Aerobic Oxidation of 5-Hydroxymethylfurfural to High-Yield 5

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Aerobic Oxidation of 5-Hydroxymethylfurfural to high yield 5-hydroxymethyl-2-furancarboxylic acid by polyvinylpyrrolidone -capped Ag Nanoparticle Catalysts Jiahuan An, Guohan Sun, and Haian Xia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05916 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Sustainable Chemistry & Engineering

Aerobic oxidation of 5-Hydroxymethylfurfural to high yield 5-hydroxymethyl-2-furancarboxylic acid by polyvinylpyrrolidone -capped Ag nanoparticle catalysts

JiahuanAn, Guohan Sun, Haian Xia* Jiangsu provincial key lab for the chemistry and utilization of agro-forest biomass, College of Chemical Engineering, Nanjing Forestry University, 159 Longpan Road, Nanjing 210037, China *Corresponding

author :Haian Xia

Tel: +86-25-85427635; Fax:+86-25-85428873 E-mail: [email protected]

1

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Abstract An efficient method for the catalytic aerobic oxidation of 5-Hydroxymethylfurfural(HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was developed using size-controlled Ag nanoparticles(NPs) in an aqueous solution. A series of AgNPs catalysts stabilized by polyvinylpyrrolidone(PVP) supported on ZrO2with different Ag loadings were prepared. XRD, TEM, XPS and FT-IR techniques were used to characterize the nature of these catalysts. The impact of the Ag loading, the molar ratio of PVP to Ag, the reaction temperature and time, and the base typeon the conversion of HMF and yield of HMFCA was investigated. The reaction results show that Ag-PVP/ZrO2 catalystswith a bigger particle size (> 10 nm) exhibited superiorcatalytic activity and HMFCA selectivity compared to Ag/ZrO2 with a smaller particle size (ca. 8.5 nm), indicating that the addition of PVP has a promoting effect on the aerobic oxidation of HMF into HMFCA. The 2.5%Ag-PVP/ZrO2(1:1) catalyst presents an excellent activity with 100% conversion of HMF and 98.2% yield of HMFCAinoxygen flowat 20°Cfor 2 h.The basicity is also very critical to obtain a high HMFCA yield, especially for the use of high concentration of HMF, i.e., Ca(OH)2 afforded a higher selectivity toward HMFCA compared to the use of NaOH or Na2CO3 under an identical reaction condition. It is revealed for the first time that the presence of a capping agent (PVP) can effectively modulate the metal-support interaction. Interestingly, it is found that the relative weak interaction between Ag NPs and the ZrO2 support has a beneficial effect on the conversion of HMF. This work provides new insights into the relationship between the metal-support interaction mediated by PVP molecule and the activity and selectivity of the oxidation reaction.

Keyword:HMF, HMFCA,PVP, Ag 2


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Introduction 5-hydroxymethylfurfural (HMF)is one of the most useful platform chemical, which can derive from carbohydrates such assugar and cellulose.1-12 Diverse products can be obtained by selective oxidation of HMF, such as 2,5-diformylfuran (DFF),13-15 5-formyl-2-furancarboxylic acid (FFCA),16,17 5-hydroxymethyl-2-furancarboxylic acid(HMFCA),18-20 and FDCA21-28(Scheme 1). HMFCA, a typical product derived from the selective oxidation of the formyl group in HMF,has been shown to be a versatile chemical. It canserve as a monomer in the synthesis of various polyesters,29 an antitumor agent,30 and a promising building block of an interleukin inhibitor.31 In contrast to the synthesis of FDCA, only veryfew studies have focused on the catalytic oxidation of HMF to HMFCA. It is not easyto acquire high yields ofHMFCA because HMFCA is only an active intermediate product and is readily further oxidized to FFCA and FDCA in the aerobic oxidation of HMF at relative high temperatures. Avelin Corma et al. developeda method to synthesize HMFCA from HMF using gold catalysts over different supports.32 Compared to Au-CeO2, Au-TiO2 and Au-C, Au-Fe2O3showed higher selectivityto HMFCA with 85% yield of HMFCA and 15% yield of FDCA at 65°Cfor 24 h.Zhang and co-workersused the K-10 clay-Mo catalyst to obtain86.9% yield of HMFCA with 100% conversion of HMF after 3 h in toluene with the flushing of oxygen at 110°C18.They also reported that Ru/CsPW gave a yield of 72.9% with a HMF conversion of 97.2% at 130°Cunder the oxygen atmosphere.19 Recently, Grunwaldtet al. developed a new method to produce HMFCA from HMF using Ag/ZrO2 as catalyst. In their work, a high selectivity towardHMFCA (≥98%) was achieved under the optimized reaction conditions with Ag/ZrO2 catalysts.20 3



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As mentioned above, it is of a great significance to use non-noble metal catalyst and more environmentally friendly solvent for the aerobic oxidation of HMF into HMFCA.Ag, one of the relative cheap metals, has attracted extensive attention in catalysis. Ag nanoparticle catalysts usually exhibit size-dependent effect, surface effect, and macroscopic quantum tunneling effect, and so on.33However,a disadvantageis that Ag NPs are easy to aggregate by the way of Ostwald ripening, especially at high temperatures, owing to their inherent properties. Therefore, a stabilizer or capping agentis needed to protect it from aggregation. Typically, Polyvinyl pyrrolidone (PVP)is one of the most prevalentcapping ligandsowingto its non-toxicity34,35 and favorable solubility in many polar solvents.36-39 Furthermore, PVP can efficiently tune the electronic status of metal NPs by a charge transfer between PVP and control the Ag particle size, thereby mediating their catalytic performances.40 It has been revealed that PVP molecules adsorb on the metal surface through its oxygen atom, resulting in the charge transfer from the metal surface to the polymer, whereas some authors argued that the interaction is through the nitrogen atom, with the charge transfer from PVP to the metal surface, which depends on the metal type and the metal particles size.36,41 However, the use of a capping agent readilycovers or encapsulates the active sites of metalNPs, thereby inhibiting the catalytic activity.42,43 In addition, the metal-support interaction has a remarkable influence on catalytic performances due to interfacial syngeneic catalysis and electronic metal-support interactions (EMSI), etc.44,45 The modulation of metal-support interactions affords a remarkable effect on catalytic reaction due to the geometric/electronic structure of interfacial effect. Wei et al. investigated interfacial synergetic catalysis of Ni@TiO2-x toward water-gas shift reaction (WGS), and revealed that the interfacial Ni species serves as the optimal active sites for H2O decomposition to produce H2.46 However, how to accurately mediate 4


