Recent advances and potential applications of flexible adsorption and separation materials: A review

Abstract

In adsorption processes of the liquid or gas phase, a particular dynamic adsorption behavior caused by the reversible mechanical deformation (expansion or contraction) of absorption materials driven by the host–guest interaction is called “flexible adsorption,” and the materials with this particular behavior can be called “flexible adsorption materials.” Flexible absorption materials, such as crosslinked polymers, have received much attention from researchers due to their flexible framework structure and specific adsorption kinetics. They have shown great potential in gas storage and other energy applications. This paper reviews recent advances in the definition, theoretical development, and classification of flexible absorption materials. Potential application examples of flexible absorption materials in gas/liquid absorption and separation, functional devices, and energy catalysis are analyzed to guide their development.

INTRODUCTION

Functional adsorption materials with microporous frameworks such as resins and coordination polymers have become a hot topic in the chemistry and materials community in the last few decades.^1–9 The boom of functional adsorption materials is attributed to their high framework designability, which provides the unique properties that chemists expect. Among the properties of these functional adsorbent materials, the dynamic adsorption behavior based on a flexible framework has attracted the interest of researchers, such as the reversible volume expansion/contraction that occurs during the adsorption process.^10–12

In adsorption processes of the liquid or gas phase, a particular dynamic adsorption behavior caused by the reversible mechanical deformation (expansion or contraction) of absorption materials driven by the host–guest interaction is called “flexible adsorption,” and the materials with this particular behavior can be called “flexible adsorption materials.” During flexible adsorption, the reversible mechanical deformation induced by the host–guest interaction can significantly affect the adsorption kinetics of the adsorbent.^13–15 For example, expanding the polymeric three-dimensional (3D) network of acrylate resins increases their oil absorption.^16–19

In addition, framework deformation is often accompanied by changes in the electrical and mechanical properties of the adsorbent.^20–22 Water absorption by flexible polymer networks leads to volume expansion and changes in electrical conductivity. Functional devices such as humidity sensors designed on this basis are also of high application value. Due to the structure's high designability, the adsorption framework's flexibility can be easily adjusted to suit various applications, offering advantages that are not available with rigid adsorption materials.

Although numerous adsorption materials have been successfully synthesized and applied in energy, catalysis, and other fields, the design and application of flexible adsorption materials have been rarely reported.^23–25 This is due to the lack of methods for researchers to understand and modulate the flexibility of the adsorption frameworks. In recent years, researchers have been interested in guest-induced dynamic adsorption processes in flexible adsorption and have tried to construct thermodynamic-based models to explore the adsorption mechanism.^26–31 Disappointingly, these attempts may not be suitable for studying flexible adsorption because these theories assume that the framework pores of adsorption materials are rigid, resulting in the inability to capture the complex deformations of frame pores during host–guest interactions. Thanks to the contributions of researchers, new advances in material design and mechanism exploration of flexible adsorption have emerged in recent years. This paper reviews the research results and related theoretical progress of flexible adsorption. Their potential application prospects are also discussed in the hope of inspiring future research.

Here, the progress in the design and application of flexible adsorption materials is summarized, followed by the current state of research on flexible adsorption mechanisms (Figure 1). We will focus on the relationship between their dynamic adsorption behavior and structure and provide a rational classification of flexible adsorption materials. Potential application examples of flexible adsorption materials are classified and presented to outline the essential features required for the target performance and the advantages of flexible adsorption materials. Finally, a summary and outlook on the development of the field are presented.

Figure 1. Schematic overview of the content of this review.15,32

FLEXIBLE ADSORPTION The definition of flexible adsorption

Since the theory of flexible adsorption is different from that of rigid adsorption, the definition of flexible adsorption is very complicated. Lu's team has researched and applied functional adsorption materials for decades.^32–36 Flexible adsorption, based on existing studies, could be defined as follows: in adsorption processes of liquid or gas phase, a particular dynamic adsorption behavior caused by the reversible mechanical deformation (expansion or contraction) of absorption materials driven by the host–guest interaction is called “flexible adsorption,” and the materials with this particular behavior can be called “flexible adsorption materials” (Figure 2). Here, the “host–guest interaction” is the unique energy factor that induces the deformation of the framework, which has different manifestations in different adsorption systems. For example, a specific liquid guest such as kerosene can cause swelling of the 3D polymer frameworks.^{17–19,37–39} The interaction between specific gas molecules such as CO₂ and the framework can also induce deformations in the framework windows of adsorbents.^40,41 Researchers attribute this adsorption-induced framework deformation to forming a thermodynamically metastable state caused by the host–guest interaction.⁴² In the desorption process, the opposite energy transfer process drives the framework to rebound. For example, the adsorption of water molecules induces the cellulose framework to swell and break the hydrogen bonds in the structure to form the metastable state.^4,43 In contrast, the reformation of hydrogen bonds in the framework at low relative humidity decreases the system's energy and drives reversible mechanical deformation of the framework (Figure 3A). The high flexibility of the framework in this process is the basis for flexible adsorption. Therefore, a highly flexible framework structure significantly lowers the barrier to forming the metastable state and efficiently induces reversible mechanical deformation.

Figure 2. Definition and illustration of flexible adsorption.

Figure 3. Examples of flexible adsorption theory research: (A) Molecular dynamics simulation of water absorption by cellulose4 (Copyright 2018, Mingyang Chen et al.). (B) Conformational changes of flexible coordination polymers under light irradiation44 (Copyright 2017, Yongtai Zheng et al.). (C) Theoretical simulation and interpretation of flexible adsorption of cellulose45 (Copyright 2019, Elsevier Ltd.). (D) C- and E-type isotherms in flexible coordination polymers42 (Copyright 2020, John Wiley and Sons).

The most important feature of flexible adsorption materials is their flexibility compared to rigid adsorbent materials with long-lasting pore channels. By modulating the flexibility of the material framework, flexible adsorption materials can break the limits of traditional adsorption theory and achieve a variety of specific application goals. Using the temperature-dependent flexibility of flexible frameworks, flexible adsorption materials can achieve selective permeation of guest molecules at specific temperatures.⁴⁶ Meanwhile, flexible adsorption materials have a significantly higher saturation adsorption capacity than rigid adsorption materials due to their highly flexible and expandable polymeric frameworks.¹⁵ For example, flexible resins can adsorb their weight of oil and water molecules several times and keep them in the polymer framework.¹⁹ This dynamically changing adsorption behavior has also been shown to significantly retard the gas adsorption equilibrium of the material and increase its saturated gas adsorption. Therefore, it is promising to explore and apply flexible adsorption materials.

Understanding and regulating framework flexibility

Since the flexibility of the adsorption framework is the basis of adsorption-induced dynamic adsorption behavior, understanding and regulating the framework flexibility are significant challenges researchers face. Although the dynamic adsorption behavior of flexible adsorption materials exhibits a significant chemical structure dependence, we still hope to find a general mechanism between framework flexibility and dynamic adsorption behavior. In general, the flexibility of adsorption materials comes from the organic chain, such as flexible ligands and side chains in the structure. In contrast, the rigidity comes from the aromatic skeleton and crosslinkers of the adsorption framework.⁴⁷ In the 3D framework of flexible adsorption materials, where the inorganic components are linked by ionic or ionic covalent bonds, the organic part is covalently bonded, hydrogen and van der Waals interactions occur between the flexible framework and the occluded species.^48,49 Structurally, accessible space to accommodate the modified steric hindrance of the flexible part and nonrigid areas are necessary factors to generate the flexible adsorption of a framework. During the host–guest interaction, the guest molecules induce distortion, bending, and other conformational changes in the flexible part of the framework. Eventually, it leads to overall distortion of the framework. In addition, although the rigid part of the framework has limited deformation latitude, its influence on the framing flexibility cannot be ignored. Recently, flexibility originating from changes in the coordination environment of metal ions and conformational changes of secondary building units containing metal ions in specific environments have also been reported, indicating the critical role of inorganic components in dynamic adsorption behavior.^50–52

In addition to the corresponding structural foundation, the dynamic adsorption behavior of the absorption framework also requires external physicochemical stimuli brought about by host–guest interactions. In the early stage of research, the dynamic behavior of adsorbents is generally accompanied by the introduction and removal of the guest molecule.⁵³ This host–guest specific interaction has been elaborated in crystalline and amorphous flexible adsorption materials.^11,15 Moreover, the dynamic adsorption behavior of the framework is also significantly modified by regulating the specific host–guest interactions, which facilitates the targeted design of the adsorption framework. For example, functional groups such as amino groups are introduced into organic resin frameworks to enhance the interaction between the framework and CO₂ molecules by formation of ammonium carbamate with CO₂.⁵⁴ In addition, by introducing photothermal sensitive functional components into the framework, the way of the host–guest interaction can be changed.^55–57 In these examples, external stimuli such as light and heat can lead to conformational changes in the functional components of the flexible framework, which can affect the framework's kinetic adsorption behavior (Figure 3B).⁴⁴ Although, in these examples, there is no direct interaction between the guest molecules and the framework, external stimuli such as light and heat still work on the framework in the form of energy, which still meets the definition of flexible adsorption.

Theoretical modeling of flexible adsorption

In addition to experimental studies of flexible adsorption materials, theoretical calculations and molecular modeling approaches have become powerful tools to explore flexible adsorption better. In the early stages of research, similar to traditional rigid adsorption theory, some theoretical, computational methods based on thermodynamic approaches considering well-defined pore structures, such as slit pores or cylinder pores, were used to study flexible adsorption.^26,27,29 However, these theoretical research methods do not apply to flexible adsorption because they cannot profile the framework's complex pore geometry and deformation during host–guest interactions.

For example, the 3D polymer pores of flexible resins are mainly morphologically distorted interchain spaces formed by disordered polymer chains, which are highly flexible and complex. Moreover, several studies have used macroscopic and continuous pore mechanics frameworks to explore flexible adsorption. The mechanical effects of adsorption were Integrated into pore mechanics by considering an additional adsorption-induced stress term. Furthermore, the adsorption-induced deformation of materials such as coal is simulated by relating the adsorption-induced stress to the excess amount of adsorbate surface.^31,58–60 Nevertheless, the method describes the adsorption process of materials by surface coverage rather than pore filling, which does not apply to flexible polymers. On this basis, Flory and Rehner proposed the Flory–Rehner theory, which builds on the concept of hybrid polymers and liquid molecules rather than distinguishing between adsorption surfaces. The Flory–Rehner theory suggests that the elastic and mixing contributions to the expansion free energy of a dry polymer network are separable and additive.⁶¹ In particular, the elastic contribution can be described by an intrinsic mechanical relationship, while the mixing contribution is evaluated by considering the mixing entropy and enthalpy. However, a solvent–polymer interaction term is introduced in this theory, which is challenging to characterize directly by experiments or molecular simulations. In addition, the variation in the free energy of the system during adsorption can be addressed by studying the adsorption isotherm because the state of the adsorption system is characterized by the molecular number and the corresponding chemical potential of the guest molecular.⁴

Following this idea, a new formulation of pore mechanics proposed by Brochard calculates the adsorption-induced stress in microporous solids as a function of the molecules number being adsorbed, rather than using the surface excess.⁵⁸ The equations derived based on this strategy are valid for nanoscale disordered porous media and can be applied to the flexible polymers' adsorption theory. Therefore, pore mechanics is highly promising for studying flexible adsorption theory as a macro–micro correlation research method. Since the coupling between deformation and adsorption occurs at the molecular level, molecular simulations such as molecular dynamics and Grand Canonical Monte Carlo (GCMC) simulations are suitable for studying flexible adsorption.^4,45 Chen et al. proposed a pore mechanics model based on molecular simulations to observe the host–guest interactions during the water absorption of cellulose, which provides insights into host–guest interaction-induced framework deformation (Figure 3C).⁴ They use the developed model to discuss the role of different mechanical properties, adsorption, and pore characteristics in the dynamic adsorption behavior and give the corresponding physical explanation.^4,62

Because the classical Brunauer–Emmett–Teller model and the ideal adsorption solution theory assume that the structure of the adsorbent is rigid and immutable, the direct application of these theories to flexible adsorption may be erroneous. Meanwhile, the isotherm classification of adsorbents defined by IUPAC is inadequate and does not reflect the dynamic adsorption behavior of flexible adsorption materials. For example, the adsorption process of water molecules on cellulose exhibits a Type II isotherm, which indicates that the adsorption process is unrestricted monolayer–multilayer in the traditional rigid adsorption theory.⁶² However, the subsequent study shows that the particular adsorption phenomenon of cellulose is due to the swelling of the micro-pores in the cellulose framework induced by water molecules. Compared to rigid adsorbents, a vital adsorption feature of flexible polymers such as crosslinked resins are the adsorption isotherm hysteresis, which is proved to be controlled by the adsorption-induced reversible framework mechanical deformation. Moreover, the dynamic adsorption behavior induced by host–guest interactions generates C- and E-type isotherms in flexible coordination polymers, which further enriches the theory of flexible adsorption (Figure 3D).⁴²

STRUCTURE AND PROPERTIES OF FLEXIBLE ABSORPTION MATERIALS Amorphous flexible adsorption materials

