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Published online on October 10, 2006, 10.1073/pnas.0602346103
PNAS | October 17, 2006 | vol. 103 | no. 42 | 15289-15294


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Polymerization Special Feature
POLYMERIZATION SPECIAL FEATURE / REVIEWS
Surface science of single-site heterogeneous olefin polymerization catalysts

Seong H. Kim*, and Gabor A. Somorjai{dagger},{ddagger}

*Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802; and {dagger}Department of Chemistry, University of California, Berkeley, CA 94720

Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved June 7, 2006 (received for review April 3, 2006)


   Abstract
Top
 Abstract
Methods
 Results and Discussion
 Conclusions
 Acknowledgements
 References
 
This article reviews the surface science of the heterogeneous olefin polymerization catalysts. The specific focus is on how to prepare and characterize stereochemically specific heterogeneous model catalysts for the Ziegler–Natta polymerization. Under clean, ultra-high vacuum conditions, low-energy electron irradiation during the chemical vapor deposition of model Ziegler–Natta catalysts can be used to create a "single-site" catalyst film with a surface structure that produces only isotactic polypropylene. The polymerization activities of the ultra-high vacuum-prepared model heterogeneous catalysts compare well with those of conventional Ziegler–Natta catalysts. X-ray photoelectron spectroscopic analyses identify the oxidation states of the Ti ions at the active sites. Temperature-programmed desorption distinguishes the binding strength of a probe molecule to the active sites that produce polypropylenes having different tacticities. These findings demonstrate that a surface science approach to the preparation and characterization of model heterogeneous catalysts can improve the catalyst design and provide fundamental understanding of the single-site olefin polymerization process.

Ziegler–Natta

Catalysts can function by being dispersed on solid surfaces (heterogeneous catalysis) or dissolved in reaction media (homogeneous catalysis). Heterogeneous catalysts are widely used in the chemical industry because they are in general easy to handle, separate, and recycle. Homogeneous catalysts are used in synthesis of specialty chemicals, offering precise control of molecular structure and reactivity. Combining the merits of these two different catalyst systems is of great interest for production of next-generation catalysts (1). In this article, we review the surface science of the heterogeneous olefin polymerization.

In the polymerization of small olefins such as ethylene and propylene, both heterogeneous and homogeneous catalytic systems are operational (Fig. 1). Heterogeneous olefin polymerization catalysts (so-called Ziegler–Natta catalysts) are used for production of more than two-thirds of the commodity polyolefins consumed in the world (2, 3). Recently, a large number of specialty polyolefins have been produced with homogeneous metallocene catalysts (4). Whereas most industrial heterogeneous catalysts producing polyethylene and polypropylene are titanium chloride-based catalysts, the homogeneous metallocene catalysts are organometallic compounds of Ti, Zn, and Hf metals in organic solvents. In both types of catalysts, the catalytic species are activated with alkyl aluminum cocatalysts to create the active sites for carbon–carbon bond formation. Triethyl aluminum (AlEt3) is widely used for heterogeneous Ziegler–Natta catalysts, whereas methylaluminoxane is typically used for homogeneous metallocene catalysts.


Figure 1
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  		Fig. 1. Schematic description of the polymerization process for a metallocene catalyst system (a) and a Ziegler–Natta catalyst system (b). Cp, cyclopentadiene; Me, methyl; Et, ethyl; MAO, methylaluminoxane.

The polymers produced with these catalysts can have a wide range of mechanical properties depending on how many monomers are connected (molecular weight) and how they are connected (microstructure). The mechanical properties of polyethylene significantly vary with the linear and branching ratio of the polymer backbone (Table 1). In the case of polypropylene, the mechanical properties depend strongly on the ordering of methyl side groups with respect to the polymer backbone. If the side groups are ordered in a single orientation with respect to the polyolefin backbone, the polymer structure is called isotactic; if they are randomly distributed along the polymer chain backbone structure, it is called atactic (Fig. 2). There are two orders of magnitude difference in the hardness (or elastic modulus) between these two polymers at the same molecular weight (Table 2). Thus, the challenge in polyolefin synthesis is to prepare a polymerization catalyst that produces polyolefin with only one type of structure and precise control of the molecular weight. These are called single-site catalysts.


