Biobased Green Process: Selective Hydrogenation of 5

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Biobased green process: Selective hydrogenation of 5hydroxymethyl furfural (HMF) to 2, 5 dimethyl furan (DMF) under mild conditions using Pd-Cs2.5H0.5PW12O40/K-10 clay Anil B Gawade, Manishkumar S. Tiwari, and Ganapati D. Yadav ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00426 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 17, 2016

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ACS Sustainable Chemistry & Engineering

ACS Sustainable Chem. Eng. Manuscript ID: sc-2016-00426b R2

Biobased green process: Selective hydrogenation of 5-hydroxymethylfurfural (HMF) to 2, 5-dimethyl furan (2,5-DMF) under mild conditions using PdCs2.5H0.5PW12O40/K-10 clay Anil B. Gawade+, Manishkumar S. Tiwari+ and Ganapati D. Yadav* Department of Chemical Engineering Institute of Chemical Technology Nathalal Parekh Marg Matunga MUMBAI-400 019 India E-mail: [email protected] Phone: +91-22-3361-1001 Fax: +91-22-3361-1020; +91-22-3361-1002

ACS Paragon Plus Environment


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ABSTRACT Conversion of biomass into valuable chemicals is highly sought after across the globe. 5Hydroxymethyl furfural (HMF) is a biobased platform chemical which could be valorized into a spectrum of fuels and chemicals. In the current work, HMF was transformed into 2,5dimethyl furan (2,5-DMF) using a number of catalysts amongst which a novel bifunctional metal-acid

catalyst,

Pd-Cs2.5H0.5PW12O40/K-10

clay,

palladium-cesium

dodeca-

tungstophosphoric acid supported on K-10 acidic clay, was found to be the best. Because of its high energy density, 2,5-DMF is a potential biofuel additive. The acidic nature of CsDTP/K-10 facilitates rapid hydrogenolysis of HMF at mild reaction condition. 2% Pd-20 % w/w CsDTP/K-10 clay designated as 2Pd-20CsDTP/K-10 gives 98% conversion of HMF with 81% selectivity to 2, 5-DMF at 90 oC and 10 atm hydrogen pressure in 2 h. The catalyst was prepared by a simple and cheap wet impregnation method and characterized, both before and after reuse, by XRD, FTIR, BET surface area, NH3-TPD, TGA, TEM and SEM. The catalyst was robust and could be used with very minimal loss in activity. The reaction mechanism and kinetics are also presented. The overall process is clean, green and sustainable.

KEYWORDS 5-Hydroxymethyl furfural (HMF), 2,5-dimethyl furan (2,5-DMF), Biofuel, Heteropoly acid, biomass, Palladium, hydrogenation, bifunctional catalyst, Green Chemistry. SYNOPSIS:

Selective hydrogenation of HMF to 2, 5-DMF

was achieved using a novel

bifunctional catalyst 2% Pd 20% w/w Cs2.5H0.5PW40/K-10 clay under mild reaction condition.

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INTRODUCTION The proven reserves of oil and natural gas are inadequate to meet the energy and material needs of the ever growing population. The current market prices of oil and natural gas are highly volatile, prone to geopolitics; thus, the future of petroleumbased chemical industry is questionable and will become unaffordable soon.1-5 The use of fossil raw materials also creates environmental issues such as greenhouse gas (GHG) emissions and air pollution. Obviously the utilization of biomass based fuels and chemicals is gaining impetus. One of the promising building blocks, 5hydroxymethylfurfural (HMF) can be synthesized from a number of bio-based feed stocks

and will be playing a key role in biorefinery. HMF is an acid catalyzed

dehydration product of carbohydrates such as glucose and fructose.6-9 A chemical tree could be derived from HMF to prominent value added chemicals such as levulinic acid,10 ɣ-valerolactone,11,12 2,5-furan-dicarboxylic acid (FDCA),13, 2,5-diformylfuran 14,15

and 2,5-dimethylfuran (2,5-DMF).16 Among the foregoing products, 2,5-DMF is the most desired one because of its

high energy density (30kJ/cm3), which is 40% higher than that of ethanol.3 It has further advantages over ethanol, such as high boiling point (92ºC), making its transportation easier, high research octane number (RON) of 119) and also it does not absorb moisture like ethanol.3,17 So it reduces the cost of distillation from water, which requires a high amount of energy. 15,16 Owing to these benefits, 2,5-DMF proves to be a better substitute for ethanol and other fossil energy sources to give a sustainable future for fuel production. 2,5-DMF is produced by hydrogenation of HMF.18-22 It can be directly obtained from fructose or glucose as a starting material using one-pot synthesis method.3,16, 23-25 A two-step process for 2,5-DMF synthesis was reported,3 in which HMF was obtained 3



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by using H2SO4 and then hydrogenation was carried out at high temperature (220°C). Thananatthanachon et al.16 have used milder reaction conditions for the synthesis of 2, 5-DMF; however, the process requires use of formic acid and H2SO4 for increasing the yield. Hu et al.20 synthesized 2, 5-DMF from HMF by using a Ru/C catalyst at 200 °C. A process for 2,5-DMF synthesis was reported at mild conditions by using carbon supported Pd-Au bimetallic catalyst in the presence of hydrochloric acid.19 Many of these processes are corrosive and toxic; the liquid acid catalysts require neutralization after reaction, which in turn, produces a large amount of waste. So, the use of heterogeneous acid catalysts along with metal sources can be beneficial to devise a clean and green process. Heteropolyacids (HPA) possess high activity due to the super acidic nature. Dodecatungstophosphoric (DTP) is reported to be most widely used HPA because of its better thermal stability and higher acidity. 26-28 The problems related to the general HPAs such as low surface area and solubility in polar solvents can be overcome by the supporting them on a proper support or converting them into insoluble salts with the metal ions. We have reported the preparation of acidic clay supported HPA, namely, 20% w/w Cs2.5H0.5PW12O40/K-10 clay (20CsDTP/K-10) by the wet impregnation method and used it in several industrially important reactions.2932

Other metal oxides and carbon supports were also used.

