Selective CO2 Capture from CO2/N2 Gas Mixtures Utilizing Tetrabutylammonium Fluoride Hydrates

Gas hydrates, a type of inclusion compound capable of trapping gas molecules within a lattice structure composed of water molecules, are gaining attention as an environmentally benign gas storage or separation platform. In general, the formation of gas hydrates from water requires high-pressure and low-temperature conditions, resulting in significant energy consumption. In this study, tetrabutylammonium fluoride (TBAF) was utilized as a thermodynamic promoter forming a semi-clathrate-type hydrate, enabling gas capture or separation at room temperature. Those TBAF hydrate systems were explored to check their capability of CO2 separation from flue gas, the mixture of CO2 and N2 gases. The formation rates and gas storage capacities of TBAF hydrates were systematically investigated under various concentrations of CO2, and they presented selective CO2 capture behavior during the hydrate formation process. The maximum gas storage capacities were achieved at 2.36 and 2.38 mmol/mol for TBAF·29.7 H2O and TBAF·32.8 H2O hydrate, respectively, after the complete enclathration of the feed gas of CO2 (80%) + N2 (20%). This study provides sufficient data to support the feasibility of TBAF hydrate systems to be applied to CO2 separation from CO2/N2 gas mixtures based on their CO2 selectivity.


Introduction
The steady increase in greenhouse gas emissions worldwide has highlighted various environmental issues, including climate change [1].The atmospheric concentration of CO 2 exceeded 415 ppm in 2022 and is expected to continue rising in the coming years [2].Consequently, to address the severity of global warming and reduce emissions of carbon dioxide, a 2050 carbon neutrality scenario has been proposed.In order to achieve this goal, research on carbon capture, utilization, and storage (CCUS) is actively underway [3].Among this, the carbon dioxide capture process incurs the highest cost, necessitating the development of economically viable and efficient capture technology.One of the recently notable technologies is the process of separating mixed gases and extracting desired gases through hydrate formation, known as hydrate-based gas separation (HBGS) [4][5][6].
Clathrate hydrates, crystalline structures formed through water molecule hydrogen bonding, possess the ability to encapsulate low-molecular-weight gas molecules within their lattice framework [7].They can effectively capture various gases, and their high gas storage capacity per unit volume makes them efficient gas storage media.Particularly notable is their non-explosive nature due to being composed of water.Research has extensively explored the hydrate-based stable storage of energy gases like natural gas [8,9], hydrogen [10][11][12], and their mixtures [13][14][15][16][17], as well as greenhouse gases such as carbon dioxide [18][19][20][21] and nitrous oxide [22][23][24].In particular, based on the gas capture capability of clathrate hydrates, hydrate-based CO 2 capture (HBCC), which separates and stores carbon dioxide in hydrate media, has garnered most attention.HBCC offers the advantages of environmental friendliness and high selectivity for carbon dioxide due to its thermodynamically stable characteristics in hydrate [25].Additionally, HBCC can be designed as a continuous process suitable for large-scale capture and enables cost-effective carbon dioxide treatment.However, traditional hydrate-based technologies typically demand low-temperature, highpressure conditions, resulting in substantial energy consumption.To address this limitation, this study explores the utilization of ionic species as thermodynamic promoters, enabling hydrate stability even at ambient temperatures.
As previously discussed, pure gas hydrates containing gas species exclusively within the hydrate medium typically necessitate high-pressure and low-temperature conditions to sustain their structural integrity.Consequently, to enhance the feasibility of hydrate-based gas storage techniques, researchers have identified and implemented various thermodynamic promoters.The three most prominent structures of clathrate hydrates are Structure I (sI, comprising two small cages (5 12 ) and six large cages (5 12 6 2 )), Structure II (sII, characterized by sixteen small cages (5 12 ) and eight large cages (5 12 6 4 )), and Structure H (sH, featuring three small cages (5 12 ), two medium cages (4 3 5 6 6 3 ), and one large cage (5 12 6 8 )).These thermodynamic promoter molecules typically occupy the large cages of sII (5 12 6 4 ) or sH (5 12 6 8 ) hydrates, stabilizing the overall structure and enabling secure storage of small gaseous guest molecules within the small cages of sII (5 12 ), or the small and medium cages of sH (5 12 , 4 3 5 6 6 3 ) hydrates under relatively lower-pressure and higher-temperature conditions.Similarly, certain ionic species can stabilize hydrate structures, leading to the formation of various types of semi-clathrate hydrates.Semi-clathrate hydrates represent unique structures capable of capturing large cations or anions within confined cages, with counterions being incorporated into the hydrate lattice.Typically, the size of the captured ionic species exceeds that of a single large cage of sI (5 12 6 2 ) or sII (5 12 6 4 ) hydrates, necessitating the partial breaking of cages to accommodate the larger ions.Due to ionic interactions between the captured species in confined cages and the incorporated counterions within the hydrate lattice, some ionic semi-clathrate hydrates exhibit exceptional thermodynamic stability, facilitating the capture of gaseous guest molecules under higher-temperature and lower-pressure conditions [26,27].This property underscores their potential utility in gas storage and separation applications.
In this study, TBAF hydrates were employed to facilitate the formation of hydrates from mixed gases containing various ratios of carbon dioxide.This study conducted experiments under conditions not previously explored and also confirmed results from a structural change perspective [28,29].Initially, we investigated the phase equilibrium conditions of TBAF hydrates with different compositions of gas mixtures.Once the temperature and pressure conditions conducive to the formation of TBAF + gas hydrates were confirmed, we conducted an analysis of their structure.Semi-clathrate hydrates exhibit three distinct crystal structures: tetragonal structure-I (TS-I) [10(5 12 )•16(5 12 6 2 )•4(5 12 6 3 )•172 H 2 O], cubic superstructure-I (CSS-I) [2(5 12 )•6(5 12 6 2 )•46 H 2 O], and hexagonal structure-I (HS-I) [3(5 12 )•2(5 12 6 2 )•2(5 12 6 3 )•40 H 2 O] [30].Since the hydration number determines the crystal structure and chemical properties of ionic hydrates, it was imperative to identify the structures of TBAF•29.7 H 2 O and TBAF•32.8H 2 O hydrates.Subsequently, we investigated the gas uptake behaviors of TBAF hydrates during the hydrate formation process.The findings of this study yield valuable insights for implementing an energy-efficient carbon dioxide capture and separation process based on semi-clathrate hydrates.

