Semi-clathrate hydrate based carbon dioxide capture and separation techniques

Highlights

● Structural and thermodynamical properties of semi-clathrate hydrate are summarized.

● Properties of quaternary salts and gas mixture hydrate are summarized.

● Challenges persist in the application of semi-clathrate hydrates for carbon capture and separation.

Abstract

CO₂ is considered as the main contributor to global warming, and hydrate enclathration is an efficient way for carbon capture and separation (CCS). Semi-clathrate hydrate (SCH) is a type of clathrate hydrate capable of encaging CO₂ molecules under mild temperature and pressure conditions. SCH has numerous unique advantages, including high thermal stability, selective absorption of gas molecules with proper size and recyclable, making it a promising candidate for CCS. While SCH based CCS technology is in the developing stage and great efforts have to be conducted to improve the performance that is determined by their thermodynamical and structural properties. This review summarizes and compares the thermodynamic and structural properties of SCH and quaternary salt hydrates with gas mixtures to be captured and separated. Based on the description of the physical properties of SCH and hydrate of quaternary salts with gas mixture, the CO₂ capture and separation from fuel gas, flue gas and biogas with SCH are reviewed. The review focuses on the use of tetra-n-butyl ammonium halide and tetra-n-butyl phosphonium halide, which are the current application hotspots. This review aims to provide guidance for the future applications of SCH.

Key words： Semi-clathrate hydrate; Tetra-n-butyl ammonium halide; Tetra-n-butyl phosphonium halide; Structure; Thermodynamical properties; CO₂ capture and separation

1 Introduction

Fossil fuels currently serve as the primary energy source for humanity, yet their combustion results in most of the greenhouse gas emission (Klara and Srivastava, 2002). From 1997 to 2020, carbon emissions produced by burning fossil fuels and manufacturing cement have almost doubled from 5.0 GtC/yr to 9.5 GtC/yr (Friedlingstein et al., 2022). With concerns on the climate change, the reduction of carbon emission has been highly valued. As climate change concerns intensify, reducing carbon emissions has become a crucial priority. The Intergovernmental Panel on Climate Change (IPCC) report highlights the need to limit global warming to 1.5 °C, requiring the peak of carbon dioxide emissions by 2030 and striving for carbon neutrality by 2060 (Yang et al., 2022). CO₂ is considered as the main contributor to the global warming and its emission have increased continuously results from human activities. Carbon dioxide (CO₂) is believed to be the primary contributor to global warming, with human activities causing a continuous increase in CO₂ emissions. According to NASA’s climate change website, CO₂ concentration in January 2023 was 419 ppm, about 33% higher than in 1958, and the rate of increase is accelerating. CO₂ capture and separation is essential to reduce greenhouse gas emission to achieve the net-zero carbon emission goal (Davis et al., 2018).

Several CO₂ capture methods have been developed, including absorption (Zhuang et al., 2016), adsorption (Ben-Mansour et al., 2016; Chen et al., 2013), membranes (Kárászová et al., 2020; Favre, 2022), cryogenics (Font-Palma et al., 2021), microbial/Algal systems and so on (Wilberforce et al., 2019; Wang et al., 2020a). Compared with the above-mentioned methods, hydrate-based CO₂ capture and separation a promising technology for large-scale applications due to its high gas storage capacity, ease of recycling, safety, and cost-effectiveness (Wang et al., 2020a). Clathrate hydrates are crystalline compounds formed by hydrogen-bonded water cages enclosing guest molecules or ions (Jeffrey and McMullan, 1967). Depending on the interaction forces between the host cage and the guest molecules, clathrate hydrate can be divided into several types. Gas hydrates are “true” clathrate hydrates in which the guest molecules with appropriate sizes stabilize the structures through the van der Waals interaction with the water cages. In peralkylonium salt hydrates, the onions participate in the host framework construction and hydrogen-bonded with the water molecules (Dyadin and Udachin, 1987). The cations fill the voids of the framework and ionically interact with the water-ionic polyhedral framework. Jeffrey categorized this type of hydrate as the semi-clathrate hydrate (SCH) since the nature of the host lattice is not necessarily changed (Jeffrey and McMullan, 1967). Semi-clathrate hydrates are stable under ambient pressure and above the ice point, even above room temperature (Dyadin and Udachin, 1984; 1987), due to the ionic and van der Waals interactions between guest cations and the water-ion polyhedral structure.

Semi-clathrate hydrates can be utilized for various applications, including gas separation (Sun et al., 2011a; Zhong and Englezos, 2012; Zhong et al., 2018), H₂ storage (Chapoy et al., 2007; Deschamps and Dalmazzone, 2010; Veluswamy et al., 2014) and air conditioning system (Stoporev et al., 2020; Rakkappan et al., 2021; Kim et al., 2022; Kim et al., 2023). Using semi-clathrate hydrates for carbon capture and storage (CCS) has several benefits, including mild operating conditions, recyclability, and high selectivity for CO₂. Captured CO₂ can be abandoned as solid clathrate hydrate, while the purified methane and hydrogen obtained from fuel and biogas can be used as clean energy sources. The salts used to form semi-clathrate hydrates can be produced on a large scale and at low cost. Furthermore, the salt solution can be recycled as no salts are consumed during the CO₂ capture and release process. Semi-clathrate hydrates can significantly simplify CCS facilities and reduce the compression cost of gas hydrate formation, which is the primary cost of CO₂ capture.

Various ionic substances can form semi-clathrate hydrates, such as tetra-n-butyl ammonium (TBA), tetra-n-butyl phosphonium (TBP) and tetra-iso-amyl ammonium (TiAA) for the cation and halide, hydroxide and carboxylate for the anion (Dyadin and Udachin, 1984;1987). TBA bromide (TBAB) (Oyama et al., 2005; Shimada et al., 2005a; 2005b; Miwa et al., 2018), TBA chloride (TBAC) (Haruo, 1982; Sato et al., 2013; Kida et al., 2019), TBA floride (TBAF) (Udachin and Lipkowski, 2002; Rodionova et al., 2008), TBP bromide (TBPB) (Muromachi et al., 2014a), and TBP chloride (TBPC) (Sakamoto et al., 2011) are the application hotspots because of the high melting temperatures around 280 K of their hydrates and will be the main content of this paper. For purpose of simplicity, we call these salts the quaternary salts.

Although CCS technology based on semi-clathrate hydrates is still in the development stage, significant efforts are underway to improve its performance. For example, selecting a proper quaternary salt can reduce hydrate formation pressure and increase the amount of captured gas. A better understanding of the phase behavior and structural properties of semi-clathrate systems is also necessary for the design of CO₂ capture processes based on hydrate technology. This review will focus on the fundamental properties of SCH and their applications in CO₂ capture and separation. The structure and thermal stability of SCH as well as the ternary hydrate of gas mixture and quaternary salts will be described. Finally, we will summarize the application of semi-clathrate hydrates in CO₂ capture and separation of fuel gas, flue gas, and biogas, analyze the challenges and future directions in this field, and draw a conclusion.

2 Structure of semi-clathrate hydrate with gas mixture

The clathrate hydrate of quaternary salts is a kind of complex semi-clathrate hydrate with combined cavities to accommodate the large guest cautions. The typical cavities of quaternary salts hydrate include the pentagonal dodecahedron (D), tetrakaidecahedron (T) and pentakaidecahedron (P) (Dyadin and Udachin, 1987). As shown in Tab.1, the unit cells of the idealized host frameworks can be described as 6T·2D·46H₂O (CS-I), 4P·16T·10D·172H₂O (TS-I), 2P·2T·3D·40H₂O (HS-I) and their super lattices. The pentagonal dodecahedron, i.e., the D cavity, is the only polyhedron that is common to all the clathrate hydrates. The host framework is formed based on D cavity with other polyhedrons through vertex linking or face sharing (Dyadin and Udachin, 1987). However, for a certain hydrate, stable structures with different host lattices or several crystal structures with the same host lattice are possible. Detailed crystal structure determination is prerequisite to characterize the clathrate hydrate of quaternary salts.

Tab.1 Hydration numbers in the cages in the semi-clathrate hydrates (Davidson, 1973; Muromachi and Takeya, 2017)

Structure	Orthorhombic	Tetragonal	Cubic
512 cages (A)	6	10	2
51262 cages (B)	4	16	6
51263 cages (C)	4	4	0
water molecules (D)	80	172	40

The host cage of SCH is formed by the anion of the semi-clathrate promoter and the water molecules through hydrogen bond, in which the cation of the semi-clathrate promoter as the guest molecule is hydrophobically included. This section summarizes and compares the cage structure and related unit cell parameters of semi-clathrate hydrates formed by tetra-n-butyl ammonium halide and tetra-n-butyl phosphonium halide. Additionally, the crystal structure of quaternary salts and gas double hydrate is also described to provide a better understanding of the performance of SCH in CCS.

2.1 Structure of semi-clathrate hydrate

2.1.1 TBAB

Under certain conditions, polyhydrates can form in the TBAB-H₂O system, where the Br ion is bonded with water molecules to incorporate the TBA cation. There are five TBAB SCH from the literature, including one orthorhombic hydrate TBAB·38H₂O (Oyama et al., 2005; Shimada et al., 2005b), three tetragonal hydrates with different stoichiometry (24, 26 and 32) (Gaponenko et al., 1984; Rodionova et al., 2013), and a trigonal hydrate with small hydration number of 21 /3 (Lipkowski et al., 2002). The structural parameters of TBAB semi-clathrate hydrates are summarized in Tab.2. By rapidly cooling of the TBAB solution, another metastable hydrate of cubic structure I was discovered from the X-ray diffraction pattern (Stoporev et al., 2021). However, the structural parameters could not be resolved because of the poor quality of the powder diffraction patterns.

Tab.2 Structural parameters of TBAB semi-clathrate hydrate

Hydrate composition	Crystal system	Space group	Unit cell (Å)	Ref.
TBAB∙21/3H2O	Trigonal	R3c	a = 16.609(1)c = 38.853(1)	Lipkowski et al. (2002)
TBAB∙24H2O	Monoclinic	C2/m	a = 28.5(2)b = 16.9(1)c = 16.5 (1)\beta = 125°	Gaponenko et al. (1984)
TBAB∙26H2O	Tetragonal	P4/mmm	a = 23.9(2)c = 50.8(5)	Gaponenko et al. (1984)
TBAB∙32H2O	Tetragonal	P4/m	a = 33.4(3)c = 12.7(1)	Gaponenko et al. (1984)
TBAB∙32H2O	Tetragonal	–	a = 23.65c = 12.50	McMullan and Jeffrey (1959)
TBAB∙36H2O	Orthorhombic	Pmmm	a = 21.3(2)b = 12.9(1)c = 12.1(1)	Gaponenko et al. (1984)
TBAB∙38H2O	Orthorhombic	Pmma	a = 21.060(5)b = 12.643(4)c = 12.018(8)	Shimada et al. (2005b)

So far, only the crystal structure of TBAB·38H₂O and TBAB∙21/ 3H₂O were solved. The ideal unit cell of TBAB∙38H₂O is 4T·4P·6D·76H₂O, with TBA cation located in the four large cages, i.e., 2T·2P cage (Shimada et al., 2005b). As shown in Fig.1(a), the host framework is constructed by Br atoms and water molecules through hydrogen bonds, which is distorted from the idealized water framework because the bond length between Br and oxygen atom is larger than that between two oxygen atoms. The TBA cautions locate at the center of four cages and part of the cage structure is broken, as shown by the dotted lines in the figure. The small D cages are empty and can encage small gas molecules in gas separation and storage applications. There are two types of D cages with distinct symmetry in the crystal lattice, namely the highly distorted D_A cages and more regular cages D_B (Muromachi et al., 2014b). The chemical formula of the empty host framework can be written as TBAB∙38H₂O∙D_A∙2D_B. The D_A cages are highly distorted because the water molecular forming the D_A cages are displaced with the TBA⁺ and pushed inward by the Br^–. Therefore, it is more suitable to incorporate the bar-shaped CO₂ molecule. The single crystal of TBPB SCH was determined to have the same structure with TBAB∙38H₂O, while no polymorphism was found (Muromachi et al., 2014a). Similar to the orthogonal TBAB SCH, two types of D cages were found in TBPB SCH that can be written as TBPB∙38H₂O∙D_A∙2D_B. The D cages in TBPB SCH show larger distortion than those in TBAB SCH because the bond length of C–P is larger than that of C–N.

