JPH04366106A - Preparation of poly(alpha-olefin) - Google Patents

Abstract

PURPOSE:To effectively produce a poly(alpha-olefin) by using a catalyst comprising a specific transition metal compound and a promoter to polymerize an alpha-olefin. CONSTITUTION:An alpha-olefin is polymerized using a catalyst which comprises a transition metal compound of the formula (wherein A<1> and A<2> each is cyclopentadienyl, indenyl, or fluorenyl; A<3> and A<4> each is a 6-20C halogenated aryl, a 1-10C alkyl, a 6-20C aryl, an alkylaryl, or H, provided that at least either of A<3> and A<4> is a halogenated aryl; R<1> and R<2> each is a halogen, H, a 1-10C alkyl, or an aryl; M is Ti, Zr, or Hf; and Q is a compound connecting A<1> to A<2> and containing C, Si, Ge, or Sn) (e.g. methyl (4-fluorophenyl) methylenecyclopentadienylfluorenylzirconium dichloride) and a promoter (e.g., aluminoxane). Thus, a poly(alpha-olefin) can be effectively produced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing poly-α-olefins. Specifically, the present invention relates to a method for producing poly-α-olefins in the presence of a catalyst consisting of a specific transition metal compound and a co-catalyst.

0002

[Prior Art] Transition metal compounds, so-called metallocene compounds, having a cyclopentadienyl group, indenyl group, fluorenyl group, or a derivative thereof as a ligand are used together with a cocatalyst such as aluminoxane to polymerize α-olefins. It is known that poly-α-olefins can be produced by this method.

JP-A-58-19309 discloses (cyclopentadienyl) 2 MeRHal (where R is cyclopentadienyl, C1 to C6 alkyl, halogen, Me is a transition metal, A method is described in which ethylene and/or α-olefin are polymerized or copolymerized in the presence of a catalyst consisting of a transition metal compound represented by (Hal is halogen) and aluminoxane.

[0004] JP-A-60-35008 describes that poly-α-olefins having a wide molecular weight distribution can be produced by using a catalyst consisting of at least two metallocene compounds and aluminoxane.

JP-A-61-130314 describes a method for producing polyolefins using a catalyst consisting of a sterically fixed zircon chelate compound and aluminoxane. The publication also describes a method for producing polyolefins with a high degree of isotacticity by using ethylene-bis-(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride as a transition metal compound. Are listed.

[0006] JP-A-64-66124 discloses a catalyst for producing stereoregular olefin polymers containing a transition metal compound having a silicon-crosslinked cyclopentadienyl compound as a ligand and aluminoxane as an active ingredient. has been done. In JP-A-2-41303, the following formula (Chemical formula 2)

[0007]

[Chemical formula 2]R”(CpRn)(CpR'm)Me
Qk (wherein each Cp is a cyclopentadienyl or substituted cyclopentadienyl ring; each Rn may be the same or different and is a hydrocarbyl residue having 1 to 20 carbon atoms; each R' m may be the same or different and are hydrocarbyl residues having 1 to 20 carbon atoms; R'' is C which provides steric rigidity to the catalyst.
is a structural bridge between the p rings; Me is a metal of group 4b, 5b, or 6b of the periodic table of the elements; each Q is 1 to 20
is a hydrocarbyl residue or halogen having a carbon atom; 0≦k≦3: 0≦n≦4: and 1≦m≦4;
and R'm are selected such that (CpR'm ) is sterically different from (CpRn ). It is described that a poly-α-olefin with good syndiotacticity can be produced by using a catalyst containing as one component.

