Article pubs.acs.org/EF
Efficient Production of Furan Derivatives from a Sugar Mixture by Catalytic Process Junhua Zhang,*,† Lu Lin,*,‡ and Shijie Liu§ †
Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡ School of Energy Research, Xiamen University, Xiamen 361005, China § Department of Paper and Bioprocess Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, New York 13210, United States ABSTRACT: We present here the results of an investigation aimed at identifying catalysts for the dehydration of glucose and xylose to 5-hydroxymethylfurfural (HMF) and furfural in a diphasic reaction system, and the subsequent conversion of HMF and furfural to 2,5-dimethylfuran (DMF) and 2-methyl furan (MF) was also investigated. For the dehydration of glucose and xylose mixture, a series of SO42−/ZrO2−TiO2 solid acid catalysts were prepared by precipitation and impregnation method. Effects of various reaction parameters and catalyst reuse cycle toward the reaction performance were studied. Experimental results indicated that the product yield could reach 30.9 mol % (for HMF) and 54.3 mol % (for furfural) under the optimal experimental conditions. The SO42−/ZrO2−TiO2 catalyst is recoverable from the resulting product mixture and reused multiple times after calcination without any substantial change on the HMF and furfural yield. Furthermore, effects of various hydrogenation parameters of HMF and furfural in n-butanol promoted by carbon-supported ruthenium (Ru/C) were discussed, and the highest yield could reach 60.3 mol % (for DMF) and 61.9 mol % (for MF) under the optimal experimental conditions by starting with HMF and furfural in pure n-butanol, but the target products of DMF and MF could not be detected when directly used by the separated organic phase as solvent and substrate. However, the DMF and MF yield could reach 32.7 and 17.5 mol %, respectively, when the separated organic phase undergoes a further purification, which indicated that the purification of the organic phase is a key step for the further hydrogenation of HMF and furfural to DMF and MF.
1. INTRODUCTION With the gradual exhaust of fossil resource and the greenhouse effect for the consumption of oil-based fuels, the sustainable development of human society is threatened. Therefore, it has become the hotspot research area for the synthesization of renewable fuel to replace oil-based energy. Although solar, wind energy, and other renewable clean energies have been industrially used in some countries, these energies cannot fully satisfy the demand of human society for the regional and climate limitation. As we all know, lignocellulose possesses some characteristics, such as it is abundant, widespread, and renewable, so it has become an important way to solve the energy shortage through the exploitation of lignocellulose to cellulose-based fuel.1,2 Cellulose-based fuel is the energy that transformed lignocellulose, such as straw, wheat straw, cornstalk, or other forest wood, into a variety of terminal energy, for instance, electric power, gas fuel, solid fuel, and liquid fuel (including bioethanol, biomethanol, and biodiesel). In recent years, an increasing effort has been devoted to finding different technologies for the catalytic transformation of lignocellulose into bio-oil (obtained by biomass flash pyrolysis)3−7 and bioethanol (obtained by biomass hydrolysis-fermentation).8 However, the traditional route, for example, the conversion of cellulose to bioethanol, has some imperfections, such as lower conversion efficiency, product extraction and purification problems, environmental pollution, etc. Therefore, the industrial application prospect for fuel ethanol derived from © 2012 American Chemical Society
lignocellulose is uncertain, and there is a highlighted urgency and critical importance to find a new path to obtain the highquality liquid fuels from lignocellulose. Lignocellulose also can be transformed into methyl furan compounds (MFs). MFs including 2-methyl furan (MF) and 2,5-dimethylfuran (DMF), always used as solvent or polymeric materials previously, and have not been linked with fuel directly. In fact, MFs have a potential large-scale application as fuel.9,10 As compared to ethanol, MFs have similar boiling points, higher octane number, energy density, are explosionproof, and are almost insoluble in water. Moreover, the purification energy consumption of MFs only is one-third of ethanol.11−13 In addition, it can be drawn from Scheme 1 that the generated xylose derived from hemicellulose should be separated previously for the conversion of lignocellulose to bioethanol. Furthermore, the conversion efficiency of bioethanol from biomass is low, one molecule of glucose only can be fermented into two molecules of ethanol and two molecules of CO2, the theory efficiency to the target product only is 0.51, and the loss rate of carbon is 33.3%. Furthermore, the oxygen ratio of the product only decreased 35.5% for bioethanol approach. On the contrary, the generated glucose and xylose derived from lignocellulose hydrolysis can be converted to MFs simultaneously through the catalytic dehydration and coupling Received: April 10, 2012 Revised: June 12, 2012 Published: June 12, 2012 4560
