Issue 43, 2019

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

Abstract

The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is “NMR-invisible” owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites via sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% H2SO4) in the zeolite.

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

Supplementary files

Article information

Article type
Edge Article
Submitted
30 May 2019
Accepted
11 Sep 2019
First published
12 Sep 2019
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2019,10, 10159-10169

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

S. Xin, Q. Wang, J. Xu, Y. Chu, P. Wang, N. Feng, G. Qi, J. Trébosc, O. Lafon, W. Fan and F. Deng, Chem. Sci., 2019, 10, 10159 DOI: 10.1039/C9SC02634G

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