In Situ ATR-FTIR Studies on MgCl2-Diisobutyl Phthalate Interactionsin Thin Film Ziegler−Natta CatalystsAjin V. Cheruvathur,†,‡ Ernie H. G. Langner,§ J. W. (Hans) Niemantsverdriet,†,‡ and Peter C. Thune*,†,‡

†Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands‡Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands§Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa

*S Supporting Information

ABSTRACT: To study the surface structure of MgCl2support and its interaction with other active components inZiegler−Natta catalyst, such as electron donors, we prepared athin film analogue for Ziegler−Natta ethylene polymerizationcatalyst support by spin-coating a solution of MgCl2 in ethanol,optionally containing a diester internal donor (diisobutyl-ortho-phthalate, DIBP) on a flat Si crystal surface. The donorcontent of these films was quantified by applying attenuatedtotal internal reflection−Fourier transform infrared spectros-copy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Changes in the interaction of DIBP with MgCl2 at varioustemperatures were monitored by in situ ATR-FTIR. Upon increasing the temperature, a shift in the (CO) band toward lowerwavenumbers was observed together with the depletion of (O−H) stretching band due to the desorption of residual ethanol. Weassign this shift to gradual redistribution of adsorbed DIBP from adsorption sites on the MgCl2 (104) surface toward the moreacidic MgCl2 (110) surface. The morphologies of MgCl2 and MgCl2/DIBP films were studied by transmission electronmicroscopy (TEM) revealing a preferential orientation of ClMgCl layers (001) parallel to the lateral film dimensions. Thisorientation becomes more pronounced upon annealing. In the absence of donor, the MgCl2 grow in to large crystals aligned inlarge domains upon annealing. Both crystal growth and alignment is impeded by the presence of donor.

■ INTRODUCTIONMost of the heterogeneous MgCl2-supported Ziegler−Nattacatalysts for stereospecific olefin polymerization contain MgCl2as a support, TiCl4 as catalyst, a trialkylaluminium as cocatalyst,and two Lewis bases acting as internal and external donors.1

This catalyst is generally described as particles of activatedMgCl2, composed of irregularly stacked Cl−Mg−Cl sandwich-like monolayers, with the microstructures terminated by the(110) and (104) lateral cuts. The reaction of MgCl2 supportwith TiCl4 leads to the adsorption of TiCl4 on the lateralcleavage surfaces of MgCl2, such as the (110) and (104)cuts.2−4 The Mg atoms at the surface are coordinated with fourchlorine atoms in the case of the (110) and five chlorine atomsin the case of the (104) surfaces, as opposed to six chlorineatoms in the bulk. On reaction with the cocatalyst (aluminumalkyls), Ti4+ is reduced to Ti3+, forming a Ti−C bond, which isessential for monomer insertion. Both an internal and anexternal donor are required for an active and stereospecifichigh-yield MgCl2-supported Ziegler−Natta catalyst for propy-lene polymerization.5,6 It is commonly accepted that theinternal donor appears to be bound to the (110) lateral cuts ofMgCl2 and thereby provides [i] stability to the more acidic,four coordinate (110) surfaces and [ii] stereoregularity to activetitanium catalysts adjacently bound to the same (110) MgCl2surface.7−11 The catalysts of the fourth generation contain alkylesters of aromatic ortho diacids as internal donors (compo-

nents of the solid catalysts) and alkoxysilanes as external donors(components of cocatalyst mixtures). Nowadays, a phthalateester such as diisobutyl phthalate (DIBP) is widely used as aninternal donor in such catalyst systems, where one of itsimportant functions is to control the amount and distributionof TiCl4.

12

Infrared spectroscopy has long been a useful tool for studyingthe chemical structure of heterogeneous catalysts. It isparticularly attractive in the field of Ziegler−Natta catalystsfor olefin polymerization, where organic complexes are formedwithin the catalyst matrix.13 The IR spectrum of the solidDIBP/MgCl2/TiCl4 catalyst is in the 1900−1550 cm−1 spectralrange and contains a complex envelope of ν(CO) bands atca. 1687 cm−1 (shifted from the 1729 cm−1 of free DIBP) aswell as two narrow bands of the ortho disubstituted benzenering of DIBP at 1595 and 1582 cm−1 (aromatic ring modes).The envelope of the carbonyl group vibrations in the DIBPcoordinated catalyst IR spectra can be resolved intocomponents.14−16 The ν(CO) bands are assigned tocomplexes of DIBP with MgCl2 and TiCl4, as well as complexesof MgCl2 with two derivatives of DIBP (these derivatives areformed due to the reaction between TiCl4 and DIBP).16−23

Received: October 11, 2011Revised: January 4, 2012Published: January 4, 2012

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Fourier transform infrared (FTIR) spectroscopy has been animportant qualitative tool for studying infrared observableadsorbates24−28 and, under certain circumstances, can be usedto determine surface concentration.29,30 The integrated Beer−Lambert relation, a = Alc, is often used to relate surfaceconcentration to IR peak areas, where a is the IR absorbancepeak area, l is the sample thickness, and c is the surfaceconcentration. A is the integrated absorption intensity whichcan also be thought of as an extinction coefficient.31−33 Aknowledge of such extinction coefficients allows the quantita-tive determination of the concentrations of surface adsorbedspecies using FTIR. When using the attenuated total internalreflection (ATR) technique, infrared radiation penetrates only afew micrometers into the bulk of a material, making it anexcellent tool for surface studies of thin films.34−38 For thinfilms (thickness less than 200 nm) the measured integratedabsorbance can be approximated to the scale linear with theconcentration of the deposited material.39

So far, X-ray photoelectron spectroscopy (XPS) has beenused to study the effect of the internal donor in flat-modelZiegler−Natta catalysts, by following the influence of theinternal donor on the electron density around titanium.40−45

This is an indirect method. In direct XPS analysis of organicadsorbents (here, electron donors), shifts in the C 1s signalcould be interpreted qualitatively, but poor deconvolution ofspectra makes quantitative analysis unreliable.46,47 Thisprompts the use of fluorine as an excellent analytical markeror doping agent for XPS, as fluorine exhibits a clear quantifiablepeak around 692 eV.31,48−51 It should be noted that XPS dataare influenced by surface topology, lateral heterogeneity anddepth from the sample surface. So, angle resolved XPS analysisis essential for accurate quantification.52

