PROCESS FOR PREPARING LINEAR ALPHA-OLEFINS FROM ETHYLENE

Abstract: Linear alpha-olefins are made by oligomerising ethylene using as catalyst a zirconium halide adduct and aluminium or zinc alkyl in the presence of a small amount of oxygen. The presence of the oxygen increases the linearity of the oligomers obtained.

Description: PROCESS FOR PREPARING LINEAR ALPHA-OLEFINS FROM ETHYLENE This invention relates to the oligomerisation of ethylene to produce linear alpha-olefins. It is known to oligomerize ethylene to produce linear alpha-olefins using a zirconium-containing catalyst, see, for example United States Patents Nos. 4486615, 4442309, 4434313, 4434312, 4410750, 4409409, 4396788, 4377720 and 4361714. Japanese Patent Application No. 60-137683 (Idemitsu Petrochemical KK - Kokai 62-000430) discloses the production of linear alpha-olefins by polymerising ethylene in the presence of a mixture consisting of a zirconium halide, an alkyl aluminium halide and a compound of sulphur or nitrogen. The present invention provides a process for the oligomerisation of ethylene to produce linear alpha-olefins having a very high degree of linearity. A high degree of linearity is important because ethylene oligomers are used as raw materials for preparing surfactants, e.g. ethoxylated linear alcohols, and linearity is necessary for the surfactants to be biodegradable. In our European Patent Application No. 88305581.6 (Publication No. 295960) we have described and claimed a process for preparing substantially linear alpha-olefins having a number average molecular weight from about 70 to 700 by oligomerizing ethylene in the presence of a homogeneous two-component catalyst, the first component being an adduct of ZrClaBrj ; , where a + b = 4 and a or b may be 0, 1, 2, 3 or 4, with an organic compound selected from the group consisting of esters, ketones, ethers, amines, nitriles, anhydrides, acid chlorides, amides or aldehydes, said organic compound having up to 30 carbon atoms and the second component being an alkyl metal catalyst selected from the group consisting of R2AIX, RA1X 2 , 3AI2X3, R3AI, and R 2 Zn wherein R is C1-C20 alkyl and X is Cl or Br, the oligomerisation being conducted in a reactor vessel at 50 to 300* C at a pressure of about 500 to 5,000 psig in a solution of a C2-C100 alphaolefin or a liquid inert solvent which is - not reactive with said catalyst and in which said two- component catalyst is soluble, the water content in the reactor vessel being such that the ratio of moles of zirconium to moles of water is at least 20 to 1. This process gives a mixture of ethylene oligomers having a high degree of linearity. It is, however, desirable still further to increase the linearity of the oligomeric product, especially in the case of the oligomers containing 12 to 18 carbon atoms. It has now been discovered that the addition of a small amount of oxygen to the ethylene feed usefully increases the linearity of the oligomeric alpha-olefin product. The amount of oxygen present in ethylene ordinarily subjected to oligomerisation is no more than three parts per million by volume. The improved linearity characteristic of the present invention is obtained by using from about 10 to about 50 pp of oxygen by volume based on the ethylene feed. While the addition of the oxygen somewhat reduces the catalyst activity, this slight disadvantage is more than compensated by the improved linearity of the product. The present invention accordingly provides a process for polymerizing ethylene to form a mixture of substantially linear alpha-olefin oligomers having a degree of polymerization from about to 3 to 30 which comprises contacting ethylene at a temperature of 50 to 250° and at a pressure of 3450 to 34500 kPa (500 to 5,000 psig) with a ' solution in an inert organic solvent of a two component catalyst in which the first component is an adduct of ZrCl a Br ];) wherein each of a and b is 0, 1, 2, 3 or 4 and a + b = 4 with an organic compound of up to 30 carbon atoms selected from the group consisting of esters, ketones, ethers, amines, nitriles, anhydrides, acid chlorides, amides and aldehydes, and a second alkyl metal component selected from the group consisting of R 2 A1X, RA1X 2/ R3AI2X 3 , R3AI, and R 2 Zn wherein R is alkyl or 1 to 20 carbon atoms and X is Cl or