*2.2. Pt-Based Catalysts*

Pt-based catalysts were initially reported to be efficient for the aerobic oxidation of HMF to produce FDCA. Recent results of the aerobic oxidation of HMF to FDCA over Pt-based catalysts are summarized in Table 2. Verdeguer et al. studied the carbon supported Pt catalysts for the oxidation of HMF [52]. The addition of Pb into the Pt/C catalyst could improve the catalytic performance with a high FDCA yield of 99%, than that with the Pd/C catalyst (81%) at ambient conditions for 2 h (25 ◦C, P = 1 bar, a O2 flow rate of 2.5 mL/s and 1.25 M NaOH solution). HMFCA was detected as the reaction intermediate, pointing to the preferred oxidation of aldehyde group of HMF with Pt-Pb catalyst. Moreover, the addition of Bi to the Pt/C catalyst also showed a beneficial effect on the FDCA yield (Table 2) [53]. An optimal Pt/Bi molar ratio of about 0.2 showed a high FDCA yield of 98% for the Pt-Bi/C at 100 ◦C, 40 bar air and the use of four equivalents of NaHCO3 after 6 h, compared to 69% for Pt/C catalyst. Both HMFCA and DFF were detected as intermediates, and the oxidation of FFCA was figured out to be the rate-determining step for this Pt-Bi/C catalyst. Introducing Bi into the Pt/C catalyst improved the catalyst stability, owing to depressing the oxygen poisoning and leaching of Pt. Similar effect was also observed in another study over TiO2 supported Pt-Bi catalyst [54].

**Table 2.** Results of the aerobic oxidation of HMF to FDCA over Pt-based catalysts.


a RGO = Reduced Graphene Oxide b EDA = ethylenediamine c PVP = polyvinylpyrrolidone. d NP-Cl = Nanoparticle with Cl ionic polymer e NP5 = Nanoparticle with C12H23O2 ionic polymer.

The catalytic performance of different Pt-based catalysts is highly dependent on the choice of support. Ramakanta et al. compared the performance of different metal oxide-supported Pt catalysts under 75 ◦C, 1 bar O2 and the use of Na2CO3 after 12 h. Pt catalysts supported on the non-reducible oxides (ZrO2, Al2O3 and C) showed high FDCA yields of above 90%, while Pt catalysts supported on reducible oxides (TiO2 and CeO2) separately gave poor FDCA yields of 2% and 8% [55]. The authors attributed the higher catalytic performance on Pt catalysts to the lower oxygen storage ability of the non-reducible oxides, which could efficiently prevent the oxidation of active metal sites. Addition of Bi into the CeO2 support could also enhance the catalytic performance of Pt/CeO2 [56]. Pt-catalysts

with bismuth (Bi) modified ceria support, the Pt/Ce0.8Bi0.2 O2-δ afforded a FDCA yield of 98% while the Pt/CeO2 only showed a FDCA yield of 20 % at 23 ◦C, 10 bar O2. Moreover, the Pt/Ce0.8Bi0.2 O2−<sup>δ</sup> catalyst showed a good catalyst stability, and could be reused for five runs with a marginal loss of its FDCA yield (from 98% in the first run to 97% in the fifth run). In this work, Pt nanoparticles were believed to first react with the hydroxyl group of HMF and form the Pt-alkoxide intermediate, which was further converted to aldehyde by β-H elimination. Bi-containing ceria contained a large amount of oxygen vacancies, which could further promote the oxygen reduction process and the cleavage of the peroxide intermediate by bismuth. Therefore, the surface electrons were used to the reduction of oxygen, and a new catalytic cycle could be smoothly continued.

