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 CRYSTAL. The zeolitic part of the system has been represented by a perfect crystalline chabazite framework. The Ti content and distribution within the model 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; the calculated adsorption energy is -34.4 (-43.6) kJ mol(-1). The stability and electronic structure of the hydrated sites are shown to be largely dependent 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 within the present model (Delta E = 35.3 W mol(-1)). The activation energy was estimated in about 84 kJ mol(-1).