메조포어 분자체 MCM-41의 표면 개질

강경구, 이현구

서울대학교 응용화학부·화학공정 신기술 연구소

Surface Modification of Mesoporous Molecular Sieves MCM-41

Kyoung-Ku Kang and Hyun-Ku Rhee

School of Chemical Engineering and Institute of Chemical Processes,

Seoul National University, Seoul 151-742, Korea

INTRODUCTION

The synthesis of M41S materials [1] has greatly expanded the application of porous materials as adsorbents, supports, and catalysts. Their synthesis and utilization have been investigated by a number of researchers because of their characteristics such as large internal surface area, favorable uniformity and easily controlled size of the pore. However, instability of the structure and hydrophilic surface nature of mesoporous molecular sieves put limitations on the range of application. In recent years, there has been a number of researches based on surface modification of a potentially useful chemical part on a mesoporous silica surface [2,3]. In some cases, there is an intentional strategy of making covalent linkages to the mesoporous silica surface. The most common of these covalent strategies generates [Si]―O―Si□ linkages (where [Si] represents a silicon atom at the silica surface) via well-known silylation methods [2,3]. However, the resulting [Si]―O―Si□ linkages may suffer from the of relatively easy solvolysis with polar solvents under certain conditions.

Surface modified silica

The direct attachment of alkyl of aryl groups to the silica surface via    Si□C bonds has been of considerable interest in the development of highly specific surface modifications for catalysis and chemically bonded stationary phase for chromatography [4,5]. Organosilica derivatives bound through [Si]□C bonds appear to be much more stable to hydrolytic decomposition than those tethered through     [Si]□O bonds [6]. Surface modified materials based on [Si]□C linkages may be attractive from the viewpoint of generating new classes of highly stable modified silica, where the pendent alkyl or aryl group may contain some desired chemical funtionality that one wishes to immobilize on a silica surface for use in, e.g., chromatography or catalysis.

Here, we report that mesoporous molecular sieves treated by surface modification show different characteristics from the ones treated by silylation. The modification of mesoporous materials is achieved by the reaction between organic species and chlorinated siliceous surface to yield a direct Si-C bond. We expect that this material may find its applications for various purposes.

EXPERIMENTS

Si-MCM-41

Si-MCM-41 was prepared under hydrothermal condition. A synthesis gel for Si-MCM-41 sample was prepared by the following procedure. The sodium silicate solution (mass composition □ SiO2 : Na2O : H2O = 9.2 :2.4 : 88.4) and ammonium hydroxide solution were combined with CTMABr (cetyltrimethylammonium bromide) solution. The composition of the resultant gel was 4SiO2 : 1CTMABr : 1Na2O : 0.15(NH4)2O : 200H2O. The gel obtained was stirred at room temperature for 1 h. The resulting mixture was placed in an autoclave for hydrothermal reaction in oven, and kept at 373K for 4days. The solid product obtained was filtered, and after the surfactant was extracted, the solid was calcined in air at 823K.

Chlorinated MCM-41

Si-MCM-41 was heated at 423K in a flask for 24 h under dry nitrogen gas. Thionyl chloride and dry benzene were added to the Si-MCM-41 in flask. The resulting suspension was stirred under dry N2(g)	protection with the flask in a 358K oil bath for 48 h. The resulting product was filtered under dry N2(g), and then the remaining thionyl chloride and benzene were removed by evacuation at room temperature.

Reaction of chlorinated MCM-41 with Grignard reagent

Chlorinated MCM-41 suspended in dry benzene was put together with 3.0M methylmagnesium bromide in diethyl ether under dry N2(g) in a flask cooled an ice-water bath. The mixture was stirred for 24 h at room temperature and then for another 4 h in a 323K oil bath. The reaction mixture was then cooled in a dry ice/acetone bath and 1.0M HCl(aq) was added to decompose excess Grignard reagent. The resulting suspension was acidified to pH 4 by addition of 1.0M HCl(aq). The resulting mixture was filtered, washed successively with deionized water, aceton, and methanol, and finally dried under vacuum at 423K for 5 h.

Surface alkylation procedure

Characterization

The structure and order of the MCM-41 were determined by X-ray powder diffraction analysis (XRD). The XRD patterns were obtained by using CuK□ radiation (□=1.5406Å). The morphology of samples were examined by SEM and TEM. Infrared spectra were recorded on Nicolet Impact 410 FT-IR instrument (in the range 4000-500cm-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

The XRD patterns of pure silica MCM-41 showed well-defined (100) reflections. The chlorination and alkylation reaction was carried out under severe condition. However, the chlorinated and the alkyl group substituted samples exhibited well-defined (100) reflections, too. The (100) peak intensity remained almost constant as in the case of pure silica MCM-41. The surface areas of all the samples were over 800 m2/g being typical of MCM-41 and the type Ⅳ isotherm, typical of mesoporous molecular sieves, is maintained. The isotherms exhibit a sharp inflection characteristic of capillary condensation within the mesopores. The sharpness of the steep rise is diminished as a result of alkyl group substituted reaction. These results indicate that the high structural intensity and the pore structure were maintained after the organic reactions.

The FT-IR spectroscopy after alkyl group substitution showed several bands: one broad near 3500 cm-1, the other sharp at 2900-3000 cm-1, and various alkyl group bands were observed. The bands at lower frequencies can be assigned to organic C-H bands of the alkyl group. Terminal Si-OH band intensity of alkyl group substituted samples decreased after the substitution reaction. The amount of surface alkyl group was measured by thermal analysis. Surface modification with bulky alkyl groups is under investigation.

CONCLUSION

Hydrophobic MCM-41 was prepared by surface modification technique. The basic idea is to bind an alkyl substituent chemically to the surface of pure silica MCM-41 via an organic reaction. The hydrophobic MCM-41 was halogenated and reacted with alkyl group. These materials were characterized by using XRD, N2 physisorption, thermal analysis, and FT-IR. Alkyl substituted samples exhibited well-defined (100) reflections and showed uniform pore size and high surface area after substitution reaction. Substitution of alkyl group to MCM-41 was confirmed by FT-IR spectroscopy and amount of alkyl group was measured by thermal analysis.

ACKNOWLEDGEMENT

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

REFERENCE

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