Br, the oligomerisation being conducted in the presence of 10 to 50 ppm by volume of oxygen based on the ethylene. The first component of the catalyst used in the present invention is an adduct of ZrCl a Brj ; , with an ester, ketone, ether, amine, nitrile, anhydride, acid chloride, amide or aldehyde, and these various adduct-forming organic compounds may have up to 30 carbon atoms. The adduct generally includes mole ratios of organic component to zirconium of from about 0.9 to 1 up to about 2 to 1. Equimolar adducts are preferred. The adduct must be soluble and stable in the solvent used as the reaction medium for the oligomerisation process of the invention. Suitable zirconium halides include ZrCl 4 , ZrBr 4 and mixed halides such as ZrClBr 3 , ZrCl 2 Br 2 and ZrCl 3 Br. Adducts of ZrCl 4 are especially preferred. The organic compound used to form the adduct is preferably an ester of the general formula RiCOCD^ wherein R^ and R 2 are each alkyl, aryl, alkaryl, aralkyl of 1 to 30 carbon atoms and R± may also be hydrogen. ^ and 2 taken together may also represent a cycloaliphatic group and the ester may be a lactone such as gammabutyrolactone or phthalide. Especially preferred are alkyl acetate esters wherein the alkyl group has 6 to 16 carbon atoms, e.g. n-hexyl acetate, n-heptyl acetate, n-octyl acetate, n-nonyl acetate, n-decyl acetate, isohexyl acetate, isodecyl acetate and the like, which have been found to form di eric equimolar adducts with ZrCl 4 . Particularly preferred adducts may be represented by the formula (ZrCl 4 . CH3COOR1 2 where R-i is Cg to C 16 alkyl or a mixture thereof. These preferred ester adducts are capable of providing highly concentrated solutions in the solvent used as the reaction medium, e.g. up to about 40 per cent by weight of ZrCl 4 when the preferred mixed isodecyl acetate esters are used. Particularly useful are mixtures of various isomers of isohexyl, isoheptyl, isooctyl, isononyl, isodecyl or isotridecyl acetate sold by Exxon Chemical Company, respectively, as Exxate R 600, Exxate R 700, Exxate 1 * 800, Exxate R 900, Exxate R 1000 and Exxate R 1300. The isohexyl acetate mixture comprises about, by weight, 36 to 38 per cent n-hexyl acetate, 18 to 20 per cent 2-methyl-1-pentyl acetate. 22 to 24 per cent 3-methyl-l-pentyl acetate and 16 to 18 per cent 4-methyl-l-pentyl acetate as principal compounds. Exxate R 1000 isodecyl acetate mixture is a complex mixture of isomers and gas chromatographic analysis shows about a hundred different isomers being present, none of which are greater than about 10 per cent by weight of the mixture. Exxate 1000 has a boiling point range of about 218°C to 250°C (425 β F to 482 β F) (95 per cent distilled). The adducts may be prepared by simple addition of the organic ester to a mixture of ZrCl 4 and the inert organic or alpha-olefin solvent. The ester is added slowly to the stirred mixture at room temperature and complete formation and dissolution of the adduct is observed within a few minutes. The dissolution is exothermic and the mixture can reach a temperature of about 50 * C during the adduct formation. Also suitable for providing soluble zirconium adducts useful as the first component in the catalysts used in the present invention are ketones, ethers and aldehydes which may be represented respectively by the formulae R 1 COR 2/ ιO 2 and R^COH wherein R^ and R2 each represent alkyl, aryl alkaryl or aralkyl and a total number of carbon atoms in R^ and R 2 is not more than about 30. Also suitable are primary, secondary and tertiary amines wherein the hydrocarbyl radicals have up to about 30 carbon atoms such as n-dodecyl amine and tri-n- hexyl amine. Also suitable are hydrocarbyl cycloaliphatic ethers and ketones having from 4 to 16 carbon atoms, e.g. cyclohexanone. Other adduct forming organic compounds useful in the present invention include nitriles and hydrides, acid chlorides and amides having up to 30 carbon atoms. These may be represented by the formulae RCN, (RCO^O, RC0C1, RC0NH 2 , RCONHR and RCONR 2 where R represents a hydrocarbyl, alkyl, aryl, alkaryl or aralkyl group of up to 30 carbon atoms. Examples are n-undecane nitrile, n-decyl succinic anhydride and n-decanoyl chloride. The second catalyst component used in the present invention is an aluminium alkyl of the formula R 2 