SYNTHESIS OF ISOPRENE FROM LINEAR BUTENES

Abstract: Isoprene is synthesized from mixed linear butenes (butene-­ 1 and butene-2) by catalytically isomerizing said mixed linear butenes to a mixture of cis and trans butene-2, hydroformylat­ ing the butene-2 mixture to 2-methylbutanal in the presence of homogeneous rhodium catalyst and organic ligand and dehy­ drating the 2-methylbutanal to isoprene in the presence of acidic heterogeneous catalyst at elevated temperature.

Description: SYNTHESIS OF ISOPRENE FROM LINEAR BUTENES This invention relates to a method of making isoprene from linear butenes. Isoprene is a key monomer for the manufacture of synthetic rubbers. It is primarily used to make cis-polylsoprene which is a stereospecific rubber having the same monomer as natural rubber. Four fundamental processes have been used commercially to construct the isoprene C5 skeleton from smaller carbon units. Only two of these processes are still in commercial operation. One of the production routes involves condensing acetylene and acetone, followed by hydrogenation and dehydration: EMI1.1 <tb> <SEP> catalyst <SEP> tCH3 <tb> <SEP> HC;C-C <SEP> -cH <tb> <SEP> KC=cH <SEP> + <SEP> cH3C0 <SEP> CH3 <SEP> > <SEP> <SEP> 3 <tb> <SEP> OH <tb> <SEP> CH3 <SEP> CH3 <tb> r <SEP> ,=CH-C-CH3 <SEP> Y <SEP> CHa=CH <SEP> -C=CH2 <tb> catalyst. <SEP> OH <tb> The other route involves, as a first step, the Prins reaction between formaldehyde and isobutylene. EMI1.2 In a subsequent step the intermediate derivative is catalytically cracked at eleVated temperatures. EMI2.1 See, e.g., French Patent 1,294,716; Chem. Absts. 57: 15309. It is generally recognized that the isobutyleneformaldehyde route represents an advance over the route using expensive acetylene feedstock. Nevertheless isobutylene also is a relatively expensive source of C4 hydrocarbon, and formaldehyde is a far more expensive source of carbon than either methanol or carbon monoxide. Accordingly, a method of making isoprene from more economically available raw mater ials would have distinct advantages. Description of the Invention The invention described and cTaimed herein provides a method for the synthesis of isoprene using low cost linear butenes and synthesis gas (CO/H2) as the raw materials. The advent of large-scale production of methyl tertiary butyl ether (MTBE) as an octane booster, particularly for premium-grade unleaded gasoline, has given rise to the availability of large quantities of linear butenes as a raffinate from the process. This availability is due to the fact that the MTBE process is presently being practiced by reacting methanol with refinery C4 streams containing a mixture of C4 olefin and paraffin isomers. A typical composition of such a stream is shown in Table 1. Table 1 Component Vol. X Is obut ene 44-49 l-butene 24-28 2-butene (cis & trans) 19-21 n-butane 6-8 isobutane 2-3 In the MTBE process only the is obutene reacts, thereby leaving a mixture of linear butenes and paraffins as a raffinate. Since no Large scale chemical uses presently exist for linear butenes, their value is currently set by their utility in fuel-related outlets, in particular as alkylate for gasoline. In order to generate the required skeleton of isoprene from a linear C4 olefin it is required that the additional carbon be added at the 2-position. It is known that the hydroformylation reaction (or oxo process) will attach a formyl group to an olefin double bond. EMI3.1 <tb> <SEP> ¯¯¯¯¯¯¯¯¯¯ <SEP> - <SEP> C <SEP> CHO <tb> + <SEP> = <SEP> + <SEP> CO <SEP> + <SEP> H2 <SEP> catalyst <tb> According to this process, discovered by Roelin of Ruhrchemie A.G. (see U.S. Patent 25327,066), alcohols are prepared from olefins, carbon monoxide and hydrogen in the liquid phase in the presence of metallic cobalt catalysts such as Raney cobalt or cobalt carbonyls at 1150-1900C and high pressures of 100-200 Atm. in a Fischer-Tropsch type reaction. The process can be carried out in two stages in which the initial stage yields principally aldehydes, which