Bridged PNPN ligand, and preparation method and application thereof

Abstract

The invention discloses a bridged PNPN ligand and a preparation method and application thereof, the ligand is simple to synthesize and only needs two steps, and a catalyst can be subjected to in-situ polymerization without advanced complexation. More prominently, when the catalyst is applied to ethylene oligomerization, the catalyst can adjust the product to be 1-octene or 1-hexene through the temperature. # imgabs0 #

Description

Technical Field

[0001] The present invention belongs to the technical field of ethylene oligomerization reaction, and particularly relates to a bridged PNPN ligand, a preparation method thereof and an application thereof. Background Art

[0002] With the rapid ramp-up of photovoltaic production capacity, the demand for photovoltaic POE is also becoming increasingly strong. As a key POE monomer, linear α-olefin has also become particularly scarce. Among them, POE products using 1-octene and 1-hexene as comonomers have good mechanical properties and processing properties, and there is a large demand in industrial production.

[0003] With the continuous development of catalysts, the activity of the catalysts has been improved, but the synthesis route is getting longer and the structure is getting more complex, making it difficult to carry out scale-up production and industrial application.

[0004] Due to the selectivity differences of the catalysts, 1-octene and 1-hexene cannot share a set of equipment and processes during industrial application, resulting in high production costs and high technical requirements. Limited by market, technology and other reasons, the supply-demand relationship of 1-octene and 1-hexene will fluctuate greatly. The investment cost of the ethylene selective oligomerization process equipment is huge, and enterprises are facing huge cost risks in production.

[0005] In the prior art, the research mainly focuses on the activity of ethylene oligomerization reaction catalysts and the selectivity for 1-octene or 1-hexene, but it cannot meet the selective production of 1-octene and 1-hexene in the same production line. Summary of the Invention

[0006] To solve the above problems, the present invention provides a bridged PNPN ligand, a preparation method thereof and an application thereof. It can be used in an ethylene oligomerization reaction catalyst. The catalyst can adjust the ratio of the products to 1-octene and 1-hexene by adjusting the reaction temperature. When the polymerization temperature is 80-100 °C, 1-octene is mainly produced. When the temperature is 40-60 °C, 1-hexene is mainly produced. The reasonable selection of the products can be realized, and the synthesis method of the catalyst is simple. The catalyst can carry out in-situ polymerization without prior complexation, and the operation is convenient.

[0007] The present invention provides a bridged PNPN ligand, and its structure is shown in Formula I:

[0008]

[0009] Among them, R1 is independently selected from aryl and its derivatives. Preferably, R1 is selected from phenyl, biphenyl, naphthyl, anthracenyl, phenyl substituted by C1-C6 alkyl, phenyl substituted by C1-C6 alkoxy, and phenyl substituted by halogen. Preferably, it is benzyl, 2-methylphenyl, 4-methylphenyl, 2,4-dimethylphenyl, 2,6-dimethylphenyl, 2-ethylphenyl, 4-ethylphenyl, 2,4-diethylphenyl, 2,6-diethylphenyl, 2-isopropylphenyl, 4-isopropylphenyl, 2,4-diisopropylphenyl, 2,6-diisopropylphenyl, 2-butylphenyl, 4-butylphenyl, 2,4-dibutylphenyl, 2,6-dibutylphenyl, 4-methoxyphenyl, o-methoxyphenyl, 4-ethoxyphenyl, o-ethoxyphenyl, 2-fluorophenyl, 3-fluorophenyl, 4-fluorophenyl.

[0010] The present invention also provides a preparation method of the ligand described above, which comprises the following steps:

[0011] (1) 7-Bromoindoline and phenyl-substituted phosphinous chloride are reacted in solvent A to prepare product II.

[0012] Preferably, the structural formula of product II is:

[0013]

[0014] (2) Product II is reacted with an alkyllithium reagent in solvent B. After the reaction is completed, dimethylchlorosilane is added, and the reaction is continued to obtain the product shown in formula I, which is a bridged PNPN ligand.

