Abstract:
There is disclosed a process for the preparation of cyclopentene which comprises selectively hydrogenating cyclopentadiene in the liquid phase by contacting cyclopentadiene with hydrogen in the presence of a hydrogenation catalyst comprising a highly dispersed form of nickel in which a polyol selected from the group consisting of 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2,3-propanetriol, 1,2-butanediol, 1,4-butanediol, 1,3-butanediol, 1,2,4-butanetriol and 1,2,3-butanetriol is employed in the reaction mixture.

Description:
BACKGROUND OF THE INVENTION 
     This invention is directed to a selective hydrogenation of dienes to monoolefins, particularly of cyclopentadiene to cyclopentene. More specifically, it is directed to a process whereby cyclopentadiene is selectively hydrogenated to cyclopentene through the use of a highly dispersed form of nickel in which a number of polyols that exhibit infinite or near infinite solubility in the water and insolubility in cyclopentadiene as used in this two-phase hydrogenation system. The polyols which exhibit such properties are 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2,3-propanetriol, 1,2-butanediol, 1,4-butanediol, 1,3-butanediol, 1,2,4-butanetriol and 1,2,3-butanetriol. 
     At the present time, substantial amounts of cyclopentadiene, usually as dicyclopentadiene, are available as a byproduct from the steam cracking of naphtha to produce primarily ethylene. Cyclopentene has been found to be useful as a monomer for the formation of general purpose elastomers by ring opening polymerization of cyclopentene. Therefore, it is desirable to convert a portion of the excess cyclopentadiene available into a more valuable raw material, such as cyclopentene. 
     The hydrogenation of cyclopentadiene to cyclopentene is not new. For instance, in U.S. Pat. No. 2,360,555, issued Oct. 17, 1944, there is disclosed a selective hydrogenation of one of the two conjugated double bonds of a cyclic diolefin to produce the corresponding cyclic monoolefin which is accomplished by conducting the hydrogenation in the liquid phase in the presence of an active hydrogenation catalyst, under moderate hydrogen pressure, such as 2 to 5 atmospheres absolute, and at relatively low temperatures, such as from 0° to 40° C. and even up to 100° C., using substantially less than the stoichiometric amount of hydrogen theoretically required to completely reduce the cyclic diene to the corresponding cyclic monoolefin. The catalyst therein disclosed is a pyrophoric nickel metal catalyst, such as Raney nickel. It is also disclosed that it is desired to conduct the reaction in dilute solution. The dilution may be affected by the addition of any solvent, stable under conditions of the process and which is not a catalyst poison and whose boiling point is such as to render it easily separable from the reaction mixture. Benzene and ethanol as well as tetralin, dioxane, isooctane, ethyl ether and diisopropyl ether are disclosed as such solvents in such process. 
     In U.S. Pat. No. 3,819,734, issued July 25, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene by bringing cyclopentadiene into contact with a catalyst consisting essentially of (1) nickel, on a magnesium or zinc oxalate support, (2) a ligand selected from the group consisting of trimethyl phosphine, triethyl phosphine, methyl ethyl propyl phosphine, trimethyl phosphite, triethyl phosphite, tributyl phosphite, triphenyl phosphite, etc., while in the presence of hydrogen, at temperatures from 0° C. and at pressures from 0 to 1000 pounds per square inch gauge. The solvent mentioned therein is ethanol. 
     In U.S. Pat. No. 3,994,986, issued Nov. 30, 1976, there is disclosed the preparation of cyclopentene from cyclopentadiene by hydrogenating cyclopentene with hydrogen gas at a ratio of 1 to 1.5 moles of hydrogen per mole of cyclopentadiene in the presence of a palladium catalyst on a carrier. 
     Also, in U.S. Pat. No. 3,857,894, issued Dec. 31, 1974, there is disclosed the hydrogenation of cyclopentadiene to cyclopentene in the presence of a palladium catalyst and a small amount of an aqueous solution of zinc salt having a water/zinc ratio of at least 1/1 by weight. 