Fig.1 Structure of SCH: (a) TBAB∙38H₂O (Shimada et al., 2005b); (b) TBPB∙38H₂O (Muromachi et al., 2014a); (c) TBAF∙29.7H₂O (Komarov et al., 2007); (d) TBAF∙5.5H₂O (Udachin and Lipkowski, 2002).

Full size|PPT slide

In the concentrated solution of TBAB and water binary system, TBAB∙21/3H₂O can crystallize in the trigonal space group R3c (Lipkowski et al., 2002). The structure of the water-anionic cluster consists of four tetragonal faces and two pentagonal faces. Unlike the traditional host framework of semi-clathrate hydrate, the water-anionic clusters can transform into guests of host cavity formed by six cross-like tetra-n-butyl ammonium cations. This means that, in the TBAB and water binary system, water molecules can both function as host and guest, depending on the concentration of the system.

It has been found that a tetragonal structure is typical for n-butyl compositions with hydration number of 32 (McMullan and Jeffrey, 1959). Despite differences in the unit cell parameters of TBAB∙32H₂O obtained in previous studies (McMullan and Jeffrey, 1959; Gaponenko et al., 1984), its detailed structure has not yet been constructed. The crystallographic parameters of all the tetragonal TBAB hydrate were determined with slightly differences, as shown in Tab.2. Rodionova et al. (2013) built structural models based on the tetragonal structure for crystal samples prepared with different concentrations of the solution. However, the structural stoichiometry was not available. A particular difficulty was encountered when growing single crystals of TBAB∙24H₂O, and it only succeeded three times during years of attempts (Gaponenko et al., 1984). Resolving the crystal structure of these metastable phase of TBAB hydrate is very challenging. It is difficult to keep the metastable state to grow single crystals with proper quality, which can be easily affected by the environment during hydrate growth.

2.1.2 Other quaternary salts

All polyhydrates of tetra-n-butyl ammonium chlorine (TBAC) have a tetragonal structure with a space group of P4₂/m, as shown in Tab.3 (Rodionova et al., 2010). The cation of quaternary ammonium salts (QAS) is located in either the 4T cavities or 3T and 1P cavity of the fused cages. The chlorine ions are hydrophilically included in the hydrate framework.

Tab.3 Structural parameters of other quaternary salts semi-clathrate hydrate

Hydrate composition	Crystal system	Space group	Unit cell (Å)	Ref.
TBAC∙24 H2O	Tetragonal	P42/m	a = 23.608±0.028c = 12.561±0.003	Rodionova et al. (2010)
TBAC∙30H2O			a = 23.733±0.013c = 12.513±0.001
TBAC∙32H2O			a = 23.737±0.011c = 12.492±0.001
TBAF∙5.5H2O	Monoclinic	C2/c	a = 16.590(3) Åb = 17.040(3) Åc = 16.510(3) Å\beta = 103.18(3)°	Udachin & Lipkowski (2002)
TBAF∙29.7H2O	Cubic	I\overline{4}3d	a = 24.375(3) Å	Komarov et al. (2007)
TBAF∙32.8H2O	Tetragonal	P42/m	a = 23.52(1) Åc = 12.30(1) Å	Rodionova et al. (2008)
TBPB∙38H2O	Orthorhombic	Pmma	a = 21.065(5) Åb = 12.657 (3) Åc = 11.992(1) Å	Muromachi et al. (2014)
TBPC∙38H2O	Tetragonal	–	a =23.7 Åc = 12.5 Å	Sakamoto et al. (2011)

The structures of clathrate hydrates formed in the TBAF-water binary system have all been characterized (McMullan et al., 1963; Udachin and Lipkowski, 2002). The hydration number depends on salt concentrations, with TBAF∙29.7H₂O and TBAF∙32.8H₂O hydrates formed under relatively dilute solutions and TBAF∙5.5H₂O hydrate found at very high salt concentration (Udachin and Lipkowski, 2002). As illustrated in Fig.1(c), TBAF∙29.7H₂O hydrate exhibits a cubic structure and the host framework is described as a distorted version of the idealized CS-I water framework with unit cell of 6T·2D·46H₂O (Komarov et al., 2007). The four T-cavities are fused to accommodate the giant cations, and the symmetry of the idealized water lattice is distorted to have a space group of I\overline{4}3d. The structure of TBAF∙32.8H₂O is similar to that of the polyhyrates of TBAC.

The TBAF∙5.5H₂O semi-clathrate hydrate has a monoclinic structure with space group of C2/c (Udachin and Lipkowski, 2002). As shown in Fig.1(d), the polyhedron formed by water molecules and the fluorine ions consists of two square faces and four pentagonal faces. The water-anionic clusters are connected with each other to form a chain surrounded by the cations. Similar to TBAB hydrate, the water molecules can function as either the guest or the host, depending on the concentration of solution. The unit volume of TBAF∙5.5H₂O is the smallest among all the quaternary salts hydrates, and the cavity in this hydrate is the smallest. As a result, this structure is not suitable for gas capture and separation.

Compared to quaternary ammonia salts, clathrate hydrate of quaternary phosphonium salts (QPS) are less investigated. Only the molecular structure of the tetrabutylphosphonium bromide (TBPB) has been resolved (Muromachi et al., 2014a). As shown in Fig.1(b), TBPB forms a simple hydrate of TBPB∙38H₂O with an orthorhombic structure. Similar to TBAB∙38H₂O, the tetrabutylphosphonium cation occupies four large cages of 2T∙2P. However, the different bond length between C-P of the TBP ion with 1.73Å and C–N of the TBA ion with 1.53Å contributes to the different thermal stability of the TBPB and TBAB semi-clathrate hydrate. The distorted D cage in TBPB hydrate is more compressed than that in TBAB, although the volumes of these cages are nearly the same, which corresponds to the different gas separation performance for specific gas molecules.

Similar to TBAC, the structure of TBPC is tetragonal with approximate molecular size (Sakamoto et al., 2011). However, the detailed structure parameters of TBAC hydrate are not available. The structure of TBPF have not been reported to date.

Among all the aforementioned polyhydrate structures, the tetragonal structure exists in all three QAS hydrates. The structure type is related to the salt concentration. In the TBAB-H₂O binary system, the orthogonal TBAB ∙38H₂O forms at dilute TBAB solution. As the salt concentration increases, the tetragonal structure appears until the water molecule transforms into a guest that enclosed by the TBA caution to form the trigonal structure. In the TBAF-H₂O binary system, structural transition occurs with an increase in concentration from the tetragonal to the cubic structure, and the water molecules transforms into guests at very high salt concentration. However, because the structural information of QPS is rather limited, the relationship between salt concentration and structure is not straightforward.

2.2 Structure of semi-clathrate hydrate with gas mixture

In quaternary salts hydrates, the host framework is formed through the hydrophilic interaction between the salt onion and water molecules. The guest cation occupies the space created by the four fused cages, leaving the small D cages vacant that can incorporate small molecules like CO₂, H₂, CH₄, N₂, etc. The guest gas molecules can be incorporated into the SCH under mild conditions, even milder than the quaternary salt hydrate because guest molecules can stabilize the SCH. Therefore, quaternary salts have been extensively studied for carbon dioxide gas capture and separation. To achieve better performance in these applications, the structure of quaternary salts and hydrate of quaternary salts with gas mixture to be captured and separated should be characterized.

TBAB is the most extensively studied quaternary salt. It can form polyhydrates with trigonal, tetragonal and orthorhombic structures. In addition to the trigonal TBAB·21/3H₂O, the other structures are relatively stable under ambient temperature. It has been found that the guest gas molecules prefer to occupy the empty cages of orthorhombic structure (Muromachi et al., 2014b; 2016a; 2016b). This may be because among all the SCH structures, the orthorhombic one has the most D cages per water molecule (Muromachi and Takeya, 2017), thus offering superior gas capacity compared to the others.

In the hydrate structures of TBAB, as well as those formed with tetra-n-butyl ammonium and tetra-n-butyl phosphonium salts, there are two distinct types of D cages with significantly different shapes. The strongly distorted D cage is referred to as D_A, while the relatively regular cages are labeled D_B and further divided into D_B1 and D_B2 (Muromachi et al., 2016b). The structures of double hydrate formed with TBAB and CO₂/CH₄/N₂ molecules were characterized and summarized in Tab.4. CO₂ gas molecules had an asymmetrical distribution among D cages, with an occupancy of 0.867 in the strongly distorted cages and an occupancy of 0.49 in the regular D cages (Muromachi et al., 2014b). The symmetry of CO₂ and TBAB double hydrate was lowered to Imma, and the unit cell in the direction of b-axis was doubled compared to the Pmma structure.

Tab.4 Summary of TBAB and gas binary hydrate crystal parameters

	TBAB + CO2	TBAB + CH4	TBAB + N2
TBAB Concentration (wt%)	0.1	0.1	0.2
PT conditions	1.08 MPa, 282.65 K	2 MPa, 284.5 K	5.8 MPa, 283.9 K
Empirical formula	TBAB·38H2O·1.85CO2	TBAB·38H2O·2.16CH4	TBAB·38H2O·1.5N2
Crystal system, space group	Orthorhombic, Imma	Orthorhombic, Pmma	Orthorhombic, Pmma
Unit cell dimensions (Å)	a = 21.0197(7)b = 25.2728(8)c = 12.0096(4)	a = 21.0329(15)b = 12.5972(9)c = 12.0333(8)	a = 21.035(4)b = 12.635(3)c = 12.021(2)
DA cage occupancy	0.867	0.174	0
DB cage occupancy	0.490	0.991	0.75
Average occupancy	0.616	0.719	0.5
Ref.	Muromachi et al. (2014b)	Muromachi et al. (2016b)	Muromachi et al. (2016a)

In contrast to CO₂, the occupancy of CH₄ in D_A and D_B was 0.174 and 0.991, respectively (Muromachi et al., 2016b). The structurally regular D_B cage hold CH₄ gas more tightly than the D_A cage with low symmetry. This finding provides guidance for CO₂ separation applications from CH₄ gas by employing ionic salts that form SCH with more asymmetric cages, thus the CO₂ separation performance can be significantly improved. As described below, the CO₂ selectivity of TBAC SCH is superior to that of TBAB SCH (Hashimoto et al., 2017b). The unit cell of tetragonal TBAC SCH can be described as 4T³P∙T⁴∙4D_L∙4D_M∙2D_N (Muromachi et al., 2022). Among all the D cages, the D_L cage can encapsulate water and chloride while rejecting CO₂ gas, and the remaining distorted D_M and D_N cages can encage CO₂ gas with slightly lower gas occupancy than D_A cages in TBAB SCH. There are more distorted D cages in TBAC SCH that show superior CO₂ selectivity than TBAB SCH.

N₂ gas molecules were observed to only occupy the regular D cages and not the highly distorted D cages in TBAB semi-clathrate hydrate with concentration of 20 wt% (Muromachi et al., 2016a). Additionally, the phase properties of TBAB and N₂ mixed hydrate were found to be different under various pressure conditions. It was observed that when the N₂ gas pressure exceeded a certain level, it could be incorporated into the TBAB hydrate. In the study by Muromachi et al. (2016a), this critical pressure was found to be 4 MPa. Therefore, to perform CO₂ separation with TBAB from flue gas, it is important to determine the critical pressure to prevent N₂ enclathration in hydrate cages. It should be noted that at a concentration of 30 wt%, a tetragonal hydrate similar to TBAB·26H₂O was found to form in the TBAB-N₂-H₂O system (Jin et al., 2019), which suggests that a high concentration of quaternary salt may not be desirable for gas separation.

