Metal chloride anion based ionic liquids: synthesis, characterization and evaluation of performance in hydrogen sulfide oxidative absorption

Three metal chloride anion based ionic liquids (MCABILs) were synthesized and characterized for high conversion of hydrogen sulfide (H2S). The MCABILs were synthesized via metathesis reaction and characterized by chemical spectroscopy such as FTIR, UV-Vis and CHNS elemental analysis. Then, the performance of those ionic liquids in H2S conversion was evaluated at various temperatures and atmospheric pressure. The results indicated remarkable efficiency of metal chloride anion based ionic liquids in H2S conversion. MCABILs managed to reach over 90% conversion efficiency at temperature as low as 50 °C, despite operating at atmospheric pressure. This occurs due to their high oxidation capability. The regeneration experiment indicated that all MCABILs can be recycled easily. Taken together, the research findings highlight the synthesis, characterization and high efficiency of MCABILs in promoting faster H2S conversion as a catalyst.


Introduction
H 2 S is a colorless gas that exists naturally, 1 and is very toxic 2 and highly ammable. Its harmful effects towards living organisms are visible at 10 ppm (ref. 3) and become fatal aer exceeding 500 ppm. 4 H 2 S is also highly corrosive to carbon steel. 5 Thus, H 2 S removal is very crucial for economic and safety reasons. Absorption, adsorption and conversion are common desulphurization technologies. LO-CAT and Claus process are most outstanding as they are capable of converting H 2 S into sulfur with excellent efficiency. 6 However, they consume a lot of energy and face catalyst deactivation. Consequently, sulfur production output is lowered and cost increased. 7 Ionic liquids (ILs) are salts with melting point lower than the water boiling point and have emerged as promising green solvents for desulphurization. 8 ILs are designer solvents because their physicochemical properties are tunable by changing the cation and anion. 9 Hence, they can be designed specically for desulphurization. Recently, many studies were targeting H 2 S conversion into sulfur using ILs. [10][11][12][13][14] Since there are unlimited possible combinations, evaluation for most efficient ILs would be very tedious and costly if carried out experimentally. 15,16 The chances to discover and select new ILs would be complicated. Thus, every potential IL must undergo a thorough and systematic screening.
This work is a continuation study of screening potential ILs for direct conversion of H 2 S by Conductor-like Screening Model for Real Solvents (COSMO-RS). 17 COSMO-RS screening was conducted to predict the thermodynamic properties of ILs for H 2 S conversion without requiring any prior experimental data. Based on COSMO-RS prediction, economical and safety reasons, three MCABILs were selected for synthesis from 300 screened. 17 Various studies have discovered potential Room-Temperature Ionic Liquids (RTILs) application for H 2 S conversion, 18,19 nonetheless, it is discovered that their capacity and solubility for H 2 S are still poor at low pressure. [20][21][22][23]

Characterization instruments
CHNS elemental analysis was performed using Leco-CHNS-932 Analyzer. FT-IR spectra of the samples were collected by Fourier transfrom infrared Thermo Nicolet is5 spectrometer. TGA curve was obtained by PerkinElmer thermogravimetric analyzer, Pyris V-3.81. DSC curve was collected by DSC Q2000 V24.11 Build 124 (Universal V4.5A TA Instruments). Density was determined by density meter DA-645 (Kyoto Electronics Manufacturing). Viscosity was obtained by AMVn Anton Paar micro viscometer. The water content analyses were conducted by V30 Mettler Toledo coulometric Karl Fischer titrator.

