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Synthesis, isolation and characterization of methyl levulinate from cellulose catalyzed by extremely low concentration acid
Journal of Energy Chemistry 22(2013)895–901
Synthesis, isolation and characterization of methyl levulinate from cellulose catalyzed by extremely low concentration acid Hui Lia , Lincai Penga,c∗ , Lu Linb∗ ,
Keli Chena ,
Heng Zhanga
a. Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, Yunnan, China; b. School of Energy Research, Xiamen University, Xiamen 361005, Fujian, China; c. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China [ Manuscript received February 28, 2013; revised March 14, 2013 ]
Abstract A direct synthesis of methyl levulinate from cellulose alcoholysis in methanol medium under mild condition (180−210 ◦ C) catalyzed by extremely low concentration sulfuric acid (60.01 mol/L) and the product isolation were developed in this study. Effects of different process variables towards the catalytic performance were performed as a function of reaction time. The results indicated that sulfuric acid concentration, temperature and initial cellulose concentration had significant effects on the synthesis of methyl levulinate. An optimized yield of around 50% was achieved at 210 ◦ C for 120 min with sulfuric acid concentration of 0.01 mol/L and initial cellulose concentration below 100 g/L. The resulting product mixture was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distill the heavy fraction. The light fraction including mainly methanol could be reused as the reaction medium without any substantial change in the yield of methyl levulinate. The chemical composition and structural of lower heavy fraction were characterized by GC/MS, FTIR, 1 H-NMR and 13 C-NMR techniques. Methyl levulinate was found to be a major ingredient of lower heavy fraction with the content over 96%. This pathway is efficient, environmentally benign and economical for the production of pure levulinate esters from cellulose. Key words cellulose; methyl levulinate; extremely low acid; catalysis; alcoholysis; isolation
1. Introduction Cellulose is abundantly available on earth, which is regarded as a promising alternative to gradual depletion of fossil fuel resources for the sustainable supply of fuels and chemicals in the future society. Much effort has been devoted to the development and research of chemical or biological transformation pathways to convert cellulose into platform chemicals like ethanol [1], 5-hydroxymethylfurfural [2], levulinic acid [3] and sorbitol [4]. Recently, the direct conversion of cellulose to levulinate esters by acid-catalyzed alcoholysis has attracted more and more concerns because the synthetic technique is simple and wastewater is minimized [5−7]. Levulinate esters are a kind of important energy chemicals and intermediates having numerous potential industrial applications. These esters are suitable to be used as additives for diesel and biodiesel transportation fuels, which have manifold excellent performances including non-toxicity, high lubricity, flashpoint stability and better flow properties under cold con-
dition [8−10]. Additionally, levulinate esters can be used either in the flavoring and fragrance industry or as substrates for chemical conversion to other meaningful chemicals by various kinds of condensation and addition reactions at the ester and keto groups [11,12]. To develop a feasible process on an industrial scale, the development and choice of a proper acid catalyst is crucial to the direct conversion of cellulose into levulinate esters. To date, several kinds of acid catalysts including mineral acid, organic acid and solid acid were employed in this study. For instance, Tominaga et al. [13] described an efficient mixedacid catalyst system consisting of Lewis and Br¨onsted acids for the conversion of cellulose in methanol at 180 ◦ C, where the highest yield of methyl levulinate reached 75%. Rataboul and Essayem [14] studied the catalytic transformation of cellulose in supercritical methanol (300 ◦ C/10 MPa) in the presence of solid acid catalysts, and the obtained yield of methyl levulinate was around 20% over CSx H3−x PW12 O40 and sulfated zirconia. Chang et al. [15] reported the direct synthesis
∗
Corresponding authors. Tel: +86-15908801086; Fax: +86-871-65920171; E-mail: [email protected] (L.Peng); [email protected] (L.Lin) This work was supported by the National Key Basic Research Program (2010CB732201) from the Ministry of Science and Technology of China, and the State Key Laboratory Open Foundation of Pulp and Paper Engineering of China (201225). Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
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of ethyl levulinate from the conversion of wheat straw catalyzed by dilute sulfuric acid using response surface methodology, and an optimized yield of ethyl levulinate in 17.91% could obtained, representing a theoretical yield of 51%. However, all these methods suffer from some of drawbacks such as serious equipment corrosion, costly catalyst, low conversion efficiency, obvious side reaction on the inter-molecular dehydration of alcohol medium to diether. In addition, these studies mainly focused on the synthesis of levulinate esters, and the investigation on products isolation is relatively few. Hence, it is very necessary to develop an efficient and economical strategy for the synthesis and isolation of levulinate esters from biomass like cellulose. In recent years, extremely low concentration acid (60.01 mol/L) has attracted considerable interest in cellulose hydrolysis process for sugar production, which has little corrosion to reactor, minimal environmental effect and low cost. Thus, the process using extremely low concentration acid also qualifies as a “green technology”. The findings have proven that its catalytic activity was comparable to that of conventional dilute acid [16−18]. In our previous research, extremely low concentration sulfuric acid catalyst system was found to be highly promising strategy for the synthesis of methyl levulinate from glucose in methanol medium, which can offer enough acid site for the completion of reaction. The optimum methyl levulinate yield was equal or even surpass those previous obtained by dilute sulfuric acid and solid acid catalysts. Most significantly, side reaction on the dehydration of methanol to dimethyl ether was slight [19]. The objective of this study was to provide/supplement technical information about the direct synthesis and isolation of methyl levulinate from cellulose catalyzed by extremely low concentration sulfuric acid. The effects of multiple process parameters including stirring rate, sulfuric acid concentration, temperature and initial cellulose concentration on the reaction performance were conducted as a function of time to optimise the yield of methyl levulinate. After the reaction was finished, the resulting product mixture was isolated by a distillation technique, and characterized with GC/MS, FTIR, 1 H-NMR and 13 C-NMR techniques.
