J. Phys. Chem. B 2006, 110, 11761-11771
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Synthesis of Thermally Stable Zirconia-Based Mesoporous Materials via a Facile Post-treatment Shih-Yuan Chen,† Ling-Yun Jang,‡ and Soofin Cheng*,† Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan, Republic of China, and Research DiVision, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China ReceiVed: January 26, 2006; In Final Form: March 22, 2006
A novel method of preparing thermally stable zirconia-based mesoporous materials was developed. The zirconiabased mesoporous materials of 2D-hexagonal structure were prepared using zirconium sulfate as the zirconium precursor and cetyltrimethylammonium (CTMA) as the pore-directing agent with the aid of salt in the synthesis solution to reduce the sulfate content in the final product and significantly improve the crystallographic ordering. Post-treatment of the mesoporous material with NaCl solution and lowering the ramping rate to less than 0.2 °C/min during the calcination process, however, were the key steps to hinder the growth of the dense zirconia phase and to retain the ordered mesostructure up to 600 °C. It was found that a portion of the surfactant (8.9-17.4 wt %) and sulfate ions (0.5-1.2 wt %) were removed during the post-treatment, which prevented the remaining sulfate groups from being reduced by the hydrogen-rich surfactant during the calcination process as confirmed by sulfur K-edge X-ray absorption near edge structure (XANES) and infrared spectroscopy. The maintenance of sulfur in the sulfate state seemed to be important in stabilizing the mesoporous structure of zirconia materials. The mesoporous zirconia materials after extraction with NaCl solution three times and calcination at 550-600 °C had the composition ZrO2-x(SO4)x with x ) 0.10-0.27. The material possesses high surface area (∼200 m2/g), large pore volume (∼0.10 cm3/g), and wormlike mesopores. In comparison with the mesoporous zirconia materials stabilized by chemical treatment, the present route was simpler and more environmentally friendly and resulted in mesoporous zirconia materials of better thermal stability.
1. Introduction Zirconium oxide is attractive owning to its excellent oxygen conductivity1 and its acid-base bifunctional catalytic activities.2 Zirconia-based materials of ordered mesoporous structure and high surface area have been synthesized in the past few years based on the surfactant templating routes.3-5 In comparison to the mesoporous silica materials, most of the nonsiliceous mesoporous materials have relatively low thermal and hydrothermal stability. The ordered mesoporous structures tend to collapse after the removal of organic templates. In the literature, there have been a few reports on the synthesis of mesoporous zirconia-based materials, but most of them have low thermal stability.6-16 In 1996, Hudson and Knowles reported the synthesis of ordered zirconia mesoporous material through a scaffolding mechanism.6 The d spacings of the mesoporous zirconia materials were finely tuned by using cationic surfactant with different chain lengths from C8TMABr to C18TMABr. However, the ordered mesostructure collapsed after the materials were calcined at 350 °C. Antonelli and Ying7 then developed the ligand-assisted synthesis route based on the coordinative binding of the surfactant headgroup with the metal centers. Mesoporous zirconia materials were synthesized by using surfactants with varied chain length and various headgroups including phosphate, sulfate, amine, and carboxylate. However, the mesoporous structures collapsed after template removal except for the one synthesized with surfactant containing * Corresponding author. Fax: +886-2-2363-6359.
[email protected]. † National Taiwan University. ‡ National Synchrotron Radiation Research Center.
E-mail:
phosphate headgroups. The phosphate group was proposed to enhance the thermal stability due to the formation of a zirconium phosphate layer over the pore wall. Another synthesis method was reported by the groups of Schuth and Stucky,8 where zirconium sulfate was used as the precursor and the sulfate group was considered a good counteranion to stabilize the positively charged surfactant head and the multivalent zirconium polyoxo cation. Both the hexagonal P6mm and cubic Ia3d mesoporous zirconia-based materials were synthesized. However, the mesostructure was stable up to 500 °C only after post-treatment of the as-made zirconium oxo-sulfate material with a suitable concentration of phosphoric acid. Furthermore, the pore diameters of the resultant materials were less than 2 nm because the pore wall was coated with a layer of zirconium phosphate (54 wt %). Recently, Lyu and co-workers12 reported the synthesis of mesoporous metal oxide materials with thermal stability up to 550 °C by using siloxane-containing gemini surfactant as the template. The result was similar to that of phosphatestabilized mesoporous materials, and the pore wall of the mesoporous materials was covered with a layer of silica species formed by oxidation of the siloxane group, as the elemental analysis showed that the Si/Zr molar ratio was ca. 0.09 in the calcined samples. In this paper, we describe the first example of the preparation of zirconia-based mesoporous materials with thermal stability up to 600 °C through post-treatment of the material synthesized from a zirconium sulfate precursor and cationic C16TMABr surfactant. An efficient post-treatment process was developed combining ion exchange and slow ramping rate calcination. The resultant mesoporous materials showed wormlike pores, high
10.1021/jp060564a CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006
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Figure 1. Small- and wide-angle XRD patterns of (a) as-made and (b) calcined (500 °C for 3 h) mZrO2(r) and mZrO2 samples.