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Scheme 1. Approach Comparison of MFs and Bioethanol Derived from Hydrolyzed Sugar
(Shanghai, China). HMF of analytical grade was obtained from Shandong Tengzhou Wutong Aromatizer Co. Other reagents and chemicals were all of analytical grade from Sinopharm Chemical Reagent Co. (Shanghai, China) and were used without further purification or treatment. Deionized water was used for all experiments. 2.2. SO42−/ZrO2−TiO2 Catalysts Preparation. All solid acid catalysts were prepared by precipitation and impregnation method. The detailed preparation procedure of SO42−/ZrO2−TiO2 catalyst was as follows: the coprecipitates of Zr(OH)4−Ti(OH)4 with various Zr− Ti mole ratios were obtained by adding concentrated NH4OH into a mixed aqueous solution of ZrOCl2·8H2O and TiCl4 with stirring until the pH value reached 9−10, and then were aged for 24 h to form Zr(OH)4−Ti(OH)4. The obtained precipitate was washed thoroughly with deionized water until the chloride ion in the filtrate could not be identified and was dried at 110 °C for 24 h. The dried precipitate was powdered below 60 mesh, then was impregnated in a 0.5 mol/L of H2SO4 solution with a solution/solid ratio of 15 mL/g and stirred at 500 rpm for 1 h. The resulting precipitated solid was then filtered using filter paper, subsequently dried at 110 °C for 12 h, and calcined at 550 °C for 3 h. Finally, a series of SO42−/ZrO2−TiO2 catalysts were obtained. 2.3. Dehydration of a Glucose and Xylose Mixture. The dehydration of glucose and xylose mixture was carried out in a 250 mL cylindrical stainless steel pressurized reactor (WHF-0.25) made by Weihai Automatic Control Reactor Ltd., China. The reactor was heated in an adjustable electric stove, and the temperature was monitored by a thermocouple connected to the reactor. Three parallel samples have been done for each set of experiment. For a typical experiment, 3.0 g of glucose, 3.0 g of xylose, 50 mL of n-butanol, 50 mL of water, and a given weight amount of solid acid catalyst were mixed to form a suspension and were poured into the reactor. The reactor was then heated to the desired temperature by external heating and shaken at 300 rpm. After the reaction was run for a desired duration, the reactor was taken from the stove and quenched in an ice cool water bath to terminate the reaction. The liquid product and solid acid catalyst were separated by filtration. For the reuse experiments of SO42−/ZrO2−TiO2 catalyst, it was recovered and calcined at 500 °C for 5 h in static air before it was reused in a new experiment under the same reaction conditions described above.
hydrogenated reduction. In particular, the energy carrier loss of carbon is zero for MFs approach, and the theory efficiency to the target products is 0.53 and 0.55. Moreover, the oxygen ratio of the product reduces significantly, decreasing 68.7% (for MF) and 63.4% (for DMF), respectively, indicating that the products have higher energy density for less oxygen ratio. It can be drawn from this comparison that the energy carrier can be preserved fully for MFs approach, and also it has higher conversion efficiency and energy density for the unit molecule; all of these advantages make MFs gradually become a substitute for gasoline and bioethanol. One of the approaches for producing MFs from lignocellulose involves its pretreatment to access its cellulosic component, followed by hydrolysis of cellulosic and hemicellulosic components to produce sugar that is subsequently converted to fuel components by either fermentation or chemical reaction.11 Because the first of these three steps is demanding, various approaches have been examined for the pretreatment of biomass.13,14 Moreover, some research pays close attention to the catalytic conversion of glucose or fructose to HMF15,16 or directly converted to DMF.17−19 However, there has been less research reported on the conversion of a sugar mixture (e.g., xylose and glucose mixture) to MFs in recent years, but this research may be conducive to the development of biofuel. So, we present a route for the conversion of a sugar mixture (glucose and xylose mixture) into MFs based on our previous studies,20−22 and the catalytic dehydration of glucose and xylose mixture was studied through the construction of a water/n-butanol diphasic reaction system. Furthermore, some key controls about HMF and furfural hydrogenation in n-butanol were also investigated, so a certain theoretical basis for the mass production of MFs liquid fuel derectly derived from lignocellulose can be provided.
