Fluorination using Solid Catalysts

A major impetus to research in this area during the past two decades (especially since the 1987 Montreal Protocol) has been the need to discover catalysts for the synthesis of alternatives to chlorofluorocarbons, specifically hydrofluorocarbons [83], which destroy the protective ozone layer by generating Cl atoms under the influence of UV radiation. 

In addition to the destruction of ozone, the CFCs have also been implicated in global warming. Hydrofluorocarbons (HFCs) have all the advantages of the CFCs (inertness, stability and low toxicity) but, unlike the CFCs, do not contain chlorine and, hence, cannot initiate a chlorine atom-catalyzed ozone destruction cycle. 

Catalyst Surface area after reaction (m g ) Temperature (°C) 320 Conv. Select. (%) (%) 350 Conv. (%) Select. (%) 370 Conv. (%) Select. (%) 

Because of the corrosive nature of HF, there are few reports of fluorination reactions in the liquid phase using solid catalysts. The use of zeolites in fluorination was reviewed in our earlier publication [1]. An unusual, room-temperature, solid-catalyzed fluorination of CH3CCI3 and related species by anhydrous HF has been reported by Thomson et al. [92-94]. He found that fluorination at ambient temperatures is possible in the presence of catalysts derived from fluorinated y-alu-mina, chromia, Fc304, and C03O4 and conditioned before reaction by treatment 

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Triorganotin fluorides can be prepared by the use of new fluorinating systems. 18-Crown-6 or dibenzo-24-crown-8 can act as solid-liquid phase transfer catalysts for CsF. Trialkyltin mercaptides can be fluorodestannylated by CsF in the presence of crown ethers or alkyl bromides358 ... 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

The most important use of /3-sultones is for the preparation of fluorinated polymers such as Nafion 64. These solid acid catalysts containing perfluorinated sulfonic acid groups have been known for many years and the presence of the electron-withdrawing F atoms increases the acid strength of the terminal sulfonic acid groups, which become comparable to that of pure sulfuric acid. Prior to the last decade, Nafion had been in use as a superacid, a fuel cell electrolyte and as a membrane-ion separator <1996CHEC-II(1B)1083>. 

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