Date of Award
Doctor of Philosophy
Chemical and Biological Engineering
Brent H Shanks
Development of a renewable liquid transportation fuel is likely to be one of the most important challenges faced by scientists during the 21st century. As biomass provides a renewable source of carbon it is ideally situated to supply this alternative to the traditional petroleum derived feedstocks. While there have been a number of different techniques used to convert biomass to liquid fuels, fast pyrolysis is particularly promising as it can quite efficiently break down biomass directly into a liquid. This resulting liquid, called bio-oil, is a very complex mixture containing a large number of oxygen functionalized compounds. Unfortunately, this oil has a number of issues that must be resolved before it can be effectively utilized as a liquid transportation fuel including acidity, reactivity, and low energy density. With this in mind, heterogeneously catalyzed C-C bond forming reactions potentially valuable for the upgrading of bio-oil were investigated.
The aldol condensation is a well known reaction in organic chemistry usually promoted through the use of strong acid or bases. However, uses of these types of catalysts will likely cause undesirable side reactions. Ideally cooperative catalysis allows for weaker acids and bases to work in tandem to promote the reaction. Use of aluminum phosphate catalysts allowed for the tuning of the acidity and basicity of the materials through a nitridation process and hence probing of this cooperative catalysis. Through performing aldol condensations using model bio-oil compounds acetaldehyde, acetone, and MEK, it was found that acid and base sites were both needed to efficiently promote the cross condensation of the aldehyde and ketone. After reaction testing, a mechanism was proposed demonstrating the benefits of using heterogeneous catalysts as it allows for the coexistence of both acid and base sites.
Ketonization of carboxylic acids is also an ideal reaction for bio-oil upgrading as it removes acidity and oxygen as well as creates C-C bonds. However, this reaction is almost always performed in the vapor phase due to the high temperatures necessary to achieve significant conversions. In order to try to engineer a more active catalyst able to perform the reaction at lower temperatures, more must be understood about ketonization. Condensed phase ketonization was examined using ceria catalysts calcined at different temperatures. It was found that the reaction proceeded either through the formation of carboxylates in the bulk or on the surface of the catalyst depending on the temperature of calcination. Moreover, through in-situ XRD, this trend was found to be true in the vapor phase as well. Kinetic studies found that the mechanism for both these routes was likely the same.
As ketonization had been claimed to be sensitive to the surface structure of the ceria catalyst, shape selective ceria nanocrystals were synthesized and examined in acetic acid ketonization both in the vapor and condensed phases. It was found that in the condensed phase the catalysts underwent carboxylate formation in the bulk thus changing the crystal structure of the materials. However, in the vapor phase this did not occur but a clear trend with surface structure was not determined. Thus it is likely the surface structure of the ceria catalysts isn't of large influence in realistic ketonization conditions. Reaction condition influences were probed as well. It was found that the temperature of ketonization greatly influenced the reaction pathway with intermediate temperature reactions resulting in metal carboxylate formation in the bulk and high temperatures promoting the reaction on the surface. Discussion of these temperature regimes and a more detailed proposed mechanism are delivered.
Lastly, ketonization using mixed metal oxides was studied. It was found that mixing of ceria with another oxide greatly changed the catalyst properties. Coupled with reaction testing, experiments determined that metal carboxylate formation and decomposition are of supreme importance for ketonization and are influenced by mixing of oxides. Along with the work using pure ceria catalysts, this research into ketonization is a significant step forward into understanding of the reaction and how it can be applied to the upgrading of fast pyrolysis bio-oil.
Ryan William Snell
Snell, Ryan William, "Carbon-carbon bond forming reactions for bio-oil upgrading: heterogeneous catalyst and model compound studies" (2012). Graduate Theses and Dissertations. 12467.