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Kinetic investigation of dilute acid hydrolysis of hardwood pulp for microcrystalline cellulose production
Carbohydrate Research 488 (2020) 107910
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Kinetic investigation of dilute acid hydrolysis of hardwood pulp for microcrystalline cellulose production
T
Nikolay Yavorova,∗, Ivo Valcheva, Greta Radevab, Desislava Todorovaa a b
Department of Pulp, Paper and Printing Arts, University of Chemical Technology and Metallurgy, 8 St. Kliment Ohridski Blvd, 1756, Sofia, Bulgaria Department of Physical Chemistry, University of Chemical Technology and Metallurgy, 8 St. Kliment Ohridski Blvd, 1756, Sofia, Bulgaria
ARTICLE INFO
ABSTRACT
Keywords: Acid hydrolysis Kinetics Microcrystalline cellulose Prout-Tompkins equation
This work presents a kinetic investigation of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp. The temperature-time dependence shows fast xylose extraction during the initial period of the process, while the glucose content increases slowly and permanently over the period. A conversion of xylose into furfural and furfural-derived chromophores is observed. It is established that only a low-brightness microcrystalline cellulose when a degree of polymerization below 300 can be obtained from hardwood pulp. The study of the acid hydrolysis kinetics, with respect to the degree of polymerization of microcrystalline cellulose, shows that the modified Prout-Tompkins equation describes most adequately the process. According to that kinetic model, the hydrolysis rate depends on a combination of chemical interaction and diffusion processes. It is evident that the activation energy does not change in the course of the process, i.e. the cellulose active centers do not change their activity.
1. Introduction Microcrystalline cellulose (MCC) has a high potential in respect to its application in various areas such as food, cosmetics and pharmaceutical industries. It has been widely used as a dispersing agent, a stabilizer, an emulsifier, a filler-binder, etc. A number of processes, such as an acid and an alkali hydrolysis, an extrusion, a steam explosion and an enzymemediated process have been employed to derive MCC from plant cell walls [1,2]. Most of the MCC production routes are based on an acid treatment of a cellulose source. The acid selectively attacks the amorphous (less-ordered) regions of the cellulose chain, thereby setting free the crystallites which constitute MCC. The cellulose polymer chains are tightly packed into ordered crystal structures by networks of hydrogen bonds, what makes the cellulose microfibrils very resistant to an acidcatalyzed treatment compared to that of the hemicelluloses [3,4]. The glycosidic bonds in the less-ordered regions split 1000–5000 times faster than those in the crystalline areas [5]. The hydrolysates from a dilute acid hydrolysis contain mainly a mixture of sugars, some of their derivatives and a number of by-products, such as furan derivatives (5-(hydroxymethyl)-2-furaldehyde (5-HMF) and 2-furaldehyde (furfural)), carboxylic acids, etc. Both 5-HMF and furfural may be obtained from the acid catalytic dehydration of hexoses (glucose, fructose) and pentoses (xylose, arabinose), respectively [6–8]. 5-HMF can readily be further transformed into levulinic and formic acids, as well as into some by∗
products (humins) under reaction conditions used in acid-catalyzed hydrolysis of cellulose [9,10]. Furfural can also be considered as an important chemical platform and a precursor for levulinic acid production [7]. It has been found earlier that the degree of polymerization (DP) of crystalline cellulose depends substantially on both the raw materials and the hydrolysis conditions [11]. The extent to which the acid-catalyzed hydrolysis depolymerizes the cellulose depends also on the severity of the treatment. Sixta et al. [12] describe an efficient way of assessment of the acid hydrolysis severity by using the P-factor on the ground of the Arrhenius-type equation. The simplest kinetic model describing the cellulose hydrolysis is developed by Saeman [13]. This kinetic model presents the process as a two-step pseudo-first order reaction. A significant development of the Saeman's model is achieved through the introduction of additional parameters aiming a more accurate description of the mechanism of the hydrolysis process. In this respect, Yan et al. [14] develop a kinetic model by incorporating parameters, reflecting the effects of the reaction limitations provided by the cellulose crystallinity, and the formation of by-products such as acid-soluble lignin-glucose and humins. The applicability of this approach to understanding the mechanism of the cellulose degradation shows a positive effect, but it should be noted that the dependencies on the raw material and the changes of the reaction conditions make these models not robust enough. The aim of the present work is to study the kinetics of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp to MCC and to analyze the kinetic regularities.
