Carbocations in Heterogeneous Catalysis Caught in the Act by (Dynamic Nuclear Polarization Enhanced) Multidimensional and Multinuclear Nuclear Magnetic Resonance



Xiao, Dong
(2020) Carbocations in Heterogeneous Catalysis Caught in the Act by (Dynamic Nuclear Polarization Enhanced) Multidimensional and Multinuclear Nuclear Magnetic Resonance. PhD thesis, University of Liverpool.

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Abstract

Probing and determining the carbocation intermediates formed during heterogenous catalytic reactions are crucial steps for understanding the reaction mechanisms, such as the methanol to hydrocarbon (MTH) reaction. Nevertheless, the observation of carbocations and their structural identification are not straightforward as they are reactive, transient, difficult to capture and exist in generally low concentrations and therefore their spectroscopic characterization is very challenging. Solid-state NMR is useful in detecting reactive carbocations on solid catalysts. However, previous NMR studies of carbocations were only limited to the use of one-dimensional (1D) NMR methods combining with some indirect methods to elucidate the carbocation structures, those methods require prior assumption of possible structures and may lead to misinterpretation. In this thesis, the carbocation intermediates formed in various zeolite catalysts under MTH conditions are directly identified via multidimensional NMR techniques which provide unambiguously experimental support for previously proposed structures and more importantly identify previously undetected ones. Moreover, the interactions between the confined carbocations and zeolite framework are quantitatively probed by multinuclear NMR methods. Since low sensitivity of NMR is also key limiting factor challenging NMR studies of carbocations, the advanced sensitivity enhancement technique known as dynamic nuclear polarization (DNP) is successfully applied to the detection of carbocations and their subsequent multidimensional and multinuclear NMR studies. These successful applications of DNP provide new opportunities for research on carbocations.

Item Type: Thesis (PhD)
Divisions: Fac of Science & Engineering > School of Chemistry
Depositing User: Symplectic Admin
Date Deposited: 08 Jun 2020 13:12
Last Modified: 10 Jun 2020 00:11
DOI: 10.17638/03084609
Supervisors:
URI: http://livrepository.liverpool.ac.uk/id/eprint/3084609