Date of Award
Doctor of Philosophy (PhD)
Chemical & Biochemical Engineering
J. Will Medlin
John L. Falconer
Douglas L. Gin
Joel L. Kaar
Conrad R. Stoldt
Oxidation of alcohols and aldehydes is one route to value-added chemicals from biomass. Because catalytic oxidation reactions on transition metals usually involve participation of oxygen, it is important to understand oxygen’s surface-level effects on these complex reactions. Surface investigations of the thermal oxidation chemistry of a range of multifunctional alcohols on quarter-monolayer oxygen-precovered Pd(111) (O/Pd(111)) is presented here.
The surface chemistry of 1,2-propanediol (PDO) on O/Pd(111) showed a strong dependency on relative concentration of PDO and O(a) (adsorbed atomic oxygen). When the concentration of O(a) is large compared to the dialkoxide produced following PDO adsorption (-OCH(CH3)CH2O-), PDO completely decomposes. Conversely, when the concentration of O(a) is small, the co-adsorbed oxygen increases the overall coverage of PDO that can undergo decomposition, which in turn strongly influences decomposition activation barriers and product distributions.
To investigate how surface oxygen participates in the reaction of important aromatic oxygenates, the surface chemistry of benzyl alcohol (PhCH2OH), benzaldehyde (PhCHO), furfuryl alcohol (C4H3OCH2OH) and furfural (C4H3OCHO) has been studied on 18O/Pd(111). In the presence of surface O, some benzaldehyde and furfural desorbed from a weakly bound ɳ1(O) aldehyde state. Benzaldehyde and furfural also react with surface oxygen to produce benzoate (PhCOO-) and furoate (C4H3OCOO-), respectively. The benzylic reactions revealed exchange occurring between surface O and the benzaldehyde and benzoate intermediates. This exchange has not been reported for other alcohols, suggesting that aromatic binding effects strongly influenced alcohol oxidation on Pd. At high coverages, deoxygenation of furfuryl alcohol to methylfuran (C4H3CH3) and benzyl alcohol to toluene (C6H5CH3) was observed simultaneously with the relevant aldehyde production, possibly through a disproportionation reaction of two alcohol molecules. Surface reactions of furan, furfural, and furfuryl alcohol on O/Pd(111) suggest the presence of surface oxygen resulted in a new, higher-temperature desorption channel for furan; this channel coincided with desorption of partial oxidation products, 2(5H)- furanone and maleic anhydride. Furfural may be decarbonylated to furan and subsequent ring opening of furan (C4H4O) through O-Cα scission was observed. Isotopic product distributions suggest that partial oxidation of furan occurs through a ring-opened carboxylate.
Williams, Rhea Marie, "Determining Oxygen’s Effects on Multifunctional Alcohol Decomposition on Pd(111)" (2014). Chemical & Biological Engineering Graduate Theses & Dissertations. 66.