Computational Insights into the Multisite Nature of the Phillips CrO_x/SiO₂ Catalyst for Ethylene Polymerization: The Perspective of Chromasiloxane Ring Size and F Modification

Abstract

The famous Phillips CrO x /SiO2 catalyst is characterized by its multisite nature and the resulting broad molecular weight distribution (MWD) of its polyethylene (PE) products. However, the multiplicity of active sites and their catalytic behavior relating to the broad MWD have scarcely been studied. Given the high and comparable efficiency of calcination and F modification in regulating the active site multiplicity of this catalyst, eight polyhedral oligomeric silsesquioxane (POSS)-based cluster models comprising four-, six-, and eight-membered chromasiloxane rings (4CR, 6CR, and 8CR) were partially or fully capped by OH or F and employed for density functional theory (DFT) calculations related to C2H4 polymerization. For the OH-capped models, the results emphasized the dominant size effect of the chromasiloxane ring (CR), where the model with a smaller CR was less abundant but could produce higher-molecular-weight (MW) PE with a higher activity. The predicted MW indicated the very broad MWD of the PE products, consistent with that of the products generated from low-temperature calcinated CrO x /SiO2. However, depending on the size of the CR, these models responded unequally to F modification. Specifically, the more abundant sites with 8CR were promoted more obviously, and the MW of their PE products was increased. This makes the polymerization rate higher and the MWD of the PE narrower, which agrees with those of the PE products synthesized by F-modified CrO x /SiO2 calcined at a low temperature. Furthermore, the origin of the effects of CR size and F modification was properly illustrated in terms of both steric and electronic variations.

The Role of In Situ/Operando IR Spectroscopy in Unraveling Adsorbate-Induced Structural Changes in Heterogeneous Catalysis

“…For example, Stiegman and co-workers adopted very simple clusters with Cr­(VI) belonging either to a strained 6CR or to a larger and less strained 10CR, the latter derived from another structure previously used by Dines and Inglis . While the O–Cr–O angle follows the same trend found by Huang et al, i.e. , the angle increases with increasing the CR size, the Cr–O bond distance does not, being larger for the 10CR model (1.753 vs 1.746 Å).…”

“…80 A similar situation has been reported by Budnick et al 107 for larger Cr(II) models, which were treated with the ONIOM embedding approach (i.e., modeling the most relevant portion of the model around the chromium center at a high level of theory, adopting quantum mechanics, and the remaining portion at a lower level of theory). In particular, the authors concentrated on two models having formula H 6 O 48 Si 22 Cr (i.e., almost three times larger than those adopted by Huang et al, 103 ). In the first one, Cr(II) species are saturating two distal Si−O dangling bonds belonging to a five-membered SiO 4 ring, generating a 6CR (vide infra model 3 in Figure 4b), while in the second, the Cr(II) species are part of a 8CR (vide infra model 4 in Figure 4b).…”

“…However, it does not explain everything. In particular, the increase in the amount of Cr A (II) sites would not explain the fact that lower polymer MWs are obtained with rising the activation temperature; according to the predictions of Huang et al, 103 the more strained Cr A (II) sites should produce high MW PE. This fact indicates that something is missing in the simplified classification of chromium sites as a function of the CR size.…”

The role of agostic interaction in the mechanism of ethylene polymerisation using Cr(iii) half-sandwich complexes: What dictates the reactivity?

“…14,15 The Cr(II)-silica-based Phillips catalyst also produces a significant amount of industrial polymers where the Cr(III)-alkyl compound is established as the active catalyst. [16][17][18] Over the last few decades, several molecules for alkene polymerisation catalysts with metal ions such as Zr, Ti, Rh, Cr, Ru, and Hf have been discovered, and lately, metal-organic frameworks that incorporate the various transition metals and organic ligands have also been found to form to perform this task. [19][20][21][22][23][24] These catalysts offer control over the molecular weight distribution, tacticity, and branching of the resulting polymers.…”

Novel Imido‐Cr/Silica Ethylene Polymerization Catalysts Modified from the Phillips Catalyst

“…[1] The vigor and vitality of this catalyst is maintained by its continuous and numerous recipe upgrades, which are generally promoted by several fundamental methods including calcination treatment, activation manipulation, as well as physical and chemical modification. [2][3][4][5] Regarding to chemical modification, it is the inorganic components, such as metal oxides, mineral acid, halogen compounds, and the combination of them, that are commonly employed, while polar organic modifiers are scarcely investigated. It probably results from the highly unsaturated and electronically deficient nature of the Cr center, which is likely to be poisoned by these polar organic components, leading to lower activities.…”