can then be reduced to the alcohol. In order to generate a molecule with the same backbone as isoprene, the above hydroformylation reaction must be stemochemically directed to the 2-position when conducted with linear butenes. In accordance with the present invention, this result is achieved by hydroformylating cis or trans butene-2 with a homogeneous rhodium catalyst in the presence of excess organic 1 igand. By contrast, cobalt catalysts of the prior art are unsuitable for this reaction since even with butene-2 as the feedstock the major a ld ehyd e isomer obtained is the linear n-pentanal. It is known from U.S. Patent 3,965,192 that rhodium catalysts with excess ligand in the hydroformylation of butene-2 can produce the aldehydes in a 10:1 ratio of 2-methylbutanal to n-pentanal. As Table 1 shows, the predominant linear butene isomer found in refinery C4 streams is butene-l. The rhodium hydroformylation catalyst with excess ligand is known to give predominantly n-pentanal with this olefin isomer (see, e.g., U.S. Patents 3,917,661 and 3,946,082). However, cis and trans butene-2 are more stable, thermodynamically, than butene-l and hence, in accordance with the present invention, isomerization of the mixed linear butenes is carried out to provide a mixture generally containing less than about 3X of butene-l. Thus, when the mixture of linear butenes Is first isomerized and then subjected to hydrofonnylatlon reaction conditions using a rhodium catalyst with excess ligand present, the product is 2-methylbutanal. In a third step of the present invention, 2methylbutanal is dehydrated to isoprene using an acidic heterogeneous catalyst, i.e., EMI5.1 <tb> <SEP> CH <SEP> CH3 <tb> <SEP> 1 <SEP> 3 <tb> CH3-CH <SEP> 2-CH-CHO <SEP> 3 <SEP> CH2=CH-C=CH2 <SEP> + <SEP> H20 <tb> Thus the invention described and claimed herein provides a new and useful route to isoprene based on linear butenes and synthesis gas which utilizes 3 steps: : EMI6.1 <tb> <SEP> isomerization <tb> <SEP> catalyst <tb> 1) <SEP> mixed <SEP> linear <SEP> butenes <SEP> ------------ > <SEP> cis <SEP> & trans <SEP> butene-2 <tb> EMI6.2 <tb> <SEP> Rhodium <SEP> CH3 <tb> <SEP> catalyst <SEP> CH3CH2CH <SEP> CHO <tb> 2) <SEP> butene-2 <SEP> + <SEP> CO <SEP> + <SEP> H2 <SEP> Catalyst <tb> <SEP> organic <tb> <SEP> ligai <SEP> 2-methylbutanal <tb> <SEP> dehydration <SEP> CH3 <tb> 3) <SEP> 2-methylbutanal <SEP> catalyst <SEP> > <SEP> CH2 <SEP> = <SEP> CH <SEP> - <SEP> C <SEP> = <SEP> CH2 <tb> i soprene The initial isomerization of butene-l to butene-2 is carried out in the presence of a double bond isomerization catalyst or a non-skeletal isomerization catalyst. By the term "non-skeletal" isomerization catalyst is meant a catalyst which does not cause any appreciable chain branching. Illustrative examples of suitable catalysts for this reaction step are alumina, aluminum sulfate, aluminum phosphate, phosphoric acid deposited on alumina, thoria, pal ladized asbestos and molybdenum sulfide as described in a treatise by F. Asinger, "Monoolefins Chemistry and Technology", Pergammon Press, 1968, at page 1021 and references cited therein. The isomerization preferably is carried out in the liquid phase at elevated temperature and pressure. Further background information on the isomerization of- butene-l to butene-2, including useful catalysts and reaction conditions, can be had by refer ence to a comprehensive review on olefin isomerization by H.N. Dunning, Ind. Etna. Chem. 45 551-564 (1953). The cetalysts and reaction conditions for sad isomerization disclosed by Dunning can be used for the isomerization step herein. An alumina and Pd catalyst combination (see, e.g. U.S. Patent 4,179,581) is particularly useful in the isomerization of butene-l to butene-2. Under a preferred set of conditions for the isomerization step, butene-l is isomerized to a mixture of cis and trans butene-2 at about 80 0C and about 300 psi (20.4 Atm.) over a fixed bed of alumina catalyst with about 0.3X Pd. A small amount of hydrogen is used