1 Introduction
Porous materials are materials with pores of different sizes and shapes. Currently, porous materials are widely used in adsorption, catalysis, separation, purification, energy storage and other fields
[1][2,3]. Typical Porous Materials include Metal-Organic Frameworks (MOFs), Porous Coordination Polymers (PCPs), and Organic Porous Materials
[4][5]. In Organic Porous materials, there are mainly Hyper-Cross-Linked Polymers (HCPs), inherently Microporous Polymers (Polymers of Intrinsic Microporosity, PIMs), Conjugated microporous Polymers (Conjugated Microporous Polymers, CMPs), Covalent Triazine Frameworks (Covalent Triazine Framework s, CTFs), Porous Aromatic Frameworks (Porous Aromatic Framework s, PAFs) and Covalent Organic Frameworks (Covalent Organic Frameworks, COFs)
[6⇓~8][9,10][11,12][13,14][15,16][17,18][19,20].
HCPs are the earliest developed porous organic materials, which can be synthesized by post-crosslinking of polymers, one-step polycondensation, or with the assistance of external crosslinking agents
[21]. The micropores of PIMs come from their own rigidity and molecular space rotation structure. By changing the precursor, insoluble PIMs network and soluble linear PIMs can be prepared
[9,10]. CMPs are a class of amorphous materials with π-conjugated structural units and three-dimensional network structure, which can be regarded as a class of HCPs
[12,22]. CTFs are formed by trimerization of rigid nitriles. The introduction of triazine ring into CMPs can increase the specific surface area of the conjugated microporous network and expand the conjugation range, which is also a hot spot in the research of CMPs
[13][23]. PAFs are a class of open framework amorphous polymers with excellent surface area (PAF-1,S
BET=5600 m
2·g
-1) and good stability, similar to CMPs. However, the conjugated structure of PAFs is destroyed by the central atom of the tetrahedral monomer
[15][5]. COFs can be seen as covalent analogs of MOFs, built up by reversible covalent bonds to form ordered, crystalline, porous skeletons. This material has developed rapidly, and conjugated COFs have been developed. On the other hand, CMPs are amorphous analogs of COFs
[24][22].
In 2007, Jiang et al. First reported the conjugated microporous polymers of Poly (aryleneethynylene) with specific surface area up to 834 m
2·g
-1[11]. Due to their unique microporous structure and potential applications, this kind of microporous materials have attracted extensive attention and research. Compared with common organic porous materials, microporous conjugated polymers have the following characteristics: (1) large specific surface area, usually close to 1000 m
2·g
-1;(2), excellent chemical stability, insoluble in acid, alkali and organic solvents
[25⇓⇓⇓~29]; (3) good thermal stability, and the decomposition temperature is more than 300 deg C; (4) because the trifurcation center molecule is required to be used as a Core and the linear group molecule is required to be used as a Linker in the process of preparing the microporous conjugated polymer,Therefore, the structure and pore size of conjugated microporous polymers can be adjusted and modularly designed by using core molecules and linker molecules with different molecular sizes, which makes it possible to design and controllably adjust the pore size and structure of microporous polymers. This review focuses on the latest research progress of CMPs, including structural design, synthesis methods and applications.
2 Design of CMPs
2.1 Topological Structure Design
Topological design of conjugated microporous polymers is a powerful tool to guide the actual synthesis. At present, there are mainly two models for the topological design of CMPs, namely the mainstream Continuous Random Network Model (CRN) and the developing Ideal Amorphous Solid Model (IAS)
[30][31].
2.1.1 Continuous random network model
The topology of conjugated microporous polymers is similar to the arrangement of atoms in glass
[32]. As shown in Figure 1, the atoms in glass are arranged in a three-dimensional extended network similar to a crystal, but the results of X-ray diffraction show that the network in glass lacks periodicity and symmetry, so the network in glass is random or disordered. Similarly, the CRN model is a network model similar to the arrangement of atoms in glass.
Ben et al. First proposed a PAF-1 model with a diamond topology in 2009, as shown in Figure 2a
[15]. The model is based on the diamond structure, and the N
2 adsorption isotherm simulated by the model (BET surface area is 5640 m
2·g
-1) is almost the same as the N
2 adsorption isotherm of PAF-1 in the experiment (BET surface area is 5600 m
2·g
-1), so the model has important reference value. The following year, Cooper et al. Developed an amorphous model of PAF-1 (fig. 2C) based on the CRN model of amorphous silicon (fig. 2b) by replacing silicon atoms with tetrahedral carbon nodes and oxygen atoms with biphenyl struts
[33]. They believe that the absence of sharp peaks in the diffraction pattern of PAF-1 indicates that the structure is amorphous, and that the theoretical density and elemental composition calculated from the amorphous model of PAF-1 are comparable to those obtained from the model with diamond topology.
2.1.2 Ideal amorphous solid model
The IAS model, proposed by Stachurski et al., is a more rigorous model of ideal amorphous solids, which is characterized by irregular packing and the absence of short-range, medium-range and long-range order. However, this model has not yet been applied to the topological design of amorphous porous organic materials
[31]. The IAS model originates from a class of atomic clusters consisting of a central atom surrounded by other nearby atoms. Specifically, the central atom is a sphere containing equidistant surface points, and a number of continuous points for coordinating the sphere are randomly distributed around the central atom, and the distributed points should be in the angular range (φ) of π/3 ≤ φ ≤ π. The central atom and the coordination sphere together form an atomic cluster. A concentric spherical shell is attached to an atomic cluster. By repeating this process, a 3-D structure containing randomly filled spheres can be obtained. If the structure is finite, it is a circular unit. If the structure is infinite, it is an ideal amorphous solid. However, precise structural models for amorphous porous organic materials are not yet available because of the inability to directly compare experimental and simulation data.
