Kinetics of Acid Hydrolysis of Arabinogalactans

Transcription

1 INTERNTINL JURNL F CEMICL RECTR ENINEERIN Volume 8 21 rticle 44 Kinetics of cid ydrolysis of rabinogalactans Bright T. Kusema Chunlin Xu Päivi Mäki-rvela Stefan Willför Bjarne olmbom Tapio Salmi Dmitry Y. Murzin Åbo kademi University, Finland, bright.kusema@abo.fi Åbo kademi University, Finland, chunlin.xu@abo.fi Åbo kademi University, Finland, paivi.maki-arvela@abo.fi Åbo kademi University, Finland, stefan.willfor@abo.fi Åbo kademi University, Finland, bjarne.holmbom@abo.fi Åbo kademi University, Finland, tapio.salmi@abo.fi Åbo kademi University, Finland, dmitry.murzin@abo.fi ISSN Copyright c 21 The Berkeley Electronic Press. ll rights reserved.

2 Kinetics of cid ydrolysis of rabinogalactans Bright T. Kusema, Chunlin Xu, Päivi Mäki-rvela, Stefan Willför, Bjarne olmbom, Tapio Salmi, and Dmitry Y. Murzin bstract The kinetics of the acid hydrolysis of arabinogalactans () was studied isothermally in a batch reactor. was hydrolyzed with hydrochloric acid and the main parameters established were the acid concentration (p), temperature and concentration. The hydrolysis rate increased with the acid concentration (p) and temperature. Complete hydrolysis of to arabinose and galactose was achieved at 9 C and p 1 without any further degradation of the sugars. first-order kinetic model including two simultaneous reactions for the formation of arabinose and galactose was successfully fitted to the experimental data. The rate constants and activation energies were calculated from the model. The decrease of the average molecular weight was also explained by the model. KEYWRDS: kinetics, acid hydrolysis, arabinogalactans This work is part of the activities of the Process Chemistry Centre (PCC) at Åbo kademi University within the Finnish Centre of Excellence Programme (26-211) by the cademy of Finland. uthors are affiliated with the Process Chemistry Centre, Laboratory of Industrial Chemistry and Reaction Engineering, Åbo kademi University, Åbo/Turku FI-25, Finland.

3 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 1 Introduction rabinogalactans () are hemicelluloses which appear in large quantities in larch species. The structural basis of (Figure 1) is a backbone of β- galactopyranose residues that are predominantly (1 3) linked and most frequently branched with D-galactopyranose, L-arabinofuranose and D-glucuronic acid side chains (Willför and olmbom 24). The average ratio of galactose, arabinose and glucuronic acid in the arabinogalactans is about 5:1:.8. The molar mass is 2 1 g/mol (Willför et al. 22). The content of is about 15 weight % of dry heartwood in larch species... Figure 1. Structural features of larch arabinogalactans. can easily be extracted from larch wood powder with water at moderate temperatures. can also be recovered in industrial scale from process waters in thermomechanical pulp mills (Person et al. 27); therefore it has a great potential to serve as a sustainable feedstock for valuable chemicals. The isolation method consists of two process steps. The suspended matter in the process water is removed by microfiltration and thereafter the hemicelluloses are concentrated by ultrafiltration, and at the same time, separated from smaller molecules and ions in the process water. Larch arabinogalactan has direct medical applications as a health product (Kelly 1999). In foods, larch arabinogalactan is used as a stabilizer, emulsifier, binder, and sweetener (Winter 1978). The monomers, L-arabinose and D- galactose have applications as specialty sugars in the food, pharmaceutical and cosmetic industries. The focus of this research is to convert into valuable products, through a sugar platform, which includes two processes: hydrolysis of to monomers as shown in Scheme 1 and transforming the sugars into biobased valuable products. The topic is timely as hydrolysis of plant-derived Published by The Berkeley Electronic Press, 21

