황산이 담지된 메조포어 지르코니아의 합성

서영웅, 이현구

서울대학교 응용화학부·화학공정 신기술 연구소

Synthesis of Mesoporous Sulfated Zirconia

Young-Woong Suh and Hyun-Ku Rhee

School of Chemical Engineering and Institute of Chemical Processes,

Seoul National University, Seoul 151-742, Korea

INTRODUCTION

Since sulfated zirconia (SZ) was first applied to the isomerization of hydrocarbons by Holm and Bailey [1], a great deal of interest has been directed to its use in the isomerization of n-butane [2], because sulfated zirconia was considered to be a superacid. However, more recent studies have shown that the catalyst most likely does not possess superacid strength [3]. Although SZ is not a solid superacid, it is still active for n-butane isomerization at the low reaction temperature.

Sulfated zirconia is typically prepared by contacting an amorphous hydrous zirconia powder with a solution of ammonium sulfate or sulfuric acid, followed by calcination at temperatures varying from 500 to 650 oC. Surface area for active sulfated zirconia ranges between 100 and 120 m2/g and it has been observed to be catalytically active when the zirconia exists as the tetragonal crystalline phase.

Therefore in order to prepare a mesoporous, high surface area sulfated zirconia catalysts, the surfactant-assisted methodology of the high surface area silica MCM-41 was utilized by several researchers [4, 5]. Schuth et al. [4] reported the formation of a mesoporous zirconia with a surface area of 280 m2/g by using zirconium alkoxide as a zirconium precursor. Knowles and Hudson [5] synthesized a mesoporous zirconia from a zirconyl chloride source created by a "scaffolding" effect of surfactant. Surface areas of ca. 300 m2/g were obtained by these authors upon removal of the surfactant. In preparing mesoporous zirconia with surfactants, however, many authors have indicated the difficulty in removing the surfactant from the organic-zirconia mesophase composite without the structural collapse.

Here, we focus on the synthesis of thermally stable mesostructured sulfated zirconia. Three synthetic routes to stabilize the ordered pore structure have been developed: (1) the organic-zirconia mesophase composite is post-treated with sulfuric acid or phosphoric acid; (2) PEO-based nonionic surfactant is first used to synthesize zirconia, based on a synthetic method of MSU-X [6]; (3) mesostructured zirconia is prepared without the surfactant by using zirconyl chloride or zirconium sulfate as a zirconium precursor.

EXPERIMENTS

Organic-Zirconia Mesophase Composite

Hexadecyltrimethylammonium bromide (CTAB; 2.5 g, 6.87 mmol) was dissolved in water (85 g), and Zr(SO4)2·4H2O (4.55 g, 0.0128 mol) dissolved in water (25 g) was added. This led to a colorless precipitate. The mixture was stirred at room temperature for 2 h and then placed in an autoclave for hydrothermal reaction in oven, and kept at 373 K for 2 days. The solid product obtained was filtered.

Post-treatment of As-synthesized Samples

The samples were treated with sulfuric acid or phosphoric acid solution before calcination to enhance the thermal stability. As-synthesized samples was added to each acidic solution (0.5 M) and the solution was stirred for 5 h. After filtration, the colorless product was dried at 373 K overnight and calcined in air at 823 K for 5 h. The calcined samples were also treated with an acidic solution again by ion-exchange or impregnation method. Then, the remaining phosphate or sulfate functional group was removed by evacuation at room temperature.

MSU-4-type Zirconia

MSU-4-type zirconia was synthesized in an open container from aqueous mixtures of zirconium precursor (Zr n-propoxide or zirconium sulfate), surfactant, and sodium mineralizer. The molar composition of each reaction mixture was Tween 20 (Polyoxyethylene sorbitan monolaurate), 0.02; Zirconium source, 0.16; NaF, 0.004, and H2O, 55.6. In a typical preparation, 1.028 g of Tween 20 was first dissolved in 50 mL of water at 308 K. Once surfactant dissolution was complete, 5.667 g of 40 wt% zirconium sulfate solution or 3.744 g of 70 wt% zirconium n-propoxide was added. The reaction mixture was stirred slowly at 308 K for 24 h. Sodium fluoride (0.84 mL of a 0.238 M NaF solution) was then added to the zirconium/surfactant solution. The solution so-obtained was aged under slow stirring at 308 K for 2 days. A white colloidal suspension appeared progressively. The powder was fitered, dried, and calcined in air at 473 K for 6 h and then at 893 K for 6 h (5 K/min).