Phase Equilibria of a Semi-Clathrate Hydrate with a Secondary Gaseous Guest
The phase equilibria of TBAF-enhanced systems consisting of TBAF•29.7/32.8H 2 O + CO 2 (20, 50, 80%) + balanced N 2 + water were measured using the customized experimental setup (Figure 1), and the detailed procedures are described in Section 4. As depicted in Figure 2, the addition of TBAF had a significant impact on the equilibrium conditions of the gas hydrate system (the detailed temperature and pressure values for each point are summarized in Table 1).Notably, the presence of TBAF caused a notable shift in the equilibrium curve of hydrate formation toward higher temperatures and lower pressures compared to hydrates formed without promoters.This shift implies that under equivalent pressure conditions, hydrate formation becomes achievable at elevated temperatures, and under equivalent temperature conditions, it becomes feasible at reduced pressures.The utilization of TBAF as a thermodynamic promoter resulted in a shift of the equilibrium curve to temperatures, exceeding 30 K higher than that of mixed gas hydrates, thereby enabling the stability of the structure even at ambient conditions.Additionally, akin to hydrates formed without promoters, an increase in the proportion of CO 2 in the gas mixture led to a rightward shift in the equilibrium curve.This consistent effect across all mixed gases suggests that TBAF exhibited reliable performance.Furthermore, it was observed that there was no significant discrepancy in the equilibrium curve shifts between TBAF•29.7 H 2 O and TBAF•32.8H 2 O hydrates.Despite the differences in hydration numbers, the thermodynamic promotion effects of TBAF hydrates remained nearly identical, indicating similar crystalline structures and secondary gaseous guest enclathration within the hydrate lattice for both semi-clathrate hydrates.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 15 conditions of the gas hydrate system (the detailed temperature and pressure values for each point are summarized in Table 1).Notably, the presence of TBAF caused a notable shift in the equilibrium curve of hydrate formation toward higher temperatures and lower pressures compared to hydrates formed without promoters.This shift implies that under equivalent pressure conditions, hydrate formation becomes achievable at elevated temperatures, and under equivalent temperature conditions, it becomes feasible at reduced pressures.The utilization of TBAF as a thermodynamic promoter resulted in a shift of the equilibrium curve to temperatures, exceeding 30 K higher than that of mixed gas hydrates, thereby enabling the stability of the structure even at ambient conditions.Additionally, akin to hydrates formed without promoters, an increase in the proportion of CO2 in the gas mixture led to a rightward shift in the equilibrium curve.This consistent effect across all mixed gases suggests that TBAF exhibited reliable performance.Furthermore, it was observed that there was no significant discrepancy in the equilibrium curve shifts between TBAF•29.7 H2O and TBAF•32.8H2O hydrates.Despite the differences in hydration numbers, the thermodynamic promotion effects of TBAF hydrates remained nearly identical, indicating similar crystalline structures and secondary gaseous guest enclathration within the hydrate lattice for both semi-clathrate hydrates.Understanding the structure of gas hydrates is crucial for accurately predicting and assessing their gas storage capabilities, particularly in applications such as gas separation and storage.In our study, we employed PXRD analysis to delve into the structural characteristics of semi-clathrate hydrates formed under varying compositions of CO2 in the feed gas.As depicted in Figure 3a, the TBAF•29.7 H2O hydrate devoid of secondary gaseous guests exhibited the anticipated cubic structure of I-43d, a well-known configuration.However, upon the introduction of CO2 and N2 molecules as secondary guests, the semiclathrate hydrate with an I-43d space group transitioned into a tetragonal structure of P42/m, as illustrated in Figure 3b-e.In contrast, the tetragonal structure of TBAF•32.8H2O hydrate remained unchanged even under gas pressurization conditions, as depicted in Figure 3f-j.These findings were consistent with the phase equilibria data of TBAF•29.7/32.8H2O + CO2 (20, 50, 80%) + balanced N2 + water (Figure 2), indicating that despite the differences in hydration numbers, both systems originated from the same crystalline structure.The cubic I-43d structure features two small cages (5 12 cages) per unit cell, suitable for accommodating secondary gaseous guests.In contrast, the tetragonal P42/m structure contains ten small cages, offering greater capacity for gaseous guest molecules within the hydrate lattice.Thus, in the presence of CO2 and N2, the semi-clathrate hydrate  Understanding the structure of gas hydrates is crucial for accurately predicting and assessing their gas storage capabilities, particularly in applications such as gas separation and storage.In our study, we employed PXRD analysis to delve into the structural characteristics of semi-clathrate hydrates formed under varying compositions of CO 2 in the feed gas.As depicted in Figure 3a, the TBAF•29.7 H 2 O hydrate devoid of secondary gaseous guests exhibited the anticipated cubic structure of I-43d, a well-known configuration.However, upon the introduction of CO 2 and N 2 molecules as secondary guests, the semi-clathrate hydrate with an I-43d space group transitioned into a tetragonal structure of P4 2 /m, as illustrated in Figure 3b-e.In contrast, the tetragonal structure of TBAF•32.8H 2 O hydrate remained unchanged even under gas pressurization conditions, as depicted in Figure 3f-j.These findings were consistent with the phase equilibria data of TBAF•29.7/32.8H 2 O + CO 2 (20, 50, 80%) + balanced N 2 + water (Figure 2), indicating that despite the differences in hydration numbers, both systems originated from the same crystalline structure.The cubic I-43d structure features two small cages (5 12 cages) per unit cell, suitable for accommodating secondary gaseous guests.In contrast, the tetragonal P4 2 /m structure contains ten small cages, offering greater capacity for gaseous guest molecules within the hydrate lattice.Thus, in the presence of CO 2 and N 2 , the semi-clathrate hydrate of TBAF•29.7 H 2 O favored the formation of the tetragonal P4 2 /m structure to accommodate more gaseous guest molecules.Following the identification of the crystalline structure of TBAF•29.7 H2O a TBAF•32.8H2O hydrates, we investigated the lattice parameters obtained from PXRD p terns.Initially, considering the molecular sizes of CO2 and N2 [35], we anticipated that lattice parameters would increase proportionally with the CO2 composition in the fe gas.However, contrary to expectations, the lattice parameters of both TBAF hydrates creased proportionally with CO2 composition up to 80%, but decreased when only C was present (i.e., 100% CO2 in the feed gas), as shown in Figure 3k,l.To elucidate th results, Raman analysis was conducted.Following the identification of the crystalline structure of TBAF•29.7 H 2 O and TBAF•32.8H 2 O hydrates, we investigated the lattice parameters obtained from PXRD patterns.Initially, considering the molecular sizes of CO 2 and N 2 [35], we anticipated that the lattice parameters would increase proportionally with the CO 2 composition in the feed gas.However, contrary to expectations, the lattice parameters of both TBAF hydrates increased proportionally with CO 2 composition up to 80%, but decreased when only CO 2 was present (i.e., 100% CO 2 in the feed gas), as shown in Figure 3k,l.To elucidate these results, Raman analysis was conducted.