2.2. Equipment and experimental procedures
2. Experimental
Amount of methyl glucoside after reaction (mol) × 100 Total amount of glucose monomer in cellulose (mol) (2) The isolated methyl levulinate from distillation was analyzed and characterized using GC/MS (Agilent 6890/5975, USA), FTIR (Bruker TNESOR 27, Germany) and NMR spectromer (Bruker AVANCE 400 MHz, switzerland), respectively.
2.1. Materials and chemicals Microcrystalline cellulose (DP≈200) with an average particle size of 100 µm from Sinopharm Chemical Reagent (Shanghai, China) was used as starting material in this study. Methyl levulinate (99% purity) and methyl glucoside (98% purity) used for calibration were obtained from Alfa Aesar (Tianjin, China) and Yangcun Chemical (Beijing, China), respectively. Other reagents and chemicals were all of analytical grade from Sinopharm Chemical Reagent (Shanghai, China) or Kemiou Chemical Reagent (Tianjin, China), and used without further purification or treatment.
The experiments were carried out in a cylindrical stainless steel pressurized reactor with 100 mL total volume made by Parr instrument company, USA. The reactor was heated in an adjustable electric stove. The temperature of the reactor contents was monitored by a thermocouple connected to the reactor. For each experiment, a 50 mL solution of sulfuric acid in methanol and a given amount of cellulose were introduced into the reactor, which was then brought to the desired temperature by external heating about 25 min and stirred for reaction. After certain reaction time, the reactor was taken from the stove and quenched in an ice cool water bath to terminate the reaction. Then, the sample take from the reactor was filtered and collected the liquid-phase fraction for analysis. The sample with a higher content of methyl levulinate was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distil the heavy products. Each fraction from distillation process was collected for further analysis. 2.3. Analytical approach The amount of methyl levulinate in the liquid-phase sample was determined by a GC (Agilent 6890 instrument, USA) equipped with an 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. Methyl glucoside was analyzed by ion chromatography (DIONEX ICS-3000, USA) equipped with a CarboPac PA1 (2 mm×250 mm) analytical column and an electrochemical detector. An aqueous solution of sodium hydroxide (80 mmol/L) was used as the eluent with a volumetric flow rate of 0.35 mL/min at 30 ◦ C. The yields of methyl levulinate and methyl glucoside on a molar base according to reaction stoichiometry were calculated as the following equations: Methyl levulinate yield (% ) = Amount of methyl levulinate after reaction (mol) × 100 Total amount of glucose monomer in cellulose (mol) (1) Methyl glucoside yield (% ) =
3. Results and discussion 3.1. Synthesis of methyl levulinate from cellulose alcoholysis To understand the alcoholysis process of cellulose in
Journal of Energy Chemistry Vol. 22 No. 6 2013
methanol medium under mild conditions (6210 ◦ C) catalyzed by extremely low concentration sulfuric acid (60.01 mol/L) and obtain the highest possible yield of methyl levulinate, effects of various process variables including stirring rate, sulfuric acid concentration, temperature and initial cellulose concentration on the reaction performance were conducted as a function of time. In our previous experiments, methyl glucoside was found to be a key intermediate product during the alcoholysis of glucose for methyl levulinate production [19]. Based on this, a target product (methyl levulinate) and an intermediate product (methyl glucoside) after the reaction were analyzed and quantified to monitor the reaction progress in this study, and the results were discussed in the following sections. 3.1.1. Ef fect of stirring rate The solid-liquid phase catalytic reaction system between cellulose and methanol solution of sulfuric acid may suffer from a certain degree of mass transfer limitation that affects the apparent reaction rate. Increasing the stirring rate might increase the contact area of the two phases, and thus removing the interfacial mass transfer resistance [20]. The experiments were conducted by changing stirring rate from zero to 800 rpm, and the results are given in Figure 1. It seems that there was no clear influence of stirring rate on the yield of methyl levulinate except the stirring rate of zero. This finding implied that the interfacial mass transfer resistance between cellulose surface and liquid phase is negligible in the stirring state, and the elevation of stirring rate is not necessary or useless for the synthesis of methyl levulinate.
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concentration ranging from zero to 0.01 mol/L on the yields of methyl glucoside and methyl levulinate with different reaction times at 190 ◦ C. It was found that there were hardly any production of methyl glucoside and methyl levulinate in the absence of exerted catalyst (i.e., without adding sulfuric acid) under our used conditions. However, a high yield of methyl glucoside of about 30% with few amount of methyl levulinate were obtained from non-catalytic degradation of cellulose in supercritical methanol under severe conditions (300−350 ◦ C and 10−43 MPa) based on previous literature reports [14,21]. This difference indicates that process conditions are an important parameter for the alcoholysis of cellulose. On the other hand, it can be observed from Figure 2 that the yields of methyl glucoside and methyl levulinate were enhanced significantly by the addition of extremely low concentration sulfuric acid as catalyst under mild conditions. It is thus clear that the use of a catalyst is much more favourable than the elevations of temperature and pressure for the synthesis of methyl levulinate from cellulose. With the augment of sulfuric acid concentration, the highest yield of methyl glucoside during the reaction and the yield of methyl levulinate at the same reaction time point increased accordingly. Higher sulfuric acid concentration (>0.01 mol/L) was not used here as it would result in serious side reaction on the inter-molecular dehydration of methanol to dimethyl ether [19]. Usually, the formation of dimethyl ether is undesired in practical applications for this technology because the recovery rate of methanol falls off and its low-down boiling point (−24.9 ◦ C) significantly increases the system pressure of requiring more high-grade reactor. In addition, it also seems that as the prolonging of reaction time, the yield of methyl glucoside increased at first and then decreased, while the yield of methyl levulinate was increasing gradually. This is due to that the polymer chains of cellulose are firstly broken down into low molecular weight fragments and then to methyl glucoside in methanol medium, which is further converted into methyl levulinate under experimental conditions. Overall, extremely low concentration sulfuric acid is found to be efficient for the alcoholysis of cellulose to methyl levulinate in methanol medium under mild conditions. An optimized yield of methyl levulinate is expected to reach by combining the following test of temperature. 3.1.3. Ef fect of temperature
Figure 1. Effect of stirring rate on the yield of methyl levulinate. Reaction conditions: initial cellulose concentration, 20 g/L; sulfuric acid concentration, 0.01 mol/L; temperature, 200 ◦ C; time, 180 min
3.1.2. Ef fect of sulfuric acid concentration Extremely low concentration sulfuric acid (60.01 mol/L) was employed as acid catalyst in the alcoholysis of cellulose to methyl levulinate. Figure 2 shows the effect of sulfuric acid
The effect of reaction temperature on the yields of methyl glucoside and methyl levulinate was investigated as a function of reaction time, and the experiments were carried out at 180, 190, 200 and 210 ◦ C, respectively. As seen from Figure 3, the temperature played a positive role in the reaction process for the synthesis of methyl levulinate. High temperature could contribute to the acceleration of chemical reaction rate and the enhancement of conversion efficiency. When the temperature was increased from 180 to 200 ◦ C, there was a significant increase in the yield of methyl levulinate after the same reaction time and the amount of methyl glucoside was decreased accordingly after the reaction time of 60 min.