[0008] The same publication also describes that syndiotactic poly-α-olefins having a wide molecular weight distribution can be produced by using two or more of the above metallocene compounds. In JP-A-2-274703, the following formula (Chemical formula 3)

[0009]

[In the formula, M1 is titanium, zirconium, vanadium, niobium or tantalum, and R1 and R2
may be the same or different from each other, hydrogen atom, halogen atom, alkyl group having 1 to 10 carbon atoms, alkoxy group having 1 to 10 carbon atoms, aryl group having 6 to 20 carbon atoms, 6 carbon atoms -20 aryloxy group, C2-10 alkenyl group, C7-40 arylalkyl group, C7-40 alkylaryl group or C8-40 arylalkenyl group R3 and R4 are different and mean mono- or polynuclear hydrocarbon radicals capable of forming a sandwich structure together with the central atom M1, and R5 is

0010

[Chemical formula 4] =BR6, =AlR6, -Ge-, -Sn-, -O
-, -S-, =SO, =SO, =NR6, =CO,
=PR6 or =P(O)R6, in which case R
6, R7 and R8 may be the same or different and each represents a hydrogen atom, a halogen atom, or a carbon atom number of 1 to 1.
0 alkyl group, fluoroalkyl group having 1 to 10 carbon atoms, fluoroaryl group having 6 to 10 carbon atoms, aryl group having 6 to 20 carbon atoms, alkoxy group having 1 to 10 carbon atoms, number of carbon atoms Alkenyl group having 2 to 10 carbon atoms, arylalkyl group having 7 to 40 carbon atoms, 8 to 4 carbon atoms
40 arylalkenyl groups or 7 to 40 carbon atoms
or R6 and R7 or R6 and R8 respectively take together with their bonding atoms to form a ring, and M2
is silicon, germanium or tin. A method for producing a high molecular weight syndiotactic polyolefin by polymerizing an olefin in the presence of a catalyst consisting of a transition metal component represented by ] and aluminoxane is described.

JP-A-2-274704 describes a method for producing a high molecular weight syndiotactic polyolefin using a similar hafnium compound. However, in these publications, zirconium or Only hafnium compounds are described, and there is no description at all about the synthesis and physical properties of the transition metal compound containing a halogen atom used in the present invention.

On the other hand, the active species of the so-called Kaminsky type catalyst as described above is [Cp'2MR]+ (where Cp'
= cyclopentadienyl derivative, M = Ti, Zr, H
Since it was suggested that transition metal cations such as those represented by f, R = alkyl), some catalyst systems that do not use aluminoxanes as promoters have been reported.

[0013] Taube et al. Organom
etall. Chem. , 347, C9 (1
988) to [Cp2 TiMe (THF)] + [B
Ph4]- (Me=methyl group, Ph=phenyl group)
Ethylene polymerization has been successfully carried out using the compound represented by

[0014] Jordan et al. Am. Che
m. Soc. , 109, 4111 (1987)
and [Cp2 ZrR(L)]+ (R=methyl group,
Zirconium complexes such as benzyl group, L = Lewis base) have been shown to polymerize ethylene.

[0015] Special Publication Publication No. 1-501950, Special Publication No. 1-501950
JP-A-502036 describes a method for polymerizing olefins using a catalyst comprising a cyclopentadienyl metal compound and an ionic compound capable of stabilizing the cyclopentadienyl metal cation.

[0016] Zambelli et al.
Ecules, 22, 2186 (1989) reports that isotactic polypropylene can be produced using a catalyst that combines a zirconium compound having a cyclopentadiene derivative as a ligand, trimethylaluminum, and fluorodimethylaluminum.

[0017]

[Problems to be Solved by the Invention] In the so-called Kaminsky-type catalyst as described above, the steric structure (stereoregularity) and molecular weight of the resulting polymer are determined by the type of transition metal compound component, and the steric structure is suitable for various uses. Synthesis of various transition metal compounds is desired to produce poly-α-olefins with (stereoregularity) and molecular weight.

On the other hand, the above-mentioned catalyst system that does not use aluminoxane has a problem that the polymerization activity is low and that a dialkyl complex, which is difficult to synthesize, must be used as the metallocene compound.

[0019]

[Means for Solving the Problems] In consideration of the above problems, the present inventors have conducted intensive studies on effective transition metal compound components of poly-α-olefins, and have found that a specific transition metal compound containing a halogen atom is It was discovered that the present invention is effective as a catalyst component for producing poly-α-olefins, and the present invention was completed.

