The oxidation of HMF is performed using highly active heterogeneous gold- and, for the first time, silver-based catalysts.

Bimetallic silver-gold alloy nanoparticles on zirconia with varying Ag/Au ratios were designed by a rational approach and tested as catalysts for the selective oxidation of the promising biomass platform molecule 5-(hydroxymethyl)furfural (HMF). For this purpose, colloidal Ag x Au 10-x particles with molar compositions x = 1/3/5/7/9 were prepared by laser ablation in liquids, a surfactant-free method for the preparation of highly pure nanoparticles, before adsorption on zirconia. In-depth characterization of the supported catalysts evidenced alloyed nanoparticles with distinct trends of the surface and bulk composition depending on the overall Ag/Au molar ratio as determined by X-ray photoelectron spectroscopy (XPS) and Xray absorption spectroscopy (XAS), respectively. To uncover the synergistic effect of the Ag/Au ratio, the catalysts were further studied in terms of the catalytic activity and selectivity in HMF oxidation. Either the aldehyde moiety or both functional groups of HMF were selectively oxidized depending on the Ag/Au composition resulting in 5-hydroxymethyl-2-furan-carboxylic acid (HFCA) or 2,5-furandicarboxylic acid (FDCA), respectively. Optimization of the reaction conditions allowed the quantitative production of HFCA over most catalysts, also after re-use. Only gold rich catalysts Ag 1 Au 9 /ZrO 2 and particularly Ag 3 Au 7 /ZrO 2 were highly active in FDCA synthesis. While Ag 3 Au 7 /ZrO 2 deactivated upon re-use due to sintering, no structural changes were observed for the other catalysts and all catalysts were stable against metal leaching. The present work thus provides fundamental insights into the synergistic effect of Ag and Au in alloyed nanoparticles as active and stable catalysts for the oxidation of HMF.

Efficient synthesis of valuable platform chemicals from renewable feedstock is a challenging, yet essential strategy for developing technologies that are both economical and sustainable. In the present study, we investigated the synthesis of 2,5-furandicarboxylic acid (FDCA) in a two-step catalytic process starting from sucrose as largely available biomass feedstock. In the first step, 5-(hydroxymethyl)furfural (HMF) was synthesized by hydrolysis and dehydration of sucrose using sulfuric acid in a continuous reactor in 34% yield. In a second step, the resulting reaction solution was directly oxidized to FDCA without further purification over a Au/ZrO2 catalyst with 84% yield (87% selectivity, batch process), corresponding to 29% overall yield with respect to sucrose. This two-step process could afford the production of pure FDCA after the respective extraction/crystallization despite the impure intermediate HMF solution. To demonstrate the direct application of the biomass-derived FDCA as monomer, the isolated product was used for Ugi-multicomponent polymerizations, establishing a new application possibility for FDCA. In the future, this efficient two-step process strategy toward FDCA should be extended to further renewable feedstock.

The utilization of biomass and development of intensified processes are essential to establish a sustainable production of chemicals in the future. Herein, we report on a strategy that allows one to directly convert the biomass-derived platform molecule 5-(hydroxymethyl)­furfural (HMF) over Au/ZrO2 in aqueous medium to 2,5-furandicarboxylic acid (FDCA), a renewable building block for biobased polymers like polyethylene furanoate. The focus lies on identifying the influence of 5-(hydroxymethyl)­furfural synthesis byproducts, like unconverted sugars, levulinic acid, and formic acid as well as the remaining inorganics, on the synthesis of 2,5-furandicarboxylic acid to save the intermediate step of HMF purification. These components were added to the reaction mixture individually and in combination to study their effect. Although most of these substances lowered the FDCA yield, the reaction conditions could be optimized to produce FDCA quantitatively. Only the addition of levulinic acid led to a severe deterioration of the production of FDCA, which was attributed to poisoning of the catalyst. In a realistic technical scenario, the direct oxidation of impure HMF from unconcentrated sugar syrup in high FDCA yield (74%) was demonstrated. Catalyst stability was investigated in the presence of sugars. On the basis of these studies, highly needed recommendations for the HMF synthesis were developed to establish a more sustainable, technically feasible, and intensified process for direct FDCA production from sugars at industrial scales.

