MgCl2-Supported Ziegler-Natta Catalysts: A DFT-D

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

MgCl-Supported Ziegler-Natta Catalysts: A DFTD ‘Flexible-Cluster’ Approach. Internal Donor Adducts Emanuele Breuza, Giuseppe Antinucci, Peter H.M. Budzelaar, Vincenzo Busico, Andrea Correa, and Christian Ehm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01500 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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The Journal of Physical Chemistry

MgCl2-Supported Ziegler-Natta Catalysts: A DFT-D ‘Flexible-Cluster’ Approach. Internal Donor Adducts Emanuele Breuza,1,2,# Giuseppe Antinucci,1,2,# Peter H. M. Budzelaar,1,2 Vincenzo Busico,1,2 Andrea Correa1,2,* and Christian Ehm1,2 1

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy. 2

Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands.

Abstract: A ‘flexible cluster’ model approach to Ziegler-Natta Catalysts for the production of isotactic polypropylene, allowing use of realistically sized MgCl2 monolayer clusters (up to 38 MgCl2 units) without any constraints, was employed to investigate the formation of adducts between the MgCl2 support and three industrially relevant internal donor (ID) classes, namely phthalates, succinates and 1,3-dimethoxypropanes. The calculated adsorption modes and thermochemical data for adducts of single donor molecules confirmed earlier literature trends only in part. Results for adducts with multiple donor molecules, in turn, did not confirm the indications of periodic models about steric repulsion between neighboring adsorbates hampering high degrees of surface coverage; as a matter of fact, such repulsions seem to be largely traceable to unnecessary constraints inherent in periodic calculations. INTRODUCTION

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In a recent paper,1 we introduced an advanced ‘flexible cluster’ model of MgCl2-supported ZieglerNatta catalysts (ZNC) for the industrial production of isotactic polypropylene (iPP). The new model, which is based on Density Functional Theory with dispersion corrections (DFT-D), does not apply any constraints upon structure optimization. Compared with periodic protocols, it is far better suited to capture the inherent features of nano-sized and highly disordered ZNC primary particles, while providing a good compromise between speed and accuracy. We have already demonstrated the ability of the model to describe the structural and thermochemical aspects of the chemisorption of TiCl4 and several probe donor molecules on regular and defective lateral terminations of monolayer MgCl 2 clusters at different degrees of Mg unsaturation.1 The results for regular terminations turned out to be consistent with state-of-the-art periodic DFT-D calculations. Regarding MgCl2 interaction with Titanium species, recently Ladas and coworkers2 studied Cl-Ti interactions by means of both theoretical and experimental methods, adding new insight on the catalytic system properties. In this paper we move forward towards the description of full ZNC formulations, by investigating cluster adducts with industrially relevant Internal Donors (ID). 3-5 These molecules represent the second largest component by weight (typically, 10 to 20%) of the pre-catalytic solid. The last four decades witnessed the trial-and-error implementation of several ZNC generations differing mainly in the nature of the ID class (Table 1).3-5 In parallel, different ex-post hypotheses on the mechanism(s) of catalyst modification by the ID were proposed and, in part, abandoned. 3-5 Currently, there is ample consensus that ID chemisorption is important to stabilize the primary particles of MgCl 2 down to the size of a few nm, which is mandatory for a compact crystal lattice to expose a surface area compatible with the required high activity in catalysis. In this respect, it is recognized that TiCl4 adsorption is not strong enough for the purpose.6-8 Moreover, in most pre-catalyst preparation protocols the ID is present when the primary MgCl2/ID/TiCl4 particles form by reaction of TiCl4 with a MgCl2 precursor compound,3-4 and it has been suggested that using an ID with a strong preference for chemisorption on a certain MgCl2 crystal surface (e.g., MgCl2(110) when the ID is a 1,3-dimethoxypropane)9-12 may steer crystallite morphology in favor of that surface.3-4, 12,


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Table 1. Typical formulations and performance ranges of MgCl2-supported ZNC for i-PP production.3-5 Generation

Internal Donor (ID)

External Donor (ED)

Productivity(a)

XS(b)

Mw/Mn

Third

Ethylbenzoate

Aromatic monoester

0.5-0.8

3-5

6-9

Fourth

Dialkylphthalate

Alkoxysilane

1-2

1-5

6-8

Fifth

2,2’-dialkyl-1,3dimethoxypropane

None or Alkoxysilane

>2

2-5

4-6

Sixth

Dialkylsuccinate

Alkoxysilane

1-2

1-5

>8

(a)

103 kg(PP)/g(Ti). (b) Xylene-Soluble Fraction in wt.-%.

