Kinetics and thermodynamic parameters of ionic liquid pretreated rubber wood biomass

    Kinetics and thermodynamic parameters of ionic liquid pretreated rubber wood biomass Amir Sada Khan, Zakaria Man, Mohammad Azmi Bustam, Chong Fai Kait, Zahoor Ullah, Asma Nasrullah, Muhammad Irfan Khan, Girma Gonfa, Pervaiz Ahmad, Nawshad Muhammad PII: DOI: Reference:

S0167-7322(16)30788-7 doi:10.1016/j.molliq.2016.09.012 MOLLIQ 6289

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

1 April 2016 27 July 2016 3 September 2016

Please cite this article as: Amir Sada Khan, Zakaria Man, Mohammad Azmi Bustam, Chong Fai Kait, Zahoor Ullah, Asma Nasrullah, Muhammad Irfan Khan, Girma Gonfa, Pervaiz Ahmad, Nawshad Muhammad, Kinetics and thermodynamic parameters of ionic liquid pretreated rubber wood biomass, Journal of Molecular Liquids (2016), doi:10.1016/j.molliq.2016.09.012

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ACCEPTED MANUSCRIPT Kinetics and thermodynamic parameters of ionic liquid pretreated rubber wood biomass Amir Sada Khan* a, Zakaria Man a, Mohammad Azmi Bustama, Chong Fai Kaitc, Zahoor Ullah a, Asma Nasrullahb, Muhammad Irfan Khana, Girma Gonfaa, Pervaiz Ahmadc, Nawshad

Center for Research in Ionic Liquids, Department of Chemical Engineering, Universiti

b

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Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia.

Fundamental and Applied Science Department, Universiti Teknologi PETRONAS (UTP), 31750

Tronoh, Perak, Malaysia.

Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,

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Muhammad*d,

Malaysia d

Interdisciplinary Research Centre in Biomedical materials, COMSATS Institute of Information

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Technology, Lahore, Pakistan

Corresponding Authors e-mail: [email protected], [email protected]

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ABSTRACT

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The impact of ionic liquids (ILs) namely 1-butyl-3-methylimidazolium chloride ([BMim][Cl]) and 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]) on rubber wood pyrolysis kinetic and

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thermodynamic parameters were investigated using thermogravimetric analysis (TGA). The ILs treated and untreated samples were characterized with FT-IR and elemental (CHNS) analyses. The activation energy for untreated and ILs treated rubber wood (RW) were determined using the

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Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Starink methods. The average activation energy calculated using FWO, KAS and Starink methods for untreated rubber wood was 120.15 KJ/mol, 117.10 kJ/mol and 117.60 kJ/mol, [BMIM][Cl] treated rubber wood was 87.32 kJ/mol, 77.73 kJ/mol, and 81.16 KJ/mol, while [BMIM][OAc] treated rubber wood was 85.64 kJ/mol, 76.63 kJ/mol, and 80.47 kJ/mol, respectively. Starink method was further used to determine the pre-exponential factor and thermodynamic parameters of untreated and ILs treated samples. The thermo kinetics and thermodynamic parameters indicate that ILs pre-treatment decreases the thermal stability of the rubber wood. From FTIR analysis, it was observed that ILs pre-treatment affected the chemical composition of rubber wood. Elemental analysis showed that ILs treated RW has a higher content of Hydrogen/Carbon ratio because of the separation of lignin and hemicellulose during pre-treatment. It was concluded that ILs pre-treatment provided a potential way to improve the thermal conversion efficiency of rubber wood. Keywords: Rubber wood, ionic liquids, thermal analysis, activation energy, thermodynamics properties 1

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Introduction The demand for energy and chemicals is increasing while the reservoirs of fossil fuels (oil,

natural gas and coal) are diminishing day by day. Owing to the fast shrinking of fossil fuel

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resources and increase in environmental pollution due to rapid industrialization, the demand for

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renewable and environmentally benign alternative energy resources is gradually increasing. Such

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enormous demand for energy and chemicals can be fulfilled by shifting from fossil resources to renewable resources. Among the various renewable resources, lignocellulosic biomass is considered as a sustainable, biodegradable and renewable source of energy and chemicals [1, 2]. Lignocellulosic biomass is mainly composed of three bio-polymeric constituents, namely,

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cellulose (semi-crystalline, present in the range of 40-55 %), hemicellulose (amorphous, present in the range of 20-30%) and lignin (amorphous, present in the range of 15-20%). Cellulose,

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hemicellulose and lignin are chemically bonded by covalent cross-linkages and non-covalent forces [3, 4]. Due to the complex structure, it is difficult to convert lignocellulosic biomass into biofuels and chemicals. Therefore, pre-treatment methods can be used to enhance the conversion

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of lignocellulosic biomass into biofuels and chemicals. Compared to physical and other chemical

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pre-treatments, ILs pre-treatment is considered as an environment friendly process. Recently, ILs have emerged as promising solvents because of their lower boiling points, high thermal stabilities,

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recyclability and have the potential to improve the thermal properties of lignocellulosic biomass [5-9]. The lignocellulosic biomass regenerated from ILs has porous texture, amorphous nature, and lower crystallinity index that contribute to their easier conversion to chemicals and biofuels. It