In the past, transmission electron microscopy (TEM) wasused for macroscopic observation of polyolefins produced onZiegler−Natta catalysts.53,54 Direct TEM imaging of α-TiCl3was also reported in literature.55,56 A microscopic analysis of theinitial stage of the polyolefin formation on a TiCl4/MgCl2catalyst was reported by Barbe et al., who used a gold sputteringtechnique for the TEM samples.57 Using high resolution TEM,the (110) lateral cut of MgCl2 in Ziegler−Natta catalyst(without electron donor) was identified.58 Another highresolution TEM study on industrial catalysts (with ester ordiester as electron donor) revealed that changes in catalystpreparation methods can affect the atomic structure of MgCl2crystals.59 In situ real-time environmental TEM studies ondonor-free TiCl4/MgCl2 catalyst identified different lateralterminations of MgCl2.

60

However, the complex nature of real catalyst prevents theacquisition of the atomic level knowledge about the activecenters for polymerization and their local geometry. The activecenters are hidden in the pore structure of the support, therebylimiting the exposed surface area for characterization. Onesolution to this problem is the development of model catalysts.There are two different approaches to model the Ziegler−Nattacatalysts. One is the computational modeling of MgCl2/TiCl4/donor bulk and surface structures by means of DFT.3,4,8,11,61−71

The other one is the experimental investigation of modelcatalysts using different characterization techniques.41,72−78 Theflat-model approach facilitates the characterization of thecatalyst by surface spectroscopy and microscopy techni-ques.41,78 However, when dealing with model catalystcomprising extremely small amounts of highly reactive surfacespecies, special care has to be devoted to creating a sufficiently

clean environment that prevents the irreversible deactivation ofthe catalyst. In our case, all components of the Ziegler−Nattacatalyst are extremely sensitive to even minute amounts ofmoisture in the reactor setup. In situ ATR-FTIR is a goodanalytical tool to study the surface chemistry of flat-modelZiegler−Natta catalyst system. It is possible to observe thechanges in the infrared features of internal donors duringthermal and chemical treatments on MgCl2, adsorption ofTiCl4, catalyst activation in the presence of Aluminum alkylsand external donors. ATR-FTIR can also be used to study theethylene and propylene polymerization as well as thecrystallinity of polymer that is produced. However, only a fewin situ infrared studies are reported in literature.23,79

The present study focuses on the in situ ATR-FTIR analysisof MgCl2/ethanol/DIBP films on a flat surface. The ν(CO)band of the MgCl2/DIBP films producing an asymmetricenvelope between 1710 and 1692 cm−1 is the prime interest forthis study. Since the TiCl4 is not present in this film, theexistence of donor−TiCl4, donor derivative−MgCl2 complexescan be excluded. The DIBP content of the films was quantifiedby combining ATR-FTIR and XPS data. Changes in the ν(CO) band of the DIBP/MgCl2 films upon heating in a flowingargon gas stream were monitored in situ, revealing the changesin the adsorption mode of DIBP on MgCl2 surface. Themorphology of MgCl2 and MgCl2/DIBP films, supported onwindow etched silicon based grids, were studied by TEM.

■ EXPERIMENTAL SECTIONMaterials. Anhydrous magnesium chloride (ball milled, 99.9%),

absolute ethanol (99.9%), titanium tetrachloride (99.9%), diisobu-tylphthalate (89%), 4-fluorophthalic anhydride (97%), isobutanol(99%), anhydrous calcium chloride, concentrated sulphuric acid(96.7%), dichloromethane, sodium bicarbonate, sodium chloride,and sodium sulfate were purchased from Aldrich Chemicals andused as received. Argon (grade 6.0) was purchased from Linde andused after passing through a Dririte/molecular sieve column (4 Å).

Synthesis of 4-Fluoro-diisobutyl Phthalate. 4-Fluoro-diisobu-tyl phthalate (FDIBP) was synthesized based on a procedure found inthe literature.80 FDIBP was obtained in high yield by the fullesterification of 4-fluorophthalic anhydride with isobutanol (Support-ing Information, Scheme S1). Obtained 4-fluoro diisobutylphthalate(FDIBP) was characterized by 1HNMR (Supporting Information,Figure S1) and IR (see Supporting Information, Figure S2).

Preparation of MgCl2/Donor Thin Films. From a bulk solutionof MgCl2 (105 mmol dm−3) in ethanol, a series of solutions withdonor/MgCl2 molar ratios of 0, 0.1, 0.15, 0.2, 0.25, and 0.50 wasprepared. All of these preparations were carried out in N2 atmosphere.In a typical procedure, a solution was spin-coated (2800 r.p.m.) underglovebox conditions onto Si crystal (for ATR-FTIR studies), Si wafer(for XPS studies), and SiO2/SiNx TEM wafers (for TEM studies).

ATR-FTIR Analysis. All FTIR spectra were collected using aNicolet Protege 460 Fourier transform infrared spectrometer equippedwith a heated HATR flow cell for Spectra-Tech ARK with a Si 45°crystal (cutoff at 1500 cm−1). The FTIR spectra of uncoordinatedDIBP and FDIBP were recorded using Nicolet Smart Golden Gateequipment with a diamond crystal (cutoff at 800 cm−1). The coated Sicrystal was mounted and sealed under a nitrogen atmosphere in anATR-cell. In order to mimic the high vacuum conditions during XPSmeasurement and simultaneously prevent moisture and oxygen fromatmosphere entering in to the system, Ar gas was flowed through thecell for 30 min at 2 bar and 30 °C. FTIR spectra were measured inabsorbance mode using a Silicon background (taken at 30 °C) with aresolution of 4 cm−1 and 32 scans per measurement. For temperatureprogrammed in situ studies, the ATR set up was gradually heatedunder argon flow with isothermal steps (10 °C difference) from 30 °Cup to 150 °C. FTIR spectra were recorded at the end of each

