Abstract:
A process for preparing isophorone nitrile is disclosed, whereby isophorone is reacted with hydrogen cyanide in the presence of a quaternary ammonium catalyst. Remaining catalyst is removed by thermal destruction and nitrogen sparging. Residual cyanide is removed by acidification and nitrogen sparging.

Description:
BACKGROUND OF THE INVENTION 
     Isophorone nitrile (IPN) or 3-cyano-3,5,5-trimethylcyclohexanone is a critical intermediate in the synthetic scheme to the diisocyanate (IPDI). Numerous prior art processes have been developed to synthesize IPN. For example, Dubreux U.S. Pat. No. 4,299,775 discloses a two-phase process for the preparation of IPN by reacting isophorone (IPH) with a cyanide in the presence of a catalytic amount of a phase-transfer agent. The quaternary ammonium catalysts are used as their chloride or bromide-salts; the chloride or bromide ion is exchanged for cyanide ion and in this cyanide form is transferred from the water to the solvent layer via the well known phase transfer mechanism. German Patent No. 1,240,854 to Scholven-Chemie discloses a process for the preparation of IPN by reacting isophorone with hydrogen cyanide in the presence of a basic catalyst such as alkali cyanide, hydroxides or alcoholates. The catalyst is removed by washing with dilute nitric acid. 
     Japanese Laid-Open specification 61-33157 to Nippon Kagako K.K. discloses a process for the preparation of isophorone nitrile by reacting isophorone with hydrogen cyanide in the presence of tetra-n-butylammonium hydroxide, tetra-n-butylphosphonium hydroxide or benzyltrimethylammonium hydroxide. The resulting reaction liquid is washed in water. 
     Acidic washing to remove catalyst produces an aqueous effluent saturated in isophorone and containing some cyanide. Disposal of such an effluent adds significantly to the cost of the product. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art have been solved by the instant invention, which provides a process for the preparation of isophorone nitrile according to the following reaction: ##STR1## 
     The isophorone nitrile is prepared by the Michael addition of HCN across the double bond of both the α and β isomers of isophorone. The instant process preferably utilizes a quaternary ammonium hydroxide as a catalyst. On a molar basis, tetramethyl, tetraethyl, and tetrabutyl ammonium hydroxides are equally effective catalysts. Because tetramethylammonium hydroxide (TMAH) has the lowest molecular weight and is also the least expensive quaternary ammonium hydroxide, TMAH is the preferred catalyst. The avoidance of the generation of large volumes of aqueous effluents can be accomplished by thermal destruction of the catalyst rather than aqueous washing to remove residual catalyst and residual HCN. The product of the thermal decomposition is trimethyl amine which can be scrubbed from the off-gases. Residual cyanide is removed by acidification of the crude product with an acid followed by sparging with an inert gas. 
     It is therefore an object of the invention to provide a process for the preparation of isophorone nitrile that uses an inexpensive, readily available catalyst. 
     It is a further object of the invention to prepare isophorone nitrile by a process that avoids generation of large volumes of aqueous effluents. 
     It is another object of the invention to provide a process for the preparation of isophorone nitrile that eliminates residual catalyst. 
     It is still another object of the invention to provide a process for the preparation of isophorone nitrile that eliminates residual cyanide. 
     It is a further object of the invention to provide a process for the preparation of isophorone nitrile that minimizes the generation of by-products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates laboratory apparatus used in the process of the instant invention. 
     FIG. 2 illustrates laboratory sparge and stripping apparatus used in the process of the instant invention. 
     FIG. 3 is a graph showing the stability of quaternary ammonium hydroxide catalysts in isophrone at various temperatures. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In preferred embodiments of the process of the instant invention, the quaternary ammonium catalyst is tetramethylammonium hydroxide (TMAH), preferably as a 25% solution in either water or methanol. The least expensive is the aqueous solution, although (in a subsequent step) the water will be stripped out of the product at the azeotropic composition 85% IPH/15% H 2  O, which will then be a process waste stream. If this is undesirable, the methanolic version of the catalyst can be used. The methanol can be condensed and separated. The non-condensable trimethylamine can be sent to a water scrubber. 
     Quaternary ammonium hydroxides pyrolyze by the Hofmann Reaction. 
     TMAH thermally decomposes to trimethyl amine plus methanol: 
     
         (CH.sub.3).sub.4 NOH→(CH.sub.3).sub.3 N+CH.sub.3 OH 
    
     Tetraethylammonium hydroxide (TEAH) decomposes to triethyl amine plus ethanol: 
     
         (CH.sub.3 CH.sub.2).sub.4 NOH→(CH.sub.3 CH.sub.2).sub.3 N+CH.sub.3 CH.sub.2 OH 
    
     Tetrabutylammonium hydroxide (TBAH) decomposes to tributyl amine plus butylene plus H 2  O: 
     
