MANUFACTURE OF HEXAFLUOROETHANE

Abstract: Disclosed is a process for the production of a highly fluorinated fluorocarbon comprising PFC-116, comprising the steps of contacting HF, an effective amount of chlorine, and a chlorofluorocarbon precursor selected from the group consisting of CFC-113, CFC-113a, CFC-114, CFC-114a, and CFC-115 in a vapor phase in the presence of a fluorination catalyst comprising Cr2O3 at a temperature of from about 300 DEG C to 500 DEG C to form a product stream, and recovering highly fluorinated fluorocarbon from the product stream. The amount of chlorine is sufficient to maintain or increase the instantaneous productivity of PFC-116.

Description: TITLE MANUFACTURE OF HEXAFLUOROETHANE CROSS REFERENCE TO RELATED APPLICATIONS This application claims the priority benefit of U.S. Provisional Application 60/030150, filed November 1, 1996 and U.S. Provisional Application 60/038659, filed February 21, 1997. FIELD OF THE INVENTION The present invention relates to a process for slowing the deactivation rate and/or eliminating deactivation of chrome oxide catalyst used in the vapor phase production of hexafluoroethane by fluorination wherein the fluorination is carried out in the presence of chlorine. The present invention further relates to processes for reducing and/or eliminating the presence of undesired by-products in such fluorinations by contacting the catalyst with chlorine. BACKGROUND OF THE INVENTION Highly fluorinated fluorocarbons are utilized as plasma etchant gases in the semiconductor industry. The highly fluorinated fiuorocarbon hexafluoroethane (CF 3 CF 3 , or PFC- 116) has found utility as a chemical vapor deposition (CND) chamber cleaning gas and as a plasma etchant gas in semiconductor device fabrication. As semiconductor manufacturers continually increase the number of functionalities per unit surface area of a semiconductor device, the increasing fineness of surface detail in turn requires greater precision and consistency in the effect the plasma arising from the etchant gas has on the semiconductor device. Etchant gases of very high purity are critical in this application. It has been found that even traces of impurities in an etchant gas may result in wide etched line width on a semiconductor device resulting in less bits (information) per chip. The presence of impurities in the etchant gas, including particulates, metals, moisture, and other halocarbons, even when present at parts- per-million levels, increases the defect rate in the production of higher density integrated circuits. As a result, there is increasing market demand for higher purity highly fluorinated etchant gases such as PFC-116. Identification of impurities and their elimination from an etchant gas are a significant aspect of preparing highly fluorinated fluorocarbons for such application in the semiconductor industry. PFC-116 has been known to be produced by allowing perhalogenated chlorofluoroethanes, e.g., trichlorotrifluoroethanes (CC1 3 CF 3 [CFC-113a] or CC1 2 FCC1F 2 [CFC-1 13]), dichlorotetrafluoroethanes (CC1 2 FCF 3 [CFC-1 14a] or CC1F 2 CC1F 2 [CFC-1 14]), and chloropentafluoroethane (CC1F 2 CF 3 [CFC-115]), among others, to react with hydrogen fluoride (HF) in a vapor phase over a solid chrome oxide catalyst. Such conventional processes for manufacture of PFC-1 16 result in progressive deactivation of the catalyst. In such processes, catalyst deactivation results in a decrease in the instantaneous productivity of PFC-116, and necessitates alteration of the reaction operating conditions to off-set the impact of the catalyst deactivation and maintain process productivity and product purity. For example, the reaction temperature may have to be incrementally increased to maintain the instantaneous PFC- 116 productivity as the catalyst deactivates. Further, conventional methods for manufacturing PFC-1 16 often produce a product stream containing significant amounts of fiuorocarbon byproducts which are difficult to separate from PFC- 1 16 by conventional separation techniques. Altering reactor conditions, such as increasing the reaction temperature to offset the catalyst deactivation, often produces concomitant increases in production of such fiuorocarbon by-products. Consequently, conventional processes for manufacture of PFC-116 require that the chrome oxide catalyst charge be replaced when it is no longer effective in obtaining the desired instantaneous PFC-116 productivity and/or when there is an unacceptable increase in by-product production. Such catalyst replacement and accompanying process interruption is costly, in terms of the material and disposition costs of