Structures and interactions of guest molecules with the silanol groups on the (010) surface of silicalite-1 : quantum chemical calculations

Abstract

Quantum mechanical calculations have been carried out to investigate structural properties and interaction between guest molecules, water and methane, and silanol group on the surface of silicalite-1. The (010) surface which 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), including MP2/6-31++G(d,p) for methane. Geometry of the silanol groups as well as those of guest 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, only one lies on the opening pore of the straight channel. The corresponding binding energy is -13.84 kcal/mol. These areas are supposed to be the first binding sites which have to be covered when water molecule approaches the surface. When water loading increases, the next favorable silanols are those of the opening pore in which the four possible complex conformations yield the binding energy of approximately -8 kcal/mol. In terms of vibrational frequency, complexation leads to red shift of the O-H stretching of the silanol group. The estimated energy barrier for water molecule to enter into the silicalite-1's pore is amount to 9.08 kcal/mol. For methane molecule, situation is different. The calculated binding energies are within thermal fluctuation at room temperature. This leads to a clear conclusion that methane molecule does not absorb on the (010) surface of silicalite-1. In addition, the entering process for methane molecule is observed to be barrier free.