Article pubs.acs.org/Macromolecules
Structural Investigation of Methylalumoxane Using Transmission Electron Microscopy Harmen S. Zijlstra,‡,§ Marc C. A. Stuart,‡ and Sjoerd Harder*,†,§ †
Inorganic and Organometallic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany Stratingh Institute, University of Groningen, Nijenborgh 4, 9747AG Groningen, Netherlands § Dutch Polymer Institute (DPI), P.O. Box 902, 5600AX Eindhoven, Netherlands ‡
S Supporting Information *
ABSTRACT: Decades after its initial discovery and application as a potent activator for single-site catalysts, methylalumoxane (MAO) still remains a mysterious and poorly understood compound. Using an adapted cryo-TEM setup, for the first time images of dried MAO solutions have been obtained. Particle analysis, size comparison, and decomposition studies yield insights into the structural nature of the observed large MAO particles and can be correlated back to different MAO clusters and their behavior. The studied MAO samples were found to consist of highly aggregated fractals with the radius of the primary particles ranging from ∼10 to 60 nm upon increasing sample age. These findings support a common particle sintering growing mechanism in which primary particles cluster together to form larger secondary particles. A repetition of this process could explain the dramatic change in structure, average molecular weight, and activity of MAO over time.
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INTRODUCTION The serendipitous discovery of methylalumoxane (MAO) has allowed for the development and commercialization of homogeneous single-site polymerization catalysts.1 MAO has become a vital component of various olefin oligomerization and polymerization processes. Although, its interaction with the catalyst is well investigated,2 the exact structure and characteristics of MAO itself remain unclear. MAO is produced by the partial hydrolysis of trimethylaluminum (Me3Al). Dissolved in toluene, this gives a mixture of different MAO oligomers and unreacted Me3Al that are in dynamic equilibrium. This already complex mixture varies with time and temperature. Aging can change the composition of MAO solutions and cause problems with reproducibility. It is well-known that upon standing at room temperature for several months a gel starts to form in the MAO solution. Furthermore, its stability is strongly dependent on concentration, all together giving for a highly variable and hard to study material. Over the past decades many studies have been carried out to solve MAO’s structural mysteries. 3 Its average composition, (Me1.4−1.5AlO0.75−0.80)n,4 has been established, and several characteristics such as the coordination number of Al (mainly four) and O (three),5,6 average molecular weight (Mw, 1200− 1800 g/mol), and cluster size (n = 20−30)7 are generally agreed upon. On the basis of these findings and other reported well-defined alumoxanes8 several different structures, containing polymeric (1), cyclic (2), and ribbon- (3−5) or cage-like (6) motives have been proposed for MAO (Figure 1).3 Despite numerous structural investigations into the soluble fraction of MAO, limited attention has been given to the © XXXX American Chemical Society
Figure 1. Selected proposed structures for MAO.
insoluble part. This gel is thought to be a highly cross-linked polymer formed through self-condensation of different MAO oligomers. This condensation most likely includes both Received: April 17, 2015 Revised: July 13, 2015
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DOI: 10.1021/acs.macromol.5b00803 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules reversible (e.g., interchain Al···O interactions)9 and nonreversible processes involving the release of CH410 (Scheme 1).
liquid nitrogen and transferred to the microscope. There it was placed in the precooled cryo-stage and transferred into the microscope with the anticontamination shield closed. The complete loading was under inert conditions. Cryo-TEM preparations of frozen solution were carried out using a nitrogen-flushed FEI vitrobox. Samples were injected on the grid in the nitrogen-flushed vitrobox, which was then quickly blotted and directly frozen into liquid nitrogen, placed in the precooled cryo-stage and transferred into the microscope with the anticontamination shield closed. Air-exposed samples were prepared by briefly exposing MAO solutions to oxygen and moisture under noninert conditions. The solutions were then quickly deposited on the copper TEM grids and prepared as previously described.
