- Email: [email protected]
Oligomerization reactions of 1-hexene with metallocene catalysts: Detailed data on reaction chemistry and kinetics
Molecular Catalysis 463 (2019) 87–93
Contents lists available at ScienceDirect
Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Oligomerization reactions of 1-hexene with metallocene catalysts: Detailed data on reaction chemistry and kinetics
T
Yury V. Kissin Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 123 Bevier Road, Piscataway, NJ, 08854-8087, USA
ARTICLE INFO
ABSTRACT
Keywords: Oligomer Metallocene catalysts Hexene Kinetics Gas chromatography
The paper describes oligomerization reactions of 1-hexene with catalysts derived from three metallocene complexes, Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2, and two types of methylalumoxane. Gas chromatographic analysis of oligomers provides detailed information on the chemical structure of individual oligomer molecules, the chemical mechanism and kinetics of early stages of the oligomerization reactions.
1. Introduction
Three zirconocene complexes, Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (nC4H9-Cp)2ZrCl2, were converted into catalysts by activating them with two types of methylalumoxane [Al(CH3)-O]i (MAO). The first type of MAO was a commercial product manufactured by reacting Al(CH3)3 with water [2,3]; it is designated as MAOO. The second type of MAO, designated as MAOSn, was produced by reacting Al(CH3)3 with an organotin compound [(n-C4H9)3Sn]2O [4]:
Oligomerization reactions of higher 1-alkenes catalyzed by some transition metal-based catalysts produce relatively short oligomer molecules which are miscible with the monomers. Chemical and kinetic studies of these reactions have several mechanistic advantages in comparison with the studies of polymerization reactions with the same catalysts. In particular, gas chromatographic (GC) analysis of the oligomer mixtures provides very detailed information both about the chemical structure of each individual “polymer” molecule, albeit the shortest molecules with the polymerization degree n up to 4–5, and about the relative content of different oligomer molecules in the reaction products. This type of information is usually inaccessible when the same catalysts are used to produce polymers with a high molecular weight (MW), with n > 100. Alkene oligomerization reactions with metallocene catalysts are a well-studied subject. Publications [1] provide a comprehensive review of the type of products formed in oligomerization reactions of propylene, 1-butene, 1-hexene, etc., average MW of the oligomers, the structure of their chain ends and their possible commercial applications. This information is adequate for general understanding of the oligomerization reactions. However, it provides relatively few details on the issues relevant for the mechanistic analysis of catalytic oligomerization reactions. This article discusses several principal chemical and kinetic features of 1-hexene oligomerization reactions with metallocene catalysts which were not investigated earlier. These data are essential for understanding of the chemical and kinetic mechanism of catalytic polymerization reactions in general.
j Al(CH3)3 + i [(n-C4H9)3Sn]2O → [Al(CH3)−O]i + 2i (n-C4H9)3Sn− CH3 (1) where j ≥ i. Main advantages of using MAOSn as a cocatalyst are ability to control its average “oligomerization degree” i and to produce MAO samples with low content of free Al(CH3)3. In general, the principal mechanistic features of alkene polymerization reactions with metallocene catalysts of the Cp2Zr2 - MAO type have been firmly established (see reviews [5–8]). The active centers in these catalysts are metallocenium cations Cp2Zr+―(monomer)n―H bearing polymer chains with different n. These active centers are initially formed by alkylation of the metallocene complexes to Cp2Zr (CH3)Cl or Cp2Zr(CH3)2 and their subsequent ionization in reactions with MAO, e.g.: Cp2Zr(CH3)2 + [Al(CH3)O]i → Cp2Zr+−CH3⋯{[Al(CH3)O]i−CH3}− (2) The metallocenium cations Cp2Zr+–(monomer)n–H are strongly associated with voluminous counter-anions {[Al(CH3)O]i−CH3}– [5,9]. These catalysts grow polymer chains by inserting C]C bonds of alkene molecules CH2]CHRʹ into the Cp2Zr+eC bond, for example, Cp2Zr+−CH3 + CH2]CHRʹ → Cp2Zr+−CH2−CHRʹ−CH3
E-mail address: [email protected]. https://doi.org/10.1016/j.mcat.2018.11.013 Received 21 October 2018; Received in revised form 2 November 2018; Accepted 20 November 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
(3)
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Polymerization reactions of ethylene with these catalysts at 40–80 °C lead to the formation of polymers with a high MW. However, when 1-hexene is used under the same conditions, these reactions produce short oligomer molecules.
Table 2 Effect of catalyst type and reaction conditions on probability of chain growth and weight-average oligomerization degree.
2. Experimental Complexes Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 were supplied by Boulder Scientific Co. Schering Berlin Polymers Company supplied the MAOO sample as solution in toluene with the total Al content 5.23 wt.-%, MAO concentration ∼10 wt.-%, free Al(CH3)3 ∼30% of the total Al content. [(C4H9)3Sn]2O, Al(CH3)3 and 1-hexene were purchased from Sigma-Aldrich. Oligomerization reactions were carried out in a 0.5-liter autoclave equipped with a magnet-driven propeller stirrer, a manometer and an external heating jacket. The reactor was dried in a nitrogen flow at 90 °C for 60 min and cooled to ∼50 °C. Then, 200 ml of neat 1-hexene was added to the reactor under a nitrogen blanket followed by ∼10 mmol of Ali-(C4H9)3 (an impurity scavenger) and a small amount of MAOO or, in sequence, Al(CH3)3 and [(C4H9)3Sn]2O. The reactor was heated to a desired temperature and solution of a metallocene complex (5.0·10−4 to 8.0·10-3 mmol) in toluene, corresponding to the [Al]/[Zr] ratio from 200 to 400, was added. In some experiments, H2 was also added to the reactor. The reaction conditions are given in Table 1. The total reactor pressure during the experiments was ∼1.4 atm. The oligomerization reactions were carried out for 2 or 4 h, they produced liquid oligomers with conversion ranging from 30 to > 80%. Productivity of the catalysts in 4-h reactions varied from 2 × 105 to 4 × 105 g/mmol Zr. GC analysis of the oligomers was carried out with a Hewlett-Packard 5890 gas chromatograph equipped with a 60-meter MXT-1 stainlesssteel column (Restek Corp.) and an FID detector. Nitrogen was used as a carrier gas at a rate of 1.1 cc/min, the split ratio was 60:1. The injection temperature was 350 °C, the column temperature was increased from 40 to 350 °C at a rate of 5 °C/min and was kept at 350 °C for 20 min. Injected volumes of all samples were 0.5 μL. Weight response factors for all GC peaks were assumed equal. In several cases, a mixture of linear alkanes was added to the oligomers to aid in product identification. GC peak assignment was based on the earlier GC analysis of branched alkanes [10,11] and branched alkenes [12].
