Applied Catalysis B: Environmental 101 (2011) 638–648 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Catalytic and structural properties of co-precipitated Mg–Zr mixed oxides for furfural valorization via aqueous aldol condensation with acetone I. Sádaba, M. Ojeda, R. Mariscal, J.L.G. Fierro, M. López Granados ∗ Institute of Catalysis and Petrochemsitry (CSIC), C/ Marie Curie 2, Campus de Cantoblanco, 28049 Madrid, Spain article info Article history: Received 4 August 2010 Received in revised form 3 November 2010 Accepted 5 November 2010 Available online 16 November 2010 Keywords: Biomass Furfural Claisen-Schmidt condensation Biofuels Furfurylidene abstract A series of MgO–ZrO 2 mixed oxides with different nominal Mg/(Mg + Zr) atomic ratios (0, 0.1, 0.3, 0.5, 0.75, 0.85 0.92) were prepared by co-precipitation and calcined at 873 K. The catalytic activity in aldol condensation of furfural with acetone was tested in aqueous medium. These solids were systematically characterized by XRD, TEM, XPS, N 2 adsorption–desorption isotherms and thermal analysis. Mg x Zr 1-x O 2-x solid solution with cubic ZrO 2 structure was the only phase observed for the mixed solids with Mg/(Mg + Zr) atomic ratio ≤0.3, which had very low activity. Two different phases were observed in solids with a nominal Mg/(Mg + Zr) atomic ratio ≥0.5: cubic MgO and c-Mg x Zr 1-x O 2-x . Moreover, the relative concentration of the latter phase decreases as the Mg concentration increases. Zr 4+ cations are present at the surface of the c-MgO, the concentration of which is lower in the Mg-richer samples. No significant differences were found between the overall catalytic activity of these samples. Therefore, three different active surface sites can be found in these solids: Mg–O–Mg sites co-existing with Mg–O–Zr sites at the surface of c-MgO, and Mg–O–Zr sites at the surface of c-Mg x Zr 1-x O 2-x . The Mg–O–Zr sites on c-MgO are much more active than the other two sites, which show similar intrinsic activity. The overall activity of each catalyst depends on the amount of each of these surface sites and on their intrinsic activity. Reutilization tests and characterization of the used catalysts suggests that the most probable causes of deactivation are fouling and/or poisoning of the surface sites by furfural derived heavy compounds, leaching of Mg 2+ and Zr 4+ cations, or hydration of the Mg oxide. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The sustainable production of liquid fuels from renewable sources is a social, political and environmental priority. Biomass can be used as an abundant, worldwide, carbon-neutral renewable energy resource to produce biofuels and biomass derived chem- icals [1–4]. Indeed, the so-called first generation biofuels include bioethanol and biodiesel obtained from a variety of biomass sources (starch, sugars, animal fats and vegetable oils). However, the use of these biofuels presents important limitations and drawbacks and accordingly, their prices cannot currently compete with those of existing fossil fuels, and only a limited reduction of net carbon diox- ide emissions is being produced. Moreover, at present, extensive production of these biofuels cannot be achieved without threat- ening human and animal food supplies, and biodiversity. Second generation biofuels that are obtained from resources and land that is not used for food purposes may resolve some of these problems. In this case, large quantities of fuel can be obtained sustainably, affordably and with improved environmental benefits. In this con- ∗ Corresponding author. Tel.: +34 91 5854937; fax: +34 91 5854760. E-mail address: mlgranados@icp.csic.es (M.L. Granados). text lignocellulosic materials appear as to be an excellent candidate as raw material for second generation biorefineries. Furfural can be obtained from C 5 carbohydrates present in ligno- celluloses [5,6] and it possesses great potential as a key compound in the valorization of the hemicellulose contained in biomass when considering the development of a modern biorefinery. Indeed, furfural can be transformed into a variety of important indus- trial chemicals by hydrogenation, oxidation, reductive amination, decarbonylation, nitration, condensations, etc. [1]. In particular, this latter valorization route of furfural through aqueous aldol con- densation with acetone has recently been proposed by Dumesic and co-workers as an intermediate step to synthesize second genera- tion biofuels obtained from lignocellulosic biomass [7–13]. Aldol condensation reactions of furfural are also of interest to form furfurylidene ketone-based compounds, which may have appli- cations as aromas in the food industry [14] and as monomers in furan derived resins [15]. In this context the challenge is to find catalysts in aldol condensation reactions that are active in aqueous media. The development of green and sustainable cat- alytic processes in biorefineries requires the use of non-toxic and inexpensive solvents. Moreover, furfural–water solutions are much cheaper as no distillation is required to separate furfural from water. 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.11.005