     The cyclopentadiene employed in the formation of cyclopentene by hydrogenation is usually obtained by depolymerizing or cracking dicyclopentadiene. In order to obtain cyclopentadiene for the hydrogenation of this invention, the depolymerization of dicyclopentadiene is accomplished by heating the dimer at a temperature above 150° C. under atmospheric pressure in a conventional cracking apparatus. The depolymerized material should be hydrogenated without substantial delay because it is also known that redimerization will occur upon standing. 
     SUMMARY OF THE INVENTION 
     According to the invention, cyclopentadiene can be selectively hydrogenated to cyclopentene in the liquid phase by contacting cyclopentadiene with hydrogen in the presence of a catalyst comprising a highly dispersed form of nickel and in which a polyol selected from the group of 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2,3-propanetriol, 1,2-butanediol, 1,4-butanediol and 1,2,3-butanetriol, is employed in the reaction mixture. 
     It has been found that in order to have a fairly selective hydrogenation of cyclopentadiene to cyclopentene, a reaction medium or diluent must be employed. Thus, according to the present invention, a polyol that exhibits near infinite or infinite solubility in water and insolubility in cyclopentadiene can be employed in the reaction mixture to form a two-phase hydrogenation system. 
     The use of these polyols as a reaction medium or in the reaction mixture has the further advantage since the cyclopentadiene and the cyclopentene are not soluble in these polyols, the process of this invention is a two-phase liquid system. This two-phase system thereby provides a method for the separation of the reaction medium or polar phase from the organic phase which contains cyclopentene and the unreacted cyclopentadiene. Another advantage is that the use of polyol reaction medium allows the cyclopentadiene feedstock which has been formed by steam cracking of dicyclopentadiene to be employed without further drying. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The temperature at which cyclopentadiene may be hydrogenated in accordance with this invention may range from 0° to 75° C. with 20° to 30° C. being most preferred. Temperatures that approach 100° C. tend to consume the cyclopentadiene in side reactions, such as dimerizations back to dicyclopentadiene and other undesirable side reactions. Generally speaking, both temperature and the pressure of hydrogen employed should be kept as low as possible consistent with reasonable rates of hydrogenation. When faster rates of reaction than that being obtained are desired, it is preferable to increase the rate of hydrogenation by means of increased hydrogen pressure rather than an increase in the temperature. 
     Extremely high pressures may be employed in the hydrogenation of this invention to effect faster rates of hydrogenation. However, it has been found that hydrogen pressure, such as 400-450 psig -- 2756-3100.5 kPa is all the pressure that is required to give reasonable rates of reaction. 
     Because the polyols of this invention are not conducive to the solubility of the cyclopentadiene and thereby cause the process to be a two-phase system, vigorous agitation is required during the hydrogenation. When this vigorous agitation is being obtained, the polyols serve as a heat sink by absorbing unwanted heat from the reaction site and, hence, moderate the hydrogenation, thereby enhancing the selectivity to cyclopentene. 
     The presence of a two-phase system when the agitation is stopped also offers the advantage that the catalyst settles to the bottom of the lower polyol layer and no residual hydrogenation of the organic layer occurs if a lengthy time is required to remove the reaction product, cyclopentene. The polyol also serves to protect the catalyst from air and thereby facilitates an easy recycling of the catalyst. 
     The catalyst employed in the present invention is a highly dispersed form of nickel. However, a Raney nickel-type catalyst is preferred. Methods for preparing the Raney nickel catalyst which are useful in this invention are known and can be found in a book entitled &#34;CATALYTIC HYDROGENATION&#34;, by Robert L. Augstine, published in 1965 by Marcel Dekker, Inc., New York, N.Y. 
     Temperatures employed to prepare Raney nickel do not vary widely and are disclosed in this reference. The author refers to these Raney nickel catalysts as W1, W2, W3, W4, W5, W6, W7, and W8. In addition to the W-type Raney nickel, a Raney nickel referred to as T-1 is preferred, or a modification of T-1 Raney nickel is preferred. 
     In the Journal of Organic Chemistry 26, 1625 (1961), there is described a process for the preparation of what the authors refer to as T-1 Raney nickel by Dominguez, Lopez and Franco. In this article, the authors state that the preparation of the T-1 Raney nickel catalyst is a modification of the procedure described by Papa, Schwenk and Whitman in the Journal of Organic Chemistry 7, 586, (1942) and Papa, Schwenk and Brieger in the Journal of Organic Chemistry, 14, 366, (1949). All of the Raney nickels described in the articles referred to above are operable in the process of this invention. 
