Investigation of the hydration of zeolite NaA by two-dimensional sodium-23 nutation NMR

Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy

The present work introduces a novel method for the selective detection of ¹ H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized ¹ H nuclei are separated due to their double nutation frequency compared to that of thermally polarized ¹ H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ ¹ H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer. The 2D nutation NMR experiment described in the present work is useful for the separation of overlapping anti-phase and in-phase signals due to hyperpolarized and thermally polarized ¹ H nuclei, respectively, which is important for research in the field of heterogeneous catalysis.

High field ENDOR as a characterization tool for functional sites in microporous materials

The determination of the details of the spatial and electronic structure of functional sites (centers) in any system, be it in materials chemistry or in biology, is the first step towards understanding their function. When such sites happen to be paramagnetic in any point of their activity cycle, the tool box offered by a variety of high resolution electron paramagnetic resonance (EPR) spectroscopic techniques becomes very attractive for their characterization. This tool box has been considerably expanded by the developments in high field (HF) EPR in general, and HF electron nuclear double resonance (ENDOR), in particular. These have led to numerous new applications in the fields of biology, physics, chemistry and materials sciences. This overview focuses specifically on recent applications of pulsed HF ENDOR spectroscopy to microporous materials, such as zeotype materials, presenting the new opportunities it offers. First, a brief description of the theoretical basis required for the analysis of the HF ENDOR spectrum is given, followed by a description of the pulsed techniques used to record spectra and assign the signals, along with a brief presentation of the required instrumentation. Next, specific applications are given, including transition metal ions and complexes exchanged into zeolite cages, transition metal substitution into frameworks of zeolites, aluminophosphate molecular sieves, and silicious mesoporous materials, the interaction of NO with Lewis sites in zeolite cages and trapped S. We end with a discussion of the advantages and the shortcomings of the method and conclude with a future outlook.

Characterization of sodium cations in dehydrated faujasites and zeolite EMT by 23Na DOR, 2D nutation, and MAS NMR

Sodium cations localized at crystallographically distinct cation sites in dehydrated zeolites were characterized using 23Na double rotation, two-dimensional nutation, and magic-angle-spinning nuclear magnetic resonance spectroscopy. The new DOR NMR technique has been applied at different magnetic field strengths to determine the quadrupole parameters of the overlapping quadrupole patterns. In the NMR spectra of dehydrated NaY and NaEMT two signals of sodium cations were identified, a low-field gaussian line at -12 +/- 1 ppm and a high-field quadrupole pattern, with an isotropic chemical shift of -8 +/- 1 ppm and a quadrupole coupling constant of about 4 MHz. By comparison of the 23Na MAS NMR intensities of these signals with the population of the cation sites determined by XRD and by calculation of the electric field gradients, the former signal was attributed to sodium cations at the sites SI and the latter one to sodium cations at the sites SI' as well as SII in faujasite and zeolite EMT. This assignment has further been confirmed by 23Na MAS NMR studies of dehydrated HNaY and BaNaY zeolites.