the catalyst and in terms of the lost production time. The inability to maintain a desired instantaneous PFC-1 16 productivity over time, the need to eventually replace the catalyst charge, and increases in the production of non-desired fiuorocarbon by-products all represent significant deficiencies of conventional processes employed in PFC-1 16 manufacture. The present inventors have discovered that the addition of chlorine to a reactant feed stream significantly retards and can essentially eliminate the deactivation of chrome oxide catalyst used in manufacture of PFC-116. The present inventors have further discovered that by adding chlorine to such feed streams it is possible to inhibit formation of undesirable fiuorocarbon by-products. The present invention solves problems associated with conventional vapor phase fluorination processes for production of PFC-116 by providing processes for mitigating chrome oxide catalyst deactivation that accompanies such processes. Further, the present invention solves problems associated with conventional vapor phase fluorination processes for production of PFC-1 16 by providing processes for reducing the concentration of undesirable fiuorocarbon by-products in the off- gas (product) stream of such fluorination processes. SUMMARY OF THE INVENTION The present invention comprises a process for the production of PFC- 1 16, comprising: contacting HF, a chlorofluorocarbon precursor selected from the group consisting of CFC-113, CFC-113a, CFC-1 14, CFC-1 14a, and CFC-115, and an effective amount of chlorine in a vapor phase in the presence of a fluorination catalyst comprising Cr O 3 at a temperature of from about 300°C to 500°C to form a product stream, and recovering PFC- 1 16 from the product stream. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plot of the reaction temperature required to maintain chrome oxide catalyst productivity over time with and without chlorine co-feed. Figure 2 is a plot of fiuorocarbon by-product concentrations over time with and without chlorine co-feed. Figure 3 is a plot of the reaction temperature required to maintain chrome oxide catalyst productivity over time with and without chlorine co-feed. Figure 4 is a plot of fiuorocarbon by-product concentrations over time with and without chlorine co-feed. DETAILED DESCRIPTION The highly fluorinated fiuorocarbon hexafluoroethane (CF 3 CF 3 , PFC-116) is conventionally produced by allowing hydrogen fluoride (HF) to react with perhalogenated chlorofluorocarbon precursors such as CFC-113, CFC-1 14 and CFC-115 in a vapor phase over a solid chrome oxide (Cr 2 O ) catalyst. Examples of such processes are described in "Catalytic Fluorination of 1,2-Dichlorotetrafluoroethane (FC-114) and of 1,1-Dichlorotetrafluoroethane (FC-114a) with HF" by Marangoni et al. in La Chimica E L Industria, Vol. 67, No. 9, September 1985, pages 467-479 and in "Heterogenous Catalytic Reactions of Chlorofluorocarbons", Blanchard et al. in Appl. Catal. (1990), 59(1), pages 123-128; the disclosures of both of which are hereby incorporated by reference. As disclosed by Marangoni et al. and Blanchard et al., allowing anhydrous hydrogen fluoride (HF) to react with at least one of 1 , 1 , 1 -trichloro- 2,2,2-trifluoroethane (CFC-113a), l,l,2-trichloro-l,2,2-trifluoroethane (CFC-113), 1,1-dichloro- 1,2,2,2 tetrafluoroethane (CFC-114a) and 1,2-dichloro- 1,1,2,2 tetrafluoroethane (CFC-114) produces a reactor off-gas (reaction product mixture) comprising chloropentafmoroethane (CFC-1 15) and hexafluoroethane (PFC- 116) and hydrogen chloride (HC1). The CFC- 115 formed and any CFC- 1 13, CFC-1 13a, CFC-1 14, and/or CFC-114a in the off-gas may be separated from the off-gas and recycled for further conversion to PFC-1 16. Any unreacted HF may also be separated from the off-gas and recycled for further reaction. The PFC-1 16 may then be recovered as product by any suitable process. PFC- 1 16 may be recovered from the reactor product stream, for example, by the process disclosed in World Intellectual Property Organization Publication No. WO96/09271 , the disclosure of which is hereby incorporated by reference. Catalyst comprising chromium oxide is desirable in conventional conversion of the perhalogenated chlorofluorocarbon precursors CFC-1 13, CFC- 1 13a, CFC-114, CFC-114a, and CFC-115 to the highly fluorinated fiuorocarbon PFC-1 16. Such catalysts have a higher activity for PFC-1 16 synthesis than non- chromium oxide catalysts, e.g., aluminum oxide (Al O 3 ) catalysts. By "higher activity" it is meant that under the same feed compositions, feed rates, operating pressure, and with a given reactor volume filled with