[0015] Preferably, in step (1), the molar ratio of 7-bromoindoline to phenyl-substituted phosphinous chloride added is 1:1-1.5.

[0016] Preferably, in step (1), solvent A is selected from one or more of tetrahydrofuran, diethyl ether, and dioxane.

[0017] Preferably, the structural formula of phenyl-substituted phosphinous chloride is:

[0018] The meaning of R1 is the same as that in formula I.

[0019] Preferably, the reaction in step (1) is carried out under the condition of -5 to 5 °C, the reaction time is 1-5 h. After the reaction is completed, water is added to quench the reaction, and the reaction solution is purified to obtain product II.

[0020] The reaction route of step (1) is schematically shown as follows:

[0021]

[0022] Preferably, in the step (2), the solvent B is selected from one or more of toluene, methylcyclohexane, and n-hexane;

[0023] Preferably, in the step (2), the molar ratio of product II: alkyllithium reagent: dimethylchlorosilane = 1: 1 - 1.2: 0.45 - 0.55.

[0024] Preferably, in the step (2), the alkyllithium reagent is selected from one or more of methyllithium, ethyllithium, propyllithium, isopropyllithium, and n-butyllithium, and preferably n-butyllithium.

[0025] The reaction between product II and the alkyllithium reagent is carried out under the conditions of -20°C to 0°C for 1 - 10 h. After adding dimethylchlorosilane, the reaction temperature is -5°C to 0°C for 1 - 5 h. After the reaction is completed, water is added to quench the reaction, and the reaction solution is purified to obtain the product shown in formula I.

[0026] The reaction route of step (2) is schematically shown as follows:

[0027]

[0028] The purification treatment in the present invention includes subjecting the reaction solution to column chromatography purification to obtain the target product and recrystallizing the target product. The height-to-diameter ratio of the chromatography column used for column chromatography purification is 5 - 10, the residence time is 10 - 60 min, and the solvent used for recrystallization is a mixed solvent of ethanol and ethyl acetate.

[0029] On the other hand, the present invention also provides the application of the PNPN ligand as an ethylene oligomerization catalyst.

[0030] An ethylene oligomerization catalyst includes a transition metal compound, the PNPN ligand of the present invention, and an alkylaluminum cocatalyst.

[0031] The transition metal compound in the present invention is selected from one or more of compounds of chromium, molybdenum, cobalt, titanium, vanadium, zirconium, nickel, and palladium, preferably compounds of chromium, zirconium, and nickel. The transition metal compound includes organic salts, inorganic salts, coordination complexes, or organometallic complexes of transition metals, such as one or more of chromium acetylacetonate, chromium chloride, tris(tetrahydrofuran)chromium(III) chloride, chromium(III) 2-ethylhexanoate, chromium(III) octanoate, hexacarbonylchromium, and (benzene)tricarbonylchromium.

[0032] The alkylaluminum cocatalyst in the present invention is selected from one or more of trimethylaluminum, triethylaluminum, triisobutylaluminum, diethylethoxyaluminum, monochlorodiethylaluminum, dichloroethylaluminum, sesquialteraluminum chloride, trioctylaluminum, methylaluminoxane (MAO), modified methylaluminoxane (MMAO), or ethylaluminoxane.

[0033] Preferably, the molar ratio of the transition metal compound, the PNPN ligand of the present invention, and the alkylaluminum cocatalyst is 1:1 - 1.5:300 - 800.

[0034] The present invention also provides the application of the above catalyst, which is used for ethylene oligomerization reaction.

[0035] In some preferred embodiments of the present invention, the method for ethylene oligomerization reaction is as follows: Before the reaction, the reaction kettle needs to be heated to 110 - 160 °C, evacuated for 1 - 4 h, replaced with nitrogen, and after the temperature is cooled to room temperature, replaced with ethylene. First, add solvent C and the alkylaluminum cocatalyst, then add the transition metal compound and the bridged PNPN ligand of the present invention. After the temperature reaches the reaction temperature, 0 - 0.8 Mpa of hydrogen and 2 MPa - 7 MPa of ethylene are successively introduced for reaction.