Carbon materials are widely used as catalyst supports owing to their good properties and easy availability. Apart from the most used active carbon, reduced graphene oxide (RGO) can be also used as support owing to its abundant surface functional groups, which can be used to anchor the metal nanoparticles. Niu et al. studied a series of RGO supported metal nanoparticles for the oxidation of HMF at 25 ◦C and 1 bar O2, with the presence of NaOH and an O2 flow rate of 50 mL/min [57]. The Pt/RGO catalyst showed a higher FDCA yield than Pd/RGO, Ru/RGO and Rh/RGO, and a FDCA yield of 84% could be obtained after 24 h. During the recycling experiments, an increase in HMFCA and a decrease in FDCA yield could be observed. Similar results that the Pt/C outperforms in catalytic activity than the Pd/C was also reported by Davis et al. [66]. Notably, Zhang et al. prepared a series of novel Fe3O4@C@Pt catalysts containing super-paramagnetic Pt-nanoparticles with a core-shell structure, and used for the oxidation of HMF (Figure 3) [58]. These novel Fe3O4@C@Pt catalysts have a spherical shape with a Fe3O4 core, a protective amorphous carbon shell and Pt nanoparticle clusters decorated on the surface. The preparation temperature showed an impact on the morphology of active Pt on the carbon shell (Figure 3i). The 110-Fe3O4@C@Pt prepared at reflux of 110 ◦C a fforded a nearly full FDCA yield at 90 ◦C after 4 h during the oxidation of HMF (Figure 3ii). In addition, this catalyst can be reused up to three times without significant loss in performance. Platinum on alumina is also studied in basic conditions (pH = 9) and high FDCA yield (99%) is reported due to strong metal-substrate interaction through π-electrons of furan nucleus [59].

**Figure 3.** (**i**) Schematic illustration of synthesis of core-shell structure X-Fe3O4@C@Pt superparamagnetic microspheres (X = Refluxing Temperature) (**ii**) (a) SEM images and model of 110-Fe3O4@C@Pt microsphere, (b) core/shell microspheres after coating with an amorphous carbon layer, and Pt-decorated magnetic core/shell microspheres: (c) 90-Fe3O4@C@Pt (d) 100-Fe3O4@C@Pt and (e) 110-Fe3O4@C@Pt synthesized at di fferent temperatures. (f) A model image of a 110-Fe3O4@C@Pt microsphere (reproduced from [58] with permission from the Royal Society of Chemistry, copyright 2016).

Most of the above examples of the aerobic oxidation of HMF over Pt catalysts are generally performed in the presence of excess base. The disadvantages of basic feeds are that product solutions

require neutralization of the base, and separation of the resulting formed inorganic salts. Base-free catalytic oxidation of HMF to FDCA has also been reported over Pt-based catalysts. Recently, Chen et al. prepared a Pt/ZrO2 by the atomic layer deposition (ALD) method and conducted the aerobic oxidation of HMF under base-free conditions [60]. A complete HMF conversion and a FDCA yield of 97.3% under mild reaction conditions (100 ◦C, O2, 4 bar, 12 h) were reported. The highly dispersed and uniform particle size of Pt particles was visualized by TEM. An improved C=O adsorption on the catalyst surface was also indicated by temperature programmed desorption of CO (CO-TPD), pointing to a strong interaction between reactants/intermediates. Both factors were attributed to the good catalytic activity for the Pt/ZrO2. Han et al. designed a novel Pt catalyst with modified C-O-Mg support for base-free aerobic oxidation of HMF to FDCA [61]. A high FDCA yield of 97% could be obtained at 110 ◦C and 10 bar O2. In addition, this catalyst could be used for ten times with little loss of activity. Even scaling up the reaction by 20 times at a large scale, a decent yield of isolated FDCA could achieve 74.9% with a high purity of 99.5%. Han et al. further developed a N-doped carbon- supported Pt for base-free aerobic oxidation of HMF to FDCA [62]. The synthesized Pt/C-EDA-x catalyst (where EDA = ethylenediamine and x = nitrogen dose) prepared by using EDA as nitrogen source showed higher catalytic activity than the counterparts using N,N-dimethylaniline (DMA), ammonia (NH3), and acetonitrile (ACH) as nitrogen source. A FDCA yield of 96% was achieved with the Pt/C-EDA-4.1 catalysts at optimal conditions (T = 110 ◦C, PO2 = 1 bar, and t =12 h). This high catalytic performance is attributed to the formation of a new kind of medium basic site due to the formation of pyridine-type nitrogen in the catalyst.

Limited examples of Pt-based bimetallic catalysts are reported for base-free aerobic oxidation of HMF to FDCA. Shen et al. prepared a Pt-Ni/AC bimetallic catalyst by atomic layer deposition of Pt nanoparticles on the surface of Ni/AC for base-free oxidation of HMF to FDCA [63]. The bimetallic Pt-Ni/AC catalyst showed a higher activity (a FDCA yield of 97.5% yield with 100% HMF conversion) even with only a 0.4 wt % Pt loading at 100 ◦C and 4 bar O2 after 15 h, compared to the monometallic counterparts with a higher Pt loading (the 5.6 wt% Ni/AC and 1.6 wt% Pt/AC). In addition, the bimetallic catalyst a fforded good reusability for at least four 15 h runs without any obvious loss of catalytic activity. The authors proposed that the addition of Ni to Pt improved the ability of Pt for C=O adsorption and oxidation, thus increasing the activity of the Pt catalyst.

Pt nanoparticles stabilized by polymers were also shown to be e ffective for base-free aerobic oxidation of HMF to FDCA. Siankevich et al. reported that polyvinylpyrrolidone (PVP) stabilized Pt nanoparticles could promote the base-free aerobic oxidation of HMF into FDCA in water [64]. A high FDCA of 95% were achieved for the Pt-PVP-GLY catalyst at mild reaction conditions (T = 80 ◦C, PO2 = 1 bar, and t = 24 h). Notably, a slight decrease of its catalytic activity was observed during five consecutive runs. The reaction mechanism was investigated for the Pt-PVP-GLY catalyst. Isotope (H218O) labeling technology was used to elaborate the reaction path, and mass spectrometric analysis of solutions after reaction verified the existence of 18O atomic levels. DFF and FFCA were detected as the reaction intermediates during this oxidation reaction, while HMFCA was not detected during base-free oxidation of HMF.

As shown in Scheme 5, the aldehyde group was proposed to undergo a rapid reversible hydration to a geminal diol by nucleophilic addition of water, and followed by proton transfer to form DFF. Similar results were also reported by other researchers under base-free conditions at relatively low pH values [45,48]. The release of two protons from the hydroxyl group in HMF could form DFF. Mass spectrometric analysis of the reaction mixture confirmed that 18O was incorporated in the oxidation products (FDCA and FFCA). Peaks with *m*/*z* 161 and 163 were attributed to three and four 18O atoms incorporated in FDCA, and peaks with *m*/*z* 143 and 145 attributed to two and three 18O atoms incorporated in FFCA. Finally, a transfer of two H to the surface of the metal occurred to form the carboxylic acid groups, and molecular oxygen reacted with the surface hydride to release H2O. Furthermore, Siankevich et al. reported Pt nanoparticles stabilized by an imidazolium-based cross-linked polymer (with chloride as the counter-anion), which could e fficiently catalyzed the

oxidation of HMF to FDCA in water with oxygen as the oxidant under mild conditions (T = 80 ◦C, PO2 = 1 bar, and t = 6 h) [65]. Various counter-anion, that is, replacing chloride by BF4<sup>−</sup>, PF6<sup>−</sup>, bis(trifluoromethylsulfonyl)imide, hexanoate, or laurate anions, in the cationic polymer has been explored. The counter-anion indeed showed an impact on the structure of the obtained platinum nanoparticles, the surface electronic properties, and their catalytic activity. The highest reaction rates were obtained with the weakly nucleophilic bis(trifluoromethylsulfonyl)imide anion, which also favored Pt in the metallic state, leading to complete conversion of the substrate and a high yield of FDCA (65%).
