Treatment of chromium oxide; use thereof in catalytic manufacture of vinyl fluoride

Abstract: Advantageous processes are disclosed for the production of vinyl fluoride. Also disclosed are advantageous methods which may be employed for the preparation of catalysts useful in such processes. Included is a method which involves reducing surface B 2 O 3 present in a bulk chromium oxide composition containing surface B 2 O 3 by treating said composition with HF at an elevated temperature. Processes are provided herein which involve contacting 1,1-difluoroethane in the vapor phase with a trivalent chromium catalyst having primarily the morphology of alpha-chromium oxide and optionally containing less than 1000 ppm alkali metal as the alkali metal oxide wherein chromium is at least 95 atom percent of the metallic cations of said catalyst, at a temperature between about 225°C and 375°C. Advantageous embodiments of these processes are disclosed wherein (i) the catalyst is prepared by reducing B 2 O 3 present in a bulk chromium oxide composition as indicated above, (ii) chromium is at least 99 atom percent of the metallic cations of the catalyst and/or (iii) the space velocity is from about 200 volumes to 2000 volumes of 1,1-difluoroethane per volume of catalyst per hour.

Description: FIELD OF THE INVENTION This invention relates to processes for the production of vinyl fluoride, and more particularly, to a catalytic process for the dehydrofluorination of 1,1-difluoroethane to vinyl fluoride. BACKGROUND Vinyl fluoride (i.e., CH 2 =CHF or VF) is a useful monomer for the preparation of fluorocarbon polymers which have excellent weathering and chemical resistance properties. Vinyl fluoride can be produced from acetylene and hydrogen fluoride using mercury catalysts. It can also be produced by the dehydrofluorination of 1,1-difluoroethane (i.e., CHF 2 CH 3 or HFC-152a). U.S. Patent No. 2,892,000 discloses a process for the manufacture of vinyl fluoride and 1,1-difluoroethane. In this process HF and acetylene are passed over a chromium catalyst (e.g., a chromium oxide or chromium salt catalyst) to obtain mixtures of VF and HFC-152a. A process is also disclosed in this patent for the conversion of product HFC-152a to VF using these catalysts. The patent describes using for this conversion of HFC-152a, catalysts which have been used for the reaction of HF and acetylene and whose activity has been reduced; and have then been treated by passing air or oxygen over the heated catalyst (e.g., at about 600 to 700°C for 1 to 3 hours) whereby it is activated for use in the dehydrofluorination of the HFC-152a to VF (e.g., at temperatures of from about 200°C to 400°C and at a rate of about 20 to about 80 volumes per volume of catalyst per hour). There is an ongoing interest in developing more efficient catalysts for the conversion of HFC-152a to VF. SUMMARY OF THE INVENTION This invention provides an advantageous process for the production of vinyl fluoride including an advantageous method for the preparation of the catalyst for this process. The invention specifically provides a process for the production of vinyl fluoride comprising: contacting 1,1-difluoroethane in the vapor phase with a trivalent chromium catalyst having primarily the morphology of alpha-chromium oxide wherein chromium is at least 95 atom percent of the metallic cations of said catalyst, at a temperature between about 225°C and 375°C; said catalyst being prepared reducing surface B 2 O 3 present in a bulk chromium oxide composition containing surface B 2 O 3 by contacting said bulk chromium oxide composition with HF at a temperature of from about 200 to 400°C. Advantageous embodiments are provided wherein chromium is at least 99 atom percent of the metallic cations of the catalyst. Also provided are advantageous embodiments wherein the space velocity is from about 200 volumes to 2000 volumes of 1,1-difluoroethane per volume of catalyst per hour. DETAILED DISCUSSION The present invention provides a process for the manufacture of vinyl fluoride by contacting 1,1-difluoroethane in the presence of a selected high purity trivalent chromium catalyst having primarily the morphology of alpha-chromium oxide (especially Cr 2 O 3 consisting essentially of alpha-chromium oxide). Included is Cr 2 O 3 prepared by pyrolysis of (NH 4 ) 2 Cr 2 O 7 . The Cr 2 O 3 catalyst prepared by the pyrolysis of ammonium dichromate suitable for the process of this invention can be prepared by any method known to the art including those disclosed in U.S. Patent Nos. 4,843,181 and 5,036,036, which are hereby incorporated herein by reference. The Cr 2 O 3 obtained in this manner may contain low levels of contaminants (e.g., potassium) which are present as a result of the manufacturing process for the original (NH 4 ) 2 Cr 2 O 7 . The level of potassium or other water soluble impurities may be reduced by a water-washing in a conventional manner. Other Cr 2 O 3 catalysts which may be used in the process of this invention include amorphous chromium oxide catalysts which have been treated to convert