Article pubs.acs.org/EF
1,3-Dimethylurea Tetrabutylphosphonium Bromide Ionic Liquids for NO Efficient and Reversible Capture Bin Jiang,†,‡,§ Weiren Lin,†,§ Luhong Zhang,*,†,§ Yongli Sun,†,§ Huawei Yang,†,§ Li Hao,†,§ and Xiaowei Tantai†,§ †
School of Chemical Engineering and Technology, and ‡National Engineering Research Center for Distillation Technology, Tianjin University, Tianjin 300072, People’s Republic of China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China ABSTRACT: To explore environmentally friendly and benign solvents to capture NO gas, a series of ionic liquids was newly synthesized by 1,3-dimethylurea and tetrabutylphosphonium bromide with different mole ratios (1:1, 2:1, and 3:1). In addition, their NO absorption and desorption properties were studied at a temperature from 30 to 70 °C and atmospheric pressure for the first time. These synthesized ionic liquids exhibited excellent reversibility and high NO solubility, such as 1.173 mol of absorbed NO/mol of ionic liquids (3:1) at 40 °C and 101.3 kPa. The solubility of NO in these synthesized ionic liquids increased with the increase of the mole ratios of 1,3-dimethylurea in ionic liquids and also with the decrease of the temperature. The studies of Fourier transform infrared spectroscopy (FTIR) and 1H and 13C nuclear magnetic resonance (NMR) spectra revealed that it is a reversible chemical process for these ionic liquids to capture NO gas; therefore, we proposed a reversible chemical absorption mechanism. These reversible and benign ionic liquids potentially offer a new opportunity to absorb NO gas.
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INTRODUCTION The major source of nitrogen oxides in the flue gas is mainly derived from the combustion process of fossil fuels and has caused serious atmospheric pollution phenomena and human health issues, such as acid rain, photochemical smog, respiratory disease, and lung tissue injury. During the combustion process under high temperatures, the radio of NO gas in nitrogen oxides comes close to about 90%, which is relatively insoluble; therefore, the reduction of emission of NO gas is extremely significant and urgently necessary.1 Recently, great efforts have been applied to the prevalent application of efficient aftertreatment technologies to limit emission of NO, including selective catalytic reduction (SCR), photocatalytic decomposition and reduction of NO, selective NO recirculation (SNR), selective non-catalytic reduction (SNCR), NO storage and reduction (NSR), etc.2−6 SCR of NO by H2, CO, or NH3 is one of the most attractive methods, promoting the direct decomposition of NO to nitrogen in the presence of excess oxygen. The efficiency of denitration can reach about 70%. However, the catalysts used in the SCR process have some serious practical difficulties in practice, such as catalyst poisoning and failure.7,8 Another extremely attractive method is the photocatalytic decomposition and reduction of NO by the application of photocatalysis at ambient temperature. After the photocatalytic decomposition, NO would direct transform to N2 and O2. However, the formation of NO2 would make the surface inactive for NO dissociation. Of course, the simplest approach is to modify the operational conditions by reducing combustion temperatures to minimize the formation of NO. Nevertheless, this procedure would increase the formation of CO and the rate of corrosion.9,10 Room-temperature ionic liquids (RTILs) have recently gained increasing attention, owing to their unique properties, including exceedingly low vapor pressures, wide liquid © 2016 American Chemical Society
temperature ranges, excellent thermal and chemical stability, and tunable properties. Because of the characteristics mentioned above, they can be applied to many chemical processes as the environmentally benign solvents, such as catalysis, extraction, and gas separations.11−13 In recent years, great efforts have focused on the experimental and theoretical studies in the field of acidic gas separation and absorption by ionic liquids (ILs), such as SO2, CO2, H2S, etc.14−16 Because of the excellent solubility of SO2, CO2, and H2S in ILs, the research of the control of NO by ILs has gained huge interest. Duan and co-workers had investigated the solubility of NO in caprolactam tetrabutyl ammonium halide ILs. ILs were synthetized by the different mole ratios of caprolactam (CPL) and tetrabutyl ammonium halides (TBAX), including tetrabutyl ammonium fluoride (TBAF), tetrabutyl ammonium