catalyst, a chromium oxide catalyst produces higher concentrations of PFC-1 16 in the reactor off-gas at a given operating temperature compared to a non-chromium oxide catalyst such as an aluminum oxide catalyst. Alternately, under the same feed compositions, feed rates, operating pressure and a given reactor volume, a chromium oxide catalyst can produce the same concentration of PFC-116 in the reactor off-gas but can do so while operating at a lower temperature compared to a non-chromium oxide catalyst such as an aluminum oxide catalyst. Chromium oxide catalysts can further typically produce PFC-116 concentrations of greater than 10 mole % of the total organics exiting in the reactor off-gas, whereas obtaining such high PFC-1 16 concentrations with non-chromium oxide catalysts such as aluminum oxide is difficult, if not impossible. Chromium oxide catalysts suitable for these conventional conversions comprise Cr 2 O formed by any process known in the art. Particularly preferred in the present process is Cr 2 O catalyst prepared such that its alkali metal content is less than 100 ppmw (parts-per-million by weight). Most preferred in the present process is Cr O 3 catalyst prepared as that disclosed in U.S. Patent No. 5,334,787, incorporated herein by reference. Although initially effective towards conversion of the perhalogenated chlorofluorocarbon precursors CFC-1 13, CFC-113a, CFC-1 14, CFC-1 14a, and CFC-1 15 to the highly fluorinated fiuorocarbon PFC-1 16, this Cr 2 O 3 catalyst deactivates over time when employed in the aforementioned conventional processes. Catalyst deactivation is manifested wherein a conventional process using the catalyst produces decreasing PFC-116 concentrations in the off-gas over time under identical feed and reaction operating conditions, i.e., where the instantaneous PFC-1 16 productivity decreases while all other reaction variables such as feed composition, feed rates, the reactor and catalyst bed volume, inlet reaction temperature and operating pressureare otherwise maintained constant. Maintaining a desired instantaneous PFC-1 16 productivity with a deactivating catalyst requires adjustments in reaction operating conditions over time to offset the effects of deactivation. In the presence of a deactivating catalyst, PFC-1 16 concentrations can be maintained in the reaction off-gas by increasing the contact time of the chlorofluorocarbon precursors with the catalyst (i.e., by reducing the reactor throughput), or by adjusting the HF/precursor feed ratio. However, it is typically only where the organic molar feed rate to the reactor is unchanged that the PFC- 116 off-gas concentration as mole % of the organics in the product stream is directly proportional to PFC-116 productivity. To maintain instantaneous PFC-1 16 productivity without changing the organic molar feed rate, where PFC- 1 16 productivity is defined as the weight of PFC- 1 16 produced per time period , more typically the precursor feed stream feed-in temperatures and/or the operating temperature of the reactor itself are increased to offset the effect of catalyst deactivation. However, such increases in temperature typically accelerate the deactivation rate of the catalyst such that loss of catalyst activity occurs faster at higher temperatures than at a lower temperature, i.e., the time required for a given percentage drop in instantaneous PFC-116 productivity becomes less. As a result, more frequent and larger increases in temperature become necessary over time to maintain the instantaneous PFC- 1 16 productivity. Such temperature increases also frequently increase the generation rate of non-1 lx fiuorocarbon by-products. By "non-1 lx" fiuorocarbon by-products is meant any compounds other than PFC-116 ("1 lx" wherein x is 6) or the chlorofluorocarbon precursors CFC-1 13 and CFC-113a ("1 lx" wherein x is 3), CFC-114 and CFC-114a ("1 lx" wherein x is 4), and CFC-115 ("1 lx" wherein x is 5). Non-1 lx fiuorocarbon by-product formation results in overall loss of PFC-116 yield from the chlorofluorocarbon precursors. By-product formation also increases the difficulty of separation and purification of PFC-116 from the reactor off-gas. For example, in a process wherein HF is contacted with chlorofluorocarbon precursors to produce PFC-1 16, the non-1 lx fiuorocarbon by-products that can be produced include fiuorocarbon 2x byproducts such as HCFC-22 ("2x" wherein x is 2) [chlorodifluoromethane] or HFC-23 ("2x" wherein x is 3) [trifluoromethane] and/or fiuorocarbon 12x byproducts such as HCFC-123 ("12x" wherein x is 3) [l,l-dichloro-2,2,2- trifluoroethane], HCFC-124 ("12x" wherein x is 4) [1-chloro- 