Article pubs.acs.org/IECR
Stability and Performance of Physically Immobilized Ionic Liquids for Mercury Adsorption from a Gas Stream Tauqeer Abbas,† Lethesh Kallidanthiyil Chellappan,†,‡ M. I Abdul Mutalib,*,† Kuah Yong Cheun,§ Syed Nasir Shah,† Salman Nazir,⊥ Amiruddin Hassan,§ Mahpuzah bt Abai,§ and Eakalak Khan∥ †
PETRONAS Ionic Liquid Centre and ‡Center for Biofuel and Biochemical Research, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak Malaysia § Process Technology R&D, Technology and Engineering Division, PETRONAS Research Sdn Bhd, 43000 Kajang, Malaysia ⊥ Buskerud and Vestfold University College, 3048 Drammen, Norway ∥ Department of Civil and Environmental Engineering, North Dakota State University, Fargo 58102, United States S Supporting Information *
ABSTRACT: Solid-supported ionic liquids (ILs) have recently received attention as a potential effective technology for mercury removal from a gas stream. However, the leaching of ILs from the solid support has not been investigated in detail. In the present study, the stability of 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) impregnated on silica and activated carbon was evaluated during elemental mercury removal (Hg0) from a gas stream. Silica- and carbon-supported [Bmim]Cl-based adsorbents were characterized before and after Hg0 adsorption by using Fourier transform infrared spectroscopy, Brunauer−Emmett−Teller surface area analysis, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and thermal gravimetric analysis. The carbon-supported adsorbent showed better stability (no leaching of ILs) compared to the silica-supported adsorbent because of the availability of substantial micropores. The lower stability of silica-supported ILs is attributed to the presence of mesopores on silica support, which holds [Bmim]Cl ineffectively in a gas flow of a high concentration of Hg0 (15 ppm). The activated carbon-supported ILs, especially in a powdered form, showed higher adsorption efficiency of Hg0 from a gas stream. The adsorption capacity of powdered carbon-supported [Bmim]Cl was 21 mg/g in 68 h of continuous adsorption. (triflimide) have 2 and 2.7 mg/g removal capacity for Hg0, respectively.10,11 Solid-supported ILs,12 particularly chemically immobilized ILs have been used for liquid stream treatment to avoid leaching of the IL from the solid support.13 However, physically immobilized ILs have been used for gas stream applications assuming that there is no leaching of the IL.10,11 In addition to leaching, the role of the solid support on the stability of the IL has not been studied in detail. There is a need to study the stability of physically immobilized ILs in gas-phase applications because of its impact on the cost and performance of the adsorbent. We studied the adsorption capacity of various ILs impregnated on different solid supports for Hg0 such as [Bmim]Cl, 1-butyl-3-methylimidazolium methanesulfate, 1butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide.14 It was found that [Bmim]Cl immobilized on the solid support provided the best adsorption capacity,14 which is in agreement with previous studies.10,11,15 In this study, the stability of the IL [Bmim]Cl immobilized on a silica and activated carbon support during removal of Hg0 from a gas (N2) stream was evaluated. Only the stability of
1. INTRODUCTION Mercury is one of the most hazardous contaminants emitted to the atmosphere because of its toxic effects on the environment and human health, persistence in the environment, and global atmospheric transport with air masses.1 The most common forms of mercury found in the environment are elemental mercury (Hg0), oxidized mercury (Hg2+), mercuric sulfide, mercuric chloride, methylmercury, and particulate mercury (Hgp).2 Hg2+ and Hgp are soluble in water and hence get deposited on local and regional scales, whereas Hg0 is present in a vapor phase and hence can be transported worldwide.3 Coal power plants and natural gas are the main emission sources of Hg0 to the environment. The hazards associated with mercury exposure are both acute and chronic.4 Exposure to Hg0 can affect the nervous system5 and at high concentrations can cause renal effects6 and pulmonary dysfunction.7 A number of technologies have been used for the removal of Hg0, which is substantially more difficult to remove from a gas stream than Hg2+.8 Bae et al. (2011)9 critically reviewed various Hg0 removal processes with capturing capacity. Recently, ionic liquids (ILs) have shown an ability to remove both Hg0 and Hg2+.10 Silica-supported ILs, which take advantage of the high adsorption capacities of the ILs and the large surface areas of the silica support, have been used for mercury removal from the flue gases of coal power plants at a laboratory scale.10,11 Silicasupported 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and silica-supported 1-butyl-1-methylpyrrolidinium bis© 2015 American Chemical Society
Received: Revised: Accepted: Published: 12114
May 13, 2015 November 8, 2015 November 12, 2015 November 12, 2015 DOI: 10.1021/acs.iecr.5b01738 Ind. Eng. Chem. Res. 2015, 54, 12114−12123
Article
Industrial & Engineering Chemistry Research
Figure 1. Apparatus for evaluation of the carbon-supported ILs for Hg0 capture (G-1, gas cylinder; V-1, V-2, V-3, and V-4, valves; I-6 and I-1, pressure gauges, P-9 and P-8, carrier gases; E-1, water bath; E-2, Hg0 source; P-2, carrier gas with Hg0; P-3, carrier gas with Hg0 for the adsorber inlet; E-4, fixed-bed adsorber; P-20, inlet stream for mercury detection; P-4, mercury outlet; P-17, outlet stream for mercury detection; F-1, mass flow controller; A-1, online mercury analyzer (Sir Galahad PSA-10.525).
