An In Situ Al K-Edge XAS Investigation of the Local Environment of H⁺- and Cu⁺-Exchanged USY and ZSM-5 Zeolites

Abstract

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Brønsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.

Visualizing Dealumination of a Single Zeolite Domain in a Real‐Life Catalytic Cracking Particle

“…After deconvolution, the average spectra of the masked zeolite regions within the fresh FCC catalyst particle closely resemble that of am easured reference zeolite Ys pectrum (Figure 2b,F igures S6-9). [33] It was found that the aging trends,a sd etermined by the increasing metals content in each FCC catalyst particle, correspond very well with ad ecrease in the amount of tetrahedrally coordinated Al (Figure 2b). [8,32] Subsequently,anon-negative linear combination least-squares fit was performed with three reference compounds:t hat is,p ure 4-coordinated Al (ZSM-5), a-alumina (6-coordinated Al, oxide), and boehmite (6-coordinated Al, oxyhydroxide).…”

The Local Environment of Cu+in Cu−Y Zeolite and Its Relationship to the Synthesis of Dimethyl Carbonate

“…To follow the effect of Al-Cu scattering distance, a systematic variation from 2.71 to 2.89 Å was made, as done in previous work. 23 2.4. Mass Spectrometry.…”

“…The presence of occluded CuCl has also been observed by Cl K-edge XANES in Cu-USY prepared in a manner similar to that used in this study. 23 Preparation of the Cu-Y by vapor-phase exchange of H-Y with CuCl required handling of H-Y in the absence of water vapor. Moreover, once exposed to water vapor, hydrated H-Y cannot be dehydrated without a loss in the crystallinity of the zeolite.…”

Effects of zeolite structure and composition on the synthesis of dimethyl carbonate by oxidative carbonylation of methanol on Cu-exchanged Y, ZSM-5, and Mordenite

“…Details of this procedure have been described elsewhere [8][9][10][11]. Freshly prepared catalysts, designated as Cu-Y, Cu-ZSM-5, and Cu-MOR, were stored in a drybox before use.…”