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Mezoporowate sita molekularne : otrzymywanie i właściwości

Kowalak S., Stawiński K.

Wiadomości Chemiczne

2000

[Z] 54, 9-10

817-843

The crystalline molecular sieves comprised almost exclusively microporous materials (zeolites and zeolite-like materials). The pore diameter of this materials was mostly in the range 0,3-0,8 nm. Despite a very narrow range of this diameter, which limits the number of molecules to be applied, the crystalline molecular sieves have attained a great commercial importance for many industrial processes (selective adsorption, ion-exchange, catalysis). The attention of many research groups was focused on syntheses of larger pore structures accessible for bulkier molecules. It was not sure, however, whether such structures could be achieved and if so, whether they would be satisfactory stable to be applied for practical purposes. The successful syntheses of crystalline structures of VPI-5, JDF-20, Cloverite, and UTD-1 indicated that extra-large diameters above 1 nm could be obtained. The structures, on the other hand, appeared not very stable and not easy to be synthesized and modified. Therefore, they have not been commercially applied. The important milestone in modem history of the molecular sieves was discovery of novel family of mesoporous, well organized materials in early ninetieths. This new family of the molecular sieves have been presented independently by Mobil and Toyota at the 9th International Zeolite Conference in Montreal. The first examples of the materials consisted exclusively of silica. Soon later the aluminosilicate mesoporous molecular sieves have been presented and then many other chemical compositions have been employed for synthesis of mesoporous molecular sieves. The materials of this kind are generally called M41S. The principle of synthesis of these materials shows some similarity with the preparation of zeolites and zeolite-like materials in respect to employing of the organic compounds as structure directing agents. The role of a template agent plays usually surfactants such as long alkyl chain amines, which form micelles in a solution. The micelle aggregates are then surrounded by inorganic precursors and the well organized array of voids (e.g. tubes) filled with surfactant molecules are formed due to condensation of an inorganic phase. The organic compounds are removed from the pores mostly by means of thermal treatment. A variety of surfactants applied allows to prepare various structures of different pore sizes (2-10 nm). The growing family of mesoporous molecular sieves contains many unique structures. The most common are: the hexagonal unit cell MCM-41, cubic MCM-48, lamellar structure MCM-50, although the number of defined structures is to date many times higher. Contrary to zeolites and other microporous molecular sieves the M41S are not crystalline materials. The XRD indicates usually only one or few reflections at very low range of 2-theta angle. The mesoporous molecular sieves show very high surface areas (usually above 1000 m2/g) and their adsorption/desorption isotherms of type IV often indicate a hysteresis loop. The transmission electron micrographs indicate a pore array in the lattice image. It has been hoped that the novel materials would show a similar catalytic activity as zeolites. These expectations have not been proven satisfactory so far. The catalytic activity of aluminosilicate mesoporous materials resembles rather activity of the amorphous aluminosilicates. The always growing variety of mesoporous structures and of their chemical compositions provides a chance of finding the materials of high and stable catalytic activity. The pore system of this materials is widely studied as a support accommodating the introduced catalytically active compounds. The mesoporous molecular sieves can be used for many other applications such as adsorption (storage of gases), matrices for microelectronic and optical devices, active fillers for polymers etc.