MgCl₂-Supported Ziegler–Natta Catalysts: a DFT-D “Flexible-Cluster” Approach to Internal Donor Adducts

Abstract

A "flexible cluster" model approach to Ziegler− Natta catalysts for the production of isotactic polypropylene, allowing the use of realistically sized MgCl 2 monolayer clusters (up to 38 MgCl 2 units) without any constraints, was employed to investigate the formation of adducts between the MgCl 2 support and three industrially relevant internal donor 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.

MgCl2‐supported Ziegler‐Natta catalysts: A DFT‐D ‘flexible‐cluster’ approach. TiCl4 and probe donor adducts

“…Notably, for CLU‐24‐104/Me 2 O adducts this turned out to be complicated by the presence of multiple local minima with very similar energy values and low separation barriers, differing primarily in the local structure at corners with terminal Mg‐bonds; reversible or irreversible reconstructions from one such minimum to another were observed upon structure re‐optimization. The problem was overcome by adsorbing H 2 O molecules at the aforementioned corners, thus saturating the Mg coordination sphere; as is well‐known, H 2 O is an ubiquitous donor with high affinity for MgCl 2 (vide infra), and a common contaminant of real MgCl 2 adducts . ‘Corner‐stabilized’ CLU‐24‐104/Me 2 O adducts did not feature any propensity to reconstruct (for details, see the SI).…”

“…The approach offers a good compromise between speed and accuracy, and has great potential in view of realistic simulations of the catalytic species. In a different paper, we highlighted model application to the adsorption of industrially relevant internal donors (i.e., diesters, 1,3‐diethers) at high coverage; this is even more difficult to achieve with periodic models without artifacts. Last but not least, Al‐alkyls and external donors (eg, alkylalkoxysilanes) will also be added to the picture.…”

Roles of Salicylate Donors in Enhancement of Productivity and Isotacticity of Ziegler–Natta Catalyzed Propylene Polymerization

“…The remaining MgCl 2 units were frozen at the bulk MgCl 2 value [54]. The flexible cluster model allows atoms to adjust their positions according to interactions with adsorbed molecules and it has been employed for the active surface of the Ziegler-Natta catalyst in several works [39,42,55]. The genuineness of the transition state was ensured by the presence of one imaginary frequency.…”

Effect of Internal Donors on Raman and IR Spectroscopic Fingerprints of MgCl2/TiCl4 Nanoclusters Determined by Machine Learning and DFT

“…Together with some experimental inconsistencies, DFT studies have definitely set aside models obtained by Molecular Mechanics investigations on binary systems TiCl 4 /MgCl 2 and Ti 2 Cl 8 /MgCl 2 [42], after puzzling DFT predictions of adsorption energies of Ti x Cl y species on regular MgCl 2 surfaces and thanks to more recent reliable descriptions of the morphology of the δ-MgCl 2 particles [43][44][45][46][47][48][49][50][51]. Theoretical calculations on MgCl 2 bulk and surface structure, exploiting periodic Density Functional Theory approaches including London interactions, pointed out that MgCl 2 crystals in the absence of adsorbates mainly expose the (104) or other pentacoordinated surfaces, whereas the picture drastically changes in the presence of adsorbates (i.e., electron donors); in fact, regular MgCl 2 crystals expose the (110) surfaces at a relatively large extent when they grow in the presence of Lewis bases (small molecules such as methanol, ethanol, and dimethyl ether) or in presence of alkoxy silanes employed as external donors [30,52].…”