The present invention relates to the chemistry of hydrocarbon compounds containing non-conjugated double bonds and, more particularly, but not exclusively, to the selective production of trans non-conjugated diolefins such as trans- 1,4-hexadiene.

Non-conjugated diolefins are useful as monomers or co-monomers in polymer synthesis. Trans-l,4-hexadiene is a reactant of choice in certain processes, such as that described in US-A- 3,565,967 which prefers a trans isomer content greater than 80% in the manufacture of ethylene-propylene- diene monomer (EPDM) elastomers. Prior art methods for producing 1,4- hexadiene with trans isomer content greater than 80% include US-A-

3,565,967 (using a soluble nickel compound) or US-A- 3,502,738 (using a rhodium catalyst). However, these prior art methods use expensive catalysts and require corrosive reagents. In contrast, cis-l,4-hexadiene can be produced with good conversion and high selectivity by the methods of US-A-

3,539,652, or 3,405,194, using a relatively inexpensive cobalt catalyst system.

The cobalt catalysts are less corrosive than either the nickel catalyst of US-A-

3,565,967 or the rhodium catalyst of US-A- 3,502,738. Various authors refer to transition metal and Group VTH metal catalyzed olefin reactions of the type: catalyst CH ₂ =CH ₂ + CHyCH ₂ =CH ₂ -CH ₃ > 2 CH _y CH=CH ₂

as metathesis or disproportionation. For clarity, the term metathesis as used here includes those reactions referred to as disproportionation, as well as those referred to as metathesis by other authors. The prior art contains numerous examples of metathesis reactions of simple olefins in the presence of various metal catalysts. In simple olefin metathesis, the cis/trans ratio of

products is found to be near the theπnodynamic equilibrium point or only slightly to favor the trans isomer.

Metathesis of acyclic 1,4-dienes has not been studied as extensively as simple olefins. However, Kawai, et al, Journal of .Molecular Catalysts, 1987, 39, 369-382 describes the synthesis of 1,4-dienes over a alumina catalyst, but the isomer content of recovered materials was not disclosed. US-A- 5,025,111 teaches metathesis of 1,4-hexadienes with ethene in the presence of Re ₂ O ₇ promoted by tetra-alkyl tins or tri-alkyl aluminums as a method to prepare 1,4-pentadienes. US-A- 3,641,174 describes metathesis of 1,4-dienes with a soluble Group VTfl transition metal complex under conditions where significant double bond position isomerization also occurred. However, the prior art has not heretofore provided a method for producing the trans non-conjugated diolefin geometrical isomer by metathesis using relatively inexpensive non-corrosive catalysts, or employing selected mixtures of three or more olefins and diolefins to produce mixtures of non-conjugated diolefins having a trans isomer content greater than 80%.

SUMMARY OF THE INVENTION

This invention relates to a method for making trans non-conjugated diolefins of structure:

H - C = C - (CH ₂ )N- C = C - R ⁴

I I I I

R ¹ H R ² R3 where N is a positive integer from 1 to 5 and, R , R , R , and R , are each hydrogen or alkyl of 1 to 5 carbons and Rl is an alkyl of 1 to 5 carbons which comprises:

a. selecting isomerizable non-conjugated diolefin with structure:

R5 - C = C - (CH ₂ )N- C = C - R ⁴

R6 H R2 R3 where N is a positive integer from 1 to 5 and , R , R , R , R , and R > are each hydrogen or alkyl of 1 to 5 carbons and R ² , R , and R ⁴ are the same as in the trans non-conjugated diolefin;

selecting a first olefin of: (1) an olefin having the structure: H- C = C - H

Rl H

(2) an olefin having the structure: H - C = C - H

Rl Rl or

(3) mixtures of olefin 1 and olefin 2 where Rl is the same as R of the trans non- conjugated diolefin;

c. selecting a second olefin which is different than the first olefin of

(1) an olefin having the structure:

H - C = C - R ⁴

I I H R3

(2) an olefin having the structure:

R ⁴ - C = C - R ⁴

I I

R3 R3

or

(3) mixtures of olefin 1 and olefin 2 where R and R ⁴ are the same as R ³ and R of the diolefin selected in step a;

d. mixing the selected diolefin with the selected first olefin and the selected second olefin in a mole ratio of first olefin to diolefin within the range of about 0.2/1 to 100/1 and a mole ratio of the selected second olefin to diolefin within the range of about 0.2/1 to 100/1 to form an olefin reaction mixture; and

e. contacting the olefin reaction mixture in a reaction vessel at a temperature in the range of -30 to 200° C at a pressure in the range of 1,070 to 14,000 kPa (140 to

2,000 psig) with a non-acidic metathesis catalyst selected from the group consisting of homogeneous metal catalysts and heterogeneous metal catalysts wherein the metal comprises either W, Mo, Co, Ta, Nb, Cr, Re, V, or mixtures thereof, to form the selected trans noncoηjugated diolefin isomer product.

