Highly Efficient and Reversible Absorption of SO2 by Aqueous

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Highly Efficient and Reversible Absorption of SO2 by Aqueous Triethylenetetramine Tetralactate Solutions Jianguo Qian,† Shuhang Ren,† Shidong Tian,† Yucui Hou,‡ Chenxing Wang,† and Weize Wu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

‡

S Supporting Information *

ABSTRACT: The capture of SO2 by ionic liquids (ILs) has drawn much attention. In this work, a series of triethylenetetramine (lactate)n ([TETA]Ln) was synthesized and triethylenetetramine tetralactate ([TETA]L4) was selected to prepare aqueous IL solutions to absorb SO2. It has been found that the aqueous IL solutions can absorb large amounts of SO2; the saturated mole ratio of SO2 to IL in the aqueous IL solution decreases with increasing temperature, and increases with increasing water content of the aqueous IL solution; the density and viscosity of the aqueous IL solution decreases with increasing temperature and water content, and the density and viscosity of aqueous IL solution after SO2 absorption is higher than that before SO2 absorption. The absorption of SO2 by the aqueous IL solution is reversible. Compared with other ILs, [TETA]L4 has many advantages in application, including inexpensive synthetic materials, uncomplicated synthetic procedures, high SO2 capacity, and excellent reversibility.

1. INTRODUCTION Sulfur dioxide (SO2), mainly emitted from the burning of fossil fuels, has caused a series of environmental pollution issues, threatening the health of human beings. Nowadays, many technologies have been developed to remove SO2. One widely used technique is flue gas desulfurization (FGD),1−3 which includes limestone, ammonia, and organic solvents scrubbing. Unfortunately, there are some obvious disadvantages with these removal processes. For example, all the absorbents are nonrenewable, meanwhile they truly result in a lot of byproducts such as calcium sulfate (CaSO4), which is not consistent with the sustainable principles. Urgently, new materials that can efficiently, reversibly, and economically capture SO2 are in development and are of extreme importance for environmental protection. The absorption of SO2 by ionic liquids (ILs) has drawn much attention and has been broadly studied. Many kinds of ILs were developed, such as guanidinium-, alkanolaminium-, imidazolium- and phosphonium-based ILs. Han et al.4 synthesized and used 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) for the removal of SO2, and they found that the mole ratio of absorbed SO2 to IL could reach 1.0 at 40 °C and 8% SO2 in nitrogen. Other kinds of ILs based on guanidinium, such as [TMG][BF4], [TMG][BTA], [TMGB 2 ][BTA], [TMG][POBF4], and [TMG][PO2BF4], were also reported for absorbing SO2.5,6 Zhang et al.7 reported new ILs based on 1,1,3,3-tetramethylguanidinium: [TMG][TE], [TMG][PHE], and [TMG][IM]. The viscosities of these ILs are much lower than that of other ILs used for absorbing SO2, and these ILs have high SO2 absorption capacities. A series of hydroxyl ammonium ILs such as monoethanolaminium lactate ([MEA]L) were also synthesized by Zhang et al.,8 and high SO2 absorption capacities were found in these ILs. Wang et al.9 synthesized a series of phosphonium-based ILs based on imidazole and triazole, and these azole-based ILs with multiple © XXXX American Chemical Society

