Porous liquids and Their Applications in Gas Capture and Separation

Dechao Wang; Yangyang Xin; Xiaoqian Li; Dongdong Yao; Yaping Zheng

doi:10.7536/PC200902

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1874-1886 DOI: 10.7536/PC200902

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Porous liquids and Their Applications in Gas Capture and Separation

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Abstract

Porous liquids(PLs), an emerging class of liquid materials with permanent porosity and good fluidity, have shown great potential in gas capture and separation. As a result, PLs used as gas capture materials have become a new research hot spot. In the present paper, the concept of PLs is firstly introduced in a nutshell. Then, the composition characteristics and the necessary conditions for constructing PLs are analyzed. Next, the synthesis progress of these three types of PLs are reviewed in detail, and their gas capture and separation performance are analyzed, especially after 2015. Lastly, the existing challenges of porous liquids and their outlooks of synthetic methods and applications in gas capture are outlined and presented.

Contents

1 Introduction

2 The classification of porous liquids and characteristics of composition

3 The synthesis progress of porous liquids and gas capture

3.1 The Type 1 porous liquids

3.2 The Type 2 porous liquids

3.3 The Type 3 porous liquids

4 Conclusion and outlook

Graphical abstract

Keywords

porous liquids / porous cavities / steric solvents / gas capture / separation

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Dechao Wang, Yangyang Xin, Xiaoqian Li, Dongdong Yao, Yaping Zheng. Porous liquids and Their Applications in Gas Capture and Separation. Progress in Chemistry, 2021, 33(10): 1874-1886 DOI:10.7536/PC200902

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1 Introduction

Porous material is a kind of solid material with a certain size and structure^[1]. According to the size, the pores can be divided into three types: macropores (> 50 nm), mesopores (2 ~ 50 nm) and micropores (< 2 nm)^[2,3]. Porous materials have been widely used in the fields of gas storage, adsorption separation and catalysis due to their diverse pore structures and easily controlled functionality^[4⇓~6]^[7,8]^[9⇓~11]^[12⇓~14]. However, in current industrial applications, porous solid materials usually have problems such as mechanical fatigue, physical aging or plasticization, and difficult continuous operation^[15,16]. For example, in the field of CO₂ capture from plant flue gas, although porous solid materials have low energy consumption in the absorption-desorption process, they are difficult to be connected to the existing pipeline flow medium system for use, while liquid absorbents (such as alcohol amine absorbents) are easy to achieve continuous operation in the carbon capture process, and the technology is becoming more and more mature^[17]^{[18⇓⇓⇓~22]}.

porous liquids (PLs) are a new type of liquid materials with stable pore structure and macroscopic flow state. In 2007, James and others first proposed the concept of "porous liquids".Porous liquids, which combine the ordered pore structure of porous solid materials with the continuity and stable mass transfer of liquids, have shown great potential applications in gas adsorption, separation and catalysis^[23]^[24⇓~26]^[27,28]^[29,30]^[31].

Due to the difficulty of synthesis, it is still in the laboratory preparation stage, but the research on porous liquids has been in constant exploration, especially after 2015, porous liquids based on different porous cavities and sterically hindered solvents have been synthesized one after another, but there is still no review report on them in the literature^[32⇓~34]. In view of this, the classification of porous liquids and the characteristics of their components are briefly introduced in this paper. Then, the synthesis methods of three kinds of porous liquids and their applications in gas trapping and separation are reviewed in detail, and the research progress of porous liquids in the past five years is emphatically expounded. Finally, the challenges and development directions of porous liquid in gas capture and separation are summarized, in order to provide a new solution for porous liquid in gas capture and separation.

2 Classification and composition characteristics of porous liquid

Since James et al proposed the concept of porous liquid, more and more researchers have studied the porous liquid^[23]. Taking the title "Porous Liquids" of the conceptual article originally published by James et al. In 2007 as the keyword, the number of papers related to Porous Liquids in each year is plotted by searching and screening in "Web of Science" (see Figure 1). It can be seen that the literature reports on porous liquids have shown an increasing trend after 2015^[23].

