Flexible o-aryl naphthenic base substituted pyridine diimine iron and cobalt complex catalyst as well as preparation method and application of flexible o-aryl naphthenic base substituted pyridine diimine iron and cobalt complex catalyst

Abstract

The invention provides a flexible o-aryl cycloalkyl substituted pyridine diimine iron and cobalt complex catalyst as well as a preparation method and application thereof, and belongs to the technical field of olefin polymerization catalysis. A series of 2, 4, 6-tricycloalkyl arylamine compounds are introduced into 2, 6-diacetylpyridine, and the molecular weight and distribution of polyethylene can be regulated and controlled along with the change of the cycloalkyl space volume; a substituent group in the complex is selected from naphthenic base, a ligand is respectively reacted with FeCl2. 4H2O and CoCl2. 6H2O in a protective gas atmosphere to obtain the complex catalyst in the scheme, the complex shows extremely high polymerization activity which is as high as 1.76 * 10 < 7 > g / mol.h, and high linear polyethylene with different molecular weight ranges is generated; and the method has good industrial potential value.

Description

Technical Field

[0001] The invention belongs to the technical field of olefin polymerization catalysis, and specifically relates to a flexible ortho-arylcycloalkyl substituted pyridinediimine iron and cobalt complex catalyst, and a preparation method and application thereof. Background Art

[0002] More than 20 years ago, Brookhart and Gibson from the University of North Carolina (UNC), DuPont, and Imperial College independently discovered bis(imino)pyridine iron(II) and cobalt(II) catalysts for ethylene polymerization and oligomerization, which have great potential for producing highly crystalline polyethylene or highly linear α-olefins with wide polydispersity. They are popular in both industry and academia because of their low cost and high activity.

[0003] However, late transition metal catalysts have the problem of poor high-temperature stability. Their chain termination rate will significantly accelerate with the increase of temperature, and the catalytic activity will gradually decrease with the increase of reaction temperature. Generally speaking, changing the steric hindrance and electronic effect of the ligand are the two main strategies for developing catalysts with application prospects in bis(imino)pyridine iron(II) and cobalt(II) catalyzed ethylene polymerization systems. Among them, the N-ortho-aryl substituents in the bis(imino)pyridine iron(II) and cobalt(II) systems play the most important role in determining the polymerization activity and the molecular weight of the produced polyethylene, and are also the most studied modification parts. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a flexible ortho-arylcycloalkyl substituted pyridinediimine iron and cobalt complex catalyst and its preparation method and application, which can regulate the polymerization activity of the catalyst in ethylene polymerization and the molecular weight and molecular weight distribution of the resulting polyethylene. The catalyst exhibits higher activity during polymerization and produces polyethylene with higher molecular weight and wider molecular weight distribution.

[0005] To achieve the above purpose, the present invention first provides a flexible o-arylcycloalkyl-substituted pyridine diimine iron and cobalt complex catalyst, wherein the complex catalyst is shown in Formula 1 below:

[0006]

[0007] Wherein, M is selected from any one of iron and cobalt, C A Selected from cycloalkyl, the structural formula is shown in Formula 2 below:

[0008]

[0009] Formula 2;

[0010] Among them, n=1, 2, 3, 4, 5, 6.

[0011] Based on a general inventive concept, the present invention also provides a method for preparing a complex catalyst, comprising the following steps:

[0012] S1. Preparation of a pyridine diimine ligand containing a 2,4,6-tricycloalkylaryl group: placing 2,6-diacetylpyridine and a tricycloalkylarylamine in a toluene solution containing p-toluenesulfonic acid, stirring at 120° C. until only one main point is shown on a thin layer chromatography plate, then evaporating the solvent under reduced pressure, diluting the remaining mixture with methanol, collecting the resulting yellow solid by filtration, and recrystallizing with dichloromethane and n-hexane to obtain a pyridine diimine ligand containing a 2,4,6-tricycloalkylaryl group;

[0013] S2. Preparation of flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalysts: Under a protective gas atmosphere, the ligand prepared in S1 is reacted with ferrous chloride tetrahydrate and cobalt chloride hexahydrate in tetrahydrofuran with stirring, the solvent is removed, and the blue or green solid powder is washed with ether and dried under vacuum to obtain a flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst.

