Catalytic fluorination of 2-chloropyridine over metal oxide catalysts in gas phase in the presence of HF

Abstract

The fluorination of 2-chloropyridine to produce selectively 2-fluoropyrydine by Cl/F exchange at 300 or 380 °C with HF as the fluorinating agent and a metal oxide as precursor of catalyst was investigated. The influence of the activation step under HF gas on the structural, microstructural and physico-chemical properties of various oxides such as MgO, CeO 2, Ce 0.5Zr 0.5O 2, LaFeO 3, Cr 2O 3, were studied. This activation step by HF led to the formation of the corresponding fluorides and/or oxyfluorides materials which were the active catalysts for the Cl/F exchanges. Moreover, this activation step modified strongly the initial properties of the starting materials. This corresponded to a strong decrease of the initial specific surface area and of the basic properties. In the same time, an increase of Lewis acid properties measured by FTIR with pyridine as probe molecule was noticed. MgO, which was initially the most basic oxide studied was also the most active for the fluorination of the 2-chloropyridine after its transformation into fluoride solid which was the active catalyst. This could be due to a lower strength Lewis acidity of the active sites formed by the activation step by HF directly correlated to the basic properties of the initial oxides. A mechanism of the 2-chloropyridine fluorination involving Lewis acid unsaturated metal site was proposed.