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the metal-support interaction remains a challenge. Herein, we develop a novel method to selectively oxidized HMF to HMFCA in an aqueous solution using the PVP-stabilized Ag NPs supported onZirconium dioxide (ZrO2) as the catalysts47. As far as we know, it is the first time that the use of Ag-PVP/ZrO2 as catalyst catalyzes the aerobic oxidation of HMF into HMFCA. It is found that the metal-support interaction between AgNPs and ZrO2 has a profound effect on the HMF conversion and HMFCA yield.

Experimental Section Materials HMF, Polyvinylpyrrolidone (PVP, K23-27), Sodium borohydride (NaBH4), Zirconium dioxide (ZrO2) were purchased from Aladdin Chemistry Co. Ltd. Sodium hydroxide (NaOH), Acetic acid were obtained from Nanjing Chemical Reagent Co., LTD. Silver nitrate (AgNO3) was supplied by Sinopharm Chemical Reagent Co., Ltd. Preparation of catalysts ZrO2 supported AgNPs were prepared by wet impregnation method with AgNO3 as the precursors and PVP as a capping agent. Ag colloid solution containing was prepared according to the following procedure. The desired amounts of PVP and AgNO3 (three Ag/PVP molar ratios were used, 0.5, 1, 2) were added to 100 mL aqueous solution at room temperature, and the solution was stirred for 2 h using a magnetic stirrer. After that, the NaBH4(0.1 mol/L, the NaBH4/Ag mole ratio was 4) aqueous solution was slowly added into the solution containing PVP and AgNO3 under vigorous stirring. Subsequently, 2 g ZrO2 was added and impregnated for 12 h and then water was removed by concentrating in vacuo to obtain various samples with different molar ratio of Ag to PVP. The samples are denoted as xwt%Ag-PVP/ZrO2(m:n),where x is the Ag 5



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loading, m:n represents as the molar ratio of Ag:PVP. For comparison, a Ag/ZrO2(2.5wt%) catalyst was fabricated as a reference sample through the standard impregnation method using AgNO3 as a precursor. In order to study the influence of the metal-support interaction on catalytic performances, the catalyst was prepared by the other procedure, which is different from the first procedure in which the Ag colloid solution was generated. The AgNO3 precursor and PVP were firstly dissolved into water, and then ZrO2 was added into the mixed solution and impregnated for 12 h. Subsequently, NaBH4was slowly added into the solution followed by removing water. The sample is denoted as xwt%Ag-PVP/ZrO2(m:n)-II. The difference between Ag-PVP/ZrO2(m:n)Ag-PVP/ZrO2(m:n)-II is that Ag-PVP/ZrO2(m:n) samples were prepared through firstly forming Ag colloid solutions prepared by NaBH4 reduction and then adding ZrO2 support, whereas Ag-PVP/ZrO2(m:n)-II was fabricated via firstlyadding ZrO2 support into aqueous solution containing Ag precursor and PVP, and then NaBH4 was added and used to reduce Ag precursor. Characterization of catalysts Powder X-ray diffraction (XRD) was recorded on Ultima IV with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA at a scanning rate of 5 ºmin-1.High resolution transmission electron microscopy (HRTEM) at 200 KV was measured JEM-2100 manufactured by JEOL. The sample was firstly dispersed in ethanol and deposited on a Cu-grid (200 mesh). X-ray photoelectron spectroscopy (XPS) was detected on an Axis Ultra DLD (SHIMADZU) with Al Kα radiation. The peak area ratio of Ag 3d5/2 to 3d3/2 of Ag0 and Ag2O was fixed to 3:2 during the deconvolution of the XPS spectra. Fourier transform infrared (FT-IR) measurements were conducted on a Nicolet 380 FT-IR spectrometer with a spectral resolution of 4 cm-1 in the wave number range of 6


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500-4000 cm-1. Catalytic reactions The catalytic reactions were performed in a 100 mL three-necked flask equipped with a reflux condenser and an oil bath. Firstly, 200 mg HMF was dissolved in 50 mL deionized water and then desired amounts of NaOH (126 mg)and 50 mg catalyst were added.Subsequently, the three-necked flask flushed with oxygen at a rate of 60 mL/min and the temperature of oil bath was 20°C.The reaction was started by starting stirring and recorded the start time. The catalytic reactions for high HMF concentration were performed in a 50 mL three-necked flask. 500 mg HMF was dissolved in 10 mL deionized water and then desired amounts of base and 125 mg catalyst were added. Other procedures were the same as above.

