Liquid-phase HF Fluorination

In 1987, the Montreal protocol led to the phase down of the chlorofluorocarbons (CFCs) because of their ozone depletion potential. These products have been replaced in a first step by HCFCs (hydrochlorofluorocarbons), which still contain chlorine, and in a further step by HFCs (hydrofluorocarbons), which are completely 

The liquid-phase fluorination processes can proceed in a noncatalytic way. This is the case for the exchange reaction by H F of the first Cl atom of 1,1,1-trichloroethane leading to l,l-dichloro-2-fluoroethane [Eq. (1)] around 100 °C with HF as the reaction solvent [1]. 

In most cases, however, the liquid-phase fluorination processes require a halogenated (Cl or F) Lewis-acid catalyst. For example, the first addition of HF on trichlorethylene followed by two subsequent Cl exchanges to reach the 1-chloro-2,2,2-trifluoroethane (F133a, which is an intermediate for F134a), are catalyzed by SbCl [Eqs. (2) and (3)]. 

Generally speaking, liquid-phase fluorination catalysts are located in groups 4 (Ti [2], etc.), 5 (Nb, Ta [3], etc.), 6 (Mo, etc.), 14 (Sn [4], etc.) and 15 (Bi, As, Sb [5], etc.) columns of the Periodic Table of the chemical elements. Among all these compounds, SbClj and more specifically the Sb(V)Cl F entities which were first synthesized by Swarts in 1895 [6], are the most active catalysts. From a mechanistic point of view, Sb(V)Cl Fj, in the presence of HF form superacidic species such as SbCl F +i H. Cl/F exchange reaction can be concerted or can proceed via a carbocation followed by addition of F [7], while the addition of HF to a double bond could proceed via two types of attack, nucleophilic (F ) or electrophilic (H ) [8]. 

SbClj is the most common liquid-phase fluorination catalyst and is used in nearly all liquid-phase fluorination processes. Therefore, improvements of such processes are made by the elimination/improvement of the drawbacks presented by SbClj  

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