Novel Supported Ionic Liquid Adsorbents for Hydrogen Sulphide Removal from Biogas

The imidazolium-based supported ionic liquids (IL) in activated carbon (AC) is an exciting strategy for developing new adsorbents for H 2 S removal from biogas. In this work, the influence of IL on AC was discovered by examining the effect of ultrasonic stirring as an impregnation method, AC particle size and IL anion type. AC300μm-[Bmim] Cl-U5 demonstrated the highest H 2 S adsorption capacity of 8.25±0.38 mg H 2 S/g and was obtained through [Bmim] Cl impregnated on 300 μm AC size through the ultrasonic stirring for five minutes at room temperature. The ad - sorption/desorption study confirmed the regeneration ability of AC300μm-[Bmim]Cl-U5 up to three cycles with a maximum adsorption capacity of 14.24±0.43 mg H 2 S/g. The SEM images confirmed the presence of IL on the AC surface and were further explained through BET analysis. TGA measurement indicated the thermal stability of pristine IL, the fresh and exhausted adsorbent. Therefore, this study proved the potential of ultrasonic-assisted supported IL as a promising adsorbent for H 2 S removal from biogas that exhibits excellent properties in high adsorption capacity and thermal stability.

Chemical absorption, utilising amine, carbonate, and ammonia solvents as absorbents, is a well-established and effective method for H 2 S removal.However, the application of chemical solvents is constrained by the concerns related to physical characteristic limitations, such as viscosity, corrosiveness, and higher energy consumption (Choi et al., 2014).Ionic liquids (IL) emerge as a promising solvent for H 2 S removal, being vapourless, thermally stable, reusable, and recognised as environmentally friendly solvents (Beigi et al., 2018).IL exist in a liquid form as molten salts and are composed of both anions and cations.Examples of cations include imidazolium, ammonium, phosphonium, pyridinium, and pyrrolidinium, while anions include tetrafluoroborate, hexafluorophosphate, bistrifluorosulfonylimide, and chloride.ILs are often referred to as 'designer' solvents due IL to their unique ability to fine-tune physicochemical properties by slightly modifying the structure of corresponding anions and cations (Taheri et al., 2021).
Besides absorption, adsorption emerges as a notably simple process, a technology with easy operation and low energy consumption, particularly well-suited for small-scale applications (Zhang H.Y. et al., 2019).While activated carbon (AC) serves as a common, cost-effective, and readily available adsorbent material, it encounters challenges regarding selectivity, breakthrough time, and limited capacity (Nurhidayah et al., 2022;Lucena et al., 2020;Plaza et al., 2010).On the other hand, IL possess issues like viscosity, high cost and dynamic separation (Bárbara Burlini Polesso et al., 2019).Combining AC and IL, however, presents an increasingly effective adsorbent solution with high adsorption capacity performance.This approach addresses the flaws of both materials and minimises IL usage, employing only a low percentage for AC-IL impregnation.
This study aimed to combine the advantages of AC with the unique properties of IL using an ultrasonic-assisted stirring method.This approach provides a new idea of a novel and greener technique for effective H 2 S removal.Previous methods of conventional stirring supported-IL were time-consuming, with required duration ranging from two to six hours (Huang et al., 2022;Ren et al., 2023).Hence, the effect of ultrasonic stirring time was studied to determine the ideal time of IL impregnation on AC.However, the pore size of the adsorbent has also been a critical issue.Previous studies of supported IL adsorbent focused more on the mesoporous material of silica and alumina (Duczinski et al., 2018  .Therefore, the interaction between IL and H 2 S and the influence of IL solubility on the H 2 S adsorption capacity were further evaluated using COSMO-RS.The outcomes propose the best AC and IL selection for the ultrasonic-assisted adsorbent and contribute to a deeper understanding of the fundamental interaction between IL and H 2 S to enhance H 2 S gas removal efficiency, specifically from biogas.

EXPERIMENTS Materials
The coconut husk-AC with a mesh size of 20×40 was used in this study.High purity of 98% IL of [Bmim]Cl and [Bmim][NTf 2 ] were purchased from Sigma Aldrich.The gas tank containing a mixture of 2000 ppm H 2 S/N 2 was obtained from Alpha Gas Solution.A column 10/10 from Cytiva was purchased and used as the adsorption column.Gas concentration was measured using Biogas Analyzer Geotech GA 5000 from QED Environmental Systems.The schematic diagram of the adsorption system setup is illustrated in Figure 1.

