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Hydrodeoxygenation of Fructose to 2,5-Dimethyltetrahydrofuran Using a Sulfur Poisoned Pt/C Catalyst Michael A. Jackson,*,† Michael Appell,‡ and Judith A. Blackburn† †

Renewable Product Technology Unit and ‡Bacterial Foodborne Pathogens and Mycology Unit, , National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University Street, Peoria, Illinois 61604, United States S Supporting Information *

ABSTRACT: Fructose has been hydrodeoxygenated to 2,5-dimethyltetrahydrofuran using a sulfided Pt/C catalyst. The reaction was carried out in a stirred reactor at 10.3 MPa H2 and 175 °C which allowed a 10% fructose solution to be converted in 2 h. The selectivity was greatly enhanced by using ethanol as solvent with 95% ethanol giving 50% DMTHF versus 9% in water. The only intermediate found along the reaction pathway was 2,5-hexanedione. This is presumed to be hydrogenated to 2,5hexanediol which then ring closes to DMTHF. Molecular simulation at the B3LYP/6-311++G(2df,2p) level was used to propose a reaction pathway from fructose to 2,5-hexanedione and then to explain the preference for the cis isomer of DMTHF. Heinen et al. studied a Ru/C catalyst at 72 °C and 1 bar H2 for the hydrogenation of fructose. They demonstrated an increase in mannitol yield from 47% to 63% when they used Sn promoted Pt/C and Pd/C catalysts.8 A second line of research for increasing the value and utility of sugars and starches, through the sugar alcohols, is the hydrogenolysis of these substrates to smaller oxygenates. Reaction conditions for hydrogenolysis are necessarily harsher than for production of the sugar alcohols. Sun and Liu worked at H2 pressures of up to 10 MPa and 240 °C to convert xylitol to ethylene glycol and propylene glycol over Ru/C.9 Ye et al. used a Ce promoted Ni/Al2O3 catalyst at 240 °C under 7 MPa H2 pressure to obtain glycerol, ethylene glycol, propylene glycol, and butylene glycol from sorbitol conversion of 90%.10 Li and Huber used Pt/SiOx-AlOx in a flow reactor operating at 220 °C and 3 MPa H2 pressure to produce a mixture of compounds from sorbitol. Their product selectivity could be managed by adjusting the weight hourly space velocity (WHSV) of the aqueous sorbitol solution over the catalyst bed. Products ranged from methanol at low WHSV to isosorbide at high WHSV and included 10% 2,5-dimethyltetrahydrofuran (DMTHF) at intermediate WHSV. DMTHF is a typically fragrant ether with features that make it attractive as a fuel component. It has a boiling point of 92 °C and limited solubility in water, it has a research octane number of 82, and its energy density of 35.5 MJ/kg is greater than that of ethanol (25 MJ/kg) and comparable to that of butanol (36 MJ/kg). It is currently, however, too expensive to burn. The recent synthesis of DMTHF from biomass by Grochowski et al. may have served to raise interest in this compound as a fuel additive.11 Simmie has investigated the thermochemical and