Corresponding author. E-mail address: [email protected] (N. Yavorov).
https://doi.org/10.1016/j.carres.2020.107910 Received 24 September 2019; Received in revised form 19 December 2019; Accepted 7 January 2020 Available online 08 January 2020 0008-6215/ © 2020 Elsevier Ltd. All rights reserved.
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
2. Experimental 2.1. Materials The investigations were performed on industrial bleached hardwood kraft pulp provided by Svilosa AD, Svishtov (Northern Bulgaria, 43°64′N 25°30′E). The ISO brightness of the pulp was 90% (ISO standard 2470–1:2016), DP – 1200 (Ph. Eur.), while the pentosans content amounted to 20% of the dry matter (TAPPI standard T 223 cm-10). Sulfuric acid of an analytical grade was purchased from Valerus Ltd (Sofia, Bulgaria). 2.2. Acid hydrolysis Dilute acid hydrolysis was performed in stainless steel laboratory autoclaves under the following conditions: the sulfuric acid charge of 2% in respect to the dry pulp; 5, 10 and 20% pulp consistency; hydrolysis temperature values of 120, 130, 140 and 150 °C; reaction time at these temperatures varying from 40 to 120 min. The pulp was fully washed with deionized water to pH above 5.
Fig. 1. Xylose extraction at different temperature values, reaction times and pulp consistencies.
2.3. A degree of polymerization DP of MCC samples was measured according to the procedures described in the European Pharmacopoeia [15]. The standard deviation of the analyses, based on triplicates, was estimated to be 0.3%. 2.4. HPLC analysis The sugars released during the acid hydrolysis were quantified on a Dionex high-pressure liquid chromatography (HPLC) system (Dionex Inc., CA, USA) equipped with a Shodex RI-101 refractive index (RI) detector (Showa Denko KK, Kawasaki, Japan) according to the NREL Laboratory Analytical Procedure [16]. The separation was performed in a Hi-Plex H column, 7.7 mm × 300 mm (Agilent Technologies, USA) at 65 °C with ultrapure water (Simplicity®, Merck KGaA, Germany) as an eluent at a flow rate of 0.5 mL min−1. The samples were filtered through a 0.2 μm filter prior to HPLC analysis. The results were calculated as a percentage (%) in respect to the dry matter. 3. Results and discussion Fig. 2. Furfural quantity at different temperature values, reaction times and 20% pulp consistency.
Due to the high pentosans content of the bleached hardwood kraft pulp, the dilute acid hydrolysis generates mainly xylose during the initial period of the process. As evident from Fig. 1, the xylose extraction increases rapidly at the beginning of the process. The degree of hydrolysis depends on the temperature and the reaction time, but it is expected to be affected to a great extent by the pulp consistency and the acid concentration. The furfural content increases significantly at the end of the acid hydrolysis and depends also on the process temperature (Fig. 2). The established transformation is very slow at a low pulp consistency and a low temperature value. The conversion of furfural to dark by-products, usually denoted as humin-like substances, proceeds also. Humins are formed by side reactions and secondary transformations of furan compounds [17]. Rosenau et al. suggest that the humins have structures similar to those of the chromophoric compounds derived from hexeneuronic acids [18]. Due to the xylose transformations to furfural the xylose kinetic curves pass through a maximum at high temperature and pulp consistency values (Fig. 1). A relationship between MCC brightness and furfural formation is obtained, which shows that the furfural-derived chromophores are responsible for the pulp brightness decrease (Fig. 3). Therefore, the minimization of furfural and humins formation is a major goal in the production of MCC of high brightness. The yield of glucose, as seen from Fig. 4, increases much slowly compared to that of xylose indicating that this parameter is not
Fig. 3. MCC brightness versus the furfural content. 2
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
Table 1 A comparison of the correlation coefficient values obtained in the description of the process investigated by different kinetic equations usually applied to the study of heterogeneous catalytic processes. R2
Equation
First order kinetic equation ln(1-α) = - k.t Power kinetic equation α = g.tχ Exponential kinetic equation α = k + a−1.lnt Modified Prout-Tompkins equation α/(1-α) = (k.t)χ Avrami-Erofeev topochemical equation α = 1 - e−(k.t)n
determining the hydrolysis process. An insignificant transformation of glucose to 5-HMF is also found at 150 °C. The degree of polymerization is a main characteristic of MCC. It is well known that DP decrease during the acid hydrolysis is basically associated with cellulose chain breakage. On the other hand, the extraction of the low molecular xylan leads to a slight DP increase. A significant decrease of DP during the initial period of the acid hydrolysis (Fig. 5) is found in the course of the present investigation. The kinetics of the process is investigated by the dimensionless quantity α, which is determined as the relative change of DP and is calculated in correspondence with Eq. (1).