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has been reported that [BMIM][Cl] and [BMIM][OAc] has good dissolving ability due to the higher hydrogen bond basicity that leads to the strong hydrogen bonds with cellulose and thereby facilitates their disruption [10, 11]. Thermochemical conversion is considered as an environment friendly disposal route, converting lignocellulosic biomass waste to biofuels and chemical. Pyrolysis is the fundamental thermochemical processes occurring in an inert atmosphere and can be used effectively to convert lignocellulosic biomass into fuels and chemicals [12-14]. TGA is a useful technique which provides quick quantitative methods for the changes in kinetics parameters of original and pretreated biomass. Among these parameters, the activation energy is a very dominating factor which gives considerable information about the wood reactivity [2, 15, 16]. The determination of kinetic parameters via TGA is based on models free methods proposed by Kissinger [17], Ozawa [18], Flynn–Wall, [19] and Friedman [20]. Till now, the very limited study is available to investigate the impact of ILs pre-treatment on lignocellulosic biomass pyrolysis kinetics and thermodynamic parameters using TG and DTG 2

ACCEPTED MANUSCRIPT data. Therefore, the current research work is especially focused on exploring the impact of ILs i.e. ([BMIM][Cl] and [BMIM][OAc]) pre-treatment on pyrolysis kinetics and thermodynamic parameters of Malaysian rubber wood. Rubber wood (Hevea Brasiliensis) is a medium-sized

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(height of up to 100 feet) tree that belongs to the family Euphorbiaceae. Presently, 95 percent of

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the Rubber plantations around the world are located in Asia. and more than 75 percent of it is produced by Malaysia, Thailand and Indonesia. In rubber wood producing countries, large

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amounts of waste material are generated as a by-product during rubber wood processing. Rubber wood wastes can be utilized as a source of biofuels and energy using thermochemical conversion processes such as pyrolysis. To investigate how ILs pre-treatment improves thermochemical

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conversion of rubber wood, thermogravimetric analysis was performed at three heating rates of 5, 7.5 and 10˚C/min using the thermogravimetric analyser. Activation energies were determined

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using Flynn-Wall-Ozawa (FWO), Kissinger and Kissinger-Akahira-Sunose (KAS) and Starink methods. The changes in thermodynamic parameters were calculated using Starink method. FTIR and CHNS analysis were performed to investigate the changes in chemical composition of the

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samples before and after treatment with ILs. This study allows us to better understand the impact

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of ILs pre-treatment on pyrolysis kinetics and thermodynamics properties of lignocellulosic

2. Experimental 2.1 Materials

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biomass and to offer new insights to further improve the efficiency of the pyrolysis process.

The rubber wood (RW) sample was selected, in this study, due to their abundance in Malaysia.

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The biomass sample was milled using planetary mill (Fristch, Germany, S.NO: 05.6000/00594) and dried for 24h in an oven at 1050C before analysis. All the chemicals used in this study were of analytical grade. Ionic liquids ([BMIM][Cl] and [BMIM][OAc]) were purchased from Merck, Malaysia. 2.2

Methods

2.2.1. Ionic liquid pre-treatment of rubber wood In a typical experiment, 0.5 g of RW and 9.5 g of IL were mixed in a round bottom flask at 110oC and reaction mixture was stirred with a magnetic stirrer for 5 hours. After the reaction, distilled water was added under vigorous stirring and the precipitate was recovered as regenerated RW. Pre-treated RW was washed five times with distilled water and centrifuged for 5min at 5000 rpm/min to completely remove the ILs. The ILs were recycled by evaporating water using vacuum rotary. The ILs pre-treated sample was dried at 1100C for 24h in an oven and was stored 3

ACCEPTED MANUSCRIPT in glass vials for further analysis. The [BMIM][Cl] and [BMIM][OAc] treated rubber wood are

Characterization

2.3.1

Thermogravimetric analysis (TGA)

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2.3.

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denoted as RW-Cl and RW-OAc.

Thermal behaviour and kinetics parameters of untreated and ILs treated RW were determined

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using thermogravimetric analyzer (ASTA 6000, Perkin Elmer, US). Samples were run in TGA from 50oC to 850˚C at heating rates of 5, 7.5 and 10˚C/min with a nitrogen flow of 20 mL/min. 2.3.2

Kinetics and thermodynamic study

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Thermal degradation kinetics was studied using iso-conversional thermo kinetics methods, which give activation energy without considering reaction mechanism. The methods used to determine

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the kinetics parameters are called model free non-isothermal methods and need a set of experimental tests at various heating rates. Here, three iso-conversional methods, namely, FWO, KAS and Starink methods were used for determination of the activation energy of RW pyrolysis.