2. EXPERIMENTS 2.1. Materials. Glucose, xylose, and furfural used for calibration with purity of over 98% were obtained from Aladdin Reagent 4561
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2.4. Hydrogenation of HMF and Furfural. The hydrogenation of HMF and furfural was carried out either using a neat mixture of HMF and furfural or by dehydration of glucose and xylose. In the latter case, the extracting solvent of n-butanol containing HMF and furfural obtained from glucose and xylose dehydration was separated with a separating funnel, and then the residual moisture in n-butanol was removed with calcium oxide and separated by filtration, so the obtained n-butanol that contains HMF and furfural was used for hydrogenation. All hydrogenation reactions were carried out in a Parr autoclave (50 mL). Three parallel samples have been done for each set of experiment. For a typical experiment, 0.75 g of HMF, 0.75 g of furfural, 25 mL of n-butanol, and a given weight amount of carbonsupported ruthenium (Ru/C) were mixed to form a suspension and then poured into the reactor. The reactor was sealed, purged three or four times with hydrogen, and pressurized to the fixed pressure with hydrogen. The reactor was then brought to the desired temperature by external heating and shaken at 300 rpm. After the reaction was run for the desired duration, the reactor was taken from the stove and quenched in an ice cool water bath to terminate the reaction. The liquid product and solid acid catalyst were separated by filtration. 2.5. Products Analysis. After each dehydration run, the aqueous and organic phases were filtered by a microstrainer. The residual glucose and xylose were determined by ion chromatography (DIONEX ICS-3000) equipped with a CarboPac PA1 (2 mm × 250 mm) analytical column and an electrochemical detector. A solution of sodium hydroxide (100 mmol) was used as the eluent with a volumetric flow rate of 0.3 mL/min at 30 °C. The content of HMF and furfural was analyzed by HPLC and quantified with calibration curves generated from commercially available standards. HPLC equipped with a pump and an Agilent Hypersil ODS column was coupled to a Gilson 118 UV/vis detector (280 nm), and the mobile phase consisted of 40% MeOH in water (flow rate is 1.0 cm3 min−1). The column temperature was maintained at 30 °C. DMF and MF were analyzed by a GC instrument (Agilent 6890) equipped with a HP-5 capillary column with dimensions of 30.0 m × 320 μm × 0.25 μm and a flame ionization detector (FID) operating at 270 °C. The carrier gas was helium with a flow rate of 1.0 mL/min. The following temperature program was used in the analysis: 40 °C (6 min)−15 °C/min−250 °C (1 min). The amount of DMF and MF in the reaction products was determined using calibration curves generated from commercially available standards.
Table 1. Effect of Catalyst Preparation Conditions on the Dehydration of a Sugar Mixturea conversion, mol % influencing factor Zr/Ti mole ratio
calcining temperature, °C
calcining time, h
yield, mol %
parameter
glucose
xylose
HMF
furfural
100% Ti 3:7 1:1 7:3 100% Zr 550 650 750 850 1 3 5 7
91.8 92.6 93.1 96.5 93.2 91.8 93.7 96.5 99.6 91.8 92.7 96.5 95.4
91.7 92.2 93.7 98.3 90.2 94.9 94.0 98.3 99.9 93.2 96.2 98.3 98.7
16.0 19.4 22.6 26.0 16.5 11.9 17.3 26.0 24.6 15.4 24.3 26.0 24.6
36.1 36.4 42.5 47.5 33.3 35.1 37.1 47.5 32.0 37.3 44.9 47.5 45.3
Other reaction conditions: reaction temperature of 170 °C, reaction time of 2 h, substrate concentration of 6 wt %, catalyst dosage of 50 wt %, agitation speed of 300 r/min.
