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

Wormhole HMS silica molecular sieves with average framework pore sizes in t he range 2.9 to 4.1 nm were assembled from 4:1 molar mixtures of TEOS as th e inorganic precursor (I degrees) and dodecylamine as the structure-directi ng surfactant (S degrees) in 63:27 (v/v) water/ethanol. Increasing the asse mbly 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 ful ly 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 chang e in the framework wall thickness (similar to1.0 nm). Analogous results wer e obtained for HMS mesostructures assembled at an I degrees /S degrees rati o of 10. Postsynthesis remodeling of as-made HMS mesostructures through dig estion in hot distilled water resulted in comparable expansions in the fram ework pore sizes. The pore expansion process, whether achieved through an i ncrease in the direct assembly temperature or through postsynthesis remodel ing 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 o f the structure-directing micelle.