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  		Table 1. Comparison of density and tensile strengths of various polyethylene grades

 


Figure 2
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  		Fig. 2. Molecular structures of isotactic (Upper) and atactic (Lower) polypropylene.


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  		Table 2. Comparison of mechanical properties of atactic and isotactic polypropylenes

 
In the case of homogeneous metallocene catalysts, the molecular engineering of the organic ligands attached to the active metal ion is used for control of the stereochemistry during the polymerization (5, 6). For certain symmetric arrangements of the large cyclic ligands attached to the active metal ion, the monomer molecules approaching the reaction center are added to the polymer chain in a specific geometry. In general, catalysts exhibiting certain Cs symmetries frequently produce syndiotactic polymers, whereas C2-symmetric catalysts typically produce isotactic polymers.

In the case of heterogeneous Ziegler–Natta catalysts, it is very challenging to produce single-site catalysts. The current generation of Ziegler–Natta catalysts is composed of TiCl4 species supported on high-surface-area magnesium chloride or magnesium ethoxide. This is the most productive form of the catalyst, evolved after several generations of formulations (7–10). The monomer molecule adsorbs on the catalyst and reacts at the active catalyst site, preformed by reaction with Et3Al. However, the chlorine and growing polymer chain ligands attached to the active site cannot control the orientation of monomer molecules added to the polymer chain. Thus the polymer produced contains a mixture of atactic and isotactic components. The stereo-chemical control of the heterogeneous Ziegler–Natta catalyst can be significantly improved by adding proper Lewis bases (organic "modifiers") during the catalyst preparation or polymerization. However, their precise roles and the structure of the modified active sites are not fully understood at the molecular level. This imposes difficulties in further improvement of the isotacticity of heterogeneous Ziegler–Natta catalysts.

In this article, we describe the use of surface science to fabricate and characterize single-site model Ziegler–Natta catalysts for polymerization of ethylene and propylene. The molecular structure of the homogeneous metallocene catalyst indicates that the stereospecific single-site heterogeneous catalyst should have an open and particularly symmetrical arrangement of ligands around the active metal ion at the catalyst surface. Irradiation by the low-energy electron during the chemical vapor deposition of model Ziegler–Natta catalyst appears to produce a "single-site" catalyst film with a surface structure that forms only isotactic polymer chains. The polymerization activities of the ultra-high vacuum (UHV)-prepared model heterogeneous catalysts compare well with those of the conventional Ziegler–Natta catalysts.

A combination of various surface science techniques are used for characterization of the surface composition, structure, and oxidation state of these model catalysts. X-ray photoelectron spectroscopy (XPS) can identify the oxidation states of Ti ions at the active sites. Temperature-programmed desorption (TPD) can distinguish the binding strength of probe molecules to the active sites that produce polypropylene with different tacticities. These findings are of great benefit to polymerization science and technology. Although the grafting of homogenous single-site catalysts on high surface area supports is a powerful way of heterogenizing metallocene catalysts (1), we suggest that single-site Ziegler–Natta systems can be prepared by modifying the surface structure and composition of the heterogeneous catalysts that are used at present in large-scale olefin polymerization.


   Methods
Top
 Abstract
 Methods
Results and Discussion
 Conclusions
 Acknowledgements
 References
 
Strategy for Preparation of Single-Site Heterogeneous Ziegler–Natta Catalysts. For preparation of single-site heterogeneous catalysts, a widely studied approach is to covalently attach homogeneous polymerization catalyst species to surface functional groups such as hydroxyl groups on solid supports (1). An alternative way is to use the molecular structure of the homogeneous polymerization catalyst as a guide and produce similar structures directly on the solid surface. This approach is taken in this surface science study. The homogeneous metallocene catalysts typically consist of two cyclopentadienyl (Cp) ligands and two chloride ligands (Fig. 1a). The Cp ligands provide a rigid framework housing the metal ion and the chloride ligands to create a space for monomer approach and polymer chain growth. One chloride ligand is removed after the activation/alkylation with the cocatalyst. The stereospecificity needed for isotactic polypropylene synthesis is rendered by the specific symmetry in the arrangement of the Cp ligands and the side groups attached to the Cp ligands. The bulky counteranions can amplify the effects of the framework symmetry character housing the active cation.

