Solvent-And Base-Free Oxidation of 5-Hydroxymethylfurfural over a PdO/AlPO4-5 Catalyst under Mild Conditions

: A solvent-free method was proposed to upgrade the biomass-derived compound 5-hydroxymethylfurfural (HMF). The oxidation of HMF to produce 2,5-furandicarboxylic acid (FDCA) has been examined in the presence of O 2 without the addition of solvent and base. Di ﬀ erent from the conversion of the aldehyde group on HMF as the initial oxidation step in H 2 O solvent, the hydroxyl group on HMF was ﬁ rst oxidized and FDCA was ﬁ nally generated without the addition of solvent. The role of O 2 is to replenish the consumption of active oxygen species on the catalyst surface. The oxidation of HMF to FDCA proceeded due to the solvent-free e ﬀ ect. A 83.6% FDCA selectivity at 38.8% HMF conversion was measured with a PdO/AlPO 4 -5 catalyst at 80 ° C for 5 h and the reaction mechanism was proposed.


■ INTRODUCTION
Biomass conversion is a highly demanding technology for generating clean fuels and value-added chemicals. 1−4 5-Hydroxymethylfurfural (HMF) is identified as a versatile platform product of the biomass-based value chain. 5−7 Abundant useful furan compounds, such as 2,5-furandicarboxylic acid (FDCA), 2,5-diformylfuran (DFF), 5-formyl-2furancarboxylic acid (FFCA) and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) are obtained from HMF oxidation reactions. 8FDCA was listed as one of the top 12 value-added compounds among the sustainable chemical chain from carbohydrates by the U.S. Department of Energy in 2004. 9−14 In addition, the furan ring of FDCA makes it easy to degrade, which leads to the feasible application of FDCA in the synthesis of biodegradable materials. 15inke et al. 16 reported the conversion of HMF to FDCA with silica-supported Pt catalyst in pH 9 water solution at 60 °C with O 2 .−22 Multiple solvents were used in previous studies, including acetic acid, dimethylformamide (DMF), acetonitrile, and H 2 O. 23,24 In particular, H 2 O as a green solvent has received much attention.In H 2 O solvent, geminal diols are measured as crucial intermediates during the reaction. 25Base promoters, such as NaOH, Na 2 CO 3 and K 2 CO 3 facilitate the hydration step that occurs between H 2 O and HMF molecules, thereby decreasing the activation barrier for the conversion of HMF to FDCA. 26Hence, studies have reported the oxidation of HMF to FDCA under basic conditions in water. 25,27vertheless, the addition of base in the reaction leads to corrosion and pollution problems.Therefore, recent studies have used solid base-supported noble-metal catalyst to perform the HMF oxidation reaction without soluble base. 28,29Gao et al. reported a hydrotalcite-activated carbon-supported Au catalyst with 99% FDCA yield in water at 100 °C for 12 h under 0.5 MPa O 2 . 30However, the stability of such catalyst needs to be improved due to the leaching of the basic sites of hydrotalcite in an aqueous solution. 31In addition, a long reaction time is generally required with solid base-supported catalysts. 32Hence, a new strategy in the oxidation of HMF to FDCA under base-free conditions is still anticipated.On the other hand, the poor solubility of FDCA in common solvents also makes the purification of FDCA with the traditional extraction method unfeasible.
Here, we report a solvent-and base-free reaction system, a PdO-based catalyst with O 2 as the oxidant without the addition of solvent and base, which achieves one-half the reaction rate compared to the recently reported Mg-modified zirconiasupported Au catalyst with NaOH additive in H 2 O solvent. 33he system here avoids the corrosion issue.The reaction pathway of the HMF oxidation is discussed under solvent-and base-free conditions.