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isothermal step (5 min), against Si background (taken at that particulartemperature).XPS Analysis. XPS measurements were performed with a VG

Escalab 200 using a standard aluminum anode (Al Kα 1486.3 eV)operating at 300 W. The coated Si wafer was transferred (using XPStransfer vessel) under nitrogen atmosphere into the XPS-antechamberand then under high vacuum conditions into the measurementchamber. Spectra were recorded at a background pressure of 1 × 10−9

mbar. Binding energies were calibrated to a Cl 2s peak at 199.5 eV.Measurements were carried out at a 0° and 60° takeoff angle relative tothe surface normal.TEM Analysis. TEM studies were carried out on an FEI Tecnai 20

(Sphera), operated at 200 kV. The custom-made TEM wafers havedimensions of 20 × 20 mm2 to allow homogeneous distribution ofsolution during spin coating. These wafers consist of 36 individualTEM grids. The grids are arranged in a square pattern and stabilizedby a silicon frame. The central part of each grid is etched away tocreate a 15−20 nm thick silicon nitride (SiNx) membrane. The size ofthis membrane (TEM window) is around 100 × 100 μm2, whichallows the electron beam to pass, thus allows imaging of supportedparticles in a sub nanometer scale. Before spin coating, the wafers werecalcined in dry air. This improves the strength of the SiNx membranedue to the formation of a 3 nm thick SiO2 surface layer. Afterdeposition of MgCl2/DIBP solution via spin coating, the TEM waferwas carefully broken in to individual TEM grids. A single TEM gridwas mounted on the sample holder. Then it was transferred to thetransmission electron microscope, under an inert atmosphere (usingTEM transfer tube). This methodology was already applied for flat-model systems with iron oxide nanoparticles in morphology studiesrelated to the carbon nanotube growth and Fischer−Tropschcatalysis81,82 and tin oxide nanoparticles produced by atmosphericpressure chemical vapor deposition.83

■ RESULTS AND DISCUSSION

Quantification of Coordinated Donor. A series ofMgCl2−DIBP mixtures in ethanol with different DIBP to Mgmolar ratio were prepared and spin coated onto the Si ATRcrystal. FTIR spectra of these MgCl2/DIBP films were recordedafter 30 min of Ar flow at 30 °C. All spectra measured at 30 °Ccontained a ν(CO) band around 1700 cm−1, which can beassigned to the ν(CO) band of coordinated DIBP. Weobserved a linear increase in peak intensity when increasing theDIBP to Mg ratio up to 0.25.A fluorine tagged DIBP homologue (4-fluoro-diisobutyl

phthalate or FDIBP) was synthesized for XPS studies. TheATR-FTIR spectra of uncoordinated DIBP and FDIBP werecompared (see Supporting Information, Figure S2 and TableS1). The ν(CO) bands of uncoordinated DIBP and FDIBP,were separated by only 3 cm−1. Apparent similarity of theν(CO) bands is highly favored when considering FDIBP as atagged analogue for DIBP, since the CO bond plays a vitalrole in coordination with metal centers. ATR FTIR spectra ofthe MgCl2/ethanol/DIBP and MgCl2/ethanol/FDIBP filmswere also compared (Supporting Information, Figure S3 andTable S1). Upon coordination, the νmax(CO) (the carbonylband peak maxima) shifts from 1725 to 1705 cm−1 (DIBP) and1728 to 1708 cm−1 (FDIBP). The Δν [difference between theνmax(CO) of uncoordinated and coordinated donor] isexactly 20 cm−1 in both cases indicating that, the influence of Fatom in the aromatic ring on the coordination behavior ofphthalate ester is negligible. This makes FDIBP an idealsubstitute for DIBP in XPS studies. MgCl2/FDIBP films wereprepared by following the same preparation method used forMgCl2/DIBP films. The same ATR-FTIR experimentalprocedure used for FDIBP/MgCl2 films was followed forFDIBP/MgCl2 complexes too. ATR-FTIR spectra of MgCl2/

DIBP films and MgCl2/FDIBP films with different donorloadings are shown in panels a and b of Figure 1, respectively.

The peak position of uncoordinated donors (in liquid phase) isalso given for comparison (dotted line). Using TQ AnalystSoftware, peak maxima (center of gravity peak location at 10%threshold) and integrated peak intensities for carbonylstretching band of DIBP and FDIBP was determined. BothDIBP and FDIBP show Beer−Lambert type behavior when

Figure 1. (a) Carbonyl and aromatic IR bands of DIBP in MgCl2/DIBP films. (a, inset) Plot of the integrated peak intensities of thecarbonyl band and the aromatic ring band (intensity multiplied by 10)versus the DIBP/Mg ratios in ethanol solution prior to spin coating.(b) Carbonyl and aromatic IR bands of FDIBP in MgCl2/FDIBP films.(b, inset) Plot of the integrated peak intensities of the carbonyl bandand the aromatic ring band (intensity multiplied by 10) versus theFDIBP/Mg ratios in ethanol solution prior to spin coating. (c) F 1sXPS spectra of the MgCl2/ethanol/FDIBP films. (c, inset) Plot of theF/Mg ratios measured by XPS (at a photo electron takeoff angle of 0°and 60°, relative to the surface normal) versus the F/Mg ratios inethanol solution prior to spin coating.