The structure of quaternary salts SCH with H₂ have not yet been determined. Raman spectroscopy showed that H₂ molecules only occupy the small D cages of TBAB SCH, and their cage occupancy is independent of TBAB concentrations (Hashimoto et al., 2006; 2008). The inclusion of gas molecule in TBAB SCH from CO₂/H₂ gas mixture was found to be affected by the gas composition. In the case of gas mixture with 40 mol% CO₂, CO₂ gas molecule was found both in the 5¹²6² and 5¹²6³ cages. However, in the case of 18 CO₂/82% H₂ and 10 CO₂/90% H₂, CO₂ gas molecules were only incorporated into the 5¹²6³ cages. In all cases, no H₂ was detected in the small cages. A similar phenomenon was discovered in stimulated biogas (45%CO₂/55%CH₄ gas mixture) by Raman spectroscopy (Xia et al., 2016). CO₂ was found to enter both the 5¹²6² and 5¹²6³ cages, while CH₄ was only incorporated into the 5¹² cages.

Single crystals of TBAB, TBAC, TBPB and TBPC with 15% CO₂/85% N₂ gas mixture were formed and characterized, as summarized in Tab.5 (Hashimoto et al., 2017b). The mixed hydrate of TBAB + 15%CO₂ + 85%N₂ had the same structure as TBAB, and the CO₂ mixed hydrate had an orthorhombic structure with space group of Imma, with symmetry lowered compared to TBAB·38H₂O semi-clathrate hydrate. This demonstrates that the symmetry lowering is dominated by CO₂, even with a mole fraction of only 15% in the gas mixture. The clathrate of TBPC and the gas mixture also had an orthorhombic structure with space group of Cmmm. Due to the poor quality of the single crystals, the TBPB and gas mixture hydrate was probably hexagonal or orthorhombic. Like the TBAB semi-clathrate hydrate, the hydrate after incorporating gas mixtures remained tetragonal.

Tab.5 Summary of quaternary salts and CO₂/N₂ mixed hydrate crystal parameters (Hashimoto et al., 2017b)

	TBAB + CO2 + N2	TBAC + CO2 + N2	TBAF + CO2 + N2	TBPB + CO2 + N2	TBPC + CO2 + N2
Concentration (wt%)	0.2	0.2	0.2	0.2	0.2
PT conditions	5.04 MPa, 284.2 K	5.03 MPa, 287.2 K	298.6K, 3.01MPa	4.93 MPa, 285.2 K	5.09 MPa, 285.2 K
Crystal system, space group	Orthorhombic, Imma	\mathrm{T}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{l},P_2^4 /m	\mathrm{T}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{l},P_2^4 /m	Hexagonal (Possibly orthorhombic)	Orthorhombic, Cmmm
Unit cell dimensions (Å)	a = 21.419(4)b = 25.833(5)c = 12.218(2)	a = 23.870(3)c = 12.497(3)	a = 23.301 (3)c = 12.179 (2)	a = 12.0602(17)c = 12.585(3)	a = 12.036(2)b = 21.145(4)c = 12.685(3)

TBAF was found to better stabilize CO₂ hydrate compared to other quaternary salts mentioned in this paper. The structure of TBAF and 12% CO₂/N₂ mixed hydrate was characterized to explain its gas separation performance (Hashimoto et al., 2020). Polyhydrates of TBAF semi-clathrate hydrate can form at different concentrations, including cubic, tetragonal and monoclinic (McMullan et al., 1963; Udachin and Lipkowski, 2002). After incorporating the CO₂/N₂ gas mixture into the TBAF semi-clathrate hydrate, only columnar-shaped crystals with tetragonal structure were found. There were ten D cages per unit cell, while only two D cages per TBAF were available for gas capture. Therefore, TBAF captured less gas than orthorhombic semi-clathrate hydrates with three D cages per salt, like TBAB, TBPB and TBPC.

3 Thermodynamical properties of semi-clathrate hydrate with gas mixture

The thermodynamic properties of semi-clathrate hydrates have been extensively studied as they are essential parameters for practical applications. One unique advantage of the semi-clathrate hydrate over gas hydrates in different application scenarios is that they can be formed at ambient pressure and relatively high temperature. Phase equilibrium diagrams, which represent the thermodynamic properties of semi-clathrate hydrates, can provide a technical foundation for industrial applications and also aid in the study of their dynamic properties. In this section, four methods for measuring the phase equilibrium conditions of semi-clathrate hydrate are introduced, including visual observation method, differential scanning calorimetry, pressure search method and the Schreinemaker’s method. Additionally, the thermodynamical properties of clathrate hydrate of quaternary salts and the mixed hydrate of quaternary salts with gases and gas mixtures are summarized.

3.1 Phase equilibrium condition measurement

The visual observation apparatus provides a simple means of measuring the equilibrium temperature of the SCH with melting temperature typically above the point (Suginaka et al., 2012). As shown in Fig.2(a), the dissociation and nucleation process can be directly observed through an optical window made of transparent glass or sapphire glass using an optical microscope (Sakamoto et al., 2011). To perform the measurement, a solution of the guest substance is filled into a glass tube, and the temperature is controlled by circulating a cooling liquid. SCH is formed by lowering the system temperature and then incrementally increase it in steps of \mathrm{\Delta }T. At each temperature step, the system temperature is held for several hours to achieve equilibrium. If SCH remains stable, the temperature is increased further, otherwise if the hydrate dissociates at a temperature of T, the equilibrium temperature of this SCH will be T–\mathrm{\Delta }T. This technique has the advantage of high accuracy, but is only suitable for measuring the equilibrium temperature above the ice point, as the optical apparatus is unable to distinguish between hydrate and ice. Additionally, this method is time-consuming, taking days to determine the equilibrium condition in a single run.

Fig.2 Schematic diagram of (a) the visual observation apparatus (Sakamoto et al., 2011), (b) isochoric pressure-search technique.

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Ye and Zhang (2014a) used a different method based on visual observation to determine phase equilibrium, but with a different heating protocol. Instead of step heating after hydrate formation, the system temperature is continuously increased at a constant speed of 0.1 K/h. When the hydrate is about to disappear, the heating rate is reduced to one-third of the initial value until hydrate completely dissociates. The corresponding temperature and pressure are considered as the phase equilibrium data. The continuous heating protocol reduces the required time for each measurement. According to the comparative study of step heating and continuous heating protocols (Tohidi et al., 2000), the error associated with continuous heating is a function of the heating rate. Since the heating rate is slow, the measurement is reliable, and the accuracy is consolidated by the phase equilibrium data of nitrogen gas which is consistent with the literature.

The visual observation apparatus typically operates at ambient pressure and can be used to investigate the phase equilibrium condition of SCH. However, in some applications that use SCH as gas hydrate promotor, the phase equilibrium determination requires relatively high pressure. In such cases, differential scanning calorimetry (DSC) is an effective technique that can determine stability conditions and provide the dissociation enthalpy, which is an important parameter in phase-changing material applications (Kharrat and Dalmazzone, 2003; Deschamps and Dalmazzone, 2009; Mayoufi et al., 2011). DSC is more efficient, but its measurement accuracy is compromised. The confidence limits are determined by the calibration results of the melting point of pure samples with known melting points, such as ice. In practical conditions, these confidence limits can be as high as ±0.4 °C (Mayoufi et al., 2010), causing systematic deviation between measurement results by different groups. Additionally, the equilibrium temperature is deemed as the onset on the DSC curve (Mayoufi et al., 2011), which is lower than the visual observation method that corresponds to the point at which all the hydrates disappear.

Different types of SCH with similar equilibrium temperatures can form in the quaternary salt-water binary system under certain temperature and ambient pressure. These SCHs are difficult to distinguish by visual observation, and their corresponding peak are always mixed together on DSC curves. Oyama et al. (2005) proposed two measuring methods to individually measure the thermophysical properties of type A and type B TBAB hydrate. The visual observation method was used to determine the lower melting temperature, while the autoclave system was adopted to obtain the higher melting temperature (Oyama et al., 2003). During the measurement, SCH was first formed under relatively low temperature and the system temperature was then raised at a constant heating rate, say 2 °C/h. If no phase changes occur, the temperature of the sample fluids will increase with the same rate. Because the hydrate dissociation is endothermic, the inflection point of the temperature curve of the sample fluids corresponds to the phase change points, i.e., the higher melting points. Oyama et al. (2003) successfully determined the congruent melting point of both type A and type B TBAB hydrate, which is 12 and 9.9 °C, respectively. This result is consistent with the results measured by visual observation method, which can only determine the congruent melting point of the type A TBAB hydrate (Shimada et al., 2003).

The isochoric pressure-search technique is commonly used to measure the phase equilibrium conditions for gas and quaternary salt solution mixtures. As shown in Fig.2(b), the system is initially cooled down, and at a certain point, the pressure drops abruptly due to hydrate formation. The temperature is then stabilized for a few hours to ensure the complete hydrate formation. Then the system is heated stepwise (Arjmandi et al., 2007; Chapoy et al., 2010) or continuously (Joshi et al., 2013; Sánchez-Mora et al., 2019) until a change in the slope formed between the temperature and pressure conditions is observed. The inflection point, labeled with a red pentagram in Fig.2(b), is the dissociation point.

Schreinemakers’ method is a powerful technique for deriving the phase diagram, yet it has been largely overlooked in the field of chemical engineering. One of the most remarkable features of this method is that it can derive the P-T diagram of a given system based solely on knowledge of the chemical compositions, even when little or no experimental data are available. However, only a few studies have taken advantage of this method to obtain phase equilibrium diagrams (Aladko and Dyadin, 1996). A comprehensive discussion on the use of Schreinemaker’s geometric approach to obtain the complete phase diagram for hydrates is presented by Mehta et al. (1996).

3.2 Thermodynamical properties of semi-clathrate hydrate

3.2.1 TBAB

The equilibrium temperature of TBAB under ambient pressure has been extensively studied by numerous research groups. In the TBAB-water binary system, the guest molecules can stabilize different frameworks under various conditions, as reported in previous studies (Gaponenko et al., 1984; Rodionova et al., 2013; Oshima et al., 2018). However, some studies have reported slightly deviating results in their measurements, as shown in Fig.3, due to various factors such as differences in measurement techniques, the volume of the salt solution used (Tohidi et al., 1994) and polymorphism.

Fig.3 (a) Equilibrium temperature of TBAB SCH under ambient pressure with no consideration of polyhydrates (Sun et al., 2008; Deschamps and Dalmazzone, 2009; Sato et al., 2013; Kobori et al., 2015a; Wang and Dennis, 2015); (b) Equilibrium temperature of Type A and Type B TBAB SCH (Darbouret et al., 2005; Oyama et al., 2005; Shimada et al., 2005a; Hashimoto et al., 2008; Ma et al., 2010).

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It is known that hydrate formation may not be repeatable, as it depends on factors such as cooling rate, water history and subcooling degree.Therefore, the phase equlibrium curve is typically obtained during hydrate dissociation. Wang and Dennis (2015) used a step cooling procedure to determine the formation temperature, which resulted in a noticeable from the other measurements. The equilibrium temperature determined by DSC refers to the onset of hydrate dissociation (Deschamps and Dalmazzone, 2009), which may be lower than that observed by optical observation (Sun et al., 2008; Sato et al., 2013; Kobori et al., 2015a), corresponding to almost complete hydrate dissociation temperature. However, the results obtained through visual observation methods deviate from each other, possibly due to differences in the volume of salt solution used. The more of the fluid, the longer time should be kept to allow the system in equilibrium. The volumes of salt solution used in Sato et al. (2013) and Kobori et al. (2015a) were 4.5 and 0.7 cm³, respectively, and their results are consistent with each other. Sun et al. used a larger volume of 80 cm³, and their equilibrium temperature was approximately 0.15–2.85 K lower than the others, with a linearly decrease in temperature difference with increasing TBAB solution.

Fig.3(a) displays the phase equilibrium behavior of TBAB as a function of its mass fraction, which exhibits an initial increase followed by a slow decrease at high concentrations. The highest temperature in this curve corresponds to the congruent melting temperature (Oyama et al., 2005), which has been reported to occur at a concentration range of 35–40 wt% in Deschamps and Dalmazzone (2009) and Kobori et al. (2015a). However, Sato et al. (2013) have shown that this range is narrower and falls between 36 and 37 wt%.

