DOI: 10.1002/cssc.201403385
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Governing Chemistry of Cellulose Hydrolysis in Supercritical Water Danilo A. Cantero, M. Dolores Bermejo, and M. Jos Cocero*[a] masses (cellulose, glucose, fructose, and wheat bran). It was found that the H + and OH ion concentration in the reaction medium as a result of water dissociation is the determining factor in the selectivity. The reaction of glucose isomerization to fructose and the further dehydration to 5-HMF are highly dependent on the ion concentration. By an increase in the pOH/pH value, these reactions were minimized to allow control of 5-HMF production. Under these conditions, the retroaldol condensation pathway was enhanced, instead of the isomerization/dehydration pathway.
At extremely low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (T = 400 8C and P = 25 MPa) to obtain a sugar yield higher than 95 wt %, whereas the 5-hydroxymethylfurfural (5-HMF) yield was lower than 0.01 wt %. If the reaction time was increased to 1 s, the main product was glycolaldehyde (60 wt %). Independently of the reaction time, the yield of 5-HMF was always lower than 0.01 wt %. To evaluate the reaction mechanism of biomass hydrolysis in pressurized water, several parameters (temperature, pressure, reaction time, and reaction medium) were studied for different bio-
Introduction Biomass exploitation as a raw material is growing as an alternative sustainable method of production of fuels and chemicals.[1] Cellulose is one of the main compounds of biomass and represents the most abundant biopolymer.[2] An important challenge in the processing of cellulosic biomass is to hydrolyze the b(14) glucose–glucose bond to produce a stream of sugars with a low concentration of byproducts, by using an efficient process.[3] These sugar streams could be further transformed into valuable chemicals such as pyruvaldehyde, glycolaldehyde,[4] 5-hydroxymethylfurfural (5-HMF),[5] organic acids, or polyalcohols.[6] Acid and enzymatic hydrolysis of cellulose are two conventional methods that need long treatment times (t 3 h) to obtain a product with poor selectivity (< 60 wt %).[7] The use of ionic liquids as both solvent and reaction medium has been intensively studied owing to the possibility of dissolving cellulose and making it more “accessible” for the hydrolysis reaction.[8] However, this kind of process takes at least 3 h of hydrolysis to obtain a selectivity near to 30 wt % of reducing sugars.[8a] These processing methods require long reaction times (hours), which will demand big reactors during the scale up. The use of pressurized water as the reaction medium for the processing of cellulosic biomass in a one-step fast process is an alternative. Total hydrolysis of cellulose can be achieved in a reaction time of 0.02 s in a supercritical water medium and produces a stream of water-soluble sugars with a low concentration of derived products (< 2 wt %).[9] This kind
of process represents an advantageous intensification that will reduce the energetic and equipment requirements in the scale up. Cellulose depolymerization in hot pressurized water has been done in different kinds of reactors (batch, semibatch, and continuous) at different temperatures and pressures, with or without catalysts.[10] The yield of sugars after biomass hydrolysis is enhanced by using supercritical water reactors operated in continuous mode at high temperature for short reaction times.[9b, 11] The combination of these two parameters is crucial for obtaining high yields of sugars. With long reaction times, the sugars are derived; at low reaction temperatures, several side reactions take place and produce many compounds. In fact, it was observed that some reactions are avoided at supercritical conditions. In particular, attention should be paid to the formation of 5-HMF. The production of 5-HMF from cellulose in pressurized water is highly dependent on the reaction temperature. In Figure 1, several experimental results are shown for cellulose hydrolysis in pressurized water from T = 300–400 8C at different pressures and reaction times. It can be observed that 5-HMF production was faster, but the yield was lower, when the reaction temperature was increased from 300 to 350 8C. The reaction time was reduced from 40 to 10 s by increasing the reaction temperature. This behavior was expected and it follows the Arrhenius law. However, an expected behavior was detected if the reaction temperature was increased over the critical point of water: the production of 5-HMF was highly avoided. Although this behavior was previously described in the literature,[4a,b, 9a, 12] a clear and quantitative explanation has not been developed yet. The different discovered behaviors can be classified into three main groups. 1) The maximum amount of 5-HMF from cellulose in pressurized water without catalyst is produced at tem-
[a] Dr. D. A. Cantero, Dr. M. D. Bermejo, Prof. Dr. M. J. Cocero Department of Chemical Engineering and Environmental Technology University of Valladolid Dr. Mergelina S/N, 47005, Valladolid (Spain) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403385.
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Full Papers actions that water participates in as a reagent or in the formation of intermediate states.[13] Another important property of water as a reaction medium is the ion product (mol2 kg2), which represents how dissociated the water molecules are (the ion concentration). If the molal concentration of OH ions (square root of the ionic product) is multiplied by the density, the molar concentration of protons or hydroxide anions in the medium is obtained. This concentration parameter includes both the variations in water volume and water dissociation. The concentration of OH ions (which is the same for H + ions) in the proximity of the critical point of water is plotted in Figure 2.
Figure 1. 5-HMF yield from cellulose hydrolysis in pressurized water against reaction time (tr). Experimental temperatures: red: 400 8C; yellow: 350 8C; and blue: 300 8C. Experimental pressures: ^: 27 MPa; &: 25/23 MPa; and ~: 23/18 MPa.
peratures lower than 300 8C.[9a, 12b,h,j,l] An increase in temperature benefits the retroaldol condensation reactions of fructose.[12l] 2) The production of 5-HMF is enhanced by increasing the availability of protons (H + ) in the reaction medium by addition of acids.[12d–g,i] 3) The production of 5-HMF is enhanced in a pressurized water medium if the pressure is increased at a constant temperature.[4a,b] The aim of this work was to study the reactions of cellulose hydrolysis, with a focus on 5-HMF production from sugars. The yields were analyzed from a chemical point of view of the reaction pathway. Several reactions were performed to obtain accurate data. Cellulose hydrolysis was tested at T = 300, 325, 350, 375, and 400 8C at 25 MPa of pressure. Also, the pressure effect was tested at T = 300, 350, and 400 8C between P = 18– 27 MPa. Studies were also conducted to analyze glucose and fructose hydrolysis in pressurized water between T = 300– 400 8C at 25 MPa. Finally, the results were contrasted with the products obtained from wheat-bran hydrolysis in supercritical water. A reaction pathway was developed, and a novel kinetic model was tested for understanding the behavior of glucose and fructose reactions in supercritical water.
