Incorporation of Functionalized Amino Acid Ionic Liquid into Highly Porous MOF-177 to Improve Post-Combustion CO₂ Capture Capacity

1. Introduction

Metal-organic-frameworks, often known as MOFs, are a unique class of porous materials which are currently the subject of substantial research for a broad variety of applications [1,2], notably the storage and separation of gases, catalysis, and the delivery of drugs. MOFs are typically three-dimensional structures that have a high surface area and porosity. These structures are formed from metal nodes that are coupled with organic linkers. MOFs have lately gained an enormous amount of popularity for applications in CO₂ capture and storage because of their high porosity as well as their great potential for capture and, most significantly, due to their tunability [2,3,4,5,6]. Although some MOFs have displayed superb adsorbance of CO₂ at high pressure, at post-combustion conditions, in which the partial pressure of CO₂ is in general only about or lower than 0.15 bar, they showed very low CO₂ uptake. To enhance CO₂ capture capability, many strategies have been deployed including open metal sites [7], grafting amine functionalities covalently to the ligands [8,9,10], impregnation of amines [11,12,13,14], and the addition of ionic liquids (ILs) [15]. Physical impregnation of amines led to higher CO₂ uptake compared to covalent grafting of amines as more amine loading can be achieved by impregnation. Nonetheless, amine-impregnated sorbents are susceptible to amine loss, degradation, and high regeneration energy [16]. Ionic liquids (ILs) as an attractive alternative to amines have been proposed partly due to their low volatility and high thermal stability, however, it has been reported that in some cases, CO₂ uptake rather diminishes for ILs modified sorbents although improved selectivity was observed [17,18,19].

The first amine-functionalized ionic liquids (AILs) were developed by Bates et al. [20], demonstrating that the CO₂ sorption capacity of ILs can be substantially enhanced by functionalizing the sorbent with amine groups (-NH₂) while at the same time retaining IL properties. Since then, numerous AILs referred to as task-specific ionic liquids (TSILs) have been reported in the scientific literature. Among them, amino acid anion-functionalized ionic liquids (AAILs) have the clear advantages of an easy synthesis process, low cost, biodegradability, and using environmentally friendly raw materials as they are sourced from naturally occurring amino acids (AAs) [21,22,23]. As a result, AAILs are deemed promising candidates to functionalize porous MOF and lead to advanced sorbents with high CO₂ capture capacity. Several researchers have demonstrated that immobilization of AAILs into solid sorbents such as MCM-41, SBA-15 and PMMA led to an increase in carbon dioxide adsorption [24,25,26,27].

Recently, we showed that AAILs containing reactive Amino Acid (AA) anions such as Glycine [Gly] and Alanine [Ala] functioned as effective guest molecules in solid ZIF-8 support, significantly increasing CO₂ uptake and CO₂/N₂ selectivity [28]. Hence, we aim to expand our study to investigate new composites by impregnating these AAILs into MOF-177 as host sorbent, which is one of the highest porous MOFs reported to date with a pore volume of 1.69 cm³/g and a BET-specific surface area of over 4000 m²/g [29,30]. MOF-177 displays a 3D crystalline framework consisting of zinc metal clusters and 1, 3 , 5-benzene tribenzoate (BTB) organic linkers. Owing to its high porosity and volume, it displayed outstanding CO₂ uptake performance (60.8 wt.%) at high pressure (50 bar); however, CO₂ uptake at low pressure (0.15 bar) was very poor (0.6 wt.%) [16]. This led us to embark on this study and incorporate AAILs into highly porous MOF-177 forming AAILs@MOF-177 composites with improved active sorption sites, hopefully leading to a higher potential for CO₂ capture at post-combustion conditions. We are unaware of any prior work involving these composites. In this study, we investigated the composites and compared them to pure MOF-177 in terms of thermal stability, crystal structure, and textural qualities. The produced composites’ potential in real-world CO₂ capture operations was investigated by investigating their CO₂ capture capacity, selectivity, and enthalpy of adsorption.