Raman Analysis of Semi-Clathrate Hydrates
Raman spectroscopy was employed to validate the stable enclathration of secondary gaseous guest molecules within the lattice structures of semi-clathrate hydrates [33].As depicted in Figure 4, confirmation of guest molecule capture in TBAF•29.7/32.8H 2 O + CO 2 (20, 50, 80%) + balanced N 2 hydrate was obtained through comprehensive Raman spectroscopy analysis.In all semi-clathrate hydrates, discernible peaks corresponding to the C−C stretching mode of the tetrabutylammonium cation (TBA + ) were observed within the range of 950 to 1400 cm −1 , along with peaks attributed to the C−H bending mode from 1400 to 1500 cm −1 , indicating the stable formation of TBAF hydrates.Additionally, the presence of CO 2 and N 2 within the small cages (5 12 cages) of semi-clathrate hydrates was evidenced by peaks observed at 1280 and 2324 cm −1 , respectively.

Raman Analysis of Semi-Clathrate Hydrates
Raman spectroscopy was employed to validate the stable enclathration of seco gaseous guest molecules within the lattice structures of semi-clathrate hydrates [3 depicted in Figure 4, confirmation of guest molecule capture in TBAF•29.7/32.8H2O (20, 50, 80%) + balanced N2 hydrate was obtained through comprehensive Raman troscopy analysis.In all semi-clathrate hydrates, discernible peaks corresponding C−C stretching mode of the tetrabutylammonium cation (TBA + ) were observed with range of 950 to 1400 cm −1 , along with peaks attributed to the C−H bending mode from to 1500 cm −1 , indicating the stable formation of TBAF hydrates.Additionally, the pr of CO2 and N2 within the small cages (5 12 cages) of semi-clathrate hydrates was evid by peaks observed at 1280 and 2324 cm −1 , respectively.To further elucidate the PXRD findings described earlier, normalization of R peak intensities of CO2 and N2 was performed using the peak at 1113 cm −1 , correspo to the C-C stretching mode of TBA + (Table 2).This normalization process was es due to the inherent variability in Raman intensity values across measurements.Wh act quantitative values from Raman intensity may pose challenges, qualitative com sons can be made.Notably, as the molar composition of CO2 to N2 in the feed g creased, the ratio of Raman peak intensity of enclathrated CO2 to N2 molecule also sh a corresponding increase in all semi-clathrates.Interestingly, when CO2 gas was th constituent in the feed gas, the Raman peak intensity of enclathrated CO2 appeared a identical compared to the hydrate formed from a feed gas composition of CO2 (80% (20%).However, in the latter case, additional enclathrated N2 was observed with hydrate lattice, resulting in a larger lattice parameter of the unit cell structure of clathrate hydrates.This finding implies that the presence of N2 within the semi-cla hydrates contributes to an augmented gas storage capacity compared to scenarios only CO2 is present.To further elucidate the PXRD findings described earlier, normalization of Raman peak intensities of CO 2 and N 2 was performed using the peak at 1113 cm −1 , corresponding to the C-C stretching mode of TBA + (Table 2).This normalization process was essential due to the inherent variability in Raman intensity values across measurements.While exact quantitative values from Raman intensity may pose challenges, qualitative comparisons can be made.Notably, as the molar composition of CO 2 to N 2 in the feed gas increased, the ratio of Raman peak intensity of enclathrated CO 2 to N 2 molecule also showed a corresponding increase in all semi-clathrates.Interestingly, when CO 2 gas was the sole constituent in the feed gas, the Raman peak intensity of enclathrated CO 2 appeared almost identical compared to the hydrate formed from a feed gas composition of CO 2 (80%) + N 2 (20%).However, in the latter case, additional enclathrated N 2 was