1. INTRODUCTION Mounting concern about the depletion of petroleum supplies and climate change offers an opportunity for the development of a biobased, sustainable economy. While most biorefineries today are ethanol and biodiesel producers, specialty chemical producers are increasing in number with products such as succinic, adipic, and levulinic acids, propane-, butane-, and pentanediols, isoprene, and p-xylene. The continued growth of this business sector may very well depend on the efficient conversion of biomass to an array of chemicals. While the current biobased products are obtained almost entirely from fermentation of sugars, an expansion into catalytic conversion methods promises to increase the number of synthetic pathways and resulting chemicals. These conversion methods could include oxidations, ketonizations, selective dehydrations, and hydrogenations. The last of which has industrial history with the hydrogenation of fructose to mannitol and glucose to sorbitol. Much of the current research on monosaccharide hydrogenation is aimed at the medicines and sweeteners market in the preparation of mannitol and sorbitol. Industrially this conversion is performed using a nickel catalyst.1 However, the low selectivity to mannitol has led to several studies looking to improve this outcome. Several catalysts have been examined under differing reaction conditions. Kuusisto et al. used CuO− ZnO at 90°−130 °C and 50 bar H2 and achieved a mannitol selectivity of 60−68% in both the presence and absence of ultrasonic radiation.2,3 Mackee et al. employed CuO−SiO2 at 90°−130 °C and 117 bar to achieve a yield of 65% mannitol.4 Mishra and Hwang examined mannitol production from mannose over Ni−TiO2 and Ru on Ni−TiO2 at 100−140 °C and 40−55 bar H2 pressure.5 They found a selectivity of 93%. Ahmed and Kadhum used Pt/C at 20−60 °C and bubbling H2 to achieve a 45% mannitol yield from fructose.6 Zhang et al. used a mixed metal hydrotalcite composed of Ni, Cu, Al, and Fe at 90−110 °C and 30 bar H2.7 They achieved almost quantitative conversion of fructose to sorbitol and mannitol. This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Received: February 26, 2015 Revised: June 26, 2015 Accepted: June 30, 2015

A

DOI: 10.1021/acs.iecr.5b00766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Table 1. Analytical Data and Surface Features of the Poisoned Pt/C Catalyst

a

catalyst

%C

%H

%N

%S

SBET

MPD, nm

Mole ratio S/Pta

%Ptb

fresh used

61.07 72.61

2.03 2.07

0.11 0.08

1.57 0.22

828 614

1.19 1.33

2.4 0.66

3.76 3.46

Determined by EDS. bDetermined by ICP.

Hydrogenations of the other substrates were performed similarly but the operating pressure was 6.9 MPa. 2.2. Analytical Methods. HPLC analyses were performed on an Aminex column and a RI detector using 0.05 M H2SO4 as mobile phase. GC−MS analyses were performed on a Shimadzu QP2010 SE GC/mass spectrometer/FID. Separations were accomplished using a Supelco Petrocol DH 50.2 (50 m × 0.2 mm × 0.5 μm) column for separations. The oven program was as follows: Initial temperature 50 °C for 2 min, ramp at 10 °C/min to 180 °C, hold for 1 min then a ramp at 20 °C/min to 300 °C with a final hold time of 6.2 min. Methyl octanoate was used as internal standard. The mass spectrometer was operated in the EI mode at 70 eV. Surface analyses were performed on a Quantachrome ASiQ (Quantachrome Instruments, Boynton Beach, FL 33426). Samples were degassed at 200 °C for 10 h prior to analysis. Surface areas were determined at −196 °C using N2 as the adsorbate in a relative pressure range of 0.025−0.30 using the BET method for total surface area calculations. Pore diameter was determined using the DR method over a pressure range of 2e−4 < p/p0 < 9e−3. Energy dispersive spectroscopy (EDS) measurements were made on a JEOL JSM-6010LA SEM with an integrated EDS attachment operating at 10 keV. 2.3. Computational Methods. All geometry optimization and transition state results are reported based on calculations carried out using density functional theory in the Spartan ’14 program.17 Initial structures were built using HyperChem v8.0.10 and the PM3 semiempirical method.18 The B3LYP exchange and correlation functional was the density functional theory method used. The 6-31+G* and 6-311++G(2df,2p) basis sets were utilized for calculations containing C, H, and O atoms. The SCF energy convergence criteria for the geometry optimization calculations was set at 1 × 10−6 hartree, maximum distance tolerance of 1.2 × 10−3 Å, and a maximum gradient tolerance of 3 × 10−4 hartrees bohr−1. The transition state geometry calculations were carried out as implemented in Spartan ’14. Thermodynamic properties were based on frequencies obtained from geometry optimized or transition state results. Density functional theory molecular dynamics (B3LYP/6-31+G*) were performed using Parallel Quantum Solutions software on cis- and trans-DMTHF for 10 picoseconds using a time step set at 1 fs and an initial temperature set at 298.15 K for a total of 10 000 steps.19

