The role of barium cations in the synthesis of low-silica LTL zeolites

The structure and extraframework atoms of the dehydrated zeolites (Ba,K)-GL (Ba10K16Si36Al36O144) and (Ba,Li)-GL (Ba18Si36Al36O144) are characterized by powder neutron diffraction. The frameworks have the LTL topology and are refined in the hexagonal space group P 6/m c c (no. 192). Barium cations fully occupy the centers of the cancrinite cages (site B) and the positions between adjacent cancrinite cages (site C). In (Ba,K)-GL, potassium cations fully occupy sites above the centers of the eight-rings (site D), which line the pores of zeolite (Ba,K)-GL, whereas in (Ba,Li)-GL, barium cations occupy two-thirds of the site D positions. The syntheses and structural refinements of LTL zeolites show that barium and potassium play analogous structure-directing roles in LTL zeolites with different Si/Al ratios. The nearly identical sizes of barium and potassium allow these cations to have optimal coordination with framework oxygens in sites B, C, and D. Barium (a divalent cation) is used in the synthesis of low-silica LTL zeolites, and its role is equivalent to potassium (a monovalent cation) in the synthesis of higher-silica (Si/Al=3) LTL zeolites where potassium also occupies sites B, C, and D. In the absence of a sufficient amount of barium in the synthesis gel, other phases are formed immediately after all the barium cations have been consumed by the zeolite GL crystals. (C) 1999 Elsevier Science B.V. All rights reserved.