2,4,9,33

Amongst the various

metals reported catalysts for the synthesis of the 2, 5-DMF from HMF, palladium seems to be more effective.

3,16,19

Hence we have chosen in this work palladium as

metal site along with solid super acid based on modified HPA for the selective hydrogenation of HMF to 2,5-DMF. Here we report an environmentally safe and green process for 2,5-DMF synthesis from HMF without using any homogeneous acid such as HCl. The use of a 4


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novel bifunctional metal-acid catalyst, 2% Pd-20% w/w Cs2.5H0.5PW12O40/K-10 (designated as 2Pd-20CsDTP/K-10) is made. In 2Pd-20CsDTP/K-10, Pd plays a role for hydrogenation while the acidic nature of 20CsDTP/K-10 activates hydrogenolysis of hydroxyl groups of bis (hydroxymethyl) furan (BHMF), which is the key intermediate formed in this reaction. In the absence of acidic media, BHMF requires higher temperatures to hydrogenate into 2, 5-DMF and longer reaction time.19,20 Hence 2Pd-20CsDTP/K-10 catalyst can play a dual role in this reaction. It gives a high conversion of HMF and excellent selectivity for 2, 5-DMF under mild reaction condition without addition of any external homogenous acid. The catalyst is stable, active, and selective and shows good reusability over many operational cycles. Effects of different process parameters were studied and reaction mechanism and kinetic model developed.

EXPERIMENTAL SECTION Chemicals All chemicals were purchased from reputed companies. 5-Hydroxymethyl-2-furfural (HMF) (99.8%) was procured from Acros Organics, Mumbai, India while cesium chloride, palladium acetate, palladium chloride, dodecatungstophosphoric acid and montmorillonite clay K-10 were purchased from Alfa Aesar, Mumbai. Tetrahydrofuran, toluene, 1,4-dioxane, methanol and n-butanol were purchased from Thomas Baker, Mumbai. Catalyst preparation

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All catalysts were synthesized by wet impregnation method. Initially, 20% (w/w) CsDTP/K10 (designated as 20CsDTP/K-10) was synthesized similarly to a process reported by Yadav et.al. 32,34-35 Preparation of Pd supported CsDTP/K10 Pd loaded catalyst was synthesized by a reported method.36 Pd supported 20CsDTP/K-10 with palladium loading from 1 to 3% (w/w) was prepared by adding a solution of dry toluene containing appropriate amount of Pd (OAc)2 in

2g

20CsDTP/K-10 and stirred at 30 oC for 4 h. Then toluene was evaporated using a rotary evaporator. During toluene evaporation, Pd (II) was reduced to Pd (0) by toluene and the solid turned black.36 After complete evaporation of toluene, the black solid was dried under vacuum at 120ºC for 12 h. The resultant material designated as XPd-20CsDTP/K-10 (where X=1% to 3% (w/w) Pd loading) was stored under vacuum in air tight bottle. The palladium loaded K-10 was prepared by the use of palladium chloride using wet impregnation and the resultant solid was calcined at 500 °C for 3 h followed by reduction of metal site at 200 °C for 2 h at 20 atm hydrogen pressure. The prepared catalyst was denoted as 2Pd/K-10.

Reaction procedure A 100 mL autoclave (Amar autoclave, Mumbai) having appropriate controllers and pitched turbine impeller was used. In a typical experiment, 0.1 M HMF made up to 20 mL with tetrahydrofuran (THF) as solvent, 4% (v/v) internal standard (n-dodecane) and catalyst loading 0.02g/cm3 were used. The autoclave was flushed 3 times with hydrogen and the mixture was agitated at 1000 rpm, at 90 oC and 10atm hydrogen pressure. Sampling was done at regular intervals of time for analysis. The mixture was 6


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analyzed by using GC (Chemito 8610) provided with BP-1 column and FID detector. The products were identified and confirmed by GC-MS (Perkin Elmer Clarus 500).

Characterization methodology:

X-ray diffraction pattern of all the catalysts including reused catalyst was performed by using X-ray diffractometer (Bruker AXS, D8 Discover, USA). The surface properties of all catalysts including reused catalyst were evaluated by Micromeritics ASAP 2000 instrument by using nitrogen adsorption–desorption isotherms. All samples were degassed at 350 ºC under vacuum for 4 h. A Fourier transform infrared spectra of all catalysts along with recycled catalyst was performed on a Perkin Elmer Spectrophotometer in the range of 400–4000 cm−1. All sample pellets were prepared by mixing 5% catalyst (by weight) in pre-dried KBr powder. Temperature programmed desorption using 10% NH3 in helium was used to find out the total acidity present in all catalysts by using Micromeritics Autochem 2920 instrument. H2-pulse chemisorption was conducted on Micromeritics Autochem 2920 instrument to calculate the active surface area of palladium. The sample was reduced at 300 °C for 1 h under 10% H2 /Ar flow and then cooled to 50°C. The chemisorption was carried out by pulse of mixture of 10% H2 /Ar. TEM analysis was carried out on a JEOL-JSM 2100 electron microscope equipped with a thermal electron emission type gun. All images were collected at 200kv using a multiscan camera.

