Topotactic Transformations Of Sodalite Cages: Synthesis And Nmr Study Of Mixed Salt-Free And Salt-Bearing Socialites

A series of mixed sodalite samples, Na-8[Al6Si6O24]Br-x.(H3O2)(2-x), with the unit cell stoichiometries varying in the 0 < x < 2 region, was made by hydrothermal synthesis and subsequently transformed into Na6+x[Al6Si6O24]Br-x.(4H(2)O)(2-x) and Na6+x[Al6Si6O24]Br-x.O2-x sodalites. Here, O refers to an empty sodalite cage. The three series, referred hereafter to as the Br/basic, Br/hydro, and Br/dry series, were characterized by powder diffraction X-ray and by Na-23, Al-27, and Br-81 magic angle spinning (MAS) NMR and high-resolution triple quantum (TQ) MAS NMR spectroscopy. We determined that incorporation of Br- anions is 130 times more preferred than incorporation of H3O2- anions during the formation of sodalite cages, which permitted precise control of the halide content in the solid. Monotonic trends in chemical shifts were observed as a function of cage occupancy, reflecting continuous changes in structural parameters. A linear correlation between Br-81 chemical shift and lattice constant with a slope of -86 ppm/Angstrom was observed for all three series. Likewise, Na-23 chemical shifts for Na+ cations in salt-bearing sodalite cages correlate linearly with the lattice constant. Both results indicate a universal dependence of the Na-23 and Br-81 chemical shifts on the Na-Br distance. The Al-27 chemical shifts of Br/basic and Br/hydro sodalites obey an established relation between delta(cs) and the average T-O-T bond angle of 0.72 ppm/degrees. Br/dry sodalites show two aluminum resonances, characterized by significantly different chemical shifts and quadrupolar interaction parameters. In that series, local symmetry distortions are evident from strong quadrupolar perturbations in the NMR spectra. P-Q values for Al-27 vary between 0.8 MHz in Br/basic sodalites and 4.4 MHz in the Br/dry series caused by deviations from the tetrahedral symmetry of the salt-free sodalite cages. For Na-23, P-Q values of 0.8, 0.8, 2.0, and 5.7 MHz were found for sodium in bromo, basic, hydro, and dry cages, respectively. In addition, both Na-23 and Br-81 spectra offer some evidence that the Br- anions in the Br/dry sodalite are displaced from the center of the expanded sodalite cage. For all three series, the spectral deconvolution of the Na-23 NMR line shapes permits an accurate determination of the mixed sodalite stoichiometry.