Structure-stability relationships for all-silica structures.

We will discuss predictions for the stability and structures of all-silica zeolitic structures. We will show that we have derived a new shell model potential, based on ab initio calculations , which gives good predictions for the structure and stability of all-silica structures. We will compare three shell model predictions with two rigid-ion model predictions for the structure and stability of silicates. We will show that shell model predictions for the relative stability of zeolites are much closer to experiment than rigid-ion model predictions, due to the canceling of covalent and electrostatic terms in the shell models which does not occur in the rigid-ion models. Using the potentials with the highest predictive power on both stabilities and structures of silicates we will discuss structure-stability relationships that have been proposed in literature. The synthesis of high silica zeolites is important for catalysis because of their thermal, hy-drothermal and acidic stabilities, good resistance to aging and in particular hydrophobicity which favors the reaction with organic molecules 1]. Despite the new high and pure-silica zeolites that have been reported 2], the relation between crystal structure and thermody-namics is still poorly understood. Insight into the factors that govern the relative stability of silica zeolites is important for their synthesis which is still a diicult process 2]. Model-ing provides a powerful tool for gaining that insight for several reasons. Many more silica zeolites, then have been synthesised yet, can be modeled so that experimentally observed relations between structure and stability can be tested more thoroughly 3..8]. The calculated data and the structure are consistent, whereas sometimes in literature 9] measured enthalpies on alumina free samples are correlated with structural data on samples which contain Al, due to the lack of structure data on pure-silica samples. Furthermore, only for a few systems there exists single-crystal data and other structural data are obtained from X-ray powder diiraction data which is considered less reliable 9]. Given a good potential, structures can be predicted which are even more accurate than the X-ray powder data 10, and this work]. In this paper we will discuss predictions for the stabilities and structures for pure-silica zeolites. Our rst aim is to make an extensive comparison between our recently developed shell model potentials 11,12], denoted as the BJS I and BJS II potential, with the shell model potential of Catlow et al. 7] denoted as the JC potential, and the rigid-ion model potentials …