Type of MAO
Temp. (°C)
Conversion (%)
Temperature effect (n-C4H9-Cp)2ZrCl2 ″-″ ″-″ ″-″ ″-″ (n-C4H9-Cp)2ZrCl2 ″-″ ″-″
MAOO ″-″ ″-″ ″-″ ″-″ MAOSn ″-″ ″-″
40 60 80 90 100 60 70 80
52.5 75.6 68.9 31.2 13.9 73.9 76.2 86.5
80 ″-″ ″-″
61.4 70.2 86.5
80 ″-″ 90 70
61.4 64.3 76.8 45.0
″-″
23.0
Effect of metallocene complex Cp2ZrCl2 MAOSn ″-″ (CH3-Cp)2ZrCl2 (n-C4H9-Cp)2ZrCl2 ″-″ Hydrogen effect Cp2ZrCl2, PH2 = 0 MAOSn ″ - ″, PH2 = 4.8 atm ″-″ ″-″ ″ - ″, PH2 = 4.2 atm (n-C4H9-Cp)2ZrCl2, MAOO PH2 = 0 ″ - ″, PH2 = 27 atm ″-″
Temp. (°C)
Propagation probability p
nweighta
Temperature effect (n-C4H9-Cp)2ZrCl2 ″-″ ″-″
MAOSn ″-″ ″-″
60 70 80
0.464 0.444 0.246
3.8 3.7 2.9
Effect of metallocene complex Cp2ZrCl2 (CH3-Cp)2ZrCl2 (n-C4H9-Cp)2ZrCl2
MAOSn ″-″ ″-″
80 ″-″ ″-″
0.285 0.376 0.272
3.0 3.4 3.0
Effect of MAO type (n-C4H9-Cp)2ZrCl2 ″-″
MAOO MAOSn
70 ″-″
0.602 0.444
4.6 3.7
Hydrogen effect Cp2ZrCl2 - MAOSn ″-″ (n-C4H9-Cp)2ZrCl2 - MAOO ″-″
PH2 = 0 PH2 = 4.8 atm PH2 = 0 PH2 = 27 atm
80 ″-″ 70 ″-″
0.286 0.246 0.602 0.400
2.6 2.5 4.6 3.1
a
Weight-average oligomerization degree, estimated with Excel.
3. Results and discussion 3.1. General features of oligomerization reactions The data on monomer conversion (Table 1) demonstrate principal features of metallocene catalysts in the oligomerization reactions. They are in general agreement with the earlier published data on 1-hexene oligomerization [1] and with the results of ethylene and propylene polymerization reactions, which are usually conducted under similar conditions [1,13]: 1 All the catalysts are quite active at temperatures from 40 to 80 °C but their performance rapidly deteriorates at higher temperatures. 2 All three metallocene complexes produce active species of similar activity. Judging by reaction conversion, the productivity ratio for Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 is ∼1:1.2:1.4. 3 Over the reaction span of 4 h, each mmol of a zirconocene-derived active species produces from 2 × 105 to 4 × 105 g of oligomers. The oligomer mixtures are very light, the average n value varies from 3 to 4 (see Table 2 below). Taking this into account, each active species produces from 8 × 105 to 1.2 × 106 oligomer molecules. 4 The presence of H2 in the reactions has practically no effect on the catalyst productivity. This insensitivity to H2 distinguish metallocene catalysts from Ti-based Ziegler-Natta catalysts which exhibit a strong H2 effect: H2 depresses their reactivity in ethylene polymerization reactions and increases it in polymerization reactions of 1-alkenes [13].
Table 1 Experimental conditions of oligomerization reactionsa. Metallocene complex
Type of MAO
Metallocene complex
3.2. Detailed chemistry of oligomerization reactions Scheme 1 shows principal chemical steps of the oligomerization reactions. This scheme is based on published experimental studies of metallocene polymerization catalysis (see reviews in [1,5,11]). GC analysis of the 1-hexene oligomers (discussed below) confirms each step in the scheme. The starting species are metallocenium cations Cp2Zr+−CH3 formed in Reaction (2). The C]C bond of a 1-hexene molecule inserts into the Zr+−CH3 bond (Reaction (3)); this reaction produces the Zr+−CH2−CH(C4H9)−CH3 species. Insertion of 1-hexene molecules into the Zr+−CH2R bond (chain propagation reactions) leads to lengthening of the alkyl chain attached to the Zr atom, i.e., the formation of Zr-Oligo1 species:
a Reactions with MAOO: [Zr] ∼5.0·10−4 mmol, [Al]/[Zr]molar ∼200; reactions with MAOSn: [Zr] ∼8.0·10-3 mmol, [Sn]/[Zr]molar ∼400.
88
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Scheme 1. Chemical steps of 1-hexene oligomerization reaction.
Cp2Zr+e[CH2−CH(C4H9)]n−CH3 [CH2−CH(C4H9)]n+1−CH3
+
CH2]CHeC4H9
→Cp2Zr+e (4)
Indeed, such reactions would produce the same Oligomer-1 and Oligomer-2 molecules as Reactions (5) and (7), respectively. However, numerous studies on kinetics of olefin polymerization and oligomerization reactions with metallocene catalysts (see a review in [13]) have shown that the dominant chain transfer reactions in metallocene catalysis have the first-order dependence on the monomer concentration, i.e., that Reactions (5) and (7) rather than the β-H elimination (Reaction (8)) are responsible for the formation of oligomer molecules with vinylidene end-bonds. GC analysis provides detailed information on the chemical structure of each oligomer molecule formed according to Scheme 1. Fig. 1 shows a part of the GC trace of the oligomer mixture produced with the (nC4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C. The range includes 1-hexene dimers (retention time 11–13 min), trimers (22–25 min), tetramers (30–34 min) and pentamers (36–40 min). Single peaks dominate the dimer and the trimer ranges. Relative retention times of these peaks (with respects to GC peaks of n-alkanes, which were run in parallel) allow an unambiguous assignment [12] of these products as CH2]C(C4H9)eC6H13 and CH2]C(C4H9)−CH2−CH(C4H9)−C6H13; the most direct proof of the primary 1-hexene insertion into the Cp2Zr+−H and the Cp2Zr+−CH2 bonds. The pentamer and the products with a higher oligomerization degree are represented by several closely spaced peaks of similar area. The catalysts used in the oligomerization reactions are not stereospecific and each oligomer molecule with higher n is a mixture of diastereomers with a slightly different GC retention time:
On occasion, a different type of reaction between the Zr+eC bond and a 1-hexene molecule takes place, the chain transfer reaction to a monomer: Cp2Zr+−CH2−CH(C4H9)n−CH3 + CH2]CH−C4H9 → Cp2Zr+ −CH2−CH2−C4H9 + CH2]C(C4H9)n-1−CH3 (5) Reaction (5) separates the oligomer molecule Oligomer-1 with an odd number of carbon atoms and starts the main catalytic cycle (the bottom cycle in Scheme 1), formation of active species Zr-Oligo2 and oligomerization products Oligomer-2 with an even number of carbon atoms (Reactions (6) and (7)): Cp2Zr+−CH2−CH2−C4H9 + n CH2]CH−C4H9 −[CH2−CH(C4H9)]n−CH2−CH2−C4H9
→
Cp2Zr+ (6)
Cp2Zr+−[CH2−CH(C4H9)]n−CH2−CH2−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2−C4H9 + CH2]C(C4H9)−[CH2−CH(C4H9)]n(7) 1−CH2−CH2−C4H9 In the earlier publications on 1-hexene oligomerization reactions with metallocene catalysts, the formation of odd- and even-C-numbered oligomer molecules is often ascribed to β-H elimination reactions in Cp2Zr+e[CH2−CH(C4H9)]neR species [1], for example: Cp2Zr+−[CH2−CH(C4H9)]n−R [CH2−CH(C4H9)]n-1−R
→
Cp2Zr+−H + CH2]C(C4H9)− (8)
89
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Fig. 2 shows the expanded GC dimer range of the same oligomer mixture. Apart from the major peak of the dimer, 2-butyl-1-octene (peak 1), several minor GC peaks are present. Two of them (peaks 2) belong to two linear dodecenes with internal C]C bonds. Their content is very low, ∼0.6%; they are formed in a rare secondary 1-hexene insertion into the Cp2Zr+eC bond, Reaction (9) (a “head-to-head” chain growth reaction in the terminology of polymer chemistry), which is immediately followed by the chain transfer reaction to a monomer, Reaction (10): Cp2Zr+−CH2−CH2−C4H9 + C4H9−CH = CH2 (C4H9)−CH2−CH2−CH2−C4H
→
Cp2Zr+−CH (9)
Cp2Zr+−CH(C4H9)−CH2−CH2−CH2−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2―C4H9 + C4H9−CH]CH−CH2−CH2−C4H9 or (10) C3H7−CH]CH−CH2−CH2−CH2−C4H9 Judging by the relative areas of GC peaks 2 and 1 in Fig. 2, the secondary insertion of a 1-hexene molecule into the Cp2Zr+eC bond is at least 2∙103 times less probable than its primary insertion. The GC peak 3 in Fig. 2 belongs to the hydrogenated dimer, CH3−CH (C4H9)−CH2−CH2eC4H9 (5-methylundecane, content ∼0.4%). The hydrogenated trimer was also identified in the oligomer mixtures. The GC peak 4 belongs to the dimer of the Oligomers-1 series (see Scheme 1), 2butyl-4-methyl-1-octene. In principle, the formation of both types of these products follows from Scheme 1: hydrogenated oligomers (Oligomers-3) are formed in the chain transfer reaction to Al(CH3)3 (this compound is an admixture in both MAOO and MAOSn) and each such reaction eventually leads to the formation of an Oligomer-1-type product. Earlier, we described another possible route to the formation of saturated oligomer chains [14]. Such products are formed with significant yields in ethylene homo-oligomerization and ethylene/1-alkene co-oligomerization reactions with metallocene catalysts at increased temperatures. The formation of the saturated products was explained by reversibility of the reaction between metallocenium active species, Cp2Zr+e[CH2−CH(C4H9)]n−H and {[Al(CH3)O]i−CH3}― anions:
Fig. 1. Gas chromatogram (C12 – C30 range) of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C.