Product analysis The products were analyzed by high performance liquid chromatography (HPLC, Agilent 1200) at 30°C using a C18 column (Elite) and a UV detector. The mobile phase consists of 1‰ aqueous acetic acid solution and pure acetonitrile (80:20, v/v) at flow rate of 0.5 mL/min was used. The maximum absorption wavelength of the UV detector was set to 265 nm. The commercial standard compounds (HMF, HMFCA, DFF, FFCA, FDCA) were purchased to identify the products of HMF oxidation by comparing their retention time. The conversion of HMF and the product yields have been calculated by the external standard method using HPLC. The HMF

conversion and HMFCA yield were calculated as follows: Conversion% = HMF mole after the reaction/HMF input the reactor *100% Yield% = Mole of HMFCA after the reaction/HMF mole before the reaction *100%;

Results and discussion XRD measurement The XRD patterns of the catalysts with various Ag loading and molar ratiosof Ag to PVP are presented in Fig. 1. The characteristic diffraction peaks at 24.0°, 28.2°, 31.4°, 34.1°, 34.4°, 35.3°, 40.7°, 49.2°, 50.1°, 55.4° are observed for all samples, which are assigned to monoclinic crystal 7



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structure of m-ZrO2 (PDF. # 37-1484). Besides the diffraction peaks of ZrO2, two weak diffraction peaks at 38.0° and 44.1°appear, which are attributed to the diffraction lines of (111) and (200) facet of metallic Ag, a face-centered cubic crystal structure.48 TEM measurement Fig. 2 shows the TEM imagestogether with their correspondingsize distributionfor Ag nanoparticlecatalysts. The Ag size distribution of as-prepared catalyst was determined by measuring the average sizes of over 200 particles. It is observed that the mean diameter of Ag NPs for the same Ag loading decreaseswith increasingthe amount of PVP: 25.8 nm for 2.5%Ag-PVP/ZrO2(1:0.5),

21.5

nm

for

2.5%Ag-PVP/ZrO2(1:1),

and

11.2

nm

for

2.5%Ag-PVP/ZrO2(1:2) (Fig. S1). It is reasonablethat the more PVP amount was used, the smaller Ag NPs would be obtained. The particle sizes of 1%Ag-PVP/ZrO2 (1:1) and 5%Ag-PVP/ZrO2(1:1) are 19.7 nm and 27.9 nm, respectively. Surprisingly, the average size of 2.5%Ag/ZrO2 catalyst without PVP is 8.5 nm. This phenomenon could be ascribed to relatively strong interaction between AgNPs and the supportZrO2, i.e.the silver ions, which firstly strongly adsorbon the surface of ZrO2, were reduced by NaBH4 leading to the generation of relatively small particles.To further corroborate the assumption, a catalyst of 2.5%Ag-PVP/ZrO2(1:1)-II was prepared by changing reduction step. It is worth noted that the preparation procedure of the catalyst 2.5%Ag-PVP/ZrO2 (1:1)-II is the same as the catalyst 2.5%Ag/ZrO2with the exception of the use of PVP.Very interestingly, it can be seen from Fig. 2C that the size of Ag NPs on the2.5%Ag-PVP/ZrO2(1:1)-II catalyst is 12.5 nm, which is between the size of Ag NPs of the 2.5%Ag/ZrO2 catalyst and 2.5% Ag-PVP/ZrO2 (1:1) catalyst, which further verify the relatively strong interaction between Ag NPs and ZrO2 support. Therefore, it is rational that the average 8


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particle size of Ag NPs protected by PVP is larger than the Ag NPs without the PVP protection. HRTEM micrograph was used to study the surface structure of Ag NPs. As can be seen from Figs. 2 and S1 (Supporting information), the Ag2O was detected by the (100) reflection, corresponding to the spacing d=0.260 nm. Besides the formation of Ag2O, metallic Ag0 was also detected by the reflections of (200) with a spacing of d=0.208 nm and (111) with d=0.234 nm.48,49 It should be noted that the surface Ag2O could be derived from the oxidation of Ag metal upon its exposure to air. XPS analysis XPS analysis was used to study the interaction of PVP and ZrO2 with the Ag surface as well as the valence status of the Ag surface, as shown in Fig. 3. As can be seen, the Ag 3d XPS spectrum of 2.5wt%Ag/ZrO2 consists of two bands corresponding to the 3d3/2 and 3d5/2 transitions, which can each be deconvoluted into two different peaks (a total of 4 peaks): at 367.8 eV and 373.8 eV, ascribed to the sliver oxides Ag2O, at 374.4 eV and 368.3 eV, assigned to the Ag0 metal.50,51For 2.5wt%Ag-PVP/ZrO2(1:1), besides the two bands associated to the Ag 3d3/2 and 3d5/2 transitions, a new peak at 367.7 eV is observed, together with another deconvoluted peak at 373.7 eV, which are attributed to the sliver oxides Ag2O. An upshift of the binding energy for Ag0 in the presence of PVP is observed, implying that a charge transfer between the PVP molecules and the Ag0 metal.A similar result that an upshift of Ag0 3d binding energy is also found for 2.5%Ag-PVP/ZrO2(1:1)-IIand other catalysts (SI, Fig. S2). Moreover, almost no shift of the binding energy of Ag+ for Ag2O takes place for these samples, suggesting that the interaction between Ag2O and PVP or ZrO2 could be neglected. To further study the interaction between Ag metal and ZrO2, the Zr 3dspectra of three 9



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representative samples areshown in Fig. 3A’-C’. For 2.5%Ag/ZrO2, the Zr 3d spectrum is consisted of two doublet with 3d5/2 and 3d3/2 located at 181.6 eV and 183.9 eV, respectively, corresponding to Zr4+.52 A downshift of the binding energy for 3d5/2 from 181.6 eV to 181.4 eV is observed for 2.5%Ag-PVP/ZrO2(1:1), suggesting that electron transfer occurs from the ZrO2 to the Ag0.52 However, for 2.5%Ag-PVP/ZrO2(1:1)-II, an upshift of the binding energy for Zr 3d5/2 from 181.6 eV to 182.0 eV is observed. Besides the two peaks at 181.4 eV and 183.7 eV, a new peak at 185.8 eV appears for 2.5%Ag-PVP/ZrO2(1:1) and 2.5%Ag-PVP/ZrO2(1:1)-II, which could be due to the interaction between PVP and ZrO2. Besides the interaction between ZrO2 and Ag metal, the interaction between PVP moleculesand Ag metal should be considered. To reveal a better insight into Ag metal-PVP interaction, the N 1s spectra are shown in Fig. 3D and 3E and Fig. S2. As can be seen, a N 1s binding energy is located at 399.6 eV, characteristic of the pyrrolidone N group.36 The N 1s binding energy at 399.6 eV is similar to that of the free PVP, suggesting that some N atoms are not involved in the coordination with or weakly anchor on the Ag NPs surface. For 2.5%Ag-PVP/ZrO2(1:1) catalyst, in addition to 399.6 eV, the other two N 1s binding energyare observed at 401.3 and 407.7 eV(Fig. 3D). Shifts to a higher binding energy are attributed to decreased electron density of the N group. Huang et al. revealed that the chemisorption of PVP to spherical Pd NPs can also crack the N-C bond in the N-C=O group, with subsequent hydrolysis produce CH2-CH2-NH2+-(CH2)3-COO-.53 On the basis of the N 1s spectral results that the high binding energies of 401.7 eV and 407.7 eV appear, we prospect that some PVP-rings were undergone ring breakage. The binding energy at 401.7 eV is assigned to positively charged amine species,53 whereas the binding energy at 407.7 eV is unidentified and might be ascribed to more 10


