Water Vapor Adsorption Properties of Ti-Containing Mesoporous Silica.

Abstract

Materials enabling intelligent humidity control were synthesized using fumed silica, tetraethylorthotitanate and quaternary alkylammonium surfactant (decyl-, dodecyl-, tetradecyl and hexadecyltrimethylammonium chlorides) as a liquid-crystal template under hydrothermal conditions. X-ray diffraction profiles and transmission electron micrographs indicated that a homogeneous hexagonal structure of SiO2-TiO2 was formed from those products. The mesostructured products had high BET surface areas from 540 to 863m2/g, with uniform pore diameters of 1.5-3nm. These physical quantities were controlled by varying the size of the organic template. Water vapor adsorption isotherms for these materials experienced a sharp increase in adsorption when the relative water vapor pressure (P/P0) was in the range of 0.4-0.6. Over this range of pressures, the maximum amount of adsorbed water was between 20 and 50mass%. The isotherms were classified as type V in the IUPAC notation, showed an unusually large hysteresis. The large hysteresis could be explained by the difference in contact angle between adsorption and desorption. For both the 2nd and 3rd. water vapor adsorption-desorption isotherms, any morphological change was not observed in the isotherms, meaning that these materials have high durability in their humidity control ability. A small shift of the broad Q4 29Si-MAS/NMR spectra indicated that a wider distribution of the Si-O-Si angles in the SiO4 tetrahedra when titanium was incorporated into the silica matrix.