Carbon Support with Tunable Porosity Prepared by Carbonizing

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A carbon with tunable porosity prepared by carbonizing chitosan for catalytic oxidation of 5-hydroxylmethylfurfural Hyejin Yu, Kyung-An Kim, Myung Jong Kang, Sung Yeon Hwang, and Hyun Gil Cha ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03775 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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ACS Sustainable Chemistry & Engineering

A carbon support with tunable porosity prepared by carbonizing chitosan for catalytic oxidation of 5hydroxylmethylfurfural Hyejin Yu,1, ‡ Kyung-An Kim, 1, ‡ Myung Jong Kang,1 Sung Yeon Hwang,1,2* Hyun Gil Cha1,* 1 Research

Center for Bio-based Chemistry, Korea Research Institute of Chemical Technology, Ulsan 44412, Republic of Korea

2 Advanced

Materials and Chemical Engineering, University of Science and Technology (UST), Daejeon 34113, Republic of Korea

Corresponding Author Sung Yeon Hwang: E-mail: [email protected] Hyun Gil Cha: Phone: E-mail: [email protected]

ACS Paragon Plus Environment

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT.

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Sustainable and efficient catalyst is needed for the aerobic oxidation of 5-

hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA).

FDCA, one of the

promising bio-based chemicals derived from cellulose or hemicellulose. Here, we report a porous carbon support prepared by the carbonization of chitosan biomass. The activated porous carbon support can be decorated with useful species with maintaining large surface area and pore volume. The morphology, structure, composition, and catalytic properties of prepared Pt-ACS-t catalyst have been systematically investigated. The catalytic performance of Pt supported on the activated porous carbon is excellent, showing aerobic oxidation activity superior to that of inactivated carbon (CS-800), having a larger surface area, high pore volume, many beneficial species and long-term durability for basic additives-free aerobic oxidation of HMF. The tunable porosity and removal of oxygenated species from the surface of chitosan in the carbonization process are believed to be responsible for the impressive aerobic oxidation reactivity of this heterogeneous catalyst.

KEYWORDS; Chitosan, Porous carbon, HMF, FDCA, Aerobic Oxidation


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Introduction Environmental contamination of petroleum derived materials has stimulated research activities for decades into the utilization of biomass derived renewable chemical resources in aspect of renewable resources.1 Lignocellulosic biomass, which is nonedible and abundant biomass source, regarded as ideal ways to develop subsequent production of high-valued chemical under mild conditions. In production of bio-based polymers, such as polyethylene-2,5-furandicarboxylate (PEF), 2,5-Furandicarboxylic acid (FDCA) has significant value as a biomass derived precursor material. Interestingly, FDCA is a viable substitute for petroleum-derived terephthalic acid for synthesizing polybutyleneterephthalate (PBT) and polyethylene terephthalate (PET) plastics.2-4 Recently, previous studies on FDCA conversion are focused on the noble-metal containing catalysts, such as Pd, Au, Ru, and Pt.5-9 The supports materials are generally activated carbon or metal oxides. However, oxide supports (mesoporous silica, γ-Al2O3, TiO2, hydrotalcite and etc.)1013

are unstable in high temperature, high pressurized aqueous conditions, which is the common

reaction condition for most of biomass transformation reactions for breaking-down structural integrity of biomass materials. This condition resulted in product contamination by leached metals contained reaction solution, loosing catalyst materials and inhibiting subsequent reuse. For example, Gupta et al.10 reported homogeneous base-free solution condition for aerobic oxidation of 5-hydroxymethylfurfural (HMF) with a hydrotalcite-supported gold nanoparticle catalyst. The yield of FDCA was almost 100% after reaction at 95 °C for 7 h reaction. However, still the stability of the catalyst remained as a problem because of the leaching out of the metal ions from hydrotalcite supporter into the aqueous solution. Therefore, the use of carbon-based catalyst supporters has been investigated widely. Carbon is a leading support material for chemical catalytic biomass transformation reactions because of its excellent electron conductivity with high


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porosity, large specific surface area and relative chemical inertness.14 In addition, it can be chemically functionalized or decorated with metallic nanoparticles to impart or improve catalytic activity. Besson et al.15 reported on FDCA production via oxidation of HMF in mild basic aqueous solutions with Pt and Pt-Bi catalysts attached on activated-carbon-supporter and Bi increases the activity and stability of the Pt/C catalyst, achieving high selectivity to FDCA during HMF oxidation reaction. For industrial processes, a homogeneous base has disadvantage on increasing reactor corrosion possibility.

