Theoretical Study on the Lewis Acidity of the Pristine AlF3 and Cl-Doped α-AlF3 Surfaces

Christian Becker,Beate Paulus,Thomas Braun

doi:10.3390/catal11050565

Christian Becker, Beate Paulus + Show 1 more

Open Access

https://doi.org/10.3390/catal11050565

Copy DOI

Abstract

In the past two decades, metal fluorides have gained importance in the field of heterogenous catalysis of bond activation reaction, e.g., hydrofluorination. One of the most investigated metal fluorides is AlF3. Together with its chlorine-doped analogon aluminiumchlorofluoride (AlClxF3−x, x = 0.05–0.3; abbreviated ACF), it has attracted much attention due to its application in catalysis. Various surface models for α-AlF3 and their chlorinated analogues (as representatives of amorphous ACF) are investigated with respect to their Lewis acidity of the active centres. First-principle density functional theory (DFT) methods with dispersion correction are used to determine the adsorption structure and energy of the probe molecules CO and NH3. The corresponding vibrational frequency shift agrees well with the measured values. With this insight we predict the local structure of the active sites and can clarify the importance of secondary interactions to the local anionic surrounding of the catalytic site.

Highlights

density functional theory (DFT)-based first principle calculations elucidate the structural and energetic properties of various surfaces occurring in nanoscopic aluminium fluoride-based materials
This study focused on the vibrational analysis of the probe molecules carbon monoxide (CO) and NH3 adsorbed on various AlF3− x Clx model systems, where x = 0.15, to improve the knowledge of the Lewis acidity of the catalytic sites in amorphous ACF
It could be confirmed that the highest shielding against adsorption of CO molecules can be reached for tetragonal coordinated aluminium centres

Summary

Introduction

The vast majority of catalysts used on industrial scale are heterogenous catalyst with a high surface-to-volume ratio to increase the number of active sites. Different chemisorption modes for NH3 on MgO [8,9] and ZnO [11] surfaces were characterized, but not much is known for the adsorption of NH3 on metal fluoride surfaces On these oxides, the adsorption of NH3 on Lewis acidic CUS resulted in a blue shift of the asymmetric bending mode. In related studies published by the same authors, the interactions with α- and β-AlF3 -surfaces were investigated with the focus on the adsorption of HF [22], H2 O [22] and CCl2 F2 [23] It can be assumed from the modelled structures by Bailey et al, that the binding motif of NH3 is similar to the “upright“ conformation of NH3 on MgO surfaces [9], where the hydrogens can interact with the neighbouring terminal fluorine atoms of the surface.

Computational Details

Results

Discussion

Conclusions