Industrial Applications Research Frontiers 2018 98 Structural change of cerium oxide-supported ruthenium catalyst during the biorefinery of platform chemicals Current requirements to reduce carbon dioxide emissions have been the driving force for biorefinery utilizing renewable resources, such as plant biomass, as carbon-neutral feedstocks for commodity chemicals [1]. To utilize biomass feedstocks in place of fossil resources, the development of highly efficient catalytic methods is strongly desired [2]. To date, much effort has been devoted to the direct carbon–oxygen (C– O) bond cleavage of high-oxygen-containing biogenic polyols to produce valuable chemicals by hydrogenolysis and deoxydehydration. For example, there have been many attempts to achieve the selective hydrogenolysis of glycerol, which is readily obtained from fats and oils, to 1,2-propanediol and 1,3-propanediol as valuable polyester monomers and solvents using copper- and platinum-based heterogeneous catalysts, respectively. On the other hand, selective cleavage of carbon–carbon (C–C) bonds has not yet been widely researched despite its great potential for extending the utility of biomass- derived oxygenates to obtain the desired carbon chain length. However, these reactions often suffer from low selectivity toward the desired chemicals, limited substrate scope, and high reaction temperatures. Therefore, the development of selective and versatile C–C bond scission catalysts that can work under milder conditions is highly desired to open new routes for industrially important chemicals from a wide range of biomass derivatives. Recently, we have found that cerium oxide- supported ruthenium (Ru/CeO 2 ) nanoparticles efficiently promoted the selective C–C bond scission of levulinic acid to 2-butanol in water [3]. Levulinic acid is one of the versatile platform chemicals obtained from biomass. There are many reports on the catalytic transformation of levulinic acid to valuable C5 chemicals, e.g., g -valerolactone, 1,4-pentanediol, and 2-methyltetrahydrofuran. However, C–C bond scission reactions into valuable C4 chemicals such as 2-butanol have seldom been reported. Furthermore, Ru/CeO 2 showed high generality for the cleavage of C–C bonds of oxygenated compounds. The results provide a simple and more environmentally friendly method for the refinery of platform chemicals from biomass that are currently produced from petroleum feedstock. The reaction of levulinic acid to 2-butanol was conducted using Ru/CeO 2 in water at 423 K under 3 MPa of H 2 . To clarify the origin of the novel catalysis of Ru/CeO 2 , several physicochemical analyses such as electron microscopy, X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS) were carried out. The XAS experiments were conducted at SPring-8 BL14B2 and BL01B1 . The morphology of Ru/CeO 2 was determined by field-emission scanning electron microscopy (FE-SEM) analysis, which clearly showed the drastic change in the morphology after the catalytic reaction. The fresh Ru/CeO 2 nanoparticles (Ru/CeO 2 _ fresh) of 3–5 m m diameters were converted into an agglomeration of rodlike structures (Fig. 1(a)) on the used Ru/CeO 2 (Ru/CeO 2 _ used). From the high-resolution TEM images of Ru/ CeO 2 _ used, Ru nanoparticles of ca. 3 nm diameter were observed on CeO 2 nanorods (Figs. 1(b,c)). The nanorods consist of fine crystalline nanowires having a line spacing of 0.27 nm, which is consistent with the {002} reflection plane of CeO 2 . In situ XRD analysis of Ru/CeO 2 