Therefore, the oxidation of HMF in homogeneous base-free

atmosphere is better for industrial applications, as well as being environmentally friendly. To avoid the disadvantages of existing carbon supported catalysts for HMF oxidation, herein, we employed chitosan, which is the common natural nitrogenous organic material, as a carbon precursor and K2CO3 as a moderate activator to prepare heterogeneous catalyst for the aerobic oxidation of HMF to FDCA under mild conditions by a simple chemical activation method. Although several previous researches have prepared porous carbons by activating biomass with K2CO3,14,

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this is the first try of employing chitosan to produce a porous support for HMF

oxidation under base-free condition. Herein, we report that the texture of the obtained porous carbons can be easily regulated by varying the activation temperature, yielding a porous carbon with very high catalytic ability for HMF oxidation and a high FDCA yield. Furthermore, the activated-carbon-supported Pt catalyst shows excellent recyclability.

EXPERIMENTAL SECTION The following chemicals were used as it is. HMF (99%), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), FDCA (97%), chitosan (D-glucosamine, low molecular weight), K2CO3, NaHCO3, polyvinylpyrrolidone (PVP, Mw = 9,000-10,000, 80% hydrolyzed), K2PtCl4, and NaBH4 were


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purchased from Sigma-Aldrich Co. 2,5-Furandicarboxaldehyde (DFF, 97%) and 5-formyl-2furancarboxylic acid (FFCA) (98%) were obtained from TCI America. HCl was obtained from SAMCHUN Chemical. Deionized water (Millipore Milli-Q Integral 10, resistivity >18 MΩ cm) was used to prepare the solutions. Preparation of activated chitosan carbon as a support First, 3 g of chitosan and 6 g of K2CO3 (ratio 1:2) were mixed in 10 mL of DI water and stirred for 30 min vigorously. The aqueous mixture was evaporated at 60 °C by an evaporator, and the mixture was dried at 100 °C in an oven overnight. Carbonization was carried out at the desired temperature (from 600–800 °C at a heating rate of 5 °C/min) under Ar flow. Then, the temperature was maintained for 2 h. After cooling to room temperature, the remaining K2CO3 was etched in 1 M HCl for 1 h with sonication, and the carbonized chitosan was dried at 80 °C in an oven overnight. The obtained materials are denoted ACS-t, where t represents the activation temperature of chitosan/K2CO3 mixture. For comparison, pure chitosan was carbonized at 800 °C without the K2CO3, denoted CS-800. Preparation of Pt-CS-800 and Pt-ACS-t catalysts Pt nanoparticles were immobilized on the support by the wet impregnation method. First, 0.2 g of ACS-t was dispersed in 20 mL of DI water, and then 0.01 g of K2PtCl4 was added to the dispersed ACS-t solution. The mixed solution was sonicated for 1 h. Then, 5 mL of NaBH4 solution (NaBH4/Pt molar ratio = 5) was slowly added and stirred for 0.5 hr. The mixture was filtered and washed with DI water and dried at 60 °C in an oven overnight. Preparation of Pt-PVP-ACS-800 catalyst First, 0.01 g of K2PtCl4 as a Pt precursor and 0.2 g of ACS-800 mixed solution was sonicated for 1 h. Then, PVP was added, and the mixture was stirred for 30 min (PVP/Pt molar ratio = 10).


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5 mL of NaBH4 solution (NaBH4/Pt molar ratio = 5) was slowly added to the mixed solution and stirred for 30 min. The mixture was filtered and washed with DI water and dried at 60° C in an oven overnight. The obtained material is denoted Pt-PVP-ACS-t.