1,2,2,2- tetrafluoroethane] or HFC- 125 ("12x" wherein x is 5) [pentafluoroethane] and/or fiuorocarbon 13x by-products such as HCFC-133a [l -chloro-2,2,2- trifluoroethane] or HFC- 134a [1,1,1,2-tetrafluoroethane]). Most of these by-products are considered detrimental in PFC-1 16, particularly in instances where the PFC-1 16 is to be used as a CVD chamber cleaning gas or as an etchant gas in semiconductor manufacture. Such by-products are particularly detrimental in instances where these non-1 lx by-products are difficult to remove from the product PFC-1 16, e.g., HFC-23, which forms an azeotrope with PFC-1 16 as disclosed in World Intellectual Property Organization Publication No. WO96/09271. Lowering the reaction operating temperature and/or increasing the HF/organic feed ratio results in less side-reaction to, and thus lower off-gas concentrations of, such non-1 lx fiuorocarbon by-products. Unfortunately, such process alterations also decrease the instantaneous PFC-116 productivity from a given reactor volume or total weight of catalyst. No method of catalyst regeneration is known which will restore deactivated chrome oxide catalyst to the activity level and durability of activity of virgin chrome oxide catalyst. Deactivated catalyst must therefore be changed out and replaced by fresh catalyst. Such catalyst changeout and accompanying process interruption is costly in terms of the material and disposition costs of the catalyst and/or in terms of the lost production time. The present invention is a process for the production of PFC-1 16, comprising: contacting HF, a chlorofluorocarbon precursor selected from the group consisting of CFC-113, CFC-113a, CFC-1 14, CFC-1 14a, and CFC-1 15, and an effective amount of chlorine in a vapor phase in the presence of a fluorination catalyst comprising Cr 2 O 3 at a temperature of from about 300°C to 500°C to form a product stream, and recovering PFC-116 from the product stream. The present inventors have discovered that carrying out, in the presence of an effective amount of chlorine, conventional processes wherein the chlorofluorocarbon precursors CFC-113, CFC-113a, CFC-1 14, CFC-114a, and CFC-1 15 are contacted with HF in the presence of a fluorination catalyst comprising Cr 2 O 3 resulting in a PFC- 116 product stream, slows, if not eliminates, the progressive catalyst deactivation observed in the conventional processes. By an "effective amount" of chlorine, is meant an amount of chlorine present during said contacting step which extends the chrome oxide catalyst life. The amount of chlorine can be relatively low and still effectively produce the unexpected effect of catalyst life extension. Chlorine concentration as low as 0.5 weight% (wt%) of the combined weight of HF and chlorine present during said contacting step extends the chrome oxide catalyst life compared to conventional processes employing no chlorine. Higher chlorine concentrations present during said contacting step, from about 2.5 wt% to 5 wt% of the combined weight of HF and chlorine, further extends the chrome oxide catalyst life and effectively stops deactivation of the catalyst. Furthermore, the beneficial effect of chlorine present during said contacting step appears to endure for a period after chlorine addition is stopped, such that the higher catalyst deactivation rate does not resume immediately upon cessation of the chlorine feed. By "extends chrome oxide catalyst life" is meant that decreases over time in instantaneous productivity of PFC-116 from the catalyst at a given set of conditions are eliminated or reduced when chlorine is present during said contacting step versus when chlorine is absent from said contacting step. By "extends chrome oxide catalyst life" is also meant that the instantaneous productivity of PFC- 116 is higher when chlorine is present during said contacting step versus when chlorine is absent from said contacting step. By "instantaneous productivity" is meant the weight of PFC-116 produced per weight of catalyst over a given time period. Typically, around 2 weeks after a specific catalyst charge begins operating, the instantaneous PFC- 1 16 productivity of a continuous conventional process will begin to sharply decrease, with the instantaneous productivity after 2 weeks typically having decreased by more than 10% compared to the instantaneous productivity obtained from the same reactor at the same reactor conditions over the first two or three days of that 2 week period of operation. Over a similar time period, the instantaneous productivity of a process wherein said contacting step is carried out in the presence of chlorine will decrease by less than 