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Concentrated levulinic acid production from sugarcane molasses Shimin Kang, Jinxia Fu, Naifu Zhou, Ribo Liu, Zhezhe Peng, and Yongjun Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03987 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018
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Concentrated levulinic acid production from sugarcane molasses
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Shimin Kang1, Jinxia Fu2, Naifu Zhou1, Ribo Liu1, Zhezhe Peng1, Yongjun Xu1*
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1
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China
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2
School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Donguan,
Hawaii Natural Energy Institute, University of Hawaii, Honolulu, HI, USA
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Abstract: Levulinic acid (LA) is generally produced from biomass through acid hydrolysis and has been
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recognized as one of the top platform chemicals. In this study, concentrated LA was produced from
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sugarcane molasses through superimposed reaction, in which the LA solution generated from hexoses
10
hydrolysis was further utilized as solvent for hydrolysis of sugarcane molasses to produce concentrated
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LA. After 3rd and 5th superimposed reactions, LA solutions with a concentration of 148 g/L and 180 g/L
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were obtained, with an average yield of 30.5 % and 23.9 %, respectively. The LA yield, however, is
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comparably low due to the increase of LA concentration, and the superimposed reaction conditions
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promote the formation of aqueous and solid byproducts.
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Keywords: Levulinic acid, sugarcane molasses, biomass hydrolysis
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1. Introduction
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Levulinic acid (LA) is a platform chemical derived from hexoses through acid catalysis and is considered
18
as one of the top value-added chemicals from biomass1, 2. LA can be utilized to produce valuable
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chemicals and fuel additives (e.g., levulinate esters, δ-amino levulinic acid, succinic acid, diphenolic acid,
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γ-valerolactone and alkanes, etc.) etc.3-6. Lignocellulosic biomass containing 40-55%7-9 of cellulose are
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usually selected for LA production. The hydrolysis process includes three major steps: (1) hydrolysis of
22
biomass to hexoses (MW = 180 g/mol), (2) dehydration of hexoses to 5-hydroxymethylfurfural (HMF)
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and (3) rehydration of HMF to form equal molar formic acid and LA (MW = 116 g/mol)2,3,10. The
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theoretical yield of LA derived from hexoses is 64.4%, and the actual LA yield is usually 50-60% of the
25
theoretical yield10-14. As high loading of lignocellulosic biomass in the reaction solution can cause low LA
26
yield13,15,16, the biomass loading concentration is usually ≤ 200 g/L. Thus, the hexoses concentration is
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generally low, ≤100 g/L, which consequently causes a low final LA concentration, ≤ 40 g/L. It is
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challenging to isolate and purify LA with low concentration due to the high solubility of LA (675 g/L)[17]
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in aqueous solution. Large amount of extraction solvents would be required when a routine solvent
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extraction technology is employed, and this may result in high manufacturing cost, high energy
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requirement and potential environmental problems. In fact, separation and purification of LA from
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aqueous solution has been regarded as a barrier for industrial production18, 19, and it was estimated that
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approximately 50–70% of the total cost for LA production would come from downstream processing.19
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The potential way to decrease the cost of downstream processing is to produce concentrated LA solution
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directly from the hydrolysis reaction.
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Molasses is a major byproduct of sugar manufacturing and accounts for approximately 30% of the
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sugar produced20. The average annual global sugar production was 174 million metric tonnes from 2012
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to 2016, and the average annual production of molasses was about 50–60 million metric tonnes21.
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Molasses is usually used for yeast and ethanol fermentation or animal feed production22, 23. Value-added
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application of molasses, therefore, is essential for the sugar production industry. Molasses is a potential
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feedstock for LA production, as its main constitute is sucrose, that can be easily hydrolyzed to hexoses
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(glucose and fructose)23.
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LA is generally stable under the acidic conditions during glucose dehydration or HMF hydration24,
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and LA can easily dissolve in water to form a co-solvent. A high LA concentration, therefore, might be
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realized in a superimposed reaction, in which the LA solution formed from the hexose hydrolysis reaction
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can be further used as the solvent for additional hexose hydrolysis to produce more LA. The LA formed
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in multiple hexose hydrolysis reactions accumulates in the reaction solution, and concentrated LA can be
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obtained. In this work, a superimposed reaction system (shown in Figure 1) was developed for LA
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production through hydrolysis of sugarcane molasses to obtain concentrated LA (> 150 g/L).