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The Flynn–Wall-Ozawa (FWO) method [14] is the isoconversional model free method used

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to calculate the apparent activation energy. The logarithmic form this method is expressed as

Where

(1)

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follows:

is the heating rate in K min−1, T is temperature in K, Ea is activation energy in kJ mol−1

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and R is universal gas constant, g represents the reaction mechanism and is considered as unity and constant at a given value of the conversion. This method allows us to calculate activation energy from the slope (-1.052Ea/R) of the plot of ln(β) versus 1/T for a series of experiments performed at different heating rates. The Kissinger–Akahira–Sunose (KAS) method [17] is based on the following expression: (2) Where α is fraction of conversion, A is pre-exponential factor and g(α) is an algebraic expression for the integral method. The activation energy was determined from the slope of straight lines by plotting ln(β/T2) versus 1/T for a set of experiments performed at various heating rates. In Starink method [21], both FWO and KAS methods can be transferred into the following formula. The Starink method is based on the following expression: (3) 4

ACCEPTED MANUSCRIPT The activation energy was calculated from the slope of each straight line by plotting versus 1/T. The pre-exponential factor (A) and the thermodynamic parameters such as enthalpy change (∆Ho),

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entropy (∆So), and Gibbs free energy (∆Go) were measured by using the following equations [22-

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24].

(5) (6) (7)

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– RT

(4)

Where KB (1.83 × 10-23 J/K) is the Boltzmann constant, h (6.36 × 10-34 J.s) is the plank constant, and Tm is the peak temperature that corresponds to the maximum mass loss in the DTG curve.

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According to the isoconversional method, the kinetics and thermodynamics parameters are not

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same for whole decomposition process but show variation with the conversion [25]. 2.3.3. Fourier Transform infrared spectroscopy (FTIR)

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The FTIR spectra of untreated and ILs treated rubber wood samples were obtained by FTIR spectrophotometer (spectrum one, Perkin–Elmer, US) using KBr pellet technique. To record FTIR spectra, biomass sample was mixed with KBr at the ratio of 1/1000. FTIR spectra were recorded within the wavenumber range of 4000cm−1 to 450 cm−1.

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2.3.4. Elemental analysis (CHNS) The elemental analysis (CHNS) of untreated and ILs pre-treated rubber wood was made using CHNS-2400 supplied by Perkin-Elmer. 3. Results and Discussion 3.1 Pyrolysis characteristics TG and DTG curves of untreated and ILs treated rubber wood at the heating rate of 5, 7.5 and 10˚C/min in the heating range of 50-800 oC is shown in Figure 1. The thermal decomposition profile showed that each sample exhibited three distinct stages of mass loss. In the first stage (dehydration), the thermal degradation ranged from 50oC to 125oC is a result of vaporization of moisture and degradation of light organic compounds [26-28]. The second stage ranged from 150oC to 380-440oC represents active pyrolysis [28]. This is the most important region because of the maximum weight loss occurrence, it plays a key role in the pyrolysis of biomass sample. In 5

ACCEPTED MANUSCRIPT this region besides the major peak, a shoulder peak at about 280 oC (Tshoulder) was observed which was assigned to thermal degradation of hemicellulose. The decrease in intensity of shoulder peak was observed with the increase in heating rate from 5 oC/min to 10 oC/min [29]. The presence of

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this distinct shoulder peak before the main degradation peak of cellulose can be attributed to the

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higher content of hemicellulose in the RW sample [1, 30, 31]. The predominant mass loss in the region from 225 oC to 340 oC is assigned to pyrolysis of cellulose [32, 33]. The third stage (active

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pyrolysis) ranges from about 410oC to 1000oC; in this stage the decomposition of lignin started and mass loss decreased slowly up to the final temperature. The thermal decomposition of lignin over a wide temperature range is due to its heterogeneity and nonexistence of a distinct primary

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structure. Thermal decomposition of lignin happened in both active and passive region of pyrolysis [20].

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However, it can be seen that ionic liquid pretreatment changes the thermal profile of RW due to the dissolution of hemicellulose and lignin. Both untreated and ILs treated biomass samples show maximum degradation in the temperature range 200-350 oC. The same type of effect was

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[Fig 1.]

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also observed by the other researchers [5].

Effect of the heating rate on DTG

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The heating rate effects the extent of thermal degradation represented TGA curves, and the position of DTG peaks. For each sample, three runs were performed at three different heating. DTG peak temperature for untreated and ILs treated RW at heating rates of 5, 7.5 and 10 oC/min

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are depicted in Table 1 and graphically shown in Figure 1. It can be seen from Table 1 that with the increase of heating rate (β) from 5oC/min to 10oC/min, the DTG curve moves towards higher temperature. The temperature of maximum loss rate (Tmax) is important being providing information about the temperature where the maximum mass loss has occurred [33, 34]. In the case of untreated RW, the maximum mass loss happened at about 322.36oC, 333.52oC and 335.71 o

C at the heating rates of 5, 7.5 and 10 oC/min, respectively. This shift of peak temperature (Tmax)

from lower to higher temperature with increase of heating rate is due to heat transfer limitation [30]. This shift in the peak position of DTG curve with an increase in heating rate has been observed by others researchers as well [18, 25]. Huang et al. proposed that the shift of peak temperature from lower to higher temperature with increasing of heating rate is due to the short pyrolysis time at higher heating rates, providing less time for the sample pyrolysis [35]. Similarly, considerable changes in the DTG curves of IL treated RW were observed (Figure1). Moreover, it is noteworthy that after ILs treatment the DTG curves for RW sample moved towards lower temperature. The Tmax observed for RW, RW-Cl and RW-OAc at 5 oC/min 6