observed within the hydrate lattice, resulting in a larger lattice parameter of the unit cell structure of semi-clathrate hydrates.This finding implies that the presence of N 2 within the semi-clathrate hydrates contributes to an augmented gas storage capacity compared to scenarios where only CO 2 is present.To evaluate the viability of utilizing TBAF hydrates for gas capture and separation processes, we conducted experiments to measure gas storage capacities during hydrate formation from mixed gases.The formation of TBAF hydrates from CO 2 and N 2 mixed gas necessitates specific temperature and pressure conditions, which were established based on data obtained from equilibrium experiments (Figure 2).Experimental parameters were set at 40 bar and 299 K with a driving force of ∆T = 3 K.Gas composition inside the reactor was continuously monitored using GC during the hydrate formation experiments, and the number of moles of gases was calculated using the measured temperature and pressure inside the reactor, following the method outlined in Section 3.5.The change in moles of gas in the vapor phase was considered indicative of the moles of gas captured within the hydrate, and thus, the moles of gas captured within the hydrate were determined based on the moles of gas in the vapor phase.
Figure 5a,b illustrates the overall gas uptake curves of the TBAF + CO 2 + N 2 hydrates during hydrate formation over a period of 200 min.The attainment of equilibrium in gas uptake confirms that sufficient hydrate formation occurred within the specified time frame.Gas uptake was expressed as the ratio of consumed gas moles to initially charged water moles.We compared the differences in gas storage capacity based on the composition of mixed gases for the same hydrate, as well as variations in hydrate numbers due to adjustments in mixed gas composition.Following hydrate formation, the total gas storage capacities were measured for each gas mixture: CO 2 (20%) + N 2 (80%), CO 2 (50%) + N 2 (50%), and CO 2 (80%) + N 2 (20%).For the TBAF•29.7 H 2 O hydrate, the gas storage capacities were 2.25, 2.33, and 2.38 mmol/mol, respectively.Similarly, for TBAF•32.8H 2 O hydrate, the gas storage capacities were 2.25, 2.34, and 2.38 mmol/mol, respectively.Comparing the two TBAF hydrates under the same gas composition revealed similar results.Moreover, this total amount of enclathrated gases corresponded to the tendencies of lattice parameter changes (Figure 3) and the Raman peak intensities of two TBAF hydrates under various mixed gas compositions (Figure 4 and Table 2).Overall, as the CO2 composition in the injected mixed gas increased, the total gas uptake also increased, indicating that CO2 is more efficiently captured within the hydrate cages compared to N2.This trend is particularly evident from the CO2 uptake results illustrated in Figure 5c,d, where the amount of CO2 captured within the hydrate increases Overall, as the CO 2 composition in the injected mixed gas increased, the total gas uptake also increased, indicating that CO 2 is more efficiently captured within the hydrate cages compared to N 2 .This trend is particularly evident from the CO 2 uptake results illustrated in Figure 5c,d, where the amount of CO 2 captured within the hydrate increases with a higher CO 2 composition in the injected gas.Additionally, the storage capacities of the two TBAF hydrates were comparable.When CO 2 (20%) + N 2 (80%), CO 2 (50%) + N 2 (50%), and CO 2 (80%) + N 2 (20%) were injected, the CO 2 storage after hydrate formation was 1.23, 1.53, and 2.19 mmol/mol, respectively, for the TBAF•29.7 H 2 O hydrate, and 1.23, 1.54, and 2.21 mmol/mol, respectively, for the TBAF•32.8H 2 O hydrate.This highlights the higher proportion of CO 2 stored within the hydrate compared to N 2 , resulting in the enhanced total gas storage capacity.