sites have very high absorption capacities of SO2. They also combined the ether-functionalized cations and tetrazolate anions to form a new kind of ILs. These ILs could both physically and chemically absorb SO2 with high absorption capacities.10 Recently, Han et al.11 synthesized [Et2NEMim][Tf2N] and [Et2NEMim][Tetz] and applied the two ILs to absorb SO2. [Et2NEMim][Tetz] showed better capacity of SO2 than [Et2NEMim][Tetz] under the same conditions. Other types of ILs such as ether-functionalized, imidazolium-based, and caprolactam tetrabutyl ammonium bromide ILs were also studied for the removal of SO2.12−14 The ILs used to absorb SO2 can be divided into two classes: normal ILs and functional ILs.15,16 SO2 absorption capacities of normal ILs are limited at low SO2 partial pressures,16 but the functional ILs have high absorption capacities of SO2 even when the partial pressure of SO2 is very low, such as that in flue gas. As a result, the functional ILs show promise of more efficient capacities than the normal ILs when they are used in real flue gas with low SO2 concentrations. ILs are composed of cations and anions from acid−base neutralization.17 Usually ILs with anions such as formate, acetate, lactate, (carboxylates generally) and dicyanamide can efficiently absorb SO2 at low pressures. However, when an IL is synthesized from a high volatile and weak acid, such as acetic acid, the IL is nonrenewable. Taking [Bmim][Ac] for example, the acetate anion will transform into acetic acid and the IL will change into [Bmim][HOSO2] during the absorption of SO2.18 As the formed acetic acid can be easily released under vacuum at elevated temperatures, the IL is nonrenewable. Therefore, it is necessary to choose an acid with high stability and low vapor Received: June 21, 2014 Revised: September 2, 2014 Accepted: September 5, 2014

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weight loss of the ILs was obtained, and the desorption efficiency could be calculated on the basis of the weight losses. 2.4. Absorption and Desorption of the Aqueous ILs Solution. The absorption and desorption of 3% SO2 in nitrogen were carried out under ambient pressure. The apparatuses mainly consisted of a N2 gas cylinder, a 3% SO2 gas cylinder, a rotameter (Beijing Forth Automation Meter Factory, China), a glass tube (with an inner diameter of 12 mm and a length of 160 mm), a tail gas absorption device, an analytical balance (BS224S, Sartorius) with a precision of 0.1 mg, a constant temperature oil bath. The temperature of the oil bath was maintained within ±0.5 °C. The apparatus also had a set of titration devices, including an acid buret, a conical flask, and one-time droppers, etc. All of the ILs ([TETA]L2, [TETA]L3, and [TETA]L4) have high viscosity which has a negative effect on the mass transfer during the absorption of SO2 by the ILs. Therefore, we mixed deionized water with [TETA]Ln to decrease the viscosity of [TETA]Ln. A variety of the aqueous [TETA]Ln solutions with different water contents was prepared. We chose the aqueous [TETA]L4 solution with 50 wt % H2O as a typical experiment. About 4.00 g of absorbent was loaded in the glass tube, and a gas stream with a flow rate of 100 cm3/min with 3% SO2 in nitrogen was bubbled through the solution at 50 °C. A small sample of solution was taken from the glass tube at regular intervals, and then the content of SO2 absorbed in the solution was analyzed. When the second or third consecutive content of SO2 absorbed in the solution was no longer changed, the absorption of SO2 from the simulated flue gas was regarded to be in equilibrium. SO2 saturation uptake was calculated on the basis of the amounts of SO2 absorbed and the IL charged. The desorption of saturated absorbent was carried out by N2 sweeping with a flow rate of 50 cm3/min at 100 °C, similarly a small desorbed sample was taken from the glass tube, and the content of SO2 desorbed in the solution was analyzed. The content of SO2 in the sampled solutions was measured using the standard iodimetry (HJ/T56-2000, a standard method of the State Environmental Protection Administration of China). The experimental method was similar as that described in our previous work.16,25 The reproducibility of the solubility measurement was better than ±2% and the uncertainty in the solubility was estimated to be ±4%. 2.5. Density and Viscosity of Aqueous [TETA]L4 Solutions before and after SO2 Absorption. The density and viscosity of the aqueous [TETA]L4 solutions were measured at ambient pressure and determined by a densitymeter (Lovis 2000M, Anton Paar, Austria) and a microviscometer (DMA 4500M, Anton Paar, Austria).