surface area, and large pore volume. The effect of residual sulfur species on the thermal stability of mesoporous zirconia materials was investigated. 2. Experimental Methods Synthesis of Mesoporous Zirconia-Based Material. The reported procedure of the synthesis of mesoporous zirconiabased materials8,17,18 was modified by introducing salts into the synthesis solutions. Generally, 2.51 g of cetyltrimethylammonium bromide (CTMABr, Acros) as pore-directing agent and 3.74 g of sodium chloride (Acros) were dissolved in 85 g of deionized water. Then, 25 g of aqueous solution containing 4.55 g of zirconium sulfate (Zr(SO4)2‚4H2O, Aldrich) was added in slowly, where the NaCl/Zr molar ratio was 5. A white precipitate appeared after ca. 15 mL of zirconium sulfate solution was added. The mixed solution was stirred at 35 °C for 1 day, then sealed in a polypropylene bottle and heated at 100 °C under static conditions for 2 days. The solid product was recovered by filtering, washing with 300 mL of deionized water, and drying at 50 °C overnight. The compositions of reactants were 1 Zr(SO4)2‚4H2O:0.5 CTMABr:5 NaCl:477 H2O. The as-made material was named mZrO2. A reference sample named mZrO2(r) was also prepared without the addition of NaCl in the synthesis solution. Post-treatment. Post-treatment with aqueous NaCl solution and calcination, noted as the soft and hard methods, respectively, were used to remove the surfactant from the as-made materials. Generally, about 4 g of the as-made material in powder form was dispersed in 400 mL of 0.5 M NaCl solution, and the suspension was stirred for 1 day. The solid product was recovered by filtering, washing with a large amount of deionized water, and drying at 50 °C overnight. This procedure was repeated several times. Calcination was performed in air atmosphere. The oven temperature was raised from room temperature to the desired calcination temperature with a ramping rate less than 0.2 °C /min, and the final calcination temperature was retained for 3 h. The post-treated materials were named mZrO2-S-x, where “S” refers to post-treatment with a 0.5 M NaCl solution for x times. The calcined materials were named Cy-mZrO2-S-x, where “C” refers to calcination and y is the calcination temperature in degrees Celsius.
Characterization. X-ray diffraction (XRD) patterns were recorded on a Philip X’pert Pro diffractometer with Cu KR radiation operated at 40 mA and 45 kV. The pore structures of the materials were analyzed by nitrogen physical sorption at liquid nitrogen temperature (77 K) using a Micrometerics TriStar 3000 system. Prior to the experiments, the materials were outgassed at 200-400 °C for 6-8 h under vacuum (10-3 Torr). The sulfur K-edge X-ray absorption near edge spectra (XANES) were recorded at the beam line 15B of National Synchrotron Radiation Research Center in Hsinchu, Taiwan. Standard operating conditions were 1.5 GeV and 120-200 mA. The photon energy was calibrated using the L-edge of pure Mo foil. The scanning electron microscopy (SEM) images were taken using a Hitachi S-800 field emission scanning electron microscope. The transmission electron microscope (TEM) experiments were performed using a Hitachi H-7100 transmission electron microscope over the sliced specimen of ca. 90 nm in thickness. The thermal analyses including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and evolved gas mass analysis were carried out using a Netzch STA-409CD system with a ramp rate of 10 °C/min in an air flow of 50 mL/ min. The elemental analysis of C, H, N, and S was measured using a Heraeus VarioEL instrument. FT-IR spectra were recorded using a BOMEM DA 3.02 spectrometer with a resolution of 2 cm-1. 3. Results and Discussion XRD Patterns of Zirconia-Based Mesoporous Materials. Well-ordered zirconia-based mesoporous materials were synthesized by adding proper amounts of salts in the synthesis solutions. Figure 1 compares the XRD patterns of the materials synthesized with and without NaCl. The (100), (110), and (200) peaks of the hexagonal P6mm arranged pore structure were much sharper and more clearly seen on the as-made mZrO2 material than that of the reference material mZrO2(r), suggesting that the mesopore ordering in the as-made mZrO2 was better than that synthesized without NaCl. The optimal amount of sodium chloride used in the synthesis was found to be ca. 3-5 for the NaCl/Zr molar ratio. Organic salts including benzyltrimethylammonium chloride (BTMACl), tetraethylammonium chloride (TEACl), and tetrapropylammonium bromide (TPABr)
Thermally Stable Zirconia-Based Mesoporous Materials
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Figure 2. Small-angle XRD patterns of the mZrO2-S-x samples.
