Integration of C–C coupling reactions of biomass-derived oxygenates to fuel-grade compounds Elif I. Gu ¨ rbu ¨ z, Edward L. Kunkes, James A. Dumesic * Department of Chemical and Biological Engineering, University of Wisconsin, Madison, WI 53706, USA 1. Introduction Concerns about diminishing reserves of petroleum resources and issues related to climatic consequences of the accumulation of CO 2 in the atmosphere have prompted the development of renewable and carbon-neutral alternative sources of energy, such as biomass. Carbohydrates derived from biomass can be converted to fuel-grade products such as light alkanes (C 1 –C 4 ), branched alkanes (C 6 –C 12 ), linear or singly branched alkanes (C 7 –C 12 ), benzene and substituted aromatics by a series of catalytic processes demonstrated in our recent work [1]. These conversion processes are initiated by de-oxygenation of sugars or polyols over a Pt-Re bi-metallic catalyst supported on carbon to obtain a hydrophobic mixture of mono-functional species, such as car- boxylic acids, ketones, alcohols and heterocycles with carbon numbers ranging from 4 to 6. The maximum carbon chain length of these species is limited to the carbon number of the sugar or polyol feed. However, higher molecular weight alkanes are required for gasoline, diesel and jet fuel, and for this purpose the hydrophobic mixture has to be subjected to C–C coupling reactions [1]. Two important coupling reactions for the upgrading of mono- functional oxygenated compounds for transportation fuel applica- tions are ketonization and aldol condensation/hydrogenation. In ketonization reactions, two carboxylic acid molecules combine to form a higher molecular weight linear ketone, CO 2 and water [2], whereas in aldol condensation/hydrogenation reactions, two ketone or secondary alcohol molecules couple to form a heavier branched ketone [3]. These ketones obtained can then be converted into corresponding fuel-grade alkanes by dehydra- tion/hydrogenation over bi-functional solid-acid supported noble metal catalysts such as Pt/NbOPO 4 [4]. It was found in our previous work that ketonization must be performed prior to aldol condensation/hydrogenation, because the active basic sites in the aldol condensation catalyst are poisoned by the presence of carboxylic acids [1,5]. Carboxylic acids in the hydrophobic mixture can be ketonized with nearly 100% yield over a Ce 1 Zr 1 O x mixed- oxide catalyst at temperatures from 623 to 673 K [1]. The aldol condensation/hydrogenation was carried out over Pd/Ce 1 Zr 1 O x with H 2 co-feed. Metal catalyzed hydrogenation of the unsaturated dehydrated aldol condensation product is required to overcome the equilibrium limitation of aldol condensation reaction and obtain high yields [3]. In addition, metal functionality is needed to catalyze the dehydrogenation reaction of alcohols to form ketones for coupling reactions [6]. In a recent work, we have suggested that ketonization and aldol condensation/hydrogenation reactions could be integrated in a single reactor with a double bed system, because the reaction conditions for both reactions are similar. Integrating the ketoniza- Applied Catalysis B: Environmental 94 (2010) 134–141 ARTICLE INFO Article history: Received 26 August 2009 Received in revised form 22 October 2009 Accepted 2 November 2009 Available online 10 November 2009 Keywords: Aldol condensation Ceria Zirconia CO 2 inhibition Water inhibition ABSTRACT Ceria-zirconia mixed oxides with different compositions including pure ceria and pure zirconia were prepared and characterized using temperature-programmed desorption (TPD) of CO 2 and NH 3 , X-ray diffraction (XRD), and BET surface area measurements. Bi-functional catalysts for C–C coupling of ketones by aldol condensation/hydrogenation were prepared by depositing palladium on these ceria- zirconia mixed oxides, and these catalysts were studied for the conversion of 2-hexanone, a representative ketone that can be derived from sugars in biomass. The Pd/ZrO 2 catalyst showed the best resistance to inhibition by CO 2 , an important factor in catalyst performance because of the presence of CO 2 in biomass-derived feed streams. Furthermore, this catalyst displayed high activity for aldol condensation, as well as good resistance to inhibition by water. These properties make Pd/ZrO 2 a desirable catalyst for integration in a single reactor of aldol condensation/hydrogenation reactions with ketonization processes, the latter of which convert carboxylic acids to ketones plus CO 2 and H 2 O. The feasibility of this integration was studied with the mixture of a carboxylic acid (butanoic acid) and a ketone (2-hexanone) in a double bed system, and the integrated process showed high activity as well as selectivity to C–C coupling products. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 608 2621095; fax: +1 608 2625434. E-mail address: dumesic@engr.wisc.edu (J.A. Dumesic). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.11.001