That is, the present invention provides a method for producing poly-α-olefin by polymerizing α-olefin, including: A)
General formula [I] (Chemical formula 5)

[0021]

embedded image (where A1 and A2 are cyclopentadienyl groups,
Indicates an indenyl group, a fluorenyl group, or a derivative thereof. A3 and A4 represent a halogenated aryl group having 6 to 20 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms, an alkylaryl group, or a hydrogen atom; At least one of them is a halogenated aryl group. R1, R
2 represents a halogen atom, a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or an aryl group. M is titanium, zirconium, or hafnium. Q is a compound containing carbon or silicon, germanium, or tin that connects A1 and A2. A method for producing a poly-α-olefin, characterized by using a catalyst consisting of a transition metal compound represented by B) and a co-catalyst.

Further, the present invention provides B) the method for producing the poly-α-olefin characterized in that aluminoxane is used as a co-catalyst component, and B) a) an organometallic compound, and b) a transition metal as a co-catalyst component. The method for producing poly-α-olefin described above is characterized by using a compound that reacts with a compound to form an ionic compound.

The transition metal compound represented by the general formula [I] used in the present invention is a new compound, and is represented by the general formula [I].
I], A1 and A2 represent a cyclopentadienyl group, an indenyl group, a fluorenyl group, or a derivative thereof. By using specific compounds for A1 and A2, stereoregularity can be imparted to the resulting polymer. As such preferable examples, A1 and A2 are respectively a cyclopentadienyl group, a fluorenyl group; a cyclopentadienyl group, a substituted fluorenyl group; an indenyl group,
Indenyl group; 1-3 substituted cyclopentadienyl group, 1
A poly-α-olefin having a syndiotactic or isotactic structure can be produced by using a ligand such as a ˜3-substituted cyclopentadienyl group. A3 and A4 are halogenated aryl groups having 6 to 20 carbon atoms or alkyl groups having 1 to 10 carbon atoms, and 6 carbon atoms.
~20 aryl groups, alkylaryl groups, or hydrogen atoms, and at least one of A3 and A4 is a halogenated aryl group. Specific examples of A3 and A4 include a hydrogen atom, a methyl group, an ethyl group, a propyl group, a phenyl group, a tolyl group, a chlorophenyl group, a dichlorophenyl group, a fluorophenyl group, and a difluorophenyl group. Q is a compound containing carbon or silicon, germanium, or tin that connects A1 and A2, and is preferably a carbon atom. R1, R2
represents a halogen atom, a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or an aryl group, preferably a chlorine atom or a methyl group. M represents titanium, zirconium, or hafnium, preferably zirconium or hafnium.

Specific examples of the transition metal compound represented by the general formula [I] include methyl(4-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride, methyl(4-fluorophenyl)methylenebisinde Nylzirconium dichloride, methyl (
4-fluorophenyl)methylenebisdimethylcyclopentadienylzirconium dichloride, bis(4-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride, bis(4-fluorophenyl)methylenebisindenylzirconium dichloride, bis(4-fluorophenyl)methylenebisindenylzirconium dichloride, -fluorophenyl)methylenedimethylcyclopentadienylzirconium dichloride, methyl(4
-fluorophenyl)methylenecyclopentadienylfluorenylhafnium dichloride, methyl(4-fluorophenyl)methylenebisindenylhafnium dichloride, methyl(4-fluorophenyl)methylenebisdimethylcyclopentadienylhafnium dichloride, bis(4-fluorophenyl)methylenebisdimethylcyclopentadienylhafnium dichloride, Examples include fluorophenyl)methylenecyclopentadienylfluorenylhafnium dichloride, bis(4-fluorophenyl)methylenebisindenylhafnium dichloride, and bis(4-fluorophenyl)methylenedimethylcyclopentadienylhafnium dichloride.