2. Results and Discussion

2.1. Characterizations of the AAILs-impregnated Sorbents

The thermogravimetric analysis of MOF-177 and AAILs@MOF-177 composites was performed to monitor the thermal stability of AAILs-supported MOF-177 composites from 25 to 800 °C in a nitrogen atmosphere. TGA thermograms of composites along with those of pure AAILs and pristine MOF-177 were presented in Figure 1a–d. The thermograms of pristine [Emim][Gly] and [Emim][Ala] indicate that both AAILs were thermally stable up to 200 °C, but beyond that temperature, a sharp weight loss was observed, indicating a very rapid decomposition. According to the derivative weight loss profile shown in Figure 1b,d, it was found that the onset decomposition temperatures (T_onset) were at about 215 °C and 225 °C, respectively. Whereas the pristine solid support MOF-177 was much more thermally stable than the pristine AAILs, with onset decomposition temperatures (T_onset) of about 421 °C in agreement with similar studies reported in the literature [30,31]. A little decrease in weight of around 2 to 4 wt.% was found in the composites at temperatures under 75 °C. This drop can be attributed to the potential evaporation of any residual methanol (BP 65 °C) that was employed during the synthesis of the composites. The thermogram of the composites revealed that the incorporated AAILs decomposed before the pristine solid support itself, as expected. However, note that there was no sharp decomposition of AAILs around the T_onset of AAILs, rather a gradual decomposition over the temperature range of 250 to 400 °C, and continued until MOF-177 itself started to decompose as displayed in Figure 1b,d. The gradual decomposition was more profound at higher loadings of AAILs such as 30-[Emim][Gly]@MOF-177 (Figure 1b) and 30-[Emim][Ala]@MOF-177 (Figure 1d). This progressive breakdown event could be attributed to the surface association that took place between MOF-177 and the incorporated AAILs.

In an attempt to determine the effect that encapsulated AAILs had on the solid support of MOF-177, the crystal structure of the composites was investigated utilizing X-ray diffraction (XRD), and the results of this investigation were illustrated in Figure 2. From the diffractogram of pristine MOF-177, the main characteristic peaks were at 5.5°, 6.2°, 9.6°, 10.4°, and 11.3°. A similar pattern was reported in the literature by Li and Yang [32], with a major peak at 5.2°, at 4.7° and 6.2 ° by Saha and Deng [31] and at 5.5° and 6.2° by Santos et al. [33]. As displayed in Figure 2a,b, the XRD of the composites revealed that the impregnation of AAILs into the MOF-177 support had a significant impact on peak intensity and position. For a small loading of 10 wt.% for both [Emim][Gly] and [Emim][Ala], the prominent characteristic peaks were at 5.5°, 6.2°, 9.6°for pristine MOF-177 and have therefore significantly declined but peaks at 11.1°and 11.7° retained their intensity with a minor shift from the original positions. It was evident that the intensity of major peaks diminished with the increment of AAILs loading and eventually almost disappeared at a loading of 30 wt.%, as displayed for both 30-[Emim] [Gly]@MOF-177 and 30-[Emim][Ala]@MOF-177, suggesting some morphological changes. The existence of the guest molecules and their interaction with the host solid support account for the observed behavior. A similar complete disappearance of all the major characteristic peaks of MOF-177 due to the presence of chloroform was reported by Saha and Deng [31] and according to the authors, the occupying guest molecules in the pore of MOF-177 caused a shift in the atomic orientation in the crystal planes. The TGA thermograms discussed in the preceding section confirmed that there was a significant interaction between AAILs and MOF-177 support. Overall, the XRD data indicated the occurrence of morphological changes in AAILs@MOF-177 composites, especially for a loading higher than 20 wt. %.

To further assess the impact of the incorporation of AAILs on the textural properties of MOF-177 support, N₂ adsorption-desorption isotherms were obtained under a liquid nitrogen environment (77 K) using the ASAP 2460 (Micromeritics) for the unmodified MOF-177 and AAILs@MOF-177 composites, and the isotherms were presented in Figure 3. The samples’ specific surface areas were determined at relative pressures (P/P0) between 0.04 and 0.1 bar using the Brunauer-Emmett-Teller (BET) model. The Langmuir correlations and calculated results are presented in Table 1. The pristine MOF-177 showed significantly higher N₂ uptake, and the isotherm profile corresponded to a reversible Type I isotherm (IUPAC classification) which was indicative of a microporous material lacking hysteresis during desorption. BET and Langmuir surface areas of pristine MOF-177 were 4172 m²·g^-1 and 4962 m²·g^-1, respectively, which were close to the reported values by Yaghi’s group [29]. Both BET and Langmuir surface areas of MOF-177 reported in the literature varied significantly from 3100 – 4962 m²·g^-1 and 4300 – 5994 m²·g^-1, respectively [29,31,32,34]. It is observed from Figure 3 that N₂ adsorption in all composites was significantly lower than in the pristine MOF-177 which was evidence of a substantial drop in surface area as well as pore volume (Table 1). For instance, upon impregnation of 10 wt.% [Emim][Gly] and [Emim][Ala], BET surface area as reduced to only 187 m²·g^-1 and 152 m²·g^-1, respectively. The surface area and pore volume of the composites were further reduced with the addition of AAILs. The BET surface areas and pore volumes obtained for a 30 wt.% loading of [Emim][Gly] have been estimated to be 27 m²·g^-1 and 0.02 cm³·g^-1, respectively. These results indicate that the pores were almost filled. It should be acknowledged that the [Emim][Gly]@MOF-177 composites exhibited somewhat greater surface area and pore volume compared to the [Emim][Ala]@MOF-177 composites. The reduction of surface area and pore volume confirmed that the AAILs were encapsulated into the pores of MOF-177. However, the severe decline in both surface area and pore volume especially for higher loading of AAILs can be attributed to possible structural and morphological changes due to the collapse of some of the MOF-177 structure which was also apparent in the XRD results discussed earlier.

2.2. CO₂ Adsorption Isotherms

MOF-177 is one of the highest porous MOFs reported to date in the open literature, and as a result, it has higher adsorption capacity at high pressure. However, its adsorption capacity at post-combustion conditions (up to 0.15 bar) was very low [16]. The current investigation involved the synthesis of composites including [Emim][Gly]@MOF-177 and [Emim][Ala]@MOF-177. These composites were prepared by introducing two amino acid anion-functionalized ionic liquids (AAILs), commonly referred to as task-specific ionic liquids (TSILs), into the MOF-177 using the wet impregnation method. These composites’ equilibrium CO₂ uptakes were determined at 30, 40, and 50 °C. Each isotherm covered pressures between 0.1 and 10 bar. Figure 4 shows the equilibrium CO₂ uptake in pure MOF-177 and [Emim][Gly]@MOF-177 for pressure spanning from 0.1 to 10.0 bar (Figure 4a,c,e) and a narrower pressure range (Figure 4b,d,f) spanning from 0.1 to 1.0 bar. The addition of [Emim][Gly] to MOF-177 increased its ability to adsorb CO₂ at low pressures between 0.1 and 1.0 bar. When compared to pristine MOF-177 and all other [Emim][Gly]@MOF-177 composites at all temperatures, CO₂ adsorption capacity increased dramatically with the incremental addition of AAILs loading and peaked at a loading of 20 wt%. At 0.2 bar and 303 K, the CO₂ adsorption for 20-[Emim][Gly]@MOF-177 was 0.45 mmol·g^-1 solid, which was three times that of the pristine MOF-177 under the same circumstances. However, increasing the TSIL amount to 30 wt.% failed to result in a noticeable rise in CO₂ uptake; rather, a decrease in CO₂ uptake to 0.26 mmol·g^-1-solid was observed when compared to 20 wt.% loading, even though it was a bit higher when compared to pure MOF-177 at the same conditions. It is noteworthy that the carbon dioxide absorption capacity of all composites consisting of [Emim][Gly]@MOF-177 exhibited a decrease in performance at the given temperature, in comparison to the pure MOF-177, when the pressure exceeded 2 bar. CO₂ uptake by all composites decreased when the temperature was elevated from 30 °C to 40 °C and 50 °C while maintaining the same pressure.

Figure 5 shows the equilibrium CO₂ uptake in the pure MOF-177 and [Emim][Ala]@MOF-177 for pressure spanning from 0.1 to 10.0 bar (Figure 5a,c,e) and a narrower pressure range (Figure 5b,d,f) spanning from 0.1 to 1.0 bar. It demonstrated that the incorporation of [Emim][Ala] had a favourable CO₂ adsorption capability for pressure below 2 bar and followed exactly a similar trend of [Emim][Gly]@MOF-177 composites. 20-[Emim][Ala]@MOF-177 outperformed all other [Emim][Ala]@MOF-177 composites and pure MOF-177 in CO₂ capture capacity. This composite reached up to 0.42 mmol·g^-1 solid at 0.2 bar and 303 K, which was nearly three times higher compared to the unaltered MOF-177 under the same conditions. When the loading of [Emim][Ala] was raised to 30 wt.%, CO₂ capture capacity dwindled, and the resulting values were in the middle of the range for capture capacity at 10 and 20 wt.% loading. It was worth noting that under identical temperature and pressure, the CO₂ adsorption capacity of [Emim][Ala]@MOF-177 composites was slightly lower than the [Emim][Gly]@MOF-177 composites with the same AAIL loading.

The introduction of [Emim][Gly] and [Emim][Ala] AAILs to the MOF-177 sorbent led to a notable enhancement in CO₂ adsorption under post-combustion circumstances (P_CO2 ≈ 0.15 bar) due to the strong attraction between CO₂ and the amino group present in AAILs. Previous research indicated that the amino group of AAILs reacted with CO₂ via a process analogous to that of amines dissolved in water [20,35,36]. Wang et al. [37] suggested that as the cation and anion of [Emim][Gly] were small in size, they could therefore come nearby an amino group and reacted to form a carbamate with a stoichiometry of 1:2. Hence, it was reasonable to assume that the amino group present in these composites reacted with CO₂ to form carbamate, resulting in a higher CO₂ capture capacity than pure MOF-177 below 1 bar.

It is noteworthy to mention that, when the pressure is below 1 bar, chemical adsorption becomes the prevailing factor. This is attributed to the strong affinity between CO₂, and AAILs. Consequently, the CO₂ adsorption capacity is enhanced. However, it is important to note that the occupation of MOF-177 pores by AAILs molecules leads to a significant reduction in the available surface area of the composites. Nevertheless, as the pressure escalated, the composites’ advantage waned, resulting in a CO₂ uptake that was lower than that of the pristine MOF-177 at pressures exceeding 2 bar. This decline can be due to the significant reduction in pore volume as well as the surface area of the composites. In the context of moderate to high pressure, the adsorption capacity of the sorbent is primarily influenced by physical adsorption sites, in addition to the active chemical adsorption sites [38].

Our experimental results revealed that there was an upper limit to the loading of AAILs which was found to be 20 wt.% and beyond the limit, CO₂ uptake decreased. This phenomenon of reduction in CO₂ capacity which is ascribed to the reduction of accessible active sites of the sorbent due to the blockage or collapse of the MOF-177 support at high AAIL concentrations, which was also evident in the XRD, BET surface area, and pore volume results of 30% AAILs@MOF-177 as discussed earlier. An analogous finding in trend was reported by Wang et al. [37] for the impregnation of [Emim][Gly] into the nanoporous structure of polymethylmethacrylate (PMMA) by varying the loading from 0 to 100 wt.% and the optimum loading was found to be 50 wt.%. In another study, Uehara et al. [26] reported that the optimum loading of [Emim][Lys] was 60 wt.% for the mesoporous silica support of SBA-15.

The stability of the composites in the CO₂ capture operation was investigated by performing multiple cycles of adsorption at 313 K and desorption cycles at 373 K at atmospheric pressure in the presence of N₂ for 20 wt.%-[Emim][Gly]@MOF-177 using the IGA microbalance. The obtained results are displayed in Figure 6. They showed that the composite sorbent can almost maintain its original adsorption capacity during multicycle operations. In addition, the CO₂ uptake process for AAILs@MOF-177 was completely reversible suggesting that the composites can be readily regenerated in the presence of flowing N₂ at 100 °C.

2.3. Selectivity for CO₂/N₂

To be deemed effective in the post-combustion capture of carbon dioxide (CO₂), a solid sorbent must have a notable level of selectivity towards CO₂ when compared to nitrogen (N₂). Consequently, the determination of N₂ adsorption isotherms at a temperature of 40 °C was undertaken to quantify the CO₂/N₂ selectivity. Each isotherm encompassed a range of pressures ranging from 0.1 to 10 bar. In our current study, we employ a particular methodology to determine the optimal selectivity. our strategy entails calculating the selectivity by comparing the molar uptakes of individual components at a given pressure, as indicated in Equation (1) [2].

$$\mathrm{S}=\frac{{\mathrm{q}}_{\mathrm{CO}2}}{{\mathrm{q}}_{\mathrm{N}2}}$$

(1)

where S denoted the selectivity and q_CO2 and q_N2 represented the molar uptakes of CO₂ and N₂, respectively. Figure 7 presents the results of the calculations made on the CO₂/N₂ selectivity for both [Emim][Gly]@MOF-177 and [Emim][Ala]@MOF-177 composites. Throughout the whole pressure spectrum, the CO₂/N₂ selectivity of pure MOF-177 fluctuated somewhere between three and five. It was discovered that the impregnation of [Emim][Gly] and [Emim][Ala] into MOF-177 boosted the selectivity for all loadings compared to the virgin MOF-177 up to a pressure of 2 bar. Regarding the composites involving [Emim][Gly]@MOF-177, it was observed that the compound 20-[Emim][Gly]@MOF-177 demonstrated the highest selectivity, reaching around 13 at a pressure of 0.2 bar and a temperature of 313 K. Nevertheless, the observed value exhibited a gradual decline as the pressure was raised, as depicted in Figure 7a. It is important to note that the increase in loading to 30-[Emim][Gly] did not increase selectivity; rather, it resulted in a selectivity that was worse than that at 10 wt.%, except at 0.1 bar. Similar behaviour was seen for [Emim][Ala]@MOF-177 composites, with the maximum selectivity of around 15 (0.1 bar) and 11 (0.2 bar) displayed by 20-[Emim][Ala]@MOF-177. It was discovered, however, that the selectivity of a 30-[Emim][Ala]@MOF-177 composite is nearly identical to that of a 20-[Emim][Ala]@MOF-177 composite.

The encapsulated amino acid-base ionic liquids contributed to the enhancement of CO₂/N₂ selectivity. This improvement was observed at pressures below 2 bars. As previously discussed, it has been proposed that the existence of amino acids leads to the formation of active chemical sorption sites for CO₂, facilitating the creation of an N-C bond. This interaction is similar to the way CO₂ interacts with an aqueous amine solution [39]. This resulted in additional CO₂ capture, although the surface area and pore volume were reduced due to the addition of the ionic liquid. On the other hand, N₂ did not have an affinity for the amino group as the adsorption was physical and depended on the available surface area. Hence, CO₂ uptake was dominant at low pressure compared to N₂, resulting in higher CO₂/N₂ selectivity. However, as the pressure increased, the physical adsorption sites also became the determining factors of the adsorption capacity besides the active chemical adsorption sites in the sorbent. Consequently, the CO₂/N₂ selectivity of composites decreased as pressure increased and became lower than the unmodified MOF-177 for pressures above 2 bar.

