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This is an author generated post-print of the article: Lee, K.M., Ngoh, G.C, & Chua, A.S.M. (2015). Ionic

liquid-mediated solid acid saccharification of sago waste: Kinetic, ionic liquid recovery and solid acid catalyst

reusability study. Industrial Crops and Products, 77, 415-423. doi:10.1016/j.indcrop.2015.09.016

Ionic liquid-mediated solid acid saccharification of sago waste: Kinetic, ionic liquid

recovery and solid acid catalyst reusability study

Kiat Moon Lee, Gek Cheng Ngoh,* and Adeline Seak May Chua

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603

Kuala Lumpur, Malaysia

* Corresponding Author:

Dr. Gek Cheng Ngoh

Postal address: Department of Chemical Engineering, Faculty of Engineering, University of

Malaya, 50603 Kuala Lumpur, Malaysia

E-mail: ngoh@um.edu.my

Telephone: +603-7967 5301

Fax: +603-7967 5319

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Abstract

Dissolved carbohydrate obtained from 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)

pretreated sago waste was saccharified by Amberlyst 15 (A15) to study the kinetics of the

ionic liquid-mediated solid acid saccharification of the lignocellulosic biomass. Besides the

kinetics of both reducing sugars production and reducing sugars degradation reactions,

emphasis was given to the recovery and reusability of the ionic liquid and solid acid catalyst.

The hydrolysis reaction follows a first order sugars production-first order sugars degradation

reaction that agreed well with the generalized Saeman kinetic model. Both the rates of

reducing sugars production and degradation varied with temperature and catalyst loading.

Higher temperature and catalyst loading promoted the rates of sugars production reaction

more than the rate of degradation reaction. This is evident from the relatively larger rate

constant of sugars production (k1) than the rate constant of degradation (k2). The activation

energy for sugars production and degradation was 125.1 kJ mol-1

and 60.8 kJ mol-1

respectively. These values are lower than the literature reported values for sulfuric acid-

catalyzed saccharification confirming its employment suitability for reducing sugars

production. The good recovery of [BMIM]Cl and good reusability of A15 further

substantiated the feasibility of the process. The recovery of ionic liquid was improved by

employing a multiple step aqueous biphasic system in this study. Approximately 60% of the

ionic liquid was recovered by 3-step aqueous biphasic system. This finding is on par with the

reported literatures using aqueous biphasic system to recover ionic liquid. Equally robust, the

solid acid catalyst regained its saccharification performance after regeneration for a total of

three cycles.

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Keywords: solid acid catalyst; ionic liquid; kinetics; starch; reducing sugars; product

recovery and reusability

1. Introduction

Sugars are abundantly available in the form of starch, cellulose and hemicellulose

carbohydrates in lignocellulosic biomass. Generally, acid-catalyzed saccharification reaction

is used to convert the carbohydrates to reducing sugars. With technological advancement,

many efficient acid catalytic systems such as strong acidic cation exchange resins (Kim et al.,

2005; Lanzafame et al., 2012; Marzo et al., 2012; Onda et al., 2008), H-type zeolites

(Lanzafame et al., 2012; Onda et al., 2008), heteropolyacids (Lanzafame et al., 2012) and

supported solid acids (Lanzafame et al., 2012; Onda et al., 2008) have been reported.

However, even with the strong acidic cation exchange resins as powerful catalysts for

saccharification (Kim et al., 2005; Kitano et al., 2009), the recalcitrant nature of biomass

prevents chemical transformation and results in low saccharification rate (Ballesteros et al.,

2008; Hu and Wen, 2008). This predicament has spurred the effort of researchers to employ

ionic liquids in biomass dissolution (Fort et al., 2007; Haykir et al., 2013; Kilpeläinen et al.,

2007; Li et al., 2010; Li et al., 2009; Mäki-Arvela et al., 2010; Sun et al., 2009; Yang et al.,

2013; Zavrel et al., 2009) to assist solid acid saccharification of biomass in the production of

reducing sugars (Dwiatmoko et al., 2010; Kim et al., 2010; Rinaldi et al., 2008; Watanabe,

2010; Lee et al., 2013).