The transition metal compound represented by the general formula [I] in the method of the present invention can be synthesized, for example, by the following route. HA1 Li + A3 A4 QX12──→
A3 A4 QX1 HA1


+ LiX1 HA2 Li + A3 A4
QX1 HA1 ────→ A3 A4 QH
A1 HA2

＋LiX1
A3 A4 QHA1 HA2 + 2n-Bu
Li ────→

A3 A4 QA1 A2 Li2 A3 A4
QA1 A2 Li2 + MX4 ────
→ A3 A4 QA1 A2 MX2


+ LiX2 or, especially when Q is a carbon atom, A3 A4 Q=A1 + HA2
Li + HCl ───→

A3 A4 QHA1 HA2 + LiCl
A3 A4 QHA1 HA2 + 2n-Bu
Li ────→

A3 A4 QA1 A2 Li2 A3
A4 QA1 A2 Li2 + MX4
───→ A3 A4 QA1 A2 MX2


+ LiX2 (where X is a halogen atom, M is titanium, zirconium or hafnium), and further the above A3 A4 QA1 A2
MX2 is RLi or RMgX (R is 1 to 10 carbon atoms
are an alkyl group and an aryl group. ) and other periodic table 1
By reacting with a metal alkyl compound of Group A or Group 2A of the Periodic Table, a compound in which at least one of X is substituted with R can be obtained.

[0026] In addition to known aluminoxanes, the co-catalyst used together with the transition metal compound during polymerization includes ionic compounds capable of forming transition metal cations, such as those described in Japanese Patent Publication No. Hei 1-501950, Omotehira 1-
Boron compounds such as those described in Japanese Patent No. 502036 can also be used as promoters. In addition, (a) organometallic compound and (b) which the present applicant previously applied for
Co-catalysts consisting of transition metal compounds and compounds capable of reacting to form ionic compounds can also be used.

As aluminoxanes, the general formula (Chemical formula 6) is used.

[0028]

It is a compound represented by [Chemical formula 6] (where R is a hydrocarbon group having 1 to 3 carbon atoms, and n is an integer of 2 or more), especially methylaluminoxane where R is a methyl group and n is 5 Of the above, preferably 10 or more are used. There is no problem even if a small amount of an alkyl aluminum compound is mixed in the above aluminoxanes.

[0029] In the present invention, the ratio of aluminoxane to the above transition metal compound is 10 to 10,000.
0 mole times, usually 50 to 10,000 times.

As the organometallic compound a) used in the present invention, a compound of a metal selected from aluminum, zinc, and magnesium is used. These organometallic compounds include halogen, oxygen, hydrogen, alkyl, alkoxy,
It has a residue such as aryl as a ligand, and these ligands may be the same or different, but at least one has an alkyl group. For example, alkyl metal compounds in which 1 to n alkyl residues having 1 to 12 carbon atoms are bonded, alkyl metal halides, alkyl metal alkoxides, and the like can be used. Among them, alkylaluminum compounds are preferably used, such as trimethylaluminum, triethylaluminum, triisopropylaluminium, tributylaluminum, dimethylaluminum chloride, diethylaluminum chloride, diisopropylaluminum chloride, diisopropylaluminium isopropoxide, ethylaluminum dichloride, ethylaluminum dichloride, and ethylaluminum dichloride. Examples include isopropoxide. The ratio of the organometallic compound to the transition metal compound used is 1 to 100,000 times by mole, usually 10 to 5,000 times by mole.

Compounds that react with the transition metal compound as component b) to form an ionic compound used in the present invention include compounds containing anions that can stabilize transition metal cations and electrophilic compounds. The following compounds can be mentioned. Examples of compounds containing anions that can stabilize transition metal cations include organic boron compound anions, organic arsenic compound anions, and organic aluminum compound anions. Preferably, the polymer does not inactivate the polymerization active species by binding to or strongly coordinating with the polymer. Examples of such suitable anions include the tetraphenyl boron anion by Taube, Jordan et al., and the tetrakis(pentafluorophenyl) boron anion described in Japanese Patent Publication No. 1-501950 and Japanese Patent Application Publication No. 1-502036. etc. can be mentioned.

The cation to form an ionic compound by pairing with these anions is not particularly limited as long as it does not react with the polymerization active species to inactivate it, and any cation that can pair with the above anions can be used. Known cations can be mentioned. Such cations include metal cations, organometallic cations, carbonium cations, trypium cations, oxonium cations, sulfonium cations, phosphonium cations, ammonium cations, and the like. More specifically, they include silver cation, dicyclopentadienyl iron cation, triphenylcarbenium cation, triphenylphosphonium cation, and tributylammonium cation.