1200 DP 1200
130 °C
140 °C
150 °C
0.9825
0.9017
0.9663
0.9758
0.9982
0.9466
0.9948
0.9973
0.9992
0.9514
0.9958
0.9980
0.9995
0.9601
0.9976
0.9994
0.9994
0.9552
0.9968
0.9989
application to solid state reactions has led to the development of numerous reaction models, even in fields other than that of solid-state chemistry. According to that kinetic model, the hydrolysis rate depends on the combination of chemical interaction and diffusion processes. The P-Т equation describes well the kinetics of solids decomposition in case of diffusion controlled heterogeneous processes starting on the easily accessible outer surface of the pulp and continuing further through a gradual penetration into the capillary system of the fiber matrix. Former investigations [19–21] show that the kinetics of the enzymatic hydrolysis of steam exploded fast-growing tree species and bleached kraft pulp can be well described by the modified topochemical equation of P-T. The observed topochemical kinetic mechanism of DP decrease during the acid hydrolysis is probably determined by the characteristic fibrous structure of the pulp. The modified equation of Prout-Tompkins can be presented in the following form:
Fig. 4. Glucose yield versus the reaction time, the reaction temperature and the pulp consistency.
=
120 °C
(1)
(1
where 1200 is the initial value of pulp DP, while DP stands for the current values at different time and temperature values. The applicability of different kinetic equations referring to different heterogeneous catalytic and topochemical processes is verified. The kinetic investigation of the process of cellulose depolymerization shows that the modified Prout-Tompkins (P-T) equation describes relatively precisely the process of dissolution of cellulose amorphous domains (Table 1). This equation is applied to solids decomposition proceeding with product nucleus branching. The interpretation given to its
)
= (kt )
(2)
where k has the meaning of an apparent rate constant, while the power factor χ (0 < χ < 1) is an invariable quantity characteristic for the system. According to that kinetic model, the hydrolysis rate depends on the combination of chemical interaction and diffusion processes. All kinetic curves are linearized in coordinates ln 1 vs. lnt (Eq. (3)) in correspondence with the logarithmic form of Eq. (2) and the linear dependences obtained are presented in Fig. 6.
ln
=
1
ln k +
ln t
(3)
The value of the power coefficient can be accepted approximately equal to 0.3. d It is of interest to determine the current rate v = dt , which changes during the process. The derivative form of the modified P-T equation (2) is presented by:
v=
d = k dt
1
(1
)
+1
(4)
Eq. (4) provides the calculation of the current rate for different values of α and the temperature. The dependence of the rate v (min−1) versus α at different temperature values is shown in Fig. 7. The temperature dependence of the current rate is followed by Arrhenius equation (Eq. (5)):
v = Ae
E RT
(5)
where A is the pre-exponential factor, while E is the activation energy. The activation energy and the pre-exponential factor are calculated at different constant values of α. The linear relation between lnv and 1/ T (Fig. 8) provides the determination of the activation energy value
Fig. 5. DP of MCC versus temperature and time values. 3
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
Fig. 6. Illustration of P-T equation linearization for the temperature values studied.
Fig. 9. A dependence of the pre-exponential factor on the kinetic variable α.
which is found equal to 136 kJ mol−1. It is evident that the activation energy does not change in the course of the process, i.e. the active centers do not change their activity. Unlike the activation energy, the pre-exponential factor changes with the increase of the kinetic variable α (Fig. 9). A correlation between the pulp brightness and DP is obtained. Its character does not depend on the temperature, the reaction time, the pulp consistency and the acid concentration (Fig. 10). The relationship illustrated in Fig. 10 shows that the one-step acid hydrolysis leads to the production of a low-brightness MCC when DP less than 300 irrespectively of the hydrolysis conditions. 4. Conclusions The kinetic investigation of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp aiming MCC production shows that the modified topochemical P-T equation describes relatively precisely the process of the cellulose depolymerization. The observed topochemical kinetic mechanism of DP decrease is probably determined by the fibrous structure, which is characteristic of the pulp. The value of the activation energy is constant indicating that the cellulose active centers do not change their activity in the course of the hydrolysis. A
Fig. 7. Current rate v versus α at different temperature values.
Fig. 8. Temperature dependences of the current rate and the rate constant at α = const.