Despite that the process scheme to produce reducing sugars via ionic liquid-mediated

solid acid saccharification of lignocellulosic biomass has been reported viable (Dwiatmoko et

al., 2010; Watanabe, 2010), the cost of ionic liquid and solid acid catalyst undermines the

economic feasibility of the process concerned. To alleviate the cost constraint, aqueous

4

biphasic systems (ABS) were attempted by researchers to separate and recover ionic liquid

(Gutowski et al., 2003). The two phase system generated by adding kosmotropic salt into

aqueous solution of ionic liquid was environmentally more benign than the conventional

extraction using organic solvents (Hatti-Kaul, 2000; Zaslavsky, 1995). On the other hand, the

reused solid acid catalyst regenerated with sulfuric acid (Rinaldi et al., 2010) to restore the

active sites for saccarification process had also indicated positive recovery performance.

The reaction kinetics of catalytic reaction is essential for process optimization and

reactor design. Hypothetically, solid acid saccharification of the dissolved carbohydrates in

ionic liquid is expected to have distinct kinetics. The well-known cellulose hydrolysis model

(Saeman, 1945) had been applied in hydrolysis of other carbohydrates such as starch (Barnali

et al., 2008) and hemicelluloses (Lu and Mosier, 2008). Based on the model, the present work

performed a kinetic study of a sequential ionic liquid-mediated solid acid catalytic reaction

on sago waste that has not been reported elsewhere. The effects of temperature and catalyst

loading on the production rate and degradation rate of reducing sugars were investigated. The

generalized Saeman kinetic model applied to fit the experimental data to determine the

kinetic parameters of the hydrolysis process. To study the overall process economic

feasibility, the recovery efficiency and reusability of ionic liquid and solid acid catalyst were

examined in this study.

2. Materials and methods

2.1. Materials

The sago waste supplied by CL Nee Sago Industries (Malaysia) was ground and

sieved into desired size of approximately 500 μm. Ionic liquid 1-butyl-3-methylimidazolium

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chloride ([BMIM]Cl, ˃98.0% purity) was from Merck (Germany), Amberlyst 15 (A15) and

potassium phosphate (≥98% purity) were purchased from Sigma-Aldrich (USA). The

ultrapure water used was generated by Arium® 611UF system (Sartorius, Germany).

2.2. Saccharification of sago waste

Sago waste (15 mg) was dissolved in a test tube containing 1 g of [BMIM]Cl using an

oil bath (MC, Julabo, Germany) at 160oC for 1.75 h. The clear solution formed was added

with 5 ml of ultrapure water and the mixture was evenly mixed. The resultant solution,

known as prehydrolysate, was saccharified with A15 of 2% (w/v) to 10% (w/v) loading in the

oil bath at a desired temperature, range of 100oC to 140

oC for 5 h. The saccharified samples

were collected at predetermined time of 5, 10, 15, 30, 60, 120, 180, 240 and 300 min. The

liquid product, known as hydrolysate, once separated from the catalyst through gravity

settlement, was determined for its reducing sugars content using 3,5-dinitrosalicylic acid

(DNS) assay (Miller, 1959). All experiments were conducted in duplicate. The reducing

sugars yield was calculated using Equation (1):

Reducing sugars yield (%) = wRS / [1.11(ws+wc) + 1.14wh] × 100% (1)

where wRS represents the weight of reducing sugars, ws, wc and wh represent the weight of

starch, cellulose and hemicelluloses in sago waste, respectively. The values 1.11 and 1.14 are

the multiplication factors that convert the respective carbohydrate to its equivalent sugars

(Sluiter et al., 2011).

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2.3. Measurement of protons concentration

The concentration of protons ([H+]) in the hydrolysate was determined by measuring

the pH of the solution according to Equation (2):

pH = - log10 [H+] (2)

The pH of the reaction solution was measured at room temperature using a pH meter (827 pH

lab, Metrohm, USA). Prior to use, the pH meter was calibrated using buffer solutions of pH

4.00, 7.00 and 9.00.

