Abstract:
A process for producing phosgene is disclosed which involves contacting a mixture comprising CO and Cl 2  (e.g., at about 300° C. or less) with carbon having an active metal content of less than 1000 ppm by weight and a high degree of oxidative stability (i.e., a weight loss of about 12 percent, or less, in the WVC Temperature Test as defined herein).

Description:
This application is a national filing under 35 USC 371 of International Application No. PCT/US96/17526 filed Nov. 1, 1996 and claims priority of U.S. Provisional Application No. 60/012,021, filed Feb. 21, 1996. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a process for the manufacture of phosgene by the reaction of chlorine with carbon monoxide in the presence of a carbon catalyst. More particularly, this invention relates to a process for the manufacture of phosgene with minimal production of the hazardous chemical, carbon tetrachloride. 
     BACKGROUND 
     The production of phosgene by the reaction of chlorine with carbon monoxide in the presence of a carbon catalyst is a well known process. The phosgene produced by this process will typically contain 400 to 500 ppm by weight carbon tetrachloride. This amount, evaluated on the basis of the total world-wide production of phosgene of about ten billion pounds (4.5×10 9  kg) corresponds to co-production of about 4 to 5 million pounds (1.8×10 6  kg to 2.3×10 6  kg) of carbon tetrachloride with the phosgene. 
     Japanese patent publication (Kokoku) No. Hei 6[1994]-29129 discloses that the amount of carbon tetrachloride produced during the phosgene manufacturing process can be reduced (e.g., by about 50%) by using an activated carbon which has been washed with an acid and which contains a total of 1.5 wt. % or less of metal components comprised of transition metals, boron, aluminum and silicon. 
     Carbon tetrachloride has been of concern in connection with ozone depletion and global warming potentials. Therefore, there is an interest in developing phosgene processes in which the amount of carbon tetrachloride impurity is minimized. 
     SUMMARY OF THE INVENTION 
     A process for producing phosgene is provided which comprises contacting a mixture comprising carbon monoxide and chlorine with carbon. In accordance with this invention, the carbon (1) has an active metal content of less than 1000 ppm by weight, and (2) loses about 12% of its weight, or less, when sequentially heated in air for the following times and temperatures; 125° C. for 30 minutes, 200° C. for 30 minutes, 300° C. for 30 minutes, 350° C for 45 minutes, 400° C. for 45 minutes, 450° C. for 45 minutes and finally at 500° C. for 30 minutes. Typically the contact is at a temperature of about 300° C., or less. 
     DETAILED DESCRIPTION 
     The present invention relates to improving the production of phosgene produced by contacting carbon monoxide and chlorine with carbon. The improvement can be employed in connection with any of those carbon-based processes used commercially or described in the art (e.g., those processes disclosed in U.S. Pat. Nos. 4,231,959 and 4,764,308). 
     Phosgene is commercially manufactured by passing carbon monoxide and chlorine over activated carbon. The reaction is strongly exothermic and is usually done in multitubular reactors to more effectively control the reaction temperature. Carbon monoxide is typically added in at least a stoichiometric amount (often in stoichiometric excess) to minimize the free chlorine content of the phosgene product. 
     As used in connection with this invention, the term &#34;active metals&#34; means metals included in the group consisting of transition metals of groups 3 to 10, boron, aluminum and silicon. Carbon which contains less than about 1000 ppm by weight of active metals is employed. Iron is considered a particularly harmful active metal (i.e., the greater the amount of iron the larger the amount of carbon tetrachloride produced). It is preferred to use carbons which not only have an active metal content of less than about 1000 ppm by weight, but also contain less than about 100 ppm by weight of iron (and more preferably less than about 80 ppm by weight iron). It is also preferable to use carbons which contain less than 200 ppm by weight of sulfur and less than 200 ppm by weight of phosphorus (and more preferably less than 100 ppm by weight each, of phosphorus and sulfur). 
     The carbons used for the process of this invention also exhibit substantial weight stability when heated in air. More particularly, when heated in air at 125° C. for 30 minutes, followed by heating at 200° C. for 30 minutes, followed by heating at 300° C. for 30 minutes, followed by heating at 350° C. for 45 minutes, followed by heating at 400° C. for 45 minutes, followed by heating at 450° C. for 45 minutes and finally followed by heating at 500° C. for 30 minutes, the carbons employed for the process of this invention lose about 12% of their weight, or less. This sequence of time and temperature conditions for evaluating the effect of heating carbon samples in air is defined herein as the &#34;WVC Temperature Test&#34;. The WVC Temperature Test may be run using thermal gravimetric analysis (TGA). Carbons which when subjected to the WVC Temperature Test lose about 12% of their weight, or less, are considered to be advantageously oxidatively stable. 
     Carbon from any of the following sources are useful for the process of this invention; wood, peat, coal, coconut shells, bones, lignite, petroleum-based residues and sugar. Commercially available carbons which may be used in this invention include those sold under the following trademarks. Bameby &amp; Sutcliffe™, Darco™, Nuchar™, Columbia JXN™, Columbia LCK™, Calgon PCB™, Calgon BPL™, Westvaco™, Norit™ and Barnaby Cheny NB™. The carbon support can be in the form of powder, granules, or pellets, or the like. 
     Preferred carbons include acid-washed carbons (e.g., carbons which have been treated with hydrochloric acid or hydrochloric acid followed by hydrofluoric acid). Acid treatment is typically sufficient to provide carbons which contain less than 1000 ppm of active metals. Suitable acid treatment of carbons is described in U.S. Pat. No. 5,136,113. 
     Particularly preferred carbons include three dimensional matrix porous carbonaceous materials. Examples are those described in U.S. Pat. No. 4,978,649, which is hereby incorporated by reference herein in its entirety. Of note are three dimensional matrix carbonaceous materials which are obtained by introducing gaseous or vaporous carbon-containing compounds (e.g., hydrocarbons) into a mass of granules of a carbonaceous material (e.g., carbon black); decomposing the carbon-containing compounds to deposit carbon on the surface of the granules; and treating the resulting material with an activator gas comprising steam to provide a porous carbonaceous material. A carbon-carbon composite material is thus formed. 
     The carbon surface area as determined by BET measurement is preferably greater than about 100 m 2  /g and more preferably greater than about 300 m 2  /g. 
     It is known from dissociation equilibria that at 100° C., phosgene contains about 50 ppm chlorine; and that at 200° C., about 0.4%, at 300° C., about 5% and at 400° C. about 20% of the phosgene is dissociated into carbon monoxide and chlorine. Also, the higher the reaction temperature, the more carbon tetrachloride is generally produced. Accordingly, the temperature of the reaction is generally about 300° C., or less (e.g., in the range of from 40° C. to 300° C.). Preferably, the temperature of the process is from about 50° C. to 200° C.; more preferably from about 50° C. to 150° C. The phosgene produced by the process of this invention typically contains about 300 ppm by weight or less of carbon tetrachloride, based upon phosgene (i.e., 300 parts by weight CCl 4  per million parts by weight COCl 2 , or less) even at a temperature of 300° C. Preferably, the reaction temperature and the carbon are chosen to provide phosgene which contains less than about 250 ppm by weight of carbon tetrachloride; and more preferably, are chosen to provide phosgene which contains less than about 100 ppm by weight of carbon tetrachloride, based upon phosgene. Of note are embodiments where the reaction time and temperature are controlled to provide a carbon tetrachloride concentration of about 100 ppm or less based upon the total product stream. 
     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, therefore, to be construed as merely illustrative, and does not constrain the remainder of the disclosure in any way whatsoever. 
    