2.2 Structure Design
At present, there is no unified standard for the theoretical model of conjugated microporous polymers. However, some progress has been made in the structural design of a specific class of CMPs. Thomas et al explored the structure-thermodynamic-property relationship of the polymer network by theoretical calculation and experiment, taking the cyanovinyl conjugated microporous polymer as an example
[34]. The construction of the network model in this study is based on the extended network material of cyanovinyl, and the structure is automatically generated. The method is implemented in Ambuild code integrated with HOOMD-blue and DL _ POLY molecular dynamics simulation code
[35,36]. For each polymer, each structural unit and solvent molecule are stoichiometrically measured, an initial simulation unit is set, and after specific terminal atoms and cap atoms are set, the network structure of the polymer is constructed. In this study, the adsorption enthalpy of carbon dioxide in the amorphous polymer network was successfully adjusted by controlling the weight percentage of cyanoethylene content, and the controllable adjustment of enthalpy was realized. In addition, the theoretical calculation of the structure design in this study provides a reference for the structure design of polymers in the future.
In addition to the structural design of CMPs, the structural design of CMPs complexes is also a hot topic in theoretical design. Xu et al. reasonably synthesized a conjugated microporous polymer containing naphthyl doped with methyl lithium under the guidance of quantum chemistry second-order Møller-Plesset (MP2) calculation, and the polymer has high hydrogen binding strength
[37]. In the past, lithium atoms tend to aggregate in the process of lithiation, and it is difficult to achieve a balance between doping amount and hydrogen storage efficiency. In order to design and synthesize lithium-doped hydrogen storage materials reasonably and effectively, more in-depth studies on the location of lithium doping, the optimal doping amount and its binding energy to hydrogen are needed. According to the theoretical calculation, when the naphthyl group is built into the microporous polymer, lithium atoms can be dispersed on both sides of the plane of the naphthyl group, which effectively improves the aggregation of lithium. In addition, the theoretical calculation of molecular binding energy shows that methyl lithium is an ideal dopant for lithium atoms, which can not only effectively bind hydrogen molecules, but also facilitate the removal of tetrahydrofuran, which disperses the precursors of lithium atoms.
For the structural design of conjugated microporous polymers, the current progress is mostly focused on the network construction and design of specific CMPs, as well as the theoretical calculation of the position and doping amount of doping atoms, while the research on the overall accurate structural model of CMPs materials is less, and the structural design and analysis of such materials still need further exploration.
3 Synthesis of CMPs
3.1 Principle of synthetic reaction
Through different synthesis reactions, the core molecule and the linker molecule are covalently connected by π conjugated bonds, thus constructing a conjugated skeleton. The chemical reactions used to prepare linear conjugated polymers can also be used for the synthesis of CMPs. Common chemical reactions for the synthesis of CMPs are shown in Figure 3. Because the structural units involved in the reaction have different geometric shapes, reactive groups and conjugated systems, the skeleton and pore structure of CMPs are rich and diverse.
3.2 Synthetic method
The traditional synthesis method is to mix the reaction monomer, catalyst and solvent, and stir them for a certain time under the protection of inert atmosphere and at a certain temperature. After the reaction is finished, collecting the precipitate, washing the precipitate with a solvent, removing the catalyst and unreacted monomers, and drying the precipitate in vacuum to obtain the CMPs after solvent molecules are completely removed.
At present, the synthesis methods of CMPs have been developed into ionothermal reaction, microwave radiation reaction and electrochemical method, which can replace the traditional methods
[38,39][40⇓~42][27,43]. Among them, the ionothermal reaction can effectively synthesize triazine-linked and phenazine-linked CMPs under high temperature conditions using metal halides as molten media
[44]. As a new way of synthesis, microwave irradiation can greatly shorten the reaction time and improve the specific surface area of the product. The electrochemical method can effectively oxidize and polymerize the material into a conjugated microporous polymer based on a monomer, and in the case of dithienothiophene (DTT), DTT-CMP is formed by electrochemical deposition on an electrode through oxidative polymerization
[27].
3.2.1 Ynthetic film
CMPs are synthesized from highly cross-linked polymer templates. However, the low solubility of the templates usually hinders the manufacture of films in favor of the formation of powder-like particles. Almost all CMPs are in the form of insoluble powders, so it is difficult to prepare CMPs films by conventional methods
[28]. Only a portion of the microporous polymer can be processed into a film. For example, a microporous polymer network is prepared by oxidative polymerization, and the product is in the form of a film. The synthesis raw materials with thiophene or carbazole functional groups are polymerized on the electrode to form a dense but still microporous film
[45]. In addition, intrinsic microporous polymers (PIMs), composed of rigid, unbranched polymer chains, can be processed directly into films in solution
[46,47].