The easily functionalized 3D frameworks of polymer resins provide large specific surface areas and high adaptability in multiple application scenarios.^63–66 Therefore, the potential of polymer resins in gas storage, gas separation, catalysis, and energy conversion has attracted strong interest from researchers. These polymer networks exhibit surprising flexibility during the host–guest interaction and can be solvated in a liquid phase system. Typically, polystyrene crosslinked resins have been extensively studied as typical flexible resins. The swelling phenomenon of polystyrene resins in organic solvents is controlled by the reduction of free energy during the solemnization of polystyrene and the elastic bonding forces resulting from the deformation of the loosely crosslinked polymer network.⁴¹ This dynamic deformation of the framework can also substantially affect the polymer resin's adsorption process. For example, a porous polymer network COP-150 with a flexible framework synthesized by Friedel–Crafts alkylation reaction exhibits unique memory adsorption characteristics during the adsorption of N₂ and CO₂ (Figure 4A).¹⁵ The high-pressure N₂ adsorption isotherm of COP-150 shows flat physical adsorption with low N₂ uptake. In contrast, the high-pressure CO₂ adsorption isotherm of COP-150 exhibits a significant hysteresis, suggesting that the host–guest interaction disturbs the COP-150 framework. Interestingly, the high-pressure N₂ adsorption isotherm of COP-150 exhibits a similar shape and hysteresis to the CO₂ adsorption isotherm after the CO₂ adsorption test and vacuum treatment, and the adsorption capacity of nitrogen increased significantly. Researchers attributed this memory adsorption phenomenon to the host–guest interaction. First, the specific interaction of CO₂ molecules with COP-150 induces framework deformation, and the 3D framework of COP-150 remains expanded after CO₂ desorption. Afterward, the N₂ molecules fill the swollen framework of COP-150 and exhibit an adsorption process similar to CO₂. Since there is no specific interaction between N₂ molecules and COP-150 to provide adsorption memory for CO₂, the swollen framework of COP-150 reverts to the ground state upon desorption of N₂. In addition, the flexible crosslinked polymer synthesized with benzene as a monomer adsorbs CO₂ by physical swelling, and the adsorption-induced frame deformation leads to an increase in CO₂ saturation adsorption, which is an advantage that other rigid adsorbent materials such as activated carbon and ZIF-8 do not have (Figure 4B).⁴⁰

Figure 4. Examples of amorphous flexible adsorption materials: (A) Flexible crosslinked polymers COP-150 synthesized by Friedel–Crafts reactions15 (Copyright 2019, Springer Nature). (B) Flexible crosslinked polymers synthesized with benzene as a monomer40 (Copyright 2019, Springer Nature). (C) Flexible oil-absorbing resin synthesized with acrylate as a monomer.67 (D) Flexible crosslinked polysulfate adsorbent (TPC-cPS) synthesized by SuFEx reaction68 (Copyright 2014, American Chemical Society).

This flexible adsorption phenomenon originates from the reversible framework distortion caused by the specific interaction between the guest molecule and the framework, in which the flexibility of the polymer network is closely related to the adsorption-induced deformation of the adsorption framework. Therefore, to obtain an intrinsic relationship between network flexibility and the adsorption kinetics of flexible polymers material, Lu et al. synthesized a series of polymer resins with flexible 3D frameworks using methods such as reverse emulsion polymerization.^{16–19,67,69–84} The water absorption test of flexible resins with different crosslinking degrees shows a typical volcano curve due to the polymer framework being too flexible at low crosslinking degrees and dissolving to a large extent during water absorption. When the crosslinking degree is high, the crosslinked framework is too rigid, and the swelling capacity of the polymer framework is limited, leading to a decrease in water absorption (Figure 4C).⁶⁷ After that, a large particle size sodium polyacrylate-type water-absorbent resin synthesized by Lu was used to investigate the effects of monomer addition, acrylic acid neutralization degree, initiator dosage, stirring speed, and dispersant on the water absorption and framework flexibility of the resin.⁷⁶ The results show that by controlling the parameters of the reaction process, the flexibility of the polymer network can be precisely regulated, and a better water absorption efficiency is obtained.

Meanwhile, Lu et al. synthesized a series of high-performance oil-absorbing resins using monomers such as acrylic acid-2-ethylhexyl ester, butyl methacrylate, dodecyl methacrylate, octyl methacrylate and their derivatives, and investigate the effect of monomer structure on their oil-absorbing properties.¹⁹ These results show that different monomers not only lead to changes in framework flexibility but also alter the interaction between the oily guest molecules and the 3D framework of flexible resin, which is caused by the differences in affinity between ester groups of different chain lengths and oily molecules. In addition, a new flexible crosslinked polysulfate adsorbent (TPC-cPS) synthesized by a sulfur fluoride exchange (SuFEx) click reaction displays significant swelling upon adsorption of organic solvents, which is due to the van der Waals forces between the nonpolar groups in the polymer and guest molecules. Meanwhile, electrostatic interactions between polar groups (O═S═O) on the polymer backbone and polar groups in organic molecules can effectively immobilize solvent molecules (Figure 4D).⁶⁸ To further explore the balance between framework flexibility and oil absorption properties, Lu used rigid/flexible coexisting crosslinkers to construct organic networks. The rigid groups can hold up the internal network space of the resin during the adsorption process, which facilitates the diffusion of the viscous oil molecules into the polymeric framework and increases the adsorption capacity of the resin for crude and refined oil. On the other hand, the flexible chain makes the network structure more resilient, allowing the material to obtain maximum expansion during adsorption and improving the adsorbent material's oil retention capacity.

Other researchers have also tried and synthesized flexible polymeric adsorbent materials based on the above research concept. Zhang et al. successfully synthesized superhydrophobic nanoporous polydivinylbenzene materials by a novel solvent thermal method.⁸⁵ The synthesized polymers have a high specific surface area, large pore capacity, controlled average pore size, superhydrophobicity, and superoleophilicity. Nanoporous poly(divinylbenzene) cannot adsorb water in a liquid or gaseous state and only swell in specific organic solvents. Hence, it has preferential selectivity for organic compound adsorption. They also successfully synthesized a series of structurally stable and flexible diphosphine ligand porous polymers and modified them with Rh nanoparticles.⁸⁶ These insoluble porous polymers can be swollen by various organic solvents and exhibit better catalytic performance than similar homogeneous catalysts in hydroformylation reactions due to the high specific surface area and high absorption rates resulting from the framework flexibility.

Crystalline flexible adsorption materials

Compared to amorphous flexible adsorption materials, the guest-induced framework deformation of crystalline flexible adsorption materials is more imperceptible. Based on the definition of flexible adsorption, the primary existing flexible crystalline adsorption materials are flexible coordination polymers, metal–organic frameworks (MOFs), and zeolites.

