Article pubs.acs.org/JPCC
Modeling Coadsorption of Titanium Tetrachloride and Bidentate Electron Donors on Magnesium Dichloride Support Surfaces Andrey S. Bazhenov,† Peter Denifl,‡ Timo Leinonen,‡ Anneli Pakkanen,‡ Mikko Linnolahti,*,† and Tapani A. Pakkanen*,† †
Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI 80101 Joensuu, Finland Borealis Polymers Oy, P.O. Box 330, FI 06101, Porvoo, Finland
‡
S Supporting Information *
ABSTRACT: Coadsorption of titanium tetrachloride and two representative bidentate electron donors on magnesium dichloride surfaces is systematically studied by means of periodic quantum chemical calculations. The two catalytically relevant surfaces in the Ziegler−Natta catalysis, (104) and (110) surfaces of the MgCl2 support, are taken into account. Adsorption of TiCl4 leads to formation of three types of mononuclear species on the magnesium dichloride surfaces. However, TiCl4 alone cannot properly stabilize the support. Coadsorption of electron donors along with TiCl4, on the other hand, is shown to significantly improve the strength of TiCl4 adsorption on the magnesium dichloride surfaces. Our findings indicate the importance of electron donors as promoters of titanium tetrachloride adsorption. The model is readily extendable to evaluate other electron donors and binuclear titanium species.
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INTRODUCTION
Supported Ziegler−Natta (ZN) catalysts account for the major part of the industrial production of stereoregular polyolefins worldwide. The latest generation catalytic ZN systems consist of (1) magnesium dichloride support, (2) titanium tetrachloride catalyst, (3) alumorganic cocatalyst, and (4) internal and (5) external electron donors. Magnesium dichloride crystallites typically expose two types of surfaces that propagate perpendicular to the [104] and [110] crystallographic directions. The two surfaces differ in coordination of the surface magnesium atoms; the (104) surface features five-coordinate magnesium atoms, whereas the (110) surface features four-coordinate magnesium atoms (Figure 1).1,2 The unsaturated magnesium atoms generate adsorption sites for titanium tetrachloride and electron donors.3 Adsorbed titanium tetrachloride transforms into polymerization active sites through activation by the cocatalyst.4−7 Electron donors, which are added during the catalyst preparation, have an influence on the morphology of the catalyst support8−15 as well as on the distribution and stereoselectivity of the active sites.16−23 Atomic-level understanding of the catalyst components and their interactions would provide valuable information regarding further development of the catalytic process. However, due to the complexity of the whole system, the picture remains incomplete. Along with state-of-the-art experimental methods, computational and modeling techniques have been exploited to address the binding of titanium tetrachloride and electron donors to the magnesium dichloride support.6,7,10,11,20−42 © 2014 American Chemical Society
Figure 1. Structure of a typical MgCl2 crystallite featuring the catalytically relevant surfaces (magnesium in yellow, chlorine in green).
Titanium tetrachloride forms mononuclear species on the (104) and (110) surfaces of the support, which are often referred to as Corradini, “slope”, and “edge” sites (Figure 2). The three sites differ in coordination of the titanium atom. There are contrary opinions regarding whether the fourcoordinate “edge” site may reconstruct to form a fivecoordinate titanium species.42 Nonetheless, the six-coordinate Corradini species have been consistently acknowledged as the most stable site on the (110) surface.27,29,30,35,38,39,41 In Received: August 28, 2014 Revised: October 22, 2014 Published: November 12, 2014 27878
dx.doi.org/10.1021/jp508693h | J. Phys. Chem. C 2014, 118, 27878−27883
The Journal of Physical Chemistry C
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Figure 2. Schematic representation of typical adsorption modes of titanium tetrachloride on the (104) and (110) surfaces of the MgCl2 support (magnesium in yellow, chlorine in green, titanium in gray). Figure 3. One-dimensional periodic models for the (104) and (110) surfaces of the MgCl2 support (magnesium in yellow, chlorine in green). The unit cells are highlighted in color.