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Differential scanning calorimetry (DSC) and thermogravimetry (TGA) were used simultaneously by using Perkin Elmer Pyris Diamond instrument to check the thermal stability of 2Pd-20CsDTP/K-10.

Scheme 1. Hydrogenation of HMF

RESULTS AND DISCUSSION Characterization of catalysts XRD patterns of different catalysts are shown in Figure 1. The loading of the crystalline DTP and CsDTP on the amorphous K-10 results in the loss of crystallinity and is well documented.34,35 The same patterns were observed here also. All these catalysts are amorphous in nature and hence CsDTP is finely dispersed on support.32 The XRD pattern of Pd/K-10 shows a sharp peak at 26º which is due to quartz. There 8


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was no significant change observed in the XRD pattern after Pd loading on CsDTP/K-10, and K-10. It may be due to the high dispersion of Pd particles on the acidic surface. Also the overlapping of the peaks related to the Cs-DTP/K-10 with the peaks of Pd

37

at 2θ=40°, 48°, 68° means that there was no change in the overall XRD

pattern of the prepared catalyst. Further, it also confirms that there is no replacement of protons with Pd in 20CsDTP/K-10 and it is well dispersed. The XRD graphs of fresh and reused catalyst were recorded and compared. There is a marginal basic difference found in the pattern which confirms that catalyst is stable. The surface properties of the virgin and used catalysts were evaluated by BET method (Table1). K-10 has a surface area around 240 m2/g which decreases with the loading of DTP as reported in an earlier reports,32,34,35 while Cs-DTP/K-10 shows increase in the surface area (Table 1, entry 2) which is due to the formation of a dense porous network between the large molecule of DTP and Cs ions.32 The surface area of 20CsDTP/K-10and K-10 was slightly decreased after Pd was loaded. It is obviously due to the presence of the Pd metal in the pore space of the catalyst. Also from Table 1 it is seen that there is almost negligible change in the pore diameters and pore volume of the virgin and used catalyst. The nitrogen adsorption-desorption isotherms shows Type IV isotherm and H3 hysteresis loop for all catalysts, which indicate the materials are mesoporous (ESI, Figure S1). The FT-IR of all catalysts were recorded and given in ESI (Figure S2). The FTIR spectrum of 2Pd-20CsDTP/K-10 was compared with bulk DTP. The peaks obtained between 810 to 1090 cm-1 correspond to stretching frequencies of W-O-W, terminal W=O and P-O in central tetrahedral of Keggin polyanion.30,32 The peak in between 1630 to 1640 cm-1 correspond to the bending frequency of –OH i.e. water of crystallization present in DTP.30,32 The loading of Pd does not affect the nature of 9



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catalyst and the Keggin structures of DTP remain intact (Figure S2).The reused catalysts also show the same IR spectra and confirm that there is no adsorbed material remaining in the pore space after reaction. The acid strength and amount of acid sites of all catalysts are calculated in terms of mmol of ammonia and given in Table 1. All catalysts have shown two distinct peaks in the temperature

range of 100-200 °C and 400-600 °C, which

confirms that the catalysts have a good amount of weak and strong acidic sites (Figure 2).The amount of acid sites shown by K-10 is due to its characteristic acidic nature, as it is prepared by the acid treatment of montmorillonite clay.32 There is a significant increase in the overall acid strength possessed by 20CsDTP/K-10 which is due to the loading of dodecatungstophosphoric acid (DTP) (H3PW12O40) which was converted into nano sized Cs2.5H0.5PW12O40 (CsDTP) in situ. K-10 shows more number of weak acidic sites as compared to the 20CsDTP/K-10 which has high amount of strong acidic sites along with weak acidic sites. The loading of Pd shows no change in the amount of acid sites of 20CsDTP/K-10 as confirmed by the NH3-TPD. Hence there is no interaction between the protons of the catalyst and palladium particles and it be concluded that presence of palladium within the pore space does not hamper the acidic sites. H2 -Pulse chemisorption of 2Pd-20CsDTP/K-10 catalyst was used to calculate the metal dispersion and the metal size. Using the stoichiometry Pds/H= 1, the concentration of surface Pd atoms was calculated to 187 µmol /g

cat

and metal

dispersion around 38%. Particle diameter was calculated as 2.9 nm and the active metal surface area per g of catalyst was around 3.4 m2/g. The TEM images of 20CsDTP/K-10 and 2Pd-20CsDTP/K-10 is presented in Figure 3. 20CsDTP/K-10 shows the multilayer structure which is a characteristic of K10


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10.38 It also confirms that loading of Cs-DTP and Pd does not affects the K-10 morphology. Diffraction pattern of 20CsDTP/K-10 does not shows any well-defined circles which confirms that the catalyst is not crystalline. This is due to the highly amorphous nature of K-10 as revealed in XRD and well reported in various literature. 32, 37

2Pd-20CsDTP/K-10 images confirm the presence of well dispersed Pd particles.

The Pd particle size are found to be in range of 2-10 nm. Thermal analysis of sample was performed in the range of 30 to 700 ºC (Figure 4). The weight loss and endotherm occurred within 100 and 150ºC which can be attributed to removal of adsorbed water. The slight weight loss after 300 oC is due to removal of constitutional water (H3O+) from DTP.39 As suggested in the earlier studies, 30,32,39 supporting DTP on the support and /or replacing the H+ ions of the DTP with metal ions enhances its thermal stability.32,39 The results obtained here are in accordance with the literature. Hence the catalysts are thermally stable up to 700 oC.

