Superoxide Ion as Oxidative Desulfurizing Agent for Aromatic Sulfur

Research Article pubs.acs.org/journal/ascecg

Superoxide Ion as Oxidative Desulfurizing Agent for Aromatic Sulfur Compounds in Ionic Liquid Media Maan Hayyan,*,†,‡ Abdulkader M. Alakrach,†,∥ Adeeb Hayyan,†,‡ Mohd Ali Hashim,†,§ and Hanee F. Hizaddin†,§ †

University of Malaya Centre for Ionic Liquids (UMCiL), ‡Institute of Halal Research University of Malaya (IHRUM), Academy of Islamic Studies, and §Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia

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S Supporting Information *

ABSTRACT: The generation of superoxide (O2•−) in ionic liquids (ILs) has been used to destroy hazardous materials. In this study, O2•− was used for the oxidative desulfurization of benzothiophene (BT) and dibenzothiophene (DBT) in two ILs as reaction media. The O2•− was released by dissolving potassium superoxide in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPyrr][TFSI] and n-methoxyethyl-n-methylmorpholinium bis(trifluoromethylsulfonyl)imide [MOEMMor][TFSI]. The highest conversion percentages for BT in [MOEMMor][TFSI] and [BMPyrr][TFSI] were 99.4 and 96.6%, respectively, and the highest conversion percentages for DBT in [MOEMMor][TFSI] and [BMPyrr][TFSI] were 98.3 and 94.3%, respectively. As the temperature was increased, the reaction of the O2•− with BT and DBT was enhanced significantly. The BT was converted easier than DBT due to the nucleophilic characteristic of the O2•− and the higher electron density of the sulfur atom in DBT than that in BT. In addition, the ILs had a clear effect on the oxidative desulfurization reaction, in which the O2•− demonstrated a higher conversion percentage in [MOEMMor][TFSI] than in [BMPyrr][TFSI]. This was the result of the stability of the O2•− in the ILs that were used in the study. In addition, the σ-potential obtained using the “Conductor-like Screening Model for Real Solvent” (COSMO-RS) indicated that [MOEMMor]+ was a superior hydrogen bond donor and had a stronger affinity for the hydrogen bond acceptor than [BMPyrr]+. The reaction mechanism was reported, and no toxic byproducts were detected. This is the first study in which O2•− in ILs was used for the conversion of BT and DBT to nonhazardous compounds, and this technique can be used for other hazardous materials. KEYWORDS: Oxidative desulfurization, Reactive oxygen species, Reaction engineering, Green solvent, Hazardous material, Dibenzothiophene, COSMO-RS, Deep eutectic solvent

1. INTRODUCTION Sulfur compounds are among the most undesirable heteroatomic components of petroleum. The high sulfur content in the fuel causes many environmental problems, since the combustion of these compounds generates toxic SOx gases (i.e., SO2 and SO3), which are major sources of acid rain and air pollution.1,2 Consequently, the European Union issued new regulations in 2010 to reduce the sulfur content of European gasoline and diesel fuels to 10 ppm or less.3 Among several approaches, the hydrodesulfurization (HDS) process is a conventional industrial method to remove sulfur compounds from petroleum. However, this method has the disadvantage of severe operation requirements, e.g., temperatures of 300−400 °C, high H2 pressure ranging from 30 to 130 atm, and high consumption of catalysts.4 In addition, the benzothiophene (BT), dibenzothiophene (DBT), and their alkylated derivatives are difficult to remove using the HDS method.4,5 For these reasons, other methods are needed as alternatives to overcome the disadvantages of the HDS process. One recent approach is © 2016 American Chemical Society

the purification of crude oil using various techniques based on oxidative,6−8 adsorptive,9,10 extractive,11 and biocatalytic desulfurization.12 Oxidative desulfurization (ODS) has become one of the most attractive desulfurization methods due to its ability to oxidize benzothiophene (BT) and dibenzothiophene (DBT) to the corresponding sulfoxides and sulfones,13 which, after reaction, can be separated by polar solvents.14 ODS requires lower temperatures and pressure than HDS for the reactions, and it does not require the use of H2,15 making the ODS process a promising alternative for the removal of sulfur from crude oil.16−18 Currently, hydrogen peroxide (H2O2) is the most popular ODS agent because it has comparatively low cost and is environmentally friendly; that is, it is nonpolluting and not strongly corrosive. In addition, experiments can be Received: October 26, 2016 Revised: December 18, 2016 Published: December 27, 2016 1854

