Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site

Abstract

The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/ SiO 2 ) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C 2 H 4 reduction of silica-supported Cr VI ions to Cr II ions in the precatalyst, exposure to ethylene results in the formation of organoCr III sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol−gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined Cr II end point. Ethylene causes rapid reoxidation of these Cr II sites to Cr III , even at low temperatures. Solid-state 13 C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO) 2 Cr III −CHCH 2 although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of Cr II via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggest that the key step is homolysis of a Cr−ethyl bond. Plausible pathways for the initiation mechanism are suggested.

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

“…EPR investigations of these catalysts showed that only a small amount of Cr 51 remained as an impurity after the reduction. [108][109][110] The non-Kramers Cr 21 species (S ¼ 2) was investigated using high field EPR at 106/212/317 GHz, and was reported to have a very small rhombicity for the ZFS tensor. 109 Upon ethylene polymerisation, two research groups reported different observations, albeit under slightly different conditions.…”

“…However, in another study the polymerisation reaction was performed at T ¼ 80 1C and the authors reported the appearance of Cr 31 signals, characterised by the broad linewidth, centred at gB1.98 with axial symmetry, together with the loss from Cr 21 signals. 109,110 Therefore, it was concluded that the Cr 21 sites were oxidised to organo-Cr 31 species by ethylene, which initiated the polymerisation.…”

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

“…One example of a long‐standing discussion is the oxidation state of the Cr active site, which has been investigated with many different spectroscopic techniques, demonstrating that the active valency lies between 2 and 3, with the true value still being questioned by varying insights from different research groups. These differences, however, can in part be ascribed to different reaction set‐ups and different catalyst materials [33, 34, 43–46, 35–42] …”

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

“…Examples include model systems [22][23][24][25][26][27][28] or well-defined catalyst systems in which Cr 6 + is reduced by e. g. CO. [13,14,[29][30][31] With an array of spectroscopic techniques revealing that the oxidation state of the Cr active site lies between 2 and 3. [13,14,[39][40][41][42][43][44][45]20,[32][33][34][35][36][37][38] However, assigning a definitive oxidation state number remains a matter of debate due to the various findings from different groups, although this could be in part explained due to different catalyst materials and reaction setups. [6,20,[47][48][49]34,[36][37][38][40][41][42]46] Fortunately, with the continuous development of advanced spectroscopic and microscopic techniques, the low Cr weightloading and relatively small portion of active sites are becoming less of a problem, exemplified by elucidation of an ethylene reduced active site.…”

“…[13,14,[39][40][41][42][43][44][45]20,[32][33][34][35][36][37][38] However, assigning a definitive oxidation state number remains a matter of debate due to the various findings from different groups, although this could be in part explained due to different catalyst materials and reaction setups. [6,20,[47][48][49]34,[36][37][38][40][41][42]46] Fortunately, with the continuous development of advanced spectroscopic and microscopic techniques, the low Cr weightloading and relatively small portion of active sites are becoming less of a problem, exemplified by elucidation of an ethylene reduced active site. [18] Even though our understanding of CO and ethylene reduced Cr/SiO 2 catalysts is increasing, research towards the effects of metal-alkyl co-catalysts on the active site structure and oxidation state has only started to gain track more recently.…”