Efficacy of various catalysts Catalysts such as 2Pd-20CsDTP/K-10, 2Pd/K-10, 20Cs-DTP/K-10 and 5Pd/C with and without HCl were used for the hydrogenation of HMF to 2,5-DMF. The catalysts were screened on the basis of the two parameters, i.e., conversion of HMF and selectivity of 2,5-DMF since the reaction is a two-step reaction in which firstly HMF is converted into the BHMF which further undergoes hydrogenolysis to give the final product 2,5-DMF (Scheme 1). From Figure 5, it is observed that almost 100% conversion of HMF after 2 h except when Cs-DTP /K-10 was used as the catalyst. Since Cs-DTP/K-10 does not have any metal site (here Pd) as compared to others, it does not initiate the reaction even though there was a hydrogen pressure in the reactor. This also confirms that the present 11



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catalyst does not play any role in current form for the hydrogenation reaction. Further it is well documented that in presence of acid catalyst,40,41 self-condensation of HMF is possible. However, from the results, almost negligible conversion of HMF was observed and through analysis it was confirmed that no self-condensed product was formed. Thus it was concluded that conditions might not be favorable to initiate the self-condensation of 5-HMF. It also confirmed that at prevalent conditions, HMF did not react. With other catalysts, 100% conversion of HMF was seen but the selectivity of 2,5DMF must be considered as the main criterion among the four catalysts having Pd. 5% Pd /C catalyst gives complete conversion of HMF but the selectivity of 2,5-DMF was very low because the major product formed was BHMF instead of 2,5-DMF. Further conversion of BHMF to 2,5-DMF usually requires high temperature and pressure. Hence under these conditions, there was poor selectivity of 2,5-DMF. In order to achieve better selectivity, like the earlier reports, we used 0.02 mmol of aqueous HCl and it helped in enhancing the rate of hydrogenolysis to form the 2, 5-DMF from the HMF. Figure 5 depicts the effect of HCl addition leading to ~86 % selectivity of 2,5DMF with Pd/C +HCl as compared to only Pd/C catalyst without HCl. This supports the finding of the reported literature16, 18, 42 which claims that with use of acids, HMF to 2, 5-DMF can be easily achieved under mild condition. Therefore the effect of use of heterogeneous acid instead of neutral supports (such as carbon, silica etc.) for this reaction was studied. Also the use of HCl have the issues related to environment and may hamper the economical aspect of the synthesis because of its corrosive nature. The process is no longer green. Therefore it was planned to explore the use of K-10 and Cs-DTP/K-10 as acidic catalysts with Pd metal for the selective synthesis of 2, 5- DMF. Figure 5 12


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shows that Pd supported on Cs-DTP/K-10 gives better selectivity and yield of the 2,5DMF as compared to Pd/K-10. To correlate the formation of 2,5-DMF with present catalyst, the acidity measurement analysis was performed byNH3 –TPD (Table 1). The acidity of K-10 was less as compared to that of Cs-DTP/K-10 and yield of 2,5-DMF was also in same order. Hence it is concluded that the high acidity of supports helps to get high yield of 2,5-DMF. Further, amongst all these catalysts 2Pd-20CsDTP/K-10 was the best catalyst for this reaction. We have also compared our catalyst with the reported literature and comparison is given in Table 2. It is seen that high temperature, pressure and long reaction time were often required to get good yield of 2,5-DMF with the use of metal catalyst and without addition of any acidic additive or acidic support. With the use of homogeneous acid (Table 2; entry 4, 7) a good yield of 2, 5-DMF at mild condition is reported; however, use of these homogenous acids may result in corrosion and disposal issues. Since

2Pd-20CsDTP/K-10 catalyst shoed

better

activity and selectivity of 2, 5-DMF at very mild condition, it was robust and hence the overall process is green.

Effect of palladium loading Pd-20CsDTP/K-10 was prepared with palladium loading in the range of 1 to 3 % (w/w). All these catalysts showed good conversion of HMF but the selectivity for 2,5DMF seems to be dependent on the wt. % of palladium loaded on the 20CsDTP/K-10. By increasing the loading from 1 to 2 % (w/w) there was an increase in the conversion of HMF and in the selectivity of 2,5-DMF; however, after 2 % (w/w) loading further ring hydrogenation of 2,5-DMF took place and major product obtained was 2,5dimethyltetrahydrofuran (DMTHF). Thus, 2 %( w/w) palladium loading was chosen as the optimum and further experiments were carried out with 2Pd-20CsDTP/K-10. 13



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Effect of solvent Hydrogenation of HMF to 2, 5-DMF in liquid phase was carried out by using various solvents such as water, tetrahydrofuran (THF), n-butanol and 1,4-dioxane (Table 3). ~100% conversion of HMF was achieved after 2 h in all solvents but the selectivity of 2,5-DMF was hampered. Effect of solvent is a very complex parameter and there are a number of factors which influence the rate of reaction and selectivity of the product.4453

Chatterjee et al.49 have correlated the HMF conversion with the δ values (difference

between the donor number and acceptor number of solvent) of different solvents and shown that with increase in δ value the rate of conversion of HMF decreases. This is due the competitive adsorption of the solvent molecule on the metal surface. It can be determined by δ value53 which states that solvents with negative δ value has very least interaction with metal catalyst, while interaction of organic solvent increases with change in δ value from negative to positive. Here we can see the value of δ increases in the following order: water < n-butanol < THF < 1, 4-dioxane (maximum). Water having low value of δ gives the high conversion while 1, 4-dioxane gives the least conversion after 1 h (Table 3). Hence our results are in the same order. Water gives high rate of conversion but the selectivity of 2,5- DMF is very low. In the presence of acidic catalyst in aqueous medium HMF tends to give levulinic acid and formic acid. Here also we got 15% of byproducts (comprising of formic acid, levulinic acid and some unknown products) along with 85% of BHMF. Moreover, as reported by Toukoniitty et al.50 with increase in dielectric constant, solubility of hydrogen decreases in organic solvent in the following order: 1, 4-dioxane>THF>nbutanol>water (least). Hence hydrogen solubility is very less in water as compared to the organic solvents and therefore only 42% selectivity of 2,5- DMF after 2 h in water. 14


