CO2-Philic [EMIM][Tf2N] Modified Silica in Mixed Matrix Membrane for High Performance CO2/CH4 Separation

Shafie, Siti Nur Alwani; Liew, Wen Xuan; Md Nordin, Nik Abdul Hadi; Bilad, Muhammad Roil; Sazali, Norazlianie; Putra, Zulfan Adi; Wirzal, Mohd Dzul Hakim

doi:https://doi.org/10.1155/2019/2924961

Advances in Polymer Technology

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Abstract Introduction Experimental Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 2924961 | https://doi.org/10.1155/2019/2924961

CO₂-Philic [EMIM][Tf₂N] Modified Silica in Mixed Matrix Membrane for High Performance CO₂/CH₄ Separation

Siti Nur Alwani Shafie,¹Wen Xuan Liew,¹Nik Abdul Hadi Md Nordin,¹Muhammad Roil Bilad,^1,2Norazlianie Sazali,^3,4Zulfan Adi Putra,¹and Mohd Dzul Hakim Wirzal¹

Academic Editor: Behnam Ghalei

Received26 Oct 2018

Revised12 Dec 2018

Accepted23 Dec 2018

Published03 Jan 2019

Abstract

Separation of carbon dioxide (CO₂) from methane (CH₄) using polymeric membranes is limited by trade-off between permeability and selectivity as depicted in Robeson curve. To overcome this challenge, this study develops membranes by incorporating silica particles (Si) modified with [EMIM][Tf₂N] ionic liquid (IL) at different IL:Si ratio to achieve desirable membrane properties and gas separation performance. Results show that the IL:Si particle has been successfully prepared, indicated by the presence of fluorine and nitrogen elements, as observed via Fourier-Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectrometer (XPS). Incorporation of the modified particles into membrane has given prominent effects on morphology and polymer chain flexibility. The mixed matrix membrane (MMM) cross-section morphology turns rougher in the presence of IL:Si during fracture due to higher loadings of silica particles and IL. Furthermore, the MMM becomes more flexible with IL presence due to IL-induced plasticization, independent of IL:Si ratio. The MMM with low IL content possesses CO₂ permeance of 34.60 ± 0.26 GPU with CO₂/CH₄ selectivity of 85.10, which is far superior to a pure polycarbonate (PC) and PC-Sil membranes at 2 bar, which surpasses the Robeson Upper Bound. This higher CO₂ selectivity is due to the presences of CO₂-philic IL within the MMM system.

1. Introduction

Natural gas is among the favorable energy sources due to its low greenhouse gases emissions than other fossil fuels. It is commonly used as fuel for heating, vehicle fuel, and electricity generation. However, natural gas extraction is accompanied by various impurities, which largely consist of carbon dioxide (CO₂) gas that can reach as high as 80% [1, 2]. The presence of CO₂ would reduce the calorific value of the natural gas and make it corrosive to the pipelines during transportation [3]. Ineffective CO₂ removal also leads to increased maintenance and processing costs. Therefore, CO₂ concentration must be lowered below 2% to minimize the pipeline corrosion [2, 4].

There are currently conventional techniques for CO₂ removal such as chemical absorptions, cryogenic, and membrane separation. Due to limitations of chemical absorption and cryogenic separation (i.e., high operating cost, process complexity and energy intensive), membrane technology for CO₂ removal is a promising technique. It offers low operating and capital costs, ease of installation and operation, minimal energy consumption, and low footprint [5]. Separation of CO₂/CH₄ commonly uses polymeric membranes due to their ease of production compared to inorganic ones [6, 7]. Gas separation in the dense membranes occurs by a solution-diffusion mechanism which is governed by diffusion and solubility coefficients [8]. Current performance of commercial polymeric membranes for CO₂/CH₄ separation still operates under the Robeson Upper Bound (that shows trade-off limit between permeability and selectivity) which restricts its widespread applicability [3].