catalysts showed that the pristine CeO 2 support was transformed into Ce(OH) 3 (Fig. 2). No diffraction peaks derived from crystalline Ru oxide and Ru metal were observed in the fresh and used catalysts, suggesting the high dispersity of Ru species, respectively. The diffraction pattern of the Ru/CeO 2 _ used catalyst without exposure to air was similar to that of Ce(OH) 3 Fig. 1. FE-SEM image of Ru/CeO 2 _used (a) and TEM images of Ru/CeO 2 _used (b) and (c) . (a) (b) (c) Research Frontiers 2018 99 (Figs. 2(d) and 2(e)), indicating reduction and a phase transition from CeO 2 to Ce(OH) 3 . After exposure to air, the Ce(OH) 3 was oxidized back to CeO 2 (Fig. 2(c)). Ru K -edge XAS analysis revealed that the Ru oxide in the fresh catalyst was reduced under the operating conditions to afford metallic Ru species (Fig. 3(a) ). FT-EXAFS and curve-fitting analysis of the peak at approximately 0.22 nm for the Ru-Ru shell revealed a Ru-Ru atomic distance of 0.267 nm and a coordination number of 8.6, indicating the formation of Ru nanoparticles with a mean diameter of ca. 3 nm [4]. The estimated particle size agreed well with that obtained from TEM observation (Figs. 3(b) and 1(b)). Ce L 3 -edge XANES analysis also confirmed the complete reduction of Ce 4+ to Ce 3+ species under the reaction conditions (Fig. 3(c)). After exposing the used Ru/CeO 2 to air, the Ce(OH) 3 nanorods were easily oxidized to CeO 2 nanorods [5]. On the other hand, neither the Ru/CeO 2 catalysts used in organic solvents nor the CeO 2 sample without the use of Ru suggested that CeO 2 was reduced to Ce(OH) 3 during the reaction. Considering the above results, we proposed the following reaction mechanism. A particular type of CeO 2 was reduced to Ce(OH) 3 nanorods under H 2 pressure in water at 423 K. The RuNPs formed on the cerium nanorods activate molecular hydrogen, and the spillover hydrogen and water promote the reduction of Ce 4+ to Ce 3+ to form Ce(OH) 3 nanorods. Water is necessary to promote the reduction of Ce species under the relatively mild conditions. The selective C–C bond cleavage was promoted by the concerted effect of the RuNPs with the basic Ce(OH) 3 nanorods. Tomoo Mizugaki Graduate School of Engineering Science, Osaka University Email: mizugaki@cheng.es.osaka-u.ac.jp Fig. 3. Ru K -edge XANES (a) and FT of Ru K -edge EXAFS (b) of Ru/CeO 2 catalysts before and after catalytic reactions (H 2 3 MPa, 423 K in water). Ce L 3 -edge XANES of Ru/CeO 2 catalyst (c) . (acac = acetylacetonate) Fig. 2. XRD patterns of (a) pristine CeO 2 , (b) Ru/CeO 2 _ fresh, (c) Ru/CeO 2 _ used after exposure to air and (d) Ce(OH) 3 and (e) Ru/C CeO 2 _ used without exposure to air. References [1] J.J. Bozell, G.R. Petersen: Green Chem. 12 (2010) 539. [2] M.L. Besson et al. : Chem. Rev. 114 (2014) 1827. [3] T. Mizugaki, K. Togo, Z. Maeno, T. Mitsudome, K. Jitsukawa, K. Kaneda: Sci. Rep. 7 (2017) 14007 . [4] L.S.R. Kumara et al .: Phys. Chem. Chem. Phys. 18 (2016) 30622. [5] C.C. Tang et al. : Adv. Mater. 17 (2005) 3005. (a) Intensity (arb. units) 2 θ (degree) (b) (c) (d) (e) 10 111 200 220 311 112 002 210 201 101 110 100 20 30 40 50 60 22070 22090 22110 22130 22150 22170 RuO 2 Ru/CeO 2 used Ru/CeO 2 fresh Ru 0 powder 0 1 2 3 4 5 6 Ru(acac) 3 Ru-O Ru-Ru ⎜ FT ⎜ Normalized Absorption (arb. units) Normalized Absorption (arb. units) 5700 5720 5740 5760 (a) (b) (c) RuO 2 Ru/CeO 2 used Ru/CeO 2 fresh Ru 0 powder Ru/CeO 2 used Ru/CeO 2 fresh Ce 2 (CO 3 ) 3 CeO 2 Energy (eV) Distance (Å) Energy (eV)
home
- JP
- EN