Selective Oxidation of Biomass-Derived Alcohols and Aromatic and

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Article Cite This: ACS Omega 2018, 3, 7944−7954

Selective Oxidation of Biomass-Derived Alcohols and Aromatic and Aliphatic Alcohols to Aldehydes with O2/Air Using a RuO2‑Supported Mn3O4 Catalyst Bhaskar Sarmah,† Biswarup Satpati,‡ and Rajendra Srivastava*,† †

Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, India Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700 064, India

‡

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S Supporting Information *

ABSTRACT: Selective catalytic oxidation of carbohydratederived 5-hydroxymethylfurfural, furfuryl alcohol, and various aromatic and aliphatic compounds to the corresponding aldehyde is a challenging task. The development of a sustainable heterogeneous catalyst is crucial in achieving high selectivity for the desired aldehyde, especially using O2 or air. In this study, a RuO2-supported Mn3O4 catalyst is reported for the selective oxidation reaction. Treatment of MnO2 molecular sieves with RuCl3 in aqueous formaldehyde solution gives a new type of RuO2-supported Mn3O4 catalyst. Detailed catalyst characterization using powder X-ray diffraction, N2 adsorption, scanning and transmission electron microscopes, diffuse reflectance UV−visible spectrometer, and X-ray photoelectron spectroscopy proves that the RuO2 species are dispersed on the highly crystalline Mn3O4 surface. This catalytic conversion process involves molecular oxygen or air (flow, 10 mL/min) as an oxidant. No external oxidizing reagent, additive, or cocatalyst is required to carry out this transformation. This oxidation protocol affords 2,5-diformylfuran, 2-formylfuran, and other aromatic and aliphatic aldehydes in good to excellent yield (70−99%). Moreover, the catalyst is easily recycled and reused without any loss in the catalytic activity. been developed.13−15 Metal complex-based processes are better with respect to green chemistry principles, such as atom economy, use of eco-friendly oxidizing agents, simplified reaction process, and so on. However, it is inferior with respect to catalyst recyclability. Therefore, efforts are being made to develop sustainable heterogeneous oxidation catalysts for this reaction. A sustainable and efficient solid catalyst should be able to catalyze this selective oxidation using O2/air at 1 atm in the absence of any base.16 RuO2 has been initially developed for the selective oxidation of alcohols into aldehydes and ketones.17,18 Further, RuO2-supported materials have been explored for the selective oxidation of alcohols.19−21 In this direction, one pioneering work is the development of Pdsupported apatite catalyst which has been successful in the selective oxidation of alcohols using O2 at atmospheric pressure.22 Later, Pd-, Pt-, Ru-, and Au-based supported catalysts have been developed.22−25 Supports such as hydrotalcites,23 hydroxyapatites,24 zeolites,21 metal oxides,25 and carbon26 have been investigated. It is also possible for metal oxides to selectively catalyze this oxidation. Certain transition-

1. INTRODUCTION Acid catalysts dominate the petrochemical industries, whereas oxidation catalysts dominate the fine chemical industries.1,2 Acid catalysts also play important roles in the production of biomass-derived alternate fuels and fuel additives.3,4 Oxidation process shares 30% of the total production in the chemical industry.5 Among the key chemicals produced by the oxidation catalysts, selective production of aldehydes is of significant importance.6 Extensive research is carried out to develop process for the selective production of aldehyde. Chemical industry demands a cost-effective but environmental-friendly process for the production of aldehydes. Even in the development of alternative fuels and fuel additives, this process plays an important role because a suitable design of the catalyst can manifest a process for the conversion of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF).7 However, such processes should adopt sustainable catalysts and oxidants. In the traditional practice, oxidizing reagents such as permanganate and dichromate are employed but these stoichiometric oxidants are expensive and/or toxic.8 Further, reactions involving these reagents produce heavy-metal-based wastes. Therefore, the development of catalytic oxidation process using O2/air as an oxidant would be the most benign approach.9−12 To achieve this objective, transition-metal complexes based on copper, palladium, and ruthenium have © 2018 American Chemical Society