At present, some progress has been made in the preparation of CMPs membranes, including solution processing, electrochemical method, interfacial reaction and so on
[48]. Based on the principle of two-step Suzuki cross-coupling reaction, Cooper et al. Synthesized soluble CMP (SCMP1) by introducing a large number of organic units (such as tert-butyl) into the periphery of CMPs network (as shown in Figure 4)
[49]. The synthesized sulfhydryl SCMP1 has micropore characteristics, and the BET specific surface area is 505 m
2·g
-1. The important feature of SCMP1 is that it can be made into thin films by solution treatment, which is a milestone. However, this method still has many shortcomings, such as the size of polymer particles is difficult to control and the specific surface area is low
[50]. Hu Aiguo et al. Proposed a confined polymerization method to control the growth of polymer through the inner diameter of the reactor to obtain soluble CMP with controllable pore size
[51]. Soluble CMP makes it possible to be molded, but the product has a low degree of polymerization and requires subsequent film-forming processing
[50]. Jiang et al. Controllably prepared conjugated microporous polycarbazole TPBCz-CMP film by electrochemical method using triphenylbenzene substituted by tricarbazole as polymerization precursor. The thickness of the film can be controlled, and it can be used as substrate or independent film
[52]. Livingston et al. Successfully prepared independent polyamide nanofilms and defect-free, highly crosslinked polyarylate nanofilms by interfacial polymerization. Studies have shown that the nanofilms prepared from twisted monomers have higher microporosity and interconnectivity
[53,54]. Tsotsalas et al. Used metal-organic framework films as movable scaffolds to grow CMP nanofilms on them by copper-catalyzed azido-alkyl cycloaddition technology, which can also be extended to other substrates
[55,56]. Liu et al. combined the processability of plastics with the rigidity and porosity of cross-linked microporous solids. In this study, 3,6-dibromophenanthrene-9,10-diammonium chloride and 3,6-dibromophenanthrene-9,10-dione were used to prepare a precursor. After a simple heating process, a soluble conjugated microporous thermosetting material (as shown in Figure 5) was prepared. The CMT membrane showed excellent selective hydrogen separation performance
[57].
3.2.2 Ynthetic monolithic structures
Because most of the CMPs are in the form of powder, they are inconvenient in storage, application and recycling
[58]. In order to solve these problems, microporous polymers can be directly coated on macroporous sponges, or special strategies such as high internal phase emulsion polymerization can be used to synthesize microporous polymer monoliths
[59,60][61,62].
At present, a large number of studies have confirmed the feasibility of the preparation of microporous polymer monoliths, and the prepared monoliths can be reprocessed to improve their properties and obtain wider applications. For example, Wu et al. Used melamine foam as a catalyst carrier to carry out CMP gelation reaction and prepared ZnP-based CMP monolithic composites
[59]. The synthesized composite monolithic material retains a layered porous structure, which is beneficial to the diffusion of the substrate in the acyl transfer reaction. Chang et al. Synthesized 1,4-diiodobenzene and 1,3,5-triethynylbenzene in gel state into compressible microporous polymer monolith by Sonogashira-Hagihara reaction
[63,64]. Liu et al. Also reported the synthesis of free-standing CMP monoliths under controlled aerobic conditions
[65]. However, the synthesis conditions of most methods are complex, and the monomers used are special and expensive. Tan et al. Developed a strategy to prepare functionalized porous polymer monoliths by linking simple aromatic compounds such as benzene or biphenyl to rigid methylene bridges via a Friedel-Crafts reaction using a somewhat reactive formaldehyde dimethyl acetal (FDA) as an external crosslinking agent
[8,66,67]. Several aromatic building blocks, such as benzene, biphenyl, TPB, methylbenzene, chlorobenzene and phenol, can be directly woven into networks with predominantly microporosity and high surface area. The method has that advantage of flexible selectable structural units, adjustable porou structure, easy control of experimental conditions, low cost of used reagents, and abundant microporous structure and high surface area of the final product.
Huang et al. Further investigated the synthesis of microporous organic nanotube networks (MONNs) by combining the formation of molecular template precursors of core-shell bottle brush copolymers with the in situ hypercrosslinking of FDA crosslinkers (Fig. 6)
[68⇓~70]. First, a bottle brush copolymer with aromatic pendant polymer chains was synthesized by a combination of atom transfer radical polymerization, cyclization polymerization, and reversible addition-fragmentation chain transfer polymerization (RAFT), resulting in a polyglycidyl methacrylate backbone (shown in black in fig. 6), an intermediate PLA layer (blue) and a functional amino base layer (green) and an outer PS shell (red). The PS (polystyrene) shell was then knitted with FDA crosslinker to form a permanent microporous hypercrosslinked network and the PLA core was removed. Finally, a hollow nanotube network with a hierarchical trimodal micropore-mesopore-macropore structure is obtained. Furthermore, by incorporating additional polymer layers with specific functional groups, such as thiols, amino, carboxyl and sulfonic acids, a large number of multi-functional MONNs have been prepared, further expanding their applications in catalysis, adsorption and rapid separation
[71][70,72][73][72].
3.3 Effects of Variables on the Synthesis of Products in the Reaction Proc
According to the different types of reactions (as shown in Figure 3), the backbone of CMPs can be divided into phenylene linkage, phenylethynyl linkage, imine linkage and phenazine linkage. In order to obtain higher adsorption capacity of CMPs in applications such as gas adsorption, many studies have been devoted to increasing the specific surface area and pore volume of CMPs. In this section, we will discuss the effects of four variables (monomer structure and ratio, reaction solvent, reaction temperature and microwave conditions, salt control) on the morphology and specific surface area of the synthesized CMPs.