2.4. Hydrolysis kinetic model

The generalized Saeman model, i.e. two consecutive first order sugars production and

sugars degradation reaction as expressed in Equation (3), was applied to determine the

kinetics of solid acid saccharification of dissolved carbohydrates in ionic liquid.

Polymers 𝑘1 Monomers

𝑘2 Degradation products (3)

The polymers and monomers expressed in the equation respectively represent the

carbohydrates and the reducing sugars, while k1 and k2 are the rate constant for production

and degradation of reducing sugars respectively.

For an irreversible first order reaction, the model shown in Equation (3) can be

described in two differential equations as expressed in Equation (4) and (5):

rS = – dS/dt = – k1S (4)

rP = dP/dt = k1S – k2P (5)

where rS is rate of degradation of carbohydrate, rP is rate of production of reducing sugars, S

is concentration of carbohydrate, P is concentration of reducing sugars and t is reaction time.

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By solving the differential equations, concentration of the reducing sugars as a function of

time can be determined using Equation (6):

P = Po × exp(–k2t) + So × k1/(k2 – k1) × (exp(–k1t) – exp(–k2t)) (6)

where the subscript o indicates the respective initial concentration.

The time required to reach maximum concentration of reducing sugars was calculated

using Equation (7). This equation is derived by setting the derivative of Saeman model,

Equation (6), to zero for maximum concentration of reducing sugars.

tmax = ln(k2/k1) / (k2 – k1) (7)

The dependence of the rate constants k on temperature can be expressed by Arrhenius

equation as in Equation (8):

k = A × exp(–Ea/RT) (8)

where A is the Arrhenius constant, Ea is the activation energy, R is universal gas constant

(8.314 J mol-1

K-1

) and T is the absolute temperature. The acid concentration and the rate

constant k at constant temperature is related as shown in the equation below (Saeman, 1945):

k = W [H+]

m (9)

where W is a constant, [H+] is the proton concentration and m is the gradient of line in

logarithm k against logarithm of [H+] plot. The kinetic parameters of the model were

determined using MATLAB R2009b (MathWorks Inc., USA).

2.5. Recovery of ionic liquid

Aqueous biphasic system (ABS) was employed to recover the ionic liquid from the

hydrolysate. Potassium phosphate solution of 80% (w/w) concentration was added to the

hydrolysate in a drop wise manner until a phase separation was observed via a change of

clear to turbid solution. The turbid solution formed two phases after centrifuged (Centrifuge

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5810R, Eppendorf, Germany). For each drop of potassium phosphate solution added, the

mixture in the test tube was shaken and the resulting solution was examined for turbidity. The

amount of potassium phosphate added was recorded and also the volumes of the two phases

formed. A portion of the bottom phase of the biphasic system was withdrawn and subjected

to second step of extraction with potassium phosphate solution. In all the extraction steps

conducted, samples from both phases were taken and analyzed using high-performance liquid

chromatography (HPLC). A Hi-Plex H column equipped with appropriate guard column

(Agilent Technologies, USA) and a refractive index detector (Waters, USA) were used to

detect and quantify the amount of [BMIM]Cl in the sample. The column temperature was

maintained at 65oC and the mobile phase was filtered, degassed ultrapure water with flow

rate at 0.6 ml/min.

The extraction efficiency of ionic liquid, %Ei in a particular extraction step, i was

calculated according to Equation (10):

%Ei = [[BMIM]Cl]Ui × VUi / {[[BMIM]Cl]Ui × VUi + [[BMIM]Cl]Bi × VBi} × 100 (10)

where [[BMIM]Cl] is the concentration of ionic liquid, subscript U and B refer to the upper

and bottom phases in the ABS respectively, and V is the volume of the particular phase. The

total extraction efficiency of ionic liquid, %E was expressed in Equation (11):

%𝐸 = BMIM Cl 𝑈𝑖× 𝑉𝑈𝑖

𝑛𝑖=1 𝑚 BMIM Cl × 100 (11)

where n is the total number of extractions, m[BMIM]Cl is the total mass of [BMIM]Cl in the

hydrolysate prior to ABS.

