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Title: Selective Oxidation of Allylic Alcohols on Palladium Catalysts.
Author: Hackett, Simon F. J.
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2008
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Abstract:
This thesis concerns the selective aerobic oxidation of allylic alcohols over practical and model palladium c~alysts. The goal was to elucidate the nature of the active catalyst site and origin of deactivation, which have hindered industrial implementation of such systems. It is hoped that understanding such fundamental aspects of the catalytic process will help optimise future· catalysts exhibiting high turnover frequencies as well as longer lifetimes on-stream. To realise these goals wet-chemical inorganic materials synthesis has been married with Ultra High Vacuum (UHV) single crystal methodologies. A series of amorphous, y-alumina supported palladium catalysts were prepared containing variable Pd concentrations. These allowed the impact of particle size on their resultant catalysis to be studied systematically. Characterising these small metal particles proved challenging, necessitating the combination of powerful bulk and surface sensitive techniques including X-ray Absorption (XAS) and Xray Photoelectron Spectroscopy (XPS) to identify the nature/structure ofthe active phase. These measurements revealed a progressive transformation from large metallic Pd clusters to small oxidised palladium clusters with decreasing precious metal loading. The concentration of surface palladium oxide correlated with the Turnover Frequency (TOF) towards the selective oxidation of both crotyl and cinnamyl alcohols. Surfactant templating routes were subsequently employed to produce high . surface, area mesoporous alumina supports, with a view to boosting the palladium dispersion and proportion of active metal oxide. Bulk and surface spectroscopies revealed similar trends in Pd cluster size and oxidation state with loading as observed for the amorphous supports, albeit with smaller more heavily oxidised palladium at the lowest loadings. High-resolution, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) revealed that mesoporous alumina was able to stabilize isolated Pd(II) atoms on its surface. Synergy between these atomically-dispersed reduced palladium centers, and improved reactant/product mass transport through the mesoporous structure resulted in extremely active catalysts for alcohol selective oxidation. Decreasing II' II J _ _~ ~ t. ', ...... '... - - - ....:-. .• the Pd concentration increased TOFs as observed for the amorphous alumina catalysts. The single-site catalyst possessed initial TOFs of4400 and 7080 hr-I for cinnamyl and crotyl alcohol oxidation to their respective aldehydes. In order to shed insight into/the un~erlying reactant/surface interaction and deactivation mechanism, UHV studies were subsequently conducted over Pd(lll) single crystal surfaces, to model the most stable (111) facets of larger metal clusters found in practical oxidation catalysts. The adsorption geometry and thermal chemistry of both benzene and bromobenzene were first studied to examine the interaction of the phenyl group in cinnamyl alcohol with Pt(111). Fast XPS and NEXAFS spectra shoW that both benzene and bromobenzene adsorb non-dissociatively, but whilst the former bonds parallel to the surface, bromobenzene shows a significant tilt angle of-60°. Thermal-desorption spectroscopy showed that 20 % of a saturated benzene monolayer desorbs intact in two states, with the remainder undergoing successive dehydrogenations depositing surface-bound hydrocarbon fragments. For bromobenzene, low temperature C-Br bond cleavage results in surface-bound -phenyl groups. A proportion ofthese dehydrogenate at 400 K, liberating hydrogen which subsequently reacts with bromine, or other surface bound phenyls, yielding HBr, benzene and H2. Approximately 30% of a bromobenzene monolayer desorbs either reversibly or as reactively-formed hydrocarbons. The chemistry of crotyl alcohol was subsequently explored on clean and oxygen covered Pd (111) surfaces. Synchrotron X-ray experiments were 'combined with laboratory mass spectrometry to elucidate the associated binding geometry and reaction network. NEXAFS and Fast XPS revealed crotyl alcohol adsorbs molecularly at 95 K, with the allyl moiety parallel to the surface. Surface_ annealing induces oxidative dehydrogenation to crotonaldehyde at sub-ambient temperatures, in competition with a minor dehydration route to butene and water. Crucially, alcohol decarbonylation was also observed at catalytically relevant temperatures, resulting in an accumulation of surface bound CO and hydrocarbon fragments over the bare metal surface. Coadsorbed oxygen suppressed this decarbonylation pathway, promoting crotonaldehyde desorption over a similar temperature regime to that observed over dispersed PdfAh03 clusters.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of York, 2008 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.490272  DOI: Not available
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