Modeling of the active site in TiCl 4 /MgCl 2 based Ziegler-Natta heterogeneous catalysts

1. Modeling of the active site in TiCl4/MgCl2 based Ziegler-Natta heterogeneous catalysts L. Petitjean and T. Ziegler University of Calgary 2500 University Drive, NW Calgary, AlbertaT2N 1N4 CANADA

2. 1. Introduction • The catalytic polymerization of -olefin, known as Ziegler-Natta catalysis, is of high interest for industry.[1] This reaction is indeed able to produce polymer from -olefin under mild conditions with high activity and is the only way to get regular polymers, such as isotactic polypropylene (iso-PP), which have a wide range of applications. Since the structure of the active species involved in this catalysis has still not been yet clearly identified experimentally, molecular modeling seems to be a good tool to understand how this catalytic system works. Some theoretical works [2-13] have already attempted to give some insight on this topic, but the complexity of the active site has been for a long time a significant problem for the calculations. • Fortunately, recent advances in theoretical methods (i.e. QM/MM methods) give us now access to larger molecular models, by treating only a small part of the system (reaction site) by quantum mechanical method and the other part by molecular mechanics potential (environment). The objective of this work is to apply such hybrid QM/MM method to investigate the structure of the active site in order to bring a new insight to this problem.

3. Diiso-butyl phtalate Ethyl benzoate Alkoxy-Silanes Alkoxy-Propane • The catalytic systems used in industrial process are basically composed by molecular TiCl4 supported on MgCl2 crystal to which is added an AlEt3 as co-catalysts. • In order to improve the activity and stereo-selectivity of the catalyst two Lewis bases have been added to initial catalytic system. (called ILB and ELB). These two compounds have a dramatic impact on the catalyst performance. Their mode of action is not clearly determined at this time but they are supposed to be a part of, or at least to interfere with, the actual active center. • It is consequently very interesting for both theoretical and experimental point of view to imagine and calculate a model, which includes the presence of a Lewis Base. Typical ILB Typical ELB

4. 2. Choice of the model • An important part of our work was to define a potential model of the active site. We have chosen to describe it as the product of the adsorption of a single TiCl4 molecule on the [100] surface of the MgCl2 support. In this structure the octahedral sphere of Ti is not filled (5-fold species). The main reasons that brought us to this choice are developed hereafter. • We have considered the formal oxidation state of the catalysts to be Ti (IV). Even if other oxydation states of Ti may be present on the active catalysts, we believe, like other authors on the basis of experimental evidences, [16] that the dominant catalytic species adopts this oxydation state. TiCl4 on [100] MgCl2 surface

5. The MgCl2 crystal gives rise principally to hexagonal shaped crystallites. This means that there is mainly two host surfaces for the adsorption of TiCl4 molecules : [100] and [110]. • On the [110] surface, it has been shown that the adsorption of TiCl4is more likely to form 6-fold species (octahedral sphere of Ti filled).[4] The reason being that a 6-fold structure is able to form 4 bonds with the surface while the 5-fold only forms 3. This stabilizes the 6-fold structure by8,1 kcal/mole with respect to the other, even if the deformation necessary to form this 6-fold species is higher than for the 5-fold one. 6-fold 5-fold

6. On the [100] surface, there can be also formation either of 6-fold or 5-fold structures. The 6-fold structure is formed via adsorption of Ti2Cl8 dimeric molecules on the surface.[11] However, the number of shared bonds with the surface per titanium atom is only 5/2 in the case of dimeric Ti2Cl8 while it is 3 for the mononeric TiCl4. Since, the deformation necessary to adsorb the original TiCl4 molecule is also larger in the case of the dimeric species, we have concluded that a 5-fold structure would be favored on the [100] surface. Since, as explained in the following a 5-fold species is more likely to form a polymerization center, we have chosen this surface as a model for MgCl2 support. 6-fold 5-fold