A1X, RA1X 2 , R3AI3X 3 , R 3 AI or a zinc alkyl of the formula R 2 Zn where lf R 2 and R 3 each represent alkyl of 1 to 20 carbon atoms and X is Cl or Br. Diethylaluminium chloride, aluminium ethyl dichloride and mixtures thereof are preferred. The process of the present invention may be conducted under generally known oligomerisation conditions of temperature and pressure, that is at a temperature from about 50 ' to 250* C and under a pressure of 3450 to 34500 kPa (500 to 5,000 psig) , preferably 6900 to 24100 kPa (1,000 to 3,500 psig) . The process is conducted in solution in an inert solvent which must be non-reactive with the catalyst system, and optionally in the presence of a solvent comprising a liquid alpha-olefin, especially a C 6 -C 10 0 alpha-olefin. Suitable solvents include aromatic or aliphatic hydrocarbons and halogenated aromatic hydrocarbons such as chlorobenzene, dichlorobenzene and chloro-toluene. Preferred solvents are toluene, xylenes and alkanes of 3 to 24 carbon atoms, especially heptane. Mixtures of these solvents may also be used. Liquid alpha-olefins may also be used as solvents for the process, and these may include, more particularly, liquid alpha- olefins which have been formed by the oligomerisation process, especially those containing 6 to 30 carbon atoms. Such alpha- olefins may be used in admixture with the aforesaid non-reactive aromatic or aliphatic solvents. A useful solvent mixture comprises a minor proportion of C 4 to C30 alpha-olefins, such as about 10 per cent by weight of Cg and C 10 alpha-olefins and 0 to 5 per cent by weight of C 4 alpha-olefins, based on the amount of the ethylene feed stock, with a balance of the solvent being xylene. The use of this solvent mixture with solvent recycle improves distillation efficiency during product recovery. The ethylene used in the present invention preferably contains not more than the following limits of impurities: acetylenic hydrocarbons less than 1 part per million by weight; dienes less than 1 part per million by weight; carbon monoxide less than 5 parts per million by weight; carbon dioxide less than 15 parts per million by weight; oxygen- containing compounds (e.g. methanol, ethanol, acetone or sec- butanol) less than 1 part per million by weight; water less than 5 parts per million by weight; hydrogen less than 1 part per million by weight; oxygen less than 3 parts per million by weight; sulphur less than 5 milligrams per cubic meter; chlorine less than 5 milligrams per cubic meter. The water content of the ethylene is preferably reduced still further to less than 20 parts per billion before it is subjected to the oligomerisation, e.g. by contacting with 3A molecular sieve. As already stated, in accordance with the present invention, the linearity of the alpha-olefin oligomers is improved by introducing into the reaction mixture from 10 to 50 parts per million by volume, preferably 20 to 40 parts per million by volume, of oxygen. The amount of catalyst used needs to be somewhat increased in order to compensate for the reduction of catalyst activity caused by the oxygen. For example, at about 40 ppm of oxygen by volume the catalyst concentration needs to be doubled to achieve the same degree of conversion as that obtained in the absence of the oxygen. At about 20 ppm of oxygen by volume, the proportion of catalyst should be increased by about 30 per cent. The ethylene and oxygen feed and the catalyst components may be introduced into the reaction vessel in any order, but preferably the ethylene, which may be pre-dissolved in the solvent, and the oxygen and the solution of zirconium tetra- halide adduct are first mixed and the second component of the catalyst, also in solution, is then added. The temperature and pressure of the oligomerisation may be varied to adjust the molecular weight and yield of the desired product. The molecular weight (Mn) may also be controlled by adjustment of the molar ratio of the second component of the catalyst to the first component (i.e. of the aluminium or zinc to the zirconium) . The preferred reaction temperature for the production of high quality linear alpha-olefin oligomers having from 6 to 20 carbon atoms is about 120" to 250'C. At these preferred temperatures, the reaction pressure should be about 6900 kPa (1,000 psig) in a