NANO LETTERS
Sulfated Tin Oxide Nanoparticles as Supports for Molecule-Based Olefin Polymerization Catalysts
2004 Vol. 4, No. 8 1557-1559
Christopher P. Nicholas and Tobin J. Marks* Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113 Received May 18, 2004
ABSTRACT Sulfated tin oxide particles with crystallite sizes of ∼5 nm were synthesized and used as effective supports for homogeneous zirconium hydrocarbyl olefin polymerization and arene hydrogenation catalysis.
Olefin polymerization is a vast, world-scale process that produces a large portion of the world’s plastic materials. A current direction of activity in this field is to use “singlesite” homogeneous organometallic catalysts in order to obtain greater control over product molecular weight distribution, greater percentages of active species, moderation of released reaction heat, and tacticity-controlling ligation around the active center.1 However, the attraction of heterogeneous polymerization catalysts is higher product molecular weights, greater catalyst stability, lower cost, and decreased reactor fouling.2 Toward this end, we recently demonstrated that group 4 hydrocarbyl chemisorption on “super” Brønsted acidic sulfated metal oxide (SMO) surfaces yields highly active catalysts having virtually 100% active sites3 for olefin polymerization and arene hydrogenation - an extremely unusual regime for supported catalysts (e.g., species A). Here, the SMO acts not only as a support, stabilizing the bound molecular adsorbate, but also as a co-catalyst/activator, converting the neutral organometallic precursor into a catalytically-active electrophile. These characteristics raise the possibility that if such catalysts could be prepared on nanoscopic SMO particles, a number of beneficial, nearhomogeneous catalytic properties might be realized. In principle, these could include better heat and mass transfer control, exposure of a greater percentage of active sites due to increased surface area, and modified surface chemical properties (e.g., greater acidity) because of the nanoscopic particle size.4 Additionally, such catalysts would have overall lower cost than typical single-site catalysts because sulfated metal oxides are cheap, readily synthesized cocatalysts versus conventional perfluoroarylboranes/borates and methylalumoxane.5 Herein, we report the use of 5 nm sulfated tin oxide (STO) nanoparticles as supports/activators * Corresponding author. E-mail:
[email protected]. 10.1021/nl049255r CCC: $27.50 Published on Web 07/16/2004
© 2004 American Chemical Society
for representative Zr hydrocarbyls and initial observations on catalytic activity for polymerization and hydrogenation. In particular, we show that cationic organozirconium species are formed upon chemisorption and that these catalysts are active for ethylene polymerization and benzene hydrogenation.
STO nanoparticles were synthesized via the literature procedure,6 wherein a tin hydroxide gel is generated following hydrolysis of SnCl4‚5H2O and treatment with NH4+OHto pH ) 8. The Sn(OH)4 gel is then washed three times with 4% NH4+OAc- solution to remove residual Cl-. Matsuhashi et al.6a speculate that OAc- functions as a “place-holder” for the SO42- groups and potentially inhibits crystallite growth. The Sn(OH)4 precipitate is then dried to yield a tin oxide gel (83% yield). Next, this gel is stirred with 3.0 M H2SO4 to exchange OAc- for SO42- before drying and calcining at 500 °C for 3 h to yield a canary-yellow solid, the color of which fades upon cooling. The STO was analyzed by powder XRD and found to be present in the SnO2 cassiterite phase7 (Figure 1). Note that STO is not a stoichiometric material, but a tin oxide having hydrosulfate groups chemically bound to the surface. Fitting of the diffraction peak widths at half-maximum to the Debye-
Figure 2. 13C CPMAS NMR (75.4 MHz) spectrum of Cp2Zr(13CH3)2 (1*)/STO chemisorption product (20 000 scans, 4 s repetition time, 7 µs contact time, 5.1 kHz spinning speed). Table 1. Ethylene Polymerization Data for the Present STO-Based Catalysts
Figure 1. X-ray diffraction pattern of nanocrystalline STO. The PDF data for cassiterite are shown immediately below. Scheme 1.
