Ruthenium nanoparticles supported on N-containing mesoporous polymer catalyzed aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF)

Applied Catalysis A: General 520 (2016) 44–52 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Ruthenium nanoparticles supported on N-containing mesoporous polymer catalyzed aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) Kajari Ghosh a , Rostam Ali Molla a,1 , Md. Asif Iqubal b,1 , Sk. Safikul Islam a,c , Sk. Manirul Islam a,∗ a Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 W.B., India Department of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India c Department of Chemistry, Aliah University, Kolkata, West Bengal 700156, India b a r t i c l e i n f o Article history: Received 31 December 2015 Received in revised form 16 March 2016 Accepted 17 March 2016 Available online 9 April 2016 Keywords: Mesoporous Poly-melamine-formaldehyde RuNPs HMF DFF Oxidation a b s t r a c t Ruthenium nanoparticles embedded on a mesoporous poly-melamine-formaldehyde material (Ru@mPMF) has been synthesized and characterized by powder XRD, HRTEM, TGA, SEM, EDS, UV–vis diffuse reflection spectroscopy (DRS), Raman spectroscopy, XPS and N2 adsorption/desorption study. The Ru@mPMF catalyst was excellent for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5diformylfuran (DFF) in toluene. Moreover, this catalyst is easily recoverable and it is reusable upto six cycles without notable decrease in its catalytic activity. Profoundly scattered and firmly bound Ru-NPs sites in mPMF might be engaged for the excellent performance of the material. Due to strong binding with the functional groups of the polymer, no indication of ruthenium leaching was there during the reaction. This suggests the actual heterogeneous nature of the material. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The ever increasing demand and continuous exploitation have caused decrease in the availability of fossil feedstock which indicates the rise in cost of such products. Thus a shifting towards the production of sustainable alternative energy and chemical feedstock has increased in recent years [1–4]. Renewable energy resources like solar, geothermal and wind have gained much attention. But these resources are unable to produce the organic chemicals which are currently obtained from fossil fuels [5–8]. But, renewable biomass that contain carbon molecules could serve as potential and sustainable feedstock for the chemical industry. The chemical industry view biomass as a promising feedstock in future and thus biomass conversion into value added chemicals is a matter of growing interest nowadays. The excessive consumption and dependence on fossil fuels and thus its depletion has led to severe impact on environment. Biomass, on the other hand has emerged as the only carbon-containing alternative, valuable, renewable and abundant feedstock that can provide bulk chemicals ∗ Corresponding author. E-mail address: manir65@rediffmail.com (Sk.M. Islam). 1 These authors contribute equally to this work. http://dx.doi.org/10.1016/j.apcata.2016.03.035 0926-860X/© 2016 Elsevier B.V. All rights reserved. and liquid fuels [9–11]. A variety of promising platform molecules can be obtained, of which 5-hydroxymethylfurfural (HMF) is synthesized by acid-catalyzed dehydration of C6-based carbohydrates [12]. HMF contains two different functional groups, hydroxyl- and aldehyde groups which accounts for its versatility in the generation of pharmaceuticals [13], fine chemicals [14–16], antifungal agents [17], plastics [18,19] and liquid fuels [20–22]. Thus, selective functional group conversion of HMF via aerial oxidation to get industrially important products, is an essential one. 2,5-Diformylfuran is an important oxidised product of HMF [23–30] which can be used as monomers for various polymer synthesis. 