Preferably the desired diolefin isomer product is separated from the reaction mixture after passing through the reaction vessel and at least a portion of the other components of the reaction mixture are returned to the reaction vessel.

In one embodiment wherein the catalyst is homogeneous, the catalyst may comprise Mo(NO)2Cl2 (PPh3)2/ETAlCl2, Mo(CO)4 (Pyridine) 2/B114NCI/ Me3Al2Cl3, various carbene complexes of Ta, Nb, Cr, and V, or mixtures thereof.

In another embodiment, wherein the catalyst is heterogeneous, the catalyst may comprise rhenium oxide (Re2θγ), tungsten oxide (WO3), cobalt oxide (CoO), molybdenum oxide (Mo03), or mixtures thereof.

In a preferred embodiment, the invention provides a method of preparing trans-l,4-hexadiene consisting essentially of contacting an olefin reaction feed mixture comprising cis-l,4-hexadiene, ethene and an olefin selected from the group consisting of propene, 2-butene, and mixtures of propene and 2-butene, with a metathesis catalyst which comprises one or more compounds selected from the group consisting of rhenium oxide (Re ₂ 0 ₇ ), tungsten oxide (WO ₃ ), cobalt oxide (CoO), or molybdenum oxide (Mo03); the selected catalyst compound being loaded onto a solid support at a loading of between 5 and 30 wt% in a fixed catalyst bed.

Although any effective metathesis catalyst may be used, an advantage of the invention is that inexpensive and non-corrosive catalysts can be used. The currently preferred catalyst is Re ₂ 0 ₇ supported on alumina. Optionally, the catalyst may be promoted by a tetraalkyl tin or a trialkyl aluminum compound. The tetraalkyl tin or trialkyl aluminum promoters will preferably have from 1 to 5 carbons in each alkyl group which may be the same or different, however, the specific identity of the alkyl groups is not critical. Tetramethyl tin is a preferred promoter. The invention provides an improved method for producing commercially important isomer mixtures of diolefins such as 1,4-hexadiene wherein the trans isomer content is greater than 80%, a preferred feed material in the manufacture of ethylene- propylene-diene monomer (EPDM) elastomers as described in US-A- 3,565,967.

DETAILED DESCRIPTION OF THE INVENTION

In general, the process of the present invention for conversion to the trans geometrical isomer is carried out by mixing a non-conjugated diolefin

feed, with a first olefin, selected so that metathesis between the diolefin feed and the first olefin at one of the double bonds can result in formation of a non-conjugated diolefin, and a second olefin, selected such that the second double bond of the diolefin feed will be regenerated after a metathesis reaction with this second olefin, under conditions that lead to selective formation of the trans geometrical isomer.

The mole ratio of non-conjugated diolefin to first and second olefins may vary, but will fall within the range of 0.2/1 to 100/1 of each olefin to diolefin. The olefin reaction feed mixture is contacted with a metathesis catalyst under conditions to promote the metathesis reaction and the reaction products are recovered. The reaction may be conducted in batches or, preferably, in a continuous reaction. The desired trans non-conjugated diolefin may be separated and some or all of the remaining components of the reaction mixture recycled for further reaction. In a model system illustrated in the examples below, the conversion of cis or mixed cis/trans 1,4-hexadienes to a reaction product having a trans content greater than 80%, is carried out by contacting an olefin reaction mixture of 1,4-hexadiene, ethene, and either propene or 2-butene with a non- acidic olefin metathesis catalyst. The product mixture is found to contain ethene, propene, trans and cis 2-butene, 1,4-pentadiene, trans and cis 1,4- hexadienes, and heavier olefins. In a preferred embodiment, the 1,4- hexadienes are separated from the reaction mixture. In an especially preferred embodiment, the other products and remaining starting materials are recycled to the metathesis reactor. The metathesis catalyst can be either homogenous or heterogeneous, provided, however, that the catalyst must not cause substantial double bond positional isomerization. In a preferred embodiment, the catalyst is in a fixed bed and the reaction is carried out as a continuous process.