Figure 2. Hydroxy ion concentration (mol L1) against temperature and pressure. pOH = log(j OH j) = pH.[14] Water density was calculated according to the International Association for the Properties of Water and Steam industrial formulation,[14a] whereas the molal ionic product of water was calculated by following the “International Formulation of Ionic Product of Water Substance”.[14b]
Important changes in the identity of the medium can be obtained if the temperature and pressure are changed at the same time. For example, the density of water at T = 300 8C and P = 27 MPa is around 750 kg m3 ; this value can be decreased to 130 kg m3 if the conditions are modified to T = 400 8C and P = 23 MPa. The H + /OH concentration varies by six orders of magnitude in the region of the critical point, which allows the possibility of working with markedly different reaction mediums. The H + /OH concentration at T = 300 8C and P = 23 MPa is around 2 106 mol L1, which means that the medium has a high concentration of ions ([H + ] and [OH]) and ionic reactions are favored.[4a, 15] The H + /OH concentration will take a value of 5.5 1012 mol L1 if the temperature and pressure are changed to 400 8C and 23 MPa; this reaction medium would favor radical reactions.[16] The reactions were assumed to follow the reaction pathway shown in Scheme 1. This reaction pathway was built by following the schemes developed in Reference [12b]. The reaction of
Results and Discussion Reaction medium and reaction pathway Supercritical water is water at temperature and pressure values above the critical points (Tc = 374 8C and Pc = 22.1 MPa). In the proximity of the critical points, the properties of water can be highly influenced by changing pressure and temperature, so the identity of the medium can be modified without changing the solvent. The medium density represents the quantity of water per volume unit (kg m3); this is a measurement of water concentration, an important factor to take into account in re-
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Full Papers To evaluate the medium effect in the selectivity, the model was solved in three different ways: 1) by considering only the concentration of cellulose and its derived products, 2) by considering also the water concentration, and 3) by considering the concentration of cellulose, its derived products, and the proton or hydroxide anion concentrations in the reactions of glucose isomerization and fructose dehydration. Therefore, the concentration of fructose (nf) was calculated according to Equation (2) or Equation (3) in Scheme 1. Main reaction pathways of cellulose hydrolysis in pressurized water. the resolution of models 2 or 3, respectively, for which nw is the glucose isomerization occurs through ring opening and keto– water concentration (mol L1), nOH-H is the concentration of enol tautomerism. These reactions take place to form transition OH or H + ions in the medium (mol L1), ng is the concentra + states with OH or H ions. Also, fructose dehydration takes tion of glucose, and kgf, kfh, and kfg are the kinetic constants of place to form transition states incorporating H + ions (one per the reactions indicated in Scheme 1. H2O molecule lost).[17] To identify these reactions in Scheme 1, dnf the symbols OH/H + have been added above the reaction ð2Þ ¼ kgf ng nw kfh nf nw kfg nf dt arrow. The production of glycolaldehyde was enhanced at supercritical conditions because the hydroxide/proton concentradnf ð3Þ ¼ kgf ng nOHH kfh nf nOHH kfg nf tion is highly decreased (pH = pOH = 13) and so is the concendt tration of fructose and its derived products. Although the reaction of glucose isomerization is avoided at low concentrations of hydroxide anions, a fructose yield of near to 10 wt % was Sugar production from cellulose hydrolysis obtained at supercritical conditions. As shown in Scheme 1, fructose can follow two main reacThe evolution of the sugar yield along with reaction time can tion pathways: fructose dehydration or retroaldol condensabe seen in Figure 3 (see also Tables S.1–S.10 in the Supporting tion. The second reaction was more beneficial than the first Information). The best conditions to obtain soluble sugars (up one because, in this way, glyceraldehyde was obtained as the main product from fructose. The maximum quantity of 5-HMF was obtained by working at T = 350 8C and P = 23 MPa (pH = pOH = 6) for a reaction time of 10 s. Under those conditions, the yield was around 15 wt %. A reaction model was built by considering cellulose as the starting material. The concentration of each compound shown in Scheme 1 was calculated against the reaction time. The difference between the calculated concentrations and the experimental ones was minimized to obtain the kinetic constant of the reactions.[9a] Equation (1) describes the evolution of compound i along with reaction time, for which ni and nj (mol L1) are the concentrations of compounds i and j, respectively, t (s) is the time, kji (s1) is the kinetic constant of the reaction in which compound j reacts to produce compound i, and kio is the kinetic constant of the reaction in which compound i reacts to produce a derived product.
n n X dni X kji nj kio ni ¼ dt 1 1
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Figure 3. Sugar yields from cellulose hydrolysis in pressurized water against reaction time. Experimental temperatures: red: 400 8C; yellow: 350 8C; and blue: 300 8C. Experimental pressures: ^: 27 MPa; &: 25/23 MPa; and ~: 23/ 18 MPa.
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Full Papers to six glucose units) from the hydrolysis of cellulose were achieved by working at T = 400 8C with extremely short reaction times (t = 0.015 s). If the reaction time was increased, the sugars were hydrolyzed and the yield decreased, as shown in Figure 4. At T = 400 8C, the yield of soluble sugars was higher
sis would have a sugar yield of between 80–98 wt % if the reaction time was between 0.015–0.2 s. The best combination of conditions to obtain a high yield of glycolaldehyde can be achieved by working at T = 400 8C and P = 23 MPa with a reaction time of 1 s. In those conditions, the yield of glycolaldehyde was around 60 wt %. Sometimes, the production of 5-HMF is undesired, especially if a microorganism postmodification process is needed.[19] At T = 400 8C, the 5-HMF production was highly avoided for all studied pressures. Independently of the residence time (0.015 s for sugars or 1 s for glycolaldehyde), the concentration of 5-HMF was lower than 0.1 wt %. Reaction model evaluation: Cellulose hydrolysis at P = 25 MPa for different temperatures The reaction pathway shown in Scheme 1 was kinetically evaluated by following Equations (1), (2), or (3). The results for the three tested models are shown in Figure 4. The behavior of the reaction models will be compared by means of the results for the kinetic constants of fructose dehydration to 5-HMF. The obtained kinetic constants for the reaction rates kog, kgg, and kfg (as indicated in Scheme 1) obtained by the resolution of model 1 follow the Arrhenius law. However, the kinetics of glucose isomerization and fructose dehydration showed a break point near the critical point of water. As can be observed in Figure 4 A, the model linearly predicts the kinetic constants of fructose dehydration at subcritical temperatures. However, near the critical point of water a break point in the kinetic was observed, which represents a deviation from the Arrhenius law. This phenomenon could be predicted because of the low concentration of 5-HMF in the products. Moreover, the concentration profiles for 5-HMF obtained at T = 400 8C were lower than those found at T = 350 or 300 8C (these concentration profiles can be seen in the Supporting Information). This deviation from the Arrhenius behavior suggests that another chemical effect is not taken into account in the kinetic evaluation. To solve this problem, the water or ion concentrations were considered as well as the reagent concentration. The reactions of glucose and fructose in near critical water are usually analyzed together with the medium density to explain the selectivity of the process.[3b, 4a,b, 9a] Qualitatively, the theory of water transition states works well: if the medium density is low, the concentration of 5-HMF will be low. To quantify the participation of water in the reaction, the water concentration was added as a reagent concentration in the reactions of glucose isomerization and fructose dehydration (model 2). Unfortunately, the obtained kinetic constants with consideration of the water concentration as a reagent concentration did not follow the Arrhenius law at sub- or supercritical water (Figure 4 B). Thus, from a quantitative point of view, the theory of water transition states would not explain the low concentration of 5-HMF at supercritical conditions. Finally, the kinetic constants of the whole reaction system followed the Arrhenius law if model 3 was resolved (Figure 4 C). After this evaluation, it can be concluded that the reactions of glucose isomerization and fructose dehydration are highly dependent on
Figure 4. Kinetic constants for fructose dehydration to 5-HMF at P = 25 MPa and T = 300–400 8C. A) Kinetic evaluation with consideration of the biomass concentration as the reagent concentration. B) Kinetic evaluation with consideration of the biomass and water concentrations as the reagent concentration. C) Kinetic evaluation with consideration of the biomass and ion concentrations as the reagent concentration. Error bars represent the experimental and fitting errors.