2,5-furandicarboxylic acid is another vital oxidised product of HMF [31–37]. However, several furan compounds like 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl2-furancarboxylic acid (FFCA), DFF, and FDCA can possibly be formed during HMF oxidation [9,39–47]. So, selective HMF oxidation to a particular product seems difficult. Now, the selective oxidation of HMF to DFF is carried out by the oxidation of hydroxyl group such that the aldehyde group (highly reactive) is not attacked, or else 5-hydroxymethyl-2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid, and finally 2,5-furandicarboxylic acid will be formed. DFF offers a large number of industrial applications and used for the preparation of polymer resins, poly schiff K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 45 Scheme 1. Schematic diagram showing the formation of mesoporous Ru◦ @mPMF. bases, composites, antifungal agents, sealants, adhesives, foams, binders, macrocyclic ligands, solvents, and organic conductors [17,47–49]. Several homogeneous and heterogeneous catalysts are reported for HMF oxidation to DFF using O2 as green oxidizing agent. Among the few works available, various vanadium-based supported catalysts were examined for HMF conversion to DFF by aerobic oxidation [23,39,48,50–53]. The reported catalysts mostly suffer from several disadvantages like difficulty in separation due to leaching of active species, excess by-product and low activity. Thus there is a requirement to develop more efficient and recyclable catalysts for the above purpose. Selective HMF oxidation using supported catalysts is an environment friendly approach. Recently, Ebitani et al. reported Ru(OH)x/HT catalyst [54] that gave 92 percent DFF yield where DMF was used as solvent which is comparable to the yield of DFF using MnO2 -based catalysts in DMSO [55]. But, the reactivity of Ru(OH)x/HT catalyst is quite low (around 3.8 h−1 estimated at ≈100% conversion). Recycled catalyst gets deactivated which may be for unstable HT surface and Ru leaching [54]. Similarly heterogeneous catalysts like covalent triazine frameworks-supported Ru catalysts [56] and an iron oxide encapsulated by ruthenium hydroxyapatite have also been used for the synthesis of DFF [24]. Therefore, developing an efficient, selective, recyclable, stable catalyst with high reactivity is still challenging. Herein, we demonstrated a catalyst achieving higher reacting efficiency and excellent recyclability at the same time. A nitrogen containing mesoporous polymer with high surface was used as the support for Ru nanoparticles, which also offers a way for 5-(hydroxymethyl) furfural to enter the porous channels to interact with RuNPs in a diffusional way. Here, we propose a safe process for the synthesis of DFF using novel Ru nano grafted mesoporous poly-melamine-formaldehyde (Ru@mPMF) material. The process is green and base free and thus environmentally friendly one. The catalyst, Ru@mPMF exhibited remarkably high activity for the synthesis of DFF from selective oxidation of HMF without the addition of homogeneous base. The condensed network structural feature of Ru@mPMF comparable to inorganic framework [57,58] containing a large number of nitrogen atoms is subjected to the superb performance of the catalytic material and is beneficial for the stabilization of RuNPs. RuNPs have larger surface areas and can be more active than the normal Ru metal. As ruthenium is less toxic thus use of it is an environmental friendly pathway for conversion of biomass derived feedstock. The conversion of HMF to DFF via base free selective oxidation using molecular oxygen is a green alternative route. 2. Experimental 2.1. Chemicals Melamine, paraformaldehyde from Sigma-Aldrich, dimethyl sulfoxide (DMSO), RuCl3 from Spectrochem, India and other reagents and solvents were purchased from Merck, India and were used without further purification. Standard procedures were followed for solvent drying and distillation. 2.2. Synthesis of mesoporous poly-melamine-formaldehyde polymer (mPMF) 3 mmol of melamine (0.378 g) and 5.4 mmol of paraformaldehyde (1.8 eq, 162 mg) were taken together in 3.36 mL DMSO (total concentration is 2.5 molar). It was taken in autoclave container and was kept in an oven for 1 h at 120 ◦ C after which it was taken out from oven for stirring on a magnetic stirrer plate. This was done to have a homogeneous solution. The mixture was again kept in an oven for 72 h at 170 ◦ C. The reaction was then removed from oven and cooled, after which it was filtered, washed with acetone then dried for 24 h at 80 ◦ C under vacuum. 