Pore size modification of mesoporous HMS molecular sieve silicas with wormhole framework structures

Wormhole HMS silica molecular sieves with average framework pore sizes in the range 2.9 to 4.1 nm were assembled from 4:1 molar mixtures of TEOS as the inorganic precursor (I degrees) and dodecylamine as the structure-directing surfactant (S degrees) in 63:27 (v/v) water/ethanol. Increasing the assembly temperature from 25 degreesC to 65 degreesC resulted in an increase in the Horvath-Kawazoe pore size (from 2.9 to 4.1 nm). Also, the ratio of fully cross-linked (Q(4)) to incompletely cross-linked (Q(3)) SiO4 sites (Q(4)/Q(3)) increased more than 2-fold from 1.96 to 4.26 with little or no change in the framework wall thickness (similar to1.0 nm). Analogous results were obtained for HMS mesostructures assembled at an I degrees /S degrees ratio of 10. Postsynthesis remodeling of as-made HMS mesostructures through digestion in hot distilled water resulted in comparable expansions in the framework pore sizes. The pore expansion process, whether achieved through an increase in the direct assembly temperature or through postsynthesis remodeling in distilled water, is consistent with a mechanism based on temperature-dependent changes in the polarity of the S degreesI degrees interface and concomitant changes in the surfactant packing parameter and self-swelling of the structure-directing micelle.