chloride (TBAC), and tetrabutyl ammonium bromide (TBAB). According to the 1H nuclear magnetic resonance (NMR) spectra, it was clearly showed that NO was physically absorbed in the ILs by hydrogen bonding. However, it was difficult for NO to form the hydrogen bonding with ILs, owing to its weak chemical polarity. Therefore, although the CPL-TBAF ILs (2:1, mole ratio) had the highest capacity for NO capture, the molar ratio of absorbed NO to ILs was just 0.16 mol/mol at 101.3 kPa and 338 K.17 Thomassen and co-workers had reported that flue gas components can be separated by supported ionic liquid phase (SILP) absorbers. They used some porous, high surface area carriers as supports for the ILs to obtain SILP absorber materials. The application of solid SILP absorbers with selected ILs can significantly improve the capacity of ILs for NO Received: August 13, 2015 Revised: November 6, 2015 Published: January 8, 2016 735
DOI: 10.1021/acs.energyfuels.5b01826 Energy Fuels 2016, 30, 735−739
Article
Energy & Fuels capture, thus making the application of ILs viable in flue gas cleaning. The results showed that NO can be reversibly and selectively absorbed by 1-butyl-3-methylimidazolium nitrate ([BMIM]NO3) ILs. This was due to the structure of ILs being well-ordered even in the liquid state with regular cavities. However, the requirements for the support material were strict. Especially, the acidity and hydrophilic nature were needed. Besides, the procedure of the preparation for the support material was also complicated.18 On the basis of that mentioned above, ILs have offered a new opportunity to develop novel NO capture systems. Therefore, it is necessary to obtain a better understanding of the solubility of NO gas in ILs. Recently, we newly synthesized a series of ILs that consisted of different mole ratios of 1,3-dimethylurea and tetrabutylphosphonium bromide (PX 4B), and their capacity for NO capture was investigated. In comparison to the traditionally used ILs, they are much cheaper and more environmentally friendly. Furthermore, no requirement for medium, 100% reaction mass efficiency, and zero emission are all achieved in the preparation process.19 The effects of different temperatures and mole ratios of 1,3-dimethylurea and PX 4B on their capacity for NO capture were systematically investigated. Fourier transform infrared spectroscopy (FTIR) and 1H and 13C NMR spectra were used to discuss the mechanism of the NO absorption process. The recycling of the synthesized ILs and the recovery of NO were also tested. The desorption process was carried out through the increase of the temperature and the decrease of the pressure.
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Figure 2. Diagram of the experimental apparatus of NO absorption in ILs: (1) NO gas cylinder, (2) N2 gas cylinder, (3) pressure relief valve, (4) gas rotor flow meters, (5) glass gas sampling tube, (6) constanttemperature water bath, and (7) residual gas absorption bottle. absorption was monitored by an electron analytical balance (Precision & Scientific FA2004, Shanghai, China) with the accuracy of 0.0001 g, through which the mole ratio of absorbed NO to ILs can be calculated. After 4 h, the weight change of the glass gas sampling tube was small in the absorption process. Therefore, the equilibrium was considered to be reached in about 4 h. To study the influence of the temperature on the NO absorption process, the temperature was set ranging from 30 to 70 °C and was maintained within ±0.5 °C by the constant temperature water bath (DF-101S, Yuhua Co., Ltd., Gongyi, China). Because the IL (3:1) was solid under 40 °C, its absorption temperature was set from 40 °C. The NO off gas was treated by hydrogen peroxide. Recycle of ILs. The experiments of the absorption (40 °C and 101.3 kPa) and desorption (80 °C and 10.1 kPa vacuum) cycles of NO gas in recycled ILs were carried out 5 times to test the reusability of ILs and the recovery of NO gas. The data errors were estimated within ±2% according to the repeatability of the measurements being better than ±2%. FTIR and 1H and 13C NMR Spectra Investigations. FTIR and 1 H and 13C NMR spectra were recorded to investigate the mechanism of NO gas dissolved in the synthesized ILs. Before and after absorption of NO gas, the FTIR spectra of ILs were measured by a Nicolet 380 spectrophotometer (Thermo Electron Co.). Meanwhile, before and after absorption and after desorption of NO gas, the 1H and 13C NMR spectra of ILs were measured by Bruker Avance III (Germany).