than 95 wt %. Similar methods of cellulose hydrolysis in pressurized water were developed in the literature with sugar selectivities of lower than 77 wt %.[11, 12j, 18] The combination of the supercritical water medium and the effective method of reaction-time control presented in this work allow cellulose hydrolysis a high yield of sugars. This is because, under those conditions, the cellulose hydrolysis kinetics are fast enough and the glucose hydrolysis kinetics are slow enough to allow our reactor to stop the reactions after total hydrolysis and before glucose degradation.[9b] It was observed that the cellulose hydroly-
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Full Papers the ion concentration in the reaction medium. This phenomenon was also observed for the reaction of methyl tert-butyl ether hydrolysis in sub- and supercritical water.[20] The kinetic constants for cellobiose hydrolysis (kh), oligosaccharides hydrolysis (kog), glucose isomerization to fructose (kgf), and glucose retroaldol condensation (kgg) are plotted in Figure 5 A–D, respectively. It can be observed that the resolution of model 3 produces a series of kinetic constants that follows the Arrhenius law for the whole system. The dotted lines in Figure 5 represent the uncertainty of the kinetic constants with consideration of the experimental errors and the fitting errors. It is important to take into account the dimension of the kinetic constant. For model 3 resolution, the kh, kog, and kgg constants have the same units (s1). However, the kgf and khmf constants have second-order units (L mol1 s1). This is because these two reactions were considered to be dependent on the concentrations of two reagents. In addition, the obtained values of the kgf and khmf constants were higher than the values of the kh, kog, and kgg constants. To describe the kinetic behavior against temperature for the different reactions, the activation energy (Ea) and the pre-exponential factor (ln ko) were calculated. These parameters are shown in Table 1 for all of the kinetics fitted. It can be also ob-
Table 1. Activation energy and pre-exponential factor for glucose reactions in pressurized water medium. Kinetic constant
Ea [kJ mol1]
Error [kJ mol1]
ln ko
Error
kh [s1] kog [s1] kgf [L mol1 s1] kgg [s1] kfh [L mol1 s1]
90.4 145.5 566.3 147.4 348.1
4.5 6.1 34.1 7.4 26.0
17.4 28.6 127.3 28.7 84.8
0.9 1.2 6.6 1.4 5.0
served in this table that the error in the Arrhenius parameters is around 5 % with consideration of the experimental and fitting uncertainties. Also, as previously analyzed for the kinetic constants of kgf and khmf, the values of the Arrhenius parameters for these constants were higher than for the others. Reaction model evaluation: Cellulose hydrolysis at different pressures and temperatures Figure 5. Kinetic constants for cellulose hydrolysis at P = 25 MPa and T = 300–400 8C. A) Kinetic constants of cellobiose hydrolysis. B) Kinetic constants of oligosaccharide hydrolysis. C) Kinetic constants of glucose isomerization to fructose. D) Kinetic constants of glucose retroaldol condensation. The dotted line represents the uncertainty of the kinetics behavior.
The evaluation of the reaction model was also tested for the reactions of cellulose hydrolysis at different pressures and temperatures. The cellulose hydrolysis was tested at T = 300 8C (P = 18, 23, and 27 MPa), T = 350 8C (P = 18, 23, and 27 MPa), and T = 400 8C (P = 23, 25, and 27 MPa). A comparison of the three model results can be seen in Figure 6 A. Once again, the outcomes of models 1 and 2 did not follow the Arrhenius law. The fructose dehydration kinetics followed the Arrhenius law if model 3 was used to solve the kinetics system. Although the results shown in Figure 6 A correspond to the P = 27 MPa ChemSusChem 0000, 00, 0 – 0
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series, the results of model 3 follow the Arrhenius law for all experimental pressures, as shown in Figure 6 B. It should be taken into account that the experiments were always done in pressurized liquid or the supercritical phase. This is because the series at P = 18 MPa was only tested at T = 300 and 350 8C. 5
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Figure 6. A) Kinetic constants of fructose dehydration with consideration of OH concentration (~), with consideration of water concentration (Cw) (&), and without consideration of the OH or water concentrations (*). P = 27 MPa. B) Kinetic constant of fructose dehydration with consideration of OH concentration at P = 27 (*), 25 (&), 23 (~), and 18 MPa (^). Error bars represent the experimental and fitting errors.
Figure 7. Glucose hydrolysis kinetic constants at T = 350, 385, and 400 8C. P = 25 MPa. A) Kinetic constants of glucose isomerization to fructose (~) and fructose dehydration (&) with consideration of the OH concentration as a reagent concentration. B) Kinetic constants of glucose retroaldol condensation (&), fructose retroaldol condensation (^), and the formation of pyruvaldehyde from glyceraldehyde (*). Error bars represent the experimental and fitting errors.
Reaction model evaluation: Glucose hydrolysis at P = 25 MPa for different temperatures
merization and fructose dehydration took similar orders of magnitude for the different analyzed situations.
The reaction pathway and the kinetic model developed for cellulose hydrolysis were also tested by analyzing glucose reactions in pressurized water. Glucose hydrolysis reactions were tested at P = 25 MPa of pressure and at temperatures around the critical point of water (T = 350, 385, and 400 8C), at which point the change in the ionic product of water is the highest. The fitted kinetic constants of kgf and khmf are plotted against temperature in Figure 7 A. The fitted kinetic constants of kgg, kgp, and kfg are plotted against temperature in Figure 7 B. As expected, the obtained kinetic constants follow the Arrhenius law if the concentration of ions is considered as a reagent concentration (model 3). The reaction mechanism proposed in this work was tested in three different ways: cellulose hydrolysis with constant pressure and changing temperature, cellulose hydrolysis with changing pressure and temperature, and glucose hydrolysis with constant pressure and changing temperature near the critical point of water. For the three situations, the kinetic constants of the glucose hydrolysis reactions follow the Arrhenius parameters if the ion concentration of the medium is taken into account. In addition, the kinetic constants of glucose iso-
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Testing the concept with modified reaction mediums and natural biomass Finally, to test the developments in the kinetics mechanism, fructose was hydrolyzed in modified reaction mediums (with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and oxalic acid). In addition, wheat bran was hydrolyzed in supercritical water for testing the production of 5-HMF from a natural biomass in supercritical water. The yields of the main products obtained after fructose hydrolysis are shown in Figure 8. The experiments were carried out in the experimental setup explained above. However, the reaction medium was modified by pumping TEMPO or oxalic acid to the reactor. TEMPO is a free radical kidnapper that is usually employed to control radical reactions in organic synthesis and polymerization.[21] Oxalic acid was used to increase the concentration of ions in the reaction medium. As expected, the production of 5-HMF from fructose at supercritical conditions (T = 400 8C and P = 23 MPa) was negligible, with pyruvaldehyde as the main product after 0.9 s of reaction time. The addition of oxalic acid to the reaction medium increased the 6
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Full Papers soluble sugars or 60 wt % of glycolaldehyde. From the viewpoint of chemistry, the selectivity of the process is governed by the ion concentration in the reaction medium. A reaction mechanism model was built and intensively tested to demonstrate its reliability. The reactions of glucose and the fructose retroaldol condensation are not very demanding of ions. In fact, these reactions are highly improved if the water molecules (reaction medium and reagent) are highly associated. On the other hand, the isomerization reactions of glucose–fructose and the dehydration reactions of these sugars are extremely diminished if the water molecules are associated. For first time, the reasons why the production of 5-HMF is highly avoided at supercritical water conditions were quantitatively explained and demonstrated. It was achieved by adding the concentration of protons or hydroxide ions as a result of water dissociation as a reagent concentration into the kinetic modeling of the reactions. The extraordinary changes in the chemical and physical properties of supercritical water allow biomass hydrolysis with the choice of desired products by simply selecting the correct reaction temperature and pressure.
