Surface science of single-site heterogeneous olefinpolymerization catalystsSeong H. Kim* and Gabor A. Somorjai†‡

*Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802; and †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)

This article reviews the surface science of the heterogeneous olefinpolymerization catalysts. The specific focus is on how to prepareand characterize stereochemically specific heterogeneous modelcatalysts for the Ziegler–Natta polymerization. Under clean, ultra-high vacuum conditions, low-energy electron irradiation duringthe chemical vapor deposition of model Ziegler–Natta catalysts canbe used to create a ‘‘single-site’’ catalyst film with a surfacestructure that produces only isotactic polypropylene. The polymer-ization activities of the ultra-high vacuum-prepared model heter-ogeneous catalysts compare well with those of conventionalZiegler–Natta catalysts. X-ray photoelectron spectroscopic analy-ses identify the oxidation states of the Ti ions at the active sites.Temperature-programmed desorption distinguishes the bindingstrength of a probe molecule to the active sites that producepolypropylenes having different tacticities. These findings dem-onstrate that a surface science approach to the preparation andcharacterization of model heterogeneous catalysts can improvethe catalyst design and provide fundamental understanding of thesingle-site olefin polymerization process.

Ziegler–Natta

Catalysts can function by being dispersed on solid sur-faces (heterogeneous catalysis) or dissolved in reactionmedia (homogeneous catalysis). Heterogeneous cata-lysts 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 chem-icals, offering precise control of molecular structure and reac-tivity. Combining the merits of these two different catalystsystems is of great interest for production of next-generationcatalysts (1). In this article, we review the surface science of theheterogeneous olefin polymerization.

In the polymerization of small olefins such as ethylene andpropylene, both heterogeneous and homogeneous catalytic sys-tems are operational (Fig. 1). Heterogeneous olefin polymer-ization catalysts (so-called Ziegler–Natta catalysts) are used forproduction of more than two-thirds of the commodity polyole-fins consumed in the world (2, 3). Recently, a large number ofspecialty polyolefins have been produced with homogeneousmetallocene catalysts (4). Whereas most industrial heteroge-neous catalysts producing polyethylene and polypropylene aretitanium chloride-based catalysts, the homogeneous metal-locene catalysts are organometallic compounds of Ti, Zn, and Hfmetals in organic solvents. In both types of catalysts, the catalyticspecies are activated with alkyl aluminum cocatalysts to createthe active sites for carbon–carbon bond formation. Triethylaluminum (AlEt3) is widely used for heterogeneous Ziegler–Natta catalysts, whereas methylaluminoxane is typically used forhomogeneous metallocene catalysts.

The polymers produced with these catalysts can have a widerange of mechanical properties depending on how many mono-mers are connected (molecular weight) and how they are con-nected (microstructure). The mechanical properties of polyeth-ylene significantly vary with the linear and branching ratio of thepolymer backbone (Table 1). In the case of polypropylene, themechanical properties depend strongly on the ordering of methyl

side groups with respect to the polymer backbone. If the sidegroups are ordered in a single orientation with respect to thepolyolefin backbone, the polymer structure is called isotactic; ifthey are randomly distributed along the polymer chain backbonestructure, it is called atactic (Fig. 2). There are two orders ofmagnitude difference in the hardness (or elastic modulus)between these two polymers at the same molecular weight (Table2). Thus, the challenge in polyolefin synthesis is to prepare apolymerization catalyst that produces polyolefin with only onetype of structure and precise control of the molecular weight.These are called single-site catalysts.

In the case of homogeneous metallocene catalysts, the mo-lecular engineering of the organic ligands attached to the activemetal ion is used for control of the stereochemistry during thepolymerization (5, 6). For certain symmetric arrangements of thelarge cyclic ligands attached to the active metal ion, the mono-mer molecules approaching the reaction center are added to thepolymer chain in a specific geometry. In general, catalystsexhibiting certain Cs symmetries frequently produce syndiotacticpolymers, whereas C2-symmetric catalysts typically produce iso-tactic polymers.