pressure, such as lactate acid, which is also environmentally friendly and relatively inexpensive. The previous work has confirmed that ILs based on lactate anion can be reused with high regeneration efficiency.8,19−22 As for cations, we have found that nearly all the cations of reported ILs contain aminogroups, whose number is a decisive factor for absorbing SO2. It was reported that polyamine-based ILs could be used to absorb CO2 at high temperatures (up to 130 °C), which suggests that the interactions between the ILs and CO2 is very strong.23 Therefore, it is expected that polyamine-based ILs have high SO2 absorption capacities. As a result, combining the advantages of polyamine cation and lactate anion, the polyamine-based ILs with lactate anion could reasonably show a satisfactory result. Hence, in this work, triethylenetetramine (lactate)n ([TETA]Ln), including [TETA]L2, [TETA]L3, and [TETA]L4, were synthesized for absorbing SO2 with a low SO2 concentration from simulated flue gas. By investigating their volatility and reversibility, we selected triethylenetetramine (lactate)4 ([TETA]L4) in aqueous solution as the most suitable for the capture of SO2. We also studied the effect of temperature and the content of IL on the absorption of SO2, and tested the regeneration of aqueous [TETA]L4 solution.

2. EXPERIMENTAL SECTION 2.1. Materials. SO2 (99.95%) and N2 (99.99%) were supplied by Beijing Haipu Gases Co., Ltd. (Beijing, China). SO2 (3%) was prepared by mixing SO2 and N2 together in a 40 dm3 high pressure cylinder. Lactic acid (85% in water) and triethylenetetramine (70% in water) were obtained from Aladdin Chemical Co., Ltd. (Shanghai, China). Soluble starch (AR) and potassium iodide (AR) were supplied by Beijing Yili Fine Chemical Co., Ltd. (Beijing, China). Hydrochloric acid (AR), sodium thiosulfate (AR), iodine (AR), and sulfuric acid (AR) were obtained from Beijing Chemical Works (Beijing, China). Ethanol was obtained from Beijing Modern Oriental Fine Chemical Co., Ltd. (Beijing, China). 2.2. Synthesis of ILs. The ILs used in this work were prepared by neutralizing polyamines in water with lactate acid of different proportions. Taking [TETA]L4 for example, the synthesis procedure is shown as follows: 0.5 mol triethylenetetramine and 100 cm3 water were added into a 500 cm3 threenecked flask. The flask was placed in an ice−water bath at 0 °C and equipped with a reflux condenser under vigorous stirring with a magnetic stirrer. Then, 2 mol lactic acid was put in a pressure funnel and was added dropwise to the flask over about 1 h. The reaction lasted for 4 h at 25 °C. The solvent was removed by evaporation under vacuum at 75 °C. Then the crude IL was swept to remove the residual water by N2 at 100 °C.24 Using a similar method, we also synthesized [TETA]L2 and [TETA]L3 for SO2 capture. 2.3. Volatility of the ILs. The test of the volatility of the ILs was carried out under ambient pressure. The apparatuses mainly consisted of a N2 gas cylinder, a rotameter (Beijing Forth Automation Meter Factory, China), a glass tube (with an inner diameter of 12 mm and a length of 160 mm), an analytical balance (BS 224S, Sartorius) with a precision of 0.1 mg, and a constant temperature oil bath. The temperature of the oil bath was maintained within ±0.5 °C. In a typical experiment, about 4.00 g of IL was loaded in the glass tube, and then the IL was swept by N2 with a flow rate of 50 cm3/min at 100 °C. The weight of the glass tube was measured at regular intervals with the analytical balance. The

3. RESULTS AND DISCUSSION 3.1. Selection of the ILs. 3.1.1. Volatility of the ILs. Figure 1 shows the weight loss of [TETA]Ln at 100 °C with N2 of 50 cm3/min as the sweeping gas, indicating that all the ILs have certain volatility. However, there is an obvious difference on the volatility of the three ILs. For example, [TETA]L2 loses 0.08% of its weight in 100 min; while the weight loss of [TETA]L4 is less than 0.005% in 100 min. The key reason for the different volatilities of the ILs is the number of anions in these ILs. For example, ethanol amine has a high volatility. When it is neutralized by lactic acid to produce ethanolamine lactate, its volatility, however, becomes very low.26 Therefore, it is reasonable that the volatility of triethylenetetramine, which contains four free amino groups, B

