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.
| Identifer | oai:union.ndltd.org:kaust.edu.sa/oai:repository.kaust.edu.sa:10754/628531 |
| Date | 08 1900 |
| Creators | Alrais, Lujain M. |
| Contributors | Basset, Jean-Marie, Physical Science and Engineering (PSE) Division, Huang, Kuo-Wei, Da Costa, Pedro M. F. J. |
| Source Sets | King Abdullah University of Science and Technology |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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