Highly Stable Mesoporous Metal Oxides Using Nano-Propping Hybrid Gemini Surfactants
Yi-Yeol Lyu,†,| Seung Hwan Yi,‡ Jeong Kuk Shon,‡ Seok Chang,† Lyong Sun Pu,†,§ Sang-Yun Lee,†
Jae Eui Yie,‡ Kookheon Char,| Galen D. Stucky,⊥ and Ji Man Kim*,‡
Electronic Materials Laboratory, Samsung AdVanced Institute of Technology, P.O. Box 111, Suwon, 440-600,Korea, Department of Applied Chemistry, Ajou UniVersity, Suwon, 442-749, Korea, Department of AdVancedMaterials, Sungkyunkwan UniVersity, Suwon, 440-746, Korea, School of Chemical Engineering, Seoul NationalUniVersity, Seoul, 151-744, Korea, and Department of Chemistry, UniVersity of California, Santa Barbara,
Received October 14, 2003; E-mail: [email protected]
Mesoporous materials, obtained by the favorable self-assembly
between organic templates and inorganic precursors, have openedmany new possibilities for applications in catalysis, separation, andnanoscience due to their large, controllable pore sizes, high surfaceareas, and easy functionalization.1,2 The nature of organic templatesis one of the most important factors for designing and synthesizingmesoporous materials.3 In general, surfactants with cationic, anionic,and neutral charges and amphiphilic block copolymers have beenutilized as organic templating agents.2 Most research in this fieldhas been focused on silica as a framework constituent. However,mesoporous materials derived from transition metal oxides insteadof the silica frameworks are expected to be quite useful for several
Figure 1. Structure of the Gemini surfactant and a schematic diagram for the nano-propping pathway.
applications. There have been several reports concerning thesynthesis of mesoporous metal oxides, such as titanium, vanadium,zirconium, tungsten, niobium, and tantalum oxides via surfactanttemplated,4 ligand-assisted,5 polymer templated,6 and nanoreplicatedpathways.7,8 However, the synthesis of mesoporous metal oxideshas been less successful as compared to those of silica materials. One difficulty lies in a facile crystallization of most metal oxides,accompanied by structural collapse, during the mesostructureformations and the removal of organic templates.
Here, we describe a new type of Gemini surfactant containing a
siloxane moiety, which can yield highly stable mesoporous metaloxides after the removal of the surfactant via the nano-proppingpathway. Figure 1 shows the structure of the surfactant and theschematic diagram for the nano-propping pathway. In the surfactant,we can control the length of the hydrophobic tail (n), the length ofthe siloxane moiety (m), and the length of the spacer between the
Figure 2. (a) XRD patterns for the mesoporous zirconium oxide obtained from C
positively charged headgroup and the siloxane moiety (k). The
18-3Si3 before and after calcination, and the corresponding (b) TEM
image and (c) SEM image of the material after calcination at 823 K, and
Gemini surfactants containing the siloxane moiety are denoted as
(d) XRD patterns for the mesoporous zirconium oxide obtained from normal
Cn-kSim. A typical synthesis procedure of C18-3Si3 surfactant is
Gemini surfactant without the siloxane moiety.
provided in the Supporting Information.
Figure 2a shows X-ray diffraction (XRD) patterns of the
Mesoporous metal oxides were prepared as follows: the Cn-kSim
mesoporous zirconium oxide obtained by using C
surfactant and distilled water were initially mixed to obtain a
as a template. As shown in Figure 2a, the zirconium oxide exhibits
homogeneous solution. An aqueous solution of metal oxide
the XRD patterns with a very intense diffraction peak and two weak
precursors such as zirconium sulfate, vanadyl sulfate, and titanium
peaks before calcination, which are characteristic of the 2-D
sulfate was added to the surfactant solution under vigorous magnetic
hexagonal (P6mm) structure.1-3 It is quite noteworthy that the
stirring. The molar composition of the synthetic mixture was 1 metal
mesoporous zirconium oxide exhibits ordered structure even after
precursors:0.05-0.5 Cn-kSim:100 H2O. The mixture was aged with
the calcination at 823 K. Figure 2b shows the transmission electron
stirring for 1 h at room temperature and subsequently heated to
microscopic (TEM) image of the mesoporous zirconium oxide after
373 K in an oven under static conditions for 24 h. The precipitates
calcination. In the TEM image, the mesopores are packed in a
were filtered, washed with distilled water, and dried at 373 K. The
hexagonal way, implying that the material has a highly ordered
product was calcined at 823 K in air flow.
2-D hexagonal structure. The scanning electron microscopic image
in Figure 2c shows that the morphology of the material is quite
Samsung Advanced Institute of Technology.
uniform. Generally, the structure constructed with zirconium oxide
is often collapsed or transferred to a poorly ordered structure by
⊥ University of California, Santa Barbara.
the thermal damage of structural integrity. For comparison,
2310 9 J. AM. CHEM. SOC. 2004, 126, 2310-2311 10.1021/ja0390348 CCC: $27.50 2004 American Chemical Society C O M M U N I C A T I O N S Figure 3. Nitrogen adsorption-desorption isothems of the mesoporous zirconia in Figure 2a and the corresponding pore size distribution curve obtained from the adsoption branch (inset).
zirconium oxide mesostructure has been obtained by the same
Figure 4. XRD patterns for the mesostructured (a) titanium oxide and (b)
synthetic procedure using a Gemini surfactant (C16H33N(CH3)2-
vanadium oxide before and after calcination, obtained from the C18-3S3
(CH2)6N(CH3)2C16H33Br2)9 without the siloxane moiety as a
template. The material thus obtained shows the highly ordered 2-Dhexagonal structure as shown in Figure 2d before calcination.