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The same trend was observed with other solvents (Table 3). However, in the case of nbutanol and 1, 4-dioxane, less selectivity of 2, 5-DMF was observed as compared to THF and can be explained on the basis of solvation effect of solvent.51 It states that in a polar medium the polar compounds remain more strongly solvated and thus results in facile adsorption of non-polar components on the catalyst surface. The vice-versa is applied for the non-polar medium. BHMF is more polar and hence more solubilized in n-butanol which results in low access to the metal catalyst. In the case of THF and 1, 4-dioxane, the polarity decreases in the same order as well as BHMF is not highly solvated and therefore more facile interaction is observed between BHMF and the metallic surface which gives better yield of 2,5-DMF in both the solvents. As stated earlier hydrogen concentration in 1, 4-dioxane is higher due to its solubility as compared to other solvents and further (ring) hydrogenation of 2,5-DMF to DMTHF occurs leading to decrease in the selectivity of 2,5-DMF. Hence THF was chosen as solvent for further screening and optimization of reaction parameters.

Stability of catalyst To ascertain the stability of the 2Pd-20CsDTP/K-10 catalyst, the reaction was stopped after 30 min and the catalyst filtered from the reaction mass completely. The reaction was continued in the absence of catalyst at the same reaction condition. There was no further increase in the yield of 2,5-DMF even after 2 h which confirmed that was no leaching of the active species in the media and the reaction was heterogeneously catalyzed. Further to support this, ICP analysis of the reaction mixture was done. A negligible Pd of 0.13 ppm was found in solvent and hence does not show any activity toward hydrogenation after filtration. Leaching of Cs-DTP in the reaction mixture was also analyzed by using well know ascorbic acid test.34,35 The reaction mixture showed 15



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no color change indicating no leaching of Cs-DTP. Thus 2Pd-20CsDTP/K-10 is a stable and active catalyst for clean synthesis of 2, 5-DMF from HMF.

OPTIMIZATION OF DIFFERENT REACTION PARAMETERS Effect of Speed of agitation To know the optimum agitation speed required to overcome the external mass transfer resistance, the stirring speed was varied from 800 to 1200 rpm. No significant change in the conversion of HMF or selectivity of 2, 5-DMF was noticed at all speeds, which meant that no mass transfer resistance was present. Therefore, 1000 rpm was considered as optimum and all further studies performed at this speed.

Effect of amount of catalyst To know the optimum amount of catalyst for this reaction, catalyst weight was varied in range of 0.2 g to 0.5 g.The concentration profiles at different catalyst loading is shown in ESI (Figure S3).The rate of hydrogenation of HMF and selectivity of 2,5DMF increase significantly with increase in weight of catalyst, which is due to proportional increase in the number of active sites of catalyst. However, the selectivity of 2,5-DMF was affected with the change of catalyst weight. Figure 6 shows that an increase in catalyst mass from 0.4 to 0.5 g, results in enhancement of reaction rate for hydrogenation of HMF to 2,5-DMF but the selectivity of 2,5-DMF decreases, due to 16


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further hydrogenation of 2,5-DMF into dimethyltetrahydrofuran (DMTHF),which is an over hydrogenated product of 2,5-DMF. Further the initial rate of hydrogenation of HMF was plotted against the amount of catalyst (Figure 7). With increase in loading the rate of reaction also increases. There is a linear relationship between the rate and the amount of catalyst. Also the Wiesz-Prater modulus44 was used to evaluate the importance of intra-particle diffusion limitation. It was far less than one which again proved that the reaction was kinetically controlled.

Effect of initial concentration of HMF The initial concentration of HMF was varied from 0.05 M to 0.15 M to study its effect on the rate of reaction of HMF and selectivity of 2, 5-DMF. The concentration profile for the reaction with time is provided in ESI (Figure S4). Rate of formation of 2, 5-DMF decreases as the concentration of HMF increases (Figure 8). This is obvious, as the concentration of HMF increases, the ratio between substrate to catalyst decreases and hence the rate also decreases. With 0.1 M initial concentration of HMF, good conversion of HMF and 81% selectivity of 2, 5-DMF was obtained after 2 h.

Effect of hydrogen pressure Effect of hydrogen pressure on the conversion of HMF and on the selectivity of the 2,5-DMF was studied. The concentration profiles at different pressure are given in ESI (Figure S5).The hydrogenation of HMF to BHMF and its further hydrogenation to 2,5-DMF seems to be greatly dependent on hydrogen pressure (Figure 9). The rate of hydrogenation of HMF increases with increase in pressure and the same can be seen with the selectivity of 2,5-DMF. As the pressure is increased the concentration of dissolved hydrogen in the reaction mixture increases and it enhances the rate of hydrogenation of HMF and further it also enhances the conversion of BHMF 17



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(intermediate) to 2,5-DMF (Figure 9). However, the further increase in pressure from 10 atm to 15 atm results in increase in concentration of over hydrogenated product (i.e. DMTHF) because of the excess of hydrogen present in the system. Hence maintaining optimum pressure is a key to obtain high yield of 2,5-DMF. Therefore, 10 atm was taken as an optimum hydrogen pressure for further studies.