[0036] The ethylene oligomerization reaction time is 10 min - 240 min, preferably 20 min - 100 min.

[0037] When the polymerization temperature is 80 - 100 °C, the selectivity for the production of 1 - octene is high, and 1 - octene is mainly produced. When the temperature is 40 - 60 °C, the selectivity for 1 - hexene is high, and 1 - hexene is mainly produced. The reaction temperature can be reasonably adjusted according to the demand for 1 - octene and 1 - hexene.

[0038] The solvent C for the ethylene oligomerization reaction is selected from one or more of n - butane, isobutane, n - pentane, cyclopentane, methylcyclopentane, methylene cyclopentane, n - hexane, cyclohexane, methylcyclohexane, n - heptane, n - octane, n - nonane, benzene, toluene, and xylene.

[0039] Preferably, after the reaction, stop introducing ethylene, quickly cool down with an ice - water bath or liquid nitrogen, slowly release the pressure, and unload the kettle to obtain the ethylene oligomerization product.

[0040] The addition amount of the catalyst is such that the molar concentration of the transition metal compound in the ethylene oligomerization reaction system is 10 - 25 μmol / L (solvent), preferably 15 - 20 μmol / L (solvent).

[0041] Compared with the prior art, the present invention has the following advantages:

[0042] The ethylene oligomerization catalyst of the present invention can selectively regulate the reaction products according to different temperatures, meet the switching of different products by controlling different process parameters and adjusting process conditions in a set of devices, and it has high selectivity and catalytic activity for both 1 - octene and 1 - hexene, and has very high industrial application value. Specific Embodiments

[0043] The following specific embodiments only illustrate the present invention, but these embodiments are only part of the present invention and do not limit the application of the present invention in other fields.

[0044] The raw materials used in the examples are all conventional raw materials in the art, and the purity specifications used are analytical pure or chemically pure.

[0045] Raw material source information:

[0046] Isopropyl PNP ligand: Shanghai Wuxi AppTec Co., Ltd.

[0047] Diphenylphosphine chloride: 98%, J&K Scientific Ltd.

[0048] Dichloromethane: 98%, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0049] 7-Bromoindoline: 99%, Shanghai Merck Chemical Technology Co., Ltd.

[0050] Diethyl ether: 99%, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0051] 1,4-Dioxane: 99%, Shanghai Aladdin Biochemical Technology Co., Ltd.

[0052] Dimethylchlorosilane: 98%, J&K Scientific Ltd.

[0053] Ethyl acetate: 99.9%, J&K Scientific Ltd.

[0054] Ethanol: Analytical pure, Sinopharm Chemical Reagent Co., Ltd.

[0055] MMAO-3a: Nouryon Chemicals B.V.

[0056] Bis(4-methoxyphenyl)phosphine chloride: Beijing Innochem Co., Ltd.

[0057] Bis(4-chlorophenyl)phosphine chloride: Beijing Innochem Co., Ltd.

[0058] Bis(3,5-di-tert-butyl-4-methoxyphenyl)phosphine chloride: Beijing Innochem Co., Ltd.

[0059] Chromium(III) acetylacetonate: Beijing Innochem Co., Ltd.

[0060] The catalytic activity of the oligomerization reaction was analyzed qualitatively and quantitatively for each component in the reaction solution, and the conditions of the GC analytical instrument used were as follows:

[0061] Instrument model: Shimadzu GC2010

[0062] Chromatographic column: DB-5 (30m 0.25mm 0.25μm)

[0063] Column temperature program: First, hold at 35°C for 10 min, then increase the temperature to 250°C at a rate of 10°C / min, and hold at this temperature for 10 min.

[0064] Detector temperature: 300°C

[0065] Carrier gas: 1 bar

[0066] Air: 0.3 bar

[0067] Fuel gas (H2): 0.3 bar

[0068] The sample mass analysis is carried out by the internal standard method. There should be:

[0069]

[0070] Where m1 is the mass of a certain product, m is the mass of the internal standard, a1 is the peak area detected by the product in the gas chromatography, and a is the peak area of the internal standard. k is a correction factor related to the substance to be measured and the detection conditions.

