The crystal structure of the synthetic zeolite L

Abstract

The crystal structure of the synthetic zeolite, Linde L, K 6 Na 3 Al 9 Si 27 O 72 · 21 H 2 O, has been determined from powder data at room temperature. The zeolite is hexagonal with unit-cell dimensions a = 18.4 Å and c = 7.5 Å. The structure has been refined assuming space group P 6/ mmm . The aluminosilicate framework is based upon the polyhedral cages which are formed by five six-membered and six four-membered rings and are found in canerinite, erionite and offretite. These cavities are linked through the planes of their upper and lower six-membered ring, thus forming columns in which hexagonal prisms and polyhedra alternate, and which run parallel to the c axis. Each column is crosslinked to three others by single oxygen bridges which form part of planar twelve-membered rings circumscribing wide channels parallel to c . Each channel can be represented as a succession of sections, circumscribed top and bottom by twelve-membered rings shared with upper and lower adjacent sections. The rings have free diameters of 7.1 to 7.8 Å. Four cation positions were found, three of which are outside the main channels and closely surrounded by framework oxygen atoms. The fourth position is in the widest part of the main channel close to the channel wall. The water molecules filling the wide channels are only loosely bound to the framework and appear to have only partial regularity of arrangement. Structures have also been developed for a considerable number of so far unknown zeolites with similarities to zeolite L. All these structures can be allocated to the chabazite family, which however on account of its size and structural variety can be divided into four subgroups.