In recent years, a composite between polymeric membrane and inorganic particle has been developed, commonly known as mixed matrix membrane (MMM). MMM attracts attention due to its improved separation performances and its potential to perform beyond the Robeson Upper Bound [9–13]. Bisphenol A polycarbonate (PC) has been established as one of the developing polymer material for gas separation. Koros et al. [14] reported on the gas transport properties of PC with CO₂ up to 8.5 barrer with CO₂/CH₄ selectivity of 25. Similarly, Hacarlioglu et al. [15] studied on the effect of various preparation parameters and thermal history on PC membrane performance and found CO₂ permeability up to 7 barrer with CO₂/CH₄ selectivity of 20. Later on, Hacarlioglu et al. [16] introduced polypyrrole as electrically conductive powder fillers incorporated into PC membrane and obtained 62 times improvement in CO₂ permeability but loss in selectivity at higher loading due to agglomeration. Silica is one of the conventional classes of inorganic fillers that has gained much attentions throughout the development of MMM. It can be categorized into nonporous and ordered mesoporous silica [17–19]. Ahn et al. [20] used nonporous silica filler and found that it affected the polymer chain packing in glassy and high free-volume polymers which further altered the molecular packing of the polymer chains, resulting in better MMM permeations. Xing & Ho [21] also incorporated fumed silica into a crosslinked polyvinylalkohol-polysiloxane which showed encouraging results in improving the CO₂ permeability and selectivity. In a more recent study, Chen et al. [22] achieved 4 times higher of CO₂ permeability using microcellular polymers incorporated with pure silica nanoparticles.

Ionic liquids (ILs) consist of ions with melting point below 100°C [23]. [EMIM][TF₂N] (1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide) is one of the many room temperature ionic liquids (RTILs). RTIL is a type of molten electrolyte that has many attractive properties such as nonflammability, high thermal stability, solubility in wide range of organic and inorganic compounds and also negligible vapor pressures [24]. Blending of ionic liquid onto polymeric membranes has long been studied and shown promising results in improving membrane permeabilities and selectivities [25–28]. Hao et al. [28] improved the CO₂ permeability and selectivity of IL-based MMM by blending [EMIM][Tf₂N] with poly(RTILs) [vBIM][Tf₂N] for separation of CO₂/N₂ and CO₂/CH₄. The resulting MMM posed CO₂ permeability of 200-300 barrer with 14-17 CO₂/CH₄ selectivity. IL-modified filler was also reported to improve gas permeation properties. Hudiono et al. [29] impregnated zeolite SAPO-34 using [EMIM][Tf₂N] and incorporated it into a poly(RTIL) membrane. They reported an increase of CO₂ and CH₄ permeability by 63% and 50% respectively, with an increase of CO₂/CH₄ selectivity by 11%. Similarly, Li et al. [30] embedded IL onto the outer surface of zeolitic imidazole framework (ZIF-8) and resulted in an increase of CO₂ permeance by 20% and CO₂/CH₄ selectivity by 75%.

It should be noted that the previous works embedded IL onto filler through pore impregnation. Hence, it is only applicable only when the fillers are porous. While incorporating IL onto nonporous silica has been reported as heavy metal adsorbent [31], their potential for MMM, specifically for CO₂/CH₄ separation, is yet to be evaluated. Hence, in this research, the IL-modified nonporous silica is utilized as inorganic fillers and the performances of the resulting MMM are evaluated. The silica fillers will first synthesized and modified with IL (at different ratio) before incorporated into PC membrane. The performance of the fabricated MMM will be evaluated with neat PC membrane for comparison. In authors’ opinion, this is the first attempt for IL-functionalized silica incorporation into MMM. Different [EMIM][TF₂N]:Silica (IL:Si) ratio were prepared to comprehend the role of IL in the resulting membrane.

2. Experimental

2.1. Materials

Polycarbonate (PC) with 254.3 g/mol and density of 1.20 g/cm³ was purchased from LG-DOW Ltd, which was then used as the polymer due to its good balance between intrinsic permeability and selectivity [32]. Dichloromethane (DCM) (Merck Ltd., boiling point of 39.6°C) was used as solvent for membrane fabrication due to its low boiling point to induce dry inversion. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf₂N] (Figure 1), 97%) as IL was purchased from Sigma Aldrich. The silica precursor consisting of tetraethyl orthosilicate (TEOS), hydrochloric acid (HCl, 37%), and ethanol (99.5% purity) was obtained from Merck Ltd. All chemicals were used without further purification.