The starting non-conjugated diolefins can be prepared by known methods such as metal catalyzed dimerization reactions between simple olefins and conjugated dienes, as are well known to those skilled in the art. For example, 1,4-hexadiene can be prepared by the reaction of ethene and butadiene in the presence of a suitable catalyst. US-A- 3,539,652 or US-A- 3,405,194 provide reaction conditions for making predominantly cis-1,4- hexadiene using a cobalt compound as a catalyst. In practicing the invention, the trans/cis ratio of the starting diolefin material is not critical. It is, of course, preferable to obtain the starting non-conjugated diolefin in good yield and with good reactant conversion, using inexpensive, non-corrosive metal compounds as catalysts to obtain the lowest-cost feed stock. Compounds of rhenium, cobalt, nickel, palladium, iron, and other Group VTfl and transition metals can be used to prepare non-conjugated dienes from lower molecular weight starting materials. US-A-4,010,216; US-A-3,792,101; and US-A- 3,927,137 teach the preparation of non-conjugated diolefins suitable as starting materials for the present method. Suitable metathesis catalyst may be either heterogeneous such as a supported Re ₂ 0 ₇ , C0O/M0O ₃ or W0 ₃ on alumina or silica or homogeneous such as Mo(NO) ₂ Cl ₂ (PPh ₃ ) ₂ /ETAlCl ₂ , Mo(CO) ₄ (Pyridine) ₂ /Bu NCl/Me ₃ Al ₂ Cl ₃ or various carbene complexes of Ta, Nb, Cr, and V.

The metathesis reaction is carried out at a temperature in the range of -30P to 200° C, preferably between -10P and 100° C, and more preferably between 0 and 30-° and at a pressure between 1,070 and 14,000 kPa (140 and 2000 psig), preferably between 2,900 and 10,400 kPa (400 and 1500 psig), and more preferably between 3,500 and 5,500 kPa (500 and 800 psig). The metathesis catalyst must not cause substantial positional isomerization. For this reason, the metathesis catalyst and its support must have essentially no acidic character. As used in this application, a catalyst is non-acidic if in use it does not cause substantial positional isomerization.

To produce a non-acidic catalyst, the metathesis catalyst can be treated so as to lower acid strength. For example, British patent 1,117,968 teaches treatment of C0O/M0O ₃ on alumina with KOH to reduce acidity of the catalyst. The amount of catalyst used is selected to carry out to the isomerization at an acceptable rate. The catalyst amount will be affected by the surface area of catalyst available, the reactor configuration, and throughput conditions. The catalyst amount is not critical and may be adjusted and selected by techniques well known to those skilled in the art. A particularly preferred group of heterogeneous catalysts are composed of Rβ ₂ θ ₇ on alumina which have been found to be especially effective. Metathesis reactions with Re ₂ 0 ₇ on alumina occur at low temperatures where the acidity of the support is low. The acidity of Re ₂ 0 ₇ on alumina can be further lowered by addition of alkali metal ions such as the techniques described in US-A-3,424,811 or British patent 1,117,968. The Re ₂ O ₇ on alumina catalysts can be prepared in a number of ways, including those described in US-A-3,676,520 or US-A-4,795,734 or British patent 1,103,976. The ratio of Re ₂ 0 ₇ to support is not critical. The catalyst loading may be in the range of 5 to 30 weight percent, typically 10 to 20 weight percent, preferably, 10 to 14 percent. Optionally, Re ₂ 0 ₇ catalyst performance may be improved by use of a promoter such as a tetraalkyl tin, according to Ware!, et al., Synthesis 1987, 935-937. The Re ₂ 0 ₇ catalyst is normally activated, for example, the air activation described in US-A- 3,641,189.

The reaction may be run continuously or in discrete batches. Inert diluents such as hexane, cyclopentane, propane, butane, cyclohexane, benzene or toluene may be used in the reaction mixture.

In a preferred embodiment, a feed comprising cis-l,4-hexadiene content greater than 50%, preferably greater than about 90%, is mixed with ethene and propene to form an olefin reaction feed mixture (or feed). The mole ratio of ethene to 1,4-hexadiene may be from 0.2/1 to 100/1, but is preferably