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positively charged amine species anchored on different Ag metal facet. According to N 1s XPS results, the PVP chemisorb on the Ag metal NPs surface mainly through the O atom and N atom of pyrrolidone cleavage ring. A similar phenomenon was also observed for the other Ag NPs catalysts (Supporting information, Fig. S3). The upshift of Ag0 3d binding energy could be due to the reason that the O atoms chemisorbed on the Ag NPs surface could strongly withdraw electrons from Ag metal nanoparticles, thereby resulting in slight electron deficient Ag metal although N atoms and ZrO2 could donor electron into Ag metals. FT-IR analysis To further analyze the interaction of PVP with Ag nanoparticles, FT-IR spectroscopy was employed (Fig. 4). From Fig. 4, the characteristic bands of the pure PVP can be observed. Thetwo bandsat 1272 and 1289 cm-1 are assigned to the stretching vibrations of the two N-C bonds, and the bands between 1300 and 1480 cm-1 are attributed to the typical C-H vibration of the pure PVP. The band at 1493 cm-1 are ascribed to the N-C stretching vibration of the N-C=O group. A broad and intense peak at 1674cm-1 of the pure PVP is also observed, which is attributed to stretching vibrations of C=O.41,53,54 For the Ag-PVP/ZrO2 catalysts shown in Fig. 4, the characteristic bands of PVP molecule can still be observed, but some changes in the their spectra also occur in these catalysts with respect to the pure PVP. Compared to the pure PVP, a band at 1657 cm-1 is observed for the Ag-PVP/ZrO2 catalysts, which is associated with the CO stretching vibration of the pyrrolidone ring interacting with the Ag metal surface.41,54 As mentioned in the Introduction, the interaction between the Ag metal and the carbonyl group of the PVP molecules is based on a charge transfer from the metal to the carbonyl group. The interaction led to a decrease in the carbonyl bond strength and shifted the 11



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characteristic band of the carbonyl group to a lower wavenumber from 1674 cm-1 to 1657 cm-1, which is in good agreement with the XPS results that the oxygen atoms could strongly withdraw electrons from surface Ag0 metal. Effect of various catalyst on HMF conversion and HMFCA yield Fig. 5 shows theHMF conversion and HMFCA yield as a function of the reaction time for the 2.5%Ag-PVP/ZrO2(1:1), 2.5%Ag-PVP/ZrO2(1:1)-II and 2.5%Ag/ZrO2catalysts.It can be seen that the catalysts produced only HMFCA, no other products such asDFF, FFCA, and FDCA were detected (Supporting Information, Fig. S5-S7). The majority of the products were trapped in the form of the HMFCA intermediate, showing that the Ag NPscannot further oxidize HMFCA into FFCA or FDCA liking other noble metal catalyst such as Pd, Pt and Au NPs.3,32 With the exception of HMFCA, a little by-products humins was formed during the aerobic oxidation of HMF. Very interestingly, the catalytic activity of 2.5%Ag-PVP/ZrO2(1:1) catalyst with a big particle size of 21.5 nm is better than that of 2.5%Ag/ZrO2 catalyst with a small average size of 8.5 nm. It has been well-known that big particle size usually affords an inferior catalytic activity and the presence of PVP blocks the active sites, thereby leading to a low catalytic activity.41 The possible interpretation for the unexpected results is that the relative strong interaction between Ag NPs and ZrO2 support could be unfavorable for the aerobic oxidation of HMF as well as the charge transfer between PVP and Ag NPs may facilitate the oxidation reaction. To further clarify our hypothesis about the interaction between Ag NPs and ZrO2 support affecting the catalytic behaviors, the catalyst 2.5%Ag-PVP/ZrO2(1:1)-II was also employed to carry out the aerobic oxidation of HMF under identical reaction conditions (Fig. 5). As expected, its reactivity is higher than that of 2.5%Ag/ZrO2 but is lower than that of 2.5%Ag-PVP/ZrO2(1:1). The turnover number (TON) of HMF of 2.5%Ag-PVP/ZrO2(1:1)-II at 2 h was 0.593 mmol/h, 12