10 %, typically less than 5 %, most often less than 2 % compared to the instantaneous productivity obtained from the same reactor at the same reactor conditions over the first two or three days of that 2 week period of operation. Alternately, where periodic temperature increases are utilized to maintain catalyst productivity and where other operating conditions may be kept constant, total temperature increases of less than 5°C are typically required over the 2 week period to maintain the PFC-1 16 productivity that had been obtained from the reactor over the first two or three days of that 2 week period of operation wherever said contacting step is carried out in the presence of an effective amount of chlorine, whereas total temperature increases of greater than 5°C are typically required over the 2 week period to maintain the productivity wherever said contacting step is carried out in the absence of an effective amount of chlorine. Further, when carrying out said contacting step in the presence of an effective amount of chlorine, PFC-116 concentrations of at least 2.5 mole%, more typically at least 5.0 mole%, most typically at least 10 mole% of the organics in the product stream may be obtained simultaneously with said extended catalyst life. Carrying out conventional PFC- 1 16 production by chrome oxide based fluorination of chlorofluorocarbon precursors in the presence of chlorine mitigates and/or avoids the need to increase the reaction temperature to maintain instantaneous PFC- 116 productivity and can delay the need to change out or regenerate the chrome oxide catalyst. The present inventors have further discovered that carrying out, in the presence of an effective amount of chlorine, conventional processes wherein chlorofluorocarbon precursors CFC-113, CFC-1 13a, CFC-114, CFC-114a, and CFC-115 are contacted with HF in the presence of a fluorination catalyst comprising Cr 2 O resulting in a PFC-1 16 product stream, results in reduced concentration of potentially undesired fiuorocarbon by-products in the reaction off-gas. In an embodiment of the present invention wherein CFC-1 14 and/or CFC-115 is allowed to react with HF in the presence of fluorination catalyst comprising Cr 2 O 3 and an effective amount of chlorine to produce PFC-116, there is reduction in the non-1 lx fiuorocarbon by-products in the off-gas. Generally, the molar feed rate of chlorine which is needed to observe this effect is at least as much, typically slightly higher than, the molar generation rate of the fiuorocarbon by-products produced in the absence of chlorine. In contrast to chlorine's effect on instantaneous PFC- 1 16 productivity, where the productivity benefit continues for at least a time even after chlorine addition is stopped, the changes in fiuorocarbon by-product generation coincide more immediately with changes in the rate of chlorine addition. The present invention therefore comprises employing chlorine to maintain or increase the instantaneous productivity of PFC-1 16 in chrome oxide- based fluorination of the chlorofluorocarbon precursors CFC-113, CFC-113a, CFC-114, CFC-114a, and CFC-115, wherein chlorine addition allows higher reaction temperature operation and thus higher reaction rates for formation of PFC-116 but without the deficiency of concomitant accelerated catalyst deactivation and increased fiuorocarbon by-product generation that is seen in conventional process in which chlorine is absent. In the process of the present invention wherein at least one chlorofluorocarbon precursor selected from CFC-1 13, CFC-1 13a, CFC-114, CFC- 114a, and CFC-1 15 is contacted with HF and an effective amount of chlorine in a vapor phase in the presence of a fluorination catalyst comprising Cr 2 O 3 resulting in a PFC-1 16 product stream, said contacting step is carried out at from about 250°C to 500°C, more preferably from about 300°C to 500°C. The contacting step may be carried out at sub- or super-atmospheric pressures; pressures of from about 0 to 150 psig are preferred. The contacting step can be carried out over a wide range of HF/organic molar feed ratios, but HF/organic molar feed ratios of between from about 0.5/1 to 6/1 are preferred. The reactor be operated over a range of throughputs (organic feed rate per weight of catalyst per unit time), but throughputs of between from about 0.1 to 2.0 pph organic feed per pound of catalyst are preferred. Recovering the PFC- 1 16 from the product stream may occur by any suitable process. For example, PFC-116 may be recovered from the reactor product stream by the distillation process disclosed in World Intellectual Property Organization Publication