According to the composition of porous liquids, porous liquids can be divided into the following three categories, as shown in Figure 2:^[23]

The first type is the intrinsic porous liquid. It is composed of rigid host units, and the rigid molecules themselves have cavity structure and fluidity. The necessary conditions for the formation are that the molecule has a suitable cavity structure and can not be self-filled between molecules; The structure of the main body unit is rigid and is not easy to collapse; It is liquid at room temperature.

The second type is the porous liquid formed by the host molecules with rigid cavity structure dissolved in the hindered solvent. The key to the design is how to maintain the porous cavity structure in the sterically hindered solvent.

The third type of porous liquid is formed by dispersing porous framework materials (such as metal-organic frameworks (MOF), zeolites, etc.) In a sterically hindered solvent. The preparation idea of the porous liquid is simple, the porous framework material only needs to be uniformly dispersed into the sterically hindered solvent with proper size, and the design difficulty is that the permanent pore structure is not occupied and destroyed by the sterically hindered solvent.

According to the reported composition and structure of porous liquids, the first type of porous liquids are generally endowed with fluidity by surface modification of porous cavities. The second and third types of porous liquids can essentially be viewed as consisting of two components: a porous cavity and a sterically hindered solvent. However, the necessary conditions for the formation of these three types of porous liquids are to maintain the existence of permanent pores and to maintain fluidity. The first type of porous liquid is usually obtained by surface modification of porous cavities or by surface modification of porous nanoparticles with organic chains. At present, the first class of porous liquid cavities have been reported, including cyclodextrins (CDs), hollow SiO₂ nanoparticles^[35]^[30]; Cavities used in the second type of porous liquids include cucurbituril, semi-cyclic aromatic hydrocarbons, and porous organic cages (POCs)^[36,37]^[38]^[4,39]. The third type of porous liquid is more reported, and the porous cavities used are mainly zeolites, porous carbons, MOFs and so on^[40]^[41,42].

3 Synthesis of Porous Liquids and Their Application in Gas Capture

3.1 Porous liquid of the first kind

The synthesis method of the first kind of porous liquid is to modify the surface of molecules or porous solid materials by chemical engineering, so that they can maintain fluidity at room temperature and maintain the existence of internal cavities.Crown ethers, hollow SiO₂ nanoparticles and other porous cavities have attracted much attention, and alkyl chains, ionic liquids and organic oligomers are often used to modify the surface of porous cavities to obtain fluidity^[43]^[26,44]^[45⇓~47]^[48⇓~50].

In 2012, James et al. Converted porous crystal materials into pure liquids by alkylating organic cages, modified the surface of porous organic cages with alkanes of different molecular chain structures (straight chain or branched chain) to prepare the first type of porous liquids, and explored in detail the effect of alkyl chain structure on the formation of porous liquids.This work provides a reference for the subsequent preparation of the first type of liquids with porous organic cages as porous cavities, but the porous liquids prepared in this study can not show a macroscopic flow state at room temperature, and the synthesis process is cumbersome and the yield is not high (Fig. 3A)^[51].

In 2014, del P Pópolo et al. Constructed a series of porous organic cage-like compounds as cavities by combining theoretical simulation with experimental synthesis, and used alkyl chains to modify POC to obtain fluidity^[52]. The results show that the porosity of porous liquid can be achieved by adjusting the size of alkyl chain, and the theoretical simulation also confirms that 30% porosity can be obtained in porous liquid. The adsorption behavior of CH₄ shows that the size of alkyl chain also affects the adsorption capacity and selectivity of gas.