Tab.2 summarizes the various polyhydrates of TBAB, with TBAB∙26H₂O and TBAB∙38H₂O being the most easily formed, also known as type A and type B hydrates (Shimada et al., 2003; 2005a). The thermodynamical properties of these two hydrates are depicted in Fig.3(b), which has been reported by several studies (Darbouret et al., 2005; Oyama et al., 2005; Shimada et al., 2005a; Hashimoto et al., 2008; Ma et al., 2010). Oyama et al. (2005) combined visual observation and isochoric continuous heating method to obtain the phase equilibrium temperature. Ma et al. (2010) used DSC to record the melting temperature of TBAB SCH with a heating rate of 0.5 °C/min. As shown in Fig.3(b), when the TBAB concentration is below 19 wt%, type B TBAB SCH is typically more stable than type A hydrate, a result that is better informed at solution concentrations above 20 wt%. The congruent melting points are approximately 40 wt% for type A and 32 wt% for type B, with melting temperature of 285.1 and 282.9 K, respectively. These findings are consistent with the literature (Gaponenko et al., 1984). The congruent melting point of 36 wt% and 37 wt% reported by Sato et al. (2013) may indicate the coexistence of these two types of TBAB SCH.

3.2.2 Other quaternary salts

The equilibrium temperature of TBAC has also been reported with some inconsistency. As shown in Fig.4, the measurements by Sun et al. (2011b) are 1.1–1.3 K lower in all concentration than those obtained by Nakayama (1987) and Sato et al. (2013). While both Sun et al. (2011b) and Sato et al. (2013) used visual observation technique, the fluid volume used was 100 and 4.5 cm³, respectively. TBAC is known to have three structures, and their melting points and stoichiometry have been reported in the literature and are summarized in Tab.6. Unlike TBAB, where different structures have markedly different melting points, all three types of TBAC SCH have tetragonal structures with slightly different melting points. The congruent melting points determined from Fig.4 are found to be between the most stable TBAC·30H₂O at 15.2 °C and the metastable phase of TBAC·24H₂O at 14.7 °C.

Fig.4 Equilibrium temperature of (a)TBAC and TBAF SCH; (b)TBPB and TBPC SCH (Nakayama, 1987; Sakamoto et al., 2008; Lee et al., 2010; Mayoufi et al., 2011; Sakamoto et al., 2011; Sun et al., 2011b; Lin et al., 2013; Mohammadi et al., 2013a; Sato et al., 2013; Suginaka et al., 2013; Zhang et al., 2013; Ye and Zhang, 2014a; 2014b; Sales Silva et al., 2016).

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Tab.6 Stoichiometry, melting point and measurement method of TBAC SCH

Stoichiometry	m.p. (°C)	Measurement method	Ref.
TBAC·(32.2±0.4)H2O	15.0	DSC	Rodionova et al. (2010)
TBAC·(32.21±0.28)H2O	15.1	Schreinemaker’s method	Aladko & Dyadin (1996)
TBAC·30H2O	15	Visual observation	Nakayama (1982)
TBAC·30H2Oa	15.2	DSC	Nakayama (1987)
TBAC·(29.7±0.4)H2O	15.1	DSC	Rodionova et al. 2010)
TBAC·(29.4±0.29)H2O	15.1	Schreinemaker’s method	Aladko & Dyadin (1996)
TBAC·(24.8±0.3)H2O	14.9	DSC	Rodionova et al. (2010)
TBAC·(24.11±0.16)H2O	14.7	Schreinemaker’s method	Aladko & Dyadin (1996)

Note: a) Hydration number is around 30.

As shown in Fig.4, the equilibrium temperature curves of TBAF coincide with each other (Sakamoto et al., 2008; Lee et al., 2010; Mohammadi et al., 2013a). The melting points are consistently reported to be 27.6 °C. This can be attributed to the fact that the polyhydrates of TBAF, namely TBAF·32H₂O and TBAF·28H₂O, have similar melting point of 27.2 °C (Dyadin et al., 1977) and 27.7 °C (Komarov et al., 2007).

As shown in Fig.4(b), there are discrepancies among the reported equilibrium data of TBPB from literature that may be attrbuted to the measurement techniques used. Suginaka et al. (2012) employed optical observation methods, while the other studies used DSC. The differences among the DSC results originate from the different dissociation points, as shown in Fig.5. Zhang et al. (2013) used the peak temperature T_peak, which better fit the data from Suginaka et al. (2012) than T_onset, which is used to determine the dissociation temperature of the isothermal phase transition. The corrected peak temperature (T_GEFTA) (Höhne et al., 1990) and the end temperature (T_end) were used to estimate the non-isothermal phase transition temperature. Lin et al. (2013) employed two DSC procedures, namely dynamic and stepwise schemes, to estimate the accuracy of different temperatures. The results show that T_end is more accurate than T_GEFTA. Mayoufi et al. (2011) considered T_GEFTA as the dissociation temperature, which resulted in lower equlibrium data than those obtained from Sales Silva et al. (2016) using T_end. Therefore, the discrepancies among the reported equilibrium data of TBPB may be attributed to differences in the measurement techniques used, as well as the dissociation points and temperature values considered.

Fig.5 Definition of characteristic peak temperature of DSC endothermic peaks, modified from (Sales Silva et al., 2016).

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The two reported equilibrium data of TBPC are consistent with each other, as both studies utilized the visual observation method (Sakamoto et al., 2011; Ye and Zhang, 2014a). Compared with TBPB, TBPC has better thermal stability, with a melting point of 11.3 °C, while the melting point for TBPB is 9.25 °C.

3.2.3 Comparison

Despite differences in the measurement techniques employed, the phase equilibrium temperature of quaternary salt semi-clathrate hydrates initially increases with concentration and then decreases. The maximum phase equilibrium temperature, also known as the congruent melting temperature (CMT), corresponds to a concentration of approximately 35 wt%. If a salt solution with a higher concentration is used, the excess ions will act as hydrate inhibitors and reduce the thermal stability of the hydrate.

The congruent points of different quaternary salts SCH are summarized in Fig.6. The stability of these hydrates are determined by the compatibility of the hydrophobic cations in the framework structures built of water molecules together with an anion (Dyadin and Udachin, 1984). The hydrophobic interaction plays a critical role in the formation process of SCH, and it depends on the size of the molecule’s hydrophobic part. As shown in the figure, with the same anion, the thermal stability of QAS SCH is higher than that of QPS SCH. This can be attributed to the different cations used, as the bond length of C–P in quaternary phosphonium salt is 1.73 Å, which is larger than the bond length of C–N (1.53 Å) in quaternary ammonium salt. As a result, the C–P bond induces larger distortion to the water molecule, which destabilizes the hydrate structure (Muromachi et al., 2014a).

Fig.6 The congruent melting point of different quaternary salts (Nakayama, 1987; Sakamoto et al., 2008; 2011; Suginaka et al., 2012; Sato et al., 2013).

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The hydrophilic interaction between water molecules and anion influences the formation conditions of the quaternary salts SCH. As illustrated in the figure, the CMP decreases with onion radius in both the QAS and QPS. The comparison data are summarized in Tab.7. The spatial complementarity between the guest molecules and the host cages, as well as the hydrophilic interaction between the anion and water molecules, contribute to the thermal stability of quaternary salt semi-clathrate hydrate (Aladko et al., 2003). In the case of QAS clathrates, the thermal stability decreases from fluorine to chloride to bromine with increase radius. Tetra-n-butyl-ammonium iodine cannot form a hydrate structure (Dyadin and Udachin, 1987), which is believed to be caused by the different distortions induced in the water molecule when halides form host cages with water molecules (Nakayama, 1983; Aladko et al., 2003). The hydrogen bond length of the host cages formed from fluorine ion with water molecule is 2.791 Å (Kobori et al., 2015b), which is very close to the hydrogen bond length of 2.8 Å in water. On the other hand, other halides with larger radii, such as chloride and bromide ions, induce larger distortions with hydrogen bond lengths of 3.1 Å (Lundgren and Olovsson, 1967) and 3.4 Å (Lundgren and Olovsson, 1968), resulting in less stable hydrates.

Tab.7 The relation between the congruent melting points and radius of the anion

Quaternary salt	CMP (K)	Concentration	Radius of anion (Å)	Ref.
TBAB	285.9	0.36–0.37	1.96	Sato et al. (2013)
TBAC	288.35	0.35	1.81	Nakayama (1987); Oshima et al. (2020)
TBAF	300.75	0.34	1.15	Sakamoto et al. (2008)
TBPB	282.4	0.35	1.96	Suginaka et al. (2012)
TBPC	283.45	0.36	1.81	Sakamoto et al. (2011)

3.3 Thermodynamical properties of semi-clathrate hydrate with gas mixture

In practical CO₂ capture and sequestration applications, the operation conditions depend on the thermodynamical properties of carbon dioxide and quaternary salt double hydrate. It is expected that CO₂ will form binary hydrate with quaternary salt (Kobori et al., 2015a), while the gas to be separated from CO₂ is kept in the gas phase. The operation conditions need to be chosen to ensure the formation of CO₂ and quaternary salts hydrate, while avoiding the formation of ternary hydrate formation of gas mixtures and quaternary salts. Therefore, the equilibrium condition of CO₂ and quaternary salts hydrate, as well as that of the ternary hydrate, need to be determined.

The equilibrium conditions for the binary hydrates of QAS/QPS and CO₂ are summarized in Fig.7 and Fig.8. Acting as hydrate promoters, quaternary salts can greatly alleviate the formation condition of the CO₂ gas hydrate. Additionally, CO₂ gas enters the empty cages of the QAS SCH, making the double hydrate more stable than QAS SCH (Sun et al., 2008). As shown in Fig.7, the CO₂-TBAB double hydrate forms at a pressure as low as 0.4 MPa and temperature of 280.2 K with a TBAB concentration of 0.05. The equilibrium temperature first increases with the concentration of the quaternary salts (Arjmandi et al., 2007; Lin et al., 2008; Deschamps and Dalmazzone, 2009; Li et al., 2010a; Joshi et al., 2013; Chazallon et al., 2014; Ye and Zhang, 2014a; Sánchez-Mora et al., 2019). However, when the salt concentration exceeds a certain value, the stability effect reduces with concentration. In the case of TBAB, the critical concentration is near 40 wt% (Ye and Zhang, 2012) and it is 35 wt% for TBAC (Ye and Zhang, 2014a) and 33 wt% for TBAF (Adisasmito et al., 1991). This behavior is similar to that of QAS SCH forming at atmospheric pressure. TBAF can better stabilize the CO₂ hydrate than TBAB and TBAC.

Fig.7 Phase equilibrium condition of CO₂ and QAS binary hydrates: (a) TBAB + CO₂; (b) TBAC + CO₂; (c) TBAF + CO₂ (Adisasmito et al., 1991; Arjmandi et al., 2007; Lin et al., 2008; Deschamps and Dalmazzone, 2009; Li et al., 2010a; Lee et al., 2012; Joshi et al., 2013; Mohammadi et al., 2013a; Chazallon et al., 2014; Ye and Zhang, 2014a; Sánchez-Mora et al., 2019; Momeni et al., 2020).

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Fig.8 Phase equilibrium condition of CO₂ and QPS binary hydrates: (a) TBAB + CO₂; (b) TBAC + CO₂; (c) TBAF + CO₂ (Zhang et al., 2013; Xie et al., 2021; Suginaka et al., 2013; Ye and Zhang, 2014b; Momeni et al., 2020).