[0033] Also, Marks et al.
r 4, 1212 (1988), electrophilic compounds such as metal oxides and metal halides that act as Lewis acids can stabilize transition metal cations, and in the present invention, this Such electrophilic compounds can be used as component b). Examples of such compounds include Al2O3,S
Examples include iO2, MgCl2, AlCl3, and the like. The ratio of the compound that reacts with the transition metal compound to form an ionic compound to the above transition metal compound is 0.1 to 10,000 times by mole, usually 0.5 to 5 times by mole.
000 moles.

What is important in the present invention is to bring a) a transition metal compound into contact with b) an organometallic compound, and then c) contact a compound that reacts with the transition metal compound to form an ionic compound. If this order is different, polymerization may not occur or
Even after polymerization, the activity is very low.

[0035] Transition metal catalyst components and/or
Alternatively, the co-catalyst can be used as is, such as SiO2, Al2O
3, MgCl2 or the like may be used by being supported on a known carrier supporting a Ziegler type catalyst.

There are no particular limitations on the polymerization method and polymerization conditions carried out in the method of the present invention, and known methods for polymerizing α-olefins may be used, such as solvent polymerization using an inert hydrocarbon medium or substantially Bulk polymerization methods and gas phase polymerization methods in which no inert hydrocarbon medium is present can also be used, and polymerization temperatures are generally from -100 to 200 DEG C. and polymerization pressures from normal pressure to 100 kg/cm@2. Preferably -50 to 100°C, normal pressure to 50 kg/cm2
It is.

Examples of the hydrocarbon medium used in the treatment or polymerization of the catalyst component in the present invention include butane, pentane, hexane, heptane, octane, nonane,
In addition to saturated hydrocarbons such as decane, cyclopentane, and cyclohexane, aromatic hydrocarbons such as benzene, toluene, and xylene can also be used.

The α-olefin used in the polymerization includes propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, 1-
Dodecene, 1-tetradecene, 1-hexadecene, 1-
Examples include α-olefins having 3 to 25 carbon atoms such as octadecene.

In the present invention, not only homopolymerization of α-olefins but also copolymers of ethylene or α-olefins having about 2 to 25 carbon atoms, such as propylene and ethylene or propylene and 1-butene, are carried out. It can also be used for

[0040]

[Synthesis of transition metal compounds]

Bis(p-fluorophenyl)fulvene 0.18mol
THF solution of cyclopentadienyl sodium 200
THF containing 38g difluorobenzophenone in ml
100 ml of the solution was added dropwise at room temperature. After stirring at room temperature for 2 hours, 500 ml of ice water and 300 ml of diethyl ether were added, the ether layer was washed with water, dehydrated with magnesium sulfate, and the solvent was distilled off under reduced pressure to obtain 41.6 g of an orange solid. By purifying this orange solid with column chromatography, 21.9 g of bis(p
-fluorophenyl) fulvene was obtained as red crystals. The elemental analysis values of this compound are shown below.

Calculated value (%) C: 81.20 H:
4.51 F: 14.29 Actual value (%)
C: 81.81 H: 4.63 F:
13.88Bis(p-fluorophenyl)cyclopentadienylfluorenylmethane 6.25 g of fluorene was dissolved in 100 ml of THF, and 37.6 mmol of methyllithium was added dropwise to this solution, followed by stirring at room temperature for 5 hours. 10 g of bis(p-fluorophenyl)fulvene synthesized above was added to the obtained red solution.
The mixture was added at 20°C and reacted overnight at room temperature. Add 3. to this reaction solution.
After adding 100 ml of 6% hydrochloric acid water and washing the organic layer with water and dehydrating it with magnesium sulfate, the solvent was distilled off under reduced pressure to obtain 15.4 g of red sticky substance. By purifying this sticky substance with column chromatography, 6.5 g of bis(
p-fluorophenyl)cyclopentadienylfluorenylmethane was obtained.