In 2020, Dai et al. Converted POC into a porous ionic liquid based on the idea of "supramolecular complexation" (Fig. 3B)^[53]. The first kind of porous ionic liquid 18-C-6-PL with POC as the anionic moiety was obtained by combining the anionic porous organic cage 18-C-6 with the porous ionic liquid KACC. Compared with the previously reported second class cage-like porous liquid 15-C-5-PL, the 18-C-6-PL porous ionic liquid has a significant advantage, for example, the thermal stability of 18-C-6-PL is enhanced due to the high bonding strength between 18-C-6 and the K⁺ and the high boiling point of 18-C-6, which makes it possible to use the porous liquid in a pressure or temperature fluctuation environment. Because the cage/crown ether ratio is the highest in the cage porous liquid system, the permanent pore cavity volume in 18-C-6-PL is the largest, which enables it to adsorb and store more gases.They believe that this supramolecular complexation strategy is universal for the preparation of porous ionic liquids, and can be used to try to use other porous molecules with anions with larger pores as porous cavities to construct the first type of porous liquids.

In addition, inspired by the construction of solvent-free nanofluids, new porous liquids have also made great progress^[54~55]^[56⇓~58]. In previous studies, the research team represented by Park et al. And Zheng Yaping of Northwestern Polytechnical University prepared a variety of solvent-free nanofluids and systematically studied their gas adsorption properties.This provides a new idea for the subsequent construction of porous liquids by grafting polymer chains through surface chemical engineering, which has become an effective and universal method for the synthesis of the first class of porous liquids, in which hollow SiO₂ nanoparticles are the most common porous cavities^[59⇓~61]^{[62⇓⇓⇓~66]}^[67]^[68]. In 2015, Dai et al. Used hollow sulfonium nanoparticles (HS) with a microporous shell as a porous cavity, first used positively charged organosilane (OS) as a neck-like layer, modified HS by dehydration condensation reaction to obtain OS @ HS, and then connected 4-nonylphenol-potassium polyethylene ether sulfonate (PEGs) as a crown layer to OS @ HS through electrostatic interaction.The HS porous liquid (HS-liquid) with fluidity at room temperature was obtained (see fig. 3C), and it was tried to be applied to the supported liquid membrane. The gas trapping and separation performance is shown in fig. 4A and B. Due to the free volume generated by the porous cavity,The CO₂ permeability of the porous liquid supported membrane is significantly improved, and the outer neck-crown oligomers can be designed for different gas separation targets, which also confirms the versatility of this construction strategy^[30].

In 2017, Zheng Yaping of Northwestern Polytechnical University and Dai of Oak Ridge Laboratory of the United States reported a porous carbon liquid with hollow carbon spheres as porous cavities and polyimidazole ionic liquids as sterically hindered solvents, as shown in Figure 3D, the basic idea is that imidazole ions can be firmly bonded to the surface of hollow carbon spheres in the form of π-cation/π-electron/π-CH.O that long-chain organic molecules can be grafted to the surface of the porous hollow carbon sphere, and the specific steps are as follows: the ionic liquid of the polyimidazole-based cationic polymer is attached to the surface of the porous hollow carbon sphere,Then the hollow porous carbon liquid is obtained by dispersing the hollow porous carbon liquid in a proper sterically hindered solvent to form a neck-shaped layer and introducing long-chain crown-shaped layer PEGs through anion exchange to liquefy the long-chain crown-shaped layer PEGs^[54]. The unique hierarchical porous structure and good chemical structure of carbon nanospheres are used as molecular sieves, which can exclude solvent molecules in solution, thus maintaining unoccupied porous cavities and showing excellent gas adsorption capacity. The gas trapping performance is shown in Figure 4 C.

Later, Zheng Yaping et al prepared the first type of porous liquid (Fig. 3e) by using hollow SiO₂ nanoparticles (HS) as the porous cavity, organosilane with sulfonic acid group as the neck layer, and polyetheramine (M2070) as the crown layer, and explored the influence of the pore structure of HS on CO₂ trapping. The larger the average pore size of HS is, the greater the CO₂ adsorption is^[55]. This study provides a more universal and low-cost method to prepare porous liquids with good CO₂ adsorption capacity, and provides a reference for the subsequent construction of porous liquids.

