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

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.

Strain Development of Selective Adsorption of Hydrocarbons in a Cu-ZSM-5 Crystal

“…ZSM-5 microcrystals were synthesized from a gel mixture consisting of tetraethyl orthosilicate, sodium aluminate (NaAlO 2 : 35% Na 2 O and 35% Al 2 O 3 ), tetrapropyl ammonium hydroxide (TPAOH), and potassium hydroxide (KOH) by heating at 200 °C for 24 h (NaAlO 2 , TPAOH, and KOH were obtained from Sigma-Aldrich). The synthesized ZSM-5 zeolites have a Si/Al molar ratio of 14.99, obtained by XPS . Their sizes are 1.5–2.5 μm in height, 2.2–3.0 μm in width, and 0.5–1.0 μm in depth.…”

“…Bragg coherent X-ray diffraction imaging (BCDI) is a unique technique to provide varied internal deformations by external parameters in an entire crystal . Most of all, the variations can be directly observed with a nondestructive method. , The results measured in in situ mode provide a means to realize the changes of internal strains during the catalytic reaction …”

“…This chemistry facilitates a catalytic reduction of nitric oxides with propene (C 3 H 6 ) and propane (C 3 H 8 ) 24 and a cracking reaction of nhexane and benzene. 25−27 Although those studies have been widely studied in the laboratories and considerably utilized in the industries, internal structural variations, including lattice deformations of the Cu-ZSM-5 crystal by hydrocarbon adsorption, 28 study because the lattice deformation might affect the channel structure. It is important to understand how the chemical interactions of the adsorption molecules on the Cu 2+ ions affect the crystalline framework of the zeolite.…”

“…29,30 The results measured in in situ mode provide a means to realize the changes of internal strains during the catalytic reaction. 28 In this study, we investigate the internal structural deviations of the Cu-ZSM-5 microcrystal during the adsorption of hydrocarbons using the BCDI technique. Four different hydrocarbons, propene (C 3 H 6 ), propane (C 3 H 8 ), n-hexane, and benzene, are compared as the absorption molecules.…”

AutoPhaseNN: Unsupervised Physics-aware Deep Learning of 3D Nanoscale Bragg Coherent Diffraction Imaging

“…This capability of BCDI to provide nanoscale structural information as well as picometer sensitivity to strain has had profound implications for the materials science, chemistry and solid-state physics communities. Examples include defect dynamics in battery electrodes (6), in-situ catalysis (7,8), photon transport (9)(10)(11), phase transformation (12)(13)(14), and plastic deformation (15)(16)(17).…”