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Green Chemistry This document is the Accepted Manuscript version of a Published Work that appeared in final form in ChemSoc.Rev, copyright © The Royal Society of Chemistry 2017 aGer peer review and technical ediIng by the publisher. To access the final edited and published work see: hKps://pubs.rsc.org/en/content/arIclelanding/2017/gc/c6gc02586b#!divAbstract Green Chemistry View Article Online PAPER View Journal | View Issue Interfacial acidity in ligand-modified ruthenium nanoparticles boosts the hydrogenation of Cite this: Green Chem.,2017,19, 2361 levulinic acid to gamma-valerolactone† Davide Albani,‡a Qiang Li,‡b Gianvito Vilé,a Sharon Mitchell,a Neyvis Almora-Barrios,b Peter T. Witte,c Núria López*b and Javier Pérez-Ramírez*a Gamma-valerolactone (GVL), a versatile renewable compound listed among the top 10 most promising platform chemicals by the US Department of Energy, is produced via hydrogenation of levulinic acid (LA). The traditional high-loading ruthenium-on-carbon catalyst (5 wt% Ru) employed for this transformation suffers from low metal utilisation and poor resistance to deactivation due to the formation of RuOx species. Aiming at an improved catalyst design, we have prepared ruthenium nanoparticles modified with the water-soluble hexadecyl(2-hydroxyethyl)dimethylammonium dihydrogen phosphate (HHDMA) ligand and supported on TiSi2O6. The hybrid catalyst has been characterised by ICP-OES, elemental analysis, 31 13 TGA, DRIFTS, H2-TPR, STEM, EDX, P and C MAS-NMR, and XPS. When evaluated in the continuous- flow hydrogenation of LA, the Ru-HHDMA/TiSi2O6 catalyst (0.24 wt% Ru) displays a fourfold higher reac- tion rate than the state-of-the-art Ru/C catalyst, while maintaining 100% selectivity to GVL and no sign of deactivation after 15 hours on stream. An in-depth molecular analysis by Density Functional Theory demonstrates that the intrinsic acidic properties at the ligand–metal interface under reaction conditions ensure that the less energy demanding path is followed. The reaction does not obey the expected Received 15th September 2016, cascade mechanism and intercalates hydrogenation steps, hydroxyl/water eliminations, and ring closings Accepted 3rd October 2016 to ensure high selectivity. Moreover, the interfacial acidity increases the robustness of the material against DOI: 10.1039/c6gc02586b ruthenium oxide formation. These results provide valuable improvements for the sustainable production rsc.li/greenchem of GLV and insights for the rationalisation of the exceptional selectivity of Ru-based catalysts. Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili 4/12/2019 2:14:54 PM. Introduction as an important bio-derived platform chemical.4–8 This com- pound can be used for the manufacture of a number of impor- The necessity of more sustainable products and processes has tant commodities such as transportation fuels, polymer inter- led to the use of new methodologies which result in a low mediates, pharmaceuticals, food ingredients, or it can be carbon footprint. This implies, in line with the 12 principles of directly applied as a renewable organic solvent. Gamma-valero- green chemistry,1,2 the utilisation of biomass-derived reactive lactone is synthesised from levulinic acid via a two-step substrates, non-toxic solvents (e.g., water), sustainable hetero- process involving selective hydrogenation and intramolecular geneous catalysts based on low precious metal loadings, and dehydration.8,9 Ruthenium-based catalysts are widely applied continuous-flow processes.3 Gamma-valerolactone (GVL), for for this reaction,10 due to the intrinsic ability of this metal to example, has been recognised by the US Department of Energy selectively hydrogenate the CvO bond without reducing other unsaturated functionalities.11 Nonetheless, conventional cata- lysts, such as 5 wt% Ru/C, have some drawbacks, including the a Institute for Chemical and Bioengineering, Department of Chemistry and Applied high metal content, possible metal leaching in water-based Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland. solvents, and oxidation to RuO that can lead to severe activity E-mail: [email protected] 2 12,13 bInstitute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, losses. In order to increase stability, some authors have 14–16 43007 Tarragona, Spain. E-mail: [email protected] proposed the utilisation of RuRe, RuSn, and RuPd alloys. cBASF Nederland BV, Strijkviertel 67, 3454PK De Meern, The Netherlands Similarly, it has been postulated that acidic moieties surround- †Electronic supplementary information (ESI) available: Additional characteriz- ing the catalytically active surface could be beneficial to reduce ation of the catalysts, reaction mechanism, and calculated properties of the extent of deactivation.17–20 Ru(0001) and Ru(0001) + HHDMA. See DOI: 10.1039/c6gc02586b and DOI: 21 22 23 10.19061/iochem-bd-1-15 Alternative active phases to Ru, such as Cu, Pt, Ni, , 24 24 25 ‡These authors contributed equally to this work. Pd, Ir, and Au, are much less selective, yielding 2-methyl- This journal is © The Royal Society of Chemistry 2017 Green Chem.,2017,19,2361–2370 | 2361 View Article Online Paper Green Chemistry tetrahydrofuran and 1,4-pentandiol, and/or require harsher sion spectroscopy, elemental analysis, scanning transmission reaction conditions to reach the same conversion level. electron microscopy, energy dispersive X-ray mapping, 31P and Applications of nanostructured hybrid materials in catalysis 13C magic angle spinning nuclear magnetic resonance, X-ray are continuously expanding due to the unique properties that photoelectron spectroscopy, diffuse reflectance infrared spec- can arise from the interaction of the organic and inorganic troscopy, and temperature-programmed reduction with H2 components.26–28 In fact, their chemical versatility makes confirms the desired interaction of the ligand and the metal these materials attractive for the possibility to combine the nanoparticle in the resulting catalyst. In order to explore the advantages of heterogeneous catalysts (e.g., easy separation of advantages of ligand modification in comparison with the the catalyst from the reaction mixture) with the intrinsic state-of-the-art Ru/C material, the catalyst performance has selectivity encountered over homogeneous catalysts.29 In the been evaluated in the continuous-flow three-phase hydrogen- last few years, a key development in the application of hybrid ation of levulinic acid to γ-valerolactone in water at different materials for selective hydrogenations was the identification temperatures, pressures, and flow rates. Density Functional of hexadecyl(2-hydroxyethyl)dimethylammonium dihydrogen Theory has been employed to derive mechanistic understand- phosphate (HHDMA) as a suitable ligand for the synthesis of ing of the high selectivity to GVL achieved over Ru-based cata- metal nanoparticles.30,31 This molecule integrates both stabi- lysts and the enhanced activity of the ligand-modified lising and reducing properties and can be applied in aqueous nanoparticles. medium, eliminating many hurdles which have long ham- pered the industrial exploitation of other routes to prepare hybrid nanoparticles (e.g., the need to use low-boiling point Experimental solvents, the fast addition of expensive or toxic reductants). Catalyst preparation This enabled the production of hybrid Pd-HHDMA and Pt- 3 HHDMA catalysts for alkyne and nitro-group hydrogenation, As depicted in Fig. 1, an aqueous solution of HHDMA (7 cm , 3 respectively, commercialised by BASF under the trademark 30%) was diluted in deionised water (200 cm ) and mixed with 32–35 3 3 NanoSelect. Due to the presence of a dihydrogen phos- an aqueous solution of RuCl3 (2 cm , 0.5 M) and HCl (0.5 cm , 3 phate anion, the HHDMA ligand acts as a pH buffer, control- 33%). The resulting mixture, containing 0.5 mg Ru per cm , was ling the local acidity. This can be utilised for attaining bene- heated to 368 K and stirred at this temperature for 2 h to form ficial effects in terms of reaction rate, while retaining high colloidal metal nanoparticles of approximately 1.5 nm. The col- selectivity levels. Since the selective hydrogenation of levulinic loidal solution was added to a suspension consisting of TiSi2O6 3 acid requires catalyst acidity to improve the performance, we (Aldrich, 30 g) and deionised water (300 cm ). The suspension decided to prepare and catalytically explore for the first time was stirred for 1 h, filtered extensively with deionised water, and HHDMA-modified Ru nanoparticles. dried overnight at 333 K. The Ru/C catalyst (Sigma-Aldrich, ref.: From a process viewpoint, the hydrogenation of levulinic 24888169) was commercially available. Prior to use, this acid to GVL has been traditionally conducted in batch material was reduced at 473 K for 1 h under flowing 10 vol% 21–25,36 3 −1 −1 reactors, mainly due to the simplicity of the equipment. H2/N2 (20 cm min ) using a heating rate of 10 K min . However, it is now widely recognised that continuous-flow Published on 03 October 2016. Downloaded by Universitat Rovira I Vigili 4/12/2019 2:14:54 PM. reactors offer important benefits in terms of process intensifi- Catalyst characterisation cation, safety, evaluation of deactivation phenomena, and The ruthenium content in the catalysts was determined by kinetic analysis in steady state.37 For these reasons, in recent inductively coupled plasma-optical emission spectrometry times, several authors have explored the selective hydrogen- (ICP-OES) using a Horiba Ultra 2 instrument equipped with a ation of levulinic acid in continuous mode.13,25,38–41 photomultiplier tube detector. The C, H, N, and P contents Herein, we report the synthesis of Ru-HHDMA colloids dis- were determined by infrared spectroscopy using a LECO playing a narrow particle size distribution centred at 1.5 nm, CHN-900 combustion furnace. Nitrogen isotherms were and the subsequent deposition onto a TiSi2O6 carrier. obtained at 77 K using a Micrometrics ASAP 2020, after Characterisation by inductively coupled plasma optical emis- evacuation of the samples at 393 K for 3 h. High-angle Fig.
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