Optimal parameters for the synthesis of the mesoporous molecular sieve [Si]-MCM-41

Abstract

Highly crystalline MCM-41 with a very narrow pore-size distribution (FWHM=1.5 Å), high surface area (1185 m2 g^-1), large grain size and thick channel walls (ca. 17 Å) was prepared in alkali-free media. The properties of the product depend on the source and concentration of the reactants, the gel aging time, the temperature and the duration of the synthesis. There is no induction period in the course of the synthesis and Ostwald's rule of successive transformations applies. The initially produced hexagonal phase is transformed into the lamellar phase and then into an amorphous phase. In the 150°C synthesis the most stable product is amorphous silica. The course of the synthesis is conveniently monitored by pH measurement. Gel aging, during which a spatial distribution of silicate polyanions and micellar cations is established, is essential for preparing high-quality MCM-41. Surfactants with the same cationic organic group but different counter-anions alter the course of the synthesis. The degree of polymerization of silica is also important. Highly basic gels favour the lamellar product; when the gel is weakly basic the quality of MCM-41 is lower as insufficient TMAOH is available to dissolve the silica. The result of excess silica is similar but even more pronounced. Purely lamellar products are made at low SiO₂ concentrations, when the gel is more strongly basic. The best quality MCM-41 is prepared from a gel of molar composition SiO₂:0.19 TMAOH:0.27 CTABr:40 H₂O (with CTABr/SiO₂=0.27, similar to the ratio in the solid product) aged at 20°C for 24 h and synthesis lasting for 48 h.

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