Article pubs.acs.org/IECR
Cyano-Containing Protic Ionic Liquids for Highly Selective Absorption of SO2 from CO2: Experimental Study and Theoretical Analysis Xiaomin Zhang, Xi Feng, He Li, Jing Peng, Youting Wu,* and Xingbang Hu* Separation Engineering Research Center, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China S Supporting Information *
ABSTRACT: The solubility of SO2 and CO2 in four cyano-containing protic ionic liquids (PILs) was experimentally measured at temperatures from 303.2 to 333.2 K and pressures up to 3.0 bar. Their physical properties, such as density, viscosity, and decomposition temperature, were also determined. It is found that [DMPANH][MOAc] and [DMAPNH][EOAc] have the best selective absorption of SO2 from CO2 at 303.2 K and 1.0 bar among the investigated PILs, and the ideal selectivities (119 and 107, respectively) of SO2/CO2 are significantly higher than those reported in literature for other ILs. The temperature-dependent Krichevsky−Kasarnovsky (K−K equation) and PR-NRTL equations are used to calculate the solubility data of SO2 and CO2, and the interactions between PILs and acid gases are analyzed thermodynamically. Quantum chemical calculations are also done to obtain the interaction configurations and energies. It is shown from the themodynamic analysis and the quantum chemical calculations that the interaction between SO2 and PILs is more energy favorable than that between CO2 and PILs, primarily due to the existence of the cyano group on the cation of PILs. The protic ionic liquids were reused for five absorption−desorption cycles without obvious loss in the absorption capacity, showing their potential as selective absorbents of SO2.
1. INTRODUCTION SO2 emission from the combustion of fossil fuels plays a main role in the formation of acid rain and air pollution, which has caused serious environmental and health issues.1 In past decades, limestone scrubbing, which was commercially widely adopted, is among the most effective techniques and commonly used methods for flue gas desulfurization (FGD) in the real industry.2 Unfortunately, there are several obvious inherent drawbacks with this removal process, such as generation of large amounts waste metal salts as byproducts, complicated technology and high energy consumption, as well as fine particle pollution (PM2.5). Accordingly, less environmentally intrusive and more sustainable gas cleaning technologies is highly required. One promising desulfurization method in the industry is to apply liquid solvents for the capture of SO2.3−8 However, conventional absorbents, such as inorganic salt solution or organic amine solution, have inherent disadvantages including low absorption capacities, strong corrosion, and high energy consumption. Therefore, novel solvents or liquid materials for the capture of SO2 is extremely desired. Ionic liquids (ILs) are considered as a candidate for this. Generally, ILs are defined as those fused salts with a melting point close to room temperature. As a class of soft materials, ILs have many impressive properties, such as extremely low volatility, high thermal stability, tunable structure, and excellent solvation power for acid gases. Therefore, ILs have received © XXXX American Chemical Society
much attention for different processes in recent years, such as gas separation and capture,9,10 catalytic reactions,11 and electrolysis.12 In particular, ILs have been widely investigated for the capture of SO2. Han and co-workers13 reported for the first time an IL, 1,1,3,3-tetramethylguanidium lactate ([TMG]L), that could absorb reversibly about 1 mol of SO2 per mole of ionic liquid at 1 bar and 40.0 °C from a N2 stream containing 8 vol % SO2. Since then, a quite number of studies reported a wide variety of ionic liquids for the capture of SO2, such as imidazolium-based ILs,14 pyridinium-based ILs,15 hydroxyl ammonium ILs,16 guanidinium-based ILs,17 etc. Employing a “dual-tuning” approach, Wang et al.18 also designed several anion-functionalized ILs, including benzoates, acetates, and phenolates, to improve the SO2 absorption capacity. Kim et al.19 designed a series of ether-functionalized imidazolium methanesulfonates as the ionic liquids for highly efficient SO2 absorption. The solubility of SO2 was found to increase with the increasing number of ether groups on the cation, and the maximum absorption at 303.2 K and atmospheric pressure could approach 6.3 mol of SO2 per mole of IL. Received: July 6, 2016 Revised: September 21, 2016 Accepted: September 29, 2016
A
DOI: 10.1021/acs.iecr.6b02588 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
PILs. Do such cyano-containing PILs have better SO2/CO2 selectivity while keep high SO2 absorption capacity? To prove the argument above and evaluate the potential application of PILs in the selective separation of SO2/CO2, four novel cyano-containing PILs, including 2-cyano-N,N-dimethylethan-1-aminium ethoxyacetate ([DMAPNH][EOAc]), 2cyano-N,N-dimethylethan-1-aminium methoxyacetate ([DMAPNH][MOAc]), 2-cyano-N-methylethan-1-aminium ethoxyacetate ([MAPNH][EOAc]), and 2-cyano-N-methylethan-1-aminium methoxyacetate ([MAPNH][MOAc]), were designed and investigated for the selective absorption of SO2 from CO2. The solubility of SO2 and CO2 was determined systematically to evaluate the ideal selectivity of SO2/CO2 in these four PILs. Quantum chemical calculations and thermodynamics modeling were also used to analyze the interactions of PILs with SO2 or CO2.