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Then, the yield of methyl levulinate was observed slightly to rise over 200 ◦ C after the reaction was completed. At lower temperature such as 180 and 190 ◦ C, the yield of methyl levulinate grew smoothly with reaction time, which may need longer time to reach the equilibrium conversion. Increasing
the temperature to 210 ◦ C, the conversion took place at a faster rate and the yield of methyl levulinate increased significantly with the prolonging of time, and the equilibrium point was almost reached by 120 min with the highest methyl levulinate yield of over 50%.
Figure 2. Effect of sulfuric acid concentration on the yields of methyl glucoside (a) and methyl levulinate (b). Reaction conditions: initial cellulose concentration, 20 g/L; temperature, 190 ◦ C; stirring rate, 400 r/min
Figure 3. Effect of temperature on the yields of methyl glucoside (a) and methyl levulinate (b). Reaction conditions: initial cellulose concentration, 20 g/L; sulfuric acid concentration, 0.01 mol/L; stirring rate, 400 r/min
3.1.4. Ef fect of initial cellulose concentration The optimal concentration of substrate is very important for the efficient use of cellulose material and the final concentration of methyl levulinate. A higher concentration is extremely favorable because it can not only enhance the production efficiency of methyl levulinate, but also cut down energy consumption in the separation of methyl levulinate. Figure 4 illustrates the effect of initial cellulose concentration ranging from 10 to 200 g/L on the concentration and yield of methyl levulinate. It can be noted that although a slight drop in methyl levulinate yield was observed, the concentration of methyl levulinate has risen significantly when initial cellulose concentration increased from 40 to 100 g/L. Further increasing substrate concentration resulted in more cellulose available for
conversion, which means higher methyl levulinate concentration to obtain. But unfortunately, the yield of methyl levulinate was found to decrease obviously and the concentration of methyl levulinate increased disproportionally when initial cellulose concentration raised from 100 to 150 and 200 g/L, indicating that cellulose material has not been utilized fully. Similar effect of initial substrate concentration on reaction performance was also observed in some previous studies [22,23]. The reason is probably due to product feedback inhibition, reactivity diminution or insufficient acid for the additional cellulose which led an incomplete reaction. Hence, compromises have to be made between concentration and yield of methyl levulinate for practical usage to reduce production cost, and reasonable initial cellulose concentration can be chosen to be around 100 g/L for the efficient synthesis of methyl levulinate.
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Figure 4. Effect of initial cellulose concentration on the concentration and yield of methyl levulinate. Reaction conditions: sulfuric acid concentration, 0.01 mol/L; temperature, 200 ◦ C; time, 180 min; stirring rate, 400 r/min
3.2. Isolation of products from the methanolysis of cellulose After the reaction was finished, the resulting product mixture was neutralized and dehydrated with calcium oxide, then filtered to collect the liquid-phase products for isolation. Based on the boiling temperatures, the liquid-phase components were isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation, and a
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corresponding schematic are presented in Figure 5. Light fraction (low boiling substances) was separated out firstly by atmospheric distillation at 80 ◦ C, which contained methanol of about 96%, methyl formate and a very small amount of methyl levulinate by GC analysis. This fraction could be reused as the reaction medium without any substantial change in the yield of methyl levulinate. Next, high boiling products in the residual component were found to be very hard to distill off even with high temperature by vacuum distillation, probably due to that they were firmly bound to the solid humins and/or methyl glucoside. For this reason, high boiling paraffin (ndodecane) was added to the residual component to help distil the heavy products, which acted as desorption driving agent for heavy products. At this stage, the component mixture was isolated by vacuum distillation at 145 ◦ C while the heavy fraction was collected. The heavy fraction was divided into two layers automatically. The upper layer containing predominantly n-dodecane (over 99%) which can be reused to help distill the heavy fraction, and the lower layer containing primarily methyl levulinate (over 96%), a very small number of levulinic acid, n-dodecane and unknown substances, were detected by GC/MS. In addition, some residual solids could be obtained after distillation. It was found that most of them (about 86%) were soluble in water. According to the result of ion chromatography, the main component was methyl glucoside. The rest of insoluble substance appeared dark-brown known as humins.