ACCEPTED MANUSCRIPT was 322, 302 and 276oC, respectively (Table 1). To better understand the impact of ILs pretreatment on DTG curves in the active pyrolysis zone, the peak position of DTG curves at lower temperature side was denoted by TmaxL and the peak position at high temperature side was denoted

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by TmaxH. After the treatment of RW with [BMIM][Cl], the peak due to hemicellulose and lignin

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almost disappeared. Besides disappearance of the shoulder peak, the main peak in the region from 320 to 336oC became narrower and shifted to a lower temperature i.e. from 322oC to 3020C at

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5oC/min. It seems that [BMIM][Cl] breaks down the hemicellulose and lignin, and causes the depolymerisation of cellulose chain, that is why [BMIM][Cl] treated RW shows the easier decomposition as compared to the untreated sample [8]. Zhang et al. reported the shift of the DTG

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peaks from higher to lower temperature for NaOH treated corn stove [11]. Whereas, in RW-OAc, the main peak in the range of 270oC - 350oC in the DTG curve could be divided into two distinct

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peaks. The appearance of the additional peak in the active zone of pyrolysis indicated the transformation of cellulose from one crystalline structure to another crystalline structure. The lignin peak in RW-OAc disappeared because of the high efficiency of [BMIM][OAC] for

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dissolution of lignin [8]. The decrease in the value of temperature of maximum loss rate (Tmax) of

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RW after ILs treatment revealed that ILs could facilitate the pyrolysis process. This decrease in Tmax of RW with ILs treatment will add some advantage in the pyrolysis process, as ILs treated

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sample will be operated at the low temperature as compared to untreated sample and this will be helpful in saving energy [36]. Ref. [27] observed that water washing and torreffaction can facilitate the pyrolysis of rice husk. Similar results were obtained by Nawshad et al. while treating

3.3

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bamboo with 1-ethyl-3-methyl imidazolium glycine ([EMIM][Gly]) [5]. [Table 1]

Kinetics study using iso-conversional methods The activation energy calculated by using KAS, FWO and Starink methods at various

conversion (α) values ranging from 10% to 70% are shown in Figures 2, 3 and 4, respectively. The non-parallel tendency was observed for conversion value below 10% and above 70%. These cases were excluded, as the lack of correlation would introduce an erroneous in the interpretation of the data. The apparent activation energies, the average activation and regression coefficients (R2) calculated for untreated and ILs treated rubber wood are presented in Table 2. The activation energy calculated for RW, RW–Cl and RW-OAC using KAS method was 120.15, 87.32 and 85.64 KJ/mol, while using FWO method was 117.10, 77.73 and 76.63 KJ/mol, respectively. Similarly, the activation energy determined for RW, RW–Cl and RW-OAC using Starink method was 117.61, 81.16 and 80.47 KJ/mol, respectively. The activation energy calculated for untreated and ILs treated rubber wood sample using KAS, FWO and Starink method show good correlation 7

ACCEPTED MANUSCRIPT with each other. The decrease in the activation energy of RW after ILs treatment could be attributed to the changes in the composition of RW samples, such as the decomposition of hemicellulose and the partial depolymerisation of cellulose and lignin. The results indicated that

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ILs pre-treatment reduced the activation energy that is needed to decompose woody biomass by

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deconstructing the tight plant cell wall structures. Jose et al. observed the same effect for agave bagasse as a function of ionic liquid pre-treatment [31]. The obtained results are almost in close

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agreement to each other’s and confirmed the predictive power of KAS, FWO and Starink models. The change in activation energy for RW, treated with various ionic liquids can be correlated with Kamlet-Traft (K-T) parameters. Among the Kamlet–Taft parameters, hydrogen bond basicity (β)

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quantifies an ability of ILs anion to donate electrons to form a hydrogen bond with a proton of lignocellulosic biomass. ILs with higher β values, tend to dissolve cellulose and lignin more

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efficiently. The decrease in the activation energy of RW treated with [BMIM][Cl] and [BMIM][OAc], correlating well with their high β values of 0.95 and 1.14, respectively. The ILs have higher β values can significantly remove lignin, reduce cellulose crystallinity, and cause

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lowering of activation energy [11].