Received: May 15, 2018 Accepted: July 3, 2018 Published: July 17, 2018 7944

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ACS Omega metal oxides, such as CeO2, have the capability to oxidize organic molecules by the lattice oxygen. Parent MnO2 and Mn3O 4 catalysts and porous polymer-supported Mn3O 4 catalysts have been explored for the catalytic oxidation of aromatic hydrocarbons and alcohols.27−29 The reduced form of the oxide, so generated, can be oxidized back to its original state by utilizing O2 that is being supplied to the reaction system, thereby, regenerating the catalyst.30 Therefore, the development of an oxidation catalyst using such support materials would be highly interesting. In recent times, selective oxidation protocol is applied to produce a wide range of biomass-derived products, especially DFF. This important chemical can be produced by the selective oxidation of HMF. DFF can be used as an important synthetic intermediate for the production of polymers,31 pharmaceuticals,32 antifungal agents,33 and renewable furan− urea resin.34 Recent literature has witnessed the development of a wide range of Ru-, V-, Fe-, and Cu-based heterogeneous catalysts for the selective oxidation of HMF to DFF.35−41 The objective of this study is to develop a versatile catalyst that has capability to oxidize biomass-derived reactants such as HMF and furfuryl alcohol to DFF and furfural, respectively, using O2/air. The developed catalyst should also be able to selectively oxidize a wide range of aromatic and aliphatic alcohols to aldehydes and ketones. In the present study, RuO2supported Mn3O4 materials were prepared and investigated in the selective oxidation using O2/air (1 atm, 10 mL/min). First, an octahedral MnO2 molecular sieve (OMS) was prepared. The Ru precursor was added to the OMS, and then, it was treated with aqueous formaldehyde to form RuO2-loaded Mn3O4. During the aqueous formaldehyde treatment, the OMS is reduced to Mn3O4 and the Ru precursor is incorporated as RuO2 on the surface of Mn3O4. Different amounts of RuO2 are loaded on the external surface of OMS, and the materials are designated as RuO2(x %)/Mn3O4. Among various catalysts investigated in this study, RuO2(3%)/ Mn3O4 exhibits better yield for aldehydes and DFF.

Figure 1. XRD patterns of OMS, Mn3O4, and RuO2(3%)/Mn3O4 prepared in this study.

process, only OMS was treated with aqueous formaldehyde. The XRD pattern of the resultant material matched well with the XRD pattern reported for Mn3O4 [2θ = 18.41 (101), 29.27 (112), 31.64 (200), 32.72 (103), 36.52 (211), 38.48 (004), 44.87 (220), 50.23 (204), 51.12 (105), 54.31 (312), 56.21 (303), 58.71 (321), 60.32 (224), 65.01 (400), and 70.05 (305)] (JCPDS no. 18-0803).46 The XRD patterns of RuO2(2%)/Mn3O4 and RuO2(1%)/Mn3O4 were similar to that of the RuO2(3%)/Mn3O4 sample (Figure S1). Textural properties and porosity present in the samples OMS, Mn3O4, and RuO2(3%)/Mn3O4 were investigated using nitrogen adsorption−desorption measurements (Figure 2). Type-II isotherm was observed for OMS. This isotherm exhibited a gradual increase in the nitrogen adsorption in the

2. RESULTS AND DISCUSSION Ruthenium trichloride hydrate, OMS, and water were mixed and stirred at ambient temperature. Then, aqueous formaldehyde was added, and the reaction mixture was heated to 373 K for 30 min. During this period, Ru was incorporated as RuO2 on the surface of Mn3O4. The details of characterization are provided below. 2.1. Physicochemical Characterization. OMS exhibited a highly crystalline X-ray diffraction (XRD) pattern with diffraction peaks corresponding to (110), (200), (310), (211), (301), (411), (600), (002), and (541) planes of OMSs reported in the literature (ICDD no. 00-29-1020) (Figure 1).42−44 The XRD pattern of ruthenium-incorporated OMS material was different from the parent OMS. ICDD database match showed that the diffraction peaks predominantly correspond to Mn3O4 and RuO2 phases. Therefore, this material was designated as RuO2(3%)/Mn3O4. This result confirmed that during the aqueous formaldehyde treatment, OMS was reduced to Mn3O4. The reducing environment was not sufficient to reduce Ru source to Ru nanoparticles, instead RuO2 was supported on the Mn3O4 surface. The diffractions located at 2θ = 27.9 (110), 34.9 (101), 39.9 (200), 44.8, 54.1 (211), 57.7 (220), 59.4 (002), 65.2 (310), 66.7 (112), 69.3 (301), and 73.9 (202) correspond to the tetragonal RuO2 (JCPDF: 03-065-2824).45 To further confirm this reduction

Figure 2. N2 adsorption−desorption isotherms of OMS, Mn3O4, and RuO2(3%)/Mn3O4 investigated in this study. The inset shows the BJH pore size distribution investigated in this study. 7945

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ACS Omega range of 0.2−0.8 (P/P0) followed by a steep increase in the N2 adsorption above 0.8 (P/P0). This steep increase in the adsorption volume can be attributed to the adsorption in the intercrystalline mesoporous−macroporous domain. Barrett− Joyner−Halenda (BJH) pore size distribution exhibited the distribution of pores in the range from 2 to 10 nm which was overlapped with the pores from 10 to 100 nm. However, the peak maximum of this overlapped pore size distribution fell in the mesopore (with peak maxima = 14.5 nm) range. Mn3O4 and RuO2(3%)/Mn3O4 also exhibited type-II isotherm similar to the parent OMS but with less adsorbed volume. This observation confirmed that during the reduction process, structural morphology would have changed. Further, loading of RuO2 was also responsible for the low adsorption volume. Bimodal pore size distribution was observed from BJH analysis. A narrow (5−18 nm) pore size distribution is followed by the mesopore−macropore overlapped domain (20−100 nm). Textural properties obtained from the N2 adsorption− desorption measurements are summarized in Table 1. RuO2(3%)/Mn3O4 exhibited low surface area and pore volume when compared to the parent OMS and Mn3O4.

Figure 3. SEM micrographs of (a) OMS, (b) Mn3O4, and (c) RuO2(3%)/Mn3O4 investigated in this study.

