Unravelling the Role of Al‐alkyl Cocatalyst for the VO_x/SiO₂ Ethylene Polymerization Catalyst: Diethylaluminum Chloride Vs. Triethylaluminum

Abstract

Vanadium-based catalysts for olefin polymerization are unique in their strong preference for Cl containing Al-alkyl cocatalysts. In this work, we performed theoretical studies on VO x /SiO 2 / diethylaluminum chloride (DEAC) and VO x /SiO 2 /triethylaluminum (TEA) with V(III) and V(IV) cluster models to provide comprehensive insight into how this catalyst system is affected by cocatalysts and why DEAC is mostly superior to TEA as activators. It is found that DEAC would hemilabily complex with the catalyst and weaken the bonds between V centers and SiO 2 , though the overall steric hindrance is enhanced as well. It ensures the faster C 2 H 4 insertion and even turns the inert V(IV) sites into active ones. Similar situation is seen when TEA is added. However, the ethyl in TEA has larger size than Cl in DEAC, and TEA also complexes with the V center more tightly, generating V-diethyl-(like) species, which have to experience energy costly reorganizations into the monoethyl configuration for insertion. Hence, the complexed TEA even slows the insertion kinetics down. Furthermore, the V-diethyl-(like) species are readily to undergo α-H elimination, yielding inert Valkylidene for subsequent enchainment via the Green-Rooney mechanism. We also investigated the effect of Al-alkyls on V(III) site rigidly constrained by an adjacent silanol, finding the grafted DEAC and TEA, though have bigger size than the silanol, just loosely restrict the V center and liberate the sites poisoned by silanol.

Chromium complexes supported by NNO-tridentate ligands: an unprecedented activity with the requirement of a small amount of MAO

“…7 Chromium-based metal complexes, both heterogeneous and homogeneous, are among the most important polyolefin catalysts and have received increasing attention because of their wide use in both ethylene polymerization and oligomerization. [41][42][43][44][45][46][47][48][49][50][51][52] However, Cr complexes bearing phenoxy-imine ligands are rare and underdeveloped. Previously, Gibson explored this ligand family for Cr-based polymerization catalysts, but only low to modest activity (usually less than 10 5 g(PE) mol −1 (Cr) h −1 ) was obtained.…”

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

“…Possibly, the complex consisting of TiBA and imido group could still interact with the Cr center in some way, just like the Al-alkyls did on Phillips and other catalysts. [25,26] Therefore, this work offers an alternative approach to synthesizing novel imido-Cr/silica catalysts simply by imidating the CrO x species of the Phillips catalyst with various isocyanates.…”

“…While overdosed TiBA decreases the activities of the Phillips catalyst evidently by destroying its Cr sites with more open structure. [25,26] The activities of the Phillips catalyst are much higher than those of the bis(imido)-Cr/silica catalysts, which is likely to result from the organic ligands that will decrease the electronic deficiency and enlarge the steric hindrance of the Cr center. Indeed, according to the review of McDaniel, the addition of donor components such as alcohols, amines, and sulfides would decrease the activ-ities of CrO x /silica greatly.…”

“…However, the copolymers produced by the modified catalysts are still featured by much higher M w and obviously broader MWD. The high temperature 13 C NMR results show that, when 10 mL 1-hexene was added (see Run 23,26, and 29 in Table 2), the content of the incorporated 1-hexene in the copolymer are 1.32, 0.48, and 0.47 mol%, respectively. It implies that the imido ligands disfavor the comonomer incorporation greatly, which is likely due to the great steric congestion of the ligands, thus impeding the insertion of 1-hexene with bigger size than C 2 H 4 .…”

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

“…Therefore, Cr­(VI) models A­(OH) † , B­(OH) † , and C­(OH) † containing 4CR , 6CR , and 8CR were constructed with geminal silanols 1 and 2, vicinal silanols 3 and 4, and isolated silanols 1 and 5 separated by three Si–O units as anchoring points, respectively (see Figure b1,d1,g1). Herein, we preferred not to introduce OH ligands directly contacting the Cr center in the models due to their poisonous effects on olefin polymerization catalysts, which should be mostly eliminated at high temperatures in practice, although some HO···Cr interactions are inevitable in the real system. − With respect to the F-modified catalyst, we assumed that the silanol groups on the silica surface were almost fully substituted by F atoms. The densities of F (1–4 F/nm 2 ) and silanol (1–3 OH/nm 2 ) groups are comparable when a normal amount of F (1–4 wt %) is added to modify the typical silica support, such as Grace Davison grade 955 for the Phillips catalyst. , The models fully modified by F were denoted A­(F) † , B­(F) † , and C­(F) † (see Figure c1, f1, and i1) and built in a manner similar to that used for the unmodified ones but with the silanols (except those serving as anchoring points) substituted by F atoms.…”