Introduction

Consumption of fossil fuels as the primary energy source significantly affects air pollution and the adverse consequences of climate change1,2. Carbon capture and storage (CCS) has emerged as a viable approach to mitigate these detrimental effects, including but not limited to the greenhouse effect, global warming, acidification of the oceans, and the spread of diseases and pests3,4. The established methods for carbon capture include adsorption5, absorption6, membrane separation7,8, and chemical capture9. The absorption of CO2 is a promising method due to its effectiveness from an economic and operational point of view. Absorption has better long-term performance and a large processing capacity on the industrial scale10,11. Aqueous amine solutions are the most frequently used reversible solvents for CO2 capture in industrial processes12. Monoethanolamine (MEA) aqueous solution is extensively utilized in contemporary industries for CO2 absorption because of its low cost, notable reactivity, high CO2 capture capacity and significant absorption rate13. However, these solvents have some inherent significant drawbacks, such as amine loss due to volatility, environmental issues, high corrosion effects, high energy consumption for the desorption process14,15,16 and degradations at high temperatures. Therefore, it is crucial to find environmentally friendly alternatives to aqueous amine-based solvents.

Ionic liquids (ILs) have been the subject of considerable research on CO2 absorption. This is primarily due to their tunable chemical structure, low vapor pressure, nonflammability, high solvation capacity, thermal stability and potential for utilization at ambient temperature17,18,19. ILs have been found to have applications in various fields, such as organic synthesis, catalysis, separation during extraction and electrochemistry20. Numerous subsequent efforts have been devoted to investigating the solubility of CO2 in ILs. Blanchard et al.21 conducted the first investigation of CO2 absorption by 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) using a high-pressure cell. [Bmim][PF6] absorbed a mole fraction of 0.6 CO2 at a temperature of 25 °C and a pressure of 8 MPa. Bates et al.22 suggested a new kind of amino group-functionalized IL with a 0.5 molar uptake of CO2 per mole of IL at a pressure of 1 atm and temperature of 295 K. Huang et al.23 documented the presence of a chemical reaction between CO2 and a basic ionic liquid (ethyltributylphosphonium succinimido ([P4442][Suc])). This reaction usually leads to the absorption of CO2 with capacities ranging from 0.5 to 1 mol of CO2 per mole of ionic liquid at a temperature of 25 °C and various partial pressures of CO2. However, ILs encountered several limitations that have precluded them from emerging as an optimal candidate for green solvents, including complex and expensive synthesis and the requirement for high purity because the presence of impurities can significantly impair the physicochemical properties of ILs24,25.

More research is also needed to determine if these solvents are environmentally friendly26,27,28,29. To overcome these drawbacks, while maintaining the beneficial characteristics of ILs, a novel class of solvents called deep eutectic solvents (DESs) has been developed.