Table 1. Physicochemical Properties of OMS, Mn3O4, and RuO2/Mn3O4 Materials Investigated in This Study material

total surface area SBET (m2/g)a

OMS Mn3O4 RuO2(2%)/Mn3O4 RuO2(3%)/Mn3O4 RuO2(4%)/Mn3O4 RuO2(5%)/Mn3O4 RuO2(3%)/Mn3O4b

146 100 89 82 75 68 80

Ru content (wt %)b

external surface area

total pore volume (cm3/g)

1.76 2.31 3.34 4.29 2.32

130 88 84 77 70 60 75

0.60 0.54 0.45 0.40 0.35 0.30 0.39

a

SBET calculated from the adsorption branch in the region of 0.05 < P/P0 ≤ 0.3. bData represents reused catalyst.

The details of surface morphology and microstructure were investigated using a scanning electron microscope (SEM). OMS exhibited a highly cross-linked nanowire-like morphology with a diameter of approximately 20 nm (Figure 3a). The morphology of Mn3O4 was somewhat different from the parent OMS morphology (Figure 3b). SEM results confirmed that the aqueous formaldehyde treatment changed the morphology of parent OMS. Furthermore, an aggregated nanocrystal morphology was observed for RuO2(3%)/Mn3O4 (Figure 3c). The presence of RuCl3 and aqueous formaldehyde together in the same reaction medium provided distinct morphology to that of parent OMS or Mn3O4, confirming the influence of Ru source in the morphological evolution of RuO2(3%)/Mn3O4. An energy-dispersive X-ray analysis (EDAX) spectrum was recorded for RuO2(3%)/Mn3O 4 while analyzing the SEM images (Figure S2). Ru, Mn, and O elements were observed in the EDAX spectrum of RuO2(3%)/Mn3O4. The highly active catalyst, RuO2(3%)/Mn3O4, was subjected to transmission electron microscopy (TEM) investigation to know the in-depth structural information about the mixed metal oxide composites (Figure 4). Figure 4a,b is the two representative TEM images of low magnification recorded from two different domains of the same specimen taken during the TEM analysis. These low-magnification TEM images

Figure 4. (a,b) Low-magnification and (c−e) high-resolution TEM images and (f) SAED pattern of RuO2(3%)/Mn3O4 investigated in this study.

further confirm the aggregated morphology obtained during the SEM investigation. The enlarged view of one such nanocrystal shows the dark domain corresponding to RuO2 (Figure 4c). High-resolution TEM images captured from two different regions of the length scale (approximately 100 nm) confirmed the absence of any nanoparticle-like morphology usually seen for the metal nanoparticles (Figure 4d,e). Less distinguishable RuO2 (dark domains) was observed on the Mn3O4 surface (Figure 4d). Very small size of ruthenium oxide (approximately 1−3 nm) is observed. Further, the highresolution TEM image shows fringes with a lattice spacing of 0.47 nm that correspond to (101) plane of Mn3O4.46 The selected-area electron diffraction (SAED) pattern obtained from the high-magnification image further confirmed the crystalline nature of the material (Figure 4f). A drift-corrected high-angle angular dark field (HAADF) image is shown in 7946

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1s spectrum demonstrated two different types of oxygen present in the sample (Figure 6d). The surface hydroxyl group appeared at a higher binding energy of 532.4 eV was originated due to the physisorbed or chemisorbed water molecules on metal or metal oxide surface.45 The peak corresponding to binding energy at 530.8 eV was attributed to the O2−/Ru−O bond reported in literature.37 Figure 6e shows the highresolution XPS spectrum for Mn 3s. The spin−orbit splitting value (ΔE) between the two doublets was 5.54 eV.55,56 This value was higher than the parent OMS sample (4.60 eV).57 This observation further confirms the change in the Mnoxidation state in the Mn3O4 phase when compared to the parent OMS.56 The average oxidation state of Mn was calculated using formula AOS = 8.95−1.13 × ΔEs, where ΔEs is the splitting energy of Mn 3s.58 The average oxidation state of Mn in RuO2(3%)/Mn3O4 was calculated to be 2.68 that matched well with the average oxidation state of Mn in Mn3O4.59 Two well-resolved doublets centered at 653.53 and 641.81 eV corresponding to Mn 2p1/2 and Mn 2p3/2, respectively, were observed in the high-resolution XPS spectrum of Mn 2p, confirming the presence of Mn2+ and Mn3+ species in Mn3O4 (Figure 6f).60 The spin−orbit splitting value (ΔE) between the two doublets was 11.74 eV that matched well with the reported value of Mn3O4.60 2.2. Catalytic Investigation. The aim of this study was to develop an efficient catalyst for the selective oxidation of biomass- and nonbiomass-derived reactants to aldehydes. The detailed study was made for the oxidation of HMF to DFF under mild reaction condition. The reactant conversion and product selectivity were confirmed using gas chromatography (GC) and GC−mass spectrometry (MS) analyses. The product was also confirmed by 1H NMR analysis (Figure S4). The aim was also to extend the application of the best catalyst in the selective oxidation of other aromatic and aliphatic compounds. The reaction condition was optimized by evaluating various influencing parameters. The details are systematically described below using HMF as a model substrate. 2.2.1. Influence of the Catalyst. Influence of the catalyst was investigated in the toluene medium at 363 K for 12 h, by flowing O2 with a flow rate of 10 mL/min (Table 2). It was found that the reaction did not proceed in the absence of

Figure 5. Elemental mapping obtained from the HAADF image shows the uniform distribution of various elements in the sample RuO2(3%)/Mn3O4 (Figure 5).