■ EXPERIMENTAL SECTION
Reagents and Instruments.HMF, DFF, FDCA, and 2,5dihydroxymethylfuran (DHMF) were obtained from Aladdin Chemicals Reagent Co. PdCl 2 was purchased from Tianjin Guangfu Co. Ltd.All chemicals were used as received.
X-ray diffraction (XRD) was recorded with a D8 Focus diffractometer.The scanning rate was set at 10°min −1 between 10 and 80 °C.The transmission electron microscopy (TEM) image was observed with a JEM-2100F.X-ray photoelectron spectroscopy (XPS) was measured with a PHI-1600 ESCA system.The FT-IR spectrum of pyridine chemisorption (Py-IR) was collected with an instrument made by Xiamen Tops Equipment Development Co., Ltd.Specifically, 20 mg of catalyst sample was weighed, pressed, and then placed in a sample chamber for vacuum treatment.Then the sample was heated at 10 °C min −1 from ambient temperature to 350 °C and kept at 350 °C for 30 min.After the sample was cooled to 100 °C, pyridine was allowed to adsorb on the sample and the adsorption process was kept for 5 min.Then the Py-IR spectrum was recorded after vacuum treatment for 10 min at the same temperature.
Catalyst Preparation.PdO/AlPO 4 -5 catalyst was synthesized with the following method.A H 2 PdCl 4 solution was prepared by dissolving palladium chloride (PdCl 2 ) into concentrated hydrochloric acid (36.5 wt % HCl) with a mole ratio of 1:2 and then diluted to 2 × 10 −2 M. Then a certain amount of AlPO 4 -5 molecular sieves was suspended in 5 mL of H 2 O and the slurry was sonicated for 1 h.Afterward, 5 mL of the as-prepared H 2 PdCl 4 solution was added dropwise (for 3 wt % Pd/AlPO 4 -5) into the AlPO 4 -5 molecular sieves slurry under stirring.Then a NaBH 4 aqueous solution (Pd/ NaBH 4 , 1:4) was added dropwise into the flask to obtain Pd NPs.The above mixture was stirred at ambient temperature for 3 h.After the mixture solution was filtrated and washed with 500 mL of distilled H 2 O, the filter cake was dried in an oven at 80 °C overnight.Subsequently, the obtained sample was calcined at 600 °C for 5 h with a heating rate of 5 °C/min, and the material was labeled as PdO/AlPO 4 -5.
Reaction Conditions.All of the reactions were carried out with a 5 mL glass bottle (9.0 mm i.d.), and the bottle was embedded into an autoclave.Typically, 1 mmol of reactant and 0.05 g of catalyst were charged into the reactor.After sealing the reactor, the air was replaced with 0.5 MPa O 2 .Then the autoclave was preheated to 80 °C for 5 h under 200 rpm stirring.After reaction was finished, the autoclave was cooled to ambient temperature.The obtained material was analyzed with an Agilent 1200 high-performance liquid chromatograph (HPLC) equipped with a UV detector operating at 280 nm.An Eclipse XDB-C18, 5 μm 4.6 × 150 mm column was used and the column oven was set at 10 °C.Acetonitrile and a 0.1 wt % acetic acid solution were mixed with a volume ratio of 3:7 and used as the mobile phase at a rate of 0.5 mL min −1 .The quantitative analysis of the products was calculated with an external standard calibration curve method.

■ RESULTS
Reaction.In Table 1, the results obtained with the PdO/ AlPO 4 -5 catalyst sample are summarized.The PdO/AlPO 4 -5 catalyst gave a FDCA selectivity of 83.6% at 38.8% HMF conversion without a base and the addition of H 2 O (entry 1).Meanwhile, DFF and FFCA were measured with selectivities of 3.2% and 13.2%, respectively.When the reaction was conducted in H 2 O solvent with Na 2 CO 3 additive, the FDCA selectivity was 86.2% at 81.4% HMF conversion.At the same time, DFF and HMFCA were measured as intermediates with selectivities of 1.1% and 5.7%, respectively (entry 2).A control experiment with H 2 O solvent was carried out, and a 45.5% selectivity of FDCA at 29.6% HMF conversion was measured.The intermediate was measured as HMFCA with a 22.8% selectivity (entry 3).If calculating the material consumption of the reaction based on the overall mass of feed, i.e., the sum of HMF, water, and base, the FDCA product mass yields are 28.8%,9.2%, and 1.8% in the three types of reactions mentioned, respectively.Apparently, the solvent-free reaction strategy has an advantage.
The effect of O 2 partial pressure was examined with the PdO/AlPO 4 -5 catalyst without H 2 O and base in Table 2.Under 0 MPa O 2 , negligible conversion of HMF was measured.In the range of 0−0.5 MPa O 2 , HMF conversions increased from 1.6% to 38.8% and FDCA selectivity improved to 83.6%, while the selectivity of the DFF intermediate decreased monotonically.However, when the O 2 pressure was increased from 0.5 to 1.0 MPa, HMF conversion was basically stable and the FDCA selectivity decreased slightly with increasing O 2 pressure.
The reaction was done with different reaction times without addition of H 2 O and base.As illustrated in Figure 1, DFF dominates the oxidation and accounts for 48.5% selectivity with 7.5% HMF conversion at 1 h.With the reaction time increased to 5 h, the HMF conversion exhibits a monotonic increased trend and reaches 38.8% at 5 h.The FFCA selectivity increases in the initial 2 h and then decreases with the extension of reaction time, while the selectivity of DFF decreases monotonically, and that of FDCA increases monotonically.After 5 h, the HMF conversions and selectivities of the different products basically approach stable values.This indicates strongly that DFF and FFCA are the intermediates and then were transformed to FDCA stepwise.