[0071] Example 1

[0072] Preparation of bridged PNPN ligand: The relevant solvents are dehydrated and deoxygenated before use.

[0073] Preparation of silicon bridge ligand 1a: Take 150 mmol of 7-bromoindoline and 150 mmol of diphenylphosphine chloride, dissolve them in 150 ml of dichloromethane, and react at 0°C in an ice-water bath for 3 h. Add water to quench the reaction, extract the reaction solution with n-hexane, remove the solvent under vacuum to obtain the product. Dissolve the product in tetrahydrofuran, add 150 mmol of n-butyllithium at 0°C in an ice-water bath, react for 1 h, add 75 mmol of dimethylchlorosilane, react for 5 h, and the product is obtained after purification.

[0074]

[0075] The NMR data of the above ligand (1a) are as follows: 1H NMR (400 MHz, CDCl3):, 6.82 - 7.48 (m, 26H), 2.65 (t, 4H), 2.15 (t, 4H), 0.5 (s, 6H)

[0076] Ethylene oligomerization 1-1: Before the reaction, heat a 300 ml reaction kettle to 150 °C, evacuate for 3 h, and replace with nitrogen three times. After cooling to room temperature, replace with ethylene twice. First, add 100 ml of dehydrated and deoxygenated solvent methylcyclohexane and 1 ml of MMAO-3a (7 wt% Al, n-heptane), then add 4.2 μmol of bridged PNPN ligand 1a and 3.5 μmol of chromium acetylacetonate (Al / Cr = 500). When the temperature is constant at 45 °C, sequentially introduce 0.5 Mpa of hydrogen and 5 MPa of ethylene to start the reaction. The reaction temperature is 45 °C and the reaction time is 60 min. After the reaction, close the ethylene inlet valve, quickly cool to below 5 °C with an ice-water bath, slowly release the pressure, and unload the kettle to obtain the ethylene oligomerization product. Analyze the product by GC. The activity is 3202 kg / gCr·h, the selectivity for 1-hexene is 87.3 wt%, and the selectivity for polymer is 0.09 wt%.

[0077] Ethylene oligomerization 1-2: The experimental conditions are the same as those of ethylene oligomerization 1-1, except that the polymerization temperature is 90 °C. Analyze the product by GC. The activity is 3006 kg / gCr·h, the selectivity for 1-octene is 92.3 wt%, and the selectivity for polymer is 0.05 wt%.

[0078] Example 2

[0079] Preparation of chlorophosphine bridge ligand 1b: Take 150 mmol of 7-bromoindoline and 150 mmol of bis(4-methoxyphenyl)phosphine chloride, dissolve them in 150 ml of dichloromethane, and react at -5 °C for 3 h. Add water to quench the reaction, extract the reaction solution with n-hexane, and remove the solvent under vacuum. Dissolve the obtained product in tetrahydrofuran, add 150 mmol of n-butyllithium at -5 °C, react for 1 h, add 75 mmol of dimethylchlorosilane, and react for 5 h. After purification, it is the PNPN ligand.

[0080]

[0081] The NMR data of the above ligand (1b) are as follows: 1H NMR (400 MHz, CDCl3): 6.82 - 7.48 (m, 22H), 3.65 (s, 12H), 2.65 (t, 4H), 2.15 (t, 4H), 0.56 (s, 6H)

[0082] Ethylene oligomerization 2-1: Before the reaction, heat a 300 ml reaction kettle to 150 °C, evacuate for 3 h, and displace with nitrogen three times. After the temperature cools to room temperature, displace with ethylene twice. First, add 100 ml of dehydrated and deoxygenated solvent methylcyclohexane and 1.6 ml of (Al / Cr = 800) MMAO-3a (7 wt% Al, n-heptane), then add 4.8 μmol of bridged PNPN ligand 1b and 3.5 μmol of chromium acetylacetonate. When the temperature is constant at 45 °C, sequentially introduce 0.5 Mpa of hydrogen and 5 MPa of ethylene to start the reaction. The reaction temperature is 45 °C, and the reaction time is 60 min. After the reaction is completed, close the ethylene inlet valve, rapidly cool to below 5 °C with an ice-water bath, slowly release the pressure, and unload the kettle to obtain the ethylene oligomerization product. Analyze the product by GC. The activity is 2908 kg / gCr·h, the selectivity for 1-hexene is 85.3 wt%, and the selectivity for polymer is 0.10 wt%.

