A METHOD OF PRODUCING l-BUTENE BY
DIMERIZATION OF ETHYLENE
This invention relates to a novel method of producing l-butene by dimerization of ethylene and to a S novel group of catalysts for use in such a method.
These catalysts are valuable because they possess high selectivity in producing l-butene from ethylene under mild conditions and in the absence of an additional catalyst modifier. The catalysts retain their high activity and high selectivity independent of whether they are used in a homogenous phase reaction or in a heterogeneous phase reaction during the method of this invention.
It is known that the low selectivity of tran-sition metal based catalysts for use in the dimeriza-iion of ethylene has been improved by the addition of various catalyst modifiers that generally have electron donating properties, for example organophosphorus com-pounds. By such use of modifiea catalysts, the formation of unwanted isomeric olefins or high molecular weight products has been reduced but the catalysts have become more complex and small changes in the catalyst compo-sition have resulted in substantial effects on catalyst productivity and selectivitv U.S. patent No. 3,686,350 issued August 22, 1972 to Yamada et al. describes the dimerization of an ~-olefin in the presence of a ternary catalyst consisting of a) an alkylaluminum compound, b) a tetra-alkyl titanate or tetra-aryltitanate of the formula Ti(O-alkyl)4 or Ti(O-Ar)4 and c) an organo phosphorus compound which is a phosphite or phosphine derivative.
U.S. patent No. 3,911,042 issued October 7, 1975 to Belov et al. describes the dimerization of ethylene to l-butene in the presence of a complex organometallic catalyst consisting of a titanium lZ98829 alcoholate of the formula Ti(OR)4 and an alkylaluminum compound of the formula AlR"2R' wherein R is alkyl of 2 to 4 carbon atoms, R' is R or H and R" is the same as R
in the presence of a specified solvent.
U.S. patent No. 4,101,600 issued July 18, 1978 to Zhukov et al. describes the dimerization of an ~-olefin by means of a two component catalyst consisting of Ti(OR)4 and AlR3 under specified reaction conditions whereby the reactants are pretreated with an ethylene-hydrogen mixture.
U.S. patent No. 4,245,131 issued January 13, 1981 to Schrock describes the preparation of l-butene from a C2 to C4 olefin by use of a tantalum or niobium catalyst.
We have now found, and herein lies our inven-tion, that a novel group of catalysts can be used for the dimerization of ethylene to l-butene and such catalysts possess high selectivity and a high reaction rate at, or close to, room temperature.
According to our invention, as claimed herein, we provide a method of producing l-butene by dimerization of ethylene in the presence of a catalyst, said catalyst being a mixture of a) an organometallic compound of the formula Am I
acac--M--Bn 5 wherein A is an alkoxy or aryloxy group;
acac is the 2,4-pentadionato anion of the formula (CH3COCHCOCH3) ;
B is A or acac, or a bidentate ligand;
m is 0, 1, 2 or 3;
i29~3829 n is 0, 1 or 2; and M is titanium, zirconium or niobium; and b) an alkylaluminum compound of the formula:
AlRxXyZz wherein x, y and z each is 0, 1, 2 or 3;
R is an alkyl group containing from one to six carbon atoms;
X is R or H; and Z is R or halogen or alkoxy group -OR.
The metal M is preferably titanium and the alkoxy group A is preferably ethoxy, propoxy or butoxy, particularly n-butoxy. The substituent B may be a bidentate ligand such as a carboxylate anion or a ~-diketonato anion. When M is titanium and n is 1 and m is 2, A is preferably alkoxy, particularly n-butoxy, and B is preferably acac. A preferred organometallic compound is Ti(n-OBu)2(acac)2.
The alkylaluminum compound preferably contains at least one alkyl group, such as methyl, ethyl, propyl or butyl, and especially two or three of such alkyl groups. A preferred alkyl group is ethyl and a parti-cularly valuable alkylaluminum compound is Al(C2H5)3.
The substituent Z may be halogen, such as chlorine, bromine or iodine, or an alkoxy group, such as ethoxy, propoxy or butoxy. Suitable alkylaluminum compounds of this type may be diethylaluminum ethoxide, ethylalu-minum dichloride and ethylaluminum sesquichloride.