a
have an obvious impact on the dehydration of glucose and xylose to HMF and furfural. It can be noted from Table 1 that HMF and furfural yields increased with the enhancement of Zr/Ti mole ratio; HMF and furfural yields reached the maximum of 26.0 and 47.5 mol % with the Zr/Ti mole ratio of 7:3. However, the HMF and furfural yields decreased significantly with further increasing the mole content of Zr. In addition, it can be perceived from the calcining temperature and calcining time influence that HMF and furfural yields will reach 26.0 and 47.5 mol % with the calcining temperature and calcining time of 750 °C and 5 h, respectively. The above results give an impression that the sulfated binary metal oxide of ZrO2−TiO2 with the Zr/Ti mole ratio of 7:3, the calcining temperature of 750 °C, and the calcining time of 5 h was clearly the most suitable catalyst and was used in the subsequent parts of this study. 3.1.2. Effect of Agitation Speed. The solid−liquid phase between the sugar mixture, double reaction system, and solid acid catalyst may suffer from severe mass transfer limitations that affect the apparent reaction rate. Increasing the agitation speed might increase the contact area of the two phases, and thus remove the interfacial mass transfer resistance.26,27 The reaction was conducted by changing the agitation speed from 0−600 rpm, and the results are given in Figure 1. It can be concluded that the yield of HMF and furfural increased rapidly as the agitation speed increased from 0 to 300 rpm, and then remained almost constant. These results indicated that the interfacial mass transfer resistance between the catalyst surface and liquid phase was negligible when the agitation speed was above 300 rpm; thus the agitation speed was set to 300 rpm in the dehydration reaction. 3.1.3. Effect of Catalyst Loading. Catalyst loading is an important parameter that needs to be optimized to increase HMF and furfural yield. The experiments were conducted at five different catalyst loadings (0, 17, 33, 50, and 67 wt %), and the results are given in Figure 2. It can be observed that HMF and furfural yields have been enhanced for higher catalyst loading. This effect was substantial; HMF and furfural yields increased from 10.2 and 14.8 mol % to 26.0 and 47.5 mol % when increasing the catalyst loading from 0 to 50 wt %. As can
3. RESULTS AND DISCUSSION 3.1. Glucose and Xylose Dehydration. 3.1.1. Comparison of Various Solid Acid Catalysts. The ZrO2−TiO2 binary oxide has been reported to exhibit a high surface acidity by a charge imbalance based on the generation of Zr−O−Ti bonding.23,24 In this Article, a series of SO42−/ZrO2−TiO2 catalysts with different preparation condition are compared for the conversion of glucose and xylose to HMF and furfural in a double reaction system (water/n-butanol), and the results are shown in Table 1. During all of the experiments, the reactants of glucose and xylose were in very small amounts or could not be detected; however, only a substantial amount of HMF (11.9−26.0 mol %) and furfural (32.0−47.5 mol %) was detected. These findings indicate that a portion of glucose and xylose was decomposed into dark-colored solids, known as humins. In the case of solid acid-catalyzed hydrolysis, the humins that form have not been characterized nearly as thoroughly, probably because the focus has been on improving selectivity to other products such as HMF and furfural. Often, all that is reported is that humins did form and in what yield. When it is reported, the elemental composition of humins is typically found to be on the order of 55.7% carbon, 4.5% hydrogen, and 30.4% oxygen.25 The experiment results from Table 1 indicated that the Zr/Ti mole ratio, calcining temperature, and calcining time 4562
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Figure 1. Effect of agitation speed on the conversion of sugar and the yields of products (other reaction conditions: reaction temperature of 170 °C, reaction time of 2 h, substrate concentration of 6 wt %, catalyst dosage of 50 wt %).
Figure 3. Effect of reaction temperature on the conversion of sugar and the yields of products (other reaction conditions: reaction time of 2 h, substrate concentration of 6 wt %, agitation speed of 300 r/min, catalyst dosage of 50 wt %).
