Pore Size Modification of Mesoporous HMS Molecular Sieve Silicas with Wormhole Framework Structures

Abstract

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 temperaturedependent 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.

Post-synthesis of Thick Wall Mesoporous Silica with Wormhole Framework Structure Using Surfactant Micelles and Silica Source Incorporated Inside the Channels of Silica Precursor with Large-mesopores

“…On the other hand, XRD pattern of LP-MPS sample does not show a certain specific symmetry of the mesostructure, suggesting that the degree of structural order of LP-MPS is lower than that of MCM-41 sample. However, a wellexpressed low angle reflection for LP-MPS is indicative of the average pore-pore correlation distance in a wormhole framework structure [16][17][18]. This is consistent with TEM image of LP-MPS as shown in Figure 2(a).…”

Pd/SiO2 catalysts: synthesis of Pd nanoparticles with the controlled size in mesoporous silicas

“…4). Metal introduction seems to disturb the local symmetry in the channel packing and leads to a loss in the integrity of the worm-like HMS framework structure [55]. In contrast, the Pd/SBA-15 composites conserve the well-ordered 2D hexagonal array of the support (Fig.…”

“…Higher order reflections were not resolved. Nevertheless, it is assumed that these materials possessing a wormhole framework structure can exhibit local hexagonal symmetry [55,57]. As the surfactant chain length increased from C 12 to C 18 , the position of the single (1 0 0) reflection shifts toward lower 2Θ values, indicating the increase of the average distance between the pore centres.…”

“…Hexagonal molecular sieves (HMS) were prepared via a neutral (S 0 I 0 ) templating route as described elsewhere [55]. The primary amines-dodecylamine and octadecylamine-were used as neutral structure directors in order to control a pore size of the resulting mesoporous silicas [16].…”

Characteristics of Au/HMS catalysts for selective oxidation of benzyl alcohol to benzaldehyde

“…HMS silicas, form a spongelike particle texture through the intergrowth of mesoscopic wormhole framework domains, and have found promising applications as heterogeneous catalysts and as supports for the immobilization of reagents [20]. A simple method for tailoring the framework pore size of HMS materials in the presence of a single amine surfactant could make them even more attractive for such applications.…”