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which was larger than that of 2.5%Ag/ZrO2(0.575 mmol/h) and was less than that of 2.5%Ag-PVP/ZrO2(1:1)(0.781 mmol/h). These results suggest that the relative strong interaction between Ag NPs and ZrO2 support is unfavorable for the transformation of HMF to HMFCA. Effect of reaction temperature The effect of reaction temperature on the conversion of HMF and selectivity of HMFCA was also studiedwith the 2.5%Ag-PVP/ZrO2(1:1) catalyst (Fig. 6). When the reaction was performed at 20 °C, HMF conversion was almost complete and selectivity toward HMFCA reached 98.2%. Nevertheless, as the temperature was increased to 40°C, the HMF conversion and yield of HMFCA decreased to 98.3% and 91.8%, respectively. When the temperaturewas further increased to 60°C, the HMF conversion (92.6%) and HMFCA yield (81.3%) continued to drop. Moreover, DFF, FFCA and FDCA were not detected at elevated temperature, showing thathigh reaction temperature could not promote the further oxidation of HMFCA but produce more humins. These observations evidently demonstrate that low temperature is beneficialfor the generation of the target product and decrease the by-product (humins) formation. Effect of the flow rate of O2 The effects of the O2 flow rate on the yield of HMFCA was also studied, and the corresponding resultsare displayed in Fig. 7. As the flow rate of O2was 30 mL/min,a HMFCA yield of 91.9% with a HMF conversion of 93.7% was obtained. As the flow rate of O2 further increased,100% conversion was able to be given at the rates of 60 and 120 mL/min. However, 60mL/min afforded a higher HMFCA yield (98.2%) compared with 120 mL/min (96.0%). Effect of the PVP/Ag molar ratio The influence of the molar ratio of Ag to PVP on the aerobic oxidation of HMF was 13



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investigated (Fig. 8). As can be seen, as the PVP/Ag molar ratio is 0.5, a HMFCA yield of 82.2% with a HMF conversion of 86.9% was obtained. As the PVP/Ag molar ratio increased to 1, a 98.2%HMFCA yield with complete HMF conversion was achieved. When the PVP/Ag molar ratio further increased to 2, the HMFCA yield reduced to 69.4% and theHMF conversion also dropped to 72.8%. The 2.5%Ag-PVP/ZrO2(1:2) catalyst has the smallest particle size among the three catalysts, but it afforded the lowest HMF conversion and HMFCA yield under the identical reaction conditions. The results clearly show that PVP has a significant effect on the catalyst activity and HMFCA yield because PVP chemically adsorbs and occupies the Ag active sites.55 Therefore, as the Ag/PVP molar ratio is 1, an optimal catalytic performance can be achieved. Effect of the loading amount Effect of various Ag loading on the HMF conversion and HMFCA yield was examined, and the corresponding result is illustrated in Fig. 9. As can be seen, HMF conversion increased from 78.5% for the Ag loading of 1.0wt% to 100% for 2.5 wt% Ag loading and then decreases to 88.3% when the loading amount of Ag is 5.0wt%. When the Ag loading is 1.0wt%, HMF was not completely converted due to low Ag loading. However, the HMF conversion decreased to 88.3% for 5.0 wt% Ag loading. The reason can be explained by the fact that the mean particle size of 5.0%Ag-PVP/ZrO2(1:1) (27.9 nm) is bigger than that of 2.5%Ag-PVP/ZrO2 (1:1) (21.5 nm) with the same Ag/PVP molar ratio, thereby resulting in a low catalytic performance. However, for 1.0%Ag-PVP/ZrO2(1:1)

(19.7

nm),

a

lower

catalytic

performance

compared

to

2.5%Ag-PVP/ZrO2(1:1) (21.5 nm) could be due to its less amount of the active sites compared to that of 2.5%Ag-PVP/ZrO2(1:1) as their both particle size is comparable. Effect of the base types 14


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The base plays an important role in the reaction ofHMF oxidation. The effects of different bases were examined and the reaction was performed with an oxygen flow rate of 60 mL/min for 2 h, and the results are summarized in Table 2. When NaOH was used for high concentration HMF oxidation, complete conversion of HMF and low selectivities toward HMFCA (< 80%) could be given, respectively (Table 2, entries 1, 2). As the mole ratio of HMF to NaOH was 1:1 and a HMF concentrationwas 50 g/L, a HMFCA selectivity of 80.4% with the HMF conversion of 97.8% were afforded (Table 2, entry 3). This suggests that high concentration of base can promote the conversion of HMF but cause more humins formation and decrease the selectivity of HMFCA. Therefore, in order to decrease the alkalinity of the base, some weak base were utilized, including Ca(OH)2, NH3·H2O and Na2CO3. As can been seen from Table 2, entries 4-6, the aerobic oxidation of HMF to HMFCA was hardly happened in the presence of NH3·H2O or Na2CO3. When Ca(OH)2 was used (entries 7-8), the yield of HMFCA increased obviously compared to NH3·H2O and Na2CO3. The weak alkalinity of Ca(OH)2 decreased the conversion of HMF but it afforded a high HMFCA selectivity under identical reaction conditions as compared to NaOH (entry 2 vs entry 7). This result clearly shows that too strong or too weak bases are unfavorable for the formation of HMFCA, especially for the use of high concentration of HMF as a substrate. Catalyst recycling To investigate the stability of catalyst, the recyclability of 2.5% Ag-PVP/ZrO2(1:1) was tested, and the result is illustrated in Fig. 10. In each successive cycle, the catalyst was centrifuged and washed with water for three times, and then dried at 363 K. The recovered catalyst was directly used for the next recycle. It can be seen from Fig. 10 that the activity of recovered catalyst had a slight decline in terms of conversion and yield after three consecutive runs. In the first and 15



Page 16 of 40

second runs, the activity did not remarkably reduce, however, the conversion of HMF dropped from 99.1% to 81.9% and yield of HMFCA dropped from 93.6% to 71.9% for three cycles, which indicates that the catalyst has a little deactivation.To test whetherthe Ag ions was leached into the aqueous solution, Ag ion concentration of the filtrate was detectedby ICP-AES. The elemental analysis results clearly shows that Ag content in the filtrate only accounts for 0.2% of catalyst, which demonstrates that the Ag leaching could be neglected. The result is good agreement with the work reported by Grunwaldt et al. that high pH value could effectively prevent the Ag ions from leaching into the aqueous solution by the formation of a Ag2O thin layer.20 In addition, to verify whether the PVP stabilizedthe Ag NPs was leached into the reaction solution, the FT-IR spectrum of the recovered catalyst was recorded (Fig. S8). The IR spectral result shows that the characteristic signals of PVP still appear, showing that the leaching of PVP can be neglected. After three cycles of reaction, the catalyst mass was lost from 125 to 78 mg, which could lead to thedecrease in the HMF conversion and HMFCA yield. In addition, it is observed that the Ag particle size of the used catalyst became larger from 21.5 nm for the fresh catalyst to 26.0 nm for the fifth recovered catalyst (Fig. S1F), which also resulted in the catalyst deactivation.

