Andrej Šmelko is a PhD student in the Zdeněk Tošner group at the Charles University, Czech Republic. His research focuses on solid-state NMR method development, particularly advanced techniques (MQMAS, D-HMQC) for characterizing quadrupolar nuclei in zeolites.

Zeolites are porous materials that catalyze chemical reactions through their tetrahedral acid sites. Because these acid sites contain quadrupolar 27Al nuclei (I = 5/2), solid-state NMR 1D 27Al spectrum is typically featureless. Therefore, we must measure the high-resolution Triple-Quantum Multiple Magic Angle Spinning (3QMAS) spectrum1. However, even this spectrum fails to resolve overlapping 27Al spectral lines, and the analysis of zeolite tetrahedral sites is only approximate. Although a similar experiment, 5QMAS, potentially provides much higher resolution, it is not routinely used because of its extremely low sensitivity. We replaced the excitation and reconversion pulses in the 5QMAS sequence by numerically optimized shaped pulses2. For zeolite tetrahedral sites, we achieved enhancement by a factor of 6, enabling 5QMAS to be used as a routine experiment. Fig. 1 demonstrates the enhanced resolution of 5QMAS in an MFI zeolite with up to 12 crystallographically distinct tetrahedral 27Al sites.

1. A. Medek, J. Am. Chem. Soc. 117, 12779–12787(1995) DOI: 10.1021/ja00156a015
2. N. Khaneja, J. Magn. Reson. 172, 296-305(2005) DOI: 10.1016/j.jmr.2004.11.004