Figure 5. Schematic of the isolation of liquid-phase products by distillation
3.3. Characterization of lower heavy fraction obtained from distillation
teristic absorbances of methyl levulinate. Figure 7 presents the 1 H-NMR spectrum of lower heavy
Some substances may not be found by GC/MS due to the limitations of capillary column used. Here, the lower heavy fraction was further characterized with FTIR and NMR spectrometry to fully confirm the sample composition and methyl levulinate purity. The result of FTIR spectrum is shown in Figure 6 and the corresponding functional groups for typical absorption characteristic peaks are given with an inset in the figure. The sharp bands at 1740 and 1720 cm−1 were assigned to the stretching vibration of C = O groups; the absorbances at 1211 and 1161 cm−1 were attributed to the C–O stretching vibration; the absorbance around 1439 cm−1 was methylene (–CH2 –) stretching vibrational band; and the absorbances around 2955 and 1362 cm−1 were attributed the stretching vibration of methyl (–CH3 ). Other than these bands detected, there was no other obvious absorption characteristic peaks in FTIR spectrum. These bands were consistent with the charac-
Figure 6. FTIR spectrum of lower heavy fraction obtained from distillation
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fraction obtained from distillation using CDCl3 as solvent. The sharp signals at 2.19 and 3.68 ppm were assigned to the protons of methyl (CH3 –). The former corresponded to methyl adjacent to a carbonyl group (–CO–) while the latter was the signal of a methyl next to an ester group (–COO–). The triplet peaks at 2.56, 2.58 and 2.60 ppm were the signals of a methylene (–CH2 –) close to an ester group (–COO–) while protons of another methylene (–CH2 –) near a carbonyl group (–CO–) showed triplet peaks at 2.74, 2.76 and 2.77 ppm. The integral ratio (i.e., hydrogen ratio) of signals at
2.19 ppm : 2.74−2.77 ppm : 2.56−2.60 ppm : 3.68 ppm was 2.98 : 2.04 : 2.02 : 3.00, which was almost identical to the actual hydrogen ratio of groups in methyl levulinate molecules (CH3 COCH2 CH2 COOCH3 ). Beyond that, several very weak peaks were observed at 1.26, 3.33, 3.43, 3.48, 3.78, 3.86 and 4.50 ppm, respectively, implying that a few impurities existed in the lower fraction. It also can be deduced that the molecular formulas of the impurities probably contained alkyl group alkenyl group and C–O linkage. A 13 C-NMR spectrometry was carried out as a
Figure 7. 1 H-NMR spectrum of lower heavy fraction obtained from distillation using CDCl3 as solvent
Figure 8.
13 C-NMR
spectrum of lower heavy fraction obtained from distillation using CDCl3 as solvent
Journal of Energy Chemistry Vol. 22 No. 6 2013
complementary analysis, and the result is shown in Figure 8. The peak at 77.0 ppm was the signal of CDCl3 solvent. In addition to this, six distinct signals of carbon were detected, which is the same as carbon number of methyl levulinate molecules and corresponded to the groups as follows. The signals at 173.2 and 206.5 ppm were assigned to the ester group carbon (–COO–) and carbonyl carbon (–CO–), respectively. The signals of two methylene carbons (–CH2 –) appeared at 27.7 and 37.9 ppm, respectively. The former was the signal of a carbon adjacent to an ester group (–COO–), and the latter was a carbon next to a carbonyl group (–CO–). The peak at 29.8 ppm was the signal of methyl carbon (–CH3 ) close to carbonyl group (–CO–) while another methyl carbon (–CH3 ) near ester group (–COO–) showed signal at 51.7 ppm. Both the results of 1 H-NMR and 13 C-NMR analysis showed good accordance with the molecular structure of methyl levulinate. Besides the characteristic peaks of methyl levulinate, other signal peaks detected were quite small compared with that of methyl levulinate, implying high purity of methyl levulinate in lower heavy fraction. 4. Conclusions The present research developed an efficient and economical strategy for the synthesis and isolation of methyl levulinate from the alcoholysis of cellulose catalyzed by extremely low concentration sulfuric acid. Experimental results indicated that sulfuric acid concentration, temperature and initial cellulose concentration had significant effects on the alcoholysis of cellulose and synthesis of methyl levulinate. Sulfuric acid concentration of 0.01 mol/L can offer enough acid sites for the completion of reaction. The formation of methyl levulinate was favored in dilute initial cellulose concentration at high temperature. A high methyl levulinate yield of around 50% could be obtained at 210 ◦ C for 120 min with sulfuric acid concentration of 0.01 mol/L and initial cellulose concentration below 100 g/L. The resulting product mixture was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distill the heavy fraction. The light fraction including mainly methanol could be reused as the reaction medium without any substantial change in products yields, and methyl levulinate was a major ingredient of lower heavy fraction with a purity over 96%. This study provides a great potential to accelerate the development of industrial conversion of abundant and inexpensive cellulose into levulinate esters as biofuels and/or high-value chemicals.
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Acknowledgements The authors are grateful to the financial support from the National Key Basic Research Program (2010CB732201) from the Ministry of the Science and Technology of China, and the State Key Laboratory Open Foundation of Pulp and Paper Engineering of China (201225).
