Crystal structures of the ammonium and hydrogen forms of zeolite rho

Abstract

To determine whether or not there is a structural basis for the unusually low n-butane sorption capacity observed for the hydrogen form of zeolite rho (H 12Al 12Si 36O 96), and to study further the surprising elasticity of the zeolite rho aluminosilicate framework (a body-centred-cubic arrangement of α-cages joined through double 8-rings), an X-ray powder diffraction study of the H + form (at 500°C) and of its precursor, the NH 4 + form (desolvated at 350°C and examined at 100°C), has been undertaken. Rietveld refinement of the NH 4—rho structure in the space group l 43m ( a = 14.821 A ̊ ) converged with R f = 0.078 and R wp = 0.105. The α-cages exhibit a tetrahedral distortion which produces oval 8-rings and a concomitant shrinkage of the double 8-ring pore openings. The twelve NH 4 + ions per unit cell are crystallographically equivalent and are located at the centre of each of these oval 8-rings. Each ammonium ion is associated with two framework oxygens at 2.89 Å, and four additional ones at 3.14 Å. The H—rho structure refined in the higher space group Im3m ( a = 14.982 A ̊ ) to R f = 0.058 and R wp = 0.108. In contrast to the NH 4—rho structure, this one displays no distortion of the α-cages and has round, fully open 8-rings. The structure offers no explanation for the low sorption observed.