2.2. Silica Particle Synthesis

The silica particle synthesis procedure was adapted from Buckley & Greenblatt (1994) [33]. The sol-gel was prepared by addition of 30 mL of TEOS and 31 mL of Ethanol into a round-bottom flask and stirred under reflux. A mixture of distilled water (38 mL) and 0.1 mL of HCL was then added dropwise to the reaction mixture under constant stirring, resulting in a cloudy mixture. The solution was stirred until homogenous mixture was obtained before it was heated to 60°C, followed by vigorous stirring for 4 h under reflux. The reaction mixture was then cooled to room temperature, followed by drying of the resulting silica sol in an oven at 60°C overnight. The collected silica sol was then crushed into fine particles, washed with distilled water to remove excess reactants, and subsequently dried in oven at 60°C for minimum of 12 h.

2.3. [EMIM][TN] Modified Silica Synthesis

Silica particles were modified via surface derivation method [31]. Known amount of synthesized silica particle was suspended in 100 mL of ethanol for 30 minutes and predetermined volume of [EMIM][Tf₂N] was then added to the solution (Table 1). The reaction mixture was stirred at room temperature for 4 h. The sample was recovered via vacuum filtration and washed with distilled water before being dried in an oven at 60°C overnight. The investigated various IL:Si ratios are listed in Table 1. It is assumed that the surface derivation method is at concentration high enough to ensure the IL:Si ratio is significant between all samples.

2.4. Dense Flat Sheet Membrane Preparation

The dense flat sheet membrane was prepared from a solution consisting of PC (20 wt%), DCM (80 wt%), and modified silica particles (3 wt% of total polymer-solvent solution, optimum loading as recommended elsewhere [34]). The PC polymer was first dried in an oven at 60°C for 24 h to remove adsorbed moistures. The modified silica particles were dispersed into the DCM solvent and 20% of polymer was added under stirring for priming purpose. The mixture was stirred for 24 h at room temperature until the polymer granules were fully dissolved, followed by addition of remaining polymer. The mixture was further stirred for another 24 h. The homogeneous polymer solution was then degassed at room temperature using an ultrasonicator to remove any air bubbles formed during preparation. The polymer solution was then hand casted using a casting bar at a wet thickness of 100 μm. The casted film was then dried at room temperature for 24 h to ensure slow evaporation of the solvent. The pristine PC and PC-Sil membrane were prepared using the same process without the filler and pristine silica particles as filler, respectively.

2.5. Characterization

Fourier-Transform Infrared Spectroscopy (FTIR, Perkin Elmer Spectrum 1) was used to analyze the functional groups present in the modified silica particles. The powder sample and KBr were grounded before mixing. The powder was then added to a die-set to form a pellet before placed in the sample holder. The spectra were studied by coaddition of 20 scans in the range of 400-4000c. Within this range, the organic component converted the radiation into vibration.

X-ray Photoelectron Spectrometer (XPS, Thermo Scientific K-Alpha) analysis was used to determine the elemental composition of the synthesized modified silica particles. Chemical state analysis was done using XPS to analyze Si, H, O, F, N elements in the modified silica particles.

Differential Scanning Calorimeter (DSC, Q2000 TA) was used to determine the glass transition temperatures () of the fabricated MMM. The analysis was done at a temperature range of 50-250°C with a heating rate of 10°C/min. The sample was first heated in the first cycle to remove the thermal history, then cooled at the same rate, and then heated again in the next heating cycle. The gas used in the analysis was nitrogen. The midpoint temperature of the transition region in the second heating cycle was determined as .

Field Emission Scanning Electron Microscopy (FESEM, Zeiss Supra 55 VP) was used to characterize the surface morphology in the prepared MMMs. The MMMs were first fractured in liquid nitrogen, coated in gold/platinum, and placed in a sample holder before analysis. The thickness of the membranes was also determined from the FESEM images.