3.1.5. Effect of Reaction Time. The effect of reaction time on the conversion of sugar mixture to HMF and furfural was investigated, and the results are shown in Figure 4. It can be
Figure 2. Effect of catalyst dosage on the conversion of sugar and the yields of products (other reaction conditions: reaction temperature of 170 °C, reaction time of 2 h, substrate concentration of 6 wt %, agitation speed of 300 r/min). Figure 4. Effect of reaction time on the conversion of sugar and the yields of products (other reaction conditions: reaction temperature of 170 °C, substrate concentration of 6 wt %, agitation speed of 300 r/ min, catalyst dosage of 50 wt %).
be seen from Figure 2, the equilibrium conversions for the formation of HMF and furfural were almost reached for the catalyst loadings of 50 wt %. Taking the cost and the efficiency into considerations, the optimal catalyst loading for this reaction was chosen to be 50 wt %. 3.1.4. Effect of Reaction Temperature. The effect of reaction temperature on the sugar mixture conversion to HMF and furfural was investigated, and the experiments were carried out at 150, 160, 170, 180, 190, and 200 °C, respectively. It can be observed from Figure 3 that temperature played a significant effect on HMF and furfural yields. When the reaction temperature was increased from 150 to 180 °C, there was a significant increase in the yields of HMF and furfural; the sugar conversion was also slight increased. Higher temperature could accelerate the rate of chemical reaction, but unwanted side reactions also appeared at the same time. When the reaction temperature rose above 190 °C, the product yield began to fall; however, the reactant of glucose and xylose cannot be detected, which indicating that more humins may occur with the increasing of reaction temperature. Therefore, elevation of temperature is unfavorable for the selectivity of desired reaction, and the optimum temperature was set to 180 °C in this experiment.
derived that with the prolonging of reaction time, the yield of HMF and furfural increased obviously. When the reaction time was increased from 0.5 to 3 h, the yield of HMF and furfural increased from 8.9 and 14.5 mol % to 30.9 and 54.3 mol %, so the optimum time was set to 3 h in this experiment. 3.1.6. Catalytic Activity. A series of solid acid catalysts including MCM-41, ZSM-5 (Si/Al = 25), H-zeolite, and SO42−/ZrO2−TiO2 as well as inert solid (blank) for the conversion of glucose and xylose to HMF and furfural in water/ n-butanol system were used, as shown in Table 2. It can be derived that the SO42−/ZrO2−TiO2 catalyst has a marked effect on the dehydration of a sugar mixture to HMF and furfural. For the blank sample, the yields of HMF and furfural were only 10.2 and 14.8 mol %. With the addition of MCM-41, ZSM-5 (Si/Al = 25), and H-zeolite, the yields of HMF and furfural increased slightly, to 13.5−18.6 mol % and 19.9−37.9 mol %, respectively. However, the yields of HMF and furfural increased significantly along with the conversion of glucose and xylose when catalyzed with SO42−/ZrO2−TiO2 catalyst, and to 26.0 and 47.5 mol %, respectively, indicating that SO42−‑/ZrO2− 4563
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Scheme 2. Lewis and Brönsted Acid Conversiona
Table 2. Dehydration Results of a Sugar Mixture in the Presence of Different Catalystsa conversion, mol %
yield, mol %
catalyst
glucose
xylose
HMF
furfural
blank MCM-41 ZSM-5 (Si/Al = 25) H-zeolite SO42−/ZrO2−TiO2 fresh recycle one time recycle two times
63.9 90.6 92.5 91.3 96.5 97.3 96.9
67. 3 93.8 94.8 91.9 98.3 95.7 97.5
10.2 18.6 13.5 17.9 26.0 24.5 25.4
14.8 37.9 23.2 19.9 47.5 44.3 46.8
a
M represents Zr or Ti.
