Synthesis of Aluminosilicate Natrolites and Control of Their Tetrahedral Atom Ordering

Natrolite (framework type NAT) is a naturally occurring zeolite with an ideal chemical composition (Na16(H2O)16) [Al16Si24O80] and a two-dimensional pore system consisting of 8-ring (2.6 × 3.9 Å) channels intersected by 9-ring (2.5 × 4.1 Å) channels.(1) This small-pore material is characterized by a completely ordered distribution of Si and Al over the three crystallographically distinct T-sites in Fdd2 (orthorhombic) symmetry with a high degree of framework flexibility.(2) Among the members of the natrolite family of zeolites, in addition, tetranatrolite, which has a similar chemical composition to that of natrolite, shows a complete disorder over the two T-sites in I42d (tetragonal) symmetry.(3) This family of zeolites is reported to be potentially useful for the selective separation of the smallest gases like H2 and He,(4) as well as of highly radioactive actinides.(5) Despite its relative abundance in nature, however, natrolite has been found very difficult to crystallize under laboratory conditions. While there are only a few papers regarding its synthesis in the literature,(6) this mineral has always been obtained together with a considerable amount of impurity phases like analcime (ANA) or gismondine (GIS).