No. WO96/09271, the disclosure of which is hereby incorporated by reference. While particular emphasis has been placed upon introducing elemental vaporous chlorine, any suitable halogen can be employed for maintaining or improving the instantaneous productivity of PFC-1 16. That is, at least one halogen such as chlorine (Cl 2 ), fluorine (F 2 ), and bromine (Br 2 ) can be employed in the contacting step of the present invention in any suitable manner, e.g., in situ generation of an elemental halogen, and injection into the feed stream, reactor or recycle stream. EXAMPLES The following examples are included to further teach the present invention and are not intended to limit the present invention. In the following Examples 1 and 2, 2 pounds of a chrome oxide catalyst prepared by the procedure of U.S. Patent No. 5,334,787 were charged to a reactor consisting of a "U"-shaped, 1 inch, schedule 40 Inconel® tube immersed in a molten salt bath, wherein the salt bath is used to maintain the reaction temperature at the selected setpoint. In the following Examples, feed rates are expressed as "kph" (kilograms per hour) and "pph" (pounds per hour), and pressures are expressed as "kPa" (kiloPascals) and psig (pounds per square inch - gauge). Example 1 In this Example, a fresh batch of the catalyst produced by the procedure of U.S. Patent No. 5,334,787 was charged to the U-tube reactor and activated by the procedure of U.S. Patent No. 5,334,787. A feed stream consisting of a CFC-114/CFC-l 15/HFC-125 mixture (69 wt% CFC-115, 29 wt% CFC-114, 2 wt% HFC- 125) was then fed to the reactor at a total organic feed rate of 0.3 kph (0.7 pph ). Hydrogen Fluoride (HF) was fed to the reactor at 0.082 kph (0.18 pph). The reactor operating pressure was 791 kPa (100 psig), and the reactor was initially operated at 375°C. The reactor operating temperature was then increased to bring the PFC-1 16 concentration in the organics exiting the reactor to between 15 and 18 mole % of the organics in the off-gas. Initially, this required that the temperature had to be adjusted to 400°C. For the rest of the catalyst runtime in this Example, the reaction temperature was adjusted as necessary to maintain the PFC-116 concentration between 15 and 18 mole % of the total organic molar concentration in the off-gas. After 1,100 hours of operation, chlorine was added to the reactor feed for 88 hours at a rate equivalent to 2.5 wt% of the HF feed rate, i.e., the chlorine was fed at 2.0 x 10 " kph (0.0045 pph). The chlorine addition was then stopped and for the next 40 hours no chlorine was added to the reactor feed. Chlorine was then again added to the reactor feed for 30 hours at a rate equivalent to 5.0 wt% of the HF feed rate, i.e., the chlorine was fed at 4 x 10 "3 kph (0.009 pph). The chlorine addition was then stopped and for the next 41 hours no chlorine was co-fed. The reactor feed was then switched to a CFC-1 14/CFC-l 15 mixture (30 wt% CFC-1 14, 70 wt% CFC-1 15), fed to the reactor at a total organic feed rate of 0.3 kph (0.7 pph). Hydrogen Fluoride (HF) was fed to the reactor at 8.2 x 10 "2 kph (0.18 pph). The reactor operating pressure was maintain at 7 1 kPa (100 psig), and the temperature was adjusted. as necessary to maintain the PFC-1 16 at 15-18 mole% of the organics in the off-gas. For the first 43 hours with this new organic mixture no chlorine was co-fed. For the final run hours, chlorine was again fed at a rate equivalent to 5.0 wt% of the HF being fed. The results of this Example are shown in Figure 1 and Figure 2. As may be seen in Figure 1, over the first 1,100 hours of operation, when no chlorine was as yet fed to the reactor, a relatively constant instantaneous PFC-1 16 productivity could be maintained only by periodically increasing the reaction temperature to offset catalyst deactivation. At any giver, temperature, the PFC-116 concentration in the off-gas was seen to eventually start to decrease. The operating temperature would then be increased, which in turn increased the PFC-116 concentration back to the desired concentration range. To maintain the PFC-116 off-gas concentration, temperature increases were required numerous times over the course of the 1,100 hours. During this initial 1,100 hours, the length of time the catalyst could produce the desired PFC-1 16 concentration at any given temperature also continued to decrease as the operating temperature required to produce that concentration increased, i.e., the catalyst could only run for increasingly shorter periods before additional temperature increases were necessary to offset the deactivation. Over this initial 1 , 100 hours prior to Chlorine addition, the