3.3.1 Structure and proportion of monomer
The framework of CMPs is composed of node units and connecting units, and as early as 2007, it has been found that the micropore size is closely related to the rigid monomer structure
[11]. Specifically, by changing the length of the connecting unit and keeping the node unit unchanged, the shorter connecting unit is beneficial to the construction of the microporous structure, while the longer connecting unit is beneficial to the generation of more mesoporous frameworks
[25].
In addition, the geometric shape of the connecting unit also has a great influence on the porous structure. When the steric hindrance of the linking unit is small, it is beneficial to the formation of the porous skeleton, while when the steric hindrance is large, the polymerization reaction can not proceed smoothly, and then the porous network can not be formed
[74].
The molar ratio of the core molecule to the linker molecule affects the degree of polymerization of the linear conjugated microporous polymer. Taking the synthesis of spirobifluorenyl CMP as an example (as shown in fig. 7), when 7,7 '-tetrabromo-9,9' -spirobifluorene is used as a node unit and para-substituted 1,4-dibromobenzene is used as a linking unit, and the ratio of the node unit to the linking unit increases from 1:0 to 1:1, 1:2, 1:3 and 1:4, the BET specific surface area of the product rapidly decreases from 1275 m
2·g
-1 to 887, 580, 457 and 156 m
2·g
-1[74]. The pore volume also decreased from 0.997 cm
3·g
-1 to 0.686, 0.415, 0.305 and 0.159 cm
3·g
-1. The pore volume of micropores has a similar trend, decreasing from 0.749 cm
3·g
-1 to 0.504, 0.335, 0.240 and 0.097 cm
3·g
-1.
The ratio of monomer to catalyst also affects the degree of polymerization of the reaction. Taking the ionothermal synthesis of triazine-based porous polymers as an example, in this reaction, the amorphous state of the product will change with the increase of the catalyst ratio, and the BET surface area and pore volume will also increase, and a series of networks with different pore volume, surface area and pore size distribution will be formed by changing the ratio of the two
[13,75]. As shown in fig. 8, the characterization result of the first synthesized porous triazine network CTF-1 shows that it is a crystal. When the amount of the catalyst ZnCl
2 is 10 equivalents, the synthesized porous triazine network is mainly micropores, and the BET surface area and pore volume are 495 m
2·g
-1 and 0.97 cm
3·g
-1, respectively; When the amount of the ZnCl
2 is 20 equivalents, the product is a mesoporous network with mesopores (8 nm) and is completely amorphous, and the BET surface area and pore volume are 1900 m
2·g
-1 and 2.25 cm
3·g
-1, respectively.
3.3.2 Reaction solvent
The influence of the reaction solvent on the CMPs is mainly reflected in two aspects: the morphology and the porous structure of the CMPs.
The results showed that the morphology of the phenylacetylene CMP was highly dependent on the reaction solvent
[76]. When the reaction solvent changed from toluene to p-xylene and mesitylene, the morphology of the products changed from spherical to tubular and flaky, while the BET specific surface area remained at about 390 m
2·g
-1. Ideally, the product should be a two-dimensional planar structure, but ultra-thin films are thermodynamically unstable, for example, graphene films often roll up or become discontinuous at a thickness of a few nanometers, so the product network is prone to roll up
[77]. Solvents such as p-xylene and mesitylene can effectively prevent the polymer from spheroidizing. When the reactant molecules of p-xylene and mesitylene polymerize and curl, they will easily form hollow tubular or plate-like structures.
The reaction solvent also plays an important role in the formation of the porous structure. For example, in the Sonogashira-Hagihara reaction of 1,3,5-triethynylbenzene with various dibromo-substituted aromatic units to prepare CMPs, DMF was found to be the most favorable solvent for the formation of microporous structure, toluene was the most unfavorable solvent for the formation of microporous structure, and THF and dioxane had similar effects, with the former tending to produce a higher proportion of micropores and the latter tending to produce a higher average specific surface area
[78].
3.3.3 Reaction Temperature and Microwave Reaction
The reaction temperature also greatly affects the porosity of the product. For example, in triazine networks prepared by trimerization of 1,4-dicyanobenzene at 400, 500, 600, and 700.d egree. C., the BET surface area increases from 920 m2·g-1 to 1600, 1750, and 2530 m2·g-1[75], respectively; The pore volumes also increased from 0.47 cm3·g-1 to 1.00, 1.58, and 2.26 cm3·g-1, respectively. Also, the reaction temperature has an important effect on the pore size distribution. The sample synthesized at 400 ℃ has a narrow pore size distribution, which is mainly concentrated in the micropore region of 1 ~ 2 nm, while the network prepared at 500 ℃ has a wide pore size distribution (1 ~ 6 nm), which includes the micropore and mesoporous regions.
Microwave reaction is a novel method for the synthesis of CMPs. Compared with the traditional heating method, microwave reaction significantly shortens the reaction time and greatly increases the porosity of the product. Still taking the triazine porous polymer shown in Figure 8 as an example, CTF-1 was synthesized by ionothermal polymerization at 400 ℃ for 40 H, with a BET specific surface area of 791 m2·g-1 and a pore volume of 0.4 cm3·g-1. Under microwave irradiation, the reaction time was shortened to 60 min, while the specific surface area and pore volume were as high as 2390 m2·g-1 and 2.52 cm3·g-1. It is worth noting that in the process of microwave reaction, the temperature can not be effectively controlled, so safety precautions need to be taken.
