Critical factors in obtaining meaningful fast MAS NMR spectra of non-integral spin quadrupolar nuclei. A review with particular emphasis on27Al MAS NMR of catalysts and minerals

Abstract

In the last decade, magic angle spinning (MAS) NMR has become an extremely important method for studying the structure of inorganic solids. Advances in NMR technology have greatly aided in understanding the structure of catalysts, minerals, clays, ceramics, glasses, etc. Obtaining meanin~ul MAS spectra of spin-1/2 nuclei such as 29Si and 31p is relatively straightforward and well understood. In contrast, obtaining meaningful MAS spectra is far from simple with non-integral spin quadrupolar nuclei such as 11B (I= 3/2), 170 (I=5/2), 23Na (I= 3/2), 27A1 (I= 5/2), 69Ga (I= 3/2), and 71Ga (I= 3/2) -to name some of the most commonly studied nuclei. Many additional factors have to be considered. This paper will deal with these factors and the utility of vely fast MAS for studying non-integral spin quadrupolar nuclei in inorganic solids.

Dealumination of HZSM-5 via steam-treatment

“…For all steamed samples, only tetrahedrally-coordinated aluminum atoms were observed in the MQMAS spectra, showing isotropic distributions as a result of variations in Al-O-Si bond angle and/or Al-O bond length [20]. The presence of aluminum in distorted tetrahedral environment caused a non-symmetric interaction with the external magnetic field, resulting in a larger quadrupolar interaction.…”

TNU‐9 Zeolite: Aluminum Distribution and Extra‐Framework Sites of Divalent Cations

“…The obtained product was washed repeatedly with deionized water and dried at ambient temperature. The chemical compositions 27 Al (3Q) and 29 Si MAS NMR spectroscopy 27 Al (3Q) and 29 Si MAS NMR spectra of the fully hydrated samples were measured on aBruker Avance III HD 500 WB/US (11.7 T) spectrometer using ZrO 2 rotors with ar otation speed of 12 kHz for 27 Al MAS NMR and 7kHz for 29 Si MAS NMR. The calcined sample was converted into the NH 4 + and Na + forms by repeated (three times) equilibration of the zeolite with a1 .0 m solution (100 mL g À1 )o fN aNO 3 and NH 4 NO 3 ,r espectively,f or 24 h. Both the powder XRD patterns (Bruker D8 diffractometer,B ruker AXS, U.S.A) and SEM images (Jeol JSM-03 SEM microscope) evidenced ah ighly crystalline zeolite (see the Supporting Information).…”

A Definitive Example of Aluminum-27 Chemical Shielding Anisotropy

“…Solid-state aluminum-27 NMR is now routinely used for the characterization of many solid materials, including zeolites, gels, minerals, clays, metal alloys, amorphous solids, and numerous coordination compounds (1)(2)(3)(4)(5). Although 27 Al is a quadrupolar nucleus, spin I ϭ 5/2, high-resolution NMR studies are feasible because 27 Al has a relatively small nuclear quadrupole moment (Q ϭ 0.14 ϫ 10 Ϫ28 m 2 ) (6).…”