reaction temperature had to be increased from the initial 400°C to over 440°C in order to maintain the target 15-18 organic mole % PFC-1 16 in the off-gas After the initial 1,100 hours, over the next 400 hours of operation, once the catalyst had been exposed to chlorine co-feed, no further increases in temperature were necessary to maintain PFC- 1 16 concentration at the desired 15-18%, indicating that the addition of the Chlorine had successfully halted, or at least significantly slowed any further deactivation. Further, the chlorine could be shut off for periods of time without the previous rapid rate of deactivation resuming, showing that the beneficial effects of the chlorine addition was maintained beyond the time chlorine was included with the reactor feed. As may be seen in Figure 2, a concomitant advantage of the chlorine addition was the reduction of the noπ-1 lx fiuorocarbon off-gas by-products. At the lower 2.5 wt% chlorine feed condition, although deactivation of the catalyst was successfully mitigated, there was little change in the total 2x + 12x + 13x fiuorocarbon by-product concentration in the off-gas. At the higher 5.0 wt% chlorine feed rate, however, the total concentration of the non-1 lx fiuorocarbon off-gas by-products was seen to decrease, as indicated by the reduction in the total 2x + 12x + 13x fiuorocarbon by-product concentration in the off-gas from the initial 2.5 organic mole% prior to chlorine addition down to 1.5 mole %. Whereas the benefits of chlorine addition in mitigating catalyst deactivation continued even after chlorine addition was stopped, the reduction in the 2x + 12x + 13x fiuorocarbon by-product concentration appeared more immediately responsive to the amount of chlorine fed. In the final period of the run with this catalyst batch, where HFC- 125 was no longer co-fed to the reactor, the same 5.0 wt% chlorine added to the HF feed reduced the total 2x + 12x + 13x fiuorocarbon by-product concentration from an initial 0.5 mole% without chlorine feed to less than 0.03 mole % of the organics in the off-gas. Example 2 In this Example, a fresh batch of the catalyst produced by the procedure of U.S. Patent No. 5,334,787 was charged to the U-tube reactor and activated by the procedure of U.S. Patent No. 5,334,787. A feed stream consisting of a CFC-114/CFC-l 15 mixture (70 wt% CFC-115, 30 wt% CFC-114) was then fed to the reactor at a total organic feed rate of 0.3 kph (0.7 pph). Hydrogen Fluoride (HF) was fed to the reactor at 8.2 x 10 "2 kph (0.18 pph). The reactor operating pressure was 791 kPa (100 psig), and the reactor was initially operated at 375°C. The reactor operating temperature was then increased to bring the PFC-1 16 concentration exiting the reactor to between 15 and 18 mole % of the organics in the off-gas. Initially, this required that the reactor temperature had to be adjusted to about 410°C. For the rest of the run time in this example, the reactor temperature was adjusted as necessary to maintain the PFC-1 16 concentration between 15 and 18 mole % of the organics in the off-gas. Initially, no chlorine was fed to this catalyst. After 250 hours of operation, chlorine was added to the reactor feed for the remainder of the run at a rate equivalent to 0.5 weight % of the HF feedrate, i.e., the chlorine was fed at about 4 x 10 "4 kph (0.0009 pph). The results of this Example are shown in Figure 3 and Figure 4. As may be seen in Figure 3, and in contrast to the catalyst activity over time in Example 1 before chlorine was added to the feed in Example 1 , the need for periodic increases in the temperature was markedly reduced with chlorine feed in Example 2, such that the temperature in Example 2 had to be increased by only 15°C to maintain the PFC-116 produced over the same 1,100 hours during which a 40°C increase had been required in Example 1. Based on this data, it was calculated that the half-life of the catalyst increased from 470 hours without any Chlorine feed to 1,400 hours with the addition of the chlorine at 0.5 wt% of the HF feedrate. Catalyst half-life is defined as the period of time required for a catalyst to deactivate to the extent that a reactor having twice the reactor volume filled with catalyst would be necessary to obtain the same instantaneous PFC-1 16 productivity that a single reactor volume had previously produced at a given starting point and under the same process operating conditions. As may be seen in Figure 4, upon chlorine addition the non-1 lx fiuorocarbon by-product concentration reduced, lowering from the initial 0.075 mole% to 1.25 mole % of the organics in the off-gas to routinely less than 0.002 mole %.