Gas permeation test was done by using a constant pressure variable volume method in a four channel permeation cells setup [34]. The membranes were tested against pure gases in a sequence from CH₄ to CO₂. The membranes were cut into 58 mm diameter circles using a circle cutter and were put into the permeation cells. The gas permeation test was done at pressures of 2 to 10 bar with incremental pressure of 2 bar at 24°C. A digital bubble flow meter (Humonics 420) was used to measure the permeate gas flow rates and the reading was repeated 3 times. The permeance, P/l (unit GPU), and permeability, P (unit barrer), are then evaluated using the following equation [34, 35]:where P/l represents the gas permeance (cm³(STP)/(seccm²cmHg)), is the permeate flow rate at standard temperature and pressure in cm³(STP)/sec, A is the effective membrane surface area (cm²), and ∆P is the pressure gradient across the feed and permeate side of the membrane (cmHg). Gas permeance is usually reported in gas permeation unit GPU and Barrer, defined as

The selectivity of the membranes, , can then be evaluated as the ratio of the more permeable gas, i, to less permeable gas, j [34]:where is the gas selectivity of gas penetrant i over gas penetrant j and and are the gas permeance of gas penetrant i and j, respectively.

3. Result and Discussion

3.1. Characterization of Modified Silica Particles

The FTIR spectra for virgin silica and the ionic liquid modified silica particles are presented in Figure 2. The FTIR spectra for virgin silica are in accordance with literature [31], confirming the successful synthesis of the silica particles. Upon IL modification, the presence of additional broad bands, noticeably at 1354 c and 742 c, is observed. Both of the bands represent the characteristic bands of [EMIM][Tf₂N] which also show good agreement with literatures [31, 36]. Thus, these results confirm the successful attachment of the IL onto the surface of the silica particles. Increasing IL:Sil ratios however shows no observable changes on the obtained spectra.

Figure 3 shows the XPS spectra of IL-modified silica particles. The elemental compositions are summarized in Table 2. The as-synthesized silica particle is rich in Si and O, without presence of fluorine or nitrogen. The additional peaks of N1s (402.08eV) and F1s (688.68eV) are observed when IL is modified onto the silica particle, regardless of the IL ratio. These peaks further confirm chemical bonding of the ionic liquid onto the surface of the silica particles. The peak areas are determined and tabulated in Table 2 for each component. Increasing silica content decreases fluorine and nitrogen elements in the modified particles, indicating a lower proportion of the IL presence. It should be noted that oxygen element decreased prominently after modification. This finding further suggests that the IL replaces the oxygen element onto the silica surface [31] and confirms the efficacy of the modification. As for 1.0:1.0 sample, the fluorine element is less than the other 4 samples. This shows that less fluorine was able to replace oxygen element onto the silica surface but still significant enough for the success of the surface derivation of silica. This result is also reflected on the shorter 734 c peak of 1.0:1.0 sample compared to other 4 modified silica sample further indicating the less amount of fluorine element.

3.2. MMM Characterization

The morphology of the fabricated membranes is presented in Figure 4, while their thicknesses are summarized in Table 4. In general, it can be observed that the membranes possess clear dense structure, especially for pure PC membrane (Figure 4(a)) and PC-Sil (Figure 4(b)). Interestingly, the presence of IL (from IL:Sil) causes the structure to has rougher cross-section (Figures 4(c)–4(g)), independent of the IL ratios. Since the filler loading was kept constant (3 wt% of total solids), this indicates the robust interface between PC matrix. Furthermore, lower amounts of silica fillers due to the presence of IL lead to rougher morphology compared to the pure PC and the PC-Sil membranes.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

The EDX elemental scan result confirms the presence of the silica on all membranes observed based on the sharp peaks of silica (Table 3). Small amount of fluorine was also detected on all PC-Sil-IL MMMs as the IL contains fluorine. The amount of fluorine gradually decreases with increasing Si ratio accordingly. In addition, EDX mapping shows an even distribution of modified silica particles for all membranes (Figure 5).

(a)

(b)

(c)

(d)

(e)

(f)

The of the prepared membranes are tabulated in Table 4. The s of the pure PC and PC-Sil membrane are 143.38°C and 141.43°C, respectively, which are comparable with previous work [34]. The presence of IL lowers the of the membrane which indicates the increase in polymer chain flexibility [37–41]. Since the filler loading was kept constant (3 wt% of total solids), this suggests that IL acts as a plasticizer, which reduces the overall rigidity and brittleness of the polymer chains, and softens the polymer matrix [38, 40, 41]. Estahbanati et al. [40] also highlighted that the addition of IL resulting in a more amorphous and less crystalline MMM.

