A long standing problem in catalysis is the identification and characterization of the active sites, i.e. an atom or an ensemble of atoms spouse on the surface of a catalyst.[Taylor1925] One relevant case, that is treated in this thesis, is constituted by the Phillips catalyst.[Hogan1958] For several reasons, even though this catalyst has be applied at industrial scale for decades and is accounted for a majority of the high density polyethylene (HDPE) production, the identification and the mechanism of the active sites are still under debate. This work was initiated in the framework of the PARACAT project, which is dedicated to study the paramagnetic species in catalysis, and focuses on 'The role of Cr paramagnetic states in olefin polymerization over Phillips catalyst.' In the course of the study, I brought this research to a larger scale which included but was not limited to the Phillips catalyst itself. Considering the relevance of the interaction between transition metal ions (TMI) and the support to the catalytic activity, I worked on systems that cover a number of oxide-supported TMIs by means of electron paramagnetic resonance (EPR) spectroscopy.
In this thesis I investigated the paramagnetic Cr(V) and Cr(III) species in the Phillips catalyst. The Cr(III) species were suggested with possible relevance to the catalytic reaction while Cr(V) species were suggested as just the spectators in the reaction.[McDaniel2010, Groppo2018] Nevertheless, Cr(V) species were used in this work as spin probes to provide more information on the overall system. Field-sweep methods including continuous wave (CW) EPR and echo detected field sweep (EDFS) showed that the there are two Cr(V) species with different local geometries. Quantitative analysis of the CW EPR showed that these two Cr(V) species have different reactivity with ethylene. The instantaneous diffusion analysis were performed on the Cr(V) species to provide information on the dispersion of the Cr on the silica surface and the results suggested clusters were formed locally. Besides studying the Phillips catalyst itself, I studied also the silica supported organometallic-Cr catalyst, Cr[CH(SiMe3)2]3/SiO2, which served as a model system to investigate the catalytic active Cr species with well-defined oxidation states and geometry. Two categories of the Cr(III) species were assigned to the active sites for olefin oligomerization and polymerization. The assignment were done by comparing their distortion of the local geometries with that of the different precursors.
On the other hand, microporous materials including zeolites and zeotype materials such as aluminophosphate (AlPO) can be engineered with different physical and chemical properties in terms of chemical composition and provide a relevant example of structure sensitivity of a heterogeneous catalyst.[Hartmann2002, Hartmann1999] Such structure sensitivity is highly relevant in catalysis and can be very well studies with EPR spectroscopy. In this regard, I investigated a series of SAPO-5 materials doped with different TMI. In the first place, the incorporation of Cr in SAPO-5 was studied focuses on the discrimination of isomorphous substitution at framework sites and extra-framework sites. In the hyperfine sublevel correlation (HYSCORE) spectrum, large hyperfine interaction (hfi) of 27Al with the matrix 31P signal provide solid evidence for the isomorphous substitution of Cr(V) at framework sites. In addition to the Cr-incorporated SAPO-5, a method to prepare a bi-metallic Mo/V-SAPO-5 system was developed and the metal-metal synergy was validated with a single electron transfer reaction and the short range hyperfine interaction. HYSOCRE spectra showed large \textit{hfi} of both 27Al and 31P and suggested the V species grafted at extra-framework sites. Moreover, the HYSCORE spectrum showed signals at low frequency region which were attributed to the 95,97Mo species with large hfi, confirming the short range interaction. Finally, the surface properties of SAPO-5 were studied by adsorbing NO radicals in the pores and investigating their interaction with the surface. Different adsorption sites of NO molecules according to different activation conditions were first discriminated by the g-factors obtained from the CW EPR. From the 27Al HYSCORE spectra, it is observed that when the activation temperature is higher the NO molecules are situated in vicinity of some defect Al sites. However, the dominant Al species were observed either in samples activated at lower temperature or by increasing the NO dosage. This is postulated as that the defect sites were blocked by residual water molecules or saturated by excessive NO molecules. The presence of water molecules were validated by 1H HYSCORE experiments and the coordination of NO-water was estimated from the hfi structure.
[Taylor1925] Taylor, H. S. Proc. R. Soc. London. Ser. A, Contain. Pap. a Math. Phys. Character 1925, 108, 105-111
[Hogan1958] Hogan, J. P.; Banks, R. L. Polymers and production thereof US Patent 2,825,721, 1958.
[McDaniel2010] McDaniel M. P. Advances in Catalysis, 1st Ed. 2010; Vol. 52, pp 123-606
[Groppo2018] Groppo, E.; Martino, G. A.; Piovano, A.; Barzan, C. ACS Catal. 2018, 8, 10846-10863
[Hartmann2002] Hartmann, M.; Kevan, L. Res. Chem. Intermed. 2002, 28, 625-695
[Hartmann1999] Hartmann, M.; Kevan, L. Chem. Rev. 1999, 99, 635-663
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:87138 |
| Date | 18 September 2023 |
| Creators | Liao, Yu-Kai |
| Contributors | Brunner, Eike, Folli, Andrea, Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1021/acs.jpcc.0c09484, 10.1016/j.jcat.2020.12.022, 10.1039/9781839162534-00001, 10.1021/acs.jpcc.3c01817 |
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