Claims: WHAT IS CLAIMED IS: 1. A process for the production of hexafluoroethane (PFC- 1 16), comprising: contacting HF, a chlorofluorocarbon precursor selected from the group consisting of 1 , 1 ,2-trichloro- 1 ,2,2-trifluoroethane (CFC- 113), 1,1,1- trichloro-2,2,2-trifluoroethane (CFC-113a), 1,1-dichloro- 1 ,2,2,2 tetrafluoroethane (CFC-1 14), l,2-dichloro-l,l,2,2 tetrafluoroethane (CFC-1 14a), and chloropentafluoroethane (CFC-115), and an effective amount of chlorine in a vapor phase in the presence of a fluorination catalyst comprising chrome oxide at a temperature of from about 300°C to about 500°C to form a product stream, and recovering hexafluoroethane from the product stream. 2. A process for maintaining or increasing the instantaneous hexafluoroethane (PFC- 1 16) productivity from fluorination catalyst used to produce highly fluorinated fiuorocarbon comprising PFC- 1 16, comprising contacting HF, an effective amount of chlorine, and a chlorofluorocarbon precursor selected from the group consisting of 1,1 , 2-trichloro- 1,2,2- trifluoroethane (CFC-1 13), l,l ,l -trichloro-2,2,2-trifluoroethane (CFC-1 13a), 1,1-dichloro- 1,2,2,2 tetrafluoroethane (CFC-1 14), l,2-dichloro-l , l,2,2 tetrafluoroethane (CFC-1 14a), and chloropentafluoroethane (CFC-1 15) in a vapor phase at a temperature of from about 300°C to about 500°C in the presence of a catalyst comprising chrome oxide. 3. The process of Claims 1 or 2 wherein the amount of chlorine present during said contacting of HF and chlorofluorocarbon precursor is from about 0.5 to 5 wt% based on the total weight of chlorine and HF. 4. The process of Claim 3 wherein the amount of chlorine present during said contacting of HF and chlorofluorocarbon precursor is from about 1 to 3 wt% based on the total weight of chlorine and HF. 5. The process of Claims 1 or 2 wherein the product stream contains less than about 1 mole% non-1 lx fiuorocarbon by-products. 6. The process of Claim 5 wherein the product stream contains less than about 0.05 mole% non-1 lx fiuorocarbon by-products. 7. The process of Claim 1 comprising maintaining an effective amount of chlorine in said contacting step such that the instantaneous hexafluoroethane (PFC-1 16) productivity is maintained or decreases by less than 10%. 8. The process of Claim 7 comprising maintaining an effective amount of chlorine in said contacting step such that the instantaneous hexafluoroethane (PFC-116) productivity is maintained or decreases by less than 5%. 9. The process of Claim 8 comprising maintaining an effective amount of chlorine in said contacting step such that the instantaneous hexafluoroethane (PFC-116) productivity is maintained or decreases by less than 2%. 10. The process of Claim 1 comprising maintaining an effective amount of chlorine in said contacting step such that the instantaneous hexafluoroethane (PFC-1 16) productivity is maintained or increases. 11. The process of Claims 1 or 2 wherein the chrome oxide catalyst is obtained by thermolysis of a dichromate consisting essentially of (NH 4 ) 2 Cr 2 O 7 . 12. The process of Claims 1 or 2 wherein said contacting is carried out at a temperature of from about 350°C to 450°C.

Inventors: MAHLER BARRY ASHER, MAHLER, BARRY, ASHER
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1997
Main classification: C07C17
IPC codes: B01J23, C07B61, C07C17, C07C19