Cp2Zr+−[CH2−CH(C4H9)]n−H⋯ {[Al(CH3)O]i−CH3}− ↔ Cp2Zr (CH3)−[CH2−CH(C4H9)]n−H + [Al(CH3)O]i (11) The interaction of neutral metallocene complexes Cp2Zr(CH3)― [CH2−CH(C4H9)]n―H with Al(CH3)3 produces analogs of Tebbe reagents, Cp2Zr(CH3)−CH2―AlMe2 and releases saturated oligomers CH3−CH(C4H9)―[CH2−CH(C4H9)]n-1―H. The GC peak 5 in Fig. 2, judging by its position, belongs to a highly branched dimer molecule. A possible route to one such product could start as a chain transfer reaction similar to Reaction (7) but with the secondary coordination of the 1-hexene molecule: Cp2Zr+−[CH2−CH(C4H9)n]−H + CH2]CH−C4H9 → Cp2Zr+−CH (CH3)−C4H9 + CH2]C(C4H9)−[CH2−CH(C4H9)]n-1−H (12) The Cp2Zr+−CH(CH3)eC4H9 species formed in Reaction (12) is stabilized by hyper-conjugation of its β−CH3 group and Zr+ but it can insert a 1-hexene molecule:
Fig. 2. The dimer range in gas chromatogram of 1-hexene oligomer produced with (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C.
Cp2Zr+−CH(CH3−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH (13) (C4H9)−CH(CH3)−C4H9
CH2]C(C4H9)−CH2−C*H(C4H9)−[CH2−CH(C4H9)]n-2−CH2−C*H (C4H9)−n-C6H13
A chain transfer converts the product of Reaction (13) into a highly branched 1-alkene:
All the dominant light oligomers in Fig. 1 belong to the Oligomer-2 products: Oligomer range: Formula: Content (%)
Dimers (C12) 2-C4H9-1-C8H15 97.2
Trimers (C18) 2,4-(C4H9)2-1-C10H18 93.5
Cp2Zr+−CH2−CH(C4H9)−CH(CH3)−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2−C4H9 + CH2]C(C4H9)−CH(CH3)−C4H9 (14) The estimation of its GC parameters using the approach outlined in [10–12] places it in the position close to that of the peak 5 in Fig. 2. Small GC peaks of such highly branched 1-alkenes can be also seen in Fig. 1. Two other reactions accompanying 1-hexene oligomerization with
Tetramers (C24) 2,4,6-(C4H9)3-1-C12H21 93.2
90
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Scheme 3. Post-reaction isomerization of vinylidene bond.
Figs. 1 and 2 are produced only if the GC analysis is carried out immediately after oligomerization reactions. Keeping untreated or alcohol-treated oligomer mixtures for several days leads to significant isomerization of vinylidene double bonds into tri-substituted C]C bonds, as shown in Scheme 3. These products are marked iso-Oligo in Fig. 3.
Fig. 3. Gas chromatogram (dimer-tetramer range) of 1-hexene oligomer produced with Cp2ZrCl2 - MAOSn catalyst at 90 °C in the presence of H2. The products were exposed to air for 24 h.
3.3. Oligomerization kinetics
zirconocene catalysts are easily detectable by GC, hydrogenation of zirconocene cations Cp2Zr+―C by H2 and post-reaction isomerization of 1-hexene oligomers. The products of both these reactions are apparent from Fig. 3. It shows the GC trace of the 1-hexene oligomer mixture which was produced with the Cp2ZrCl2 - MAOSn catalyst at 90 °C in the presence of H2 and was exposed to air for 24 h. In this case, the formation of large quantities of hydrogenated oligomers (marked H2-Oligo in Fig. 3) shows that H2 readily interacts with any Cp2Zr+―C bond. This reaction leads to an additional catalytic cycle shown in Scheme 2. All oligomerization reactions carried out in the presence of H2 (Table 1) yielded large quantities of hydrogenated oligomers: Metallocene complex
Reaction conditions
Cp2ZrCl2 Cp2ZrCl2 (n-C4H9-Cp)2ZrCl2
80 °C, PH2 = 4.8 atm 90 °C, PH2 = 4.2 atm 70 °C, PH2 = 27 atm
Each active species derived from a zirconocene complex produces, on average, ∼1·106 oligomer molecules over a 4-hour reaction time, i.e., the formation time of a single oligomer molecule is about 0.01–0.02 s. Therefore, concentrations of all reactants, 1-hexene, the active species and Al(CH3)3, could be regarded constant during this time interval. The normalized formation probability of an oligomer molecule with n monomer units starting from n = 2 (dimer) and, therefore, the content of the [Oln] oligomer molecules under stationary conditions is: [Oln] = (1-p)∙pn−2 or [(1-p)/p2]∙pn
(15)
where p is the probability of chain growth and ∑[Oln] = 1 for n > 1. In the simplest case, when an oligomerization cycle is limited to Reactions (6) and (7), which are both first-order reactions with respect to the monomer concentration CM,
Hydrogenated products, % Dimers Trimers Tetramers 40 22 – 39 25 26 91 78 54
p = kp/(kp + ktM)
(16)
where kp is the rate constant of the chain growth reaction, Reaction (6), and ktM is the rate constant of the chain transfer reaction to the monomer, Reaction (7). If an oligomerization reaction is carried out in the presence of H2, the expression for the chain growth probability is:
The Cp2Zr+―n-C6H13 species (see Schemes 1 and 2) is also easily hydrogenated; this reaction produces n-hexane. GC analysis of several 1-hexene oligomers after their exposure to air and moisture showed that a residue of MAO in these products is readily converted into a potent acidic catalyst that isomerizes end-double bonds in the oligomer molecules. “Clean” gas chromatograms as that in
p = kp∙CM/(kp∙CM + k tM∙CM + k tH2∙CH2) where k t
H2
is the rate constant of chain transfer reaction to H2 (see
Scheme 2. Oligomerization of 1-hexene in the presence of H2. 91
(17)
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Fig. 4. Distribution of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 MAOO catalyst at 70 °C.
Fig. 5. Modeling oligomer distribution in coordinates log([Yieldn]weight/n) vs. n (see Supplement). Parameters: p = 0.5, kpʹ/kp = 5, kp″/kp = 3, kp″ʹ/kp = 1.5, kt″/kt = 4, kt″ʹ/ kt = 2.5.