DOI: 10.1021/acssuschemeng.6b02573 ACS Sustainable Chem. Eng. 2017, 5, 1854−1863

Research Article

ACS Sustainable Chemistry & Engineering

dose, and provide flexible storage, thereby facilitating the transport and shipment requirements.59

conducted at mild conditions, and this is considered to be one of the most favorable advantages of using H2O2 as an oxidative agent.19−21 However, the use of H2O2 also has some disadvantages, such as the production of water after reaction, handling issues, and storage problems.22 Superoxide (O2•−), which is a good oxidizing agent, is generated by the reduction of O2 to produce an oxygencentered radical.23 O2•− was first reported in 1934 by Haber and Weiss; they generated the ion through the decomposition of H2O2 and during the oxidation reactions of ferrous ions (Fe2+) by oxygen in aqueous solutions.24,25 However, due to the O2•− reaction tendency toward H2O (eq 1), O2•− was generated in nonaqueous solutions, such as aprotic solvents, e.g., dimethyl sulfoxide (DMSO), acetonitrile (AcN), propylene carbonate, pyridine, and acetone,26,27 though the utilization of aprotic solvents in the generation process of O2•− in industrial applications was limited due to their volatility, low boiling points, and the negative ecological effects.28 2O2•− + H 2O → O2 + HOO− + HO−

2. RESULTS AND DISCUSSION 2.1. Process of Converting Sulfur Compounds Using O2•− in ILs. The process of refining crude petroleum has two stages. The first stage is the extraction of sulfur compounds by ILs followed by conversion of sulfur compounds into nontoxic byproducts using O2•− in the second stage. Hence, the conversion was studied for sulfur compounds that were dissolved in ILs. The purpose of this reaction was to convert the toxic chemicals to nonhazardous compounds. As a result, the process would be practical for industrial scale applications. The tendency for superoxide to react with sulfur compounds is considerably high, taking into account the nucleophilic characteristic of O2•−. Figure 1 and Table S1 show the sulfur

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Recently, ILs, as neoteric solvents, have attracted the attention of people in both academia and industry because they have many advantages over conventional solvents. For instance, ILs have negligible vapor pressure, high thermal stability from below ambient to well over 300−400 °C,29,30 wide electrochemical windows (4.5−6 V),31 and the ability to dissolve several inorganic and organic compounds. In addition, they are recyclable.29,32,33 Currently, ILs and their analogous deep eutectic solvents are used extensively in research concerned with the extractive desulfurization of liquid fuels.34−43 For instance, Domańska et al. (2014) investigated the utilization of ILs in the extraction of sulfur compounds from gasoline and diesel oil.3 They used pyridinium-, pyrrolidinium-, morpholinium-, imidazolium-, piperidinium-, phosphonium-, and ammonium-based ILs. Pyrrolidinium provided the best extraction efficiency, and it was followed by morpholinium and pyridinium, which showed high selectivity and high distribution coefficient for the extraction of sulfur compounds. However, the sulfur removal selectivity from fuels is of significant importance, as it is directly linked to the removal mechanisms. As a result, the selectivity is rather difficult to obtain unless the mechanism is fully understood.44 The ODS using ILs has also been explored recently.7,34,45−49 IL may act as a multitask fluid, since it can be used as extractant to sulfurs or as catalyst to accelerate conversion reaction, or it can serve as both extractant and catalyst.50 In addition, the research on the oxidizing agents’ stability in ILs as reaction media is of great importance.34 AlNashef et al. (2001, 2002) first reported the generation of a stable O2•− in 1-methyl-3-n-butylimidazolium hexafluorophosphate.51,52 Consequently, numerous studies have been dedicated to investigating the stability of O2•− in ILs.24,27,53−57 In this work, O2•− was examined as a potential oxidizing agent for the ODS of BT and DBT. The ILs, 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMPyrr][TFSI] and n-methoxyethyl-n-methylmorpholinium bis(trifluoromethylsulfonyl)imide [MOEMMor][TFSI], were used as media to release O2•− by dissolving potassium superoxide (KO2). These ILs demonstrated that the O2•− had good stability, as reported in previous studies.53,58 The superoxide salts, particularly KO2, can reduce the required reactor volume considerably, reduce the weight of the oxidant