continuous stirred tank reactor. This produces about 20 per cent conversion of ethylene and limits the production of high molecular weight polyethylene to less than about 0.1 weight per cent of the product. In a tubular reactor, conversions of 65 to 80 per cent of ethylene at about 120 * to 250"C can be achieved at pressures of about 20700 kPa (3,000 psig), depending upon the exact configuration of the reactor. The amount of catalyst used is conveniently expressed as the weight ratio of the ethylene feed to the zirconium in the catalyst. Generally, from about 10,000 to 120,000 parts by weight of ethylene are used per part by weight of zirconium in the catalyst, the preferred amount being from 25,000 to 35,000 parts by weight of ethylene per part by weight of zirconium and most preferably about 31,000 parts by weight of ethylene. As already noted, the amount of water present in the reaction system should be reduced as far as possible since the catalyst is particularly sensitive to water. Small amounts of water tend to increase the production of the undesired high molecular weight polyethylene and reduce conversion to the desired linear alpha-olefins. The relative amounts of the two catalyst components used in the process of the invention can be varied, but a mol ratio of the second component to the first component from about 1 : 1 up to about 50 : 1 is generally used, the preferred ratio being from about 10 : l to about 25 : 1. During the reaction the mol ratio of the ethylene feed to the oligomerisation product should be maintained at about 0.8 in order to minimize copolymerisation reactions which might interfere with the achievement of the desired high degree of linearity of the product. Preferably this ratio is greater than 2. The linear alpha-olefin oligomerisation product may be isolated by conventional procedures, e.g. use of an aqueous caustic catalyst quench followed by water washing and recovery of the final product by distillation. The invention is illustrated by the following Examples: EXAMPLES A series of ethylene oligomerizations were conducted in a 1-liter stirred autoclave at the temperatures and pressures indicated in the Table below. Reactor volume was controlled at about 500 cc by a dip leg which served as the reactor exit. The autoclave was electrically heated and oil cooled. Pressure and temperature were automatically controlled. Polymer grade ethylene was compressed to about 200 barg from a bank of 60 barg cylinders. After compression, the ethylene gas was passed over a bed of molecular sieves to remove water to less than 1 ppmv. The moisture content was monitored continuously using aluminium oxide sensors. The oxygen content of the ethylene was also continuously monitored using a on-line oxygen meter and was less than 3ppmv. Ethylene was fed continuously at a measured rate to the reactor during the test runs. Reaction solvent was dried over molecular sieves to less than 1 ppmw and then etered continuously into the reactor. Catalyst and co-catalyst solutions were prepared in a dry box using heated and evacuated glassware to ensure minimum water contamination. The zirconium catalyst was diluted in dry solvent (solvent dried to less than 1 ppmw over molecular sieves) to a concentration of about 20 x 10~ 6 gram moles of zirconium per gram of solution. The solutions were then transferred to the reactor feed tanks and held under a nitrogen blanket. The Zr catalyst solution was fed to the reactor at 10 to 100 cc/hr. The aluminium co-catalyst solutions were prepared from concentrated stock solutions obtained from a commercial supplier. Again, dilution solvent was dried to less than 1 ppmw water content before use. Co- catalyst was generally diluted to about 200 x 10" 6 gram moles of aluminium per gram of solution. The diluted solution was transferred to the reactor at 10 to 100 cc/hr. A test run was started by feeding solvent, ethylene, and co-catalyst to the reactor during a heat-up period lasting up to several hours. Then, the Zr catalyst feed was started. A run balance period for data collection was started after steady state was achieved, generally 1-2 hours after the oligomerization was initiated, as noted by the reaction temperature. The catalyst used was a equimolar complex of Zirconium tetrachloride and isodecyl acetate isomers (Exxate R 1000 of Exxon Chemical Company) and aluminum diethyl chloride. The ethylene solvent ratio was 1:1 by weight. The following Table shows for for 25 runs carried out in the absence of added oxygen: the reaction temperature, the reaction pressure in bars gauge, the aluminum: zirconium molar ratio, the weight percent conversion of ethylene into oligomers, the catalyst activity in terms of kilograms of oligomer product per gram of Zirconium in the catalyst per hour, and the number average molecular weight of the oligomeric product. T A B E 1 RUN T P Al/Zr Conv. Catalyst Activity NO "C barα molar wt % Kα/α Zr/h Hn of product 1 172 187 13.8 77 225 102 2 171 188 14.8 52 168 102 3 171 188 14.8 68 184 103 4 171 188 14.3 40 158 100 5 171 187 12.7 73 177 105 6 171 186 13.9 82 152 114 7 172 187 16.3 84 182 110 8 171 185 18.8 55 183 100 9 171 186 17.8 74 193 102 10 171 189 16.0 77 189 104 11 171 189 13.6 69 182 105 12 171 188 13.6 82 187 107 13 170 188 12.3 85 171 113 14 170 185 14.0 62 160 109 15 171 190 14.9 74 199 107 16 170 187 12.2 83 179 112 17 175 188 14.6 80 168 110 18 171 188 13.0 85 177 116 19 171 189 13.7 82 164 113 20 171 186.5 11.6 73.5 169 110 21 171 188 12.2 66.5 141 107 22 171 190 12.3 87 165 112 23 171 188 10.8 80.5 151 110 24 171 188.5 11.5 72.5 159 110 25 171 188.5 11.1 70.0 147 112 For each run the proportions of linear alpha-olefins, linear internal olefins, and branched olefins were determined. The proportions of branched olefins obtained in each run are shown in the following Table: TABLE 2 FRACTION OF BRANCHED ISOMERS, WT% based on total oligomers of the indicated carbon number In the further series of experiments oxygen was added to the reaction mixture in a proportion of 37, 21 or 23.5 parts per million by volume based on the ethylene. The reaction conditions were otherwise identical except that the Zr and Al catalyst concentration had to be doubled in the case of 37 ppm by volume of oxygen and had to be increased by 30% in the case of 21 or 23.5 ppm by volume of oxygen at achieve comparable conversion levels. This oxygen was deliberately added to ethylene feed and continuously monitored during the runs. The following Table shows, for each run, the proportion of added oxygen, the reaction temperature, the pressure in bars gauge, the mol ratio of aluminium to zirconium in the catalyst, the weight percentage conversion of the ethylene feed, the activity of the catalyst in kilograms of oligomeric product per gram of zirconium per hour, and the number average molecular weight of the oligomers obtained: The data for Run no. 26 are included for comparison. They show that, in the presence of 37 ppmw of oxygen, if too small an amount of the catalyst is used, the percentage conversion becomes very small. The following Table shows the proportions of branched isomers obtained for each of runs 27 to 33: TABLE 4 FRACTION OF BRANCHED ISOMERS. WT% bas b In general, the proportion of linear olefins obtained decreases and that of branched olefins obtained increases as the conversion rate of the ethylene increases. The amount of internal linear olefins which can be calculated by subtracting the total amount of alpha and branched isomers of a given carbon number from 100, remains the same with or without oxygen being present. For any particular conversion rate, the addition of oxygen to the reaction mixture in accordance with the present invention increases the alpha linearity of the oligomers obtained. This is illustrated in the accompanying -awings. Figures 1 and 2 show the weight percentage of linear alpha-olefins plotted against weight percentage conversion of the ethylene feed and: Figures 3 and 4 show the weight percentage of branched olefins plotted against the weight percentage conversion of the ethylene feed. In each Figure the solid lines represent the weight percentage of linear (Figures 1 and 2) or branched (Figures 3 and 4) olefins to be expected in the absence of oxygen. The plotted points show the results obtained in the presence of oxygen as given above in Tables 3 and 4. Figures 1 and 3 relates to oligomers having 8, 12 or 18 carbon atoms and Figures 2 and 4 relates to oligomers having 6,10, 14 and 16 carbon atoms. In each case the increased linearity or reduction in branching achieved by the presence of the oxygen is clearly shown.

