(217ad) Adsorption/Desorption and Transport of Water in Two-Dimensional Hexagonal Mesoporous Silica

The recent development of synthesis technique of porous materials makes it possible to control pore sizes and pore structures in the mesoporous range. Detail analysis of adsorption/desorption and transport of water in well-ordered mesopores could be useful not only to understand the fundamental of transport phenomena in mesopores but also to design porous materials suitable for specific engineering applications such as adsorption, filtration, separation and catalysis techniques.

In this study, we focus on mesoporus silica as desiccant material. Water uptake and release of a Zr doped two-dimensional hexagonal mesoporous silica with pore diameters of 3.8 nm (Zr-MPS) was measured by a gravimetric method in a flow system at atmospheric pressure. The adsorption-desorption isotherms and transport of water in Zr-MPS were measured at 283, 288, 293 and 298 K. The mass of sample particles and total flow rate were determined to be about 3.2 mg and 200 sccm, respectively, by try and error to maintain the stable measurement. The sample was degassed at 573 K for 8 h below 8 × 10^-3 Pa before the first water vapor adsorption measurement. Similarly, the second water vapor adsorption measurement was performed after the once-measured samples had been degassed at 413 K for 8 h.

Type V adsorption-desorption isotherm with significant hysteresis was obtained. The adsorption layer was formed on the pore surface at low relative pressure of water vapor, and capillary condensation was initiated around 0.4–0.6 relative pressure, and then pore filling was completed near the saturated vapor pressure. The relaxation rate of water uptake in the capillary condensation state was much slower than that in the layer adsorption state or pore filling state. In the desorption process, the isotherms and relaxation rates had a similar trend. The effects of the hydrophilicity of mesopores and temperature on the adsorption-desorption kinetics suggested that water transport mechanism in mesopores was not simple molecular diffusion due to concentration gradient, but surface diffusion. In the capillary condensation and evaporation states, the relaxation curve could be estimated by the Fickian diffusion equation at a small stepwise change in relative humidity but it could not be estimated at a large stepwise change in relative humidity. The relaxation curves at a large stepwise change in relative humidity suggested that the transport mechanism could be a liquid water flow due to capillary action.