2.6. Regeneration and reusability of solid acid catalyst

The solid acid catalyst Amberlyst 15 (A15) was regenerated according to the

procedure suggested by Rohm and Haas Company, USA. The spent A15 was collected and

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loaded into a glass tube with a perforated end to create a catalyst bed. To initiate the

regeneration, A15 catalyst bed was flushed with sufficient amount of ultrapure water to

remove any unwanted chemical. The water was drained followed by pumping 10% (v/v)

sulfuric acid solution at a rate of 4 bed volumes (BVs) per hour until a total of 1.5 BVs of

sulfuric acid was passed through the catalyst bed. The rinsing of the bed was repeated with

ultrapure water to remove excess sulfuric acid from the catalyst. Rinsing was carried out at

the same flow rate as the regeneration flow rate until approximate 1 or 2 BVs of ultrapure

water had passed through the bed. The flow rate of the rinsing water was then increased to 12

BVs/h and continued until the effluent registered a pH of above 4. After the water had been

drained, catalyst was placed in an oven at 105oC overnight to remove water moisture. The

dried regenerated A15 was subsequently used for solid acid saccharification.

3. Results and discussion

3.1. Effects of temperature and catalyst loading on the rate of production and degradation

of reducing sugars

In this study, the sago waste sample contained 36.5 ± 1.2% (w/w) starch, 40.0 ± 2.2%

(w/w) cellulose, 13.1 ± 0.1% (w/w) hemicellulose and 4.5 ± 0.1% (w/w) lignin (Lee et al.,

2014). In view of its high carbohydrates content, sago waste is favorable for reducing sugars

production. The concentrations of reducing sugars during the course of hydrolysis at different

operational conditions are shown in Figure 1. An increase in reducing sugars concentration

was observed initially for all experimental conditions. This suggests a net production of

reducing sugars due to the rate of reducing sugars production was greater than the

degradation reaction. However, when the saccharification reaction was conducted for longer

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than 180 min at 100oC, the concentration of reducing sugars remained constant, indicating the

production and degradation of sugars were progressed at the same rate. In the present study,

the hydrolysis of carbohydrate was performed in a batch test. Thus, the carbohydrates

decreased with the progression of hydrolysis as it gradually converted into reducing sugars.

Since reaction rate is proportional to reactant concentration, the rate of reducing sugars

production was reduced proportionally to the carbohydrate concentration. As hydrolysis of

carbohydrates produced more reducing sugars, rate of degradation would thus be increased.

Therefore, further reduction in the reducing sugars concentration had occurred. This

phenomenon was obvious at 120oC and 140

oC as shown in Figure 1 (b) and (c) respectively.

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Fig. 1. Concentration of reducing sugars at different temperature and catalyst loading: (a)

100oC; (b) 120

oC; and (c) 140

oC.

At the same temperature, different catalyst loadings gave different profiles in reducing

sugars concentration over a period of time. At low temperature of 100oC and 120

oC,

concentration of reducing sugars reached maximum at a shorter time for a higher catalyst

loading. For instance, 10% catalyst loading took about 2 h to achieve maximum

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concentration of reducing sugars, but 6% catalyst loading required 3 h at the same

temperature of 100oC (refer to Figure 1(a)). At higher temperature of 140

oC, three

investigated catalyst loadings reached maximum concentration of reducing sugars in less than

30 min. Due to degradation, the concentration of reducing sugars decreased as hydrolysis

reaction continues to take place. Higher catalyst loading experienced a greater extent of

degradation and resulted in lowest concentration of reducing sugars at the end of reaction

(refer to Figure 1(c)).

The study showed that different temperatures and different catalyst loadings produced

different profiles of reducing sugars concentration, which in turn suggests that these

parameters affect both the rates of production of reducing sugars and degradation of sugars.