Figure 1. Effect of temperature on the conversion percentage of BT and DBT by O2•− as oxidizing agent.

conversion percentages of BT and DBT, and they clearly show that O2•− exhibited significant capability for converting the aromatic sulfur compounds. The percentages of the original sulfur compounds removed in the process were 99.4 and 98.3% for BT and DBT, respectively. It took 2.532 mmol of O2•− to oxidize 0.112 and 0.109 mmol of BT and DBT, respectively. This indicated that the ratio of sulfur to KO2 was 1:22.7 for BT and 1:23.3 for DBT. Figure S1 shows the chromatograms of BT and DBT before and after reaction with O2•− in ILs. The selection of suitable ILs as reaction media is an important aspect of the process because the ILs may have a direct effect on the stability of the O2•− generated in the ILs. The stability of the O2•− in [MOEMMor][TFSI] and [BMPyrr][TFSI] was studied by Hayyan et al. (2012, 2015), and they demonstrated that the O2•− ions had high stability in both IL systems.53,60 However, they were more stable in [MOEMMor][TFSI] than in [BMPyrr][TFSI], with consumption rates 0.651 × 10−3 and 4.049 × 10−3 mM/min, respectively. Figure 2 shows the effect of the structure of the ILs on the conversion results. However, it is noteworthy that the conversion of BT in [BMPyrr][TFSI] was higher under mild conditions (i.e., temperatures below 50 °C) than the conversion in [MOEMMor][TFSI]. This was thought to be due to the viscosity of the ILs, since highly viscous ILs will slow the dissolution rate of the superoxide salt. The viscosity of 1855


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that crown ethers are used to stimulate the solubility of superoxide salts for the O2•− reaction with organic molecules.23 A higher conversion percentage was observed for [MOEMMor][TFSI] than for [BMPyrr][TFSI], but the difference was small, i.e., 3% for BT and 4% for DBT. The COSMO-RS calculation was performed to investigate the effect of the structure of the ILs on the conversion percentage using the σ-profile and the σ-potential analysis, as illustrated in Figure 3. Based on the σ-profile (Figure 3a), both cations had profiles

Figure 2. Effect of the structure of the ILs on the conversion percentage of BT and DBT after reaction with the O2•−.

[MOEMMor][TFSI] (257.6 mPa s)61 was higher than that of [BMPyrr][TFSI] (68.16 mPa s)62 at room temperature (25 °C). However, the viscosity of [MOEMMor][TFSI] was reduced significantly as the temperature increased (i.e., 77.98 mPa s at 50 °C),61 and this facilitated the dissolution and diffusion of the reaction products. A linear trend was observed between the conversation rate and the increase in the temperature. Ultimately, at 180 °C, the percentages of conversion of BT in [MOEMMor][TFSI] and [BMPyrr][TFSI] reached 99.4 and 96.6%, respectively, and the percentages of conversion of DBT in [MOEMMor][TFSI] and [BMPyrr][TFSI] reached 98.3 and 94.3%, respectively. These results indicated that the conversion of BT was easier than that of DBT, and this difference was attributed to two factors, i.e., the electron density around the sulfur atom and the steric hindrance. This is in agreement with a recent study conducted on the conversion of thiophene and 2-methylthiophene using O2•−.63 The electron densities around the sulfur atoms in BT and DBT were calculated by Otsuki et al. (2000) as 5.739 and 5.758, respectively.64 Many previous studies have reported that the desulfurization performance decreased as the electron density decreased when using H2O2 in ODS.65−67 However, in this study, DBT was more difficult to convert than BT, and this was thought to be attributable to the nucleophilicity of the O2•− and the electron density of the S atom. This is in accordance with the study of Roberts et al. (1981), in which it was observed that O2•− ions are nucleophilic in solvents that do not provide protons.68 In addition, the solubility of BT is higher than that of DBT in all ILs (Table S2). The structures of the cations and anions of the ILs and their sizes affect the solubility of the sulfur compounds.69 This result indicated that, in addition to the ILs’ main role as the media in which these reactions occur, the solubility of the sulfur compounds in the ILs also affected the reactions between the various sulfur compounds and the O2•− ions. ILs can be regenerated by washing with organic solvents such as diethyl ether.70,71 There are several methods that can be used to extract K+ following superoxide release from its salt, one of which is the use of crown ethers as metal ion extractant to extract K+. Since the discovery of this class in 1967, crown ethers have been widely explored for metal ion extraction in liquid/liquid separation processes.72−74 It is also noteworthy