Any modeling study aimed at providing better catalyst understanding must consider the fate of the ID after pre-catalyst activation. Upon reaction with the AlR 3/ED co-catalyst (ED = External Donor; usually, an alkylalkoxysilane), two different scenarios are possible. 3-5 Some pre-catalysts release all or part of the ID, due to a reversible or irreversible interaction with the Al-alkyl (e.g., when ID = phthalate or succinate);3-5, 13, 14 this is associated with a vast ID/ED exchange on the catalyst surface. 3-5 Some others instead (e.g., when ID = 1,3-dimethoxypropane) retain the ID, which then contributes to define the active pocket of the catalytic species, and therefore catalyst stereoselectivity. 3-5, 9-11 Notably, in both scenarios a massive adsorption of Al-alkyl species on the catalytic surfaces takes place, with detectable consequences on the observed polymerization behavior;4 whether this entails a competition of such species with the ID and/or ED for access to the surface, or the formation of adducts with donor molecules while adsorbed, is not yet clear. In view of the above, useful computational models must be able to evaluate quantitatively the crossinteractions and competitive adsorption of all components of an active ZNC formulation. Therefore, all relevant processes must be considered using the same calculation protocols and level of theory, ending up with absolute (or at least strictly comparable) thermochemical data. Whereas we greatly value the conclusions of several seminal papers in the previous literature, in particular with respect to ID adsorption modes and possible preference for certain MgCl2 crystallite terminations,3-5, 8-22 here we aim to attach a quantitative meaning to such conclusions. Moreover, we note that real-world ZNC are characterized by a


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complex adsorbate pool, with several different species (i.e. Ti and Al alkyl chlorides, ID, ED) contributing to reach the necessary high degree of surface coverage. 3-5 Evaluating said pool with periodic calculations is virtually impossible, whereas our flexible cluster model is well-suited to the job. As a matter of fact, in the last part of this paper we will introduce a first example of MgCl 2/ID cluster adduct with a realistically high ID surface coverage, and comment the substantial differences with respect to previous attempts of periodic model descriptions. 23 For the purpose of the present study, dimethylphthalate (DMP), 2,3-dimethyl-dimethylsuccinate (DMS), and 2,2-dimethyl-1,3-dimethoxypropane (DMMP) were used as ‘minimum models’ of the three selected ID classes. THEORETICAL METHODS All geometries were fully optimized using the Turbomole 6.4 package,24 employing the TPSS functional (defined in Turbomole as a Slater-Dirac LDA functional + TPSS for the exchange part, and a Perdew-Wang 1992 LDA functional + TPSS for the correlation part),25-28 and grid size m4.29-30 The electronic configuration of all atoms was described by a split valence basis set plus polarization functions (def2-SVP).31-32 The RI approximation33-34 (universal auxiliary basis set)30, 35-36 combined with the MARIJ accelerator37 was used for this part of the work. All structures were characterized by a vibrational analysis (numerical frequencies) to confirm the absence of imaginary frequencies. Thermal contributions (enthalpy and entropy, at 25°C) were also calculated at this level. Following recently published protocols,38-40 the optimized structures were then used for the evaluation of binding energies, by means of basis set superposition error (BSSE)41 corrected single point energy calculations with the TPSS functional and the larger basis sets def2-TZVPP.31 Grimme’s D2 dispersion corrections were applied.42 If not indicated otherwise, binding energies (Eads) were calculated as follows: Eads = ECLU-ID – ECLU – EID + BSSE correction where ECLU-ID, ECLU, EID are the energy values of the adduct, of the naked MgCl2 cluster, and of the most stable conformer of the ID considered, respectively. Similar expressions hold for Hads and Gads.


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RESULTS AND DISCUSSION Chemisorption of single ID molecules. In our previous paper,1 we introduced two families of monolayer MgCl2 model clusters, denominated CLU-n-104 and CLU-n-110 (n = number of MgCl2 units). The former class features dominant edges with penta-coordinate Mg atoms, and therefore locally mimics regular -MgCl2(104) lateral crystal terminations. The latter class, in turn, features dominant edges with tetra-coordinate Mg atoms, and therefore locally mimics regular -MgCl2(110) lateral crystal terminations. In this part of the work, two such clusters, namely CLU-24-104 and CLU-27-110 (Figure 1), were employed to investigate the chemisorption of the three prototypical ID molecules mentioned in the Introduction (namely DMP, DMS, and DMMP).