Scheme 2) and CH2 is the concentration of H2 in solution. The area under a GC peak of a particular oligomer, Arean, is proportional to the weight amount of the oligomer molecule containing n monomer units. Thus, the Arean/n ratio is proportional to the [Oln] value in Eq. (15) and the GC data can be linearized as: Log(Arean/n) ∼ Const + n∙log(p)
kinetics. The scheme assumes that the propagation rate constant kp and the chain transfer rate constant kt do not depend on the oligomerization degree n. Fig. 4 shows that this assumption holds well for n > 4 but it is not satisfactory when the formation of the lightest oligomer molecules is examined. A more detailed kinetic scheme (Scheme 4) takes these effects into account. Scheme 4 assumes that the values of kp and kt vary with n at the earliest stages of oligomer chain formation:
(18)
Fig. 4 shows the data on the distribution of 1-hexene oligomers produced with the (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C in the coordinates of Eq. (18). Indeed, most of the points on the plot are on a straight line indicating applicability of Eq. (15) for the description of the oligomer yield distribution vs. n. However, a noticeable deviation from the power law of Eq. (15) is obvious for the lightest oligomers with n = 2, 3 and 4. Similar deviations from the power distribution were observed for 1-hexene oligomers produced with all the metallocene catalysts. These results show that the basic reaction sequence in Scheme 1 is not fully suitable for a precise kinetic description of the oligomerization
- for species Zr-Ol1 (species Cp2Zr+―C6H13) - kpʹ (chain transfer on Zr-Ol1 species is a nul-reaction). - for species Zr-Ol2 (species Cp2Zr+−CH2−CH(C4H9)―C6H13) - kp″ and ktM″. - for species Zr-Ol3 (species Cp2Zr+―[CH2−CH(C4H9)]2―C6H13) kpʹ″ and ktMʹ″.
Scheme 4. Kinetic scheme of early stages in oligomerization reaction. 92
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
features, the nature of the Cp rings attached to the Zr atom and the structure of the MAO counter-anion [5,13]. Both the association degree i and the structure of the MAO counter-anion in these active species depend on the conditions of MAO synthesis, whether it is produced in a reaction of Al(CH3)3 with water (the standard reaction of MAO synthesis) or in a reaction of Al(CH3)3 with an organotin compound (Reaction (1)). 4. Conclusions 1 GC analysis of the lightest products formed in 1-hexene oligomerization reactions with several metallocene catalysts, complexes Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 activated with two types of methylalumoxane, provides the most detailed information on the chemical structure of each individual oligomer molecule. The data demonstrate the dominance of primary 1-hexene insertion into the Cp2Zr+eH and the Cp2Zr+−CH2 bonds, provide the estimation of the probability of secondary 1-hexene insertion into the same bonds and the formation probability of hydrogenated analogs of the oligomers in the absence and in the presence of H2 in the oligomerization reactions. 2 GC analysis of the oligomers shows that their exposure to atmosphere for prolonged periods of time converts residue of methylalumoxane into potent acidic catalyst species which readily isomerize end-C]C bonds in the reaction products into internal tri-substituted C]C bonds. 3 Distribution of product yields with respect to the oligomerization degree confirms the stochastic nature of the oligomerization reactions. Significant deviations from the expected power-law distribution were found only for the lightest products, dimers-tetramers. The deviations are explained by steric effects in the formation reactions of the shortest growing oligomer chains.
Fig. 6. Distribution of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 MAOO catalyst at 70 °C in the absence of H2 (1) and at PH2 = .27 atm (2).
- for all other species Zr-Oln (species Cp2Zr+―[CH2−CH(C4H9)]nM 1―C6H13) - kp and kt . Kinetic expressions for relative yields of oligomers with different n values are given in Supplement. One example of numerical modeling of the oligomer distribution based on Scheme 4 is shown in Fig. 5 for p = 0.5. Oligomer yields were calculated with Excel using equations in Supplement. The modeling takes into consideration obvious steric effects in bimolecular reactions with the participation of a 1-hexene molecule: kpʹ > > kp because its insertion into the Cp2Zr+―C bond of the active center Zr-Ol1 with a short linear alkyl group [i.e., Cp2Zr+―(CH2)5−CH3] proceeds faster than the same reaction in the case of any branched Zr-Oln center, the rates of all reactions are higher when they involve active centers Zr-Oln with shorter and less branched chains, etc. Table 2 gives the probability of chain growth p on active centers ZrOln with relatively long chains, n > 4, for different catalysts and for different oligomerization conditions. In the case of “true” polymerization reactions, this probability is very high, p > 0.99 [13], and the effect of polymerization conditions on the p value is difficult to evaluate with good precision. In contrast, the p value in the 1-hexene oligomerization reactions is low, between 0.2 and 0.6, and it can be evaluated more dependably. Table 2 also lists weight-average chain lengths of the oligomers. The effects of the reaction parameters follow the expected pattern. As typical for all alkene polymerization reactions with metallocene catalysts, an increase of reaction temperature decreases the probability of chain growth (Eq. (16)) and decreases the average “MW of the polymer” (in this case, the weight-average oligomerization degree) because the activation energy of the chain transfer reaction to the monomer (Reaction (7)) is higher than the activation energy of the chain propagation reaction (Reaction (6)). Introduction of hydrogen to the oligomerization reaction invariably decreases the probability of chain growth (Eq. (17)) because of an additional chain transfer reaction (Scheme 2). One example of this difference is shown in Fig. 6. Both the type of the metallocene complex and the source of the cocatalyst, MAOO vs. MAOSn, also affect the oligomerization kinetics. The activity and the kinetic properties of reaction centers Cp2Zr+― (monomer)n―H… {[Al(CH3)O]i−CH3}―obviously depend on their two
Conflict of interest The author declares no conflict of interest. References [1] (a) C. Janiak, K.C.H. Lange, P. Marquardt, J. Mol. Catal. A: Chem. 180 (2002) 43–58; (b) C. Janiak, Coord. Chem. Rev. 250 (2006) 66–94; (c) C. Janiak, F. Blank, Macromol. Symp. 236 (2006) 14–22; (d) C. Janiak, K.C.H. Lange, P. Marquardt, R.-P. Kruger, R. Hanselmann, Macromol. Chem. Phys. 203 (2002) 129–138; (e) Y.V. Kissin, F.S. Schwab, J. Appl. Polym. Sci. 111 (2008) 273–280. [2] D.B. Malpass, R. Hoff (Ed.), Handbook of Transition Metal Polymerization Catalysts, 2nd edition, Wiley, New York, 2018, pp. 1–30. [3] (a) E.Y. Chen, T.J. Marks, Chem. Rev. 100 (2000) 1391–1434; (b) H. Zijlstra, S. Harder, Ber (2015) 2014–2020. [4] Y.V. Kissin, A.J. Brandolini, Macromolecules 36 (2003) 18–28. [5] Y.V. Kissin, Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, Amsterdam, 2008 Chapter 6. [6] A.G. Alt, A. Koppl, Chem. Rev. 100 (2000) 1205–1222. [7] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100 (2000) 1253–1346. [8] M. Bochmann, Organometallics 29 (2010) 4711–4740. [9] L. Negureanu, R.W. Hall, G. Leslie, L.G. Butler, L.A. Simeral, J. Am. Chem. Soc. 128 (2006) 16816–16826. [10] Y.V. Kissin, G.P. Feulmer, J. Chromatogr. Sci. 24 (1986) 53–61. [11] Y.V. Kissin, G.P. Feulmer, W.P. Payne, J. Chromatogr. Sci. 24 (1986) 64–169. [12] Y.V. Kissin, J. Chromatogr. Sci. 24 (1986) 278–285. [13] Y.V. Kissin, Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, Amsterdam, 2008 Chapter 5. [14] Y.V. Kissin, A.S. Goldman, Macromol. Chem. Phys. 210 (2009) 1942–1956.
