Titanium Neopentyl supported on KCC-1 and Al-modified KCC-1 and its Catalytic Application for Ethylene polymerization

Abstract
A new generation of Titanium based catalysts for ethylene polymerization has been developed through the Surface Organometallic Chemistry (SOMC) methodology using a novel type of silica support having a 3D fibrous morphology, KCC-1. The first type of Tibased catalyst was obtained by reacting isolated silanol surface groups, ≡SiOH of KCC-1 (dehydroxylated at 700 ⁰C under high vacuum, 10-5 bar) with titanium (IV) tetraneopentyl, Ti(CH2tBu)4 to produce [(≡SiO)Ti(CH2tBu)3]. The second type of Ti-based catalyst was generated by using an Al-modified KCC-1. The peculiarity of this support is due to the presence of tetra-coordinated aluminum-bound hydroxyl group, [(≡Si-O-Si≡)(≡SiO)2Al- OH] that can be used as a Lewis Acid anchor sites and generate new catalytic properties. The well-defined [(≡Si-O-Si≡)(≡SiO)2Al-OH] was obtained by reacting diisopropylaluminum hydride with KCC treated at 700 °C followed by a thermal treatment at 400 °C and oxidation with N2O. IR spectra of pyridine adsorbed on the Al sites show that these were strong Lewis acid sites (constituting 80% of the total Al sites). Thus, the highly electrophilic support surface was used to create a single well-defined surface organo-titanium fragment [(≡Si–O–Si≡)(≡Si–O–)2Al–O–Ti(CH2tBu)3] by the reaction of the surface [(≡Si–O–Si≡)(≡Si–O)2Al–OH]) groups with Ti(CH2-tBu)4 at room temperature for 4 h in dry pentane.

The performance of each Ti-supported catalyst assessed for ethylene polymerization. It was found that Al-modified support (highly electrophilic) provide better activity compared to the unmodified one. Indeed, the productivity of the catalyst [(≡Si–O– Si≡)(≡Si–O–)2Al–O–Ti(CH2tBu)3] was found to be 67.8 g of PE/ 1mmol Ti/ 1h with molecular weight of 3208408 g/mol; polydispersity was found to be 2.3, and (HDPE) high-density polyethylene was obtained. In contrast, [(≡SiO)Ti(CH2tBu)3] (unmodified one) produces lower molecular weight polymer 989843 g/mol, higher polydispersity (PD) 6.7 and low-density polyethylene (LDPE) productivity was found to be 14.670 g PE/1mmol Ti /1h. These results demonstrate that modification of the oxide ligands on silica through a generation of Al Lewis acid site opens up new catalytic properties, markedly enhancing the catalytic performance of supported organotitanium species. We also demonstrate how the silica mesostructure (2D vs 3D ) affects the catalytic activity in ethylene polymerization. While SBA15 (2D) could limit the accessibility of the active sites resulting in lower yield. In contrast, KCC-1 (3D) are more active in ethylene polymerization, because the active sites reside on the external surface are fully accessible to the substrate.

Citation
Alrais, L. M. (2018). Titanium Neopentyl supported on KCC-1 and Al-modified KCC-1 and its Catalytic Application for Ethylene polymerization. KAUST Research Repository. https://doi.org/10.25781/KAUST-PS182

DOI
10.25781/KAUST-PS182

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