Compared with the structures of metallocene catalysts, the metal ions at the MgCl2, TiCl2, and TiCl3 solid surfaces are densely packed with chloride ions (Fig. 1b). For these compounds, the thermodynamically most stable surface is the (001) basal plane of the rhombohedral crystal structure (9, 11). This surface is terminated with a close-packed chloride layer and the metal ion is shielded beneath this chloride layer (12–14). Because of this structure, the (001) surface has a very low reactivity and is difficult to activate for polymerization (15, 16). The next most stable surface structures are the (100) and (110) planes of the rhombohedral crystal structure. Although the metal ion is partially exposed in these planes, the chloride ion arrangement around the metal ion does not have the correct symmetry to produce isotactic polypropylene (11). The thermodynamics of the chloride compound structure therefore render it difficult to obtain the open and specific ligand structure around the active metal ion needed for preparation of the single-site polymerization site.

Synthesis of Heterogeneous Ziegler–Natta Model Catalysts Under UHV Conditions. To create a solid surface with metal ions surrounded by more loosely packed anions in a low-symmetry arrangement, we used charged particles such as electrons to overcome this difficulty (17–19). These charged particles can be generated in a separate source in UHV and impinged on the catalyst surface. Because of their high cross-sections in interactions with the condensed phase, these charged particles interact mostly with surface species. In the case of electrons, the surface chloride ions are removed by electron-induced desorption (20).

We have prepared two Ziegler–Natta model catalysts with and without electron irradiation and compared the polymerization activity and product stereochemistry. The first model system is a thin film of TiClx/MgCl2, which mimics the structure of typical heterogeneous Ziegler–Natta catalyst. The TiClx/MgCl2 thin film can be produced by simultaneous deposition of metallic Mg and TiCl4 on an Au substrate (Fig. 3Left). Partial reduction of TiCl4 to TiClx and oxidation of Mg to MgCl2 takes place during this reaction (21, 22). The low solubility of TiClx in MgCl2 leads to the formation of a TiClx monolayer on top of MgCl2 multilayers. The oxidation states of the titanium species have a distribution of 4+, 3+, and 2+. When these films are irradiated with low-energy electrons and ions, some fraction of surface chlorine ions are desorbed into vacuum, leaving under-coordinated metal ions at the surface (17, 20). However, these defective surfaces are readily converted to the stable structure by diffusion of chloride ions from the underlayers (20).


Figure 3
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  		Fig. 3. TiClx/MgCl2 (Left) and TiCly (Right) model catalyst films. The TiClx/MgCl2 film is produced by simultaneous dose of Mg (flux = {approx}6 x 1012 atoms per cm2·s) and TiCl4 (pressure = 2 x 10–7 Torr) on an Au substrate held at 300 K. The TiCly film is produced by electrons irradiation (flux = 1 x 1014 electrons per cm2·s) at an Au substrate (100 K) during exposure to TiCl4 vapor (pressure = 1 x 10–7 Torr).

The second model system uses electrons, instead of Mg, to produce more chloride deficiency (Fig. 3 Right). In this method, the substrate is continuously irradiated by an electron beam with an energy of {approx}500–1,000 eV during the TiCl4 exposure. The transiently adsorbed TiCl4 molecules on Au are dissociated upon interaction with electrons and form chloride-deficient species. These species are eventually accumulated into a TiCly film. Angle-resolved XPS analysis indicates that the TiCly film consists of a monolayer of Ti4+ species (chemisorbed TiCl4) on top of TiCl2 layers (19).