Figure 3. (a) σ-Profile and (b) σ-potential of sulfur compounds and ions comprising the ILs that were used in this study.

that were complementary to those of BT and DBT, which explains the high percentage of conversion. However, based on the σ-potential (Figure 3b), the [MOEMMor] cation is a better hydrogen bond donor and has a better affinity for the hydrogen bond acceptor than the [BMPyrr] cation. In addition, the [MOEMMor] cation can also function as a hydrogen bond acceptor despite being a weak one. Probably, the tendency of the [MOEMMor] cation to function as both a hydrogen bond donor and a hydrogen bond acceptor (weak one), and also the fact that [MOEMMor] is a better HBD than [BMPyrr], may contribute to the higher conversion percentage of the sulfur compounds. This supported the catalytic activity role of the IL as reported earlier.63 The HOMO and LUMO orbitals can also be rough indicators of the interaction between species in the complex. Ideally, we want the HOMO and LUMO to come from opposite molecules to indicate a favorable interaction. For example, if the HOMO (or LUMO) comes from either the IL 1856


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Scheme 1. HOMO and LUMO for BT Interaction with (a) [MOEMMor][TFSI] and with (b) [BMPyrr][TFSI]

Scheme 2. HOMO and LUMO for DBT Interaction with (a) [MOEMMor][TFSI] and with (b) [BMPyrr][TFSI]

cation or anion, and the LUMO (or HOMO) comes from the sulfur compound, this indicates a favorable interaction between the IL and the sulfur compound is expected. But, if both HOMO and LUMO come from the IL or both come from the sulfur compound, it cannot be concluded whether there is a

favorable interaction between the two species (i.e., IL-sulfur compound). For the complex [MOEMMor][TFSI]-BT, the HOMO comes from BT, whereas the LUMO comes from the cation. This indicates that there is a strong interaction between the IL and BT. In contrast, for the complex [BMPyrr][TFSI]1857


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effect of the reaction rate on the conversion process. The conversion of sulfur compounds increased as the temperature increased (Figure 1) and as the time increased (Figure 4). Again, this result can be explained by the increase in the solubility of KO2 in the ILs and by the decrease in the viscosities of the ILs as the temperature increased. In contrast, under mild conditions, Ahmed et al. (2015) did not obtain high conversion of sulfur compounds when they investigated the use of O2•− in five phosphonium-based ILs.67 They used the O2•− to convert two sulfur compounds (i.e., thiophene and DBT). The results were only 15% conversion of thiophene, and DBT did not react with the O2•− over a period of 2 h. In addition, numerous studies have pointed out the effect of temperature and time on the conversion of sulfur compounds. For instance, carbon nanotubes were used as catalysts to activate oxygen for ODS of three sulfur compounds, i.e., BT, DBT, and 4,6dimethyldibenzothiophene.22 A complete conversion of the sulfur compounds to the corresponding sulfones was achieved at 150 °C with a maximum time of 180 min. 2.2. Reaction Mechanism. Scheme 3 illustrates the proposed mechanism. Several studies have reported similar mechanisms for the oxidation of thiophenes and aromatic sulfides.16,75,76 Attar et al. (1978) reported that aromatic sulfur compounds are oxidized to the corresponding sulfoxides and then to sulfones (eq 2). The oxidation products begin to decompose as the temperature increases (eq 3).4

BT, both the HOMO and the LUMO come from BT. This is indicative of a weak interaction between the IL and BT (Scheme 1). However, for complexes that contained DBT, both the HOMO and the LUMO came from the DBT in the complexes [MOEMMor][TFSI]-DBT and [BMPyrr][TFSI]DBT (Scheme 2). In this case, the result is inconclusive; that is, the HOMO and the LUMO location does not indicate which IL would have the stronger interaction with DBT. To study the effect of the reaction rate and temperature, the reaction process was investigated as a function of time (i.e., 0, 3, 6, 9, 12, and 15 h) with different temperatures (i.e., 27, 40, 50, 60, 80, 100, 120, 140, 160, and 180 °C). Figure 4 shows the

[O]

[O]

R‐S‐R′ ⎯→ ⎯ R‐SO‐R′ ⎯→ ⎯ R‐SO2 ‐R′ Figure 4. Effect of time on the percentage of conversion of BT and DBT by O2•− at 180 °C.