Scheme 1. (a) Chemical Irreversible Self-Condensation of MAO Clusters and (b) Physical Reversible SelfCondensation of MAO Clusters
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RESULTS AND DISCUSSION Because of its extremely high sensitivity toward air and moisture, great precautions need to be taken when handling MAO solutions. To keep oxidation and hydrolysis to a minimum, all samples were prepared inside a glovebox under inert atmosphere and only thoroughly dried solvents and apparatuses were used. Stock solutions of MAO were dropped onto the copper TEM grids and blotted dry. After preparation, the samples were put into sealed containers and kept in liquid nitrogen. They were then transferred to the TEM, loaded, and measured under cryo-TEM conditions. Care should be taken at all points during preparation and transportation, as even slight contamination with air results in sample decomposition. As MAO particles increase in size over time, the initial experiments were carried out with aged MAO (Crompton, 5 years old) to have large, clearly visible particles. After finetuning the experimental setup, TEM images showing regular aggregates were obtained. The primary particles are spherical and form fractal aggregates through particle sintering in a pearlchain-like motive containing multiple side branches (Figure 2a, b). It is well-known that particles coalescing in a random fashion form such aggregates, and similar structural motives are therefore often observed in partially combusted materials such as aerosols and diesel soot.12,13 Because MAO is in essence
Although the gel is still a mildly active cocatalyst, almost all literature studies have focused on the soluble part of MAO. One of these few studies that includes large, poorly soluble MAO clusters was reported by Stellbrink et al., in which they find discrepancy for hydrolytically or nonhydrolytically produced MAO.9 Using small-angle neutron scattering, they found the majority of their nonhydrolytically produced MAO samples to consist of long linear polymers with a Mw of up to 20 000 g/mol and an approximate radius of gyration of 46 Å. The small remaining fraction of MAO was found to be consistent of larger particles whose size could not be determined with light scattering. Follow-up studies on hydrolytically produced MAO gave Mw and size results that are much closer to those reported using solution-based methods (Mw ≈ 1800 g/mol; radius of gyration ≈ 9 Å).11 If adapted carefully, transmission electron microscopy (TEM) could be a potential means to visually investigate these larger MAO gels or precursors thereof. Herein we report an adapted TEM setup that allows for the studying of the highly volatile and sensitive MAO mixtures without complete decomposition. For the first time, a structural TEM investigation into the nature, size, and dispersion of the larger MAO particles in differently aged samples is described.
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EXPERIMENTAL SECTION
General Procedures. MAO was obtained as a 10% solution in toluene from Crompton (2007) and Chemtura (2012) and stored at room temperature inside the glovebox. Toluene (p. a. ≥ 99.7% from Sigma-Aldrich) was degassed using N2 and dried over activated aluminum oxide. All other glassware and materials were oven-dried and stored inside a glovebox before use to ensure proper dryness. Samples were observed using a FEI Tecnai 20 transmission electron microscope operating at 200 keV. A gatan model 626 cryo-stage was used for the observation under inert conditions. Elemental analysis was performed with an Oxford Instruments Tmax 80T SDD detector. Images were recorded on a slow-scan CCD camera. Sample Preparation. Inside the glovebox, the original 10% MAO solution was diluted with dry toluene to 1%. The 1% solution of MAO in toluene was carefully deposited on a carbon-coated 400 mesh copper TEM grid using a micropipette. The grid was blotted dry with predried filter paper and left in the box to allow complete evaporation of the solvent. The sample was then placed in a grid holder and sealed in an airtight vessel. Upon transferring the vessel out of the glovebox, the grid holder containing the grid was carefully dropped in a bath of
Figure 2. TEM image of aged MAO samples (a,b) and an EDX spectrum (c). B
DOI: 10.1021/acs.macromol.5b00803 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
were not dried completely but that the toluene solution is frozen and studied as is. The sample insertion, however, cannot be done under perfect inert conditions and that is why images are blurred by gas bubbles (presumably CH4 formed by accidental hydrolysis). One can, however, recognize sphere-like primary particles and cluster/chains thereof. This means that pearl-chain structures already exist in solution and are not only due to drying. To further study the formation of these aggregates, we performed similar experiments as previously described with a batch of fresh MAO (Chemtura, 6 months old). If the size of MAO clusters increases over time, limiting the time between synthesis and measurement should give for smaller particles. As expected, the background in the TEM images of fresh MAO shows particles with a similar pearl-chain-like structure but smaller size (Figure 4). Size analysis of the particles indicates
nothing more than partially hydrolyzed Me3Al, the similarities between the obtained images and those reported for other partially combusted substances can be expected and support the idea that we are seeing MAO and not its decomposition products. EDX analysis of the samples shows the presences of Al, O, and C (Figure 2c). Because of the varying trace amounts of oxygen and carbon on the carrier, it has, however, not yet been possible to accurately and consistently estimate the average composition of the measured samples (see Supporting Information). However, it was found that the denser particles had a significantly higher Al/O ratio then the less dense material. This indicates that the Al/C ratio decreases with increasing size, a process that most likely happens through the release of methane. The highly aggregated MAO particles observed seem to be indicative of the cross-linked gels proposed by others.10a Higher magnification gives close-up images that can be analyzed in a straightforward manner to obtain an estimation of the primary particle size. Doing so gives an average radius of 44 nm (Figure 3).14 This value is much higher than the 1 to 2 nm generally observed using solution based techniques and the 4 to 5 nm observed by light scattering.3,9,11
Figure 4. TEM image of fresh MAO.
