Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO₂ Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study

Abstract

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization since its invention in 1953. The uniqueness of this catalyst is related to its ability to produce broad molecular weight distribution (MWD) PE materials as well as that no co‐catalysts are required to attain activity. Nonetheless, co‐catalysts in the form of metal‐alkyls can be added for scavenging poisons, enhancing catalyst activity, reducing the induction period, and tailoring polymer characteristics. The activation mechanism and related polymerization mechanism remain elusive, despite extensive industrial and academic research. Here, we show that by varying the type and amount of metal‐alkyl co‐catalyst, we can tailor polymer properties around a single Cr/SiO2 Phillips catalyst formulation. Furthermore, we show that these different polymer properties exist in the early stages of polymerization. We have used conventional polymer characterization techniques, such as size exclusion chromatography (SEC) and 13C NMR, for studying the metal‐alkyl co‐catalyst effect on short‐chain branching (SCB), long‐chain branching (LCB) and molecular weight distribution (MWD) at the bulk scale. In addition, scanning transmission X‐ray microscopy (STXM) was used as a synchrotron technique to study the PE formation in the early stages: allowing us to investigate the produced type of early‐stage PE within one particle cross‐section with high energy resolution and nanometer scale spatial resolution.

Controlling Polyethylene Molecular Weights and Distributions Using Chromium Complexes Supported by SNN-Tridentate Ligands

“…Generally, different classes of polyethylenes result from the use of specific metal-based catalysts, of which the design and synthesis are expensive, time-consuming, and not always as rewarding as expected. Therefore, the development of tunable catalytic systems that could produce different polyethylenes is highly desirable but challenging. , One of the industrially most relevant catalyst is the chromium-based Phillips catalyst (CrO x /SiO 2 ). − Discovered in 1951 by Hogan and Banks at Phillips Petroleum, this heterogeneous catalyst allows the manufacture of one-third of the high-density polyethylene (HDPE) commercialized, which is the most widely used polymer . Although the Phillips catalyst has been studied for over 70 years by both academic and industrial scientists, the nature of the initiation process, the coordination environment and the oxidation state of the metal in the active species, and the catalytic mechanism remain a hot and sometimes controversial topic. − Furthermore, much effort has been dedicated to the development of well-defined homogeneous Cr-based catalysts, ,− that are more amenable to detailed spectroscopic and structural studies than heterogeneous catalysts.…”

“…H NMR (400 MHz, CDCl 3 ) δ: 8.56 (s, 1 H), 8.47 (d, J = 8 5. Hz, 1 H), 8.29 (d, J = 8 5. Hz, 1 H), 8.10 (d, J = 8 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

“…As a result, the catalysts prepared by methods ( 1) and ( 2) produce polymers with different properties. 3,51 With method (3), only CO 2 is produced during Cr(VI) reduction, and this CO 2 is immediately released from the catalytic system. It was also experimentally verified that the Cr(VI) precursors were mostly reduced to Cr(II) sites.…”

“…Regarding method (2), the added Al-alkyl cocatalyst is also kept in the catalyst system and interacts with the active sites. As a result, the catalysts prepared by methods (1) and (2) produce polymers with different properties. , With method (3), only CO 2 is produced during Cr­(VI) reduction, and this CO 2 is immediately released from the catalytic system. It was also experimentally verified that the Cr­(VI) precursors were mostly reduced to Cr­(II) sites. , Moreover, the Cr­(II) produced by method (3) is able to synthesize PE with properties similar to those of the Cr­(II) produced by the conventional method (1).…”

Comparison of Support Effects on Phillips and Metallocene Catalysts

“…At about 400 • C, the chromium is oxidized into the hexavalent form, which spreads out and becomes anchored onto the support as chromate or dichromate surface esters [3,[5][6][7][8][9][10][11][12][13][14]. These species are then reduced by ethylene to a lower valence, expanding the potential coordination sphere, alkylating the Cr, and forming the coordinatively unsaturated active sites [3][4][5][15][16][17][18][19][20]. Consequently, and somewhat unusually for inorganic industrial catalysts, the active sites are individually attached to the support.…”