Flexible coordination polymers

Porous coordination polymers, as porous platforms with highly designable structures, has various metal sites and ligand species in their structures that can significantly impact the framework's flexibility.^87–89 Flexible porous coordination polymer {[Zn(ip)(ebpy)]}_n (CID-1) prepared by Tanaka et al. exhibited a two-step adsorption process in the adsorption isotherm of methanol (Figure 5A).⁹⁰ They suggest that this particular adsorption kinetics is induced by the interaction between the methane molecules and the coordination polymers. CID-1 reached the first methanol adsorption saturation at low pressure (P/P₀ = 0.1). With increasing pressure, the pores were opened and reached the second adsorption saturation due to the interaction between the guest molecules and the pores. This dynamic adsorption behavior is further shown to be influenced by the crystal size. The methanol adsorption isotherms and adsorption kinetics show that the adsorption curves of CID-1 nanocrystal at (P/P₀ = 0.067–0.073) possess a more significant hysteresis than bulk CID-1, and CID-1 nanocrystal's absorption rate is faster than bulk CID-1. This suggests that the host–guest interaction and adsorption kinetics during adsorption can be modulated by changing the size and surface morphology of the flexible coordination polymer.

Figure 5. Examples of crystalline flexible adsorption materials: (A) Flexible coordination polymers nanocrystals (CID-1) prepared by nonaqueous inverse microemulsion with ultrasonication90 (Copyright 2010, Nature Publishing Group). (B) Flexible coordination polymers PCP-N and PCP-C prepared by the reactions of Hppt and Hdpt with Fe2+ ions, respectively91 (Copyright 2015, American Chemical Society). (C) Dynamic adsorption behavior of BUT-8(Cr)A92 (Copyright 2017, Yang et al.). (D) Flexible chabazite samples with different Si/Al ratios and cation types93 (Copyright 2013, American Chemical Society).

In subsequent studies, researchers achieved the geometric transformation of the structure near the metal site by adjusting the ligand flexibility of the coordination polymer and significantly increasing the flexibility of the polymer structure.^94,95 For example, flexible coordination polymers {[Zn₂(tp)₂(L²)]·2.5DMF·0.5H₂O} containing secondary structural units were constructed using elongated axial ligands.⁵² In situ x-ray diffraction and gas adsorption isotherms indicate that {[Zn₂(tp)₂(L²)]·2.5DMF·0.5H₂O} undergo a reversible crystallographic transition and exhibit typical flexible adsorption characteristics during the gas absorption process, which result from the reversible breakage/formation of metal–ligand bonds within the framework. Subsequently, Susumu et al. constructed two flexible coordination polymers [Fe(ppt)₂] (PCP-N) and [Fe(dpt)₂] (PCP-C) based on pyrazinyl and pyridyl ligands (Figure 5B).⁹¹ Experimental and characterization results show that due to the weaker coordination ability of the pyrazine group than the pyridine group, PCP-N exhibits significant flexibility during the adsorption of various gases. In contrast, PCP-C exhibits typical rigid adsorption characteristics. In addition, the coordination polymer Zn-DPA-2H₂O containing the helical ligand DPA undergoes rotational rearrangement of the helical structure due to the interaction between the guest molecules and the ligand during CO₂ adsorption, which results in a slight deformation of the framework structure and exhibits flexible adsorption characteristics.⁹⁶

Density functional theory calculations theoretically illustrate the flexible adsorption phenomenon of flexible coordination polymers. For example, three kinds of pore structures in flexible coordination polymers Cu(OPTz) are involved in the CO₂ adsorption process: 1D channel I, intermediate I and II pores, and small pore II.⁹⁷ The adsorption process of 1D channel I is not controlled by the crystal structure distortion. However, when the crystal deformation induced by the adsorption of 1D channel I reach a certain level, it will cause the opening of I and II mesopores and tiny pores II. Therefore, the adsorption of Cu(OPTz) on gas molecules exhibits a significant temperature dependence.

Flexible MOFs

“Flexible MOFs” describes MOFs materials that possess guest-induced structural change properties, which can achieve a variety of dynamic behaviors, such as stretching of the framework, opening and closing of the pores, and reversible changes in physicochemical properties.⁹⁸ Therefore, flexible MOFs have great potential for storage, separation, sensing and many other applications.^96,99 Although strategies and methods for the targeted construction of flexible MOFs have been well developed in recent studies, the rational construction of flexible MOFs remains a significant challenge. An overview of the advances in the understanding of flexibility in MOFs is presented in this section.

Like flexible coordination polymers, the ligand-metal structure of flexible MOFs forms a complex coordination framework. The metal–inorganic part mainly provides the rigidity of the framework, and the organic ligands provide flexibility. Therefore, designing the structure of flexible ligands using grafting and substitution is a basic guideline for regulating the framework flexibility of MOFs. For example, introducing two or four methyl groups into the framework can open the pores more quickly in the presence of liquid without significantly reducing the framework expansion.¹⁰⁰ Yang et al. used ligands modified with high-density sulfonic acid (–SO₃H) to synthesize BUT-8(Cr)A, whose –SO₃H group on the inner surface of the pore channel can induce reversible mechanical deformation of the framework during interaction with the guest molecules (Figure 5C).⁹² The functionalization of organic ligands as a powerful mean to modulate the flexibility of MOF frameworks can even confer a certain degree of flexibility to rigid MOF materials. Typically, modified MOF-5 by ligands alkyl functionalization showed guest and temperature-induced structural deformation (up to 17% volume shrinkage).¹⁰⁰

Recently, examples of modulating the flexibility of the MOF framework by other means have been reported. Doping of metals in the MOF material framework significantly affects the conformation of the metal–ligand structure and changes the framework flexibility.¹⁰¹ In addition, Susumu et al. reported a unique example of flexible MOFs whose dynamic behavior was influenced by the crystal size, known as the “shape memory effect.”¹⁰² By reducing the size of the flexible MOF crystal, the “open” state of the flexible MOF crystal could be maintained, and the open-door effect caused by the phase transition between “open” and “closed” could be eliminated.

Flexible zeolites

Some examples of zeolites also showed surprising flexibility in the adsorption process. The adsorption of various chemicals could cause slight changes in the zeolite crystal framework, resulting in changes of up to 2.3% per unit of cell volume.¹⁰³ These guest-induced structural distortions can significantly affect the adsorption kinetics of porous zeolites. Fractional-order kinetic simulations were used to study the effect of framework flexibility on the adsorption diffusion of aromatic hydrocarbons in MIF-type zeolites.¹⁰⁴ The results show that the flexibility of the zeolite framework reduces the energy barrier between the zeolite framework and the lower energy states, increasing the diffusion rate of aromatic compounds through the channels compared to that of rigid zeolites. Similar to other flexible adsorption materials, the framework distortion induced by the guest molecule is an essential factor affecting the kinetics of flexible adsorption.