Figure 5. Drift-corrected HAADF image and the elemental mapping recorded from the region marked by yellow box in the HAADF image investigated in this study.

The diffuse reflectance ultraviolet (DRUV)−visible study provided evidence for the transformation of OMS to Mn3O4 during aqueous formaldehyde treatment (Supporting Information). However, it did not provide convincing evidence about the oxidation state of Ru present in RuO2/Mn3O4. To study the oxidation states of Ru and Mn in RuO2(3%)/Mn3O4, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Ru, Mn, and O were observed in the surface XPS profile of RuO2(3%)/Mn3O4 (Figure 6a). Ru 3p exhibited two wellresolved doublets corresponding to Ru 3p1/2 and 3p3/2 located at 484.8 and 462.6 eV, respectively, confirming the formation of RuO2 (Figure 6b).47−49 In the case of Ru0, Ru 3p1/2 and Ru 3p3/2 should have observed at 486.18 and 464 eV, respectively (Figure 6c).50 Thus, confirming the formation of RuO2 on the external surface of Mn3O4. Further, two prominent peaks at 281 and 285 eV corresponding to 3d5/2 and 3d3/2 of RuO2, respectively, were observed in the high-resolution Ru 3d spectrum (Figure 6c).51 It may be noted that the C 1s peak observed at 284.5 eV was due to the surface carbonaceous contamination that often overlaps with Ru 3d3/2 (Figure 6c).52 Two weak satellite peaks at 282.2 and 286.9 eV were also observed that correspond to RuO2.53,54 The high-resolution O

Figure 6. XPS spectrum of RuO2(3%)/Mn3O4 (a) full survey spectrum and high-resolution spectra corresponding to (b) Ru 3p, (c) Ru 3d, (d) O 1s, (e) Mn 3s, and (f) Mn 2p investigated in this study. 7947

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Reaction condition: HMF (1 mmol), toluene (8 mL), catalyst (50 mg), temperature (363 K), O2/air flow (10 mL/min), reaction time (12 h). bAir flow (20 mL/min). cCatalytic activity data after fifth cycle. The product was also confirmed from 1H NMR spectroscopy. Trace amount of benzaldehyde was also obtained as a side product (entries 11−13).

coupled plasma−atomic emission spectroscopy (ICP−AES) and surface area measurements. The surface area and pore volume of the resultant material were found to be 70 m2/g and 0.32 cm3/g. The Ru content was found to be 2.91%. Using this catalyst at the optimum condition, the HMF conversion of 60.7% and DFF selectivity of 100% were obtained. On the basis of this experiment, we can say that even though the Ru content was more in this case, but the activity was lower to that of the catalyst prepared under the method described in the Experimental Section. This experiment confirms that the RuO2 dispersion and the textural properties of resultant material play key roles in achieving higher activity. 2.2.2. Influence of O2 Pressure. Having optimized RuO2(3%)/Mn3O4 as the best catalyst, influence of O2 pressure was investigated. A high-pressure liquid-phase reactor (Parr, USA) was used to carry out reactions performed at pressure higher than 1 bar. It was interesting to observe that at any pressure higher than 1 bar, an additional side product was observed (Table 2, entry 11). This side product was not observed when the reaction was performed under O2/air flow. The side product was the dimer of HMF [5,5-oxy-(bismethylene)-2-furaldehyde] which was formed by the condensation of two HMF molecules. Influence of O2 pressure is shown in Figure 7a. With the increase in the O2 pressure from 2 to 8 bar, HMF conversion was increased but the DFF selectivity was decreased. It may be noted that using Mn3O4 alone, at 2 bar O2 pressure, somewhat higher HMF conversion but lower DFF selectivity was obtained when compared to the