Supported IL preparation
The AC was sieved to achieve 300 and 600 µm uniform particle sizes.Next, the AC was dried at 90 ℃ for 24 hours to eliminate the moisture and stored in an airtight bottle before use.Adsorbent preparation was started with the dilution of 0.20 g of [Bmim]Cl in 10 ml of ethanol.The solution was allowed to dissolve for a few minutes before being added to a universal bottle filled with 2.0 g of AC.The AC-IL solution was then subjected to ultrasonic stirring at 50 Hz for 0, 5, 10, 20, and 30 minutes at room temperature.Then, the ethanol was removed using a rotary evaporator at 75℃.Lastly, the AC-IL adsorbent was dried at 90℃ for 24 hours in an oven to remove excess solvent.These procedures were replicated for different AC particle sizes and IL.Each adsorbent sample was labelled ACXµm-[IL]-UY, where X represents AC particle size (300, 600), and Y denotes the ultrasonic duration in minutes.

H 2 S adsorption and desorption
Approximately 2.00 g of absorbent was measured and packed with glass wool in a column with 10 × 10 mm (length x inner diameter).The adsorption tests were conducted at ambient temperature and pressure (1 bar).The fed concentration was 2000 ppm H 2 S diluted in N 2 and flowed at 0.10 L/min.The adsorption time and outlet concentration were recorded for each five minutes until the breakthrough and saturation points (for adsorption/desorption tests).A Geotech GA 5000 biogas analyser was used to measure the real-time H 2 S outlet concentration.The adsorption/desorption test was carried out using the optimum adsorbent under the same conditions (room temperature and 1 bar) for three cycles.The exhausted sample was heated at 150 °C for 30 minutes to desorb the gas before each cycle.Each test was duplicated for better accuracy.Next, the H 2 S adsorption capacities of supported IL-AC adsorbents were calculated at one ppm (as the breakthrough time).T B and C values were recorded in minutes and ppm, respectively, and applied as in Equation 1 where: q is referred to adsorption capacity (mg H 2 S/g), Q is the flow rate (L/min), T B is the breakthrough time at one ppm (min), C 0 is the H 2 S gas concentration (ppm), MW H2S is H 2 S molecule mass (mg/mol), V m is the molar volume at S.T.P (L/mol), and m adsorbent is the final mass of AC-IL sample (g).
The maximum adsorption capacity, Q max of the adsorbent in each cycle was calculated as in Equation 2, as modified from Equation 1: where: m final is the mass of exhausted adsorbent, m initial is the mass of desorbed absorbent.
The degradation of each cycle was calculated as the percentage differences as in Equation 3 (Zulkefli et al., 2022a), where N is the previous cycle and N+1 is the current cycle.
The adsorbent performance was calculated as the removal efficiency as in (Kalidhasan et al., 2012), when C inlet and C outlet are referred to inlet and outlet H 2 S concentration in ppm.
Characterisation of the adsorbent X-ray diffraction (XRD) analyses of the fresh and exhausted (after H 2 S exposure) adsorbents were performed using Bruker D8 Advance, with Cu Kα (40 kV, 40 mA) at λ = 0.154 nm and range temperature of 256-2054 °C.On the basis of Bragg's Law, XRD powder patterns (~10 mg) were shown in the 2θ range of 5-80°.Scanning Electron Microscopy (SEM) analyses using ZEISS Supra 55VP determined the sample morphology and elemental content.The structure of the sample surface can be portrayed at 10x to 300Kx magnification with a maximum of 10nm of sample thickness.Brunauer-Emmett-Teller (BET) theory in Micrometritics TriStar II Plus version 3.03 was applied to calculate the specific surface area and porosity based on nitrogen adsorption/desorption isotherm at 77 K. Around 0.20 g of sample were placed under nitrogen flow at 250 ℃ for three hours.The surface area, pore volume, and pore size were calculated by the BET as well as Barrett-Joyner-Halanda (BJH) at p/p 0 = 0.95.Thermal gravimetric analysis (TGA) data were obtained through the NETZSCH STA 449 F3 Jupiter model.The samples were heated from 30-600 ℃ at 20 ℃/min under 20 ml/min nitrogen flow.Before the measurement, the sample of <5.0 mg was pre-treated at 120℃ for two hours in the oven to remove the moisture.