[0014] Preferably, the stirring time in step S1 is 72 hours, and the molar ratio of 2,6-diacetylpyridine to tricycloalkyl aromatic amine is 1:2.5.

[0015] Preferably, in step S2, the molar ratio of the 2,4,6-tricycloalkylaryl pyridine diimine ligand to ferrous chloride tetrahydrate or cobalt chloride hexahydrate is 1:1.

[0016] Preferably, the stirring time in step S2 is 12 hours, and the protective gas is selected from argon or nitrogen.

[0017] Based on a general inventive concept, the present invention also provides an application of a flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst in catalyzing ethylene polymerization in a polymerization solvent.

[0018] Preferably, the catalyst in the ethylene polymerization reaction is a composite catalyst composed of a flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst and a co-catalyst.

[0019] Preferably, the cocatalyst comprises any one of alkylaluminum, aluminoxane and alkylaluminum chloride, and the molar ratio of the flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst to the cocatalyst is 1:500-2000.

[0020] Preferably, the aluminoxane includes methylaluminoxane or modified methylaluminoxane, and the alkylaluminum chloride includes diethylaluminum chloride.

[0021] Preferably, the reaction temperature of the catalytic ethylene polymerization reaction is 20-80° C., the reaction time is 10-60 minutes, the reaction pressure is 0.4-1.5 MPa, and the polymerization solvent is at least one of toluene, n-hexane, cyclohexane and heptane.

[0022] The catalytic mechanism of the catalyst prepared in this scheme is:

[0023] The general synthesis formula of the flexible o-arylcycloalkyl substituted pyridine diimine iron (II) and cobalt (II) catalyst prepared in this scheme is as follows:

[0024]

[0025] Under the catalysis of p-toluenesulfonic acid, 2,6-diacetylpyridine reacted with 2,4,6-tricycloalkylarylamine to generate the target bis(imino)pyridine ligands: pyridine diimine iron (II) and cobalt (II) (L1 and L2) in high yields (83%-87%). Ligands L1-L2 were reacted with an equal amount of ferrous chloride tetrahydrate in tetrahydrofuran (THF) at room temperature to generate complexes Fe1-Fe2 (blue) in excellent yields (91%-95%). Similarly, ligands L1-L2 were reacted with an equal amount of cobalt chloride hexahydrate in tetrahydrofuran to easily prepare the corresponding cobalt (II) complexes Co1-Co2 (green) in high yields (91%-94%).

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] By adjusting the size of the cycloalkyl ring, the polymerization activity of the catalyst in ethylene polymerization and the molecular weight and molecular weight distribution of the resulting polyethylene can be regulated. The smaller the ring, the more conducive it is to the coordination and insertion of ethylene. The catalyst exhibits higher activity during polymerization and produces polyethylene with higher molecular weight and wider molecular weight distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 This is the hydrogen nuclear magnetic resonance spectrum of the 2,4,6-tricyclopentylaryl pyridine diimine ligand prepared in Example 1;

[0030] Figure 2This is the hydrogen nuclear magnetic resonance spectrum of the 2,4,6-tricyclohexyl aryl pyridine diimine ligand prepared in Example 2;

[0031] Figure 3 These are single crystal images of the complex catalyst prepared in Example 1, a is a single crystal image of the complex catalyst Fe1, and b is a single crystal image of the complex catalyst Co1. DETAILED DESCRIPTION

[0032] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0033] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Without departing from the spirit and substance of the present invention, modifications or substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0034] Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art; unless otherwise specified, the reagents used in the examples are commercially available.

[0035] Reagents and raw materials used in the examples: All metal organic reactions were carried out under nitrogen protection, and all solvents were dried and deoxygenated. Anhydrous methanol and ethanol were analytically pure and used directly. Toluene was dehydrated by molecular sieve, refluxed with sodium metal under nitrogen protection, and evaporated before use.