DESs are formed by combining a hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA). Since DESs can easily be synthesized, they are practical and economical alternatives to ILs. The first DES was synthesized by Abbott et al.30 using choline chloride (ChCl) and urea with the molar ratio of 1:2. Both of these components are biodegradable and non-toxic. Some of recent investigations have focused on the corrosion behavior of DES based on choline chloride, representing ammonium quaternary salts. These studies have recognized the high stability of the ammonium salt, which can maintain its stability without decomposition or deactivation under severe electrochemical conditions and in the presence of negative electrical potentials. Consequently, a considerable body of literature exists on the application of choline chloride-based DESs as corrosion inhibitors in aqueous environments31. Ammonium quaternary salts are the most commonly employed HBAs due to their accessibility, affordability and low toxicity32,33. Another advantage of DESs is that HBD and HBA concentrations may be modified to customize their properties for a specific purpose34,35,36,37. Also, DESs have emerged as a viable substitute for ILs for CO2 absorption38,39,40,41. Li et al.42 effectively synthesized several choline-based DESs and utilized them for CO2 absorption for the first time. They demonstrated that at a pressure of 12.5 MPa and a temperature of 40 °C, ChCl-urea (1:2) absorbed CO2 with a mole fraction of 0.309. Leron et al.43 investigated the impact of varying temperatures and pressures on the CO2 solubility in ChCl-urea (1:2) at temperatures ranging from 303.15 to 343.15 K and pressures of up to 6.0 MPa. They demonstrated that CO2 solubility in ChCl-urea (1:2) DES increased with increasing pressure and decreased with increasing temperature. Additionally, they examined how the molar ratio of salt and HBD affects the solubility of CO2. The solubility of CO2 in various ammonium and phosphonium DESs was examined by Sarmad et al.44 at temperatures of 298.15 K and pressures of up to 2 MPa. They reported that the CO2 solubility of 15 synthesized DESs is higher than that of conventional ILs. It should be mentioned that the renewal of the absorbent in practical applications is important in any absorption process including CO2 capture. Zhang et al.45 demonstrated that after six absorption–desorption cycles, the regeneration efficiency of [TETA]Cl-DG (1:2) and [TETA]Cl-EG (1:3) drops from 100 to 97.5%. Yan et al.46 examined the solubility of CO2 in various superbase IL-based DESs. The 1,8-diazabicyclo-[5,4,0]undec-7-ene imidazole/Ethylene glycol ([HDBU][Im]/EG) with a mass ratio of 7:3, exhibited the maximum CO2 absorption capacity of 0.141 g CO2 per g DES at 100 kPa and 40 °C. Additionally, the CO2 absorption capacity of DES remained stable after five absorption and desorption cycles. Recently, several articles have investigated the effect of viscosity on the solubility of CO2 in DESs47,48. The solvent's viscosity is an important physical property that can substantially impact the mass transfer49. An enhancement in the solvent's capacity to capture CO2 can result from a reduction in viscosity50. Viscosity, also, impacts the energy needed to manufacture and move materials51. Temperature, kind of HBA and HBD, and their respective molar ratios all affect the viscosities of DESs52. The viscosity of DES increases during absorption, resulting in a decrease in the absorption rate46,49. Some studies have documented the viscosities of amine-based DESs in their pure form and the viscosities of the DESs after CO2 absorption45,53. For a solvent to be considered suitable in the gas absorption industry, multiple factors beyond absorption capacity must be evaluated. For solvents with relatively low toxicity, considerations include the potential for long-term use, the absence of solvent loss, the energy required for solvent recovery, and the regenaration efficiency after multiple absorption and desorption cycles. Aqeous amine solutions which chemically absorb CO2, present challenges such as low biodegradability, volatility, and high energy requirements for regeneration, which are costly and environmentally detrimental.

Our study is focuses on experimentally investigating the utilization of tetrapropylammonium bromide (TPAB) and a naturally occurring carboxylic acid to form a DES for CO2 capture. To accomplish this goal, two DESs were synthesized employing TPAB as a hydrogen bond acceptor (HBA) and formic acid (Fa) as a hydrogen bond donor (HBD) in molar ratios of 1:1 and 1:2. The presence of hydrogen bond between TPAB and Fa, and physical absorption of CO2 were confirmed through FT-IR and NMR spectra. Experiments were conducted at the temperatures of 25.0, 35.0 and 45.0 °C and pressures of approximately up to 35.000 bar. The impact of variations in pressure, temperature, and viscosity on CO2 absorption was investigated. Using the CO2 solubility data, Henry’s law constant and the enthalpy of dissolution were obtained. To model the vapor–liquid equilibrium of the CO2-DES system, the Peng–Robinson equation of state (PR EOS)54 and the nonrandom two liquid (NRTL)55 activity coefficient model were employed. Furthermore, five cycles of regeneration experiments were performed on the DES with better performance under vacuum and at 65.0 °C condition.

Experimental procedure

Materials

The substances used in this study and their corresponding molecular structures, sources, and purities are presented in Table 1. They were used as received.

Table 1 Names of chemicals and their purities used in this study.
Full size table

Synthesis of DESs

The DESs were synthesized by the heating method. HBA and HBD were mixed at a precise molar ratio and temperature. TPAB and Fa were introduced into a stainless steel two-shell reactor autoclave. Two DESs were made with the molar ratios of TPAB-Fa (1:1) and TPAB-Fa (1:2) using a balance (Precisa XT220A with the precision of ± 10−4 g). The mixture was then agitated using a magnetic stirrer (Fisher, Cat. No. 14–511-113) and heated using a circulating water bath (LAUDA Alpha RA 8, 248–373 K) for five hours at 70.0 °C. The obtained liquid was a clear and uniform phase.