The relationship between the metal-support interaction and catalytic properties

Grunwaldt et al. assumed that reduced Ag NPs are responsible for the aerobic oxidation of HMF to HMFCA.20 In the present work, the Ag-PVP/ZrO2 catalysts were reduced by NaBH4, which is different from Ag/ZrO2 catalyst prepared by calcination at 350 oC.20 XPS results clearly show that there exist two Ag species, i.e. Ag0 and Ag2O species over these Ag nanoparticle catalysts. The Ag2O could be generated from the oxidation of Ag0 atoms upon exposure in air. Moreover, it is observed that a high HMFCA yield of 98.2% was achieved over 2.5% 16


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Ag-PVP/ZrO2 (1:1) at 20 oC, which is lower than the optimized reaction temperature at 50 oC over Ag-PVP/ZrO2 (1:1) in the work reported by Grunwaldt et al.20 This also indicates that the Ag NPs exhibit better catalytic performances in the oxidation of HMF after the modification with PVP.

It is found that the Ag particle sizes have a significant effect on the catalytic performances in the aerobic oxidation of HMF to HMFCA. The Ag/ZrO2 catalyst does not exhibit better catalytic activity although it has a smaller particle size (8.5 nm) as compared to Ag-PVP/ZrO2(1:1)(21.5 nm), which is completely different from the well-known conclusion that a smaller metal particle size should have a higher catalytic activity.41 Besides the reason that a charge transfer between PVP and Ag NPs favors the conversion of HMF, the other reason is that the interaction between Ag NPs and ZrO2 support. Moreover, we succeeded to control the Ag particle size by tuning the metal-support interaction with the use of PVP. It is observed that the change of the metal-support interaction can effectively modulate the catalytic activity in the aerobic oxidation of HMF into HMF. To explain our hypothesis reasonably, a schematic illustration is presented in Scheme 2. As shown in Scheme 2, we prospect that the PVP molecules interact with the Ag NPs can efficiently decrease the interaction between Ag NPs and ZrO2 as compared to Ag/ZrO2 without the presence of PVP. Therefore, Ag/ZrO2 catalyst has the strongest metal-support interaction among the three catalysts (Ag/ZrO2, Ag-PVP/ZrO2(1:1), Ag-PVP/ZrO2 (1:1)-II). This is because the first step in the synthesis of Ag-PVP/ZrO2(1:1) is the production of Ag colloid solution, in which more PVP molecules could freely interact with Ag NPs in comparison with the synthesis of Ag-PVP/ZrO2 (1:1)-II that PVP have to competitively adsorb with ZrO2 support. Thus, we propose that Ag-PVP/ZrO2(1:1) has more PVP molecules interacted with Ag NPs than those of Ag-PVP/ZrO2 (1:1)-II, thereby resulting in the weakest interaction among the three catalysts. The different 17



Page 18 of 40

metal-support interaction leads to different Ag particle size, thereby influencing the catalytic activity in the aerobic oxidation of HMF. It seems that a weak metal-support interaction is beneficial for the oxidation of HMF. However, further mechanistic studies need to be undertaken on the relationship between the metal-support interaction and the catalytic performances.

CONCLUSION In summary, a novel, efficient and green approach for the synthesis of HMFCA from HMF with ZrO2 supported PVP-assisted Ag NPs as catalysts has been developed. The catalysts with different molar ratios of Ag to PVP and various Ag loadings were synthesized. An excellent yield (98.2%) of HMFCA with a complete conversion (100%) of HMF was achieved over the 2.5%Ag-PVP/ZrO2(1:1) catalyst at 20 oC. The alkalinity has a remarkable effect on the HMF conversion and HMFCA yield, especially with high concentration of HMF as a substrate, i.e., Ca(OH)2 gave a higher selectivity to HMFCA compared to other bases including NaOH or Na2CO3. The presence of PVP can control the particle size of Ag NPs andmodulate the metal-support interaction by adjusting its amount and changing the synthesis procedure, thereby tuning the catalytic performances. More interestingly, it is revealed that a weak interaction between the ZrO2 support and AgNPs has a beneficial effect onthe conversion of HMF and HMFCA selectivity. This work provides a new route in tuning the metal-support interaction by using capping agents, i.e. changing the interaction between metal and support through a capping agents coordinated to metal NPs, thereby tailoring the catalytic performances. ASSOCATION CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 18


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TEM micrograph, XPS, and FT-IR spectra of these catalysts, as well as HPLC result of the reaction product. AUTHOR INFORMATION Corresponding Author *E-mail:[email protected]

Notes The authors declare no competing financial interest.

Acknowledge This work was financially supported by the Natural Science Foundation of Jiangsu province (grant no. BK20171452). This work was also financially supported by the Natural Science Foundation of Jiangsu Higher Education Institutes of China (16KJB220003).

