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Active Site Formation in MgCl2-Supported Ziegler-Natta Catalysts. A Density Functional Theory Study Denis V. Stukalov* and Vladimir A. Zakharov BoreskoV Institute of Catalysis, Russian Academy of Sciences, NoVosibirsk 630090, Russia ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: October 18, 2009
All types of chemical reactions (reduction, alkylation, and complexation of Ti species by organoaluminum compounds) leading to formation of active sites in heterogeneous Ziegler-Natta catalysts have been studied within density functional theory. A generally accepted mechanism of active site formation was found to be less preferable as compared to the alternative ways considered. Based on the calculation implemented, a whole active site formation scenario was postulated for the different active center precursors on the MgCl2 surface. The mechanism proposed allows us to rationalize the domination of Ti3+ over Ti2+ under reduction of Ti4+ surface species by AlR3, the absence of an electron spin resonance signal for the TiCl4/MgCl2 + AlR3 system with low Ti content, the stronger reduction ability of Al(i-Bu)3 than AlEt3, the deactivation effect of AlR2Cl, and the reactivation ability of AlR3. 1. Introduction Active centers of MgCl2-supported Ziegler-Natta catalysts are generally suggested to be alkylated Ti3+ and to some degree Ti2+ species located on the (104) or/and (110) MgCl2 surface which are produced upon interaction of TiCl4 adsorbed on activated MgCl2 with trialkylaluminum, for example, AlEt3 or Al(i-Bu)3.1,2 The structures of active site precursors (surface TiCl4 species) and resulting active centers have been studied in many contributions,3-19 while the transformation mechanism of the former into the latter has attracted far less attention, and so it is mainly based on a priori deductions rather than experimental evidence. The structures of active center precursors in the catalytic system are more unambiguous than the structures of active centers in that, and therefore some insight into the process of active site formation certainly will promote the drawing of a real active site nature. Moreover, the presence of AlR3 cocatalyst during polymerization influences catalyst functioning obviously due to a great number of chemical reactions between AlR3 and precatalyst.1,2,20-22 It should be noted that despite the many theoretical contributions that have been performed to study catalyst functioning at a molecular level,23-47 the role of AlR3 in that is nebulous so far. Since both experimental and theoretical studies show that the Ti atoms in an octahedral Cl coordination are the most preferable in the TiCl4/MgCl2 system,8,11,16 mononuclear TiCl4 complex on the (110) MgCl2 surface and dinuclear Ti2Cl8 complex on the (104) MgCl2 surface can be considered as the most plausible precursors of active centers, and so reaction of such Ti species with trialkylaluminum must generate the basic variety of active centers in the catalyst. Interaction of the surface Ti(IV) species with trialkylaluminum includes the following processes: reduction of Ti4+ species to Ti3+ and Ti2+ species, alkylation of Ti species with formation of Ti-C bonds, and complexing between the reduced Ti species and AlClxR3-x compounds. To determine the fundamental correlations between catalyst structure and polymer properties, many questions about the state of resulting catalytic system require clarification, such as the peculiarities * To whom correspondence should be addressed. E-mail: stukalov@ catalysis.ru. Fax: +7 383 330 96 87.