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As shown in Fig.8, the stabilization of CO₂ with QPS also increase first with concentration and then decrease above a certain value. The critical concentration of TBPB and TBPC is both near 35 wt%. The equilibrium condition of CO₂ and TBPB is slightly milder than that of CO₂ and TBPC with the same salt concentration. It has been demonstrated that TBAB and TBPB with stoichiometric concentration of TBAB∙26H₂O and TBPB∙32H₂O have a competent stabilization effect on CO₂ hydrates (Mayoufi et al., 2010). As described in the previous section, the equilibrium temperature of TBAC is higher than that of TBPC, due to the smaller distortion to the water molecule from the C–N bond. However, after incorporation of CO₂, this behavior partly changes. When the mass fraction w = 0.05, 0.10 and 0.20, the equilibrium temperature of CO₂ + TBAC is higher than that of CO₂ + TBPC double hydrates at relatively low pressure, while it is the opposite at higher pressure (Ye and Zhang, 2014a). To explain these findings, the morphology evolution of TBAC and TBPC with and without CO₂ was compared. The morphology of TBAC with and without CO₂ was different at dilute solution. TBAC hydrate initially appeared as a needle-like shape and then evolved into a columnar shape, while TBAC hydrate with CO₂ initially showed two quite different morphologies of needle-like and hexagonal plate shape and changed into a columnar shape after a short period. The morphology of TBPC with and without CO₂ was the same, showing similar morphology to TBAC hydrate. Thus, the structural variation of TBAC after incorporation of CO₂ gas may contribute to the thermodynamical difference between TBAC and TBPC hydrate with CO₂.

Therefore, from the perspective of operation conditions in CO₂ gas capture and sequestration, TBAF with concentration of 34 wt% is preferred due to the greatest stability enhancement it provides. However, there are also other factors that affect the efficiency of a hydrate-based CO₂ removal process, such as the amount of gas entrapped in a given mass of semi-clathrate hydrate and the gas selectivity of the gas mixture. These factors must also be considered when selecting the optimal operating conditions for CO₂ gas capture and sequestration.

The phase equilibrium conditions of CO₂/H₂ mixture and quaternary salt hydrate are summarized in Fig.9 (Li et al., 2010b; Kim et al., 2011; Mohammadi et al., 2013b; Park et al., 2013; Babu et al., 2014c). CO₂ hydrate (Adisasmito et al., 1991) has better thermal stability than H₂ hydrate (Chapoy et al., 2010), thus the thermal stability of the gas mixture increases with the proportion of CO₂. The addition of quaternary salts can shift the phase equilibrium of CO₂/H₂ mixture hydrate to higher temperature and lower pressure, with TBAF having the highest enhancement of the thermal stability. The thermal stability improvement also increases with the concentration of the quaternary salts. However, this trend has an inflection point that occurs at a TBAB concentration between 3 and 3.7 mol%, similar to the phase equilibrium condition of TBAB hydrate. At TBAB concentration higher than 3.7 mol%, the excess amount of TBAB molecules remains as free ions and inhabits hydrate formation. The inflection point was not discovered in the other quaternary salts, as the measurement data were limited. However, it can be estimated that the existence of this inflection point based on the phase equilibrium curve of the quaternary salts.

Fig.9 Phase equilibrium condition of CO₂/H₂ mixture and quaternary salt hydrate: (a) TBAB + CO₂ + H₂ (Li et al., 2010b; Kim et al., 2011; Mohammadi et al., 2013b; Park et al., 2013); (b) TBAF + CO₂ + H₂ (Park et al., 2013); (c) TBANO₃ + CO₂ + H₂ (Babu et al., 2014c).

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The thermodynamical properties of CO₂/N₂ mixture and quaternary salt hydrate have been collected in Fig.10 (Duc et al., 2007; Deschamps and Dalmazzone, 2009; Meysel et al., 2011; Belandria et al., 2012; Majumdar et al., 2012; Mohammadi et al., 2012; Hashimoto et al., 2017a; 2017b; Sánchez-Mora et al., 2019; Hashimoto et al., 2020; Kim et al., 2020). The equilibrium pressure of N₂ hydrate is much higher than the CO₂ hydrate (Sánchez-Mora et al., 2019), thus the CO₂ in the gas mixture can increase the thermal stability of gas mixture hydrate. The more CO₂ there is, the higher the thermal stability. The enhancement of quaternary salt to gas hydrate is similar to that of CO₂/H₂ gas mixtures. However, since higher concentrations of quaternary salts have not been measured in the literature, the inflection point indicating the optimum concentration have not been determined. It is estimated to have the same trend as in the case of CO₂/H₂ gas mixtures. The thermodynamic stability of tetra-iso-amyl ammonium bromide (TiAAB) semi-clathrates with CO₂/N₂ mixture was also investigated (Majumdar et al., 2012). Since TiAAB has even better thermal stability than TBAF, the ternary hydrate of TiAAB + CO₂ + N₂ has the highest thermal stability among all the other quaternary salts shown in the figure.

Fig.10 Phase equilibrium condition of CO₂/N₂ mixture and quaternary salt hydrate: (a) TBAB + CO₂ + N₂; (b) TBAC/TBAF/TBPB/TBPC/TiAAB + CO₂ + N₂ (Adisasmito et al., 1991; Duc et al., 2007; Deschamps and Dalmazzone, 2009; Meysel et al., 2011; Belandria et al., 2012; Majumdar et al., 2012; Mohammadi et al., 2012; Hashimoto et al., 2017a; 2017b; Sánchez-Mora et al., 2019; Hashimoto et al., 2020; Kim et al., 2020).

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The phase equilibrium curve of CO₂/CH₄ mixture and quaternary salt hydrate were obtained to design the separation of CO₂ from the biogas (Deschamps and Dalmazzone, 2009; Acosta et al., 2011; Fan et al., 2013; Mohammadi et al., 2013b; Li et al., 2017; Zang and Liang, 2017; Yan et al., 2019). As illustrated in Fig.11, the thermal stability of CO₂ and CH₄ gas mixtures slightly increases with the proportion of CO₂. When the concentration of the TBAB solution is as low as 1 wt%, the promotion effect of TBAB on the gas hydrate is negligible. Significant improvements in the thermal stability of gas mixture hydrates occur when the concentration of quaternary salt increases to 4.3 wt%. The operating pressure can be greatly lowered, achieving 2MPa at ambient temperature using TBAB with a concentration of 40 wt%.

Fig.11 Phase equilibrium condition of CO₂/CH₄ mixture and quaternary salt hydrate with different gas compositions: (a) CO₂ composition 60%, blue curve; CO₂ composition 50%, black curve; (b) CO₂ composition 33%, blue curve; CO₂ composition 40%, black curve (Deschamps and Dalmazzone, 2009; Acosta et al., 2011; Fan et al., 2013; Mohammadi et al., 2013b; Li et al., 2017; Zang and Liang, 2017; Yan et al., 2019).

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4 Application in carbon dioxide capture and separation

Carbon dioxide is considered the main contributor to global warming, and as human societies develop, the demand for energy is increasing. Conventional fossil fuels are currently the dominant energy resources, and CO₂ emissions during the fossil fuel combustion are the primary causes of the CO₂ concentrations in the atmosphere. CO₂ capture can be applied either before or after fuel combustion, namely, pre-combustion (Babu et al., 2015) and post-combustion (Wang and Song, 2020). Pre-combustion involves capturing CO₂ from fuel gas generated by Integrated Coal Gasification Cycle (IGCC) plants, which typically containing 40% CO₂ and 60% H₂. Post-combustion deals with CO₂ capture from flue gas, consisting of approximately 15%–20% CO₂ and 5% O₂, with the remaining gas being N₂.

Hydrate formation is a feasible way for capturing CO₂ from nitrogen, hydrogen, or other waste gases from coal combustion and biogases, among others (Ma et al., 2016). Howerver, the stringent conditions of relatively high pressure and low temperature required to form CO₂ hydrate hinder its application. The presence of quaternary ammonium salts can shift the equilibrium condition by formation a double hydrate with CO₂ to milder conditions of higher temperature and lower pressure, in which the CO₂ molecules enter the empty cages of the hydrate. In this case, CO₂ capture and separation from other gases consume less energy and show great application potential. This section summarizes the performance of CO₂ capture from fuel gas, flue gas and biogas using various quaternary salts.

4.1 Mechanism of CCS with SCH

Essentially, the principle of CCS with quaternary salts is to take advantage of the thermodynamical difference between the salt with CO₂ and the other gas that needs to be separated. As shown in Fig.12, the binary hydrate of TBAB and CO₂ exhibits higher thermal stability than that of TBAB and H₂/N₂/CH₄ hydrates. Therefore, CO₂ molecules can be enclathrated in the hydrate cage, leaving the gas molecules to be separated in the gas phase. The purified H₂ from fuel gas can be further utilized as clean energy.

Fig.12 Phase equilibrium condition of TBAB and H₂/N₂/CH₄/CO₂ hydrate with mass concentration of 0.1 (Arjmandi et al., 2007; Mohammadi et al., 2011; Joshi et al., 2013; Jin et al., 2019).

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The phase equilibrium curve of CO₂ hydrate and CH₄ hydrate are pretty close to each other, with CO₂ having slightly better thermal stability than CH₄ (Aladko et al., 2003; Lee et al., 2012). Therefore, to separate CO₂ from CO₂/CH₄ gas mixtures, the optimal operation conditions exist in the gap between the P–T curve of CO₂ and CH₄ gas hydrates that corresponds to CO₂ hydrate formation while CH₄ remains in the gas phase (Babu et al., 2014b). Delicate control of the operation conditions is required in the practical applications.

4.2 Performance parameters of CCS

First, the parameters that characterize carbon dioxide capture and separation performance are described. The number of moles of gas consumed at any time during hydrate formation can be calculated by the following equation:

(1)\mathrm{\Delta }n= n_0- n_t.

In some cases, the normalized gas uptake is defined as:

(2)\mathrm{\Delta }{n}_{nor}=\frac{\mathrm{\Delta }n}{n_w},

where n_w is the number of moles of the water utilized in the experiment. \mathrm{\Delta } n_{nor} is related to the conversion of water into gas hydrates or semi-clathrate hydrates.

The split fraction of carbon dioxide, i.e., CO₂ recovery, in gaseous and hydrate phase is calculated as follows (Linga et al., 2007a):

(3)\text{S.Fr.}=\frac{n_{\text{C}{\text{O}}_{\text{2}}}^{\text{H}}}{n_{\text{C}{\text{O}}_{\text{2}}}^{\text{Feed}}},

where n_{\text{C}{\text{O}}_{\text{2}}}^{\text{H}} is the number of moles of CO₂ in hydrate phase at the end of the experiment and n_{\text{C}{\text{O}}_{\text{2}}}^{\text{Feed}} is the number of moles of CO₂ in the initial feed gas.

Another important parameter is the separation factor that is defined as (Linga et al., 2007a):

(4)\text{S.F.}=\frac{n_{\text{C}{\text{O}}_{\text{2}}}^{\text{H}}/n_{\text{C}{\text{O}}_{\text{2}}}^{\text{gas}}}{n_{\text{X}}^{\text{H}}/n_{\text{X}}^{\text{gas}}},

where X denote the other gas from the feed gas mixture that will be separated from CO₂, n_{\text{X}}^{\text{H}} is the number of moles of X in hydrate phase, n_{\text{C}{\text{O}}_{\text{2}}}^{\text{gas}} and n_{\text{X}}^{\text{gas}} are the number of moles of CO₂ and X in the gas phase at the end of experiment.

4.3 CO₂ capture and separation from fuel gas

The gas mixture from IGCC power plants typically contain 40% CO₂ and 60% H₂. CO₂ can be captured for further disposal, while the resulting H₂ can be utilized as clean energy. It should be noted that quaternary salts are also promising candidates for hydrogen storage (Chapoy et al., 2007; Hashimoto et al., 2008), although this is beyond the scope of this work. Quaternary salts are potential promoters for CO₂ capture from fuel gas through hydrate formation, and their performance are summarized in Tab.8. After one stage separation with the quaternary salt solution, the CO₂ concentration in the hydrate slurry can be enriched to more than 90%. Meanwhile, the operation pressure can be moderated to less than 5 MPa. Since the fuel gas pressure from the ICGC plants is 2.5–5 MPa (Park et al., 2013), there is no need to compress the gas mixture, which can significantly reduce the cost of the CO₂ capture process.