Kumar et al. prepared a porous liquid by grafting a long organic polymer chain onto an anisotropic hollow silicon shell nanorod as a porous cavity (Figure 4E)^[69]. The method comprises the following steps of: firstly, preparing three kinds of hollow silicon shell nanorods with different length-diameter ratios; then, carrying out surface modification and grafting of organosilane; and finally, adsorbing organic chains onto the organosilane through electrostatic action, wherein the grafted organic long chains do not block pores on the hollow silicon shell nanorods. The gas separation performance test of the porous liquid shows that the porous liquid also has good CO₂ separation performance at 0 deg C, as shown in fig. 4D.

Recently, Chen Zhengan et al used hollow SiO₂(HS) as porous cavities (average pore size is 10 nm), and used electrostatic-assisted strategy to graft organic chains on its surface to prepare porous liquids, as shown in Figure 4 f^[70]. The content of HS in the porous liquid was about 13 wt% and exhibited an excellent CO₂ adsorption capacity. Liu et al. adopted a method similar to that of Dai et al. for the preparation of hollow SiO₂ porous liquids, using MOF UiO-66 as a porous cavity, using the hydroxyl groups of organosilane (OS) to dehydrate and condense with the hydroxyl groups on the surface of UiO-66 to obtain OS-modified Ui0-66 (Ui0-66 @ OS), and then through the electrostatic interaction between anion-exchanged PEGs and OS to obtain UiO-66 porous liquids^[71]^[30]. The test results show that the small pore size (0. 8 and 1. 1 nm) of UiO-66 is not occupied, and the porosity is maintained. The polyionic liquid mainly plays a transitional role to form a neck-shaped layer, and then introduces long chains through anion exchange to endow the porous cavity with fluidity. It is worth noting that the structural diversity of ionic liquids expands the selectivity of the use of porous liquid sterically hindered solvents.

In addition, some scholars have also carried out theoretical simulations to explore the influence of the structure of porous liquid on gas capture. Qiao et al. Used theoretical simulations to explore the adsorption behavior of porous liquids for different gases^[72]. The calculated results show that the amount of gas stored in each cage does not only change with the size of the cage, but also is affected by the non-electrostatic interaction (dispersion force) between the gas and the organic cage.At the same time, it is affected by the diameter of gas molecules, the former controls the force of gas entering the organic cage, and the latter determines whether the space in the cage can be occupied by gas.

It can be seen from the above reports of the first type of porous liquids that the use of polymer chains to chemically modify the surface of porous cavities to give them fluidity is a universal method, and the richness of organic chain structures provides more possibilities for design. In addition, the polymer chain can be functionalized to further enhance the chemical selectivity of the gas so as to meet the trapping requirements of different gas components. However, there are not many porous cavity materials based on the first kind of porous liquid, especially the advanced porous materials (APMs), such as MOFs, metal-organic cages (MOCs), covalent organic frameworks (COFs) and so on^[25]. As a kind of porous solid material with highly porous structure and adjustable surface chemical properties, APMs have great potential as the first type of porous liquid cavity materials. Because the viscosity of the porous liquid obtained by surface chemical engineering grafting polymer chains is relatively large, how to adjust the viscosity of the porous liquid to meet the actual needs is also a problem worthy of consideration, such as selecting polymer chains with different molecular weights and different structures, adjusting the density of grafting, and so on^[48,64].

3.2 The second kind of porous liquid

The second type of porous liquid is formed by dissolving host molecules with a rigid cavity structure in a steric solvent. The reports of this type of porous liquid mainly come from the research teams represented by James et al and Cooper et al^[73⇓~75]^{[76⇓⇓~79]}.