catalyst (Table 2, entry 1). OMS afforded moderate HMF conversion and produced DFF as the only product (Table 2, entry 2). At the similar reaction condition, Mn3O4 was also active and produced moderate HMF conversion with 100% selectivity for DFF (Table 2, entry 3). With an aim to improve the catalytic activity, Ru was incorporated in the catalyst. The process adopted for Ru incorporation led to form RuO2-loaded Mn3O4 materials. RuO2(1%)/Mn3O4 exhibited improved activity when compared to the parent OMS and Mn3O4 (Table 2, entry 4). With an increase in RuO2 loading from 1 to 3%, the DFF yield improved. However, with further increase in the RuO2 loading to 4%, the DFF yield marginally decreased (Table 2, entry 9). With the increase in RuO2 loading from 1 to 3% (input), catalytic activity was linearly increased (Figure S5). With further increase in RuO2 loading, catalytic activity was marginally decreased. High dispersion of RuO2 on Mn3O4 support results into the highly exposed sites for reactants to adsorb and catalyze the reaction. With further increase in the RuO2 loading, aggregation of RuO2 takes place that results in the lowering of exposed sites for the reactant to adsorb and catalyze the reaction. Therefore, catalytic activity was reduced with loading of large amount of RuO2. This observation was also consistent with lower surface area and pore volume of RuO2(4%)/Mn3O4 and RuO2(5%)/Mn3O4 when compared to RuO2(3%)/Mn3O4. Therefore, RuO2(3%)/Mn3O4 was optimized as the best catalyst for this reaction. Using this procedure, the output Ru content was low when compared to input Ru content (Table 1); therefore, another strategy was adopted to prepare the catalyst with higher Ru loading. After the formaldehyde treatment at reflux condition for 30 min, the reaction mixture was dried at 423 K. The resultant solid was washed with water and dried at 373 K for 12 h. The resultant material was subjected to inductively

Figure 7. (a) Influence of O2 pressure on DFF selectivity. Influence of temperature investigated at two conditions, (b) O2 flow (10 mL/min) and (c) O2 pressure (2 bar). Activation energy calculation by the (d) plot of ln k vs (1/T). Reaction condition: HMF (1 mmol), toluene (8 mL), catalyst (50 mg), time (12 h).

Table 2. Catalytic Activity Data Obtained in the Transformation of HMF to DFFa

product selectivity (%) s. no.

catalyst

HMF conv. (%)

O2

1 2 3 4 5 6 7 8 9 10 11 12 13c

none OMS Mn3O4 RuO2(1%)/Mn3O4 RuO2(2%)/Mn3O4 RuO2(3%)/Mn3O4 RuO2(3%)/Mn3O4 RuO2(3%)/Mn3O4 RuO2(4%)/Mn3O4 RuO2(5%)/Mn3O4 RuO2(3%)/Mn3O4 Mn3O4 RuO2(3%)/Mn3O4

0 30.2 33.4 49.2 57.4 65.7 55.9 65.4 64.3 62.7 75.2 38.4 73.2

O2 flow O2 flow O2 flow O2 flow O2 flow O2 flow air flow air flowb O2 flow O2 flow 2 bar 2 bar 2 bar

major minor 100 100 100 100 100 100 100 100 100 85 78 85

15 22 15

a

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ACS Omega reaction performed under O2 flow (Table 2, compare entries 3 and 12). 2.2.3. Influence of Mass Transfer. In order to investigate the influence of mass transfer, reactions were performed in a closed pressure reactor under different stirring rates (200, 400, 600, and 800 rpm) for a stipulated time period. Further reactions were also performed at different stirring rates (300, 500, 700, and 900 rpm) in a round-bottom flask. Under both the conditions, no significant change in the HMF conversion/ DFF selectivity was obtained. These experiments confirmed that under the optimum reaction condition, mass transfer had no significant influence on the catalytic activity. 2.2.4. Influence of Temperature. Influence of temperature was investigated at two different O2 pressures. Maintaining O2 flow (10 mL/min) constant, the temperature was increased from 333 to 363 K which resulted in an increase in the HMF conversion from 22 to 65% (Figure 7b). It may be noted that with further increase in the temperature, the solvent started evaporating; therefore, the study was limited up to 363 K. In this temperature range (333−363 K), DFF was the only product. When the same reaction was performed at 2 bar pressure in the liquid-phase reactor in the temperature range of 343−393 K, HMF conversion was increased from 43 to 96%, but the DFF selectivity was reduced from 85 to 79% (Figure 7c). Considering the 100% selectivity for DFF and influence of pressure and temperature, the kinetic studies were carried out at different temperatures (333−363 K) with O2 flow of 10 mL/min. From the Arrhenius plot of ln k versus 1/T, the activation energy Ea was calculated (Figure 7d). The activation energy for this reaction using RuO2(3%)/Mn3O4 as a catalyst was calculated to be 47.7 kJ/mol. 2.2.5. Influence of the Solvent. The influence of the solvent was studied at 363 K under O2 flow (10 mL/min). Solvent played an important role in obtaining a good yield of DFF. Less polar and high boiling solvents such as toluene, p-chloro toluene, and xylene afforded higher DFF yield when compared to high polar and high boiling solvents such as DMSO and DMF. Further, H2O, 1,4-dioxane, and methyl isobutyl ketone afforded very low DFF yield (Figure 8a). Considering the economy of the process and higher yield of DFF, toluene was selected as the optimum solvent for further study. Influence of solvent suggests that the polar solvents have produced the lower yield when compared to the nonpolar solvents.