Figure 1. Top view of the two cluster models used in this study: CLU-24-104 (left) and CLU-27-110 (right). Only surficial atoms are colored: Mg in violet, Cl in green. In CLU-24-104, penta-coordinate corner Mg atoms were stabilized by chemisorbed H2O molecules (see text and Ref 1).

Individual ID molecules in a suitable conformation (vide infra) were adsorbed on the dominant termination of each cluster far (> 6 Å) from the heavily distorted corners, and the structure of the adduct was fully optimized without any constraints. In the case of CLU-24-104, corner stabilization by means of H2O adsorption was necessary to overcome the multiple-minima problem, as previously described. 1 After applying thermal corrections, adsorption enthalpies (Hads) and Gibbs free energies (Gads) were calculated. It is important to note that enthalpy and Gibbs free energy changes (in kcal per mol of ID)


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refer to adsorption on a predefined edge (cluster and a single ID molecule originally at infinite distance are approached and ID adsorption takes place); therefore, calculated values for the adsorption of the same ID on different clusters (e.g. CLU-n-104 vs CLU-n-110) are not informative on the ability of said ID to favor the formation of one MgCl2 termination over another upon pre-catalyst preparation. Moreover, when comparing Hads and Gads values pertaining to different adsorption modes of a given ID on the same cluster termination, one should keep in mind that the chelating mode engages one single Mg atom per ID molecule, whereas the bridging mode engages two. We will return to the consequences of these simple concepts in the following discussion. All three considered ID can access multiple conformations, which represents a complication in view of the chemisorption study. In the case of DMP, out of several identified conformers (Figure 2), two (labeled as A and B) turned out to be much lower in energy. Preliminary lower-level calculations pointed out that this high negative Grel carries over to the corresponding adsorbate structures; therefore, only A and B conformers were retained for high-level calculations (Table 2, and Figures 3, 4). Out of these two, moreover, DMP-B is the only one with both carbonyl groups in the proper orientation to bind to Mg in chelating or bridging mode; the former is clearly favored on MgCl2(110)-like terminations and is similar to coordination modes observed for phthalate complexes of Ti, Mo, Ba. 43-48 DMP-A, on the other hand, needs to use one carbonyl and the ether O of the other ester group; this leads to a weaker chelating mode on MgCl2(110)-like terminations, and the downgrading of bridging into monodentate binding on MgCl2(104)-like terminations; not surprisingly, no examples of such a coordination mode have ever been reported.


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Table 2. Calculated values of adsorption enthalpy (Hads) and Gibbs free energies (Gads), in kcal/mol, for the adsorption of DMP on CLU-27-110 and CLU-24-104.

Adduct CLU-27-110 + DMP-1 CLU-27-110 + DMP-2 CLU-27-110 + DMP-3 CLU-27-110 + DMP-4 CLU-27-110 + DMP-5

Binding mode chelating chelating bridging bridging monodentate

Hads -41.5 -33.7 -37.8 -36.3 -27.2

Gads -27.8 -19.7 -23.8 -20.6 -16.0

CLU-24-104 + DMP-1 CLU-24-104 + DMP-2 CLU-24-104 + DMP-3

bridging bridging monodentate

-38.9 -37.8 -25.3

-24.2 -23.2 -13.6

Figure 2. Several DMP conformers with relative values of Gibbs free energy (in kcal/mol). C in grey, O in red, H in white.


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Figure 3. Cutouts of the fully optimized structures for the adducts of DMP with CLU-27-110 (see also Table 2). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.

Figure 4. Cutouts of the fully optimized structures for the adducts of DMP with CLU-24-104 (see also Table 2). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.


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Moving on to DMS, the conformational space of the molecule is wide to the point that a plethora of conformers of very similar Gibbs free energies were identified (for details, see SI). In the present study we considered only those structures that may adopt the chelating and/or the bridging adsorption mode (examples in Figure 5).

Figure 5. Several DMS conformers with relative values of Gibbs free energy (in kcal/mol). C in grey, O in red, H in white. See SI for more structures and details.