(heat)

R‐SO2 ‐R′ ⎯⎯⎯⎯⎯→ hydrocarbons + SO2

(2)

(3)

Scheme 3. Proposed Reaction Mechanisms of BT and DBT with O2•−

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Figure 5. FTIR chromatograms for DBT before and after conversion by O2•− in [MOEMMor][TFSI].

Figure 6. Mass spectra of the oxidation product of DBT and biphenyl.

to identify the new functional groups in the products of the reactions of BT and DBT with O2•− ions in the IL media. Figure 5 shows the FTIR spectra of BT and DBT before and after reaction with O2•− ions at 100 °C, and several new peaks were present in the spectra after the reactions. For instance, there is a sharp peak centered at ∼1060 cm−1 in the spectra after reaction with the O2•− that was not present in the spectra of pure DBT. This peak results from the stretching of the SO bond in sulfoxide. Furthermore, two other peaks, centered at about 1350 and 1140 cm−1, may represent the asymmetric and symmetric stretches of the OSO of sulfones.78 Hence, the

In addition, Chan (2010) reported that SO2, which is produced when sulfur compounds are oxidized by O2•− in ILs, can be oxidized to sulfate (SO4−2).77 Sulfates can be precipitated by adding BaCl2 solution, and a white precipitate of barium sulfate (BaSO4) is produced. In this study, a similar result was obtained, as was verified by adding barium chloride to the sample. Zhang et al. (2014) studied the catalytic effect of carbon nanotubes to convert molecular oxygen to active oxygen (i.e., O2•−), thereby degrading aromatic sulfur compounds.22 Our results were compatible with their results, in that DBT was oxidized to the corresponding sulfones. FTIR analysis was used 1859


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taken before and after the addition of KO2 by dissolving 0.1 g of the IL−sulfur mixture in 1 g of AcN. This procedure was repeated, and more KO2 was added until the sulfur peak either was not detected or did not change. This reaction was conducted at room temperature and at 40, 50, 60, 80, 100, 120, 140, 160, and 180 °C for both BT and DBT. This temperature range was selected because the temperatures were lower than the boiling points of BT (221 °C) and DBT (332 °C).80 The FTIR spectra were identified by FTIR spectrophotometry (Bruker TENSOR 27) using KBr disks at room temperature. The spectrophotometer was used to characterize the new products in the range 650−4000 cm−1. The gas-chromatograph/mass spectrometer (GCMS-QP 2008, Shimadzu QP-5050A) with a DB-5 column (30 m × 0.32 mm × 1 μm) was used to identify BT and DBT before and after reaction with the O2•− at 180 °C and to identify the products of the reaction. The injector temperature was set at 275 °C. The initial temperature was set at 35 °C for a 10 min holding time, and then, the temperature was ramped to 250 °C at a heating rate of 10 °C min−1. 4.3. Solubility of Sulfur Compounds in the ILs. The solubility of BT and DBT in the ILs was measured at room temperature by adding an excess amount of the sulfur compound to a glass vial that contained the IL with vigorous stirring. Then, the mixture settled to form a two-phase system. After reaching equilibrium, a sample was withdrawn carefully from the saturated IL phase and diluted in AcN for analysis using HPLC.

FTIR analysis suggested the presence of the SO of sulfoxides and the OSO of sulfones as products of the conversion of the sulfur compounds. The GC/MS chromatograms of nonconverted and converted sulfur compounds are shown in Figure S2. The GC peaks of sulfur compounds in the oxidized samples were smaller than the peaks in the nonoxidized samples. Consequently, the chromatograms indicated that the sulfur compounds were preferentially converted by this reaction. A new peak of biphenyl in the chromatogram after the DBT conversion process is observed in Figure 6, which is in accordance with the results of the FTIR analysis, as shown in Figure 5.

