Abstract:
Disclosed herein is a process for the reductive alkylation of primary amines to form secondary amines, by high pressure reaction of the primary amine with an alkylating agent and hydrogen in the presence of a catalyst which comprises metallic palladium. A process of the present invention is characterized in having high conversion rates and high selectivities, both greater than 95% on a first pass through the reactor. According to a preferred embodiment, the secondary amine produced comprises at least one 2-alkyl group bonded to the nitrogen atom of the primary amine product.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to catalytic processes for alkylations of amines, and in more particular to selective formation of secondary amines from primary amines. A process according to the invention is especially well-suited for preparing 2-alkyl substituted amines from primary amines.  
         BACKGROUND INFORMATION  
         [0002]    Processes for producing organic amines have been known in the art for quite some time. For example, U.S. Pat. No. 3,994,975 relates to the reductive amination of unsaturated cyclic ketones; to cyclic amines prepared thereby; and to uses thereof For example, when isophorone is reductively aminated, trimethyl cyclohexylamines and cyclohexenamines are obtained. The invention includes uses of the amine products as fuel additives for stabilizing distillate fuels.  
           [0003]    U.S. Pat. No. 4,521,624 provides a process for the reductive amination of a saturated cyclic ketone which comprises reacting a saturated ketone with an amine in the presence of hydrogen and a hydrogenation catalyst, whereby said saturated cyclic ketone is converted to the corresponding cyclic amine.  
           [0004]    U.S. Pat. No. 5,001,267 relates to secondary alkyl amine derivatives of ethylenediamine which are formed in a one-step reaction when ethylenediamine is substantially simultaneously continuously mixed with a methyl alkyl ketone to form a reaction mixture which is continuously hydrogenated in the presence of a hydrogenation catalyst and hydrogen. The alkyl group contains 1 to 4 carbon atoms.  
           [0005]    U.S. Pat. No. 5,093,528 teaches a process for producing a secondary amine terminated polyether from a primary amine terminated polyether in which the primary amine terminated polyether is reacted at elevated temperature with a carbonyl compound in the presence of hydrogen and a catalyst composition comprising nickel, ruthenium and at least one other transition metal selected from the second or third row transition metals. The amount of carbonyl compound employed is in excess of the stoichiometric amount required to react completely with the ether. The secondary amine terminated polyethers are useful in the formation of polyurea elastomers.  
           [0006]    While the prior art is replete with processes for producing organic amines, there has been no work concerned with producing secondary amines selectively, for example, secondary amines derived from isophorone diamine using acetone in the presence of hydrogen and a catalyst to form secondary amines. When base metal catalysts (Ni, Co, etc.) are used, a high acetone to amine ratio is required and even under these conditions a low conversion to amine is observed. Also, amine dimer (amine coupling product) is formed in substantial amounts.  
         SUMMARY OF THE INVENTION  
         [0007]    We have discovered that by using a catalyst of palladium on carbon, that high conversion and selectivity to secondary amine is obtained when reacting isophorone diamine with acetone in the presence of hydrogen. Palladium has been known previously as a reductive amination catalyst; however, in the known cases this catalyst was only found useful for making tertiary amines using ketones, as the alkylation according to prior art methods would not cease at the secondary amine stage, but would proceed uncontrollably to a tertiary amine product. Thus, the present invention relates to a process to selectively produce secondary amines, in high yield, by reacting a primary amine with a ketone in the presence of hydrogen using a catalyst comprising palladium on carbon.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0008]    The present invention is concerned with the reaction:  
                         
