DETERMINATION OF THE QUADRUPOLE COUPLING-CONSTANT OF THE INVISIBLE ALUMINUM SPINS IN ZEOLITE HY WITH H-1/AL-27 TRAPDOR NMR

The transfer of populations in double resonance (TRAPDOR) NMR method is applied to the H-1 and Al-27 nuclei of Bronsted acid sites in dehydrated zeolite HY. In the experiment, the intensity of a H-1 magic angle spinning (MAS) echo signal is reduced by Al-27 irradiation applied during the evolution period of the echo sequence. A. semiquantitative theory is proposed to explain the data. Analysis of the signal reduction as a function of the irradiation-frequency offset allows a determination of the quadrupole coupling constant of the Al-27 nuclei as well as an estimation of the asymmetry parameter and the geometry of the Al-H vector. This technique provides a means of extracting spin-interaction parameters of quadrupolar nuclei that are otherwise invisible by MAS NMR. The method extends the capabilities of NMR to the characterization of catalytically interesting, but typically invisible, aluminum atoms such as those located at the surfaces of alumina supports and at acid sites.