New Experimental Approaches for DNP Surface Enhanced Quadrupolar NMR Spectroscopy at Very High Magnetic Field
Final Report Abstract
During the two last years, I investigated surface properties of catalytic materials and a class of support materials known as aluminosilicates. The goal was to understand in detail their synthesis process as well as their surface properties. The catalysts investigated are used in the important industrial conversion of ethene-to-propene and propane-to-propene. Aluminosilicates are popular catalysts and support materials whose performance is correlated to the nature of their surface sites. Therefore, understanding the atomic-level structure of these sites is of great interest. Improved signal quality attained by Dynamic Nuclear Polarization Surface Enhanced solid-state NMR spectroscopy (DNP SENS) makes it possible to characterize these few nanometers thick surfaces in great detail within a reasonable amount of time. In this work, I investigated silica nanoparticles coated with aluminum oxide layers of varying thickness via Atomic Layer Deposition (ALD). Firstly, improved pulse programs focused on 1H-27Al polarization transfer were developed, considering that quadrupolar lineshapes are often distorted by widely used cross-polarization through spin-lock. Secondly, the acidic nature of the catalytic surface sites was studied by adsorbing basic molecule pyridine on the materials surfaces and analyzing the acid-base interaction through DNP SENS. The chemical shifts of adsorbed 15N-labelled pyridines appear in a wide range of 200 to 317 ppm that provides the opportunity to deconvolute spectral line shapes with minimal ambiguity. Ni was grafted on the aluminum oxide layer, and thus the obtained catalyst for ethene-to-propene conversion was also characterized after the pyridine adsorption step. Identifying the signals in 15N DNP enhanced NMR spectra led to discerning the presence and strength of Lewis and Brønsted acid sites. Finally, the developed methodology was also applied to (GaAl)2O3 mixed-oxide catalysts for propane dehydrogenation. This work concretely associates the acidic nature of surface sites to the performance of catalytic materials of future.
Publications
- Atomic-Scale Structure and Its Impact on Chemical Properties of Aluminum Oxide Layers Prepared by Atomic Layer Deposition on Silica. Chem. Mater. 2021, 33, 9, 3335–3348
Monu Kaushik, César Leroy, Zixuan Chen, David Gajan, Elena Willinger, Christoph R. Müller, Franck Fayon, Dominique Massiot, Alexey Fedorov, Christophe Copéret, Anne Lesage, and Pierre Florian
(See online at https://doi.org/10.1021/acs.chemmater.1c00516)