A large number of strongly bound bidentate adsorbate structures were identified for both CLU-27-110 (Table 3 and Figure 6) and CLU-24-104 (Table 4 and Figure 7). In the latter case, binding occurs exclusively via the bridging mode, whereas in the former case the chelating mode is also possible and usually favored. For some conformations, monodentate binding can be observed; not unexpectedly, this leads to weaker binding than for both bidentate modes. Nonetheless, the donor binds strongly to the


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surface even in monodentate fashion (Gads between -10 and -15 kcal/mol, rather than -20 to -30 kcal/mol). Table 3. Calculated values of adsorption enthalpy (Hads) and Gibbs free energies (Gads), in kcal/mol, for the adsorption of DMSP on CLU-27-110.

Adduct CLU-27-110 + DMSM-1 CLU-27-110 + DMSM-2 CLU-27-110 + DMSM-3 CLU-27-110 + DMSM-4 CLU-27-110 + DMSM-5 CLU-27-110 + DMSM-6

Binding mode chelating chelating bridging bridging bridging monodentate

Hads -43.5 -43.3 -41.2 -41.3 -41.6 -26.2

Gads -29.7 -28.7 -25.7 -25.6 -24.9 -14.7

CLU-27-110 + DMSR-7 CLU-27-110 + DMSR-8 CLU-27-110 + DMSR-9 CLU-27-110 + DMSR-10 CLU-27-110 + DMSR-11 CLU-27-110 + DMSR-12 CLU-27-110 + DMSR-13 CLU-27-110 + DMSR-14

chelating chelating chelating bridging bridging bridging bridging monodentate

-44.1 -42.4 -41.6 -43.9 -42.9 -43.4 -37.6 -27.3

-29.8 -28.3 -27.6 -28.0 -27.9 -27.7 -22.4 -10.4

CLU-27-110 + DMSS-15 CLU-27-110 + DMSS-16

chelating bridging

-41.1 -40.5

-27.8 -27.0

Table 4. Calculated values of adsorption enthalpy (Hads) and Gibbs free energies (Gads), in kcal/mol, for the adsorption of DMS on CLU-24-104.

Adduct CLU-24-104 + DMSM-1 CLU-24-104 + DMSM-2 CLU-24-104 + DMSM-3 CLU-24-104 + DMSM-4

Binding mode bridging bridging bridging bridging

Hads -37.0 -37.1 -35.2 -35.1

Gads -21.7 -21.6 -19.9 -19.7

CLU-24-104 + DMSrac-5 CLU-24-104 + DMSrac-6 CLU-24-104 + DMSrac-7 CLU-24-104 + DMSrac-8 CLU-24-104 + DMSrac-9 CLU-24-104 + DMSrac-10 CLU-24-104 + DMSrac-11

bridging bridging bridging bridging bridging monodentate monodentate

-39.6 -39.3 -38.9 -37.6 -36.6 -25.6 -24.1

-25.5 -24.8 -23.8 -23.1 -21.3 -13.4 -11.4


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Figure 6. Cutouts of the fully optimized structures for some adducts of DMS with CLU-27-110 (see also Table 3). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.

Figure 7. Cutouts of the fully optimized structures for some adducts of DMS with CLU-24-104 (see also Table 4). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.


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Last but not least, we come to the case of DMMP. A preliminary conformational analysis of the isolated (gas phase) molecule was carried out, aiming to identify conformers with an orientation of the two ether functions potentially suited to chemisorb in chelating or bridging mode; out of four lowest energy conformations (A-D in Figure 8), only A and D matched the aforementioned pre-requisite. The optimized structures of the most stable adducts on the two clusters are shown in Figure 9 and 10. The corresponding values of adsorption enthalpies and Gibbs free energies are reported in Table 5. In agreement with previous literature findings, 8-11 this donor clearly favors chelating over bridging chemisorption on CLU-27-110 (by more than 10 kcal/mol). Whereas the above is not enough to conclude tout-court that MgCl2(110)-like terminations are dominant in 1,3-dimethoxypropane-containing precatalysts, it does lend support to such a hypothesis.12

Figure 8. Several DMMP conformers with relative values of Gibbs free energy (in kcal/mol). C in grey, O in red, H in white.


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Table 5. Calculated values of adsorption enthalpy (Hads) and Gibbs free energies (Gads), in kcal/mol, for the adsorption of DMMP on CLU-27-110 and CLU-24-104.

