NMR crystallography of zeolites: How far can we go without diffraction data?

Abstract

Nuclear magnetic resonance (NMR) crystallography-an approach to structure determination that seeks to integrate solid-state NMR spectroscopy, diffraction, and computation methods-has emerged as an effective strategy to determine structures of difficult-to-characterize materials, including zeolites and related network materials. This paper explores how far it is possible to go in determining the structure of a zeolite framework from a minimal amount of input information derived only from solid-state NMR spectroscopy. It is shown that the framework structure of the fluoride-containing and tetramethylammonium-templated octadecasil clathrasil material can be solved from the 1D Si NMR spectrum and a single 2D Si NMR correlation spectrum alone, without the space group and unit cell parameters normally obtained from diffraction data. The resulting NMR-solved structure is in excellent agreement with the structures determined previously by diffraction methods. It is anticipated that NMR crystallography strategies like this will be useful for structure determination of other materials, which cannot be solved from diffraction methods alone.

Shedding light on the atomic-scale structure of amorphous silica–alumina and its Brønsted acid sites

“…Again, this is in direct contrast to zeolites, whose well-ordered structures can be determined by advanced SSNMR spectroscopic methods. [24][25][26][27][28][29] The recent development of efficient high-field dynamic nuclear polarization (DNP) [30][31][32][33] has provided new opportunities for the precise structural characterization of surface sites by SSNMR. DNP operates by partially saturating an allowed, or forbidden, electron paramagnetic resonance transition with high-power microwaves, and subsequently transferring the electrons' high polarization to the nuclear spins.…”

“…A particular application of DNP concerns the hyperpolarization of surfaces wherein the radical polarizing agent is introduced near the surfaces of the materials by incipient wetness impregnation. 31,32 The methods' enhanced sensitivity has already enabled the acquisition of challenging 29 Si- 27 Al correlation spectra that have provided new insights into the structures of ASAs. 21 DNP has also been applied to identifying dilute SiO 4 sites that were deposited on alumina surfaces through atomic layer deposition as well as chemical liquid deposition.…”

“…42,43 Herein, we used DNP-enhanced SSNMR to determine the long-range ordering of atoms in ASAs prepared by flame-spray pyrolysis (FSP). 44 Using 29 Si and 27 Al homonuclear correlation experiments, the potential clustering, or lack thereof, of Si and Al sites could be addressed. 29 Si{ 27 Al} resonance-echo saturationpulse double-resonance (RESPDOR) 45 experiments were then used to tackle questions regarding the overall ordering of these sites in the material.…”

“…44 Using 29 Si and 27 Al homonuclear correlation experiments, the potential clustering, or lack thereof, of Si and Al sites could be addressed. 29 Si{ 27 Al} resonance-echo saturationpulse double-resonance (RESPDOR) 45 experiments were then used to tackle questions regarding the overall ordering of these sites in the material. A novel simulation model was designed that could directly yield, from a single 29 Si{ 27 Al} RESPDOR curve, the fraction of Si atoms in the clustered (silica) phase and the local Al concentration surrounding Si atoms in the mixed silicaalumina phase, as well as provide conclusive evidence of the validity of Loewenstein's rule in ASAs.…”

Enhanced 1H-X D-HMQC performance through improved 1H homonuclear decoupling

“…[7][8][9] SSNMR's main structural handle is the acquisition of 2D homonuclear and heteronuclear correlation (HETCOR) data, which yield information regarding the bonding networks of the material as well as the proximities, and specific distances, between the various sites. While, in principle, the three-dimensional structure of any material can be solved through the acquisition of extensive correlation data, [10][11][12][13][14][15][16][17] experimental challenges, especially poor sensitivity, prevent the practical realization of this approach. Multiple classes of dipolar-based (through-space) HETCOR techniques have thus been developed over the last few decades to both improve the sensitivity and information content of spectra.…”