Tab.8 Performance of quaternary salts for CO₂ capture from fuel gas

Gas mixture	Solution	Concentration (mol)	P (MPa)	T (K)	Reactor volume (mL)	Aqueous volume (mL)	Induction time (min)	∆n (mol)	CO2 in hydrate	S.F.	S.Fr.	Ref.
CO2(39.2)/H2(60.8)	H2O	–	7.5	273.7	323	150	5.3a	0.095	86.5	98.7	0.42	Linga et al. (2007b)
CO2(40)/H2(60)	H2O	–	8	276	250	80	–	0.02c	95	–	–	Park et al. (2013)
CO2(40.2)/H2(59.8)	TBAB	0.29%	3.9	273.5	527	260	1	0.05	86	15.7	0.41	Gholinezhad et al. (2011)
CO2(40)/H2(60)	TBAB	0.29%	3.08	274.15	1350	180	10.2	0.133	96.4	–	0.145	Xu et al. (2013)
CO2(40)/H2(60)b	TBAB	0.29%	6	279.2	142	53	20.1	0.01c	95.2	49.5	–	Babu et al. (2014a)
CO2(40.4)/H2(59.6)	TBAB	0.29%	5	277.4	863	300	19	–	82	–	0.64	Horii & Ohmura (2018)
CO2(39.2)/H2(60.8)	TBAB	0.29%	3	278.15	336	180	–	0.105	96.85	136.08	0.67	Li et al. (2011)
CO2(40.1)/H2(59.9)	TBAB	1%	3	280.15	–	–	25.53	0.03	89.07	25.99	0.241	Kim et al. (2011)
CO2(40)/H2(60)	TBAB	0.6%	8	283.8	250	80	–	0.0025c	95	–	–	Park et al. (2013)
CO2(80.5)/H2(19.5)	TBAB	0.29%	3.31	276.7	527	260	–	–	96	15	0.35	Gholinezhad et al. (2011)
CO2(18)/H2(82)	TBAB	0.29%	3.25	274.15	1350	180	21	0.101	92.8	–	0.163	Xu et al. (2013)
CO2(10)/H2(90)	TBAB	0.29%	4.52	274.15	1350	180	36.4	0.067	96.4	–	0.192	Xu et al. (2013)
CO2(40)/H2(60)	TBAF	3.3%	8	288	250	80	–	0.0035c	95	–	–	Park et al. (2013)
CO2(40.6)/H2(59.4)	TBAF	3.38%	4	298	142	53	167.56	0.008	93.1	–	–	Zheng et al. (2017)
CO2(40)/H2(60)b	TBANO3	1%	6	274.2	142	53	0.55	0.0132c	93.75	32.21	–	Babu et al. (2014c)
CO2(35)/H2(65)	TBAC	2.7%	3.028	288.2	100	30	12	0.011	96	61	0.29	Muromachi (2021)
CO2(34.5)/H2(65.5)	TBPB	2.2%	3.032	284.1	100	30	28	0.019	95	26	0.45	Muromachi (2021)
CO2(34.7)/H2(65.3)	TBPC	2.5%	2.982	284	100	30	17	0.014	97	23	0.34	Muromachi (2021)

Note: a) Memory water of 3 h after hydrate decomposition is used; b) Batch experiments were conducted; c) Normalized gas uptake.

The CO₂ separation from fuel gas involves two phases: gas dissolution and hydrate enclathration. Among all the parameters that affect the CO₂ separation performance, the induction time and the volume ratio of the gas mixture and the aqueous solution mainly contribute to the gas dissolution stage. The hydrate enclathration process is largely affected by the pressure and temperature conditions, the selected quaternary salt and its concentration. The S.F. of different quaternary salts in fuel gas separation is summarized in Fig.13, and the performance comparison among different quaternary salts for CO₂ separation from fuel gas is summarized in Tab.8.

Fig.13 S.F. of different quaternary salts in fuel gas separation (Gholinezhad et al., 2011; Kim et al., 2011; Li et al., 2011; Babu et al., 2014a; 2014c; Muromachi, 2021).

Full size|PPT slide

TBAB is the mostly studied QAS in this application area. Li et al. (2011) carried out a comparative study of different TBAB concentrations ranging from 0.14 to 1 mol% with the same driving force and found that the critical concentration with the optimum CO₂ separation performance was 0.29 mol%. The gas uptake first increased with concentration until 0.29 mol% and then decreased, as did the CO₂ concentration in the hydrate slurry. A high separation factor of 136.08 was obtained, which was the highest among all the quaternary salts in the literature. However, the results from Kim et al. (2011) showed that the critical concentration was 1 mol%, which was different from the aforementioned experiments. The S.F. and S.Fr. in 1 mol% TBAB were 25.99 and 0.241, respectively. The experimental temperature in the latter study was approximately 1 K higher than that in study of Li et al. (2011). In another experiment carried out by Babu et al. (2014a), TBAB solution with concentration of 0.5, 1, 1.5, 2 and 3 mol% were compared. The best performance was found in the 0.3 mol% TBAB solution, which is similar to study of Li et al. (2011), while the maximum separation factor in their experiments was 49.5.

Generally speaking, in case of a dilute TBAB solution, such as 0.29 or 0.3 mol%, the introduction time is much longer than in other cases, allowing more gas to dissolve into the solution. The solubility of CO₂ is much larger than that of H₂, resulting in the formation of more CO₂ and TBAB binary hydrate. TBAB solutions with high concentrations can degrade the CO₂ separation performance. First, the equilibrium temperature of TBAB and H₂ binary hydrate increases with concentration (Li et al., 2010b), and more H₂ tends to enter into SCH structure under certain temperature conditions. Secondly, TBAB SCH forms under higher concentrations before the gas dissolves into the liquid, and the agglomeration of TBAB hydrate hinders gas dissolution. This has been verified by visual observation of CO₂ separation experiments (Zheng et al., 2017).

Polymorphism is one of the key factors that influence gas capture properties. As is known, TBAB forms five polyhydrates under different conditions (Gaponenko et al., 1984; Lipkowski et al., 2002). It has been found that two types of TBAB hydrates, namely, tetragonal type A and orthorhombic type B hydrate, are preferentially formed (Shimada et al., 2003). In the presence of CO₂, type B hydrate is preferred over the type A structure (Muromachi et al., 2014b). The different gas capacity of these two types of TBAB semi-clathrate hydrates has been confirmed and quantified (Zhou and Liang, 2019). CO₂ adsorption experiments were conducted with type A TBAB hydrate particles that were formed in advance and carefully weighed to quantify the volume ratio of CO₂ capture. Based on in situ Raman spectroscopy and powder X-ray diffraction, nCO₂·TBAB·26H₂O transformed into nCO₂·TBAB·38H₂O and TBAB·21 /3H₂O under pressure of 2 MPa. The volume ratio of CO₂ in the standard state absorbed by TBAB·26H₂O hydrate particles changed from 10 v/v at 1 MPa to 67 v/v at 2 MPa. The trapped CO₂ molecules destroyed the TBAB·26H₂O hydrate structure, inducing the formation of a new structure that was more compatible to CO₂ molecules. Muromachi et al. observed irregular CO₂ capture with a TBAB aqueous solution under different subcooling temperatures, the CO₂ capture amount under subcooling of 3.3–4.5 K was three times higher than that under a subcooling temperature of 2.2–3.3 K (Muromachi, 2021). Since the gas capacity of type A TBAB hydrate is less than that of type B TBAB hydrate, the phases observed under different subcooling conditions are probably type A and B TBAB hydrate, respectively.

By using a one-stage separation with TBAB solution, the optimum separation factor and the split fraction of TBAB can achieve 136.08 and 0.67. This means that 67% of CO₂ can be recovered, and hydrate slurries with more than 97 mol% CO₂ can be obtained, indicating the applicability of TBAB in this area. However, under normal conditions, a certain amount of CO₂ still exists in the residual gas after single-stage separation, and multi-stage separation, where the gas mixture from the previous stage is used as the feeding gas should be conducted. With two-stage separation, a composition of 80.5 mol% CO₂ in the first stage can be enriched to more than 95 mol% using TBAB with a concentration of 0.29 mol% (Gholinezhad et al., 2011). To simulate the multi-stage process, an initial gas mixture of 90% H₂/10% CO₂ was prepared for separation, and only 3.6 mol% of CO₂ was left in the residual gas after the separation (Xu et al., 2013).

Since TBAF can better stabilize the CO₂ hydrate than TBAB (Li et al., 2010a), the CO₂ separation performance of TBAF were investigated (Park et al., 2013; Zheng et al., 2017). Based on the kinetics of H₂-TBAF and CO₂-TBAF binary hydrate, Trueba et al. indicated the feasibility of using TBAF for the purification of flue gases (Trueba et al., 2013). However, it was found that the TBAF may not be applicable for CO₂ separation due to its poor performance. The gas uptake was much less than that of TBAB solution (Park et al., 2013; Zheng et al., 2017). This is because about 79.5% of the 5¹² cages in TBAF hydrate framework are filled with water (Komarov et al., 2007; Manakov et al., 2011), which limit the amount of gas that can be absorbed to form binary hydrate. The influence of pressure and temperature conditions, i.e., the driving force of TBAF and CO₂ binary hydrate on CO₂ separation performance, has been studied (Zheng et al., 2017). The experiments were conducted with TBAF at the stoichiometric concentration of 3.38 mol% under pressure of 2, 4, 6 MPa and temperature of 298, 292 and 286 K. The induction time at 298 K was the highest among the three temperatures because more gas dissolved into the liquid solution. The dissolved gas also increased with pressure due to higher gas solubility and fugacity. However, the mole concentration of CO₂ in hydrate slurry in 6 MPa is lower than 4 MPa because at higher pressure, more H₂ molecules occupy the hydrate cages.

Tetra-n-butyl ammonium nitrate (TBANO₃) has also been evaluated as a promoter for precombustion capture of CO₂ (Babu et al., 2014b; Babu et al., 2014c). With 1 mol% TBANO₃, the molar concentration of CO₂ in the hydrate slurry achieved 93%, with a normalized gas uptake of 0.0132 and a separation factor of 32.21, which was comparable to that of TBAB measured with the same apparatus (Babu et al., 2014a). A subsequent study was conducted to investigate the influence of operation temperature and pressure on the gas separation performance. The gas uptake capacity of TBANO₃ was found to be higher than that of TBAB or TBAF at comparable driving forces (Babu et al., 2014b). However, it should be noted that the experiments were conducted with a batch reactor, which was a closed system. The gas uptake was largely affected by the initial feed gas and the liquid (Wang et al., 2020a) and thus was not a proper metric to evaluate the separation performance in this case.

To achieve better CO₂ gas selectivity, it would be ideal if the clathrate hydrate of quaternary salts could reject H₂ molecules. This was found to be the case for TBPB and TBPC semi-clathrate hydrate (Muromachi, 2020). The conclusion was based on the equilibrium condition of quaternary salts solutions and gas mixture. The equilibrium temperature of TBAB/TBAC + CO₂ + H₂ hydrate was higher than that of the TBAB/TBAC + CO₂ hydrate with identical CO₂ fugacity, demonstrating the H₂ inclusion in the hydrate framework that increases its thermal stability. Evidence of H₂ gas entering into the 5¹² cages of TBAB was provided by ¹H nuclear magnetic resonance (NMR) measurements (Park et al., 2013). The equilibrium temperature of TBPB and TBPC SCH with gas mixtures was lower than that with only CO₂ gas, showing their rejection of H₂ gas. It was found that TBPB was favorable in terms of gas storage capacity (Deschamps and Dalmazzone, 2010; Mayoufi et al., 2010). Although TBPC can better stabilize CO₂ hydrate than TBPB (Sakamoto et al., 2011; Iino et al., 2014), TBPB was considered a superior promoter for CO₂ capture from fuel gas in terms of CO₂ separation performance and simple phase behavior (Muromachi, 2021).

All the quaternary salts solutions employed as promoters in fuel gas separation in the literature have resulted in low separation factors and high gas consumption (Park et al., 2013). Therefore, efforts to find the proper quaternary salts as promoters are still necessary. Several factors should be considered for their application. The structure of QAS/QPS SCH is the most important factor that limits gas capacity. Among all the structures described in the previous section, the orthorhombic structure with the most D cages per quaternary salt, shows the highest gas capacity. Better thermal stability is preferred to lower separation cost. The polymorphism of the QAS/QPS should be avoided because it requires delicate control of the operation conditions, which is challenging in practice.

