Quantum chemical calculations on the stability of different conformations of silicate building block structures in relevance to zeolite synthesis

Abstract

The relative influence of various geometric parameters such as bond lengths, bond angles and dihedral angles on the energetics of silicate structures is studied. The influence of dihedral angles, in particular, on the preferred conformation and energy of silicate building blocks is highlighted. The possibility of formation of ring structures and the diameters of the rings will depend on these Si-O-Si-O and O-Si-O-Si dihedral angles. Different conformations were obtained for a dimer cluster model by varying the two types of dihedral angles and their relative stability was studied using quantum chemical calculations. The study of variation of the dihedral angles on the stability of silicate structures is further extended to larger cluster models. The importance of dihedral angles is discussed in the context of formation of silicate building blocks during the synthesis of zeolites. The combinations of Si-O-Si-O and O-Si-O-Si dihedral angles, such as 0° and 60°, 120° and 300° etc., leading to staggered adjacent SiO₄ groups are energetically preferred. The energetic preference of the staggered structures over eclipsed structures is an order of magnitude higher for pentamers cluster models than for dimers cluster models.