Abstract:
A process for preparing high-purity cyclopropyl methyl ketone from compositions which contain cyclopropyl methyl ketone, 4,5-dihydro-2-methylfuran, addition products of 4,5-dihydro-2-methylfuran by a continuous rectification process in which at least a portion of the composition to be purified is introduced into a rectification apparatus via a side feed located above the bottom portion of the apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for preparing high-purity cyclopropyl methyl ketone (CPMK) from compositions, e.g., reaction discharges or mixtures, that contain CPMK, 4,5-dihydro-2-methylfaran (DHMF), addition products of DHMF, and, optionally, impurities which are produced in the synthesis of CPMK from 2-acetylbutyrolactone (ABL). 
     2. Description of the Background 
     CPMK is a valuable intermediate which is required in high purity for the synthesis of a variety of agricultural and pharmaceutical active compounds. A purity of &gt;99% by weight is required for the use of CPMK as a building block for synthesis in the agricultural and pharmaceutical sectors. 
     CPMK can be particularly advantageously prepared from ABL by cyclizing ABL to give CPMK with elimination of CO 2  under alkali metal halide catalysis [Chemistry Letters 11, pp. 1149-52 (1975)]. It may be prepared continuously by introducing the alkali metal halide in a solvent at high temperature and continuously adding the ABL. Since, in the reaction, in addition to the main product CPMK, DHMF is also formed at a content of usually 5 to 30% by weight, as product of value of the reaction, a mixture of CPMK and DHMF is always continuously distilled off simultaneously from the reaction. Corresponding preparation processes are disclosed, for example, in EP-A 0 552 586 or DE-A 195 03 241. However, the CPMK obtained in this manner does not comply with the desired purity requirements because of the unacceptably high DHMF content. 
     DHMF is sufficiently more volatile than CPMK and can therefore in principle be separated off from CPMK by distillation without problems, in order to obtain CPMK of high purity. However, in the presence of acid, DHMF reacts with numerous nucleophiles, such as water, alcohols, amines etc., to give high-boiling addition products (cf Houben/Weyl: Methoden der organischen Chemie [Methods in Organic Chemistry], Vol. VI/3, 1965, p. 698, incorporated herein by reference). The addition of water to DHMF forms, for example, acetopropanol, which itself can in turn add to DHMF. These addition reactions are reversible, so that at high temperatures acetopropanol or its addition product to DHMF or other addition products to DHMF can release DHMF again by elimination or cyclization. 
     In a crude mixture of CPMK and DHMF, small amounts of water or other nucleophilic impurities are also always present, which reversibly convert DHMF into high-boilers in the manner described above. Purification of CPMK by distillation to separate off DHMF, therefore leads, depending on the bottom temperature, to recleavage of the higher-boiling DHMF addition products and thus again to contamination of CPMK with DHMF in the final distillation fraction. In the purification of CPMK by distillation, there is therefore always the problem of contamination of the CPMK by recleavage of the addition products of DHMF as a function of the bottom temperature. Because of the bottom temperatures required in conventional distillations, e.g., 110 to 180° C., which are necessary to comply with the purity requirements, there is the necessity of separating off further DHMF from the first runnings and final fractions of the distillation. However, this requires a high technical complexity and reduces the CPMK yield. 
     Lowering the bottom temperature by applying a vacuum and thus preventing recleavage of DHMF addition products and the thus ensuing contamination of CPMK can be accomplished. However, the technical complexity for condensing the relatively low-boiling CPMK is too great, making this process unfeasible. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a simple process for preparing high-purity CPMK in very good yields, by purifying CPMK from a composition, e.g., a reaction discharge or mixture, which contains CPMK, DHMF, addition products of DHMF, and, optionally impurities which are produced during the synthesis of CPMK from 2-acetylbutyrolactone (ABL). 
     The object of the present invention and others is achieved with a process for preparing high-purity cyclopropyl methyl ketone from a composition which contains CPMK, DHMF, and addition products of DHMF, by 
     (a) introducing the composition into a rectification apparatus having a bottom and a side feed above the bottom through the side feed; and 
     (b) rectifying the composition to produce purified cyclopropyl methyl ketone, 
     where steps (a) and (b) are conducted simultaneously. 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Surprisingly, using the process according to the present invention, high-purity CPMK having a purity of &gt;99% by weight can be prepared in high yield. The purity may be 99.3% by weight or higher, such as 99.5% by weight. Yields of high-purity CPMK of over 90% (based on the CPMK content in the starting mixture to be worked up according to the invention) can be achieved according to the invention. Preferably, the yield is at least 93% by weight, more preferably at least 94% by weight, and, most preferably, at least 95% by weight. 