kinetic parameters of DMTHF to establish framework data for further evaluation.12 Activated carbon is a commonly used catalyst support for liquid phase hydrogenation reactions using Pt and Pd and others as active metals. Carbons offer high surface areas, stability in most solvent systems, and ease of precious metal recovery after use by combustion. However, carbons are prepared from various sources such as wood, peat, coconut shells, and other biomaterials that tend to lead to inconsistent preparations. Activation of the carbon by controlled heating adds further to the uncertainty of the composition by the addition of the oxygen functionality in a random manner. The activated carbon also contains hydrogen, nitrogen, sulfur, and minerals. Each of these varies with each batch making reproducibility a challenge. Sulfur is known to be an effective catalyst poison, usually in a detrimental way. Its strong affinity for Pt and Pd in three-way catalysts used to clean automotive exhaust has driven the need for ultralow sulfur gasoline and diesel. On the other hand, sulfur and other poisons can also affect reaction selectivity by imparting electronic or orientation changes to a catalytic metal center or by reducing the effective size of metal crystallites.13 The selective hydrogenation of crotonaldehyde to crotyl alcohol by a sulfur-poisoned copper catalyst appears to be controlled by changes in how the aldehyde adsorbs onto the copper surface.14 Mori et al.15 demonstrated that the hydrogenation of chalcone, a molecule containing carbonyl and olefin functionalities, is completely hydrogenated by Pd/C unless the reaction is run in the presence of diphenylsulfide. Under conditions of 0.01 mol % Ph2S the olefin was hydrogenated to alkane while the carbonyl group was untouched. Carbon-supported PtS will selectively reduce 2,5dichloronitrobenzene to 2,5-dichloroanaline without hydrodehalogenation.16 This paper describes the selective hydrodeoxygenation of fructose to DMTHF using a sulfided platinum on carbon catalyst in aqueous ethanol. The hydrogenation of possible intermediates along the reaction pathway is also described.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 2.1.1. Chemicals. Fructose, 2,5-hexanedione, 2,5-hexanediol, and 2,5-dimethylfuran were sourced from Sigma-Aldrich and ethanol from Fisher Scientific. The 5% Pt/C-sulfided catalyst was supplied by Engelhard. 2.1.2. Typical Procedure for the Hydrodeoxygenation of Fructose. The HDO reactions were performed in a Parr Instruments (Parr Instruments, Inc., Moline, IL) 300 mL T316 stainless steel reactor with a glass liner. The liner was loaded with 50 mL of a 0.55 molar fructose solution and 500 mg of catalyst, the reactor was sealed, purged with H2 to remove air, pressurized to 2 MPa H2, and then heated to the reaction temperature with stirring at 265 rpm. The reactor was then further charged with H2 to 10.3 MPa. This was taken as the reaction start time. Reaction time was 2 h. Samples of about 2 mL were withdrawn through a sampling tube. These were filtered through a 0.45 μm nylon filter prior to analysis.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The surface properties and chemical composition of the poisoned Pt/C catalyst both fresh and used are shown in Table 1. The surface area and pore diameter of the fresh catalyst is typical of an activated carbon support. The micropore surface area as determined by the v−t plot was about 70% of the total surface area. The 1.57 wt % sulfur in the fresh catalyst led to a S/Pt mole ratio of 2.4 from an EDS measurement. The catalyst used in this work was presumably prepared as described in the 1966 United States patent issued to Engelhard for A Method of Preparing a Sulfided Platinum on Carbon Catalyst.20 The patent describes a B



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position pathways of glucose and fructose that aprotic solvents or those with higher proton affinities than water would suppress furanose dehydration.31 To examine the possible benefit of using ethanol as part of the solvent system, we performed the hydrodeoxygenation of fructose in ethanol solutions and found that DMTHF selectivity increased dramatically with increasing ethanol concentrations. This is shown in Figure 1.