Zr Hydrocarbyls Used as Catalyst Precursors
Scherrer equation (Figure S1, Supporting Information) reveals the STO to have crystallite sizes of ∼5 nm. Broadening due to strain is ignored as a first approximation, while instrumental broadening was corrected using a reference LaB6 powder sample. From the literature, XRD, SEM, and TEM data on STOs indicate that STO nanocrystal dimensions determined by XRD are ∼1/2 those determined by TEM.6d Literature SEM images show what appear to be loosely agglomerated nanoparticles, and our SEM results are in accord with these findings. BET analysis (N2 desorption) reveals the present STO samples to have surface areas of 90 m2/g (Figure S2), which is consistent with formation of STO.6a Nonsulfated tin oxide samples typically have surface areas as low as 10-15 m2/g, although small crystallite sizes may still be present.6b The nanocrystalline STO thus formed was used in protonolytic chemisorption of metal-alkyl precursor complexes via the very strong solid Brønsted acidity (H0 ) -18)6 of this material. The organometallic complexes (Scheme 1) Cp2Zr(CH3)28 (1) Cp2Zr(13CH3)29 (1*), (Me5Cp)2ZrMe210 (2), (Me5Cp)ZrMe311 (3), and ZrBz412 (4) were synthesized via literature procedures. Then, in a two-sided, fritted reaction vessel interfaced to a high vacuum line (∼10-6 Torr), 10 mL of pentane was transferred from Na/K alloy onto wellmixed, measured quantities of the organometallic precursor complex (in stoichiometric excess) and STO. The resulting slurry was next stirred for 1.0 h and then filtered. The impregnated support was collected on the frit, washed three times with pentane to remove physisorbed hydrocarbyl, and finally dried in vacuo. ICP analysis of 3/STO indicates 0.18 Zr/nm2 and 0.49 S/nm2. Control experiments with sulfated zirconia supported Zr hydrocarbyls show that adsorbed hydrocarbyl concentration does not vary appreciably with precursor.13 1558
precursor
polymerization activitya
polyethylene Tmb
1 2 3 4
8.6 × 103 1.5 × 104 9.8 × 104 6.6 × 105
135.2 135.5 136.1 135.1
a Units in g PE/mol Zr h-1. b Melting temperature in °C determined by DSC.
Of initial interest here was the question of whether the STO nanoparticles have sufficiently strong Brønsted acidity as a support/activator to produce cationic organozirconium species (eq 1, species A) rather than catalytically inert µ-oxo species (eq 1, species B). The use of weak Brønsted acids, e.g., silica, favors the formation of µ-oxo species.14 The chemisorption of isotopically enriched Cp2Zr(13CH3)2 (1*)/ STO was studied by 13C CPMAS NMR. Previously, we have shown that δZr-13CH3 responds to environmental changes by upfield displacement from δ30.45 ppm when a neutral µ-oxo complex or other neutral Lewis base-stabilized complex is formed (eq 1, species B), and by downfield displacement when a cationic species is formed (eq 1, species A).15 As can be seen from the NMR data (Figure 2), chemisorption of complex 1* on STO nanoparticles predominantly yields a cationic species of moderate electrophilicity (Zr-13CH3, δ35.7; Cp, δ113.8) and what appears to be a small fraction of a µ-oxo complex (Zr-13CH3, δ20.5). The Cp resonance of the µ-oxo complex is visible as a shoulder on the main Cp feature. The ethylene polymerization behavior of the present complexes was studied by charging 30 mg of the supported catalyst into a medium pressure reactor along with 2.5 mL dry toluene. The reactor was then interfaced to the high vacuum line followed by charging with ethylene to 150 psi. After warming to 60 °C, the slurry was rapidly stirred. After a measured time interval, polymerization was quenched with methanol and the polymeric product