Framework density distribution of zeolite-type tetrahedral nets

Abstract

The porosity of zeolite-type frameworks can be expressed in terms of the framework density (FD), defined as the number of tetra-hedral atoms per 1,000 ų. For the known zeolites and zeolite-type materials, FD values range from 12.5 to 20.2. Here we show that the range of the observed FD values depends on the type and relative number of the smallest rings in the tetrahedral networks. The frameworks of lowest density are those with a maximum number of 4-rings. The minimum framework density increases with the size of the smallest rings in the network. There is a clear gap in the FD values between zeolite and dense frameworks, and this also depends on the type of smallest rings present. These findings are of considerable predictive value in the important field of zeolites and molecular sieves; in particular, the minimum possible FD can be predicted for categories of frameworks defined by the smallest rings. The results of this study can serve as a guide for future efforts to synthesize low-density framework structures.