     The present invention includes a continuous rectification step in an apparatus having a bottom and a side feed located above the bottom. In the present process, at least a portion of the impure mixture containing CPMK is added to the rectification by means of the side feed, by means of which the amount of the mixture to be fractionated introduced in the bottom of the apparatus is significantly decreased in comparison with conventional, purely distillation, processes. A fraction of the impure composition may be added via the side feed. Alternatively, all of the impure composition is introduced through the side feed. The rectification step according to the invention each time of only a small portion of the starting mixture to be worked up according to the invention enables the bottom temperature, during the entire work-up of the starting mixture, possibly including further distillation process steps, to be lowered to less than or equal to 160° C., preferably less than or equal to 140° C., particularly preferably less than or equal to 120° C. This suppresses the recleavage of addition products of DHMF and facilitates the production of high-purity CPMK. The bottom temperature may be 110, 112, 115, 118° C., or lower. 
     The process according to the invention is very simple to carry out, since conventional rectification apparatuses can be used for the rectification step according to the invention. As already mentioned, the process according to the invention for the work-up of the starting mixture can comprise, in addition to this rectification step, other, e.g. distillation, process steps. For a general discussion of distillation processes, see Kirk-Othmer Encyclopedia of Chemical Technology, Volume 8, pp. 311-358, 1993, incorporated herein by reference. 
     In a preferred embodiment of the process according to the invention, initially, for example, a portion of the DHMF, which is more volatile than CPMK, may be distilled off from the starting mixture to be fractionated. This distillation may be performed continuously or batchwise at bottom temperatures preferably below 120° C. The resulting crude CPMK, which is still contaminated primarily with high-boiling addition products of DHMF and with other compounds which can result from the synthesis of CPMK from 2-acetylbutyrolactone (ABL), is fed into a distillation column at the side above the bottom. At the top of the column, in the steady state, CPMK is taken off at a purity of &gt;99% by weight, with the bottom temperature not exceeding 160° C., preferably not exceeding 140° C., particularly preferably not exceeding 120° C. Based on the CPMK content in the starting mixture to be fractionated, yields of high purity CPMK by distillation of greater than 95% can be achieved. 
     In another particularly preferred embodiment, the process according to the invention comprises one single continuous rectification step for preparing high-purity CPMK. The starting mixture to be fractionated may be fed to the rectification apparatus in this case at the side roughly in the center of the column in such a manner that the ratio of the number of theoretical plates of the enrichment portion of the column to the number of theoretical plates of the stripping portion is 1 to 2. This ratio range includes all specific values and subranges therebetween, including 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, and 1.9. At the top of the column, the low-boiling DHMF may be continuously separated off, whereas in a sidestream takeoff just above the distillation bottom phase, the CPMK may be obtained at a high purity &gt;99% by weight and high yield of customarily greater than 90%. The sidestream takeoff of the CPMK is preferably performed immediately above the boiling liquid of the bottom; however, it can alternatively be performed in the lower third, based on the number of theoretical plates, of the stripping column in the vicinity of the bottom. Some of the bottom product can be taken off during the distillation, if required. 
     In practice, to carry out this last-mentioned particularly preferred process embodiment, distillation columns having arranged distillation packings have proved useful. Good results have been achieved, for example, using a column having a separation capacity of 25 theoretical plates, arranged in 3 sections, or a column having a separation capacity of 35 theoretical plates, arranged in 4 sections. The sidestream takeoff of the high-purity CPMK is performed here either below or above the 1 st packing above the bottom; the feed was arranged either at the center or near the bottom above the first of the total of 3 packings. The last-described single-stage preferred embodiment is suitable, in particular, for industrial application, because of its simple handling and practicability. 
     The process according to the invention is expediently carried out under atmospheric pressure, but can also be carried out at a pressure of 300 to 1050 mbar, preferably at 500 to 1030 mbar, particularly preferably at 800 to 1020 mbar. 
     Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
    