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To further verify the FDCA formation pathway without addition of H 2 O and base, O 2 was replaced by 0.5 MPa N 2 during the reaction, while no HMF conversion was measured.2,5-Dihydroxymethylfuran (DHMF) and DFF were tested as reactants, and the results are listed in Table 3. DHMF had a conversion of 43.4% with oxidation of its alcohol groups to generate sequentially HMF, DFF, and FDCA with selectivities of 21.4%, 10.3%, and 56.3%, respectively; 48.6% DFF was converted to FFCA and FDCA with selectivities of 13.2% and 73.2%, respectively.Similarly, no DHMF and DFF were converted when the reactions were operated under a N 2 atmosphere.
Characterization.The XRD pattern and TEM image of the catalyst sample are presented in Figure 2. As the PdO content is relatively low, all of the XRD peaks are attributed to AlPO 4 -5 molecular sieves and no PdO diffraction peak is observed.In the TEM image, the PdO nanoparticles are uniformly distributed on AlPO 4 -5 crystallite and the particle size is mainly distributed at 2−5 nm.
In Figure 3, X-ray photoelectron spectroscopy (XPS) was recorded to observe the chemical states of Pd on the catalyst before and after the reaction.In both Pd 3d spectra, the Pd 0 and Pd 2+ valence states were confirmed.The binding energies at 337.6 and 342.9 eV correspond to Pd 2+ , while the peaks located at 336.8 and 342.2 eV are assigned to Pd 0 . 34,35ccording to the peak-fitting results, the ratio of Pd 0 species was increased from 23.1% to 53.4% and the Pd 2+ species was decreased from 76.9% to 46.6% after the reaction.
The nature of the acid sites on the fresh catalyst surface was characterized with FT-IR of pyridine chemisorption (Py-IR), and the result is given in Figure 4. Lewis acid sites are detected as major on the catalyst surface. 36

■ DISCUSSION
The data in Table 2 reveal that HMF oxidation is sensitive to the O 2 partial pressure at a low value range without addition of solvent, and the effect of O 2 pressure becomes weak above 0.5