Figure 8. Yields of fructose hydrolysis at T = 400 8C, P = 23 MPa, and t = 0.9 s. The reaction medium was modified with TEMPO or oxalic acid.
availability of protons in the medium, which would promote the fructose dehydration reaction. In fact, if the reaction medium was acidified, the production of 5-HMF in supercritical water was enhanced to 15 wt %. The same behavior was observed with TEMPO as the reaction-medium modifier. This free radical kidnapper has an acid role as a result of the dissociation of the OH group bonded to the nitrogen atom. Once again, an acid medium promoted the production of 5-HMF, which corroborated the fact that 5-HMF production is highly dependent on the proton availability in the medium. Wheat bran was also hydrolyzed in the aforementioned experimental setup. The reaction temperature was set at 400 8C with a reactor pressure of 25 MPa. The reaction time was varied from 0.19 s to 0.69 s. Fortunately, as can be seen in Figure 9, the yields of 5-HMF were lower than 0.05 wt %. A detailed description of wheat-bran hydrolysis for sugar and lignin production can be found in a previous work.[22]
Experimental Section The experiments were carried out in a continuous pilot plant that was able to work at temperatures up to 425 8C and pressures up to 30 MPa. A schematic diagram of the process is shown in Figure 10. Two streams continuously fed a microreactor: a cellulose
Figure 10. Schematic diagram of the supercritical water hydrolysis facility.
stream and a supercritical water stream. Strict control of the reaction times was achieved by a combination of three factors: 1) rapid heating by supercritical water injection of the cellulose suspension stream, 2) rapid cooling by sudden depressurization down to atmospheric pressure and T 100 8C by using a micrometering valve that was able to stand temperatures up to T = 425 8C, and 3) selection of a series of tubular reactors of different accurately determined volumes. The volume of the used reactors varied from 0.12– 64.5 mL, which in combination with flows of between 1–2 g s1 and taking into account the density of water under the experimental conditions, gives reaction times of 0.004–40 s. A detailed description of the experimental setup is given in the Supporting Information. The reactor is fed from two streams (biomass and water); however, no extra water is needed in the process once the steady state is achieved. As can be seen in Figure 10, after the reactor, a flash chamber separator produces two streams: vapor
Figure 9. Yields of wheat-bran hydrolysis at T = 400 8C and P = 25 MPa for reaction times of between t = 0.19 and 0.69 s.
Conclusions The process presented in this work shows an efficient alternative to hydrolyze cellulose selectively. The control of the reaction time is the key to obtain yields higher than 95 wt % of ChemSusChem 0000, 00, 0 – 0
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[10] D. A. Cantero, M. D. Bermejo, M. J. Cocero, J. Supercrit. Fluids 2015, DOI: 10.1016/j.supflu.2014.1007.1003. [11] M. Sasaki, T. Adschiri, K. Arai, AIChE J. 2004, 50, 192 – 202. [12] a) M. Sasaki, B. Kabyemela, R. Malaluan, S. Hirose, N. Takeda, T. Adschiri, K. Arai, J. Supercrit. Fluids 1998, 13, 261 – 268; b) M. Sasaki, K. Goto, K. Tajima, T. Adschiri, K. Arai, Green Chem. 2002, 4, 285 – 287; c) M. Sasaki, Z. Fang, Y. Fukushima, T. Adschiri, K. Arai, Ind. Eng. Chem. Res. 2000, 39, 2883 – 2890; d) M. J. Antal, Jr., W. S. L. Mok, G. N. Richards, Carbohydr. Res. 1990, 199, 91 – 109; e) F. S. Asghari, H. Yoshida, Ind. Eng. Chem. Res. 2007, 46, 7703 – 7710; f) F. S. Asghari, H. Yoshida, Carbohydr. Res. 2010, 345, 124 – 131; g) M. Bicker, D. Kaiser, L. Ott, H. Vogel, J. Supercrit. Fluids 2005, 36, 118 – 126; h) D. Brçll, C. Kaul, A. Krmer, P. Krammer, T. Richter, M. Jung, H. Vogel, P. Zehner, Angew. Chem. Int. Ed. 1999, 38, 2998 – 3014; Angew. Chem. 1999, 111, 3180 – 3196; i) V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko, V. Nikolakis, N. S. Marinkovic, A. I. Frenkel, S. I. Sandler, D. G. Vlachos, J. Am. Chem. Soc. 2013, 135, 3997 – 4006; j) K. Ehara, S. Saka, J. Wood Sci. 2005, 51, 148 – 153; k) F. Jin, Z. Zhou, T. Moriya, H. Kishida, H. Higashijima, H. Enomoto, Environ. Sci. Technol. 2005, 39, 1893 – 1902; l) D. Klingler, H. Vogel, J. Supercrit. Fluids 2010, 55, 259 – 270; m) X. L, S. Saka, J. Supercrit. Fluids 2012, 61, 146 – 156; n) T. Sakaki, M. Shibata, T. Miki, H. Hirosue, N. Hayashi, Energy Fuels 1996, 10, 684 – 688. [13] M. Akizuki, T. Fujii, R. Hayashi, Y. Oshima, J. Biosci. Bioeng. 2014, 117, 10 – 18. [14] a) W. Wagner, R. J. Cooper, A. Dittmann, J. Kijima, J. H. Kretzschmar, A. Kruse, R. Mares, K. Oguchi, H. Sato, I. Stocker, O. Sifner, Y. Takaishi, I. Tanishita, J. Trubenbach, T. Willkommen, J. Eng. Gas Turbines Power 2000, 122, 150 – 182; b) W. L. Marshall, E. U. Franck, J. Phys. Chem. Ref. Data 1981, 10, 295 – 304. [15] a) N. Akiya, P. E. Savage, Chem. Rev. 2002, 102, 2725 – 2750; b) A. Kruse, A. Gawlik, Ind. Eng. Chem. Res. 2003, 42, 267 – 279; c) H. Weingrtner, E. U. Franck, Angew. Chem. Int. Ed. 2005, 44, 2672 – 2692; Angew. Chem. 2005, 117, 2730 – 2752. [16] C. Promdej, Y. Matsumura, Ind. Eng. Chem. Res. 2011, 50, 8492 – 8497. [17] R. S. Assary, T. Kim, J. J. Low, J. Greeley, L. A. Curtiss, Phys. Chem. Chem. Phys. 2012, 14, 16603 – 16611. [18] a) Y. Zhao, W.-J. Lu, H.-T. Wang, Chem. Eng. J. 2009, 150, 411 – 417; b) S. Kumar, R. B. Gupta, Ind. Eng. Chem. Res. 2008, 47, 9321 – 9329. [19] C. Schacht, C. Zetzl, G. Brunner, J. Supercrit. Fluids 2008, 46, 299 – 321. [20] J. D. Taylor, J. I. Steinfeld, J. W. Tester, Ind. Eng. Chem. Res. 2001, 40, 67 – 74. [21] C. J. Hawker, Acc. Chem. Res. 1997, 30, 373 – 382. [22] D. A. Cantero, C. Martinez, M. D. Bermejo, M. J. Cocero, Green Chem. 2015, 17, 610 – 618.
Acknowledgements The authors thank the Spanish Ministry of Economy and Competitiveness for the Projects CTQ2013-44143-R, CTQ2011-27347, and ENE2012-33613. The authors thank Repsol for technical support. M.D.B. thanks the Spanish Ministry of Economy and Competitiveness for the Ramn y Cajal research fellowship RYC-2013-13976 Keywords: biomass · carbohydrates · hydrolysis · kinetics · water chemistry [1] A. J. Ragauskas, C. K. Williams, B. H. Davison, G. Britovsek, J. Cairney, C. A. Eckert, W. J. Frederick, J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz, R. Murphy, R. Templer, T. Tschaplinski, Science 2006, 311, 484 – 489. [2] D. Klemm, B. Heublein, H. P. Fink, A. Bohn, Angew. Chem. Int. Ed. 2005, 44, 3358 – 3393; Angew. Chem. 2005, 117, 3422 – 3458. [3] a) J. Tollefson, Nature 2008, 451, 880 – 883; b) K. Arai, R. L. Smith, Jr., T. M. Aida, J. Supercrit. Fluids 2009, 47, 628 – 636; c) K. Barta, P. C. Ford, Acc. Chem. Res. 2014, 47, 1503 – 1512. [4] a) T. M. Aida, Y. Sato, M. Watanabe, K. Tajima, T. Nonaka, H. Hattori, K. Arai, J. Supercrit. Fluids 2007, 40, 381 – 388; b) T. M. Aida, K. Tajima, M. Watanabe, Y. Saito, K. Kuroda, T. Nonaka, H. Hattori, R. L. Smith, Jr., K. Arai, J. Supercrit. Fluids 2007, 42, 110 – 119; c) B. M. Kabyemela, T. Adschiri, R. M. Malaluan, K. Arai, Ind. Eng. Chem. Res. 1999, 38, 2888 – 2895; d) B. M. Kabyemela, T. Adschiri, R. M. Malaluan, K. Arai, H. Ohzeki, Ind. Eng. Chem. Res. 1997, 36, 5063 – 5067. [5] a) A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411 – 2502; b) R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres, J. G. de Vries, Chem. Rev. 2013, 113, 1499 – 1597. [6] a) A. M. Ruppert, K. Weinberg, R. Palkovits, Angew. Chem. Int. Ed. 2012, 51, 2564 – 2601; Angew. Chem. 2012, 124, 2614 – 2654; b) C. Luo, S. Wang, H. Liu, Angew. Chem. Int. Ed. 2007, 46, 7636 – 7639; Angew. Chem. 2007, 119, 7780 – 7783. [7] a) L. Shuai, X. Pan, Energy Environ. Sci. 2012, 5, 6889 – 6894; b) C. E. Wyman, B. E. Dale, R. T. Elander, M. Holtzapple, M. R. Ladisch, Y. Y. Lee, Bioresour. Technol. 2005, 96, 2026 – 2032. [8] a) R. Rinaldi, R. Palkovits, F. Schth, Angew. Chem. Int. Ed. 2008, 47, 8047 – 8050; Angew. Chem. 2008, 120, 8167 – 8170; b) S. Morales-delaRosa, J. M. Campos-Martin, J. L. G. Fierro, Chem. Eng. J. 2012, 181 – 182, 538 – 541. [9] a) D. A. Cantero, M. D. Bermejo, M. J. Cocero, J. Supercrit. Fluids 2013, 75, 48 – 57; b) D. A. Cantero, M. D. Bermejo, M. J. Cocero, Bioresour. Technol. 2013, 135, 697 – 703.