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plotting the integrated intensity (absorbance mode) of the CO stretching band and aromatic ring bands versus Donor to Mgratio (shown in the insets of panels a and b of Figure 1). Thisindicates that donor/MgCl2 ratio in films reproduce the donorto MgCl2 ratio in the spin coating solution.XPS is used to confirm the ATR-FTIR quantification of the

donor concentration in the film. A fluorinated derivative ofDIBP (FDIBP) was used as the internal donor in the films forXPS studies. Using XPS, the constituent atoms in the FDIBP/MgCl2 films were quantified and the lateral heterogeneity of thefilms was established. The combined XPS spectrum (F 1sregion) of MgCl2/Ethanol/FDIBP films, with donor/Mg molarratios ranging from 0.05 to 0.25 is shown in Figure 1c. Angleresolved XPS experiments showed that the Mg to F ratioremained unchanged, irrespective of the photoelectron takeoffangle 0° or 60° (relative to the surface normal). This is anindication of homogeneous DIBP distribution throughout theMgCl2 film. In a previous study, we determined the thickness ofMgCl2 film around 11 ± 2 nm.41 Figure 1c inset represents theplot of actual FDIBP to Mg ratio calculated using XPS versusinitial FDIBP to Mg ratio in the spin coated solution. XPSproves the one-to-one correlation between concentration ofdonor in solution and film. Therefore, ATR-FTIR can be usefor quantitative estimation of the donor in MgCl2/donor films.In Situ ATR-FTIR Studies during Annealing of the

DIBP/MgCl2 Films under Argon Flow. The spectra recordedbefore starting the Ar flow, represents the state of MgCl2/filmas spin coated. The ν(CO) band of DIBP at that state wassuper imposable to uncoordinated DIBP (around 1725 cm−1),which lead to the conclusion that the donor was notchemisorbed on Mg. The IR signal of ethanol (remnantsfrom the impregnation solution) was also observed. Within the30 min of Ar flow at 30 °C, the ν(CO) band broadened andshifted to lower wavenumber (around 1700 cm−1) indicatingthe coordination of DIBP to Mg surface. Simultaneously, weobserved the partial removal of residual ethanol from the film.The νmax(CO) at room temperature was changed from 1704to 1715 cm−1, when the DIBP loading was increased from 5%to 50% of MgCl2. This is probably due to the incompleteconversion of uncoordinated donor to coordinated donor.When the temperature is increased from 30 to 150 °C, all of theDIBP/MgCl2 films showed a clear and progressive shift inνmax(CO) of DIBP to a lower wavenumber (around 1690cm−1). Further removal of residual ethanol from the film wasobserved. In the films with DIBP concentration above 20 mol% with respect to MgCl2, partial desorption of DIBP was alsoobserved. As an example, the ATR-FTIR spectra recorded inbetween the in situ experiment on MgCl2/DIBP film withdonor to Mg ratio of 0.15 is shown in Figure 2. The bandν(O−H) represents the O−H stretching mode of ethanol. Thisband disappeared at 150 °C due to ethanol removal. At 30 °C,the band ν(C−C) represents the C−C stretching modes ofethanol and donor. The ethanol leaves the surface at hightemperature and shape of the band changes. At 150 °C, theband ν(C−C) solely represents C−C stretching modes ofDIBP. XPS studies on MgCl2/ethanol films showed that theamount of residual ethanol in the film at 30 °C is around 13%with respect to MgCl2 (see the Supporting Information, FigureS4). Based on this, the amount of residual ethanol with respectto MgCl2 in the MgCl2/ethanol/DIBP film was estimated,which is around 18.5 mol % before Ar flow, 9 mol % after 30min of Ar flow, and 1 mol % after annealing at 150 °C. Thepeak maxima of ν(CO) shifted from 1705 to 1688 cm−1.

After cooling the ATR system to room temperature, thecarbonyl band at 1688 cm−1 remains stationary, indicating anirreversible shift. In situ ATR-FTIR spectra of DIBP/MgCl2films at different temperatures and different loadings werecompared. The surface concentration of DIBP at eachtemperature was calculated using the quantification method(based on Beer−Lambert relation) discussed in the previoussection. Donor coverage as the function of temperature isshown in Figure 3. The maximum amount of DIBP in the

MgCl2/DIBP film declines from 38% at 30 °C to 19% at 150°C. The average peak maxima of the spectra at this stage werearound 1692 cm−1. In the literature, the broad peak around1687 cm−1 was attributed as a superposition of bands due toDIBP coordinated on different surface sites of MgCl2.

14 Theshift in νmax(CO) occurs at elevated temperature can beexplained as the changes in the coordination sites on MgCl2 orchanges in the coordination mode of DIBP. The assignmentsfor the phthalate ester peak found in literature are summarizedin Table 1. It is believed that the four coordinated MgCl2 (110)site is more acidic than the five coordinated MgCl2 (104) site.

Figure 2. Comparative ATR FTIR spectra of the DIBP/MgCl2 film(with a donor to Mg ratio of 0.15) at different stages of in situexperiment. ν(O−H) is the O−H stretching band of ethanol, ν(C−C)is the C−C stretching band of ethanol and donor, and ν(C−O) is thecarbonyl stretching of DIBP.

Figure 3. Thermal desorption pattern of DIBP (calculated fromintegrated peak intensities of DIBP carbonyl band).

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The interaction of electron donor with the MgCl2 (110) sitewill be stronger than that of the (104) site, which results in alower wavenumber infrared band for electron donorcoordinates on the MgCl2 (110) site.The experimental ATR-FTIR spectra (recorded at different

donor concentrations and temperatures) of the MgCl2/DIBPfilms were simulated as a summation of Gaussian−Lorentzianfunction. The best fit for the entire spectra was achieved whenthree components with peak maxima around 1725, 1705, and1685 were used. For the details of simulations (peak fitting), werefer to the Supporting Information, Table S2 and Figure S5.Based on the information from Table 1, the observed ν(CO)band can be explained as a combination of three overlappingcomponents corresponding to the DIBP at three differentchemical environments/phases: [i] uncoordinated DIBP, [ii]DIBP coordinated on MgCl2 (104) site, and [iii] DIBPcoordinated on MgCl2 (110) site. The relative amounts of theDIBP at the above-mentioned environments were calculatedusing the quantification method (discussed above). XPSmeasurements confirmed that the molar extinction coefficientof the ν(CO) envelope does not change upon annealing(Supporting Information, Figures S6 and S7). The relativeamounts of the three DIBP components in the MgCl2 films as afunction of temperature are plotted in Figure 4. DIBP remains

