1547
I n d . Eng. Chem. Res. 1989,28,1547-1549
Aerosol Direct Fluorination of C1 and C2 Chlorocarbons James L. Adcock,* Sastry A. Kunda, and Donald R. Taylor Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600
Mario J. Nappa and Allen C. Sievert E. I. du Pont de Nemours & Co., Inc., Jackson Laboratory, Chambers Works, Deepwater, New Jersey 08023
Aerosol direct fluorination of chlorinated methanes and ethanes has been found to produce perfluorinated analogues in good yields. The focus of this study was the fluorination of one- and two-carbon alkyl chlorides, with the goal of understanding the nature of fluorine reactivity with these compounds under the lower acidity, fluoride ion rich conditions prevalent in the aerosol fluorination process. Aerosol fluorination was viewed as a potential avenue of synthesis of the industrially important chlorofluorocarbons (CFC’s) and hydrochlorofluorocarbons (HCFC’s) such CFC-11 (CFCl,), HCFC-22 (CHF2C1), and CFC-113 (CF2C1CFC12). Direct fluorination of alkyl chlorides has received relatively little attention in the literature. Early work by Bigelow et al. (1937) demonstrated gas-phase fluorination of C-Cl bonds by forming CC12FCC12F(CFC-112) from C2C&. Bockemuller (1933),and later Bigelow et al. (1937), reported the earliest observations of the addition of Fzto olefins, with the formation of CFC-112 from tetrachloroethylene. Later Bigelow and co-workers (1939) obtained CF4,CF3C1, CFZ=CCl2, and CF3C1from the fluorination of ethyl chloride. In a paper concerning the solution-phase fluorination of chlorocarbons, Miller (1940) observed the formation of CFC1, from chloroform as well as from the addition of F2to tetrachloroethylene. Miller’s work with chloroolefins (Miller and Dittman, 1956; Miller and Koch, 1957; Miller et al., 1947, 1967) has become a classic study of these substrates. More recently, Adcock (Adcock et al., 1983; Adcock and Evans, 1984), using the aerosol reactor system with photochemical finishing, found that primary alkyl chlorides are stable to chloride elimination and could be fluorinated cleanly in many cases to the corresponding perfluorinated primary alkyl chlorides. However, secondary, and particularly tertiary, alkyl chlorides undergo 1,2-chlorineshifts to afford perfluorinated primary alkyl chlorides. For example, n-propyl chloride afforded F-1-chloro-propane in 85% yield while 2-chloro-2-methylpropaneafforded F-lchloro-2-methylpropane in 47 % yield. These uncomplicated results are in contrast with the observation by Schmeisser and co-workers (1967) that although the gasphase fluorination of 1,2-dichloropropane at 120-200 OC afforded a mixture of the desired CF2C1CFC1CF3as well as C1CFzCFzCF2C1in low yield, they reported significant amounts of tri- and tetrachlorinated products, C3Fs and fragments such as CF4 and C2F,. Electrochemical fluorination of methyl chloride has been reported by Nagase et al. (1966); a mixture of fluoromethanes and chlorofluoromethanes was obtained. The work reported herein is an extension of our studies of direct fluorination with the aerosol fluorination method, which has been reviewed by Adcock and Cherry (1987). In aerosol direct fluorination, the substrate is deposited on a thermally generated aerosol of sodium fluoride. The resulting particles (average radius ca. 17.5 A) are then fluorinated in a multistage reactor in which the fluorine concentration and/or temperature is increased in each stage. The major process variables in these reactions are the F2/substrate ratio (reaction stoichiometry), fluorine concentration, residence time, temperature, and presence of the NaF aerosol. In addition, the reaction can be finished photochemically; this final step is particularly ef08S8-5S85/S9/2628-1547$01.50/0
fective if perfluorination is the objective of the experiment. The focus of this study was the fluorination of one- and two-carbon alkyl chlorides with the goal of understanding the nature of fluorine reactivity with these compounds under the lower acidity, fluoride ion rich conditions prevalent in the aerosol fluorination process. Aerosol fluorination was viewed as a potential avenue of synthesis of the industrially important chlorofluorocarbons (CFC’s) and hydrochlorofluorocarbons (HCFC’s) such CFC-11 (CFClJ, HCFC-22 (CHF,Cl), and CFC-113 (CF2ClCFC12).