of Ti oxidation state distribution (particularly, between the (104) and (110) MgCl2 surfaces), degree of Ti species alkylation, nuclearity of the Ti species, and degree of Ti species complexing with AlClxR3-x. First of all, the mechanism of interaction of Ti surface species and AlClxR3-x compounds should be expected to determine the features listed above. Analysis of literature data on this question exhibits some contradictions and incomprehensible facts. The main problem arises from the application of some results obtained for the reaction between TiCl4 and organoaluminum compounds in solution toward the heterogeneous reaction in the TiCl4/MgCl2 + AlR3 system. Interaction between TiCl4 and different organoaluminum compounds such as Al(C2H5)2Cl, Al(CH3)3, Al(C2H5)3, and Al(i-C4H9)3 was abundantly studied in solutions of n-alkanes.48-52 For example, in case of Al(C2H5)2Cl formation of solid TiCl3, Al(C2H5)Cl2, ethane, and ethylene were found.51 Since all attempts made to isolate any titanium alkyls fail, it has been suggested that reduction of TiCl4 by organoaluminum compounds proceeds via alkyltitanium halides, e.g., TiCl3R and TiCl2R2, which decompose to give lower titanium halides.49 As a result, the interaction involved was described as a sequence of two reactions: alkylation (eq 1) and disproportion (eq 2).48,51
TiCl4+Al(C2H5)2Cl f Cl3TiC2H5+Al(C2H5)Cl2
(1)
2Cl3TiC2H5 f 2TiCl3+C2H6+C2H4
(2)
In turn, interaction of TiCl4 chemisorbed on the MgCl2 surface with AlR3 has been investigated largely with respect to transformation of Ti surface complexes. The electron spin resonance (ESR) and chemical analysis data indicate that under polymerization conditions the main part of Ti4+ is reduced to polynuclear species of Ti3+ and far less to mononuclear Ti3+ complexes and Ti2+ species.3-7 However, the other products and mechanism of Ti reduction were taken by analogy with reaction in solution: the alkylation step was postulated to precede the reduction stage. Alkylation of Ti(IV) surface species was considered within density functional theory (DFT) by Puhakka et al. and later by
10.1021/jp907812k CCC: $40.75 2009 American Chemical Society Published on Web 11/04/2009
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Figure 1. Capability of Ti(IV) species to be alkylated by triethylaluminum: (a) five-coordinated Ti(IV) complex on the (104) MgCl2 surface can be alkylated through Cl vacancy at Ti center; (b) it is unclear whether six-coordinated Ti(IV) complex on the (110) MgCl2 surface can be alkylated or not because Cl vacancy at Ti atom is absent.
Champagne et al.53,54 However, Puhakka and Champagne calculated only exothermicity of alkylation reactions, the relative alkylating strength of different Et2Al-R cocatalysts being postulated starting from the differences in enthalpies,54 while kinetic aspects were out of consideration. In turn, we believe that speculation about the alkylating strength of different AlR3 compounds on the basis of thermodynamics is not correct, and so in the present work we aim to consider potential barriers for alkylation of TiCl4 species. As for the mechanism of Ti species alkylation, it was suggested that Ti(IV) surface species had a vacancy by which alkyl radical of trialkylaluminum exchanges with Cl atom of TiCl4 species (Figure 1a).53 However, it is not clear whether the Ti(IV) species with Ti atoms in octahedral Cl surroundings without any vacancies (Figure 1b), which can be supposed to be the most preferable Ti state in the TiCl4/MgCl2 system, can be alkylated. In addition, it is not clear how alkylated Ti(IV) surface species can be reduced by the disproportion reaction (eq 2) since the distance between Ti species is at least 3.6 Å on the (104) MgCl2 surface and 6.3 Å on the (110) MgCl2 surface. To clarify the process of active site formation, in the present work reduction, alkylation, and complexation reactions between organoaluminum compounds and Ti species were studied within DFT. The data obtained allow us to rationalize the domination of Ti3+ over Ti2+ under reduction of Ti4+ surface species by AlR3,1-7 the absence of an ESR signal for the TiCl4/MgCl2 + AlR3 system with low Ti content,55 and stronger reduction ability of Al(i-Bu)3 than AlEt3.4 The whole active site formation scenario was postulated. 2. Computational Details All calculations were carried out within DFT with hybrid three-parameter Becke’s exchange-correlation functional B3LYP.56 The all-electronic basis 6-31G* was employed for atoms H, C, Al, Ti, and Cl. Due to the open-shell character of reduced Ti species, a spin-unrestricted formalism was used. The calculations were performed using the Gaussian 98 package.57 The following reduction paths for TiCl4 species by triethylaluminum were considered: reduction of TiCl4 molecule by one AlEt3 molecule with formation of ethyl chloride EtCl (Table 1, path I), reduction of TiCl4 molecule by two AlEt3 molecules with formation of butane C4H10 (Table 1, path II), reduction of TiCl4 molecule by two AlEt3 molecules with formation of ethane C2H6 and ethylene C2H4 (Table 1, path III), and reduction of two TiCl4 molecules located at the distance 6.3 Å (which was chosen as the distance between the neighboring adsorption sites on the (110) MgCl2 surface) by AlEt3 with formation of Ti3+ species (Table 1, path IV). In paths I-III the resulting Ti state was Ti2+(triplet) as more stable than Ti2+(singlet). The transition states for these mechanisms were approximated through a linear