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DE % =

Rs − Rt 100 Rs

(1)

where Rs is the mole ratio of SO2 to IL when the IL is saturated, Rt is the mole ratio of SO2 to IL at the determination time. For example, [TETA]L2 shows the highest absorption capacities of SO2 with 2.45 mol SO2 per mole IL (0.616 g of SO2 per gram of IL), but the DE is less than 45%; [TETA]L3 shows absorption capacities of SO2 with 2.04 mol SO2 per mole IL (0.379 g of SO2 per gram of IL), and the DE can reach 75%; [TETA]L4 shows absorption capacities of SO2 with 1.44 mol SO2 per mole IL (0.212 g of SO2 per gram of IL), and the DE of SO2 can nearly reach 100%. The solubility of 3% SO2 in deionized water at different temperatures was also measured in this work, and it was found to be less than 0.0009 mol SO2 per mole water at 30 °C, negligible compared with SO2 absorption capacities of [TETA]Ln. The main reason for the different absorption capacities and desorption efficiencies is the number of anion groups in these ILs. When an IL has more free amino groups, it is more basic, which results in much stronger interactions between the IL and SO2. Therefore, an IL with more free amino groups can absorb more SO2 and release less. On the basis of the overall consideration of absorption and desorption, [TETA]L4 was selected as the best absorbent for the capture of SO2 from simulated flue gas. The viscosity of [TETA]L4 is high, so we prepare aqueous [TETA]L4 solution with different water contents to absorb SO2 from simulated flue gas, because of the hydrophilicity of the IL and the moisture (water vapor) in flue gas, which is between 8% and 20%. 3.2. Effect of Temperature on the Absorption of 3% SO2. Figure 4 shows the absorption of 3% SO2 in aqueous

Figure 1. Volatility of several kinds of [TETA]Ln at 100 °C under atmospheric pressure: ■, [TETA]L2; ●, [TETA]L3; ▲, [TETA]L4.

will gradually lower with an increase of neutralized amino groups by lactic acid. 3.1.2. Absorption and Desorption of 3% SO2 in Nitrogen by Aqueous ILs Solution. Figures 2 and 3 show the absorption

Figure 2. Absorption of 3% SO2 in nitrogen based on mole ratio by three kinds of aqueous [TETA]Ln solutions with 50 wt % H2O under atmospheric pressure at 50 °C: ■, [TETA]L2; ●, [TETA]L3; ▲, [TETA]L4.

Figure 4. Absorption of 3% SO2 in nitrogen based on mole ratio by aqueous [TETA]L4 solution with 50 wt % H2O under atmospheric pressure at different temperatures: ■, 40 °C; ●, 50 °C; ▲, 60 °C.

[TETA]L4 solution with 50 wt % water at different temperatures. As can be seen from Figure 4, the temperature has significant influence on the absorption of SO2 by aqueous [TETA]L4 solution. The mole ratio of saturated SO2 to [TETA]L4 increases with the decrease of temperature. For instance, when the temperature is 60 °C, the mole ratio of saturated SO2 to [TETA]L4 can reach 1.21 (0.178 on mass ratio). However, when the temperature decreases to 40 °C, the mole ratio of saturated SO2 to [TETA]L4 increases to 1.64 (0.242 on mass ratio). 3.3. Effect of Water Content on the Absorption of 3% SO2. Figure 5 shows the absorption capacity of 3% SO2 in

Figure 3. Desorption of SO2 by nitrogen sweep in three kinds of aqueous [TETA]Ln solutions under atmospheric pressure at 100 °C: ■, [TETA]L2; ●, [TETA]L3; ▲, [TETA]L4.

of 3% SO2 in nitrogen in the three ILs at 50 °C and the desorption of SO2 at 100 °C, respectively. The results indicate that all the ILs can absorb and desorb SO2. However, the saturated mole ratios of SO2 to ILs and the desorption efficiency (DE) are significantly different. The DE is defined as the following eq 1. C

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[TETA]L4 + nH 2SO3 ⇌ [TETA]L4 − n[HSO3]n + nHL (3)