In the case of heterogeneous Ziegler–Natta catalysts, it is verychallenging to produce single-site catalysts. The current gener-ation of Ziegler–Natta catalysts is composed of TiCl4 speciessupported on high-surface-area magnesium chloride or magne-sium ethoxide. This is the most productive form of the catalyst,evolved after several generations of formulations (7–10). Themonomer molecule adsorbs on the catalyst and reacts at theactive catalyst site, preformed by reaction with Et3Al. However,the chlorine and growing polymer chain ligands attached to theactive site cannot control the orientation of monomer moleculesadded to the polymer chain. Thus the polymer produced con-tains a mixture of atactic and isotactic components. The stereo-chemical control of the heterogeneous Ziegler–Natta catalyst

Conflict of interest statement: No conflicts declared.

This article is a PNAS direct submission.

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

‡To whom correspondence should be addressed. E-mail: somorjai@socrates.berkeley.edu.

© 2006 by The National Academy of Sciences of the USA

Fig. 1. Schematic description of the polymerization process for a metal-locene catalyst system (a) and a Ziegler–Natta catalyst system (b). Cp, cyclo-pentadiene; Me, methyl; Et, ethyl; MAO, methylaluminoxane.

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can be significantly improved by adding proper Lewis bases(organic ‘‘modifiers’’) during the catalyst preparation or poly-merization. However, their precise roles and the structure of themodified active sites are not fully understood at the molecularlevel. This imposes difficulties in further improvement of theisotacticity of heterogeneous Ziegler–Natta catalysts.

In this article, we describe the use of surface science tofabricate and characterize single-site model Ziegler–Natta cat-alysts for polymerization of ethylene and propylene. The mo-lecular structure of the homogeneous metallocene catalyst in-dicates that the stereospecific single-site heterogeneous catalystshould have an open and particularly symmetrical arrangementof ligands around the active metal ion at the catalyst surface.Irradiation by the low-energy electron during the chemical vapordeposition of model Ziegler–Natta catalyst appears to producea ‘‘single-site’’ catalyst film with a surface structure that formsonly isotactic polymer chains. The polymerization activities ofthe ultra-high vacuum (UHV)-prepared model heterogeneouscatalysts compare well with those of the conventional Ziegler–Natta catalysts.

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

MethodsStrategy for Preparation of Single-Site Heterogeneous Ziegler–NattaCatalysts. For preparation of single-site heterogeneous catalysts,a widely studied approach is to covalently attach homogeneouspolymerization catalyst species to surface functional groups suchas hydroxyl groups on solid supports (1). An alternative way isto use the molecular structure of the homogeneous polymeriza-tion catalyst as a guide and produce similar structures directly on

the solid surface. This approach is taken in this surface sciencestudy. The homogeneous metallocene catalysts typically consistof two cyclopentadienyl (Cp) ligands and two chloride ligands(Fig. 1a). The Cp ligands provide a rigid framework housing themetal ion and the chloride ligands to create a space for monomerapproach and polymer chain growth. One chloride ligand isremoved after the activation�alkylation with the cocatalyst. Thestereospecificity needed for isotactic polypropylene synthesis isrendered by the specific symmetry in the arrangement of the Cpligands and the side groups attached to the Cp ligands. The bulkycounteranions can amplify the effects of the framework sym-metry character housing the active cation.

Compared with the structures of metallocene catalysts, themetal ions at the MgCl2, TiCl2, and TiCl3 solid surfaces aredensely packed with chloride ions (Fig. 1b). For these com-pounds, the thermodynamically most stable surface is the (001)basal plane of the rhombohedral crystal structure (9, 11). Thissurface is terminated with a close-packed chloride layer and themetal ion is shielded beneath this chloride layer (12–14). Be-cause of this structure, the (001) surface has a very low reactivityand is difficult to activate for polymerization (15, 16). The nextmost stable surface structures are the (100) and (110) planes ofthe rhombohedral crystal structure. Although the metal ion ispartially exposed in these planes, the chloride ion arrangementaround the metal ion does not have the correct symmetry toproduce isotactic polypropylene (11). The thermodynamics ofthe chloride compound structure therefore render it difficult toobtain the open and specific ligand structure around the activemetal ion needed for preparation of the single-site polymeriza-tion site.