glycosidic hydroxyl of glucose and xylose will be attacked by SO42−/ZrO2−TiO2 catalyst, so the glucose and xylose loop will be opened at C1, and the optical activity of glucose will be changed and isomerized to β-glucopyranose anomer accordingly. next, fructose (Scheme 3a) and D-xylulose (Scheme 3b) can be generated via the isomerization, which would be rapidly dehydrated to HMF and furfural under the reaction conditions. Simultaneously, the formation HMF may be further converted into levulinic acid and formic acid under the water existing circumstances.33 On the basis of the aforementioned factors, a double reaction system consisting of water and n-butanol has been constructed for the dehydration of glucose and xylose, so the generated HMF and furfural can be extracted into an organic layer quickly, which can effectively prevent the further hydrolysis for HMF and furfural and improve the product yield eventually. 3.2. HMF and Furfural Hydrogenation. The hydrogenation of pure HMF and furfural was investigated in detail using Ru/C as the catalyst and pure n-butanol as the reaction sysytem. The effect of reaction temperature on conversion of HMF and furfural to DMF and MF was investigated, and the experiments were carried out at 180, 200, 220, 240, and 260 °C, respectively. It can be observed from Figure 5 that temperature played a significant effect on DMF, especially on MF yield. When the reaction temperature was increased from 180 to 200 °C, there was a significant increase in the yields of DMF and MF. With the reaction temperature further increased to 220 °C, or even to 260 °C, the DMF yield remained almost constant. However, the MF yield increased obviously with the prolonging of reaction temperature. These results indicated that HMF has higher hydrogenation activity, and the DMF yield derived from HMF can reach maximum under lower temperature. Furthermore, the effect of reaction time on the hydrogenation was discussed (Figure 6),; it can be derived that with the prolonging of reaction time, the yields of DMF and MF increased obviously. When the reaction time was increased from 0.5 to 1.0 h, the yields of DMF and MF increased from 46.5 and 17.8 mol % to 60.0 and 40.9 mol %. When the reaction time further increased to 1.5 h, the DMF yield remained almost constant; however, the MF yield increased obviously, and at the reaction time of 1.5 h, the MF yield reached the maximum. These results also indicated that HMF has a higher hydrogenation activity than that of furfural. In addition, the effect of agitation speed on the hydrogenation reaction was monitored by changing the agitation speed from 0−500 rpm, and the results are given in Figure 7. It can be concluded that the yields of DMF and MF increased accordingly as agitation speed increased from 0 to 300 rpm, and then remained almost constant. These results indicated that the interfacial mass transfer resistance between the catalyst surface and liquid phase was negligible when the agitation speed was above 300 rpm; thus the agitation speed was set to 300 rpm in the hydrogenation reaction.
a Other reaction conditions: reaction temperature of 170 °C, reaction time of 2 h, substrate concentration of 6 wt %, catalyst dosage of 50 wt %, agitation speed of 300 r/min.
TiO2 can be used as an efficient catalyst for the dehydration of a sugar mixture to furfural compounds. Furthermore, long-term stability of heterogeneous catalyst is an extremely important characteristic for practical usage to reduce production cost. So, the catalytic stability of SO42−/ ZrO2−TiO2 catalyst was investigated by repeatedly using this catalyst for the dehydration under the same reaction conditions. For a typical recycle experiment, the spent SO42−/ZrO2−TiO2 catalyst was separated from the liquid products. It was found that the color of the catalyst surface became dark gray, which resulted from the adsorbed humins during the dehydration reaction. Several studies have indicated that the recovered catalyst without any treatment was deactivated significantly in successive runs.28 Fortunately, the deactivated catalyst can be regenerated by the method of calcination to burn away these deposited humins.29,30 For these reasons, in our catalyst reuse cycle test, the recovered catalyst was calcined at 500 °C for 5 h in static air before it was reused in a new experiment under similar reaction conditions. The results of HMF and furfural yields are presented in Table 2. It can be perceived that the yields of HMF and furfural are almost unchanged in the subsequent second and third cycles, indicating that the thermally regenerated catalyst was found to remain better catalysis-active. 