as a catalyst promoter in this reaction and the hydrogen is methanated to preclude catalyst poisoning by CO. It will be appreciated that the linear butenes used in the isomerization reaction need not be separated from the accompanying paraffins in the C4 refinery stream since only the isomerized butenes will react in the next step. The resulting butene-2 mixture, or butene-2 enriched mixture, is then hydroformylated to 2-methyl butanal in the presence of a homogeneous rhodium catalyst and an excess of organic ligand. The rhodium catalyst should be soluble in the reaction medium along with the reactants. Illustrative examples of suitable rhodium catalysts for the hydroformylation step are rhodium salts such as rhodium sulfate, rhodium acetate, rhodium nitrate and, less desirably, rhodium halides such as rhodium trichloride and rhodium tribromide (see, e.g., U.S. Patent 4,110,404). Also suitable are rhodium carbonyl catalysts such as rhodium carbonyl chloride LRh(CO)2 C1 2 (see, e.g., U.S. Patent 4,292,448-); rhodium chelates such as rhodium acetylacetonate (see, e.g., U.S. Patent 4,110,404; rhodium catalysts complexed with phosphine such as Wilkinson's catalyst - chlorotris (triphenylphosphine)Rh - RhCl(PPh3)3 (see, e.g., U.S. Patent 3,946,082); and rhodium carbonyl complexes with phosphines such as carbonylhydrotris(triphenylphosphine)Rh- RhH(CO)(PPh3)3 (see, e.g., U.S. Patents 3,965,192 and 4,200,591-592), and tricarbonylbisStriphenylphosphine)- rhodiumPl+) tetraphenylborate - Rh(C03)(PPh3)2BPh4 (see e.g., U.S. Patent 4,052,461). Other suitable rhodium catalysts are the organic complexes of rhodium with organic molecules such as ethylene, propylene, 1,5-cyclooctadiene, 1,5-hexadiene and bicyclo-2.2.1hepta-2,5-diene. Illustrative of such organic rhodium complexes is rhodium dflcylo-1,5-octadiene (see, e.g., U.S. Patent 4,210,608); di-u-chlorobis [(1,2,5,6-k )-l ,5cyclobtctadiene3 di-rhodium or dimeric cyclooctadienyl rhodium chloride (see, e.g., Ger. Offen. 2,051,319); and [(1,2,5,6-# )-1 ,5-cyclooctadiene] bis(triphenylphosphine)rhodium(l) tetraphenylborate (see, e.g., U.S. Patent 4,052,461). The preferred rhodium catalysts are non-halide containing rhodium compounds. The organic ligand should be capable of binding to the rhodium ion and may be firmly bound as in the above rhodium complexes and/or may remain uncoordinated as a free compound. The tertiary organo phosphorus, arsenic and antimony compounds are particularly useful as such ligands for the rhodium ion. Illustrative examples of these 1 igands for use with the rhodium catalyst are the organic phosphines and phosphites (see, e.g., U.S. Patent 3,801,646). Especially useful are the trialkylated phosphines such as triethyl phosphine, tributylphosphine and triphenylphosphine and other such C3-C90 alkyl-, aryl- and alkaryl phosphines as well as the corresponding phosphites such as, e.g., triethylphosphite, tributylphosphite and triphenylphosphite. Organic arsines and stibines also can be used as the ligand but the stibines are less preferred than the corresponding phosphines and arsines. An exemplary rhodium catalyst and organic 1 igand combination for use in the hydroformylation step of the present invention is a mixture of Rh(cycloöctadiene)- acetylacetonate with an excess of tri phenyl phcsph ine. This-catalyst/ligand combination has resulted in greater than 98X selectivity to 2-methylbutanal as described in a more detailed example hereinafter. The catalyst generally is present in an amount of from about 10-1 to about 1Q moles (Rh basis) per mole of butene-2 feed and the volume percent of CO/H2 generally ranges from about 10/90 to about 90/10 CO/H2 in the hydroformylation step herein. Under preferred conditions of the hydroformylation step, the cis and trans butene-2 mixture is reacted with synthesis gas (CO:H2 = 1:1) at a temperature of 0 from about 70 C to about 1500C and a pressure of from about one to about 100 atmospheres in the presence of from about 60 to about 150 moles of triphenylphoshine per