CO2 absorption experimental setup

The equipment used for the absorption experiments, which is shown in Fig. 1, is the same as used in the author’s previous publications56,57,58,59. It is comprised of a CO2 gas cylinder, a gas container (182 ml) connected to the CO2 cylinder via valve V1, an autoclave reactor (37 ml) connected to the gas container through valves V4 and V6, a magnetic stirrer (Fisher, Cat. No. 14–511-113), a circulating water bath (LAUDA Alpha RA 8, 248–373 K) for temperature adjustment, temperature sensors (TS1, TS2) (K-type, with the precision of ± 0.1 K), and pressure sensors (PS1, PS2) (Sensys, Model: M5156-11700X-050BG, with the precision of ± 2.5 kPa). These sensors measure and control the temperature and pressure of the gas container and autoclave reactor. They are connected to a computer for data analysis, display, and storage. V2, V7, and V8 are the valves that release CO2 from the system. V3 and V5 valves connect the gas container and autoclave reactor to PS1 and PS2.

Fig. 1
figure 1

Schematic of the experimental setup for CO2 absorption–desorption: (1) CO2 cylinder, (2) gas container, (3) autoclave reactor, (4) magnetic stirrer, (5) circulating water bath, and (6) computer.

Full size image

FT-IR, NMR, and viscosity analyses

To investigate the hydrogen bond interaction between HBA and HBD and the production of DES, we employed analytical techniques such as FT-IR (Perkin Elmer Spectrum RX1, Canada) and NMR (Bruker DRX-500, operating at 500 MHz). Furthermore, to determine the absorption mechanism, these investigations were carried out before and after CO2 absorption. The viscosity of the eutectic solvent was measured using the dynamic shear rheometer (DSR) SmartPave 102e at temperatures of 25.0 and 45.0 °C for each molar ratio of DES.

CO2 absorption and desorption experiments

A precise amount of DES was injected into the autoclave reactor, then, the gas container and the autoclave reactor were purged of gases using a vacuum pump. CO2 was supplied into the gas container from the cylinder. After the temperature and pressure of the gas container were fixed, the quantity of CO2 entering the gas container (\(n_{{co_{2} }}^{e}\)) was determined by applying Eq. (1). Subsequently, CO2 was delivered into the autoclave reactor. Equations (2), (3) were employed to calculate the number of moles of CO2 that remained in the CO2 container (\(n_{{co_{2} }}^{r}\)) and entered into the autoclave reactor (\(n_{{co_{2} }}^{g}\)).

$${n}_{{CO}_{2}}^{e}=\frac{{P}_{e}{V}_{gc}}{{Z}_{{CO}_{2}}^{e}R{T}_{e}}$$
(1)
$${n}_{{CO}_{2}}^{r}=\frac{{P}_{r}{V}_{gc}}{{Z}_{{CO}_{2}}^{r}R{T}_{r}}$$
(2)
$${n}_{{CO}_{2}}^{g}={n}_{{CO}_{2}}^{e}-{n}_{{CO}_{2}}^{r}=\frac{{V}_{gc}}{R}(\frac{{P}_{e}}{{Z}_{{CO}_{2}}^{e}{T}_{e}}-\frac{{P}_{r}}{{Z}_{{CO}_{2}}^{r}{T}_{r}})$$
(3)

In the above equations P, T, and Z represent pressure, temperature, and compressibility factor, respectively. Superscripts and subscripts e and r refer to entering and remaining in the gas container, respectively. \({n}_{{CO}_{2}}^{g}\) denotes the number of gas molecules entering the autoclave reactor. R and \({V}_{gc}\) represent the universal gas constant and the volume of the gas container, respectively. Equations (4), (5) were employed to calculate the amount of CO2 molecules absorbed in the DES (\({n}_{{CO}_{2}}^{l}\)) and the amount that remained in the autoclave reactor after reaching equilibrium (\({n}_{{CO}_{2}}^{eq}\)). Superscript eq refers to the phase equilibrium condition, \({V}_{DES}\) refers to the volume of the solvent and V is the volume of the autoclave reactor.

$${n}_{{CO}_{2}}^{l}={n}_{{CO}_{2}}^{g}-{n}_{{CO}_{2}}^{eq}$$
(4)
$${n}_{{CO}_{2}}^{eq}=\frac{{P}_{{CO}_{2}}^{eq}(V-{V}_{DES})}{{Z}_{{CO}_{2}}^{eq}R{T}_{{CO}_{2}}^{eq}}$$
(5)

All the compressibility factors were calculated by the PR EOS54.