The concentration of [TETA]L 4 , H 2 SO 3 , [TETA]L4‑n[HSO3]n and HL are represented by CA, CB, CC, and CD. The simulated flue gas with 3% SO2 was bubbled through the absorbent constantly; therefore based on reaction 1, the concentration of H2SO3 is constant with the change of water content of aqueous [TETA]L4 solution, namely CB is not changed. Meanwhile, CA, CC, and CD change obviously with the change of water content, which will result in an equilibrium shift of reaction 2 according to Le Chatelier’s principle. Compared with CA, CC, and CD, CB increases relatively when there is more water in the absorbent, causing the reversible reaction 2 to move right. In other words, the solubility of SO2 in the aqueous [TETA]L4 solution increases with the increase of water content of aqueous [TETA]L4 solution. 3.4. Density and Viscosity of the Aqueous [TETA]L4 Solution Before and After SO2 Absorption. Density and viscosity of absorbents are important parameters for the SO2 absorption process. The density and viscosity of aqueous [TETA]L4 solution before and after SO2 absorption with different water contents at different temperatures were investigated and the results are shown in Figures 7 and 8,

Figure 5. Absorption capacity of 3% SO2 in nitrogen based on mole ratio by aqueous [TETA]L4 solution with different water contents at 50 °C under atmospheric pressure.

aqueous [TETA]L4 solution with different water contents at 50 °C. The saturated mole ratio of SO2 to [TETA]L4 increases with the increase of water content. For example, when the water content is 40 wt %, the saturated mole ratio of SO2 to [TETA]L4 can reach 1.34 (0.197 on mass ratio). However, when the water content increases to 90 wt %, the saturated mole ratio of SO2 to [TETA]L4 increases to 1.81 (0.267 on mass ratio). The above result suggests that the water content of aqueous [TETA]L4 solution has a significant effect on SO2 absorption. To explain the above result, the mechanism of SO 2 absorption by aqueous [TETA]L4 solution was studied by FT-IR. The FT-IR spectra of aqueous [TETA]L4 solution and aqueous [TETA]L4 solution-SO2 are shown in Figure 6.

Figure 7. Density of aqueous [TETA]L4 solution before (hollow symbols) and after (solid symbols) absorption as a function of temperature at different water mass fractions: ☆ and ★, 30%; ○ and ●, 40%; ◊ and ⧫, 50%; □ and ■, 60%;. △ and ▲, 70%.

respectively. It can be found in Figure 7 that the density of the aqueous [TETA]L4 solution decreases with the increase of Figure 6. FT-IR spectra of aqueous [TETA]L4 solution before and after SO2 absorption: (a) aqueous [TETA]L4 solution, (b) aqueous [TETA]L4 solution −SO2.

Compared with the FT-IR spectrum of aqueous [TETA]L4 solution, the spectrum of aqueous [TETA]L4 solution-SO2 shows new absorption bands at 1172 and 971 cm−1, which can be assigned to sulfate SO and S−O stretches, respectively. On the basis of the study of FT-IR, we can determine the mechanism of the SO2 absorption by aqueous [TETA]L4 solution: first, sulfurous acid (H2SO3) was produced by SO2 reacting with H2O, and then H2SO3 reacts with [TETA]L4 at different mole ratios. The key reactions are shown as follows: SO2 + H 2O ⇌ H 2SO3

Figure 8. Viscosity of aqueous [TETA]L4 solution before (hollow symbols) and after (solid symbols) absorption as a function of temperature at different water mass fractions: ☆ and ★, 30%; ○ and ●, 40%; ◊ and ⧫, 50%; □ and ■, 60%;. △ and ▲, 70%.