These rates increased with temperature and acid concentration. The effect of the temperature

can be explained by the endothermic reaction of the hydrolysis process, whereby high

temperature favors the reaction (Jin et al., 2011). Meanwhile, an increase in the acid

concentration had increased the amount of protons available for hydrolysis at higher catalyst

loading. To elucidate the temperature and acid concentration effect on the reaction rates, a

detailed kinetic study was performed.

3.2. Determination of kinetic rate constants of reducing sugars production and

degradation

Having the sago waste dissolved in ionic liquid, the dissolved carbohydrates will be

uniformly hydrolyzed by the A15 during solid acid saccharification as each of the glycosidic

bonds in the carbohydrates has equal accessibility to the catalyst. To examine the kinetics of

solid acid saccharification of the dissolved carbohydrate, the generalized Saeman model

expressed in Equation (3) was applied. As shown in Figure 1, the experimental data was well

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fitted into the Saeman model and the finding suggests that solid acid saccharification of the

dissolved carbohydrate is a pseudo-homogeneous irreversible first order reactions.

Even though the hydrolysis reaction follows that of the Saeman model, solid acid

saccharification of dissolved carbohydrate is expected to have different values for their

kinetic parameters. The kinetic rate constants, k1 and k2 in Equation (6) were determined by

fitting the experimental data to the equation with the aid of MATLAB. The values obtained

for each operational condition were tabulated in Table 1. The accuracy of the rate constants

generated by MATLAB were verified by the low values of the sum of square (less than 0.15)

between the concentration of the reducing sugars predicted by the model and the

concentration of the reducing sugars determined from the experiment. The rate constants of

sugars production (k1) are higher than the rate constants of sugars degradation (k2) implies

that the operational conditions favored the reducing sugars production over the reducing

sugars degradation.

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Table 1

Rate constants for reducing sugars production and degradation

Temperature

(oC)

Catalyst loading

(%, w/v)

k1 × 102

(min-1

)

k2 × 103

(min-1

)

k1/k2

100 2 0.545 0.221 24.6

6 1.32 0.224 58.9

10 1.63 0.228 71.6

120 2 4.39 0.312 141

6 9.87 0.379 260

10 11.8 0.405 291

140 2 23.1 0.771 300

6 59.5 1.79 332

10 105 2.56 410

k1: rate constant of sugars production reaction; k2: rate constant of sugars degradation

reaction.

It can be seen from Table 1 that the higher the reaction temperature and acid

concentration, the higher is the kinetic rate constant. This is as previously explained in which

the endothermic reaction and the high availability of protons promoted the hydrolysis

reaction. The rate constant for sugars production (k1) increased more with the increase in

temperature than the increase in catalyst loading. An increase of 20oC from either 100

oC to

120oC or 120

oC to 140

oC at any catalyst loading, a five-fold to nine-fold increment in k1

values was obtained. Nevertheless, an increment of approximately two-fold was observed

when catalyst loading increased from 2% to 6% or 6% to 10% at all temperature investigated.

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Hence, elevation of the operational temperature is a better option to enhance the production

of reducing sugars.

The selectivity factor, i.e. the ratio of reducing sugars production rate constant to

degradation rate constant (k1/k2) presented in Table 1, was used to evaluate the efficiency of

different operational conditions in saccharification. The highest selectivity factor of 410 was

observed at 140oC and 10% catalyst loading. The hydrolysis conducted at the mentioned

conditions achieved maximum concentration of reducing sugars at the shortest time of 6 min

but did not give the highest concentration at the end of the experiment as shown in Figure 1.