Computational study of COSMO-RS
COSMO-RS was used to predict the thermodynamic properties and interaction behaviour of H 2 S and IL upon mixing.It was assumed that the absorption performance could be correlated with the solubility of H 2 S in IL, based on the fugacity of H 2 S molecules in vapour and liquid phases, as described by Henry's law.The solubility values of H 2 S in various ILs were gathered from the current study and existing literature to benchmark the accuracy of the COSMO-RS model.Imidazolium-based IL, such as 1-hexyl-3-methylimi-

RESULTS AND DISCUSSION
Effect of ultrasonic stirring time on H 2 S removal AC, with a particle size of 300 μm (AC300μm), serves as the solid material for the impregnation of [Bmim]Cl facilitated by ultrasonic stirring.Ultrasonic stirring is chosen for its capability to provide uniform and effective stirring as well as further enhance the adsorption efficiency (Mullick and Neogi, 2019).The effect of ultrasonic stirring time was systematically investigated across durations of 0, 5, 10, 20, and 30 minutes at room temperature.As depicted in Figure 2, the most significant H 2 S adsorption capacity was achieved within a short 5-minute ultrasonic impregnation through the sample designated as AC300μm-[Bmim]Cl-U5.AC300μm-[Bmim]Cl-U5 exhibited highest adsorption capacity of 8.25±0.38 mg H 2 S/g.The efficiency of ultrasonic stirring lies in its ability to promote the opening of additional micropores within the AC structure, thereby increasing the surface area for interaction between AC and the IL.It is noteworthy that prolonged stirring times resulted in lower adsorption capacities due to the possibility of AC pore blockage, which reduces the available surface area (Zhang et al., 2018).The shorter stirring time proves advantageous for mass transfer as rapid formation of microbubbles and loss facilitate the process.In addition, Dou et 2019), both found that ultrasonic stirring can increase the AC surface area and pores.

Effect of AC particle size on H 2 S removal
The effect of AC particle size was studied using [Bmim]Cl impregnated on AC using ultrasonic stirring for 5 minutes at room temperature.Figure 3a shows the H 2 S adsorption capacity of AC-[Bmim] Cl-U5 at different sizes, 300 and 600 µm AC.It demonstrates that AC with a smaller particle size possesses a greater capacity for gas adsorption than its larger counterparts.Figure 3b shows the breakthrough curves obtained at ambient temperature and pressure at 0.10 L/min flow rate.The breakthrough time increases at decreasing AC particle size, consistently with the higher adsorption capacity, according to the data presented in Figure 3b  ).On the basis of the BET analysis conducted on raw AC (without impregnation) presented in Table 1, a smaller AC particle, AC300µm, possesses a higher surface area and total pore volume than AC600µm, which gives more exposed sites on the particle where adsorption can occur.Cl-U5 at different particle sizes under ambient temperature and pressure

