Synthesis of OFF-type zeolite in a quasi non aqueous medium:: structure directing role of p-dioxane and alkaline cations

Syntheses were performed at 150 degrees C with the starting composition 10 SiO2:1 Al2O3:a Na2O:b A(2)O:c p-dioxane:d H2O where A represents an alkaline cation (K, Rb, Cs and Li). In the presence of potassium a pure OFF-type zeolite is obtained for a Na/K molar ratio equal to 2, independently of the p-dioxane content. The OFF-type solids were characterized by XRD, SEM Si-29 and Al-27 MAS NMR spectroscopy. Using C-13 CP MAS NMR spectroscopy, thermal and chemical analysis, one molecule of p-dioxane was found per unit cell; it is probably trapped in the gmelinite cage (1 cage per unit cell) as a complex with sodium. Indeed, in the absence of sodium, an LTL-type zeolite free of p-dioxane crystallizes. Concerning the other alkaline cations (Rb, Cs and Li), an OFF-type material was obtained with rubidium, cesium directs the synthesis toward analcime, whereas no real structure-directing effect was observed for lithium. Indeed, in the latter case, a mixture of GIS-, MAZ-, MOR- or SOD-type zeolites was obtained. (C) 1999 Elsevier Science B.V. All rights reserved.