Enhanced Productivity of a Supported Olefin Trimerization Catalyst

Letter pubs.acs.org/acscatalysis

Enhanced Productivity of a Supported Olefin Trimerization Catalyst Aaron Sattler, Dinesh C. Aluthge, Jay R. Winkler, Jay A. Labinger,* and John E. Bercaw* Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: Treatment of dry silica with methylaluminoxane (MAO) followed by (FI)TiCl3 (FI = (N-(5-methyl-3-(1adamantyl)salicylidene)-2′-(2″-methoxyphenyl)anilinato) gives a heterogeneous supported ethylene trimerization catalyst, s(FI)Ti, which exhibits productivity more than an order of magnitude higher than its homogeneous analogues. This increase in productivity is attributed to a decreased rate of catalyst decomposition, a process that is proposed to occur via comproportionation to an inactive TiIII species; immobilization retards this process. In addition, s(FI)Ti catalyzes trimerization of α-olefins with high selectivity. Based on regioisomer distributions, catalysis by s(FI)Ti involves the same active species as the previously reported homogeneous systems (FI)TiR2Me/B(C6F5)3 (R = Me, CH2SiMe3, CH2CMe3). KEYWORDS: selective olefin trimerization, supported catalysis, heterogeneous catalysis, catalyst decomposition, titanium

T

he production of 1-hexene by selective ethylene trimerization has been and continues to be a significant research effort, due to its use as a comonomer for the industrial synthesis of linear low-density polyethylene (LLDPE).1 A number of catalysts have been developed,2,3 most of which are activated by aluminum reagents, usually methylaluminoxane (MAO) in large excess. We have been interested in a system initially reported by Fujita and co-workers, consisting of a precatalyst, (FI)TiCl3 (FI = (N-(5-methyl-3-(1-adamantyl)salicylidene)-2′-(2″-methoxyphenyl)anilinato), activated by addition of 10 000 equiv of MAO.4 We were able to achieve comparable performance starting with a precatalyst, (FI)TiMe3, that can be activated by a stoichiometric reagent, B(C6F5)3.5 The absence of large quantities of activator facilitates detailed study of the mechanism of trimerization; we were able to elucidate, among other features: (i) an experimentally consistent mechanism for initiation; (ii) the identity of the main decomposition product, a TiIII species; and (iii) relative rate constants for the key steps of initiation (ki), trimerization (kt), and decomposition (kd).5 Furthermore, (FI)TiMe3/B(C6F5)3 was found to trimerize α-olefins, a reaction not reported for the original Fujita catalyst and quite uncommon with other ethylene trimerization catalysts.6−11 More recently, we synthesized an alternate precatalyst, (FI)Ti(CH2CMe3)2Me, with the expectation of faster initiation and consequent higher concentration of the active catalyst.12 As depicted by the simulated data in Figure 1, a greater initiation rate does indeed lead to higher concentrations of the active © XXXX American Chemical Society

Figure 1. Kinetics simulations13 of trimerization catalysis using (FI)TiMe3 (left, slow initiation) and (FI)Ti(CH2CMe3)2Me (right, fast initiation) as precatalysts. The top traces show TON as a function of time; the latter precatalyst gives about 10× the initial activity as the former (note the different scales of the abscissa) but the same overall productivity. The bottom traces show the concentrations of key species determined from the simulations: cyan, precatalyst (i.e., (FI)TiR2+); red, active catalyst; blue, decomposition product (i.e., TiIII).

catalyst, but the overall productivity, reported as total turnover number (TON) of ethylene monomers (mmol) per Ti (mmol) after cessation of activity, is unchanged. Indeed, the ultimate Received: November 18, 2015

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DOI: 10.1021/acscatal.5b02604 ACS Catal. 2016, 6, 19−22

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ACS Catalysis

covalently. Nonetheless, washing with toluene resulted in no detectable extracted material and no reduction of catalytic activity. This supported system does indeed catalyze ethylene trimerization: a rapidly stirred suspension of s(FI)Ti (200 mg, 3.6 μmol Ti) in toluene (15 mL) under a constant ethylene atmosphere (1 atm) at 25 °C produces primarily 1-hexene and C10 olefins (Scheme 2); small amounts of C14 olefins (∼5%)