The elemental analysis values of this compound are shown below. Calculated value (%) C: 86.11 H:
5.09 F: 8.80 Actual value (%)
C: 86.02 H: 4.97 F: 8
．． 94bis(p-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride6.
5g of the above synthesized bis(p-fluorophenyl)cyclopentadienylfluorenylmethane was dissolved in 150ml of THF.
30 mmol of n-BuLi was added dropwise to this solution. After reacting overnight at room temperature, the solvent was distilled off under reduced pressure and the solid obtained was washed with pentane.
350 ml of a dichloromethane slurry containing g of zirconium tetrachloride was added at -78°C. After stirring at -78°C for 4 hours, the mixture was heated to room temperature and further reacted overnight. The reaction slurry was filtered and the filtrate was concentrated to approximately 50 ml. Adding 100 ml of pentane to this concentrate yields 1.8 g of bis(p-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride.The elemental analysis values of this compound are shown below. Calculated value (%) C: 62.83 H: 3.38 F:
6.42 Cl: 11.96 Actual value (%) C: 63.
44 H: 3.50 F: 6.14 Cl: 11
．． 32 [Polymerization] 1.0 mg of the above-synthesized bis(p-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride and methylaluminoxane manufactured by Tosoh Akzo Co., Ltd. (polymerization degree 17.7) 0.5 g was charged. After adding 0.65 liters of liquid propylene, the temperature was raised to 40°C, and polymerization was carried out at this temperature for 1 hour. The amount of syndiotactic polypropylene powder obtained was 24.6 g. The intrinsic viscosity (hereinafter abbreviated as [η]) of the powder measured in a tetralin solution at 135°C is 2.56 dl/g,
The syndiotactic pentad fraction determined from the peak intensity observed at about 20.2 ppm in 13C-NMR measurement was 0.873.

Comparative Example 1 1.0 mg of diphenylmethylenecyclopentadienylfluorenylzirconium dichloride described in JP-A-2-274703 as a transition metal compound component
Polymerization was carried out in the same manner as [Polymerization] in Example 1 except that . The amount of syndiotactic polypropylene powder obtained was 42.0 g. Powder [η] is 4.1
8 dl/g, syndiotactic pentad fraction 0.
It was 893. By carrying out the method of the present invention in this manner, poly-α-olefins having different physical properties can be produced.

Example 2 Example 1 was placed in a 1.0 liter autoclave which was sufficiently purged with nitrogen.
2.0 mg of bis(p-fluorophenyl)methylenecyclopentadienylfluorenylzirconium dichloride synthesized in step 1, 70 mg of triethylaluminum and 20 ml of toluene were added, and then 15 mg of triphenylcarbenium tetrakis(pentafluorophenyl)boron was added.
mg was added. After adding 0.65 l of liquid propylene 4
The temperature was raised to 0°C, and polymerization was carried out at this temperature for 1 hour. The obtained syndiotactic polypropylene powder was 38.
It was 7g. The powder [η] is 2.23 dl/g,
The syndiotactic pentad fraction was 0.861.

[0045]

Effects of the Invention By carrying out the method of the present invention, poly-α-olefins can be effectively produced and are of great industrial value.

Claims (3)

[Claims] Claim 1: Polymerization of α-olefin to produce poly-α-
In the method for producing an olefin, A) general formula [I]
(Chemical formula 1) [Chemical formula 1] (Here, A1 and A2 are cyclopentadienyl groups,
Indicates an indenyl group, a fluorenyl group, or a derivative thereof. A3 and A4 represent a halogenated aryl group having 6 to 20 carbon atoms, an alkyl group having 1 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms, an alkylaryl group, or a hydrogen atom; At least one of them is a halogenated aryl group. R1, R
2 represents a halogen atom, a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, or an aryl group. M is titanium, zirconium, or hafnium. Q is a compound containing carbon or silicon, germanium, or tin that connects A1 and A2. A method for producing a poly-α-olefin, characterized by using a catalyst consisting of a transition metal compound represented by B) and a co-catalyst. 2. The method for producing a poly-α-olefin according to claim 1, characterized in that B) aluminoxane is used as the promoter component. 3. B) The poly-α- according to claim 1, characterized in that a) an organometallic compound and b) a compound that reacts with a transition metal compound to form an ionic compound are used as co-catalyst components. Method for producing olefins.