uncoordinated in the film after spin coating but prior to argonflow. A large excess of ethanol in the film indicates that thelayered MgCl2 crystallites may not yet have formed. Non-layered MgCl2−ethanol adduct formation from a saturatedsolution of MgCl2 in ethanol is reported in the literature.84 Asthe ethanol desorbs in the Ar flow, crystalline MgCl2 forms andDIBP starts to coordinate on MgCl2 (104) and (110) sites. Theexistence of crystalline MgCl2 in nonannealed films has beenconfirmed by TEM (see below). Within the 30 min of Ar flow,the majority of the DIBP coordinates on MgCl2 (104) and(110) sites. At low donor loading (DIBP to Mg mol ratio<0.15), DIBP prefers to coordinate on the MgCl2 (104) site. Athigh donor loading (DIBP to Mg mol ratio >0.15), DIBP doesnot have any preference between the MgCl2 (104) or (110)site. The amount of residual ethanol remaining in the film (after30 min of Ar flow) is decreases with increasing amounts ofDIBP in the film. The above two observations lead to the pointthat ethanol preferably coordinates on MgCl2 (110) site andDIBP coordinates on the remaining surface sites. On thermalannealing, ethanol desorbs from the MgCl2 (110) surface.Simultaneously, DIBP migrates from MgCl2 (104) sites toMgCl2 (110) sites. In the case of high DIBP loadings, therelative abundance of DIBP on MgCl2 (110) sites also increasewith increasing temperature, up to a point at which the totalintensity of the carbonyl band starts decrease. This decrease inintensity is only observed in films with donor to Mg ratio abovethan 0.20. The maximum donor loading observed on MgCl2decreases with increasing temperature (assuming that themaximum donor loading corresponds to fully covered MgCl2surface). This implies that the crystals sinter at hightemperature, even in the presence of internal donor. Themaximum amount of donor coordinated to the MgCl2 surfacesites was around 18 mol %.The outcome of this peak fitting suggests the conversion of

uncoordinated DIBP into DIBP coordinated on MgCl2 (104)and (110) sites at 30 °C, followed by the migration of DIBPfrom MgCl2 (104) sites to MgCl2 (110) sites at elevated

Table 1. Peak Assignments for Phthalate Esters, Reported inthe Literature

ν(CO) bandassigned νmax(CO) in

cm−1

uncoordinated phthalate ester (condensed state)/physisorbed to 6-fold Mg ion at (001) plane

1732,18 1730,17

172815,16,19,20 1733,14

172221

phthalate ester bound to 5-fold Mg ion at (104)edge

1705,22 1704,20 1700,16

169915,21

phthalate ester bound to 4-fold Mg ion at (110)edge

1692,16 1684,20

1672,15,21 166222

phthalate ester bound to 3-fold Mg ion at corner 1656,20 165015,21

Figure 4. Changes in distribution of DIBP among different components with respect to DIBP loading and temperature (the range of error due to thecovariance of different components is shown along with each data point). The approximate amount of ethanol derived from the intensity of the (O−H) stretching band is included. The ethanol amounts are only a rough estimate due to the uncertainty of the XPS quantification of ethanol in MgCl2films.

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temperatures. On spin coating, a thin film of MgCl2, ethanol,and DIBP is formed. The chemical state of MgCl2 at this stageis either as nonlayered MgCl2-ethanol adducts or MgCl2crystals (with lateral surface sites coordinated by ethanol)formed from this adduct. At this stage, DIBP remainsuncoordinated in the film. During Ar gas flow, a large amountof ethanol removes from the film, and DIBP coordinates to thevacancies created by ethanol on MgCl2 (104) and (110) sites.Upon annealing, DIBP transfers from MgCl2 (104) sites toMgCl2 (110). The transfer of DIBP occurs in the temperaturerange of 50−90 °C, which is the same temperature range atwhich most of the ethanol removes from the surface. It can beassumed that MgCl2 (110) sites become vacant on ethanolremoval, and DIBP migrates from less acidic MgCl2 (104) siteto more acidic MgCl2 (110) site. It is already known thatMgCl2 (110) sites are unstable in the absence of adsorbentmolecule, because of the high surface energy of the emptysurface.3 SEM-EDX studies on Ostwald ripened crystals ofMgCl2 prepared from DIBP/MgCl2/ethanol solution (DIBP toMg ratio of 0.1), pointed out the formation of crystallites with120° and 90° edge angles, which indicates the presence of(104) and (110) edge surfaces of MgCl2 with five and 4-foldMg ions respectively.85 The coexistence of MgCl2 (104) and(110) sites in the MgCl2 crystal (in the presence of DIBP) is inline with the results of peak fitting.

MgCl2 Film Morphology. Transmission electron micros-copy was used to study the morphology of MgCl2 and MgCl2/DIBP films. The TEM images of annealed (at 150 °C, for 1 h)and nonannealed (as spin coated) MgCl2 and MgCl2/DIBPfilms were compared (Figure 5). The concentration of DIBP inthe precursor solution for MgCl2/DIBP film preparation was 15mol % with respect to Mg. Note that the nonannealed films arealready went through the vacuum conditions of TEMmeasurements. Therefore we expect these samples to becomparable to the ATR films after a 30 min treatment inflowing argon. In all cases, MgCl2 platelets were observed. Incontrast to films prepared by chemical vapor deposition in ultrahigh vacuum on metallic surfaces,74,75,77 these platelets werearranged parallel to the surface normal (edge-on orientation).This may be due to the chemical affinity of coordinativelyunsaturated crystal side edges with the underlying SiO2 layer ofTEM grid. The diffraction patterns of these MgCl2 and MgCl2/DIBP films (annealed and nonannealed) were compared(Supporting Information, Figure S8). The most intense signalin the diffraction pattern of all these films corresponds to a dspacing of 5.9 Å, which represent the basal (001) plane ofMgCl2.

4,60 The TEM images reveals that, MgCl2 crystals arealready present in the film at 30 °C itself. In the nonannealedfilms, the crystalline regions are small, both in the lateraldimensions of the MgCl2 platelets as well as in the degree ofstacking along the (001) direction. After annealing, we observe

Figure 5. TEM images of MgCl2 and MgCl2/DIBP films: (a) MgCl2 film at 30 °C. (b) MgCl2 film after annealing at 150 °C for 1 h; FFT view of thesame as inset. (c) MgCl2/DIBP film at 30 °C. (d) MgCl2/DIBP film after annealing at 150 °C for 1 h; FFT view of the same as inset.