TON should depend only on the relative rates of trimerization and decomposition, not that of initiation, in this system. In principle, increasing catalyst productivity could be accomplished by either increasing kt or decreasing kd; however, since the active species in this system appears to be independent of precatalyst,12 the former would necessitate design of an entirely new ligand or metal complex. On the other hand, we observed that decomposition occurs by the formation of a TiIII complex, a process we believe occurs by comproportionation of TiIV and TiII intermediates in the catalytic cycle, thereby forming two inactive TiIII species. We postulated that restricting catalyst mobility by supporting it on a surface might retard such a bimolecular decomposition pathway.14 Of course, supporting homogeneous catalysts is quite common, because of the desirable properties of heterogeneous catalysts for industrial processes;15,16 however, doing so typically results in decreased activity relative to the soluble analogue. A comprehensive review states that “with few exceptions, in myriad cases, the activity of the supported catalyst is half to a tenth that of the soluble catalyst”;15 two recent studies on selective trimerization found that supporting the catalyst reduced activity by 5−100 fold.14b,c In contrast, the supported ethylene trimerization catalyst we report here gives an overall productivity more than an order of magnitude higher than the analogous homogeneous system, while maintaining the ability to trimerize α-olefins with TONs above 6000. The solid material we chose to try was MAO/silica, which we hoped would serve to both activate and support the Ti catalyst.17 A slurry of dry silica in toluene was treated with MAO, in the ratio of 8.3 mmol Al per gram of SiO2. Because the latter contains approximately 2.2 mmol of Si−OH groups per gram,18 that corresponds to about 4 Al/SiOH. The slurry was stirred at room temperature for 2 h, followed by stirring for 1 h with (FI)TiCl3 at a mol ratio of ca. 300:1 Al:Ti (Scheme 1).19 Removal of volatiles gave a free-flowing light yellow

Scheme 2. 1-Hexene and Three Major C10 Isomers Obtained in Selective Trimerization Catalysis

are also observed, with only trace amounts of polyethylene (∼3%) formed. The C 10 and C 14 olefins arise from cotrimerization of ethylene with the 1-hexene produced in situ; the relative degree of higher olefin production depends on the catalyst concentration, degree of conversion, and relative rates of ethylene vs 1-hexene incorporation.5 The overall selectivity for olefin trimers is greater than 95%, with an overall TON of ca. 1.2 × 105 mmol ethylene oligomerized/mmol Ti. For comparison, the TON of the homogeneous (FI)TiMe3/ B(C6F5)3 system at 1 atm was 8.3 × 103. Under the same conditions, we obtained a TON around 9.0 × 103 (and over 10 times more polyethylene) with the original (FI)TiCl3/10 000 equiv MAO system; extrapolation of the previously reported high-pressure data4 to 1 atm gives a similar value, 1.3 × 104. Thus, supporting the catalyst increases the productivity by an order of magnitude. The same three major C10 olefin isomers (Scheme 2) are found for (FI)TiMe3/B(C6F5)35 and s(FI)Ti catalysis, indicating the same (or very similar) active species in the two systems. Because s(FI)Ti is preactivated (already in the cationic methylated form) and requires no additional reagent (unlike (FI)TiCl3/MAO and (FI)TiMe3/B(C6F5)3, which both require combining two reagents for catalyst activation), solventless catalysis should be possible. Indeed, exposure of dry s(FI)Ti to ethylene (1 atm) produced 1-hexene and C10 olefins, resulting in gradual wetting of the solid. Although these experiments were not optimized, one could imagine further increasing productivity through use of fixed-bed or trickle-bed reactors. The kinetics of trimerization catalysis (Figure 3) were analyzed in the same way as the homogeneous systems of

Scheme 1. Synthetic Preparation of Supported Catalyst

powder, with a calculated loading of 0.018 mmol Ti/g; the powder is pyrophoric, being extremely reactive toward oxygen and water. The product presumably has the structure represented in s(FI)Ti (Figure 2), where the precatalyst is bound to the support via electrostatic interactions, not

Figure 3. TON versus time for ethylene trimerization by s(FI)Ti. Red circles are experimental values; blue line is simulated values based on parameters in Table 1.13

Figure 2. Representation of catalyst supported on MAO-treated silica, s(FI)Ti. 20

DOI: 10.1021/acscatal.5b02604 ACS Catal. 2016, 6, 19−22

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ACS Catalysis Figure 1, and the resulting parameters are shown in Table 1.13 Both kd and ki are lower for the supported catalyst. It is not