[0036] Example 1 Preparation of Flexible O-Arylcycloalkyl Substituted Pyridine Diimine Iron (II) and Cobalt (II) Catalysts

[0037] S1. Preparation of a pyridine diimine ligand containing a 2,4,6-tricycloalkylaryl group: 2,6-diacetylpyridine (1.0 mmol, 1.0 equivalent) and cyclopentylarylamine (2.5 mmol, 2.5 equivalent) were placed in a toluene (20 mL) solution containing p-toluenesulfonic acid (20 mg), and stirred at 120°C for 72 h until only one main point was shown on the thin layer chromatography plate. The solvent was then evaporated under reduced pressure. The remaining mixture was diluted with methanol (20 mL). The resulting yellow solid was collected by filtration and recrystallized using dichloromethane and n-hexane to obtain a pyridine diimine ligand containing a 2,4,6-tricyclopentylaryl group with a yield of 83%;

[0038] The NMR analysis data of the ligand prepared by S1 are:

[0039] 1H NMR (600MHz, CDCl3) δ8.47 (d, J=7.7Hz, 2H, Py-H), 7.93 (dq, J=11.6, 6.4, 5.9Hz, 1H, Py-H), 7.05 (s, 4H, Ar-H), 3.03-2.96 (m, 2H, p-Cp-CH), 2. 89-2.77(m,4H,o-Cp-CH),2.25(s,6H,N=C-CH3),2.11(td,J=7.6,3.8Hz,4H,Cp-CH2),2.04-1.98(m,4H,Cp-CH2),1.89–1.47(m,40H,Cp-CH2). 13 C NMR (151MHz, CDCl3) δ167.35(C=N),155.43,145.92,141.08,136.85,133.43,122.26,122.18,46.10(p-Cp-CH),40.47(o-Cp -CH),34.85(Cp-CH2),34.07(Cp-CH2),34.04(Cp-CH2),25.91(Cp-CH2),25.87(Cp-CH2),25.58(Cp-CH2),17.39(N=C-CH3).

[0040] The mass spectrometry data of the ligand prepared by S1 are: HRMS (m / z): calcd for C51H68N3: 722.5413, found: 722.5328 [M+H]+.

[0041] The H NMR spectrum of the ligand prepared by S1 is shown in Figure 1 As shown;

[0042] S2. Preparation of flexible o-arylcyclopentyl-substituted pyridinediimine iron (II) and cobalt (II) complex catalysts: Under a nitrogen atmosphere, the ligand prepared in S1 (0.2 mmol) was stirred with ferrous chloride tetrahydrate (0.2 mmol) and cobalt chloride hexahydrate (0.2 mmol) in tetrahydrofuran (10 mL) for 12 h, and the solvent was removed. The blue (Fe complex) and green (Co complex) solid powders were washed with ether and dried under vacuum to obtain the corresponding flexible o-arylcyclopentyl-substituted pyridinediimine iron complex Fe1 (yield 95%) and flexible o-arylcyclopentyl-substituted pyridinediimine cobalt complex Co1 (yield 94%).

[0043] The iron complex and the cobalt complex obtained in step S2 were subjected to elemental analysis:

[0044] Elemental analysis: Fe1: calc.for C 51 H 67Cl 2 FeN 3 :C,72.16;H,7.96;N,4.95.Found:C,72.24;H,7.85;N,4.89;Co1:calc.for C 51 H 67 Cl 2 CoN 3 :C,71.90;H,7.93;N,4.93.Found:C,71.84;H,7.86;N,4.85.

[0045] Mass spectrometry analysis was performed on the iron complex Fe1 and the cobalt complex Co1 obtained in step S2:

[0046] Fe1:MALDI-TOF-MS(m / z):calcd for C 51 H 67 ClFeN 3 :812.44, found:812.58[M-Cl]+; Co1: MALDI-TOF-MS(m / z):calc.for C 51 H 67 CCO 3 :815.44,found:816.00[M-Cl]+.