3.3. Gas Permeation

3.3.1. Influence of Modified Silica on Membrane Performance

The fabricated pure PC membrane possesses CO₂ permeance of 13 GPU and CO₂/CH₄ selectivity of 18.5, comparable with other [34]. Upon incorporation of the synthesized silica (PC-Sil), a slight decrease in the CO₂ permeance to 7.57 GPU is observed while prominent increased in CO₂/CH₄ selectivity to 25.96. The results can be attributed to the presence of silica within the MMM that contributes to higher solubilities and interactions of CO₂ with OH groups in silica particles [34, 42]. As a result, improvement in the selectivity with the expense of CO₂ permeance is observed.

Figure 6(a) shows the gas permeance of the PC-Sil-IL MMMs at feed pressure of 2 bar. Upon incorporating the IL-modified silica particles (PC-Sil-IL), both CO₂ permeance and CH₄ permeance increase prominently compared to the PC-Sil MMM. Remarkable improvements are shown for MMM with IL:Si ratio of 1.0:2.5, where the CO₂ and CH₄ permeance increase to 34.60 ± 0.26 GPU and 0.40 ± 0.001 GPU, respectively. The increase in gas permeance can be attributed to two main factors, namely, diffusivity and solubility. The presence of IL results in a more amorphous and flexible structure of MMM as observed by the changes (Table 3), facilitating transport of gas molecules [37]. In addition, the relaxation of the polymer matrix and higher FFV with addition of IL also enhances gases diffusivity [38, 40]. The increment of diffusivity of gases is more prominent for gases with larger kinetic diameters, which explains the larger increase of diffusion for CH₄ than CO₂, which leads to an increase of permeability of CH₄ at higher IL loadings.

(a)

(b)

On the other hand, the increase of CO₂ permeability is also contributed by the increased solubility of CO₂ to the membrane through the presence of IL. The IL has high affinity towards CO₂; thus its presence leads to an increase of CO₂ solubility higher than CH₄ [36, 43]. The solubilities of CO₂ and CH₄ for [EMIM][Tf₂N] IL are 50 atm and 1200 atm in Henry constant at STP [36, 44]. The lower value of Henry’s constant for CO₂ gas justifies the enhanced permeability of CO₂ due to the higher CO₂ solubility effect [36]. Similar trends have also been reported elsewhere [26, 28, 36, 38]. Thus, the combined effect of diffusivity and solubility increases gas permeance, in which CO₂ has more prominent increment than CH₄ owing to the presence of CO₂-philic IL.

The CO₂/CH₄ selectivity data of PC-Sil-IL membranes are shown in Figure 6(b). In general, the higher the IL content, the higher the CO₂/CH₄ selectivity due to the presence of higher CO₂-philic sites within the MMM. Interestingly, the highest IL content (sample IL:Si ratio of 1.0:0.5) shows the lowest selectivity of 19.90 ± 1.43, significantly lower than the PC-Sil MMM of 25.96. Only IL:Si ratio of 1.0:2.5 and 1.0:2.0 is superior to PC-Sil MMM. We propose that there are competition effects between solubility (from IL content) and diffusivity (from IL-induced plasticization). At high IL content (1.0:0.5), the CO₂ permeance increases by 832%, while CH₄ permeance increased by 11-fold as compared to PC-Sil. This indicates that the diffusivity factors induced by the IL-plasticization are superior thanks to the increased in the solubility, causing the more prominent increase in CH₄ permeance than CO₂. As the IL content is minimized, the effect of IL-induced plasticization is suppressed, as observed by increased in (relative to 1.0:0.5). As a result, the IL-induce plasticization is less severe and the solubility factor becomes more dominant. Thus, the sample with lowest IL content (1.0:2.5) suppresses CH₄ permeance (decreased by 96%) while favors CO₂ to permeance (increased by 357%) relative to PC-Sil. The ideal selectivity of 1.0:2.5 is 85.10, which is 228% more superior to the PC-Sil.