    
     EXAMPLES 
     General Catalyst Testing Procedure 
     A 1/2&#34; (1.27 mm) O.D×15&#34; (381 mm) Inconel® 600 nickel alloy tube containing a 100 mesh (0.015 mm) Monel® nickel alloy screen was used as the reactor. The reactor was charged with about 2.5 mL to about 8 mL of carbon catalyst and heated to 300° C. This was the temperature used for all the examples. 
     A 1:1 molar ratio mixture of carbon monoxide and chlorine was passed over the catalyst. The contact times were between 8 to 12 seconds. The experimental results are shown in Table 1. 
     The comparative examples were done in the same way as described above. The results are shown in Table A. 
     General Analytical Procedure 
     The reactor effluent was sampled on-line with a Hewlett Packard HP 5890 gas chromatograph using a 105 m long, 0.25 mm I.D. column containing Restak™ RTX-1 Crossbond 100% dimethyl polysiloxane. Gas chromatographic conditions were 50° C. for 10 minutes followed by temperature programming to 200° C. at a rate of 15° C./minute. The smallest amount of carbon tetrachloride that could be quantitatively identified was about 80 ppm by weight. 
     Thermal Analysis Procedure 
     Thermal gravimetric analysis (TGA) was done using a TA Instruments analyzer. The TGA experiments were done in air at a flow rate of 80 mL/min. The carbon sample was heated in air for the following times and temperatures; 125° C. for 30 minutes, 200° C. for 30 minutes, 300° C. for 30 minutes, 350° C. for 45 minutes, 400° C. for 45 minutes, 450° C. for 45 minutes and finally at 500° C. for 30 minutes. The weight loss was measured at each interval and finally after completion of the heating cycle. The percentage weight loss after completion of the heating cycle at 500° C. is shown in the tables. 
     Legend 
     Carbon Sample 
     A-D. Porous carbonaceous material. 
     E. HCl and HF washed sample of coconut shell carbon. 
     R. Commercial sample (1) of coconut shell carbon. 
     S. HCl washed sample of Carbon Sample R. 
     T. Commercial sample (2) of coconut shell carbon. 
     V. Calcined petroleum coke. 
     