CHARACTERIZATION X-ray diffraction (XRD) measurements were performed using an Ultima IV Rigaku analyzer equipped with a X-ray tube (40 kV, 30 mA) using Cu Kα radiation (Kα = 1.54056 A). Raman spectroscopic measurements were performed using a Thermo Fisher DXR Raman microscope. A 532 nm of green laser was used as the excitation source, and the spectra were scanned between 800 and 2000 cm-1. Field emission scanning electron microscope (FE-SEM) measurements were performed by using a TESCAN Mira 3 operated at 20 kV. Transmission electron microscopy (TEM) measurements were performed using a JEM 2010 electron microscope operated at 200 kV. The sample were suspended in ethanol and dispersed ultrasonically for preparing TEM measurements. The suspension was dropped on a carbon coated copper grid. X-ray photoelectron spectroscopy (XPS) measurements were performed using monochromated X-ray source (Al Kα radiation, 1846.6 eV) and fitted by an iterative least squares algorithm (CasaXPS software) using a Gaussian–Lorentzian (70/30) peak shape and applying the Shirley background correction. The porosity of the samples was investigated by Nitrogen gas adsorption at 77 K using an Atosorb IQ gas sorption analyzer (Quantachrome). The Brunauer-Emmett-Teller (BET) equation was used for calculating specific area of catalysts. The amount of adsorbed nitrogen at a relative pressure of P/Po = 1.0 was used to determine the total pore volume (VT). The micropore volume (VDR) was calculated by the Dubinin–Radushkevich equation.

The volume of mesopore (Vmes) was

calculated from the difference between VT and VDR. The distribution of pore size was also


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calculated by the quenched-solid density functional theory method (QSDFT) with the isotherm results. Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements were performed by using a Thermo Scientific iCAP 7400 Duo to measure the actual loading amounts of Pt or other metals.

Catalytic reaction The aerobic oxidation was carried out in a batch-type reactor (20 mL) equipped with a pressure gauge. The vessel was charged with 10 mL of aqueous 50 mM HMF solution and 0.04 g of catalyst. The reactor was then purged three times with oxygen gas, leaving the vessel at the desired pressure, stirred for 450 rpm, and raised to the required temperature for the required time. After the reaction, the reactor was rapidly cooled to room temperature, and a known volume of 0.1 M NaHCO3 solution was added to dissolve the products that may have precipitated. The solid catalyst was recovered by filtration, and the liquid product was analyzed by high-performance liquid chromatography (HPLC, YL9100 chromatograph system with an ultraviolet-visible detector (YL9120, 265 nm). 5 mM of Sulfuric acid was used as the mobile phase in isocratic mode. Before the first injection, one hour of equilibration was performed. The samples and standards (10 µL) were injected directly onto a 300 mm × 7.7 mm Hi-Plex H column purchased from Agilent Technologies with a 0.5 mL/min of flow rate at 65 °C. Additionally, the products were identified by comparison with known commercially pure samples. The external calibration method was used for the quantification of the amounts of reactants consumed and products generated. The HMF conversion and the product yield were calculated using eqs (1) and (2). Conversion (%) = Yield (%) =

𝑚𝑜𝑙 𝑜𝑓 𝐻𝑀𝐹 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑚𝑜𝑙 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝐻𝑀𝐹

𝑚𝑜𝑙 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 𝑚𝑜𝑙 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝐻𝑀𝐹

(1)

× 100%

× 100%

(2)


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RESULTS AND DISCUSSION

Catalyst characterization As shown Figure 1, the sample morphology was investigated by SEM characterization. In comparing the SEM images of the samples, we could not observe macropores in CS-800. The chitosan particles have an irregular, stone-like morphology in Figure 1(a). However, threedimensional interconnecting macropores were formed by the activation reaction at 800 °C under Ar atmosphere with K2CO3 as the activator, as shown in Figure 1(b). Thus, the morphology of the chitosan particles changed to that of porous carbon. The activation mechanism of chitosan by K2CO3 has been proposed as below; 17 K2CO3 + -CH2 → K2O + 2CO + H2

(1)

K2CO3 + 2-CH → K2O + 3CO + H2 (2) K2O + -CH2 → 2K + CO + H2 (3) 2K2O + 2-CH → 4K + 2CO + H2 (4) The amount of CO and H2 generated during the activation procedure depends on the activation temperature. The porous carbon structure is beneficial, providing a large surface area for catalytic reactions. The pore structures of the CS-800 and ACS-t samples were investigated by the N2 isotherm adsorption–desorption method (Table 1). The parameters for pore structure of the samples activated at various temperatures with same amount of activator. The nitrogen adsorption-