93
Contents lists available at ScienceDirect
Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
Oligomerization reactions of 1-hexene with metallocene catalysts: Detailed data on reaction chemistry and kinetics
T
Yury V. Kissin Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 123 Bevier Road, Piscataway, NJ, 08854-8087, USA
ARTICLE INFO
ABSTRACT
Keywords: Oligomer Metallocene catalysts Hexene Kinetics Gas chromatography
The paper describes oligomerization reactions of 1-hexene with catalysts derived from three metallocene complexes, Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2, and two types of methylalumoxane. Gas chromatographic analysis of oligomers provides detailed information on the chemical structure of individual oligomer molecules, the chemical mechanism and kinetics of early stages of the oligomerization reactions.
1. Introduction
Three zirconocene complexes, Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (nC4H9-Cp)2ZrCl2, were converted into catalysts by activating them with two types of methylalumoxane [Al(CH3)-O]i (MAO). The first type of MAO was a commercial product manufactured by reacting Al(CH3)3 with water [2,3]; it is designated as MAOO. The second type of MAO, designated as MAOSn, was produced by reacting Al(CH3)3 with an organotin compound [(n-C4H9)3Sn]2O [4]:
Oligomerization reactions of higher 1-alkenes catalyzed by some transition metal-based catalysts produce relatively short oligomer molecules which are miscible with the monomers. Chemical and kinetic studies of these reactions have several mechanistic advantages in comparison with the studies of polymerization reactions with the same catalysts. In particular, gas chromatographic (GC) analysis of the oligomer mixtures provides very detailed information both about the chemical structure of each individual “polymer” molecule, albeit the shortest molecules with the polymerization degree n up to 4–5, and about the relative content of different oligomer molecules in the reaction products. This type of information is usually inaccessible when the same catalysts are used to produce polymers with a high molecular weight (MW), with n > 100. Alkene oligomerization reactions with metallocene catalysts are a well-studied subject. Publications [1] provide a comprehensive review of the type of products formed in oligomerization reactions of propylene, 1-butene, 1-hexene, etc., average MW of the oligomers, the structure of their chain ends and their possible commercial applications. This information is adequate for general understanding of the oligomerization reactions. However, it provides relatively few details on the issues relevant for the mechanistic analysis of catalytic oligomerization reactions. This article discusses several principal chemical and kinetic features of 1-hexene oligomerization reactions with metallocene catalysts which were not investigated earlier. These data are essential for understanding of the chemical and kinetic mechanism of catalytic polymerization reactions in general.
j Al(CH3)3 + i [(n-C4H9)3Sn]2O → [Al(CH3)−O]i + 2i (n-C4H9)3Sn− CH3 (1) where j ≥ i. Main advantages of using MAOSn as a cocatalyst are ability to control its average “oligomerization degree” i and to produce MAO samples with low content of free Al(CH3)3. In general, the principal mechanistic features of alkene polymerization reactions with metallocene catalysts of the Cp2Zr2 - MAO type have been firmly established (see reviews [5–8]). The active centers in these catalysts are metallocenium cations Cp2Zr+―(monomer)n―H bearing polymer chains with different n. These active centers are initially formed by alkylation of the metallocene complexes to Cp2Zr (CH3)Cl or Cp2Zr(CH3)2 and their subsequent ionization in reactions with MAO, e.g.: Cp2Zr(CH3)2 + [Al(CH3)O]i → Cp2Zr+−CH3⋯{[Al(CH3)O]i−CH3}− (2) The metallocenium cations Cp2Zr+–(monomer)n–H are strongly associated with voluminous counter-anions {[Al(CH3)O]i−CH3}– [5,9]. These catalysts grow polymer chains by inserting C]C bonds of alkene molecules CH2]CHRʹ into the Cp2Zr+eC bond, for example, Cp2Zr+−CH3 + CH2]CHRʹ → Cp2Zr+−CH2−CHRʹ−CH3
E-mail address: [email protected]. https://doi.org/10.1016/j.mcat.2018.11.013 Received 21 October 2018; Received in revised form 2 November 2018; Accepted 20 November 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
(3)
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Polymerization reactions of ethylene with these catalysts at 40–80 °C lead to the formation of polymers with a high MW. However, when 1-hexene is used under the same conditions, these reactions produce short oligomer molecules.
Table 2 Effect of catalyst type and reaction conditions on probability of chain growth and weight-average oligomerization degree.
2. Experimental Complexes Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 were supplied by Boulder Scientific Co. Schering Berlin Polymers Company supplied the MAOO sample as solution in toluene with the total Al content 5.23 wt.-%, MAO concentration ∼10 wt.-%, free Al(CH3)3 ∼30% of the total Al content. [(C4H9)3Sn]2O, Al(CH3)3 and 1-hexene were purchased from Sigma-Aldrich. Oligomerization reactions were carried out in a 0.5-liter autoclave equipped with a magnet-driven propeller stirrer, a manometer and an external heating jacket. The reactor was dried in a nitrogen flow at 90 °C for 60 min and cooled to ∼50 °C. Then, 200 ml of neat 1-hexene was added to the reactor under a nitrogen blanket followed by ∼10 mmol of Ali-(C4H9)3 (an impurity scavenger) and a small amount of MAOO or, in sequence, Al(CH3)3 and [(C4H9)3Sn]2O. The reactor was heated to a desired temperature and solution of a metallocene complex (5.0·10−4 to 8.0·10-3 mmol) in toluene, corresponding to the [Al]/[Zr] ratio from 200 to 400, was added. In some experiments, H2 was also added to the reactor. The reaction conditions are given in Table 1. The total reactor pressure during the experiments was ∼1.4 atm. The oligomerization reactions were carried out for 2 or 4 h, they produced liquid oligomers with conversion ranging from 30 to > 80%. Productivity of the catalysts in 4-h reactions varied from 2 × 105 to 4 × 105 g/mmol Zr. GC analysis of the oligomers was carried out with a Hewlett-Packard 5890 gas chromatograph equipped with a 60-meter MXT-1 stainlesssteel column (Restek Corp.) and an FID detector. Nitrogen was used as a carrier gas at a rate of 1.1 cc/min, the split ratio was 60:1. The injection temperature was 350 °C, the column temperature was increased from 40 to 350 °C at a rate of 5 °C/min and was kept at 350 °C for 20 min. Injected volumes of all samples were 0.5 μL. Weight response factors for all GC peaks were assumed equal. In several cases, a mixture of linear alkanes was added to the oligomers to aid in product identification. GC peak assignment was based on the earlier GC analysis of branched alkanes [10,11] and branched alkenes [12].
Type of MAO
Temp. (°C)
Conversion (%)
Temperature effect (n-C4H9-Cp)2ZrCl2 ″-″ ″-″ ″-″ ″-″ (n-C4H9-Cp)2ZrCl2 ″-″ ″-″
MAOO ″-″ ″-″ ″-″ ″-″ MAOSn ″-″ ″-″
40 60 80 90 100 60 70 80
52.5 75.6 68.9 31.2 13.9 73.9 76.2 86.5
80 ″-″ ″-″
61.4 70.2 86.5
80 ″-″ 90 70
61.4 64.3 76.8 45.0
″-″
23.0
Effect of metallocene complex Cp2ZrCl2 MAOSn ″-″ (CH3-Cp)2ZrCl2 (n-C4H9-Cp)2ZrCl2 ″-″ Hydrogen effect Cp2ZrCl2, PH2 = 0 MAOSn ″ - ″, PH2 = 4.8 atm ″-″ ″-″ ″ - ″, PH2 = 4.2 atm (n-C4H9-Cp)2ZrCl2, MAOO PH2 = 0 ″ - ″, PH2 = 27 atm ″-″
Temp. (°C)
Propagation probability p
nweighta
Temperature effect (n-C4H9-Cp)2ZrCl2 ″-″ ″-″
MAOSn ″-″ ″-″
60 70 80
0.464 0.444 0.246
3.8 3.7 2.9
Effect of metallocene complex Cp2ZrCl2 (CH3-Cp)2ZrCl2 (n-C4H9-Cp)2ZrCl2
MAOSn ″-″ ″-″
80 ″-″ ″-″
0.285 0.376 0.272
3.0 3.4 3.0
Effect of MAO type (n-C4H9-Cp)2ZrCl2 ″-″
MAOO MAOSn
70 ″-″
0.602 0.444
4.6 3.7
Hydrogen effect Cp2ZrCl2 - MAOSn ″-″ (n-C4H9-Cp)2ZrCl2 - MAOO ″-″
PH2 = 0 PH2 = 4.8 atm PH2 = 0 PH2 = 27 atm
80 ″-″ 70 ″-″
0.286 0.246 0.602 0.400
2.6 2.5 4.6 3.1
a
Weight-average oligomerization degree, estimated with Excel.