Fig. 10. ISO brightness versus DP of MCC. 4
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
transformation of xylose to furfural and furfural-derived chromophores is verified. It leads to a significant MCC brightness decrease. It is found that only a low-brightness MCC when DP below 300 can be obtained from bleached hardwood kraft pulp. The temperature-time dependence of the degree of the acid-catalyzed hydrolysis of bleached hardwood kraft pulp can be used for the process simulation and control.
11.023. [7] A. Morone, M. Apte, R.A. Pandey, Levulinic acid production from renewable waste resources: bottlenecks, potential remedies, advancements and applications, Renew. Sustain. Energy Rev. 51 (2015) 548–565, https://doi.org/10.1016/j.rser.2015.06. 032. [8] W. Sangarunlert, P. Piumsomboon, S. Ngamprasertsith, Furfural production by acid hydrolysis and supercritical carbon dioxide extraction from rice husk, Korean J. Chem. Eng. 24 (2007) 936–941, https://doi.org/10.1007/s11814-007-0101-z. [9] B. Girisuta, L.P.B.M. Janssen, H.J. Heeres, Kinetic study on the acid-catalyzed hydrolysis of cellulose to levulinic acid, Ind. Eng. Chem. Res. 46 (2007) 1696–1708, https://doi.org/10.1021/ie061186z. [10] C. Liang, Y. Hu, Y. Wang, L. Wu, W. Zhang, Production of levulinic acid from corn cob residue in a fed-batch acid hydrolysis process, Process Biochem. 73 (2018) 124–131, https://doi.org/10.1016/j.procbio.2018.08.002. [11] H. Håkansson, P. Ahlgren, Acid hydrolysis of some industrial pulps: effect of hydrolysis conditions and raw material, Cellulose 12 (2005) 177–183, https://doi. org/10.1007/s10570-004-1038-6. [12] H. Sixta, A. Potthast, A.W. Krotschek, Chemical pulping processes, in: H. Sixta (Ed.), Handbook of Pulp, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2006, pp. 109–509. [13] J.F. Saeman, Kinetics of wood saccharification - hydrolysis of cellulose and decomposition of sugars in dilute acid at high temperature, Ind. Eng. Chem. 37 (1945) 43–52, https://doi.org/10.1021/ie50421a009. [14] L. Yan, A.A. Greenwood, A. Hossain, B. Yang, A comprehensive mechanistic kinetic model for dilute acid hydrolysis of switchgrass cellulose to glucose, 5-HMF and levulinic acid, RSC Adv. 4 (2014) 23492–23504, https://doi.org/10.1039/ C4RA01631A. [15] European pharmacopoeia (Ph. Eur.), 8.0 (07/2013), pp. 1824–1828. [16] A. Sluiter, B. Hames, R. Ruiz, C. Scarlata, J. Sluiter, D. Templeton, Determination of Sugars, Byproducts, and Degradation Products in Liquid Fraction Process Samples, NREL, Golden, 2008 Technical Report NREL/TP-510-42623. [17] I.V. Sumerskii, S.M. Krutov, M.Y. Zarubin, Humin-like substances formed under the conditions of industrial hydrolysis of wood, Russ. J. Appl. Chem. 83 (2010) 320–327, https://doi.org/10.1134/S1070427210020266. [18] T. Rosenau, A. Potthast, N.S. Zwirchmayr, H. Hettegger, F. Plasser, T. Hosoya, M. Bacher, K. Krainz, T. Dietz, Chromophores from hexeneuronic acids: identification of HexA-derived chromophores, Cellulose 24 (2017) 3671–3687, https://doi. org/10.1007/s10570-017-1397-4. [19] E. Valcheva, S. Veleva, I. Valchev, I. Dimitrov, Kinetic model of xylanase action on kraft pulp, React. Kinet. Catal. Lett. 71 (2000) 231–238, https://doi.org/10.1023/ A:1010310706612. [20] I. Valchev, N. Yavorov, S. Petrin, Topochemical kinetic mechanism of cellulase hydrolysis on fast-growing tree species. COST Action FP1105, Holzforschung 70 (2016) 1147–1153, https://doi.org/10.1515/hf-2016-0030. [21] S. Petrin, N. Yavorov, P. Tzvetkov, I. Valchev, G. Radeva, Kinetic aspects of enzyme hydrolysis of cellulose fiber material, Bulg. chem. commun. 47 (2015) 30–33.