Results and Discussion The results for fluorination of methyl chloride are shown in Table I. The desired reaction .CH,Cl+ 2Fz CHF&l+ 2HF (1) presents a particular challenge to the direct fluorination technique since the goal was to produce HCFC-22 selectively starting with methyl chloride. This seemed to be feasible based on the general trend that the rate of fluorination of C-H bonds decreases with increasing fluorine and/or chlorine substitution due to a combination of bond polarity and lone-pair electron-repulsioneffects. From the data in table I, it is apparent that HCFC-22 could not be selectively prepared in the aerosol reactor system even in the absence of photochemical finishing. Run 1shows an interesting bimodal distribution of products, which was evident even though a less than stoichiometric amount of fluorine was fed to the reactor. That is, the distribution of products from the reaction has a minimum at HCFC-22. This indicates that HCFC-22 is reacting at a faster rate than would be anticipated for a molecule with geminal fluorine and chlorine substituents. Hence, in addition to promoting Cl/F exchange, as indicated by the formation of FC-14 (CF,) and HFC-32 (CHzF2),the NaF aerosol appears to catalyze C-H fluorination. It is interesting in this regard that HFC-32 and FC-14 were observed in the products but that HFC-23 (CHF,) was not. Feeding a stoichiometric amount of fluorine to the reactor drove the reaction to mostly CFC-13 (CF3C1)and FC-14 (see run 2). Use of a more gradual fluorine concentration gradient by adding another reactor module had little effect on the product distribution (runs 3 and 4). The best selectivity to HCFC-22 was obtained in the absence of the NaF aerosol; i.e., by running the reaction as a simple gas-phase fluorination with a fluorine concentration gradient (runs 5-7). In this case, a maximum HCFC-22 yield of 25% was obtained with large amounts of CFC-13 and FC-14 still present. Consistent with our -+
0 1989 American Chemical Society
1548 Ind. Eng. Chem. Res., Vol. 28, No. 10, 1989 Table I. Direct Fluorination of Methyl Chloride run Id
FZ/CH&l" 1
4d
2 1 2
50 6ef 7ed.h
3 2.7 2.7
2d
3d
temp, 'Cb -50, -50 -50, -50 -50, -50, -50 -50, -50, -50 25 85, 85 85, 85
14 9.0 30.5 6.6 24 17.6 13.0 14.3
13 18 46.7 9.6 44
64 38.0 29.9
products, 22 2.4 3.0 1.2 2.5 10.1 25.0 7.4
%'
31 45 12 46 18 3.6 10.0 0
40
32 3.7 7.8 5.0 10 4.6
22 0
32 1 0 0
0 0
35.4
" Molar ratio of F2 to CH&l fed to the reactor. bReactortemperature; more than one entry indicates the temperatures of each module in a multistage reaction. Products are designated by their fluorocarbon code numbers (see Experimental Section). dSubstrate deposited on an NaF aerosol; no photochemical finishing step was employed. eReaction run without NaF aerosol; CH&l introduced as the vapor. f Also obtained a 14% relative yield of unidentified material. gHF injected into main carrier stream during this reaction. hAlso obtained a 13% relative yield of unidentified material. Table 11. Aerosol Fluorination of Methylene Chloride products, run F7/CH7C17" temp, OCb 14 13 12 21 30 Sd 2.2 -40, -30 tr tr 67 0 0 9e 4 -20,-IO 0 25 59 13 2.8 "Molar ratio of F2 to CH2C12fed to reactor. bTemperature of each reactor module. CTheproducts are designated by their fluorocarbon code numbers (see Experimental Section). dNo photochemical finishing step was employed. A 67% yield isolated in 98% purity from reactor, 33% not recovered due to aerosol losses in product trap. 'A photochemical finishing step was employed. Mass recovery near quantitative.
Scheme I CHzCh
+
F'
-
'CHC12
+