In 2015, James and Cooper reported on Nature that the second porous liquid with crown ether (15-crown-5) as the sterically hindered solvent and the organic cage modified by crown ether as the porous cavity (as shown in Figure 5A)^[24]. The results show that this kind of porous liquid is different from the first kind of porous liquid with crown ether as porous cavity, which has excellent fluidity at room temperature, and the sterically hindered solvent can not enter the porous cavity, so the porous cavity rate is more than 500 times higher, and the gas adsorption performance test shows that it has 8 times higher methane adsorption capacity. These studies have opened up a new way to synthesize the second type of porous liquids, which are expected to be applied in chemical processes such as gas capture and catalysis. Later, James and Cooper improved the above research (as shown in Fig. 5B) and synthesized the second type of porous liquid with scrambled cage molecules as porous cavities^[80]. In order to reduce the viscosity of disordered cage based PLs, crown ether-functionalized cages were replaced by disordered cages, and hexachloropropene (HCP) was used as a sterically hindered solvent. The results showed that the solubility of different gas components was significantly enhanced, and the recycling of the porous liquid could be achieved by ultrasonic treatment to desorb the adsorbed gas. This indicates that the dynamic covalent interference strategy can be used to increase the solubility of porous organic cages, which provides a good idea for the subsequent synthesis of porous liquids with disordered cages as porous cavities.

In 2020, Cooper et al explored the design of the second type of porous liquid in detail. Because little was known about the design considerations of the second type of porous liquid before this study, they assisted the design synthesis by using high-throughput automated screening (Fig. 5C)^[81]. This involves the design of a high-throughput workflow that, for example, designs a new porous liquid that includes a solvent that is economical and easy to recycle and process, rather than the first solvent HCP of the previous work, and produces more cavity structures, regardless of the size of the solvent.

In 2019, Cooper et al. Reported that the adsorption selectivity of porous liquid to gas was achieved by regulating the size of POC molecular window (see Figure 5d)^[82]. The porous organic cage CC15-R is an analog of CC3-R with smaller pore windows (1.77 Å and 4.00 Å, respectively), which can be partially blocked by methyl groups, and CC15-R can still adsorb smaller gas molecules. The change of POC window and size provides a new way to control the gas selectivity of PLs.

In addition, some scholars have applied the second type of porous liquid to the field of membrane separation. Peng Xinsheng et al. Of Zhejiang University prepared the second type of porous liquid by using metal organic cage MOP-18 as the porous cavity and 15-crown-5 as the sterically hindered solvent (see Figure 5e)^[29]. 15-crown-5 was chosen as the solvent mainly because it is a liquid at room temperature and its molecular diameter is larger than the window size of MOP-18, which can ensure the existence of permanent pores. In previous studies, porous liquids have been shown to have great potential in the field of gas storage. There are few reports on the separation and collection of gases by porous liquid membranes. Therefore, Peng Xinsheng et al. Prepared a graphene oxide supported porous liquid membrane (GO-SPLM). The MOP-18 porous cavity in the porous liquid weakens the barrier to gas diffusion.Considering the diversity of metalloorganics, polyhedra and various bulky solvents (such as ionic liquids), this method enriches the second class of porous liquids and brings new opportunities for natural gas storage and separation.

From the above, it can be seen that the second type of porous cavity materials mainly focus on porous organic cages; There are few studies based on APMs, for example, there is no report on porous liquids with three-dimensional covalent organic frameworks (COFs) as porous cavities, and how to control the viscosity of the second type of porous liquids is also worthy of attention^[83,84]^[85,86].

3.3 The third kind of porous liquid

Compared with the first two types of porous liquids, the third type of porous liquids has been studied more, and the key to its design is to disperse the porous framework material in the sterically hindered solvent and maintain permanent pores.Therefore, researchers have adopted a variety of strategies to prevent sterically hindered solvents from entering the porous cavity to maintain permanent porosity, such as surface pre-wrapped polymer layer, electrostatic interaction, π-π complexation, volume effect and other strategies^[87,88]^[89,90]^[91,92]^[54,93,94].