3.3.4 Salt Regulation
In that reaction proces, a proper amount of salt is adde to the monomer, so that the microporous structure of the product can be effectively regulated and controlled. Studies have shown that the addition of salt can effectively change the pore size distribution and narrow the pore size distribution to the micropore range, while the pores of PTPA without salt are widely distributed in the micropore, mesopore and macropore range (as shown in Figure 9)
[79]. In addition, the specific surface area of PTPA decreases as the radius of salt ions increases. This study suggests that the effect of salt on porosity is due to the adjustment and optimization of the Hansen solubility parameter of the solvent.
4 Application of CMPs
In general, CMPs have three main characteristics, namely, nanoporous, π-conjugated, and can be designed and controlled at the molecular level. Based on the above three characteristics, CMPs have been widely used in the fields of gas adsorption and storage, heterogeneous catalysis, luminescent materials, chemical sensors, electric energy storage and biological hybrids
[11,56,80,81][82⇓⇓~85][86⇓~88][89⇓~91][44,92,93][52,94]. In this section, we will review the latest research progress of CMPs in several major application areas.
4.1 Gas adsorption and storage
In the field of gas adsorption and storage, CMPs are mainly used in the storage of H
2, CO
2, CH
4 and other gases
[95⇓~97]. Carbon dioxide is a greenhouse gas that contributes to global warming, sea level rise and ocean acidity, so it needs to be effectively absorbed by CO
2. The storage of hydrogen and methane is mainly for use as clean fuels. At present, the improvement of gas adsorption and storage capacity of CMPs is mainly achieved through the following two aspects: one is to control the synthesis method to improve the specific surface area and porosity of CMPs materials, so that the pore diameter of materials can be well matched with the dynamic diameter of gas; The other is to improve the pore environment by introducing functional units into the surface of the pore wall of the material, so as to effectively improve the adsorption capacity and selectivity for specific gases. For low pressure gas adsorption of CMPs, the material must have high adsorption heat and large binding sites. For high-pressure gas storage of CMPs, the material needs to have a high surface area. However, if the binding sites of the material increase, its specific surface area will inevitably decrease. Therefore, the trade-off and combination between the excellent specific surface area and large binding sites of CMPs materials remains a major challenge.
In the aspect of hydrogen adsorption, CMPs have shown some excellent performance
[98]. Due to its high calorific value, high energy density, and environmentally friendly gas, hydrogen has become a new energy source and has attracted wide attention. However, these remarkable results obtained at low temperatures are still insufficient for practical applications. In order to solve this bottleneck, it should be considered to increase the enthalpy of hydrogen adsorption in molecular design. The hydrogen storage capacity of some CMPs is summarized in Table 1.
Among the various technologies for capturing CO2, CMPs adsorption is a feasible and efficient method. Due to the large specific surface area, high porosity, molecular design and pore surface modification, CMPs can further enhance the interaction with the CO2 and improve the adsorption capacity. The CO2 storage capacity of some CMPs is summarized in Table 2.
As the main component of natural gas, methane is an abundant energy source. Compared with other gases, there are few studies on the CH
4 adsorption of CMP. Since methane can be stored in carbonaceous materials at room temperature, the adsorption capacity of organic porous materials for methane is promising. The CH
4 absorption capacity of COP-1 is 29 mg·g
-1 at 298 K and 18 bar.After loading lithium ions on COP-1, the absorption of Li @ COP-1 increases significantly to 39 mg·g
-1, but the absorption capacity is still very low compared with MOF and COF
[112]. At present, the highest adsorption capacities of COF (COF-102) and MOF (MOF-210) for CH
4 are 187 mg·g
-1 and 220 mg·g
-1, respectively
[113][114]. The methane storage capacity of some CMPs is summarized in Table 3.
4.2 Gas separation
Similar to gas adsorption, CMPs are also gradually applied to gas separation. At present, large-scale gas separation includes olefin/paraffin separation, steam/steam separation, hydrogen recovery, air separation, natural gas purification, CO
2 separation and so on. The main methods of commercial gas separation are cryogenic distillation, pressure swing adsorption, temperature swing adsorption and chemisorption
[115]. Taking CO
2 separation as an example, amine absorption technology is the most widely used, but amine materials are more toxic
[116]. In order to solve this problem, the application of various porous materials in the field of gas separation has been widely studied. Although MOF has excellent gas storage performance, most of them can not be used in practice because of their poor chemical stability
[117]. CMPs not only have abundant micropores, which can be used to modify the material and pore surface, but also have excellent chemical stability and thermal stability, so the separation of different gases can be achieved by adjusting the performance of CMPs. There are two major bottlenecks in the wide application of CMPs for gas separation, one is that most of the synthesis process of CMPs needs to add expensive transition metals as catalysts, and the other is that most of the CMPs are insoluble powders, so in the process of gas separation, a large number of CMPs need to be added to ensure the smooth separation. At present, the main research trend is soluble CMPs, and the preparation of CMPs membrane for gas separation. Compared with other mainstream processes, membrane-based gas separation technology does not need heat-driven phase change, and can greatly reduce energy consumption.