For the production of chemicals from biomass, new selective processes are required. The selective oxidation of 5-(Hydroxymethyl)furfural (HMF), a promising platform molecule in fine chemistry, to 2,5-furandicarboxylic acid (FDCA) is considered a promising approach and requires the oxidation of two functional groups. In this study, Au/ZrO 2 catalysts with different mean particle sizes were prepared by a chemical reduction method using tetrakis(hydroxymethyl)phosphonium chloride (THPC) and tested in HMF oxidation. The catalyst with the smallest mean particle size (2.1 nm) and the narrowest particle size distribution was highly active in the oxidation of the aldehyde moiety of HMF, but less active in alcohol oxidation. On the other hand, increased activity in FDCA synthesis up to 92% yield was observed over catalysts with a larger mean particle size (2.7 nm), which had a large fraction of small and some larger particles. A decreasing FDCA yield over the catalyst with the largest mean particle size (2.9 nm) indicates that the oxidation of both functional groups require different particle sizes and hint at the presence of an optimal particle size for both oxidation steps. The activity of Au particles seems to be influenced by surface steps and H bonding strength, the latter particularly in aldehyde oxidation. Therefore, the presence of both small and some larger Au particles seem to give catalysts with the highest catalytic activity.Catalysts 2020, 10, 342 2 of 13 oxidation of HMF has gained great interest in recent years. Selective oxidation of HMF can for example give 5-hydroxymethyl-2-furancarboxylic acid (HFCA) or 2,5-diformylfuran. One of the most important oxidation products is 2,5-furandicarboxylic acid (FDCA), [11,12] which is produced via the oxidation of both functional groups of HMF (Scheme 1). The structural similarity of FDCA to terephthalic acid led to its consideration as one of the twelve important molecules that can be produced from sugar-containing biomass feedstock [11,12]. Therefore, FDCA may be used in future bio-based polymers thus overcoming the need of petrochemical-based terephthalic acid [13,14]. Synthetic approaches for FDCA synthesis include stoichiometric oxidation reactions [15] as well as catalytic routes with mostly molecular oxygen. The latter range from bio-[16,17] to electro-[18-20] and metal catalysis. Current FDCA production is based on the homogeneously catalyzed AMOCO process, which is carried out in acetic acid solvent at 125 • C and 70 bar air in the presence of a Cu/Mn/Br catalyst [21]. In addition, heterogeneous metal catalysts have been applied in HMF oxidation. Especially supported noble metals like Pt [22][23][24] or Pd [25][26][27] are catalytically active in this reaction. Among others, supported gold-based catalysts show high activity in the oxidation of HMF with oxygen to FDCA [28][29][30][31][32][33][34][35]. For example, Au supported on CeO 2 nanoparticles allowed the production of FDCA in 96% yield after 5 h at 130 • C [28].Catalysts 2020, 10, x FOR PEER REVIEW 2 of...

Silver-catalyzed oxidation of 5-(hydroxymethyl)furfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was investigated using Ag/ZrO₂ and Ag/TiO₂ catalysts. The reaction proceeded very selectively without formation of the dicarboxylic acid in the presence...

In alloyed nanoparticles, synergistic electronic and/or geometric effects may enhance the catalytic properties compared to their monometallic counterparts. Herein, we address the synthesis of bimetallic Au/Cu nanoparticles with different compositions by wet chemical reduction in ionic liquids. The nanoparticles were successively supported on carbon. The ionic liquid could be recycled after synthesis. Annealing of the carbon‐supported NPs at 400 °C led to NPs of the ordered intermetallic L10 AuCu phase. The nanoparticle‐derived catalysts were characterized by X‐ray diffraction analysis, transmission electron microscopy, X‐ray photoelectron spectroscopy and optical emission spectroscopy with inductively coupled plasma. Oxidation of biomass‐derived furans is a prominent process for biomass transformation into value‐added chemicals. Herein, the oxidation of 5‐hydroxymethyl‐2‐furfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) was chosen as a model reaction to evaluate the effect of Cu addition and intermetallic structure on the catalytic performance. Particularly Au/Cu nanoparticles with an Au/Cu ratio of 3 : 1 showed very high conversion to FDCA.