CO2 absorption investigations were conducted at three temperatures (25.0, 35.0, and 45.0 °C) and, as mentioned before, two TPAB to Fa molar ratios of 1:1 and 1:2. Six equilibrium pressures (ranging from 1.650 to 35.125 bar) were determined at each temperature for each TPAB to Fa molar ratios. Five regeneration cycles were conducted at 65.0 °C and atmospheric pressure for the best TPAB to FA ratio, which is specified in the next sections. The efficiency of solvent regeneration (\({\eta }_{reg}\)) was determined using Eq. (6) in which the number of moles of CO2 absorbed during the i-th regeneration and the initial absorption are represented by the variables \({n}_{i}\) and \({n}_{0}\), respectively.

$$\eta \%=\frac{{n}_{i}}{{n}_{0}}$$
(6)

CO2 absorption thermodynamic modeling

The γ-φ approach was adopted for thermodynamic modeling. The solubility of CO2 was determined by using the NRTL55 model in conjunction with the PR EOS. The PR EOS54 is presented by Eqs. (7), (8), (9), (10).

$$P=\frac{RT}{v-b}-\frac{a}{v\left(v+b\right)+b(v-b)}$$
(7)
$$a=\frac{0.45724{(R{T}_{C})}^{2}}{{P}_{C}}{\left[1+m(1-{{T}_{r}}^{0.5})\right]}^{2}$$
(8)
$$b=0.0778\frac{R{T}_{C}}{{P}_{C}}$$
(9)
$$m=0.37464+1.5422\omega -0.2699{\omega }^{2}$$
(10)

where a is the parameter representative of attractive forces between molecules and b denotes the co-volume parameter. v, \({T}_{C}\), \({P}_{C}\), \(\omega\), and \({T}_{r}\) are the volume, the critical temperature, the critical pressure, the acentric factor, and reduced temperature, respectively. Equations (11), (12), (13) represent the NRTL55 activity coefficient model.

$$ln{\gamma }_{i}=\frac{{\sum }_{j}{x}_{j}{\tau }_{ji}{G}_{ji}}{{\sum }_{k}{x}_{k}{G}_{ki}}+{\sum }_{j}\frac{{x}_{j}{G}_{ji}}{{\sum }_{k}{x}_{k}{G}_{ki}}+\left({\tau }_{ij-}\frac{{\sum }_{m}{x}_{m}{\tau }_{mj}{G}_{mj}}{{\sum }_{k}{x}_{k}{G}_{kj}}\right)$$
(11)
$${G}_{ij}=\text{exp}(-\alpha {\tau }_{ji})$$
(12)
$${\tau }_{ji}=\frac{{b}_{ji}}{RT}$$
(13)

\(\alpha\) and \(b_{ji}\) are the non-randomness and binary interaction parameters, respectively.

Results and discussion

Structure of DESs

FT-IR analysis of synthesized DESs

Figure 2 displays the FT-IR spectra of Fa, TPAB, TPAB-Fa (1:1), and TPAB-Fa (1:2). The spectra of Fa exhibits two peaks at 2941 and 3106 cm−1, which are attributed to the –OH stretching vibration29,60. In contrast, the spectra of TPAB shows peaks at 2870, 2926, and 2963 cm−1, which are related to the -CH stretching vibration61. After blending the substances, the spectral peaks at 2941 cm−1 and 3106 cm−1 of Fa and the peak at 2926 cm−1 of TPAB are eliminated for both DESs. The removal can be attributed to the establishment of hydrogen bonds between TPAB and Fa. Furthermore, the TPAB peaks at 2870 and 2963 cm−1 have shifted to 2960 and 2975 cm−1, respectively, with decreased intensity. This phenomenon can be attributed to a modification in the intensity of the hydrogen bonds. The combination of two separate peaks at 1458 and 1487 cm−1 in the TPAB spectrum into a solitary peak at 1476 cm-1, detected in the spectra of both DESs, provides more insight into the creation of hydrogen bond62.

Fig. 2
figure 2

FT-IR spectra of HBA, HBD, and DESs. (a) Fa, (b) TPAB, (c) TPAB-Fa (1:1), (d) TPAB-Fa (1:2).

