Oxidation of Sugars and Polyols for Sustainable Production of Value-added Chemicals
University of Kansas
Chemical & Petroleum Engineering
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Biomass has attracted tremendous research attentions in the last few decades as one of the most promising renewable option to produce chemical products. To achieve efficiency and productivity similar to that demonstrated for the fossil-based routes, development of active and selective catalysts for biomass-to-chemicals conversion are highly demanded. In this dissertation, investigations on catalysis and kinetics of aqueous phase oxidation of ethylene glycol and glucose are presented as examples of biomass conversion to chemicals. Current state of development in these processes is reviewed in the Chapter 1 that demonstrates the research gap in designing a highly active oxidation catalyst for the selective production of carboxylic acid, especially under base-free conditions. The screening of monometallic catalysts for the aqueous phase oxidation of ethylene glycol to glycolic acid was investigated as presented in the Chapter 2. The study employs an integrated approach of combining activity tests, surface characterization, and computational modeling. The effects of alkali promotion and the role of O2/water on ethylene glycol oxidation in the aqueous phase are discussed. NaOH as a base alone was found to catalyze EG oxidation, as also acting as a promoter in the presence of metal catalysts. O2 was involved in producing OOH* species that reacts with deprotonated EG to form glycolic acid, while producing H2O2 in the aqueous phase as a byproduct. Based on computational modeling, predictions of a volcano-type correlation between adsorption energy and reaction activity for various metals are presented that show trends consistent with the experimental results. In the Chapter 3, experimental study on a highly active and selective bimetallic Pt-Fe alloy catalyst on CeO2 support is presented along with screening of various bimetallic catalysts for the aqueous phase oxidation of ethylene glycol. Selectivity with respect to glycolic acid as a product is discussed. The Pt-Fe nanoparticles are highly alloyed with a fcc type of crystal structure and a chemical state of Pt0/Fe0, as confirmed from XRD and EXAFS characterizations, respectively. Compared to the monometallic Pt catalyst prepared by an identical method, the Pt-Fe catalyst shows more than a 17-fold higher TOF achieving complete EG conversion in 4 hours at 70 oC and ambient O2 pressure under alkaline conditions, with a selectivity of glycolic acid up to 62% on a carbon basis. From parametric effect studies, it can be concluded that the synergistic bimetallic effect occurs due to significantly changing the O2 adsorption-dissociation characteristics on the catalyst surface. The addition of a base shows a promotional effect on both Pt and Pt-Fe catalysts at low NaOH concentrations but an inhibition effect on both catalysts is observed at sufficiently high NaOH concentrations. In contrast, HCO3- and CO32- ions were found to have an inhibition effect on the EG oxidation. Oxidation of glucose to gluconic and glucaric acid using molecular O2 in aqueous solution is an environmentally friendlier alternative to the conventional method that uses nitric acid as an oxidant. However, obtaining a satisfactory yield of the desirable product, glucaric acid, especially under base-free conditions, is still a challenge. In the Chapter 4, optimization of Pt based mono and bimetallic catalysts is reported by tuning four factors: support type, synthesis method, reductant used in synthesis, and choice of the second metal. All these four factors influence the glucaric acid selectivity. Among the tested combinations, the Pt-Cu/TiO2 bimetallic catalyst showed ~60% glucaric acid selectivity in one-step glucose oxidation under base-free conditions at 90 oC and 15 bar O2. The catalyst consists of Pt metal particles (~2.8 nm diameter on average) with a dominant presence of the alloyed Pt-Cu phase, as confirmed by XRD and TEM analyses. These results provide valuable insights for rational design of glucose oxidation catalysts. It is of great interest to study the shape dependency on the catalytic activity of Pt catalysts in oxidation reactions. In the Chapter 5, one-pot synthesis strategy of supported Pt catalyst is discussed for synthesis of monodispersed nanocrystals enclosed by single lattice planes. The Pt and Pt-Cu nanocrystals with cubic, octahedral, and stellar morphologies were successfully synthesized, which were evaluated for their performance in glucose oxidation. It was found that the crystal facets of Pt-Cu nanoparticles play an important role in determining the catalytic activity and selectivity in the glucaric acid production. In addition, significant bimetallic effect was demonstrated an enhancement in the selective formation of glucaric acid (TOF increased for about 10 times regarding to glucaric acid production).
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