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2 Experimental Section
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2.1 Materials
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Sugarcane molasses was obtained from Donta group, Dongguan, China, and its composition and
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properties are listed in Table S1.
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2.2 Reaction for LA formation
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The reactions in this work include one initial batch reaction and subsequent superimposed reactions
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reusing the reaction solution. The reactions were conducted in a 100 mL polytetrafluoroethylene (PTFE)
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reactor. After loading the samples, the PTFE reactor was placed in an air-circulated oven at given
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temperatures. The reactor temperature reached the given temperatures (± 2 °C) in about 60 min, and the
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reaction time was recorded afterwards. The reactor was cooled down using tap water after the reaction.
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The sequential superimposed reaction process for reusing the reaction solution is shown in Figure 1.
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In the first batch of the reactions, 40.0 mL of sugarcane molasses solution (0.2 mol/L H2SO4, 184.0 g/L
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sugarcane molasses) was added in a 100 mL PTFE reactor, in which the concentration of sugars (sucrose,
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glucose and fructose) is 100 g/L. The reaction was conducted at 180 ± 2°C for 3 h and the reaction
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solution was separated from the solid residues by filtration afterwards. The solid residues were first
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soaked in 100 ± 10 mL mL DI water for about 1 h, and then separated from the aqueous solution by
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filtration and continuous transferring another 100 ± 10 mL DI water for washing. The 200 ± 20 mL
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aqueous solutions collected were labeled as washing solution. 7.36 ± 0.01 g fresh sugarcane molasses was
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then added in the reaction solution for the consequent superimposed reaction following the same
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procedure mentioned above. As ~10% of initial H2SO4 (0.8 mmol H2SO4) was lost in the washing
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solution, 0.8 mmol fresh H2SO4 was added back to the reaction solution before each superimposed
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reaction. This process was repeated until a desired LA concentration was achieved. The yields of solid
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residues, LA and formic acid were calculated based on the initial weight of sugarcane molasses.
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2.3. Analysis
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The concentrations of LA and formic acid in the aqueous solutions were analyzed using high performance
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liquid chromatograph (HPLC, Shimadzu, Japan) with a C18 reversed-phase column. The aqueous
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products were extracted by methylene dichloride and analyzed by Shimadzu QP 2010 Plus gas
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chromatography−mass spectrometer (GC-MS). The functional groups were analyzed using Tensor 27
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Fourier transform infrared spectroscopy (FT-IR, Bruker, Karlsruhe, Germany), and the surface
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morphology of samples was studied using a JEOL JSM-6701F environmental scanning electron
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microscopy (SEM, Tokyo, Japan).
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3. Results and Discussion
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3.1. Conventional batch reaction
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In the conventional batch reaction, the hydrolysis products were separated from the reaction solution
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after the test. Figures 2-4 show the influences of reaction conditions on the concentration of LA and
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formic acid and the yield of solid residues. The acid concentration has more impact on the final LA and
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formic acid concentrations in comparison with reaction time, and a higher acid concentration improves
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the LA formation and results in a higher LA concentration (Figure 2 A). When the acid concentration was
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too low (e.g. 0.05 mol/L H2SO4), the concentration of LA would be very low (65 g/L) even at short reaction time (e.g. 2-3 h). High yields of solid residues, however,
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occurred when no catalyst H2SO4 or low concentration H2SO4 (e.g. 0.05 mol/L) were employed for the
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reaction (Figure 2 C and D). These solid residues were probably produced from hydrothermal
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carbonization due to lack of acid catalyst 25. Thus, a relative high concentration H2SO4 (i.e. 0.2 mol/L)
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was utilized as the catalyst for the following works.
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Temperature is another critical factor that affects the product distribution (Figure 3). Elevated
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temperature (180 oC) increases the LA concentration and accelerates the reaction. The LA concentration,
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however, may decrease when the reaction time is too long or the reaction temperature is too high (e.g. 190
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o
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realized when the temperature is relative mild, 150-160 oC, but a longer reaction time is required, >6 h.