collected by filtration, dried overnight under high vacuum at 80 °C, and weighed. Ethylene polymerization data for complexes 1/STO, 2/STO, 3/STO, and 4/STO are summarized in Table 1. It can be seen that as coordinative and electronic saturation around the Zr center is decreased, polymerization activity increases. A similar trend was previously observed for other SMOsupported organozirconium systems as well.3,15,16 The present ethylene polymerization activities of the STO supported catalysts are moderate, but not as great as systems supported on sulfated zirconia or sulfated alumina.3,15 That STOsupported catalysts are less active than sulfated zirconia supported ones is consistent with the slight upfield shift of Nano Lett., Vol. 4, No. 8, 2004
the Zr-CH3 peak from 36.0 ppm for 1*/sulfated zirconia to 35.7 ppm for 1*/STO. Nonetheless, the present STO nanoparticle-based catalysts are significantly more active than those derived from bulk sulfated iron oxide or sulfated titania.16 That the polyethylene formed in the present experiments cannot be removed from the catalyst support, even by extraction with 1,2,4-trichlorobenzene at 140 °C, is indicative of ultrahigh molecular weight polyethylene. Such products are typical of ethylene polymerizations mediated by zirconium hydrocarbyls supported on alumina,17 or other sulfated metal oxides.3,15 Furthermore, IR spectra of the present polymers as hot-pressed films exhibit all the characteristic signatures of high-density/linear polyethylene18 such as ν(CH2) at 2933-2825 cm-1, ν(CH2) at 1469 cm-1, a CH2 wagging mode at 1367 cm-1, and a CH2 rocking mode at 716 cm-1, respectively (Figure S3). The present organozirconium-STO catalysts are also active for benzene hydrogenation to cyclohexane. For these reactions, the medium pressure reactor, connected to a 500 mL gas ballast, was flame-dried under high vacuum and charged in the glovebox with 60 mg of 3/STO and 1.0 mL (1.1 × 10-2 mol) of benzene dried over Na/K. The apparatus was removed from the glovebox and attached to the high vacuum line. After thorough evacuation (10-5 Torr) of the reactor at -78 °C, the reactor was warmed to room temperature and pressurized to 1.0 atm of H2. The reactor was then immersed in an oil bath maintained at 25(0.1) °C and stirred rapidly. The consumption of H2 was measured with a digital pressure transducer. Preliminary experiments with 3/STO yield Nt ∼10 mol C6H6/mol Zr h, significantly less than for 3/sulfated zirconia15 and 3/sulfated alumina3, suggesting that arene hydrogenation is significantly more demanding than olefin polymerization for these catalysts. We conclude that nanocrystalline STO provides a high surface area support and co-catalyst/activator that facilitates predominant formation of catalytically-active chemisorbed cationic organozirconium species (eq 1, species A) as judged by catalytic properties and 13C CPMAS NMR. Such cationic species yield high molecular weight polyethylene and are active for the hydrogenation of benzene to cyclohexane. Acknowledgment. We thank the DOE for support of this research (grant number DE-FG02-86ER13511). C.P.N. thanks Mr. C. Sheets for help with X-ray data collection and analysis of the Debye-Scherrer measurements and Mr. K. Popp for help with and discussion of the BET measurements. This work made use of Central Facilities supported by the MRSEC program of the NSF (DMR-007-6097) at the Materials Research Center of Northwestern University. Supporting Information Available: Debye-Scherrer analysis of X-ray data, BET results, and IR spectrum of polyethylene formed. This material is available free of charge via the Internet at http://pubs.acs.org.