[0083] Ethylene oligomerization 2-2: The experimental conditions are the same as those of ethylene oligomerization 2-1, except that the polymerization temperature is 90 °C. Analyze the product by GC. The activity is 2750 kg / gCr·h, the selectivity for 1-octene is 87.3 wt%, and the selectivity for polymer is 0.15 wt%.

[0084] Example 3

[0085] Preparation of bridged PNPN ligand: The relevant solvents are dehydrated and deoxygenated before use.

[0086] Preparation of chlorophosphine bridge ligand: Take 150 mmol of 7-bromoindoline and 180 mmol of bis(4-chlorophenyl)phosphine chloride, dissolve them in 150 ml of dichloromethane, and react under an ice-water bath at 0 °C for 3 h. Add water to quench the reaction, extract the reaction solution with n-hexane, and remove the solvent under vacuum. Dissolve the obtained product in tetrahydrofuran, add 150 mmol of n-butyllithium under an ice-water bath at 0 °C, react for 1 h, add 75 mmol of dimethylchlorosilane, and react for 5 h. After purification, it is the PNPN ligand.

[0087]

[0088] The NMR data of the above ligand (1a) are as follows: 1H NMR (400 MHz, CDCl3): 6.82~7.48 (m, 22H), 2.65 (t, 4H), 2.15 (t, 4H), 0.56 (s, 6H)

[0089] Ethylene oligomerization 3-1: Before the reaction, heat a 300 ml reaction kettle to 150 °C, evacuate for 3 h, and displace with nitrogen three times. Wait for the temperature to cool to room temperature, displace with ethylene twice, first add 100 ml of dehydrated and deoxygenated solvent methylcyclohexane and 0.6 ml of (Al / Cr = 300) MMAO-3a (7 wt% Al, n-heptane), then add 3.8 μmol of bridged PNPN ligand 1c and 3.5 μmol of chromium acetylacetonate. Wait for the temperature to stabilize at 45 °C, and then sequentially introduce 0.5 Mpa of hydrogen and 5 MPa of ethylene to start the reaction. The reaction temperature is 50 °C and the reaction time is 60 min. After the reaction is completed, close the ethylene inlet valve, quickly cool it to below 5 °C with an ice-water bath, slowly release the pressure, and unload the kettle to obtain the ethylene oligomerization product. Analyze the product by GC, the activity is 3401 kg / gCr·h, the selectivity of 1-hexene is 83.3 wt%, and the selectivity of the polymer is 0.12 wt%.

[0090] Ethylene oligomerization 3-2: The experimental conditions are the same as those of ethylene oligomerization 3-1, except that the polymerization temperature is 95 °C. Analyze the product by GC, the activity is 2902 kg / gCr·h, the selectivity of 1-octene is 90.3 wt%, and the selectivity of the polymer is 0.09 wt%.

[0091] Example 4

[0092] Preparation of bridged PNPN ligand: Dehydrate and deoxygenate the relevant solvents before use.

[0093] Preparation of chlorophosphine bridge ligand: Take 150 mmol of 7-bromoindoline and 150 mmol of bis(3,5-di-tert-butyl-4-methoxyphenyl)phosphine chloride, dissolve them in 150 ml of dichloromethane, and react under an ice-water bath at 0 °C for 3 h. Add water to quench the reaction, extract the reaction solution with n-hexane, and remove the solvent under vacuum. Dissolve the solid in tetrahydrofuran, add 150 mmol of alkyllithium under an ice-water bath at 0 °C, react for 1 h, add 75 mmol of dimethylchlorosilane, and react for 5 h. The ligand 1d is obtained through separation and purification.

