Cation Location in Dehydrated Na−Rb−Y Zeolite:  An XRD and IR Study

High-quality powder XRD data of a dehydrated Na−Rb−Y zeolite with Si/Al = 2.7, collected at the ESRF beam line BM16, are analyzed. The quality of the Rietveld refinement is very satisfactory, as confirmed by the low Rwp and Rp values of 7.77% and 6.02%, respectively, and by the comparison of the Rb+/Na+ ratio (1.36(6)) with the experimental one (1.38) obtained by analytical methods. We have found that 9.3(3) Na+ cations are located in site SI, 8.3(6) Na+ and 5.4(6) Rb+ cations in site SI‘, and 2.9(6) Na+ and 22.4(6) Rb+ cations in site SII. No cations have been observed in site SIII. Less than 4 cations out of 51.8 remain to be located in the unit cell, which has a cell parameter of a = 24.7673(4) Å, (V = 15192.8(7) Å3). The low occupancy by Na+ of sites in the supercage (site SII) is confirmed by a parallel IR study of CO and N2 adsorbed at low temperature, where carbon monoxide and dinitrogen have been found to interact mainly with the Rb+ cations. The spectroscopic study of the stretching vibrations of Na+···CO and Na+···N2 adducts allows the estimation of a Rb+/Na+ ratio of approximately 9 in site SII, in close agreement with the ratio obtained from the Rietveld refinement, 8(2).