Thermodynamic characteristic of IL and H 2 S
The prediction for gas solubility was conducted through the COSMO-RS computational program.Before the prediction, validating the COSMO-RS using experimental data is crucial to confirm the reliability and accuracy of the model.Hence, the available experimental solubility data were collected from this study and the literature as well as compared against the COSMO-RS predicted solubility values.The experimental and predicted H 2 S solubility, Henry's law constant (H) and excess enthalpy (H E ) of six chosen IL from the literature are presented in Table 2. Figure 5a (Mutalib et al., 2022).
The interactions between the IL and H 2 S were analyzed by the σ-profile and σ-potential curves portrayed in Figure 6a and Figure 6b.There are three main regions in σ-profile and σ-potential: hydrogen bond acceptor (σ > 0.01 eÅ 2 ), hydrogen bond donor (σ < -0.01 eÅ 2 ), and nonpolar (-0.01 eÅ 2 < σ < 0.01 eÅ 2 ).For H 2 S, it can be observed that the peaks appear at the range of -0.013 to 0.013 e/Å 2 , in which the polar peaks formed at 0.005 e/Å 2 and -0.010 e/Å 2 correspond to S and H atoms, respectively.On the other hand, the curve in the nonpolar region is contributed by the partial screening charge densities between the two S-H bonds.Hence, H 2 S can be said to have polar and nonpolar properties.The σ-profile of H 2 S is in agreement by the σ-potential where H 2 S has the lowest negative value in the hydrogen bond acceptor region, which induces high affinity towards [Bmim]Cl.The σ-profile of [Bmim]Cl falls in the range of -0.017 to 0.020 e/Å 2 , indicating an affinity for the molecules with nonpolar and hydrogen bond acceptor properties.Additionally, the Cl-anion peaks at 0.019 e/Å represent a strong hydrogen bond acceptor capability (Mutalib et al., 2022).The σ-profile is in line with the σ-potential of [Bmim]Cl, in which the Cl -anion shows strong affinity to hydrogen bond donor, as reflected by the curve of σ >-0.010 e/Å 2 .In addition, [Bmim] + cation shows a narrow curve with negative µ values on the right side, reflecting a relevant affinity towards a hydrogen bond-accepting molecule, i.e., the S atoms of H 2 S.Meanwhile, the σ-profile of [Bmim][NTf 2 ] falls in the range of -0.017 to 0.016 e/Å 2 , with the largest peak formed at +0.002 e/ Å 2 , indicating the strong nonpolar property of the alkyl-fluorine group.The peak of NTf 2 -anion appears at 0.011 e/Å 2 indicating a weaker affinity to  hydrogen bond acceptor than Cl -.This is consistent with the σ-potential of NTf 2 -anion at the value of -0.009 e/Å 2 illustrating the affinity toward hydrogen bond donor and rejecting hydrogen bond acceptor molecule (Azlan Kassim et al., 2020).

Regeneration study
The regeneration study involved assessing adsorption capacity until saturation by the chosen adsorbent, AC300μm-[Bmim]Cl-U5. Desorption of the exhausted adsorbent was conducted at 150°C for 30 minutes.The adsorption/desorption profile, depicted in the Figure 7a revealed that AC300μm-[Bmim]Cl-U5 reached the saturation point at the same C/C 0 ratio for the first three cycles.Although the total adsorption time spanned 690 minutes, the adsorption time for cycles 1 to 3 at C/C 0 = 0 gradually shortened.This indicates a decrease in durability, aligning with the Q max values in the Figure 7b and Table 3.
Calculation using Equation 2 revealed a high Q max of 14.24 mg H 2 S/g in the first cycle as presented in Figure 7b, attributed to the presence of pores and the number of active sites on the AC surface.However, the degradation percentage was 42.25%, with an adsorption time of 35 minutes at C/C 0 = 0, shorter than in the first cycle (65 minutes).Throughout the adsorption/desorption tests, the degradation percentage stays on average between 33.17% and 42.25%, indicative of a decreasing effectiveness of the adsorbent.
Therefore, optimising regeneration conditions, such as temperature and heating time, becomes crucial for prolonging the shelf life of this adsorbent.In conclusion, AC300μm-[Bmim]Cl-U5 demonstrated the potential for regeneration through heating at 150 °C for 30 minutes for at least three cycles, with an acceptable reduction in Q max as observed in previous studies (Ma et al., 2019).

XRD
The analysis of crystallinity and amorphous properties was conducted on various samples, including raw AC, fresh and exhausted adsorbents, AC300μm, AC300μm-[Bmim]Cl-U5(B), and AC300μm-[Bmim]Cl-U5.The high carbon peak (002) at 2θ = 24° indicates the amorphous structure of carbon, while the broad peak signifies the positive impact of effective IL dispersion on the AC surface.Table 4 revealed that AC300µm, in its pristine state without impregnation, exhibits amorphous characteristics with a    2016) agreed that ultrasonic radiation strongly effects adsorbent properties by promoting the impregnation process and increasing the crystallisation of the particles.The higher crystallinity percentage of AC300μm-[Bmim]Cl-U5(B) proved the even dispersion of IL active component on the AC surface, which has positively affected the adsorption capacity (Dou et al., 2015).