[0094]

[0095] The NMR data of the above ligand (1a) are as follows: 1H NMR (400 MHz, CDCl3): 6.82~7.48 (m, 14H), 3.65 (s, 12H), 2.65 (t, 4H), 2.15 (t, 4H), 1.15 (s, 72H), 0.57 (s, 6H)

[0096] Ethylene oligomerization 4-1: Before the reaction, heat a 300 ml reaction kettle to 150 °C, evacuate for 3 h, and displace with nitrogen three times. After cooling the temperature to room temperature, displace with ethylene twice. First, add 100 ml of dehydrated and deoxygenated solvent methylcyclohexane and 1 ml of (Al / Cr = 500) MMAO-3a (7 wt% Al, n-heptane), then add 4.2 μmol of bridged PNPN ligand 1d and 3.5 μmol of chromium acetylacetonate. When the temperature is constant at 45 °C, sequentially introduce 0.5 Mpa of hydrogen and 5 MPa of ethylene to start the reaction. The reaction temperature is 55 °C and the reaction time is 60 min. After the reaction is completed, close the ethylene inlet valve, rapidly cool to below 5 °C using an ice-water bath, slowly relieve the pressure, and unload the kettle to obtain the ethylene oligomerization product. Analyze the product by GC. The activity is 2805 kg / gCr.h, the selectivity for 1-hexene is 80.3 wt%, and the polymer selectivity is 0.11 wt%.

[0097] Ethylene oligomerization 4-2: The experimental conditions are the same as those in ethylene oligomerization 4-1, except that the polymerization temperature is 100 °C. Analyze the product by GC. The activity is 3028 kg / gCr.h, the selectivity for 1-octene is 92.3 wt%, and the polymer selectivity is 0.05 wt%.

Claims

1. A bridged PNPN ligand, characterized in that, Its structure is as shown in Formula I: Among them, R1 is independently selected from aryl and its derivatives. Preferably, R1 is selected from phenyl, biphenyl, naphthyl, anthracenyl, phenyl substituted by C1-C6 alkyl, phenyl substituted by C1-C6 alkoxy, phenyl substituted by halogen, preferably benzyl, 2-methylphenyl, 4-methylphenyl, 2,4-dimethylphenyl, 2,6-dimethylphenyl, 2-ethylphenyl, 4-ethylphenyl, 2,4-diethylphenyl, 2,6-diethylphenyl, 2-isopropylphenyl, 4-isopropylphenyl, 2,4-diisopropylphenyl, 2,6-diisopropylphenyl, 2-butylphenyl, 4-butylphenyl, 2,4-dibutylphenyl, 2,6-dibutylphenyl, 4-methoxyphenyl, o-methoxyphenyl, 4-ethoxyphenyl, o-ethoxyphenyl, 2-fluorophenyl, 3-fluorophenyl, 4-fluorophenyl.

2. The preparation method of the ligand according to claim 1, characterized in that, It comprises the following steps: (1) 7-bromoindoline and phenyl-substituted phosphinous chloride react in solvent A to prepare product II; Preferably, the structural formula of product II is: (2) React product II with an alkyllithium reagent in solvent B. After the reaction, add dimethylchlorosilane and continue the reaction to obtain the product shown in Formula I, which is the bridged PNPN ligand.

3. The preparation method according to claim 2, wherein In the said step (1), the molar ratio of 7-bromoindoline to phenyl-substituted phosphinous chloride added is: 1:1 - 1.5; Preferably, the structural formula of phenyl-substituted phosphinous chloride is: R1 has the same meaning as in formula I.

4. The preparation method according to claim 2 or 3, characterized in that, In the said step (1), solvent A is selected from one or more of tetrahydrofuran, diethyl ether, and dioxane; Preferably, the reaction in step (1) is carried out at -5 to 5 °C, the reaction time is 1 - 5 h. After the reaction, add water to quench the reaction, and purify the reaction solution to obtain product II.