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FULL PAPERS Just add water: A reaction mechanism for cellulose hydrolysis that can explain the huge selectivity of biomass hydrolysis in supercritical water is presented. The model of the reaction mechanism has been validated by several experiments carried out in a continuous pilot plant capable at various conditions. It was found that the proton and hydroxide anion concentration in the medium due to water dissociation (represented by the ionic product of water, Kw) is the determining factor in the selectivity of the process.
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D. A. Cantero, M. D. Bermejo, M. J. Cocero* && – && Governing Chemistry of Cellulose Hydrolysis in Supercritical Water
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Governing Chemistry of Cellulose Hydrolysis in Supercritical Water Danilo A. Cantero, M. Dolores Bermejo, and M. Jos Cocero*[a] masses (cellulose, glucose, fructose, and wheat bran). It was found that the H + and OH ion concentration in the reaction medium as a result of water dissociation is the determining factor in the selectivity. The reaction of glucose isomerization to fructose and the further dehydration to 5-HMF are highly dependent on the ion concentration. By an increase in the pOH/pH value, these reactions were minimized to allow control of 5-HMF production. Under these conditions, the retroaldol condensation pathway was enhanced, instead of the isomerization/dehydration pathway.
At extremely low reaction times (0.02 s), cellulose was hydrolyzed in supercritical water (T = 400 8C and P = 25 MPa) to obtain a sugar yield higher than 95 wt %, whereas the 5-hydroxymethylfurfural (5-HMF) yield was lower than 0.01 wt %. If the reaction time was increased to 1 s, the main product was glycolaldehyde (60 wt %). Independently of the reaction time, the yield of 5-HMF was always lower than 0.01 wt %. To evaluate the reaction mechanism of biomass hydrolysis in pressurized water, several parameters (temperature, pressure, reaction time, and reaction medium) were studied for different bio-
Introduction Biomass exploitation as a raw material is growing as an alternative sustainable method of production of fuels and chemicals.[1] Cellulose is one of the main compounds of biomass and represents the most abundant biopolymer.[2] An important challenge in the processing of cellulosic biomass is to hydrolyze the b(14) glucose–glucose bond to produce a stream of sugars with a low concentration of byproducts, by using an efficient process.[3] These sugar streams could be further transformed into valuable chemicals such as pyruvaldehyde, glycolaldehyde,[4] 5-hydroxymethylfurfural (5-HMF),[5] organic acids, or polyalcohols.[6] Acid and enzymatic hydrolysis of cellulose are two conventional methods that need long treatment times (t 3 h) to obtain a product with poor selectivity (< 60 wt %).[7] The use of ionic liquids as both solvent and reaction medium has been intensively studied owing to the possibility of dissolving cellulose and making it more “accessible” for the hydrolysis reaction.[8] However, this kind of process takes at least 3 h of hydrolysis to obtain a selectivity near to 30 wt % of reducing sugars.[8a] These processing methods require long reaction times (hours), which will demand big reactors during the scale up. The use of pressurized water as the reaction medium for the processing of cellulosic biomass in a one-step fast process is an alternative. Total hydrolysis of cellulose can be achieved in a reaction time of 0.02 s in a supercritical water medium and produces a stream of water-soluble sugars with a low concentration of derived products (< 2 wt %).[9] This kind
of process represents an advantageous intensification that will reduce the energetic and equipment requirements in the scale up. Cellulose depolymerization in hot pressurized water has been done in different kinds of reactors (batch, semibatch, and continuous) at different temperatures and pressures, with or without catalysts.[10] The yield of sugars after biomass hydrolysis is enhanced by using supercritical water reactors operated in continuous mode at high temperature for short reaction times.[9b, 11] The combination of these two parameters is crucial for obtaining high yields of sugars. With long reaction times, the sugars are derived; at low reaction temperatures, several side reactions take place and produce many compounds. In fact, it was observed that some reactions are avoided at supercritical conditions. In particular, attention should be paid to the formation of 5-HMF. The production of 5-HMF from cellulose in pressurized water is highly dependent on the reaction temperature. In Figure 1, several experimental results are shown for cellulose hydrolysis in pressurized water from T = 300–400 8C at different pressures and reaction times. It can be observed that 5-HMF production was faster, but the yield was lower, when the reaction temperature was increased from 300 to 350 8C. The reaction time was reduced from 40 to 10 s by increasing the reaction temperature. This behavior was expected and it follows the Arrhenius law. However, an expected behavior was detected if the reaction temperature was increased over the critical point of water: the production of 5-HMF was highly avoided. Although this behavior was previously described in the literature,[4a,b, 9a, 12] a clear and quantitative explanation has not been developed yet. The different discovered behaviors can be classified into three main groups. 1) The maximum amount of 5-HMF from cellulose in pressurized water without catalyst is produced at tem-
[a] Dr. D. A. Cantero, Dr. M. D. Bermejo, Prof. Dr. M. J. Cocero Department of Chemical Engineering and Environmental Technology University of Valladolid Dr. Mergelina S/N, 47005, Valladolid (Spain) E-mail: [email protected] Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201403385.
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Full Papers actions that water participates in as a reagent or in the formation of intermediate states.[13] Another important property of water as a reaction medium is the ion product (mol2 kg2), which represents how dissociated the water molecules are (the ion concentration). If the molal concentration of OH ions (square root of the ionic product) is multiplied by the density, the molar concentration of protons or hydroxide anions in the medium is obtained. This concentration parameter includes both the variations in water volume and water dissociation. The concentration of OH ions (which is the same for H + ions) in the proximity of the critical point of water is plotted in Figure 2.
