Observation of spinning sidebands in the 27Al magic-angle spinning nuclear magnetic resonance spectra of FAU and EMT structure-type zeolites.
Wide-band (1 MHz) 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded for zeolite Y (Si/Al approximately 2.7) and high-silica (Si/Al approximately 3.8) FAU, EMT and mixed FAU-EMT structure-type zeolites synthesized using 15-crown-5 and/or 18-crown-6 ethers as templating agents. Spinning sidebands (related to first-order quadrupolar effects affecting satellite transitions) are observed in the spectra of the four calcined and fully rehydrated samples, reflecting a high symmetry of the framework aluminium atom surrounded by four oxygens. It is shown that the intensity of the spinning sidebands progressively increases after calcination and rehydration of the samples, indicating that the restoration of the high local order around the 27Al nuclei is rather slow. On the other hand, second-order quadrupolar interactions rapidly decrease upon rehydration of the calcined samples which is achieved within one day, as indicated by thermogravimetry. Some hypotheses are proposed to explain such a difference, and the role of water is also discussed.