mole of rhodium catalyst (Rh basis). The hydroformylation reaction is exothermic and most of the heat of reaction can he removed by vaporizing the reactor effluent. Coarse reactor temperature control also can be achieved hy adjusting the reactor pressure. The 2-methylbutanal product recovered from the hydroformylation step is then dehydrated to isoprene at elevated temperature in the presence of an acidic heterogeneous catalyst. Useful catalysts for this purpose are acid dehydration catalysts such as phosphoric acid anhydride, boric acid anhydride, acid silicon dioxides and acid titanium oxides, and other such acidic compounds. A preferred catalyst for the dehydration step in this invention is boron phosphate which is described in British Patent 1,385,348, Example 8, (Chem. Absts. 79: 106002) and USSR Patent 721,116 (Chem. Absts. 98: 9348). According to the latter patent, the catalyst is prepared by mixing concentrated H3P04 and boric acid, evaporating the mixture, grinding, calcining and then processing with a mixture of steam and air containing 15-75 vol X steam at 450-6500C for 1-8 hours. Other particularly desirable catalyst materials for the dehydration step are silica-beads having surface areas #100mê/gm, such as those available commercially from PPG Industries, e.g., PPG Silica Beads Type MOll, 1/16 inch, having the following characteristics: Surface Area = 50-60m2/gm 0 Average Pore Diameter = 800 A Average Pore Volfime = 0.6cc/gm The dehydration reaction is endothermic and under preferred conditions the reaction is performed in the vapor phase over a fixed bed of the catalyst at elevated temperature of from about 2000C to about 4000C. The desired isoprene product is recovered in high yield, e.g. selectivity to isoprene as high as 85X at 80% conversion of the 2-methylbutanal as described in a more detailed example hereinafter. The following examples will further illustrate the invention although it should be understood that the invention is not limited to these specific examples. Example 1 A C4 fed refinery stream having a component composition within the following approximate range of concentrations is used as a source of linear butenes. Table 2 Compound Vol. X Isobutene 44-49 butene-l 24-28 butene-2 (cis and trans) 19-21 n-butane 6-8 i sobutane 2-3 The C4 refinery stream is fed to the bottom of a butene isomerization reactor having a fixed bed of alumina catalyst with 0.3X Pd. A small amount of methanated hydrogen is fed to the top of the reactor as a catalyst promoter. The isomerization reaction takes place at 80 0C and 300 psi (20.4 Atm). The resulting isomerized cis and trans butene-2 can be separated from the other C4 components and recovered or can be used directly without being sepÅarated from admixture with the entrained paraffins before proceeding to the next step. Butene-2 (cis and trans) which can be obtained from a refinery stream as aboue, or which is otherwise available as merchant butene-2, can be hydroformylated to 2-methyl butanal as follows: In this example, merchant butene-2 (1732 gm) was charged to an autoclave together with Rh(cyclooctadiene) acetylacetonate (4.08 gm) and triphenylphosphine (170.37 gm). The autoclave was pressured to 1000 psig with a 1:1 mixture of CO:Hz and then heated to stirring to 1100C. The reaction temperature was maintained for four hours. The autoclave was cooled and vented. The resulting product was analyzed by gas chromatography and shown to contain greater than 98% 2-methylbutanal. 2-methylbutanal (15% in helium) was passed over a solid crystalline boron phosphate catalyst (liquid hourly space velocity, 0.7 hr 1) at 350 C. The gaseous product was analyzed by a gas chromatographic technique and it was found that the selectivity to isoprene was 85X at 80Z conversion of the 2-methylbutanal. Example 2 A C4 fed refinery stream processed by a MTBE plant (which removes isobutylene) has the following composition: Table 3 Component Mole X n-butane 7.5 is obu tane 7.5 isobutylene 2.0 butene-l 41.5 cis butene-2 16.5 trans butene-2 25.0 The refinery stream is fed to the isomerization reactor and the butene-2 enriched stream is recovered as in Example 1, above,without separation of the paraffins. Butene-2 (cis and trans) from the isomerization reaction step is hydroformylated with CO and H2 in a 1:1 ratio (synthesis gas) at 1150C and 117 psi (8 Atm.) with a homogeneous rhodium catalyst and 1 igand as in Example 1. 