   Results and Discussion
Top
 Abstract
 Methods
 Results and Discussion
Conclusions
 Acknowledgements
 References
 
Polymerization Activity and Single-Site Propylene Polymerization Test. Polymerization activity.  The polymerization activity of thin film catalysts having a nominal surface area of {approx}1 cm2 is monitored nondestructively and continuously by using a laser reflection interferometry technique (Fig. 4) (23, 24). In this method, a laser beam is reflected from the catalyst surface. As the polymer layer grows, the beam reflected from the polymer/gas interface interferes with the beam reflected from the catalyst/polymer interface. The periodic interference fringes can be used to calculate the thickness of the growing polymer film, which can then be converted to the monomer consumption rate. Once activated by a brief exposure to AlEt3 vapor, both TiClx/MgCl2 and TiCly model catalysts can polymerize ethylene and propylene. The initial polymerization rates of the TiClx/MgCl2 model catalysts synthesized in UHV are {approx}2–4 x 10–8 g C3H6 molecules per cm2 catalyst per s for propylene and {approx}5–9 x 10–7 g C2H4 molecules per cm2 catalyst per s for ethylene. These polymerization rates correspond to nominal turnover frequencies of {approx}3–6 x 1014 C3H6 molecules per cm2 catalyst per s and {approx}1–1.8 x 1016 C2H4 molecules per cm2 catalyst per s, respectively. The ethylene polymerization rate is {approx}30 times faster than the propylene polymerization rate on the same catalyst. If the number of the active sites is assumed to be only 10% of the surface Ti ions ({approx}1 x 1013 sites per cm2), then the turnover frequency (the number of monomers reacted per site) is estimated to be {approx}30–60 C3H6 molecules per s and {approx}1,000–1,800 C2H4 molecules per s.


Figure 4
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  		Fig. 4. Laser reflection interferometry experiment showing growth of polyethylene film. (Left) Schematic description of a laser reflection interferometry experiment. (Center) Recorded data for a laser reflection interferometry experiment. (Right) Growth of a polyethylene film as a function of time during the ethylene polymerization on a TiClx/MgCl2 model catalyst. Polymerization was performed with 900 Torr of ethylene. The reactor temperature was kept at 340 K.

These polymerization activities of the model catalysts can be compared directly with those of industrial catalysts. Typical industrial catalysts have a surface area of {approx}50 m2/g. The polymerization activity of the model catalyst calculated for a 1-cm2 surface area would correspond to {approx}75 g polypropylene per g catalyst per hr·atm and {approx}1,400 g polyethylene per g catalyst per hr·atm for catalysts with a surface area of 50 m2/g. Industrial catalysts produce {approx}100–500 g polypropylene per g catalyst per hr·atm and {approx}2,000–10,000 g polyethylene per g catalyst per hr·atm (9). The fact that the model catalysts exhibit activities similar to the industrial catalysts suggests that the surface properties of the model catalysts prepared in UHV are relevant to those of industrial catalysts (25, 26).

Single-site propylene polymerization.  The stereochemistry of the two model catalysts (TiClx/MgCl2 and TiCly) is compared for propylene polymerization. The former represents the conventional Ziegler–Natta catalyst. The latter is produced by the electron beam irradiation method to mimic the open ligand structure of the metallocene catalysts. When used for propylene polymerization, the TiClx/MgCl2 model catalyst produces a mixture of atactic and isotactic polypropylenes, whereas the electron-irradiated TiCly model catalyst produces exclusively isotactic polypropylene (27). Fig. 5 shows the topographic images and the C-C chain helix vibrational peaks of the as-grown polypropylene films for TiClx/MgCl2 and TiCly. The polypropylene film on TiCly is much rougher than that on TiClx/MgCl2, implying the presence of crystalline domains of isotactic polypropylene. In the infrared spectroscopic analysis, the crystalline isotactic polypropylene shows a characteristic isotactic helix vibration peak at 998 cm–1. This peak is much more prominent for the polypropylene film grown on TiCly than that on TiClx/MgCl2. In solvent extraction experiments, the atactic polypropylene fraction was negligible for films grown on TiCly, whereas the film grown on TiClx/MgCl2 contains a large amount of atactic polypropylene. These results indicate that the TiCly catalyst produced by the electron-induced TiCl4 deposition is a single site from a stereochemistry point of view, producing only isotactic polypropylene, whereas the TiClx/MgCl2 catalyst produced by codeposition of Mg and TiCl4 contains multiple active sites and produces a mixture of atactic and isotactic polypropylene.


