Flexibility in zeolites:²⁹Si NMR studies of ZSM-5 frame transitions

Abstract

Zeolites are most often perceived as rigid solids. Recent evidence has demonstrated that the temperature or the adsorption of molecules whose dimensions approach the pore dimensions induce changes in the solid structure, i.e., a flexing of the solid lattice. For pentacil zeolites, as ZSM-5, a transition between monoclinic and orthorhombic forms of the crystalline structure is found with a change in temperature or the adsorption of ring containing molecules. We find that the temperature at which this transition occurs depends on the Si/Al ratio within ZSM-5 and the discrete, reversible transition can be measured by²⁹Si NMR, confirming prior X-ray studies. A similar dependence is found for ZSM-11. Phosphorous modification of the ZSM-5 does not change the transition temperature; however, steam treating of the zeolite does. The implications of these measurements to the flexibility and to the potential for transport is discussed. An analogy between transport and flexibility in one-dimensional polymers and that in the three-dimensional, open, solid network of zeolites is suggested.