Initially, Palomino et al. found that Rho zeolites successfully separate CO₂ from CH₄ with very high selectivity and demonstrated that this phenomenon was related to structural changes in the zeolite induced by CO₂ adsorption.¹⁰⁵ Subsequently, Webley et al. conducted an in-depth study of the dynamic adsorption behavior on flexible zeolites and synthesized a series of flexible Rho zeolites for gas adsorption.^{12,93,106–108} They proposed the “molecular trapdoor” effect, in which gas molecules induce reversible distortion of cations within the flexible skeleton of zeolite, thus changing the adsorption kinetics of the guest molecule. Subsequently, a series of Rho zeolites with different monovalent cations were synthesized. In situ x-ray diffraction analysis and zero-length column chromatography demonstrate that guest-induced structural distortions occur mainly in the cations of the zeolite framework, and changes in cation species lead to different CO₂ adsorption kinetics (Figure 5D).⁹³ Choi et al. found a significant cation-dependent hysteresis in the CO₂ adsorption isotherm of Na⁺, K⁺, and Rb⁺ small-pore zeolite with Si/Al ratio of 3.0. Solid-state NMR and x-ray diffraction analyses suggest that additional skeletal cations have coordination interactions with the skeletal oxygen, the guest molecule, or both, thereby controlling skeletal dynamics and adsorption behavior.⁸⁷ In addition, researchers pointed out that the actual mechanism of CO₂ selective adsorption on intermediate-silica small-pore MER zeolites can vary from cation-gated to co-cation-gated breathing to breathing, depending on the combination of their topological structure and compositional flexibility.⁸⁸

APPLICATION OF FLEXIBLE ADSORPTION MATERIALS Application of flexible adsorption materials in fossil energy storage and separation

Recently, the demand for fossil energy, such as oil and coal, has significantly increased worldwide, causing a severe energy crisis.¹⁰⁹ Therefore, solid-gas separation and storage technology has received much attention from researchers as an efficient and straightforward energy storage technology. Solid gas separation and storage technology can separate important industrial gases, such as CO₂, from mixed waste gases and store them in solid media for backup.¹¹⁰ However, conventional rigid adsorption materials such as porous carbon have poor gas separation and storage properties due to the lack of active sites for interaction with the guest molecules.⁴⁰ In contrast, flexible adsorption materials have more potential for gas storage and separation applications due to their large specific surface area and diversity of chemical structures.

The dynamic adsorption behavior induced by the host–guest interaction is a unique theoretical advantage of flexible adsorption materials compared to rigid adsorbents. There are two main strategies for applying flexible adsorption materials to gas separation and adsorption: (1) modulation of host–guest interactions and (2) modulation of external physicochemical stimulations. In the first case, the specific interaction on the guest molecule needs to be enhanced by adjusting the structure and composition of the adsorption framework. In the second case, the focus is the introduction of light- and heat-sensitive groups into the flexible framework to improve the sensitivity of the framework.

In flexible adsorption, the host–guest interaction, as the initiator of the mechanical deformation of the framework, has an essential influence on the kinetics of guest molecule adsorption. For example, adsorption of CO₂ by polymers such as polystyrene causes their 3D frameworks to swell, so the researchers hypothesized that porous supercross-linked polymers would adsorb CO₂ via a chemically specific swelling mechanism. As a typical example of the first case, flexible porous crosslinked polymers synthesized with benzene as monomer underwent significant swelling during CO₂ adsorption, resulting in delayed CO₂ adsorption equilibrium and increased CO₂ saturation adsorption. The CO₂ adsorption capacity of this polymer network is 15.32 mmol/g, which is significantly higher than that of conventional adsorbent materials such as ZIF-8 (Figure 6A).⁴⁰ Moreover, due to the specific interaction between the benzene framework and CO₂ molecules, this flexible adsorbent shows better selectivity for N₂ and H₂ in CO₂/N₂ and CO₂/H₂ separation.

Figure 6. Examples of the gas/liquid separation and storage applications of flexible adsorption materials: (A) CO2 adsorption and separation in polymer 140 (Copyright 2010, Nature Publishing Group). (B) Storage of CO2 in PCN-12355 (Copyright 2012, American Chemical Society). (C) N2 adsorption in 1⊃AB46 (Copyright 2012, American Chemical Society). (D) Oil–water separation in porous PU@rGO111 (Copyright 2011, RSC Publishing).

In addition, the flexible adsorption framework functionalized with amino, carboxyl, and fluorine groups can enhance the interaction with CO₂ through Lewis acid–base interactions, hydrogen bonding, and quadrupole interactions.^112–114 For example, polyamide adsorption materials with large specific surface areas were synthesized by polymerization of melamine and binary/tertiary benzoic acid. The interaction between the framework and CO₂ molecules was adjusted by changing the nitrogen content in the polymer to obtain better CO₂ adsorption/separation performance.¹¹⁵ Meanwhile, introducing functional groups in the framework by post treatment to achieve optimal isometric adsorption heat is also a common modification strategy. The construction of amine-functional adsorbent resins using organic amine molecules (polyethyleneimine and tetraethylenepentamine) impregnated with carbonyl-rich polymeric frameworks effectively enhances the interaction between CO₂ molecules and flexible frameworks.^54,116,117 In addition, flexible polymers as carriers for amine functional adsorbents have adjustable pore size, high adsorption capacity, and a variety of adsorption and loading active sites, further contributing to their gas adsorption and separation efficiency. Various flexible adsorption materials have been designed for gas adsorption and separation applications based on a similar guest-induced dynamic adsorption principle.^115,118 Besides, the migration of cationic adsorption sites in the flexible zeolite structure also promotes the adsorption and separation of CO₂. Typically, cations in the window positions between the cages of the zeolite Rho structure block the adsorption of CH₄.¹⁰⁶ However, in the presence of CO₂, the cations undergo a minor migration and adsorb CO₂ due to the interaction between the host and guest. The adsorption of CO₂ can be further enhanced by adjusting the cation species and content in zeolite, which is due to the difference in the interaction of different cations in the CO₂ molecule and framework.⁹³

Moreover, some flexible adsorption materials exhibit dynamic behaviors when subjected to external stimuli such as light and heat. For example, Park et al. synthesized a photoconvertible flexible adsorbent based on ligand 2-(phenyldiazenyl)terephthalate. Due to the photosensitivity of its framework side chains, the pore structure of PCN-123 can be adjusted by controlling the trans and cis conformations of the ligand during photochemistry treatment, which further modulates the CO₂ adsorption capacity of PCN-123 (Figure 6B).⁵⁵ Meanwhile, Zheng et al. introduced azobenzene into the flexible polymer framework and synthesized a new type of flexible photosensitive adsorbent. Azobenzene is the photosensitive functional component that successfully induced a reversible cis/trans isomerization structural transition of the leading flexible skeleton by light stimulation and modulated the gas adsorption. In addition to the above examples, inducing reversible framework deformation and improving the gas adsorption capacity of materials through external thermal effects is also a common strategy (Figure 6C).⁴⁶ Flexible polymers with flexible ligands [Mn(bdc)(dpe)] were shown to selectively adsorb CO₂ from CO₂/C₂H₂ at specific temperatures due to changes in overall (distortion of the crystal structure) and local (tilt of the ligand benzene ring) flexibility at different temperatures.¹¹