Figure 1. 5-HMF yield from cellulose hydrolysis in pressurized water against reaction time (tr). Experimental temperatures: red: 400 8C; yellow: 350 8C; and blue: 300 8C. Experimental pressures: ^: 27 MPa; &: 25/23 MPa; and ~: 23/18 MPa.
peratures lower than 300 8C.[9a, 12b,h,j,l] An increase in temperature benefits the retroaldol condensation reactions of fructose.[12l] 2) The production of 5-HMF is enhanced by increasing the availability of protons (H + ) in the reaction medium by addition of acids.[12d–g,i] 3) The production of 5-HMF is enhanced in a pressurized water medium if the pressure is increased at a constant temperature.[4a,b] The aim of this work was to study the reactions of cellulose hydrolysis, with a focus on 5-HMF production from sugars. The yields were analyzed from a chemical point of view of the reaction pathway. Several reactions were performed to obtain accurate data. Cellulose hydrolysis was tested at T = 300, 325, 350, 375, and 400 8C at 25 MPa of pressure. Also, the pressure effect was tested at T = 300, 350, and 400 8C between P = 18– 27 MPa. Studies were also conducted to analyze glucose and fructose hydrolysis in pressurized water between T = 300– 400 8C at 25 MPa. Finally, the results were contrasted with the products obtained from wheat-bran hydrolysis in supercritical water. A reaction pathway was developed, and a novel kinetic model was tested for understanding the behavior of glucose and fructose reactions in supercritical water.
Figure 2. Hydroxy ion concentration (mol L1) against temperature and pressure. pOH = log(j OH j) = pH.[14] Water density was calculated according to the International Association for the Properties of Water and Steam industrial formulation,[14a] whereas the molal ionic product of water was calculated by following the “International Formulation of Ionic Product of Water Substance”.[14b]
Important changes in the identity of the medium can be obtained if the temperature and pressure are changed at the same time. For example, the density of water at T = 300 8C and P = 27 MPa is around 750 kg m3 ; this value can be decreased to 130 kg m3 if the conditions are modified to T = 400 8C and P = 23 MPa. The H + /OH concentration varies by six orders of magnitude in the region of the critical point, which allows the possibility of working with markedly different reaction mediums. The H + /OH concentration at T = 300 8C and P = 23 MPa is around 2 106 mol L1, which means that the medium has a high concentration of ions ([H + ] and [OH]) and ionic reactions are favored.[4a, 15] The H + /OH concentration will take a value of 5.5 1012 mol L1 if the temperature and pressure are changed to 400 8C and 23 MPa; this reaction medium would favor radical reactions.[16] The reactions were assumed to follow the reaction pathway shown in Scheme 1. This reaction pathway was built by following the schemes developed in Reference [12b]. The reaction of
Results and Discussion Reaction medium and reaction pathway Supercritical water is water at temperature and pressure values above the critical points (Tc = 374 8C and Pc = 22.1 MPa). In the proximity of the critical points, the properties of water can be highly influenced by changing pressure and temperature, so the identity of the medium can be modified without changing the solvent. The medium density represents the quantity of water per volume unit (kg m3); this is a measurement of water concentration, an important factor to take into account in re-
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Full Papers To evaluate the medium effect in the selectivity, the model was solved in three different ways: 1) by considering only the concentration of cellulose and its derived products, 2) by considering also the water concentration, and 3) by considering the concentration of cellulose, its derived products, and the proton or hydroxide anion concentrations in the reactions of glucose isomerization and fructose dehydration. Therefore, the concentration of fructose (nf) was calculated according to Equation (2) or Equation (3) in Scheme 1. Main reaction pathways of cellulose hydrolysis in pressurized water. the resolution of models 2 or 3, respectively, for which nw is the glucose isomerization occurs through ring opening and keto– water concentration (mol L1), nOH-H is the concentration of enol tautomerism. These reactions take place to form transition OH or H + ions in the medium (mol L1), ng is the concentra + states with OH or H ions. Also, fructose dehydration takes tion of glucose, and kgf, kfh, and kfg are the kinetic constants of place to form transition states incorporating H + ions (one per the reactions indicated in Scheme 1. H2O molecule lost).[17] To identify these reactions in Scheme 1, dnf the symbols OH/H + have been added above the reaction ð2Þ ¼ kgf ng nw kfh nf nw kfg nf dt arrow. The production of glycolaldehyde was enhanced at supercritical conditions because the hydroxide/proton concentradnf ð3Þ ¼ kgf ng nOHH kfh nf nOHH kfg nf tion is highly decreased (pH = pOH = 13) and so is the concendt tration of fructose and its derived products. Although the reaction of glucose isomerization is avoided at low concentrations of hydroxide anions, a fructose yield of near to 10 wt % was Sugar production from cellulose hydrolysis obtained at supercritical conditions. As shown in Scheme 1, fructose can follow two main reacThe evolution of the sugar yield along with reaction time can tion pathways: fructose dehydration or retroaldol condensabe seen in Figure 3 (see also Tables S.1–S.10 in the Supporting tion. The second reaction was more beneficial than the first Information). The best conditions to obtain soluble sugars (up one because, in this way, glyceraldehyde was obtained as the main product from fructose. The maximum quantity of 5-HMF was obtained by working at T = 350 8C and P = 23 MPa (pH = pOH = 6) for a reaction time of 10 s. Under those conditions, the yield was around 15 wt %. A reaction model was built by considering cellulose as the starting material. The concentration of each compound shown in Scheme 1 was calculated against the reaction time. The difference between the calculated concentrations and the experimental ones was minimized to obtain the kinetic constant of the reactions.[9a] Equation (1) describes the evolution of compound i along with reaction time, for which ni and nj (mol L1) are the concentrations of compounds i and j, respectively, t (s) is the time, kji (s1) is the kinetic constant of the reaction in which compound j reacts to produce compound i, and kio is the kinetic constant of the reaction in which compound i reacts to produce a derived product.
n n X dni X kji nj kio ni ¼ dt 1 1
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Figure 3. Sugar yields from cellulose hydrolysis in pressurized water against reaction time. Experimental temperatures: red: 400 8C; yellow: 350 8C; and blue: 300 8C. Experimental pressures: ^: 27 MPa; &: 25/23 MPa; and ~: 23/ 18 MPa.
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sis would have a sugar yield of between 80–98 wt % if the reaction time was between 0.015–0.2 s. The best combination of conditions to obtain a high yield of glycolaldehyde can be achieved by working at T = 400 8C and P = 23 MPa with a reaction time of 1 s. In those conditions, the yield of glycolaldehyde was around 60 wt %. Sometimes, the production of 5-HMF is undesired, especially if a microorganism postmodification process is needed.[19] At T = 400 8C, the 5-HMF production was highly avoided for all studied pressures. Independently of the residence time (0.015 s for sugars or 1 s for glycolaldehyde), the concentration of 5-HMF was lower than 0.1 wt %. Reaction model evaluation: Cellulose hydrolysis at P = 25 MPa for different temperatures The reaction pathway shown in Scheme 1 was kinetically evaluated by following Equations (1), (2), or (3). The results for the three tested models are shown in Figure 4. The behavior of the reaction models will be compared by means of the results for the kinetic constants of fructose dehydration to 5-HMF. The obtained kinetic constants for the reaction rates kog, kgg, and kfg (as indicated in Scheme 1) obtained by the resolution of model 1 follow the Arrhenius law. However, the kinetics of glucose isomerization and fructose dehydration showed a break point near the critical point of water. As can be observed in Figure 4 A, the model linearly predicts the kinetic constants of fructose dehydration at subcritical temperatures. However, near the critical point of water a break point in the kinetic was observed, which represents a deviation from the Arrhenius law. This phenomenon could be predicted because of the low concentration of 5-HMF in the products. Moreover, the concentration profiles for 5-HMF obtained at T = 400 8C were lower than those found at T = 350 or 300 8C (these concentration profiles can be seen in the Supporting Information). This deviation from the Arrhenius behavior suggests that another chemical effect is not taken into account in the kinetic evaluation. To solve this problem, the water or ion concentrations were considered as well as the reagent concentration. The reactions of glucose and fructose in near critical water are usually analyzed together with the medium density to explain the selectivity of the process.[3b, 4a,b, 9a] Qualitatively, the theory of water transition states works well: if the medium density is low, the concentration of 5-HMF will be low. To quantify the participation of water in the reaction, the water concentration was added as a reagent concentration in the reactions of glucose isomerization and fructose dehydration (model 2). Unfortunately, the obtained kinetic constants with consideration of the water concentration as a reagent concentration did not follow the Arrhenius law at sub- or supercritical water (Figure 4 B). Thus, from a quantitative point of view, the theory of water transition states would not explain the low concentration of 5-HMF at supercritical conditions. Finally, the kinetic constants of the whole reaction system followed the Arrhenius law if model 3 was resolved (Figure 4 C). After this evaluation, it can be concluded that the reactions of glucose isomerization and fructose dehydration are highly dependent on
Figure 4. Kinetic constants for fructose dehydration to 5-HMF at P = 25 MPa and T = 300–400 8C. A) Kinetic evaluation with consideration of the biomass concentration as the reagent concentration. B) Kinetic evaluation with consideration of the biomass and water concentrations as the reagent concentration. C) Kinetic evaluation with consideration of the biomass and ion concentrations as the reagent concentration. Error bars represent the experimental and fitting errors.