(2) D

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temperature and water content. For example, the density of aqueous [TETA]L4 solution (wIL = 50%) decreases from 1.1519 g/cm3 to 1.1462 g/cm3 when temperature increases from 30 to 40 °C. And when water content increases from 50 wt % to 70 wt %, the density of aqueous [TETA]L4 solution decreases from 1.1403 g/cm3 to 1.0797 g/cm3 at 50 °C. Meanwhile the absorbed SO2 can also influence the density of aqueous [TETA]L4 solution. The density of the solution increases after SO2 absorption. For example, the density of aqueous [TETA]L4 solution (wIL = 50%) is 1.1403 g/cm3 at 50 °C, while it is 1.1733 g/cm3 after SO2 absorption. Figure 8 shows the viscosity of aqueous [TETA]L4 solutions as a function of temperature with different water contents. It can be found in Figure 8 that the viscosity of aqueous [TETA]L4 solutions decrease with an increase of temperature and water content obviously. And the viscosity of aqueous [TETA]L4 solutions increases after the absorption of SO2. For example, the viscosity of the solutions (wIL = 50%) is 11.08 mP· s and 13.84 mP·s at 50 °C before and after SO2 absorption. These low viscosities indicate that the absorbents are suitable for transportation and absorption of SO2. 3.5. The Reuse of Aqueous [TETA]L4 Solution. The absorption was carried out at 60 °C for 240 min, and the desorption was carried out at 100 °C for 300 min. The absorption/desorption cycles of [TETA]L4 + 30% water are shown in Figure 9. It can be seen from the figure that no

Figure 10. FT-IR spectra of aqueous [TETA]L4 solution before and after cyclic test: (a) before cyclic test, (b) after cyclic test.

reversibility, we found that aqueous [TETA]L4 solution is the most suitable one for the capture of SO2. The results indicate that [TETA]L4 has the lowest volatility among the three ILs, and its weight loss is less than 0.005% in 100 min; SO2 absorption by [TETA]Ln decreases with the increase of “n”, but the DE of [TETA]Ln increases with the increase of “n”, and can reach to 99%. Aqueous [TETA]L4 solution captures SO2 well, and the mole ratio of saturated SO2 to IL in the aqueous [TETA]L4 solution decreases with the increase of temperature, and increases with the increase of water content of aqueous [TETA]L4 solution. The absorption of SO2 by the aqueous [TETA]L4 solution is reversible, and no obvious loss of SO2 absorption capacity of the IL was observed after five cycles. The density and viscosity of the aqueous [TETA]L4 solution have been measured. The viscosities of the absorbents are very low even after the absorption of SO2. The aqueous [TETA]L4 solution is a promising absorbent for the removal of SO2 from the flue gas.

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ASSOCIATED CONTENT

S Supporting Information *

Graphical representations of the absorption of 3% SO2 in nitrogen based on mass ratio at different conditions, and the absorption and desorption of 3% SO2 in nitrogen based on mass ratio by aqueous [TETA]L4 solution. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Absorption and desorption of 3% SO2 in nitrogen based on mole ratio by aqueous [TETA]L4 solution with 30 wt % water at 60 °C under atmospheric pressure: solid column, absorption; striped column, desorption.

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obvious loss of SO2 absorption capacity of the IL was observed after five cycles, and the mole ratio of SO2 to [TETA]L4 from 1 to 5 cycles are (1.23, 0.05), (1.21, 0.04), (1.23, 0.05), (1.22, 0.04), and (1.21, 0.05) for absorption and desorption, respectively. The FT-IR spectra of aqueous [TETA]L4 solution before and after five cyclic tests have also been studied. As shown in Figure 10, there is no obvious change in FT-IR spectra of ILs before and after the cyclic test. In other words, there is no obvious degradation of as-obtained ILs. The result suggests that the aqueous ILs solution can be used as recyclable absorbents for the capture of SO2.

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 10 64427603. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project is financially supported by the Natural Science Foundation of China (21076138, 21306007) and Research Fund for the Doctoral Program of Higher Education of China (20130010120005). The authors thank Professors Zhenyu Liu and Qingya Liu for their helpful discussion and suggestions.

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4. CONCLUSION In this work, triethylenetetramine (lactate)n ([TETA]Ln), including [TETA]L2, [TETA]L 3, and [TETA]L4, were synthesized for absorbing SO2 with 3% concentration from simulated flue gas. After investigating their volatility and

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dx.doi.org/10.1021/ie502503t | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX