Highly Efficient Absorption of CO2 by Protic Ionic Liquids-Amine Blends at High Temperatures.

In view of the increasingly serious harm of CO2 to the environment, it is highly desirable to develop effective CO2 absorbents. In this work, we demonstrated an efficient absorption of CO2 by blends of protic ionic liquids (PILs) plus amines. The density and viscosity of investigative four PILs-amine mixtures were measured. By systematically studying the effects of the solution ratio, temperature, CO2 partial pressure, and water content on the absorption of CO2, it is found that the 3-dimethylamino-1-propylamine acetate ([DMAPAH][OAc]) plus ethanediamine (EDA) mixture shows the highest CO2 uptake of 0.295 g CO2 per g absorbent at 50 °C and 1 bar and a further increase in the absorption of CO2 to 0.299 g/g by adding water with a mass fraction of 20%. Furthermore, the absorption mechanism of CO2 in the presence and absence of water has also been investigated by FTIR and NMR spectra.

Introduction

It is well known that over emission of CO2 is one of the primary causes of global warming and has led to serious environmental problems.[1] Flue gases from fossil fuel combustion are the most critical pathway for CO2 emissions, and the development of CO2 capture and storage technologies is one of the most promising ways to reduce the impact of greenhouse gases on environmental change.[2,3] However, the burning flue gas produces a CO2 yield of less than 15%, while the high temperature of the flue gas and the low partial pressure of CO2 result in a low thermodynamic driving force, which poses a great challenge for the development of efficient and low-cost CO2 capture technologies.[4−6]

Ionic liquids (ILs) have many attractive natures, especially functionalized ILs with outstanding attributes like low heat capacity, high thermal stability, negative vapor pressure, outstanding CO2 affinity, etc.[7−10] However, the use of ILs in industrial processes is limited by the high price and high viscosity of ILs.[11,12] Protic ionic liquids (PILs), a class of low-cost ILs, can avoid the limitations of functionalized ILs for CO2 capture. Many amines thus are paired with carboxylic acids, phenol derivatives, or imidazole derivatives to form PILs.[13−15] For instance, a series of diamine carboxylate PILs with CO2 absorption capacity were synthesized, with the CO2 uptake reaching 0.33 mol CO2/mol IL.[16] Li et al. reported the formation of three PILs from the ultra-strong base tetramethylguanidine (TMG) with imidazole (Im), pyrrole, and phenol, where [TMGH][Im] shows a maximum CO2 absorption of 0.177 g CO2/g IL at 30 °C and 1 bar.[15] Before this, we reported the use of 3-dimethylamino-1-propylamine (DMAPA)-based PILs to capture CO2, acquiring a maximum absorption of 3.99 mol CO2/kg IL.[17] However, it cannot be avoided that the viscosity of the ILs increases dramatically after absorbing CO2, and the absorption capacity of CO2 also needs to be increased at high temperatures in the above processes.

Preliminary research studies show that adding amines can reduce the viscosity of ILs, resulting in faster absorption and mass transfer. Especially, the absorption of CO2 can be enhanced by constructing mixed absorbents of ILs plus amine due to the good affinity between amine and CO2.[18−20] After extensive studies,[21−23] DMAPA as an organic amine has a unique advantage in absorbing CO2. In particular, DMAPA-based ionic liquids can effectively absorb CO2 through intramolecular proton transfer.[24] But more than that, amine must have the advantages of low viscosity, high CO2 capacity, fast adsorption rate, and so on when it is used to modify IL. Therefore, we note that EDA has preponderance and is widely praised for capturing CO2. More importantly, EDA has the smallest molecular weight, which results in the high CO2 capacity per unit mass.[25,26] Inspired by these,[18,19,27,28] we reported here four mixed PILs-amine absorbents for CO2 capture at high temperatures, in which ethanediamine (EDA) and diethylenetriamine (DETA) were selected to confect the mixed absorbents along with PILs 3-dimethylamino-1-propylamium acetic acid [DMAPAH][OAc] or 3-dimethylamino-1-propylamium lactic acid [DMAPAH][LA]. The effects of the temperature, CO2 partial pressure, and water content on the absorption of CO2 in well-chosen PILs-amine blends were investigated. Furthermore, the absorption mechanism of CO2 in the presence and absence of water has also been investigated by FTIR and NMR spectra.