J. Phys. Chem. C, Vol. 113, No. 51, 2009 21377 scan of potential energy surface along reaction coordinate with a step of 0.1 Å. For paths I-III at each step of the scan the spin state of the system was triplet. The reaction coordinate was defined as a distance between the CR atom of the AlEt3 molecule and the Cl atom of the TiCl4 molecule for path I, the CR atom of the first AlEt3 molecule and the CR atom of the second AlEt3 molecule for path II, and the Hβ atom of the first AlEt3 molecule and the CR atom of the second AlEt3 molecule for paths III and IV (see Figure 2). To consider the possibility of alkylated Ti(IV) species to be reduced, disproportion of two TiCl3Et molecules (Table 1, path V) and reduction of TiCl3Et by AlEt3 (Table 1, path VI) were studied analogously to path III (see Figure 2). To compare the efficiency of organoaluminum compounds to reduce Ti(IV) species, reduction of TiCl4 molecule by AlEt2(i-Bu) (VII) and AlEt2Cl (VIII) with formation of ethane and ethylene was examined similarly to reaction III (see Table 1). The transition state for alkylation of the TiCl4 species by AlEt3 was approximated through a linear scan of potential energy surface along the distance between the CR atom of the AlEt3 molecule and the Ti atom of the TiCl4 molecule with a step of 0.1 Å. The frequency analysis was carried out for each transition state approximation obtained. 3. Results and Discussion The active centers are generally supposed to form as a result of the alkylation-reduction reaction sequence between surface Ti(IV) species and trialkylaluminum, for example, AlEt3.1,2 To study these reactions by means of quantum-chemical calculations, it is necessary to chose a reasonable model for surface Ti(IV) species. This model cannot be big enough since a number of different chemical reactions should be considered in order to build a whole scenario of active site formation. Since a number of Ti(IV) complexes on the MgCl2 surface are potential and there are no reliable experimental data on what type of them is real and dominant, we decided to consider generalized abstract Ti(IV) speciessa TiCl4 molecule. Incidentally, four-coordinated Ti(IV) species (see Figure 3) that resemble the TiCl4 molecule were previously proposed as a potential active site precursor on the MgCl2 surface.16 It is reasonable to think that regularities observed on one type of Ti(IV) complex, for example, on the four-coordinated TiCl4 species, would be qualitatively the same for the other TiCl4 species (five- and six-coordinated ones) since support effect is most likely to influence the different reaction paths to a close extent within one type of Ti species. Moreover, we observed that geometry of Ti species (Ti-Cl distances, Cl-Ti-Cl angles) was invariable in the course of the increased potential energy along the reaction coordinate in contrast to AlR3 geometry. It implies that breaking bonds in AlR3 species determine largely the nature of activation energy for the reduction reactions. Because of these reasons, a TiCl4 molecule is an appropriate object to compare different paths of active center formation. In addition, such an object has an advantage: one of the most reliable theory level B3LYP/6-31G* can be provided in that case. Thus, in the present work the reduction reactions will be considered within a TiCl4 molecule since such approach suffices for qualitative deductions on an active site formation scenario. First of all, we envisage the reduction stage because for octahedral Ti(IV) species, the most stable Ti(IV) complexes on the MgCl2 surface,16 it is unclear whether they can be alkylated or not in view of the absence of Cl vacancy through which Et and Cl groups have been suggested to be exchanged (see Figure 1).53
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TABLE 1: Energetics for Reduction of Ti(IV) Species by Organoaluminum Compoundsa label
a
transition state
reaction
products
TiCl4+Al(C2H5)3 f TiCl2 · Al(C2H5)2Cl + C2H5Cl
42.0
25.0
II
TiCl4+2Al(C2H5)3 f TiCl2 · 2Al(C2H5)2Cl + C4H10
39.2
-57.2
III
TiCl4+2Al(C2H5)3 f TiCl2 · 2Al(C2H5)2Cl + C2H4+C2H6
36.6
-31.5
IV
2TiCl4+2Al(C2H5)3 f 2[TiCl3 · Al(C2H5)2Cl] + C2H4+C2H6
34.6
-42.0
V
2TiCl3(C2H5) f Ti2Cl6+C2H4+C2H6
67.3
2.8
VI
TiCl3(C2H5) + Al(C2H5)3 f TiCl2 · Al(C2H5)2Cl + C2H4+C2H6
51.0
8.3
VII
TiCl4+2Al(C2H5)2(i-Bu) f TiCl2 · 2Al(C2H5)2Cl + i-C4H8+i-C4H10
32.9
-37.8
VIII
TiCl4+2Al(C2H5)2Cl f TiCl2 · 2Al(C2H5)Cl2+C2H4+C2H6
44.7
-19.5
I
Energy in kcal/mol.