[Fig. 2] [Fig. 3] [Fig. 4] [Table 2]

The relationship between the apparent activation energy and the conversion rate (range from 10% to 70%) is listed in Table 2 and graphically shown in Figure 5. The activation energy is not

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constant for all the values of α ranging from 10% to 70%, variation in activation energy was observed with the progress of conversion (α) [37]. The activation energy for RW almost remains same and there is no considerable change with the increase of the value of α from 40% to 60%. This region is kinetically important and corresponds to the temperature ranging from 330 to 400 0

C. In this region major degradation of biomass is taking place due to unzipping of cellulose

molecules. As a result of this fast unzipping of cellulose molecules the original structure of cellulose molecules disappears. The constant values of activation energy from α = 40% to 60% , indicate the possible single reaction mechanism and change in mechanism might be taking place at high conversion value (α ≥ 0.7) [38]. The values of apparent activation energies calculated for RW using KAS, FWO and Starink methods were in the range of 125.05-77.67, 122.66-71.04 and 118.79-79.66 KJ/mol, respectively. In the case of RW-Cl, the activation energy increase when conversion increase from 10% to 30% and then decrease up to a minimum level at 50% conversion. The value of activation energy 8

ACCEPTED MANUSCRIPT increases with the increase of conversion value behind 50% conversion. The apparent activation energy calculated for RW-Cl using KAS, FWO and Starink methods were in the range of 37.6192.93, 30.92-88.19 and 31.26-82.71 KJ/mol, respectively. For the RW-OAc, the activation energy

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was linearly increasing with the increase of conversion value from 10% to 50% and then

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decreases up to a minimum level at the conversion of 60%. At 70% conversion, the maximum value of apparent activation energy was observed. The apparent activation energy calculated for

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RW-Cl using KAS, FWO and Starink methods were in the range of 74.84-114.99, 70.00-103.37 and 70.33-109.15 kJ/mol, respectively.

It is observed that the apparent activation energy for RW, RW-Cl and RW-OAc have not the

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similar values for all conversion rates; representing large numbers of series and parallel reactions in the solid state. The reaction mechanism for both untreated and ILs treated rubber wood was not

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same for all conversion ranges and apparent activation energy depends on conversion values and also on the nature of ILs. This difference in trends of variation of activation energy could be related to the difference in nature of the interaction of different ILs with biomass components. It

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can be concluded that the decrease in the activation energy of RW treated with ILs containing

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same cations and different anions could be related to different interaction mechanism of anions with hemicellulose, cellulose, and lignin of lignocellulosic biomass.

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3.4

[Fig 5.]

Thermodynamic parameters Table 3 shows the pre-exponential factor (A) of untreated and ILs treated RW based on

Starink method. The values of pre-exponential factor calculated for RW, RW-Cl and RW-OAc is

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in the range of 4.42×104 to 1.62×109 s–1, 0.980427 to 5.16×106 s–1, 5.92×103 to 2.34×107 s-1

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respectively. The parameters show variation to conversion rate and depend on the structural integrity of biomass. The variation in values of pre-exponential factor confirmed the existence of complex composition in samples and occurrence of complex reactions during biomass pyrolysis process [39]. The value of pre-exponential factor for ILs treated RW is lesser than untreated RW. These lower values of A for ILs treated sample indicate a quicker and easier degradation [40, 41]. Table 4 shows the thermodynamic parameters (∆Ho, ∆So and ∆Go) of untreated and ILs treated rubber wood based on Starink method. The change in enthalpy (∆Ho) values is an important thermodynamic function which ascertains that the reaction will be endothermic or exothermic. The average change in enthalpy calculated for RW, RW-Cl, and RW-OAc are 97.371, 76.211, and 75.514 KJ/mol, respectively. The positive values of enthalpies show that energy is needed to decompose the biomass. Based on the obtained calculated values of enthalpy, the lower values of the enthalpy change for ILs treated samples corresponds to the need of lower energy for 9

ACCEPTED MANUSCRIPT their thermal decomposition as compared to untreated RW. The variation in the enthalpy change with the values of conversion is linked to the difference in nature of untreated and ILs treated RW. Wang et al [42]. also observed the variation in activation enthalpy as a function of conversion

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value. A similar trend in enthalpy change of the red pepper waste as a function of conversion

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value was also observed which were between 23.37 kJ/mol and 142.21 kJ/mol [42]. Change in entropy (∆So) calculated for untreated and ILs treated rubber wood is shown in

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Table 4. The average ∆So calculated for RW, RW-Cl, and RW-OAc are -131.56, -167.82, and 168.692 J/mol, respectively. Both untreated and IL treated rubber wood have negative values. The ∆So of RW varied from -299.96 J/mol to -170.029 J/mol and showed lower average values than

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RW-Cl and RW-OAC i.e. -167.824 J/mol and -168.692 J/mol, respectively. Thus, the ILs treated RW samples have higher values of entropy change which correspond to more disorderness in their

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structure as compared to the untreated sample. Comparatively, the higher value of ∆S means that the ILs treated samples are more activated, disordered and the degrees of freedom of rotation, as well as vibration, are more to the untreated rubber wood [41, 42]. The increase in disorderness of

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ILs treated samples suggested to correlate with their lower thermal stabilities.

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Gibbs free energy (∆Go) is an important state function used to conclude the degree and spontaneity of reactions. The change in Gibbs free energy for untreated and ILs treated samples is

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listed in Table 4. There is a slight difference in ∆Go of both ILs treated and untreated samples which shows similar nature of spontaneity in their decomposition reaction. Similar variation in ∆Go was also observed in the thermal decomposition of red pepper waste [23, 41].