Results and Discussion

Characterization of Absorbents

The presented PILs were characterized by NMR and FTIR (Figures S1–S3). It was clearly shown that we successfully synthesized the PILs. Thermostability was also investigated (Figure S4). As shown in the figure, the thermostability of both ionic liquids is greater than 150 °C. The water content of the PILs was measured by a Karl Fischer method, and the PILs are almost anhydrous after deep dewatering (Table S1). In addition, the purities of [DMAPAH][OAc] and [DMAPAH][LA] are 99.0 and 94.5%, respectively, which were analyzed by 1H NMR.

Physical properties of absorbents are essential characteristic parameters for industrial applications, so the densities and viscosities were measured at different temperatures and the results are shown in Figure and listed in Tables S2 and S3.

Figure 1

(a) Densities and (b) viscosities of four PILs-amine blends at various temperatures.

Measurements of the four PILs-amine blends in a temperature range of 30–70 °C showed that the density decreased almost linearly with increasing temperature (Figure a). The viscosity of four PILs-amine blends decreased exponentially with the increase in temperature (Figure b). Densities and viscosities of four PILs-amine blends were fitted by eqs and 2:[17]where a0, a1, and η0 are undetermined parameters, Ea is the activation energy, and ρ and η are the densities and viscosities of the mixed absorbents, respectively. These parameters were obtained by fitting the experimental data into eqs and 2, and the results are shown in Table . The Ea value decreased in the order [DMAPAH][LA]-DETA > [DMAPAH][LA]-EDA > [DMAPAH][OAc]-DETA > [DMAPAH][OAc]-EDA, which is consistent with the viscosity variation trend of absorbents.

Table 1

Fitting Results of Densities and Viscosities for the Four PILs-Amine Blends

	parameters
absorbents	a0	a1	Ea (kJ·mol–1)	ln η0
[DMAPAH][OAc]-EDA	1.12	–0.57	10.41	–1.91
[DMAPAH][OAc]-DETA	1.13	–0.51	19.74	–4.28
[DMAPAH][LA]-EDA	1.15	–0.57	12.67	–2.54
[DMAPAH][LA]-DETA	1.19	–0.59	26.42	–6.46
[DMAPAH][OAc]:EDA = 1:0.5	1.12	–0.51	28.50	–8.07
[DMAPAH][OAc]:EDA = 0.5:1	1.11	–0.61	16.80	–5.33

The viscosities of four PILs-amine blends are in the range of 9.34–56.27 mPa·s at 30 °C, which are lower than most ILs CO2 absorbents previously reported in the literature.[29,30] Notably, the viscosities of the PILs-amine blends are much lower than those of pure PILs. The low viscosity is more conducive to the mass transfer process for the absorption of carbon dioxide.

Effect of PILs and Amines on CO2 Absorption

To explore the effect of PILs and amines on CO2 absorption, CO2 absorption was carried out at 30 °C and 1 bar for four different PILs-amine blends. As shown in Figure , the CO2 absorption in [DMAPAH][OAc]-EDA, [DMAPAH][OAc]-DETA, [DMAPAH][LA]-EDA, and [DMAPAH][LA]-DETA reached the balance with the absorption amounts of 0.260, 0.188, 0.233, and 0.156 g CO2/g absorbents, respectively. The absorption of CO2 is higher than those of absorbents reported previously.[30,31] Since the acetate ion has a higher alkalinity than the lactate ion, therefore CO2 uptake in [DMAPAH][OAc] is higher than that in [DMAPAH][LA].[32] Noteworthily, it was found that various degrees of foaming are clearly observed during the absorption processes, which leads to an increase in the apparent viscosity of the liquid, resulting in a decrease in the rate of CO2 absorption, especially under low temperature conditions. According to the above results, [DMAPAH][OAc]-EDA, as a candidate, was selected to further investigate the effects of the temperature, CO2 partial pressure, and water content on the CO2 absorption.

Figure 2

CO2 absorption in four PILs-amine blends at 30 °C and 1 bar.

The properties of [DMAPAH][OAc] and EDA blends with different mass ratios were studied. It is obvious that the introduction of EDA is beneficial to CO2 capture (Figure a). The addition of EDA greatly enhances the CO2 adsorption rate of the adsorbents. In addition, the viscosity of the absorbents decreased significantly with the increase in the EDA content (Figure b), which was favorable. This greatly reduces the mass transfer resistance of CO2 in the absorbent. In general, EDA is an excellent CO2 absorbent. However, it is limited by low thermal stability. Therefore, it is more attractive to enhance its thermal stability by making blends with ionic liquids. The TG curve clearly indicated an increase in the thermal stability of absorbents (Figure c). Interestingly, the absorbent obtains better thermal stability with adding a little IL ([DMAPAH][OAc]:EDA = 0.5:1), as shown in Figure d. In particular, the loss temperature of EDA therein is significantly improved (from about 85 to 115 °C). Overall, this demonstrates the potential of the protic ionic liquids-amine blends.