Competitive adsorption of solvent molecules and HMF on the active sites could be the possible reason for this difference in the activity. Water can be easily adsorbed on the catalyst site because of hydrogen bonding and provides less sites for HMF to adsorb; therefore, significantly low yield of DFF was obtained in H2O. Further, polar solvents have affinity to be adsorbed on the catalyst sites; therefore, the catalyst exhibited low activity in the polar solvents. HMF will have preferential adsorption on the catalyst site when compared to nonpolar solvents (such as toluene); therefore, more DFF yields were obtained in the nonpolar solvent medium. 2.2.6. Influence of the Oxidant. Catalytic reactions were performed at atmospheric pressure to investigate the influence of oxidant. Almost similar DFF yields were obtained when the reactions were performed in O2 flow (10 mL/min) and O2 (2 bar) (Table 2, compare entries 6 and 11). However, somewhat lower DFF yield was obtained when the same reaction was carried out using O2-filled balloon (Figure 8b). Also, when the reaction was performed in air at 1 atm (air-filled balloon), lower DFF yield was obtained (Figure 8b). Further, the reaction using air flow (10 mL/min) produced lower DFF yield (Table 2, compare entries 6 and 7). When the air flow rate was increased to 20 mL/min, the DFF yield was almost similar to the DFF yield obtained using O2 with 10 mL/min flow rate (Table 2, compare entries 6 and 8). This shows that even cheaper oxidant such as air can be used to get good DFF yield. It is interesting to note that the reaction proceeded well even with H2O2 and tert-butylhydroperoxide. However, these oxidants produced less yield of DFF when compared to the reaction performed in O2/air flow (10 mL/min). Considering the low cost and easy handling of O2/air in flow condition, it can be concluded that O2/air would be the best oxidant for this reaction. 2.2.7. Influence of the Substrate. A wide range of aromatic and aliphatic substrates such as furfuryl alcohol, 5-hydroxymethyl furfural, cinnamyl alcohol, benzyl alcohol, cyclohexanol, allyl alcohol, and 1-octanol were investigated in O2 flow (10 mL/min) and O2 (2 bar) conditions (Table 3). Moderate yield (60−80%) was obtained when the reaction was performed for 12 h in O2 flow (10 mL/min). Yield was further improved when the reaction duration was extended to 18 h. For each of the substrates, except for HMF and 1-octanol, the corresponding partially oxidized product aldehyde was obtained as the only product under O2 flow and pressure (2 bar) conditions. When compared to HMF, furfuryl alcohol provided lower product yield (Table 3, entries 1−2). However, the yield was improved by performing the reaction at elevated temperature (383 K) at 2 bar pressure (Table 3, entry 3). However, at the higher temperature, even in the case of furfuryl alcohol, dimer was obtained as a minor product. Cinnamyl alcohol, benzyl alcohol, and cyclohexanol afforded more than 90% yield for aldehyde/ketone when the reaction was performed at 2 bar (Table 3, entries 4−6). In the case of 1octanol in addition to partially oxidized product 1-octanal, fully oxidized product 1-octanoic acid was also obtained (Table 3, entry 8). Reactivity varies by varying the substrate. For example, furfuryl alcohol exhibited less activity when compared to HMF. Furfuryl alcohol and HMF have one common furan ring in both the substrates. Because of the presence of an active functional group (−CHO) in HMF, it withdraws electron from the ring; therefore, the O−H bond becomes weaker; therefore, oxidant can easily oxidize the carbon bearing O−H bond to aldehyde group. If one compare the activity difference between

Figure 8. (a) Effect of solvents and (b) oxidants investigated in this study. 7949

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ACS Omega Table 3. Influence of Substrates during Oxidation under O2 Flow and Pressure (2 bar) Conditiona

a

Reaction condition: substrate (1 mmol), toluene (8 mL), RuO2(3%)/Mn3O4 (50 mg), reaction temperature (363 K), time (12 h). bO2 (10 mL/ min). cTime (18 h). dO2 (2 bar). e383 K and 2 bar pressure. Trace amount of benzaldehyde was also obtained as side product at 2 bar (entry 1−8).

to DFF using lattice oxygen present in Mn3O4/OMS, and in this process, reduction of Mn4+/Mn3+ to Mn3+/Mn2+ takes place (Scheme 1a). Molecular O2 present in the reaction system reoxidizes Mn3+/Mn2+ to Mn4+/Mn3+ followed by replenishment of consumed lattice oxygen (oxygen vacancy) (Scheme 1a).61 RuO2(3%)/Mn3O4 has adsorbed water molecules that provide Ru−OH sites at the catalyst surface for the adsorption of HMF molecules through the hydroxyl group of HMF (Scheme 1b).20 Under the reaction condition, dehydration takes place and Ru surface bound to HMF is formed as an intermediate (I). It undergoes intramolecular cyclization to afford 2,5-diformyl furan (DFF) and generates ruthenium hydride species (II). Molecular O2 present in the reaction system oxidizes the ruthenium hydride species to ruthenium peroxide intermediate (III) and finally completes the cycle and regenerates the catalyst. High pressure and temperature facilitate the self-condensation of HMF molecules to form the dimer (Scheme 1b) as a side product. 2.2.9. Catalyst Recyclability. Recyclability of the catalyst RuO2(3%)/Mn3O4 was investigated in the oxidation of HMF to DFF at two different O2 pressures. First, the reaction was carried out using HMF (1 mmol), toluene (8 mL),

