Catalysts are of incredible importance to maintaining and advancing our society, being used in the production of fuels, plastics, and a number of high-value chemicals. The study of these materials and their properties is therefore of great interest, so that better understanding can inform the design of novel high-performance catalysts. Reactants containing multiple functional groups, such as those derived from biomass, provide a substantial challenge due to the multiple reactions that can occur, often resulting in low product selectivity. One method of improving the selectivity of heterogeneous catalysts is to modify the surface with organic ligands. In this thesis, we employ phosphonic acids as surface modifiers on supported noble metal catalysts, and investigate the role of these modifiers in enhancing rates of deoxygenation and hydrogenation chemistries.
First, phosphonic acids having either alkyl or carboxylic acid tail groups were deposited onto a commercial Pd/Al2O3 catalyst for the deoxygenation of different biomass-derived oxygenates. The deoxygenation of these compounds is important for the upgrading of lignocellulosic bio-oil to renewable, combustible fuels. Catalysts were characterized by Fourier-transform infrared spectroscopy, which confirmed the presence of the modifiers on the surface, and indicated that all modifiers bind through the phosphonic acid group, with the alkyl or carboxylic acid tails remaining suspended above the surface. Spectra were also collected following the adsorption of pyridine, which confirmed the hypothesis that phosphonic acid modification creates surface Brønsted acid sites. Reactor studies showed that all modifiers enhanced deoxygenation rates, though the extent to which the rates improved was found to be dependent on both the tail moiety, and the chain length. A combination of temperature-dependent pyridine adsorption experiments, along with density functional theory calculations, revealed that the highest rates occurred on the catalysts having the strongest acid sites. Furthermore, geometry optimizations showed that the strong acid sites formed by the short carboxylic acid tails were the result of intramolecular stabilization of the deprotonated acid site. An unexpected result from this study is that the modifiers were present not only on the catalyst support, but on the metal surface as well. Control experiments confirmed that deoxygenation was enhanced at the metal-support interface, while modifiers present on the Pd surface appeared to assist in ring hydrogenation.
The applicability of phosphonic acid modifiers was explored by examining the effect of the catalyst support. In this next study, Pt catalysts were prepared by incipient-wetness impregnation onto Al2O3, TiO2, CeO2, and SiO2-Al2O3 supports. Phosphonic acids containing either alkyl or carboxylic acid tails were then used to modify each of the supported Pt catalysts. Modification improved deoxygenation rates for the model compound benzyl alcohol when applied to Pt supported by Al2O3, TiO2, and CeO2. For the Pt/SiO2-Al2O3 support, phosphonic acid modification resulted in decreased rates of deoxygenation, but higher toluene selectivities. The relative strength of Brønsted acid sites were measured on the modified and unmodified catalysts, which suggested that modification only improves deoxygenation rates when providing new or stronger Brønsted acid sites.
In the third study, focus was returned to the Pd/Al2O3 system to investigate the role of phosphonic acids in enhancing ring hydrogenation. To determine if the observed increase in rate was simply due to the presence of modifiers on the Pd surface or a more specific interaction, both thiol and phosphonic acid modifiers were studied. For each type of modifier, two tail structures were used to achieve comparably low and high coverages of the modifiers on the Pd surface. When used for the ring hydrogenation of 2-methyl furan, all modifiers were found to decrease the rate, and the loss of activity was correlated to the apparent modifier coverage, as measured by relative rates of H2/D2 exchange. When using a polar reactant, furfuryl alcohol, thiols again suppressed ring hydrogenation but phosphonic acids were found to enhance the rate. Density functional theory calculations showed that this result can likely be attributed to a surface hydrogen bonding interaction between modifier and reactant, present when the reactant contains an appropriate substituent group, as with furfuryl alcohol. This result indicates that the nature of the phosphonic acid binding group may be a useful tool in controlling surface chemistry beyond the creation of new active sites. More broadly, this work shows that surface modifiers can be designed to introduce desirable intermolecular surface interactions, providing an additional degree of freedom for directing catalyst reactivity.
|Advisor:||Medlin, J. Will|
|Commitee:||Griffin, Michael B., Schwartz, Daniel K., Musgrave, Charles B., Bruns, Carson J.|
|School:||University of Colorado at Boulder|
|Department:||Chemical and Biological Engineering|
|School Location:||United States -- Colorado|
|Source:||DAI-B 82/3(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Organic chemistry, Chemistry|
|Keywords:||Catalyst Modification, Deoxygenation, Heterogeneous Catalysis, Phosphonic Acids|
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