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[Table 3] [Table 4] Spectroscopic analysis of untreated and ILs treated rubber wood FT-IR analysis was used to investigate the effect of ILs pre-treatment on surface functional

groups of rubber wood. The FTIR spectra for untreated and ILs treated RW samples are shown in Figure 6. Spectra show that ILs treatment affected the fingerprinting and crystallinity of RW. The broad peak in the region of 3600 cm−1 to 3100 cm−1 is assigned to O-H stretching vibration [43]. The presence of a band at 2929 cm-1 is assigned to axial deformation of C–H group. The change in this peak position appeared in the 3600 cm−1 to 3100 cm−1 regions gives information about hydrogen bonding and ultimately cellulose crystallinity [6, 44]. Decrease in intensity of the band at 2900 cm−1 (corresponding to C-H vibration) confirmed the increase in the amorphous domain of RW with ILs treatment. The peak at 1737 cm-1 is assigned to a carbonyl (C=O) group between hemicelluloses and lignin. The disappearance of the peak at 1737 cm-1 in ILs treated samples indicates the removal of hemicellulose and breakdown of the side chain of lignin [45, 46]. Similarly, the band intensity of peaks at 1500 cm−1 (C=C stretching vibration) and 1250 cm-1 (C10

ACCEPTED MANUSCRIPT O stretching in hemicellulose and lignin) is greatly reduced with all ILs pre-treatment, which reflects the delignification. This decrease is due to the breakage of the ester linkage in lignin and hemicellulose due to ILs pre-treatment [45]. The decrease in the intensity of the peak at the 1500

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cm−1 for RW-Cl and RW-OAc samples is due to the significant removal of hemicelluloses after

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[BMIM][Cl] and [BMIM][OAc] treatments [44, 47, 48]. The anions of [BMIM][Cl] and [BMIM][OAc] interact with O–H groups of biomass and disrupt the hydrogen bonds present

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within and between the lignocellulosic substrates. FTIR spectra in the 1800–800 cm-1 region were characteristic of the cellulose structure. The bands at 1426 cm-1 and 896 cm-1, assigned to CH2 scissoring motion and C–O–C stretching in cellulose, respectively, are quite sensitive to the

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amount of crystalline and amorphous cellulose. The intensity of peak located at 896 cm-1 is relatively stronger in untreated RW in comparison to ILs treated samples. It is reported that the

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intensity of this peak increases with decreases of crystallinity of cellulose in biomass sample and change in crystal lattice from cellulose I to cellulose II [20, 49]. [Fig 6.]

Elemental analysis of untreated and ILs treated rubber wood

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Elementary analysis (CHNS) for untreated and ionic liquid-treated sample is provided in Table 5. The Hydrogen (H) content for RW is increasing after pretreatment with [BMIM][Cl] and

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[BMIM][OAc]. ILs pretreatment cause increase in H/C ratio of RW. The RW-Cl and RW-OAc sample contain more hydrogen which helps in the co-processing (i.e. co-pyrolysis, co-firing, and co-gasification), and lowers the CO2 emission per unit energy production [16]. This increase in H/C ratio for RW-Cl and RW-OAc is due to dissolution of lignin and hemicellulose with ionic

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liquid pretreatment, cellulose possesses a higher H/C ratio than hemicellulose and lignin [50]. On another hand, by IL pretreatment, the sulphur content decrease which can considerably decrease the inventory of SOx [51]. [Table 5] 4.

Conclusion The effect of ILs treatment on pyrolysis and thermodynamics behaviour of rubber wood was

studied thoroughly. Thermogravimetric analysis shows the decrease in thermal stability of the rubber wood samples after treatment with ionic liquids. Activation energy calculated by FlynnWall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS) and Starink methods for untreated and ILs treated sample shows the reduction of activation energy compared to untreated rubber wood sample respectively. The value of pre-exponential factor calculated for RW, RW-Cl and RW-OAc is in the range of 4.42×104 to 1.62×109 s–1, 0.980427 to 5.16×106 s–1, and 5.92×103 to 2.34×107 s-1 , respectively. The average change in enthalpy calculated for RW, RW-Cl, and RW-OAc are 11

ACCEPTED MANUSCRIPT 97.371, 76.211, and 75.514 KJ/mol, respectively which indicated that the untreated RW sample required more heat as compared to ILs treated samples in their endothermic decomposition process. The ILs treated RW samples have higher values of entropy change which corresponded

The FTIR analysis confirms the change in chemical composition and

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untreated sample.

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to more disorderness in their structure and thereby low thermal stability as compared to the

crystallinity of the rubber wood with ILs treatment. The ILs treated sample shows higher values of

SC R

H/C which suggested to assist in the co-processing (i.e. co-pyrolysis, co-firing, and cogasification), and lower the CO2 emission per unit energy production. Acknowledgement

NU

The authors gratefully acknowledge the Ministry of Higher Education (MOHE) for funding the research work under Exploration Research Grant Scheme (ERGS) and Centre of Research in

MA

Ionic Liquids, Universiti Teknologi PETRONAS, 31750 Tronoh, Perak, Malaysia for supporting this project. We are especially thankful to Dr. Asrul Mustafa (Malaysian rubber wood) for providing rubber wood sample.