adsorption branch of the isotherm using the Barrett−Joyner− Halenda method. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX) were performed on uncoated silica and carbon, silica-supported [Bmim]Cl [before and after Hg0 adsorption (batch experiment)], and carbon-supported [Bmim]Cl [before and after Hg0 adsorption (batch and continuous experiments)] using a Zeiss-SUPRA 55VP microscope at different magnifications (100×, 1000×, 5000×, and 10000×) to study morphological changes. Thermal gravimetric analysis (TGA) was conducted on uncoated and [Bmim]Cl-coated carbon and silica before and after Hg0 adsorption, using a thermal gravimetric analyzer (PerkinElmer, Pyris V-3.81). The samples were heated in a N2 environment from 50 to 400 °C at a heating rate of 10 °C/min. The uncertainty of the measurement was ±1 °C. The solid-state 199Hg NMR spectra were recorded at a magnetic field strength of 11.75 T using a Bruker Avance 500 spectrometer. Simulations of chemical-shift powder patterns of static samples and of magic-angle-spinning spectra of spinning samples using the Herzfeld−Berger19 technique were performed with the solids simulation package (“solaguide”) in the TopSpin (version 3.2) NMR software program from Bruker BioSpin. 2.3. Mercury Adsorption Apparatus and Experimental Procedures. A laboratory-scale fixed-bed continuous-flow adsorber used for evaluation of the sorbents is shown in Figure 1. The rig contains a glass wool reactor (7 in. in length and 1/4 in. in diameter), a mercury permeation vessel, flowmeters, and pressure gauges. The adsorbent (0.1 g) was loaded into the glass wool reactor. A mercury permeation bottle was sealed and immersed in a temperature-controlled water bath at 50 °C. Hg0 vapor generated was carried by N2 to the fixed-bed adsorber. The gas flow rate to the adsorption column was maintained at 60 mL/min through a mass-flow controller. Two types of experiments (batch and continuous) were conducted for Hg0 adsorption. For batch experiments, a single particle of silica- and carbon-supported [Bmim]Cl (particle sizes of 2 and 4 mm) was kept in the glass wool reactor under the continuous flow of a Hg0-vapor-containing gas (N2) for 24 h, as shown in the Supporting Information (Figure S1a). Batch
[Bmim][Cl]-coated adsorbents was studied because of their high mercury removal efficiency compared with other ILs such as 1-butyl-3-methylimidazolium methanesulfate and 1-butyl-3methylimidazolium bis(trifluoromethylesulfonyl)imide.14 A continuous fixed-bed adsorption column was used to evaluate the stability of the IL and Hg0 removal by different solid supports. The study provides a better understanding of the solid support choices for the immobilization of ILs to get promising results in air pollution control applications.