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Figure captions Scheme 1The route for the selective oxidation of 5-HMF. Figure 1XRD patterns of the catalyst: (a) 2.5%Ag-PVP/ZrO2 (1:1)-II; (b) 2.5%Ag-PVP/ZrO2 (1:0.5); (c) 2.5%Ag-PVP/ZrO2 (1:1); (d) 2.5%Ag-PVP/ZrO2 (1:2); (e) 2.5%Ag/ZrO2; (f) 1%Ag-PVP/ZrO2 (1:1); and (g) 5%Ag-PVP/ZrO2 (1:1). Figure 2TEM andHRTEM images and particle sizes distributions: (A,D) 2.5%Ag-PVP/ZrO2 (1:1);(B, E) 2.5%Ag/ZrO2;(C, F) 2.5%Ag-PVP/ZrO2 (1:1)-II. Figure 3X-ray photoelectron spectra for Ag 3d and Zr 3d of the catalysts: (A, A’) 2.5%Ag/ZrO2, (B, B’)2.5%Ag-PVP/ZrO2 (1:1), and (C, C’) 2.5%Ag-PVP/ZrO2 (1:1)-II as well as N 1s spectra of (D)2.5%Ag-PVP/ZrO2 (1:1), and (E) 2.5%Ag-PVP/ZrO2 (1:1)-II. Figure 4FT-IR spectra of the catalysts: (a) 2.5%Ag/ZrO2,(b) 2.5%Ag-PVP/ZrO2 (1:0.5), (c) 2.5%Ag-PVP/ZrO2

(1:1),(d)

2.5%Ag-PVP/ZrO2

(1:2),(e)

1%Ag-PVP/ZrO2

(1:1),(f)

5%Ag-PVP/ZrO2 (1:1), and (g) 2.5%Ag-PVP/ZrO2 (1:1)-II. Figure5 HMF conversion and HMFCA yield as a function of time over three various catalysts.Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h. Figure 6 Impact ofreaction temperature on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 2 h. Figure 7 Effect ofO2 flow rate on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 20°C, 2 h. 24


Page 25 of 40 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


Figure8 Effectof the molar ratio of Ag and PVP on HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h. Figure 9 Influence ofdifferent Ag loading on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.20 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h. Figure 10The recycling experiments in the selective oxidation of HMF to HMFCA over 2.5%Ag-PVP/ZrO2 (1:1). Reaction condition: 0.50 g HMF, 0.317 g NaOH, 10 mL H2O, 0.125 g catalyst, 60 mL/min O2, 20°C, 2 h.

Scheme 2 Schematic illustration of the relationship between the metal-support interaction and catalytic performances of Ag NPs catalysts.

25



Page 26 of 40

Table 1Results of average particle size and the binding energy of Ag0 and Ag2O of the catalysts NPs mean size Entry

sample (nm)a

aAg

the binding energy of 3d5/2(eV)

Ag0

Ag2O

1

2.5% Ag /ZrO2

8.5±2.3

368.3

367.8

2

2.5% Ag-PVP/ZrO2 (1:0.5)

25.8±8.8

371.0

367.8

3

2.5% Ag-PVP/ZrO2 (1:1)

21.5±5.3

369.5

367.7

4

2.5% Ag-PVP/ZrO2 (1:2)

11.2±3.9

369.4

368.7

5

1% Ag-PVP/ZrO2 (1:1)

19.7±5.8

372.0

369.4

6

5% Ag-PVP/ZrO2 (1:1)

27.9±11.9

370.8

367.5

7

2.5% Ag-PVP/ZrO2 (1:1)-II

12.5±3.7

368.6

368.0

particle size was determined by TEM.

26


Page 27 of 40 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


Table 2.The effect of the bases on the conversion of HMF to HMFCA

Entry

HMF: Base

Temperatur

Concentration

Conversion

Yield

Selectivity

e

(g/L)

(%)

(%)

(%)

(℃)

1

HMF : NaOH = 1:2

20

10

100.0

70.0

70.0

2

HMF : NaOH = 1:2

20

50

100.0

77.3

77.3

3

HMF : NaOH = 1: 1

20

50

97.8

78.7

80.4

4

HMF : NH3H2O = 1 :2

20

50

3.5

/

/

5

HMF : Na2CO3 = 1: 1

20

50

0.9

/

/

6

HMF : Na2CO3 = 1: 4

20

50

8.5

2.1

24.7

7

HMF : Ca(OH)2 = 1: 2

20

50

74.9

69.8

93.2

8

HMF : Ca(OH)2 = 1: 4

20

50

82.5

67.5

81.9

Reaction condition: 50 mg 2.5%Ag-PVP/ZrO2 (1:1) catalyst, 60 mL/min O2, 2 h.

27



O

HO O HMF

[O]

on ati up d i o HO x e o gr tiv ydic c e l h Se alde of

Sel e of h ctive ydr oxid oxy atio l gr n oup

O

Page 28 of 40

O OH

HMFCA

[O]

O

OH

O

O

O

O

O [O]

FFCA

O

O

OH

HO FDCA

O DFF

Scheme 1 The reaction pathway for the aerobic oxidation of 5-HMF.

28


*

** *

g

Ag (200)

* Intensitity (a.u.)

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


Ag (111)

Page 29 of 40

* monoclinic ZrO2

**

f e d c b a 10

20

30

40

50

60

70

80

2 theta(degree) Figure 1XRD patterns of the catalyst: (a) 2.5%Ag-PVP/ZrO2 (1:1)-II; (b) 2.5%Ag-PVP/ZrO2 (1:0.5); (c) 2.5%Ag-PVP/ZrO2 (1:1); (d) 2.5%Ag-PVP/ZrO2 (1:2); (e) 2.5%Ag/ZrO2; (f) 1%Ag-PVP/ZrO2 (1:1); and (g) 5%Ag-PVP/ZrO2 (1:1).

29



(A)

40

Mean = 21.5 ± 5.3 nm

Frequency (%)

30

20

10

0

10

20 Particle Size (nm)

30

(E)

(B)

40

Mean = 8.5 ± 2.3 nm

Frequency (%)

30

20

10

0

3

6

9 12 Particle Size (nm)

15

18

(F)

(C)

PVP

Mean = 12.5 ± 3.7 nm

30

Percentage (%)

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

Page 30 of 40

d=0.208 nm Ag (200)

20

10

0

d=0.320 nm ZrO2 5

10

15 20 Particle Size (nm)

25

Figure 2 TEM and HRTEM images and particle sizes distributions: (A,D) 2.5%Ag-PVP/ZrO2 (1:1);(B, E) 2.5%Ag/ZrO2;(C, F) 2.5%Ag-PVP/ZrO2 (1:1)-II.

