Synthesis and crystal structure of cyanate sodalite |Na8(OCN)2|[Al6Si6O24]

Cyanate sodalite was prepared by different methods: hydrothermal alkaline transformation of various aluminosilicates in the presence of sodium cyanate salt and high temperature intra-cage anion oxidation reaction of well known cyanide sodalite. Influence of temperature, reaction time as well as NaOH- and cyanate concentration was checked during the hydrothermal runs using kaolinite or zeolites A, X and LSX as source materials. Parameters of intra-cage cyanide oxidation reaction were temperature, heating rate and gas atmosphere (air or CO2). All products were characterized by X-ray powder diffraction and IR-spectroscopy. Crystallization in the cyanate containing system was mainly ruled by dissolution kinetics of the different aluminosilicate source materials hand in hand with progress of hydrolysis of the cyanate anions. The experiments clearly show that hydrothermal synthesis of pure cyanate sodalite is not possible due to rapid hydrolysis of the template salt. Thus only sodalites with low cyanate content were obtained from mild hydrothermal syntheses. The degree of cage filling with OCN- was not higher than 45% and most of the remaining cages were filled with water and some also with carbonate or hydroxide. In contrast, nearly pure cyanate sodalite was observed as product of intra-cage reaction from heating up cyanide sodalite in carbon dioxide atmosphere. The structural features of cyanate sodalite were further investigated by Rietveld refinement of X-ray powder data. The Si and Al atoms of the aluminosilicate framework are completely ordered and the average Al-O and Si-O bond lengths were calculated to 175 and 160 pm, respectively. Whereas the carbon atoms of the cyanate anions are located at the centre of the sodalite cages the oxygen- and nitrogen atoms were refined on undistinguishable positions x, 0, 0 and -x, 0, 0. Guest anions are positionally disordered, but dynamic averaging of the cyanate positions is suggested as known from thiocyanate sodalite. The aim of this work was to prepare an interesting material for future studies of intra-cage reactions of cyanate inside a sodalite host matrix and to develop new applications of sodalites as reservoir mineral for special guest anions. (C) 2003 Elsevier Inc. All rights reserved.