SEM-EDX
The SEM images in Figure 8    Cl exhibited uniform and deeper adherence to the AC particles.Consequently, this short five-minute ultrasonic stirring process effectively avoided IL leaching from the AC surface.After the H 2 S adsorption test, the surface of AC300μm-[Bmim]Cl-U5 reveals the presence of adsorbed sulphur gas within its pores.Table 5 indicates the composition in weight percentage (wt %) for each element in the H 2 S adsorbent.AC-[Bmim]Cl-U5 exhibited an average sulphur adsorption of 3.70 wt % which indicated increased surface area and pore volume.
AC300μm-[Bmim]Cl-U5(B) recorded a high impregnation of 2.5 wt % Cl on the AC surface and this has highlighted the effectiveness of the ultrasonic stirring method in achieving a well-distributed dispersion of IL on the AC surface.

BET
According to the IUPAC classification of pore dimensions, pores are categorised into three types: micropores (d < 2 nm), mesopores (d = 2-50 nm), and macropores (d > 50 nm) (Thommes et al., 2015).The AC300μm-[Bmim]Cl-U5 adsorbent falls under the micropore category with a size below 2 nm.Micropore adsorbents are well-known as excellent materials of choice for the adsorption of H 2 S, as conducted in a previous study (Chaves and Jiménez, 2018).Table 6 shows a reduction in the surface area value of AC300μm-[Bmim]Cl-U5(B), decreasing from 583.00 to 540.99 m 2 /g as adsorption occurs.Specifically, the micropore area decreases from 460.54 m 2 /g before adsorption to 400.52 m 2 /g after adsorption, indicating a 13.03% reduction due to gas content filling the pores.These reductions in surface area and pore volume align with the findings from a previous study (Choo et al., 2013).The N 2 adsorption-desorption method is employed to determine the adsorption type and the pore structure of the adsorbent.The N 2 adsorption profile reveals a Type 1 isotherm (IUPAC), signifying multilayer adsorption on micropore adsorbents (Thommes et al., 2015).Both the AC300μm-[Bmim]Cl-U5(B) and AC300μm-[Bmim]Cl-U5 samples exhibit similar behaviour, as evidenced by the graph lines.Apart from the information presented in Table 6, these materials distinctly showcase their micropore properties, particularly in the low relative pressure range (p/ p 0 < 0.1).Regarding N 2 adsorption, AC300μm-[Bmim]Cl-U5(B) demonstrates a high adsorption quantity, reaching up to 190 cm 3 /g STP.Similarly, AC300μm-[Bmim]Cl-U5 maintains a significant adsorption capacity, even after H 2 S adsorption, with the ability to adsorb up to 175 cm 3 /g STP.

TGA
The TGA and DTG profiles of AC300μm-[Bmim]Cl-U5(B), AC300μm-[Bmim]Cl-U5, and [Bmim]Cl are presented in Figure 9a and Figure 9b, showcasing the temperature derivative from room temperature to 600 °C at a heating rate of 20°C/min in a nitrogen atmosphere.Mass losses and their rate were meticulously recorded across temperature derivatives of 30-100 °C, 100-400 °C, and 400-600°C in Figure 9a and Figure 9b as well as detailed in Table 7 below.Notably, in the 100-400 °C temperature range, AC300μm-[Bmim]Cl-U5(B), AC300μm-[Bmim]Cl-U5, and [Bmim]Cl exhibited the highest mass losses compared to the derivative of 400-600 °C and 30-100 °C.The mass loss of the prepared adsorbent, AC300μm-[Bmim]Cl-U5, is substantially lower than that of the pure IL, [Bmim]Cl and the mass loss rate (%/min) of AC300μm-[Bmim]Cl-U5 is higher compared to [Bmim]Cl.This can be attributed to the successful impregnation effect, which synergistically combines the unique properties of AC and IL and leads to higher H 2 S adsorption capacity.Consequently, the thermal stability of this H 2 S adsorbent has markedly improved, proven by a low degradation rate ranging from 0.91% to 8.74% (Table 7).However, Zulkefli et al. (2019) produced an H 2 S adsorbent from ZnAc 2 -AC, exhibiting a higher mass loss ranging from 4-16% at the initial temperature derivatives of 29-100°C.It is acknowledged that this temperature range is usually associated with moisture loss inherent in the adsorbents.The actual IL loading (%) on AC can also be calculated from this analysis through Equation 6 below, and it revealed that a high amount of 76.7% IL was successfully impregnated on the AC surface.

