Conversion of HMF into FDCA was carried out by a simple and green process based on alkaline ionic liquid (IL) modified Pd/C catalyst (Pd/C-OH−). Alkaline ionic liquids were chosen to optimize Pd/C catalyst for special hydrophilicity and hydrophobicity, redox stability, and unique dissolving abilities for polar compounds. The Pd/C-OH− catalyst was successfully prepared and characterized by SEM, XRD, TG, FT-IR, and CO2-TPD technologies. Loading of alkaline ionic liquid on the surface of Pd/C was 2.54 mmol·g−1. The catalyst showed excellent catalytic activity in the HMF oxidation after optimization of reaction temperature, reaction time, catalyst amount, and solvent. Supported alkaline ionic liquid (IL) could be a substitute and promotion for homogeneous base (NaOH). Under optimal reaction conditions, high HMF conversion of 100% and FDCA yield of 82.39% were achieved over Pd/C-OH− catalyst in water at 373 K for 24 h.
Currently, fuel and chemicals needed in society are largely derived from fossil fuels. Rapid depletion of fossil resources has generated widespread interest in the utilization of renewable resources for the sustainable production of fuel and chemicals. Renewable energy, such as wind energy, solar energy, and geothermal energy, cannot be used to produce organic chemicals that are currently based on fossil fuels. In contrast, biomass resources with a large proportion of carbohydrates are extensive. Through selective dehydration or hydrogenation processes, biomass can produce liquid fuels and organic chemicals [
In the process of producing FDCA, first of all, Miura et al. used traditional oxidant KMnO4 to synthesize FDCA [
Among Pd, Pt, Ru, and Au-based catalysts, the catalytic efficiency of supported Pd nanoparticles is remarkable [
The oxidation of HMF requires addition of homogeneous base while utilizing noble metal-based catalysts. Without base, FDCA may not be produced in large quantities. It is now pointed out that the base can remove the HMF acidic oxidation product accumulated on the catalyst surface and allow the reaction to proceed continuously. For example, Nie et al. demonstrated a base-free process for no FDCA synthesis using activated carbon loaded Ru metal nanoparticles (Ru/C) catalyst in toluene with 95.8% 2,5-diformylfuran (DFF) yield [
In this work, we report a much more environmentally benign and safe process for aqueous HMF oxidation using solid base modified activated carbon-supported palladium nanoparticle catalyst (Pd/C-OH−). Conversion of HMF into FDCA could be carried out by a simple and green process based on a novel ionic liquid (IL) modified Pd/C catalyst and then exchanged Cl− in ionic liquid with OH− (Scheme
Reaction of HMF oxidation to FDCA.
5-Hydroxymethyl-2-furancarboxylic acid (HFCA), 5-formyl-2-furancarboxylic acid (FFCA), HMF, and FDCA were purchased from the J&K Chemical Co., Ltd. (Shanghai, China). Palladium(II) chloride (PdCl2, 59% Pd) was purchased from Aladdin Chemicals Co., Ltd. (Beijing, China). Activated carbon was purchased from Sigma-Aldrich (Beijing, China). 1-Methylimidazole, 3-(Chloropropyl)triethoxysilane, and all solvents were purchased from Sinopharm Chemical Reagents (Shanghai, China).
Pd/C was prepared by incipient wetness impregnation of activated carbon with aqueous solution of PdCl2. 1 g PdCl2 (5.64 mmol) was added to a 250 mL conical flask with 50 mL deionized water, and then the mixture was diluted to 100 mL with deionized water and shaken well. 3 g activated carbon was added to the PdCl2 solution with 1000 rpm stirring condition for 10 h, followed by evaporation and drying at 383 K in air oven overnight. The obtained powder was reduced in a flow of 20% H2 in N2 at 623 K for 4 h to form final Pd/C catalyst.
20 g 1-methylimidazole and 62 g 3-(chloropropyl)triethoxysilane (molar ratio 1 : 1.08) were added to a 250 mL round bottom flask; the mixture was sealed and heated in oil bath at 368 K with 1000 rpm magnetic stirring for 24 h to form ionic liquid. After that, the reaction mixture was cooled down to room temperature and the formed ionic liquid was reserved for further use.
