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 o ) and dodecylamine as the structure-directing surfactant (S°) in 63:27 (v/v) water/ethanol. Increasing the assembly temperature from 25 °C to 65 °C 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 ) SiO 4 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 (∼1.0 nm). Analogous results were obtained for HMS mesostructures assembled at an I o /S° 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°I o interface and concomitant changes in the surfactant packing parameter and self-swelling of the structure-directing micelle.