Abstract:
A method of removing water from wet organoborane by dissolving the wet organoborane in an organic solvent in which water is incompletely soluble, decanting any insoluble water, and distilling the organic phase to remove water which may be contained therein.

Description:
FIELD OF THE INVENTION 
     The present invention concerns a process for drying wet organoborane compounds. 
     BACKGROUND OF THE INVENTION 
     Certain organoborane compounds (boron hydride compounds in which hydrogen is substituted with organic moieties) are known to be useful as promoters in hydrocyanation reactions. A commercially important hydrocyanation reaction that utilizes these promoters is the conversion of pentenenitrile compounds to adiponitrile. For example, U.S. Pat. No. 3,496,218 describes a process for hydrocyanation of non-conjugated ethylenically unsaturated organic compounds, such as 3-pentenenitrile, in the presence of a nickel/triarylphosphite catalyst and a triorganoborane compound promoter, such as triphenylborane, to produce adiponitrile. Adiponitrile is an intermediate in the production of hexamethylene diamine, a nylon-6,6 component. Adiponitrile is also an intermediate in the production of caprolactam and nylon-6. 
     Organoborane compounds are easily hydrolyzed. For example, triphenylborane can be hydrolyzed to diphenylborinic and phenylboronic acids in the presence of water. Even at low temperatures (for example, below 10° C.), triphenylborane will slowly hydrolyze to diphenylborinic and phenylboronic acids with trace amounts (e.g. 10 ppm) of water present. At elevated temperatures, the hydrolysis rate significantly increases. It is important to dry organoborane compounds which are used as promoters in hydrocyanation reactions, because many hydrocyanation reactions occur at elevated temperatures. Current drying methods include exposure of organoborane compounds to hot nitrogen and molecular sieves. Use of hot nitrogen leads to degradation of some organoborane to undersirable organoborinic and boronic acids. As a result, there is a need in the art for an improved method of drying organoborane compounds that minimizes the production of such undesirable degradation products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The Drawing consists of two figures in which like reference numerals are used to indicate like elements. 
     FIG. 1 depicts apparatus for carrying out the method of the present invention. 
     FIG. 2 depicts a distillation column for carrying out the method of the present invention. 
    