Effect of temperature Influence of reaction temperature on the conversion of HMF and on the selectivity of 2,5-DMF was studied and the concentration profiles at different temperatures is shown in ESI (Figure S6). With increase in temperature the rate of hydrogenation of HMF increases but the selectivity of 2,5-DMF gets affected with temperature because of subsequent reaction to ring hydrogenation. The conversion of HMF increases with rise in temperature which evidence that the reaction is kinetically controlled and there is no influence of any mass transfer resistance on the reaction rate. As shown in Figure 10, the yield of 2,5-DMF increases with increase in temperature from 80 to 90°C but thereafter further hydrogenation of furan ring occurs and gives DMTHF. Hence 90 °C was considered as the optimum temperature for better yield of 2,5-DMF under specified reaction condition.

Reusability of catalyst After completion of reaction, the catalyst was filtered and refluxed in methanol for 2 h to wash out any adsorbed reactant from the surface and pores of the catalyst. Then it was filtered, dried and used in the next experiment. In the reusability experiment, on the basis of weight of recovered catalyst, a fresh catalyst as make-up was added (~ 5% of total weight) to make up the total weight to 0.4 g. Remaining conditions were the 18


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same. As shown in Figure 11, the conversion slightly decreases but the selectivity of 2,5-DMF is maintained throughout the reuse cycle of catalyst (3 times). Hence it can be inferred that the catalyst is reusable with very slight decrease in its activity. Further the characterization of the recovered catalyst after washing was done as stated earlier. It also confirmed that there was a marginal loss in activity of catalyst after reuse and its fidelity was intact.

Kinetics of hydrogenation of HMF Mechanism Hydrogenation of HMF takes place in a series of steps. Both the metallic and acidic sites of 2Pd-20CsDTP/K-10 are useful in the hydrogenation of HMF to 2,5-DMF. Initially hydrogen gets dissociatively adsorbed on the metal site and HMF gets weakly adsorbed on the acidic site. Firstly the aldehyde group is hydrogenated into hydroxyl group which gives BHMF as an intermediate (Figure S7). The acidic site of catalyst is useful for fast hydrogenolysis of BHMF into 2,5-DMF through MFA. Finally, 2, 5DMF gets desorbed and catalyst surface is free for the next cycle. A reaction mechanism is postulated in Scheme 2.

Development of a Mathematical Model Based on Schemes 1 and 2, a mathematical model was developed for the hydrogenation of HMF using dual site LHHWs mechanism. The detail derivation is provided in supplementary information (ESI).

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Scheme 2: Postulated mechanism for hydrogenation of HMF to 2,5-DMF.

The rate of reaction for the different steps involved for the hydrogenation is given here. Rate of hydrogenation of HMF (A) can be written as: −

k1 K AC A K H 2 pH 2 w dC A = dt [1 + K AC A + K BCB + K D CD + K E CE ] 1 + K H pH 2 + KW CW 

(1)

Rate of reaction of BHMF (B) to 2,5-DMF (D) can be written as -

[ k1K AC A − k2 K BCB ] K H 2 pH 2 w dCB = dt [1 + K AC A + K BCB + K D CD + K E CE ] 1 + K H pH 2 + KW CW 

(2)

20


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Rate of reaction of 2,5-DMF (D) can be written as

[ k 2 K B C B − k3 K D C D ] K H 2 p H 2 w dCD = dt [1 + K AC A + K BCB + K DCD + K E CE ] 1 + K H pH 2 + KW CW 

(3)

Rate of formation of DMTHF (E) can be written as: k3 K D C D K H 2 p H 2 w dCE = dt [1 + K AC A + K BCB + K D CD + K E CE ] 1 + K H pH 2 + KW CW 

(4)

Where, w= catalyst loading, g.cm-3 Using above equations, rate constants (k) and adsorption constants (Ki) were calculated. The adsorption constants values for different products and reactants were calculated and given in Supplementry Information (Table S1). The rate constant values of each step at different temperature are given in Table 4. Thus Arrhenius plot was made to calculate the activation energy. The activation energy for different steps are 14.7, 8.8 and 18.1 kcal/mol, respectively. The lower activation energy of second step indicates that low temperature is favored for this reaction. This is in concurrence with the experimental findings as higher yield of 2, 5-DMF is observed at lower temperature and with increase in temperature the yield of DMTHF increases as it has high activation energy as calculated. For the validation of predicated model, we have calculated the concentration of different species using the different constants. These values ae then plotted as a dotted line (ESI, Figure S3-S8) along with experimental values. The plot shows the best fitting of the experimental and

theoretical value, hence

validates the model very well.

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CONCLUSIONS 2% w/w Pd supported on 20% w/w CsDTP/K-10 (2Pd-20CsDTP/K-10) prepared by two step method shows better catalytic activity for 2,5-DMF synthesis than 5% Pd/C under

homogeneous acidic

condition.

The

acidic

support

helps for

rapid

hydrogenolysis of BHMF into 2,5-DMF. The catalyst was well characterized by different analytical techniques and reused thrice with very marginal loss in activity. The optimum reaction condition was obtained by studying all reaction parameters. A detailed mathematical model was developed using a duel site LHHW mechanism and used to calculate various constants. The activation energy was evaluated for each step. The process is green and novel.