This is because the rate of reaction is only affected by the rate constant and the concentration

of reactant. As no reducing sugars can be produced when all the carbohydrates had been

completely utilized with the possible degradation of sugars continued to take place, highest

concentration of reducing sugars would not be obtained toward the end of reaction. In brief, a

high selectivity factor promotes the production of reducing sugars provided that the reaction

time is kept within the maximum production of reducing sugars to avoid degradation. Table 2

shows the maximum time required to achieve the highest concentration of reducing sugars

(tmax) at different reaction conditions. The values of tmax are good references for selecting

suitable operational conditions to achieve high reducing sugars yields. In this study, it is

advantageous to conduct the hydrolysis reaction at high temperature as it gave maximum

amount of reducing sugars at relatively shorter time range, i.e. 6 min to 25 min when

hydrolysis was conducted at 140oC compared to 48 min to 114 min for 120

oC and a much

longer time range of 265 min to 613 min was required at 100oC.

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Table 2

Time taken to achieve maximum concentration of reducing sugars

Temperature

(oC)

Catalyst

loading

(%, w/v)

Time to achieve

maximum concentration

of reducing sugars, tmax

(min)

Maximum

concentration of

reducing sugars

(mg/ml)

Maximum

reducing

sugars yield

(%)

100 2 613 2.18 72.8

6 314 2.33 77.8

10 265 2.35 78.5

120 2 114 2.41 80.5

6 57 2.45 81.8

10 48 2.45 81.8

140 2 25 2.45 81.8

6 10 2.46 82.1

10 6 2.46 82.1

3.3. Determination of kinetic parameters in the rate constants

Arrhenius equation as expressed in Equation (8) was used to study the effect of

temperature on the rate constants. To determine the kinetic parameters, i.e. the Arrhenius

constants and activation energies of the reactions, natural logarithmic of rate constant (ln k)

versus reciprocal of absolute temperature (1/T) were plotted as illustrated in Figure 2. Based

on the figure, activation energy and Arrhenius constant for both the sugars production and the

sugars degradation reactions were determined and listed in Table 3. The average activation

energy of sugars production and sugars degradation was 125.1 ± 7.0 kJ mol-1

and 60.8 ± 19.1

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kJ mol-1

respectively. Due to the higher activation energy for sugars production, the rate of

sugars production increased in a greater extent than the rate of sugars degradation when

temperature is increased. With higher activation energy, the rate of reaction is more

temperature-sensitive (Fogler, 2006). Therefore, the selectivity increased with temperature as

it prefers the sugars production reaction as shown in Table 1.

Fig. 2. Arrhenius plots at various catalyst loadings for (a) sugars production rate constant, k1;

and (b) sugars degradation rate constant, k2.

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Table 3

Activation energies and Arrhenius constants for sugars production and degradation rate

constants at various catalyst loadings

Catalyst

loading

(%, w/v)

Sugars production reaction Sugars degradation reaction

Arrhenius

constant, A

(min-1

)

Activation

energy, Ea

(kJ mol-1

)

R2 Arrhenius

constant, A

(min-1

)

Activation

energy, Ea

(kJ mol-1

)

R2

2 3.82 × 1014

120.2 0.9987 7.23 × 10 39.7 0.9223

4 1.62 × 1015

122.1 1.0000 3.18 × 105 65.9 0.9087

6 6.57 × 1016

133.1 0.9966 1.02 × 107 76.8 0.8988

Average 125.1 60.8

The activation energy for sugars production in this study was close to the one reported

by Kumoro et al. (2008) for sago waste that was hydrolyzed with diluted sulfuric acid. When

compared to other researchers’ works employing different biomass, the reported activation

energy was 171.6 kJ mol-1

, 151.5 kJ mol-1

and 179.5 kJ mol-1

for dilute acid hydrolysis of

corn stover (Bhandari et al., 1984), cellulose (Girisuta et al., 2007) and Douglas fir (Saeman,

1945) respectively. In comparison, the present study has lower activation energy for sugars

production. This could have been attributed to the variation in structural and chemical

composition of the lignocellulosic biomass applied (Canettieri et al., 2007; Ranganathan et

al., 1985). In sago waste, approximately 40% of its total carbohydrate is starch and the

remaining is of cellulose and hemicellulose (Lee et al., 2014). Starch is composed of

branched polymeric chains of D-glucose linked by α-1,4-glycosidic or α-1,6-glycosidic bonds,

whereas the building block of cellulose is long linear polymeric D-glucose units linked by β-

1,4-glycosidic bonds. The branched polymeric chains of starch can be readily hydrolyzed

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compared to the long linear polymeric chains of cellulose. Thus, starchy lignocellulosic

biomass like sago waste is more susceptible to hydrolysis reaction which requires lower

activation energy for reducing sugars production.