60 moles of triphenylphosphine ligand per mole of Rh is used. Zinc oxide packing is used to remove traces of chloride from the synthesis gas feed and molecular sieve packing is used to remove traces of water from the butene-2 feed. The resulting 2-methylbutanal is separated from unreacted material and recovered. 2-methylbutanal from the hydroformylation reaction step is dehydrated to isoprene in an endothermic reaction at 400 C and 100 psi C6.8 Atm.) over a fixed bed of silica beads. 10 moles of steam dilution per mole of the aldehyde are used. Ths resulting isoprene is separated from the reaction zone and recovered. Various other examples will be apparent to the ordinary person skilled in the art after reading the present disclosure without departing from the spirit and scope of the invention and it is intended that all such further examples be included within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method of synthesizing isoprene from linear butenes comprising (a) isomerizing mixed linear butenes in the presence of a double bond isomerization catalyst to a mixture of cis and trans butene-2, (b) hydroformylating said butene-2 mixture with CO and H2 in the presence of homogeneous rhodium catalyst and an excess of organic ligand to 2-methylbutanal, (c) and dehydrating said 2-methylbutanal at elevated temperature in the presence of acidic heterogeneous catalyst to isoprene. 2. The method of Claim 1 in which the isomerizationcatalyst is alumina. 3. The method of Claim 1 in which the isomerization is carried out in the liquid phase at elevated temperature of about 80 C and pressure of about 300 psi over a fixed bed of alumina catalyst with about 0.37, palladium. 4. The method of Claim 1 in which the homogeneous rhodium catalyst is Rh(cyclooctadiene)acetylacetonate and the organic ligand is triphenylphosphine. 5. The method of Claim 1 in which the hydroformylation is carried out with synthesis gas at a temperature of from about 700C to about 150 C and a pressure of from about one to about 100 atmospheres in the presence of from about 60 moles to about 150 moles of triphenylphosphine per mole of rhodium catalyst. 6. The method of Claim 1 in which the acidic heterogeneous dehydration catalyst is boron phosphate. 7. The method of Claim 1 in which the acidic heterogeneous dehydration catalyst is silica beads. 8. The method of Claim 1 in which the dehydration is carried out over a fixed bed of catalyst in the vapor phase at a temperature of from about 200 C to about 400 C. 9. The method of Claim 1 in which the isomerization catalyst is alumina, the homogeneous rhodium catalyst is Rh(cyclooctadiene)acetylacetonate, the organic ligand is triphenylphosphine and the acidic heterogeneous catalyst is boron phosphate. 10. The method of Claim 1 in which the somerization is carried out in the liquid phase at elevated temperature of about 80 C and pressure of about 300 psi over a fixed bed of alumina catalyst with about 0.3% paltadiuns the hydroformylation is carried out with synthesis gas at a temperature of from about 70 C to about 1500C and a pressure of from about one to about 100 atmospheres in the presence of from about 60 moles to about 150 moles of triphenylphosphine per mole of rhodium catalyst, and in which the dehydration is carried out over a fixed bed of catalyst in the vapor phase at a temperature of from about 2000C to about 4000C.

Inventors: BARKER, GEORGE EDWARD, FORSTER, DENIS
Applicants: MONSANTO COMPANY
Application year: 1982
Main classification: C07C11
IPC codes: C07C11, B01J27, B01J21, C07B61, C07C45, B01J31, C07C67, C07C5, C07C47, C07C1, B01J23