entry 4 and entry 7, the mesomeric effect of phenyl group (−M effect) in cinnamyl alcohol withdraws the electron density from O−H via extended conjugation, thereby weakening the O−H bond. Thus, oxidant can facilitate the oxidation of cinnamyl alcohol to cinnamaldehyde readily when compared to allyl alcohol. Similar effect was observed for benzyl alcohol (entry 5). It is well known that ketone is more stable than aldehyde. Aldehyde has the tendency to oxidize further to form acid; therefore, in the case of 1-octanol, octanoic acid is obtained as a side product. However, in cyclohexanol oxidation (2° alcohol), alpha C−H is more susceptible for oxidation because of the strain effect when compared to long alkyl C−H bond of 1-octanol; therefore, higher yield of cyclohexanone was obtained. 2.2.8. Proposed Mechanism for Partial Oxidation. Table 2 demonstrates that the oxidation took place even with parent OMS and Mn3O4. However, RuO2(3%)/Mn3O4 exhibited better activity than Mn3O4. It suggests that two sites (RuO2 and Mn3O4) are playing independently/cooperatively in the oxidation reaction. Mn3O4 is a spinel structure which is composed of both [Mn2+] and [Mn3+]. Similarly, OMS contains [Mn4+] and [Mn3+]. Mn4+/Mn3+ oxidizes the HMF 7950

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atm). Amount of Ru in the recovered catalysts was found to be 2.28 and 2.32 wt %. To further confirm the metal leaching, a hot filtration test was performed. Reaction was conducted for 4 h under O2 flow (1 atm). The reaction was stopped, and the catalyst was removed using a centrifuge machine. A small aliquot of reaction mixture was withdrawn and analyzed by GC. HMF conversion and DFF selectivity were 28.4 and 100%, respectively. Reaction was further continued in the absence of catalyst for remaining 8 h in O2 flow. After 12 h of the reaction, HMF conversion and DFF selectivity were 28.7 and 100%, respectively. The hot filtration experiment confirmed that active species were not leached; therefore, no increase in HMF conversion was observed. Elemental analysis was performed for the reaction mixture obtained after 12 h. Ru and Mn species were not detected which confirmed that Ru and Mn were not leached into the solution and those Ru/Mn sites which were present in the catalyst catalyzed the reaction as a heterogeneous catalyst to produce the desired product. Comparative catalytic activity data suggest that the present catalyst demonstrated better activity (compare TOF) than various reported ruthenium-based catalysts under mild reaction condition for the conversion of HMF to DFF (Table S1).7,35,61−64 In some cases, although the yield of DFF/TOF is high, however, the catalyst synthesis procedure is complicated and the reactions were performed at higher temperature and pressure.7,35,61−64

Scheme 1. Proposed Mechanism for the Synthesis of DFF and HMF Dimer from HMF Using (a) OMS or Mn3O4 as a Catalyst and (b) RuO2(3%)/Mn3O4 as a Catalyst Investigated in This Study

3. CONCLUSIONS In this work, a facile and sustainable conversion of biomassderived HMF and nonbiomass-derived alcohols to aldehydes under mild reaction condition was demonstrated. This conversion was achieved using a RuO2-supported Mn3O4based catalyst. The catalyst was prepared by the reaction of octahedral molecular sieve MnO2, RuCl3, and aqueous formaldehyde. At reflux temperature, MnO2 was reduced to Mn3O4 but Ru metal nanoparticles were not formed. TEM study confirmed that small RuO2 particles were uniformly distributed over the external surface of Mn3O4. XRD and XPS analyses further confirmed that RuO2 and Mn3O4 were formed. RuO2-supported Mn3O4 offered an extremely simple and highly efficient heterogeneous catalyst system for the oxidation of alcohols using oxygen/air. This sustainable catalytic oxidation process did not involve any additives and cocatalysts that make it an ideal environmentally benign chemical process for potential industrial application. This catalytic oxidation was facilitated by the Mn2+/Mn3+ sites of Mn3O4 support along with RuO2 sites present in the nanocomposite catalyst. Among the catalysts investigated in this study, RuO2(3%)/Mn3O4 exhibited the best catalytic activity under O2/air flow condition and afforded 100% selectivity for the aldehyde product in good to excellent yield for a wide range of important chemicals including DFF, 2-formylfuran, benzaldehyde, cinnamaldehyde, and so forth. The catalyst was recycled, and no appreciable loss in the catalytic activity was observed even after five cycles. The use of sustainable oxidant such as molecular O2/air (10 mL/ min) and facile and easy recovery of catalyst and the desired product from the reaction mixture make the process economically viable. Efforts are in progress in our laboratory to make more efficient catalyst system for the direct transformation of biomass-derived carbohydrates to DFF and other value-added fine chemicals and fuels.

temperature (373 K), time (12 h), O2 flow (10 mL/min), and catalyst (50 mg). After the first use, the catalyst was separated from the reaction mixture using a centrifuge machine. The recovered catalyst was washed with toluene and dried in an oven at 373 K for 10 h for the next cycle. Recycling study shows no significant decrease in the activity even after five cycles (Figure 9a). Furthermore, when the reaction was carried

Figure 9. Catalytic performance of RuO2(3%)/Mn3O4 during the recycling experiments up to five cycles in (a) 1 atm (10 mL/min) and (b) 2 bar O2 condition investigated in this study.