Funding This work was supported by the Bulgarian Ministry of Education and Science under the National Research Programme “Young scientists and postdoctoral students” approved by DCM № 577/17.08.2018. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D. Trache, M.H. Hussin, C.T.H. Chuin, S. Sabar, M.R.N. Fazita, O.F.A. Taiwo, T.M. Hassan, M.K.M. Haafiz, Microcrystalline cellulose: isolation, characterization and bio-composites application-A review, Int. J. Biol. Macromol. 93 (2016) 789–804, https://doi.org/10.1016/j.ijbiomac.2016.09.056. [2] M.H. Hussin, N.A. Husin, I. Bello, N. Othman, M.A. Bakar, M.K.M. Haafiz, Isolation of microcrystalline cellulose (MCC) from oil palm frond as potential natural filler for PVA-LiClO4 polymer electrolyte, Int. J. Electrochem. Sci. 13 (2018) 3356–3371, https://doi.org/10.20964/2018.04.06. [3] A. Pinkert, K.N. Marsh, S. Pang, M.P. Staiger, Ionic liquids and their interaction with cellulose, Chem. Rev. 109 (2009) 6712–6728, https://doi.org/10.1021/ cr9001947. [4] L.Q. Jiang, Z. Fang, F. Guo, L.B. Yang, Production of 2,3-butanediol from acid hydrolysates of Jatropha hulls with Klebsiella oxytoca, Bioresour. Technol. 107 (2012) 405–410, https://doi.org/10.1016/j.biortech.2011.12.083. [5] H. Krässig, J. Schurz, R.G. Steadman, K. Schliefer, W. Albrecht, M. Mohring, H. Schlosser, Cellulose, in: B. Elvers (Ed.), Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2012, pp. 279–332. [6] C. Rong, X. Ding, Y. Zhu, Y. Li, L. Wang, Y. Qu, X. Ma, Z. Wang, Production of furfural from xylose at atmospheric pressure by dilute sulfuric acid and inorganic salts, Carbohydr. Res. 350 (2012) 77–80, https://doi.org/10.1016/j.carres.2011.
5
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Kinetic investigation of dilute acid hydrolysis of hardwood pulp for microcrystalline cellulose production
T
Nikolay Yavorova,∗, Ivo Valcheva, Greta Radevab, Desislava Todorovaa a b
Department of Pulp, Paper and Printing Arts, University of Chemical Technology and Metallurgy, 8 St. Kliment Ohridski Blvd, 1756, Sofia, Bulgaria Department of Physical Chemistry, University of Chemical Technology and Metallurgy, 8 St. Kliment Ohridski Blvd, 1756, Sofia, Bulgaria
ARTICLE INFO
ABSTRACT
Keywords: Acid hydrolysis Kinetics Microcrystalline cellulose Prout-Tompkins equation
This work presents a kinetic investigation of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp. The temperature-time dependence shows fast xylose extraction during the initial period of the process, while the glucose content increases slowly and permanently over the period. A conversion of xylose into furfural and furfural-derived chromophores is observed. It is established that only a low-brightness microcrystalline cellulose when a degree of polymerization below 300 can be obtained from hardwood pulp. The study of the acid hydrolysis kinetics, with respect to the degree of polymerization of microcrystalline cellulose, shows that the modified Prout-Tompkins equation describes most adequately the process. According to that kinetic model, the hydrolysis rate depends on a combination of chemical interaction and diffusion processes. It is evident that the activation energy does not change in the course of the process, i.e. the cellulose active centers do not change their activity.
