Determination of quadrupolar coupling strength of metals in zeolite materials

Nuclear Magnetic Resonance (NMR) has been used to study molecular structure for 50 years, with most of the emphasis having been on chemical shift and, more recently, dipolar coupling. However, for nuclei with a quadrupolar moment, I > 1/2, measurements of quadrupolar coupling can be informative about the local structure around a nucleus. Quadrupolar coupling measurements are applied to an important class of zeolite materials. Zeolites are used as catalysts, adsorbents, gas storage materials and more. Quadrupolar coupling measurements herein yield information on cation distortion and acid site structure, as well as the regeneration and stability of acid sites.; An optimized and simplified rotor assisted population transfer (RAPT) pulse sequence for the determination of quadrupolar coupling (CQ) is reported. The adiabatic transfer experiment is optimized for four parameters and discussed in terms of the adiabaticity parameter of Vega. This method excels for moderate to small CQ values and samples for which second-order quadrupolar lineshape analysis fails due to other broadening mechanisms.; Investigation of zeolite A shows that the CQ at the aluminum acid site depends on the fit of an incorporated cation in the 6-membered ring of the zeolite. K+ substitution, a good fit for this ring, gives the smallest distortion (CQ), while Cs+ and Li+ both show significantly larger values.; Quadrupolar coupling measurements confirm that hydration plays a large role in the relaxation of the framework around aluminum in the zeolite ZSM-5. Large differences in aluminum site local distortion between the vacuum dried and fully hydrated ZSM-5 zeolite indicate a role for water in this phenomenon. We also measure, for the first time, an intermediate hydration state in the frequently studied H-ZSM-5 catalyst. This hydration state, containing 1-2 water molecules per aluminum site, possesses an intermediate level of distortion and may reflect the geometry of the acid catalytic sites in the presence of gas phase adsorbates.; Quadrupolar anisotropy measurements enable the assignment of four aluminum sites in zeolite HY. Determination of CQ allows quantitative analysis of aluminum site distribution under various catalyst treatment conditions. The CQ measurements and calculations of isotropic chemical shift demonstrate that the peak often assigned as penta-coordinate aluminum is aluminum in a distorted tetrahedral environment.; The moderate acidity of boron zeolites is demonstrated to arise from weak bridging hydroxyl boron acid sites. We directly observe two distinct overlapping boron sites in hydrated SSZ-42 using quadrupolar coupling measurements. The two sites have very similar isotropic chemical shifts, but very different quadrupolar coupling constants and therefore local geometries.