When CMPs membranes are used for gas separation, two parameters, gas permeability and selectivity, are mainly used as performance evaluation indexes. In addition, the introduction of other groups into CMPs can also effectively improve their gas adsorption capacity. Zhong et al. Synthesized a triazine-based material with both ultramicropores and high nitrogen content, which used the deep overlap of potential caused by small pore size to produce a larger interaction, as well as the Lewis acid-base electrostatic interaction between nitrogen atoms and carbon dioxide, so that the material had both high CO
2 capture capacity and selectivity
[118]. At present, the main difficulties in the field of CMPs membrane-based gas separation are that most CMPs are insoluble powders and can not be processed into films; CMPs that can be made into thin films are not suitable for commercial production because of their high synthesis and film-making costs. Therefore, it is necessary to explore the CMPs membrane-based materials with high gas permeability and good gas selectivity.
4.3 Adsorb heavy metals, dyes, solvents and other chemicals
In addition to gases, a large number of micropores in CMPs can also effectively adsorb heavy metals, dyes, organic solvents, fullerenes and toxic chemicals.
Heavy metal ions released into the environment pose a serious threat to human health because they can accumulate in the human body and ecosystem and are usually not biodegradable
[119]. Based on the theory of soft-hard acids and bases, thiol groups have strong affinity for some metal ions. In some studies, thiol groups were introduced into CMP to enhance its adsorption capacity for metal ions. Huang et al. Developed a two-dimensional sulfurized COF with imine linkage, which forms a Hg-S coordination bond and can efficiently capture Hg (II)
[120]. In the study of Sun et al., COF-V was synthesized by imine condensation, and then modified by introducing 1,2-ethanedithiol in the N
2 environment
[121]. The product was COF-S-SH, and the removal rate of Hg (Ⅱ), Pb (Ⅱ) and Cu (Ⅱ) was more than 99% in 10 min. The maximum adsorption capacity of COF-S-SH for Hg (Ⅱ) is as high as 1350 mg·g
-1. X-ray absorption fine structure spectroscopic analysis shows that each Hg is bound by only two S through intramolecular synergy. In addition, according to the latest research progress, Chen et al. Used CMPs with different structures to have different pore sizes to achieve controllable adjustment of material adsorption capacity
[122]. The adsorption mechanisms of heavy metal ions by CMP mainly include: coordination interaction, acid-base interaction, electrostatic interaction, chemical bonding, diffusion and Van der Waals force.
Organic dyes have toxicity, teratogenicity and mutagenicity, so the treatment of dye-containing wastewater is particularly important for the ecological environment and human health. Organic dyes are generally divided into anionic dyes and cationic dyes. The screen effect plays a major role in the application of COF for dye removal. For example, Ning et al. synthesized a COF based on salicylidene-aniline, named SA-COF, which can quickly adsorb and remove some organic dyes based on size-dependent separation and charge-selective separation
[123]. CMPs can have different properties by introducing units with different properties in the synthesis process. For example, the introduction of hydrophobic units can make CMPs have hydrophobic pores, while hydrophilic pores can enhance the hydrophilicity of porous materials. Cooper et al synthesized 1,3,5-triethynylbenzene and dibromobenzene with different substituents, and tested these CMPs with water-soluble methyl orange dye. The results showed that only CMPs with hydroxyl groups in the skeleton could absorb methyl orange, while CMPs without hydrophilic groups could not capture methyl orange
[124]. In addition, Deng et al showed that HCMP can effectively adsorb organic solvents and oils due to its superhydrophobicity
[125].
Effective removal of toxic chemical waste is a major challenge. Porous materials offer a cutting-edge green approach to the removal of chemical waste, especially toxic chemicals, and are superior to traditional methods in many ways. Take the chemical amine as an example, amine is an important chemical, but it can cause serious pollution to the environment and directly threaten the health of organisms. Jiang et al. Synthesized a series of metal-porphyrin-based CMPs for the adsorption of amines. The study found that the adsorption capacity of the material mainly depends on the type of metal. The metal ion centered on the porphyrin ring affects the electron density and conjugated skeleton of the porphyrin ring, thereby affecting the absorption of amines by the material
[126]. Therefore, controlling the electron density of CMPs is a good research direction. Liu et al. Designed and synthesized thiophene-based POF with high specific surface area and stability by using tetraphenylmethane as a node, and introduced sulfone groups into the material by post-oxidation of m-chloroperbenzoic acid
[127]. The obtained POF-SO has sulfone functional groups, and the pore size reduction is negligible.The introduction of sulfone groups makes the adsorption capacity of the material for amine increase rapidly from 3.5 mmol·g
-1 to 5.8 mmol·g
-1. However, on the whole, there is still much room for improvement in the removal efficiency of toxic chemical wastes by materials.