Figure 3

(a) CO2 capture at 30 °C and 1 bar. (b) Viscosities at various temperatures. (c) TGA and (d) DTG of the [DMAPAH][OAc] and EDA blends with different mass ratios.

Effect of Temperature on CO2 Absorption

The effect of temperature on CO2 absorption by [DMAPAH][OAc]-EDA was investigated from 20 to 60 °C under atmospheric pressure. As shown in Figure , the CO2 uptake of [DMAPAH][OAc]-EDA is 0.260 g CO2/g absorbent at 30 °C and the absorption of CO2 further increases to 0.295 g CO2/g by enhancing the temperature to 50 °C. The absorption rate increased gradually with increasing temperature, and the absorption rate reached a maximum when the temperature is 60 °C. Particularly, blistering can be weakened observably by enhancing the temperature (Figure S5), which greatly reduced the mass transfer resistance during CO2 absorption. The results above indicated that the influence of mass transfer is larger than the temperature during CO2 absorption.

Figure 4

Absorption of CO2 by [DMAPAH][OAc]-EDA at different temperatures and 1.0 bar.

Effect of CO2 Pressure on CO2 Absorption

The partial pressure of CO2 in the waste gas produced by the industrial application is low and CO2 generally coexists with other gases, so it is necessary to study the effect of CO2 partial pressure on the absorption of CO2 by the PILs-amine. The CO2 absorption performance of [DMAPAH][OAc]-EDA was thus investigated at 30 °C with different partial pressures of CO2. As shown in Figure , the CO2 uptake in [DMAPAH][OAc]-EDA decreased slightly with the decrease in the partial pressure of CO2. It is worth noting that the CO2 uptake at 0.1 bar CO2 can reach 0.207 g CO2/g absorbent, which shows a higher CO2 absorption capacity than the functionalized ILs or some formulated absorbents reported in the literature.[15,33]

Figure 5

CO2 absorption by [DMAPAH][OAc]-EDA at 30 °C and different partial pressures of CO2.

The absorption capacities of CO2 in some functionalized ILs or some formulated absorbents are listed in Table . For these absorbents reported in the literature, the CO2 capacities are usually below 0.23 g/g.[30,34] Compared with these absorbents, the PILs-amine blends exhibited the better absorption performance of CO2, especially under high temperature conditions.

Table 2

Comparison of the CO2 Absorption Capacity of the Different Absorbents

absorbents	T (°C)	p (bar)	CO2 uptake (g/g)	ref
[DMAPAH][OAc]-EDA	30	1	0.260	this work
[DMAPAH][OAc]-EDA	30	0.1	0.207
[DMAPAH][OAc]-EDA	50	0.1	0.295
[DMAPAH][For]	30	1	0.083	(16)
[DMAPAH][F]	30	1	0.159	(31)
[DMAPAH][4F-PhO]	30	1	0.176	(17)
[DMAPAH][Ac]	20	1	0.082	(16)
[DMAPAH][Py]	23	1	0.225	(34)
90% [DMAPAH][EOAc]	30	1	0.107	(30)
90% [DMEDAH][EOAc]	30	1	0.102	(30)
[P4444][PhO]	40	1	0.096	(35)
[N2222][PhO]	50	1	0.126	(36)
Li(TEPA)Tf2N + Li(TEG)Tf2N	80	0.1	0.060	(37)
[P4442][Suc]	60	0.1	0.039	(38)
[N1111][Gly]	25	0.65	0.051	(33)
[N1111][Gly]-H2O (70 wt %)	25	0.64	0.178	(33)
[TMGH][Im]	30	0.1	0.050	(15)
[P66614][2-Op]	20	0.1	0.100	(39)

Effect of the Water Content on CO2 Absorption

The industrial flue gas is characterized not only by a high temperature and low CO2 partial pressure but also by the fact that it contains water vapor, so it is also important to study the effect of the water content on the CO2 absorption. As shown in Figure , when the water contents are in a range of 3–10 wt %, the absorption capacity and absorption rate of [DMAPAH][OAc]-EDA-H2O changed slightly with the increase in the water content. When the content of water is 20 wt %, a maximal CO2 absorption of 0.299 g/g is afforded. The appropriate water content can reduce the viscosity of the absorbent and can promote the mass transfer of CO2. The absorbent also gained a higher CO2 capture rate. In terms of absorption products, bicarbonate with a higher thermal stability was produced when water was present, while the anhydrous [DMAPAH][OAc]-EDA generated carbamate, which was the reason for the increased viscosity of the absorbent.[15,40]

Figure 6

Effect of the water content on the CO2 absorption in [DMAPAH][OAc]-EDA-H2O at 30 °C and 1 bar.