5. The preparation method according to any one of claims 2-4, characterized in that, In the said step (2), solvent B is selected from one or more of toluene, methylcyclohexane, and n-hexane; Preferably, in the said step (2), the molar ratio of product II: alkyllithium reagent: dimethylchlorosilane = 1:1 - 1.2:0.45 - 0.55; Preferably, in the said step (2), the alkyllithium reagent is selected from one or more of methyllithium, ethyllithium, propyllithium, isopropyllithium, and n-butyllithium, preferably n-butyllithium.

6. The preparation method according to any one of claims 2-5, characterized in that, The reaction of product II with the alkyllithium reagent is carried out at -20 °C - 0 °C, the reaction time is 1 - 10 h, the reaction temperature after adding dimethylchlorosilane is -5 - 0 °C, the reaction time is 1 - 5 h. After the reaction, add water to quench the reaction, and purify the reaction solution to obtain the product shown in Formula I.

7. Application of the PNPN ligand according to claim 1 or the PNPN ligand prepared by the preparation method according to any one of claims 2 - 6 as an ethylene oligomerization catalyst.

8. An ethylene oligomerization catalyst, comprising a transition metal compound, the PNPN ligand according to claim 1 or the PNPN ligand prepared by the preparation method according to any one of claims 2 - 6, and an alkylaluminum cocatalyst; Preferably, the transition metal compound is selected from one or more of compounds of chromium, molybdenum, cobalt, titanium, vanadium, zirconium, nickel, and palladium, preferably compounds of chromium, zirconium, and nickel; Preferably, the alkylaluminum cocatalyst is selected from one or more of trimethylaluminum, triethylaluminum, triisobutylaluminum, diethylethoxyaluminum, monochlorodiethylaluminum, dichloroethylaluminum, sesquialethylaluminum chloride, trioctylaluminum, methylaluminoxane, modified methylaluminoxane or ethylaluminoxane; Preferably, the molar ratio of the transition metal compound, the PNPN ligand described in claim 1 or the PNPN ligand prepared by the preparation method described in any one of claims 2-6 to the alkylaluminum cocatalyst is 1:1 - 1.5:300 - 800.

9. Use of the ethylene oligomerization catalyst according to claim 8, which is used for ethylene oligomerization reaction; Preferably, the method for the ethylene oligomerization reaction is as follows: before the reaction, the reaction kettle needs to be heated to 110 - 160 °C, evacuated for 1 - 4 h, replaced with nitrogen, and after the temperature is cooled to room temperature, replaced with ethylene. First, add solvent C and the alkylaluminum cocatalyst, then add the transition metal compound and the bridged PNPN ligand of the present invention. After the temperature reaches the reaction temperature, introduce 0 - 0.8 Mpa of hydrogen and 2 MPa - 7 MPa of ethylene to carry out the reaction; The ethylene oligomerization reaction time is 10 min - 240 min, preferably 20 min - 100 min; The ethylene oligomerization reaction solvent C is selected from one or more of n-butane, isobutane, n-pentane, cyclopentane, methylcyclopentane, methylenecyclopentane, n-hexane, cyclohexane, methylcyclohexane, n-heptane, n-octane, n-nonane, benzene, toluene, xylene; Preferably, after the reaction is completed, stop introducing ethylene, quickly cool down with an ice-water bath or liquid nitrogen, slowly release the pressure, and unload the kettle to obtain the ethylene oligomerization product; The addition amount of the catalyst is such that the molar concentration of the transition metal compound added in the ethylene oligomerization reaction system is 10 - 25 μmol / L (solvent), preferably 15 - 20 μmol / L (solvent).

10. The application according to claim 9, characterized in that, When the polymerization temperature is 80 - 100 °C, the selectivity for the reaction of 1-octene is high, and mainly 1-octene is produced. When the temperature is 40 - 60 °C, the selectivity for 1-hexene is high, and mainly 1-hexene is produced.

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