them to exhibit alpha-chromium oxide morphology as determined by x-ray diffraction. While several methods for treatment might be available, a useful method is to heat amorphous chromium oxide to an elevated temperature (e.g., between 400 and 500°C) in air for a sufficient period of time (usually from 24 to 48 hours). Bulk chromium oxide compositions containing B 2 O 3 may be heated in oxygen, in air or in other oxygen-containing environments at an elevated temperature (e.g., between 400 and 500°C) for a time sufficient to enrich the B 2 O 3 on the surface of the composition. It is generally desirable to enrich the B 2 O 3 on the surface in this manner by at least a factor of two compared to the surface analysis of the untreated bulk composition. This method may be used, for example, for enriching surface B 2 O 3 in bulk chromium oxide compositions wherein Cr is from about 95 to 99 atom percent of the metallic cations of the composition and B is from about 0.1 to 5 atom percent of the metallic cations of the composition. When the starting bulk chromium oxide composition is amorphous, this heat treatment can be used to simultaneously provide alpha-chromium oxide morphology within the composition. The amount of B 2 O 3 present on the surface of the chromium oxide composition is then be reduced by treatment with HF in the vapor phase at an eleveated temperature of from 200 to 400°C. For example, Guignet's green (a commercially available green pigment and which has typical compositions as follows: Cr 2 O 3 79 to 83%, H 2 O 16 to 18%, B 2 O 3 1.5 to 2.7%) can be treated by this method of surface enrichment to convert it essentially to the alpha-form. The boron present as B 2 O 3 can be removed after this heat treatment by treatment of the heat-treated material with HF at an elevated temperature (usually from 200 to 300°C) to remove the boron as BF 3 . During this treatment with HF minor amounts of the chromium oxide may be converted to chromium oxyfluorides. Prior to treatment the Guignet's green is considered to have little, if any, alpha-chromium oxide morphology. After the above described heat treatment to remove boron, the resulting chromium catalyst has an x-ray pattern typical of alpha-chromium oxide. The structure of the catalyst is not critical and may, for example, include pellets, powders or granules. Preferably, the trivalent chromium catalyst contains less than 1000 ppm alkali metal, as the alkali metal oxide, and less than 2000 ppm boron, as B 2 O 3 . It has been found particularly advantageous for converting 1,1-difluoroethane, to employ catalysts wherein the metallic cations of the catalyst are at least about 99 atom percent chromium, more preferably about 99.5 atom percent or more chromium. The Cr 2 O 3 catalyst is treated with HF before use. It is thought that this converts some of the surface chrome oxide to chromium oxyfluorides. This pretreatment can be accomplished by placing the Cr 2 O 3 in a suitable container, which can be the reactor to be used to perform the reaction of the instant invention; and thereafter, passing HF over the dried Cr 2 O 3 so as to partially saturate the Cr 2 O 3 with HF. This is conveniently carried out by passing HF over the Cr 2 O 3 for a period of time (e.g., about 15 to 300 minutes) at a temperature of about 200°C to about 400°C. The reaction temperature will normally be within the range from about 225°C to 375°C. The 1,1-difluoroethane is passed over the catalyst at a rate of preferably about 200 volumes to about 2000 volumes per volume of catalyst per hour; more preferably 400 volumes to 1000 volumes per volume of catalyst per hour. The reaction pressure can be subatmospheric, atmospheric or superatmospheric. Generally, near atmospheric pressures are preferred. The dehydrofluorination of 1,1-difluoroethane to vinyl fluoride and hydrogen fluoride is an equilibrium reaction. According to published literature the following equilibrium concentrations of vinyl fluoride (VF) have been determined; about 13% VF at 227°C, about 40% VF at 327°C and about 99% VF at 427°C. Unreacted starting material can be recycled to the reactor for the production of additional CH 2 =CHF. Vinyl fluoride (b.p. -72°C) may be recovered from the reaction product and any unreacted 1,1-difluoroethane (b.p. -25°C) by conventional procedures such as distillation. The process of this invention can be carried out readily in the vapor phase using well known chemical engineering practice. The reaction zone and its associated feed lines, effluent lines and associated units should be constructed of materials resistant to hydrogen fluoride. Typical materials of construction, well-known to the fluorination art, include stainless steels, in particular of the austenitic type, the well-known high nickel alloys, such as Monel® nickel-copper alloys, Hastelloy® nickel-based alloys and, Inconel® nickel-chromium alloys, and copper-clad steel. Silicon carbide is also suitable for reactor fabrication. Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following preferred specific embodiments are to be construed as illustrative, and not as constraining the remainder of the disclosure in any way whatsoever. EXAMPLES General Procedure for Catalyst Evaluation Catalyst evaluation was done in a fixed-bed reactor consisting of a 12" (30.5 cm) X 1/2" (1.3 cm) carbon steel tube maintained in a fluidized sandbath. The temperature was measured at the center of the bed. The feed materials were sent upflow through the reactor. The reactor was charged with the catalyst to be evaluated. It was then heated to about 250°C in a flow of nitrogen (50 cc/min) for about 30 minutes. The temperature was reduced to 175°C and a HF:Nitrogen flow in the ratio of 1:1 (total flow 100 cc/min) was passed through. After HF was observed in the reactor exit, the HF/Nitrogen ratio was changed to 4:1 (total flow 100 cc/min) and the temperature of the reactor gradually increased to 350-400°C. The reactor contents were kept at 350-400°C for about 30 minutes. The reactor contents were then brought to the desired operating conditions for catalyst evaluations. General Procedure for Product Analysis The following general procedure is illustrative of the method used. Part of the total reactor effluent was. sampled on-line for organic product analysis using a Hewlett Packard HP 5890 gas chromatograph equipped with a 20' (6.1 m) long x 1/8" (0.32 cm) diameter tube containing Krytox™ perfluorinated polyether on an inert carbon support. The helium flow was 35 mL/min. Gas chromatographic conditions were 70°C for an initial hold period of three minutes followed by temperature programming to 180°C at a rate of 6°C/minute. Unless indicated otherwise, the reported results are in mole%. The bulk of the reactor effluent containing organic products and also inorganic acid HF was treated with aqueous caustic to neutralize the acid prior to disposal. Legend 1141 is CH 2 =CHF F152a is CH 3 CHF 2 SV is Space Velocity (volume F152a per volume catalyst per hour) Comparative EXAMPLE Dehydrofluorination of F152a Catalyst: Calcined Guignet's Green Cr 2 O 3 5 cc, 3.3 g, 10 to 20 mesh (2.0 to 0.84 mm) A commercially available sample of Guignet's Green Cr 2 O 3 was calcined in air at 500°C for 72 hours prior to use. X-ray examination showed that it was converted essentially to alpha-chromium oxide. To the reactor was fed 25 cc/min (SV was 300) of F152a. The catalyst was not activated with HF prior to use. The results reported are in area%. Results under a variety of conditions are shown in the table. Time (Hours) Temp. (°C) F1141 F152a 1.0 250 0.4 99.6 2.0 260 0.9 99.1 3.0 280 31.3 68.7 4.0 280 21.1 78.9 5.0 280 30.3 69.7 6.0 280 31.5 68.5 8.0 290 39.1 60.9 9.0 300 46.1 53.9 9.5 310 53.5 46.5 10.0 320 60.5 39.5 10.5 330 66.9 33.0 11.0 340 72.9 27.0 12.0 350 77.3 22.6 EXAMPLE Dehydrofluorination of F152a Catalyst: Calcined Guignet's Green Cr 2 O 3 5 cc, 3.3 g, 10 to 20 mesh (2.0 to 0.84 mm) A commercially available sample of Guignet's Green Cr 2 O 3 was calcined in air at 500°C for 72 hours and then activated in a stream of HF to 400°C prior to use. During the course of the activation considerable amounts of boron trifluoride were produced which hydrolyzed to boric acid in water and was identified as boric acid by standard methods. Examination of the catalyst surface via x-ray photoelectron spectroscopy showed that about 10% of the catalyst surface contained boric oxide (expressed as boron) showing an enrichment of boron on the catalyst surface from the bulk of the catalyst during calcination at 500°C in air (compared to about 3% before calcination) before treatment with HF. After HF treatment, there was no boron detected by the same technique. Results under a variety of conditions are shown in the table. Temp °C F152a cc/min SV F1141 F152a 250 25 300 20.1 79.9 280 25 300 35.8 64.2 300 25 300 49.0 51.0 320 25 300 62.3 37.6 340 25 300 74.0 25.7 350 25 300 78.5 21.1 350 50 600 78.2 21.7 350 75 900 75.7 24.3 350 100 1200 71.6 28.2 There were small quantities of unidentified products at higher temperatures.

Claims: A process for the production of vinyl fluoride comprising: contacting 1,1-difluoroethane in the vapor phase with a trivalent chromium catalyst having primarily the morphology of alpha-chromium oxide wherein chromium is at least 95 atom percent of the metallic cations of said catalyst, at a temperature between 225°C and 375°C; said catalyst being prepared by reducing surface B 2 O 3 present in a bulk chromium oxide composition containing surface B 2 O 3 by contacting said bulk chromium oxide composition with HF at a temperature of from 200 to 400°C. The process of Claim 1 wherein chromium is at least 99 atom percent of the metallic cations of the catalyst. The process of Claim 2 wherein the space velocity is from a 200 volumes to 2000 volumes of 1,1-difluoroethane per volume of catalyst per hour.

Inventors: CHRISTOPH FRANK J, COULSTON GEORGE W, RAO VELLIYUR NOTT MALLIKARJUNA, CHRISTOPH, FRANK J., COULSTON, GEORGE W., RAO, VELLIYUR NOTT MALLIKARJUNA
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1996
Main classification: B01J23
IPC codes: B01J37, B01J27, B01J23, C07C17