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desorption isotherms for CS-800, ACS-600, ACS-700, and ACS-800 are shown in Figure S1. All samples showed isotherms that are a combination of type I and type-IV behaviors. Hysteresis loops can be seen for the CS-800, ACS-600, ACS-700, and ACS-800 samples within a broad relative pressure (P/Po) range of 0.4-0.9; this is due to capillary condensation in the mesopores structure. The K2CO3 activation process is the reaction between the carbon and K2CO3, removing carbon atoms and playing a critical role in increasing the microporosity of carbon, leaded an increased specific surface area of carbon. Thus, the specific surface area of the carbon supporters increased through activation process, from 1154 m2g-1 for ACS-600 to 1979 m2g-1 for ACS-700 and, finally, to 2908 m2g-1 for ACS-800, with increasing activation temperature. Furthermore, the total pore volume change is well matched with that of the specific surface area, which means that the formation of highly porous structure is caused from activation process by K2CO3. The percentage micropore volumes compared to volume of the total pore of the K2CO3 activated carbon supporter materials (88% for ACS-600, 88% for ACS-700 and 93% for ACS-800) are increased than that of activated carbon (35%) without K2CO3. This result implies that gasification by K2CO3 contributed on the increased microporosity. The pore sizes investigated by the Hgoorvath-Kawazoe (HK) methods. The size of meso and micro pore were reduced from 1.3 and 0.77 nm for CS-800 and ACS-800, respectively. The decrease in the pore sizes of the activated carbon supporter samples with the K2CO3 activation are in accordance with the increase of the micropore volume percentage. Figure 2(a) presents XRD data for the porous carbons prepared at different activation temperatures. Note that two broad peaks, corresponding to the (100) and (002) planes at around 24° and 42°, respectively, were observed for all samples, indicating the graphitic structure of carbonized chitosan. The broad diffraction pattern and low intensity of the (002) reflections in the


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ACS-t samples compared with those of CS-800 represent the poorly ordered graphitic structure arising from the increasing microporous structure with increasing activation temperature. The ratio of the height of the (002) Bragg peak to the background, defined as R, is shown in Figure 2(a). The R-values decrease as following the increase of single layer contents in the carbon, as reported by Dahn et al.18 An increase of R-value means that a higher degree of graphitization of porous carbon, which indicates a more significant concentration of parallel single graphitic layers in the porous carbon. The R-values of ACS-600 and ACS-700 are a little larger than that of ACS800 because a relatively higher activation temperature during the CO and H2 gas flow lead to orientation of some edge and reduce the nonparallel single layers concentration. Thus, a slightly higher degree of graphitization of the porous carbon resulted by the K2CO3 activation. For the ACS-t samples, the R-values decreased with increasing activation temperature. Notably, the Rvalue becomes close to 1 with increasing activation temperature because of the graphene sheets was distributed randomly in the structure. As a result, a breakdown of aligned structural domains in the carbon matrix is caused by K2CO3 activation process, because of the potassium compounds, which is intercalated. The further specific structure is elucidated based on Raman spectroscopy. As shown in Figure 2(b), the peaks of the breathing mode of k-point phonons with A1g symmetry present at around 1320 cm-1 (D-bands), which corresponding to defective graphitic structures or disordered carbons. The peaks at around 1580 cm-1 (G-bands) are assigned to the E2g phonons of sp2 carbon atoms, correspond to the tangential vibrations of the carbon atoms which are a characteristic feature of graphitic layers. Thus, the intensity ratios of these two peaks partially depend on the degree of graphitization, which are consistent with the XRD results. The XPS analysis was performed on CS-800 and ACS-t samples prepared at different activation temperatures, and the results are shown in Figure 3. The surface nitrogen and oxygen spectra are