    
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a method of removing water from a wet organoborane compound, comprising: 
     (a) forming a mixture by mixing the wet organoborane compound with a solvent in which water is incompletely soluble and which solvent comprises a nitrile compound; 
     (b) allowing the mixture of step (a) to separate into an aqueous phase and an organic phase, which organic phase comprises substantially all of the organoborane compound; 
     (c) separating the aqueous phase from the organic phase; and 
     (d) distilling the organic phase to remove water which may be contained therein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention concerns a method of removing water from wet organoborane compound. The expression “wet organoborane compound” means an organoborane compound which is associated with water. Typical wet organoborane compounds can be associated with 5% to 25% water by weight. Preferred organoborane compounds are triphenylborane, tri(paratolyl)borane, tri(metatolyl)borane, tri(orthotolyl)borane, tri(biphenyl)borane, tri(paramethoxyphenyl)borane, tri(parachlorophenyl)borane, tri(paraflorophenyl)borane, phenylboroxin, diphenyl(phenoxy)borane, and phenyl(diphenoxy)borane. An especially preferred organoborane is triphenylborane. 
     The wet organoborane compound is dissolved in a solvent comprising a nitrile compound to produce an organoborane solution. Preferably, the solvent should have a boiling point of 20 to 200 C. The solvent should be one that has low water solubility. Preferred nitrile compounds include 3-pentenenitrile, 4-pentenenitrile, and 2-pentenenitrile. A mixture of two or more of the pentenenitrile compounds may be used. These mixtures are referred to herein as “pentenenitriles,” “pentenenitrile mixtures” or “mixtures of pentenenitriles.” Other nitriles, such as methyl butenenitriles, butenenitriles, pentanenitriles, methyl butanenitriles, butanenitriles, and acetonitrile, also may be used. This solvent may also contain additional compounds that are miscible with the nitrile compound. These additional compounds serve as co-solvents. Preferred co-solvents are cycloheptane, cyclohexane, methylcylopentane, cyclopentane, heptane, hexane, pentane, toulene, and benzene. 
     An especially preferred co-solvent is cyclohexane. In the present invention, an especially preferred solvent comprises a mixture of pentenenitriles (including 3-pentenenitrile) and cyclohexane. Preferably, the ratio of cyclohexane to pentenenitrile is 0 to 10 by weight. A preferred ratio of cyclohexane to pentenenitrile is 2 to 5 by weight. In order to reduce hydrolysis of the organoborane compound before decantation and distillation, the solution may be maintained at a reduced temperature, preferably between 0 to 20° C. The weight percent of the organoborane compound in solvent is not critical and typically ranges from 1 to 10% 
     Water that is not miscible in the organoborane solution is decanted. Any residual water in the remaining organoborane solution is then removed by distillation. 
     By decanting most of the water prior to distillation, hydrolysis of the organoborane compound during distillation is minimized. 
     Referring now to FIG. 1, there is shown apparatus  10  that can be used for carrying out the present method. The apparatus  10  comprises a dissolving tank  12 , a decanting tank  14 , and a distillation column  16 . Dissolving tank  12  contains a stirrer  18 . 
     Wet organoborane compound crystals  20 , solvent  22 , and optional co-solvent  24  are fed to dissolving tank  12 , in which the crystals are dissolved by means of stirrer  18 . In the case of triphenylborane, the solvent preferably is a mixture of pentenenitriles, and the co-solvent is preferably cyclohexane. Preferably the contents of dissolving tank  12  are kept at low temperatures, such as 10 to 15 C. 
     The contents of dissolving tank  12  are then fed to decanting tank  14 , where the contents are allowed to separate into an aqueous phase  26  (containing most of the water present in the dissolving tank  12 ), and an organic phase  28 . The organic phase  28  contains substantially all the organoborane compound dissolved in the organic solvent and co-solvent, and a minor portion of the water initially present in the contents of dissolving tank  12 . The aqueous phase  26  is decanted and treated as waste material. The organic phase  28  is fed as a stream  30  to the top portion of distillation column  16 . In distillation column  16 , water and most of the co-solvent are vaporized and removed as overhead stream  32 , which is condensed in condenser  34  to produce a recycle stream  36 , which is fed into dissolving tank  12 . Organoborane compound and solvent are removed from the bottom of distillation column  16 . Referring now to FIG. 2, distillation column  16  is shown in greater detail. The function of distillation column  16  is to remove the final traces of water from the organoborane and to concentrate it in the product. Water and cyclohexane are removed overhead in stream  32 . The organic phase  30  is introduced into the top portion of column  16  in which a feed distributor  38  causes the organic phase  30  to disperse across the cross-section of the column  16 . The column  16  is divided into two broad zones. An upper zone  40  contains a random packing material, such as Norton 15 IMTP high efficiency packing rings. A lower zone  44  contains a structured packing material, such as Norton HS-10 ISP. Dividing the two zones is a liquid collector and redistributor  42 . Below the lower zone  44  is a vapor distributor  46 . The column  16  is heated at its bottom portion by the output of a reboiler, which, itself, is heated by steam. A stream  48  containing organoborane compound and solvent is withdrawn from the bottom of column  16 . A portion  50  of stream  48  is fed to a reboiler  52  in which the contents of stream  50  are partially vaporized and returned to the lower portion of column  16  below vapor distributor  46 . The desired product is recovered as stream  56 , which contains the organoborane compound, substantially free of water, dissolved in the solvent. 