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MPa.In the low partial pressure range, the reaction may be controlled by the initial HMF activation step, so that the adsorption/surface concentration of O 2 favors the overall reaction rate.However, when the O 2 partial pressure is high enough, the reaction may be controlled by the following conversion steps of the intermediates.On one hand, the PdO/ AlPO 4 -5 catalyst was surrounded by a high concentration of HMF in the absence of solvent, which limited the activation step of O 2 .On the other hand, the intermediates or FDCA product were difficult to desorb from the active sites on the catalyst surface without solvent, which may also limit the further oxidation process.Therefore, a further increase of the O 2 partial pressure does not improve the conversion.This is consistent with the previous literature. 37DCA can be generated from HMF without the addition of solvent and base.However, a significantly different product distribution is obtained.When the reaction is carried out in H 2 O solvent, attachment of the H 2 O molecule to the −CHO group in HMF leads to the formation of intermediate diols as shown in Scheme 1.−40 Thus, HMFCA has been often referred to as the main intermediate in H 2 O solvent.Besides, the addition of Na 2 CO 3 facilitates the hydration step between HMF and H 2 O toward FDCA.Moreover, oxidation of the C−H bond on the alcohol side was proposed as the slow step during the HMF oxidation reaction.Oxidation of the C−H bond on the alcohol side is not involved in the conversion of HMF to HMFCA in the reaction with H 2 O solvent in the presence of base.However, when the reaction was carried out without addition of H 2 O and base as Scheme 1 shows, DFF as the main intermediate was formed at the beginning with a reaction time of 1 h.HMF was first oxidized to DFF and involved oxidation of the C−H and O−H bonds on the alcohol side, suggesting that the HMF oxidation was facilitated without the addition of H 2 O.
In Figure 1, HMFCA was not measured without base and the addition of H 2 O, indicating that the carbon with a hydroxyl group in HMF is preferentially oxidized instead of the carbon of the aldehyde group.When the reaction time is between 1.5 and 2.5 h, FFCA selectivity is first increased and then decreased with a moderate increase of HMF conversion.These results mean that the aldehyde group in DFF oxidizes faster than that in FFCA.Meanwhile, the C−OH bond in HMF was first oxidized to −CHO and then to −COOH.Without the addition of H 2 O, oxidation of HMF to FDCA undergoes three steps, via DFF, FFCA, and FDCA.Though a H 2 O molecule was produced as a byproduct during HMF oxidation, the formation of HMFCA was not favored at a high concentration of HMF molecules without base as a promoter.
The XPS peak-fitting results reveal that the content of Pd 0 on the PdO/AlPO 4 -5 catalyst surface was increased after the reaction, indicating that part of Pd 2+ was reduced to Pd 0 .The consumption of lattice oxygen is involved in the oxidation reaction, which leads to the increase of oxygen vacancies on the surface and lattice of PdO. 41,42In addition, the high chemical potential of HMF around the PdO site limits the reoxidation of Pd 0 to Pd 2+ .
The high dispersion of PdO nanoparticles on the support leads to the increased available active sites to facilitate the reaction and restrain the aggregation of small metal clusters.Besides, as the proton abstraction of HMF is regarded as the initial step for the formation of FDCA, the acid property of the catalyst plays a significant role in HMF conversion due to the adsorption ability of the reactant. 43Specifically, the hydroxyl group of HMF can adsorb on the Lewis acid sites by sharing the lone pair electrons, leading to the transformation of the hydroxyl group on the catalyst interface. 44These facts may According to the experimental results, a Mars−van Krevelen mechanism, as shown in Scheme 2, can be proposed to describe the oxidation of HMF to FDCA.First, the hydroxyl group is adsorbed on the PdO/AlPO 4 -5 catalyst, and then C− OH in HMF is oxidized to form the aldehyde group assisted by surface oxygen.Meanwhile, Pd 2+ is reduced to Pd 0 partially.Subsequently, the oxygen in the PdO phase is replenished by the dissociative adsorption of O 2 and the diffusion of the O atom in the PdO phase. 45Afterward, the aldehyde group on DFF is oxidized to a carboxyl group in FFCA.Consumption of the surface oxygen on PdO/AlPO 4 -5 and its replenishment both occurred during the redox cycle.Finally, the aldehyde group in FFCA is further oxidized to generate FDCA.

■ CONCLUSIONS
In summary, the selective oxidation of HMF to FDCA was carried out without the addition of water and base in the presence of gaseous O 2 on a PdO/AlPO 4 -5 catalyst.The HMF oxidation reaction is facilitated and proceeded via three major steps with the initiation of C−OH oxidation through DFF and FFCA intermediates to form FDCA. Molecular oxygen plays a role in the replenishment of the lattice and surface oxygen of the active PdO phase.Although HMF oxidation studies performed under base-free conditions with noble-metalsupported catalysts are in progress, the oxidation of HMF to FDCA conducted without the addition of a solvent and base may provide a new strategy in upgrading the biomass-derived chemicals.
XRD patterns of used catalyst and FT-IR characterization (PDF) ■

Figure 3 .
Figure 3. XPS spectra of Pd in PdO/AlPO 4 -5 sample before and after use.

Table 1 .
HMF Oxidation Reaction Results with the PdO/ AlPO 4 -5 Catalyst MPa O 2 , reaction time 5 h, temperature 80 °C.b Other conditions were the same but with 1 mL of H 2 O and 0.01 g of Na 2 CO 3 .c Reaction was done adding 1 mL of H 2 O as solvent but without Na 2 CO 3 .

Table 3 .
Results of Control Experiments a a Reaction conditions: 1 mmol of reactant, 0.05 g of PdO/AlPO 4 -5 catalyst, reaction time 5 h, temperature 80 °C.