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shown in Figure 3(a) and 3(b), respectively. Likewise, the contents of surface nitrogen in the ACSt samples decreased from 7.8 wt% to close to zero with increasing activation temperature. The fast decomposition of the N species during the K2CO3 activation process released N2 gas. In the case of surface oxygen species, with increasing activation temperature, the surface oxygen content decreased from 16.6 to 5 wt%, as calculated by XPS analysis and summarized in Table 2. The deconvolution of the O 1s spectra showed the four peaks as follow: quinines derived carbonyl oxygen for peak 1 (531.1 eV); esters and anhydrides derived carbonyl oxygen atoms and hydroxyl groups derived oxygen atoms corresponding to peak 2 (532.3 eV); esters and anhydrides derived non-carbonyl (ether-type) oxygen atoms matched with peak 3 (533.3 eV); and water absorbed and/or oxygen for peak 4 (534.2 eV). These results are consistent with previous reports.

Catalytic performance of the activated-carbon-supported catalysts The aerobic oxidation of HMF usually follows the mechanism shown in Scheme 1.19-20 The hydroxyl group of HMF is oxidized to a formyl group firstly. After it, the two formyl groups in DFF are oxidized to biacid form successively. The whole reaction pathway follows route A pathway. This route is same with most previous reports of catalytic oxidation in aqueous condition without a base. HMF can be converted to HMFCA rapidly in the presence of hydroxide ions in a basic solution by the formation of a geminal diol via nucleophilic addition of a hydroxide ion to the aldehyde with a proton transferred from water to the intermediate of alkoxy ion. HMFCA is then turned to FFCA by the deprotonation of an alcohol to an alkoxy group. In base-free conditions, without the assistance of hydroxide ions, the HMF is rapidly converted into DFF by the oxidation of hydroxymethyl groups of HMF into the formyl group of DFF on Pt active sites. Then, one formyl group of DFF is subsequently converted into a carboxyl group, forming FFCA. Finally,


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the remaining formyl group of FFCA is oxidized to the carboxyl group, and the FDCA is formed. This last step is considered the rate-determining step of the overall oxidation of HMF into FDCA. Plenty of previous studies reported that oxygen-containing functional groups such as carboxyl, carbonyl/quinone, phenol, ester groups and anhydride, on carbon surfaces can influence the catalytic behavior of carbon-based catalysts.21-22 An oxygenated surface of ACS-t has been reported to be unfavorable for the FDCA production via aerobic oxidation of HMF in the base condition over a Pt/AC catalyst, whereas our study presented that the carbonyl/quinone groups on ACS-t enhanced the formation of FDCA with a Pt/ACS-t catalyst within base-free conditions. To obtain further insights into the possible effects of the functional groups on the ACS-t surfaces, we investigated the Pt/ACS-t catalysts with chitosan activated at different temperatures. The oxidation of HMF to FDCA with the Pt-ACS-t catalysts was performed at 1.0 MPa O2 and 110 °C without any additive bases such as NaOH, Na2CO3, or NaHCO3. The Pt-ACS-t (t = 600800) samples were used to investigate the effect of the carbonization temperature on the aerobic oxidation of HMF. For comparison, Pt-CS-800 catalysts were also used for HMF oxidation under the same conditions. As shown in Figure 4, HMF was converted completely to FDCA, and the highest FDCA yield was achieved by Pt-ACS-800. Finally, a 75% yield of FDCA was achieved, and a 25% yield of FFCA was detected as an intermediate. Pt-ACS-600 showed lower catalytic activity than Pt-ACS-800, whereas Pt-CS-800 hardly showed any catalytic activity for the FDCA production via aerobic oxidation of HMF. Thus, HMF oxidation is highly related to the activation temperature used combined with the use of K2CO3 as an activator. Activated carbon can adsorb many PtCl42- cations. As shown by the ICP analysis, the Pt loading was 2.24 wt% in Pt-ACS-800 but 1.95 and 1.95 wt% in Pt-ACS-700 and Pt-ACS-600, respectively. The adsorption of Pt is related to the surface charge and porosity of ACS-t. In the case of ACS-


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600, there are few sites for Pt adsorption because of the relatively low surface area and negative charge because of the abundant oxygen groups, as detected by XPS analysis. On the other hand, the samples prepared at increasing activation temperature, such as ACS-800, showed larger surface areas and lower oxygen contents than those prepared at a lower temperature, such as ACS-600. Therefore, ACS-800 readily adsorbed PtCl42- anions on the carbon surface.

