Structure and energetics of water-silanol binding on the surface of silicalite-1: quantum chemical calculations

Quantum mechanical calculations have been carried out to investigate the structural properties and the interaction between water molecules and silanol groups on the surface of silicalite-1. The (010) surface, which is perpendicular to the straight channel, has been selected and represented by three fragments taken from different parts of the surface. Calculations have been performed using different levels of accuracy: HF/6-31G(d,p), B3LYP/6-31G(d,p), HF/6-31++G(d,p), and B3LYP/6-31++G(d,p). The basis set superposition error has been taken into account. The geometry of the silanol groups and that of the water molecules have been fully optimized. The results show that the most stable conformation takes place when a water molecule forms two hydrogen bonds with two silanols, with only one silanol lying on the opening of the pore of the straight channel. The corresponding binding energy is -48.82 kJ/mol. These areas are supposed to be the first binding sites which have to be covered when the water molecule approaches the surface. When the water loading increases, the next favorable silanols are those of the opening of the pore in which the four possible complex conformations yield a binding energy between -25.62 and -37.41 kJ/mol. It was also found that the calculated O-H bond length of the silanol in the free form was slightly shorter than that in the complex. In terms of the stretching frequency, the complexation leads to a red shift of the O-H stretching of the silanol group.