C), indicating existence of side reactions. It is worth noting that a desirable LA concentration can also be
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Theoretically, formic acid is formed along with LA in equivalent molar yield. Formic acid, however,
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is unstable under elevated temperature conditions26. The concentration of formic acid increases at the
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beginning of the reaction, but then it decreases with increase of temperature and/or reaction time (Figure
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3B). It should be noted that the formic acid concentration began to decrease after 4 h at 180 oC with the
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presence of 0.2 mol/L H2SO4 (Figure 3B). This is consistent with the trend of LA concentration change
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(Figure 3A), indicating that the hydrolysis reaction completes after 4 h (0.2 mol/L H2SO4 at 180 oC) and
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further increase of reaction time leads to low concentration of formic acid due to decomposition. The
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solid residues generated from acid hydrolysis were mainly humins, that formed by polymerization of
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hydrolysis intermediates (e.g., glucose and 5-hydroxymethylfurfural)27, and its yield increases with
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reaction time and temperature regardless of the acid concentration level (Figures 2 and 3). These solid
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residues are usually considered as low-value-added byproducts28, even though the recent studies reported
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that humins can be utilized for the preparing adhesive and carbon materials29,30.
112 113
The impacts of process severity on the production of LA, formic acid and solid residues are expressed by the severity factor.
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Severity factor =log (t×exp((T-100)/14.75))-pH.
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The severity factor is a combination of temperature (T, °C), reaction time (t, min), and solution acidity
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(pH), and has been widely employed for evaluating the biomass hydrolysis process31. Figure S1 shows the
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influence of severity factor on the concentrations of LA and formic acid and the yield of solid residues. A
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relatively high severity factor (2.8-5.0) generally accelerates the polymerization reactions and results in
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higher yield of solid residues. The concentration of LA first increased with severity factor but then
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decreases with further increase of severity factor over ~4.3. A relative high LA concentration (>65 g/L)
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can be achieved when the severity factor is in a range of 3.8-4.5. There is, however, no direct relationship
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between the severity factor and the formic acid concentration, as formic acid is unstable, especially at
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high temperatures (e.g., >180 oC).
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As expected, a relative high LA and formic acid concentration can be realized through increasing the
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initial concentration of sugarcane molasses (shown in Figure 4). A high LA concentration, 67 g/L, 85 g/L,
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95 g/L and 113 g/L, was achieved when the initial concentration is high, 184 g/L, 277 g/L, 368 g/L, 552
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g/L, respectively. The increased LA concentration, however, was realized by significantly sacrificing the
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LA yield. The LA yield in the reaction with the above mentioned four initial concentrations of sugarcane
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molasses was 36.5 %, 29.2 %, 24.8% and 18.1%, respectively (seen Table 1). The LA yield decreased
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approximately 50% when the initial sugarcane molasses increased from 184 g/L to 552 g/L. Similar
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phenomena were also observed for formic acid. The formic acid yield has to be sacrificed in order to
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achieve a high concentration of formic acid. Thus, it is not desirable to increase the LA concentration
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through the increase of sugarcane molasses concentration, and superimposed reaction was conducted in
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this investigation as discussed below.
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3.2 Superimposed reaction
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The superimposed reaction was conducted at 180 oC for 3 h with 0.2 mol/L H2SO4 solution (severity
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factor = 4.2). Figure S2 shows the stability of LA in 0.2 mol/L H2SO4 solution at 180 oC. Similar as the
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results reported in previous works24, LA (100 g/L) is stable under the acidic aqueous reaction conditions
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and LA-water co-solvent may be a potential solvent for sugarcane molasses hydrolysis. A high LA
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concentration, therefore, can be realized through reusing the reaction solution containing the acid catalyst
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and LA formed in previous runs. As listed in Tables 1 and 2, the superimposed reaction process can
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effectively increase the LA concentration without significantly sacrificing the LA yield. For example,
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148.1g/L of LA was obtained after the 3rd superimposed reaction with an average yield of 30.5 %, and
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180.2 g/L of LA was achieved after the 5th superimposed reaction with an average yield of 23.9 %. It is
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worth noting that 180.2 g/L is the highest LA concentration reported so far for carbohydrate conversion in
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acidic solution. Similarly, a high concentration of formic acid (>50 g/L) was also obtained in the
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superimposed reactions (listed in Table 2).
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The superimposed reaction has significant advantages in comparison with the one-pot reaction. For
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example, when the sugarcane molasses concentration is 552 g/L, the LA concentration and yield after the
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superimposed reaction were 1.6 times (180 vs. 113) and 1.7 times (30.5% vs. 18.1%) higher than that
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obtained from direct one-pot reaction (Table 1). It is worth noting that approximately 90% of the H2SO4
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catalyst was left in the reaction solutions and can be reused to catalyze the hydrolysis reaction without
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separation processes or other treatment (shown in Figure 1). The mineral acid, however, is usually
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neutralized with alkali (e.g. CaO) and removed as gypsum (CaSO4) after conventional batch reaction 23.