Similarly, Babu et al. synthesized a flexible adsorption framework using a butterfly-type ligand composed of isophthalic acid and phenothiazine-5,5-dioxide (OPTz), whose ligand structure possesses good local motion flexibility. Since the material framework undergoes distortions under external thermal stimulation, its guest adsorption is significantly temperature-responsive, favoring the kinetic separation of O₂/Ar.⁴⁴

In addition to CO₂, storing and separating of hydrocarbons is also attractive to the energy industry. Lin et al. reported a microporous flexible adsorption material [Zn(dps)₂(SiF₆)] (UTSA-300) that exhibits a pore opening-closing transition upon activation, forming a closed pore skeleton UTSA-300a locked by multiple hydrogen bonds.⁹⁴ The gas adsorption results show that UTSA-300a has an extremely high C₂H₂ adsorption capacity (76.4 cm³ g⁻¹) and can completely expel C₂H₄ and CO₂ from the C₂H₄/C₂H₂ and CO₂/C₂H₂ mixtures, owing to the strong host–guest interactions between the UTSA-300a framework and C₂H₂ molecules. Moreover, the flexible MOF (NTU-65) reported by Dong et al. can selectively separate C₂H₄ in a three-phase gas mixture (C₂H₂/CO₂/C₂H₄) because the framework structure of NTU-65 changes slightly at different temperatures, and its pore size and adsorption active sites can be adjusted according to temperature.¹¹⁹ Zeng et al. suggested that an increase in gas pressure was also shown to induce deformation of the flexible adsorbent material framework, leading to changes in the accessibility of the metal adsorption active site in the structure, further affecting acetylene adsorption kinetics.¹²⁰ In addition, the dynamic adsorption behavior of flexible adsorption materials has proven to be of great potential and value for the storage and separation of fossil energy sources such as CH₄ and C₃H₆.^44,121–123

Application of flexible adsorption materials in liquid adsorption and separation

In addition to the storage and separation of gases, flexible absorption materials also show excellent potential in liquid storage and separation due to the expandable 3D network structure. From the viewpoint of chemical structure, the presence of various functional groups (hydroxyl, carboxyl, and amide groups) inside the 3D polymer framework provides specific interactions between the adsorbent and the liquid molecules.

Lu's team has been working on the design and synthesis of flexible resin adsorbent materials with high efficiency in water and oil absorption. For example, flexible polymer resins synthesized by suspension polymerization of diethylhexyl enolate and butyl methacrylate in inert solvents can adsorb kerosene to a high amount of 10.2 g/g and benzene 18.8 g/g.⁸³ The interaction between the oleophilic groups of flexible polymer framework and the oil molecules triggers the swelling of the resin, and the swollen 3D polymer network wraps the oil molecules inside, effectively improving the oil retention performance of the resin. Lu et al. designed and synthesized a series of flexible oil-absorbing resins with different monomer ratios, and types, crosslinker amounts and types to deeply explore the effect of framework flexibility on oil absorption performance.⁸³ Typically, due to the difference in chain length between butyl methacrylate and 1,2-ethylhexyl acrylate, the different monomer ratios lead to changes in framework flexibility and affect the oil absorption efficiency of the resin. Flexible resins copolymerized from dodecyl methacrylate and various monomers exhibited different oil absorption efficiencies due to the affinities between the ester groups and the oily molecules. The results show that 2-ethylhexyl acrylate has the longest ester group chain and the most robust interaction with oily molecules. The ester groups with side chains are less lipophilic than the straight-chain ester groups with the same carbon atoms.⁸² In addition, as rigid groups, benzene rings can support flexible polymer chains and form 3D spaces to accelerate the diffusion and adsorption rate of oil molecules. Therefore, the number of benzene rings in the side chain structure of the monomer also significantly affects the oil absorption performance of the resin. The amount of crosslinking agent directly affects the crosslinking degree and framework flexibility of the 3D resin framework. The crosslinker type significantly affects the resin's network structure and the affinity between the oil molecule and the resin. For example, the oil absorption efficiency of copolymer resins synthesized with 1,4-butanediol diacrylate as a crosslinking agent is higher than that of pentaerythritol triacrylate, glycidyl methacrylate and 1,6-hexanediol diacrylate due to the complete network structure and high lipophilicity of 1,4-butanediol diacrylate.⁸¹ Moreover, a higher crosslinking degree makes the framework less flexible and reduces the oil absorption performance of the resin. A lower crosslinking degree makes the flexible framework too loose to provide a 3D framework for oil absorption and retention.¹⁷ Therefore, reasonable regulation of the framework flexibility and chemical composition is essential when designing high-performance oil-absorbent resins.

In addition to high-performance oil-absorbing flexible resins, Lu et al. also explored the application of flexible adsorption resins for water absorption. Like high-performance oil-absorbing resins, reasonable framework flexibility and chemical structure are the basis for obtaining high water absorption and retention properties. Unlike traditional rigid absorbent materials that rely on the capillary force of the porous structure to adsorb water molecules, flexible resins adsorb water molecules through the physical infiltration of the flexible framework and hydrogen bonding. Therefore, they possess a stronger water absorption and retention capacity. For example, the adsorption amounts of deionized water and 0.9% NaCl in 10 min were approximately 1500 and 120 g/g for the sodium polyacrylate flexible absorbent resin synthesized by inverse emulsion polymerization.⁶⁷ The monomer ratio, initiator dosage, and other factors significantly influence its water absorption performance due to the flexible adsorption 3D framework's different compositions and physical properties. A flexible framework with good flexibility and chemical structure can lock water molecules within the polymer network to form cured water to improve water absorption and retention efficiency. Meanwhile, the flexible absorption materials designed and synthesized by Lu have tremendous potential for oil–water separation applications. For example, a superhydrophobic-superlipophilic material with good elasticity and hydrophobic–lipophilic solid properties was prepared by modifying reduced graphene oxide on polyurethane foam, which can selectively adsorb oily molecules such as toluene and chloroform in oil–water mixtures (Figure 6D).¹¹¹

