Interaction of Ti-zeolites with water. A periodic ab initio study

The interaction of water molecules with Ti sites in Ti-containing zeolites has been studied by means of the ab initio periodic Hartree-Fock program CR YSTAL. The zeolitic part of the system has been represented by a perfect cr ystalline chabazite framework. The Ti content and distribution within the m odel structure have been designed so as to keep the maximum symmetry. Water physisorption has been accomplished by adding one (two) water molecule to each symmetry equivalent Ti-site; the geometry has been fully optimized; th e calculated adsorption energy is -34.4 (-43.6) kJ mol(-1). The stability a nd electronic structure of the hydrated sites are shown to be largely depen dent on the framework flexibility. The hydrolysis of the Ti-O-Si bridge to yield Ti-OH and Si-OH has also been studied. This reaction is endothermic w ithin the present model (Delta E = 35.3 W mol(-1)). The activation energy w as estimated in about 84 kJ mol(-1).