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As most of H2SO4 catalyst was reused in the superimposed reaction, the superimposed reaction is
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considered as a desirable method for producing LA with high concentration.
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Interestingly, the increase of LA and formic acid concentration lead to a slight decrease of LA yield
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in the superimposed reactions (Table 1). Different from formic acid, LA is stable in the acidic water
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solution at 180 oC. The decrease of LA selectivity in the superimposed reactions is probably a major cause
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of the LA yield decrease. The byproducts in the aqueous solution and solid residues formed were also
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analyzed. Under Brønsted acid catalytic condition, the conversion of sucrose includes (I) hydrolysis of
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sucrose to equivalent amounts of glucose and fructose, (II) dehydration of glucose and fructose into HMF,
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and (III) rehydration of HMF to LA23. It should be noted that sucrose can be easily hydrolyzed and form
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the fructofuranosyl cation directly in a biphasic system containing both Lewis and Brønsted acids.32 Pure
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glucose and fructose, therefore, were also employed to react in the aqueous and LA rich-in solution, and
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compared with that of sugarcane molasses. As shown in Figure S3, LA is the major product in the
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conversion of glucose, fructose and sugarcane molasses. The huge peak in the GC-MS spectra results
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from the existence of high concentration LA, and the reason to select the extracted solution with high
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concentration is identify other byproducts with comparably low concentration. Four aqueous byproducts,
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i.e.
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3-methyl-1,2-cyclopentanedione, were detected in the reaction solution after the 5th superimposed
(1)
2-methyl-2-cyclopentenone,
(2)
2,5-hexanedione,
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(3)
gamma.-valerolactone,
(4)
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reaction of sugarcane molasses. Few of (1)-(3) and a relative low content of (4) were detected in the 1st
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run of the superimposed reaction. None of the four aqueous byproducts, however, were found in the
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reactions with glucose and fructose (Figure S3 A and B), indicating that the four byproducts were not
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directly formed from glucose, fructose or LA. Although the formation mechanism of the four byproducts
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is still under investigation, the existence of these byproducts demonstrates the occurrence of side
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reactions in the superimposed reaction system.
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Table S2 shows the results of glucose reactions with solvents containing 100-150 g/L LA or 45 g/L
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formic acid. The presence of LA in the glucose solution inhibits the further LA formation, and the yield of
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newly formed LA decreases with the increase of initial LA concentration in the reaction solution. The
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presence of formic acid in the glucose solution also has similar impacts on the LA formation. In fact,
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more glucose was converted to solid residues in the LA or formic acid rich solutions (Table S2). This is
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consistent with the results listed in Table 2 that the yield of solid residues increased in the sequential
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superimposed reactions. The SEM (Figure 5) and FT-IR (Figure S4) analysis illustrate that all the solid
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residues are accumulated microspheres and have similar functional groups. The microspheres (diameter
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1.5-5 um) after the 5th batch, however, are bigger and accumulates more closely in comparison with the
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microspheres (diameter 1-3 um) after the 1st batch, meaning that the superimposed reactions promote the
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growth of solid residues.
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4. Conclusion
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A superimposed reaction was developed for high concentration LA production from sugarcane molasses.
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A LA solution with 148 g/L and 180 g/L were obtained in the 3rd and 5th superimposed reactions, with an
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average yield of 30.5% and 23.9%, respectively. The superimposed reaction was found to be an effective
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method to realize the LA production with high concentration, but reusing the LA rich reaction solution
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causes the formation of aqueous byproducts and solid residues and sacrifices the LA yield.
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Acknowledgments:
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This article was made possible by Grant Number 21606045 from the National Natural Science
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Foundation of China, Grant Number 2017A030313084 from Natural Science Foundation of Guangdong
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Province of China, and Grant Number 2013508140001 from International Science & Technology
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Cooperation Project of Dongguan.
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Supporting Information
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Figure 1. The superimposed reaction process.
