Applications of multinuclear solid-state NMR spectroscopy to the characterisation of industrial catalysts
Abstract
This thesis describes applications of advanced multinuclear solid-state nuclear
magnetic resonance (NMR) experiments to the characterisation of industrially-relevant catalyst materials. Experiments on γ-Al₂O₃ introduce the use of solid-state
NMR spectroscopy for the investigation of disordered solids. The existence of Al(V)
sites on the surface of this material is demonstrated, showing that removal of adsorbed
H₂O may facilitate a rearrangement effect in γ-Al₂O₃ that promotes the formation of
these Al environments. A range of aluminium oxide-based supported metal catalysts
has been investigated. Studies of these systems by ¹H and ²⁷Al solid-state NMR
spectroscopy indicate that a metal-support interaction (MSI) exists between surface
cobalt oxide crystallites and the γ-Al₂O₃ support, and is strongest for materials
containing small, well dispersed Co oxide crystallites. It is shown that the hygroscopic
nature of γ-Al₂O₃ allows the extent of the MSI to be visualised by ¹H MAS NMR, by
observing the extent of the proton-metal oxide interaction resulting from the presence
of adventitious adsorbed H₂O. The surface/bulk chemistry of Co spinel aluminate
materials is also investigated. ¹H, ²⁹Si, ²⁷Al and ¹⁷O solid-state NMR techniques are
used to gain insight into the structural nature of silicated alumina catalysts. The
combination of isotopic enrichment and dynamic nuclear polarisation (DNP) surface-enhanced NMR spectroscopy can provide a definitive and fully quantitative
description of the surface structure of Si-γ-Al₂O₃ (1.5 wt% Si), and the role of
adventitious surface water is highlighted. Analysis of silicated aluminas prepared by
“sequential grafting” and “single shot” approaches shows that silica growth on γ-Al₂O₃ follows two distinct morphologies. ¹⁷O gas exchange enrichment is also shown to be
successful in facilitating ¹⁷O solid-state NMR studies of these materials. It is
demonstrated that double (²⁹Si and ¹⁷O) enrichment of Si-γ-Al₂O₃ (1.5 wt% Si) can
facilitate access to ²⁹Si-¹⁷O 2D correlation experiments, even at low silica loading. An
exploratory investigation of Ti-alumina model catalysts has also been carried out using ¹H, ²⁷Al and ¹⁷O solid-state NMR spectroscopy. These studies indicate that Ti-γ-Al₂O₃
and Ti-Al M50 may be structurally distinct materials.
Type
Thesis, PhD Doctor of Philosophy
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