Mapping the oxygen structure of \u03b3-Al2O3 by high-field solid-state NMR spectroscopy

Qiang Wang,Xiaoling Wang,Feng Deng,Zhehong Gan,Wenzheng Li,Ivan Hung,Frederic Mentink-Vigier,Guodong Qi,Jun Xu,Xiang Wang

doi:10.1038/s41467-020-17470-4

Abstract

γ-Al2O3 is one of the most widely used catalysts or catalyst supports in numerous industrial catalytic processes. Understanding the structure of γ-Al2O3 is essential to tuning its physicochemical property, which still remains a great challenge. We report a strategy for the observation and determination of oxygen structure of γ-Al2O3 by using two-dimensional (2D) solid-state NMR spectroscopy at high field. 2D 17O double-quantum single-quantum homonuclear correlation NMR experiment is conducted at an ultra-high magnetic field of 35.2 T to reveal the spatial proximities between different oxygen species from the bulk to surface. Furthermore, 2D proton-detected 1H-17O heteronuclear correlation NMR experiments allow for a rapid identification and differentiation of surface hydroxyl groups and (sub-)surface oxygen species. Our experimental results demonstrate a non-random distribution of oxygen species in γ-Al2O3.

Highlights

Optimization and rational design of related heterogeneous catalysts rely on detailed knowledge of the structure–property relationship
The formation of AlV can be related to the dehydroxylation of surface AlVI, and a relatively higher treatment temperature would result in more AlV on the surface γ-Al2O3
This is demonstrated by the overlapping 17O signals and insufficient spectral resolution even in the 2D 17O 3QMAS spectrum recorded at high field (Fig. 1b, c)

Summary

Introduction

Optimization and rational design of related heterogeneous catalysts rely on detailed knowledge of the structure–property relationship. The oxygen speciation (e.g., hydroxyl or defects) of γ-Al2O3 impacts its surface properties (i.e., acidity/basicity), and in many cases the proposed catalytic mechanisms are closely related to the local environment of oxygen atoms[21]. Determination of oxygen species in γ-Al2O3 is prerequisite for understanding its structure and physicochemical property, which has remained a great challenge. 17O magic-angle spinning (MAS) NMR has been utilized in recent years to probe the local environments of oxygen atoms in various oxygen-containing materials[27,28,29,30,31,32]. Taking advantage of dynamic nuclear polarization (DNP) technique[35,36,37,38], Pruski and coworkers showed the direct observation of Brønsted acid sites at natural abundance by 17O DNP surface-enhanced

Methods

Results

Conclusion