Separation Efficiency of Semi-Clathrate Hydrates-Based Gas Separation
In our quest to comprehend gas separation and storage utilizing semi-clathrate hydrates, we meticulously monitored the changes in gas composition and pressure within the reactor throughout the hydrate formation process. Figure 6 depicts the dynamic evolution of gas composition during hydrate formation from the initial mixed gas for each hydrate, while the gradual decrease in pressure over time attests to the stable capture of gas within the hydrate lattice.This phenomenon underscores the efficacy of gas capture within the hydrate matrix.CO2 and N2 within the hydrate structure [36][37][38], ultimately impacting separation efficiency.Similarly, TBAF•32.8H2O hydrates exhibited a comparable phenomenon, with CO2 preferentially captured initially, followed by N2 (Figure 6d-f).Moreover, Figure 7 demonstrates the final CO2 concentrations in both the vapor and hydrate phases post-semi-clathrate hydrate formation.Across all compositions, it was revealed that the CO2 composition decreased in the vapor phase while concurrently augmenting within the hydrate phase.Upon pressurizing the TBAF•29.7 H 2 O hydrate with a mixed gas containing 20% CO 2 , an intriguing sequence of events unfolded: initially, CO 2 was stored within the hydrate lattice, causing a noticeable reduction in CO 2 composition in the gas phase, as illustrated in Figure 6a.However, around the 160 min mark post-initiation of formation, a discernible uptick in N 2 uptake appeared, leading to a subsequent increase in CO 2 composition within the gas phase.Analogous trends were observed for other mixed gases (Figure 6b,c).As the proportion of CO 2 in the mixed gas increased, the diminishment in CO 2 concentration within the gas phase became more pronounced, yet over time, N 2 depletion also became more conspicuous.This intricate interplay elucidates the differential capture kinetics of CO 2 and N 2 within the hydrate structure [36][37][38], ultimately impacting separation efficiency.
Similarly, TBAF•32.8H 2 O hydrates exhibited a comparable phenomenon, with CO 2 preferentially captured initially, followed by N 2 (Figure 6d-f).Moreover, Figure 7 demonstrates the final CO 2 concentrations in both the vapor and hydrate phases post-semiclathrate hydrate formation.Across all compositions, it was revealed that the CO 2 composition decreased in the vapor phase while concurrently augmenting within the hydrate phase.This disparity underscores the selective separation of CO 2 from the injected gas via hydrate formation, with a conspicuous augmentation in separation efficiency as the CO 2 proportion in the mixed gas increased.