30


Page 31 of 40

Ag 3d Ag 3d3/2 367.8 373.8

368.3

374.4

378

376

374

372

370

368

366

188

186

369.5

376

374

372

370

368

366

(B')

364

183.7

188

186

368.6

376

374

372

370

368

366

184.2

Normalized Intensity (a.u.)


(C')

Data Fit Ag (0) Ag (I)

374.0

378

364

188

N 1s

401.3 407.7

404

402

400

Zr 3d

Zr 3d5/2

Data

184

182

180

178

398

Zr 3d3/2

Zr 3d5/2

176

Zr 3d

182.0

Data

Fit

186

184

182

180

178

176

396

394

N 1s

(E) 399.6


399.6

406

Zr 3d3/2

Binding Energy (eV)

(D)

408

176

186.0

Binding Energy (eV)

410

178

185.8

Ag 3d

368.0

374.5

180

Binding Energy (eV)

Ag 3d5/2 Ag 3d3/2

182

Fit

Binding Energy (eV)

(C)

184

181.4




373.7

378

Fit

183.9

364

Ag 3d

367.7

375.5

Data

Binding Energy (eV)

Ag 3d5/2 Ag 3d3/2

Zr 3d

Zr 3d3/2

Binding Energy (eV)

(B)

Zr 3d5/2

181.6

(A')


Ag 3d5/2



(A)


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


407.7

410

408

406

Binding Energy (eV)

401.7

404

402

400

398

396

394

Binding Energy (eV)

Figure 3 X-ray photoelectron spectra for Ag 3d and Zr 3d of the catalysts: (A, A’) 2.5%Ag/ZrO2, (B, B’)2.5%Ag-PVP/ZrO2 (1:1), and (C, C’) 2.5%Ag-PVP/ZrO2 (1:1)-II as well as N 1s spectra of (D)2.5%Ag-PVP/ZrO2 (1:1), and (E) 2.5%Ag-PVP/ZrO2 (1:1)-II 31


1493 1465 1398 1383 1289 1272

Page 32 of 40

a b c d e f

Absorbance(a.u.)

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

1674 1657


g PVP*0.2

2000

1800

1600

1400

-1

1200

1000

Wavenumber(cm ) Figure 4 FT-IR spectra of the catalysts: (a) 2.5%Ag/ZrO2, (b) 2.5%Ag-PVP/ZrO2 (1:0.5), (c) 2.5%Ag-PVP/ZrO2 (1:1), (d) 2.5%Ag-PVP/ZrO2 (1:2), (e) 1%Ag-PVP/ZrO2 (1:1), (f) 5%Ag-PVP/ZrO2 (1:1), and (g) 2.5%Ag-PVP/ZrO2 (1:1)-II.

32


Page 33 of 40

100 90 80 70

2.5%Ag-PVP/ZrO2 (1:1) Conv.

%

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


2.5%Ag-PVP/ZrO2 (1:1) Yield.

60

2.5%Ag/ZrO2 Conv. 2.5%Ag/ZrO2 Yield.

50

2.5%Ag-PVP/ZrO2 (1:1)-II Conv. 2.5%Ag-PVP/ZrO2 (1:1)-II Yield.

40 30 0.5

2

4

6

9

Time(h) Figure 5 HMF conversion and HMFCA yield as a function of time over three various catalysts.Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h.

33



HMF conversion HMFCA yield

100 80 60

%

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

Page 34 of 40

40 20 0 20

40

60

o Temperature( C)

Figure 6 Impact ofreaction temperature on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 2 h.

34


Page 35 of 40

HMF Conversion HMFCA Yield

100

90

%

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


80

70

30

60

120

O2 (mL/min) Figure 7 Effect of O2 flow rate on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1). Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 20°C, 2 h.

35



HMF Conversion HMFCA Yield

100

90

80

%

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

Page 36 of 40

70

60

50

1：0.5

1：1

1：2

Ag:PVP Figure 8 Effect of the molar ratio of Ag and PVP on HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.2 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h.

36


Page 37 of 40

HMF conversion HMFCA yield

100 80

%

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


60 40 20 0 1.0%

2.5% Ag loading (wt%)

5.0%

Figure 9 Influence of different Ag loading on the HMF conversion and HMFCA yield over 2.5%Ag-PVP/ZrO2 (1:1).Reaction condition: 0.20 g HMF, 0.126 g NaOH, 50 mL H2O, 0.05 g catalyst, 60 mL/min O2, 20°C, 2 h.

37



HMF Conversion HMFCA Yield 100

80

%

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

Page 38 of 40

60

40

20

0 0

1

Cycles

2

3

Figure 10 The recycling experiments in the selective oxidation of HMF to HMFCA over 2.5%Ag-PVP/ZrO2 (1:1). Reaction condition: 0.50 g HMF, 0.317 g NaOH, 10 mL H2O, 0.125 g catalyst, 60 mL/min O2, 20°C, 2 h.

38


Page 39 of 40 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


OH O

O

HO

O H

O2/ 20 oC

O

OH

base PVP

PVP Ag Ag Ag Ag

ZrO2 I: Ag/ZrO2

ZrO2

ZrO2

II: Ag-PVP/ZrO2-in situ

The metal-support interaction: I > II > III TON : I< II < III

Scheme 2 Schematic illustration of the relationship between the metal-support interaction and catalytic performances of Ag NPs catalysts.

39



A high yield of renewable, value-added bio-chemicals–HMFCA was achieved from oxidation of biomassderived 5-HMF over PVP-capped Ag nanoparticle catalysts. 105x86mm (96 x 96 DPI)


Page 40 of 40

Aerobic Oxidation of 5-Hydroxymethylfurfural to High-Yield 5

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