4.4 Heterogeneous catalysis
There are three kinds of CMPs in the field of catalysis: one is to construct heterogeneous catalysts by embedding metal-ligand catalysts into the material skeleton through the "bottom-up" strategy
[128]; Condly, the material is used as a carrier, and metal nanoparticles are loaded by a post-modification mode to construct a heterogeneous catalyst; Third, through the "bottom-up" strategy, metal-free organic small molecule catalysts are embedded into the material skeleton to construct catalysts. Because CMPs can integrate various functional units into its framework, catalytic sites can be inserted into the framework of CMPs to form a nanoreactor with built-in catalytic sites and micropores. At present, metalloporphyrin units, iridium sites, platinum sites, palladium sites, titanium ions and pyridine units are used as active sites to complex with CMPs. Zhao et al. Developed a one-pot synthesis of a series of Pd/CMPs catalysts, which mainly involved Pd-catalyzed Suzuki coupling reaction and in-situ pyrolysis of Pd precursors
[129]. Due to the spatial confinement effect of micropores in CMP, the size of Pd sites can be effectively controlled to be less than 2 nm. And Pd/CMP show excellent catalytic activity for that reduction of p-NPh. The results showed that the high catalytic activity of Pd/PTT was attributed not only to the reduction of Pd size, but also to the retention of pores after Pd loading. As another example, FeP-CMP has metalloporphyrin units serving as junctions in a tetragonal framework
[130]. The material is composed of dense catalytic sites and has a specific surface area of up to 1270 m
2·g
-1. Both the substrate and the reactant can enter the micropores to form a heterogeneous catalytic system. FeP-CMP has high catalytic activity for the oxidation of sulfides, and the conversion rate is as high as 97%. In addition, the material has high activity, can be used for the oxidation of various substrates, such as alkyl, aromatic and cyclic sulfides, and has high chemical selectivity in the oxidation process. FeP-CMP also showed high efficiency (99%) and high activity (TON > 107) in the epoxidation of olefins. Like most conjugated microporous polymers, FeP-CMP is insoluble and can be separated from the reaction mixture by direct filtration. The FeP-CMP can also be cycled repeatedly with little change in catalytic activity. Cooper et al. developed CMP with iridium sites that catalyze reductive amination reactions. The catalytic activity of CMP-CpIr-3 was high, and the reaction yield was up to 90%
[131].
Wang et al. Successfully applied the free radical thiol-alkyne coupling strategy to the post-modification of conjugated microporous polymers with triple bonds, which provided a universal and simple post-modification method for the efficient design of photocatalysts based on CMP
[132]. A large number of amino groups are incorporated into the backbone of the polymer without compromising the optical band gap and morphology. Under visible light irradiation, the photocatalytic activity of the modified CMPs-SC
2NH
2 for the decomposition of H
2 was higher than that of the original CMPs, and the stability factor and AQY of the BBT-SC
2NH
2 at 420 nm reached 272μmol·h
-1 and 3. 3%, which were 27. 2 and 47. 1 times of that of BBT, respectively. It can be speculated that the introduction of amino groups not only makes the CMPs-SC
2NH
2 more hydrophilic, but also accelerates the separation of photogenerated charges and prolongs the electron lifetime, thus significantly improving the efficiency of subsequent proton reduction. In addition, this study shows that the nitrogen in the introduced NH
2 group is important for improving the photocatalytic activity, and the aliphatic chain length between sulfur and nitrogen is also important for the performance.
Most photo-redox catalysts are homogeneous catalysts and utilize rare transition metals
[133]. CMPs have many advantages as heterogeneous photo-redox catalysts, the most significant of which is to reduce the use of expensive metals and simplify the recovery and reuse of catalysts. In the CMP material system, the conjugated structure makes it have excellent photoelectric properties, the high specific surface area provides an interface for the separation of electrostatic charges, and the abundant pore structure allows the transport of ions
[134]. Meier et al. studied the effect of the spacer length of the covalent triazine skeleton (CTF) on photocatalytic hydrogen production from water, synthesized materials with phenylene to tetraphenylene spacer units by trifluoromethanesulfonic acid-catalyzed Suzuki-Miyaura polycondensation, respectively, and in the presence of a hole scavenger,The photocatalytic properties of all materials were systematically studied under visible light, and the results showed that both synthesis routes produced CTFs with similar hydrogen release rates but different optical properties. In the mixture of water and triethanolamine with platinum promoter, the highest hydrogen release rate in the cyclotrimer series was found for CTF-2, with an apparent quantum yield of 1.6% at 420 nm
[135]. According to (TD-) DFT calculations, CTF-1 can be expected to have the highest performance, and this difference can be explained by the trade-off between increased light absorption and reduced thermodynamic driving force. Zhang et al. Developed a series of carbazole-based conjugated microporous polymers with adjustable redox potential, and studied their photocatalytic properties in C-3 formylation and thiocyanation of indole.The results showed that the photo-redox potential of the series of materials could be adjusted by changing the nature of the core structure (phenyl or triazine unit).The photocatalytic efficiency was the highest at room temperature, and the yield of CMP-CSU6 @ 70 was as high as 90%.The series of photocatalysts were easy to recycle, and the photocatalytic activity was still high after being reused for five times
[136]. Wang et al. Synthesized the photocatalyst CMPN using terminal alkynes and 1,3,5-triethynylbenzene, which can directly decompose water into H
2 and O
2 under visible light
[137].
In general, the conjugated and microporous characteristics of CMPs have led to their rapid development in the field of catalysis. In addition, the modular nature of CMPs makes it easier to introduce chiral and metal cocatalysts. However, conjugated COFs, which are similar to them, have more constraints in design because they are crystalline materials, and the design of multi-component catalysts is more complex and difficult.