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

We have prepared two Ziegler–Natta model catalysts with andwithout electron irradiation and compared the polymerizationactivity and product stereochemistry. The first model system isa thin film of TiClx�MgCl2, which mimics the structure of typicalheterogeneous Ziegler–Natta catalyst. The TiClx�MgCl2 thinfilm can be produced by simultaneous deposition of metallic Mgand TiCl4 on an Au substrate (Fig. 3 Left). Partial reduction ofTiCl4 to TiClx and oxidation of Mg to MgCl2 takes place duringthis reaction (21, 22). The low solubility of TiClx in MgCl2 leadsto the formation of a TiClx monolayer on top of MgCl2 multi-layers. The oxidation states of the titanium species have adistribution of 4�, 3�, and 2�. When these films are irradiatedwith low-energy electrons and ions, some fraction of surfacechlorine ions are desorbed into vacuum, leaving under-coordinated metal ions at the surface (17, 20). However, thesedefective surfaces are readily converted to the stable structure bydiffusion of chloride ions from the underlayers (20).

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

Table 1. Comparison of density and tensile strengths of variouspolyethylene grades

Polyethylenegrade Density, g/cm3

Tensilestrength,

MPa

High density 0.941–0.965 43Low density 0.910–0.925 24Linear low density 0.900–0.939 37

Table 2. Comparison of mechanical properties of atactic andisotactic polypropylenes

PolypropyleneElastic modulus,

GPaHardness,

MPa

Isotactic 1.09 125Atactic 0.15 1.4

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The second model system uses electrons, instead of Mg, toproduce more chloride deficiency (Fig. 3 Right). In this method,the substrate is continuously irradiated by an electron beam withan energy of �500–1,000 eV during the TiCl4 exposure. Thetransiently adsorbed TiCl4 molecules on Au are dissociated uponinteraction 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 consistsof a monolayer of Ti4� species (chemisorbed TiCl4) on top ofTiCl2 layers (19).

Results and DiscussionPolymerization Activity and Single-Site Propylene PolymerizationTest. Polymerization activity. The polymerization activity of thin filmcatalysts having a nominal surface area of �1 cm2 is monitorednondestructively and continuously by using a laser reflectioninterferometry technique (Fig. 4) (23, 24). In this method, a laserbeam is reflected from the catalyst surface. As the polymer layergrows, the beam reflected from the polymer�gas interfaceinterferes with the beam reflected from the catalyst�polymerinterface. The periodic interference fringes can be used tocalculate the thickness of the growing polymer film, which canthen be converted to the monomer consumption rate. Onceactivated by a brief exposure to AlEt3 vapor, both TiClx�MgCl2and TiCly model catalysts can polymerize ethylene and pro-pylene. The initial polymerization rates of the TiClx�MgCl2model catalysts synthesized in UHV are �2–4 � 10�8 g C3H6

molecules per cm2 catalyst per s for propylene and �5–9 � 10�7

g C2H4 molecules per cm2 catalyst per s for ethylene. Thesepolymerization rates correspond to nominal turnover frequen-cies of �3–6 � 1014 C3H6 molecules per cm2 catalyst per s and�1–1.8 � 1016 C2H4 molecules per cm2 catalyst per s, respec-tively. The ethylene polymerization rate is �30 times faster thanthe propylene polymerization rate on the same catalyst. If thenumber of the active sites is assumed to be only 10% of thesurface Ti ions (�1 � 1013 sites per cm2), then the turnoverfrequency (the number of monomers reacted per site) is esti-mated to be �30–60 C3H6 molecules per s and �1,000–1,800C2H4 molecules per s.

These polymerization activities of the model catalysts can becompared directly with those of industrial catalysts. Typicalindustrial catalysts have a surface area of �50 m2�g. Thepolymerization activity of the model catalyst calculated for a1-cm2 surface area would correspond to �75 g polypropylene perg catalyst per hr�atm and �1,400 g polyethylene per g catalyst perhr�atm for catalysts with a surface area of 50 m2�g. Industrialcatalysts produce �100–500 g polypropylene per g catalyst perhr�atm and �2,000–10,000 g polyethylene per g catalyst perhr�atm (9). The fact that the model catalysts exhibit activitiessimilar to the industrial catalysts suggests that the surfaceproperties of the model catalysts prepared in UHV are relevantto those of industrial catalysts (25, 26).Single-site propylene polymerization. The stereochemistry of the twomodel catalysts (TiClx�MgCl2 and TiCly) is compared for pro-pylene polymerization. The former represents the conventionalZiegler–Natta catalyst. The latter is produced by the electronbeam irradiation method to mimic the open ligand structure ofthe metallocene catalysts. When used for propylene polymer-ization, the TiClx�MgCl2 model catalyst produces a mixture ofatactic and isotactic polypropylenes, whereas the electron-irradiated TiCly model catalyst produces exclusively isotacticpolypropylene (27). Fig. 5 shows the topographic images and theC-C chain helix vibrational peaks of the as-grown polypropylenefilms for TiClx�MgCl2 and TiCly. The polypropylene film onTiCly is much rougher than that on TiClx�MgCl2, implying thepresence of crystalline domains of isotactic polypropylene. In theinfrared spectroscopic analysis, the crystalline isotactic polypro-pylene shows a characteristic isotactic helix vibration peak at 998cm�1. This peak is much more prominent for the polypropylenefilm grown on TiCly than that on TiClx�MgCl2. In solventextraction experiments, the atactic polypropylene fraction wasnegligible for films grown on TiCly, whereas the film grown onTiClx�MgCl2 contains a large amount of atactic polypropylene.These results indicate that the TiCly catalyst produced by the