Materials
Ultrahigh-purity deionized water was acquired from a Direct-Q3 water purifica unit (Millipore, Burlington, MA, USA).The gas mixture of CO2 (20, 50, 80%) + bala N2 used for this study was supplied by Seoul Specialty Gas Co. (Seoul, Republic of Ko TBAF solution with a concentration of 75% in water was purchased from Sigma-Ald (St. Louis, MA, USA).

Phase Equilibrium Measurements of a Semi-Clathrate Hydrate
The conventional isochoric method was employed to determine the phase equ rium conditions of TBAF•29.7 H2O and TBAF•32.8H2O with CO2 (20, 50, 80%) + bala N2 [43].The prepared TBAF solutions having a hydration number of TBAF•29.7 H2O TBAF•32.8H2O were loaded into the stirring reactor used for phase equilibrium meas ments.The reactor was placed in a circulating water bath (RW-2025G, Jeio Tech., Dae Republic of Korea) capable of temperature control, and mixed gas was pressurized the reactor to the desired pressure using a syringe pump.The internal temperature o Further quantitative analysis yielded two pivotal parameters to gauge the separation efficiency of TBAF hydrates, as tabulated in Table 3.The separation factor presents the extent to which CO 2 was sequestered relative to other gases in the initial injected gas, while CO 2 recovery (%) quantifies the proportion of CO 2 stored from the initial injected gas into the hydrate phase.Encouragingly, the calculation results unveiled highly favorable values for both parameters across various CO 2 compositions in the mixed gas, with the two TBAF hydrates displaying strikingly similar values.Typically, a separation factor exceeding 10 is deemed substantial [39][40][41][42], underscoring the profound potential application of TBAF hydrates in gas separation and capture technologies, as substantiated by our findings.

Materials
Ultrahigh-purity deionized water was acquired from a Direct-Q3 water purification unit (Millipore, Burlington, MA, USA).The gas mixture of CO 2 (20, 50, 80%) + balanced N 2 used for this study was supplied by Seoul Specialty Gas Co. (Seoul, Republic of Korea).TBAF solution with a concentration of 75% in water was purchased from Sigma-Aldrich (St. Louis, MA, USA).