Theoretically, for polymer crosslinked networks, the conformational change of polymer chain segments gives the framework high flexibility.⁴⁷ The polymer chains in the framework spontaneously tend to a curled conformation without an external energy contribution. When external liquid molecules enter the polymer network, the interaction of guest molecules and functional groups disrupts the hydrogen bonding of the network, leading to rotational or stretching distortions in the conformational phase of the polymer chain segments, forming a thermodynamically metastable state. In this process, the flexible chains with higher charge density and more robust interaction with liquid molecules are more likely to undergo conformational changes and obtain higher liquid adsorption efficiency. This theory emphasizes the importance of the flexible (disordered) crosslinker, which is stretched to a conformation with low entropy during elastic expansion. In contrast, the stretched flexible crosslinker returns to the ground state during framework recovery and contributes positive entropy to the reorganization of the hydrogen bonding in the polymer framework. Similar structures are also found in many proteins, where the amorphous domain enables the rigid domain to alter its functions dynamically.^124,125

Application of flexible adsorption materials in functional devices

In nature, changes in environmental humidity can lead to volume distortions in materials such as hair and leaves, accompanied by changes in electrical properties. Based on this phenomenon, flexible adsorption materials with high framework flexibility have been used to build various functional devices. They have attracted a great deal of interest from researchers.

Cellulose, as a flexible adsorption polymer with an organic 3D network, undergoes expansion of the framework structure when interacting with water molecules.⁴ Moreover, the electrical properties of cellulose are sensitive to the environmental humidity, which is caused by the interaction of the adsorbed water molecules with the hydroxyl groups in the framework and the formation of conductive paths. Safari et al. prepared a humidity-sensitive device by combining cellulose with carbon nanotubes based on this particular property of cellulose and studied the change in conductivity of the device under different humidity conditions (Figure 7A).²¹ The results show that the conductivity of the devices is enhanced with increasing humidity due to the opposite effect of water vapor on the electrical properties of carbon nanotubes and cellulose. On the one hand, charge transport between water vapor molecules and carbon nanotubes leads to electron-hole complexation on the surface of carbon nanotubes. On the other hand, the decrease in electrical conductivity is compensated by the formation of conductive paths and a plasmon leap to the cellulose surface, thus increasing the electrical conductivity of carbon nanotubes. In addition to the change in electrical properties, the interaction between the water molecules and the functional groups of the cellulose framework leads to a significant volume expansion.⁴ When cellulose is constructed as a spring-like cylindrical coil, the water molecule adsorption-induced volume expansion provides the device with a muscle-like stretching motion. This artificial muscle assembly has a significantly elevated tensile stress in a humid environment due to the interaction of water molecules with hydroxyl and amide groups on the cellulose framework forming hydrogen bonds (Figure 7B).¹¹⁶ Natural textiles integrated with this model can respond intelligently in humid environments by expelling hot and humid air, thus effectively relieving the wearer's physical discomfort during sweating. In addition to cellulose, some other organic polymers also possess this unique guest-induced framework deformation property. Lee et al. constructed an efficient humidity sensor using agarose as a hydrophilic matrix and carbon nanotubes as a conductive filler. The composite microfiber composed of an agarose matrix swells or shrinks with changes in ambient humidity, resulting in changes in the junction density between the hydrophilic matrix and the conductive fillers.²² In addition, the adsorption of water molecules by the carbon nanotube fillers can further increase the resistance of the microfiber sensor. Real-time monitoring of the microfiber resistance shows that the sensor responds reliably and reversibly to changes in humidity with a reasonable response rate.

Figure 7. Examples of the functional devices and energy catalysis applications of flexible adsorption materials: (A) Humidity sensor based on CNT/NCC composite21 (Copyright 2016, American Chemical Society). (B) Moisture-responsive natural fiber coil-structured artificial muscles126 (Copyright 2018, American Chemical Society). (C) Detection of aromatic VOCs in the porous flexible framework [Zn2(bdc)2(dpNDI)]n127 (Copyright 2011, Takashima et al.). (D) Catalytic reaction of CO2 and epoxides in PCN-700-Me2128 (Copyright 2016, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

In addition to constructing functional devices based on the electrical signal changes generated during flexible adsorption, specific molecular detection through optical signal changes generated by structural distortion of organic molecules is also of great research value. For example, Takashima et al. constructed a novel entangled porous flexible framework [Zn₂(bdc)₂(dpNDI)]_n by embedding photoactive organic module naphthalene diimide in a flexible adsorption framework. Information about the different benzene ring substituents of the aromatic guest is decoded into the corresponding visible light emission (Figure 7C).¹²⁷ The interaction of the aromatic guest molecule confined in the nanopore with the photosensitive molecule leads to a conformational change in the fluorescent photosensitive organic module and induces deformation of the adsorption framework. The resulting enhanced visible emission detectable to the naked eye is used to accomplish fluorescence sensing of different molecules. Similarly, introducing the fluorescent reporter stilbene into a flexible 2D network Zn₂(terephthalate)₂(triethylenediamine)_n bridged by triethylenediamine can also accomplish fluorescence sensing of CO₂ and acetylene, demonstrating that the flexible adsorption phenomenon resulting from the host–guest interaction plays a vital role in the conversion of gas adsorption into detectable output signals.⁸⁸

Application of flexible adsorption materials in energy catalysis

Unfortunately, due to the particular structural design ideas and research methods, flexible adsorption materials have not been studied more intensively and extensively in catalysis. A typical example, Bharadwaj et al. synthesized a new flexible framework and performed Knoevenagel condensation reactions in the framework pores.¹²⁹ In situ x-ray diffraction analysis demonstrates that the generation of products during the catalytic process in the pores leads to a subtle deformation of the framework. Thus, the unique flexibility of adsorption framework allows researchers to monitor chemically important reactions, which facilitates further exploration of the reaction mechanisms by researchers. Moreover, Yuan et al. designed and synthesized a series of flexible framework systems and investigated the effect of guest-induced structural distortions on catalytic efficiency (Figure 7D).¹²⁸ Single-crystal x-ray diffraction shows that the modulation of framework flexibility can be achieved using organic linkers with different functional groups. Furthermore, in the cycloaddition reaction of CO₂ with epoxides, the activity of Lewis acid sites in the PCN-700-Me₂ structure can be turned on and off by guest-induced framework distortion. Before framework deformation, the reaction substrate can approach both pairs of symmetrically independent –OH⁻/H₂O groups on the Zr₆ cluster distributed in the c- and b- direction of the PCN-700 crystal structure. In contrast, after frame deformation, the change in crystal structure leads to the stacking of ligands and clusters in the c-direction, causing the reaction substrate to approach the –OH⁻/H₂O groups only from the b-direction and the closure of the reaction site. Such switchable catalysts originate from the unique framework flexibility of flexible adsorption materials, which have more excellent research value and application prospects. In addition, some potential flexible adsorption materials have also shown better activity in energy catalysis.^130–132 However, the intrinsic connection between flexible adsorption phenomena and catalytic mechanisms needs further exploration.