Figure 3. Four-coordinated TiCl4 species on the (110) MgCl2 surface.
Figure 2. Different paths of Ti(IV) species reduction. The double arrows represent reaction coordinates.
Reduction of Ti(IV) Species. There can be three types of organic products upon TiCl4 reduction by AlEt3: C2H5Cl (path
I), C4H10 (path II), and C2H4 + C2H6 (path III). For the sake of simplicity, first reduction reactions of single TiCl4 species with formation of Ti2+ species were examined (see Table 1, reactions I-III). It should be noted that the resulting TiCl2 species formed stable complexes with one or two AlR2Cl molecules, whereas the initial Ti(IV) species formed very weak complexes with AlR3 molecules (energy gain was about 1 kcal/ mol). By that reason the activation energies for these reactions summarized in Table 1 were related to the separated initial components. The corresponding transition states of the reduction reactions will not be presented at all since they were tangled owing to complicated steric orientation of alkyl radicals. Potential barriers of reactions I-III were 42.0, 39.2, and 36.6 kcal/mol, respectively. However, to compare the preference of paths I, II, and III it is necessary to take into account not only the potential barriers but also the different molecularity of these reactions with regard to trialkylaluminum: reaction I has the first order to trialkylaluminum, while reactions II and III have the second order to trialkylaluminum. The estimation of reaction rates under typical polymerization conditions (concentration of AlEt3 ) 10-2 mol/L, 70 °C) indicates that all these paths are possible since the rates of reactions I-III were comparable. Actually, formation of ethane, butane, ethyl chloride, and ethylene was observed upon reduction of TiCl4 by AlEt2Cl in
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Figure 5. Optimized structure of Et-bridge dimer Al2Et6.
Figure 4. Dinuclear Ti(IV) species on the (104) MgCl2 surface (a) and greatly separated mononuclear Ti(IV) species on the (110) MgCl2 surface (b).
refs 51 and 52, thus pointing to rationality of the calculations implemented. A significant difference is observed in thermodynamics of these processes: path I is endothermic, whereas paths II and III are highly exothermic (see Table 1). It seems that a small portion of ethyl chloride is found at the interaction of TiCl4 and AlEt2Cl.51,52 Since the dominant organic gaseous product is known to be ethane,48-52 further potential paths of Ti(IV) species reduction were considered necessarily with transfer of Hβ atom from the first organoaluminum molecule to the CR atom of the second one (see Figure 2). The next aim is to consider the generation of Ti3+ species being the dominant Ti oxidation state after reduction of the TiCl4/MgCl2 system by trialkylaluminum.1-7 To this end, reduction of two TiCl4 molecules by AlEt3 was examined (Table 1, path IV). The potential barrier for reaction IV is close to that for reaction III, and so the ratio between the reduction rates of Ti4+ to Ti3+ and Ti4+ to Ti2+ seems to be determined largely by a steric factor. We believe that location of two Al atoms within one TiCl4 surface species is less preferable than within two TiCl4 species due to less steric repulsion (see Figure 2, paths III and IV). If this is the case, reduction of Ti4+ to Ti3+ is more favorable than that of Ti4+ to Ti2+, thus explaining that Ti3+ is a main Ti oxidation state in the catalyst.1-7 However, the probability of Ti2+ formation can be increased if the distance between the neighboring Ti4+ species is large for two Et radicals from AlEt3 molecules at these Ti species to be able to collide. Such a situation is highly unlikely on the (104) MgCl2 surface where dinuclear Ti species (Figure 4a) are favorable, but it would take place on the (110) MgCl2 surface where mononuclear Ti species are generally supposed (Figure 4b).8,11,12,16,19 Indeed, path IV is impossible for greatly separated Ti4+ species on the (110) MgCl2 surface (Figure 4b), and then path III should be believed to be realized in that case. This fact is confirmed by the absence of an ESR signal for the TiCl4/ MgCl2 + AlEt3 system with the low Ti content.55 Obviously, in such a system Ti4+ species on the (110) MgCl2 surface can be reduced only to the ESR silent Ti2+ species, which can be catalytic centers too since atomic activity in ethylene polymerization even increases upon the decrease of Ti concentration on the MgCl2 surface.55,58-61 The reactions considered so far are applied to unalkylated Ti4+ complexes, but, for example, in the case of Ti4+ surface species with Cl vacancy the first stage may be alkylation and therefore reduction of alkylated Ti4+ species should be taken into account too. In literature it is suggested that TiCl3Et species interact with each other producing Ti3+, ethane, and ethylene.48-50 We modeled this reaction (Table 1, path V) and found that the
corresponding potential barrier is high enough (67.3 kcal/mol), thus doubting the realizability of such a reduction path. Consequently, an alternative path including interaction between TiCl3Et species and AlEt3 molecule (Table 1, path VI) was proposed. This way proved to be 16.3 kcal/mol more favorable than path V, but it has a disadvantage: such a scheme implies solely formation of Ti(II) species whose portion in the catalyst is far less than Ti(III) species.1-7 Because of that, and also taking into account that path IV is more preferable as compared to path VI, it should be concluded that reduction of TiCl3Et species is most likely to proceed similarly to that of TiCl4 species (through path IV): ethyl radical of TiCl3Et does not partake in this process. Thus, in contrast to the received opinion, for any Ti(IV) species on the MgCl2 surface reduction occurs with direct participation of organoaluminum compound. This unexpected result makes it possible to apply all statements obtained for unalkylated Ti(IV) species to alkylated ones. To elucidate the influence of alkyl radical structure in AlR3 cocatalyst on its ability to reduce Ti4+, interaction of TiCl4 and Al(C2H5)2(i-Bu) with formation of i-C4H8 and i-C4H10 was considered (Table 1, reaction VII). The potential barrier for this path was noticeably lower than the potential barrier for path III. This difference can be associated only with the structural features of hydrocarbon radicals in the trialkylaluminum molecules used. The essence of reduction reactions III and VII lays in the Hβ transfer from the Cβ atom of the first trialkylaluminum molecule to the CR atom of the second trialkylaluminum molecule (Figure 2, path III), and so attention should be given to the structure of CR and Cβ centers in the hydrocarbon radicals involved. The nearest surroundings of CR centers are identical for ethyl and isobutyl radical, whereas the environment of Cβ centers significantly differs: the Cβ atom of isobutyl radical has three adjacent C atoms, while the Cβ atom of ethyl radical has one adjacent C atom. Because of the latter reason, electron density delocalization can be expected to be far larger in the case of isobutyl radical (reaction VII) than in the case of ethyl radical (reaction III). This effect seems to facilitate dissociation of the Cβ-H bond, leading to the decreased potential barrier of reaction VII in comparison with reaction III. These results allow us to conclude that Al(i-Bu)3, which is also usually used in the polymerization process, is a more effective reducing agent than AlEt3 and finally rationalize the fact that maximum intensity in the ESR spectrum is observed at higher mole ratio Al/Ti in the case of AlEt3 than in the case of Al(i-Bu)3.4 Although, of course, not only the reactivity of Al(i-Bu)3 itself can determine its reduction ability. It is worth taking into account the fact that Al(i-Bu)3 is monomeric, whereas AlEt3 forms the Et-bridged dimer Al2Et6 (an optimized structure is presented in Figure 5). Due to kinetic difficulties (the increased screening of Al atoms by Et radicals), dimeric structure should be believed to be less reactive as a reductive agent as compared to monomeric one. Besides, formation of reactive hydride Al(i-
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Figure 6. Alkylation mechanism of Ti(IV) species by AlEt3.
