Time-resolved in situ visualization of the structural response of zeolites during catalysis

Jinback Kang ,Aymeric Robert ,Wonsuk Cha ,Kyuseok Yun ,Jesse N Clark ,Yong S Chu ,Kyung Byung Yoon ,Nguyen Huu Thanh ,Dong-Jin Kim ,Myungwoo Chung ,Marcin Sikorski ,Xiaojing Huang ,Jaeseung Kim ,Yong Nam Choi ,Heeju Lee ,Ross Harder ,Sanghoon Song ,Mee Song ,Jérôme Carnis ,Ian Robinson ,Gukil An ,Hyun-Jung Kim

doi:10.1038/s41467-020-19728-3

Abstract

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. The inhomogeneous distribution of the cations affects their catalytic performances because it influences the intra-crystalline diffusion rates of the reactants and products. However, the structural deformation regarding inhomogeneous active regions during the catalysis is not yet observed by conventional analytical tools. Here we employ in situ X-ray free electron laser-based time-resolved coherent X-ray diffraction imaging to investigate the internal deformations originating from the inhomogeneous Cu ion distributions in Cu-exchanged ZSM-5 zeolite crystals during the deoxygenation of nitrogen oxides with propene. We show that the interactions between the reactants and the active sites lead to an unusual strain distribution, confirmed by density functional theory simulations. These observations provide insights into the role of structural inhomogeneity in zeolites during catalysis and will assist the future design of zeolites for their applications.

Highlights

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations
We show how the inhomogeneities affect the crystal strain in Cu(II)-exchanged ZSM-5 during the catalytic process of deoxygenation of nitrogen oxides (NOx) to N2 with propene as the reducing agent by employing time-resolved in situ Bragg geometry CDI (BCDI) measurements with X-ray free electron lasers (XFELs)
The initial parameters for the phase retrieval process were set from the 3D Coherent X-ray diffraction (CXD) data for the crystal under N2 (Supplementary Fig. 1)

Summary

Introduction

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. Zeolites have been extensively used as catalysts for various reactions, including refinement of crude oil[1] and reduction of nitrogen oxides (NOx) in vehicle exhausts[2], adsorbents for various molecules including carbon dioxide[3], cation exchangers[4], size-selective separation of molecules[5], and many others[6] These unique properties of zeolites arise from their sub-nanometer scale pores, which vary in size and shape depending on the type of zeolites. It has been known that the adsorbed molecules are not evenly distributed within the zeolite crystals[11] In other words, their inhomogeneous distributions within crystals sensitively affect their performances because these factors influence the intra-crystalline diffusion rates of the reactants and products[12]. Time-resolved in situ visualization of the active regions in zeolite crystals during the chemical process and the subsequent elucidation of the factors within zeolite crystals provide important information to maximize the usage of zeolite as catalysts and to design and synthesize zeolite catalysts with enhanced performances[13,14]

Methods

Results

Conclusion