The mesoporous materials based on titanium and vanadium
However, the ordered structure is completely collapsed upon
oxides can also be synthesized using the present Gemini surfactants
calcination at 673 K. A widely used organic template for the
and also demonstrate high thermal stability as shown in Figure 4.
synthesis of mesoporous materials, cetyltrimethylammonium bro-
In the case of mesoporous titanium oxide, the mesostructure thus
mide,1,2 also results in the formation of ordered mesoporous
obtained is somewhat disordered. We are currently optimizing the
zirconium oxide before calcination and the complete structural
synthetic conditions to obtain the highly ordered materials. Figure
deconstruction after calcination (not shown). These results indicate
4b indicates that the vanadium oxide mesostructure has a well-
that the siloxane moiety introduced in the Gemini surfactants in
defined bicontinuous cubic symmetry (Ia3d), similar to the MCM-
the present work acts as a stabilizer for the zirconium oxide
48 structures.1 However, the mesostructure almost disappears upon
calcination, which is believed to be due to the weak thermal stability
The self-assembly of cationic zirconium species, which are
present in the reaction mixture, with the surfactants is known to be
In conclusion, Gemini surfactants containing a siloxane moiety
mediated by sulfate anions because the zirconium species are
have been designed and successfully synthesized in the present study
protonated under this condition.10 The sulfate anion may be an
and are utilized as templates for mesoporous metal oxides such as
excellent mediator between cationic metal precursors and cationic
zirconia, titania, and vanadia. The siloxane moiety is believed to
surfactants due to its large size and divalent charge. Consequently,
play an important nano-propping role during the surfactant removal
metal sulfates generally yield highly ordered mesoporous materials
by direct calcination, yielding thermally stable mesoporous metal
in the presence of cationic surfactants. However, the presence of
oxides. It is believed that the synthesis strategy described here can
sulfate groups in the inorganic framework prevents full condensa-
be applied to the synthesis of robust nanostructured materials such
tion, and this leads to the major framework disruption because of
as nanoparticles and nanorods in addition to mesoporous materials.
the sulfate removal upon calcination. The siloxane species in thepresent surfactant molecules seem to play a nano-propping role, to
Acknowledgment. This work was supported by the Korea
maintain the mesostructure during the calcination, as shown in
Research Foundation Grant (KRF-2003-015-C00319). K.C. also
Figure 1. Elemental analysis of the calcined zirconium oxide
acknowledges the financial support from the National Research
indicates that a small amount of silica species exists within the
Laboratory Program (Grant M1-0104-00-0191).
framework (Si/Zr ) 0.09) after calcination. Stabilization of the
Supporting Information Available: Detailed synthesis conditions
mesostructured metal oxide by the silica species is similar to the
for the C18-3Si3 surfactant (PDF). This material is available free of
effects of phosphate or chromate which can improve the stability
charge via the Internet at http://pubs.acs.org.
of the materials so that the materials retain the mesoporosity aftercalcination.11 We also found that the lattice contraction of the
materials after the calcination at 823 K is dependent on the number
(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.
of siloxane groups within the Gemini surfactant. C18-3Si2, C18-3Si3,
Nature 1992, 359, 710.
(2) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999,
18-3Si4 surfactants give 30%, 24%, and 20% of the lattice
38, 56 and references therein.
contractions, respectively, which also indirectly confirms the nano-
(3) Kim, J. M.; Sakamoto, Y.; Hwang, Y. K.; Kwon, Y.-U.; Terasaki, O.;
propping role of the siloxane moieties.
Park, S.-E.; Stucky, G. D. J. Phys. Chem. B 2002, 106, 2552.
Figure 3 shows the nitrogen adsorption and desorption isotherms
(4) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew.Chem., Int. Ed. Engl. 1996, 35, 541.
for the mesoporous zirconium oxide obtained after calcination at
(5) Antonelli, D.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426.
823 K. A small stepwise increase appears in the adsorption isotherm
(6) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature1998, 396, 152.
0.2, indicating capillary condensation in the
(7) Kang, M.; Yi, S. H.; Lee, H. I.; Yie, J. E.; Kim, J. M. Chem. Commun.
mesopores. No hysteresis loop is obtained in this region; however,
there is a hysteresis that appears at P/P )
(8) Dong, A.; Ren, N.; Tang, Y.; Zhang, Y.; Hua, W.; Gao, Z. J. Am. Chem.Soc. 2003, 125, 4976.
due to the interparticular spacing between agglomerated particles
(9) Huo, Q.; Leon, R.; Petroff, P. N.; Stucky, G. D. Science 1995, 268, 1324.
(see Figure 2c). A BJH pore size curve obtained from the adsorption
(10) Schu¨th, F. Chem. Mater. 2001, 13, 3184 and references therein.
branch of the isotherm indicates that the material has well-defined
(11) Kim, J. M.; Shin, C. H.; Ryoo, R. Catal. Today 1997, 38, 221. J. AM. CHEM. SOC. 9 VOL. 126, NO. 8, 2004 2311
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