Claims: CLAIMS 1. Process for a polymerizing ethylene to form a mixture of substantially linear alpha-olefin oligomers having a degree of polymerization from about 3 to 30 which comprises contacting ethylene at a temperature of 50 to 250"C and at a pressure of 3450 to 34500 kPa (500 to 5,000 psig) with a solution in an inert organic solvent of a two component catalyst in which the first component is an adduct of ZrCl a Brj-,, wherein each of a and b is 0, 1, 2, 3 or 4 and a + b = 4 with an organic compound of up to 30 carbon atoms selected from the group consisting of esters, ketones, ethers, amines, nitriles, anhydrides, acid chlorides, amides and aldehydes, and the second component is an alkyl metal component selected from the group consisting of R2AIX, RA1X 2 , 3AI2X3, R 3 AI, and R 2 Zn wherein R is alkyl or 1 to 20 carbon atoms and X is Cl or Br, the oligomerisation being conducted in the presence of 10 to 50 ppm of oxygen by volume based on the ethylene. 2. Process according to claim 1 in which the proportion of oxygen is 20 to 40 ppm by volume based on the ethylene. 3. Process according to claim 1 or 2 wherein the weight ratio of ethylene to zirconium is about 10,000 : 1 to 120,000 : 1. 4. Process according to claim 3 wherein the said ratio is from 25,000 : 1 to 35,000 : 1. 5. Process according to any one of the preceding claims wherein, in the said first component, the said organic compound is an ester of the formula RICOOR2 wherein each of Rl and R2 is alkyl, aryl, alkaryl, aralkyl and Rl may also be hydrogen, the total number of carbon atoms in Rl and R2 being from 1 to 30. 6. Process according to any one of claims l to 4 wherein the said first component is an adduct of ZrCl 4 and an acetate ester of the formula CH3COO 1 wherein Rl is alkyl or 6 to 16 carbon atoms. 7. Process according to claim 6 wherein the said acetate ester is a mixture of isomers of isodecyl acetate. 8. Process according to any one of the preceding claims wherein the said inert solvent is toluene, xylene, or an alkane of 3 to 24 carbon atoms. 9. Process according to any one of the preceding claims wherein the second catalyst component is diethyl aluminium chloride or ethyl aluminium dichloride or a mixture thereof. 10. Process according to any one of the preceding claims wherein the oligomerization is effected at a temperature of 120 to 250 β C and under a pressure of 6900 to 24100 kPa (1,000 to 3,500 psig).

Inventors: MATHYS GEORGES MARIE KAREL, MATHYS, GEORGES, MARIE, KAREL
Applicants: EXXON CHEMICAL LTD, EXXON CHEMICAL LIMITED
Application year: 1990
Main classification: C07C2
IPC codes: B01J31, C07B61, C07C2, C07C11, C08F4, C08F10