ASSOCIATED CONTENT Supporting Information N2 adsorption-desorption isotherms of different catalyst, FT-IR spectra of catalysts, Concentration profile of hydrogenation of HMF with time for different reaction parameters NMR of BHMF, Mathematical model for the hydrogenation of HMF AUTHOR INFORMATION Corresponding Author *Ganapati D. Yadav. E-mail: [email protected]; Phone: +91-22-3361-1001, Fax: +91-22-3361-1020; +91-22-3361-1002 Author Contributions 22


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+

These authors have contributed equally.

Notes The authors declare no competing financial interest. .ACKNOWLEDGEMENTS A.B. Gawade acknowledges University Grants Commission (UGC) for the award of BSR Senior Research Fellowship under its Green technology program in Chemistry. M.S. Tiwari acknowledges University Grants Commission (UGC) for the award of BSR Senior Research Fellowship under its SAP program in Centre for Advanced Studies in Chemical Engineering. G.D. Yadav acknowledges support from R.T. Mody Distinguished Professor Endowment and J.C. Bose National Fellowship of Department of Science and Technology, Govt. of India.

NOMENCLATURE A

5-Hydroxymethyl furfural

A.S1

chemisorbed species A

B

bis (hydroxymethyl)furan

B.S

chemisorbed species B

C

hydroxymethyl furan

CA

concentration of A, (mol/L or M)

CB

concentration of B, (M)

CC

concentration of C (M)

CD

concentration of D (M)

CW

concentration of water, (M) 23



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CAS1

concentration of A at catalyst surface, (M)

CBS1

concentration of B at catalyst surface,( M)

CCS1

concentration of C at catalyst surface,( M)

CDS1

concentration of D at catalyst surface,( M)

CHS2

concentration of hydrogen at catalyst surface, (M)

CH2

concentration of hydrogen, (M)

C S1

concentration of vacant acidic sites, (M)

C S2

concentration of vacant metallic sites, (M)

CT1

total concentration of acidic sites, (M)

CT2

total concentration of metallic sites, (M)

C.S1

chemisorbed species C

D

dimethyl furan (2,5-DMF)

D.S1

chemisorbed species D

E

dimethyltetrahydrofuran (DMTHF)

E.S1

chemisorbed species E

H.S2

chemisorbed hydrogen

k

reaction rate constant

K

equilibrium constant for the adsorption of different species (L/mol)

KH2

equilibrium constant for the adsorption reaction of hydrogen (L/mol)

M

mol/L

-r

rate of surface reaction (mol L-1 min-1)

S1

active acidic site

S2

active metallic site

Page 24 of 49

24


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w

catalyst loading (g/L)

W

water

ACRONYMS HMF

5-hydroxymethyl furfural

BHMF

bis (hydroxymethyl)furan

2,5-DMF

2,5-dimethyl furan

DMTHF

dimethyl tetrahydrofuran

DTP

dodeca-tungstophosphoric

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List of Tables Table 1: Surface area, pore volume, pore diameter and acid strength analysis Surface

Pore

Pore

area

volume

diameter

(m2/g)

(cm3/g)

(nm)

Total acidity No

Catalyst

(mmol/g)

1

K-10

240

0.39

5.9

0.97

2

20Cs-DTP/K-10

212.5

0.29

5.7

1.52

3

2Pd/K-10

228

0.35

5.6

0.95

4

2Pd-20CsDTP/K-10

206.6

0.27

5.4

1.50

201.7

0.26

5.4

1.44

Reused 2Pd-20CsDTP/K5 10

33



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Table 2: Hydrogenation of HMF to 2,5-DMF Sr.

Hydrogen Catalyst

No

T

t

Conversion

Yield

(°C)

(h)

(%)

(%)

Solvent source

ref.

1

CuRu/C

H2

n-butanol

220

10

100

61

3

2

Ru/C

H2

THF

200

2

100

95

18

3

PtCo@HCS

H2

n-butanol

180

2

100

98

34

4

PdAu/C +HCl

H2

THF

60

6

100

96

17

5

Ru/Co3O4

H2

THF

130

24

100

93

16

6

Ru-NaY

H2

THF

220

1

100

78

35

7

Pd/C/H2SO4

formic acid

THF

70

15

100

95

14

8

Ni/Co3O4

H2

THF

130

24

99

76

36

9

Pd/Zn/C

H2

THF

150

6

99

85

32

10

NiSi-PS

H2

130

3

100

72.9

37

220

4

100

58

38

90

2

98

81

1, 4dioxane 211

Ru-HT

H2 propanol

2PdThis 12

20CsDTP/K-

H2

THF

work 10

34


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Table 3: Effect of solvents on the conversion of HMF and selectivity of 2,5-DMF. No Solvents

Conversion of HMF (%) 1h

2h

Selectivity of 2,5-DMF after 2 h (%)

1

Water

78

100

42

2

n-Butanol

66

100

46

3

THF

60

98

83

4

1,4-Dioxane

52

91

67

Condition : HMF=0.1 M, catalyst= 0.4g, temperature 90°C, speed of agitation 1000rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h. Table 4: Rate constants for different steps at different temperature

Sr. No

k1 x103

k2x103

k3x104

(L2mol−1g−1s−1)



T (K)

1.

353

0.9

0.42

0.21

2.

363

1.8

0.6

0.43

3.

373

2.8

0.96

0.86

4.

383

5.1

1.1

1.7

35



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Figure Captions and Figures Figure 1

XRD of different catalysts (a)2Pd/K-10, (b) 20CsDTP/K-10, (c) Fresh 2Pd20CsDTP/K-10, (d) Reused 2Pd-20CsDTP/K-10.

