Abstract
Well-ordered as-synthesized siliceous MCM-41 mesoporous molecular sieves can be prepared from the sol of colloidal silica (Ludox HS-40 and AS-40) only under optimized conditions. Two processing parameters have a significant effect on the structure. First, the period of stirring the mixture of Ludox AS-40 or HS-40 and tetraethylammonium hydroxide should be extended to 22 h. Second, the following temperature regime should be used: the reaction mixture consisting of colloidal silica, tetraethylammonium hydroxide, hexadecyltrimethylammonium chloride and water is being first aged at ambient temperature for 20 h, then the temperature is gradually increased and finally the mixture is kept at 100 °C for 40 h. As-made MCM-41 prepared even by an optimized procedure contains after calcination some proportion of an amorphous phase. Extraction with water of the as-made material is a suitable procedure for the improvement of the phase purity as less well-ordered and amorphous particles are preferentially dissolved and thus removed. In this way, a substantial improvement in the quality of MCM-41 molecular sieves can be achieved.
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