Mechanism of CO2 Absorption in Mixed Systems

To investigate the mechanism of CO2 absorption, FTIR was employed to investigate CO2 absorption in [DMAPAH][OAc]-EDA and [DMAPAH][OAc]-EDA-H2O (20 wt %). As shown in Figure a, the new peaks appeared after the absorption of CO2 by [DMAPAH][OAc]-EDA. Concretely, new peaks at 1384 and 1044 cm–1 can be attributed to the C–O stretching vibration in carbamate.[41] The peaks at 1178 and 1429 cm–1 can be attributed to the deformation vibration of C–N and −NH3+,[42,43] respectively. The new peak at 1623 cm–1 is due to the stretching vibration of the C=O bond in the formation of carbamate after the absorption of CO2, and the results suggested that carbamate is formed between the basic nitrogen atom of DMAPA and CO2.[44,45] In addition, the appearance of a broader peak at 3135 cm–1 suggests the possible formation of zwitterions (NH2+COO–) during the absorption of CO2.[46] As shown in Figure b, the characteristic peaks of bicarbonate can be found at 1047 and 1157 cm–1 after capturing CO2 with [DMAPAH][OAc]-EDA-H2O (20 wt %).[15,47] Furthermore, the gradually broadening peaks in a range of 1419–1573 cm–1 are due to the continuous formation of −NH3+ during the reactions.[43]

Figure 7

FTIR spectra of the (a) [DMAPAH][OAc]-EDA and (b) [DMAPAH][OAc]-EDA-H2O systems before and after absorption of CO2.

To further obtain the absorption mechanism of CO2, NMR spectroscopy was employed. As shown in Figure , after [DMAPAH][OAc]-EDA absorbed CO2, a new carbon signal at 167.00 ppm was attributed to the carbonyl carbon in the generated carbamate.[48] In contrast, two peaks at 166.83 and 162.93 ppm were observed after CO2 absorption in [DMAPAH][OAc]-EDA-H2O (20 wt %), the former attributed to carbonyl carbon in carbamate and the latter due to bicarbonate formation.[49−51] These results are in good agreement with FTIR.

Figure 8

13C NMR spectra of the [DMAPAH][OAc]-EDA and [DMAPAH][OAc]-EDA-H2O (20%) systems before and after CO2 absorption.

From FTIR and 13C NMR analyses and combined with experimental results, we have presented a possible CO2 absorption pathway for CO2 absorption by [DMAPAH][OAc]-EDA and [DMAPAH][OAc]-EDA-H2O. As shown in Figure , [DMAPAH][OAc] rearranged automatically into its balanced structure and the protons on the primary amine are transferred to the tertiary amine followed by exposure of the primary amine group and finally combining with EDA to rapidly absorb CO2 and form zwitterions.[16,29,52] If there is a small amount of water in the reaction system, the zwitterions transfer into carbamate and the protonated EDA is generated. In contrast, the transfer rate of the zwitterions is slightly slower than that of the aqueous absorbent due to the absence of water assistance.[27,50] Carbamate can be further hydrolyzed to produce bicarbonate, which usually occurs in the presence of excess water. Furthermore, this step quickly and automatically takes place in the presence of water.[53] It is worth noting that the generation of primary amines by hydrolysis of carbamate makes more primary amines available for CO2 absorption, further illustrating the speedy absorption rate and high CO2 loading of the [DMAPAH][OAc]-EDA-H2O mixture.

Figure 9

Schematic diagram of the proposed CO2 absorption mechanism for the [DMAPAH][OAc]-EDA and [DMAPAH][OAc]-EDA-H2O systems.

Recycling of Absorbents

To study the regeneration performance of absorbents, [DMAPAH][OAc]-EDA-H2O (20 wt %) first absorbed CO2 to reach saturation at 30 °C and 1 bar, and then the mixture of absorbents was desorbed by bubbling N2 at 110 °C for 100 min. It can be seen from Figure that the aqueous [DMAPAH][OAc]-EDA solution can be recycled. After the absorbent can be recycled three times, the absorption of CO2 still exceeds 0.20 g/g. The reduced absorption performance may be due to insufficient desorption of the absorbent and oxidative degradation of EDA.

Figure 10

Recycling of the aqueous [DMAPAH][OAc]-EDA solution (mass fraction of water is 20%).

Conclusions

In conclusion, we demonstrated an effective CO2 absorption using PILs-amine blends as the reversible absorbents. The experimental systems were used to study the effects of the absorbent composition, temperature, pressure, and water content on the CO2 absorption performance. Good CO2 absorption capacity in both the high-temperature and low-pressure range was acquired. Furthermore, TG, FTIR, and NMR have shown that there is a synergistic effect between [DMAPAH][OAc] and EDA for enhancing the absorption of CO2. This work suggests that PILs-amine blends are promising candidates in the treatment of industrial high-temperature CO2 gas.

Experimental Section

Materials

Chemicals of DMAPA (>99.0%), DETA (>99.0%), EDA (>99.0%), acetic acid (>99.0%), and lactic acid (>85%) were purchased from Energy Chemical Technology (Shanghai) Co., Ltd., China. CO2 (>99.99%) and N2 (>99.999%) were supplied by Guiyang Sanhe Special Gas Center, China. All chemicals were used directly without further purification. All mass measurements were performed on an electronic balance with an accuracy of ±0.1 mg (Sartorius BS224S). FTIR spectra were recorded on a Nicolet iS50 FTIR spectrometer. 13C NMR and 1H NMR spectra were recorded on a JNM-ECZ-400 spectrometer. The thermal stability was measured using an STA 449F5 simultaneous thermal analyzer with a heat rate of 10 °C·min–1 in a N2 atmosphere at a flow rate of 20 mL·min–1.

Synthesis and Characterization of PILs

The PILs [DMAPAH][OAc] and [DMAPAH][LA] were prepared by dropping DMAPA into acetic acid or lactic acid at an ambient environment with strong stirring for 24 h, respectively, in a molar ratio of 1:1. PILs need to be vacuum dried at 60 °C for 48 h before use to remove water as much as possible. The structures of amines and PILs are shown in Scheme .

Scheme 1

Structures of Ethanediamine (EDA), Diethylenetriamine (DETA), [DMAPAH][OAc], and [DMAPAH][LA]

Preparation of PILs-Amine Blends

PILs-amine blends were prepared by mixing amine with the desired PILs in a 1:1 mass ratio. Typically, [DMAPAH][OAc]-EDA was obtained by intensive mixing of equal quantity of [DMAPAH][OAc] and EDA at room temperature for more than 24 h. In addition, the absorbents were prepared with various mass ratios of IL and amine as 1:0.5 and 0.5:1. According to the previously reported method,[22] the density and viscosity of the PILs-amine blends were determined by a pycnometer method and a Ubbelohde viscometer method, respectively. We used secondary water and ethanol to calibrate before measuring the absorbents. The above operation was repeated six times, and the difference was no more than 0.2 s each measurement. The average value is T, which is the outflow time of the absorbents. The aqueous solutions of [DMAPAH][OAc]-EDA were also prepared for the study of CO2 absorption, and the mass fractions of water were 3, 10, 20, 40, and 70%, respectively.

Absorption and Desorption of CO2

The absorption experiments were performed in a glass container with an inner diameter of 2 cm, and the installation diagram is shown in Figure . In a typical procedure, CO2 gas was bubbled at a flow rate of about 50 mL·min–1 through the absorbent in a glass vessel (10). The glass vessel was immersed in a water bath (8) at a temperature required. The CO2 uptake was measured periodically with an electronic balance. Solvent loss caused by gas entrainment can be neglected due to the low gas flow rate. The influence of temperature on the CO2 absorption was studied by varying the temperature from 20 to 60 °C at 1 bar. During the absorption of CO2 under reduced pressure, the CO2 partial pressure was controlled by adjusting the flow rates of pure CO2 gas and N2 gas. To study the effect of the water content on CO2 absorption, CO2 was bubbled through the mixed absorbent with different water contents. For the desorption of CO2, absorbents after absorbing CO2 were heated under a N2 atmosphere to release CO2. Reflux condensation avoided the loss of absorbents during regeneration, and the next absorption cycle used the regenerated absorbent directly.

Figure 11

Experimental graph of CO2 absorption. (1) N2 gas cylinder; (2) CO2 gas cylinder; (3) and (4) valve; (5) and (6) gas mass flow-meter; (7) gas premixing tank; (8) water bath; (9) magnetic stirrer; and (10) glass tube with round bottom.