Full size image

NMR analysis

The 1HNMR and 13CNMR spectra of HBA, HBD, and DES were obtained using deuterium oxide (D2O) as the solvent. Figure 3 displays the 1HNMR spectra of Fa, TPAB, and TPAB-Fa (1:2). TPAB-Fa (1:2) was subjected to 13CNMR and 1HNMR analysis as the superior DES for CO2 absorption. Upon mixing, it is evident that the peak of Fa at δ = 7.41 ppm, associated with the –CH group, has shifted to δ = 8.11 ppm, showing a change of the C–H bond in Fa as an HBD. However, the change in the peak at δ = 4.85 ppm, related to D2O, is insignificant63. The peak related to OH has been removed due to the presence of D2O in the system. Fa and TPAB have formed a hydrogen bond connection using the hydrogen atom in the OH group of Fa. Furthermore, the TPAB exhibits peaks at δ = 3.11, 3.17, and 1.72 ppm, which have shifted to 2.98, 1.51, and 0.77 ppm, respectively. This shift indicates that the C–H stretching of TPAB, as a hydrogen bond acceptor, has changed. Based on the explanations provided, it can be deduced that Fa and TPAB effectively carried out their roles as the HBD and HBA, respectively. A primary concern in application of the resulting eutectic solvent in gas absorption, is its potential release into the environment. NMR and FTIR analyses of the raw materials and the DES indicate that hydrogen bonds form between the raw materials without any chemical reactions. This characteristic reduces the solvent's volatility that significantly diminishes the risk of atmospheric dissemination.

Fig. 3
figure 3

1HNMR spectra of Fa, TPAB, TPAB-Fa (1:2).

Full size image

CO2 solubility

The results of CO2 solubility at three different temperatures, two molar ratios of HBA to HBD, and six pressures are shown in Fig. 4. It is observed that the molar fraction of CO2 increases by increase in pressure and decreases with increasing temperature. Initially, the impact of the HBA to HBD molar ratio was examined, which showed that the CO2 solubility of TPAB-Fa (1:2) was higher than that of TPAB-Fa (1:1) at the same temperature. The TPAB-Fa (1:2) had the maximum CO2 absorption (\({x}_{{CO}_{2}}\)) at pressures higher than 30.000 bar and temperatures of 25.0, 35.0, and 45.0 °C, with values of 0.218, 0.156, and 0.137, respectively, whereas the TPAB-Fa (1:1) showed solubility values of 0.169, 0.147, and 0.127 at the same temperatures. Higher absorption in the TPAB-Fa (1:2) could be attributed to the existence of more location for CO2 absorption because of one more –OH group, as shown by the 1HNMR study, which also revealed that TPAB and Fa had a hydrogen bond from the –OH side of Fa.

Fig. 4
figure 4

Experimental mole fraction of absorbed CO2 in two molar ratios of DES at 25.0, 35.0, and 45.0 °C and pressures up to 35.000 bar.

Full size image

Furthermore, the justification for this observation can be the viscosity of DESs. As mentioned previously, the viscosity of any solvent plays a crucial role in absorption. The viscosity values of TPAB-Fa (1:2) at 25.0 and 45.0 °C are 53.98 and 21.62 mPa·s, respectively, whereas the viscosity of TPAB-Fa (1:1) are 946.16 and 222.55 mPa·s at the same temperatures. As the HBD ratio increased, the eutectic solvent viscosity decreased.

Our results showed that the solvation of CO2 in the DES with the higher HBD to HBA ratio was higher at the same temperature. In other words, the relationship between viscosity and temperature is inversely proportional. Higher temperatures lead to less absorption, whereas lower viscosities increase the solubility of CO2. At 25.0 and 35.0 °C temperatures, TPAB-Fa (1:1) demonstrated more capture than TPAB-Fa (1:2) at temperatures of 35.0 and 45.0 °C, respectively. This suggests that the impact of temperature outweighs the impact of viscosity. Figure 4 also demonstrates that the difference in CO2 absorption at varying temperatures under low pressures is negligible.

Nevertheless, with increasing pressure, the effect of temperature on absorption becomes more significant. Desorption of CO2 may be done by lowering pressure to a vacuum and relying less on raising temperature to high levels. The desorption operation can make advantage of this observation to achieve high desorption efficiency with less energy consumption.

