Characterization of Extra-Framework Cation Positions in Zeolites NaX and NaY with Very Fast 23Na MAS and Multiple Quantum MAS NMR Spectroscopy

Abstract

23Na MAS and two-dimensional multiple quantum MAS (MQMAS) NMR techniques have been used to identify the sodium cations in zeolites NaX and NaY, and to investigate changes in the distribution of the sodium cations on adsorption of hydrofluorocarbon-134 (HFC-134, CF2HCF2H), -134a (CF3CFH2), -125 (CF3CF2H), and -143 (CF2HCFH2). 23Na NMR parameters determined from the MQMAS NMR spectra (acquired with high 23Na radio frequency power) were used for quantitative analysis of the 23Na MAS NMR spectra, obtained at very fast spinning frequencies (21 kHz). At room temperature, four (I, II, I‘, and hydrated sodium cations) and five (I, two III‘, II, and I‘) sodium sites were identified in NaY and NaX, respectively. Cation occupancies for the different sites were also obtained following sorption of HFC-134, again from the MQMAS and fast MAS spectra. For zeolite NaY, the distribution of the extra-framework cations changed significantly, the SI‘ cations migrating into the SI site and into the supercages (resulting in occupancy of the SIII‘ sites), presumably to optimize cation−HFC interactions. No significant change in the SI‘ cation population was detected following sorption of HFC-134 on zeolite NaX. However, two SI sites, due to different local environments for the cations, within the hexagonal prisms, were observed. Variable-temperature 23Na MAS and 2D MQMAS NMR experiments were performed on bare NaY, to probe rearrangement of the sodium cations above room temperature. The SI resonance was observed to decrease in intensity above 150 °C, and a new resonance was seen. A two-dimensional 23Na-exchange NMR experiment, performed at 250 °C, revealed considerable cation mobility involving the supercage cation sites.