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40 (B)
0.2 mol/L H2SO4
80 (A)
0.1 mol/L H2SO4
70
Formica acid concentration (g/L)
LA concentration (g/L)
50 40 30 20
0.1 mol/L H2SO4 0.05 mol/L H2SO4
30 25 20 15 10 5
10
0
0 1
255
0.2 mol/L H2SO4
35
0.05 mol/L H2SO4
60
2
3
4 5 Reaction time (h)
6
7
1
8
2
3
4 5 Reaction time (h)
6
7
8
27.5 (D)
30 (C) 0.2 mol/L H2SO4
25.0
0.1 mol/L H2SO4
25
Without H2SO4 addition
22.5
0.05 mol/L H2SO4
20.0
20
Yield of residue (wt%)
Yield of solid residue (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
15 10 5
17.5 15.0 12.5 10.0 7.5 5.0 2.5
0
0.0
1
2
3
256
4
5
6
7
8
1
Reaction time (h)
2
3
4
5
6
7
8
Reaction time (h)
257
Figure 2. Impacts of reaction time and acid concentration on the conversion of 184 g/L cane molasses
258
solution at 180 oC.
259
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Energy & Fuels
260 70
28 (B) 26 24
(A)
65
Formic acid concentration (g/L)
LA concentration (g/L)
60 55 o
190 C o 180 C o 170 C o 160 C o 150 C
50 45 40 35 30
20 1
261
22 20 18 16 14 12 10 8 6
o
190 C o 180 C o 170 C o 160 C o 150 C
4 2 0
25
2
3
4
5
6
7
8
1
2
Reaction time (h)
3
4 5 Reaction time (h)
6
7
8
20 (C) 18 16
Yield of solid residue (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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14 12 10
o
190 C o 180 C o 170 C o 160 C o 150 C
8 6 4 2 0 1
262
2
3
4
5
6
7
8
Reaction time (h)
263
Figure 3. Impacts of reaction time and temperature on the conversion of 184 g/L cane molasses solution
264
in a 0.2 M H2SO4 solution
265
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120 (A)
184 g/L
277 g/L
368 g/L
55 (B)
552 g/L
184 g/L
277 g/L
368 g/L
552 g/L
50
100
45
Formic acid concentration (g/L)
LA concentration (g/L)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
80 60 40 20
40 35 30 25 20 15 10 5 0
0
266 267
2
3
4 5 Reaction time (h)
6
7
2
3
4
5
6
7
Reaction time (h)
Figure 4. Influences of cane molasses concentration at 180 oC with 0.2 mol/L H2SO4.
268
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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
269 270 271
Figure 5. SEM image of solid residues from the 1st run (A) and 5th run (B) of the superimposed reactions.
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Page 15 of 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
272
Table 1. Comparison of one-pot batch reaction and superimposed reactions. Reaction conditions
The first run of superimposed
Total cane molasses used for 1 L
Average yield of LA
Average yield of formic
reaction solution (g)
(wt%)
acid (wt%)
184
36.5 ± 0.4
13.5 ± 0.3
368 (=184×2)
33.8 ± 0.5
11.6 ± 0.4
552 (=184×3)
30.5 ± 0.7
10.1 ± 0.4
736 (=184×4)
26.6 ± 1.1
8.1 ± 0.4
920 (=184×5)
23.9 ± 1.3
6.7 ± 0.4
277
29.2 ± 0.9
11.2 ± 0.3
368
24.8 ± 1.0
9.1 ± 0.4
552
18.1 ± 1.1
7.8 ± 0.4
reaction The first two runs of the superimposed reaction The first three runs of the superimposed reaction The first four runs of the superimposed reaction The first five runs of the superimposed reaction One-pot batch reaction of 277 g/L cane molasses One-pot batch reaction of 368 g/L cane molasses One-pot batch reaction of 552 g/L cane molasses
273 274
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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
275
276 277 278
Table 2. Concentrations of LA, formic acid and yield of solid residues in the superimposed reactions. Runs of LA in Formic acid in Yield of LA in Formic acid in superimposed reaction washing solid residue washing reaction reaction. solution solution (g/L)1 (wt%) solution solution (g/L)1 (g/L)1 (g/L)1 1st 64.1 ± 0.3 0.6 ± 0.1 24.1 ± 0.1 0.2 ± 0.1 14.4 ± 0.3 2nd 113.2 ± 0.5 1.6 ± 0.1 39.2 ± 0.2 0.5 ± 0.1 17.7 ± 0.3 3rd 148.1 ± 0.7 1.8 ± 0.1 50.0 ± 0.3 0.5 ± 0.1 20.4 ± 0.4 4th 165.3 ± 1.0 2.1 ± 0.1 51.1 ± 0.3 0.5 ± 0.1 21.3 ± 0.4 5th 180.2 ± 1.3 2.5 ± 0.1 51.8 ± 0.3 0.5 ± 0.1 25.8 ± 0.5 1 The volumes of all reaction solutions and washing solutions are 40 mL and 200 mL, respectively.
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