INSIGHTS INTO ZEOLITE BEHAVIOR FROM THE RIGID UNIT MODE MODEL

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