Pt size effect on HMF oxidation The effect of the PVP capping agent on the aerobic oxidation of HMF was investigated, and the results are plotted in Figure 5(a). When Pt-PVP-ACS-800 was used as a catalyst for HMF oxidation, the conversion of HMF reached 99% (yield of FDCA) over 5 h at 110 °C and 1.0 MPa. In contrast, the Pt-ACS-800 sample yielded only 75% FDCA. This result shows the influence of the size of the Pt particles on HMF oxidation. Based on the TEM study in Figure S2, the average size of Pt nanoparticles without PVP capping agent was 3.93 nm, and that with PVP capping agent was 2.02 nm. These results indicate that the PVP-capped Pt nanoparticles become smaller and the particle size distribution becomes narrower than that of the PVP-free Pt nanoparticles. This property of PVP-capped Pt nanoparticles provided more active sites, resulting in high conversion activity for HMF oxidation. Figure 5(b) shows the time profiles of the aerobic oxidation of HMF oxidation over Pt/PVP-ACS-800. In a short reaction time, a low concentration of DFF was detected, which was quickly converted into FFCA. The conversion of FFCA to FDCA was much slower.

This result is in good agreement with the reaction pathway considering the rate-

determining step of HMF oxidation to FDCA. The effect of O2 pressure and reaction temperature of HMF oxidation was investigated, and the results are shown in Figures 5(c) and 5(d).


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With increasing reaction temperature, the FDCA yield increased significantly, and 99% FDCA yield was recorded at 110 °C, with a negligible amount of FFCA was detected. The influence of the O2 pressure on HMF oxidation is plotted in Figure 5(d). At low pressure of 0.5 MPa, the intermediate product, FFCA, was detected, corresponding that the incomplete oxidation process was occur. On higher pressure in the reaction process, 1.0 MPa, the catalytic activity of the PtPVP-ACS-800 catalyst was increased with observing higher FDCA yield. At O2 pressures greater than 1.0 MPa, some of the product disappeared. Thus, we determined the optimum conditions to be 110 °C, 1 MPa O2, and 5 h.

Catalyst reusability The stability and reusability of a catalyst are important factors for industrial use. Therefore, the Pt/PVP-ACS-800 catalyst was used 10 times to test its reusability. The results are plotted in Figure 6(a). This catalyst was facilely reused after precipitation by centrifuge, washing with water, and drying in an oven at 60 °C overnight before further reaction. HMF was entirely converted in all runs, with achieving 97% FDCA yields until run five. With further reaction cycles, the yield of FDCA decreased, and, at the same time, the yield of FFCA increased. As shown in Fig. 6(b), TEM observation of the 10-times-used Pt/PVP-ACS-800 shows the aggregation of Pt nanoparticles. This phenomenon might cause a reduction in the catalyst activity.

CONCLUSIONS A highly porous carbon support was prepared by the carbonization of chitosan, which is the second most abundant polysaccharide found in nature. The activated porous carbon support (ACS800) not only has a large surface area (2908 m2g-1) and high pore volume (1.418 cm3g-1) but also


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shows enhanced FDCA production via aerobic oxidation of HMF because of the easily adsorbed PtCl42- anions and low content of oxygen-containing groups. PVP-capped Pt nanoparticles on the surface of ACS-800 showed a significant increase in the catalytic activity for HMF oxidation because of the reduction in the size of the Pt nanoparticles. The conversion of HMF and yield of FDCA were both higher than 99%, which was achieved using Pt/PVP-ACS-800 for 5 h at 1.0 MPa O2 pressure and 110 °C. The Pt/PVP-ACS-800 catalyst showed good stability for HMF oxidation over five reaction cycles without significant loss of catalytic activity. These results demonstrate the promise for the biomass conversion of activated porous carbon catalysts derived from chitosan biomass. Thus, Pt-PVP-ACS-800 is an optimal catalyst for HMF oxidation reaction under basefree mild conditions given both the high stability and high activity.

