Dipolar-Based 27Al .fwdarw. 29Si Solid-State NMR Connectivity Experiments in Zeolite Molecular Sieve Frameworks

Abstract

In this work we present the results of a systematic investigation of a set of dipolar-based 27Al -29Si connectivity experiments for a series of representative zeolite molecular sieve frameworks: zeolites A, X, Y, and 52. Coherence transfer from quadrupolar 27Al (I = 5/2) to 29Si (I = '/2) nuclei through cross polarization and transferred-echo double-resonance (TEDOR) experiments is observed, and an experimental investigation of the parameters controlling the efficiency of both experiments is presented. Rotational-echo double-resonance (REDOR) and dipolar-dephasing difference experiments are also demonstrated. These experiments utilize the heteronuclear dipolar couplings, whose magnitudes are proportional to the inverse third power of the internuclear distance and are therefore extremely sensitive to the separation of the coupled spins. In addition, we demonstrate that for these dipolar-based experiments, there is a relative enhancement of a given resonance which is approximately linear with the number of aluminum atoms in the neighboring tetrahedral sites. Twodimensional extensions of the cross-polarization and TEDOR experiments are also possible and will be of use for complex mixtures or where several resonances are observable in the one-dimensional spectra of the two nuclei.

Structural information from quadrupolar nuclei in solid state NMR

“…Also, with only a few assumptions, the cross-polarization dynamics in this regime may be described in terms of fictitious spin-1/2 operators, and a simplified analysis of variable contact time experiments is therefore possible (62,85,86). Variable contact time experiments have also been used to great effect by Fyfe and colleagues (87,88) to assign 27 Al and 29 Si resonances in the aluminophosphates VPI-5 and AlPO-8 and 27 Al and 29 Si resonances in zeolite frameworks.…”

29Si NMR in solid state with CPMG acquisition under MAS

“…However, such experiments are inherently challenging due to the low efficiency of the CP process involving quadrupolar nuclei [66,67] and low concentrations of 27 Si spin pairs, especially in materials with high Si/Al ratios. The feasibility of 29 Si{ 27 Al} NMR methods has nevertheless been demonstrated in 1D and 2D studies of zeolites [68], minerals [69], and ceramic materials [70]. In the latter study, the sensitivity was enhanced by increasing the population inversion across the central transition using RAPT [71].…”

Double Resonance NMR Echo Spectroscopy of Aluminosilicates

“…Al internuclear distances in a ZSM-5 [13] and a Cuexchanged ZSM-5, [14] respectively. The 29 Si± 27 Al REDOR experiment was exploited to obtain the framework internuclear connectivity and to distinguish silicons with different numbers of connected aluminums in a series of zeolites A, X, Y, and W. [15] 15 N± 27 Al REDOR was used to probe the adsorption site of absorbed monomethylamine in an HY zeolite. [16] TRAPDOR was used to detect 1 H± 27 Al and 15 N± 27 Al internuclear connectivity and the quadrupole coupling constant in HY zeolites.…”

“…The 29 Si± 27 Al REDOR experiment was exploited to obtain the framework internuclear connectivity and to distinguish silicons with different numbers of connected aluminums in a series of zeolites A, X, Y, and W. [15] 15 N± 27 Al REDOR was used to probe the adsorption site of absorbed monomethylamine in an HY zeolite. [16] TRAPDOR was used to detect 1 H± 27 Al and 15 N± 27 Al internuclear connectivity and the quadrupole coupling constant in HY zeolites. [6,16] These techniques are also extremely useful in the study of glasses, e.g., TRAPDOR of aluminosilicate glasses [17,18] and REDOR of silicate and aluminoborate glasses.…”