3. CONCLUSIONS In this work, the conversion of BT and DBT by O2•− was investigated in [MOEMMor][TFSI] and [BMPyrr][TFSI]. The O2•− was generated by dissolving KO2 in ILs. The results showed that BT was easily converted than DBT. The ILs had a clear effect on the conversion process. The O2•− produced higher conversion percentages in [MOEMMor][TFSI] than in [BMPyrr][TFSI] for both of the sulfur compounds. Increasing the temperature substantially affected the conversion process by reducing the viscosity of the IL and increasing the solubility of the superoxide salt, thereby accelerating the reaction rate. The ILs showed promise as media for the generation of O2•− and for the conversion of sulfur compounds.

5. COMPUTATIONAL DETAILS 5.1. COSMO-RS Model. The “Conductor-like Screening Model for Real Solvents” (COSMO-RS) is a dielectric model in which molecules are placed in a conductor as the reference state. The interaction energy in COSMO-RS is represented in terms of its polarization charge density, σ and σ′, which include the electrostatic misfit energy (Emisfit), hydrogen bond interaction (Ehb), and van der Waals interaction (EvdW).81 The most important descriptor in the COSMO-RS model is the local screening charge density, σ, because it is the only descriptor that is used to determine the interaction energies and, thus, all other statistical thermodynamic properties, such as chemical potential and activity coefficient. This descriptor can be calculated using any quantum chemical program in the form of “.cosmo f ile” output. In the .cosmo f ile, the surface of the species is divided into segments with a certain surface charge density. By applying a local averaging algorithm on the surface charge densities over effective contact segments, a probability function (σ-profile) can be plotted. The details of the local averaging algorithm are available in ref 81. 5.2. Geometry Optimization and COSMO-RS Implementation. The job of optimizing the geometry was performed for each individual species (each sulfur compound, each cation, and each anion), for the ionic liquid complex, and for the complexes of the sulfur compounds with the ILs. Initial structures for each job were drawn, and the job was executed to perform geometry optimization at the Hartree−Fock level, and the 6-31G* basis was set using Turbomole version 4.0.1 software.82 The HOMO and LUMO orbitals were generated from the results of the calculations. Then, the optimized geometries of the compounds were used for a single point calculation to generate the .cosmo files using the density functional theory combined with the Becke−Perdew functional and the triple-ζ valence potential (TZVP) basis set using the Turbomole 4.0.1 software. Then, the COSMOthermX software with the parametrization file BP_TZVP_C30_1401.ctd was used to generate the σ-profile and the σ-potential graphs.83

4. EXPERIMENTAL SECTION 4.1. Materials. Scheme 4 illustrates the chemical structures of the ILs, i.e., [BMPyrr][TFSI] (98%) and [MOEMMor][TFSI] (synthesis

Scheme 4. Chemical Structures of the ILs Ions and Sulfur Compounds

grade with a purity of 98%). The sulfur compounds, i.e., BT (99%) and DBT (99%) were supplied by Merck. Potassium superoxide (KO2) (99.9%) was purchased from Sigma-Aldrich, HPLC grade acetonitrile (AcN) (99.9%) was purchased from UNICHROM, and diethyl ether (99%) was supplied by Merck. 4.2. Oxidative Desulfurization of BT and DBT in ILs. The ILs were dried overnight in a vacuum oven at 50 °C, as described previously.28,79 Appropriate amounts of the sulfur compounds were added to a labeled vial containing the ILs. The IL−sulfur mixture was stirred for 30 min with a magnetic stirrer. A sample was then withdrawn and diluted with AcN, and analyzed using HPLC, as described before.71 Next, KO2 was added gradually to the vial that contained the IL−sulfur mixture with vigorous stirring. Samples were

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02573. HPLC and GCMS chromatograms (PDF) 1860


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]; Tel/Fax No.: + 6-03-7967-5311. ORCID

Maan Hayyan: 0000-0002-2179-0801 Present Address ∥

(Abdulkader M. Alakrach) Center of Excellence Geopolymer and Green Technology (CEGeoGTech), Faculty of Engineering Technology (FETech), Universiti Malaysia Perlis (UniMAP), UniCITI Alam Campus, Perlis 02100, Sungai Chucuh, Padang Besar, Malaysia. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the University of Malaya HIR MOHE (D000003-16001) and UMRG (RP037B-15AET) is gratefully acknowledged.

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