4.4 CO₂ capture and separation from flue gas

The task of capturing flue gas from power plants using hydrates is to separate CO₂ from a CO₂/N₂ mixture at ambient pressure, in which the CO₂ molar content is approximately 15%–20%. As shown in Fig.14, with the same initial CO₂ composition, different quaternary salts have similar separation performance in terms of the CO₂ composition in the hydrate slurry. The obtained CO₂ composition in the hydrate increases with its initial composition, while it is still lower than that of CO₂–H₂ gas mixture after one stage of separation. In fuel gas separation by quaternary salt solutions, the CO₂ composition in the hydrate slurry after one stage of separation is typically larger than 90%. The difference is caused by several factors. First of all, the CO₂ concentration in the flue gas is between 3%–15% and it is 40% in the fuel gas. Higher initial CO₂ concentration induces a higher CO₂ concentration in the SCH. Meanwhile, as shown in Tab.9, the diameter of N₂ is similar to that of a CO₂ molecule, and it can be incorporated in the semi-clathrate cages when the pressure is above a certain value (Muromachi et al., 2016a). However, H₂ can rarely incorporate into the hydrate cages (Muromachi, 2021).

Tab.9 Diameters of different gas molecules

Gas molecule	H2	N2	CH4	H2S	CO2
Diameter (nm)	0.27	0.41	0.44	0.46	0.51

Fig.14 SCH-based separation for CO₂-N₂ gas mixture with various initial CO₂ compositions (Duc et al., 2007; Fan et al., 2009; Herri et al., 2014; Ye and Zhang, 2014b; Kim and Seo, 2015; Hashimoto et al., 2017a; Komatsu et al., 2019; Hashimoto et al., 2020).

Full size|PPT slide

The details of the CO₂ capture performance from flue gas of QAS/QPS in Fig.14 are shown sequentially in Tab.10. Except for the extraordinarily high separation factor of 118.7 obtained from separation of a CO₂-rich gas mixture of 66.5% with TBAB solution, the S.F. of QAS/QPS range from 4 to 30. The extra-high S.F. was achieved by TBAB solution with a mass concentration of 0.112, and the gas mixture to be separated is rich in CO₂ with 66.5%. The CO₂ in the hydrate slurry after separation is as high as 98.9%. With the same initial CO₂ composition of approximately 12% and the same mass concentration of 0.2, TBAC has the highest S.F. of 10.2 while TBPC has the lowest S.F. value of 4.

Tab.10 Summary of the literature data for flue gas separation measurements

Salts	Initial composition (%)	Concentration (wt%)	P (MPa)	T (K)	\mathrm{\Delta }T (K)	n_{\text{C}{\text{O}}_{\text{2}}}^{\text{H}}	S.F.	S.Fr.	Ref.
TBAB	13.3	0.2	1	281.2	3.1	25.8	7.2	–	Hashimoto et al. (2017a)
	15.24	0.32	1	282.2	3.5	76.1	21.9	–	Hashimoto et al. (2017b)
	16.6	0.05	4.03	277.65	–	36.53	9.82	0.53	Fan et al. (2009)
	20	0.05	2	277	–	55.5	9.8	0.54	Komatsu et al. (2019)
	31.7	0.218	5.2	288.6	–	90.9	29.34	–	Herri et al. (2014)
	37.4	0.05	2	285.2	–	78.9	17.6	0.68	Duc et al. (2007)
	66.5	0.112	2.5	283.4	–	98.9	118.7	–	Herri et al. (2014)
TBAC	12.5	0.2	5	286.2	2.5	51.8	10.2	–	Hashimoto et al. (2017a)
	20	0.345	3	284.7	5	60	8	–	Kim and Seo (2015)
TBAF	12	0.2	1.01	295.2	–	37.4	5.2	0.4	Hashimoto et al. (2020)
	16.6	0.04	2.46	277.65	–	56.55	36.98	0.56	Fan et al. (2009)
	20	0.338	3	284.7	5	56	–	–	Kim and Seo (2015)
TBPB	12.1	0.2	5.02	284.2	3.6	31.2	4.8	–	Hashimoto et al. (2017a)
	17	0.05	3.5	277.5	–	61.71	13.25	–	Ye and Zhang (2014b)
	61.71	0.05	3	277.5	–	91.28	16.23	–	Ye and Zhang (2014b)
TBPC	12	0.2	5	284.2	3.7	28.6	4	–	Hashimoto et al. (2017a)

The structures of the QAS/QPS semi-clathrate hydrates play an important role in their CO₂ capture and separation performance. According to the comparative study of the CO₂ separation from 15% CO₂/N₂ gas mixture by TBAB, TBAC, TBPB and TBPC semi-clathrate hydrate (Hashimoto et al., 2017b), the CO₂ selectivity of TBAC with a tetragonal structure was the highest, while the total amount of captured CO₂ by TBAC hydrate was the least, corresponding to 60%–90% of the other hydrates. With polycrystalline structures of orthorhombic and tetragonal, TBAB hydrate had moderate performance and was superior in gas selectivity than TBPB and TBPC hydrate with an orthorhombic structure. The type A TBAB hydrate with a tetragonal structure had better CO₂ selectivity than type B TBAB hydrate with an orthorhombic structure (Rodriguez et al., 2020).

The different gas capacity among different hydrate structures can be explained with the hydration numbers in different structures, which are summarized in Tab.1 (Muromachi and Takeya, 2017). Since guest gas molecules are trapped in the small cages of the SCH, the smallest ratio of the number of water molecules with the 5¹² cages in the orthorhombic hydrate has the highest gas capacity. The cubic structure with the highest ratio is assumed to have the poorest gas capture capability. This prediction was consistent with the experimental study of flue gas separation with TBAF hydrate with a cubic structure (Kim and Seo, 2015; Hashimoto et al., 2020).

Based on CO₂ gas separation experiments with TBAB, TBAC and TBAF under the same driving force of 5 K, it was concluded that the CO₂ selectivity was independent of the types of QAS used (Kim and Seo, 2015). The initial gas mixture with 20% CO₂ was enriched to approximately 60% CO₂ in all QAS hydrate slurry. The difference between Kim and Seo (2015) and Hashimoto et al. (2017b) may result from different experimental methods. In the former case, a semi-batch operation with continuous gas feeding was utilized, while an isolated reactor was adopted in the latter. Kim et al. adopted a batch mode where the reactor was isolated, thus the driving force in the isolated reactor kept decreasing during the gas separation process, which may degrade the separation performance (Zhong and Englezos, 2012; Hashimoto et al., 2017a).

The experimental conditions and the concentration of quaternary salts may influence the performance of CO₂ capture and separation. The pressure of the gas mixture was found to significantly alter the gas separation with TBAB and TBAF solution (Fan et al., 2009). The S.F. first increased and then decreased with pressure because more N₂ accompanied with CO₂ was incorporated into the hydrate cages. The CO₂ recovery remained almost unchanged with pressure. The gas separation performance for TBAB hydrate was found to be better at a pressure of 1 MPa than at high pressure of 2 and 3 MPa (Hashimoto et al., 2017a). They also found that a low temperature was beneficial to engage CO₂ in TBAF semi-clathrate hydrate. Dense TBAF aqueous solution reduced both the CO₂ selectivity and gas capacity, and this tendency was also found for TBAB, TBAC and TBPB hydrates (Hashimoto et al., 2020). Moreover, at higher mass concentration, polymorphism of the SCH may appear and show inconsistent gas separation performance (Hashimoto et al., 2017a; 2020). The volume of the reactor and quaternary salt solution also affects gas separation performance, which was quantified by the gas liquid volume ratio R_v (Li et al., 2009). It was found that a lower R_v corresponded to a higher ratio of gas to hydrate, which is beneficial to CO₂ gas separation.

To improve the practical application of QAS/QPS semi-clathrate hydrates for CO₂ capture, research should be focused on finding appropriate quaternary salts and determining the best physical conditions to capture as much CO₂ gas as possible from gas mixtures. By optimizing the experimental conditions, such as pressure, temperature, mass concentration, and gas-liquid volume ratio, the CO₂ selectivity and gas capacity of QAS/QPS semi-clathrate hydrates can be enhanced. Moreover, the development of new quaternary salts with better gas separation performance and thermal stability is also necessary for the practical application of QAS/QPS semi-clathrate hydrates in CO₂ capture.

4.5 CO₂ capture and separation from biogas

Biogas, which is usually used as a fuel, contains methane, carbon dioxide and small proportion of H₂S gas. To avoid the generation of corrosive sulfurous acid, H₂S needs to be removed (Kamata et al., 2005). Additionally, CO₂ should be separated to purify the natural gas. Results have shown that H₂S can be captured along with CO₂ in the same process, and the CO₂ separation from CO₂/CH₄ gas mixture should be considered accordingly (Castellani et al., 2014). The proportion of CO₂ in biogas generally ranges from 30%–40%.

The separation of CO₂ from CH₄ is challenging from a thermodynamical perspective because their phase equilibrium conditions are pretty close to each other, as shown in Fig.12. On the other hand, CO₂ and CH₄ molecules have similar size and both tend to occupy the small cages in quaternary salts SCH (Lee et al., 2012; Tang et al., 2013). The incorporation of CH₄ or CO₂ molecules can both improve the equilibrium temperature of the quaternary salts. For instance, the equilibrium temperature of TBAB at a mass concentration of 20% is 283.2 K (Sato et al., 2013), it is 287.55 K at a CH₄ gas pressure of 1.421 MPa and 286.9 K at a CO₂ gas pressure of 1.52 MPa, respectively. For single CH₄ and CO₂ gases, the equilibrium temperature is 259.1 K at 1.648 MPa and 274.3 K at 1.42 MPa (Adisasmito et al., 1991). In other words, the temperature difference between TBAB/CH₄ and TBAB/CO₂ is lowered compared to that between CH₄ and CO₂ hydrate. The different gas occupancy of CH₄ and CO₂ in TBAB hydrates may contribute to this phenomenon. As is known, CH₄ prefer to occupy the regular distorted D_B cages (Muromachi et al., 2016b), while CO₂ occupies the highly distorted D_A cages (Muromachi et al., 2014b). The chemical composition of the crystal unit cell was determined to be TBAB·38H₂O·2.14 CH₄ at 284.5 K and 1.92 MPa. It is TBAB·38H₂O·1.79 CO₂ at a similar temperature and pressure condition of 287.3 K and 1.6 MPa. This means that more CH₄ enters into the hydrate cage, thus showing a better stabilization effect on TBAB hydrate.

Despite the difficulties, the separation of CH₄ from CO₂ with quaternary salts is feasible and attempts have been made to find the best salt and optimal conditions (Lee et al., 2011; Fan et al., 2013; Li et al., 2015; Fan et al., 2016; Wang et al., 2016; Xia et al., 2016; Li et al., 2017; Yue et al., 2018; Li et al., 2019; Yan et al., 2019; Wang et al., 2020b). The investigations have been summarized in Tab.11. All quaternary salts can significantly improve the stability of CO₂ and CH₄ hydrate, as shown in Fig.11 (Deschamps and Dalmazzone, 2009; Acosta et al., 2011; Fan et al., 2013; Mohammadi et al., 2013a; Li et al., 2017; Zang and Liang, 2017; Yan et al., 2019). Lee et al. measured the stability of the ternary CO₂ + TBAB + H₂O and CH₄ + TBAB + H₂O system with different salt concentrations of 0.6, 3.7 and 7.7 mol%, and the highest stabilization effect was observed at a concentration of 3.7 mol% (Lee et al., 2011). This concentration corresponded to the stoichiometric concentration of type A TBAB hydrate (Oyama et al., 2005). Yan et al. (2019) investigated the thermodynamical properties of TBAB + CO₂ + CH₄ mixed hydrates at TBAB concentrations of 0.29, 0.62, 1.38 and 2.57 mol%. The thermal stability of the mixed hydrate increased with the TBAB concentration. Therefore, there is an optimum TBAB concentrations to stabilize CO₂/CH₄ hydrate, above which the extra ions from the salt will inhibit the hydrate formation.