Nano Lett., Vol. 4, No. 8, 2004
References (1) (a) Suzuki, Y.; Terao, H.; Fujita, T. Bull. Chem. Soc. Jpn. 2003, 76, 1493-1517. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-315. (c) DeSouza, R. F.; Casagrande, O. L., Jr. Macromol. Rapid Comm. 2001, 22, 1293-1301. (d) Pedeutour, J.-N.; Radhakrishnan, K.; Cramail, H.; Deffieux, A. Macromol. Rapid Comm. 2001, 22, 1095-1123. (e) Mecking, S. F. Angew. Chem., Int. Ed. Eng. 2001, 40, 534-40. (f) Blom, R., Follestad, A., Rytter, E., Tilset, M., Ystenes, M., Eds. Organometallic Catalysts and Olefin Polymerization: Catalysts for a New Millenium, Springer-Verlag: Berlin, 2001. (2) Hlatky, G. Chem. ReV. 2000, 100, 1347-76 and references therein. (3) Nicholas, C. P.; Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 4325-31. (4) For examples where reducing particle size to nanoscopic dimensions modifies catalytic properties, see: (a) Moreno-Man˜as, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638-43. (b) Haruta, M. J. Nanoparticle Res. 2003, 5, 3-4. (c) Bell, A. T. Science 2003, 299, 1688-91. (d) Johnson, B. F. G. Top. Catal. 2003, 24, 147. (e) Stark, W. J.; Pratsinis, S. E.; Baiker, A. Chimia 2002, 56, 485-9. (f) Somorjai, G. A.; Borodko, Y. G. Catal. Lett. 2001, 76, 1-5. (g) Rao, C. N. R.; Cheetham, A. K. J. Mater. Res. 2001, 11, 2887-94. (5) (a) Chen, M.-C.; Roberts, J. A.; Marks, T. J. Organometallics 2004, 23, 932. (b) Metz, M. V.; Schwartz, D. J.; Stern, C. L.; Marks, T. J.; Nickias, P. N. Organometallics 2002, 21, 4159-68. (c) Li, L.; Stern, C. L.; Marks, T. J. Organometallics 2000, 19, 3332-7. (d) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 1001531. (e) Andresen, A.; Cordes, H. G.; Herwig, J.; Kaminsky, W.; Merck, A.; Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H. J. Angew. Chem. 1976, 88, 689-90. (6) (a) Matsuhashi, H.; Miyazaki, H.; Kawamura, Y.; Nakamura, H.; Arata, K. Chem. Mater. 2001, 13, 3038-42. (b) Patel, A.; Coudurier, G.; Essayem, N.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1997, 93, 347-53. (c) Arata, K. Appl. Catal. A 1996, 146, 3. (d) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffman, G. P. Energy Fuels 1991, 5, 497-507. (7) Matched to the pattern calculated from the structure studied in: Seki, H.; Ishizawa, N.; Mizutani, N.; Kato, M. Yogyo Kyokaishi (J. Ceram. Soc. Jpn.) 1984, 92, 219. (8) Wailes, P. C.; Weigold, H.; Bell, A. P. J. Organomet. Chem. 1972, 34, 105-164. (9) Synthesized from 13CH3Li‚LiI prepared from13CH3I (99% 13C, Cambridge Isotopes) using methods analogous to those of ref 8. (10) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100, 2716-2724. (11) Wolczanski, P. T.; Bercaw, J. E. Organometallics 1982, 1, 793799. (12) Zucchini, U.; Albizzati, E.; Giannini, U. J. Organomet. Chem. 1971, 26, 357-372. (13) Unpublished observations. Ahn, H.; Marks, T. J. (14) Finch, W. C.; Gillespie, R. D.; Hedden, D.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 6221-32. (15) (a) Ahn, H.; Nicholas, C. P.; Marks, T. J. Organometallics 2002, 21, 1788-1806. (b) Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 1998, 120(51), 13533-13534. (c) Marks, T. J. Acc. Chem. Res. 1992, 25, 57-65. (16) Nicholas, C. P.; Marks, T. J., submitted to Langmuir. (17) Work from DuPont has shown that tetraneophyl zirconium supported on alumina also forms high molecular weight polyolefins: Ittel, S. D. J. Macromol. Sci.-Chem. 1990, A27 (9-11), 1133-46. Collette, J. W.; Tullock, C. W.; MacDonald, R. N.; Buck, W. H.; Su, A. C. L.; Harrell, J. R.; Mulhaupt, R.; Anderson, B. C. Macromol. 1989, 22, 3851-3858. (18) Tashiro, T.; Sasaki, S.; Kobayashi, M. Macromolecules 1996, 29, 7460-7469, and references therein.
NL049255R
1559