catalyst prepared using a carbon from a softwood of surface area greater than 800 m2/g. The catalyst had a platinum loading of 5 wt % and was poisoned by bubbling H2S through an aqueous slurry of the catalyst resulting in a final catalyst that was 1−3 wt % sulfur. These characteristics are consistent with the catalyst we describe here. The catalyst was reevaluated following a single use. The used catalyst contained 85% less sulfur and 8% less platinum than the fresh catalyst. The increase in carbon and the C:H ratio indicates there was some coke deposition resulting in a lower BET surface area. The increase in pore diameter also suggests a broad structural change occurs as well. 3.2. Hydrodeoxygenation of Fructose. The catalysts used for the HDO reactions were used as is and not reduced prior to loading into the reactor. Our initial efforts were a catalyst screening for activity toward the hydrogenation or hydrogenolysis of glucose, while at the same time we wanted to avoid the conversion to hydroxymethylfurfural (HMF) and the sugar alcohols sorbitol and mannitol since there is extensive research into the production of these important products. The hydrogenation of 15 wt % glucose or fructose in water over 1 g of Pt/C at 175 °C and 3.4 MPa hydrogen pressure resulted in a mixture of alcohols and diols, the most prominent of which are shown in Table S1 in the Supporting Information. These solutions were a lightly colored pale yellow indicating the lack of polymeric materials common to sugar decomposition. The alcohols have been reported before as products from the hydrogenolysis of cellulose21−23 and sugar alcohols.24−26 It should be noted that we do not see HMF from reactions that are catalyzed by platinum on carbon. Alumina-supported catalysts, however, produced enough HMF and subsequent insoluble polymeric material that these catalysts were not pursued further.27 The differences in yields between poisoned and nonpoisoned catalysts shown in the table offer the first indication that the sulfur-poisoned catalyst can alter the selectivity of the reaction, particularly with fructose as the substrate. The conversion of 18% of fructose to 2,5hexanedione using the sulfided catalyst is a notable change in selectivity. This same catalyst gave a trace of 2,5-hexanedione from glucose as well. Since we see the isomerization of glucose and fructose under the reaction conditions, it is reasonable to conclude that glucose gave 2,5-hexanedione only after isomerization to fructose. Therefore, fructose became the sugar we examined in detail. It should be noted that the level of DMTHF produced remained low with the poisoned catalyst under the low hydrogen pressure conditions of the initial screens. Increasing the pressure to 10.3 MPa served to increase DMTHF yields but only to about 9%. Also, the catalyst we selected for further study was the Pt/C-sulfided, and the reused samples of this catalyst are hereafter referred to as nonpoisoned reflecting the loss of sulfur as shown in Table 1. 3.2.1. Effect of Ethanol as Cosolvent. There are published results suggesting the use of ethanol for novel fructose chemistry. Tucker et al. used ethanol as a phase modifier to increase selectivity toward HMF from fructose dehydration over Amberlyst 70.28 Flood et al.29 showed that ethanol effects the equilibrium concentration of the fructose isomers as well as the rate of mutarotation. The rate of conversion of the crystalline form, β-D-fructopyranose, to the fructofuranose forms is five times faster in water than in a 90% ethanol solution. Also, it is known that it is the furanose forms that dehydrate leading to HMF.30 Furthermore, Assary and Curtiss concluded from a quantum chemical study of the decom-

Figure 1. Effect of ethanol concentration on DMTHF selectivity. Reaction conditions were 0.55 M fructose in 50 mL of solvent with 500 mg of Pt/C-sulfided, 175 °C, 10.3 MPa hydrogen pressure, and reaction time of 2 h.

To investigate the idea that fructose is stabilized by ethanol, we followed its dehydration to HMF under the conditions used for hydrogenation reactions but in 48−90% ethanol solutions. It can be seen in Figure 2 that fructose is more stable in the

Figure 2. Decomposition of fructose in solutions of aqueous ethanol. Conditions: 0.55 M fructose, 175 °C, 4 MPa Hydrogen. Solid circles represent fructose and open circles represent HMF concentrations.

solutions of higher ethanol concentration which also results in lower concentrations of HMF. The dehydration of fructose is a first order reaction so we can use the method of half-lives to determine rough reaction rates of 0.019 min−1 in 48% EtOH, 0.011 min−1 in 57% EtOH, and 0.0035 min−1 in 90% EtOH. These results differ from those of Kuster who showed that fructose dehydration catalyzed by HCl at 95 °C was enhanced C