than 95 wt %. Similar methods of cellulose hydrolysis in pressurized water were developed in the literature with sugar selectivities of lower than 77 wt %.[11, 12j, 18] The combination of the supercritical water medium and the effective method of reaction-time control presented in this work allow cellulose hydrolysis a high yield of sugars. This is because, under those conditions, the cellulose hydrolysis kinetics are fast enough and the glucose hydrolysis kinetics are slow enough to allow our reactor to stop the reactions after total hydrolysis and before glucose degradation.[9b] It was observed that the cellulose hydroly-
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Full Papers the ion concentration in the reaction medium. This phenomenon was also observed for the reaction of methyl tert-butyl ether hydrolysis in sub- and supercritical water.[20] The kinetic constants for cellobiose hydrolysis (kh), oligosaccharides hydrolysis (kog), glucose isomerization to fructose (kgf), and glucose retroaldol condensation (kgg) are plotted in Figure 5 A–D, respectively. It can be observed that the resolution of model 3 produces a series of kinetic constants that follows the Arrhenius law for the whole system. The dotted lines in Figure 5 represent the uncertainty of the kinetic constants with consideration of the experimental errors and the fitting errors. It is important to take into account the dimension of the kinetic constant. For model 3 resolution, the kh, kog, and kgg constants have the same units (s1). However, the kgf and khmf constants have second-order units (L mol1 s1). This is because these two reactions were considered to be dependent on the concentrations of two reagents. In addition, the obtained values of the kgf and khmf constants were higher than the values of the kh, kog, and kgg constants. To describe the kinetic behavior against temperature for the different reactions, the activation energy (Ea) and the pre-exponential factor (ln ko) were calculated. These parameters are shown in Table 1 for all of the kinetics fitted. It can be also ob-
Table 1. Activation energy and pre-exponential factor for glucose reactions in pressurized water medium. Kinetic constant
Ea [kJ mol1]
Error [kJ mol1]
ln ko
Error
kh [s1] kog [s1] kgf [L mol1 s1] kgg [s1] kfh [L mol1 s1]
90.4 145.5 566.3 147.4 348.1
4.5 6.1 34.1 7.4 26.0
17.4 28.6 127.3 28.7 84.8
0.9 1.2 6.6 1.4 5.0
served in this table that the error in the Arrhenius parameters is around 5 % with consideration of the experimental and fitting uncertainties. Also, as previously analyzed for the kinetic constants of kgf and khmf, the values of the Arrhenius parameters for these constants were higher than for the others. Reaction model evaluation: Cellulose hydrolysis at different pressures and temperatures Figure 5. Kinetic constants for cellulose hydrolysis at P = 25 MPa and T = 300–400 8C. A) Kinetic constants of cellobiose hydrolysis. B) Kinetic constants of oligosaccharide hydrolysis. C) Kinetic constants of glucose isomerization to fructose. D) Kinetic constants of glucose retroaldol condensation. The dotted line represents the uncertainty of the kinetics behavior.
The evaluation of the reaction model was also tested for the reactions of cellulose hydrolysis at different pressures and temperatures. The cellulose hydrolysis was tested at T = 300 8C (P = 18, 23, and 27 MPa), T = 350 8C (P = 18, 23, and 27 MPa), and T = 400 8C (P = 23, 25, and 27 MPa). A comparison of the three model results can be seen in Figure 6 A. Once again, the outcomes of models 1 and 2 did not follow the Arrhenius law. The fructose dehydration kinetics followed the Arrhenius law if model 3 was used to solve the kinetics system. Although the results shown in Figure 6 A correspond to the P = 27 MPa ChemSusChem 0000, 00, 0 – 0
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series, the results of model 3 follow the Arrhenius law for all experimental pressures, as shown in Figure 6 B. It should be taken into account that the experiments were always done in pressurized liquid or the supercritical phase. This is because the series at P = 18 MPa was only tested at T = 300 and 350 8C. 5
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Figure 6. A) Kinetic constants of fructose dehydration with consideration of OH concentration (~), with consideration of water concentration (Cw) (&), and without consideration of the OH or water concentrations (*). P = 27 MPa. B) Kinetic constant of fructose dehydration with consideration of OH concentration at P = 27 (*), 25 (&), 23 (~), and 18 MPa (^). Error bars represent the experimental and fitting errors.
Figure 7. Glucose hydrolysis kinetic constants at T = 350, 385, and 400 8C. P = 25 MPa. A) Kinetic constants of glucose isomerization to fructose (~) and fructose dehydration (&) with consideration of the OH concentration as a reagent concentration. B) Kinetic constants of glucose retroaldol condensation (&), fructose retroaldol condensation (^), and the formation of pyruvaldehyde from glyceraldehyde (*). Error bars represent the experimental and fitting errors.
Reaction model evaluation: Glucose hydrolysis at P = 25 MPa for different temperatures
merization and fructose dehydration took similar orders of magnitude for the different analyzed situations.
The reaction pathway and the kinetic model developed for cellulose hydrolysis were also tested by analyzing glucose reactions in pressurized water. Glucose hydrolysis reactions were tested at P = 25 MPa of pressure and at temperatures around the critical point of water (T = 350, 385, and 400 8C), at which point the change in the ionic product of water is the highest. The fitted kinetic constants of kgf and khmf are plotted against temperature in Figure 7 A. The fitted kinetic constants of kgg, kgp, and kfg are plotted against temperature in Figure 7 B. As expected, the obtained kinetic constants follow the Arrhenius law if the concentration of ions is considered as a reagent concentration (model 3). The reaction mechanism proposed in this work was tested in three different ways: cellulose hydrolysis with constant pressure and changing temperature, cellulose hydrolysis with changing pressure and temperature, and glucose hydrolysis with constant pressure and changing temperature near the critical point of water. For the three situations, the kinetic constants of the glucose hydrolysis reactions follow the Arrhenius parameters if the ion concentration of the medium is taken into account. In addition, the kinetic constants of glucose iso-
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Testing the concept with modified reaction mediums and natural biomass Finally, to test the developments in the kinetics mechanism, fructose was hydrolyzed in modified reaction mediums (with 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and oxalic acid). In addition, wheat bran was hydrolyzed in supercritical water for testing the production of 5-HMF from a natural biomass in supercritical water. The yields of the main products obtained after fructose hydrolysis are shown in Figure 8. The experiments were carried out in the experimental setup explained above. However, the reaction medium was modified by pumping TEMPO or oxalic acid to the reactor. TEMPO is a free radical kidnapper that is usually employed to control radical reactions in organic synthesis and polymerization.[21] Oxalic acid was used to increase the concentration of ions in the reaction medium. As expected, the production of 5-HMF from fructose at supercritical conditions (T = 400 8C and P = 23 MPa) was negligible, with pyruvaldehyde as the main product after 0.9 s of reaction time. The addition of oxalic acid to the reaction medium increased the 6
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Full Papers soluble sugars or 60 wt % of glycolaldehyde. From the viewpoint of chemistry, the selectivity of the process is governed by the ion concentration in the reaction medium. A reaction mechanism model was built and intensively tested to demonstrate its reliability. The reactions of glucose and the fructose retroaldol condensation are not very demanding of ions. In fact, these reactions are highly improved if the water molecules (reaction medium and reagent) are highly associated. On the other hand, the isomerization reactions of glucose–fructose and the dehydration reactions of these sugars are extremely diminished if the water molecules are associated. For first time, the reasons why the production of 5-HMF is highly avoided at supercritical water conditions were quantitatively explained and demonstrated. It was achieved by adding the concentration of protons or hydroxide ions as a result of water dissociation as a reagent concentration into the kinetic modeling of the reactions. The extraordinary changes in the chemical and physical properties of supercritical water allow biomass hydrolysis with the choice of desired products by simply selecting the correct reaction temperature and pressure.