in the range of 1/1 to 10/1 and, more preferably, 2/1 to 4/1. It is desirable to use sufficient ethene to limit formation of heavier metathesis products, but not so much that the composition of 1,4-hexadiene in the product becomes economically unattractive. Propene or 2-butene or mixtures of propene and 2-butene are used in a mole ratio to 1,4-hexadiene of 0.2/1 to 100/1, preferably in the range of 1/1 to 10/1 and, more preferably, in the range of 2/1 to 4/1. The amount of propene or 2-butene is selected to maximize the composition of 1,4-hexadienes relative to 1,4-pentadienes in the product mixture. A preferred catalyst system is 10-14 weight percent Re ₂ 0 ₇ on alumina catalyst, which is activated in a flowing air stream at 500° C for 2-5 hours. The catalyst is then contacted under anhydrous conditions with ethene, propene and cis-l,4-hexadiene at -30 to 200° C and at a pressure of 1,070 to 14,000 kPa (140 to 2,000 psig). In a still more preferred embodiment, the 10- 14 weight percent Re ₂ 0 ₇ on alumina catalyst is placed in a fixed bed and the mixture of mono and diolefin is passed through the bed at a weight hourly space velocity ("WHSV") of 0.5 to 12 g-feed/g-catalyst/hour. The flow rate through the bed is preferably at a WHSV of 0.5 to 5 g-feed/g-catalyst/hour, and most preferably, at a WHSV of 0.5 to 3 g-feed/g-catalyst/hour. When using a fixed bed catalyst, the reactor pressure ranges are as stated above. The reactor temperature is preferably between -Iff and 100° C, but more preferably, between 0 and 7ff C, and, most preferably, between 0 and 3ff C. In an especially preferred embodiment, a 14% Re ₂ 0 ₇ on alumina catalyst is activated in a flowing stream of air at 50ff C for 5 hours in a fixed bed. The reactor is purged and the catalyst is contacted under anhydrous conditions with ethene, propene, and cis-l,4-hexadiene at a temperature between ff and 3ff C at a pressure of 3,500 to 5,500 kPa (500 to 800 psig), with a WHSV of 0.5 to 3 g-feed/g-catalyst/hour.

In a further embodiment of the invention, the trans diolefin product is separated from the other metathesis products by any convenient means, such as distillation. Optionally, all other components of the reaction mixture are returned to the metathesis reaction vessel. Optionally, the return stream is mixed with incoming fresh starting olefin feed mixture and the total feed mixture is passed through a fixed bed reactor. For example, 1,4-hexadienes from the metathesis reaction described above are separated from the other products by distillation, gas chromatography, counter current extraction, column chromatography or other separation method and the 1,4-pentadienes, heavy olefins, ethene, propene, and 2-butene are returned to the metathesis reactor. In a more preferred embodiment, the return stream of 1,4- pentadiene, heavy olefins, ethene, propene, and 2-butene is mixed with fresh 1,4-hexadiene, ethene, propene or 2-butene and again passed through a fixed bed of Re ₂ O ₇ on alumina catalyst under the conditions recited above. In the preceding illustration, 2-butene may be substituted for propene in the reaction mixture, if desired, or a mixture of propene and 2-butene may be used.

The example below shows the various reaction conditions and feed ratios needed to obtain product with a trans isomer content greater than 60% preferably greater than about 80%. Trans diolefins are kinetically and thermodynamically favored over cis. Cis content greater than 50% is indicative of low per pass conversion conditions.

EXAMPLES

In runs 1-7, 14 wt. % Re ₂ O ₇ catalyst supported on alumina, and mixed with quartz, was placed in a 25.4 cm(10 in.) x 1.02 cm (0.4 in.) tubular stainless steel reaction vessel, brought to the desired temperature, and contacted with ethene, propene, and 99.9% cis-l,4-hexadiene under the conditions summarized in Table la below. The reaction products were determined by gas chromatography and identified as trans/cis-l,4-hexadiene, 1,4-pentadiene, trans/cis-2-butene and heavier alkenes. The conversion of cis-l,4-hexadiene to trans-l,4-hexadiene is observed in each case. In these runs, no positional isomerization was observed.

Table lb illustrates the conversion of 1,4-pentadiene to trans/cis-1,4- hexadiene in run 8, in the same reaction vessel as runs 1-7. Table lc presents data for batch operation and shows, in run 10, that trans-l,4-hexadiene as a feed stock yields predominately trans product. Run 9 shows the conversion of cis-l,4-hexadiene to 80/20 trans/cis-l,4-hexadiene under the same reaction conditions. Runs 9 and 10 were performed in a 50 ml stirred reaction vessel.

TABLE

¹ Over the 21 hours, the recovery increased from 22% to 26%, but the trans/cis ratio remained in the narrow range between 80.5/19.5 to 82/18.

TABLE (Continued)

Rim 9 feed was 99% cis-l,4-hexadiene. Run 10 feed was 98% trans -1,4-hexadiene.