References [1] Geddes C C, Nieves I U, Ingram L O. Curr Opin Biotechnol, 2011, 22(3): 312 [2] Wang P, Yu H B, Zhan S H, Wang S Q. Bioresour Technol, 2011, 102(5): 4179 [3] Wettstein S G, Alonso D M, Chong Y X, Dumesic J A. Energy Environ Sci, 2012, 5(8): 8199 [4] Yang P F, Kobayashi H, Hara K, Fukuoka A. ChemSusChem, 2012, 5(5): 920 [5] Garves K. J Wood Chem Technol, 1988, 8(1): 121 [6] Olson E S, Kjelden M R, Schlag A J, Sharma R K. ACS Symp Ser, 2001, 784: 51 [7] Peng L C, Zhuang J P, Lin L. J Nat Gas Chem, 2012, 21(2): 138 [8] Windom B C, Lovestead T M, Mascal M, Nikitin E B, Bruno T J. Energy Fuels, 2011, 25(4): 1878 [9] Joshi H, Moser B R, Toler J, Smith W F, Walker T. Biomass Bioenerg, 2011, 35(7): 3262 [10] Lou W Y, Cai J, Duan Z Q, Zong M H. Chin J Catal (Cuihua Xuebao), 2011, 32(11): 1755 [11] Hayes D J. Catal Today, 2009, 145(1-2): 138 [12] G¨urb¨uz E I, Alonso D M, Bond J Q, Dumesic J A. ChemSusChem, 2011, 4(3): 357 [13] Tominaga K, Mori A, Fukushima Y, Shimada S, Sato K. Green Chem, 2011, 13(4): 810 [14] Rataboul F, Essayem N. Ind Eng Chem Res, 2011, 50(2): 799 [15] Chang C, Xu G Z, Jiang X X. Bioresour Technol, 2012, 121: 93 [16] Kim J S, Lee Y Y, Torget R W. Appl Biochem Biotechnol, 2001, 91-93(1-9): 331 [17] Ojumu T V, AttahDaniel B E, Betiku E, Solomon B O. Biotechnol Bioprocess Eng, 2003, 8(5): 291 [18] Zhuang X S, Qi W, Yuan Z H, Wang Q, Tan X S. J Biobased Mater Bioenergy, 2010, 4(1): 35 [19] Peng L C, Lin L, Li H. Ind Crop Prod, 2012, 40: 136 [20] Devulapelli V G, Weng H S. Catal Commun, 2009, 10(13): 1638 [21] Ishikawa Y, Saka S. Cellulose, 2001, 8(3): 189 [22] Girisuta B, Janssen L P B M, Heeres H J. Ind Eng Chem Res, 2007, 46(6): 1696 [23] Peng L C, Lin L, Zhang J H, Zhuang J P, Zhang B X, Gong Y. Molecules, 2010, 15(8): 5258
Synthesis, isolation and characterization of methyl levulinate from cellulose catalyzed by extremely low concentration acid Hui Lia , Lincai Penga,c∗ , Lu Linb∗ ,
Keli Chena ,
Heng Zhanga
a. Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, Yunnan, China; b. School of Energy Research, Xiamen University, Xiamen 361005, Fujian, China; c. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China [ Manuscript received February 28, 2013; revised March 14, 2013 ]
Abstract A direct synthesis of methyl levulinate from cellulose alcoholysis in methanol medium under mild condition (180−210 ◦ C) catalyzed by extremely low concentration sulfuric acid (60.01 mol/L) and the product isolation were developed in this study. Effects of different process variables towards the catalytic performance were performed as a function of reaction time. The results indicated that sulfuric acid concentration, temperature and initial cellulose concentration had significant effects on the synthesis of methyl levulinate. An optimized yield of around 50% was achieved at 210 ◦ C for 120 min with sulfuric acid concentration of 0.01 mol/L and initial cellulose concentration below 100 g/L. The resulting product mixture was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distill the heavy fraction. The light fraction including mainly methanol could be reused as the reaction medium without any substantial change in the yield of methyl levulinate. The chemical composition and structural of lower heavy fraction were characterized by GC/MS, FTIR, 1 H-NMR and 13 C-NMR techniques. Methyl levulinate was found to be a major ingredient of lower heavy fraction with the content over 96%. This pathway is efficient, environmentally benign and economical for the production of pure levulinate esters from cellulose. Key words cellulose; methyl levulinate; extremely low acid; catalysis; alcoholysis; isolation
1. Introduction Cellulose is abundantly available on earth, which is regarded as a promising alternative to gradual depletion of fossil fuel resources for the sustainable supply of fuels and chemicals in the future society. Much effort has been devoted to the development and research of chemical or biological transformation pathways to convert cellulose into platform chemicals like ethanol [1], 5-hydroxymethylfurfural [2], levulinic acid [3] and sorbitol [4]. Recently, the direct conversion of cellulose to levulinate esters by acid-catalyzed alcoholysis has attracted more and more concerns because the synthetic technique is simple and wastewater is minimized [5−7]. Levulinate esters are a kind of important energy chemicals and intermediates having numerous potential industrial applications. These esters are suitable to be used as additives for diesel and biodiesel transportation fuels, which have manifold excellent performances including non-toxicity, high lubricity, flashpoint stability and better flow properties under cold con-
dition [8−10]. Additionally, levulinate esters can be used either in the flavoring and fragrance industry or as substrates for chemical conversion to other meaningful chemicals by various kinds of condensation and addition reactions at the ester and keto groups [11,12]. To develop a feasible process on an industrial scale, the development and choice of a proper acid catalyst is crucial to the direct conversion of cellulose into levulinate esters. To date, several kinds of acid catalysts including mineral acid, organic acid and solid acid were employed in this study. For instance, Tominaga et al. [13] described an efficient mixedacid catalyst system consisting of Lewis and Br¨onsted acids for the conversion of cellulose in methanol at 180 ◦ C, where the highest yield of methyl levulinate reached 75%. Rataboul and Essayem [14] studied the catalytic transformation of cellulose in supercritical methanol (300 ◦ C/10 MPa) in the presence of solid acid catalysts, and the obtained yield of methyl levulinate was around 20% over CSx H3−x PW12 O40 and sulfated zirconia. Chang et al. [15] reported the direct synthesis
∗
Corresponding authors. Tel: +86-15908801086; Fax: +86-871-65920171; E-mail: [email protected] (L.Peng); [email protected] (L.Lin) This work was supported by the National Key Basic Research Program (2010CB732201) from the Ministry of Science and Technology of China, and the State Key Laboratory Open Foundation of Pulp and Paper Engineering of China (201225). Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
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of ethyl levulinate from the conversion of wheat straw catalyzed by dilute sulfuric acid using response surface methodology, and an optimized yield of ethyl levulinate in 17.91% could obtained, representing a theoretical yield of 51%. However, all these methods suffer from some of drawbacks such as serious equipment corrosion, costly catalyst, low conversion efficiency, obvious side reaction on the inter-molecular dehydration of alcohol medium to diether. In addition, these studies mainly focused on the synthesis of levulinate esters, and the investigation on products isolation is relatively few. Hence, it is very necessary to develop an efficient and economical strategy for the synthesis and isolation of levulinate esters from biomass like cellulose. In recent years, extremely low concentration acid (60.01 mol/L) has attracted considerable interest in cellulose hydrolysis process for sugar production, which has little corrosion to reactor, minimal environmental effect and low cost. Thus, the process using extremely low concentration acid also qualifies as a “green technology”. The findings have proven that its catalytic activity was comparable to that of conventional dilute acid [16−18]. In our previous research, extremely low concentration sulfuric acid catalyst system was found to be highly promising strategy for the synthesis of methyl levulinate from glucose in methanol medium, which can offer enough acid site for the completion of reaction. The optimum methyl levulinate yield was equal or even surpass those previous obtained by dilute sulfuric acid and solid acid catalysts. Most significantly, side reaction on the dehydration of methanol to dimethyl ether was slight [19]. The objective of this study was to provide/supplement technical information about the direct synthesis and isolation of methyl levulinate from cellulose catalyzed by extremely low concentration sulfuric acid. The effects of multiple process parameters including stirring rate, sulfuric acid concentration, temperature and initial cellulose concentration on the reaction performance were conducted as a function of time to optimise the yield of methyl levulinate. After the reaction was finished, the resulting product mixture was isolated by a distillation technique, and characterized with GC/MS, FTIR, 1 H-NMR and 13 C-NMR techniques.