Tab.11 CO₂ gas separation performance with quaternary salts from biogas

Gases	Promoters	PT condition	Separation performance	Ref.
33% CO2/CH4	TBAB(0.05 wt%) + [BMIm]BF4	3 MPa278 K	Methane in the residual gas increased and the time required to reach balance was significantly shortened by addition of [BMIm]BF4. The highest CH4 concentration in the residual gas phase was 84.0%, maximum CO2 separation factor of 10.3.	Li et al. (2015)
33% CO2/CH4	TBAB(0.1, 0.293, 0.9) mol%	1.14 MPa281.3 K	TBAB concentration of 0.293 mol% has the optimum separation performance with maximum gas uptake, CH4 fraction in the residual gas phase, CO2 separation factor, and CH4 separation and recovery factor.	Fan et al. (2016)
40% CO2/CH4	TBPB(5–33.2)wt%	2.8 MPa278.1–284.2 K	Maximum CO2 separation factor at 33.2 wt% TBPB solution was (31.7 ± 3.3), 8.9 times of 1% THF. CO2 recovery was 0.41 and the CO2 concentration in hydrate achieved 70.8%.	Li et al. (2017)
45% CO2/CH4	DMSO + THF/TBAB	2.5 MPa283.25-285.96 K	DMSO will increase the solubility of CO2 in solution and CO2 separation factor increased from 40 with only TBAB to 59.22 with TBAB and DMSO.	Xia et al. (2016)
33% CO2/CH4	TBAB + [C8min] BF4	4 MPa276.15–279.15 K	The combination of TBAB and [C8min] BF4 could increase the mole friction of CH4 in residual gas, while the CH4 in hydrate also increased, and the recovery of CH4 from biogas decreases.	Yue et al. (2018)
40% CO2/CH4	TBAB	2.8 MPa, 278.8 K	CO2 separation factor is 85.5, CO2 recovery is 0.344, and CO2 concentration in hydrate is 96.5%	Li et al. (2019)
	TBPB	2.8 MPa, 278 K	CO2 separation factor is 33.8, CO2 recovery is 0.293, and CO2 concentration in hydrate is 92.1%
40% CO2/CH4	TBAB(0.29, 0.62, 1.38, 2.57) mol %	2.8 MPa280.1–284.8 K	0.62 mol% TBAB gave highest gas consumption and highest CO2 recovery of 0.502. The highest gas separation factor of 36.5 and the CO2 concentration in hydrate was 73% achieved at concentration of 2.57 mol%.	Wang et al. (2020b)

The optimum concentration of quaternary salt that corresponds to the highest thermal stability of mixed quaternary salt and CH₄/CO₂ mixed hydrate may not be consistent with that providing the optimal separation performance. The optimal separation performance with maximum gas uptake, CH₄ fraction of 93.52 mol% in the residual gas phase, CO₂ separation factor of 42.17, and CH₄ separation and recovery factor of 1.43 was found at a TBAB concentration of 0.293 mol%, compared to concentrations of 0.1 and 0.9 mol% (Fan et al., 2016). The TBAB concentration of 0.293 mol% was also found to provide the best separation performance in fuel gas separation (Li et al., 2010c). This may be because higher concentrations make no further contribution to CO₂ capture since TBAB cations and CO₂ both occupy the large cages in semi-clathrate hydrate (Fan et al., 2016). However, different results were obtained in a recent investigation with different TBAB concentrations of 0.29, 0.62, 1.38 and 2.57 mol %. The highest gas consumption and highest CO₂ recovery of 0.502 happened at a 0.62 mol% TBAB solution, while the highest gas separation factor of 36.5 and CO₂ concentration in hydrate of 73% were achieved at 2.57 mol% (Wang et al., 2020b). This may be because at a concentration of 0.62 mol%, more CH₄ was incorporated in the hydrate cages. The occupation of different gas molecules in semi-clathrate cages at the molecular level is essential to interpret the inconsistent experimental observations.

In addition to TBAB, the potential of other quaternary salts has been evaluated. There is a range between the CO₂ and CH₄ phase equilibrium curve in which CO₂ is enclathrated in the hydrate cages while CH₄ remains in the gas phase. Quaternary salts can enlarge this range and Fan et al. (2013) found that TBAF induced a larger extent of the range than TBAC and TBAB. Compared to the tetragonal TBAB hydrates, the orthorhombic TBPB semi-clathrate hydrates are believed to have a superior CO₂ storage capacity (Muromachi et al., 2014a). Experimental results showed that the thermal stability of TBPB + 40%CO₂ + 60%CH₄ mixed hydrate increased as the TBPB concentration increased from 5 to 33.2 wt% (Li et al., 2017). The highest CO₂ separation factor of 31.7\pm3.3, CO₂ recovery of 0.422 and CO₂ fraction of 73.8% in the hydrate phase were achieved at a TBPB concentration of 33.2 wt% and a high temperature of 284.2 K, demonstrating its applicability in CO₂ gas separation. However, the same group made a further investigation on the formation kinetics of TBAB/TBPB solution and CO₂/CH₄ gas mixtures and found that the TBPB semi-clathrate hydrate grew laterally at the gas/liquid interface and quickly covered the interface, hindering mass transfer and gas consumption (Li et al., 2019). Meanwhile, the TBAB semi-clathrate hydrate initially formed around the gas/liquid interface and then sank to the reactor bottom. As a consequence, TBAB had a better CO₂ separation performance than TBPB, with a CO₂ separation factor as high as 85.5, CO₂ recovery of 0.352, and CO₂ concentration in hydrate is 96.5%.

The combination of quaternary salts and other additives has been proposed for CO₂ separation from CH₄. 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF₄), which may increase the CO₂ solubility (Iarikov et al., 2011) was adopted together with 5 wt% TBAB solution for 33 mol% CO₂/67.0 mol% CH₄ gas separation (Li et al., 2015). Results demonstrated that the additional additive increased the methane fraction in the residual gas and significantly shortened the time required to reach balance. However, the highest CO₂ gas separation factor was only 10.3. Another additive, dimethyl sulfoxide (DMSO), which can also enhance the solubility of CO₂ (Xia et al., 2012), was proposed to separate simulated biogas of 45.0 mol% CO₂/CH₄ binary mixture with TBAB (Xia et al., 2016). The promotion effect of DMSO was verified with improved CO₂ fraction in the hydrate phase and decreased CO₂ fraction in the residual gas, thus increasing the CO₂ separation factor from 40 with only TBAB to 59.22 with TBAB and DMSO. Another homologous compound of omidazolium-based ionic liquid, 1-octyl-3-methylimidazolium tetrafluorborate ([C₈min] BF₄), was applied for the first time in simulative biogas (37.0 mol%CO₂/CH₄) separation accompanied with TBAB (Yue et al., 2018). Although the addition of [C₈min] BF₄ increased the mole fraction of CH₄ in the residual gas, the CH₄ in the hydrate also increased and the recovery of CH₄ from biogas decreased.

5 Challenges and prospects

Currently, there is a lack of structural information of SCH and hydrate of CO₂ and quaternary salts under different conditions (Rodionova et al., 2022). The polymorphism of SCH further complicates this situation, as it is difficult to maintain the metastable state to grow high-quality single crystal. The environment during hydrate growth can easily affect crystal growth, and incorporating gas molecules into SCH may induce structure transformations that are operation condition-dependent. For example, Zhou et al. found that different types of TBAB hydrate were formed under various supercooling conditions, with markedly different gas storage capacity (Zhou and Liang, 2019). However, optimizing the CCS process by manipulating this process is challenging. Therefore, it is crucial to have a clear understanding of the type of hydrate and the gas content that will form during a given process when implementing gas separation processes using SCH.

The kinetics of semi-clathrate hydrate formation play a critical role in the CCS process. One problem related to quaternary salts hydrate is the supercooling effect, whereby the salts crystallize at a temperature much lower than their equilibrium temperature and the corresponding temperature difference is called the degree of supercooling. For TBAB hydrate, the maximum allowable degree of supercooling can be as high as 17.7 K (Sugahara and Machida, 2017). Memory effect, a phenomenon in which recrystallization occurs at a lower degree of supercooling or within a shorter induction time, has been proposed to suppress supercooling (Oshima et al., 2010; Machida et al., 2018; 2020). While the lifetime of the memory effect sharply decreases when the temperature is 2 K higher than the equilibrium temperature of TBAB hydrate. Nucleators, or nucleating agents, have also been proposed to reduce the degree of supercooling. Halloysite clay nanotubes were tested as nucleating agents of TBAB hydrate, and adding 1 wt% of halloysite increased the onset temperature of hydrate crystallization by 2 °C (Stoporev et al., 2020).

Another common challenge in hydrate related applications is the slow kinetics of formation. Surfactants are commonly used as promoters for hydrate formation, and they have been extensively studied in this area (Mohammadi et al., 2018; Zheng et al., 2018; Xie et al., 2021). In addition to surfactants, numerous studies have been conducted to enhance hydrate formation by physically increasing the surface area available through reactor medium such as sand packs, silica gels, foams, nanoparticles, hydrogels (Stoporev et al., 2020) and so on (Linga and Clarke, 2017). The kinetics of CO₂ hydrate formation are also affected by the driving force, stirring speed, agitator geometry, initial volume of water and vessel size (Liu et al., 2022).

Since the cation of quaternary salts occupy most of the cage space, the low gas capacity of SCH is a major obstacle in CCS applications. Selecting an appropriate quaternary salt to reduce hydrate formation pressure and increase the amount of captured gas is key to CCS applications. Besides quaternary ammonium and phosphonium salts with halide anions, the potential of halogen-free salts has attracted much attention (Shimada et al., 2019). Quaternary salts with carboxylic acid anions in the guest compounds, which are more environment friendly than halide anions, have been studied (Muromachi et al., 2015; Yamauchi et al., 2017a; 2017b; Shimada et al., 2018; Koyama et al., 2020; Miyamoto et al., 2020). Previous research on semi-clathrate hydrate based on carboxylic acid anions mainly focused on their potential applications as thermal energy storage materials, and their phase equilibrium conditions and dissociation heat have been determined. The melting points of this group of semi-clathrate hydrate are between 7 and 22 °C, which are compatible with the SCH of quaternary salts with halide anions. Moreover, the hydroxycarboxylates that are formed by adding a hydroxy group to carboxylates can also form semi-clathrate hydrates. The properties of hydroxycarboxylates based SCH can be tailored by crystal engineering technique to modify the onions with hydroxy groups (Muromachi and Takeya, 2018; 2019). However, their performance on CO₂ gas capture and sequestration have not yet been investigated.

6 Conclusions

This paper reviews the structure and thermodynamical properties of SCH and SCH with inclusion of the gas molecules for separation. Quaternary salts have the ability to incorporate CO₂ gas molecules at mild pressure and temperature conditions, thus reducing the costs associated with gas mixture compression. Among all the quaternary salts considered in this review, TBAF has been found to better stabilize CO₂ gas molecules, although its gas storage capacity is limited because about 79.5% of the 5¹² cages are occupied by water molecules. SCH with an orthogonal structure is superior in terms of gas storage capacity because the ratio of water molecules with the 5¹² cages is smallest. Polymorphism plays an important role to the performance of CCS with SCH, and precise manipulation of hydrate structure is currently difficult. The performance of CCS is also influenced by the operation conditions, the reactor volume, the quaternary salt concentrations. The main obstacles of the CCS with SCH are low gas storage capacity and low formation kinetics. Efforts should be conducted to find the most appropriate quaternary salt to achieve better gas storage capacity as well as high thermal stability. Ways to improve the kinetic behaviors of CO₂ and quaternary salt hydrate formation are necessary to scale up its industrial application.

Acknowledgements

This work was funded by the financial support from the China Geological Survey (No. DD20230063) and the Guangdong Major Project of Basic and Applied Basic Research (No. 2020B0301030003).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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