CONCLUSIONS
AC-IL adsorbents were successfully synthesized through ultrasonic stirring at room temperature.Characterisation using XRD, SEM-EDX, BET, and N 2 -sorption analyses confirmed the successful impregnation of IL onto AC and revealed the contrasts between the fresh and exhausted adsorbents.In terms of adsorption capacity, the   (UKM-YSD) Chair for Sustainability.We would also like to thank the Government of Malaysia and Universiti Kebangsaan Malaysia for funding this work through Malaysia Research University Network (MRUN) Grant (LRGS MRUN/ F2/01/2019/4/2).

Figure 1 .
Figure 1.Schematic diagram of adsorption experiment system al. (2016) used ultrasonic-assisted stirring to impregnate Fe-Zn-Cu in char for simultaneous removal of H 2 S and CO 2 .The findings prove the ultrasonic stirring increased the surface area and pore volume, contributing up to 17.62 g S/100 g adsorption capacity (Dou et al., 2015).This also agrees with the previous studies by Jiang et al. (2020) and Zhang et al. ( (Santiago et al., 2019).AC600µm-[Bmim]Cl-U5 demonstrated a shorter breakthrough time of 58.5 minutes than AC300µm-[Bmim]Cl-U5 while achieving 88.15% removal efficiency at 105 minutes(Dou et al., 2016).Smaller particle size facilitates better dispersion of IL through increased surface area to volume ratio of the adsorbent(Zulkefli et al., 2022b

Figure
Figure 4a and Figure 4b demonstrated the H 2 S adsorption capacity of AC300-U5 with the impregnation of [Bmim]Cl and [Bmim]NTf 2 .As it can be observed in Figure 4a, AC300µm-[Bmim] Cl-U5 exhibits much greater H 2 S adsorption

Figure 2 .Figure 3 .
Figure 2. H 2 S adsorption capacity of AC300-[Bmim]Cl at different ultrasonic stirring time sizes under ambient temperature and pressure and Figure5b

Figure 7 .
Figure 7. (a) Profile and (b) maximum adsorption capacity of three H 2 S adsorption/desorption cycle of AC300μm-[Bmim]Cl-U5 71.1% and Amorphous: Crystallinity (A: C) ratio of 2.26:1.00.In contrast, the fresh and exhausted adsorbents of AC300μm-[Bmim]Cl-U5(B) and AC300μm-[Bmim]Cl-U5 display A: C ratios of 1.10:1.00and 1.02:1.00,respectively.The crystallinity properties of AC300μm-[Bmim]Cl-U5 are notably enhanced compared to the pristine AC300μm, primarily attributed to the ultrasonic stirring effect applied during the impregnation process.The high-frequency vibrations generated by ultrasonic waves have effectively modified the crystallinity of AC.Dou et al. ( demonstrated the morphological structures of AC300μm-[Bmim] Cl-U5(B) and AC300μm-[Bmim]Cl-U5.A white composition is observed along the surface walls and within the pores.This observation aligns with the expected distribution of IL composition on the AC surfaces which was achieved by applying ultrasonic stirring.The particles of [Bmim]

Figure 8 .
Figure 8.The morphology of (a) AC300μm-[Bmim]Cl-U5(B) and (b) AC300μm-[Bmim]Cl-U5 at the magnification scale of 5.0 Kx (2µm) pairing of AC300μm with [Bmim]Cl exhibited superior performance compared to [Bmim][NTf 2 ].The COSMO-RS study emphasises the super H 2 S solubility of [Bmim]Cl relative to [Bmim][NTf 2 ] and other ILs under consideration.Consequently, AC300μm-[Bmim]Cl-U5 emerged as the most effective adsorbent, performing the highest adsorption capacity and good regeneration capability.Remarkably, with only five minutes of stirring time and without additional heating, AC300μm-[Bmim]Cl-U5 demonstrated the ability to adsorb up to 8.25 ±0.38 mg H 2 S/g.Furthermore, it exhibits the capability for regeneration through at least three cycles with acceptable degradation and demonstrates stability in the adsorption profile throughout each cycle.AC300μm-[Bmim]Cl-U5 achieved an impressive 83.7% H 2 S removal efficiency, decreasing the concentration from 2000 to 326 ppm in the first cycle.These findings validate the synthesis approach and highlight the excellent adsorption performance as well as high removal efficiency of AC300μm-[Bmim]Cl-U5 as a promising candidate for H 2 S removal applications.