2.2. Equipment and experimental procedures
2. Experimental
Amount of methyl glucoside after reaction (mol) × 100 Total amount of glucose monomer in cellulose (mol) (2) The isolated methyl levulinate from distillation was analyzed and characterized using GC/MS (Agilent 6890/5975, USA), FTIR (Bruker TNESOR 27, Germany) and NMR spectromer (Bruker AVANCE 400 MHz, switzerland), respectively.
2.1. Materials and chemicals Microcrystalline cellulose (DP≈200) with an average particle size of 100 µm from Sinopharm Chemical Reagent (Shanghai, China) was used as starting material in this study. Methyl levulinate (99% purity) and methyl glucoside (98% purity) used for calibration were obtained from Alfa Aesar (Tianjin, China) and Yangcun Chemical (Beijing, China), respectively. Other reagents and chemicals were all of analytical grade from Sinopharm Chemical Reagent (Shanghai, China) or Kemiou Chemical Reagent (Tianjin, China), and used without further purification or treatment.
The experiments were carried out in a cylindrical stainless steel pressurized reactor with 100 mL total volume made by Parr instrument company, USA. The reactor was heated in an adjustable electric stove. The temperature of the reactor contents was monitored by a thermocouple connected to the reactor. For each experiment, a 50 mL solution of sulfuric acid in methanol and a given amount of cellulose were introduced into the reactor, which was then brought to the desired temperature by external heating about 25 min and stirred for reaction. After certain reaction time, the reactor was taken from the stove and quenched in an ice cool water bath to terminate the reaction. Then, the sample take from the reactor was filtered and collected the liquid-phase fraction for analysis. The sample with a higher content of methyl levulinate was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distil the heavy products. Each fraction from distillation process was collected for further analysis. 2.3. Analytical approach The amount of methyl levulinate in the liquid-phase sample was determined by a GC (Agilent 6890 instrument, USA) equipped with an 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. Methyl glucoside was analyzed by ion chromatography (DIONEX ICS-3000, USA) equipped with a CarboPac PA1 (2 mm×250 mm) analytical column and an electrochemical detector. An aqueous solution of sodium hydroxide (80 mmol/L) was used as the eluent with a volumetric flow rate of 0.35 mL/min at 30 ◦ C. The yields of methyl levulinate and methyl glucoside on a molar base according to reaction stoichiometry were calculated as the following equations: Methyl levulinate yield (% ) = Amount of methyl levulinate after reaction (mol) × 100 Total amount of glucose monomer in cellulose (mol) (1) Methyl glucoside yield (% ) =
3. Results and discussion 3.1. Synthesis of methyl levulinate from cellulose alcoholysis To understand the alcoholysis process of cellulose in
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methanol medium under mild conditions (6210 ◦ C) catalyzed by extremely low concentration sulfuric acid (60.01 mol/L) and obtain the highest possible yield of methyl levulinate, effects of various process variables including stirring rate, sulfuric acid concentration, temperature and initial cellulose concentration on the reaction performance were conducted as a function of time. In our previous experiments, methyl glucoside was found to be a key intermediate product during the alcoholysis of glucose for methyl levulinate production [19]. Based on this, a target product (methyl levulinate) and an intermediate product (methyl glucoside) after the reaction were analyzed and quantified to monitor the reaction progress in this study, and the results were discussed in the following sections. 3.1.1. Ef fect of stirring rate The solid-liquid phase catalytic reaction system between cellulose and methanol solution of sulfuric acid may suffer from a certain degree of mass transfer limitation that affects the apparent reaction rate. Increasing the stirring rate might increase the contact area of the two phases, and thus removing the interfacial mass transfer resistance [20]. The experiments were conducted by changing stirring rate from zero to 800 rpm, and the results are given in Figure 1. It seems that there was no clear influence of stirring rate on the yield of methyl levulinate except the stirring rate of zero. This finding implied that the interfacial mass transfer resistance between cellulose surface and liquid phase is negligible in the stirring state, and the elevation of stirring rate is not necessary or useless for the synthesis of methyl levulinate.
