Insights into zeolite behaviour from the rigid unit mode model

Abstract

Kinetic processes in zeolites are heavily affected by the inherent flexibility of the underlying frameworks that zeolitic materials are composed of. Such frameworks support vibrational modes that propagate through crystals without distorting the constituent tetrahedra at all, these being “Rigid Unit Modes” (RUMs). Zeolites are unusual in that they support such modes at all possible wave vectors k. This means that “local RUMs” can be formed from the continuum of Bloch (non-local) RUMs. A local RUM can give rise to a localised, non-periodic, distortion in a framework that costs very little energy. These distortions enable molecules to diffuse through a framework more easily, and provide better binding sites for adsorbed ions. In particular local RUMs enable ions and molecules to change the environment of their cages to give more favourable conditions where effective catalysis can occur. Local RUMs also enable one to discover which parts of a framework are most flexible and hence more likely to be catalytically active.