3. Results and discussion 3.1. General features of oligomerization reactions The data on monomer conversion (Table 1) demonstrate principal features of metallocene catalysts in the oligomerization reactions. They are in general agreement with the earlier published data on 1-hexene oligomerization [1] and with the results of ethylene and propylene polymerization reactions, which are usually conducted under similar conditions [1,13]: 1 All the catalysts are quite active at temperatures from 40 to 80 °C but their performance rapidly deteriorates at higher temperatures. 2 All three metallocene complexes produce active species of similar activity. Judging by reaction conversion, the productivity ratio for Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 is ∼1:1.2:1.4. 3 Over the reaction span of 4 h, each mmol of a zirconocene-derived active species produces from 2 × 105 to 4 × 105 g of oligomers. The oligomer mixtures are very light, the average n value varies from 3 to 4 (see Table 2 below). Taking this into account, each active species produces from 8 × 105 to 1.2 × 106 oligomer molecules. 4 The presence of H2 in the reactions has practically no effect on the catalyst productivity. This insensitivity to H2 distinguish metallocene catalysts from Ti-based Ziegler-Natta catalysts which exhibit a strong H2 effect: H2 depresses their reactivity in ethylene polymerization reactions and increases it in polymerization reactions of 1-alkenes [13].
Table 1 Experimental conditions of oligomerization reactionsa. Metallocene complex
Type of MAO
Metallocene complex
3.2. Detailed chemistry of oligomerization reactions Scheme 1 shows principal chemical steps of the oligomerization reactions. This scheme is based on published experimental studies of metallocene polymerization catalysis (see reviews in [1,5,11]). GC analysis of the 1-hexene oligomers (discussed below) confirms each step in the scheme. The starting species are metallocenium cations Cp2Zr+−CH3 formed in Reaction (2). The C]C bond of a 1-hexene molecule inserts into the Zr+−CH3 bond (Reaction (3)); this reaction produces the Zr+−CH2−CH(C4H9)−CH3 species. Insertion of 1-hexene molecules into the Zr+−CH2R bond (chain propagation reactions) leads to lengthening of the alkyl chain attached to the Zr atom, i.e., the formation of Zr-Oligo1 species:
a Reactions with MAOO: [Zr] ∼5.0·10−4 mmol, [Al]/[Zr]molar ∼200; reactions with MAOSn: [Zr] ∼8.0·10-3 mmol, [Sn]/[Zr]molar ∼400.
88
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Scheme 1. Chemical steps of 1-hexene oligomerization reaction.
Cp2Zr+e[CH2−CH(C4H9)]n−CH3 [CH2−CH(C4H9)]n+1−CH3
+
CH2]CHeC4H9
→Cp2Zr+e (4)
Indeed, such reactions would produce the same Oligomer-1 and Oligomer-2 molecules as Reactions (5) and (7), respectively. However, numerous studies on kinetics of olefin polymerization and oligomerization reactions with metallocene catalysts (see a review in [13]) have shown that the dominant chain transfer reactions in metallocene catalysis have the first-order dependence on the monomer concentration, i.e., that Reactions (5) and (7) rather than the β-H elimination (Reaction (8)) are responsible for the formation of oligomer molecules with vinylidene end-bonds. GC analysis provides detailed information on the chemical structure of each oligomer molecule formed according to Scheme 1. Fig. 1 shows a part of the GC trace of the oligomer mixture produced with the (nC4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C. The range includes 1-hexene dimers (retention time 11–13 min), trimers (22–25 min), tetramers (30–34 min) and pentamers (36–40 min). Single peaks dominate the dimer and the trimer ranges. Relative retention times of these peaks (with respects to GC peaks of n-alkanes, which were run in parallel) allow an unambiguous assignment [12] of these products as CH2]C(C4H9)eC6H13 and CH2]C(C4H9)−CH2−CH(C4H9)−C6H13; the most direct proof of the primary 1-hexene insertion into the Cp2Zr+−H and the Cp2Zr+−CH2 bonds. The pentamer and the products with a higher oligomerization degree are represented by several closely spaced peaks of similar area. The catalysts used in the oligomerization reactions are not stereospecific and each oligomer molecule with higher n is a mixture of diastereomers with a slightly different GC retention time:
On occasion, a different type of reaction between the Zr+eC bond and a 1-hexene molecule takes place, the chain transfer reaction to a monomer: Cp2Zr+−CH2−CH(C4H9)n−CH3 + CH2]CH−C4H9 → Cp2Zr+ −CH2−CH2−C4H9 + CH2]C(C4H9)n-1−CH3 (5) Reaction (5) separates the oligomer molecule Oligomer-1 with an odd number of carbon atoms and starts the main catalytic cycle (the bottom cycle in Scheme 1), formation of active species Zr-Oligo2 and oligomerization products Oligomer-2 with an even number of carbon atoms (Reactions (6) and (7)): Cp2Zr+−CH2−CH2−C4H9 + n CH2]CH−C4H9 −[CH2−CH(C4H9)]n−CH2−CH2−C4H9
→
Cp2Zr+ (6)
Cp2Zr+−[CH2−CH(C4H9)]n−CH2−CH2−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2−C4H9 + CH2]C(C4H9)−[CH2−CH(C4H9)]n(7) 1−CH2−CH2−C4H9 In the earlier publications on 1-hexene oligomerization reactions with metallocene catalysts, the formation of odd- and even-C-numbered oligomer molecules is often ascribed to β-H elimination reactions in Cp2Zr+e[CH2−CH(C4H9)]neR species [1], for example: Cp2Zr+−[CH2−CH(C4H9)]n−R [CH2−CH(C4H9)]n-1−R
→
Cp2Zr+−H + CH2]C(C4H9)− (8)
89
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Fig. 2 shows the expanded GC dimer range of the same oligomer mixture. Apart from the major peak of the dimer, 2-butyl-1-octene (peak 1), several minor GC peaks are present. Two of them (peaks 2) belong to two linear dodecenes with internal C]C bonds. Their content is very low, ∼0.6%; they are formed in a rare secondary 1-hexene insertion into the Cp2Zr+eC bond, Reaction (9) (a “head-to-head” chain growth reaction in the terminology of polymer chemistry), which is immediately followed by the chain transfer reaction to a monomer, Reaction (10): Cp2Zr+−CH2−CH2−C4H9 + C4H9−CH = CH2 (C4H9)−CH2−CH2−CH2−C4H
→
Cp2Zr+−CH (9)
Cp2Zr+−CH(C4H9)−CH2−CH2−CH2−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2―C4H9 + C4H9−CH]CH−CH2−CH2−C4H9 or (10) C3H7−CH]CH−CH2−CH2−CH2−C4H9 Judging by the relative areas of GC peaks 2 and 1 in Fig. 2, the secondary insertion of a 1-hexene molecule into the Cp2Zr+eC bond is at least 2∙103 times less probable than its primary insertion. The GC peak 3 in Fig. 2 belongs to the hydrogenated dimer, CH3−CH (C4H9)−CH2−CH2eC4H9 (5-methylundecane, content ∼0.4%). The hydrogenated trimer was also identified in the oligomer mixtures. The GC peak 4 belongs to the dimer of the Oligomers-1 series (see Scheme 1), 2butyl-4-methyl-1-octene. In principle, the formation of both types of these products follows from Scheme 1: hydrogenated oligomers (Oligomers-3) are formed in the chain transfer reaction to Al(CH3)3 (this compound is an admixture in both MAOO and MAOSn) and each such reaction eventually leads to the formation of an Oligomer-1-type product. Earlier, we described another possible route to the formation of saturated oligomer chains [14]. Such products are formed with significant yields in ethylene homo-oligomerization and ethylene/1-alkene co-oligomerization reactions with metallocene catalysts at increased temperatures. The formation of the saturated products was explained by reversibility of the reaction between metallocenium active species, Cp2Zr+e[CH2−CH(C4H9)]n−H and {[Al(CH3)O]i−CH3}― anions:
Fig. 1. Gas chromatogram (C12 – C30 range) of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C.