     In a preferred mode, wherein the organoborane is triphenylborane, the dissolving tank  12  is operated as follows. Sufficient solvent, (preferably pentenenitriles) is added to the dissolving tank  12 , such that after the triphenylborane solution is dried in the distillation column  16 , the concentration of triphenylborane is about 25 wt %. The pentenenitriles concentration is about 70 wt %, with the remainder being primarily co-solvent (cyclohexane). The pentenenitriles concentration in the distillation column stream  56  is such that the triphenylborane remains in solution at ambient temperature. The cyclohexane to pentenenitriles weight ratio in the dissolving tank is about 80/20. Preferably, the contents of dissolving tank  12  are kept at 15 C., which minimizes the hydrolytic degradation of triphenylborane. 
     In a preferred mode, wherein the organoborane is triphenylborane, the decanting tank  14  is operated as follows. The temperature should be kept low, preferably about 15 C. to prevent hydrolytic degradation of the triphenylborane. In the decantation step, the cyclohexane (a low water solubility co-solvent) serves to drive most of the water into the decanted aqueous stream and minimize water fed to the distillation column  16 . Cyclohexane also minimizes the loss of organics (cyclohexane, pentenenitrile, and triphenylborane) to the stream  30 . Other co-solvents, such as benzene, hexane, and other low water solubility solvents, should work similarly to cyclohexane. The ratio of recycle stream  36  to pentenenitrile feed stream  22  may vary over a wide range. An 80/20 ratio is preferred. 
     In a preferred mode, wherein the organoborane is triphenylborane, the distillation column  16  is operated as follows. Overhead stream  32  has about 5wt % pentenenitrile, with the balance being cyclohexane and water. Preferably, the distillation column  16  is operated at 2 psig, but can be operated at lower pressures if so desired. The 2 psig value minimizes column temperatures and maintains positive pressure operation to prevent air leaks into the process. The temperature at the top of distillation column  16  preferably is about 85-86 C., and the temperature of the bottom of the column is about 128-129 C. The reboiler exit temperature is 142-143 C. The temperature profiles in each of zones  40 ,  44  is virtually constant. The temperature of upper zone  40  is 85-86 C., and the temperature of lower zone  44  is 86 to 87 C. The useful temperature range in the distillation column  16  depends on the residence time in the upper zone  40 . Packed columns are especially preferred, because they combine short liquid residence times with proper vapor-liquid contact necessary for distillation. Use of multiple zones of packing versus a single zone of packing is not critical as long as short liquid residence time and proper vapor-liquid contact for distillation staging is maintained. Use of random packing versus structured packing is also not critical, as long as the residence time for the liquid in the upper zone  40  is short. Random packing is preferred for the upper zone  40 , so that if the decanting is incomplete and any residual two-phase liquid (aqueous/organic) feed reaches the column, the random packing in the upper zone  40  will minimize the possibility of free water falling down to the base of the column, where it could cause hydrolytic degradation of the triphenylborane. 
     The stream  30  is distributed across the top of the upper zone  40  through the distributor  38 . For packed columns, it is essential for good vapor-liquid contact that liquid be distributed evenly over the packing. The liquid then travels down the upper zone  40  of distillation column  16  through random packing. A suitable packing is Norton 15 IMTP high efficiency packing rings. (Norton Chemical Process Products Corporation, P.O. Box 350, Akron, Ohio. 44309-0350) The liquid then is redistributed by liquid collector and redistributor  42 , located below the random packing, before entering the lower zone  44  which contains structured packing (Norton HS-10 ISP). 
     A thermal siphon reboiler using 175 psig steam is used to boil-up the stream  50  in the base of distillation column  16 . Vapor from the reboiler is distributed evenly across the bottom of the structured packing section using vapor distributor  46 . Varying the steam flow controls the temperature of stream  54 . 
     Experiments were conducted to compare the effectiveness of the present method with that of the prior art molecular sieves/hot nitrogen technology for their respective abilities to dry wet triphenylborane. The resulting dried products were compared for water content (ppm) and mole ratio of diphenylborinic acid to triphenylborane. The results are indicated in the table below and show that the product of the present invention was drier and contained less degradation product than that of the prior art method. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Method 
                 Mole Ratio 
                 H 2 O (ppm) 
               
               
                   
                   
               
             
             
               
                   
                 Present Method 
                 0.011 
                 156 
               
               
                   
                 Prior Art 
                 0.032 
                 338