out at 2 bar O2 pressure, after fifth cycle, DFF yield of 63.4% was obtained (Figure 9b). XRD (Figure S6), SEM (Figure S7), and textural properties (Table 1) confirm that the catalyst was stable after the recycling experiments. Elemental analysis was performed for the recovered catalysts after performing the reaction for five cycles at 2 bar O2 pressure and O2 flow (1 7951

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4. EXPERIMENTAL SECTION 4.1. Material and Methods. Potassium permanganate (99%), manganese sulfate monohydrate (98%), and urea (98%) were purchased from Loba Chemie Pvt. Ltd. Mumbai, India. Ruthenium trichloride hydrate (RuCl3·nH2O, 99.99%, Sigma-Aldrich), nitric acid (SD Fine, 69%), and aqueous formaldehyde (37%, SD Fine Chemical Ltd. India.) were used as such without any purification. Solvents and other reagents used in the synthesis and catalysis were procured from Merck, India Limited. 4.2. Catalyst Synthesis. OMS was prepared by following the reported procedure.42 Different amounts of RuO2 were supported, and the resultant materials are designated as RuO2(x %)/Mn3O4, where x = 1, 2, 3, 4, and 5. For illustration, the procedure for the synthesis of RuO2(3%)/ Mn3O4 is described as follows. RuCl3·nH2O (30.8 mg) was dissolved in deionized water (50 mL) in a 100 mL roundbottom flask. After complete dissolution, 540 mg of OMS was added to the above dark black solution and allowed to stir overnight (8−10 h) under ambient temperature. Then, 4.8 mL of reducing agent (aqueous formaldehyde, 2.5 mM) was added, and stirring was continued for additional 20 min at ambient temperature. Finally, the resulting mixture was heated at 373 K for 30 min. During reflux, the color of the reaction mixture changed from dark black to brown. After cooling to ambient temperature, the resulting mixture was centrifuged, washed thoroughly with deionized water 2−3 times, and dried at 373 K for 12 h. Ru contents in RuO2(3%)/Mn3O4 were estimated from ICP−AES (Table 1). Though the aim was to support Ru on the external surface of OMS, but the synthesis procedure adopted in this study provided RuO2 supported on Mn3O4. Physicochemical characterization discussed below confirmed the formation of RuO2-supported Mn3O4 nanocomposites. 4.3. Procedure of Catalytic Reaction. Catalytic reactions were performed at atmospheric pressure by flowing oxygen/air during the reaction. For example, HMF (1 mmol), catalyst (50 mg), and 8 mL of toluene were added into a 25 mL doublenecked round-bottomed flask and stirred at 363 K for 12 h. During the reaction, O2/air was charged at a flow rate of 10 mL/min. After performing the reaction at desired time period, the reaction was stopped and the flask was cooled to ambient temperature. The catalyst was removed using centrifuge machine, and the reaction mixture was analyzed using GC (Yonglin 6100; BP-5; 30 m × 0.25 μm × 0.25 μm). The products of the reaction were confirmed using GC−MS (Shimadzu GCMS-QP 2010 Ultra; Rtx-5 Sil Ms; 30 m × 0.25 μm × 0.25 μm) and 1H NMR (400 MHz, JEOL). Various products synthesized in this study are commercially available. They were procured from Merck, India. Standard mixtures of these compounds were prepared and analyzed by GC. Calibration curves were constructed by plotting the concentration and area under the peak obtained from the GC analysis (Standard addition method). The conversion was determined based on the results obtained from the GC analysis and calibration curves. Each sample was measured three times, and the standard deviation was calculated. The standard deviation was in the range of 0 to ±2.0 (more precisely in the range of ±0.2 to ±1.6). The average values (to the nearest integer) are provided in tables and Figures 7−9. Furthermore, these reactions were also performed at elevated pressures in a liquid-phase pressure reactor (100 mL

Parr Reactor). HMF (1 mmol), catalyst (50 mg), and 8 mL of toluene were charged into the reactor. The reactor was pressurized with different O2 pressures (2, 4, 6, and 8 bar), and all of these reactions were conducted at 363 K for 12 h.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01009. Catalyst characterizations; XRD patterns of Mn3O4 and different wt % RuO2 supported on Mn3O4 samples; DRUV−vis spectra of OMS, Mn3O4, and different wt % ruthenium oxide-supported Mn3O4 samples; EDAX spectrum RuO2(3%)/Mn3O4; XRD; SEM of recycled catalyst; 1H NMR spectrum of DFF; and comparative catalytic activity data of RuO2(3%)/Mn3O4 over various reported ruthenium-based heterogeneous catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-1881-242175. Fax: +91-1881-223395 (R.S.). ORCID

Biswarup Satpati: 0000-0003-1175-7562 Rajendra Srivastava: 0000-0003-2271-5376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support provided by Director IIT Ropar through grant 9-35/2009/IITRPR/43 dated 09/01/2018 is gratefully acknowledged. B. Sarmah sincerely thanks UGC, New Delhi, for SRF fellowship.

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REFERENCES

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