1. Introduction Microcrystalline cellulose (MCC) has a high potential in respect to its application in various areas such as food, cosmetics and pharmaceutical industries. It has been widely used as a dispersing agent, a stabilizer, an emulsifier, a filler-binder, etc. A number of processes, such as an acid and an alkali hydrolysis, an extrusion, a steam explosion and an enzymemediated process have been employed to derive MCC from plant cell walls [1,2]. Most of the MCC production routes are based on an acid treatment of a cellulose source. The acid selectively attacks the amorphous (less-ordered) regions of the cellulose chain, thereby setting free the crystallites which constitute MCC. The cellulose polymer chains are tightly packed into ordered crystal structures by networks of hydrogen bonds, what makes the cellulose microfibrils very resistant to an acidcatalyzed treatment compared to that of the hemicelluloses [3,4]. The glycosidic bonds in the less-ordered regions split 1000–5000 times faster than those in the crystalline areas [5]. The hydrolysates from a dilute acid hydrolysis contain mainly a mixture of sugars, some of their derivatives and a number of by-products, such as furan derivatives (5-(hydroxymethyl)-2-furaldehyde (5-HMF) and 2-furaldehyde (furfural)), carboxylic acids, etc. Both 5-HMF and furfural may be obtained from the acid catalytic dehydration of hexoses (glucose, fructose) and pentoses (xylose, arabinose), respectively [6–8]. 5-HMF can readily be further transformed into levulinic and formic acids, as well as into some by∗
products (humins) under reaction conditions used in acid-catalyzed hydrolysis of cellulose [9,10]. Furfural can also be considered as an important chemical platform and a precursor for levulinic acid production [7]. It has been found earlier that the degree of polymerization (DP) of crystalline cellulose depends substantially on both the raw materials and the hydrolysis conditions [11]. The extent to which the acid-catalyzed hydrolysis depolymerizes the cellulose depends also on the severity of the treatment. Sixta et al. [12] describe an efficient way of assessment of the acid hydrolysis severity by using the P-factor on the ground of the Arrhenius-type equation. The simplest kinetic model describing the cellulose hydrolysis is developed by Saeman [13]. This kinetic model presents the process as a two-step pseudo-first order reaction. A significant development of the Saeman's model is achieved through the introduction of additional parameters aiming a more accurate description of the mechanism of the hydrolysis process. In this respect, Yan et al. [14] develop a kinetic model by incorporating parameters, reflecting the effects of the reaction limitations provided by the cellulose crystallinity, and the formation of by-products such as acid-soluble lignin-glucose and humins. The applicability of this approach to understanding the mechanism of the cellulose degradation shows a positive effect, but it should be noted that the dependencies on the raw material and the changes of the reaction conditions make these models not robust enough. The aim of the present work is to study the kinetics of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp to MCC and to analyze the kinetic regularities.
Corresponding author. E-mail address: [email protected] (N. Yavorov).
https://doi.org/10.1016/j.carres.2020.107910 Received 24 September 2019; Received in revised form 19 December 2019; Accepted 7 January 2020 Available online 08 January 2020 0008-6215/ © 2020 Elsevier Ltd. All rights reserved.
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
2. Experimental 2.1. Materials The investigations were performed on industrial bleached hardwood kraft pulp provided by Svilosa AD, Svishtov (Northern Bulgaria, 43°64′N 25°30′E). The ISO brightness of the pulp was 90% (ISO standard 2470–1:2016), DP – 1200 (Ph. Eur.), while the pentosans content amounted to 20% of the dry matter (TAPPI standard T 223 cm-10). Sulfuric acid of an analytical grade was purchased from Valerus Ltd (Sofia, Bulgaria). 2.2. Acid hydrolysis Dilute acid hydrolysis was performed in stainless steel laboratory autoclaves under the following conditions: the sulfuric acid charge of 2% in respect to the dry pulp; 5, 10 and 20% pulp consistency; hydrolysis temperature values of 120, 130, 140 and 150 °C; reaction time at these temperatures varying from 40 to 120 min. The pulp was fully washed with deionized water to pH above 5.
Fig. 1. Xylose extraction at different temperature values, reaction times and pulp consistencies.
2.3. A degree of polymerization DP of MCC samples was measured according to the procedures described in the European Pharmacopoeia [15]. The standard deviation of the analyses, based on triplicates, was estimated to be 0.3%. 2.4. HPLC analysis The sugars released during the acid hydrolysis were quantified on a Dionex high-pressure liquid chromatography (HPLC) system (Dionex Inc., CA, USA) equipped with a Shodex RI-101 refractive index (RI) detector (Showa Denko KK, Kawasaki, Japan) according to the NREL Laboratory Analytical Procedure [16]. The separation was performed in a Hi-Plex H column, 7.7 mm × 300 mm (Agilent Technologies, USA) at 65 °C with ultrapure water (Simplicity®, Merck KGaA, Germany) as an eluent at a flow rate of 0.5 mL min−1. The samples were filtered through a 0.2 μm filter prior to HPLC analysis. The results were calculated as a percentage (%) in respect to the dry matter. 3. Results and discussion Fig. 2. Furfural quantity at different temperature values, reaction times and 20% pulp consistency.