Phase Equilibrium Measurements of a Semi-Clathrate Hydrate
The conventional isochoric method was employed to determine the phase equilibrium conditions of TBAF•29.7 H 2 O and TBAF•32.8H 2 O with CO 2 (20, 50, 80%) + balanced N 2 [43].The prepared TBAF solutions having a hydration number of TBAF•29.7 H 2 O and TBAF•32.8H 2 O were loaded into the stirring reactor used for phase equilibrium measurements.The reactor was placed in a circulating water bath (RW-2025G, Jeio Tech., Daejeon, Republic of Korea) capable of temperature control, and mixed gas was pressurized into the reactor to the desired pressure using a syringe pump.The internal temperature of the reactor was maintained at a temperature where hydrate formation does not occur, and air existing inside the cell was flushed out by continuous injection of feed gas for each sample.The stirrer started stirring at 100 rpm.Then, sufficient time was allowed for stabilization without changes in temperature and pressure before starting the temperature changes.
Once the equilibrium state was confirmed without temperature and pressure change, the temperature was gradually lowered to initiate and facilitate the hydrate formation process.After the sudden pressure drop due to the gas enclathration and the completion of the formation process, the temperature was increased very slowly at a rate of 0.1 K/h to provide a sufficient time for an equilibrated dissociation state at each temperature.Pressure and temperature changes during the hydrate formation and dissociation processes were measured using a pressure transducer (PSHB0250BCPG, Sensys Co., Ansan-si, Republic of Korea, full-scale accuracy of ±0.15%, 0-250 bar) and a four-wire type Pt-100 Ω probe (full-scale accuracy of ±0.05%, 13-923 K), respectively, and the data were monitored in real time using a DAQ system (LabVIEW).This data were then utilized to plot the formationdissociation curve and identify the hydrate equilibrium point at each initial temperature and pressure condition.This procedure was repeated multiple times for each sample to determine the phase equilibrium line of TBAF•29.7 H 2 O and TBAF•32.8H 2 O with various feed gas compositions.

Structure and Guest Enclathration Analysis of Semi-Clathrate Hydrates
The semi-clathrate hydrate (TBAF + CO 2 + N 2 ) samples formed at 40 bar were prepared for the PXRD and Raman spectroscopic analysis.Prior to retrieving the semi-clathrate hydrate powder samples, the reactor was rapidly cooled using liquid nitrogen to minimize sample damage during the recovery of samples.After a brief period, the reactor was depressurized, and the semi-clathrate hydrates were collected and finely ground to a 40 µm size using a sieve immersed in liquid nitrogen.
Powder X-ray diffraction (PXRD) was utilized to analyze the crystalline structure of semi-clathrate hydrates.The PXRD data were collected at the supramolecular crystallography beamline (2D) facility located at the Pohang Accelerator Laboratory (PAL) in the Republic of Korea.A synchrotron radiation source with a wavelength of 0.900 Å and an ADSC Quantum 210 CCD diffractometer (Pohang, Republic of Korea) were employed for data acquisition.The sample powder, stored in liquid nitrogen, was promptly transferred into pre-cooled polyimide tubing (Cole-Parmer, Vernon Hills, IL, USA) to minimize potential sample damage, and the experiments were conducted at approximately 93 K.The crystal structures of the samples were determined using the Le Bail fitting method, which involves profile matching implemented in the FullProf Program (version 2.00).
Raman spectroscopy was employed to determine the guest enclathration within semiclathrate hydrates.The Raman spectra of the semi-clathrate hydrates were obtained using a dispersive Raman spectrometer (ARAMIS, Horiba Jobin Yvon Inc., Palaiseau, France), with a 514.53 nm Ar ion laser serving as the excitation source.The scattered light was dispersed by the 1800 grating of the spectrometer and detected by a charge-coupled device (CCD) with electrical cooling (203 K).The sample temperature was maintained at 93 K using a THMSG 600 unit (Linkam Scientific, Redhill, UK).

Gas Composition Analysis
The prepared 100 mL of TBAF solution was injected into the stirring reactor with a volume of 200 mL.Subsequently, the reactor was placed in a circulating water bath.The solution was stirred at a speed of 100 rpm, and mixed gas was pressurized into the reactor to conduct the hydrate formation experiment.The conditions for hydrate formation were set at a pressure of 40 bar, with the formation temperature determined to be 299 K, corresponding to a driving force of ∆T = 3 K at the specified pressure.
To analyze the gas composition inside the hydrate, real-time gas composition in the reactor was measured using a gas chromatograph (GC) (YL6500 GC, Young Lin Instrument Co, Anyang, Republic of Korea) during the hydrate formation process.Gas composition in the reactor was analyzed at 20 min intervals for 200 min, from the pressurization of mixed gas into the reactor to the completion of hydrate formation.To minimize pressure loss due to GC measurements, a metering pump (Eldex, Napa, CA, USA) was used.The constructed hydrate formation system is illustrated in Figure 1.