2. MATERIALS AND METHODS 2.1. Materials and IL Coating. [Bmim]Cl and dichloromethane (DCM) were purchased from Sigma and Fischer Scientific, whereas silica and carbon supports were purchased from Johnson Matthey and Atlas Chemicals, respectively. The coating was performed according to a previously reported procedure,11 with the following modifications to get homogeneous immobilization of the IL on a solid support. A solution of [Bmim]Cl (7.5 g) in DCM (100 mL) was added to the solid support (22.5 g). The reaction mixture was stirred at room temperature at 300 rpm for 24 h. DCM was removed using a vacuum rotary evaporator, and the adsorbent was further dried in a vacuum oven at 90 °C for 24 h to remove the remaining solvent and moisture. Using a previously reported procedure,16,17 which relies on the weights of solid supports with the IL and bare solid supports, it was found that the entire 7.5 g of [Bmim]Cl was successfully immobilized on both solid supports. 2.2. Characterization of Supported ILs. The Fourier transform infrared (FTIR) spectra for the fresh and spent adsorbents were recorded using a Spectrum One/BX PerkinElmer spectrometer at a wavenumber range of 450− 4000 cm−1. The spent adsorbents for FTIR analysis were taken from continuous (breakthrough) experiments. The surface area and pore size of uncoated and [Bmim]Cl-coated silica and carbon were analyzed by N2 adsorption−desorption isotherms at 77 K in a Micromeritics ASAP 2020 apparatus.18 Before adsorption−desorption experiments, samples were outgassed at 50 °C and a pressure of 500 μmHg. The surface and micropore areas were calculated from the adsorption isotherms using the Brunauer−Emmett−Teller (BET) and t-plot methods, respectively, whereas the pore-size distribution was obtained from the 12115
DOI: 10.1021/acs.iecr.5b01738 Ind. Eng. Chem. Res. 2015, 54, 12114−12123
Article
Industrial & Engineering Chemistry Research
[Bmim]Cl. Similarly, the pore volume of [Bmim]Cl-coated silica was also more than 60% lower than uncoated silica. This suggests that [Bmim]Cl partially filled the pores, but there was sufficient accessible pore area and volume, and the accessible pore volume and surface area have a characteristic diameter close to that of uncoated silica. For the carbon support, the BET surface area of [Bmim]Cl-coated carbon was reduced more than 95% compared to uncoated carbon. The micropore area of uncoated carbon was reduced by 98% after the coating of [Bmim]Cl, which means that the IL fills or blocks the micropores significantly.20 The overlapping adsorption potentials of the opposite pores walls make the adsorption energies much stronger in the micropores than those in the mesopores and macropores.21 As a result, [Bmim]Cl would be attracted to micropores and block them, while leaving the mesopores open. This explains why [Bmim]Cl-coated carbon had larger pores compared with fresh activated carbon. The adsorption isotherm for activated carbon is shown in Figure 2a. The adsorption isotherm for activated carbon is a type I adsorption isotherm. The graph depicts monolayer adsorption that is common for adsorption of N2 on charcoal. This can be easily explained using the Langmuir adsorption isotherm.22 The adsorption isotherm for carbon-supported [Bmim]Cl is shown in Figure 3a. The isotherm can be depicted as a type IV isotherm having multilayer adsorption, pore condensation, and hysteresis. Hysteresis is an indication of capillary condensation that occurs in the mesopores.23 The attachment between [Bmim]Cl and carbon is likely dominated by chemisorption, as evidenced by more pore size reduction during the desorption cycle compared to silica-supported [Bmim]Cl, as shown in Table 1. Figures 2b and 3b show the N2 adsorption−desorption isotherm of uncoated and [Bmim]Cl-coated silica supports. Both isotherms are the same type. The isotherms represent a type V isotherm according to the definition developed by IUPAC having c > 1.24 Silica-supported [Bmim]Cl lies in the mesoporous range; that is why both capillary condensation and a hysteresis loop were observed. At low pressure, monolayer formation occurred, followed by multilayer formation, which occurred at medium pressure. At higher pressure, hysteresis was observed because of capillary condensation in meso- and macropores. 3.2. Mercury Adsorption Capacity. The adsorption capacities of silica- and carbon-supported [Bmim]Cl were 1 and 3.61 mg/g of the adsorbent in 24 h, respectively. Carbonsupported [Bmim]Cl showed more than 3 times the adsorption capacity of silica-supported [Bmim]Cl. The adsorption capacity of room temperature ILs impregnated on the silica support for Hg0 removal from flue gases was studied, and the adsorption capacity of silica (mesoporous)-supported [Bmim]Cl was 2.2 mg/g.10 Moreover, Ji et al. (2008)11 found that the adsorption capacity of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide coated on mesoporous silica for Hg0 was 2.7 mg/g. To further investigate the adsorption capacity difference, a breakthrough experiment was performed. Surprisingly, complete saturation/breakthrough (Cout/Cin = 1) was reached within 30 min in the case of silica-supported [Bmim]Cl, resulting in a poor adsorption capacity (