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selectivity. Scheme 1 shows a possible pathway for the conversion of fructose to DMTHF including two routes from fructose converging at 2,5-hexanedione. The upper route shows the dehydration of fructose and the subsequent hydrogenation to 2,5-DMF. 2,5-DMF would be in equilibrium with 2,5hexanedione through the addition and loss of water. The lower route depicts the hydrodeoxygenation of acyclic fructose to the 1,3,6-trideoxyfructose. This species could dehydrate at C4 and rearrange through its keto−enol forms to 2,5-hexanedione. Neither the deoxyfructose species nor HMF has been detected in the reaction mixture, however. The only transient species detected during fructose hydrodeoxygenation in aqueous ethanol was 2,5-hexanedione. At a H2 pressure of 10.3 MPa this species was present at levels of about 20 mol % and decreased as DMTHF increased and ultimately was completely converted. The time course of the hydrogenation of 2,5-hexanedione over the poisoned and nonpoisoned catalysts is shown in Figure 3. Over the poisoned catalyst, this species is converted to DMTHF at a yield of 70 mol % with about 4 mol % being reduced to 2-hexanol. 2Hexanone reached about 3 mol %, and 2,5-DMF was detected early but at levels that remained below 1 mol %. Whereas over the used catalyst, the yields of both DMTHF and 2,5hexanediol were 45 mol %. The sulfur appears to increase DMTHF selectivity from 2,5hexanedione by increasing the rate at which 2,5-hexanediol ring closes. Here it should be pointed out that there is some preference toward the cis isomer of DMTHF. DMTHF from fructose is 71% in the cis configuration. 2,5-Hexanedione leads to DMTHF that is also 71% cis from the poisoned catalyst but a slightly higher 76% cis when the nonpoisoned catalyst is used. Furthermore, the ring closing of racemic 2,5-hexanediol catalyzed by the poisoned catalyst gives the cis and trans isomers in equal amounts. Contrast this with the results of Yamaguchi et al. who have shown that chiral 2,5-hexanediol ring closes in hot water saturated and acidified with CO2 to give up to 85% cis isomer.34 Taken together, these results suggest the cis preference originates with the hydrogenation of the dione to the diol with chiral centers which would result from the relative orientation of the two ketone functionalities on the catalyst surface. In addition, it is of interest to note the energetic preference between the cis and trans isomers of DMTHF (see Figure 4). The cis form is energetically favored over the trans form in terms of electronic energy and the free energy. Density functional molecular dynamics simulation at the B3LYP/6-31+G* level in vacuo suggest that the trans form limits the number of corners of the ring that can pucker, or deform the furan ring. This decreased flexibility makes the trans isomer more energetic.

by the addition of polyethylene glycol, whereas the rehydration of HMF to levulinic acid was slightly decreased.32 Kuster’s results were later bolstered by semiempirical PM3 quantum mechanics/molecular mechanics calculations which suggested that water displacement by other solvents would accelerate HMF formation.33 3.2.2. Effect of Catalyst Loading. Table 2 shows the dependence of DMTHF yield on the mass of catalyst used in Table 2. Effect of Catalyst Loading on Selectivity toward DMTHF. Reaction Conditions Were 0.55 M Fructose in 90% Ethanol, 175° C, 10.3 MPa H2, 2−5 h to Complete Conversion of Fructose mass catalyst, mg

