The Influence of the Gold Particle Size on the Catalytic Oxidation of 5-(Hydroxymethyl)furfural

Abstract

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...

Oxidation of 5-Hydroxymethylfurfural on Supported Ag, Au, Pd and Bimetallic Pd-Au Catalysts: Effect of the Support

“…At the same time, as shown in our previous work [24], the dependence of the catalytic activity, in particular TOF, on the average size of gold nanoparticles can be volcano type behavior, indicating the optimal size of gold nanoparticles required to achieve the highest TOF value. Analysis of the data allows suggesting that in the present study as in [62], a similar dependence of the catalytic activity on the average size of bimetallic nanoparticles can be observed. Thus, the oxidation of HMF to FDCA is a structure-sensitive reaction requiring an optimal bimetallic nanoparticle size of about 4 nm.…”

Selective Aerobic Oxidation of 5‐(Hydroxymethyl)furfural over Heterogeneous Silver‐Gold Nanoparticle Catalysts

“…The optimum of FDCA production over Ag 3 Au 7 , may be attributed to two effects: particle size or alloy composition. In general, the particle size is an important factor in pure Au catalysis, although Au catalysts in the given size range (≈7–11 nm, Table 1) were all found active in FDCA production [51,54] . Note, that for CO oxidation, a reduced effect of particle size was found for alloy formation with Ag [55] .…”

“…In general, the particle size is an important factor in pure Au catalysis, although Au catalysts in the given size range ( � 7-11 nm, Table 1) were all found active in FDCA production. [51,54] Note, that for CO oxidation, a reduced effect of particle size was found for alloy formation with Ag. [55] Thus, the particle composition seems to have a stronger effect on FDCA production, with an optimal Ag/Au composition for both aldehyde and alcohol oxidation.Generally, the catalytic tests uncovered a pronounced effect of the catalyst composition on the product distribution at different temperatures.…”

Design of bimetallic Au/Cu nanoparticles in ionic liquids: Synthesis and catalytic properties in 5‐(hydroxymethyl)furfural oxidation

“…Noble metals (such as Au, Pd, Pt or Ru) exhibit high activity and FDCA selectivity under mild reaction conditions with temperatures ranging from room temperature to 165 °C [21a,22] . Au particles supported on various materials (e. g. ZrO 2 , [23] TiO 2 , [24] C, [24] CeO 2 , [25] Ce x Zr 1‐x O 2 [26] ), for example, have been explored for the aerobic HMF oxidation. Bimetallic catalysts of Au such as Au/Cu catalysts were further shown to exhibit a higher FDCA yield with respect to their monometallic Au and Cu counterparts ,[9d,27] and are thus a good model reaction.…”