Controlled Assembly of a Heterogeneous Single‐Site Ethylene Trimerization Catalyst as Probed by X‐ray Absorption Spectroscopy

Abstract

X-ray absorption spectroscopy at the Cr K- and L(2,3)-edges was used to study the assembling process of a heterogeneous Cr-based single-site catalyst. The starting point was a Phillips-type system with monochromate species anchored on a silica surface, which was first reduced to a variety of different surface Cr(II) species. The reduced sample was modified with a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand in the presence of CH(2)Cl(2) as solvent to yield a heterogeneous single-site Cr-based catalyst active in the trimerization of ethylene. The molecular structure of the resultant catalytic material consists of distorted octahedral Cr(III) species. The extended X-ray absorption fine-structure (EXAFS) spectroscopy fitting procedure in R space up to 2.5 A showed that the synthesis leads to coordination with a TAC ligand. The fit also shows that it was possible to complete the six-fold environment around Cr(III) with two oxygen atoms and one chloride ligand. This chloride ligand is formed in a redox process from the solvent and is responsible for the oxidation of surface Cr(II) to Cr(III). The obtained geometry and the local environment of the surface complex are discussed in light of its homogeneous counterpart and confirm the single-site characteristics of the prepared catalytic material.

Polyethylene with Reverse Co‐monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding

“…To the best of our knowledge,wepresent in Figure 3athe first Cr L 2,3 -edge XAS spectrum of Phillips-type catalyst using STXM. Thespin-orbit splitting of Cr causes distinct L 2 and L 3 regions in the spectral range of 572-592 eV.I nc omparison with the literature reference spectra, [50] it can be concluded that Cr is most probably in adistorted O h state in the form of ad ispersed Cr 2 O 3 -like phase.T he oxidation number of the activated catalyst, calcined in dry O 2 ,i se xpected to be + 6. Therefore,t he lower oxidation state of 3 + can be explained by the exposure of the activated catalyst to air,w ater and epoxy resin during the required preparation procedure for the STXM measurements,w here Cr 6+ has been partially converted into Cr 3+ .…”

Surface chromium single sites: open problems and recent advances

“…In this respect, the above-mentioned annealing method (Groppo et al 2005d) is a way to fine tune the relative populations of Cr II sites, and thus the molecular weight distribution of the polyethylene product. More recently, based on the addition of specific surface modifying agents, finer methods have been adopted (Nenu & Weckhuysen 2005;Nenu et al 2006Nenu et al , 2007. In addition, the possibility of changing the catalytic activity of Cr/SiO 2 by altering the chemical composition of the support has been known since the 1970s.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[207] It is noteworthy that (SiO)TaMe 2 Cl 2 catalyzes the trimerization of ethylene without ac o-catalyst to yield 1-hexene as the major product (> 80 %) along with 1-butene (< 10 %), albeit with al ow productivity (ca. [207] It is noteworthy that (SiO)TaMe 2 Cl 2 catalyzes the trimerization of ethylene without ac o-catalyst to yield 1-hexene as the major product (> 80 %) along with 1-butene (< 10 %), albeit with al ow productivity (ca.…”