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concentration ranging from zero to 0.01 mol/L on the yields of methyl glucoside and methyl levulinate with different reaction times at 190 ◦ C. It was found that there were hardly any production of methyl glucoside and methyl levulinate in the absence of exerted catalyst (i.e., without adding sulfuric acid) under our used conditions. However, a high yield of methyl glucoside of about 30% with few amount of methyl levulinate were obtained from non-catalytic degradation of cellulose in supercritical methanol under severe conditions (300−350 ◦ C and 10−43 MPa) based on previous literature reports [14,21]. This difference indicates that process conditions are an important parameter for the alcoholysis of cellulose. On the other hand, it can be observed from Figure 2 that the yields of methyl glucoside and methyl levulinate were enhanced significantly by the addition of extremely low concentration sulfuric acid as catalyst under mild conditions. It is thus clear that the use of a catalyst is much more favourable than the elevations of temperature and pressure for the synthesis of methyl levulinate from cellulose. With the augment of sulfuric acid concentration, the highest yield of methyl glucoside during the reaction and the yield of methyl levulinate at the same reaction time point increased accordingly. Higher sulfuric acid concentration (>0.01 mol/L) was not used here as it would result in serious side reaction on the inter-molecular dehydration of methanol to dimethyl ether [19]. Usually, the formation of dimethyl ether is undesired in practical applications for this technology because the recovery rate of methanol falls off and its low-down boiling point (−24.9 ◦ C) significantly increases the system pressure of requiring more high-grade reactor. In addition, it also seems that as the prolonging of reaction time, the yield of methyl glucoside increased at first and then decreased, while the yield of methyl levulinate was increasing gradually. This is due to that the polymer chains of cellulose are firstly broken down into low molecular weight fragments and then to methyl glucoside in methanol medium, which is further converted into methyl levulinate under experimental conditions. Overall, extremely low concentration sulfuric acid is found to be efficient for the alcoholysis of cellulose to methyl levulinate in methanol medium under mild conditions. An optimized yield of methyl levulinate is expected to reach by combining the following test of temperature. 3.1.3. Ef fect of temperature
Figure 1. Effect of stirring rate on the yield of methyl levulinate. Reaction conditions: initial cellulose concentration, 20 g/L; sulfuric acid concentration, 0.01 mol/L; temperature, 200 ◦ C; time, 180 min
3.1.2. Ef fect of sulfuric acid concentration Extremely low concentration sulfuric acid (60.01 mol/L) was employed as acid catalyst in the alcoholysis of cellulose to methyl levulinate. Figure 2 shows the effect of sulfuric acid
The effect of reaction temperature on the yields of methyl glucoside and methyl levulinate was investigated as a function of reaction time, and the experiments were carried out at 180, 190, 200 and 210 ◦ C, respectively. As seen from Figure 3, the temperature played a positive role in the reaction process for the synthesis of methyl levulinate. High temperature could contribute to the acceleration of chemical reaction rate and the enhancement of conversion efficiency. When the temperature was increased from 180 to 200 ◦ C, there was a significant increase in the yield of methyl levulinate after the same reaction time and the amount of methyl glucoside was decreased accordingly after the reaction time of 60 min.
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Then, the yield of methyl levulinate was observed slightly to rise over 200 ◦ C after the reaction was completed. At lower temperature such as 180 and 190 ◦ C, the yield of methyl levulinate grew smoothly with reaction time, which may need longer time to reach the equilibrium conversion. Increasing
the temperature to 210 ◦ C, the conversion took place at a faster rate and the yield of methyl levulinate increased significantly with the prolonging of time, and the equilibrium point was almost reached by 120 min with the highest methyl levulinate yield of over 50%.
Figure 2. Effect of sulfuric acid concentration on the yields of methyl glucoside (a) and methyl levulinate (b). Reaction conditions: initial cellulose concentration, 20 g/L; temperature, 190 ◦ C; stirring rate, 400 r/min
Figure 3. Effect of temperature on the yields of methyl glucoside (a) and methyl levulinate (b). Reaction conditions: initial cellulose concentration, 20 g/L; sulfuric acid concentration, 0.01 mol/L; stirring rate, 400 r/min
3.1.4. Ef fect of initial cellulose concentration The optimal concentration of substrate is very important for the efficient use of cellulose material and the final concentration of methyl levulinate. A higher concentration is extremely favorable because it can not only enhance the production efficiency of methyl levulinate, but also cut down energy consumption in the separation of methyl levulinate. Figure 4 illustrates the effect of initial cellulose concentration ranging from 10 to 200 g/L on the concentration and yield of methyl levulinate. It can be noted that although a slight drop in methyl levulinate yield was observed, the concentration of methyl levulinate has risen significantly when initial cellulose concentration increased from 40 to 100 g/L. Further increasing substrate concentration resulted in more cellulose available for
conversion, which means higher methyl levulinate concentration to obtain. But unfortunately, the yield of methyl levulinate was found to decrease obviously and the concentration of methyl levulinate increased disproportionally when initial cellulose concentration raised from 100 to 150 and 200 g/L, indicating that cellulose material has not been utilized fully. Similar effect of initial substrate concentration on reaction performance was also observed in some previous studies [22,23]. The reason is probably due to product feedback inhibition, reactivity diminution or insufficient acid for the additional cellulose which led an incomplete reaction. Hence, compromises have to be made between concentration and yield of methyl levulinate for practical usage to reduce production cost, and reasonable initial cellulose concentration can be chosen to be around 100 g/L for the efficient synthesis of methyl levulinate.
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Figure 4. Effect of initial cellulose concentration on the concentration and yield of methyl levulinate. Reaction conditions: sulfuric acid concentration, 0.01 mol/L; temperature, 200 ◦ C; time, 180 min; stirring rate, 400 r/min
3.2. Isolation of products from the methanolysis of cellulose After the reaction was finished, the resulting product mixture was neutralized and dehydrated with calcium oxide, then filtered to collect the liquid-phase products for isolation. Based on the boiling temperatures, the liquid-phase components were isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation, and a
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corresponding schematic are presented in Figure 5. Light fraction (low boiling substances) was separated out firstly by atmospheric distillation at 80 ◦ C, which contained methanol of about 96%, methyl formate and a very small amount of methyl levulinate by GC analysis. This fraction could be reused as the reaction medium without any substantial change in the yield of methyl levulinate. Next, high boiling products in the residual component were found to be very hard to distill off even with high temperature by vacuum distillation, probably due to that they were firmly bound to the solid humins and/or methyl glucoside. For this reason, high boiling paraffin (ndodecane) was added to the residual component to help distil the heavy products, which acted as desorption driving agent for heavy products. At this stage, the component mixture was isolated by vacuum distillation at 145 ◦ C while the heavy fraction was collected. The heavy fraction was divided into two layers automatically. The upper layer containing predominantly n-dodecane (over 99%) which can be reused to help distill the heavy fraction, and the lower layer containing primarily methyl levulinate (over 96%), a very small number of levulinic acid, n-dodecane and unknown substances, were detected by GC/MS. In addition, some residual solids could be obtained after distillation. It was found that most of them (about 86%) were soluble in water. According to the result of ion chromatography, the main component was methyl glucoside. The rest of insoluble substance appeared dark-brown known as humins.