7. + AlEt3 • After the adsorption on the surface and prior to the olefin insertion, the active center must get in some way its first Ti-C bond. Experimentally, in most of the case this activation step is done by reaction with AlEt3. • In the case of 6-fold models,[11-12] in which the octahedral sphere of the Ti is filled, this step involves the abstraction of a Cl atom before the chlorine/alkyl exchange with AlEt3 can take place. This reaction is quite energetically expensive. • With a 5-fold structure, the activation involves only a chlorine/alkyl exchange, which is quite cheap. • A 5-fold species is consequently more likely to form an actual polymerization center. DE = -9,3 kcal/mole DE = -11,3 kcal/mole + AlEt2Cl Alkyl exchange with AlEt3

8. It is known experimentally that by adding an ELB before the AlEt3 the molecule instead of activating the catalyst kills its activity.This effect is very difficult to understand with a 6-fold site model in which the octahedral sphere of the Ti is filled. In that case, no easy reaction is possible before the activation. • By contrast, with a 5-fold model we can imagine that a ELB will be able to complex easily the TiCl4 center. This will fill the octahedral sphere of the Ti and hinder further activation by AlEt3. + DE = -16,8 kcal/mole

9.  ES site Isotactic Polymer  CE site Syndiotactic Polymer  C1 site Isotactoïd (Low Regular) Polymer 3. Building the QM/MM model • High Resolution 13C NMR spectra can be best fitted using a 3-sites type statistics.[15] This statistical model corresponds to the mixing of an enantiomorphic model site (ES), a chain-end model site (CE), and a more general model site for polymerization called C1. It corresponds to the fact that the polymer is composed by 3 different stereo-blocks : highly-isotactic, syndiotactic and isotactoïd (i.e. isotactic with low regularity) corresponding respectively to ES, CE and C1 models.

10. We have tried to interpret the NMR features in terms of molecular structure and imagined a molecular model, which is able to have 3 possible reaction centers. The idea is that the presence of ELB or ILB molecules around the Ti active site would create an asymmetry, which would be at the origin of 3 reactive centers (A, B, C). The purpose of the QM/MM calculations has been to check if those 3 sites could be related to the stereo-blocks described by NMR. Bulk Bulk Bulk Bulk Reaction Center A Reaction Center B Reaction Center C

11. Our hypothesis can be associated to many molecular structures. Indeed, it is generally accepted that the active center lies in many different environment leading to a distribution of properties of the product. Unfortunately, for the calculations we had to choose one. We decided to pick up the one in which reaction centers B and C would be most differentiating. QM system QM system

12. 4. Computational details • We have used a QM/MM code implemented at the University of Calgary[18] using the ADF density functional package developed by Baerends et al.[19] The molecular mechanics part has been treated using the Sybyl force field [20] for C, N, O, H and Cl atoms. Van der Waals parameters, which are not present in Sybyl, have been added from Dreiding [21] for Ti and Mg atom type. • For the quantum part, the electronic configuration of the molecular systems were described by a triple- basis set on Ti atom for 3s, 3p, 3d and 4s, while double- STO basis set with polarization functions were applied for Mg, Cl, C and H atoms. [22]The 1s electron of C atoms and the 1s-2p electrons for Mg, Cl and Ti atoms were treated as frozen core. The auxiliary s, p, d, f and g STO functions, [23] centered on all nuclei, were used to fit the electron density and the Coulomb and exchange potentials in each SCF cycle. The B-LYP exchange-correlation functional [24]was used in all the calculations. • In all the calculations, the atoms of the surface model have been fixed using the X-ray parameters published in the literature [17] for the crystal bulk. For the QM part, the surface model is Mg2Cl4, which may be seen as modest. However, the use of a larger and much more realistic cluster model (Mg9Cl4H6) for the MgCl2 layer didn’t affect dramatically the results.