         (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2).sub.4 NOH→CH.sub.3 CH=CHCH.sub.3 +(CH.sub.3 Ch.sub.2 CH.sub.2 CH.sub.2).sub.3 N+H.sub.2 O 
    
     These reactions can be applied in the instant process to remove the catalyst from IPN, thereby circumventing an aqueous wash and its concomitant generation of large volumes of effluent. FIG. 3 is a graphical illustration of the thermal decomposition of TMAH and tetraethylammonium hydroxide in an isophorone system at temperatures from 110° C. to 150° C. Most of the catalyst can be removed from the crude product at 140°-150° C. Prolonged heat up and hold periods at 150° C. during the catalyst destruction step create color bodies in the solids. Heating from 110° C. to 150° C. in a few minutes and holding at 150° C. or 140° C. for about an hour enabled white IPN to be isolated. Preferably, the thermal decomposition is carried out at about 140° C. for about 90 minutes, with sparging with an inert gas, preferably N 2 , to remove volatile decomposition product. The sparging can be carried out anywhere from 100°-200° C., preferably about  110° C. However, if the temperature is raised to strip out IPH or distill IPN, the sparging can be carried out at that raised temperature (e.g., about 140°-150° C., or if a vacuum is present, as high as about 200° C). During sparging, isophorone/IPN solution is condensed and can be recycled to the next batch. Residual free cyanide is also destroyed in this step. 
     Alternatively, the catalyst can be removed by neutralization with an acid, such as phosphoric, nitric, or sulfuric acid, followed by removal of the resulting insoluble quaternary ammonium salts by filtration of the cooled isophorone solution. Typically the IPH/IPN solution is cooled to about 20°-30° C. for filtration of the TMAH salts. 
     The problem of cyanide contamination of all products, whether crude solutions or solids obtained by precipitation in a hydrocarbon and washing followed by vacuum drying to remove the hydrocarbon, or by vacuum distillation to isolate IPN itself, will now be discussed. It is believed that most of the residual HCN is free HCN or CN -   rather than combined HCN, and probably exists in solution as the cyanide salt of TMAH. Neutralization with a strong mineral acid or an organic acid converts the cyanide to free HCN, which can then be eliminated by sparging with an inert gas, such as carbon dioxide or nitrogen, preferably nitrogen. Glacial acetic acid, phosphoric acid and sulfuric acid are suitable, with acetic acid being especially preferred. Phosphoric acid is the preferred mineral acid, as the phosphate anion has minimal impact on a subsequent hydrogenation step. The acid is preferably used in an amount equivalent to or greater than the residual catalyst on a molar basis. The sparging is carried out at a temperature of about 110° C. for about 1-2 hours. This procedure of mitigating cyanide contamination can be used in addition to or instead of the aforementioned catalyst pyrolysis. Specifically, if the catalyst is not destroyed by pyrolysis, acidification with an organic acid, preferably acetic acid, will result in the formation of a TMAH salt which is soluble in the IPN/isophorone solution, such as the acetate salt of TMAH. The salt does not have to be removed prior to the subsequent hydrogenation step, as it does not interfere in that hydrogenation. Since the resulting isophorone diamine (IPDA) must then be purified by distillation, the catalyst could be removed at that point. Alternatively, if the catalyst is not destroyed by pyrolysis but instead is neutralized by, e.g. phosphoric acid, the catalyst can be removed by filtration of the insoluble TMAH phosphate salt from the cooled isophorone/IPN solution. 
     A suitable minimum temperature for the reaction of isophorone to IPN is about 110° C.-115° C., preferably about 110° C. The sensitivity of the catalyst to temperature is a factor in selecting the minimum reaction temperature. At a mole ratio IPH:HCN:TMAH of 2.00:1.00:0.01 and a temperature of 110° C., the minimum HCN feed time is about 1 hour. 
     Isophorone is both a reactant and a solvent. To obtain high yields, isophorone must be used in stoichiometric excess to the HCN added. A ratio of 2 moles isophorone:1 mole HCN is preferred, but marginally higher yields are obtained with larger excesses of isophorone. As little as about 0.001 moles of catalyst (per mole HCN) can be used, although product yield may suffer. Preferably about 0.005 to 0.01, most preferably about 0.01 moles of catalyst are used. Larger amounts can be used, the limitation being cost. 
     Generally, the IPN product is needed free of isophorone. This can be accomplished by vacuum stripping off the isophorone to give a crude molten product which can then either be converted to a solid (e.g., on a chiller/flaker or a chilled belt) or dissolved in the solvent used for the subsequent hydrogenation to IPDA. Alternatively, the stripped melt can be vacuum distilled to produce a highly pure IPN. 
     In some manufacturing situations, the IPN dissolved in isophorone is the desired product form, e.g., as a 35% solution in isophorone. In this instance, the initial IPH charge is preferably increased to an amount sufficient to dilute the product solution to, e.g., 35% IPN. Since yield increases with higher IPH excesses, this procedure is preferred to adding IPH as a diluent to the product. 
     The generation of by-products can be mitigated by avoiding the addition of catalyst until the reaction temperature is reached, and by immediately starting the HCN feed once the catalyst is added. This procedure also results in increased yield of product. 
     Unless otherwise indicated, the apparatus used for the following Examples is illustrated in FIGS. I and II. The critical units are a one Liter Parr stirred autoclave and a Harvard Apparatus syringe pump equipped with a 100 ml stainless steel syringe. With this pump and syringe, the HCN could be delivered at precisely known and constant rates without any danger of loss of HCN. 
     EXAMPLE 1 
     The apparatus is shown in FIG. 1. To the one liter stainless steel autoclave was charged 4.00 moles of isophrone. The head space was purged with nitrogen, then the autoclave was sealed. The temperature was raised to 80° C. and 0.02 moles of TMAH catalyst were added. The temperature was further increased to and held at 110° C. Two moles of anhydrous HCN were pumped into the isophrone by means of the stainless steel syringe pump at constant but different rates for several experiments. The batch was held for 1 hr at 110° C., sampled, and analyzed for %IPN and %HCN. 
     