2.4. Equilibrium Isotherm Modeling

To accurately represent the outcomes of the experimental investigations pertaining to the design of adsorption and desorption processes, it was imperative to construct a model of the equilibrium isotherm. The presence of encapsulated AAILs within the pore structure of MOF-177 resulted in the composite material under investigation exhibiting binding sites of varying strengths. After conducting a comparative analysis with various existing models, it was determined that the Dual-Site-Langmuir (DSL) model [18,40] had a high level of suitability. The model incorporates the Langmuir adsorption mechanism, which involves two distinct adsorption sites. The overall adsorption is determined by the cumulative effect of adsorption occurring at each individual site, as represented by Equation (2) [39].

$${\mathrm{N}}_{\mathrm{e}}=\frac{N_A{b}_{A}P}{1+b_{A}P}+\frac{N_B{b}_{B}P}{1+b_{B}P}$$

(2)

In the DSL model, the equilibrium intake of CO₂ (N_e) is denoted in millimoles per gram of solid, while the equilibrium pressure (P) is given in bar. The parameters N_A, N_B, b_A, and b_B were utilized in this model. The regressed parameters for the composites are presented in Table 2 and Table 3. These tables include the parameters obtained from the regression analysis for different loadings of ionic liquids (ILs) at various temperatures. Figure 8 and Figure 9 provide visual representations of the fitting curves generated by the DSL model. Due to the R² values exhibiting a high degree of proximity to unity, it can be inferred that the model effectively achieved a remarkable level of conformity with the experimental data. Consequently, the data obtained from the model will be utilized to compute the heat of adsorption in the subsequent section.

2.5. Isosteric heat of adsorption (Q_st)

The determination of the adsorption enthalpy of carbon dioxide (Q_st), referred to as the isosteric heat of adsorption, plays a pivotal role in the adsorption process. It displayed the gas molecules’ affinity for the adsorbents and the degree of their interaction with them. The energy requirements for the adsorption-desorption process were therefore quantified. CO₂ isotherms at 303, 313, and 323 K were used to calculate the adsorption enthalpy (Q_st). At first, the DSL model was employed to establish a suitable match for the isotherms, as elucidated in the preceding section. Subsequently, the Clausius-Clapeyron equation (3) was utilized [39].

$${\left(\ln P\right)}_N=-\left(\raisebox{1ex}{$Q_{st}$}\!\left/ \!\raisebox{-1ex}{$R$}\right.\right)\left(\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$T$}\right.\right)+C$$

(3)

In the given context, the symbol P denotes the pressure in units of bar, N represents the extent of CO₂ adsorption, T signifies the temperature measured in Kelvin (K), and R denotes the universal gas constant. The equation was utilized to produce graphs depicting the natural logarithm of the partial pressure (ln P) as a function of the reciprocal of temperature (1/T) while maintaining a constant rate of carbon dioxide consumption. The value of Q_st was then determined by calculating the slope of these plots. The findings are depicted in Figure 10, illustrating the outcomes for both pure MOF-177 and AAILs@MOF-177.

Q_st values for the pure MOF-177 remained relatively stable at 13 kJ·mol^-1, in contrast, a significant rise in Q_st was observed for the composites, particularly at low levels of CO₂ uptake. When compared to other [Emim][Gly] composites, 20-[Emim][Gly]@MOF-177 showed the highest values for Qst, reaching a maximum of -28 kJ·mol^-1 at 0.2 mmol·g^-1 CO₂ uptake. This value was double that of pure MOF-177 under identical conditions [Figure 10a]. Similarly, it was observed that the composite material 20-[Emim][Ala]@MOF-177 displayed the most significant Q_st values compared to other composites of [Emim][Ala]@MOF-177 [Figure 10b]. The significant increase in Q_st can be ascribed to the strong intermolecular forces between carbon dioxide (CO₂) and the ionic liquids that have been incorporated within the pores of MOF-177. There exists a hypothesis suggesting that the anions of AAILs, containing the -NH₂ group, undergo a reaction to form an N-C bond. This reaction is believed to contribute to the higher heat release observed after the adsorption of CO₂ [39]. The decrease in the quantity of available adsorption sites can be attributed to the observed decline in the Q_st value across all composites, which coincided with an increase in CO₂ uptake. A similar observation was reported for a composite of MIL-100 (Fe) modified with DETA [11]. As expected, Q_st for both 30%-[Emim][Gly]@MOF-177 and 30%-[Emim][Ala]@MOF-177 were in between 10 and 20 wt.% loading, which confirmed the CO₂ adsorption isotherm and selectivity pattern observed and discussed in the previous section.