and inorganic salts such as NaBr were found to have an effect similar to NaCl, but Na2(SO4) would interfere with mesopore ordering. To the best of our knowledge, this is the first example of applying salts in the synthesis to improve the pore ordering of nonsiliceous mesoporous materials. Among the salts, the effect of NaCl was most obvious, and it is also the most economical and least hazardous salt used. The addition of salt was found to increase the thermal stability of mesostructured ZrO2 slightly. Figure 1b shows that the sample calcined at 500 °C (termed C500-mZrO2) retains a weak diffraction at 2θ ∼3.8°, while the reference material has no diffraction peak in the small-angle region. In the high-angle region, a broad peak around 2θ ∼30.5° corresponding to the crystalline ZrO2 phase was observed on the calcined reference material, but no peak was observed on C500-mZrO2. This demonstrates that although template removal from mesostructured ZrO2 material by calcination would destroy the mesopores, the sample synthesized with NaCl could still retain the ordering of the mesostructure slightly after calcination at 500 °C. In comparison to high-temperature calcination, extraction by solvent is a relatively soft method and an alternative path to remove the surfactant species from the mesoporous materials.19 Hereafter, the extraction treatment was carried out on the mesostructured zirconia materials synthesized with NaCl because better crystallographic ordering was observed on the asmade samples. In a preliminary experiment, the mesoporous structure was found to collapse after the sample was extrated with a boiling ethanol solution for 30 min. This implies that the mesostructure is very sensitive to the removal of the CTMA surfactant, and the extraction rate was probably too fast by using boiling ethanol. Accordingly, a mild extraction method was developed by using aqueous NaCl solution as the extraction agent. By varying the concentration of the NaCl solution, it was found that the optimal concentration of NaCl around 0.5 M was needed in order to retain the mesostructure after 550 °C calcination. Above this concentration, the mesostructure would collapse after extraction due to too much surfactant being removed. Below this concentration, the efficiency of surfactant removal was poor and the mesostructure also collapsed after calcination.
Figure 3. Small- and wide-angle XRD patterns of mZrO2 and mZrO2S-x samples calcined at (a) 550 °C, (b) 600 °C, and (c) 650 °C.
The small-angle XRD patterns of mesostructured zirconia before and after extraction with NaCl solution are shown in Figure 2. In comparison to the as-made mZrO2, the intensity of the (100) peak decreased and the (110) and (200) diffraction peaks gradually disappeared with the increase in the number of extractions. The d(100) also shifted toward smaller spacings from the original value of 42.1 Å to 41.4, 39.9, and 37.6 Å after for one, two, and three extractions, respectively. The structural contraction in (100) was ca. 10% after the as-made mZrO2 was extracted with NaCl solution three times. The XRD patterns of the calcined zirconia-based materials are shown in Figure 3. All the extracted samples retained a diffraction peak in the low-angle region of 2θ ∼3-4°, corresponding to d(100) spacings of 25.5, 26.9, and 27.1 Å for C550-
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TABLE 1: Textural Properties of Calcined Zirconia-Based Materials materiala C550-mZrO2(r) C550-mZrO2 C550-mZrO2-S-1 C550-mZrO2-S-2 C550-mZrO2-S-3 C600-mZrO2-S-1 C600-mZrO2-S-2 C600-mZrO2-S-3 C650-mZrO2-S-1 C650-mZrO2-S-2 C650-mZrO2-S-3
SBET (m2/g) VTotal (cm3/g) 20 15 89 185 225 98 201 239 93 144 142
0.010 0.007 0.052 0.089 0.108 0.061 0.121 0.151 0.066 0.100 0.104
Φb (nm)
Wtc (nm)
1.1 (