H 2 S absorption measurement
H 2 S absorption measurement was conducted at atmospheric pressure and various temperatures at 25 C, 50 C, 75 C and 100 C as depicted in Fig. 2. Raw materials inlet into the system consists of three gas path way and the gas ow rate was controlled by the ow meter and mass ow controller. A gas mixture of oxygen and 100 ppm concentration of H 2 S was passed through the 25 mL stainless steel reactor in which the MCABILs were loaded. N 2 gas was used as the carrier gas and purging gas to ensure all the system is free of H 2 S before and aer the reaction happened. The reaction temperature was between room temperature until 100 C and regulated by the heating mantel around the reactor. The concentration of H 2 S at both inlet and outlet gas was detected by H 2 S gas analyzers. The residual H 2 S gas was absorbed by the concentrated NaOH solution in the scrubber tanks. In this test, a maximum 10 ppm of the initial concentration of H 2 S was dened as the breakthrough concentration. ] display several peaks at 1465 cm À1 , 1410 cm À1 , 1110 cm À1 and 720 cm À1 associated with P-C stretching vibration as depicted in Fig. 3. The broad peak that exists in region between 2960 cm À1 to 2850 cm À1 are assigned to aliphatic moieties of C-H stretching vibration. The bands located at 985 cm À1 and 810 cm À1 are attributed to CH]CH bending vibrations. A single peak at 1375 cm À1 is associated with CH 2 -CH 3 stretching vibration, while the peak at 1210 cm À1 is corresponded to C-H bending vibration. Based on spectra analysis, there is no presence of alkyl halides, C-Cl bond. Hence, this conrms the structure of MCABILs as shown in Fig. S1-S3. † show the density and viscosity of MCABILs, respectively. The densities varied from 0.98139 to 1.05059 g cm À3 and in reported range of 0.9 to 1.7 g cm À3 . 27 No signicant decrease in viscosity was observed aer the temperature reached 60 C. 28 The viscosity and density of  MCABILs decrease as the temperature increases. At 20 C, MCABILs exhibited viscosity between 543.11 to 880.42 cP and density between 0.98139 and 1.00274 g cm À3 as shown in Tables S1 and S2. † Since MCABILs possess lower viscosity and density at higher temperature, they can be excellent catalysts for direct H 2 S conversion.   4 ] being the most stable. They were fully decomposed at a high temperature of 500 C because phosphonium based ILs are very stable thermally. 29 Since each MCABILs possesses T onset higher than 400 C, it is concluded that each of them is very stable ILs thermally. 30 The melting point for each MCABILs are shown in Table S4. † The melting points of MCABILs increases as the molecular weight (MW) of the anions increases. An increase in MW of the anions side chains resulted in stronger intermolecular reaction which contributes to the rise in melting points. Since they share similar cation, the melting points only vary slightly.

H 2 S removal performance of MCABILs
As depicted in Table 1     In this work, H 2 S absorption was performed in dynamic conditions and the partial pressure was also relatively low. However, the MCABILs were still able to perform well while maintaining high efficiency of H 2 S removal at relatively low pressure. The H 2 S conversion percentages were calculated using eqn (1): where, x H 2 S,in and x H 2 S,out are the concentration of H 2 S at the inlet and outlet, respectively. Fig. 6 depicted the FT-IR spectra of MCABILs before and aer H 2 S conversion. Since it does not show any signicant difference, that means no chemical reaction between H 2 S and ILs molecules have occurred. The presence of metal chlorides in the ILs matrix enhance the desulfurization abilities of pure ILs which have low physical and chemical solubility of H 2 S.