3. Materials and Methods

3.1. Materials

Sigma Aldrich supplied the methanol (CAS 67-56-1), [Emim][Gly] (CAS 766537-74-0), [Emim][Ala] (CAS 766537-81-9) and the MOF-177 (Basolite Z377, CAS: 676593-65-0). Before beginning sample preparation, MOF-177 was allowed to dry overnight at 110 ^oC. All other compounds were used just as supplied. To reduce the amount of time that the samples were exposed to moisture, the ionic liquid, MOF-177, and the produced composites were stored in a glovebox (Clean Tech LLC) filled with argon gas. Praxair Inc. Canada was the supplier of the CO₂ and N₂ that had a high purity level (99.99 vol.%).

3.2. Preparation of AAIL@MOF-177 composite

Using the wet impregnation approach and methanol as a solvent, amino acid ionic liquids [Emim][Gly] and [Emim][Ala] were effectively embedded within the porous MOF-177 support. In summary, the desired amount of AAILs was added to 4 ml of methanol in a small glass vial, and the mixture was made homogenized by shaking it for 30 minutes. The AAIL-methanol solution was carefully added in a dropwise manner to the pre-weighed dehydrated MOF-177, which was contained in a separate glass vial. The resulting combination was then agitated for 1 hour. Following 24 hours of solvent evaporation under ambient conditions, any residual solvent was then removed by subjecting the composite to a drying process at a temperature of 80 °C for 2 hours. The created composite samples were stored in an argon-filled glovebox to prevent exposure to moisture and were labelled as X-AAILs@MOF-177, in which X represents the weight % of AAILs used; for instance, 10 wt.% [Emim][Gly] will be referred to as 10-[Emim][Gly]@MOF-177.

3.3. Characterization

The thermogravimetric analysis (TGA) of [Emim][Gly], [Emim][Ala], pristine MOF-177, and all synthesized AAILs@MOF-177 composites was conducted with a Shimadzu Thermal Gravimetric Analysis device (TGA-50). The analysis was performed using a nitrogen flow rate of 50 mL/min with the temperature gradually increasing to 800 ^oC at a rate of 10 ^oC/min. In each of the analyses, a sample size of roughly 10 to 12 mg was utilized. To analyze the crystal structure of pure MOF-177 support as well as AAILs@MOF-177 composites, a tabletop X-ray diffraction (XRD) device (Rigaku Miniflex-II) was employed. The Cu Ka radiation used in this experiment had a wavelength of 1.5418 Å. The examination was carried out at a scanning step of 1.2 ^oC/min between 2θ values of 2 and 20 degrees, a temperature of 77 K and using an instrument manufactured by Micromeritics ASAP. The N₂ adsorption and desorption isotherms of MOF-177 and as-synthesized composites were determined. Based on the data from the N₂ isotherm, the textural features of each sample, such as the surface area (BET and Langmuir), as well as the pore volume, were computed.