Scheme 3. Major Isomer Obtained from Selective Trimerization of α-Olefins

Table 1. Simulated Kinetics Parameters13 for Ethylene Trimerization Catalystsa

a

catalyst system

ki

kt

kd

(FI)TiMe3/B(C6F5)3 (FI)Ti(CH2CMe3)2Me/B(C6F5)3 s(FI)Ti

3.0 × 10−4 3.0 × 10−3 5.8 × 10−5

6.2 6.2 6.2

2.4 × 10−3 2.4 × 10−3 1.8 × 10−4

the two catalyst systems, a further indication that the active species are the same or very similar. In contrast, the original Fujita system [(FI)TiCl3/MAO] does not give selective trimerization of α-olefins in our hands; instead, a distribution of oligomers, mostly dimers, was obtained, demonstrating another advantage for supporting the catalyst in this system. As discussed previously,5 a selective process for oligomerizing αolefins could provide an efficient route to higher value products such as jet fuel,22 diesel fuel, and synthetic motor lubricants (e.g., Castrol Edge, Mobil 1),23 by alleviating the need for expensive and energy intensive separations. In conclusion, we have prepared a supported version of the homogeneous trimerization catalyst (FI)TiCl3/MAO. s(FI)Ti is a precatalyst for the selective catalytic trimerization of ethylene to 1-hexene, as well as of α-olefins. Analysis of the regioisomers produced in the cotrimerization of 1-hexene and ethylene, and in the homotrimerization of 1-hexene, indicates that the active species in both the supported and stoichiometric systems are essentially the same. Most importantly, the supported catalyst system exhibits substantially increased productivities, believed to result from slowing the rate constant of decomposition to TiIII species. This finding stands in strong contrast to the majority of supported catalysts, which are typically less productive than their homogeneous analogues.

Rate constants are all pseudo-first order, with units s−1.13

unreasonable that ki, the rate constant of initiation, should differ somewhat between (FI)TiMe3/B(C6F5)3 and s(FI)Ti, as the interactions of the [(FI)TiMe2]+ precatalyst with the two different anions ([MeB(C6F5)3]− vs [MAO/SiO2]−) could affect coordination and insertion of ethylene, the first steps of the reductive initiation process.5 The main decomposition pathway for the supported system is again formation of a TiIII species, as shown by EPR spectroscopy (Figure 4). The (FI)TiMe3/B(C6F5)3 system,

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Figure 4. EPR spectra of TiIII species produced upon catalyst decomposition. Black, (FI)TiMe3/B(C6F5)3 in toluene solution (25 °C); red, (FI)TiMe3/B(C6F5)3 in toluene glass (77 K); blue, s(FI)Ti suspension in toluene (25 °C). Isotropic = 1.958, Axial g⊥ = 1.972, g|| = 1.930.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02604. Experimental details (PDF)

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after catalytic activity has ceased, gives an isotropic EPR signal in solution at 25 °C (black, Figure 4), which becomes anisotropic (axial) upon cooling to 77 K to form a glass (red, Figure 4). Similar TiIII signals have been observed for the decomposition products of the (FI)TiCl3/MAO system.20 The corresponding spectrum of a thick toluene suspension of s(FI) Ti after catalysis (blue, Figure 4) resembles but is even broader than that of the glass, reflecting the slow rate of tumbling and heterogeneity of the silica/MAO surface. Thus, the active catalyst formed from s(FI)Ti appears to decompose by the same pathway as its homogeneous analogues, a process that has indeed been slowed by supporting, as intended, but not stopped altogether.21 In addition to the catalytic trimerization of ethylene, s(FI)Ti is also capable of trimerizing α-olefins (1-pentene, 1-hexene, and 1-decene) with TONs over 10× higher (ca. 6.0 × 103 for 1hexene) than those achieved with homogeneous (FI)TiMe3/ B(C 6 F 5 ) 3 , 5 again consistent with a decreased rate of decomposition. (The kinetics parameters determined for 1hexene trimerization and experimental data are shown in the Supporting Information; the ratio of kt/kd is about 15x smaller, accounting for the substantially lower TON achieved.) Additionally, the high regioselectivity (the regioisomer shown in Scheme 3 comprises ca. 85% of all trimers) is very similar for

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Harry B. Gray for helpful discussions. This work was funded by BP through the XC2 program. REFERENCES

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