 
           [0009]    in which R may be any alkyl or alkylaryl group, straight-chain, branched, or cyclic; R′ and R″ may each independently be hydrogen or any C 1 -C 20  alkyl group, straight-chain, branched, or cyclic, which is carried out in the presence of a catalyst comprising palladium on carbon, and which is carried out at any temperature in the range of between 80° C. to 200° C. and at any pressure in the range of between 100 psig to 3000 psig.  
           [0010]    Reductive alkylations of amines which use catalysts and which are also carried out according to prior art methods typically yield a reaction product mixture which contains substantial amounts of tertiary amine products, coupling products, and unreacted starting materials. We now report our discovery that using a catalyst comprising palladium on carbon in the presence of hydrogen for the reductive alkylation of a primary amine with a ketone results in nearly complete selectivity of the primary amine to a 2-alkyl substituted secondary amine product, with only negligible amounts of impurities, in a single pass through a tubular reactor.  
           [0011]    A process according to one preferred embodiment the invention is concerned with alkylation of isophorone diamine according to the reaction scheme:  
                         
 
           [0012]    using a catalyst comprising palladium on carbon.  
           [0013]    More generally, according to the invention, secondary amines can be produced by alkylating corresponding primary amines with ketones or higher aldehydes. Surprisingly, the high conversions observed when carrying out a process according to the invention were obtained with relatively low acetone to amine ratio, and an insignificant amount of corresponding tertiary amine was obtained. Further, no amine coupling product was observed to be present in the reaction product.  
           [0014]    The alkylating agent used in a process according to the present invention may be either an aldehyde or ketone, represented by the structure:  
                         
 
           [0015]    in which R′ and R″ may each independently be selected from the group consisting of hydrogen; C 1 -C 10  alkyl, whether straight-chain or branched; or alkylaryl, subject to the proviso that both A and B cannot simultaneously be hydrogen.  
           [0016]    While a process according to the present invention may be carried out in either a batch process or continuous process fashion, it is most preferred that the process be carried out in a continuous fashion. According to one preferred form of the invention, the reaction is carried out in a tubular reactor, the use of which is well known to those skilled in the art. In practice of such embodiment, the reactor comprises a tube which is charged with particles of the catalyst material, and in which said tube is surrounded by a heat transfer medium, such as a fluid sold under the DOWTHERM® trademark by the Dow Chemical Company, to control the temperature of the reactor. Typically, the tubular reactor is oriented in the vertical direction and the reactants of alkylating agent, hydrogen, and amine are fed into the bottom of the tube, and the product stream emerges from the top portion of the tube and is collected. If desired or necessary, the effluent of the tubular reactor is further processed (e.g., distilled) to yield a purified product. One of the unexpected benefits of the present invention, owing to the high selectivity and conversion discovered, is that for most practical end uses, the reactor effluent of a process carried out in accordance with the invention requires no further purification steps. This alleviation of further purification leads to a less resource-intensive overall process for producing a particular alkylated amine product, the cost savings of which may be readily passed on to consumers of the products generated, and thus to the public who enjoys the end use products made from such amines, including polyurethane and polyurea products, since the amine products of a process according to the present invention will in may cases be used as catalysts in producing polyurethanes and polyureas.  
           [0017]    While the present invention has been constructively reduced by virtue of the foregoing, the inventors now provide accounts in the form of the following written examples, for the convenience of the reader in furtherance of appreciating the scope of the present invention. As such, these examples are thus presented to be exemplary of the inventive process, and not delimitive in any way of the aforesaid disclosure: 
       
    
    