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a more extensive stacking of the MgCl2 crystals in both films. Inthe annealed, donor free MgCl2 film (Figure 5b), the plateletsalso grow in their lateral dimensions leading to large domains ofslightly bent and twisted MgCl2 platelets in parallel alignment.These observations demonstrate that the nanocrystalline MgCl2platelets reorganize and sinter upon heating. The annealedMgCl2/DIBP film features comparatively smaller crystals, whichleads to smaller degree of platelet stacking as compared to theannealed MgCl2 film. We attribute this effect to DIBPstabilizing the coordinatively unsaturated magnesium sites atthe perimeter of MgCl2 platelets. The lateral dimensions of theMgCl2 crystals in the annealed DIBP/MgCl2 film was around4−8 nm. This corresponds to approximately 25−15% surfaceMg ions available for donor coordination, which is in goodqualitative agreement with the concentration of DIBP onMgCl2 surface sites in the film, as shown by ATR experiments.

■ CONCLUSIONS

A sensitive method was developed for the investigation ofcoordinated donor on flat-model Ziegler−Natta catalysts bymeans of in situ ATR-FTIR. The method is based on the linearrelationship between the carbonyl stretching band integratedpeak intensity of DIBP and concentration of DIBP (concen-tration of DIBP in the MgCl2/DIBP adduct prepared) inMgCl2 films. The results were calibrated by XPS studies on a Flabeled homologue of DIBP, which is the 4-fluoro-diisobutylph-thalate (FDIBP). The newly synthesized FDIBP displayed awell-quantifiable fluorine 1s peak during XPS-studies of spin-coated MgCl2/ethanol/donor surfaces, as opposed to thehitherto used, poorly resolved carbon signals of DIBP. ATR-FTIR spectra of the flat surfaces showed sufficient similarity incoordination behavior between FDIBP and DIBP to regard 4-fluoro-diisobutylphthalate as a possible substitute for DIBP infuture XPS studies on flat-model catalyst surfaces. Changes inthe coordination behavior of DIBP on MgCl2 were followed byin situ ATR-FTIR spectroscopy. Directly after impregnation,DIBP shows no interaction with MgCl2. However, as someethanol desorbs at 30 °C in flowing argon, DIBP binds to theMgCl2. We tentatively explain the observed changes of theν(CO) adsorption bands as a combination of threeoverlapping components assigned to uncoordinated DIBP,DIBP adsorbed to MgCl2 (104), and DIBP adsorbed to MgCl2(110). The molecular level structure of the coordination sites(surface site structure, DIBP coordination mode and DIBPcoverage on surface sites) and the corresponding wavenumbersfor the carbonyl bands of adsorbed DIBP can only be found byDFT calculations, which is in progress.Based on our tentative assignment, we derive the following

conclusions:

• Before annealing, we observe a slight preference of theDIBP donor for the MgCl2 (104) site. This preference isprobably induced by the presence of coadsorbed ethanol.

• Upon annealing, DIBP transfers from MgCl2 (104) sitesto MgCl2 (110) sites as these adsorption sites becomevacant due to desorption of ethanol from MgCl2 (110)sites.

• The saturation loading of DIBP after annealing at 150 °Cis 18 mol % with respect to Mg. The maximum donorloading observed on MgCl2 decreases with increasingtemperature (assuming that the maximum donor loadingcorresponds to fully covered MgCl2 surface). Thisimplies that the crystals sinter at high temperature,

even in the presence of internal donor. Excess donor isremoved from the film under Ar flow.

TEM studies show that sintering of MgCl2 films occursduring annealing. In the absence of donor this sintering leads tovery large MgCl2 platelets that offer only a very small numberof coordination sites. In the presence of donor, MgCl2 plateletsremain much smaller due to the stabilization of the surface sitesby adsorbed DIBP. The observed lateral dimensions of theMgCl2 platelets (4−8 nm) agree well with the size require-ments for DIBP adsorbed on MgCl2 assuming a full coverageon both MgCl2 (104) and (110).The results show the feasibility of preparing Ziegler−Natta

catalysts with different donor concentration on different MgCl2surface sites. Since the activity and selectivity of the catalyst isheavily depends on the coordination behavior of internal donor,it is interesting to compare the activity and selectivity of thesecatalysts. The quantification method discussed in this articlecan be applied to different types of donors used in Ziegler−Natta catalysis. Moreover, this quantitative information will beuseful for the in situ studies on the further steps of catalystpreparation, such as treatment with TiCl4 and aluminum alkyls.

■ ASSOCIATED CONTENT

*S Supporting InformationSynthesis of 4-fluoro-diisobutyl phthalate (FDIBP); 1HNMRspectra of FDIBP; comparative studies on DIBP and FDIBPcoordination; quantification of ethanol in the MgCl2/ethanoland MgCl2/ethanol/donor films; peak fitting procedure; molarextinction coefficient calibration; electron diffraction pattern ofMgCl2, DIBP/MgCl2 films. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: p.c.thuene@tue.nl. Telephone: +31 40 247 4997.

■ ACKNOWLEDGMENTSThis research forms part of the research program of the DutchPolymer Institute (DPI) under Project 712. The authors aregrateful to Prof. Vincenzo Busico of University of Naples forthe valuable discussions and comments.