Figure 5. Schematic of the isolation of liquid-phase products by distillation
3.3. Characterization of lower heavy fraction obtained from distillation
teristic absorbances of methyl levulinate. Figure 7 presents the 1 H-NMR spectrum of lower heavy
Some substances may not be found by GC/MS due to the limitations of capillary column used. Here, the lower heavy fraction was further characterized with FTIR and NMR spectrometry to fully confirm the sample composition and methyl levulinate purity. The result of FTIR spectrum is shown in Figure 6 and the corresponding functional groups for typical absorption characteristic peaks are given with an inset in the figure. The sharp bands at 1740 and 1720 cm−1 were assigned to the stretching vibration of C = O groups; the absorbances at 1211 and 1161 cm−1 were attributed to the C–O stretching vibration; the absorbance around 1439 cm−1 was methylene (–CH2 –) stretching vibrational band; and the absorbances around 2955 and 1362 cm−1 were attributed the stretching vibration of methyl (–CH3 ). Other than these bands detected, there was no other obvious absorption characteristic peaks in FTIR spectrum. These bands were consistent with the charac-
Figure 6. FTIR spectrum of lower heavy fraction obtained from distillation
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fraction obtained from distillation using CDCl3 as solvent. The sharp signals at 2.19 and 3.68 ppm were assigned to the protons of methyl (CH3 –). The former corresponded to methyl adjacent to a carbonyl group (–CO–) while the latter was the signal of a methyl next to an ester group (–COO–). The triplet peaks at 2.56, 2.58 and 2.60 ppm were the signals of a methylene (–CH2 –) close to an ester group (–COO–) while protons of another methylene (–CH2 –) near a carbonyl group (–CO–) showed triplet peaks at 2.74, 2.76 and 2.77 ppm. The integral ratio (i.e., hydrogen ratio) of signals at
2.19 ppm : 2.74−2.77 ppm : 2.56−2.60 ppm : 3.68 ppm was 2.98 : 2.04 : 2.02 : 3.00, which was almost identical to the actual hydrogen ratio of groups in methyl levulinate molecules (CH3 COCH2 CH2 COOCH3 ). Beyond that, several very weak peaks were observed at 1.26, 3.33, 3.43, 3.48, 3.78, 3.86 and 4.50 ppm, respectively, implying that a few impurities existed in the lower fraction. It also can be deduced that the molecular formulas of the impurities probably contained alkyl group alkenyl group and C–O linkage. A 13 C-NMR spectrometry was carried out as a
Figure 7. 1 H-NMR spectrum of lower heavy fraction obtained from distillation using CDCl3 as solvent
Figure 8.
13 C-NMR
spectrum of lower heavy fraction obtained from distillation using CDCl3 as solvent
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complementary analysis, and the result is shown in Figure 8. The peak at 77.0 ppm was the signal of CDCl3 solvent. In addition to this, six distinct signals of carbon were detected, which is the same as carbon number of methyl levulinate molecules and corresponded to the groups as follows. The signals at 173.2 and 206.5 ppm were assigned to the ester group carbon (–COO–) and carbonyl carbon (–CO–), respectively. The signals of two methylene carbons (–CH2 –) appeared at 27.7 and 37.9 ppm, respectively. The former was the signal of a carbon adjacent to an ester group (–COO–), and the latter was a carbon next to a carbonyl group (–CO–). The peak at 29.8 ppm was the signal of methyl carbon (–CH3 ) close to carbonyl group (–CO–) while another methyl carbon (–CH3 ) near ester group (–COO–) showed signal at 51.7 ppm. Both the results of 1 H-NMR and 13 C-NMR analysis showed good accordance with the molecular structure of methyl levulinate. Besides the characteristic peaks of methyl levulinate, other signal peaks detected were quite small compared with that of methyl levulinate, implying high purity of methyl levulinate in lower heavy fraction. 4. Conclusions The present research developed an efficient and economical strategy for the synthesis and isolation of methyl levulinate from the alcoholysis of cellulose catalyzed by extremely low concentration sulfuric acid. Experimental results indicated that sulfuric acid concentration, temperature and initial cellulose concentration had significant effects on the alcoholysis of cellulose and synthesis of methyl levulinate. Sulfuric acid concentration of 0.01 mol/L can offer enough acid sites for the completion of reaction. The formation of methyl levulinate was favored in dilute initial cellulose concentration at high temperature. A high methyl levulinate yield of around 50% could be obtained at 210 ◦ C for 120 min with sulfuric acid concentration of 0.01 mol/L and initial cellulose concentration below 100 g/L. The resulting product mixture was isolated by a distillation technique that combines an atmospheric distillation with a vacuum distillation where n-dodecane was added to help distill the heavy fraction. The light fraction including mainly methanol could be reused as the reaction medium without any substantial change in products yields, and methyl levulinate was a major ingredient of lower heavy fraction with a purity over 96%. This study provides a great potential to accelerate the development of industrial conversion of abundant and inexpensive cellulose into levulinate esters as biofuels and/or high-value chemicals.
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Acknowledgements The authors are grateful to the financial support from the National Key Basic Research Program (2010CB732201) from the Ministry of the Science and Technology of China, and the State Key Laboratory Open Foundation of Pulp and Paper Engineering of China (201225).
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