Synthesis of (Alumino) Silicate Materials Using Organic Molecules and Self-Assembled Organic Aggregates as Structure-Dir
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MARK E. DAVIS, CONG-YAN CHEN, SANDRA L. BURKETT AND RAUL F. LOBO Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125
ABSTRACT Microporous zeolites can be synthesized using organic structure-directing molecules whose function is to organize inorganic species into particular topologies that then spontaneously self-assemble into the final crystalline materials. Extension of the zeolite assembly process to the use of organic molecular aggregates as structure-directing agents yields ordered mesoporous materials like MCM-41 and MCM-48. A unifying picture of the assembly processes ocurring in the syntheses of micro- and mesoporous materials is presented.
INTRODUCTION Zeolites and other crystalline, microporous solids have found great utility as catalysts and sorption materials. One of the reasons why this is so is because these crystalline oxides contain large internal surface areas accessible through uniformly sized pores. To date, the largest proven pore size is approximately 12-13 A in the material called VPI-5.' The quest for larger pore, crystalline materials is ongoing. Recently, workers at Mobil prepared the first ordered mesoporous materials. 2,3 These (alumino)silicate solids contain pores of uniform size in the range of - 15-100 A. Thus, for the first time, solids possessing mesopores with size uniformity comparible to that observed in crystalline, microporous materials can be prepared. Two types of mesoporous materials can be synthesized: (i) (alumino)silicates containing hexagonally arranged one-dimensional pores of tunable diameter from -15 to 100 A (denoted MCM-41) and (ii) (alumino)silicates possessing a three-dimensional pore system of size -30 'Aand revealing an X-ray diffraction pattern that can be indexed with cubic symmetry (space group : Ia3d). 2,3 These novel materials are prepared using organic surfactant molecules and a liquid crystal templating mechanism is proposed. 2 ,3 This mechanism involves the formation of organic liquid crystals that participate as structure-directing agents in the preparation of the final organic-inorganic composite material. There is a long history of "templating" or structure-direction in the synthesis of crystalline, microporous materials. However, the organic-inorganic interactions are thought to occur between inorganic species and single organic molecules. With the mesoporous materials, the organic molecules must first self-assemble into an aggregate of some type to function as a structure-directing agent. We have analyzed the synthesis of MCM-41 and have proposed a mechanism for its formation. 4 ,5 Our mechanistic proposal is discussed below. 831 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
The objectives of this work are to: (i) review several salient features of structuredirection in the synthesis of crystalline, microporous materials, (ii) develop an overall mechanism for the synthesis of ordered mesoporous solids (MCM-41 and MCM-48) and (iii) show the relationships between the mechani
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