addition, titanium tetrachloride molecules may form binuclear species on the magnesium dichloride surfaces.27,28,31,33,39,40 Computational studies have indicated stronger binding of the binuclear sites compared to the mononuclear analogs.27 However, there are opposite points of view, supporting favorable mononuclear adsorption of titanium tetrachloride36 and even decomposition of binuclear species upon complexation with the support surfaces.38 Coadsorption of titanium tetrachloride and electron donors has been shown to have several effects. Donors present in excessive amounts may lead to effective poisoning of the polymerization active sites via direct binding to the titanium atoms, therefore affecting molecular weights of the polymeric products.34,37 In addition, donors bound to the magnesium dichloride surfaces in the proximity of the titanium species may alter their steric environment and even improve their stereospecificity.25,32 Moreover, donors bound to the adjacent sites may provide the active sites with the electron density transmitted through the magnesium dichloride support.29,30 Herein, we report a computational study on coadsorption of titanium tetrachloride and electron donors to understand the effect of donors on the titanium sites. We systematically evaluate the possible coadsorption modes using the three mononuclear titanium sites in combination with dimethyl phthalate and 2,2-dimethyl 1,3-dimethoxy propane donors, which have been frequently used in industry.
plain (104) and (110) surfaces belong to the P211 and P112/m rod groups, respectively. The symmetry of the (104) surface remains unchanged upon saturation by adsorbates, whereas the symmetry of the (110) surface lowers to the P1̅ rod group. The choice of the models is based on the previous evaluation of the MgCl2/donor systems.48 The relative stability of a MgCl2 surface, ΔES, can be evaluated using the infinite α-MgCl2 sheet as the reference ΔES = 0.5 × (ES − mEsheet) × a−1
(1) −1
where ES and Esheet are total electronic energies (kJ mol ) of the surface and the two-dimensional MgCl2 sheet, m is an integer that equalizes the number of MgCl2 units in the unit cells, and a is the lattice parameter of the surface unit cell. The relative stability, being a one-dimensional analog to the surface energy, essentially indicates energetic expenses of formation of an MgCl2 surface of a unit length. The relative stability of the surface changes upon incorporation of adsorbates. One could operate with adsorption energy to address the degree of interaction; however, when multiple adsorbates are considered at once, the adsorption energy becomes an insufficient measure. The relative stability of the complex, ΔE∑, is thus a more descriptive quantity, which is calculated as
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COMPUTATIONAL DETAILS The PBE0 density functional theory method,43,44 as implemented in the CRYSTAL09 code, 45,46 was employed throughout the study in combination with the def-TZVP47 basis set (Supporting Information) optimized for solid-state calculations. The choice of the method and the basis set was directly based on the previously reported evaluation.48 Calculations were carried out in a one-dimensional periodic approach. The shrinking factor of 3 (SHRINK) was applied to generate a Monkhorst−Pack grid of k-points in the reciprocal space. Tightened tolerance factors of 8, 8, 8, 8, and 16 were used for the evaluation of the Coulomb and exchange integrals (TOLINTEG). Each structure was optimized within the respective symmetry group using default optimization convergence thresholds and extra-large integration grid for the density functional part (XLGRID). The lattice parameter was kept constant during optimization (FIXCELL). Basis set superposition error was accounted using the counterpoise technique.49
ΔE∑ = 0.5 × (E∑ − mEsheet −
∑ kjEj) × a−1
(2)
−1
where E∑ is total electronic energy (kJ mol ) of the whole complex and Ej is total electronic energy (kJ mol−1) of the jth adsorbate present in the amount of kj molecules in the unit cell. The relative stabilities of a complex, ΔE ∑ , and the corresponding plain surface, ΔES, may be compared to quantify the degree of stabilization of the surface, ΔΔE, upon saturation with selected adsorbates. ΔΔE = ΔE∑ − ΔES
(3)
Because the relative stabilities defined by eqs 1−3 are divided by the lattice parameter, they are directly comparable regardless of the size of the unit cell and the number and size of incorporated adsorbate molecules, making the relative stability a generic quantity to address effects of electron-donating compounds on the electron-deficient MgCl2 surfaces. Adsorption of Electron Donors. Bidentate electron donors, namely 2,2-dimethyl 1,3-dimethoxy propane (de) and dimethyl phthalate (dmp), were considered to bind the (104) surface in bridging mode and the (110) surface in chelating mode (Figure 4). The two donors were selected to study due to their distinctly different energetics on the magnesium dichloride surfaces. Figure 5 presents the relative stability of
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RESULTS AND DISCUSSION Surface Models. The catalytically relevant (104) and (110) surfaces were represented as one-dimensional periodic ribbons of five atomic layers thick, which were constructed from the two-dimensional α-MgCl2 sheet (Figure 3). The models for the 27879
dx.doi.org/10.1021/jp508693h | J. Phys. Chem. C 2014, 118, 27878−27883
The Journal of Physical Chemistry C
Article
Figure 4. Packing of the studied donors in the bridging binding mode on the (104) surface (top) and in the chelating binding mode on the (110) surface (bottom) of the MgCl2 support (magnesium in yellow, chlorine in green, donors in blue). Part of the surface models is hidden for clarity.