D

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AC

CE P

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D

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NU

SC R

IP

T

[42]

15

7.5 C/min

40

-0.4

-0.6

D

20

-20 300

400

500 0

Temperature [ C]

600

700

SC R

-1.5 -2.0

40

-0.8

-2.5

o

a b c

20

5 C/min o

7.5 C/min

0

-3.5 100

200

300

400

500

600

o

Temperature [ C]

100

(c)

a c b

80

0

-2

o

a b c

60

5 C/min

-4

o

7.5 C/min o

10 C/min

-6

40 -8 20

-10

0 100

200

-3.0

o

10 C/min

800

CE P

200

Weight loss [%]

100

TE

0

60

o

o

10 C/min

-1.0

-dx/dT (mg/ C)

o

-0.5

o

5 C/min

a b c

80

300

400

500 o Temperature [ C]

600

700

-12 800

Fig 1. TGA and DTG curve for (a) RW (b) RW-Cl and (c) RW-OAc.

16

0.0

-dx/dT (mg/ C)

-0.2

o

a b c

60

(b)

NU

b c

100

0.0

Weight loss [%]

80

(a)

o -dx/dT (mg/ C)

a

MA

100

AC

Weight loss [%]

IP

T

ACCEPTED MANUSCRIPT

700

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

10

-8.7

30 40 50

-9.3

-2

ln [K ]

MA

-9.0

60

-9.9

CE P

TE

D



70

-9.6

-10.2

-10.5 0.90

AC  -2  ln [K ]

1.05

1.20

1.35

1.50

50

40

30

20

10

(c)

60

-9.6

70

-9.8

-10.0 -10.2 -10.4

1.28

1.36

1.44

1.65

1000/T [1/K]

-9.2 -9.4

1.52

(b)

20

1.60

1.68

1.76

1.84

1.92

1000/T [1/K]

Fig 2 . KAS plot for (a) RW (b) RW-Cl and (c) RW-OAc.

17

2.00

1.80

1.95

2.10

3.5

10

3.3

60

(b)

50

40

30

20

10

3.30

ln [K/min]

3.2

3.15

MA

3.1 3.0 2.9 2.8

3.00

2.6 1.52

1.60

1.68

1.76

1.84

TE

1.44

1.92

2.70

2.00

0.90

1.05

1.20

1.35

1000/T[1/K]

1.50

60 50

70

3.4

40

30 20

(c)

10

3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6

1.28

1.36

1.65

1000/T [1/K]

3.5

ln [K/min]

1.36

CE P

1.28

D

2.85

2.7

AC

ln [K/min]

70

3.45

NU

3.4

(a)

50 40 30 20

60

70

SC R

IP

T

ACCEPTED MANUSCRIPT

1.44

1.52

1.60

1.68

1.76

1.84

1.92

1000/T[1/K]

Fig 3 . FWO plot for (a) RW (b) RW-Cl and (c) RW-OAc.

18

2.00

1.80

1.95

2.10

-8.70

50

10

20

(a)

-2

[K ]



ln

-9.30

-9.60 1.65

1.70

1.75

1.80

1.85

1.90

TE

1.60

50 60 70

-9.6

-9.9

-10.5 0.90

1.95

1.05

1.20

1.35

1000/T [1/K]

1.50

40

-8.8

60

30

20

10

(c)

50



-2

[K ]

-9.0

70

-9.2 -9.4 -9.6 -9.8

-10.0 1.3

1.4

1.65

1000/T [1/K]

CE P

-8.6

ln

1.55

30

-10.2

D

-9.45

1.5

(b)

-9.3

MA

-9.15

AC

ln



-2

[K ]

70

10

40

-9.0

-9.00

1.50

20

-8.7

NU

60

-8.85

30

40

SC R

IP

T

ACCEPTED MANUSCRIPT

1.6

1.7

1.8

1.9

2.0

1000/T [1/K]

Fig 4. Starink plot for (a) RW (b) RW-Cl and (c) RW-OAc.

19

2.1

1.80

1.95

2.10

180

(a)

Ea, KAS Ea, FWO Ea, Starink

120

MA

120 100 80

10

20

30

40

TE

40

50

60

80 60 40 20

D

60

E a [KJ/mol]

100

0 10

70

20

30

CE P

50

60

70

(c)

Ea, KAS Ea, FWO Ea, Starink

140

40

Conversion [%]

Conversion [%]

E a [KJ/mol]

120

AC

E a [KJ/mol]

140

(b)

Ea, KAS Ea, FWO Ea, Starink

140

NU

160

SC R

IP

T

ACCEPTED MANUSCRIPT

100

80

60

40 10

20

30

40

50

60

70

Conversion [%]

Fig 5. Variation of apparent activation energy with conversion for (a) RW (b) RW-Cl and (c) RW-OAc.