radii in the range of 8 to 16 nm. This is about one-third of the diameter observed for the old MAO, clearly indicating an increase in size with age. This supports the idea that the larger, gel-like species in MAO solutions consist of highly cross-linked aggregates that are formed by the repetitive aggregation of smaller particles into bigger particles. Because of the high sensitivity of MAO, it is almost impossible to completely prevent partial decomposition. The bigger high-density spots (100−1000 nm) in Figure 3 most likely represent partially decomposed material due to partial reaction with trace amounts of air during sample preparation. EDX analysis of the new samples could again not give an accurate composition but showed a lower Al:O ratio for the smaller particles, indicating a higher Al/C and thus less oxidized MAO (see Supporting Information). Furthermore, similarly to the old MAO samples the Al:O ratio in the larger, denser particles was higher than that of the less dense particles. To confirm the intactness of the smaller pearl-chain-like particles, we took TEM images of MAO solutions that had been deliberately exposed to air (Figure 5). The images show the conversion of the pearl-chain like particles into a material consisting of large and irregular clusters (ca. 10 μm) containing particles that partially show sharp edges as expected for crystalline material. We presume this is Al2O3, but we were not able to measure clear diffraction patterns of this semicrystalline material.
Figure 3. Magnified TEM image of aged MAO and accompanying size distribution diagram for the primary spherical particles.
Aggregation through particle sintering is a well-established mechanism of particle growth for fractal aggregates formed after partial combustion processes and has been explored in detail.15 On the basis of these observations it can be postulated that the larger particles in MAO form through extensive selfaggregation of smaller primary particles (Scheme 2). This Scheme 2. Self-Aggregation of Smaller Primary Particles into Larger Secondary Particles and Repetition Thereof
aggregation process is likely to be continuous and can be visualized to occur in a repetitive fashion. This could explain the progressive increase in Mw, eventually resulting in the formation of insoluble polymeric gels. To investigate whether the chain-like structures are formed upon drying or already exist in MAO solution, we were interested in cryo-TEM studies on frozen solutions of MAO in toluene (see Supporting Information). This means that samples
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CONCLUSION A TEM investigation on different MAO mixtures has been performed. Through the means of an adapted cryo-TEM setup, it has been possible to observe the highly sensitive MAO mixture without complete decomposition. Images obtained C
DOI: 10.1021/acs.macromol.5b00803 Macromolecules XXXX, XXX, XXX−XXX
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(3) Zijlstra, H. S.; Harder, S. Eur. J. Inorg. Chem. 2015, 1, 19−43. and references therein. (4) Imhoff, D. W.; Simeral, L. S.; Sangokoya, S. A.; Peel, J. H. Organometallics 1998, 17, 1941−1945. (5) (a) Babushkin, D. E.; Semikolenova, N. V.; Panchenko, V. N.; Sobolev, A. P.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 1997, 198, 3845−3854. (b) Zakharov, V. A.; Talsi, E. P.; Zakharov, I. I.; Babushkin, D. E.; Semikolenova, N. V. Kinet. Catal. 1999, 40, 836− 850. (6) Bryant, P. L.; Harwell, C. R.; Mrse, A. A.; Emery, E. F.; Gan, Z.; Caldwell, T.; Reyes, A. P.; Kuhns, P.; Hoyt, D. W.; Simeral, L. S.; Hall, R. W.; Butler, L. G. J. Am. Chem. Soc. 2001, 123, 12009−12017. (7) (a) Von Lacroix, K.; Heitmann, B.; Sinn, H. Macromol. Symp. 1995, 97, 137−142. (b) Rocchigiani, L.; Busico, V.; Pastore, A.; Macchioni, A. Dalton Trans. 