[0047] The single crystal image of the flexible o-arylcyclopentyl substituted pyridine diimine iron complex obtained in this example is as follows: Figure 3 (a) as shown:

[0048] The single crystal image of the flexible o-arylcyclopentyl substituted pyridine diimine cobalt complex obtained in this example is as follows: Figure 3 (b) as shown:

[0049] Example 2 Preparation of Flexible O-Arylcycloalkyl Substituted Pyridine Diimine Iron (II) and Cobalt (II) Catalysts

[0050] S1. Preparation of a pyridine diimine ligand containing a 2,4,6-tricycloalkyl aryl group: 2,6-diacetylpyridine (1.0 mmol, 1.0 equivalent) and cyclohexyl arylamine (2.5 mmol, 2.5 equivalent) were placed in a toluene (20 mL) solution containing p-toluenesulfonic acid (20 mg), and stirred at 120 ° C for 72 h until only one main point was shown on the thin layer chromatographic plate. The solvent was then evaporated under reduced pressure. The remaining mixture was diluted with methanol (20 mL). The resulting yellow solid was collected by filtration and recrystallized using dichloromethane and n-hexane to obtain a pyridine diimine ligand containing a 2,4,6-tricyclohexyl aryl group with a yield of 83%.

[0051] The NMR analysis data of the ligand prepared by S1 are:

[0052] 1 H NMR (400MHz, CDCl3) δ8.37(d,J=7.8Hz,2H,Py-H),7.92(t,J=7.8Hz,1H,Py-H),6.98(s,4H,Ar-H),2.49(td,J=9.7,8.3,3.0Hz,2H,p-Cy-CH),2 .36(td,J=10.1,8.7,2.9Hz,4H,o-Cy-CH),2.26(s,6H,N=C-CH3),1.82(ddd,J=71.7,30.2,11.7Hz,30H,Cy-CH2),1.57–1.14(m,30H,Cy-CH2). 13 C NMR (101MHz, CDCl3) δ167.40(C=N),155.78,144.39,142.82,136.98,134.63,122.03,121.94,44.57(p-Cy-CH),39.35(o-Cy-CH),34.88(Cy- CH2),33.76(Cy-CH2),33.55(Cy-CH2),27.33(Cy-CH2),27.24(Cy-CH2),27.20(Cy-CH2),26.47(Cy-CH2),26.42(Cy-CH2),17.49(N=C-CH3).

[0053] Mass spectrometry analysis of the ligand prepared by S1: HRMS (m / z): calcd for C57H80N3: 806.6347, found: 806.6257 [M+H]+.

[0054] The H NMR spectrum of the ligand prepared by S1 is shown in Figure 2 As shown;

[0055] S2. Preparation of flexible o-arylcyclohexyl-substituted pyridinediimine ligand iron (II) and cobalt (II) complex catalysts: Under a nitrogen atmosphere, the ligand prepared in S1 (0.2 mmol) was stirred with ferrous chloride tetrahydrate (0.2 mmol) and cobalt chloride hexahydrate (0.2 mmol) in tetrahydrofuran (10 mL) for 12 h, and then the solvent was removed. The blue (Fe complex) and green (Co complex) solid powders were washed with ether and dried under vacuum to obtain the corresponding flexible o-arylcyclopentyl-substituted pyridinediimine iron complex Fe2 (yield 91%) and flexible o-arylcyclopentyl-substituted pyridinediimine cobalt complex Co2 (yield 91%).

[0056] The iron complex Fe2 and the cobalt complex Co2 obtained in step S2 were subjected to elemental analysis:

[0057] Fe2:calc.for C57H79Cl2FeN3:C,73.38;H,8.53;N,4.50.Found:C,73.45;H,8.36;N,4.61;Co2:calc.for C57H79Cl2CoN3:C,73.14;H,8.51;N,4.49.Found:C,73.26;H,8.49;N,4.32.