2.3. Synthesis of Ru@mPMF catalyst In typical synthesis, 500 mg mesoporous polymer in water (10 mL) was stirred along with 40 mg RuCl3 at room temperature for 10 h. It was then evaporated to dryness in a rotary evaporator. Ethylene glycol solution (20 mL) was added to the dry composite. It was then refluxed at 180 ◦ C for 6 h. When solid settled down, it was washed with distilled water. Then the solution was filtered and washed with methanol and distilled water (Scheme 1). The obtained material was dried for 12 h at 60 ◦ C. Ruthenium loading to the mPMF was determined to be 4.20% by AAS. 46 K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 Table 1 Effect of solvent on HMF oxidation catalyzed by Ru◦ @mPMF. Entry Solvent Conversion HMF (%) DFF selectivity (%) FFCA selectivity (%) 1 3 4 5 ACN Toluene DMSO H2 O 35.5 99.6 84.3 88.2 67 85 76 69 8 3 14 23 Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL solvent, 2 MPa O2, 12 h, 105 ◦ C. Fig. 1. Powder XRD pattern of mPMF and Ru@mPMF materials. 2.4. Catalytic reaction The selective oxidation of HMF with molecular O2 was performed in a 50 mL stainless-steel teflon-lined autoclave. Ru@mPMF (50 mg) and HMF (2 mmol) were added into the reactor that was pre-charged with toluene (10 mL) followed by the introduction of O2 pressure (2 MPa) at 105 ◦ C for 12 h. When the system reached the desired temperature, vigorous stirring was initiated and fixed at 600 rpm. Then after a fixed time, the reaction was brought to an end and the reactor was cooled to room temperature. After the reaction, samples of the reaction mixture were taken and, after filtering off the catalyst, analyzed by GC (Varian-430 with a flame ionization detector) and HPLC (Agilent 1200 series, using a reverse-phase C18 column). The quantity of furan compounds was sensitive to a UV detector at a wavelength of 280 nm. The mobile phase was composed of acetonitrile and 0.1 wt% acetic acid aqueous solution in a volume ratio of 30:70, and the flow rate of 1.0 mL min−1 was employed to identify possible products according to calibration with standard solutions of the products and reactants. 3. Results and discussion 3.1. Characterization of catalyst Fig. 1 shows the powdered XRD of mPMF and Ru nanoparticles dispersed on it (Ru@mPMF). Mesoporous mPMF shows its characteristic broad diffraction peak centered at 2␪= 21.75 ◦ [59]. The decrease of the overall reflection intensity with respect to the parent, Ru@mPMF is a consequence of the inclusion of guest ruthenium nanoparticles in its pores and surfaces. In the XRD pattern of the Ru@mPMF, the additional prominent diffraction peaks at 2␪ = 39.4, 46.4 and 77.3◦ represents (100), (101) and (110) FCC crystal planes due to Bragg’s reflection respectively [60]. HR-TEM images of Ru@mPMF is demonstrated in Fig. 2 which suggests that the material have ordered foam-like interconnected porous network structure containing high electron density dark spots throughout the specimen [61]. The spherical natures of the nanoparticles are revealed in a closer view of the individual nanoparticles (Fig. 2B). These spherical particles are assigned to ruthenium-nanoparticles in Ru@mPMF (Fig. 2C) and nanoparticles were homogeneously dispersed with an average particle size of ≈3.3 nm (Fig. 2D). TEM-EDX obtained from the nano material (Fig. 2E) depicts the presence of the expected elements in the structure of the catalyst, namely ruthenium, carbon and nitrogen. Fig. 3A and B shows the scanning electron micrographs of the Ru@mPMF. The FE-SEM images indicate uniform submicron-sized spherical morphology of the material. Large assembly of particles are formed from aggregation of the spherical paticles. These are again inter connected with each other. The presence of carbon and nitrogen are indicated in elemental mapping (Fig. 3C and D). The N2 adsorption-desorption isotherm of the Ru@mPMF (Fig. 4A) exhibits a type IV isotherm that demonstrates the mesoporous structure of the material. Again, the sharp increase in uptake of nitrogen and the hysteresis loop (P/P0 above 0.80) indicated the interparticle spaces. The BET surface area of the Ru@mPMF was found to be 190 m2 /g. In