The absorption process mechanism was identified by FT-IR and 13CNMR analyses. Figure 5 presents the FT-IR analysis of DESs before and after CO2 absorption. A small new peak at 2200 cm−1 is observed after absorption in TPAB-Fa (1:1); indicating the presence of asymmetric O = C = O stretching in the eutectic solvent. In addition, considerable peaks are seen at 3600–3800 cm−1, identifying CO2 combination bands64,65. These combination bands arise from the interactions between different vibrational modes, enhancing the utility of FTIR spectroscopy in studying and quantifying CO2 in diverse applications65. No substantial change is observed in peaks of TPAB-Fa (1:2) after absorption. This may be due to the low viscosity of the DES, which causes CO2 to desorb quickly before FT-IR and NMR analyses can be conducted, also indicating no chemical reactions between the DES and CO2. This issue was further validated by the 13CNMR analysis depicted in Fig. 6. It is evident that after absorption, both the number and quantity of 13CNMR peaks are not changed in TPAB-Fa (1:2), suggesting that the CO2 was absorbed physically66. We may reach to the conclusion that it is due to the physical nature of the absorption mechanism that absorption amount increases with increasing pressure.

Fig. 5
figure 5

FT-IR analysis of DES before and after absorption: (a) before absorption, (b) after absorption, (1) TPAB-Fa (1:1), (2) TPAB-Fa (1:2).

Full size image
Fig. 6
figure 6

13CNMR analysis of TPAB-Fa (1:2), (a) before absorption, (b) after absorption.

Full size image

Thermodynamic modeling of vapor–liquid equilibrium

The DES-CO2 phase equilibrium was correlated using PR EOS54 and NRTL model55 for the vapor and liquid phases, respectively. It was assumed that the experimental vapor pressure of DES is negligible. Hence, the vapor phase consists of pure CO2. Henry’s law constant was calculated using the slope of the fugacity-mole fraction diagram of experimental data at points where the mole fraction was less than 0.08. The \(\gamma\)-\(\varphi\) approach was used to fit the NRTL55 binary interaction parameters. The value of \(\alpha\) was set to 0.3. Parameters were adjusted using the MATLAB67 software (version 2022b 1.0.0.1) for both DESs at each temperature. The objective function (OF) was defined by Eq. (14).

$$\text{OF}\boldsymbol{\%}=\mathit{min}\left(\frac{\sum_{i=1}^{n}(\left|{x}_{iexp}-{x}_{ical}\right|)}{n}\right)\times 100$$
(14)

where \(n\) represents the number of data points. \({x}_{exp}\) and \({x}_{cal}\) denote the experimental and calculated mole fractions, respectively. Table 2 introduces the Henry’s law constants, OF, \(({b}_{ij},{b}_{ji})\).

Table 2 Calculated Henry’s law constant and NRTL parameters for the DES-CO2 system, assuming α = 0.3, component j is CO2.
Full size table

In Fig. 7, the modeling results of the \(\gamma\)-\(\varphi\) approach and the experimental solubility data are compared. As it is observed, the calculation results are extremely close to the experimental data.

Fig. 7
figure 7

Calculated and experimental mole fractions as a function of pressure at different temperatures: (a) TPAB-Fa (1:1) at 25.0 °C, (b) TPAB-Fa (1:1) at 35.0 °C, (c) TPAB-Fa (1:1) at 45.0 °C, (d) TPAB-Fa (1:2) at 25.0 °C, (e) TPAB-Fa (1:2) at 35.0 °C, (f) TPAB-Fa (1:2) at 45.0 °C.

Full size image

Desorption of CO2

Critical considerations when identifying stable solvents for industrial applications require the degree to which the solubility decreases after a series of absorption–desorption steps. Based on the data presented in Fig. 4, there is no significant difference in CO2 absorption at low pressures between 25.0 and 45.0 °C. Therefore, it can be inferred that in the desorption process, which occurs at higher temperatures and lower pressures, the temperature increase does not play a significant role in desorption compared to the pressure decrease. In contrast to previous studies that considered a desorption temperature of 100°C66,68,69, this study reached high desorption efficiency at approximately vacuum pressure and a temperature of 65.0 °C. This practical study demonstrates high desorption efficiency at a temperature considerably lower than other studies. The mole fraction of CO2 in TPAB-Fa (1:2) is shown in Fig. 8 after six successive absorptions at 25.0 °C and desorption at 65.0 °C in vacuum condition. During the initial absorption after regeneration, the solvent’s efficiency decreased by approximately 1% at the maximum pressure. The amount of experimental CO2 absorption during the second to fifth cycle, at the same pressures as the first absorption–desorption cycle, was determined using regression between values of pressure. However, the solvent’s efficiency remained relatively stable in the subsequent four cycles. Table 3 displays regeneration efficiency (\({\eta }_{reg}\)) of DESs at each cycle. At low pressures, the percentage was approximately 90–95%. However, as the pressure increased, the percentage also increased. Eventually, at the highest pressure, regeneration efficiency was 99%, demonstrating the low volatility and high stability of the resulting DES. Additionally, the high reversibility of this DES minimizes the need for replenishment.