[0058] The iron complex F2 and the cobalt complex Co2 obtained in step S2 were subjected to mass spectrometry analysis:

[0059] Fe2:MALDI-TOF-MS(m / z):calcd for C 57 H 79 ClFeN 3 :896.53, found:896.83[M-Cl]+; Co2: MALDI-TOF-MS(m / z):calc.for C 57 H 79 CCO 3 :899.53,found:899.83[M-Cl]+.

[0060] Experimental Example 1: The effect of Fe1 and Co1 prepared in Example 1 on catalytic polymerization of ethylene

[0061] Take a 350 ml thick-walled pressure-resistant glass reactor, dry it at 90 ° C for 3 hours, and then connect it to a high-pressure gas pipeline. Then, adjust the reactor to the required polymerization temperature (80 ° C), add 48 mL of toluene and 1 mmol of methylaluminoxane (MAO) to the reactor under ethylene atmosphere, and then inject 1 micromole of catalyst in 2 ml of dichloromethane into the polymerization system with a syringe. Under rapid stirring, pressurize the reactor and maintain the ethylene pressure at 6 atmospheres. After 10 minutes, the pressure reactor is vented, and the polymer is precipitated in ethanol, filtered, and dried under vacuum conditions at 50 ° C for at least 24 hours.

[0062] Table 1 below shows the experimental conditions for ethylene polymerization using the preferred catalyst provided in this experimental example; polymerization result data such as polymer molecular weight (Mw), polymer molecular weight distribution (PDI), etc.

[0063] Table 1 Effect of the catalyst prepared in Example 1 on ethylene polymerization

[0064]

[0065]

[0066] In the table: activity is 10 6 g / (mol Cat·h), Mw is the number average molecular weight, and Mw / Mn is the molecular weight distribution.

[0067] Experimental Example 2: Effect of Fe2 and Co2 prepared in Example 2 on catalytic polymerization of ethylene

[0068] Take a 350 ml thick-walled pressure-resistant glass reactor, dry it at 90 ° C for 3 hours and connect it to a high-pressure gas pipeline. Subsequently, adjust the reactor to the desired polymerization temperature (80 ° C). Under an ethylene atmosphere, add 48 mL of toluene and 1 mmol of methylaluminoxane (MAO) to the reactor, and then inject 1 micromole of catalyst in 2 ml of dichloromethane into the polymerization system with a syringe. Under rapid stirring, pressurize the reactor and maintain the ethylene pressure at 6 atmospheres. After 10 minutes, the pressure reactor is vented, the polymer is precipitated in ethanol, filtered and dried under vacuum conditions at 50 ° C for at least 24 hours.

[0069] Table 2 below shows the experimental conditions for ethylene polymerization using the preferred catalyst provided in this experimental example; polymerization result data such as polymer molecular weight (Mw), polymer molecular weight distribution (PDI), etc.

[0070] Table 2 Effect of catalyst prepared in Example 2 on ethylene polymerization

[0071] Entry Cat. T / ℃ Yield / g <![CDATA[Act. b ]]> <![CDATA[M w c ]]> <![CDATA[M w / M n c ]]> <![CDATA[T m (℃) d ]]> 1 Fe2 20 0.67 4.02 47.0 6.23 134 2 Fe2 40 1.33 7.98 26.5 4.70 132 3 Fe2 60 1.25 7.50 21.6 4.14 130 4 Fe2 80 0.44 2.64 11.9 4.12 128 5 Co2 20 0.60 3.60 3.4 2.66 108 6 Co2 40 0.71 4.26 2.8 2.61 109 7 Co2 60 0.71 4.26 2.3 2.31 107 8 Co2 80 0.63 3.78 2.1 2.13 106

[0072] In the table: activity is 10 6 g / (mol Cat·h), Mw is the number average molecular weight, and Mw / Mn is the molecular weight distribution.

[0073] The results in Tables 1 and 2 show that all the Fe(II) and Co(II) complexes exhibited extremely high polymerization activities (up to 1.76×10 7 g / mol·h), and produced highly linear polyethylene with different molecular weight ranges (weight average molecular weight Mw = 2.1-232.8 kg / mol).