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8. Corro, G.; Cebada, S.; Pal, U.; Fierro, J. L. G., Au0–Au3+ bifunctional site mediated enhanced catalytic activity of Au/ZnO composite in diesel particulate matter oxidation. Journal of Catalysis 2017, 347, 148-156, DOI 10.1016/j.cat.2017.01.011. 9. Morales-Flores, N.; Pal, U.; Sánchez Mora, E., Photocatalytic behavior of ZnO and Ptincorporated ZnO nanoparticles in phenol degradation. Applied Catalysis A: General 2011, 394 (1), 269-275, DOI 10.1016/j.apcata.2011.01.011. 10. Gupta, N. K.; Nishimura, S.; Takagaki, A.; Ebitani, K., Hydrotalcite-supported goldnanoparticle-catalyzed highly efficient base-free aqueous oxidation of 5-hydroxymethylfurfural into 2, 5-furandicarboxylic acid under atmospheric oxygen pressure. Green Chem. 2011, 13 (4), 824-827, DOI 10.1039/C0GC00911C. 11. Tao, F. F.; Shan, J.-j.; Nguyen, L.; Wang, Z.; Zhang, S.; Zhang, L.; Wu, Z.; Huang, W.; Zeng, S.; Hu, P., Understanding complete oxidation of methane on spinel oxides at a molecular level. Nat. Commun. 2015, 6, 7798, DOI 10.1038/ncomms8798. 12. Wang, Y.; Yu, K.; Lei, D.; Si, W.; Feng, Y.; Lou, L.-L.; Liu, S., Basicity-Tuned Hydrotalcite-Supported Pd Catalysts for Aerobic Oxidation of 5-Hydroxymethyl-2-furfural under Mild Conditions. ACS Sustain. Chem. Eng. 2016, 4 (9), 4752-4761, DOI 10.1021/acssuschemeng.6b00965. 13. Mishra, D. K.; Lee, H. J.; Kim, J.; Lee, H.-S.; Cho, J. K.; Suh, Y.-W.; Yi, Y.; Kim, Y. J., MnCo2O4 spinel supported ruthenium catalyst for air-oxidation of HMF to FDCA under aqueous phase and base-free conditions. Green Chem. 2017, 19 (7), 1619-1623, DOI 10.1039/C7GC00027H. 14. Fan, X.; Zhang, L.; Zhang, G.; Shu, Z.; Shi, J., Chitosan derived nitrogen-doped microporous carbons for high performance CO2 capture. Carbon 2013, 61, 423-430, DOI 10.1016/j.carbon.2013.05.026. 15. Rass, H. A.; Essayem, N.; Besson, M., Selective aqueous phase oxidation of 5hydroxymethylfurfural to 2, 5-furandicarboxylic acid over Pt/C catalysts: influence of the base and effect of bismuth promotion. Green Chem. 2013, 15 (8), 2240-2251, DOI 10.1039/C3GC40727F. 16. Fujiki, J.; Yogo, K., The increased CO2 adsorption performance of chitosan-derived activated carbons with nitrogen-doping. Chem. Commun. 2016, 52 (1), 186-189, DOI 10.1039/C5CC06934C. 17. Lu, C.; Xu, S.; Liu, C., The role of K2CO3 during the chemical activation of petroleum coke with KOH. J. Anal. Appl. Pyrolysis 2010, 87 (2), 282-287, DOI 10.1016/j.jaap.2010.02.001. 18. Liu, Y.; Xue, J.; Zheng, T.; Dahn, J., Mechanism of lithium insertion in hard carbons prepared by pyrolysis of epoxy resins. Carbon 1996, 34 (2), 193-200. DOI 10.1016/00086223(96)00177-7. 19. Artz, J.; Palkovits, R., Base‐Free Aqueous‐Phase Oxidation of 5‐Hydroxymethylfurfural over Ruthenium Catalysts Supported on Covalent Triazine Frameworks. ChemSusChem 2015, 8 (22), 3832-3838, DOI 10.1002/cssc.201501106. 20. Cha, H. G.; Choi, K.-S., Combined biomass valorization and hydrogen production in a photoelectrochemical cell. Nat. Chem. 2015, 7 (4), 328-333, DOI 10.1038/nchem.2194. 21. Figueiredo, J.; Pereira, M.; Freitas, M.; Orfao, J., Modification of the surface chemistry of activated carbons. Carbon 1999, 37 (9), 1379-1389, DOI 10.1016/S0008-6223(98)00333-9. 22. Gallezot, P., Conversion of biomass to selected chemical products. Chem. Soc. Rev. 2012, 41 (4), 1538-1558, DOI 10.1039/C1CS15147A.