DMTHF selectivity,%

100 500 1000

17 41 47

the reaction. Reaction conditions were 10% fructose in 50 mL of a 90% ethanol solution at 175 °C and 10.3 MPa H2 pressure for 2−5 h, which resulted in complete conversion of fructose. Since the selectivity increases with catalyst loading, the selectivity depends on a competition with reactions not catalyzed by the platinum metal centers. These reactions could include the dehydration to HMF, although neither this nor its reaction products, such as levulinic acid, were detected. 3.2.3. Effect of Sulfur Poison. The hydrodeoxygenation of fructose was performed over fresh, sulfided catalyst and used catalyst which was substantially depleted in sulfur. The sulfided catalyst gave DMTHF as the main product while the used catalyst produced a mixture of mannitol and sorbitol. Yields were rather low and each also gave small amounts of other hydrogenated products. (see Supporting Information Table S2). Carbon balances were at best 70%. The sulfur from the catalyst was lost as H2S as was determined qualitatively by venting the reactor over filter paper saturated with lead acetate and observing the brown PbS precipitate. We attempted to recreate the original catalyst by the addition of sulfide to the spent catalyst. Following the patent description, catalyst was suspended in water and treated with either H2S or Na2S. Neither of these restored the activity of the catalyst despite increasing the sulfur content of the catalyst back to 0.62 wt % or about 40% of the original loading. These two sulfides, as well as di-tert-butyldisulfide, sodium ethylsulfate, and thiophene, were also added to the reaction mixture as in situ poisons without returning the catalytic activity. At this point, we began to formulate a mechanism for the conversion and perhaps the role of sulfur in effecting catalyst

Scheme 1. Possible Reaction Pathways for the Hydrodeoxygenation of Fructose through the Sequential Dehydration and Hydrogenation Steps

D



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Figure 3. Time course of the reaction of 2,5-hexandione over the poisoned catalyst, (A) and the used, nonpoisoned catalyst (B). Conditions: 0.19 mmol/mL 2,5-hexanedione in 90% ethanol/water, 175 °C, 6.9 MPa H2, 250 mg of catalyst.

Figure 4. B3LYP/6-311+G(2df,2p) geometry optimized structures of the cis- and trans-DMTHF and the calculated energy differences.

of water, and 2,5-DMF can be formed from the hydrogenation of HMF.35 Therefore, we examined the hydrogenation of 2,5DMF over the poisoned and used Pt/C catalysts. The time course of the two hydrogenations is shown in Figure 6 where the differences resulting from the two catalysts can be seen (see also Supporting Information Table S3 for the product distribution from these reactions). Here we can see the effect of the sulfur poison in the doubling of the yield of DMTHF, whereas the used catalyst produces 2-hexanol as the primary product. Over the poisoned catalyst, shown in 6A, 2,5hexanedione builds-up to 30 mol % in solution before falling. This intermediate has two paths it can follow: the productive path to DMTHF that requires reduction to 2,5-hexanediol, or the dead-end path to 2-hexanol through 2-hexanone. 2,5Hexanediol never reaches a detectible level in solution showing that the ring closing occurs on the catalyst surface. In 6B, the results from the used catalyst are shown. The reaction initially is much faster than over the poisoned catalyst which is typical of poisoned catalysts where increased selectivity comes at the expense of reaction rate. However, after 10 min the reaction slows dramatically. In the first 20 min, 2,5-hexanedione builds to only 5 mol % while 2-hexanone reaches 22 mol % and 2hexanol has started its rise to 51 mol %. Unlike the poisoned

The reaction coordinate for the spontaneous cyclization of 2,5-hexanediol to 2,5-DMTHF is shown in Figure 5. It is interesting to note that both the spontaneous cyclization to cisand trans-2,5-DMTHF proceeds through a transition state barrier exceeding 256 kJ mol−1 and the free energy of the products plus water is lower than the 2,5-hexanediol starting material. It should also be noted that (2R,5S)-2,5-hexanediol is a meso compound and the product cis-2,5-DMTHF exists in one form. In contrast, trans-2,5-DMTHF consists of an enantiomeric pair with (2R,5R)-2,5-hexanediol giving trans(2R,5R)-2,5-DMTHF and (2S,5S)-2,5-hexanediol leading to trans-(2S,5S)-2,5-DMTHF. The reaction coordinate for (2S,5S)-2,5-hexanediol to trans-(2S,5S)-2,5-DMTHF is energetically equivalent to the (R,R) form shown in Figure 5. Contributions from the solvent effects on the catalysts, intermediates, transition states, and any direct role of solvent in the cyclization will also impact the cisvs trans-2,5-DMTHF ratio. In addition, these results also suggest the stereochemistry of the 2,5-hexanediol formed from the reduction of 2,5hexanedione influences the preference of the formation of cisvs trans-2,5-DMTHF. Working backward through the reaction scheme, 2,5hexanedione can be produced from 2,5-DMF by the addition E



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Figure 5. Reaction coordinate intermediates of cis- and trans-2,5-DMTHF formation from stereoisomers of 2,5 hexanediol at the B3LYP/6-311+ +G(2df,2p) level.