Recently, Li Tao et al. Of Shanghai University of Science and Technology reported a universal method to construct MOFs porous liquid, as shown in Figure 6a^[20]. The key of this study is how to form a stable dispersion when the surface of MOFs is incompatible with PDMS. In this regard, the problem is solved by using a previously proposed strategy of growing a polymer layer on the surface of MOFs with universality, and the polymer chain with PDMS end capping is polymerized by Atom transfer radical polymerization (ATRP).ATRP), and the thickness and the number of layers of the polymer are controllable. This surface modification step increases the compatibility between the MOFs particles and the sterically hindered solvent PDMS, and then the UiO-66 wrapped by the polymer layer is dispersed in the sterically hindered solvent PDMS as a porous cavity to prepare a porous liquid^[88]. PDMS can maintain liquid fluidity at extremely high molecular weight, which can ensure that PDMS will not enter the UiO-66 pore channels and retain its porosity. At the same time, PDMS has the characteristics of good thermal stability, low vapor pressure and good economy, which makes it an ideal porous liquid steric hindrance solvent. Dispersing the UiO-66 particles in the PDMS solvent allows the porous liquid to flow at room temperature while retaining the porosity of UiO-66, and still flow at a low temperature of -35 ° C, as shown in fig. 6d (D1, D2). By adjusting the molecular weight and polymer chain structure of PDMS, the pore structure in PDMS solvent can be maintained permanently. This paper provides a new idea to promote the rapid development of porous liquids and the application of MOFs materials in the field of gas capture.

In addition, Zheng Yaping and Dai first reported the third type of porous liquid with hydrogenated zeolite as the porous cavity, as shown in Fig. 6B. The difficulty of this work lies in the high crystallinity of zeolite.There are few reactive hydroxyl sites on the surface, so it is difficult to pre-modify the surface by grafting organic chains through surface chemical engineering methods, and small molecular chains are easy to enter the cavity structure^[40]. Considering the existence of Bronsted sites in hydrogenated zeolites, they used branched ionic liquids to interact with hydrogenated zeolites through hydrogen bonds to construct porous zeolite liquids. Compared with pure zeolite molecular sieve, the porous liquid has better gas adsorption performance and selectivity, as shown in Figure 6e, and the pore structure of zeolite can be designed to regulate the gas adsorption-desorption process.

Xing Huabin et al of Zhejiang University and Dai et al reported a method to synthesize the third type of porous liquid through porous framework (ZIF-8, ZSM-5, Silicalite-1) and bulk ionic liquid ([DBU-PEG][NTf₂]IL)) (see Figure 6 C)^[95]. The permanent porosity of these porous liquids was confirmed by Positron Annihilation Lifetime Spectroscopy (PALS) and Positron Annihilation Lifetime Spectroscopy adsorption tests. Due to the successful retention of the inherent porosity of the ZIF-8, ZSM-5 and Silicalite-1 nanoparticles, the porous liquids exhibited easily regulated Positron Annihilation Lifetime Spectroscopy adsorption capacity. A significant hysteresis loop was observed in the Positron Annihilation Lifetime Spectroscopy desorption curves of the porous liquids under special environmental conditions (Fig. 6 f). The combination of controlled adsorption and hysteretic desorption behavior of porous liquids demonstrates their potential to reasonably control the adsorption behavior of gases. Pore channels and active functional sites, such as uncoordinated metal sites and basic amino groups, inherent in the body of the porous cavity can be retained even after the porous liquid is made. In addition, the precise control of thermodynamic factors (such as adsorbate concentration) and kinetic factors (such as diffusion rate) can be achieved by designing appropriate ionic liquids as sterically hindered solvents.

Li et al. Constructed a novel porous MOF liquid (UiO-66-liquid) based on the principle of similar miscibility and steric hindrance effect by using polyetheramine (D2000) modified MOF (UiO-66) as a porous cavity and highly stable ionic liquid [M2070] [IPA] as a steric hindrance solvent (Fig. 7A)^[41]. After [M2070] [IPA] polyionic liquid was synthesized by one-step method, the surface of UiO-66 porous cavity with carboxyl group was modified by polyether amine D2000 with terminal amine group. Because D2000 and M2070 have similar polyether structure, UiO-66 modified by D2000 can be well dispersed in [M2070] [IPA], so the synthesized porous liquid UiO-66-liquid has good stability at room temperature. Since the size of [M2070] [IPA] is larger than that of the UiO-66 porous cavity, the porous cavity is well preserved, showing a high adsorption value of 7.32 wt% CO₂ at 10 bar, as shown in fig. 7d.