Similarly, lower activation energy of sugars degradation reaction in the present study

compared to those reported by Saeman (1945) and Girisuta et al. (2007) respectively at 139.9

kJ mol-1

and 152.2 kJ mol-1

. When compare the rate constant k2, the literature reported values

0.0029 min-1

to 0.00795 min-1

(Aguilar et al., 2002) and 0.00585 min-1

to 0.118 min-1

(Saeman, 1945) are higher than the values obtained in this study (0.0002 min-1

and 0.0026

min-1

). The lower k2 values indicates that reducing sugars degrading rate is relatively slow.

This suggests that the problem of sugars degradation at high temperature encountered in

sulfuric acid saccharification can be resolved by employing solid acid saccharification on

ionic liquid dissolved carbohydrates. Such employment not only improves the sugars yield

from biomass but also assists the subsequent fermentation as inhibitory effect on

microorganism by degraded sugars products is minimized (Delgenes et al., 1996; Kootstra et

al., 2009; Larsson et al., 1999; Sreenath and Jeffries, 2000; Taherzadeh et al., 1999).

To investigate the effect of acid concentration on reducing sugars yield, different

catalyst loadings were applied to the saccharification reaction. As reported by Dwiatmoko et

al. (2010), the presence of ionic liquid solution induces the release of protons (H+) from the

SO3H functional groups attached to the A15 solid acid catalyst. Since the mobile H+ ions

released into the hydrolysate are responsible for the homogeneous saccharification reaction, it

was based on when investigate the effect of acid concentration on the rate constant. The

amount of H+ ions released was determined by measuring the pH of the hydrolysate. Table 4

presents the amount of acid sites, the quantity of H+ ions in the hydrolysate and the

percentage of H+ released into the hydrolysate at specific catalyst loading.

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Table 4

Protons (H+) release from A15 at various catalyst loadings

Catalyst

loading

(%, w/v)

Amount of

acid sites a

(mmol)

pH of resultant

solution

Amount of H+ ions in

hydrolysate

(mmol)

Percentage of

H+ released

b

(%)

2 0.23 1.240 0.14 62.6

6 0.69 0.815 0.38 55.5

10 1.15 0.605 0.62 54.0

a Concentration of acid sites in Amberlyst 15 = 4.6 mmol/g

b Percentage of H

+ released = Amount of H

+ ions in hydrolysate / Amount of acid sites × 100

Table 4 shows that the amount of H+ ions released was lower than the number of acid

sites present in the catalyst. This implies that not all the H+ bonded to the catalyst was

released into the hydrolysate. The percentage of H+

ions released into the hydrolysate

decreased as the catalyst loading increased. This could be due to insufficient ionic liquid in

the hydrolysate to induce all the H+ ions to be released from the acid sites at a high catalyst

loading. As stated earlier, the H+ ions used for saccharification should be of the mobile H

+

ions that present in the hydrolysate. Therefore, only the concentration of H+ ions in

hydrolysate was taken into consideration to study the effect of acid concentration on kinetic

rate constants.

The plot of logarithm k versus logarithm of H+ concentration as shown in Figure 3

was used to correlate the acid concentration to sugars production and degradation rate

constants. The kinetic parameter, m, that describes the effect of H+ concentration towards rate

constant was determined from the gradient of the graph (refer to Equation (9)) and the values

obtained at various operating conditions are tabulated in Table 5. All the m values shown are

less than 2, which is comparable with those reported in the literature (Saeman, 1945; Lu and

21

Mosier, 2008). The m values for k2 are lower than the corresponding value of k1 suggests that

acid concentration might not have affected the sugars degradation significantly compared to

its effect on the sugars production reaction.

