Selective Oxidation of Biomass-Derived Chemicals

Abstract

Due to depleting fossil resources, alternative feedstock for obtaining chemicals is of great importance. Biomass provides a promising alternative renewable carbon source; however, new sustainable processes for conversion of biomass are required to sufficient implementation into the industry. These processes should be able to compete with the established processes based on fossil resources. Glycolaldehyde is an often-observed by-product formed from degradation of larger sugars. Due to competing ecological and economical aspects of the well-established processes for extraction and conversion to chemicals from fossil resources, limited amounts of waste chemicals i.e. by-products are allowed in the processes of conversion of biomass and biomass-derived compounds. It is therefore important to develop methods for conversion of these simple by-products that are environmentally benign and of low cost. The objective of this dissertation was to develop new, alternative and sustainable methods for oxidative catalytic upgrading of biomass-derived compounds, with focus on oxidation of glycolaldehyde and simple alcohols as model substrates for larger sugars. Supported gold nanoparticle were studied for the selective oxidation of glycolaldehyde to glycolic acid, which has found applications in various industries. Limitations by competing reactions and catalyst deactivation was observed, affording up to 68% of glycolic acid at mild and aqueous conditions. The green oxidant, molecular oxygen, was applied for these oxidations and the reaction took place under base-free conditions. Oxidation of glycolaldehyde was further studied in the formation of formic acid. Efficient release of hydrogen from formic acid has proven formic acid a viable precursor for hydrogen, facilitating safe transportation and storage. Hydrogen has found application in various industries including energy conversion by fuel cells. High yields of formic acid was obtained from oxidation of glycolaldehyde over a supported ruthenium catalyst. The effect of the size of the support was studied, however, minor differences was observed. A slightly better yield of formic acid was obtained from nanoparticulate ceria. Vanadium-substituted Keggin polyoxometalates have proven efficient catalysts for conversion of various biomass compounds into formic acid. The Wells-Dawson-type heteropolyoxometalates are less thoroughly studied for biomassviconversion. Tungsten-based Wells-Dawson heteropolyoxometalates were synthesized and examined for the oxidation of alcohols and sucrose. However, successful oxidations of the alcohols and sucrose were not observed. The search for application of the synthesized Wells-Dawson heteropolyoxometalate is ongoing in collaboration with the group of Prof. Wasserscheid at FAU in Germany. It was shown, that glycolaldehyde can be selectively converted into glycolic acid or formic acid, depending on the catalyst applied, under mild, aqueous and base-free conditions. High product stability in the ceria-supported ruthenium hydroxide catalyzed oxidation to formic acid, together with the good catalyst activity observed when reusing the catalysts, rendered the developed procedure for formic acid production highly applicable for industrial use. Furthermore, the synthesized Wells-Dawson polyoxometalate catalysts afforded low catalytic oxidative activity.