A particularly preferred catalyst is a mixture of Ti(n-OBu)2(acac)2 and Al(C2H5)3. This mixture may be used in the molar proportion of Ti(n-OBu)2(acac)2 to n-Al(C2H5)3 wherein n is within the range of from about 1 to about 200, more particularly in the range of from about 1 to about 15, and especially in the range of from about 2 to about 6. About 3 is most preferred for n.
i298829 Thus a particularly valuable catalyst is a mixture of Ti(n-OBu)2(acac)2 and n-Al(C2~I5)3 wherein n is from about 2 to about: 6 and especially where n is about 3.
The method may be conveniently carried out in the presence of an inert organic solvent, especially a hydrocar-bon solvent such as iso-pentane, n-pentane, n-hexane, n-heptane, n-octane, decane, dodecane, an isomer of any of the foregoing, benzene, toluene, xylene or ethylbenzene, or a mixture of two or more of such solvents. The preferred sol-vent is toluene or n-hexane.
The method of dimerization of this invention may be carried out at a temperature of from about -10C to about 100C but a preferred temperature range is from about 0C to about 50C and a temperature of choice is from about 10C to about 30C.
The dimerization reaction rate depends upon the ethylene concentration and thus a higher ethylene partial pressure in the dimerization reactor favours the formation of l-butene. The dimerization may take place within a broad range of ethylene partial pressure from about atmos-pheric pressure up to an elevated pressure of about 8 MPa.
A preferred ethylene partial pressure ranges from about 0.2 MPa to about 3 MPa.
The concentration o~ organometallic compound, par-ticularly a titanium compound, in the reaction medium has aneffect upon the rate of reaction in that an increase in the concentration of said titanium compound in the solvent med-ium increases the overall consumption of ethylene and thus produces more l-butene over a given time period. However, the dimerization reaction is a strongly exothermic reaction and with higher catalyst concentrations, or when reaction occurs under higher ethylene pressure, there may be some dif-ficulty in controlling the resultant reaction temperature. It is therefore recommended that the concentration of catalyst ~Z98829 used in the dimerization be such that the organometallic compound is within the range of from about 10 6 to about 10 1 mole of titanium per litre of solvent, and pre-ferably from about 10 5 to about 10 2 mole of titanium per litre of solvent. It is to be understood however that higher or lower concentrations outside these ranges may be used, depending upon the available equipment for regulating such an exothermic reaction.
When the catalyst is to be used in the presence of an inert organic solvent, such as a hydrocabon sol-vent, for example toluene or n-hexane, it may be pre-pared in different ways according to equipment available.
Thus, the catalyst may be prepared directly in the dimerization reactor by blending the organometallic compound and the alkylaluminum compound in the organic solvent in the reactor and then introducing ethylene thereto. Alternatively, the two compounds may be mixed and the catalyst thus prepared outside the reactor may be added to the reactor containing the desired solvent, or the catalyst solution in the solvent may be prepared outside the reactor and then be added to the reactor.
Another possibility is to add part of the alkylaluminum compound to the reactor and mix the remainder of the alkylaluminum compound with the organometallic compound and then add that mixture to the reactor. It is pre-ferred to add the solvent and alkylaluminum compound to the reactor first and thereafter add the organometallic compound.
The catalyst used in the method of this invention may alternatively be deposited on an inert solid support. Such a solid support may be for example, silica, alumina, silica*alumina (a natural or synthetic gel containing both oxides or natural aluminosilicates), magnesium oxide, titanium dioxide, calcium oxide, a natural zeolite, a synthetic zeolite, a synthetic resin or a macromolecular compound.
~29aa~9 The catalyst used for dimerization may be deposited on an inert solid support and the reaction may then be carried out either in a solvent medium with the supported catalyst suspended therein, or in the gas phase.
The reaction may be a single batch reaction or it may be a continuous flow system.
The catalyst used for dimerization may be deposited on an inert solid support and the reaction may then be carried out in a reactor wherein the sup-ported catalyst is in a bed, such as a fixed bed, a fluidized bed or a trickle bed.