Application of flexible adsorption materials in environment fields

With the rapid industrialization in various countries, industrial emissions containing carbon monoxide, hydrocarbons, nitrogen oxides, and sulfur oxides are being released into the environment from power plants and other energy industries, causing severe harm to the ecological environment and human health.^133–135 At the same time, the large amount of water-soluble organic waste discharged has become an important factor in water pollution. Due to the strong interaction between the subject and the object, flexible adsorption materials have shown high application value for rapidly removing environmental pollutants.^136,137 This section details examples of the application of flexible adsorption materials for environmental restoration and looks forward to their development.

The highly designable framework of flexible adsorption materials leads to easy modulation of host–guest interactions and enhanced adsorption of contaminant molecules. For example, Dai et al. synthesized a series of porous polymers by radical polymerization of ligands such as pyridine. POP-Py has an SO₂ capacity of 10.8 mmol g⁻¹ with good reversible adsorption ability. And POP-Py also showed excellent performance for separating SO₂ in SO₂/CO₂/N₂ ternary mixtures.¹³⁸ The results of Monte Carlo (GCMC) simulations and density flooding theory calculations showed that the nitrogen atoms on the pyridine and bipyridine rings were the main sites for SO₂ adsorption, while the carbon atoms on the pyridine and bipyridine rings were the weaker adsorption sites (Figure 8A). Furthermore, the main contribution is the interaction between the positively charged S atom in the SO₂ molecule and the N atom in the polymer framework. In addition, the ionic ultramicroporous polymers (IUPs) designed by Suo et al. can effectively remove SO₂ and have good SO₂ selectivity (>5000) for SO₂/CO₂ mixture. This was attributed to the strong interaction between OS…Br in the flexible frame and SO₂ and the framework swelling of IUPs during adsorption (Figure 8B).¹³⁹ Similarly, enhanced host–guest interactions can effectively improve the material's affinity for NH₃ and SO₂ by functionalizing the microporous polymeric framework with carboxylic acid and amidoxime functional groups.¹⁴⁰

Figure 8. Examples of environmental restoration applications of flexible adsorption materials: (A) SO2 adsorption and separation in POP-Py138 (Copyright 2022, American Chemical Society). (B) SO2 adsorption and separation in IUPs139 (Copyright 2020, John Wiley and Sons). (C) BPA adsorption and degradation in perylene-based bifunctional micelle141 (Copyright 2022, John Wiley and Sons). (D) volatile organic compounds (VOCs) adsorption in high surface area porous polymers142 (Copyright 2021, Elsevier B.V.).

Removal of specific contaminant molecules by directional modulation of the chemical composition of the flexible adsorption framework is a straightforward, efficient strategy. For example, polyaniline and its composites exhibit high NO₂ removal efficiency due to the interaction between amino groups and NO₂ molecules.^143–146 Moreover, Zhao et al. designed and synthesized two low-cost molecularly imprinted polymers to adsorb NO₂ from flue gas using different templates.¹⁴⁷ The materials can effectively adsorb NO₂ from mixed flue gas due to the strong interaction between the framework and the nitrogen dioxide molecules. And due to the large number of hydrophobic groups in the polymers –(CH₂–CH)_n– and the hydrogen bonding force between them, the H₂O molecules have a less negative effect on NO₂ adsorption. Further studies have shown that the –NH₂, –C═O group in the backbone of flexible adsorbent materials can interact with nitrogen dioxide, producing surface-bound nitrates and enhancing pollutant adsorption.¹⁴⁸

For the adsorption and degradation of organic pollutants in the liquid phase, it is also very important to construct strong host–guest interactions. For example, Lu et al. designed and synthesized an “in situ adsorption/catalytic” perylene-based bifunctional micelle for efficient, accurate, and rapid adsorption and catalytic degradation of low concentrations of the organic pollutant bisphenol A (BPA) (Figure 8C).¹⁴¹ Due to π–π, hydrophobic interactions and hydrogen bonding between the flexible adsorption micelles and BPA, the micelles exhibit rapid, high capacity and highly selective adsorption of BPA. Moreover, Shang et al. developed a series of novel high surface area porous polymers by molecular expansion strategy. Due to the multiple C–H…O, C–H…Cl, O–H…N, and C–H…π interactions between volatile organic compounds and polymer framework, this type of flexible adsorption materials has high application value in the field of environmental restoration (Figure 8D).¹⁴²

In summary, through the rational design and regulation of the host–guest interaction of the adsorption framework, flexible adsorption materials can effectively remove and separate pollutant molecules originating from the energy industry, which has very high research and application value. Subsequent studies should focus on exploring flexible adsorption materials' structural functionalization and adsorption mechanism.

SUMMARY AND OUTLOOK

In this paper, recent advances in flexible adsorption materials are reviewed. In adsorption processes of the liquid or gas phase, a particular dynamic adsorption behavior caused by the reversible mechanical deformation (expansion or contraction) of absorption materials driven by the host–guest interaction is called “flexible adsorption,” and the materials with this particular behavior can be called “flexible adsorption materials.” Through an in-depth discussion of flexible adsorption phenomena and the classification/application of flexible adsorption materials, this review provides new ideas for designing and applying flexible adsorption materials. Recent progress in this field has shown a rise in the number of reports on flexible adsorption materials, with advanced characterization techniques and theoretical, computational models being developed to explore the mechanisms of flexible adsorption. As a result, flexible adsorption materials are beginning to be designed and constructed in a targeted manner to suit different potential applications. Researchers have also gained a deeper and more comprehensive understanding of the flexible adsorption theory.

Although remarkable progress has been achieved, this field still has many opportunities and challenges. For example, the exploration of flexible adsorption theory has always relied on traditional adsorption models that cannot take into account the stress and deformation of the adsorption framework, which is unconvincing or even wrong. More advanced characterization techniques, such as in situ technology methods and more rational theoretical, computational models, are urgently needed to effectively explore the flexible adsorption mechanism. By accomplishing these goals, researchers can quickly and precisely modulate the material framework's flexibility and chemical structure to suit various applications. In addition, it is essential to extend and deepen the application of flexible adsorption materials. Existing studies, such as energy catalysis, still fail to link the dynamic adsorption behavior of flexible adsorption materials with catalysis theory. Due to the unique dynamic structure and adsorption mechanism of flexible adsorption materials, their applications in selective adsorption and catalysis are still worthy of further study. In summary, a brighter future based on the current research results on flexible adsorption materials can be expected.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support provided by the National Key R&D Program of China (2020YFC1818401 and 2017YFC0210906), National Natural Science Foundation of China (21938006, 21776190, and 21978185), Basic Research Project of Leading Technology in Jiangsu Province (BK20202012), Suzhou Science and Technology Bureau Project (SYG201935), and the project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).