4 2 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 polysaccharides to platform sugars in good yields is an increasingly important issue... ydrolysis Cl + Scheme 1. cid hydrolysis of to arabinose and galactose. In general, the acid hydrolysis of hemicelluloses is influenced by their structures, the conformation of the individual sugar units and the acidic medium (Lai 21). In hydrolysis or depolymerization of the polysaccharide, the bonds between the sugar units are cleaved to form simple sugars and partially hydrolyzed oligomers. Depolymerization of the polysaccharide can be accomplished through chemical, thermal (Severian 25), enzymatic (Tayal et al. 1999) and ultrasonic processes (Tayal et al. 2), which are dependent on the structures and conformation of the polymers and the reaction medium. In chemical degradation, acid hydrolysis, cleavage of the bonds in the polysaccharide is the main mechanism for the chain scission (batzoglou and Chornet 1998). The challenge is to identify the reaction conditions and catalysts to convert the polysaccharide to monomers and at the same time to avoid further degradation of sugars to products such as furfural and hydroxymethylfurfural (MF) which are undesirable. The acid hydrolysis of other hemicellulose, such as water-soluble -acetyl galactoglucomannan (M) has been studied recently (Xu et al. 28). The kinetics of the acid hydrolysis of M was investigated at temperatures up to 9 C in the p range of 1-3. The molar mass of M decreased considerably with the treatment time at temperatures exceeding 7 C and p below 2. first order kinetic model was applied and the activation energy E was 15 kj mol -1 for the acid hydrolysis of spruce M. The aim of this study is to investigate the kinetics of acid hydrolysis of. Previous kinetic studies on acid hydrolysis of various hemicelluloses claim that the rate constant of the hydrolysis is independent of the acid, but dependent only on the proton concentration [ + ] (Blecker et al. 22). In this work, the hydrolysis of was studied by using hydrochloric acid. This investigation was undertaken to develop a kinetic model and to determine the reaction parameters specifically applicable for the acid hydrolysis of. The effects of process variables such as p, temperature and concentration on the kinetics of acid hydrolysis were investigated.

5 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 3 Experimental Materials Larix sibirica wood samples were separately splintered, freeze-dried and ground in a Cyclo-Tec mill (Tecator Inc.) producing particles smaller than 3 mesh (.5 mm). The wood powder was extracted in a Soxhlet apparatus with methyl tertbutyl ether (MTBE), to remove lipophilic extractives. Figure 2 shows the procedure used for isolation of water-soluble hemicelluloses, arabinogalactans from wood tissues (Willför and olmbom 24). The extract was purified by precipitation in ethanol. The precipitate was air dried and in a vacuum drier. The raw obtained was of high purity, +95 % dry weight. The yield of recovered by extraction after precipitation in ethanol was around 8 % based on processed wood. The solutions for acid hydrolysis were prepared by dissolving the required amount of in distilled water at room temperature (weight %). cid hydrolysis The acid hydrolysis of arabinogalactan was conducted isothermally in a batch stirred reactor. 1 ml of.5 weight % solution was prepared by dissolving.5 g of in 1 ml distilled water. The solution was preheated and the p was adjusted by adding 1M Cl to the required p values of 1, 2 and 3. The solution was loaded to the reactor and the temperature set to the desired value. This moment was considered as the initial starting point of the experiment. The samples for analysis, about 5 ml, were periodically withdrawn from the reactor and transferred to glass tubes and the temperature was immediately lowered to stop the reaction by placing them in ice water. The samples were further neutralized with 1 M Na solution to a neutral p of 6 7. nalysis The monosaccharides were analyzed by gas chromatography (C) equipped with a flame ionization detector (FID) on a 25 m x.2 mm i.d. column coated with cross-linked methyl polysiloxane (P-1) after direct silylation of the sample (Sundberg et al. 1996; Bertaud et al. 22). The column oven parameters were 1 C raised at 2 C/min to 17 C, and 12 C/min to 29 C (4 min); carrier gas 2 (4 ml/min); split injector 26 C (1:15); injection volume 1 µl. Published by The Berkeley Electronic Press, 21

6 4 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 Wood Drying and grinding Wood powder Extraction with MTBE Extractive free wood powder Mixing with distilled water and stirring for 1.5 h, 2 C Filtration Wood soluble substances Precipitation in ethanol rabinogalactans Figure 2. Procedure for isolation of water-soluble polysaccharides, arabinogalactans, from wood.