         ______________________________________                % IPN Yield ppm HCN atExpt. No.   HCN Feed Time                From HCN    End of Feed______________________________________1       2       hr       88.4      5012       1       hr       95.2      4063       30      min      93.4      5154       25      min      74.2      1932   after 1 hr hold  85.4      3105       1       hr       94.3      541______________________________________ 
    
     EXAMPLE 2 - Thermal destruction of TMAH 
     IPN solutions in isophrone were prepared by the same procedure as in Example 1 but with a constant 1 hr HCN addition time. The solutions were heated to 120°, 130°, 140°, or 150° C. and sparged with N 2 . The IPN/isophrone solutions were analyzed at about 15 min time increments for residual TMAH by titration with standard HCl. 
     
         ______________________________________Sparging  Minutes to Obtain ppm TMAHTemperature     Minimum Residual TMAH                       Initial  Final______________________________________120° C.     142               143      42130° C.     120               163      27140° C.      85               163      22150° C.      67               150      19______________________________________ 
    
     EXAMPLE 3 - Acidification/sparging to remove HCN 
     A solution of IPN in isophrone was prepared by the procedure of Example 1 using a 1 hr HCN feed and a 15 min hold period after the end of the HCN addition. 615 g was transferred to the sparging apparatus (FIG. 11). By titration, this solution contained 260 ppm of residual TMAH, or 17.5 milli moles. To this solution was added 2.7 g of 85% H 3  PO 4 , or 23.4 milli moles. The solution was heated to 110° C. and sparged with N 2 . Samples were analyzed periodically for HCN. 
     
         ______________________________________Before acidification      426 ppm HCNAfter sparging for          30 min     299          60          49          90          0______________________________________ 
    
     EXAMPLE 4 
     Four batches of IPN solution in isophrone were prepared in a 30 gallon stainless steel reactor fitted with a reflux condenser. To the reactor was charged 172 lb (1.22 lb moles) of isophrone via vacuum. The vacuum was then broken by bleeding in N 2  to 0 psig. The isophrone was heated to 110° C. and 1.48 or 1.11 lb of 25% aqueous TMAH (0.004 or 0.003 lb moles) was added. Addition of 11.0 lb of HCN (0.41 lb moles) was started immediately and finished about 1 hr later. During the HCN addition the batch temperature was maintained at 110°-115° C. The batch was then sampled and analyzed for IPN, TMAH, and HCN. Glacial acetic acid was added and the batch sparged with N 2  at 110°-115° C. for 30 min, at which time no HCN could be detected in any of the four batches. The solution was then cooled to about 30° C. and diluted with isophrone to about 35% IPN. 
     
         ______________________________________                 Glacial AceticYield (%)   TMAH      Acid AddedAfter   Final  Catalyst     Mole/Mole                                    ppm HCNRun  HCN     Pro-   mole/mole    of TMAH After HCNNo.  Feed    duct   of HCN  lb   Charged Feed______________________________________1    100.4   98.5   0.01    0.363                            1.51    2352     99.7   97.2   0.01    0.290                            1.21    n.d.*3     99.2   97.2   0.0075  0.220                            1.22    n.d.*4     97.3   93.4   0.0075  0.220                            1.22    n.d.*______________________________________ *Not determined