3.3.2. Influence of Pressure on Membrane Performance

Each prepared MMM was tested at different feed pressures to study the effect of pressure on its performance. The results are depicted in Figure 7. For all MMMs, both CO₂ permeance and CH₄ permeance show an overall decline with increasing pressure. This is in accordance with the dual-sorption model, common for glassy polymers, such as PC. At low pressure, Henry and Langmuir sites are not saturated and the presence of microvoids allows for higher permeations [45, 46]. As pressure increases, the saturation of Henry and Langmuir sites would drastically reduce the pathway of permeation which then decreases the permeability of the MMM effectively. For all fabricated MMMs, there was no indication of plasticization phenomena as no obvious rise in permeance was observed at pressures up to 10 bar. This could be due to the fact that the strong ionic interactions from the IL could counteract the volume dilation induced by the absorbed CO₂.

(a)

(b)

All MMMs show an overall decreasing trend in selectivity with increasing pressure. This is due to the larger decrease in permeance of CO₂ gas compared to CH₄ gas from 2 bar to 10 bar, which affects the overall selectivity of the MMMs. This is justified by the saturation of sites at higher pressures affecting largely on the CO₂ permeability due to the higher solubility effect of CO₂ relative to CH₄ gas. Thus, increasing the concentration of the CO₂ penetrant would greatly reduce its permeability [34, 47]. Hassanajili et al. stated that, as hydrostatic pressure increases, the free volumes decrease, thus declining permeance and overall selectivity [47].

3.4. Comparison with 2008 Robeson Upper Bound

The gas permeation performances of the fabricated MMMs are plotted on the Robeson’s upper bound (Figure 8) by converting the permeation unit to barrer using (2) (the thickness of each membrane was based on the thickness observed via morphology (Table 4)). From the plot, it can be observed that all PC-Sil-IL MMMs at 2 bar performances fall in the attractive region, especially for PC-Sil-IL with IL:Si ratio of 1.0:2.5. Similar results also reported by Ban et al. [48] (CO₂ permeability of ~300 barrer and CO₂/CH₄ selectivity of 38) and Li et al. [30] (CO₂ permeability of ~100 barrer and CO₂/CH₄ selectivity of 36). The presence of [EMITf₂] impregnated into their porous filler contributed to superior CO₂ permeability and CO₂/CH₄ selectivity, exceeding the Robeson upper bound. However, with increasing pressure from 2 to 10 bar, the performance falls linearly towards below the line to the typical region, which obeys the dual-mode sorption theory, similar to that highlighted by previous researches [34, 47]. Overall results suggest that modification of silica particles with [EMITf₂] and embedding in PC matrix imparts superior gas separation properties at low pressure. However, further investigation of PC-Sil-IL should be conducted to study its performance at high pressures.

4. Conclusion

The effect of IL:Si ratio on the modified silica particles incorporated in PC polymer matrix was thoroughly investigated and discussed. The IL-modified silica has been successfully prepared as indicated from fluorine functional group from XPS and FTIR spectra. The presence of IL in the membrane resulted in rough morphology on the cross-section that is likely caused by the less robust interaction between PC and the fillers. Moreover, the ILs prompt higher polymer chain relaxation as observed in the decreased . These results suggest that the presence of IL affects the resulting MMM properties, even at low concentrations. Gas permeation results show that increasing IL:Si ratio on the fillers reduces CO₂ permeability while increasing CO₂/CH₄ selectivity, with ratio of 1.0:2.5, gives the most promising results. The fabricated MMMs also show impressive capabilities of transcending the Robeson’s upper bound at low pressure of 2 bar. An increment of 3-fold in CO₂/CH₄ selectivity was observed for the MMM with IL:Si ratio of 1.0:2.5 ratio. MMM with IL:Si of 1.0:2.5 ratio shows a desirable performance but further investigation with regard to the performance at high pressures should be considered.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge the financial support provided by Universiti Teknologi PETRONAS (UTP) under Short Term Internal Research Fund (STIRF) (0153AA-F74) to carry out this research work. The authors also acknowledge the technical assistance provided by Chemical Engineering Department and Centralised Analytical Lab UTP during the experimental works.

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Copyright © 2019 Siti Nur Alwani Shafie et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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