3.4. Adsorption Isotherms

An intelligent gravimetric analyzer (IGA-003), manufactured by Hidden Isochema Ltd., was utilized to acquire CO₂ adsorption data at 303, 313, and 323 K, as well as N₂ adsorption data at 313 K, throughout a wide span of pressures ranging from 0.1 bar to 10 bar. The IGA consisted of a computer-controlled microbalance that measured the weight in real-time with a precision of 1 µg. Between 50 and 70 mg of material were deposited into a sample bucket for each isotherm. After heating the sample chamber to 80 °C using a water bath and vacuuming to 10 mbar with a diaphragm and turbo-pump (Pfeiffer), the sample weight stayed constant for 1 hour, indicating that all the solvent, moisture, and contaminants were removed. Following the completion of the outgassing process, the temperature of the water bath was adjusted to the predetermined isotherm temperature, and subsequently, the sample was permitted to attain the designated temperature. Once the sample was ready, pressure levels were pre-set to a value ranging from 0.1 to 10 bar in the IGASwin software, and the isotherm measurements were initiated. A mass flow controller (MFC) was used to regulate the quantity of CO₂ or N₂ injected into the chamber to maintain the desired pressure. The IGASwin program kept track of the real-time measurements of mass, temperature, and pressure. After allowing each pressure level to establish equilibrium for at least two hours and recording the results, more CO₂ or N₂ was injected via the MFC at the following pressure level. At a given temperature, this procedure was repeated for each of the predetermined pressures. After the experiment was complete, the buoyancy effect was accounted for in the real-time adsorption data.

3.5. Cyclic adsorption-desorption test

At 40 °C and 1 bar, the IGA was used to perform cyclic CO₂ adsorption and desorption studies. As explained earlier, before the adsorption sample was degassed and the temperature adjusted to 40 °C. After 30 minutes of temperature stabilization, the injection of carbon dioxide (100 mL/min) was carried out to commence the process of adsorption maintained for 60 minutes. To facilitate desorption, the sample was thereafter exposed to thermal treatment at a temperature of 100 °C under a continuous flow of nitrogen gas (100 mL/min) for 150 minutes. This process aimed at eliminating the adsorbed carbon dioxide from the sample. These processes of adsorption and desorption were carried out for five cycles.

4. Conclusions

Enhanced CO₂ adsorption and selectivity were observed in composites made by encapsulating two amino acid-based ionic liquids (AAILs) within the highly porous metal-organic framework, MOF-177. The composite material with a loading of 20 wt.% [Emim][Gly] demonstrated the highest recorded CO₂ uptake of 0.45 mmol·g^-1 solid at a pressure of 0.2 bar and 303 K. The composite material with 20 wt.% [Emim][Ala] exhibited a CO₂ uptake of 0.42 mmol·g^-1 solid, of 3 and 2.8 times higher than the CO₂ uptake of pure MOF-177. The introduction of AAILs resulted in an enhancement of the CO₂/N₂ selectivity, with values increasing from 5 (for the pure MOF-177) to 13 for [Emim][Gly] and 11 for [Emim][Ala] at a pressure of 0.2 bar and a temperature of 313 K. The interaction between carbon dioxide and the amino (-NH₂) functional group, facilitated by the anion of the amino acid ionic liquids (AAILs) resulted in increased adsorption enthalpy (Q_st) values. Q_st values for the pure MOF-177 remained relatively stable at 13 kJ·mol^-1, in contrast, a significant rise in Q_st was observed for the composites, particularly at low levels of CO₂ uptake. When compared to other [Emim][Gly] composites, 20-[Emim][Gly]@MOF-177 showed the highest values for Qst, reaching a maximum of -28 kJ·mol^-1 at 0.2 mmol·g^-1 CO₂ uptake. The present investigation also revealed that the ideal loading of AAIL was 20 wt.%, whereas any subsequent increase in loading to 30 wt.% was inadvisable. At a loading of 30 wt.% the decline in CO₂ absorption could potentially be attributed to a reduction in the availability of active sites on the sorbent. This reduction may be caused by the blockage or the collapse of the MOF-177 structure, as evidenced by a decrease in both surface area and pore volume. The X-ray diffraction (XRD) examination revealed that the introduction of higher amounts of AAIL led to a loss in the structural integrity of the original support material.

This study provides insights into the structural integrity of AAILs@MOF-177 composites, as well as their performance in terms of CO₂ capture and CO₂/N₂ selectivity, and adsorption enthalpies in post-combustion CO₂ capture processes.