13. 5. Results • Here is a summary of the QM/MM calculations carried out on the active site model with 2,2,6,6 tetra-methyl piperidine (TMP) as ELB (p.11). We want to be able to compare qualitatively the results of the calculations with the NMR data. This means that we have to determine the type polymer produced by each reaction center A,B,C. • A first step to evaluate the type polymer produced by each center is to determine the selectivity of the insertion at each of the potential position of coordination of the olefin (A1,A2,B1,B2,C1,C2). For each position, this involves the calculations of two paths corresponding to the two face of the olefin (re or si). By comparing the energy of the transition states we will know which path is favorable. • In a second step, we have to imagine a succession of insertions. In the case of center B and C, we have postulated that the polymerization occurs via a chain migratory insertion, which means that the insertion takes place alternatively in position 1 or position 2. Indeed, by looking at the -H agostic resting states it can be seen that the Ti stays in a pseudo octahedral conformation between two insertions.

14. However, this is not true in the case of center A, in which the Ti adopts a tetrahedral conformation. This involves a movement of the chain in an intermediate position between the 1 and 2 (which correspond to octahedral environment). Moreover the two possible conformations found for the resting-state have the same energy and can be considered to interconvert easily. Two conformations of canter A resting-state

15. Selectivity of Center A : • The presence of TMP molecule induces an asymmetry between position 1 and position 2. It turns out that the transitions states corresponding to insertion with olefin in position 2 are lower in energy. It seems then reasonable to think that a majority of insertion will occur through a path which involves insertion with olefin in position 2. • Additionally, due to the presence of surface atoms the chain cannot rotate (pointing down in the figure). When the olefin is in position 2, the re insertion will occur by trans approach of propylene while si one will happen by cis approach. The insertion will be favored when occurring through a trans approach (here re). • Consequently, we believe that Center A will produce isotactic polymer. re (E=+3.4) si (E=+5.6 ) re (E= +3.0) si (E= 0) Position A1 Position A2

16. Selectivity of Center B : • When the olefin is in position 1, due to the presence of surface atoms nearby, the chain cannot rotate to avoid the pressure of the incoming olefin. The re insertion will occur by trans approach of propylene while si one will happen by cis approach. The more favorable corresponds to the trans approach insertion (here re). • By contrast, when the olefin is in position 2, the chain can easily rotate to avoid the pressure of the olefin. Both re and si insertions will occurs favorably by the less sterically demanding trans approach. The insertion is almost not selective. • Consequently, one can think that B would produce a hemi-isotactic type of polymer. re (E=0) si (E=+2.6) re (E=0) si (E=+0.5) Position B1 Position B2

17. Selectivity of Center C : • As in the case of B and due to the presence of atoms from both surface and TMP molecule nearby, the chain cannot rotate to avoid the pressure of the incoming olefin when the olefin is in position 1. As already explained, the more favorable insertion corresponds to the trans approach insertion, in that case for the si face. • When the olefin is in position 2, the presence of TMP molecule hinders the rotation of the chain. Again it cannot avoid the pressure of the incoming olefin. The favorable trans approach insertion is here the re one. • Consequently, center C can be considered to produce syndiotactic polymer. re (E=+4.6) si (E=0) re (E=0) si (E=+2.9) Position C1 Position C2

18. 6. Conclusions • We have presented here a new approach for the modeling of the active site of heterogeneous Ziegler-Natta catalysts. This model consists in a TiCl4 molecule adsorbed on a [100] MgCl2 surface surrounded by two molecules of ELB. These molecules create an asymmetric environment that makes possible to imagine a structure with 3 possible reaction centers. • QM/MM calculations have been carried out on this type of model with 2,2,6,6 tetramethyl piperidine (TMP) as ELB. We have been able to identify the type of polymer produced by 3 reaction centers (A, B and C). A produces higlhy isotactic polymer, B seems to produce hemi-isotactic polymer, while C could produce syndiotactic polymer. • Our model seems in fairly good quliative agreement with NMR datas describing the formation of stereo-blocks in polypropylene. By this way, it gives some insight on the influence of the ELB on the catalysts selectivity. The presence of ELB molecules is at the origin of the formation isotactic polymer by reaction center A. Syndiotactic blocks formed by center C would not exist without the presence of the ELB. • In our future works, we will focus on describing the evolution of stereospecificity of the catalyst using a series of ELB molecules. With our model we will try to understand what are the mechanism that explain the variations of stereospecificy in the family of R,R'-dimethoxy propane molecules.

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