     EXAMPLE 1  
       [0018]    Preparation of N-isopropyl-N′,N′-dimethyl-1,3-propylenediamine  
         [0019]    Into a jacketed stainless steel tubular upflow reactor (volume=600cc) having inside diameter of 1.338″ and containing a thermowell fabricated from 0.25 O.D. tubing extended upward into the reactor were simultaneously fed 580 g per hour of dimethylaminopropylamine (“DMAPA”) and 420 g per hour of acetone were simultaneously fed (upflow). The reactor tube was full of a packed-bed of nickel catalyst such as that disclosed in U.S. Pat. Nos. 3,151,112 and 3,654,370. Hydrogen was fed at about 100% in excess of the stoichiometric amount. The reaction was conducted under a pressure of 1500 psig and at a temperature of 135° C. After a single pass through the approximate 30″ length of the tubular reactor, the reactor effluent was analyzed and showed about 85% N-isopropyl-N′,N′-dimethyl-1,3-propylenediamine, 6% unreacted DMAPA; 1% di-isopropyl DMAPA; and 2% coupling product. Also, di-isopropyl DMAPA failed to separate from DMAPA during the subsequent distillation of the product.  
       EXAMPLE 2  
       [0020]    Preparation of N-isopropyl-N′,N′-dimethyl-1,3-propylenediamine  
         [0021]    The procedure of Example 1 was repeated, with the exception that the nickel catalyst used previously was replaced by a 1% palladium on carbon catalyst (Engelhard) and the reaction temperature was reduced to 120° C. The effluent was analyzed and gave about 97% yield of N-isopropyl-N′,N′-dimethyl-1,3-propylenediamine. No significant amounts of di-isopropyl DMAPA or coupling products were detected. The effluent was distilled to give 99.5% pure N-isopropyl-N′,N′-dimethyl-1,3-propylenediamine.  
       EXAMPLE 3  
       [0022]    Preparation of N,N′-Diisopropylisophorone Diamine  
         [0023]    About 300 g per hour of isophorone diamine and 450 g per hour of acetone were fed upflow into a 600 cc packed-bed reactor filled with a nickel catalyst as described in example 1 above. Hydrogen was fed at about 100% in excess. The reaction was conducted at 2000 psig and 140° C. Lights were stripped out of the reactor effluent under reduce pressure. The resulting product was analyzed to contain 8.78 meq/g of total amine and 3.557 meq/g of primary amine. This result indicates that a significant amount (40.5%) of primary amine group was not alkylated. Also, GC analysis showed about 2.00% of “heavies” were present. In other words, amine coupling had occurred.  
       EXAMPLE 4  
       [0024]    Preparation of N,N′-Diisopropylisophorone Diamine  
         [0025]    A 200 cc DOWTHERM® heated stainless steel tubular upflow reactor which has an inside diameter of 0.815″ and a thermostat fabricated from 0.25″ OD tubing extend upwardly into the reactor was used. The reactor was filled with a 1.0% palladium on carbon catalyst (Engelhard). About 100 g per hour of isophorone diamine and 135 g per hour of acetone was fed into the tubular reactor, simultaneously, along with hydrogen at about 100% in excess. The reaction was conducted at 2000 psig and 150° C. Lights were stripped out of the reactor effluent under reduce pressure. The resulting product was analyzed to contain 7.89 meq/g of total amine, 0.07 meq/g of primary amine, and 0.04 meq/g of tertiary amine. GC analysis also showed no evidence of coupling. This result indicates, with Pd/C catalyst, high conversion and selectivity were achieved.  
       EXAMPLE 5  
       [0026]    Comparative  
         [0027]    The procedure of example 4 was followed. However, equal mass amounts of isophorone diamine and acetone were fed, and space velocity was 1.3 g/hr total liquid feed per cc catalyst. The resulting product was analyzed to contain 7.83 meq/g of total amine, and 0. 16 meq/g of tertiary amine. No primary amine was detected. In other words, the product consisted of greater than 99 wt. % secondary amine and less than 0.2 wt. % tertiary amine. Again, the result demonstrated that, with Pd/C catalyst, high conversion and high selectivity can be obtained even at lower ketone to amine ratio.  
         [0028]    The present invention may be carried out using such reactants as all known primary amines, and including dimethylaminopropylamine, monoethanolamine, DGA® agent (available from Huntsman Petrochemical Corporation of Austin, Tex.) and JEFFAMINE® amine products, also available from Huntsman Petrochemical Corporation.  
         [0029]    The preferred catalysts useful in a process according to the invention are palladium on carbon.  
         [0030]    Consideration must be given to the fact that although this invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the claims appended hereto. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims which follow.