Figure 6. Packing of titanium tetrachloride on the MgCl2 surfaces in different sites (magnesium in yellow, chlorine in green, titanium in gray). Part of the surface layers is hidden for clarity.
Figure 5. Stability of the plain MgCl2 surfaces (eq 1) and surfaces saturated by the studied electron donors (eq 2) relative to the infinite MgCl2 sheet.
Figure 7. Stability (eq 2) of the MgCl2 surfaces saturated by titanium tetrachloride adsorbed in the mononuclear binding modes. The stabilities are given relative to the infinite sheet. The solid lines represent the relative stabilities of the plain MgCl2 surfaces (eq 1).
the surface/donor complexes in comparison to the plain surfaces, wherefrom it is seen that both donors greatly stabilize both MgCl2 surfaces. Nonetheless, the donors show different behavior toward binding to the MgCl2 surfaces. In particular, spatially demanding and rigid diether preferably stabilizes the four-coordinate surface, whereas more compact and flexible diester binds nearly equally to both MgCl2 surfaces.22,29−32 Adsorption of Titanium Tetrachloride. Figure 6 presents the MgCl2 surfaces saturated by titanium tetrachloride in three mononuclear forms, namely the Corradini (C), “slope” (S), and “edge” (E) sites. The relative stability (eq 2) of the titanium sites (Figure 7) appears to be connected to the coordination of the titanium atom in each adsorbed species; the six-coordinate C site is the most stable, followed by the five-coordinate S site. The four-coordinate E site, with physisorbed TiCl4, is the least stable. Overall, titanium tetrachloride alone cannot provide sufficient stabilization for the MgCl2 surfaces, as shown in Figure 7 by the highly positive values of the relative stability that are given relative to the completely saturated MgCl2 sheet. The stabilization provided by TiCl4 with respect to the plain (104) and (110) surfaces (eq 3) is 8.1 kJ mol−1 Å−1 for the C site, 1.4 kJ mol−1 Å−1 for the S site, and 1.3 kJ mol−1 Å−1 for the
E site. In comparison, the donors discussed above stabilize the surfaces (eq 3) in the range from 10.0 to 20.0 kJ mol−1 Å−1. Adsorbed titanium species could thus become desorbed in real conditions of catalyst preparation. Coadsorption of Titanium Tetrachloride and Electron Donors. Construction of the coadsorption modes was based on two principles: (1) binding of titanium tetrachloride and donors to the MgCl2 surfaces adjacent to each other and (2) additional binding of donors to the titanium atom whenever titanium site has free vacancies (Figure 8). Being six-coordinate, the C site prevents binding of the donors to the titanium atom. Therefore, we considered complexes where donors are bound to the MgCl2 surface in a proximity of the titanium species. Figure 9 shows optimized C1 complexes for both de and dmp. The C1/dmp complex was found to be more stable than the C1/de complex by 3.4 kJ mol−1 Å−1, which may be explained by better fitting of the more compact and flexible dmp molecules near the titanium species. The stabilization (eq 3) of the magnesium dichloride surface provided by the coadsorption complex C1 is 12.3 kJ mol−1 Å−1 in the case of dmp and 8.9 kJ mol−1 Å−1 in the case of de. 27880
dx.doi.org/10.1021/jp508693h | J. Phys. Chem. C 2014, 118, 27878−27883
The Journal of Physical Chemistry C
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Figure 10. Stability (eq 2) of the coadsorption complexes on the (104) and (110) surfaces of the MgCl2 support relative to the infinite MgCl2 sheet. The solid lines represent the relative stabilities of the corresponding MgCl2/TiCl4 complexes (eq 2).
de cannot make a proper fit around the titanium species, and the relative stability of E1−E3 complexes spans over the range of 5.8 kJ mol−1 Å−1 wide. In the case of the five-coordinate S site, the greatest stabilization is achieved when titanium tetrachloride and the donors form complex S1, where the donors are bound to the adjacent surface sites (Figure 11). Binding of the donors to the titanium atom (complex S2) leads to a significant stabilization as well. The difference in the relative stability of the complexes S1 and S2 is
