Heterogenisation of zirconocene catalysts for ethylene polymerisation

Abstract

A study of amorphous silica gel supported showed a decrease in the silanols content from 5.7 to 2.4 molecules per nm² for an increase in the calcination temperature from 200 to 800^oC. Infrared spectroscopy characterised different silanols types: at 200^oC, isolated and H-bonded silanols were present on the surface while at 800^oC only isolated silanols remained.

Supported Cp₂ZrCl₂ and Cp₂ZrMe₂ have given identical Zr K-edge EXAFS models where the zirconium centre was supported via two oxygens at 2.1 Å. The carbons of the Cp ligands remained present at 2.5 Å. Direct Si-Me bondings were present with a resonance at δ-0.1 in ¹H MAS NMR. Gas titration following the deposition of Cp₂ZrMe₂ showed that 2 methane molecules were evolved per zirconium. No gas was evolved for Cp₂ZrCl_2. EDE and in situ mass spectroscopy indicated that Cp ligands desorption under N₂ arose at 300^oC for Cp₂ZrCl₂ and 450^oC for Cp₂ZrMe₂ but carbon residues remained on silica. The difference in decomposition temperatures was suggested to be due to HCl presence in the supported Cp₂ZrCl₂. The decomposed samples gave an orthorhombic ZrO₂Zr K-edge EXAFS model.

According to gas titration, the GaMe₃ and AlMe₃ were supported via evolution of 2, 1 and 0.33 of methane molecules respectively for ratios of 1/3, 1 and 3 Ga/Al per silanol available. The gas production was independent of the silica calcination temperature. Direct Si-Me, Si-OMe interactions were also observed by CP-MAS ¹³C NMR with resonances at δ-1 and δ 50 and Ga K-edge EXAFS indicated a 4 co-ordinated Ga centre by light scatterers, oxygen or carbon. A 1:1 ratio in silica calcined at 800^oC produced monocoverage of the silica surface by GaMe₃ and AlMe₃. Their decomposition under N₂ showed a two-stage process: a first evolution of methane corresponded to the loss of gallium co-ordinated methyls while the slower second stage represented the decomposition of adsorbed hydrocarbonaceous species. In the decomposed samples, some carbon residues remained for both Al and Ga derivatives. The gallium centre was oxygen co-ordinated gallium aggregation on the surface. For 3:1 ratios, the aggregation observed was larger (Ga co-ordination of 1.5).

Finally, Cp₂ZrCl₂ and GaMe₃/AlMe₃ were supported. Models derived from Ga and Zr K-edge EXAFS, production of methane, HCl presence, Si-Me and Si-OMe bondings were all similar to previous results. Decompositions under nitrogen were not clean and proceeded via a two-stage process for methane evolution and 1 broad process for Cp ligand decomposition. Differences were observed as 2 types of Cp ligands were characterised by ¹³C CP-MAS NMR in the supported samples: 1 corresponding to the Cp₂ZrCl₂ supported via 2 oxygens and the other representing the presence of Zr-Me interaction witnessed by ¹³C CP-MAS NMR with a resonance at δ 32. This model was predominant for supported Cp₂ZrCl₂/AlMe₃ samples and its Zr K-edge EXAFS indicated no oxygen shell at 2.1 Å.

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