     Other nickel catalysts useful in the invention can be obtained by the use of new techniques known to the catalyst art for depositing metals on suitable supports in a highly dispersed form. These nickel catalysts would exhibit catalytic properties similar to the properties exhibited by the Raney nickel catalysts. 
     In the article by Dominguez et al, the authors state that the T-1 Raney nickel is prepared as follows: 
     To a 1-liter 3-neck flask containing 600 milliliters (ml) of a 10 percent sodium hydroxide solution, 40 grams of Raney nickel aluminum alloy (50 percent nickel) were added in small portions over a period of 20 to 30 minutes with mechanical stirring. The temperature was kept at 90°-95° C. during this addition. The mixture was stirred for an additional hour period at which time the stirring was stopped and the nickel was allowed to settle, and the solution decanted. The metal was washed five times with 200-ml portions of water and then five times with 50-ml portions of ethanol in such a manner that the nickel was always covered with liquid. The catalyst was then stored under ethanol and refrigerated for further use. 
     The Raney nickel employed in some of the examples of this invention and termed by the present inventor as Modified T-1 Raney nickel was prepared by a slight modification of Dominguez et al&#39;s procedure as follows: 
     A solution of 2 grams of sodium hydroxide in 50 ml of water was heated to its boiling point and then there was added 2 grams of Raney nickel aluminum alloy (1 gram of Raney nickel) as rapidly as the hydrogen evolution would permit. This mixture was then digested at 95° to 100° C. for 1/4 hour (reflux) and the water was continually replaced as it evaporated. The solution was decanted from the Raney nickel and the metal washed with three 250-ml portions of cold water. This catalyst was employed without washing with ethanol. 
     The ratio of catalyst to cyclopentadiene is not too critical. It has been found satisfactory results are obtained when about one part by weight of catalyst per 500 parts by weight of cyclopentadiene are employed. When a catalyst to cyclopentadiene weight ratio greater than about 1 to 33 is employed, the catalyst is being wasted. 
     The amount of polyol employed should range from about a volume ratio of polyol to cyclopentadiene of about 1/1 to about 4/1. 
     The present invention can be applied to continuous or batch process. While the ratio of catalyst to cyclopentadiene set forth is more applicable to batch processing, those skilled in the art could readily adapt the reactants to the catalyst and reaction conditions to continuous processing. 
     The practice of this invention is further illustrated by reference to the following examples which are intended to be representative rather than restrictive of the scope of the invention. Unless otherwise indicated, all parts and percentages are by weight. 
     EXAMPLE 1 
     A 1-liter stainless steel reactor was swept with nitrogen and charged with 200-ml of ethylene glycol containing 1.0 gram of modified T-1 Raney nickel suspended by swirling. Sixty-six grams (1.0 mole) of cyclopentadiene containing 5 ml of pentane as an internal standard was then added under nitrogen. The sealed reactor was then charged with 450 psig -- 3100.5 kPa of hydrogen with stirring. The reactor was held at 25° C. with internal cooling coils and the reaction was stopped when 85-95% of the theoretical amount of hydrogen had been consumed (57 min). The reaction was stopped by stopping the stirring and venting the hydrogen pressure to one atmosphere. The top cyclopentene layer was withdrawn for gas chromatographic analysis. The analysis revealed a 90.7 percent conversion of cyclopentadiene and a 96.7 percent selectivity to cyclopentene, with a 3.3 percent selectivity to cyclopentane. 
     EXAMPLE 2 
     A reaction was carried out under the conditions of Example 1 except that 100-ml of ethylene glycol was employed and the reaction was carried out for 75 minutes. Gas chromatographic analysis revealed: 
     89.5% conversion cyclopentadiene 
     90.5% selectivity cyclopentene 
     8.3% selectivity cyclopentane 
     EXAMPLE 3 
     A reaction was carried out under the conditions of Example 1, except that 300-ml of ethylene glycol was employed and the reaction was carried out for 75 minutes. Gas chromatographic analysis revealed: 
     85.6% conversion cyclopentadiene 
     92.4% selectivity cyclopentene 
     1.4% selectivity cyclopentane 
     While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.