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

Fig. 3. TiClx�MgCl2 (Left) and TiCly (Right) model catalyst films. The TiClx�MgCl2 film is produced by simultaneous dose of Mg (flux � �6 � 1012 atomsper cm2�s) and TiCl4 (pressure � 2 � 10�7 Torr) on an Au substrate held at 300K. The TiCly film is produced by electrons irradiation (flux � 1 � 1014 electronsper cm2�s) at an Au substrate (100 K) during exposure to TiCl4 vapor (pressure �1 � 10�7 Torr).

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electron-induced TiCl4 deposition is a single site from a stere-ochemistry point of view, producing only isotactic polypro-pylene, whereas the TiClx�MgCl2 catalyst produced by codepo-sition of Mg and TiCl4 contains multiple active sites andproduces a mixture of atactic and isotactic polypropylene.

Relationship Between Active-Site Properties and Polymerization Be-haviors. Being able to synthesize the well characterized modelcatalyst gives an unprecedented opportunity to study the corre-lation between the surface properties of the catalyst and thepolymerization activity and stereochemistry. What are the oxi-dation states of the active sites? What controls the stereochem-istry of the propylene polymerization? These questions arestudied with XPS and probe-molecule TPD.Oxidation state of the activated Ti species. The electronic state of thepolymerization-active Ti species can be found in the changes inthe oxidation-state distribution of the model catalysts before andafter the AlEt3 activation (27). Fig. 6 compares the high-resolution XPS of the Ti 2p3/2 peak for the TiClx�MgCl2 andTiCly films. The former represents the conventional Ziegler–Natta system, and the latter is the single-site model catalyst. Bothsystems show the same changes after the activation. Uponreaction with AlEt3, the Ti4� peak intensity decreases and theTi2� intensity increases significantly. The Ti3� peak intensityshows only a minor change. Because both catalysts have similaroxidation-state distributions but give much different tacticity inpropylene polymerization, the Ti oxidation-state distributiondoes not seem to be an important factor in stereoregularity. TheAl peak is not detected in the XPS of the activated catalystsurface in both systems. This result rules out the bimetallicmechanism in which the Al-containing species bonded to the Tiactive site is claimed to be responsible for the stereochemistrycontrol.

Another important aspect of the XPS results is that the Ti2�

species appear to be the active species for polymerization. Thisresult is somewhat contrary to the previous belief that the onlyTi3� species are catalytically active (28–32). The Ti3� activespecies was inferred from ESR studies. However, it should benoted that the Ti2� and Ti4� ions are not ESR active becausethey are not paramagnetic and �80% of total Ti3� ions in theZiegler–Natta catalysts are ESR silent because of interactionswith 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 withthe Ti3� concentration.Surface structure of the model catalysts. The adsorption-site distri-bution on the model Ziegler–Natta catalyst surfaces can bedetermined from TPD of an inert probe molecule (34, 35). Agood probe molecule is mesitylene, which weakly adsorbs on thecatalyst surface and desorbs at �190–300 K without altering thecatalyst surface. Fig. 7 displays the mesitylene TPD results for awell characterized MgCl2 support film produced by thermalevaporation of MgCl2 on Au (20). From structural informationon the MgCl2 film (12–14), the �200-K desorption site isattributed to the (001) basal plane structure where the chlorideions at the outermost layer are close-packed and the metal ionsunder the chloride layer are coordinated to six chloride ions. Thehigh-temperature desorption peak can be attributed to thenonbasal planes or defects in the ionic lattice of the basal planewhere the surface chloride ions are not close-packed and themetal ions beneath the chloride layer are undercoordinated.