EXPERIMENTAL SECTION
Materials. Tetrabutylphosphonium bromide (CAS Registry Number 3115-68-2, C16H36PBr) and 1,3-dimethylurea (CAS Registry Number 96-31-1, C3H8N2O) were purchased from J&K Chemical, Ltd., China. Their purity were up to 99%. All chemicals were used as received. NO gas (≥99.9%) and N2 (≥99.99%) gas were supplied by Tianjin Shengtang Specialty Gases Co., Ltd. The ILs were synthesized according to the literature procedure as showed in Figure 1.20
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Figure 1. Synthetic route to the 1,3-dimethylurea tetrabutylphosphonium bromide ILs.
RESULTS AND DISCUSSION Characterization of ILs. 1H NMR spectra of 1,3dimethylurea tetrabutylphosphonium bromide ILs (1:1, mole ratio) are presented. 1H NMR (500 MHz, CDCl3) δ: 5.83 ppm [s, 2H, (HN)2], 2.63 ppm [s, 6H, (HNCH3)2], 2.33−2.28 ppm [ m , 8 H , P + ( C H 2 ) 4 ] , 1 . 4 7− 1 . 4 3 p p m [ m , 1 6 H , P + (CH 2 CH 2 CH 2 ) 4 ], and 0.91−0.88 ppm [t, 12H, P+(CH2CH2CH2CH3)4]. Absorption Behaviors of NO Gas in ILs. The effect of the temperature on the solubility of NO in synthesized ILs is shown in Figure 3. It was clear that, as the temperature increased, the capacity of ILs for NO capture decreased gradually. Furthermore, the increase of the mole fraction of 1,3-dimethylurea in the ILs contributed to the NO solubility, which can be explained by more absorption sites being available through the increase of the 1,3-dimethylurea concentration in the ILs. For example, the mole ratio of absorbed NO to ILs (2:1, mole ratio) was 0.918 at 30 °C, while it was 0.465 at 60 °C. In addition, at 30 °C, the mole ratio of absorbed NO to ILs (1:1, mole ratio) was 0.663 lower than 0.918. As we can see, the ILs (3:1, mole ratio) had
All ILs were dried under vacuum at 60 °C for more than 24 h to reduce the possible content of water. The water content was less than 0.2 wt % determined by a Karl Fischer moisture titrator (AKF-2010, HOGON Co., Ltd.). Experimental Apparatus and Procedures. The experimental apparatus for the absorption of NO by these ILs mainly consisted of the NO and N2 gas cylinders, a glass gas sampling tube (inner diameter of 10 mm), a constant temperature water bath, and a residual gas absorption bottle. The absorption process of NO in different mole ratios ILs (1:1, 2:1, and 3:1) was operated at a temperature from 30 to 70 °C and 101.3 kPa. The diagram of the experimental apparatus is shown in Figure 2. In a typical absorption experiment, a kind of IL (1.5−2 g) was loaded in a glass gas sampling tube. After checking the air tightness of the experimental apparatus, N2 gas was released at a rate of 30 mL/ min for 30 min to drive away the air in the experimental apparatus as far as possible. Then, N2 gas was cut off by closing the valve. After that, NO gas was bubbled through the glass gas sampling tube at a rate of 10 mL/min. It was clear that the gas absorption device did not leak. The weight change of the glass gas sampling tube before and after 736
DOI: 10.1021/acs.energyfuels.5b01826 Energy Fuels 2016, 30, 735−739
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Figure 3. Mole ratio of absorbed NO in synthesized ILs as a function of the temperature. Conditions: 101.3 kPa. Figure 5. FTIR spectra of the ILs before absorption and after absorption of NO.
the highest capacity for NO capture and the mole fraction solubility of NO in this ILs was 1.173 at 40 °C and 101.3 kPa. Therefore, in this work, the ILs (3:1, mole ratio) can be regarded as the most potential absorbent of NO to develop novel NO capture systems. Recycle of ILs. The reusability of the ILs for NO capture is illustrated in Figure 4.
Figure 6. 1H NMR spectra (500 MHz) of the ILs before and after absorption and after desorption of NO in CDCl3. Figure 4. Reusability of ILs in NO absorption. Conditions: 40 °C and 101.3 kPa (absorption) and 80 °C and 10.1 kPa vacuum (desorption).