Fig. 3. Logarithmic plots of rate constant versus H+ concentration at various temperature for

(a) sugars production rate constant, k1; and (b) sugars degradation rate constant, k2.

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Table 5

The m values for sugars production and degradation rate constants at various temperatures

Temperature

(oC)

Sugars production reaction Sugars degradation reaction

m R2 m R

2

100 0.7732 0.9789 0.0202 0.9284

120 0.6973 0.9741 0.1814 0.9931

140 1.0239 0.9976 0.8267 0.9989

3.4. Recovery of ionic liquid and recyclability of solid acid catalyst

In this study, 80% potassium phosphate solution was used to create an aqueous

biphasic system (ABS) to separate and recover the ionic liquid used for the dissolution of

carbohydrates. The separation performance is presented in Table 6. Estimated 39.6% of

[BMIM]Cl was being extracted from the upper phase after the first extraction. The ionic

liquid extraction efficiency obtained was low compared to those reported in the literatures

which range between 60% to 75% (Wu et al., 2008). This can be explained by the different

ionic liquid solution concentrations used in the reported work whereby potassium phosphate

solution concentration was manipulated according to the targeted concentration of ionic

liquid solution. Whereas the ionic liquid solution used in this study was resulted from the

dissolution-saccharification reaction at fixed concentration which only requires a fixed

concentration of potassium phosphate solution to create an ABS. This might have resulted in

poor extraction efficiency of [BMIM]Cl. Due to the constraint of applying different

concentrations of potassium phosphate, a multiple step extraction of ionic liquid was

attempted to enhance its recovery. After three extractions, 59.7% of [BMIM]Cl was

23

recovered. This signifies the adoption of multiple step ABS to recover the [BMIM]Cl from

the hydrolysate is feasible.

Table 6

Recovery of [BMIM]Cl in the multistep aqueous biphasic system

No. of extraction, i Extraction efficiency

In particular extraction step, i (%Ei) Total (%E)

1 39.6 ± 1.1 39.6

2 16.1 ± 1.8 49.3

3 20.5 ± 4.2 59.7

Once the ionic liquid pretreated sago waste had been saccharified, the solid acid

catalyst was regenerated with sulfuric acid. Figure 4 shows that the saccharification

performance of the regenerated solid acid catalyst was similar to that of the fresh catalyst;

whereas the saccharification performance of the non-regenerated catalyst had decreased after

each cycle. The regenerated catalyst can be reused for three times with its saccharification

performance unaltered. This finding agrees well with the result reported by Rinaldi et al.

(2010). The regeneration process regained the saccharification performance of the catalyst

through the mechanism in which the H+ ions from the sulfuric acid was displaced the ionic

liquid cations that attached to the SO3- groups to retrieve its sulfonic groups (Dwiatmoko et

al., 2010). Therefore, the used solid acid catalyst can maintain the saccharification efficiency

with proper regeneration.

24

Fig. 4. Reducing sugars recovery from sago waste by regenerated A15 and unregenerated

A15.

4. Conclusions

Solid acid saccharification of the ionic liquid pretreated sago waste follows the first order

sugars production-first order sugars degradation reaction. The hydrolysis favors the

production of sugars monomers over the competing sugars degradation reaction. Higher

saccharification temperature and catalyst loading gave higher rates of reactions with

temperature having a greater effect in both rates of reactions. Even though reducing sugars

degradation is inevitable, its degree of degradation compared to that of the sulfuric acid

saccharification is considerably insignificant. The study also demonstrated that the used ionic

25

liquid and solid acid catalyst could be recovered and reused. Approximately 60% of ionic

liquid can be recovered using a 3-step ABS, while the regenerated solid acid catalyst

exhibited same saccharification performance after being used for three cycles. The high ionic

liquid recovery and good catalyst reusability signify the economic feasibility of ionic liquid-

mediated solid acid saccharification system.

Acknowledgement

Financial support provided by University of Malaya under grant no. RP002B-13AET is

greatly acknowledged.

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