Adduct CLU-27-110 + DMMP-1 CLU-27-110 + DMMP-2 CLU-27-110 + DMMP-3 CLU-27-110 + DMMP-4 CLU-27-110 + DMMP-5 CLU-27-110 + DMMP-6 CLU-27-110 + DMMP-7

Binding mode chelating chelating chelating bridging bridging monodentate monodentate

Hads -43.5 -42.1 -39.4 -29.1 -27.3 -26.6 -27.1

Gads -29.4 -28.6 -26.1 -14.6 -11.9 -14.9 -14.4

CLU-24-104 + DMMP-1 CLU-24-104 + DMMP-2 CLU-24-104 + DMMP-2 CLU-24-104 + DMMP-4

bridging bridging bridging monodentate

-34.9 -32.6 -32.2 -20.6

-19.2 -17.3 -16.4 -7.7

Figure 9. Cutouts of the fully optimized structures for the adducts of DMMP with CLU-27-110 (see also Table 5). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.


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Figure 10. Cutouts of optimized structures for the adducts of DMMP with CLU-24-104 (see also surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.

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Table 5

). Only

Chemisorption of multiple DMMP molecules. The modeling study of the previous section was aimed to identify the characteristic chemisorption modes of the investigated classes of ID, and quantify comparatively their adsorption strength. On the other hand, we have already noted in the Introduction that real-world pre-catalysts are characterized by high degrees of surface coverage, which calls for cluster models with multiple donor adsorption. In this section, we will report on adducts between a model CLU38-110 cluster and three DMMP molecules bound to adjacent tetra-coordinate Mg atoms of a MgCl2(110)-like termination. The cluster is large enough to feature the two terminal DMMP molecules in the string at a fairly large distance (> 6 Å) from the nearest corner. We note that adsorbing H2O molecules at the two corners of the opposite cluster termination was beneficial to eliminate numerical noise in lowfrequency vibration modes upon structure optimization. In view of the results for the adsorption of single DMMP molecules on CLU-27-110 (Table 5), only DMMP-A and DMMP-D conformers chemisorbed in chelating mode were considered. The fully optimized structures of the adducts are shown in Figure 11. Calculated values of adsorption enthalpies and Gibbs free energies are reported in Table 6; these include


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average values for all three molecules in a string, and individual values for the central molecule in each given string (marked with an asterisk).

Figure 11. Top and side views of cutouts of optimized structures of the adducts of CLU-38-110 with three DMMP molecules (see also Tables 6 and 7). Only surficial atoms are colored: Mg in violet, Cl in green, C in grey, O in red, H in white.


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Table 6. Calculated values of adsorption enthalpy (Hads) and Gibbs free energies (Gads), in kcal/mol, for the chelating adsorption of three molecules of DMMP in conformation A or D on CLU-38-110 (see text and Figure 11).

Adduct

*ads

G*ads

Hads>

Gads>

CLU-38-110 + 3DMMP-1 CLU-38-110 + 3DMMP-2 CLU-38-110 + 3DMMP-3 CLU-38-110 + 3DMMP-4 CLU-38-110 + 3DMMP-5 CLU-38-110 + 3DMMP-6

-43.9 -41.2 -41.6 -41.7 -40.0 -39.5

-29.2 -27.6 -27.6 -27.5 -26.4 -24.5

-43.8 -42.9 -43.1 -42.7 -42.1 -42.4

-29.3 -28.8 -28.8 -28.3 -28.0 -27.8

The above results clearly indicate that adjacent DMMP adsorption on a lateral termination of a monolayer MgCl2 model cluster can occur with hardly any difference with respect to the individual molecule at low surface coverage. As a matter of fact, the calculated adsorption enthalpies and Gibbs free energies, respectively, are virtually identical. This seems to disagree with the conclusions of periodic DFT(-D) calculations, according to which steric repulsion between neighboring adsorbate molecules would severely hamper the achievement of high degrees of surface coverage. 22 In our opinion, the disagreement is only apparent, and partly due to the limitations in the protocol of typical periodic calculations that we have already highlighted in a previous paper. 1 By definition, periodic codes assume strict long-range periodicity in two or three dimensions, which corresponds to crystalline order. In the specific case of adsorbate overlayers, adsorbed molecules in adjacent unit cells are copies, repeating identically along the translation axes. This inhibits all possibilities of individual positional or conformational rearrangements that may alleviate steric repulsion between neighboring adsorbates. In principle, the use of very large supercells could represent a solution to the problem, but – as we have already noted elsewhere1 – this would have unbearable computational costs. Moreover, modeling the very small and highly disordered primary ZNC particles as extended crystals is conceptually incorrect. In view of all this, we suggest that all indications of a low degree of surface coverage for ZNC models based on periodic calculations should be critically reconsidered.