2 g Pd/C catalyst and 6.46 g ionic liquid (20 mmol) prepared in 2.3 were added to a 250 mL round bottom flask with 50 mL toluene and were heated in oil bath at the reflux temperature with 1000 rpm magnetic stirring for 14 h and then the solid catalyst was filtered and washed with water and ethanol, respectively. Finally, the crude product was dried in a vacuum oven at 323 K overnight (Scheme
The schematic diagram of alkaline ionic liquid (IL) modified Pd/C catalyst (Pd/C-OH−) synthesis.
The cation exchange of Cl− in Pd/C-Cl− with OH− was described as follows. 2 g catalyst prepared in 2.4 and 3.2 g NaOH were added to a 150 mL Erlenmeyer flask with 50 mL H2O. The reaction system was sealed and stirred at room temperature for 24 h. Finally, the catalyst was filtered with water and dried in a vacuum oven at 323 K overnight.
The substrate HMF (0.08 mmol, 10 mg) was firstly dissolved in 4 g water. A certain amount of Pd/C-OH− catalyst (20 mg, 40 mg, 60 mg, and 80 mg) was added to the reaction mixture quickly and oxygen was passed into the reaction mixture for 5 min. At the end, the reaction mixture was sealed immediately. Finally, the reaction mixture was immersed in an oil bath at desired temperature (313 K, 333 K, 353 K, 373 K, and 393 K) with magnetic stirring at a constant rate of 500 rpm in high-pressure cylinders. After reacting for desired time (2 h, 4 h, 8 h, 12 h, and 24 h), 10
The furan compounds were analyzed by HPLC (Shimadzu Technology, model 20AB) equipped with a UV detector. The samples were separated at the wavelength of 280 nm using a reverse-phase C18 column (200 mm × 4.6 mm). The mobile phase was composed of acetonitrile and 0.1 wt.% acetic acid aqueous solution at the flow rate of 0.5 mL·min−1. The contents of HMF, HFCA, FFCA, and FDCA were obtained by the calibration curves of the standard material. The specific calculation was as follows: HMF conversion = moles of converted HMF/moles of starting HMF × 100%. HFCA yield = moles of HFCA/moles of starting HMF × 100%. FFCA yield = moles of FFCA/moles of starting HMF × 100%. FDCA yield = moles of FDCA/moles of starting HMF × 100%.
Scanning electron microscope (SEM) images were produced on a JSM-7500F instrument and obtained at 30 KV with 1.4 nm image resolution. X-ray powder diffraction (XRD) patterns were measured on a Bruker D8 Advance powder diffractometer. XRD was collected in the 2
SEM images of Pd/C and Pd/C-OH− catalysts were shown in Figure
SEM images of (a) Pd/C and (b) Pd/C-OH−.
FTIR spectra of (a) Pd/C and (b) Pd/C-OH−.
As shown in Figure
TG spectra of Pd/C-OH−.
XRD patterns of the Pd/C, Pd/C-Cl−, and Pd/C-OH− catalysts were shown in Figure
XRD patterns of (a) Pd/C, (b) Pd/C-Cl−, and (c) Pd/C-OH−.
The CO2-TPD profile for Pd/C-OH− was shown in Figure
CO2-TPD profile for Pd/C-OH−.
In this experiment, different catalyst amounts were used to establish the effect of catalyst dosage on the oxidation of HMF with keeping the other operation conditions constant. The effect of catalyst loading was varied over the range of 0–0.08 g/cm3 on the basis of total volume of the reaction mixture [
The effect of catalyst dosage on the HMF oxidation.
Entry | Catalyst dosage (mg) | HMF conversion (%) | HFCA yield (%) | FFCA yield (%) | FDCA yield (%) |
---|---|---|---|---|---|
1 | 0 | 8.10 | — | — | — |
2 | 20 | 80.78 | 66.52 | 8.36 | 5.54 |
3 | 40 | 100 | 26.61 | 11.13 | 61.52 |
4 | 60 | 100 | 35.48 | 4.73 | 59.54 |
5 | 80 | 100 | 30.64 | — | 45.89 |
It has been reported that solvents were very important in the chemical reaction, because solvent has different properties, such as polarity, dielectric constant, steric hindrance, and acid base [
Effect of the solvents on the HMF oxidation. Reaction conditions: Pd/C-OH− (40 mg), HMF (10 mg), 353 K, and 12 h.