comparison to the mPMF the surface area decreases from 536 (Fig. S8) to 190 m2 g−1 , and the mesopore diameter decreases from 6.2 nm to ≈ 1.4 nm. This may be due to the deposition of ruthenium nanoparticles at the outer surface as well as on the pores of mPMF. Fig. 5 displays the Raman spectra of Ru@mPMF where bands at 1412 and 1551 cm−1 have been shown. The band at 1412 cm−1 is the D band showing the disorder or defect in carbon atom. The band at 1551 cm−1 corresponds to G band and depicts the sp2 in plane vibration of carbon atom. Raman spectroscopy of mesoporous Ru@mPMF showed different intensity peaks of each component. XPS spectrum of Ru@mPMF is depicted in Fig. 6. The spectra show that ruthenium species in the Ru@mPMF is in the metallic form (Ru◦ ). For Ru@mPMF, three peaks of Ru 3d5/2 , Ru 3d3/2 and Ru 3p3/2 are intensified at 281.8 eV, 286.2 eV and 461.9 eV respectively. It is clear from the XPS data that the Ru(0) species are produced during reduction and are dispersed in an uniform manner throughout the mesoporous object. These binding energy values are agreeable with the values for ruthenium (0) state accounted in the literature [62]. 3.2. Catalytic activity Aerobic oxidation of HMF Recently, we have reported various mesoporous polymer supported catalysts exhibiting high performance for a great variety of industrially relevant methods [63–65]. In polymer confined RuNPs (Ru@mPMF) material in the field of selective oxidation of HMF using molecular oxygen as the sole oxidant (Scheme 2). 5-Hydroxymethylfurfural (HMF) is oxidized to get various products. Selectivity of a particular product is dependent on the reaction parameters and nature of the catalyst used. Recently, we have reported a catalytic system for aerobic oxidation of benzyl alcohols where molecular oxygen was used as the sole oxidant [66]. Now we have performed aerobic oxidation of HMF using ruthenium based catalyst and oxygen as the sole oxidant. At first, HMF oxidation was carried out by taking 0.2 mmol HMF, 50 mg Ru@mPMF in a reactor using acetonitrile (10 mL) as the solvent at 80 ◦ C under 1 MPa pressure of O2 for 3 h, but the reaction did not progress beyond K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 47 Fig. 2. HR-TEM images (A–C), distribution plot (2-D), EDX (2E) of Ru@mPMF material. 38% conversion of HMF. The reaction was carried out in different solvents, to select the best one for the same (Table 1). Thus, the selective HMF oxidation was carried out in acetonitrile, toluene, dimethylsulfoxide (DMSO) and water. Effects of various solvents on the conversion of HMF and selectivity of DFF using Ru@mPMF catalyst are summarized in (Table 1, entries 1–5). Selectivity of DFF is depends on the polarity of solvents. As the polarity of solvent increase gradually conversion of HMF to DFF decreases. Among the various solvents, aromatic less polar solvent, toluene was found to be the best for the transformation in comparison to other solvents such as DMSO, DMF and water. Usually it is assumed that the activity of the catalyst is affected by solvent polarity, but there is no general agreement regarding this phenomenon on the conversion and selectivity [67]. The reason might be that more side reactions like over oxidation can possibly occurr at higher polar solvents [68]. Literature study shows that in polar solvent medium favour the formation of FDCA than DFF. Remarkably, as evident from entry 6 of Table 1, the selectivity was much higher in toluene than in water. DFF selectivity in water even declined sharply to 69%, the corresponding FFCA selectivity being 23% and conversion of HMF being 88.2%. This emulates that in HMF and DFF, the aldehyde group hydration to geminal diols [69] is facilitated by water that subsequently gets oxidized to carboxylic acids (Scheme 3). The aerobic oxidation of HMF was also carried out under different reaction time and catalyst amount to obtain the optimum reaction conditions(Table 2). When the amount of catalyst was 10 mg in the above reaction, DFF (66%) was formed as a major product along with the FFCA (2%) and HMF conversion was 19% at 5 h but 48 K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 Fig. 3. FE-SEM (A & B) images