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Looking at the structures in Figure 11, one may be surprised to see how similar to a crystalline packing the arrangement of adjacent DMMP molecules is (e.g. models 1 and 5). In reality, one should not be surprised: an ordered arrangement is essential to fill the available space effectively and maximize attractive intermolecular interactions. On the other hand, the ability of each molecule, independently of the others, to minimize repulsive interactions by means of small positional and/or conformational adjustments, as well as that of the cluster to assist by undergoing slight deformations, is essential to overcome the aforementioned limitations imposed by a strict periodicity.

CONCLUSIONS In this paper, we applied our novel flexible cluster model of ZNC for iPP production to investigate the formation of adducts between realistically sized MgCl 2 monolayer clusters and three molecules (namely dimethylphthalate

(DMP),

dimethyl-2,3-dimethylsuccinate

(DMS),

and

2,2-dimethyl-1,3-

dimethoxypropane (DMMP)) belonging in as many industrially relevant ID classes. The chemisorption of single and multiple ID molecules on MgCl2 monolayer edges exposing tetra-coordinate and pentacoordinate Mg atoms, reminiscent – respectively – of -MgCl2(110) and -MgCl2(104) terminations, was studied without any constraints at the stage of adduct structural optimization, and the thermochemistry of the process was determined quantitatively. Concerning the chemisorption of single ID molecules, our calculations turned out to be partly in line with the previous literature.8-22 In particular, the strong preference of DMMP for chelating over bridging adsorption modes on tetra-coordinate Mg was confirmed. On the other hand, a lower but still appreciable preference for chelating and/or bridging chemisorption on tetra-coordinate Mg over bridging chemisorption on penta-coordinate Mg was predicted for the other two molecules (i.e. DMP and DMS) as well; this is reasonable, considering the higher Lewis acidity of tetra-coordinate Mg. In general, calculated adsorption enthalpies, typically in the range of -30 to -40 kcal/mol, confirm the ability of all such molecules to effectively stabilize lateral MgCl 2 crystallite terminations. Compared with TiCl4 adsorption, 1, 6, 7 even monodentate ID chemisorption was estimated to be stronger; therefore, giving credit


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to our (and others’) calculations, the ability of TiCl4 to co-adsorb with the ID may be traced to the fact that, in typical pre-catalyst preparation protocols,3 the former is used in very large excess over the latter, which in turn is dosed more or less stoichiometrically with respect to the targeted amount of exposed Mg. Our results are not directly informative about the possible tendency of a given ID to favor adsorption on a specific MgCl2 crystallite termination rather than distributing on more. On the other hand, in our opinion the calculations suggest that the range of adsorbate structures accessible to the ID on a given termination, directly proportional to the multiplicity of different local surface environments and decreasing from DMS to DMP to DMMP, represents a most important parameter to relate ID structure to PP polydispersity (Table 1). Recent solid state NMR studies are consistent with this view. 49 The hypothesis may hold valid even in case the ID is released during polymerization,3-5 assuming that the thus formed surface vacancies can affect the mode(s) according to which incoming ED or Al-alkyl species can adsorb. As far as high-coverage adducts are concerned, we believe that the results presented in the previous section clearly demonstrate the advantages of the proposed flexible cluster approach compared with periodic protocols. This will become even more evident in forthcoming papers, when high-coverage models of ZNC primary particles with a mixed co-adsorbate pool will be introduced.

ASSOCIATED CONTENT Supporting Information Conformational analysis on DMS and more tests about models stability, as well as, xyz coordinates of all optimized systems, can be found in the supplementary data associated with this article via http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].


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Phone: +39 081674357 Fax: +39 081674057 This two authors equally contributed to the paper# ORCID Andrea Correa 0000-0003-4238-2732 Christian Ehm 0000-0002-2538-5141 Peter H. M. Budzelaar 0000-0003-0039-4479 Vincenzo Busico 0000-0001-7079-1651 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work is part of the Research Programme of the Dutch Polymer Institute, Eindhoven, The Netherlands, Projects #754 and #793. EB and AC thank CINECA (ISCRA grant) for the availability of high performance computing resources and support. Preliminary work was carried out at KAUST (King Abdullah University of Science and Technology) by Dr. Emanuele Breuza. EB thanks Prof. Luigi Cavallo for useful discussion.


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Graphical Abstract for the paper MgCl2 Supported Ziegler-Natta Catalysts: A DFT-D Flexible Cluster Approach. Internal Donor Adducts.


MgCl2-Supported Ziegler-Natta Catalysts: A DFT-D

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