The absorption process lasted 4 h at 40 °C and 101.3 kPa, and the desorption process lasted 4 h at 80 °C and 10.1 kPa vacuum. The experiments of the absorption and desorption cycles of NO gas were carried out 5 times. Obviously, after 5 cycles, the capacity of the ILs for NO capture is without evident loss. Mechanism of NO Absorption. To obtain some details about the mechanism between NO and ILs, FTIR and 1H and 13 C NMR spectra were recorded. Before and after absorption of NO, FTIR spectra of the ILs are shown in Figure 5. Before and after absorption and after desorption of NO, 1H and 13C NMR spectra of the ILs are shown in Figures 6 and 7, respectively. According to the FTIR spectra of the ILs after absorption of NO, we can probably infer N−H (3300 cm−1), C−H in −CH3 (2960 cm−1), C−H in −CH2 (2925 and 2860 cm−1), intermolecular H···Br− in the anion (1710 cm−1), CO (1660 cm−1), N−H (1560 cm−1), intermolecular O···H in the anion (1520 cm−1), NO (1240 cm−1), and C−N (1190 cm−1). In comparison to the FTIR spectra of the ILs before
Figure 7. 13C NMR spectra (500 MHz) of the ILs before and after absorption and after desorption of NO in CDCl3.
absorption of NO, the spectra of the ILs after absorption of NO emerge three new absorption bands at 1710, 1520, and 1240 cm−1, which can be assigned to the formation of H···Br−, O···H, and NO, respectively. Besides, according to the 1H NMR spectra of the ILs after absorption, there are two new 737
DOI: 10.1021/acs.energyfuels.5b01826 Energy Fuels 2016, 30, 735−739
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Figure 8. Proposed reaction between the synthesized ILs and NO.
resonances that can be observed at δ = 2.97 and 3.09 ppm, which are possibly consistent with CH3−N and CH3−NH in the reaction product. This phenomenon can be explained by the reduction of the shielding effect in the methyl group. After the reaction, the content of the electron-withdrawing group in the 1,3-dimethylurea increased, resulting in the shifting of resonances of the two methyl groups, from δ = 2.63 to 2.97 and 3.09 ppm, respectively. Furthermore, according to the 13C NMR spectra of ILs after absorption, a new resonance can be observed at δ = 27.17 ppm, which is probably consistent with CH3−N in the reaction product. The 1H and 13C NMR spectra indicate that the ILs would return to the original state after desorption. With deduction from the 1H and 13C NMR spectra and reusability experiment, it is unambiguous that the reaction between the ILs and NO was a reversible chemical reaction. However, the reaction product was unstable and easy to dissociate, especially in the solvent, leading to new resonances in the 13C and 1H NMR spectra not being strong enough, as expected. The reaction of NO with amines to produce zwitterions had been known for many years, such as the structure of diethylenetriamine/NO adduct being demonstrated. It was clearly shown that the reaction mechanism involved the sequential addition of NO.21 This mechanism can closely relate to the free radical property of NO, which make NO able to either obtain or lose the lone pair electrons. On the basis of this, we propose a most likely reaction between the ILs and NO in our view, which is shown in Figure 8. We can probably infer that NO reacts with the group of −NH in the anion of the ILs and forms the N−N bond. After the desorption of ILs at 80 °C and 10.1 kPa vacuum, the reversible N−N bond breaks down, resulting in absorbed NO being released.
can be applied to the NO removal process as the environmentally benign solvents.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (21336007).
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REFERENCES
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CONCLUSION A series of ILs was newly synthesized by 1,3-dimethylurea and tetrabutylphosphonium bromide with different proportions and had offered an new method to capture NO gas efficiently. The solubility of NO in different ILs was investigated at the temperature from 30 to 70 °C and atmospheric pressure. The results of this work indicated that the capacity of the synthesized ILs for NO gas capture was notable. For example, the mole fraction solubility of NO gas in ILs (3:1, mole ratio) was 1.173 at 40 °C and 101.3 kPa. The experimental results revealed that the decrease of the temperature and the increase of mole ratios of 1,3-dimethylurea contributed to the NO solubility in the ILs. Through the FTIR and 1H and 13C NMR spectra investigation, we had a better understanding of the mechanism of NO gas dissolved in the synthesized ILs and proposed a reversible chemical absorption mechanism between the ILs and NO. Furthermore, the synthesized ILs can be reused, with the absorption capacity of the ILs for NO without evident loss after 5 cycles. Consequently, these synthesized ILs 738
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