■ REFERENCES(1) Moore, E. P., Jr. Polypropylene Handbook: Polymerization; HanserPublishers: New-York, 1996; Chapter 2.(2) Giannini, U. Makromol. Chem. Suppl. 1981, 5, 216.(3) Busico, V.; Causa, M.; Cipullo., R.; Credendino, R.; Cutillo, F.;Friederichs, N.; Lamanna, R.; Segre, A.; Castelli, V. V. A. J. Phys. ChemC 2008, 112, 1081−1089.(4) Correa, A.; Piemontesi, F.; Morini, G.; Cavallo, L. Macromolecules2007, 40, 9181−9189.(5) Sacchi, M. C.; Tritto, I.; Locatelli, P. Prog. Polym. Sci. 1991, 16(2−3), 331−360.(6) Sacchi, M. C.; Forlini, F.; Tritto, I.; Locatelli, P.; Morini, G.;Noristi, L.; Albizzati, E. Macromolecules 1996, 29 (10), 3341−3345.(7) Taniike, T.; Terano, M. Macromol. Rapid Commun. 2008, 29,1472−1474.(8) Boero, M.; Parrinello, M.; Huffer, S.; Weiss, H. J. Am. Chem. Soc.2000, 122, 501−509.(9) Brambilla, L.; Zerbi, G.; Piementosi, F.; Nascetti, S.; Morini, G. J.Mol. Catal. A: Chem. 2007, 263, 103−111.(10) Ribour, D.; Monteil, V.; Spitz, R. J. Polym. Sci. A: Polym. Chem.2008, 46, 5461−5470.

Langmuir Article

dx.doi.org/10.1021/la203972k | Langmuir 2012, 28, 2643−26512649

(11) Vanka, K.; Singh, G.; Iyer, D.; Gupta, V. K. J. Phys. Chem. C2010, 114 (37), 15771−15781.(12) Chadwick, J. C. Macromol. Symp. 2001, 173, 21−35.(13) Terano, M.; Kataoka, T. J. Polym. Sci. A: Polym. Chem. 1990, 28,2035−2048.(14) Zohuri, G.; Ahmadjo, S.; Jamjah, R.; Nekoomanesh, M. Iran.Polym. J. 2001, 10 (3), 149−155.(15) Potapov, A. G.; Bukatov, G. D.; Zakharov, V. A. J. Mol. Catal. A:Chem. 2006, 246 (1−2), 248−254.(16) Kissin, Y. V.; Liu, X. S.; Pollick, D. J.; Brungard, N. L.; Chang,M. J. Mol. Catal. A: Chem. 2008, 287 (1−2), 45−52.(17) Arzoumanidis, G. G.; Karayannis, N. M. Appl. Catal. 1991, 76,221−231.(18) Yang, C. B.; Hsu, C. C.; Park, Y. S.; Shurvell, H. F. Eur. Polym. J.1994, 30 (2), 205−214.(19) Makwana, U.; Naik, D. G.; Singh, G.; Patel, V.; Patil, H. R.;Gupta, V. K. Catal. Lett. 2009, 131 (3−4), 624−631.(20) Singh, G.; Kaur, S.; Makwana, U.; Patankar, R. B.; Gupta, V. K.Macromol. Chem. Phys. 2009, 210 (1), 69−76.(21) Potapov, A. G.; Bukatov., G. D.; Zakharov., V. A. J. Mol. Catal.A: Chem. 2010, 316 (3−4), 95−99.(22) Stukalov, D. V.; Zakharov, V. A.; Potapov, A. G.; Bukatov, G. D.J. Catal. 2009, 266 (1), 39−49.(23) Ystenes, M.; Bache, Ø.; Bade, O. M.; Blom, R.; Eliertsen, J. L.;Nirisen, Ø.; Ott, M.; Svendsen, K.; Rytter, E. In Future technology forPolyolefin and Olefin Polymerization catalysis; Terano, M., Shiono, T.,Eds.; Technology and education Publishers: Tokyo, 2002; pp 184−191.(24) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61 (11), 1504−1512.(25) Arai, H.; Tominaga, H. J. Catal. 1976, 43 (1−3), 131−142.(26) Srinivas, G.; Chuang, S. S. C.; Debnath, S. J. Catal. 1994, 148(2), 748−758.(27) Chuang, S. S. C.; Brundage, M. A.; Balakos, M. W.; Srinivas, G.Appl. Spectrosc. 1995, 49 (8), 1151−1163.(28) Balakos, M. W.; Chuang, S. S. C.; Srinivas, G.; Brundage, M. A.J. Catal. 1995, 157 (1), 51−65.(29) Rasband, P. B.; Hecker, W. C. J. Catal. 1993, 139 (2), 551−560.(30) Srinivas, G.; Chuang, S. S. C. J. Catal. 1993, 144 (1), 131−147.(31) Nakahama, K.; Kawaguchi, H.; Fujimoto, K. Langmuir 2000, 16(21), 7882−7886.(32) Cavanagh, R. R.; Yates, J. T. J. Chem. Phys. 1981, 74 (7), 4150−4155.(33) Tan, C. D.; Ni, J. F. J. Chem. Eng. Data 1997, 42 (2), 342−345.(34) Bugay, D. E. Adv. Drug Delivery Rev. 2001, 48 (1), 43−65.(35) Planinsek, O.; Planinsek, D.; Zega, A.; Breznik, M.; Srcic, S. Int.J. Pharm. 2006, 319 (1−2), 13−19.(36) Tickanen, L. D.; Tejedor-Tejedor, M. I.; Anderson, M. A.Langmuir 1991, 7, 451−456.(37) Leewis, C. M.; Kessels, W. M. M.; van de Sanden, M. C. M.;Niemantsverdriet, J. W. (Hans) J. Vac. Sci. Technol. 2006, 24 (2), 296−304.(38) Harrick, N. J. J. Opt. Soc. Am. 1965, 55, 851−857.(39) Muller, M.; Keßler, B. Langmuir 2011, 27 (20), 12499−12505.(40) Kaushik, V. K.; Gupta, V. K.; Naik, D. G. Appl. Surf. Sci. 2006,253 (2), 753−756.(41) Andoni, A.; Chadwick, J. C.; Milani, S.; Niemantsverdriet, J. W.(Hans); Thune, P. C. J. Catal. 2007, 247 (2), 129−136.(42) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, J. W. (Hans);Thune, P. C. Macromol. Rapid Commun. 2007, 28 (14), 1466−1471.(43) Da Silva, A. A.; Alves, M. D. M.; dos Santos, J. H. Z. J. Appl.Polym. Sci. 2008, 109 (3), 1675−1683.(44) Zhao, X.; Zhang, Y.; Song, Y.; Wei, G. Surf. Rev. Lett. 2007, 14(5), 951−955.(45) Mori, H.; Hasebe, K.; Terano, M. J. Mol. Catal. A: Chem. 1999,140, 165−172.(46) Xiao-hong, G.; Guang-hao, C.; Chii, S. J. Environ. Sci. 2007, 19,438−443.