A preferred method for the dimerization of ethylene to l-butene in the presence of a catalyst is the use of a catalyst being a mixture of a) an organometallic compound of the formula:
Ti(n-oBu)2(acac)2 wherein acac is the 2,4-pentadionato anion of the formula (CH3COCHCOCH3) and b) an alkylaluminum compound of the formula Al(C2H5)3 in the presence of an inert organic hydro-carbon solvent such as toluene or n-hexane preferably at a reaction temperature from about 10C to about 30C and wherein the catalyst is used in the molar proportion of Ti(n-OBu)2(acac)2 to Al(C2H5)3 in the range of from about 1:1 to about 1:15.
Such a method is conveniently carried out wherein the amount of the organometallic compound in the solvent is in the range of from about 10 5 to about 10 mole of titanium per litre and wherein the absolute pressure in the dimerization reactor during the dimeri-zation is from about 500 KPa to about 3 MPa.
A method of choice for the production of 1-butene comprises the dimerization of ethylene in the presence of a catalyst, the catalyst being a mixture of a) an organometallic compound of the formula:
Ti(n-OBu)2(acac)2 wherein acac is the 2,4-pentadionato anion of the formula (CH3COCHCOCH3) ; and b) an alkylaluminum compound of the formula Al(C2H5)3 the catalyst being used in the molar proportion of Ti(n-OBu)2-(acac)2 to n Al(C2H5)3 wherein n is in the range of from about 2 to about 6, preferably about 3, and wherein the di-merization is carried out in the presence of toluene or n-hexane as solvent and at a temperature of from about 10C
to about 30C, the organometallic compound being present in the solvent at a concentration in the range of from about 10 5 to about 10 2 mole of titanium per litre and the abso-lute pressure in the dimerization reactor is from about 500 KPa to about 3 MPa.
As a further feature of the invention we provide a novel catalyst which is a mixture of a) an organometallic compound of the formula:
Am acac--M--Bn wherein A is an alkoxy or aryloxy group;
acac is the 2,4-pentadionato anion of the formula (CH3COCHCOCH3) ;
B is A or acac, or a bidentate ligand;
m is 0, 1, 2 or 3;
n is 0, 1 or 2; and M is titanium, zirconium or niobium; and b) an alkylaluminum compound of the formula:
AlRxXyZz wherein x, y and z, each is 0, 1, 2 or 3;
R is an alkylgroup containing from one to six carbon atoms;
X is R or H; and Z is R or halogen or an alkoxy group -OR.
The metal M is preferably titanium, A is preferably alkoxy r B is preferably acac and m and n are preferably 2 and 1, respectively.
The preferred catalyst ls a mixture of Ti(n-OBu)2-(acac)2 and Al(alkyl)3 wherein alkyl is preferably ethyl. A
catalyst of choice is Ti(n-oBu)2(acac)2~n~Al(c2H5)3 wherein n is in the range of from 1 to 15, preferably from 2 to 6, and especially about 3.
It is recommended that the solvent to be used in the reaction and the organometallic compound be degassed and stored under an inert atmosphere in order to avoid an unneces-sary increase in the use of the alkylaluminum compound. The organometallic compound is sensitive to air and moisture and thus this compound, and the catalyst prepared therefrom, is liable to decompose in the presence of air and moisture.
The invention is illustrated by, but not limited by, the following Examples.
Example 1 A stainless steel, jacketed reactor (450 ml capa-city) equipped with an internal cooling coil and a stirrer (800 rpm) is evacuated, under vacuum, and then purged with purified nitrogen. There is then added to the reactor, a solution of 100 ml purified solvent (in this case, toluene) containing 0.19 g (1.67 mmole) of triethylaluminum resulting in an aluminum concentration of 1.67 x 10 mol/litre in the reactor. After five minutes, a toluene solution, containing 0.2 g of Ti(n-OBu)2(acac)2 is added, to provide an aluminum to titanium molar ratio equal to 3.21:1. The Ti(n-OBu)2-(acac)2 is a commercial product supplied by Strem Chemicals under the name titanium (di-n-butoxy)bis(acetyloacetonate).
The reactor contents are cooled to 10C and the reactor is then pressurized with ethylene to a total pressure of 2 MPa.
The reaction temperature is maintained at 10~C in the reac-tor using a temperature regulator and a cooling medium flow-ing through the jacket and the internal cooling coil.
1298~329 The dimerization reaction is run for 30 minutes at a constant temperature and at constant pressure, which is maintained by an on-line pressure regulator. The flow of ethylene to the reactor duing the dimerization reaction is monitored by a mass flow meter with digital display.