Figure 2

NH3- TPD patterns of different catalysts (a) 20CsDTP/K-10, (b) 2Pd20CsDTP/K-10, (c) Reused 2Pd-20CsDTP/K-10

Figure 3

TEM images of (a)& (b) 20CsDTP/K-10, (c) & (d)2Pd-20CsDTP/K-10

Figure 4

DSC-TGA analysis of 2Pd-20CsDTP/K-10

Figure 5

Effect of various catalysts on conversion of HMF and on the selectivity of products. HMF 0.1 M, catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.

Figure 6

Effect of catalyst loading on conversion of HMF and on the selectivity of products. HMF =0.1M, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.

Figure 7

initial rate Vs catalyst weight

Figure 8

Effect of initial concentration of HMF on conversion of HMF and on the selectivity of products. catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3,reaction time 2 h.

Figure 9

Effect of hydrogen pressure on conversion of HMF and on the selectivity of products. HMF =0.1 M, catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, total volume 20 cm3, reaction time 2 h.

Figure10

Effect of temperature on conversion of HMF and on the selectivity of products. HMF=0.1M, catalyst wt. 0.4g, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.

Figure 11

Reusability of catalyst. HMF= 0.1 M, catalyst= 0.4g, temperature 90°C, speed of agitation 1000rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h. 36 ACS Paragon Plus Environment

Page 37 of 49

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Figure 1: XRD of different catalysts (a)2Pd/K-10, (b) 20CsDTP/K-10, (c) Fresh 2Pd20CsDTP/K-10, (d) Reused 2Pd-20CsDTP/K-10.

37 ACS Paragon Plus Environment


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Figure 2:NH3- TPD patterns of different catalysts (a) 20CsDTP/K-10, (b) 2Pd20CsDTP/K-10, (c) Reused 2Pd-20CsDTP/K-10


Page 39 of 49

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Figure 3: TEM images of (a) & (b) 20CsDTP/K-10, (c) & (d)2Pd-20CsDTP/K-10



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Page 40 of 49

Figure 4: DSC-TGA analysis of 2Pd-20CsDTP/K-10


Page 41 of 49

100

Conversion /Selectivity (%)

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

60

40

20

0

Catalyst

Conversion of HMF

Selectivity of 2,5-DMF

Figure 5: Effect of various catalysts on conversion of HMF and on the selectivity of 2,5-DMF. HMF =0.1 M, catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h..



100 Conversion /Selectivity (%)

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 42 of 49

80

60

40

20

0 0.2

0.3 0.4 Catalyst loading (g)

0.5

Conversion of HMF


Selectivity of BHMF

Selectivity of DMTHF

Figure 6: Effect of catalyst loading on conversion of HMF and on the selectivity of products. HMF= 0.1M, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.


Page 43 of 49

2.5

Initial rate X 104 (mol L-1 min-1)

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


R² = 0.9921

2

1.5

1

0.5

0 0

0.1

0.2 0.3 0.4 Catalyst weight (g)

0.5

0.6

Figure 7: initial rate Vs catalyst weight




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Page 44 of 49

80

60

40

20

0 0.05 0.1 0.15 Initial concentration of HMF (M) Conversion of HMF

Selectivity of DMF

Selectivity of BHMF


Figure 8: Effect of initial concentration of HMF on conversion of HMF and on the selectivity of Products. catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3,reaction time 2 h.


Page 45 of 49


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 60 40 20 0 5

10 15 Hydrogen pressure (atm)

Conversion of HMF


Selectivity of BHMF


Figure 9: Effect of hydrogen pressure on conversion of HMF and on the selectivity of products. HMF=0.1M, catalyst wt. 0.4g, temperature 90°C, speed of agitation 1000 rpm, total volume 20 cm3, reaction time 2 h.



100 Conversion /Selectivity(%)

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 46 of 49

80 60 40 20 0 80

90 100 Temperature (°C)

110

Conversion of HMF

Selectivity of 2,5- DMF

Selectivity of BHMF


Figure 10: Effect of temperature on conversion of HMF and on the selectivity of products. HMF=0.1M, catalyst wt. 0.4g, speed of agitation 1000 rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.


Page 47 of 49

100 Conversion/Selectivity (%)

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

60

40

20

0 Fresh

1st Reuse

Conversion of HMF

2nd Reuse

3rd Reuse


Figure 11: Reusability of catalyst. HMF=0.1M, catalyst= 0.4g, temperature 90°C, speed of agitation 1000rpm, hydrogen pressure 10 atm, total volume 20 cm3, reaction time 2 h.



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Page 48 of 49

Graphical abstract Biobased green process: Selective hydrogenation of 5-hydroxymethyl furfural (HMF) to 2, 5 dimethyl furan (DMF) under mild conditions using Pd-Cs2.5H0.5PW12O40/K-10 clay Anil B. Gawade, Manishkumar S. Tiwari and Ganapati D. Yadav*

Selective hydrogenation of HMF to 2, 5-DMF was achieved under mild reaction condition and without adding external homogenous acid using a novel multifunctional catalyst based on palladium and Cs2.5H0.5PW12O40/K-10 clay.


Page 49 of 49

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

1


Graphical abstract

2

Biobased green process: Selective hydrogenation of 5-hydroxymethyl

3

furfural (HMF) to 2, 5 dimethyl furan (DMF) under mild conditions

4

using Pd-Cs2.5H0.5PW12O40/K-10 clay

5

Anil B. Gawade+, Manishkumar S. Tiwari+ and Ganapati D. Yadav*

6 7 8 9

Selective hydrogenation of HMF to 2, 5-DMF was achieved under mild reaction condition and without adding external homogenous acid using novel bifunctional catalyst.

10


Biobased Green Process: Selective Hydrogenation of 5

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