Surface Chemistry of Mesoporous Materials: Effect of Nanopore Confinement
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Surface Chemistry of Mesoporous Materials: Effect of Nanopore Confinement Yifeng Wang1, Charles Bryan1, Huifang Xu2, Huizhen Gao1 1 2
Sandia National Laboratories, Carlsbad, New Mexico 88220; Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116;
Abstract Acid-base titration and metal sorption experiments were performed on both mesoporous alumina and alumina particles under various ionic strengths. It has been demonstrated that surface chemistry and ion sorption within nanopores can be significantly modified by a nano-scale space confinement. As the pore size is reduced to a few nanometers, the difference between surface acidity constants (∆pK = pK2 – pK1) decreases, giving rise to a higher surface charge density on a nanopore surface than that on an unconfined solid-solution interface. The change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on that nanopore surfaces.
1. Introduction Functional materials that can effectively remove specific ions from aqueous solutions are of great interest for chemical separation and environmental cleanup applications. Recent progress in the synthesis of nanostructured materials opens a new arena for developing such materials. Mesoporous materials synthesized using supramolecular templating processes (1) have attracted particular attention, due to their large specific surface areas and controllable nano-scale pore size and geometry. Mesoporous silica with a monolayer of thiol (-SH) group grafted on its pore surface displays a high sorption capacity for removing mercury from aqueous solutions (2, 3). Uncalcined mesoporous silicate materials synthesized with hexadecyltrimethylammonium bromide as a template are able to remove significant amounts of trichlorethylene and tetrachloroethylene from water (4), while calcined mesoporous silicates or titanosilicates were found to have a capability for removing copper, lead, and uranyl ions from aqueous solutions (5 - 7). Furthermore, an ordered mesoporous anion-exchange inorganic/organic hybrid resin has been suggested for radionuclide separation (8). A mechanistic understanding of ion sorption by mesoporous materials is yet to be understood. It is generally known that a condensed phase could exhibit different physical and chemical properties than its bulk phase as its dimension is reduced to a nanometer scale (9, 10). In this paper, we want to demonstrate that surface chemistry and ion sorption within nanopores can be significantly modified by a nano-scale space confinement, as a result enhancing ion sorption onto nanopore surfaces.
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2. Experiments To isolate the possible effect of nanopore confinement, we conducted acid-base titration and Zn sorption experiments on both mesoporous and non-mesoporous alumina materials under various solution ionic strengths (11). The mesoporous alumina was purchased from Aldrich Chemical Company, Inc. This material has a “worm hole-like” structure with a pore size of ~ 2 nm x 2 nm x 10 nm
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