7 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 5 The total carbohydrate composition of the polysaccharide was analyzed by acid methanolysis followed by silylation and gas chromatography (C-FID) of the silylated sugar monomers. The C oven parameters were 1 C raised at 4 C/min to 175 C, and at 12 C/min to 29 C (5 min) (Sundberg et al. 1996; Bertaud et al. 22). The average weight molar mass (Mw) and the molar mass distribution (MWD) of the polysaccharide were determined by high pressure size exclusion chromatography (PLC-SEC) in online combination with a multiangle laser light scattering (MLLS) instrument and a refractive index (RI) detector according to Xu et al. 28. two column system, 2 x UltrahydrogelTM linear 7.8 x 3 mm columns in series was used..1 M NaN 3 was used as the elution solvent and the flow rate of.5 ml/min was applied. The samples were filtered through a.2 µm nylon syringe filter before injection. The injection volume was 1 µl. Low molecular products such as furfural and hydroxymethylfurfural (MF) which appear as a result of further degradation of sugars were identified by C-MS with MSD 5973 P detector on a 25 m x 2 µm x.11 µm, 5% phenyl methyl siloxane column (P-5). The column oven parameters were 45 C raised at 1 C/min to 135 C, and 3 C/min to 2 C; carrier gas e (5 ml/min); split ratio (5:1) 25 C; injection volume 1 µl. Iso-propanol was used as an internal standard. Results and Discussion Qualitative kinetics In the acid hydrolysis of, the two main monosaccharides, L-arabinose and D- galactose were obtained at temperatures up to 1 C, p = 1, 2 and 3, and the initial concentration C =.5, 2.5 and 5. weight %. Typical kinetic curves of the hydrolysis and the released monomers are shown in Figure 3. complete conversion of was achieved after 14 min., no was detected. It can be noted from the results that arabinose was released totally within 12 min, whereas there was a continuous gradual increase of the galactose concentration during 14 min. s can be seen, the monomers, arabinose and galactose were stable at the above mentioned conditions. There was no decrease in the quantity of arabinose shown by a straight curve in Figure 3 and no furfural was detected. The molar ratio of the structural units of arabinose and galactose in the initial was 1:6. The yield of arabinose and galactose in the final products of hydrolysis was 95%. The molar mass (Mw) reduction of with reaction time is shown in Figure 4. There was a sharp decrease in molar weight within the first 12 min, which is explained by the presence of the two simultaneous reactions until Published by The Berkeley Electronic Press, 21

8 6 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 complete release of arabinose followed by the gradual hydrolysis of galactose residue. 1 8 rabinose alactose C, mg/g Time, min Figure 3. Typical kinetic curves of hydrolysis to arabinose and galactose at 9 C and p Mw, kda Time, min Figure 4. Molar mass reduction of during acid hydrolysis at 9 C and p 1. The results of hydrolysis at 9 C and different p values are shown in Figure 5. The residual concentrations were plotted against the reaction times for the different reaction conditions. In each of the experiments, the p value of the solution was adjusted to 1, 2 and 3, respectively. The experimental results show a very strong dependence of the hydrolysis rates on the acid concentration (p). complete conversion of was achieved at p 1 after 14 min with only 25% and less than 1% conversion at p 2 and 3, respectively. t p 1, was completely hydrolyzed to monomers achieving the maximum quantities of

9 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 7 free sugar units from the polysaccharide. In the case of p 2, there was almost complete release of arabinose from the polysaccharide with small amounts of galactose monomers leaving a partially hydrolyzed. The slowest hydrolysis rates of with hardly any galactose sugar units being cleaved from the chain were observed at p 3. Under these conditions, the obtained results suggest that the lowest acid concentration was not sufficient to hydrolyze the bonds of. Thus we conclude here that it is easy to hydrolyze the arabinose unit from the side chains of followed by the release of galactose from the main chain. 1 8 C, mg/g p=1 p=2 p= Time, min Figure 5. Effect of p on the hydrolysis. The temperature dependence of the hydrolysis is demonstrated in Figure 6. s expected, the reaction rate increased with temperature. The hydrolysis rate of at temperatures lower than 7 C was very slow. t 8 C, only complete release of arabinose was achieved, but partially hydrolyzed galactose residue was left. The conversion of was 43%. complete conversion of to monomers was achieved at 9 C and 1 C. t these conditions no was detected. It can be noted that after the hydrolysis at 1 C, traces of degradation products such as furfural were observed. For this reason, the temperature for hydrolysis shall not exceed 1 C. The kinetic parameters of hydrolysis were calculated based on the temperature dependent results in the kinetic modelling section. Published by The Berkeley Electronic Press, 21

