The acidic nature of “NMR-invisible” tri-coordinated framework aluminum species in zeolites

Abstract

A unique insight into the acidic nature of the tri-coordinated framework aluminum (AlFR) in H-ZSM-5 zeolite catalysts has been provided using multi-nuclear and multi-dimensional solid-state NMR spectroscopy in conjunction with TMPO probe molecules.

Deactivation of Cu‐Exchanged Automotive‐Emission NH3‐SCR Catalysts Elucidated with Nanoscale Resolution Using Scanning Transmission X‐ray Microscopy

“…[29][30][31] The distorted local environment and large quadrupolar coupling constant of tri-coordinated Al results in NMR invisibility, so a probe molecule was employed to certify their fine struc-ture. [32,33] The most direct observation of this Al coordination is achieved by XAS via the presence of the aforementioned pre-edge feature that is indicative of the presence of empty mixed s, d, and p orbitals, that is, the nature of the unsaturated Al. [25] Previous observations of tri-coordinated Al were reversible in zeolite H-Beta and H-Mordenite, such that it could be restored to tetrahedral Al when exposed to air or wet helium.…”

Deactivation of Cu‐Exchanged Automotive‐Emission NH3‐SCR Catalysts Elucidated with Nanoscale Resolution Using Scanning Transmission X‐ray Microscopy

“…Tri‐coordinated Al was proposed to form by dehydroxylation of Brønsted acid sites followed by dealumination in zeolites, but it was mostly deduced indirectly by combining 27 Al MAS NMR, EPR, and FT‐IR spectra recorded after steaming or heating . The distorted local environment and large quadrupolar coupling constant of tri‐coordinated Al results in NMR invisibility, so a probe molecule was employed to certify their fine structure . The most direct observation of this Al coordination is achieved by XAS via the presence of the aforementioned pre‐edge feature that is indicative of the presence of empty mixed s, d, and p orbitals, that is, the nature of the unsaturated Al .…”

“…[29][30][31] The distorted local environment and large quadrupolar coupling constant of tri-coordinated Al results in NMR invisibility, so a probe molecule was employed to certify their fine structure. [32,33] The most direct observation of this Al coordination is achieved by XAS via the presence of the aforementioned pre-edge feature that is indicative of the presence of empty mixed s, d, and p orbitals, that is, the nature of the unsaturated Al. [25] Previous observations of tri-coordinated Al were reversible in zeolite H-Beta and H-Mordenite, such that it could be restored to tetrahedral Al when exposed to air or wet helium.…”

Accurate heteronuclear distance measurements at all magic-angle spinning frequencies in solid-state NMR spectroscopy

“…[45][46][47] The multiple types of acid sites are critical to its catalytic performance. 48,49 Investigation of the acidic nature of H-ZSM-5 can reveal vital details of the catalytic process, and help to manipulate the acidity in ZSM-5 zeolite synthesis and post-synthesis process. In solid-state NMR, weak bases, such as trimethylphosphine oxide, have been mostly used for the characterization of zeolite acidity.…”