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FIGURES.

Figure 1. SEM images of (a) CS-800 and (b) ACS-800.


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Figure 2. (a) XRD and (b) Raman spectra of the CS-800 and ACS-t samples prepared at different activation temperatures.


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Figure 3. XPS N 1s and O 1s spectra of the CS-800 and ACS-t samples. B.E. is binding energy.


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Figure 4. Comparison of catalytic properties in base-free HMF conversion using Pt-CS-800 and Pt-ACS-t prepared at different activation temperatures. Reaction conditions: HMF (0.5 mmol), H2O (10 ml), catalyst (40 mg), O2 pressure (1.0 MPa), time (5 h), and temperature (110 °C).


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Figure 5. (a) Comparison of the catalytic properties of Pt-ACS-800 and Pt-PVP-ACS800. (b) Time profiles of HMF oxidation over Pt/PVP-ACS-800. (c) and (d) Effect of O2 pressure and temperature on HMF oxidation by Pt-PVP-ACS-800, respectively.


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Figure 6. Reusability tests of Pt/PVP-ACS 800 (a) and a TEM image of the used Pt/PVP-ACS 800 catalyst (b). Reaction conditions: HMF (0.5 mmol), H2O (10 ml), catalyst (40 mg), O2 pressure (1.0 MPa), time (5 h), and temperature (110 °C).


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Scheme 1. Possible reaction mechanism for the aerobic oxidation of HMF to FDCA.


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Table 1. Pore structure parameters of the activated samples.

Samples

Textural properties SBET (m2/g)

Vtot (cm3/g)

Vmicro (cm3/g)

DHK (nm)

CS-800

57

0.091

0.032

1.325

ACS-600

1154

0.566

0.502

0.577

ACS-700

1979

0.862

0.799

0.668

ACS-800

2908

1.418

1.317

0.773

Vtot: Total pore volume at P/P0 = 0.99 Vmicro: Analyzed by the t-plot method DHK: median micropore size by the HK method.


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Table 2. Results of the fits of the O 1s region. The values are given as the percentage of total intensity.

Samples CS-800 ACS-600 ACS-700 ACS-800

531.11) 41.6 20.9 33.7

Binding energy (eV) 532.32) 533.33) 34.4 16.7 30.9 47.8 37.8 38.9 31.6 28.5

534.24) 7.3 21.3 2.4 6.2

Chemical composition (wt%) C O N 82.1 9.74 8.15 75.6 16.6 7.8 91.2 7.4 1.4 94.6 5 -

1)531.1

eV, C=O, carbonyl/quinone groups at 531.1 eV.

2)532.3

eV, carbonyl oxygen atoms in esters and anhydrides and oxygen in hydroxyls at 523.3

eV. 3)533.3

eV, ether oxygen atoms in esters and anhydrides at 534.2 eV.

4)534.2

eV, the oxygen atoms in carbonyl groups at 534.2 eV.


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ASSOCIATED CONTENT Supporting Information. N2 adsorption-desorption isotherms, Barrett-Joyner-Halenda (BJH) pore size distribution curves, and TEM results are provided as a Supporting Information

AUTHOR INFORMATION Corresponding Author Sung Yeon Hwang: E-mail: [email protected] Hyun Gil Cha: Phone: E-mail: [email protected]

Notes The author declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Korea Research Institute of Chemical Technology (KRICT) core project (SI1809&KK1806). S.Y.H. acknowledges funding from the Ministry of Trade, Industry and Energy (MOTIE, Korea) through the Technology Innovation Program (10070150).


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Table of content For Table of Contents Use Only Biomass derived porous carbon support Pt catalyst is very efficient for aerobic oxidation of HMF into valuable chemical without base additives.


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Carbon Support with Tunable Porosity Prepared by Carbonizing

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