However, CO2, regarded as the chief greenhouse gas, appears simultaneously with SO2 in most industrial cases. Most ILs designed specifically for SO2 absorption in literature are problematic, since these ILs are strongly alkaline and cannot differentiate the two acid gases. In addition, the partial pressures of SO2 in the industrial gas streams are always much lower than those of CO2. Therefore, the selective separation of SO2 from CO2 is highly important and necessary for the chemical processes. It is shown from currently available literature data that the ideal selectivity of SO2/CO2 in most ILs ix very poor, although some researchers have simulated the difference for SO2 and CO2 in some functional ionic liquids through theoretical investigation.20,21 Protic ionic liquids (PILs) are a subset of ionic liquids that are formed through the transfer of a proton from a Brønsted acid to a Brønsted base.22 They are low in cost and can be easily synthesized through the neutralization of corresponding acids and bases. To the best of our knowledge, aprotic ionic liquids have been favorably investigated for the selective absorption of SO2 from CO2 in literature. For example, Han et al.14 investigated the selective absorption of SO2 and CO2 in two tertiary-amine functioned imidazolium-based ionic liquids, and the selectivity of SO2/CO2 in these two ILs were 39 and 40, respectively. Zhang et al.15 designed several pyridiniumbased ILs for high efficient SO2 uptake, with the best selectivity of SO2/CO2 being 56. Till now, there is still little effort toward developing PILs as selective absorbents of SO2. However, we argue that PILs may have better selective absorption of SO2 from CO2, since the existence of protic cations is theoretically helpful in weakening greatly the interaction between CO2 and ionic liquids while influencing little that between SO2 and ionic liquids. The basicity of PILs still may be too high since SO2 is a Lewis Acid and it can interact with PIL through Lewis acid− base interactions. For example, [TMG][L], as the first reported PIL for SO2 absorption, is a strong base−weak acid salt that has a high basicity to prefer SO2 absorption rather than SO2 desorption (complete SO2 desorption cannot be achieved although temperatures are carried out at 90 °C).23 Moreover, H2O also appears simultaneously with SO2 at most industrial cases. In this basic environment, the thermally stable sulfite would be formed. This process was almost irreversible. The question arises whether we can introduce a substituent on the cation that tunes the basicity of the PILs to further differentiate SO2 and CO2. We know that cyano is a strong electronwithdrawing group that can be introduced easily into the cation of PILs to weaken the basicity. For instance, the pKa value of the conjugate acid of dimethyl-n-propylamine is 9.99, while the pKa value of the conjugate acids of 3-dimethylaminopropionitrile (DMAPN) is 7.0.24,25 However, when the basicity of the amines is weakened, how could we know that the amines are still basic to fully deprotonate the acids. Ideally in PILs, the complete proton transfer has been expected. However, it is unlikely since proton transfer may be less than complete when neutral species can occur.22 It can be estimated by the ΔpKa (pKa(base) − pKa(acid)) how complete the proton transfer is.26 For instance, the ΔpKa (pKa(DMAPN) − pKa(MOAc)) is 3.47 for [DMAPNH][MOAc].27 The estimated ionization fraction is above 95%, showing that iconicity is acceptable. In fact, organic acid-based PILs are mostly a mixture of PIL and a few neutral species. But in the IL research field, they are still accepted as