Mesostructured Zirconia Prepared without Surfactant

The samples of hydrous zirconia were prepared via the conventional route. 28-30 % ammonia was slowly added to 100 ml of a 0.4 M solution of zirconyl chloride or zirconium sulfate under vigorous stirring until the target pH value of 10.0 was attained in each preparation. The pH adjusted precipitated slurries were then stirred under reflux at 363 K during the 20 h digestion period. Afterwards, the hydrous zirconia was recovered via vacuum filtration and washed with excess water to remove chloride ions, and was then dried at 373 K. All hydrous zirconia samples were subsequently sulfated by contact with 0.5 M sulfuric acid solution (5 ml/g) for 1 h under slow stirring. The sulfated zirconias were recovered via vacuum filtration without washing and dried at 373 K. The catalysts were calcined at 823 K under dry air for 3 h.

Characterization

The structure and order of mesostructured zirconia samples were determined by X-ray powder diffraction analysis (XRD). The XRD patterns were obtained by using CuK□ radiation (□=1.54056Å). The morphology of samples were examined by SEM and TEM. Infrared spectra were recorded on Nicolet Impact 410 FT-IR instrument (in the range 4000-500 cm-1) using the self support wafer in-situ techniques and diffuse reflectance method. The specific surface areas and average pore diameters were determined by nitrogen physisorption at liquid nitrogen temperature using a Micrometrics ASAP 2010.

RESULTS AND DISCUSSION

We investigated the coordination interaction between positively charged surfactant molecules and zirconium sulfate as the inorganic precursor species, leading to mesostructured MCM-41 analogues. Via the sulfate groups, charge interactions with the surfactant head groups can occur. The XRD pattern of the zirconium sulfate surfactant composite shows only reflections in the low-angle region and very large lattice parameters, which is typical for mesostructured materials, as shown in Fig. 1.

Figure 1. XRD patterns of (a) as-synthesized sample prepared by zirconium sufate and CTAB, and (b) calcined sample.

For many other transition metal oxides, the calcination of the surfactant composites results in a structure collapse. The same holds for composites synthesized from CTAB and zirconium sulfate (Fig. 1, b). Hence, a post-synthetic treatment of the as-synthesized material has been developed; the mesostructured Zr(SO4)2-surfactant composite is treated with 0.5 M phosphoric acid or sulfuric acid solution. Figure 2 shows XRD patterns of the samples treated by different methods. When as-made material is treated with H2SO4 solution followed by calcination at 823 K, the structure is collapsed (Fig. 2, e). On the other hand, the XRD pattern after the phosphoric acid treatment is similar to the as-made composite material, except for a small 0.1 nm shift of the reflections (Fig. 2, a). After calcination at 823 K, the materials show highly intensive (100) reflections and weak (110) and (200) reflections (Fig. 2, b). During calcination a strong shrinkage of the pores occurs which can be seen in the shift of the (100) reflections to smaller d spacings of more than 1 nm. This observation may be interpreted in terms of connecting the uncondensed ZrOH groups and replacing sulfate groups by phosphate groups during the treatment, because phosphate groups (□= 50%) show stronger complexing ability than sulfate [7].

Under present study, our aim is to develop sulfated zirconia with high surface area. Therefore, H3PO4-treated sample should be treated again with sulfuric acid solution by ion-exchange or impregnation method. In case of using ion-exchange method, a substantial structure collapse is observed (Fig. 2, d) whereas the materials impregnated with sulfuric acid solution exhibits an intense (100) reflections, similar to calcined H3PO4-treated sample.

The material so-obtained was analyzed by FT-IR (not shown here). In the as-synthesized sample, a broad intense IR absorption around 1060 cm-1 indicated the presence of phosphate ions in the material, which remained in the final product after calcination. At the same time, the IR absorption bands due to C-H vibrations at 2925 and 2852 cm-1 remained unchanged, indicating that the surfactant was not extracted during the treatment.

Figure 2. XRD patterns of the samples post-treated by different methods; (a) H3PO4- treated sample, (b) calcined H3PO4-treated sample, (c) calcined H3PO4- treated and H2SO4-impregnated sample, (d) calcined H3PO4-treated and H2SO4-treated sample, (e) calcined H2SO4-treated sample.

MSU-4-type zirconia and mesostructured zirconia prepared without surfactant are under investigation.

ACKNOWLEDGEMENT

The financial aid from the Brain Korea 21 Program supported by the Ministry of Education is greatly appreciated.

REFERENCES

[1] V.C.F. Holm, G.C. Bailey, US Patent 3,032,599 (1962), to Philips Petroleum.

[2] B.H. Davis, R.A. Keogh, R. Srinivasan, Catal. Today 20 (1994) 219.

[3] X. Song, A. Sayari, Catal. Rev. Sci. Eng. 38 (1996) 329.

[4] U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schuth, Angew. Chem., Int. Ed. Engl. 35 (1996) 539.

[5] J.A. Knowles, M.J. Hudson, J. Chem. Soc. Chem. Commun. (1995) 2083.

[6] S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia, Science 269 (1995) 1242.

[7] J. Livage, M. Henry, C. Sachez, Prog. Solid State Chem. 18 (1988) 259.