3.1.7. Reaction Process for the Conversion of Glucose and Xylose. In this Article, SO42−/ZrO2−TiO2 was used as the catalyst for the dehydration reaction. The conversion of glucose and xylose to HMF and furfural is a complex multistep process. It can be noted that S6+ ions exist in the [SO4] with SO bonds in the structure, and the metal atoms of Zr and Ti are connected by the top angle oxygen ion, which results in the coordination structure, and then Lewis and Brönsted acids can be formed. Moreover, SO42−/ZrO2−TiO2 can be kept as a kind of amorphous porous loose structure for the different oxygen ligands (the oxygen coordination numbers of Ti and Zr are 6 and 8, respectively),31 so the SO42−/ZrO2−TiO2 catalyst will present strong acid. Meanwhile, the Lewis and Brönsted acids of SO42−/ZrO2−TiO2 catalyst can be mutually transformed through the adsorption of H2O (Scheme 2). On the basis of the above transformation, the dehydration catalytic activity of SO42−/ZrO2−TiO2 catalyst will be improved through the collaborative catalytic role of Lewis and Brönsted acids. According to the experimental results and the related literature,32 a reaction pathway for the acid-catalyzed dehydration in the water/n-butanol reaction system is proposed, as illustrated in Scheme 3. In the initial step, 4564
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Scheme 3. Proposed Mechanism for the Conversion of Sugar Mixtures to Furfural Catalyzed by SO42−/ZrO2−TiO2
Figure 5. Effect of reaction temperature on the yields of furans and the conversion of substrates (other reaction conditions: substrate concentration (the mass ratio of F:HMF was 1:1) of 6 wt %, agitation speed of 500 r/min, catalyst dosage of 20 wt %, reaction time of 1.5 h, hydrogen dosage of 0.01475 mol/g substrate).
Figure 6. Effect of reaction time on the yields of furans and the conversion of substrates (other reaction conditions: substrate concentration (the mass ratio of F:HMF was 1:1) of 6 wt %, agitation speed of 500 r/min, catalyst dosage of 20 wt %, reaction temperature of 260 °C, hydrogen dosage of 0.01475 mol/g substrate).
Eventually, the effect of hydrogen dosage on the hydrogenation reaction was considered by changing the hydrogen dosage from 0−0.003687 mol/g substrates, and the results are given in Figure 8. It can be observed that the yield of DMF and MF increased rapidly as hydrogen dosage increased from 0 mol/g substrates to 0.003687 mol/g substrates, and then reduced gradually with the further increasing of hydrogen dosage. These results indicated that superfluous hydrogen dosage will result in the further hydrogenation for the
generated DMF and MF; thus the hydrogen dosage was set to 0.003687 mol/g substrates in the hydrogenation reaction. On the basis of the above study, the hydrogenation of HMF and furfural obtained from glucose and xylose dehydration were done, and the results were compared to those obtained using neat HMF and furfural. During all experiments, the reactants of HMF and furfural were in very small amounts or could not be detected; however, only a substantial amount of DMF (60.3 mol % yield) and MF (61.9 mol % yield) was detected. These 4565
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reached 60.3 and 61.9 mol % with the substrate of neat HMF and furfural, and used neat n-butanol as solvent. However, the target products of DMF and MF could not be detected when directly used in the separated organic phase as substrate and solvent (Table 3, extracted 1). Furthermore, the target products of DMF and MF also could not be detected when using extracted 1 as solvent and neat HMF and furfural as substrate. These results indicated that the separated organic phase may have an obvious effect on the hydrogenation of HMF and furfural. On the basis of the above hypothesis, the separated organic phase (Table 3, extracted 1) underwent a reduced pressure distillation. Furthermore, because the solubility of NaCl in anhydrous organic solvents is lower than that in water, removal of water causes the salt to precipitate out of solution; thus, the water and NaCl can be separated from HMF and n-butanol,9 and the purified organic solvent was marked as extracted 2. Next, through a comparison experiment that used extracted 2 as solvent and neat HMF and furfural as substrate, it can be observed that the DMF and MF yield only was 32.7 and 17.5 mol %, far below that of the neat run. All of these results indicated that the purification of the organic phase is a key step for the further hydrogenation of HMF and furfural. Further detailed investigations of the purification are still in progress.
Figure 7. Effect of agitation speed on the yields of furans and the conversion of substrates (other reaction conditions: substrate concentration (the mass ratio of F:HMF was 1:1) of 6 wt %; reaction time of 1.5 h, catalyst dosage of 20 wt %, reaction temperature of 260 °C, hydrogen dosage of 0.01475 mol/g substrate).