10 8 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle T=1 C T=9 C T=8 C C, mg/g Time, min Figure 6. Effect of temperature on hydrolysis. Kinetic modelling kinetic model based on the experimental results was developed. The hemicellulose structure was considered to be composed of two distinct fractions, one that is relatively easy to hydrolyze and the other more difficult (Lloyd and Wyman 23; Maloney et al. 1986). The arabinose residue, because of its easier accessibility might release faster, while the galactose units would be produced more slowly. The kinetic model was therefore set into the reaction scheme described. pplying the first order kinetics with respect to the functional groups for both reactions, the following rate equations can be obtained: r k c c 1, 1 = (1) r k c c 2, 2 = (2) where r 1 and r 2 are the rates of hydrolysis to arabinose and galactose respectively, k 1 and k 2 are the kinetic rate constants, c = [ + ], the acid concentration (mol L -1 ), and c, and c, denote the arabinose and galactose fractions in the polysaccharide, i.e. easy to hydrolyze (arabinose) and more difficult to hydrolyze (galactose) hemicellulose fractions, respectively. The temperature dependence of the acid hydrolysis was modelled using the rrhenius equation,

11 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 9 Ea1 k1 = k1 exp (3) RT Ea2 k2 = k2 exp (4) RT where k 1 and k 2 are the dimensionless pre-exponential constants, E a1 and E a2 the activation energies (J mol -1 ), R is the universal gas constant ( mol -1 K -1 ), and T is the reaction temperature (K). The mass balances of the functional groups (i) in the constant-volume batch reactor are written in the form: dc i = r (5) i dt Eq. (5) assumes that the liquid-phase volume remains constant during the reaction. The concentrations of arabinose () and galactose () units liberated in the hydrolysis are thus obtained from the differential equations dc dt dc dt = k c c (6) 1, = k c c (7) 2, where c, and c, give the concentrations of the arabinose and galactose units in arabinogalactan. The following total balance is valid for the monomer units c c, = co, + (8) c c, = co, + (9) which implies that the total amounts of the arabinose and galactose units remain constant. This assumption is justified, since the amounts of the degradation products were negligible. fter inserting the relations (8)-(9) into the differential equations (6)-(7), the expressions Published by The Berkeley Electronic Press, 21

12 1 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 dc dt dc dt = k c c c ) (1) 1 ( o, = k c c c ) (11) 2 ( o, are obtained. n the other hand, it should be remembered that the initial amounts correspond to the final amounts of arabinose and galactose in the case of complete hydrolysis, i.e. c, =c and c, =c. These relations are inserted in the mass balances (1)-(11), which are integrated with the initial condition t=, c = and c =. The logarithmic functions are obtained: ln( 1 c / c ) = k c t (12) 1 ln( 1 c / c ) = k c t (13) 2 The validity of the proposed model can be checked by plotting the right-hand sides of eqs (12)-(13) versus the reaction time. Furthermore, division of the above equations gives ln( 1 c / c ) = ( k1 / k2 )ln(1 c / c ) (14) i.e. a double logarithmic plot ln(1-c /c ) vs. ln(1-c /c ) should give a straight line, if the model is valid. The slope gives the ratio between the rate constants k 1 /k 2 which is independent of the acid catalyst concentration (c ). The test plots corresponding to eqs (12)-(13) are shown in Figure 7a. s revealed by the figure, the lines are straight, which indicates the validity of the model. The double logarithmic plot, eq. (14) is shown for various catalyst concentrations in Figure 7b. The plots obtained with different acid catalyst concentrations coincide as predicted by the model. The test plots gave preliminary values for the rate constants.