Cp2Zr+−[CH2−CH(C4H9)]n−H⋯ {[Al(CH3)O]i−CH3}− ↔ Cp2Zr (CH3)−[CH2−CH(C4H9)]n−H + [Al(CH3)O]i (11) The interaction of neutral metallocene complexes Cp2Zr(CH3)― [CH2−CH(C4H9)]n―H with Al(CH3)3 produces analogs of Tebbe reagents, Cp2Zr(CH3)−CH2―AlMe2 and releases saturated oligomers CH3−CH(C4H9)―[CH2−CH(C4H9)]n-1―H. The GC peak 5 in Fig. 2, judging by its position, belongs to a highly branched dimer molecule. A possible route to one such product could start as a chain transfer reaction similar to Reaction (7) but with the secondary coordination of the 1-hexene molecule: Cp2Zr+−[CH2−CH(C4H9)n]−H + CH2]CH−C4H9 → Cp2Zr+−CH (CH3)−C4H9 + CH2]C(C4H9)−[CH2−CH(C4H9)]n-1−H (12) The Cp2Zr+−CH(CH3)eC4H9 species formed in Reaction (12) is stabilized by hyper-conjugation of its β−CH3 group and Zr+ but it can insert a 1-hexene molecule:
Fig. 2. The dimer range in gas chromatogram of 1-hexene oligomer produced with (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C.
Cp2Zr+−CH(CH3−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH (13) (C4H9)−CH(CH3)−C4H9
CH2]C(C4H9)−CH2−C*H(C4H9)−[CH2−CH(C4H9)]n-2−CH2−C*H (C4H9)−n-C6H13
A chain transfer converts the product of Reaction (13) into a highly branched 1-alkene:
All the dominant light oligomers in Fig. 1 belong to the Oligomer-2 products: Oligomer range: Formula: Content (%)
Dimers (C12) 2-C4H9-1-C8H15 97.2
Trimers (C18) 2,4-(C4H9)2-1-C10H18 93.5
Cp2Zr+−CH2−CH(C4H9)−CH(CH3)−C4H9 + CH2]CH−C4H9 → Cp2Zr+−CH2−CH2−C4H9 + CH2]C(C4H9)−CH(CH3)−C4H9 (14) The estimation of its GC parameters using the approach outlined in [10–12] places it in the position close to that of the peak 5 in Fig. 2. Small GC peaks of such highly branched 1-alkenes can be also seen in Fig. 1. Two other reactions accompanying 1-hexene oligomerization with
Tetramers (C24) 2,4,6-(C4H9)3-1-C12H21 93.2
90
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Scheme 3. Post-reaction isomerization of vinylidene bond.
Figs. 1 and 2 are produced only if the GC analysis is carried out immediately after oligomerization reactions. Keeping untreated or alcohol-treated oligomer mixtures for several days leads to significant isomerization of vinylidene double bonds into tri-substituted C]C bonds, as shown in Scheme 3. These products are marked iso-Oligo in Fig. 3.
Fig. 3. Gas chromatogram (dimer-tetramer range) of 1-hexene oligomer produced with Cp2ZrCl2 - MAOSn catalyst at 90 °C in the presence of H2. The products were exposed to air for 24 h.
3.3. Oligomerization kinetics
zirconocene catalysts are easily detectable by GC, hydrogenation of zirconocene cations Cp2Zr+―C by H2 and post-reaction isomerization of 1-hexene oligomers. The products of both these reactions are apparent from Fig. 3. It shows the GC trace of the 1-hexene oligomer mixture which was produced with the Cp2ZrCl2 - MAOSn catalyst at 90 °C in the presence of H2 and was exposed to air for 24 h. In this case, the formation of large quantities of hydrogenated oligomers (marked H2-Oligo in Fig. 3) shows that H2 readily interacts with any Cp2Zr+―C bond. This reaction leads to an additional catalytic cycle shown in Scheme 2. All oligomerization reactions carried out in the presence of H2 (Table 1) yielded large quantities of hydrogenated oligomers: Metallocene complex
Reaction conditions
Cp2ZrCl2 Cp2ZrCl2 (n-C4H9-Cp)2ZrCl2
80 °C, PH2 = 4.8 atm 90 °C, PH2 = 4.2 atm 70 °C, PH2 = 27 atm
Each active species derived from a zirconocene complex produces, on average, ∼1·106 oligomer molecules over a 4-hour reaction time, i.e., the formation time of a single oligomer molecule is about 0.01–0.02 s. Therefore, concentrations of all reactants, 1-hexene, the active species and Al(CH3)3, could be regarded constant during this time interval. The normalized formation probability of an oligomer molecule with n monomer units starting from n = 2 (dimer) and, therefore, the content of the [Oln] oligomer molecules under stationary conditions is: [Oln] = (1-p)∙pn−2 or [(1-p)/p2]∙pn
(15)
where p is the probability of chain growth and ∑[Oln] = 1 for n > 1. In the simplest case, when an oligomerization cycle is limited to Reactions (6) and (7), which are both first-order reactions with respect to the monomer concentration CM,
Hydrogenated products, % Dimers Trimers Tetramers 40 22 – 39 25 26 91 78 54
p = kp/(kp + ktM)
(16)
where kp is the rate constant of the chain growth reaction, Reaction (6), and ktM is the rate constant of the chain transfer reaction to the monomer, Reaction (7). If an oligomerization reaction is carried out in the presence of H2, the expression for the chain growth probability is:
The Cp2Zr+―n-C6H13 species (see Schemes 1 and 2) is also easily hydrogenated; this reaction produces n-hexane. GC analysis of several 1-hexene oligomers after their exposure to air and moisture showed that a residue of MAO in these products is readily converted into a potent acidic catalyst that isomerizes end-double bonds in the oligomer molecules. “Clean” gas chromatograms as that in
p = kp∙CM/(kp∙CM + k tM∙CM + k tH2∙CH2) where k t
H2
is the rate constant of chain transfer reaction to H2 (see
Scheme 2. Oligomerization of 1-hexene in the presence of H2. 91
(17)
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
Fig. 4. Distribution of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 MAOO catalyst at 70 °C.
Fig. 5. Modeling oligomer distribution in coordinates log([Yieldn]weight/n) vs. n (see Supplement). Parameters: p = 0.5, kpʹ/kp = 5, kp″/kp = 3, kp″ʹ/kp = 1.5, kt″/kt = 4, kt″ʹ/ kt = 2.5.
