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1.1.4.Fluorination of alkenes with hydrogen fluoride in the presence of catalysts Hydrofluorination in gas and liquid phases in the presence of catalysts is widely used in production of various chladones. A data file on these processes is vast and is not under consideration in this review. Some examples of such a fluorination in connection with transition to production of ozone-safety chladones not containing chlorine and bromine atoms we present in Table 7. Here chlorine-containing olefins are used as the initial substrates. Table 7
These processes were taken on special significance in recent years due to development of new ozone-safety freons containing atoms of C,F,H only and in connection with the Vienna convention about protection of the ozone layer (1985). Prohibition of production and consumption of chlorofluorocarbons was a ground for development of studies and technologies for production of new freons. The main attention was paid to freons which could completely replace chlorofluoro-containing freons. Table 8 presents the data on such freons. Table 8
Gas-phase hydrofluorination is most effective over catalysts, the process runs under elevated temperatures and pressure. As a rule, alkenes with a different number of chlorine atoms are used as substituents. So, 1,1-dichloroethylene (50-70oC, 10-30 Bar) gives 1,1-dichloro-1-fluoroethane [149]. Trichloroethylene in a fluidized catalyst ( chromo-magnesium fluoride) in gas-phase hydrofluorination under the influence of anhydrous hydrogen fluoride gives at first 1,1,1-trifluoro-2-chloroethane (Freon 133a)[64,150]. This stage is practically irreversible. As it follows from experimental data and thermodynamic calculations of the process, the content of Freon 133a in organic part of the synthesis products can be 90-98% by volume. The optimal conditions for obtaining high yield of Freon 133a are as follows: mole ratio of HF: C2HCl3=4-7:1, a temperature of 250-350oC, a contact time of 5-15 s [64,150].
The influence of pressure on the 1 and 2 stages of the process has been investigated [151]. It was found that in the first stage a pressure increase up to 6 atm resulted in an increase in conversion of trichloroethylene ( 95-97%), when pressure was increasing up to 3 atm the selectivity of Freon133a was increasing and further practically did not change. In the second stage a pressure increase results in a reduction of freon133a conversion : a pressure change from 1 to 3 atm brings to a change in selectivity from 22.3 to 15.8%, at higher pressures the conversion remains almost constant (14.6- 14.8%). Nevertheless industrial production of Freon 134a according to this scheme is perspective [64]. Gas phase hydrofluorination of perchloroethylene and hydrogen-containing ethanes at elevated temperatures (330-465oC) and under pressure in an excess of anhydrous hydrogen fluoride in the presence of a catalyst containing a chromium compounds and magnesium fluoride in amount of 8-24% of the catalyst mass at a mole ratio of HF:alkene= 4-40:1 and a contact time of 5-60 sec results in formation of fluorine-containing ethanes [152]. Perchloroethylene with HF in the presence of a catalyst based on Cr2O3 at 350oC and 1 bar at the contact time of 12.5 sec gives 1,1,1,2,2-pentafluoro-ethane (Freon 125) as the main reaction product [153,154]. Propylene derivatives behave similarly over a catalyst based on fluorinated aluminum: 1-chloro-3,3,3-trifluoropropylene and 1,3,3,3-tetrafluoropropylene give 1,1,1,2,2-pentafluoropropane [155]. This Freon is obtained also in hydrofluorination of 1,1,3,3-tetrachloropropylene and 1,3,3,3-tetrachloropropylene over SbF5 catalyst at a pressure of 10kg/cm2 (20 hours) [156]. Hexafluoropropylene in gas phase is hydrofluorinated under the influence of HF over catalysts based on compounds of trivalent chrome with additions of aluminum fluorides at 210oC and a ratio of reagents of 4-20:1 (HF:hexafluoropropylene) with formation of 2H-heptafluoropropane (R- 227ea) in 98.5% yield [157,158]. R-227 is a perspective ozone-safety Freon which can be used as an effective fire-extinguishing agent. In hydrofluorination of fluoroalkenes under the influence of anhydrous hydrogen fluoride over an active catalyst, an activated carbon promoted by fluorides of alkali metals (NaF, KF, CsF, RbF), fluorocarbons are formed [159]. So, in gas phase hydrofluorination the most effective are catalysts based on compounds of aluminum [160] and chromium [161-163], magnesium, bismuth [164,165] whereas many tri- and tetrahalogen-substituted olefins add hydrogen fluoride in the presence of BF3 (60-180oC), fluorides and chlorides of tin and antimony. The reactions of halogen-containing olefins with hydrogen fluoride in the presence of catalysts are of great industrial importance and a large data file has been collected for these processes [30]. Liquid-phase hydrofluorination requires a lower temperature in comparison with the gas phase process and as catalysts there are used fluorides of transition metals in the higher oxidation degree, BF3 and super acids in HF medium. Catalytic hydrofluorination is more effective [132]. Tetrachloroethylene is not influenced by hydrogen fluoride at 160oC and reacts with them only in the presence of a catalyst, for example TaF5 [166].
Perfluorinated alkenes are also subjected to the effect of anhydrous hydrogen fluoride in liquid phase over catalysts. Thus hydrofluorination of tetrafluoroethylene in the presence of
SbF5 and tertiary amines results in formation of pentafluoroethane [167]. So at a content of
SbF5 of 0.02 mole% and at mole ratios of tetrafluoroethylene:SbF5=15:1 and
tetrafluoroethylene:SbF5 =1:10 , the contact time of 4 h, temperature
90oC there was obtained a mixture of products with the following content:
C2HF5 84.5%, C2F6 0.3%, C2F4 15.2%. Hydrofluorination of hexafluoropropylene in liquid phase in the presence of Lewis acids ( fluorides of transition metals
TaF5, NdF5, SbF5) gives 1,1,1,2,3,3,3-heptafluoropropane [167a]. The same compound is formed when tributylammonium hydrofluoride
(Bu3N HFx (2<x<3)) is used as a catalyst [167b]. To be continued |
