Synthesis, X-ray diffraction and MAS NMR characteristics of nitrate cancrinite Na 7.6[AlSiO 4] 6(NO 3) 1.6(H 2O) 2

Abstract

Investigations on the hydrothermal formation and the crystal structure of nitrate cancrinite have been carried out in the system Na 2O– xSiO 2–Al 2O 3–NaNO 3–H 2O, 1< x<6. Various starting materials have been examined to find the best reaction system for cancrinite formation under low temperatures and pressures (473 K, autogenous pressure). Additional experiments were performed at elevated conditions at 773 K and 0.15 GPa. Polycrystalline nitrate cancrinite and various amounts of unchanged educts were obtained in all low-temperature experiments. In contrast, in the high-temperature/high-pressure experiments a crossover of sodalite and cancrinite formation was observed as a function of the Si/Al ratio within the starting system for x=1.0, 2.0, 4.0 and 6.0 (i.e. at Si/Al ratios of 0.5, 1.0, 2.0 and 3.0). Structural investigations of nitrate cancrinite were performed by IR and MAS NMR spectroscopy of the nuclei 29Si, 27Al and 23Na. The results confirmed the alternating Si, Al ordering of the alumosilicate framework for a Si/Al ratio of 1.0 for all samples, independent of the Si/Al ratio of the educts as well as the temperature and pressure of crystallization. A distribution of the quadrupolar interaction of the sodium cations caused by the enclathrated water molecules and motional effects can be suggested from the 23Na MAS NMR spectrum. The single line with its slightly asymmetric shape makes it impossible to distinguish the sodium cations inside the structure channel from those located in the ε-cages of the cancrinite structure. In order to complete the study on nitrate cancrinite a single crystal structure refinement was carried out. As in the case of carbonate cancrinite, NO 3 − anions are distributed over two nonequivalent positions in the hexagonal channel of the framework. All the crystal water molecules were found to be arranged in the ε-cages of the cancrinite structure.