OPTIMAL PARAMETERS FOR THE SYNTHESIS OF THE MESOPOROUS MOLECULAR-SIEVE [SI]-MCM-41

Highly crystalline MCM-41 with a very narrow pore-size distribution (F WHM = 1.5 Angstrom), high surface area (1185 m(2) g(-1)), large grain size and thick channel walls (ca. 17 Angstrom) was prepared in alkali- free media. The properties of the product depend on the source and con centration of the reactants, the gel aging time, the temperature and t he duration of the synthesis. There is no induction period in the cour se of the synthesis and Ostwald's rule of successive transformations a pplies. The initially produced hexagonal phase is transformed into the lamellar phase and then into an amorphous phase. In the 150 degrees 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 micella r cations is established, is essential for preparing high-quality MCM- 41. Surfactants with the same cationic organic group but different cou nter-anions alter the course of the synthesis. The degree of polymeriz ation of silica is also important. Highly basic gels favour the lamell ar product; when the gel is weakly basic the quality of MCM-41 is lowe r as insufficient TMAOH is available to dissolve the silica. The resul t of excess silica is similar but even more pronounced. Purely lamella r products are made at low SiO2 concentrations, when the gel is more s trongly 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 degrees C for 24 h and synthesis lasting for 48 h.