Bu)2H in the case of Al(i-Bu)3 decomposition62 can also promote reduction of Ti(IV) species by Al(i-Bu)3. The next reduction agent AlEt2Cl was chosen to understand the influence of substituents to reduce Ti4+ species. The corresponding reaction VIII had significantly higher potential barrier than reactions III and VII (see Table 1), pointing out that AlEt2Cl is a soft reducing agent. It can be obviously associated with the positive mesomeric effect of Cl that leads to the Al-C bond becoming weaker (since two resonance structures are possible for AlEtCl+ cation). This agent is of great interest since it is known that interaction of TiCl4 with high excess of AlEt2Cl in solution results mainly in formation of Ti3+, whereas the other reduction agents (AlEt3, AlBu3) lead to formation of compounds with Ti in the lower oxidation levels.50 This fact apparently is not associated with higher potential barrier of Ti2+ formation in comparison with Ti3+ since it was demonstrated that both the paths have the close potential barriers (Table 1, paths III and IV). We therefore suppose that such a feature of AlEt2Cl is connected with the ability of AlEt2Cl to form strong complexes with Ti3+ species (in detail such processes will be considered below) which prevent subsequent reduction of Ti3+ species. The analogous effect is worth anticipating in the catalyst because a huge excess of trialkylaluminum is used under polymerization (typical Al/Ti molar ratio is 100), but the amount of Ti2+ is far less than that of Ti3+. Thus, interaction of Ti4+ species on the MgCl2 surface with AlR3 results mainly in formation of Ti3+ species rather than Ti2+ species since (i) the former path is more preferable due to steric reasons, as pointed above, and (ii) the Ti3+ surface species formed are coordinated with AlR2Cl molecules that prevent other transformations of these species. Further, we consider the possibilities of such screened Ti3+ species to be converted into the active sites for R-olefin polymerization. Alkylation of Ti Species. Another important function of trialkylaluminum leading to creation of the active centers is alkylation of Ti species with formation of a Ti-C bond. Due to a higher stability of the six-coordinated Ti(IV) species,16 it was more logical to deal with the reduction of TiCl4 species at the first place as the absence of a vacancy at six-coordinated Ti atoms casts a shadow on the possibility of such species to be alkylated. Nevertheless, the experimental data known do not allow us to exclude completely formation of unsaturated Ti(IV) species on the MgCl2 surface, and so the alkylation of TiCl4 molecule by AlEt3 was also taken into account. The energetic diagram for this reaction is presented in Figure 6. The potential barrier (9.7 kcal/mol) is significantly lower than that for the reduction reactions with the same components (see Table 1). It
means that for Ti4+ species having chlorine vacancies the alkylation step by far precedes the reduction step as it is generally supposed in literature. In turn, for surface Ti4+ species in the octahedral surroundings of Cl atoms, the reduction stage may take priority over the alkylation stage by the reason pointed above, and so in that case active sites should be generated by alkylation of Ti3+ species. To consider their alkylation by AlEt3, we chose the TiCl3 molecule as a model of Ti3+ surface species. This approach by far provides a certain overestimation in the binding strength of AlR3 to Ti3+ center since Lewis acidity of real Ti(III) species is less because of their strong coordination to the MgCl2 surface. However, the absolute value of binding energy is not critical in order to compare qualitatively different paths of TiCl3 · nAlEt3 complex dissociation (see Figure 7) that lead finally to alkylation of Ti(III) species. Alkylation of TiCl3 species by AlEt3 implies two steps: complex formation and its dissociation (Figure 7). It is interesting that several AlEt3 molecules can be bound to TiCl3. In the present work we considered complexes of TiCl3 with one and two AlEt3 molecules to find the most optimal way for Ti3+ species to be alkylated. Both complexes were stable enough (binding energy is 17.6 kcal/mol for TiCl3 · AlEt3 and 18.5 kcal/ mol for TiCl3 · 2AlEt3) evidently due to high reactivity of TiCl3 having an unpaired electron. The energy difference between these complexes (0.9 kcal/mol) being small enough points to an equilibrium occurring between them. The final goal was to obtain alkylated TiCl2Et species, and therefore decomposition of TiCl3 · AlEt3 and TiCl3 · 2AlEt3 complexes was taken into account. The former complex gives TiCl2Et and AlEt2Cl, whereas the latter produces TiCl2Et and a new complex AlEt2Cl · AlEt3. The second path is 11.4 kcal/mol more favorable than the first one since a new stable AlEt2Cl · AlEt3 complex is formed in that case. Besides, the first decomposition way is less preferable than the reverse dissociation of TiCl3 · AlEt3 with formation of TiCl3 and AlEt3. Obviously, an additional AlEt3 molecule promotes elimination of AlEt2Cl bound with Ti3+ species due to formation of AlEt2Cl · AlEt3 complex and so facilitates its alkylation. Because of the same reason AlEt3 is able to decrease catalyst deactivation related to coordination of AlEt2Cl and AlEtCl2 with active sites. Thus, these effects allow us to rationalize the fact that in polymerization a huge excess of trialkylaluminum is employed. In conclusion, based on the results obtained the most probable way of active center formation in MgCl2-supported Ziegler-Natta catalysts can be postulated (see Figure 8): at first Ti4+ centers in six-coordinated Cl surroundings are reduced by AlEt3 with
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Figure 7. Alkylation mechanism of Ti(III) species by AlEt3. Enthalpies ∆H are given in kcal/mol.