[0074] The size of the cycloalkyl ring and the type of metal have a significant effect on ethylene polymerization. Smaller cyclopentyl substituents are more favorable for increasing the polymerization activity of iron(II) and cobalt(II) complexes and the molecular weight of the resulting polyethylene than the corresponding cyclohexyl groups. For complexes with the same ligand, iron(II) species produce polyethylene with higher molecular weight and broader molecular weight distribution than the corresponding cobalt(II) species, all other conditions being equal.

[0075] This type of complex catalyst has shown great potential for industrial application, which also fully demonstrates that this type of complex catalyst is extremely worthy of in-depth research and further development.

[0076] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. For those skilled in the art, improvements and changes obtained without departing from the technical concept of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A flexible o-arylcycloalkyl substituted pyridine diimine iron and cobalt complex catalyst, characterized in that: The complex catalyst is shown in Formula 1 below: Wherein, M is selected from any one of iron and cobalt, C A Selected from cycloalkyl, the structural formula is shown in Formula 2 below: Among them, n=1, 2, 3, 4, 5, 6.

2. A method for preparing the complex catalyst according to claim 1, characterized in that: The following steps are involved: S1. Preparation of a pyridine diimine ligand containing a 2,4,6-tricycloalkylaryl group: placing 2,6-diacetylpyridine and a tricycloalkylarylamine in a toluene solution containing p-toluenesulfonic acid, stirring at 120° C. until only one main point is shown on a thin layer chromatography plate, then evaporating the solvent under reduced pressure, diluting the remaining mixture with methanol, collecting the resulting yellow solid by filtration, and recrystallizing with dichloromethane and n-hexane to obtain a pyridine diimine ligand containing a 2,4,6-tricycloalkylaryl group; S2. Preparation of flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalysts: Under a protective gas atmosphere, the ligand prepared in S1 is reacted with ferrous chloride tetrahydrate and cobalt chloride hexahydrate in tetrahydrofuran with stirring, the solvent is removed, and the blue or green solid powder is washed with ether and dried under vacuum to obtain a flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst.

3. The preparation method according to claim 2, characterized in that: The stirring time in step S1 is 72 hours, and the molar ratio of 2,6-diacetylpyridine to tricycloalkyl aromatic amine is 1:2.

5.

4. The preparation method according to claim 2, characterized in that: In the step S2, the molar ratio of the 2,4,6-tricycloalkylaryl pyridine diimine ligand to ferrous chloride tetrahydrate or cobalt chloride hexahydrate is 1:

1.

5. The preparation method according to claim 2, characterized in that: The stirring time in step S2 is 12 hours, and the protective gas is selected from argon or nitrogen.

6. Use of the flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst as claimed in claim 1 or the flexible o-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst prepared by the preparation method according to any one of claims 2 to 5 in catalyzing ethylene polymerization in a polymerization solvent.

7. The use according to claim 6, characterized in that: The catalyst in the ethylene polymerization reaction is a composite catalyst composed of a flexible ortho-aryl cycloalkyl-substituted pyridine diimine iron and cobalt complex catalyst and a co-catalyst.

8. The use according to claim 8, characterized in that: The cocatalyst comprises any one of alkylaluminum, aluminoxane and alkylaluminum chloride, and the molar ratio of the flexible ortho-arylcycloalkyl-substituted pyridinediimine iron and cobalt complex catalyst to the cocatalyst is 1:500-2000.

9. The use according to claim 8, characterized in that: The aluminoxane includes methylaluminoxane or modified methylaluminoxane, and the alkylaluminum chloride includes diethylaluminum chloride.

10. The use according to claim 6, characterized in that: The reaction temperature of the catalytic ethylene polymerization reaction is 20-80° C., the reaction time is 10-60 minutes, the reaction pressure is 0.4-1.5 MPa, and the polymerization solvent is at least one of toluene, n-hexane, cyclohexane, and heptane.

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