(612f) Molecular Dynamics Study of Water Vapor Adsorption into Ordered Mesoprous Silica

Microporous and mesoporous materials have been applied as adsorbents. The knowledge of the structural and dynamical properties of adsorbates is essential for the design of adsorbents, but the properties of adsorbates have not yet been clarified. In our previous study [1], the properties of water molecules in zeolite NaX (Si/Al = 1.0) and in zeolite NaY (Si/Al = 2.0) were investigated with molecular dynamics (MD) simulation. The calculation results show that in zeolite NaX, the water vapor adsorption can be expressed by using the Langmuir model with two adsorption sites, but in zeolite NaY, it is not Langmuir-type adsorption. In this study, the MD simulations of hydrated ordered mesoporous silicas were performed using the all atom model. The calculation cell is 39.31x34.04x43.24 Å3, which is derived from 512 orthorhombic unit cell of α-quartz (Si3O6), with the 3-dimentional periodic boundary condition. The diameter of the ordered mesoporous silica is about 2 nm with 15.3/nm2 SiOH groups on the surface. The simulations were performed in NVT ensemble with 0, 48, 96, 144, 192, 240, 288, 336 water molecules in the ordered mesoporous silica. Despite the several previous studies [2, 3], the adsorption phenomena inside the ordered mesoporous silicas have not yet been clarified on the molecular level. The objective of this study is to clarify the structural and dynamical properties of water molecule inside the ordered mesoporous silicas and to obtain the molecular pictures of the adsorption process. The calculation results show that the averaged interaction energy between water molecules are larger than that of the dimer of water molecules, even in a low hydration state, water molecules form clusters, and in an intermediate hydration state, the condensation in the gas phase and the adsorption on the surface occur simultaneously. The radial distribution functions and the running integration numbers support this description of adsorption process. In a low hydration state, the first peak position of the radial distribution functions between oxygen atoms of water molecules (OW?OW) is the same to that of liquid water, implying that the clusters of water molecules appear in the gas phase at low hydration. And the running integration number between oxygen atoms of water molecules?oxygen atoms of mesoporous silica (OW?OM) increases even in a high hydration state, implying that the adsorption on the surface occurs at high hydration, in other words, the condensation occurs before fully occupying the adsorption sites on the surface. For the thermodynamic properties, the differential heat of adsorption was calculated from the derivative of the potential energy with respect to the number of water molecules [1]. The potential energy can be decomposed into three terms: the potential energies between water?mesoporous silica, water?water, and mesoporous silica?mesoporous silica. The derivatives of the potential energy with respect to the number of water molecules can also be decomposed in the same manner. The differential heat of adsorption is almost constant in the whole hydration states, but the differential heat of adsorption due to the interaction between water?mesoporous silica decreases sharply with increasing the number of water molecules from low hydration state to intermediate hydration state but keeps constant at high hydration. On the other hand, the differential heat of adsorption due to the interaction between water?water has the opposite trend. This calculation result suggests that although the total differential heat of adsorption is independent of the hydration states, the adsorption mechanism varies with the hydration states.

References (1) K. Shirono, A. Endo and H. Daiguji J. Phys. Chem. B 109 (2005) 3446-3453. (2) J. Puibasset and R. J.-M. Pellenq, Phys. Chem. Chem. Phys. 6 (2004) 1933-1937. (3) P. Gallo, M. Rapinesi, and M. Roverea, J. Chem. Phys. 117 (2002) 369-375.