Figure 9 .
Figure 9. Profile analysis of (a) TGA and (b) DTG of AC300μm-[Bmim]Cl-U5(B), AC300μm-[Bmim]Cl-U5 and [Bmim]Cl at temperature range of 30-600°C ; Bárbara B. Polesso et al., 2019; Bárbara Burlini Polesso et al., 2019).Hence, two AC particle sizes of 300 and 600 µm were chosen as IL support further to investigate the particle size effect on H 2 S removal.Lastly, the IL character of different anions was investigated through the hydrophilic IL of [Bmim] Cl and hydrophobic IL of [Bmim][NTf 2 ].The hydrophobic/hydrophilic nature of IL was believed to significantly affect selective gas removal (Caglayan et al., 2023).However, limited investigation has been conducted in this regard.Hence, [Bmim]Cl and [Bmim]NTf 2 were opted in this work as promising candidates for H 2 S adsorption.Furthermore, the high solubility of both IL, as examined in previous works, is believed to contribute to their performance as effective adsorbents (Cheng et al., 2021; Guanhua et al., 2019; Mutalib et al., 2022; Bárbara B. Polesso et al., 2019; Zhou et al., 2016 ] (Huang et al., 2016; Rahmati-Rostami et al., 2009) and [Emim][BF 4 ] (Jalili et al., 2019).The collected data from this investigation and the literature served to cross-check and validate the predictive capabilities of the COS-MO-RS model.Thermodynamic properties of the mixtures were calculated based on the generated and optimised molecular structure of the species involved, which is imported into COSMOtherm as the .cosmofile.The generation of the .cosmofile was conducted through a single-point calculation using DFT with Becke-Perdew and the Triple-ζ Zeta Valence Potential (TZVP) basis set.The solubility of H 2 S in each IL was predicted under 1 bar pressure and calculated using COSMO-RS based on Equation 5 (Fariza et al., 2022):

Table 1 .
(Caglayan et al., 2023;Zhao et al., 2014)tion, [Bmim]Cl possesses better H 2 S selectivity than [Bmim][NTf 2 ] due to its polar nature, which is mainly associated with imidazolium cation.It was reported that the presence of [Bmim]Cl with 10% loading on silica was more effective than 50% of [Bmim][NTf 2 ] on the same support material of silica (Bárbara B. Polesso et al., 2019).Other than higher selectivity, fluorinated anions with smaller sizes can provide greater access for adsorption and offer better solubility than larger anions, such as NTf 2 -(Li et al., 2023).In addition, the hydrophilic nature of [Bmim]Cl contributes to its ability to adsorb a high capacity of H 2 S. H 2 S is known as a weak acid gas.Hence, it is very water-soluble, where the solubility reported to be ~80 mM at 37°C as an equilibrium between molecules(Caglayan et al., 2023;Zhao et al., 2014).Not only that, the IL anion of [Cl -] performed better selectivity and activity than other anions, such as [BF 4 AC particle size (µm)BET surface area, S BET (m 2 /g) Total pore volume (cm 3 /g) Micropore area (m 2 /g) Pore size (A°) capacity and longer breakthrough time than AC300µm-[Bmim][NTf 2 ]-U5.The breakthrough time of AC300µm-[Bmim][NTf 2 ]-U5 is shorter than AC300µm-[Bmim]Cl-U5, consistently with a lower adsorption capacity of 6.48±0.18mg H 2 S/g.Within 105 minutes of adsorption time, AC300µm-[Bmim]Cl-U5 achieved up to 89.00% removal efficiency of H 2 S, while AC300µm-[Bmim][NTf 2 ]-U5 exhibited lower but still comparable removal efficiency of 87.60%.
(Mohan et al., 2022)E against H 2 S solubility.It was observed that [Bmim]Cl gave the highest H 2 S solubility compared to other imidazolium-based IL at lower H (0.301 bar) and H E (-3.393 kcal/mol).The lower H and H E values of the solute in the solvents indicate that the solvent exhibits a high tendency to interact and dissolve the solute(Mohan et al., 2022).The highly electronegative Cl -significantly enhances the polarity of the whole IL system, thus making it a suitable hydrogen bond acceptor.The R value is considered insignificantly critical aligning with the purpose of the benchmarking study.

Table 2 .
List of experimental and COSMO-RS predicted H 2 S solubility, H and H E of six chosen IL at 1 bar