Liu et al. prepared a porous liquid by dispersing ZIF-8 in an ionic liquid [Bpy][NTf₂], and the results showed that preventing the agglomeration of ZIF-8 before dispersion was beneficial to the formation of a uniform and stable porous liquid (see Figure 7C), and the breakthrough experiment also confirmed the gas trapping ability of the porous liquid (shown in Figure 7e)^[96]. There are still many reports on the exploration of ZIF porous liquids. For example, Longley et al., on the basis of liquid phase ZIF-4, explored the changes of structure and gas adsorption performance when two kinds of MOFs porous liquids were mixed^[34]. By mixing ZIF-4[M(Im)₂] with ZIF-62[Zn(Im)_1.75(bIm)_0.25](M=Co²⁺, Zn²⁺,Im:C₃H₃N^2-,bIm:C₇H₅N^2-), an interconnected glassy structure was formed between the two MOF porous liquids (Fig. 7B). Recently, Costa Gomes et al. prepared porous liquids with (ZIF-8 and Zn(MeIm)₂) as the porous cavity and ionic liquid [P_6,6,6,14][NTf₂] as the sterically hindered solvent, respectively. Compared with the pure ionic liquid, the porous liquid containing 5 wt% of ZIF-8 has a reversible adsorption capacity of 150% of N₂ and 100% of CH₄^[97].

To sum up, compared with the first two kinds of porous liquids, the third kind of porous liquids has been studied more, and there have been attempts to use APMs as new porous liquids with porous cavities. In order to prevent the sterically hindered solvent from occupying the permanent pores, researchers have tried to maintain the permanent pores by enhancing the force between the porous cavity and the solvent or adjusting the volume effect of the sterically hindered solvent. The gas trapping performance tests show that these porous liquids have great potential for application. The viscosity of the third kind of porous liquid is relatively low, such as the UiO-66/PDMS porous liquid reported by Li Tao of Shanghai University of Science and Technology, which shows great advantages. However, most of the studies only use a single porous solid material as a cavity, and there is no report on the third type of porous liquid with a composite porous solid material as a cavity. In addition, as an excellent gas trapping material, most of the current studies only focus on the adsorption-desorption of gas, and there are few reports on the comprehensive performance evaluation of porous liquids applied to the adsorption column of pressure swing adsorption process^[32].

4 Conclusion and prospect

Since the concept of porous liquid was proposed in 2007, the research of porous liquid has been in the initial stage due to the difficulty of synthesis, complex process and difficult control of conditions. It was not until 2015 that a large number of porous liquids were reported, and foreign research teams represented by Dai et al., Cooper et al., and James et al.The research team represented by Zheng Yaping of Northwestern Polytechnical University in China has designed and prepared different types of porous liquids through various methods, and explored their application potential in gas capture and separation, which has attracted wide attention of scholars to this new type of "porous solid-liquid homogeneous mixed material". However, due to the difficulty of synthesis and the complexity of the process, the challenges of porous liquids and the next research directions are mainly as follows:

(1) Optimal design and synthesis of porous cavities. How to increase the proportion of cavities in the porous liquid and create more effective space for gas capture while maintaining fluidity is a problem worthy of discussion. At present, although a variety of porous cavities have been used to prepare porous liquids, the research on porous liquids based on APMs as porous cavities has not been carried out in depth. These APMs mainly include MOCs, COFs, etc. In addition, the research on the first type of porous liquids based on MOFs as porous cavities has not been carried out in depth^{[98⇓⇓~101]}^[102]^[103,104]. Moreover, at present, the synthesis of porous liquids is limited to a porous solid material as a cavity, and there is no report on a porous solid material with a composite structure as a porous cavity.

(2) Optimal design of sterically hindered solvents. Gas trapping is divided into physical adsorption and chemical adsorption. In order to enhance the interaction between gas and porous liquid, groups that can react with specific target gas can be introduced into the sterically hindered solvent to further improve the selective trapping capacity of gas. At present, the viscosity of synthetic porous liquids is relatively high, especially the first kind of porous liquids, most of which can not meet the requirements of pipeline flow media.Therefore, how to optimize the design of sterically hindered solvents to meet the requirements of low-viscosity porous liquids is particularly necessary, and low-viscosity ionic liquids or PDMS and other commonly used raw materials can be considered for further optimization^[20].