20

SC R

IP

T

ACCEPTED MANUSCRIPT

NU

b

MA

Transmittance (%)

a

4500

CE P

TE

D

c

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

AC

Fig 6. FTIR spectra of (a) rubber wood and rubber wood treated with (b) [BMIM][Cl] (c) [BMIM][OAc]

21

SC R

IP

T

ACCEPTED MANUSCRIPT

Table 1. Influence of ILs treatment on thermal stability of rubber wood. RW Tmax (oC) 322

RW-Cl (Tmax) 302

RW-OAc TmaxL TmaxH 276 328

7.5

333

306

284

333

10

336

309

300

346

D

MA

NU

Heating rate (oC/min) 5

α

KAS

CE P

Sample

TE

Table 2. Activation energy (kJ/mol) for ILs treated and untreated rubber wood using KAS, FWO and Starink methods

Ea

10

R2

Ea

R2

0.988

126.66

0.987

11.79

0.948

123.31

0.992

120.54

0.994

121.45

0.965

140.99

0.990

138.90

0.990

125.27

0.989

40

127.14

0.997

124.08

0.998

129.26

0.987

50

124.86

0.999

121.46

0.998

124.19

0.988

60

124.64

0.998

121.01

0.974

124.61

0.985

70

77.63

0.979

71.04

0.950

79.667

0.989

AC

30

Average

RW-Cl

Ea

Starink

125.05

20 RW

R2

FWO

120.15

117.10

117.61

10

37.61

0.996

30.93

0.987

31.26

0.992

20

88.27

0.995

79.31

0.994

83.74

0.989

30

103.42

0.992

86.38

0.990

101.99

0.991

40

96.73

0.981

93.89

0.998

91.26

0.990

50

90.88

0.985

86.40

0.998

83.73

0.988

60

101.38

0.979

79.03

0.974

93.41

0.985

22

ACCEPTED MANUSCRIPT 92.93

88.20

0.950

Average

87.32

10

74.84

0.989

70.00

0.987

70.33

0.978

20

78.87

0.992

70.18

0.994

74.15

0.975

30

80.77

0.990

71.76

0.990

40

86.22

0.997

76.90

0.998

81.28

0.987

50

84.64

0.999

75.00

0.998

79.27

0.988

60

79.19

0.998

69.23

0.974

73.22

0.985

70

114.99

0.979

103.37

0.950

109.15

0.989

Average

85.64

SC R

CE P

TE

D

MA

NU

76.63

23

0.989

81.16

75.84

IP

77.73

82.73

T

0.982

AC

RW-OAc

70

80.47

0.989

ACCEPTED MANUSCRIPT

Table 3. Pre-exponential factor for RW, RW-Cl, and RW-OAc based on Starink method. RW

RW-Cl

RW-OAc

10

6.84

26.31

20

116.50

78.79

30

120.32

97.04

70.89

40

124.31

86.31

76.33

50

119.24

78.78

74.32

60

119.66

88.46

68.27

70

74.718

77.781

104.20

65.38 69.20

MA

NU

SC R

IP

T

Conversion

Conversion

RW-Cl

RW-OAc

6.84

26.31

65.38

116.50

78.79

69.20

30

120.32

97.04

70.89

40

124.31

86.31

76.33

50

119.24

78.78

74.32

60

119.66

88.46

68.27

70

74.718

77.781

104.20

97.37

76.21

75.51

RW-Cl

RW-OAc

10

AC

∆Ho (KJ/mol)

CE P

20

RW

TE

Property

D

Table 4. Thermodynamics parameters for ILs treated and untreated rubber wood based on Starink

Average ∆Ho Property

∆Go (KJ/mol)

Average ∆Go

Conversion

RW

10

185.36

180.54

176.53

20

173.82

175.66

176.26

30

173.67

174.69

176.15

40

173.51

175.24

175.81

50

173.71

175.66

175.94

60

173.70

175.12

176.33

70

175.91

175.73

174.35

175.67

176.09

175.91

24

ACCEPTED MANUSCRIPT Conversion

RW-Cl

RW-OAc

-299.96

-259.14

-186.75

20

-96.32

-162.77

-179.89

30

-89.64

-130.47

-176.87

40

-82.68

-149.42

-167.15

50

-91.53

-162.79

-170.74

60

-90.79

70

-170.03 -131.56

AC

CE P

TE

D

MA

NU

Average ∆So

T

10

IP

∆So (J/mol)

RW

SC R

Property

25

-145.61

-181.56

-164.57

-117.87

-167.82

-168.69

ACCEPTED MANUSCRIPT

Table 5. Elemental analysis of untreated and ILs treated rubber wood H

N

O

S

47.55

6.87

2.55

43.03

RW-Cl

35.81

5.60

0.13

58.46

RW-OAc

36.25

5.92

0.16

57.67

AC

CE P

TE

D

MA

NU

SC R

RW

26

H/C 4.163

0.144

0.29

0.156

0.77

0.163

T

C

IP

Sample

ACCEPTED MANUSCRIPT Highlights

CE P

TE

D

MA

NU

SC R

IP

T

The effect of [BMIM][Cl] and [BMIM][Ac] on Rubber wood was investigated. Pyrolysis Kinetics and thermodynamics properties were evaluated. The activation energy was determined using the isoconversional models of kinetics Enthalpies, entropies, and Gibbs free energy were correlated with thermal stability Ea and thermodynamics properties’ values were lower for ILs treated samples

AC

    

27