2013, 42, 9104−9111. (c) Trefz, T. K.; Henderson, M. A.; Wang, W. Y.; Collins, S.; McIndoe, J. S. Organometallics 2013, 32, 3149−3152. (8) (a) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1993, 115, 4971−4984. (b) Harlan, C. J.; Mason, M. R.; Barron, A. R. Organometallics 1994, 13, 2957−2969. (c) Harlan, C. J.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1995, 117, 6465−6474. (9) Stellbrink, J.; Niu, A.; Allgaier, J.; Richter, D.; Koenig, B. W.; Hartmann, R.; Coates, G. W.; Fetters, L. J. Macromolecules 2007, 40, 4972−4981. (10) (a) Kaminsky, W.; Strübel, C. J. Mol. Catal. A: Chem. 1998, 128, 191−200. (b) The addition of water could also lead to condensation of clusters. Surface Al-Me might react with water to form Al−OH + CH4, which after reaction with Al−Me could form Al−O−Al and CH4. In fact, this is something that might even happen during the synthesis of MAO itself (which means that it is not unthinkable that pearl-chainlike structures already exist in very fresh batches of MAO); however, considering the fact that gelling takes place under inert conditions, we think that self-condensation by the formation of Al-CH2-Al bridges might be more obvious. (11) Ghiotto, F.; Pateraki, C.; Tanskanen, J.; Severn, J. R.; Luehmann, N.; Kusmin, A.; Stellbrink, J.; Linnolahti, M.; Bochmann, M. Organometallics 2014, 32, 3354−3362. (12) Wentzel, M.; Gorzawski, H.; Naumann, K. H.; Saathoff, H.; Weinbruch, S. J. Aerosol Sci. 2003, 34, 1347−1370. (13) Sorensen, C. M. Aerosol Sci. Technol. 2011, 45, 765−779. (14) All image analyses were performed using ImageJ v. 1.48 and functions therein. (15) Eggersdorfer, M. L.; Kadau, D.; Herrmann, H. J.; Pratsinis, S. E. Langmuir 2011, 27, 6358−6367.
Figure 5. TEM images of air exposed MAO samples, showing transformation of the original pearl-chain structure into larger aggregates consisting of semicrystalline material (most likely Al2O3).
from both old and new samples show similar fractal aggregates with differently sized particles. This allowed the larger gel-like particles to be visually observed and analyzed for the first time. The highly aggregated pattern indicates a cross-linked polymer that consists of spherical primary particles that are linked together like in a pearl chain-like fashion. This structure remains intact with age, while the size of the particles increases. To exclude complete sample decomposition, cryo-TEM and air exposed samples were also investigated. Cryo-TEM images show similar aggregation, whereas the air-exposed images show transformation of the chain-like structures into a larger amorphous material containing semicrystalline particles. Thus, it can be concluded that the gelling that is often observed in MAO solutions is caused by the formation of pearl-chain like aggregates that grow larger over time through a series of selfaggregation processes.
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ASSOCIATED CONTENT
S Supporting Information *
TEM images of the cryo-TEM experiments and EDX analysis of the observed particles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00803.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is part of the research program of the DPI, Eindhoven, Netherlands (project # 728) REFERENCES
(1) Baier, M. C.; Zuideveld, M. A.; Mecking, S. Angew. Chem., Int. Ed. 2014, 53, 9722−9744. (2) For recent papers on the catalyst interaction of MAO, see, for example: (a) Hirvi, J. T.; Bochmann, M.; Severn, J. R.; Linnolahti, M. ChemPhysChem 2014, 15, 2732−2742. (b) Trefz, T. K.; Henderson, M. A.; Linnolahti, M.; Collins, S.; McIndoe, J. S. Chem. - Eur. J. 2015, 21, 2980−2991. D
DOI: 10.1021/acs.macromol.5b00803 Macromolecules XXXX, XXX, XXX−XXX