13 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 11 -ln(1-c/c ) a. 4, 3,5 3, 2,5 2, 1,5 1,,5, Time, min rabinose alactose -ln(1-c /c ) b. 4, 3,5 3, 2,5 2, 1,5 1,,5,,,1,2,3,4,5 -ln(1-c /c ) p 1 p 2 p 3 Figure 7. (a.) Test plots for the hydrolysis of. (b.) Double logarithmic plot for the determination of the ratio of the rate constants (k 1 /k 2 ). The final values of the rate constants, along with their temperature dependencies were obtained with non-linear regression analysis, which was applied to the differential equations (1)-(11). The model was treated with ModEst, the software for parameter estimation, simulation and optimization (aario 27). The differential equations were solved in situ with the backward difference method implemented in ModEst. The kinetic parameters were estimated by using a combined Simplex-Levenberg-Marquardt method, which minimizes the residual sum of squares between the estimated and the experimental concentrations with non-linear regression (Marquardt 1963). In this case, the objective function which was minimized in the non-linear regression is given by = ( c ) 2 i c Q (15) i t, t exp i, t where c i,t denotes the concentrations calculated from the model and c i,texp gives the experimentally recorded concentrations of the arabinose and galactose units i at the reaction time t. The parameter estimation results are summarized in Table 1 obtained from the kinetic model of the experimental data. The degree of explanation is used for describing the adequacy of the model to fit the experimental data. The model fits the experimental results well, having an explanation factor of 98%. Published by The Berkeley Electronic Press, 21

14 12 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 Table 1. Results from the parameter estimation. Parameter Value Relative standard error, % E a1, kj mol E a2, kj mol k k The contour plots of rrhenius law with the two parametrizations for the same data are shown in Figure 8. The contour plots show more globally the sensitivity of the optimal points for the activation energies and the pre-exponential constants for arabinose (E a1, k 1 ) and galactose (E a2, k 2 ) respectively. The identifiability of the parameters was well defined. Figure 8. Contour plots for rrhenius parameters k, E a. Modelling of the average molecular weight The average molecular weight measured experimentally (Figure 4) can be related to the concentrations of the arabinose and galactose units. simplified treatment

15 Kusema et al.: Kinetics of cid ydrolysis of rabinogalactans 13 based on the average numbers is presented here. The average molecular weight (M) is M N, M + N, = M (16) where N, and N, are the average numbers of the corresponding monomer units in the macromolecule; M and M are the molecular weights of arabinose and galactose, respectively. fter the reaction has been progressing a while, the number of units is diminished to N, N, N / N = (17) N, N, N / N = (18) where N and N B denote the numbers of arabinose and galactose monomers formed. N is the initial number of macromolecules, each of them having originally N, and N, arabinose and galactose units in average. These relations are inserted in the definition of the average molecular weight, eq. (16), which obtains the form ( N, N / N ) M + ( N, N / N M = ) M (19) rearrangement gives M = N ( N (2), M + N, M N M + N M ) / where N, M + N, M is de facto the initial average molecular weight (M ). The vogadro number (N) is introduced (N =N n and N =N n, where n and n are the molar amounts). Eq. (2) becomes M = M N / N )( n M + n M ) (21) ( fter introducing the concentrations (n =c V L and n =c V L ), we get M = M NV / N )( c M + c M ) (22) ( L which can be written in a compact form Published by The Berkeley Electronic Press, 21

16 14 International Journal of Chemical Reactor Engineering Vol. 8 [21], rticle 44 M M = β ( αc + c ) (23) where α=m /M and β=nv L M /N. Eq. (23) implies that a decrease of the average molecular weight is proportional to c +αc, α is known and a test plot M -M versus c +αc should give a straight line with the slope β. The test plot is shown by Figure 9. s the figure shows, the decrease of the average molecular weight is proportional to the sum of the arabinose and galactose concentrations. This confirms that the simple model for the change of the molecular weight can be used for the hydrolysis of arabinogalactan M -M αc +c Figure 9. The dependence of average molecular weight on the concentrations of the monomers. Conclusions The kinetics of acid (Cl) hydrolysis of arabinogalactan was studied isothermally in a batch reactor. Experimental results showed that the hydrolysis rate increased with both the acid concentration and the reaction temperature. Under mild operating conditions, i.e. at low temperatures and Cl concentrations, only complete release of arabinose took place, leaving the galactose fraction as partially hydrolyzed residue. rabinogalactan was fully hydrolyzed to arabinose and galactose without further degradation of the monomers at the optimum conditions of p=1 and a temperature of 9 C with respect to the functional units. first order kinetic model was found to describe the kinetic data. The activation energies for the release of arabinose and galactose were 126 kj mol -1 and 135 kj mol -1, respectively. The decrease of the average molecular weight was explained by the model.

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