Quantitative Analysis of Gas Separation Performance
The gas storage capacity within the hydrate was calculated using the gas composition obtained from experiments of semi-clathrate hydrate formation conducted with an in situ GC system.Initially, the critical temperature, critical pressure, and acentric factor of the entire mixed gas were determined by multiplying the factors of each component obtained from the gas composition of the vapor phase analyzed by GC. ) T C and P C are the critical temperature and pressure, respectively, and ω is the acentric factor.These are used to determine the reduced temperature and pressure of the entire mixed gas, as shown below: P r = P P C T r and P r present the reduced temperature and pressure, respectively.T and P present the temperature and pressure obtained through experiment, respectively.We calculated Z by putting the previously obtained parameters into Pitzer's correlation.
B 0 and B 1 are the virial coefficients and Z presents the compressibility factor.Z 0 and Z 1 are variables related to the correlation for compressibility factors.We used the calculated Z values to determine the total gas storage capacity.
(∆n) t = n 0 − n t = PV zRT 0 − PV zRT t (∆n) t presents the moles of gas trapped inside the hydrate up to time t.R is the universal gas constant, and V is the volume of the gas (mL).n t , T t , P t , and Z t present the moles of gas (mol), temperature (K), pressure (bar), and compressibility factor at time t, respectively.n 0 , T 0 , P 0 , and Z 0 present the initial moles of gas (mol), temperature (K), pressure (bar), and compressibility factor of the feed gas, respectively.We used the obtained moles of gas from the above calculation to determine the concentration of CO 2 in the gas phase and in the hydrate phase after hydrate formation.
x H CO 2 = n 0 gas y 0 CO 2 × n 1 gas y 1 CO 2 n 0 gas × n 1 gas (11) x v CO 2 and x H CO 2 represent the concentration of CO 2 in the vapor phase and in the hydrate phase, respectively.n 0 gas is the initial moles of the mixed gas, n 1 gas is the moles of the mixed gas after hydrate formation, y 0 CO 2 is the initial moles of CO 2 , and y 1 CO 2 is the moles of CO 2 after hydrate formation.Additionally, two parameters were calculated to evaluate the efficiency of gas separation in the mixed gas.× 100(%) (13) where S is separation factor, and R is CO 2 recovery %.n H CO 2 and n H N 2 present the moles of CO 2 and N 2 in the hydrate phase, respectively, n gas CO 2 and n gas N 2 present the moles of CO 2 and N 2 in the vapor phase, respectively, and n f eed CO 2 presents the moles of CO 2 in the feed gas.

Conclusions
This study investigated the potential of semi-clathrate hydrates containing tetrabutylammonium fluoride (TBAF) for CO 2 /N 2 gas capture and separation.Structural analysis conducted through PXRD unveiled structural transitions in TBAF•29.7 H 2 O hydrates upon gas capture, suggesting a shift toward structures with a higher number of small cages to accommodate secondary gaseous guest molecules.Raman spectroscopy corroborated the stable enclathration of guest molecules within TBAF hydrates.Moreover, the gas uptake capacities of TBAF hydrates during formation from CO 2 and N 2 mixed gases were evaluated.Experimental findings revealed varying gas storage capacities contingent upon the ratio of CO 2 and N 2 mixtures, with notably superior storage capacity observed for CO 2 .These results underscore the potential efficacy of TBAF hydrates in the gas separation and capture of CO 2 and N 2 mixed gases.Additionally, the separation efficiency of TBAF hydrates was examined, with the observed similarity between the two types of TBAF hydrates underscoring their consistent performance.Hence, this research underscores the significance of TBAF as a promising additive in advancing the practical utilization of semi-clathrate hydrate for gas separation and storage technologies.

Figure 1 .
Figure 1.(a) Schematic diagram of the overall semi-clathrate hydrate formation system.(b) Schematic procedure of TBAF hydrate-based gas separation.

Figure 1 .
Figure 1.(a) Schematic diagram of the overall semi-clathrate hydrate formation system.(b) Schematic procedure of TBAF hydrate-based gas separation.
of TBAF•29.7 H2O favored the formation of the tetragonal P42/m structure to accommod more gaseous guest molecules.

Table 1 .
Phase equilibrium data for the TBAF + CO 2 + N 2 systems.

Table 2 .
Raman peak intensity of the guest molecules in semi-clathrate hydrates.

Table 3 .
Separation performance of the semi-clathrate hydrates.

Table 3 .
Separation performance of the semi-clathrate hydrates.