Due to the high pentosans content of the bleached hardwood kraft pulp, the dilute acid hydrolysis generates mainly xylose during the initial period of the process. As evident from Fig. 1, the xylose extraction increases rapidly at the beginning of the process. The degree of hydrolysis depends on the temperature and the reaction time, but it is expected to be affected to a great extent by the pulp consistency and the acid concentration. The furfural content increases significantly at the end of the acid hydrolysis and depends also on the process temperature (Fig. 2). The established transformation is very slow at a low pulp consistency and a low temperature value. The conversion of furfural to dark by-products, usually denoted as humin-like substances, proceeds also. Humins are formed by side reactions and secondary transformations of furan compounds [17]. Rosenau et al. suggest that the humins have structures similar to those of the chromophoric compounds derived from hexeneuronic acids [18]. Due to the xylose transformations to furfural the xylose kinetic curves pass through a maximum at high temperature and pulp consistency values (Fig. 1). A relationship between MCC brightness and furfural formation is obtained, which shows that the furfural-derived chromophores are responsible for the pulp brightness decrease (Fig. 3). Therefore, the minimization of furfural and humins formation is a major goal in the production of MCC of high brightness. The yield of glucose, as seen from Fig. 4, increases much slowly compared to that of xylose indicating that this parameter is not
Fig. 3. MCC brightness versus the furfural content. 2
Carbohydrate Research 488 (2020) 107910
N. Yavorov, et al.
Table 1 A comparison of the correlation coefficient values obtained in the description of the process investigated by different kinetic equations usually applied to the study of heterogeneous catalytic processes. R2
Equation
First order kinetic equation ln(1-α) = - k.t Power kinetic equation α = g.tχ Exponential kinetic equation α = k + a−1.lnt Modified Prout-Tompkins equation α/(1-α) = (k.t)χ Avrami-Erofeev topochemical equation α = 1 - e−(k.t)n
determining the hydrolysis process. An insignificant transformation of glucose to 5-HMF is also found at 150 °C. The degree of polymerization is a main characteristic of MCC. It is well known that DP decrease during the acid hydrolysis is basically associated with cellulose chain breakage. On the other hand, the extraction of the low molecular xylan leads to a slight DP increase. A significant decrease of DP during the initial period of the acid hydrolysis (Fig. 5) is found in the course of the present investigation. The kinetics of the process is investigated by the dimensionless quantity α, which is determined as the relative change of DP and is calculated in correspondence with Eq. (1).
1200 DP 1200
130 °C
140 °C
150 °C
0.9825
0.9017
0.9663
0.9758
0.9982
0.9466
0.9948
0.9973
0.9992
0.9514
0.9958
0.9980
0.9995
0.9601
0.9976
0.9994
0.9994
0.9552
0.9968
0.9989
application to solid state reactions has led to the development of numerous reaction models, even in fields other than that of solid-state chemistry. According to that kinetic model, the hydrolysis rate depends on the combination of chemical interaction and diffusion processes. The P-Т equation describes well the kinetics of solids decomposition in case of diffusion controlled heterogeneous processes starting on the easily accessible outer surface of the pulp and continuing further through a gradual penetration into the capillary system of the fiber matrix. Former investigations [19–21] show that the kinetics of the enzymatic hydrolysis of steam exploded fast-growing tree species and bleached kraft pulp can be well described by the modified topochemical equation of P-T. The observed topochemical kinetic mechanism of DP decrease during the acid hydrolysis is probably determined by the characteristic fibrous structure of the pulp. The modified equation of Prout-Tompkins can be presented in the following form:
Fig. 4. Glucose yield versus the reaction time, the reaction temperature and the pulp consistency.
=
120 °C
(1)
(1
where 1200 is the initial value of pulp DP, while DP stands for the current values at different time and temperature values. The applicability of different kinetic equations referring to different heterogeneous catalytic and topochemical processes is verified. The kinetic investigation of the process of cellulose depolymerization shows that the modified Prout-Tompkins (P-T) equation describes relatively precisely the process of dissolution of cellulose amorphous domains (Table 1). This equation is applied to solids decomposition proceeding with product nucleus branching. The interpretation given to its
)
= (kt )
(2)
where k has the meaning of an apparent rate constant, while the power factor χ (0 < χ < 1) is an invariable quantity characteristic for the system. According to that kinetic model, the hydrolysis rate depends on the combination of chemical interaction and diffusion processes. All kinetic curves are linearized in coordinates ln 1 vs. lnt (Eq. (3)) in correspondence with the logarithmic form of Eq. (2) and the linear dependences obtained are presented in Fig. 6.