Scheme 2) and CH2 is the concentration of H2 in solution. The area under a GC peak of a particular oligomer, Arean, is proportional to the weight amount of the oligomer molecule containing n monomer units. Thus, the Arean/n ratio is proportional to the [Oln] value in Eq. (15) and the GC data can be linearized as: Log(Arean/n) ∼ Const + n∙log(p)
kinetics. The scheme assumes that the propagation rate constant kp and the chain transfer rate constant kt do not depend on the oligomerization degree n. Fig. 4 shows that this assumption holds well for n > 4 but it is not satisfactory when the formation of the lightest oligomer molecules is examined. A more detailed kinetic scheme (Scheme 4) takes these effects into account. Scheme 4 assumes that the values of kp and kt vary with n at the earliest stages of oligomer chain formation:
(18)
Fig. 4 shows the data on the distribution of 1-hexene oligomers produced with the (n-C4H9-Cp)2ZrCl2 - MAOO catalyst at 70 °C in the coordinates of Eq. (18). Indeed, most of the points on the plot are on a straight line indicating applicability of Eq. (15) for the description of the oligomer yield distribution vs. n. However, a noticeable deviation from the power law of Eq. (15) is obvious for the lightest oligomers with n = 2, 3 and 4. Similar deviations from the power distribution were observed for 1-hexene oligomers produced with all the metallocene catalysts. These results show that the basic reaction sequence in Scheme 1 is not fully suitable for a precise kinetic description of the oligomerization
- for species Zr-Ol1 (species Cp2Zr+―C6H13) - kpʹ (chain transfer on Zr-Ol1 species is a nul-reaction). - for species Zr-Ol2 (species Cp2Zr+−CH2−CH(C4H9)―C6H13) - kp″ and ktM″. - for species Zr-Ol3 (species Cp2Zr+―[CH2−CH(C4H9)]2―C6H13) kpʹ″ and ktMʹ″.
Scheme 4. Kinetic scheme of early stages in oligomerization reaction. 92
Molecular Catalysis 463 (2019) 87–93
Y.V. Kissin
features, the nature of the Cp rings attached to the Zr atom and the structure of the MAO counter-anion [5,13]. Both the association degree i and the structure of the MAO counter-anion in these active species depend on the conditions of MAO synthesis, whether it is produced in a reaction of Al(CH3)3 with water (the standard reaction of MAO synthesis) or in a reaction of Al(CH3)3 with an organotin compound (Reaction (1)). 4. Conclusions 1 GC analysis of the lightest products formed in 1-hexene oligomerization reactions with several metallocene catalysts, complexes Cp2ZrCl2, (CH3-Cp)2ZrCl2 and (n-C4H9-Cp)2ZrCl2 activated with two types of methylalumoxane, provides the most detailed information on the chemical structure of each individual oligomer molecule. The data demonstrate the dominance of primary 1-hexene insertion into the Cp2Zr+eH and the Cp2Zr+−CH2 bonds, provide the estimation of the probability of secondary 1-hexene insertion into the same bonds and the formation probability of hydrogenated analogs of the oligomers in the absence and in the presence of H2 in the oligomerization reactions. 2 GC analysis of the oligomers shows that their exposure to atmosphere for prolonged periods of time converts residue of methylalumoxane into potent acidic catalyst species which readily isomerize end-C]C bonds in the reaction products into internal tri-substituted C]C bonds. 3 Distribution of product yields with respect to the oligomerization degree confirms the stochastic nature of the oligomerization reactions. Significant deviations from the expected power-law distribution were found only for the lightest products, dimers-tetramers. The deviations are explained by steric effects in the formation reactions of the shortest growing oligomer chains.
Fig. 6. Distribution of 1-hexene oligomers produced with (n-C4H9-Cp)2ZrCl2 MAOO catalyst at 70 °C in the absence of H2 (1) and at PH2 = .27 atm (2).
- for all other species Zr-Oln (species Cp2Zr+―[CH2−CH(C4H9)]nM 1―C6H13) - kp and kt . Kinetic expressions for relative yields of oligomers with different n values are given in Supplement. One example of numerical modeling of the oligomer distribution based on Scheme 4 is shown in Fig. 5 for p = 0.5. Oligomer yields were calculated with Excel using equations in Supplement. The modeling takes into consideration obvious steric effects in bimolecular reactions with the participation of a 1-hexene molecule: kpʹ > > kp because its insertion into the Cp2Zr+―C bond of the active center Zr-Ol1 with a short linear alkyl group [i.e., Cp2Zr+―(CH2)5−CH3] proceeds faster than the same reaction in the case of any branched Zr-Oln center, the rates of all reactions are higher when they involve active centers Zr-Oln with shorter and less branched chains, etc. Table 2 gives the probability of chain growth p on active centers ZrOln with relatively long chains, n > 4, for different catalysts and for different oligomerization conditions. In the case of “true” polymerization reactions, this probability is very high, p > 0.99 [13], and the effect of polymerization conditions on the p value is difficult to evaluate with good precision. In contrast, the p value in the 1-hexene oligomerization reactions is low, between 0.2 and 0.6, and it can be evaluated more dependably. Table 2 also lists weight-average chain lengths of the oligomers. The effects of the reaction parameters follow the expected pattern. As typical for all alkene polymerization reactions with metallocene catalysts, an increase of reaction temperature decreases the probability of chain growth (Eq. (16)) and decreases the average “MW of the polymer” (in this case, the weight-average oligomerization degree) because the activation energy of the chain transfer reaction to the monomer (Reaction (7)) is higher than the activation energy of the chain propagation reaction (Reaction (6)). Introduction of hydrogen to the oligomerization reaction invariably decreases the probability of chain growth (Eq. (17)) because of an additional chain transfer reaction (Scheme 2). One example of this difference is shown in Fig. 6. Both the type of the metallocene complex and the source of the cocatalyst, MAOO vs. MAOSn, also affect the oligomerization kinetics. The activity and the kinetic properties of reaction centers Cp2Zr+― (monomer)n―H… {[Al(CH3)O]i−CH3}―obviously depend on their two
Conflict of interest The author declares no conflict of interest. References [1] (a) C. Janiak, K.C.H. Lange, P. Marquardt, J. Mol. Catal. A: Chem. 180 (2002) 43–58; (b) C. Janiak, Coord. Chem. Rev. 250 (2006) 66–94; (c) C. Janiak, F. Blank, Macromol. Symp. 236 (2006) 14–22; (d) C. Janiak, K.C.H. Lange, P. Marquardt, R.-P. Kruger, R. Hanselmann, Macromol. Chem. Phys. 203 (2002) 129–138; (e) Y.V. Kissin, F.S. Schwab, J. Appl. Polym. Sci. 111 (2008) 273–280. [2] D.B. Malpass, R. Hoff (Ed.), Handbook of Transition Metal Polymerization Catalysts, 2nd edition, Wiley, New York, 2018, pp. 1–30. [3] (a) E.Y. Chen, T.J. Marks, Chem. Rev. 100 (2000) 1391–1434; (b) H. Zijlstra, S. Harder, Ber (2015) 2014–2020. [4] Y.V. Kissin, A.J. Brandolini, Macromolecules 36 (2003) 18–28. [5] Y.V. Kissin, Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, Amsterdam, 2008 Chapter 6. [6] A.G. Alt, A. Koppl, Chem. Rev. 100 (2000) 1205–1222. [7] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100 (2000) 1253–1346. [8] M. Bochmann, Organometallics 29 (2010) 4711–4740. [9] L. Negureanu, R.W. Hall, G. Leslie, L.G. Butler, L.A. Simeral, J. Am. Chem. Soc. 128 (2006) 16816–16826. [10] Y.V. Kissin, G.P. Feulmer, J. Chromatogr. Sci. 24 (1986) 53–61. [11] Y.V. Kissin, G.P. Feulmer, W.P. Payne, J. Chromatogr. Sci. 24 (1986) 64–169. [12] Y.V. Kissin, J. Chromatogr. Sci. 24 (1986) 278–285. [13] Y.V. Kissin, Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier, Amsterdam, 2008 Chapter 5. [14] Y.V. Kissin, A.S. Goldman, Macromol. Chem. Phys. 210 (2009) 1942–1956.
93