Fig. 8
figure 8

TPAB-Fa (1:2) absorption–desorption cycle: absorption at 25.0 °C and approximately 2.000–35.000 bar, desorption at 65.0 °C and under vacuum.

Full size image
Table 3 Regeneration efficiency (%) at 65.0 °C of TPAB-Fa (1:2) for each cycle of DES recovery at the six pressure steps.
Full size table

Comparison to other DESs

Table 4 presents the solubility of CO2 in several DESs at varying pressure and temperature ranges compared to the DESs utilized in this study. The findings indicated that TPAB-Fa (1:2) and TPAB-Fa (1:1) outperformed most other solvents. Moreover, it should be taken into account that formic acid is an affordable organic acid with minimal risks to humans and environment due to its natural character. Similarly, TPAB is a cost-effective quaternary ammonium salt. Consequently, the chemicals used in preparing these DESs are more economical than most commonly employed substances in other DESs.

Table 4 Comparison of CO2 solubility range of several well-known DESs with the DESs employed in this work.
Full size table

To determine the energy required for the process and the subsequent recovery of the solvent, it is necessary to estimate the enthalpy of solvation (ΔHsol), defined as the strength of the intermolecular interaction between DES and CO2. The ΔHsol was determined at a fixed mole fraction (\({x}_{{CO}_{2}}=0.1\)) using the Clausius–Clapeyron equation at 25.0 to 45.0 °C. The results are presented in Table 5. This is then compared to several other DESs and solvents.

Table 5 The partial molar enthalpy of CO2 solution in DESs employed in this study, other DESs, and various physical and chemical solvents across temperature and pressure ranges.
Full size table
$${\Delta H}_{sol}=R{\left(\frac{\partial \text{ln}P}{\partial \left(\frac{1}{T}\right)}\right)}_{{x}_{{CO}_{2}}}$$
(15)

Due to the negative values of ΔHsol, the absorption process of DESs is exothermic. This is the reason why the absorption amount decreases with increasing temperature. While the values of ΔHsol are within the range of other physical and chemical solvents, they are notably lower than aqueous MEA solution. This behavior suggests a weaker interaction between CO2 and DES molecules, resulting in improved regeneration capability for the DESs used in this study. In other words, the DES requires less energy for the regeneration process, which leads to a reduction in the consumption of non-renewable energy sources and a reduction in the environmental impact caused by their consumption.

Conclusions

This research aimed to investigate the CO2 absorption capacity of TPAB-Fa (1:1) and TPAB-Fa (1:2) DESs at three different temperatures (25.0, 35.0, and 45.0 °C) and pressures up to more than 35.000 bar. The FT-IR spectra validated the hydrogen bond between HBA and HBD and confirmed the physical absorption of CO2 in DES. TPAB-Fa (1:2) demonstrated the highest CO2 solubility (\({x}_{{CO}_{2}}=0.218\)) at 25.0 °C and the pressure of 35.200 bar. The solubility of CO2 displayed an inverse relation with temperature and viscosity while exhibited a direct relation with pressure. The NRTL activity coefficient model and PR EOS accurately modeled the experimental CO2 solubility data. Henry’s law constant was obtained from experimental data at each temperature. Its minimum value of 15.09 MPa was related to TPAB-Fa (1:2) + CO2 mixture at 25.0 °C, showing the maximum solubility. The TPAB-Fa (1:2) solvent was regenerated five times under vacuum conditions at 65.0 °C, resulting in a marginal 1% decrease in efficiency. The Clausius–Clapeyron equation was employed to calculate the ΔHsol. According to the values of ΔHsol, the exothermic nature of CO2 absorption was proved. We may conclude that this study has introduced an environmentally sustainable DES which is distinguished by its cost-effectiveness, higher absorption efficiency, and good recyclability.