(3) Testing and characterization of porous liquids^[32]. One of the keys to prove the success of porous liquid preparation is to determine the existence of permanent pores, but some traditional methods for pore characterization of porous solid materials are not suitable for porous liquids, and the test of adsorption isotherms at low pressure often leads to the loss of volatile sterically hindered solvents. Positron annihilation lifetime spectroscopy is an effective means, but it is difficult for most research teams to obtain such instruments for testing, so it is worth exploring whether other conventional testing and characterization methods (such as Raman spectroscopy) can be used^[32].

(4) The economics of porous liquids. At present, the synthesis of porous liquids is mostly in the laboratory stage, without in-depth consideration of the economy of practical application, and the synthesis process is cumbersome, so it is particularly necessary to use raw materials and reagents with good economy, and to facilitate access to the current chemical flow pipeline process^[41].

(5) The microscopic mechanism of porous liquid trapping gas. At present, that gas capture by the porou liquid mainly focuses on the experimental t, and the reports of the theoretical simulation are few, so the method of combining the experiment and the theoretical simulation can be considered in the next step,To guide the experiment of directional synthesis of porous liquids to meet the separation requirements of different gas components, and to explore the "structure-property" relationship between cavity structure and sterically hindered solvents, and the relationship between porous cavity and gas trapping performance^[105]. Then, according to the experimental results, the mechanism of porous liquid trapping gas is analyzed from the microscopic level by theoretical simulation. In addition, the evaluation of the gas trapping performance of porous liquid is mainly focused on the test of gas adsorption capacity at present, and there are few reports on the gas penetration experiment.The next step is to explore the gas penetration experiment of porous liquid, in order to provide theoretical support for the adsorption kinetics of gas trapping by porous liquid.

(6) Regeneration of porous liquid. At present, most of the research focuses on the synthesis of porous liquids, and then test the gas trapping performance of porous liquids. But there is no report about that regeneration of the porou liquid, and the next step is to discuss the regeneration and recycle of the porou liquid through different regeneration technologies such as variable temperature, variable pressure and variable voltage technologies, so as to further utilize the captured gas,For example, after the porous liquid is used as a CO₂ trapping agent, if the CO₂ can be regenerated, the CO₂ can be further used to synthesize basic chemical raw materials and realize chemical poly-generation, which is also in line with the current concept of sustainable development.

(7) Porous liquids as fillers for hybrid matrix membranes (MMMs). Porous liquids have been extensively studied as a gas trapping agent and a good filler for mixed matrix membranes, but few reports have been published. The cavity structure and the fluidity of the porous liquid can be used for solving the problems of less effective volume in the mixed matrix film and nanometer filler agglomeration; It is also worth exploring to use the surface groups of porous liquid to form a cross-linked structure with the polymer matrix to construct a mixed matrix membrane with excellent mechanical properties and high porous liquid filler, so as to improve the gas separation and capture performance.

(8) Expanding other applications of porous liquids. On the one hand, the cavity of porous liquid can not only absorb a large amount of gas, but also enhance gas transmission, but at present, most of the research is focused on CO₂ gas, and there is little research on other types of gas such as low-carbon olefins, alkanes and so on, which can expand the application of other gas capture. On the other hand, the fluidity and permanent pores of porous liquids show some potential in catalysis, and the use of porous liquids in catalytic reactions is expected to narrow the gap between homogeneous catalysis and heterogeneous catalysis^[32]. If biocompatible porous cavities and sterically hindered solvents can be designed, the application of porous liquids in drug delivery fluids can also be attempted^[32]. In addition, porous liquid is expected to be used as a continuous flow phase in the existing pipeline flow system, so it is very important to study the effects of porous liquid microstructure and physicochemical properties on the flow state before scale-up.

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