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Experimental and computational studies of aqueous-phase hydrogenation of biorenewable aromatic chemicals

Jin Yang, University of Massachusetts Amherst

Abstract

Production of basic chemical feedstocks from biomass, as opposed to petroleum, can contribute significantly towards a sustainable future. A promising approach for obtaining aromatic chemicals from biomass is through a hybrid process that combines the high selectivity of biological reactions with the high reactivity of thermochemical reactions. One such process of interest starts with glucose, which can be enzymatically hydrolyzed with high selectivity to a "bridge molecule", myo-inositol. Myo-inositol can subsequently be dehydrated via conventional chemical catalysis to highly oxygenated aromatics, such as phloroglucinol. This thesis focuses on the downstream catalytic processing (hydrogenation/hydrodeoxygenation) of phloroglucinol to produce higher value chemicals, such as phenol or resorcinol. Noble metals, such as platinum and palladium, are widely used as hydrogenation catalysts. Using a platinum catalyst, we find that the first step in upgrading phloroglucinol is hydrodeoxygenation to resorcinol and phenol, followed by hydrogenation of the aromatic ring to produce 1,3-cyclohexanediol, cyclohexanone, and cyclohexanol. Carbon-carbon σ—bond cleavage is not observed. Complementary ab initio Molecular Dynamics calculations in the aqueous phase confirm that hydrodeoxygenation is thermodynamically preferred over direct hydrogenation of the aromatic ring due to the steric hindrance caused by hydrogen bonding between phloroglucinol and surrounding water molecules. A more detailed study of aqueous phase hydrogenation of phenol--a product of the hydrogenation of phloroglucinol--is undertaken to examine the role of solvent on reaction thermodynamics and kinetics. Density Functional Theory (DFT) calculations are performed to elucidate all the intermediate steps along the reaction pathway that are normally inaccessible in experiments. Transition state calculations of two key reaction steps are undertaken and the activation barriers are found to be in good agreement with our experimental data. The solvent, in particular, is found to significantly affect the reaction heat of these two key steps. By combing our modeling and experimental results, we show that the solvent plays a critical role in aqueous phase hydrogenation reactions.^

Subject Area

Organic chemistry|Chemical engineering

Recommended Citation

Yang, Jin, "Experimental and computational studies of aqueous-phase hydrogenation of biorenewable aromatic chemicals" (2013). Doctoral Dissertations Available from Proquest. AAI3603177.
http://scholarworks.umass.edu/dissertations/AAI3603177

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