Figure 8. Probable mechanism for active site formation schematically illustrated by the example of mononuclear TiCl4 species on the (110) MgCl2 surface.
formation of complexes TiCl3 · AlEt2Cl; then AlEt3 eliminates AlEt2Cl due to formation of complexes AlEt2Cl · AlEt3 and at the same time complexes TiCl3 · AlEt3 and TiCl3 · 2AlEt3 are formed; finally TiCl3 · 2AlEt3 complexes are decomposed with formation of TiCl2Et speciessthe structures generally supposed as active sites for R-olefin polymerization. Of course, we realize that it is possible for the alkylation stage to take priority over the reduction stage even in the case of six-coordinated Ti(IV) species, but it cannot modify the main principles that govern the process of Ti reduction since Ti reduction should occur with direct participation of alkylaluminum. Thus, an alternative scheme provides for the following stage sequence: at first TiCl4 centers are alkylated with formation of TiCl3Et species; then AlEt3 reduces TiCl3Et species with formation of complexes TiCl2Et · AlEt2Cl; finally, AlEt3 eliminates AlEt2Cl due to formation of complexes AlEt2Cl · AlEt3, leading to creation of TiCl2Et species. Finally, we want to make some points concerning applicability of the computational results to a more complicated systemsMgCl2supported Ziegler-Natta catalysts for propylene polymerization. These catalysts comprise additionally Lewis bases (for example, diesters or diethers) adsorbed on the support surface which provide high catalyst stereoselectivity. The presence of relatively bulky diester or diether molecules on the MgCl2 surface results in the separation of Ti species on the support surface.15 In turn, it would promote formation of Ti(II) species because two AlEt3 molecules at adjacent Ti(IV) species cannot collide similarly
to the situation for the system with the low Ti content considered above (see Figure 4b). Of course, this effect is likely to be insignificant on the (104) MgCl2 plane where dimeric Ti species are favorable,16 but it could be crucial on the (110) MgCl2 plane where monomeric Ti species seem to dominate.16,19 Besides, it is worth expecting that the presence of bulky Lewis base molecules on the support surface creates steric difficulties for interaction of Ti(IV) species with AlR3, thus preventing Ti(IV) centers from reduction. However, another processsremoving Lewis base due to complexation with AlR3 molecule1,2,63scould assist in overcoming this obstacle. 4. Conclusions All types of chemical reactions (reduction, alkylation, and complexation of Ti species by organoaluminum compounds) leading to formation of active sites in heterogeneous Ziegler-Natta catalysts were studied within DFT. Analysis of Ti4+ species reduction by different organoaluminum compounds in a variety of ways shows the following: (i) All Ti4+ surface species with Ti atom in different coordination environments (having or not having Cl vacancies) are reduced with direct participation of alkylaluminum. (ii) Three types of organic gaseous product (ethane and ethylene, butane, ethyl chloride) under reduction are possible; taking into account different molecularity of these paths and calculated potential barriers, it was concluded that the rates of
