid was recovered by filtration, washed abundantly with
6
0
1
2
3
4
5
7
8
9 10 11
2
θ /degree
Figure 1. The XRD patterns of (A) HMS (before calcination),
(B) Nb-HMS (before calcination, Nb/Si = 1:100 (molar ratio)),
(C) HMS (after calcination), (D) Nb-HMS (after calcination,
Nb/Si = 1:100 (molar ratio)).
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.11 (2003)
993
tion of propylene with both H2O2 and TBHP as oxidants. Al-
though Nb-MCM-41,Ti-HMS, and Ti-MCM-41 are also effec-
tive in the oxidation of propylene with H2O2 and TBHP, the
activities and PO yields of them are lower than those of Nb-
HMS. It seems that Nb is preferable to Ti and HMS is preferable
to MCM-41 in the epoxidation of propylene. TS-1 showed high
activity and PO yield in the oxidation of propylene with H2O2,
but it reasonably showed the lowest activity with TBHP as the
oxidant because the limited size of the silicalite pore
SiO 4−
4
Si−O−Nb
SiO 4−
4
B
A
ꢀ
(5:6 ꢂ 5:3 A) seriously restricted TBHP to enter into the pores
of TS-1. Nb-HMS is a highly active catalyst for the oxidation
of propylene irrespective of the nature of the oxidants. Our syn-
thetic approach therefore provides new opportunities for the de-
sign of mesoporous catalysts in the oxidation of organic com-
pounds.
1400
1200
1000
Wave number/cm−1
800
600
In conclusion, we have synthesized Nb-HMS using n-dode-
cylamine as surfactant molecule at room temperature. Infrared
spectra and UV–vis spectra proved that Nb ions had been incor-
porated into the HMS framework uniformly in Nb-HMS. The re-
sulting Nb-HMS material exhibited excellent catalytic activity
and selectivity for PO in the epoxidation of propylene with hy-
drogen peroxide and alkyl peroxides as oxidants.
Figure 2. FT-IR spectra of calcined samples dispersed in KBr
pellets. A: HMS, B: Nb-HMS (Nb/Si = 1:100 (molar ratio)).
sorption isotherms. Infrared spectra of HMS and Nb-HMS after
calcination are shown in Figure 2. Both of the samples exhibit
the symmetric stretching vibration bond at around 810 cmꢁ1
and the anti-symmetric vibration band at around 1100 cmꢁ1 of
the tetrahedral SiO4 structural units. HMS exhibits no band at
6,9
We gratefully acknowledge the financial support from the
NEDO.
around 960 cmꢁ1
,
but Nb-HMS exhibits an infrared band at
around 960 cmꢁ1 like in the infrared spectrum of Nb-MCM-
41.16 The band at around 960 cmꢁ1 has been widely used to char-
acterize the incorporation of metal ions in the silica framework
as the stretching Si–O vibration mode perturbed by the neighbor-
ing metal ions.16,17 The presence of an infrared band at around
960 cmꢁ1 is the evidence for the isomorphous substitution of
Si by Nb ions in Nb-HMS. The isomorphous substitution has
been also proved by UV–vis spectroscopy. The UV–vis spec-
trum of Nb-HMS shows a single adsorption edge at 220 nm,
where the formation of Nb–O–Si bonds is suggested.18
References
1
C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and
J. C. Beck, Nature, 359, 710 (1992).
2
P. T. Tanev, M. Chibwe, and T. J. Pinnavaia, Nature, 368,
321 (1994).
3
4
R. Mokaya and W. Jones, Chem. Commun., 1996, 981.
A. Tuel, S. Gontier, and R. Teissier, Chem. Commun., 1996,
651.
5
6
S. Gontier and A. Tuel, Microprous Mater., 5, 161 (1995).
Z. Fu, J. Chen, D. Yin, L. Zhang, and Y. Zhang, Catal. Lett.,
66, 105 (2000).
The catalytic activity and selectivity in the oxidation of pro-
pylene over various catalysts are shown in Table 1. The designed
metal amounts of the samples were 1:100 for the M/Si molar ra-
tio. The actual metal amounts in the products were measured by
ICP analyses and the results were consistent with the designed
metal amounts in the samples. Propylene was oxidized to pro-
duce propylene oxide as a main product, together with propylene
glycol and methoxypropanol as by-products. The efficiencies of
the oxidants (H2O2 and TBHP) were larger than 98% over all
catalysts tested in this study (not shown in Table 1). Nb-HMS
showed the highest activity and selectivity for PO in the oxida-
7
8
9
H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto,
M. Che, and M. Anpo, Chem. Commun., 2001, 435.
T. R. Pauly, Y. Liu, T. J. Pinnavaia, S. J. L. Billinge, and T.
P. Rieker, J. Am. Chem. Soc., 121, 8835 (1999).
Y. Liu, K. Suzuki, S. Hamakawa, T. Hayakawa, K. Murata,
T. Ishii, and M. Kumagai, Catal. Lett., 66, 205 (2000).
10 Y. Liu, K. Murata, M. Inaba, and N. Mimura, Catal. Lett., 89,
49 (2003).
11 I. Nowak and M. Ziolek, Chem. Rev., 99, 3603 (1999).
12 J. Xin, J. Suo, X. Zhang, and Z. Zhang, New J. Chem., 24,
569 (2000).
Table 1. Catalytic oxidation of propylene over various catalysts
at 323 Ka
13 I. Nowak, B. Kilos, M. Ziolek, and A. Lewandowska, Catal.
Today, 78, 487 (2003).
14 M. G. Clerici, G. Bellussi, and U. Romano, J. Catal., 129,
159 (1991).
15 T. Sato, J. Dakka, and R. A. Sheldon, J. Chem. Soc., Chem.
Commun., 1994, 1887.
16 V. Parvulescu, C. Anastasescu, C. Constantin, and B. L. Su,
Catal. Today, 78, 477 (2003).
Catalysts
Oxidation with H2O2 Oxidation with TBHP
Conv./% Select./% Conv./% Select./%
Nb-HMSb
Nb-MCM-41b
Ti-HMSc
Ti-MCM-41c
TS-1c
41.3
32.5
28.3
26.2
40.5
98.3
96.7
95.4
96.3
98.1
42.7
34.6
26.7
24.8
13.2
98.6
97.3
96.2
95.9
90.4
aAutoclave: 50 mL;catalyst: 0.2 g;propylene: 0.8 MPa;30%
H2O2 or 70% TBHP: 6.5 mmol;methanol: 10 mL;reaction time:
4 h. bNb/Si = 1:100 (molar ratio). cTi/Si = 1:100 (molar ratio).
17 P. Wu, T. Tatsumi, T. Komatsu, and T. Yashima, J. Catal.,
202, 245 (2001).
18 L. Zhang and J. K. Ying, AIChE J., 43, 2793 (1997).
Published on the web (Advance View) October 6, 2003;DOI 10.1246/cl.2003.992