Figure 5
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  		Fig. 5. Atomic force microscopy (AFM) and infrared spectroscopy analysis results for the polypropylene films produced with TiClx/MgCl2/Au (Left) and TiCly/Au (Right) catalysts. The AFM image size is 20 x 20 µm. The height full scale is 2.1 µm.

Relationship Between Active-Site Properties and Polymerization Behaviors. Being able to synthesize the well characterized model catalyst gives an unprecedented opportunity to study the correlation between the surface properties of the catalyst and the polymerization activity and stereochemistry. What are the oxidation states of the active sites? What controls the stereochemistry of the propylene polymerization? These questions are studied with XPS and probe-molecule TPD.

Oxidation state of the activated Ti species.  The electronic state of the polymerization-active Ti species can be found in the changes in the oxidation-state distribution of the model catalysts before and after the AlEt3 activation (27). Fig. 6compares the high-resolution XPS of the Ti 2p3/2 peak for the TiClx/MgCl2 and TiCly films. The former represents the conventional Ziegler–Natta system, and the latter is the single-site model catalyst. Both systems show the same changes after the activation. Upon reaction with AlEt3, the Ti4+ peak intensity decreases and the Ti2+ intensity increases significantly. The Ti3+ peak intensity shows only a minor change. Because both catalysts have similar oxidation-state distributions but give much different tacticity in propylene polymerization, the Ti oxidation-state distribution does not seem to be an important factor in stereoregularity. The Al peak is not detected in the XPS of the activated catalyst surface in both systems. This result rules out the bimetallic mechanism in which the Al-containing species bonded to the Ti active site is claimed to be responsible for the stereochemistry control.


Figure 6
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  		Fig. 6. Ti 2p3/2 XPS spectra of the TiClx/MgCl2 (Left) and TiCly (Right) catalysts that produced the polypropylene of Fig. 5.

Another important aspect of the XPS results is that the Ti2+ species appear to be the active species for polymerization. This result is somewhat contrary to the previous belief that the only Ti3+ species are catalytically active (28–32). The Ti3+ active species was inferred from ESR studies. However, it should be noted that the Ti2+ and Ti4+ ions are not ESR active because they are not paramagnetic and >80% of total Ti3+ ions in the Ziegler–Natta catalysts are ESR silent because of interactions with the adjacent Ti3+ ions (28, 29). Freund and coworkers (33) recently used ESR to analyze model Ziegler–Natta catalysts and found that the polymerization reactivity is not correlated with the Ti3+ concentration.

Surface structure of the model catalysts.  The adsorption-site distribution on the model Ziegler–Natta catalyst surfaces can be determined from TPD of an inert probe molecule (34, 35). A good probe molecule is mesitylene, which weakly adsorbs on the catalyst surface and desorbs at {approx}190–300 K without altering the catalyst surface. Fig. 7 displays the mesitylene TPD results for a well characterized MgCl2 support film produced by thermal evaporation of MgCl2 on Au (20). From structural information on the MgCl2 film (12–14), the {approx}200-K desorption site is attributed to the (001) basal plane structure where the chloride ions at the outermost layer are close-packed and the metal ions under the chloride layer are coordinated to six chloride ions. The high-temperature desorption peak can be attributed to the nonbasal planes or defects in the ionic lattice of the basal plane where the surface chloride ions are not close-packed and the metal ions beneath the chloride layer are undercoordinated.


Figure 7
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  		Fig. 7. TPD of mesitylene adsorbed on an MgCl2 film deposited on Au.

The mesitylene TPD can also be used to examine the structural distribution of surface site on the model catalyst before and after the AlEt3 activation (Fig. 8) (27). The mesitylene TPD for the TiClx/MgCl2 model catalysts reveals a surface-site distribution similar to the MgCl2 film–basal plane sites (198 K) and nonbasal plane sites (245 K). In the case of the electron-bombarded TiCly catalyst, only one mesitylene TPD peak at 247 K is observed. This finding indicates that all Ti species at the catalyst surface are undercoordinated with chloride ions, which is expected to be the desired structure from the comparison with homogeneous metallocene catalysts.