and elemental mapping (C & D) of Ru@mPMF. Scheme 2. Selective oxidation of 5-hydroxymethylfurfural towards 2,5-diformylfuran (DFF). when it was increased to 50 mg the conversion of HMF increased upto 99.6% (Table 2, entry 5). The noted difference in activity may be due to RuNPs loading that again, may be involved in forming various active species. The active sites may possess varied reactivity and accessibility in the porous network of the mPMF support. On the other hand, increasing the reaction time upto 12 h resulted K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 49 Scheme 3. Probable mechanism for the oxidation of HMF. Ru@mPMF In t en s i t y (a.u ) D band G Band 1250 1500 1750 2000 Wavenumber (Cm -1) Fig. 5. Raman spectroscopy of Ru@-mPMF. Table 2 Effect of reaction time and catalyst amount on HMF oxidation. Entry Reaction time (h) Ru◦ @mPMF(mg) Conversion(%) 1 2 3 4 5 6 5 8 10 12 12 14 10 20 30 40 50 60 19 32 59 81 99.6 99.6 Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2 , 105 ◦ C Fig. 4. N2 adsorption/desorption isotherms (A) and pore size distribution (B) of Ru@mPMF. in almost full conversion of HMF (99.6%), clearly indicating that the conversion of the reaction depends on the reaction time. These conditions did not give the full substrate conversion. To find the appropriate reaction conditions reaction time and catalyst amount were screened (Table 2). The identification of DFF in the reaction product was confirmed by 1 H NMR (Fig. S2). The aerobic oxidation of HMF was carried out under various temperatures (Fig. 7) where it was observed that the reaction temperature affected the oxidation efficiency and also the products distribution. Under 70 ◦ C, HMF conversions were not more than 65% in 12 h. Noticeably, as the temperature was raised from 70 ◦ C to 105 ◦ C, there was a gradual increase in HMF conversion upto 99.6%. But beyond this temperature, selectivity (%) of the DFF decreased with increasing the temperature from 105 ◦ C to 115 ◦ C and the DFF yield decreased from 85% to 82.5%. Beyond 105 ◦ C FFCA began to form. FFCA selectivity was 3% at 105 ◦ C temperature. These results show that Ru◦ @mPMF catalyst is worthwhile for selective oxidation of HMF. Its catalytic activity can be increased at higher temperatures without the loss of its high DFF selectivity. 50 K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 Table 3 Effect of oxidant on HMF conversion and selectivity. Entry Oxidant (%) HMF conversion DFF selectivity (%) 1 2 3a 4a 5b H2 O2 TBHP Air O2 O2 11.4 100 51 69 99.6 38 9 68 73 85 Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2, 12 h, 105 ◦ C; a (1 MPa), b (2 MPa). notable effect both on conversion of HMF and yield of DFF (Table 3). HMF conversion reached only upto 11.4% (Table 3, entry 1) when H2 O2 was used as the oxidant. H2 O2 decomposed to oxygen while following the given condition. Upto 100% conversion of HMF was obtained after 12 h when TBHP was used as the oxidant. But in this case DFF yield was very low, only 9.0% (Table 3, entry 2). Moreover, other oxidation products like FDCA and HMFCA were not observed, which might be for the furan ring breaking by t-BuOOH [69]. Comparatively meager reactivity in air may be ascribed to the low O2 concentration. Admirable capability of O2 activation for selective HMF oxidation to DFF was shown by the catalyst. Excellent conversion of HMF (99.6%) and 85% of DFF yield were obtained. Influence of the support Fig. 6. X-ray photoelectron spectroscopy (XPS) analysis of Ru@mPMF. HMF conversion (%) DFF selectivity (%) Conversion/ Selectivity (%) 100 90 80 70 60 70 80 90 100 110 120 Temperature (0C) Fig. 7. Effect of temperature on oxidation of HMF. Reaction conditions: 2.0 mmol HMF, 50 mg catalyst, 10 mL toluene, 2 MPa O2, 12 h. Effect of oxidants For the identification of the optimum condition for the oxidation of HMF, various oxidants were screened and they displayed a As we discussed before that mPMF support contain large number of nitrogen atoms which can bind metal on its surface and its pore. Small sized RuNPs can disperse on the mPMF support. This result clearly showed that the support plays a key role for the selective oxidation of HMF to DFF. Generally, a