Theoretical Studies on VPI - 5 . 3 . The MS - Q

Aluminophosphate zeolite is an artiicial material which shows strange hydrophilic-ity. We have been investigating the reason by quantum mechanics, and found that the hydrophilicity of this zeolite depended on the local geometric deformation, and speculated that the site-speciic hydrophilicity might be explained by diierences in the stiiness of the local deformation determined by the location with the pore structure. In order to test these ideas, we construct a reliable force eld based on the new variable charge, morse stretch (MS-Q) method developed by Demiralp, C aa ggn, and Goddard. Our force eld well reproduces the experimental structure of VPI-5. Aluminophosphate zeolite (AlPO 4) 1] is an artiicial material in which tetrahedral AlO 4 and PO 4 are linked alternately in three dimensions to be a regular pore structure with no defect. This structure makes the AlPO 4 electronically neutral with neither the counter cations nor Bronstead acid sites like standard natural zeolites. Consequently, AlPO 4 is expected to have adsorption, catalysis, and separation properties diierent from standard zeolites. Based on the cluster model from quantum mechanics (QM) calculations 2], we have used molecular dynamics (MD) simulations to study the hydrophilicity of AlPO 4. Despite the lack of a hydrophilic site, AlPO 4 has a strong hydrophilicity as connrmed by isothermal adsorption experiments 3]. One might assume that this hydrophilicity is merely a consequence of the diierence in electronegativity between Al(1.5) and P(2.1). However, recent experimental data 4] shows that some aluminum sites (denoted as a site) can adsorb two water molecules whereas other sites (denoted as b site) adsorb none. This implies that structural character must play a role in hydrophilicity of AlPO 4. In the AlPO 4 frame, the a site is located at the intersection of two rings with four oxygen atoms, and the b site is located at the intersection of a ring with four oxygen atoms and a ring with six oxygen atoms. To investigate the hydrophilicity of AlPO 4 , we made cluster models from the hy-drate 4] and dehydrate 5] forms of VPI-5 6] (an AlPO 4 containing rings with 18 oxygen atoms). We carried out ab initio QM calculations (Hartree-Fock level with basis set su-perposition error correction) on the reaction of a water molecule with a cluster model of VPI-5. We found the following results which were connrmed quantitatively using the energy decomposition method 7]: (1) No water molecule is adsorbed at the …