Catalytic Production and Upgrading of Biomass Derived Monofunctional Hydrocarbons

Abstract

We have studied the deoxygenation/ reforming of biomass derived carbohydrates to yield monofunctional hydrocarbons that can be utilized as fuels, commodity chemicals and solvents. Additionally, we have developed C-C coupling processes to upgrade these monofunctional species into fuel grade components that can be utilized in the current transportation infrastructure. We have demonstrated the catalytic conversion of biomass-derived carbohydrates, including monosaccharides and sugar alcohols, into hydrophobic mixtures of monofunctional C4-C6 hydrocarbon species containing alcohols, ketones, heterocyclic compounds and carboxylic acids. The conversion occurs over carbon supported Pt-Re catalysts and removes more than 80% of the initial oxygen content of the sugars and polyols, yielding a spontaneously-separating organic phase. This process operates at moderate pressures (20-30 bar) and temperatures (283-523 K) and utilizes highly concentrated aqueous feeds (40-60%) of sorbitol or glucose. At 503 K and 18 bar, Pt-Re/C showed excellent stability for longer than one month time-on-stream and yielded an organic stream containing ~ 50% of the carbon found in the 60 wt% sorbitol feed. A yield of 70% of the maximum possible conversion of the carbon in sorbitol to monofunctional species was obtained, corresponding to the production of 1 kg of organic for every 4 kg of sorbitol. Furthermore, we have studied catalytic C-C coupling processes to convert functional species (carboxylic acids, alcohols and ketones ) derived from carbohydrate conversion into C7-C12 ketones, that can be converted into diesel grade alkanes via deoxygenation over solid acid supported metal catalysts such as Pt/NbOPO4. These processes include ketonization in which two carboxylic acid molecules combine to form a linear ketone, CO2 and water, and aldol condensation/hydrogenation in which two ketone or secondary alcohol molecules combine to form a singly-branched ketone. We have studied ketonization of the aforementioned carbohydrate derived organic species over a CeZrOx catalyst at 648-673K and found near 100% conversion of carboxylic acids into C7-C11 linear ketones. The aldol condensation/hydrogenation process occurs on bi-functional catalysts that contain acid/basic functionality as well as metal sites to dissociate hydrogen. Aldol condensation/hydrogenation was studied over a low loading (0.25 wt%) Pd/CeZrOx catalyst at 623K, and was found to convert ~60% of the condensable ketones and alcohols found in the aforementioned carbohydrate-derived mixtures into C8-C12 branched ketones. Further investigation of the aldol condensation/hydrogenation reaction was performed by examining the reactivity of a representative ketone - 2-hexanone over Pd/CeZrOx and CeZrOx catalysts at temperatures between 573 and 673 K, and pressures of 5 to 26 bar. Reaction kinetics studies show that in addition to the expected C12 condensation product (7-methyl-5-undecaone), the CeZrOxbased catalysts produce C18 and C9 secondary species, along with light alkanes (<C7). Low loadings of Pd (e.g., 0.25 wt. %) lead to optimal activity and selectivity for the production of C12 species. The high activation energy of C9 formation (140 kJ/mol) compared to the formation of C12 and C18 species (15 and 28 kJ/mol, respectively) indicate that these species may be formed as a result of the decomposition of heavier condensation products. The self-coupling of 2-hexanone was found to be positive order in both 2-hexanone and hydrogen. The addition of primary alcohols and carboxylic acids as well as water and CO2 to the feed was found to reversibly inhibit the self-coupling activity of 2-hexanone.