ln
=
1
ln k +
ln t
(3)
The value of the power coefficient can be accepted approximately equal to 0.3. d It is of interest to determine the current rate v = dt , which changes during the process. The derivative form of the modified P-T equation (2) is presented by:
v=
d = k dt
1
(1
)
+1
(4)
Eq. (4) provides the calculation of the current rate for different values of α and the temperature. The dependence of the rate v (min−1) versus α at different temperature values is shown in Fig. 7. The temperature dependence of the current rate is followed by Arrhenius equation (Eq. (5)):
v = Ae
E RT
(5)
where A is the pre-exponential factor, while E is the activation energy. The activation energy and the pre-exponential factor are calculated at different constant values of α. The linear relation between lnv and 1/ T (Fig. 8) provides the determination of the activation energy value
Fig. 5. DP of MCC versus temperature and time values. 3
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Fig. 6. Illustration of P-T equation linearization for the temperature values studied.
Fig. 9. A dependence of the pre-exponential factor on the kinetic variable α.
which is found equal to 136 kJ mol−1. It is evident that the activation energy does not change in the course of the process, i.e. the active centers do not change their activity. Unlike the activation energy, the pre-exponential factor changes with the increase of the kinetic variable α (Fig. 9). A correlation between the pulp brightness and DP is obtained. Its character does not depend on the temperature, the reaction time, the pulp consistency and the acid concentration (Fig. 10). The relationship illustrated in Fig. 10 shows that the one-step acid hydrolysis leads to the production of a low-brightness MCC when DP less than 300 irrespectively of the hydrolysis conditions. 4. Conclusions The kinetic investigation of dilute sulfuric acid hydrolysis of bleached hardwood kraft pulp aiming MCC production shows that the modified topochemical P-T equation describes relatively precisely the process of the cellulose depolymerization. The observed topochemical kinetic mechanism of DP decrease is probably determined by the fibrous structure, which is characteristic of the pulp. The value of the activation energy is constant indicating that the cellulose active centers do not change their activity in the course of the hydrolysis. A
Fig. 7. Current rate v versus α at different temperature values.
Fig. 8. Temperature dependences of the current rate and the rate constant at α = const.
Fig. 10. ISO brightness versus DP of MCC. 4
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transformation of xylose to furfural and furfural-derived chromophores is verified. It leads to a significant MCC brightness decrease. It is found that only a low-brightness MCC when DP below 300 can be obtained from bleached hardwood kraft pulp. The temperature-time dependence of the degree of the acid-catalyzed hydrolysis of bleached hardwood kraft pulp can be used for the process simulation and control.
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Funding This work was supported by the Bulgarian Ministry of Education and Science under the National Research Programme “Young scientists and postdoctoral students” approved by DCM № 577/17.08.2018. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D. Trache, M.H. Hussin, C.T.H. Chuin, S. Sabar, M.R.N. Fazita, O.F.A. Taiwo, T.M. Hassan, M.K.M. Haafiz, Microcrystalline cellulose: isolation, characterization and bio-composites application-A review, Int. J. Biol. Macromol. 93 (2016) 789–804, https://doi.org/10.1016/j.ijbiomac.2016.09.056. [2] M.H. Hussin, N.A. Husin, I. Bello, N. Othman, M.A. Bakar, M.K.M. Haafiz, Isolation of microcrystalline cellulose (MCC) from oil palm frond as potential natural filler for PVA-LiClO4 polymer electrolyte, Int. J. Electrochem. Sci. 13 (2018) 3356–3371, https://doi.org/10.20964/2018.04.06. [3] A. Pinkert, K.N. Marsh, S. Pang, M.P. Staiger, Ionic liquids and their interaction with cellulose, Chem. Rev. 109 (2009) 6712–6728, https://doi.org/10.1021/ cr9001947. [4] L.Q. Jiang, Z. Fang, F. Guo, L.B. Yang, Production of 2,3-butanediol from acid hydrolysates of Jatropha hulls with Klebsiella oxytoca, Bioresour. Technol. 107 (2012) 405–410, https://doi.org/10.1016/j.biortech.2011.12.083. [5] H. Krässig, J. Schurz, R.G. Steadman, K. Schliefer, W. Albrecht, M. Mohring, H. Schlosser, Cellulose, in: B. Elvers (Ed.), Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2012, pp. 279–332. [6] C. Rong, X. Ding, Y. Zhu, Y. Li, L. Wang, Y. Qu, X. Ma, Z. Wang, Production of furfural from xylose at atmospheric pressure by dilute sulfuric acid and inorganic salts, Carbohydr. Res. 350 (2012) 77–80, https://doi.org/10.1016/j.carres.2011.
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