(47) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, J. W. (Hans);Thune, P. C. Macromol. Symp. 2007, 260, 140−146.(48) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.;Smith, L. M.; Hamers, R. J. Langmuir 2002, 18 (4), 968−971.(49) De Giglio, E.; Cometa, S.; Cioffi, N.; Torsi, L.; Sabbatini, L.Anal. Bioanal. Chem. 2007, 389, 2055−2063.(50) Ahmed, S. F.; Mitra, M. K.; Chattopadhyay, K. K. Appl. Surf. Sci.2007, 253 (12), 5480−5484.(51) Snow, A. W.; Foos, E. E.; Coble, M. M.; Jernigan, G. G.;Ancona, M. G. Analyst 2009, 134 (9), 1790−1801.(52) Thomas, H. R.; O’Malley, J. J. Macromolecules 1979, 12 (2),323−329.(53) Kakugo, M.; Sadathoshi, H.; Yokoyama, K.; Kojima, K.Macromolecules 1989, 22, 547−551.(54) Kakugo, M.; Sadathoshi, H.; Sakai, J.; Yokoyama, M.Macromolecules 1989, 22, 3172−3177.(55) Murry, R. T.; Pearce, R.; Platt, D. J. Polym. Sci., Polym. Lett. Ed.1978, 16, 303−308.(56) Higuchi, T.; Liu, B.; Nakatani, H.; Otsuka, N.; Terano, M. Appl.Surf. Sci. 2003, 214, 272−277.(57) Barbe, P. C.; Noristi, L.; Baruzzi, G.; Marchetti, E. Makromol.Chem. Rapid. Commun. 1983, 4, 249−252.(58) Mori, H.; Sawada, M.; Higuchi, T.; Hasebe, K.; Otsuka, N.;Terano, M. Macromol. Rapid Commun. 1999, 20, 245−250.(59) Mori, H.; Higuchi, T.; Otsuka, N.; Terano, M. Macromol. Chem.Phys. 2000, 201, 2789−2798.(60) Oleshko, V. P.; Crozier, P. A.; Cantrell, R. D.; Westwood, A. D.J. Electron. Microsc. 2002, 51 (Supplement), S27−S39.(61) Boero, M.; Parrinello, M.; Terakura, K. J. Am. Chem. Soc. 1998,120, 2746−2752.(62) Boero, M.; Parrinello, M.; Terakura, K. Surf. Sci. 1999, 438, 1−8.(63) Boero, M.; Parrinello, M.; Weiss, H.; Huffer, S. J. Phys. Chem. A2001, 105, 5096−5105.(64) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. J. Phys. Chem. A1997, 101, 6063−6068.(65) Seth, M.; Margl, P. M.; Ziegler, T. Macromolecules 2002, 35,7815−7829.(66) Taniike, T.; Terano, M. Macromol. Rapid Commun. 2007, 28,1918−1922.(67) Mukopadhyay, S.; Kulkarni, S. A.; Bhaduri, S. J. Mol. Struct.THEOCHEM 2004, 673, 65−77.(68) Mukopadhyay, S.; Kulkarni, S. A.; Bhaduri, S. J. Organomet.Chem. 2005, 690, 1356−1365.(69) Stukalov, D. V.; Zakharov, V. A. J. Phys. Chem. C. 2009, 113,21376−21382.(70) Stukalov, D. V.; Zilberberg, I. L.; Zakharov, V. A.Macromolecules2009, 42, 8165−8171.(71) Stukalov, D. V.; Zakharov, V. A.; Zilberberg, I. L. J. Phys. Chem.C 2010, 114, 429−435.(72) Siokou, A.; Ntais, S. Surf. Sci. 2003, 540, 379−388.(73) Magni, E.; Somorjai, G. A. Surf. Sci. 1997, 377−379, 824−827.(74) Magni, E.; Somorjai, G. A. J. Phys. Chem. B. 1998, 102, 8788−8795.(75) Kim, S. H.; Somorjai, G. A. Surf. Interface Anal. 2001, 31, 701−710.(76) Kim, S. H.; Somorjai, G. A. J. Phys. Chem. B 2002, 106, 1386−1391.(77) Kim, S. H.; Tewell, C. R.; Somorjai, G. A. Kor. J. Chem. Eng.2002, 19 (1), 1−10.(78) Thune, P. C.; Niemantsverdriet, J. W. (Hans) Surf. Sci. 2009,603, 1756−1762.(79) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, J. W. (Hans);Thune, P. C. Catal. Lett. 2009, 130 (3−4), 278−285.(80) Skrzypek, J.; Sadlowski, J. Z.; Lachowska, M.; Turzanski, M.Chem. Eng. Process. 1994, 33 (6), 413−418.(81) Moodley, P.; Loos, J.; Niemantsverdriet, J. W. (Hans); Thune,P. C. Carbon. 2009, 47, 2002−2013.(82) Moodley, P.; Scheijen, F. J. E.; Niemantsverdriet, J. W. (Hans);Thune, P. C. Catal. Today. 2009, 154 (1−2), 142−148.

Langmuir Article

dx.doi.org/10.1021/la203972k | Langmuir 2012, 28, 2643−26512650

(83) Mannie, G. J. A.; van Deelen, J.; Niemantsverdriet, J. W. (Hans);Thune, P. C. Appl. Phys. Lett. 2011, 98, 051907.(84) Sozzani, P.; Bracco, S.; Comotti, A.; Simonutti, R.; Camurati, I.J. Am. Chem. Soc. 2003, 125, 12881−12893.(85) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, J. W. (Hans);Thune, P. C. J. Catal. 2008, 257, 81−86.

Langmuir Article

dx.doi.org/10.1021/la203972k | Langmuir 2012, 28, 2643−26512651