Figure 6. Time course of the hydrogenation of 2,5-dimethylfuran over (A) the poisoned catalyst and (B) the used or nonpoisoned catalyst. Conditions: 0.19 mmol/mL 2,5-DMF in 90% ethanol/water, 175 °C, 6.9 MPa H2, 250 mg of catalyst.

mechanism is occurring in our system along with the ring closing of 2,5-hexanediol, then this would explain the elevated yield of the cis isomer on the used catalyst. The sulfur poison affects the hydrogenation of both 2,5hexanedione and 2,5-DMF but in different ways. In each of these reactions the selectivity toward DMTHF is higher than that found when starting with fructose. The high DMTHF selectivity from these reactants, coupled with the disparate results from the HDO of fructose over the two catalysts, suggests the effect occurs prior to 2,5-hexanedione formation. This made us consider an alternative mechanism in which the acyclic form of fructose is the active substrate in the HDO reaction over the poisoned catalyst. This is the lower branch of Scheme 1. Crystalline fructose is exclusively in the β-Dfructopyranose form, but in solution it exists as five isomers. The pyranose and furanose forms each exist as two epimers

version of the catalyst, 2,5-hexanediol appears to be a dead-end product as it climbs throughout the course of the reaction. In comparing these two plots, it appears that the sulfur minimizes the asymmetrical hydrogenation of the diketone to 2-hexanone while also increasing the rate at which the diol ring-closes. The difference in ring closing could result from a lack of surface acidity in the used catalyst compared to the fresh catalyst. The DMTHF produced from 2,5-DMF over the poisoned catalyst was 71% cis, whereas over the used catalyst, the configuration was 80% cis. This suggests that over the used catalyst another pathway may be available. This pathway would be similar to that proposed by Hansen et al. for the hydrogenation of HMF over a copper catalyst in which they suggest the orientation of the intermediate 2,5-DMF on the catalyst directs the preferential formation of the cis isomer.36 They reported DMTHF was produced as 87% in the cis form. If this F



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designated α and β. These interconvert through an acyclic keto intermediate. At elevated temperatures and in ethanol solutions, the β-D-fructopyranose isomer is converted to the β-D-fructofuranose form.29 This equilibrium would make for a low but steady concentration of the acyclic isomer that could be hydrogenated directly to 2,5-hexanedione. Details of a computational reaction coordinate and energies of intermediates and products of this mechanism are given in the Supporting Information.

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4. CONCLUSION 2,5-Dimethyltetrahydrofuran can be prepared from fructose over a sulfur-poisoned Pt/C catalyst in aqueous ethanol at yields approaching 50%. The ethanol solvent appears to stabilize fructose against dehydration to HMF allowing the hydrodeoxygenation to take place. The ring-opening of fructofuranose leads to 2,5-hexanedione as the only isolated intermediate in the reaction. This is further hydrogenated to 2,5-hexanediol that ring closes to DMTHF. The catalyst used here loses sulfur during the reaction and so cannot be reused and cannot even be regenerated by poisoning. This work does show that the conversion of fructose to DMTHF is a promising reaction for production of a biobased product if a reusable catalyst can be developed.

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ASSOCIATED CONTENT

* Supporting Information S

Tables showing the product distribution from the HDO of fructose and hydrogenation of 2,5-DMF; detailed simulations of two pathways that were considered for the fructose to DMTHF conversion. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.iecr.5b00766.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 309-681-6255. Fax: 309-681-6524. Notes

Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Arthur Thompson of ARS/NCAUR for EDS analyses, Kim Ascherl of ARS/NCAUR for ICP analyses, and Dr. Charles Mullen of ARS/ERRC for elemental analyses.

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REFERENCES

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