Tracking the reasons for the peculiarity of Cr/Al2O3 catalyst in ethylene polymerization

Abstract

Looking to the past, heading to the future. In this contribution we explain the reasons why the Cr/Al 2 O 3 Phillips catalysts exhibit a faster kinetics profile in ethylene polymerization reaction with respect to Cr/SiO 2 . Diffuse reflectance UV-Vis and FT-IR spectroscopies unequivocally demonstrate that, albeit several types of reduced Cr sites are stabilized by the Al 2 O 3 support, only the 4-fold coordinated Cr 2+ sites are active precursors in ethylene polymerization, as for Cr 2+ /SiO 2 . Nevertheless, kinetic experiments indicate that ethylene polymerization is 15 times faster on COreduced Cr/Al 2 O 3 than on CO-reduced Cr/SiO 2 . The difference is even more striking (two order of magnitude) when the reaction rates per active Cr sites are compared. Our experimental results suggest two reasons behind the faster polymerization kinetic of Cr/Al 2 O 3 : 1) the higher ionic character of the Cr-O-Al bond with respect to the Cr-O-Si one; 2) the nature of the ancillary ligands in the coordination sphere of the Cr active sites (which are mainly carbonates for CO-reduced Cr/Al 2 O 3 and siloxane bridges for CO-reduced Cr/SiO 2 ).

The Effect of Al-Alkyls on the Phillips Catalyst for Ethylene Polymerization: The Case of Diethylaluminum Ethoxide (DEALE)

“…the first Cr-C bond is already present). However, it should be noticed that a similar fast kinetic profile is shown also by CO-reduced Cr/Al 2 O 3 in the absence of any activator [5], and recently ascribed to the presence of Al(III) sites in close proximity of the Cr(II) sites [27], a situation very similar to that found for Cr(II)/SiO 2 +DEALE. As a general conclusion, the results discussed in this work demonstrate once more that the intrinsic activity of the Cr sites in Phillips-type catalysts does depend not only on their molecular structure, but also on the concurrence of several other phenomena involving the support and the surroundings of the active sites [60].…”

“…The kinetics of ethylene polymerization was studied by sending 200 mbar of ethylene at room temperature over 0.2 g of catalyst inside a quartz reactor of known volume, and recording the ethylene pressure as a function of time, as already done in the past [26,27]. A similar experiment was repeated for the catalyst inside the FT-IR cell, collecting the spectra as a function of time.…”

Influence of Pore Structure and Metal‐Node Geometry on the Polymerization of Ethylene over Cr‐Based Metal–Organic Frameworks

“…[12][13][14][15][16][17] This has been elegantly exemplified by the linear correlation between catalytic CO 2 photoreduction, as am odel reaction, and the electronic structure of the linker,s howing that functional groups on the organic linkerd irectly affect the reactivity. [15][16][17][18][19][20] This method is comparable to conventional strategies for heterogenous catalyst design,s uch as using dopants (promoters) or tuning the support, [3,[21][22][23][24][25][26] but with the added value of as traightforward characterization and understanding of the material. The ability to change the pore structure,m etal-node and electronic structure with relative ease, make MOFs increasingly prominent candidates forh eterogenized single-site catalysis.…”

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

“…Another extraordinary aspect, and long‐standing point of discussion, is the large variety of PEs that can be produced with the Phillips catalyst and the multitude of strategies, such as calcination temperature, support type, co‐catalyst addition and catalyst pre‐treatment, one can employ in tailoring the final PE product properties [43, 47–55] . The diversity of produced PEs, in terms of PE‐type as well as the broad molecular weight distributions (MWD), is ascribed to the large variety of Cr surface sites.…”