4. CONCLUSIONS The present study described a catalytic process for the feasibility of converting a glucose and xylose mixture to MFs through catalytic dehydration and coupling hydrogenated reduction. Glucose and xylose were almost totally consumed during the dehydration reaction catalyzed by SO42−/ZrO2− TiO2, and the product yield could reach 30.9 mol % (for HMF) and 54.3 mol % (for furfural) under the optimal experimental conditions. The SO42−/ZrO2−TiO2 catalyst is recoverable from the resulting product mixture and reused multiple times after calcination without any substantial change in the HMF and furfural yield. Although solid acid catalyst used here tends to give low product yield, heterogeneous catalysts show unparalleled advantages in comparison to the traditional homogeneous acids in terms of recycle and equipment corrosion. Hydrogenation of HMF and furfural in n-butanol promoted by Ru/C produced a series of products, and in particular MFs, a product of particular interest because of its high-energy content. The highest yield could reach 60.3 mol % (for DMF) and 61.9 mol % (for MF) under the optimal experimental conditions by starting with pure HMF and furfural, but the target products of DMF and MF could not be detected when the separated organic phase was directly used as solvent. However, the DMF and MF yields could reach 32.7 and 17.5 mol %, respectively, when the separated organic phase underwent a further purification. All of these results indicated that the purification of the organic phase is a key step for the further hydrogenation of HMF and furfural.
Figure 8. Effect of hydrogen dosage on the yields of furans and the conversion of substrates (other reaction conditions: substrate concentration (the mass ratio of F:HMF was 1:1) of 6 wt %, reaction time of 1.5 h, catalyst dosage of 20 wt %, reaction temperature of 260 °C, agitation speed of 500 r/min).
findings indicate that a portion of HMF and furfural decomposed into some other resultants. As the purpose of this step was to produce DMF and MF, the determinations of other resultants’ detailed yields are not presented here. The data shown in Table 3 indicate that the source of HMF and furfural has an obvious effect on the target products yields. It can be noted from Table 3 that the DMF and MF yields Table 3. Hydrogenation of HMF and Furfural Using Four Sources of HMF and Furfurala conversion, mol %
yield, mol %
run
substrate (HMF and furfural)
HMF
furfural
DMF
MF
1 2 3 4
neat extracted 1 extracted 1 + neat HMF and furfural extracted 2
99.8 94.3 90.7 93.4
99.8 93.1 92.2 92.8
60.3
61.9
32.7
17.5
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.Z.);
[email protected] (L.L.).
a
Other reaction conditions: substrate concentration (the mass ratio of F:HMF was 1:1) of 6 wt %, reaction time of 1.5 h, catalyst dosage of 20 wt %, reaction temperature of 260 °C, hydrogen dosage of 0.01475 mol/g substrate, agitation speed of 500 r/min.
Notes
The authors declare no competing financial interest. 4566
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Energy & Fuels
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Article
(29) Tyagi, B.; Mishra, M. K.; Jasra, R. V. J. Mol. Catal. A: Chem. 2010, 317, 41−45. (30) Sun, H.; Ding, Y.; Duan, J.; Zhang, Q.; Wang, Z.; Lou, H.; Zheng, X. Bioresour. Technol. 2010, 101, 953−958. (31) Ito, K.; Kakino, S.; Ikeue, K.; Machida, M. Appl. Catal., B 2007, 74, 137−143. (32) Yan, H.; Yang, Y.; Tong, D.; Xiang, X.; Hu, C. Catal. Commun. 2009, 10, 1558−1563. (33) Girisuta, B.; Janssen, L.; Heeres, H. J. Ind. Eng. Chem. Res. 2007, 46, 1696−1708.
ACKNOWLEDGMENTS We are grateful for the financial support from the Science Foundation of Zhejiang Sci-Tech University (ZSTU) (1101821-Y), the Young Researchers Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education (ZSTU) (2011QN08), the National Key Basic Research Program (2010CB732201) from the Ministry of Science and Technology of China, the Natural Science Foundation of China (2106121, U0733001), the Basic Foundation for Scientific Research of Universities (2010121077) from the Ministry of Education of China, and the Provincial R&D Program (1270-K42004) from the Economic and Trade Committee of Fujian Province of China.
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