As the results shown in Table
The effect of reaction temperature on the HMF oxidation.
Entry | Reaction temperature (K) | HMF conversion (%) | HFCA yield (%) | FFCA yield (%) | FDCA yield (%) |
---|---|---|---|---|---|
1 | 313 | 83.12 | 43.46 | 3.75 | 35.50 |
2 | 333 | 100 | 40.97 | 8.44 | 50.52 |
3 | 353 | 100 | 26.61 | 11.13 | 61.52 |
4 | 373 | 100 | 9.76 | 12.81 | 76.79 |
5 | 393 | 100 | — | — | 78.55 |
In order to give more insights into the aerobic HMF oxidation over Pd/C-OH− catalyst, the oxidation of HMF at different reaction time point was recorded, and the results were shown in Figure
Effect of the reaction time on the HMF oxidation. Reaction conditions: Pd/C-OH− (40 mg), HMF (10 mg), H2O (4 g), and 373 K.
Base played an important role in the oxidation of HMF into FDCA. One important reason for the use of base was that base can be used to neutralize the FDCA, avoiding FDCA adsorbed on the surface of catalyst. The salt of FDCA was dissolved in the reaction solution and kept the catalyst activity stable [
The effect of base on the HMF oxidation.
Entry | Base (mmol) | HMF conversion (%) | HFCA yield (%) | FFCA yield (%) | FDCA yield (%) |
---|---|---|---|---|---|
1 | 0 | 36.79 | — | 10.08 | 21.75 |
2 | 0.10 (solid base) | 100 | 26.61 | 11.13 | 61.52 |
3 | 0.10 (solid base) | 100 | 9.76 | 12.81 | 76.79 |
4 | 0.10 (homogeneous base) | 99.90 | 62.09 | 18.70 | 18.62 |
5 | 0.10 (homogeneous base) | 100 | 49.67 | 11.42 | 37.64 |
To verify the recyclability and stability of heterogeneous catalysts, the recycling experiments of the Pd/C-OH− were carried out at 373 K with 40 mg catalyst for 12 h. After the reaction, the supernatant was removed. The catalyst was washed with water and ethanol twice. Finally, the catalyst was dried in a vacuum oven at 323 K. Then the spent catalyst was reused for the next cycle under the same reaction conditions. The results were shown in Figure
Recycle experiments of the catalyst. Reaction conditions: Pd/C-OH− (40 mg), HMF (10 mg), 373 K, H2O (4 g), and 12 h.
In summary, the Pd/C-OH− catalyst was successfully prepared and characterized by SEM, XRD, TG, FT-IR, and CO2-TPD technologies. The catalyst showed high catalytic activity in the HMF oxidation and reaction temperature, reaction time, catalyst amount, and solvent greatly affected the oxidation of HMF. Under optimal reaction conditions, high HMF conversion of 100% and FDCA yield of 82.39% were achieved over Pd/C-OH− catalyst in water at 373 K for 24 h. A much more environmentally benign and green process for aqueous HMF oxidation was reported using solid base modified activated carbon-supported palladium nanoparticle catalyst (Pd/C-OH−).
The authors declare that the received funding did not lead to any conflicts of interest regarding the publication of this manuscript.
This work was funded by the National Natural Science Foundation of China (nos. 31170672, 21406093, and 21676125), the Natural Science Foundation of Jiangsu Province (BK20140529), Key University Science Research Project of Jiangsu Province (14KJB530001), China Postdoctoral Science Foundation (2014T70489, 2013M541621, 1302152C, and 2014M550271), Six Talent Peaks Project in Jiangsu Province (2015-NY-016), Foundation for the Eminent Talents and Research Foundation for Advanced Talents of Jiangsu University (12JDG074 and 14JDG014), and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.