The mesitylene TPD can also be used to examine the struc-tural distribution of surface site on the model catalyst before andafter the AlEt3 activation (Fig. 8) (27). The mesitylene TPD forthe TiClx�MgCl2 model catalysts reveals a surface-site distribu-tion similar to the MgCl2 film–basal plane sites (198 K) andnonbasal plane sites (245 K). In the case of the electron-

Fig. 5. Atomic force microscopy (AFM) and infrared spectroscopy analysisresults for the polypropylene films produced with TiClx�MgCl2�Au (Left) andTiCly�Au (Right) catalysts. The AFM image size is 20 � 20 �m. The height fullscale is 2.1 �m.

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

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

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bombarded TiCly catalyst, only one mesitylene TPD peak at 247K is observed. This finding indicates that all Ti species at thecatalyst surface are undercoordinated with chloride ions, whichis expected to be the desired structure from the comparison withhomogeneous metallocene catalysts.

After activation with the AlEt3 cocatalyst, the desorptiontemperature of the mesitylene probe molecule decreases by �4K for the basal plane sites and �15 K for the undercoordinatedsites. These changes of the mesitylene desorption temperatureimply chemical and structural changes on the catalyst surface.Because the basal-plane sites are fully coordinated with chlorideions, the alkylation reaction will be a replacement of one chlorideion with a C2H5 group (36, 37). This replacement reactioninduces only a minor change in the mesitylene-surface interac-tions. For the undercoordinated sites of TiClx�MgCl2 and TiCly,alkylation by AlEt3 can occur by addition of one C2H5 group tothe active metal ions, causing a larger decrease in the mesitylenedesorption temperature (36, 37).

The comparison of the tacticity of the polypropylene producedand the mesitylene TPD of the activated model catalysts pro-vides direct evidence for a correlation between the structures ofthe catalyst surfaces and the stereospecificity of propylene

polymerization (36, 38–45). The active sites originating fromalkylation of the undercoordinated Ti2� sites are stereochemi-cally specific, whereas those originating from the close-packedbasal plane are stereochemically nonspecific. The alkylation ofthe open-structure metal ion on the Ziegler–Natta catalystseems to create the polymerization site structure similar to theactivated metallocene catalyst.

ConclusionsProcess optimization based on empirical data for heterogeneousolefin polymerization without a fundamental understanding ofthe molecular processes for the polymer formation has reachedthe limit. Further improvements of the olefin polymerizationsystem will require catalyst design and investigation of themolecular mechanism of the polymerization. The surface scienceresults reported here prove the potential of such catalyst design.Although the surface science approach proves the concept of thismolecular engineering of catalytic materials, it cannot be usedfor preparation of bulk materials. Therefore, more intenseresearch on producing molecularly engineered bulk materials isrequired. The heterogenization of metallocene catalysts on theexterior of inactive solid materials has been a major researchdirection in this field. Recently, significant progress has beenmade in anchoring these metallocene catalysts in the meso-porous oxide materials (1). Another direction would be tosynthesize an organo-chloride solid materials that combine thefunction of organic modifiers with 3D chloride networks. Anexample is MgCl2–C2H5OH complexes (46).

The bonding of olefins at the AlEt3-activated catalytic sites isstill unclear. What are the structures of the ethylene andpropylene molecules initially adsorbed at these activated sites?What arrangement of the various sites control their activities andstereospecificities? These questions might be answered withreal-space atomic-scale microscopy and surface-specific spec-troscopy under reaction conditions. These include high-pressurescanning tunneling microscopy (STM), atomic force microscopy(AFM), sum-frequency-generation (SFG) vibrational spectros-copy, extended x-ray absorption fine-structure (EXAFS) spec-troscopy, etc. STM and AFM can give site-specific informationof various surface sites under the reaction conditions (47). TheSFG vibrational spectroscopy has a unique advantage of beingable to probe surface species without interference from gas-phase and bulk species (48). EXAFS spectroscopy can providethe structural information with precise distance and arrange-ment of ligands around the metal ion (49). If these techniques arecombined with the single-site catalyst preparation, it should bepossible to detect reaction intermediates leading to polymerchain growth and chiral orientation.

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.

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