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corresponding chemical reactions are comparable, but formation of ethyl chloride is unfavorable by thermodynamics. (iii) The reduction reactions resulting in formation of Ti3+ and Ti2+ had close potential barriers, but the former seems to be more preferable due to steric reasons, thus explaining domination of Ti3+ over Ti2+ (complexation of Ti3+ species by AlR2Cl would prevent subsequent reduction). (iv) Chemical structure of organoaluminum compounds can significantly influence its ability to reduce Ti4+ species; Al(iBu)3 is predicted to be the stronger reduction agent, whereas AlEt3Cl is the less effective reduction agent. Consideration of Ti species alkylation by triethylaluminum indicates the following: (i) Ti4+ species are alkylated by exchange of Cl and Et between TiCl4 species and AlEt3 with considerable potential barrier. (ii) Alkylation of TiCl3 species includes two steps: formation of complex TiCl3 · AlEt3 and its dissociation. The dissociation is promoted by additional AlEt3 molecule since it facilitates elimination of AlEt2Cl bound to Ti3+ species due to formation of complexes such as AlEt2Cl · AlEt3. Because of that reason AlEt3 is able to decrease the catalyst deactivation associated with adsorption of AlEt2Cl or AlEtCl2 on the active sites. Acknowledgment. The authors gratefully acknowledge support from the Siberian Supercomputer Center SB RAS through grant no. 26 (2008) “Mathematical models, numeral methods and parallel algorithms for solving large application problem of SB RAS and their realization on multiprocessor SuperComputers”. References and Notes (1) Barbe, P. C.; Ceccin, G.; Noristi, L. AdV. Polym. Sci. 1987, 81, 1. (2) Albizzati, E.; Giannini, U.; Collina, G.; Noristi, L.; Resconi, L. Catalysts and Polymerizations. In Polypropylene Handbook; Moore, E. P., Jr., Ed.; Hanser-Gardner Publications: Cincinnati, OH, 1996; Chapter 2. (3) Zakharov, V. A.; Makhtarulin, S. I.; Poluboyarov, V. A.; Anufrienko, V. F. Makromol. Chem. 1984, 185, 1781. (4) Sergeev, S. A.; Poluboyarov, V. A.; Zakharov, V. A.; Anufrienko, V. F.; Bukatov, G. D. Makromol. Chem. 1985, 186, 243. (5) Brant, P.; Speca, A. Macromolecules 1987, 20, 2740. (6) Brant, P.; Speca, A. N.; Johnston, D. C. J. Catal. 1988, 113, 250. (7) Chien, J. C. W.; Hu, Y. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 897. (8) Potapov, A. G.; Kriventsov, V. V.; Kochubey, D. I.; Bukatov, G. D.; Zakharov, V. A. Macromol. Chem. Phys. 1997, 198, 3477. (9) Potapov, A. G.; Zakharov, V. A.; Mikenas, T. B.; Sergeev, S. A.; Volodin, A. M. Macromol. Chem. Phys. 1997, 198, 2867. (10) Potapov, A. G.; Terskikh, V. V.; Zakharov, V. A.; Bukatov, G. D. J. Mol. Catal. A: Chem. 1997, 145, 147. (11) Brambilla, L.; Zerbi, G.; Piemontesi, F.; Nascetti, S.; Morini, G. J. Mol. Catal. A: Chem. 2007, 263, 103. (12) Busico, V.; Corradini, P.; Martino, L.; Proto, A.; Savino, V. Makromol. Chem. 1985, 186, 1279. (13) Busico, V.; Causa, M.; Cipullo, R.; Credendino, R.; Cutillo, F.; Friederichs, N.; Lamanna, R.; Segre, A.; Castelli, V. J. Phys. Chem. C 2008, 112, 1081. (14) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, J. W.; Thune, P. J. Catal. 2008, 257, 81. (15) Stukalov, D. V.; Zakharov, V. A.; Potapov, A. G.; Bukatov, G. D. J. Catal. 2009, 266, 39. (16) Monaco, G.; Toto, M.; Guerra, G.; Corradini, P.; Cavallo, L. Macromolecules 2000, 33, 8953. (17) Martinsky, C.; Minot, C.; Ricart, J. M. Surf. Sci. 2001, 490, 237. (18) Taniike, T.; Terano, M. Macromol. Rapid Commun. 2007, 28, 1918. (19) Taniike, T.; Terano, M. Macromol. Rapid Commun. 2008, 29, 1472. (20) Nitta, T.; Liu, B.; Nakatani, H.; Terano, M. J. Mol. Catal. A: Chem. 2002, 180, 25. (21) Liu, B.; Nitta, T.; Nakatani, H.; Terano, M. Macromol. Chem. Phys. 2002, 203, 2412. (22) Liu, B.; Nitta, T.; Nakatani, H.; Terano, M. Macromol. Chem. Phys. 2003, 204, 395.
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