Mechanism of absorption
The reaction mechanism of H 2 S oxidative absorption in this research involves redox reaction between MCABILs and H 2 S. A study conducted by DeBerry et al. 32 Fig. 7. The result reported a higher conversion at higher temperature. This demonstrates that the conversion of H 2 S is an endothermic reaction, which is supported by literature. 34 Since it is an endothermic process, it limits the thermodynamic conversion at lower temperature.
However, since MCABILs were designed specically to perform excellently at low temperature and pressure, MCABILs still can achieve high conversion as high as 90% at 50 C and 1 atm. At higher temperature, the conversion of H 2 S in MCABILs is much higher. It can be concluded that the performance of MCABILs is optimum at higher temperature.
3.6.2. Effects of viscosity of MCABILs. Tables 2 and 3 shows the desulfurization performance of MCABILs with respect to their viscosity at 50 C and 100 C, respectively. Among all three MCABILs, P 66614 GaCl 4 demonstrated a lower H 2 S conversion at lower temperature. This is because P 66614 GaCl 4 has the highest viscosity followed by P 66614 FeCl 4 and P 66614 SnCl 3 . It is very viscous at lower temperature and unable to perform the conversion efficiently. Meanwhile, P 66614 SnCl 3 which possess a low viscosity at lower temperature showed a remarkable performance in H 2 S conversion. Hence, lower viscosity ILs are the best choice for H 2 S conversion.
3.6.3. Effects of reaction with time. Fig. 8 shows the percentage of H 2 S conversion into elemental sulfur with respect to reaction time, using P 66614 FeCl 4 . At the beginning, the reaction was rather slow because of mixing and dispersion of H 2 S,  O 2 and ILs phases. For example, conversion of H 2 S into elemental sulfur by P 66614 FeCl at 100 C and 1 bar. The obtained results were 79.07%, 83.69%, 86.4%, 88.94%, 91.25% and 92.83% at 60, 120, 180, 240, 300 and 360 min respectively. The conversion rate increases with time because of the increase in mass transfer between the mixture gases of H 2 S and O 2 and the catalyst, metal chloride anion based ILs and completed aer a certain period of time.
Liu and Wang 35 reported that as the reaction was prolonged until 10 h, the percentage of H 2 S conversion in PMo 10 V 2 / BmimCl decreases to below 75% which demonstrated that PMo 10 V 2 /BmimCl is only acting as solvent. Meanwhile, MCA-BILs maintains a high conversion percentage above 90% until 600 min without any performance drop and signies its role as a catalyst for desulfurization reaction.
The experimental results indicated that the percentage of conversion (%) increases with increasing reaction time. As the reaction reaches 360 min, the conversion percentage of H 2 S has reached its peak at 93.51% and remain stagnant aerwards until reaches 600 min. Hence, 360 min can be deduced as the optimum reaction time for conversion of H 2 S into elemental sulfur.

Comparison of desulfurization performance for MCABILs
As shown in Fig. 9, all MCABILs with phosphonium cations demonstrated a high rate of desulfurization, reaching above 90% during 6 h of reaction at 100 C. The performance of all MCABILs at 50 C were also depicted in Fig. 10. Comparatively, the performance of MCABILs at lower temperature were a bit lower compared to their performance at 100 C. This is probably due to higher viscosity of MCABILs at lower temperature.
Since MCABILs consist of similar cation, it is concluded that the metal chloride anions and their oxidation number play a signicant role in affecting the desulfurization performance. MCABILs were playing role as catalysts for the removal of H 2 S through absorptive oxidation. Comparatively, they are more stable and efficient compared to the conventional ILs reported 36     as the viscosity of MCABILs decreased. As a result, P 66614 SnCl 3 demonstrated the best desulfurization performance.  Fig. 11. The efficiency of H 2 S removal was observed aer the absorption took place for 3 h. The rates of H 2 S removal for the studied MCABILs were found to be higher than 90% within 5 cycles thus, signifying an outstanding regeneration performance of MCABILs for H 2 S removal.

Conclusions
The contribution of this paper is twofold; to report the synthesis and characterization of three new MCABILs via combination of trihexyl(tetradecyl)phosphonium cation with metal chlorideanions and to evaluate their performance in H 2 S conversion. The yields for all ionic liquids were obtained in range of 90% to 99%. The synthesized ILs exist as liquid at room temperature and very stable thermally, possessing high decomposition temperatures ranging between 468.77 C and 480.34 C. Overall, MCABILs show desirable physical properties which make them excellent catalysts for absorptive oxidation of H 2 S. Those MCABILs also shown a remarkable desulfurization performance with over 90% efficiency at atmospheric pressure and can be regenerated for multiple usage.

Conflicts of interest
There are no conicts to declare.