                       TABLE 1______________________________________                      TGA  Active    CCl.sub.4 CCl.sub.4 Wt. Metal Fe Surface   Carbon Conc..sup.1 Conc..sup.2 Loss.sup.3 Content.sup.4 Content Area                                         Ex. Sample ppm ppm wt. %                                        ppm ppm m2/g______________________________________1    A       100     161   0.86  27    13    --  2 E 100 158 9.63 125 21 --  3 B  90 140 1.15 358 28 350  4 C  90 139 9.15 836 64 409  5 D &lt;50 &lt;90 3.37 516 37 500______________________________________ .sup.1 By weight as ppm of the product stream. The values shown are averages taken over 7 hours and are highend estimates. .sup.2 By weight as ppm of the phosgene produced. The values shown are averages taken over 7 hours and are highend estimates. .sup.3 The carbon sample was heated in air for the following times and temperatures; 125° C. for 30 minutes, 200° C. for 30 minutes, 300° C. for 30 minutes, 350° C. for 45 minutes, 400° C. for 45 minutes, 450° C. for 45 minutes and finally at 500° C. for 30 minutes. .sup.4 Active metals consist of transition metals of groups 3 to 10, boron, aluminum and silicon. 
    
     COMPARATIVE EXAMPLES 
     
                       TABLE A______________________________________                      TGA  Active    CCl.sub.4 CCl.sub.4 Wt. Metal Fe Surface   Carbon Conc..sup.1 Conc..sup.2 Loss.sup.3 Content.sup.4 Content Area                                         Ex. Sample ppm ppm wt. %                                        ppm ppm m2/g______________________________________A    S       400     640   14.94                            659    81   --  B R 320 500 89.83 4900 360 1012  C T 220 340 88.20 1516 130  835  D V 120 320  7.54 2934  50 0.58______________________________________ .sup.1 By weight as ppm of the product stream. The values shown are averages taken over 7 hours and are highend estimates. .sup.2 By weight as ppm of the phosgene produced. The values shown are averages taken over 7 hours and are highend estimates. .sup.3 The carbon sample was heated in air for the following times and temperatures; 125° C. for 30 minutes, 200° C. for 30 minutes, 300° C. for 30 minutes, 350° C. for 45 minutes, 400° C. for 45 minutes, 450° C. for 45 minutes and finally at 500° C. for 30 minutes. .sup.4 Active metals consist of transition metals of groups 3 to 10, boron, aluminum and silicon.