Figure 8
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  		Fig. 8. Mesitylene TPD profiles for the TiClx/MgCl2 (Left) and TiCly (Right) catalysts that produced the polypropylene of Fig. 5. Mesitylene exposures were 0.2, 0.6, 1.0, and 1.4 liters.

After activation with the AlEt3 cocatalyst, the desorption temperature of the mesitylene probe molecule decreases by {approx}4 K for the basal plane sites and {approx}15 K for the undercoordinated sites. These changes of the mesitylene desorption temperature imply chemical and structural changes on the catalyst surface. Because the basal-plane sites are fully coordinated with chloride ions, the alkylation reaction will be a replacement of one chloride ion with a C2H5 group (36, 37). This replacement reaction induces only a minor change in the mesitylene-surface interactions. For the undercoordinated sites of TiClx/MgCl2 and TiCly, alkylation by AlEt3 can occur by addition of one C2H5 group to the active metal ions, causing a larger decrease in the mesitylene desorption temperature (36, 37).

The comparison of the tacticity of the polypropylene produced and the mesitylene TPD of the activated model catalysts provides direct evidence for a correlation between the structures of the catalyst surfaces and the stereospecificity of propylene polymerization (36, 38–45). The active sites originating from alkylation of the undercoordinated Ti2+ sites are stereochemically specific, whereas those originating from the close-packed basal plane are stereochemically nonspecific. The alkylation of the open-structure metal ion on the Ziegler–Natta catalyst seems to create the polymerization site structure similar to the activated metallocene catalyst.


   Conclusions
Top
 Abstract
 Methods
 Results and Discussion
 Conclusions
Acknowledgements
 References
 
Process optimization based on empirical data for heterogeneous olefin polymerization without a fundamental understanding of the molecular processes for the polymer formation has reached the limit. Further improvements of the olefin polymerization system will require catalyst design and investigation of the molecular mechanism of the polymerization. The surface science results reported here prove the potential of such catalyst design. Although the surface science approach proves the concept of this molecular engineering of catalytic materials, it cannot be used for preparation of bulk materials. Therefore, more intense research on producing molecularly engineered bulk materials is required. The heterogenization of metallocene catalysts on the exterior of inactive solid materials has been a major research direction in this field. Recently, significant progress has been made in anchoring these metallocene catalysts in the mesoporous oxide materials (1). Another direction would be to synthesize an organo-chloride solid materials that combine the function of organic modifiers with 3D chloride networks. An example is MgCl2–C2H5OH complexes (46).

The bonding of olefins at the AlEt3-activated catalytic sites is still unclear. What are the structures of the ethylene and propylene molecules initially adsorbed at these activated sites? What arrangement of the various sites control their activities and stereospecificities? These questions might be answered with real-space atomic-scale microscopy and surface-specific spectroscopy under reaction conditions. These include high-pressure scanning tunneling microscopy (STM), atomic force microscopy (AFM), sum-frequency-generation (SFG) vibrational spectroscopy, extended x-ray absorption fine-structure (EXAFS) spectroscopy, etc. STM and AFM can give site-specific information of various surface sites under the reaction conditions (47). The SFG vibrational spectroscopy has a unique advantage of being able to probe surface species without interference from gas-phase and bulk species (48). EXAFS spectroscopy can provide the structural information with precise distance and arrangement of ligands around the metal ion (49). If these techniques are combined with the single-site catalyst preparation, it should be possible to detect reaction intermediates leading to polymer chain growth and chiral orientation.


   Acknowledgements
Top
 Abstract
 Methods
 Results and Discussion
 Conclusions
 Acknowledgements
References
 
This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division of the U.S. Department of Energy under Contract DE-AC03-76SF00098.


   Footnotes
 

Abbreviations: UHV, ultra-high vacuum; XPS, x-ray photoelectron spectroscopy; TPD, temperature-programmed desorption; Cp, cyclopentadienyl.

{ddagger}To whom correspondence should be addressed. E-mail: somorjai{at}socrates.berkeley.edu

Author contributions: G.A.S. designed research; S.H.K. performed research; and S.H.K. wrote the paper.

Conflict of interest statement: No conflicts declared.

This article is a PNAS direct submission.

© 2006 by The National Academy of Sciences of the USA


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