Formation of active sites in TS-1 by hydrolysis and inversion

The interaction of water molecules with Ti sites in titanium silicalite has been studied using a hybrid quantum mechanical/molecular mechanical approach. We have examined the structure and stability of a number of structurally different sites, many of which are based on the hydrolysis and inversion of tetrahedral sites in the zeolitic framework. The hydration of all considered Ti centers is found to be exothermic, which is in good agreement with experiment and previous theoretical work. Under such conditions, the most stable configuration considered is the bis(aquo) tripodal model, indicating that hydrolysis and inversion of tetrahedral sites also helps to increase the reactivity of the Ti site toward water. Our results further show that the insertion of one water molecule increases the coordination of the Ti center from four to five in the tetrapodal, tripodal, bipodal, and titanyl configurations and from five to six in the 2MR structure. In the presence of two water molecules, however, only one of the molecules binds directly to the Ti center; thus, the coordination remains the same as the mono(aquo) complex. In the bis(aquo) complexes, there is also a strong hydrogen-bonding network between the hydrogen of the first water molecule and the oxygen of the second water molecule. We find that the local structural features of the resulting series of tetra- and tripodal Ti moieties are in good agreement with data from EXAFS studies.