Figure 8. Yields of fructose hydrolysis at T = 400 8C, P = 23 MPa, and t = 0.9 s. The reaction medium was modified with TEMPO or oxalic acid.
availability of protons in the medium, which would promote the fructose dehydration reaction. In fact, if the reaction medium was acidified, the production of 5-HMF in supercritical water was enhanced to 15 wt %. The same behavior was observed with TEMPO as the reaction-medium modifier. This free radical kidnapper has an acid role as a result of the dissociation of the OH group bonded to the nitrogen atom. Once again, an acid medium promoted the production of 5-HMF, which corroborated the fact that 5-HMF production is highly dependent on the proton availability in the medium. Wheat bran was also hydrolyzed in the aforementioned experimental setup. The reaction temperature was set at 400 8C with a reactor pressure of 25 MPa. The reaction time was varied from 0.19 s to 0.69 s. Fortunately, as can be seen in Figure 9, the yields of 5-HMF were lower than 0.05 wt %. A detailed description of wheat-bran hydrolysis for sugar and lignin production can be found in a previous work.[22]
Experimental Section The experiments were carried out in a continuous pilot plant that was able to work at temperatures up to 425 8C and pressures up to 30 MPa. A schematic diagram of the process is shown in Figure 10. Two streams continuously fed a microreactor: a cellulose
Figure 10. Schematic diagram of the supercritical water hydrolysis facility.
stream and a supercritical water stream. Strict control of the reaction times was achieved by a combination of three factors: 1) rapid heating by supercritical water injection of the cellulose suspension stream, 2) rapid cooling by sudden depressurization down to atmospheric pressure and T 100 8C by using a micrometering valve that was able to stand temperatures up to T = 425 8C, and 3) selection of a series of tubular reactors of different accurately determined volumes. The volume of the used reactors varied from 0.12– 64.5 mL, which in combination with flows of between 1–2 g s1 and taking into account the density of water under the experimental conditions, gives reaction times of 0.004–40 s. A detailed description of the experimental setup is given in the Supporting Information. The reactor is fed from two streams (biomass and water); however, no extra water is needed in the process once the steady state is achieved. As can be seen in Figure 10, after the reactor, a flash chamber separator produces two streams: vapor
Figure 9. Yields of wheat-bran hydrolysis at T = 400 8C and P = 25 MPa for reaction times of between t = 0.19 and 0.69 s.
Conclusions The process presented in this work shows an efficient alternative to hydrolyze cellulose selectively. The control of the reaction time is the key to obtain yields higher than 95 wt % of ChemSusChem 0000, 00, 0 – 0
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Full Papers (water) and liquid (dissolved sugars). The vapor is almost pure water that can be recycled into the system.
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Acknowledgements The authors thank the Spanish Ministry of Economy and Competitiveness for the Projects CTQ2013-44143-R, CTQ2011-27347, and ENE2012-33613. The authors thank Repsol for technical support. M.D.B. thanks the Spanish Ministry of Economy and Competitiveness for the Ramn y Cajal research fellowship RYC-2013-13976 Keywords: biomass · carbohydrates · hydrolysis · kinetics · water chemistry [1] A. J. Ragauskas, C. K. Williams, B. H. Davison, G. Britovsek, J. Cairney, C. A. Eckert, W. J. Frederick, J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz, R. Murphy, R. Templer, T. Tschaplinski, Science 2006, 311, 484 – 489. [2] D. Klemm, B. Heublein, H. P. Fink, A. Bohn, Angew. Chem. Int. Ed. 2005, 44, 3358 – 3393; Angew. Chem. 2005, 117, 3422 – 3458. [3] a) J. Tollefson, Nature 2008, 451, 880 – 883; b) K. Arai, R. L. Smith, Jr., T. M. Aida, J. Supercrit. Fluids 2009, 47, 628 – 636; c) K. Barta, P. C. Ford, Acc. Chem. Res. 2014, 47, 1503 – 1512. [4] a) T. M. Aida, Y. Sato, M. Watanabe, K. Tajima, T. Nonaka, H. Hattori, K. Arai, J. Supercrit. Fluids 2007, 40, 381 – 388; b) T. M. Aida, K. Tajima, M. Watanabe, Y. Saito, K. Kuroda, T. Nonaka, H. Hattori, R. L. Smith, Jr., K. Arai, J. Supercrit. Fluids 2007, 42, 110 – 119; c) B. M. Kabyemela, T. Adschiri, R. M. Malaluan, K. Arai, Ind. Eng. Chem. Res. 1999, 38, 2888 – 2895; d) B. M. Kabyemela, T. Adschiri, R. M. Malaluan, K. Arai, H. Ohzeki, Ind. Eng. Chem. Res. 1997, 36, 5063 – 5067. [5] a) A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411 – 2502; b) R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres, J. G. de Vries, Chem. Rev. 2013, 113, 1499 – 1597. [6] a) A. M. Ruppert, K. Weinberg, R. Palkovits, Angew. Chem. Int. Ed. 2012, 51, 2564 – 2601; Angew. Chem. 2012, 124, 2614 – 2654; b) C. Luo, S. Wang, H. Liu, Angew. Chem. Int. Ed. 2007, 46, 7636 – 7639; Angew. Chem. 2007, 119, 7780 – 7783. [7] a) L. Shuai, X. Pan, Energy Environ. Sci. 2012, 5, 6889 – 6894; b) C. E. Wyman, B. E. Dale, R. T. Elander, M. Holtzapple, M. R. Ladisch, Y. Y. Lee, Bioresour. Technol. 2005, 96, 2026 – 2032. [8] a) R. Rinaldi, R. Palkovits, F. Schth, Angew. Chem. Int. Ed. 2008, 47, 8047 – 8050; Angew. Chem. 2008, 120, 8167 – 8170; b) S. Morales-delaRosa, J. M. Campos-Martin, J. L. G. Fierro, Chem. Eng. J. 2012, 181 – 182, 538 – 541. [9] a) D. A. Cantero, M. D. Bermejo, M. J. Cocero, J. Supercrit. Fluids 2013, 75, 48 – 57; b) D. A. Cantero, M. D. Bermejo, M. J. Cocero, Bioresour. Technol. 2013, 135, 697 – 703.
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Received: December 9, 2014 Published online on && &&, 0000
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FULL PAPERS Just add water: A reaction mechanism for cellulose hydrolysis that can explain the huge selectivity of biomass hydrolysis in supercritical water is presented. The model of the reaction mechanism has been validated by several experiments carried out in a continuous pilot plant capable at various conditions. It was found that the proton and hydroxide anion concentration in the medium due to water dissociation (represented by the ionic product of water, Kw) is the determining factor in the selectivity of the process.
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D. A. Cantero, M. D. Bermejo, M. J. Cocero* && – && Governing Chemistry of Cellulose Hydrolysis in Supercritical Water
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