Controlled assembly of a Cr-based heterogeneous single-site ethylene trimerisation catalyst

Abstract

One of the most illustrative examples of heterogeneous catalyst complexity is the Phillips-type Cr/SiO2 catalyst, which is industrially used for the production of more than 40% of all polyethylene worldwide. It is considered that (i) the amount of active Cr sites is less than 10% of the total amount of surface Cr sites; (ii) almost every oxidation state between 2 and 6 has been proposed to be active in the polymerisation of ethylene and (iii) families of Cr sites are present, showing different polymerisation characteristics. A potential approach to tackle the associated problems when characterising the Phillips catalyst could be: (i) to find a way to order the surface of the catalyst material, if possible by creating a well-defined single-site system on which structural and mechanistic investigations can be undertaken and (ii) to develop more powerful analytical methodologies either by combining several complementary techniques or by refining the analytical methods in order to remove potential interfering effects. Both routes have been explored in this PhD thesis. A heterogeneous Cr-based polymerisation catalyst was converted into a highly active and selective ethylene trimerisation catalyst by using a special assembling approach. It was determined that the TAC ligand is coordinated to a surface CrIII site and that two chloride ligands are also present in the coordination sphere of Cr. At low ethylene pressure, this catalyst can produce highly branched oligomers where 1-hexene is partially incorporated. At higher pressures, crystalline polymers enriched with a monoclinic phase are obtained. Moreover, it was discovered that, during the controlled assembly of the ethylene trimerisation site, the dispersive nature of the heterogeneous system is gradually removed. The following process is envisaged to take place: (i) Anchoring CrII species onto a silica surface leads to the formation of a catalytic system characterised by a broad dispersity in active ethylene polymerisation sites. (ii) CH2Cl2 determines the internal oxidation of some CrII species to more uniform CrIII species, thus, leading to a partial reduction of the system dispersity. However, other possible surface Cr species (CrIIC-type, CrIII, CrIV, CrV present in the system) are insignificantly affected by the presence of CH2Cl2. Polymerisation tests on the CH2Cl2-modified Cr/SiO2 system have proved that this catalyst presents now a medium active site dispersity. Moreover, it has been shown that a lower number of more defined active sites is responsible for the increased polymerisation activity. (iii) The additional presence of the TAC ligand determines the almost complete conversion of surface Cr species into CrIII sites; i.e., a surface TAC-CrCl2/SiO2 complex. The TAC ligand forces all surface chromium sites to adopt a unique conformation due to its rigid configuration induced by its cyclic structure. In this way, the poly-dispersity of the system is almost completely removed and the assembled system becomes mono-disperse. This unique surface trimerisation site is responsible for the PI value of 1.87.