catalyst support may offer better dispersions of active sites. Another important effect of the support is the enrichment of the adsorption of the reacting species and the reaction intermediates. This effect becomes particularly vital for the heterogeneous catalytic conversion of organic molecules. Ru@mPMF has large surface area which may influence the adsorption of HMF; on the other hand, porous channels of mPMF support enhanced the activity of the catalyst. Various inorganic metal oxides have been used as a support, more acidic support such as ZSM led to poorer activity, especially very little DFF selectivity. More basic support may produce unidentified degradation and polymerization byproducts. Thus Nie et al. showed that for the efficient synthesis of DFF by selective oxidation of HMF, the dormant part of the support whose acidity and basicity are weak gets favourable [38]. So, mPMF was chosen as support as it contains high amount of aminal ( NH CH2 NH-) groups. The dual roles of a Brønsted acid and Lewis base are played by the triazine rings [61]. Catalytic activity of the nanoparticles greatly depends on size of nanoparticles. The diameter of most reported ruthenium nanoparticles were larger than 5 nm. The characteristics of small nanoclusters, especially those below 5 nm, could be considerably different from larger ones due to the quantum size effects. The difficulty in the synthesis of small sized Ru clusters makes it challenging. Successful synthesis of ruthenium nanoclusters with controlled core size with different procedures [60] have been reported recently. Here, we synthesized 3.2 ± 0.3 nm ruthenium nanoparticles. On the other hand, we have used mPMF as the support, which is highly porous. So, very small sized ruthenium nanoparticles can be easily incorporated into it. Thus, Ru@mPMF catalyst is highly reusable and effective for the oxidation of HMF reaction. Porous nature of Ru@mPMF material increased the catalyst novelty. 3.3. Catalyst reusability The recyclability of Ru@mPMF material in selective oxidation of HMF to DFF (Fig. 8) was studied as the stability and reusabil- K. Ghosh et al. / Applied Catalysis A: General 520 (2016) 44–52 51 References Fig. 8. Catalyst reusability test of the Ru@mPMF catalyst. ity of heterogeneous catalytic system is an important requirement. The catalyst was recovered by simple filtration after reaction completion. The recovered catalyst was washed with ethyl acetate, acetone and dried at 50 ◦ C. The regained catalyst was used in the next cycle under the optimized reaction conditions with further addition of substrates in appropriate amount. Almost no deterioration was observed upto six catalytic cycles which confirms great stableness of the heterogeneous catalyst for the mentioned procedure. Atomic absorption spectroscopy (AAS) was used to determine the ruthenium concentration in the filtrate. Ruthenium leakage (0.0021%) was negligible and thus it was clear that ruthenium metal remained perfect to a notable degree with the heterogeneous support (mPMF). Any noticeable amount of leaching was not observed for the reaction. 4. Conclusions In this study, nitrogen enriched organic polymer embedded ruthenium nanoparticles, Ru@mPMF catalyst was synthesized and well characterized. It was successfully applied in selective oxidation of HMF to DFF under O2 atmosphere. Nitrogen containing, porous organic polymer proved crucial support for this aerobic oxidation. The solvent system, temperature and oxygen pressure were the important reaction parameters. The catalyst is synthesized from low priced and economically obtainable starting materials. Ru@mPMF is very much stable in moisture and air. This catalyst is recyclable upto six consecutive cycles without loss of its catalytic activity. Studies are in progress to know the exact mechanistic pathway for this reaction and also to apply the catalytic system for other organic transformation. Acknowledgments S.M.I acknowledges DST-SERB, UGC and DST-W.B. for financial support. R.A.M. acknowledges UGC, New Delhi, India for his Maulana Azad National Fellowship (F1-17.1/2012-13/MANF2012-13-MUS-WES-9628/SA-III). 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