The reaction run is terminated by stopping the ethylene flow and quickly depressurizing the reactor, collecting all the gaseous products in the gas tank. The total gas volume and exact gas composition is determined for the gas mixture from the tank. The composition of the liquid phase in the reactor is analyzed by GC after adding a small amount of diluted hydrochloric acid. After two-phase separation, the solvent containing phase is evaporated to determine whether or not any polymeric material has been formed and the amount, if any.
During this run, no polyethylene was formed.
The yield of l-butene was 6.1 kg per gram of Ti per hour.
Selectivity of the reaction was 92.8~; n-butane and C6 hydrocarbons were formed as byproducts.
Example 2 By using the same reactants and following the procedure outlined in Example 1, the dimerization of ethylene is carried out at 24C instead of 10C. The Ti concentration is 5.2 x 10 3 mol/litre and the alumi-num to titanium molar ratio is 3.21`1 After a 30 minutes run, the yield is 17.34 kg of l-butene per gram of Ti per hour. The selectivity of the dimerization of ethy-lene to 1-butene is 99.5%; the remaining .5% is a result of n-butane formation.
Examples 3 and 4 By following the procedure outlined in Example 1, dimerization was carried out at two different temp-eratures. The results can be compared with those of Examples 1 and 2.
~298829 Example ~ Ti conc. Al:TiPolyethylene Yield g of Selectivity molar l-butene/g C mmol/l ratio g/l f Ti per hr %
3 50 6.9 3.21 2.7 1470 47.4 4 8~ 6.9 3.21 2.2 1404 49.1 It is noted that, under constant pressure and con-stant Al:Ti ratio, the dimerization reaction is temperature sensitive. The dimerization reaction yield and selectivity decreases when the reaction temperature increases above 25C.
5 However, only a slight formation of polyethylene was observed at these higher temperatures.
Examples 5 to 8 By following the procedure outlined in Example 1, Examples 5 to 8 show the influence of alkylaluminum compound 10 concentration, the nature of the solvent, and the variation of reaction temperature on ethylene dimerization reaction.
Example Ten~p Ti conc. Al:Ti Polyethylene Yield g of Selectivity Solvent molar l-butene g C ~mol/l ratio g/lf Ti per hr %
6.9 6.42 0 14373 76.9 Toluene 6 50 6.9 6.44 22.9 4715 34.4 Toluene 7 80 6.9 6.44 19.5 1468 16.4 Toluene 8 50 6.9 5.08 11.2 9785 69.6 Hexane It is noted that a higher Al:Ti ratio accelerates all possible reactions. Even at 10C, a high ethylene con-version to l-butene is possible but, at the same t~me, the 15 selectivity of the dimerization is lower than in the case of a reaction carried out with a lower alkylaluminum compound concentration. Further increases in the reaction temperature significantly decreases yield and selectivity. The dimeriza-tion reaction medium has an influence on the selectivity and 20 l-butene yield. When n-hexane is used the selectivity and reaction yield increase compared to those obtained when using toluene as the solvent.
Examples 9 to 15 The following Examples show the effect of the cata-25 lyst procedure, the influence of the nature of the alkylalu-minum compound, the addition of a catalyst modifier (an elec-tron donor), tetrahydrofuran (THF), and the reaction tempera-ture.
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iZ988Z9 In Examples 9 to 15, the catalytic mixture was first prepared outside the pressure reactor. The reactor was filled with the required amount of solvent (toluene or n-hexane) and half of the total amount of alkylaluminum compound. The cata-lyst was then prepared by mixing an additional 5 ml of toluenewith a corresponding amount of the titanium compound and, if required, with a corresponding amount of tetrahydrofuran for five minutes. The remaining alkylaluminum compound was then added to this solution. The solution changed colour from light orange to dark blue and was then transferred to the re-actor.
Under standard conditions at 25C, the most effect-ive alkylaluminum compound is triethylaluminum. With an in-crease in the number of chlorine atoms in the alkylaluminum compound, the l-butene yield decreases and ethylene polymer-ization plays the dominant role. Adding tetrahydrofuran as the electron donor decreases conversion of ethylene to 1-butene. This effect is observed under the entire temperature range.