    
     EXAMPLES 
     Comparative Example 
     
         ______________________________________Distillation    1239.3 gfeedstock:Composition:    CPMK: 1080.7 g (87.2% by weight)    DHMF: 137.6 g (11.1% by weight)    Water: 7.4 g (0.6% by weight)Apparatus:    0.5 m Multifil distillation column    21 distillation flaskDistillation:    The mixture to be rectified is introduced into the dis-    tillation stillpot. The mixture is heated to boiling using    an oil bath. DHMF is first distilled off at a reflux    ratio of 20:1. Then, &gt;99% by weight pure CPMK is    produced at a reflux ratio of 5:1.    As a final fraction, at an elevated bottom temperature, a    CPMK fraction contaminated with DHMF is obtained.______________________________________ 
    
     The fractions listed in Table 1 are obtained. 
     
                                           TABLE 1__________________________________________________________________________    Oilbath    temperature     Bottom temp.            Top temp.                 Reflux                     Mass                         GC (% by weight)Fraction    (° C.)     (° C.)            (° C.)                 (R/D)                     (g) DHMF                             CPMK__________________________________________________________________________1   132-134     107-110            72-80                 20:1                     28.9                         80.1                             2.42   134-138     110-111            81-82                 20:1                     21.6                         91.0                             4.63   138-150     112-113             84-102                 20:1                     56.8                         71.1                             27.64   152-158     113-114            102-112                 20:1                     76.0                         13.4                             85.75   159   114-116            112   5:1                     940.0                         0.4 99.16   160-220     116-199            108-109                  5:1                     65.3                         8.6 86.2Residue                   43.4Total                     1232.0__________________________________________________________________________ 
    
     Fraction 5 is the product of value, i.e., contains CPMK. The distillation yield of 99% by weight pure CPMK is 86%, based on a starting amount of CPMK of 1080.7 g. As a final fraction, a CPMK fraction which is again heavily contaminated with DHMF (fraction 6) is obtained. 
     Example 1 
     According to the Invention, Two-Stage Distillation 
     
         ______________________________________Distillation    1593.5 gfeedstock:Composition:    CPMK: 1362.4 g (85.5% by weight)    DHMF: 200.8 g (12.6% by weight)    Water: 15.9 g (1.0% by weight)Apparatus:    11 distillation flask with side outlet, two heatable 0.5 m    Multifil columns mounted one on top of the other,    with central feed.______________________________________ 
    
     a) DHMF/Water Separation 
     Procedure 
     Approximately 500 g of the distillation feedstock are introduced into the k-k-k-- distillation flask. At a bottom temperature of 109 to 113° C. and a reflux ratio of 10:1, a DHMF/water mixture is first distilled off up to a top temperature of 83° C. The remaining distillation feedstock is then continuously fed into the center of the column at a rate of approximately 150 gfh. The overflowing distillation bottom phase is collected in a storage vessel. 
     Continuous Distillation Conditions: 
     Bottom temperature: 113-114° C. 
     Top temperature: 82-83° C. 
     Refklux (Rk/D): 12:1 
     After completion of the continuous distillation, the following are present: 
     
         ______________________________________1.  Distillate:   214.5 g    Composition:  CPMK:     20.3 g (9.5% by weight)             DHMF:     180.2 g (84.0% by weight)             Water:    14.0 g (6.5% by weight)2.  Distillation bottom             1378.0 g    product:    Composition:  CPMK:     1342.0 g (97.4% by weight)             DHMF:     20.8 g (1.5% by weight)             Water:    1.9 g (0.1% by weight)             Residue:  13.3 g (1.0% by weight)______________________________________ 
    
     b) Purification of CPMK by Distillation (Rectification) 
     Distillation feedstock: Distillation bottom product from the preceding distillation (1378.0 g) 
     Procedure 
     Approximately 400 g of the distillation bottom product from the preceding DHMF/water separation are introduced into the above-described apparatus, the bottom outlet being closed. The purification by distillation is begun at a bottom temperature of 113° C. and a top temperature of 111° C. and the feed of the remaining distillation bottom product (150 gk/h) into the column center is then begun. 
     Continuous rectification conditions: 
     Bottom temperature: 113-114° C. 
     Top temperature: 112° C. 
     Reflux (R/D): 5:10 
     Two Fractions are Obtained as a Result 
     
         ______________________________________Fraction 1:   50.0 g comprising:                  98.0% by weight of DHMF and                  1.5% by weight of CPMKFraction 2:   1287.0 g comprising:                  99.5% by weight of CPMKResidue:   40.0 g______________________________________ 
    
     The yield of 99.5% by weight pure CPMK is thus 94.4%, based on the mass of CPMK in the original distillation feedstock. 
     Example 2 
     According to the Invention, Single-Stage Rectification 
     
         ______________________________________Distillation    3971.9 gfeedstock:Composition:    CPMK: 3654.1 g (92.2% by weight)    DHMF: 278.1 g (7.0% by weight)    Water: 35.7 g (0.9% by weight)Apparatus:    11 distillation three neck flask having two heatable 0.5 m    Multifil columns mounted one on top of the other, with    central feed and a distillation bridge mounted directly    above the bottom.______________________________________ 
    
     Procedure 
     At an oilbath temperature of 128° C., the distillation feedstock is fed into the center of the column at a rate of 150 g/h. DHMF is separated off at the top of the column (fraction K1), while CPMK is distilled over the distillation bridge. 
     Distillation conditions 
     Bottom temperature: 110-116° C. 
     Top temperature K1: 79-82° C. 
     Top temperature K2: 113° C. 
     Reflux R1, R/D: 50:1 
     Reflux R2, R/D: 1:2 
     
         ______________________________________Fraction K1:   304.0 g comprising                87.2% by weight of DHMF, 1.6% by                weight of CPMK and                11.1% by weight of waterFraction K2:   3418.8 g comprising                99.5% by weight of CPMKResidue:   245.0 g______________________________________ 
    
     The overall yield of 99.5% by weight pure CPMK is thus 93.6%, based on the mass of CPMK in the distillation feedstock. 
     German Patent Application 197 10 879.2, filed Mar. 15, 1997, is incorporated herein by reference in its entirety. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.