2. EXPERIMENTAL SECTION 2.1. Materials. SO2 and CO2 were supplied from Nanjing Tianze Gas Co., Ltd. (China) with a minimum purity of 99.99 mol %. 3-Dimethylaminopropionitrile (98 wt %), 3-methylaminopropionitrile (98 wt %), ethoxyacetic acid (98 wt %), and methoxyacetic acid (98 wt %) were all purchased from Aladdin Chemical Reagent Co. and used without further purification. 2.2. Preparation and Characterization of PILs and Starting Materials. The four PILs investigated in this work were 2-cyano-N,N-dimethylethan-1-aminium ethoxyacetate ([DMAPNH][EOAc]), 2-cyano-N,N-dimethylethan-1-aminium methoxyacetate ([DMAPNH][MOAc]), 2-cyano-Nmethylethan-1-aminium ethoxyacetate ([MAPNH][EOAc]), and 2-cyano-N-methylethan-1-aminium methoxyacetate ([MAPNH][MOAc]) (Figures 1). They were all synthesized
Figure 1. Chemical structures of the four protic ionic liquids investigated in our work.
according to the literature procedures.28 Briefly, the ethanol solution of corresponding acid was added dropwise to the ethanol solution of base. The reaction was stirred for 24 h in an ice bath. Ethanol was then removed by evaporation at 318.2 K under reduced pressure. The obtained liquid was dried under vacuum at 333.2 K for 72 h to remove trace of solvent to offer a yellow liquid. The chemical structures of the four PILs were confirmed by using 1H NMR, 13C NMR, and FT-IR spectra. 1H NMR and 13C NMR spectra were measured on a Bruker DPX 300 MHz spectrometer, using d6-CDCl3 as a solvent with TMS B
DOI: 10.1021/acs.iecr.6b02588 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. Density (a) and viscosity (b) of [DMAPNH][EOAc] (■), [DMAPNH][MOAc] (●), [MAPNH][EOAc] (▲), and [MAPNH][MOAc] (▼) at different temperatures.
acidic gas uptake, n(PS), can thus be calculated using the following equation:
as the internal standard to confirm the structures of the four PILs. FT-IR spectra were recorded on NEXUS870 FTIR spectrometer. The water content was measured by SARTORIUS MA150, which displayed that the mass fractions of water in the samples were all no more than 1000 ppm. The decomposition temperatures were determined by TGA (NETZSCH, STA 449 C) from room temperature to 400 °C under N2 atmosphere at a scanning rate of 10 °C/min. The density was determined using an Anton Par DMA 5000 type automatic densimeter with a precision of 0.00001 g/cm3. The viscosity was acquired on a Brookfield LVDV-II1Pro viscometer with an uncertainty of 1%. To furtherly investigate how complete the protons transfer is from acid to base, the chemical structures of the four starting materials were also confirmed by using 1H NMR, 13C NMR, and FT-IR spectra. The NMR and FT-IR spectral data of PILs and starting materials can be found in the Supporting Information. 2.3. Determination of Gas Absorption. The apparatus and methodology for the determination of gas absorption in PILs is the same as those in our previous work.29 The whole device consists of two 316 L stainless steel chambers whose volumes are 121.025 cm3 (V1) and 47.073 cm3 (V2), respectively. The bigger chamber, named as gas reservior, isolates gas before it contacts the IL samples in the smaller chamber. The smaller chamber used as equilibrium cell is equipped with a magnetic stirrer. The temperatures (T) of both chambers are controlled by a water bath with an uncertainty of ±0.1 K. The pressures in the two chambers are monitored using two pressure transducers (WIDEPLUS PRECISION INSTRUMENTS CO., Ltd.) of ±0.2% uncertainty (in relation to the full scale). The pressure transducers are connected to a Numeric Instrument (WP-D821-200-1212-N-2P) to record the pressure changes online. In a typical run, a known mass (w) of pure IL sample was placed into the equilibrium cell, and the air in the two chambers was evacuated (