4.5 Luminescent material
By introducing specific groups into the three-dimensional skeleton of CMPs, the π-conjugation can be further expanded, and the CMPs can become a class of efficient luminescent materials. Cooper et al. Have successfully adjusted the emission wavelength of the material by changing the composition of the pyrenyl CMPs, and the YPy powder synthesized with 1,3,6,8-tetrabromopyrene as the monomer emits red fluorescence centered at 620 nm
[86]. While when the hydrazone unit is linked to a phenylene or biphenylene unit, the resulting CMPs emit light at shorter wavelengths. Jiang et al. developed a highly luminescent CMP using tetraphenylethylene (TPE) as a precursor
[87]. Because the CMP is highly crosslinked, the TPE-CMP can effectively inhibit the rotation of the TPE unit, and the interwoven CMP structure can further promote pi conjugation, promote excitation migration, and improve the luminescent activity, so that the material can emit light in both liquid phase and solid phase. In addition, due to the cross-linked nature of the network, TPE-CMP emits strongly in various solvents such as methanol, dioxane, THF, dichloromethane, chloroform, hexane, DMF, benzene and water. Bildirir et al. Showed that DTT-CMP formed by electrochemical oxidation polymerization was also an interesting luminescent material, and spectroelectrochemical measurements were carried out on indium tin oxide electrodes.It was found that DTT-CMPs had a blue shift in the process of forming free radical cations, and the absorbance decreased at 450 nm, and new absorption bands appeared at 855 nm and 1100 nm. When the voltage was further increased, the blue shift occurred again
[27]. According to spectroelectrochemical measurements, a significant color change was observed during the redox process, from orange-red (0 V) to light green (1.2 V) and finally light blue (1.4 V). The color change is completely reversible in the redox cycle, and the transmittance measurement at 1074 nm shows that the switching time is 0. 84 s when the potential changes from 0 to 1. 4 V. In recent years, core-shell strategy, as a new luminescence strategy, can achieve adjustable and controllable luminescence color while ensuring high luminous efficiency, and has also attracted wide attention.
At present, researchers have developed many fluorescent adsorbents for the detection and adsorption of various gases, organic pollutants and heavy metal ions
[138][139][140,141]. Uranium, for example, is an important element in the nuclear industry, but when released into the environment, it poses a potential health risk to humans. Xu et al. designed and synthesized a fluorescent conjugated microporous polymer (CMPAO), as shown in Figure 10. The amidoxime ligand on CMPAO enables it to selectively bind to uranium, while the hydrophilic carboxylic acid bond greatly enhances the dispersibility of the material in water and can fully contact with uranyl ions
[142]. The best adsorption capacity of CMPAO can reach 251.9 mg·U·g
-1. In addition, the adsorption of uranium on the material results in significant fluorescence quenching, allowing sensitive and selective detection of uranyl ions by fluorescence chromatography. Moreover, due to the signal amplification of the conjugated backbone, the detection limit of CMPAO for uranyl ion in deionized water is low, which is 1.7×10
-9M, far below the maximum pollution standard for drinking water (6.3×10
-8M) of the World Health Organization.
4.6 Chemical Sensor
Conjugated microporous polymers can be used as a class of effective chemical sensors to detect various chemical substances due to their π-conjugated and luminescent properties. The main principle of its application as a chemical sensor is as follows: specific sites that can interact with target compounds are introduced into the conjugated chain, thus enhancing the interaction interface and effectively improving the signal transduction activity. CMPs have a large specific surface area, which can provide a broad interface for the interaction of compounds. Cao et al. developed two CMP (COP-3, COP-4) consisting of 1,3,5-tris (4-bromophenyl) benzene and 2,4,6-tris (4-bromophenyl) benzene, in which the [1,3,5] triazine block acts as a fluorescent chemosensor
[143]. The material can effectively detect para-nitroaromatic explosives such as picric acid (PA) and 2,4,6-trinitrotoluene (TNT) with a detection limit of less than 1 ppm. Liu et al synthesized a new type of hollow tubular organic conjugated microporous polymers (HTCMPs), which can directly detect peroxynitrite anions (ONOO
-),ONOO
- is the main reactive nitrogen (RNS) in human cells, and the abnormal expression of RNS is related to many diseases
[144]. Their results showed that the sensor made from HTCMPs had a wide linear detection range (4.62 nM ~ 95.1 μM), a low detection limit (1.54 ± 0.5 nM) and a high sensitivity (196.9μA·μM
-1·cm
-2). More importantly, this study is the first to successfully detect and quantify a mixture of H
2O
2 and ONOO
- separately at a selected detection potential. The method is also used for electrochemically detecting two important molecules of reactive oxygen and reactive nitrogen in glioma cells. This work opens up a new way to effectively assess the effects of oxidative stress in biological systems and is expected to be used in clinical diagnosis.
4.7 Electric Energy Storage
Due to their high specific surface area, excellent physical and chemical stability, and conjugated structure extending along the molecular chain, CMPs have great application prospects in lithium ion battery electrode materials. Jiang Jiaxing et al. Synthesized a pyrene-based conjugated microporous polymer PyDB with high specific surface area by Suzuki coupling reaction using tetrabromopyrene and p-phenylenediboronic acid as building units.At the current density of 50 mA·g
-1, the discharge capacity reaches 163 mA·h·g
-1, and even at the current density of 3000 mA·g
-1, it still has 62 mA·h·g
-1 reversible capacity.It still has a capacity of 167 mA·h·g
-1 after 300 cycles at a current density of 100 mA·g
-1[50]. When the polymer is used as a negative electrode material, the discharge capacity reaches 495 mA·h·g
-1 at a current density of 50 mA·g
-1, and the polymer still has the capacity of 245 mA·h·g
-1 after being cycled for 300 times at a current density of 200 mA·g
-1. The excellent electrochemical performance of PyDB is mainly attributed to its extended conjugated structure and porous structure with high specific surface area.The large conjugated structure is conducive to the doping reaction and electron conduction of molecular chains, and the porous structure with high specific surface area is conducive to providing a large number of active sites and promoting the migration of ions.
CMPs are also used to improve the cycle performance of lithium-sulfur batteries. Lithium-sulfur batteries have low capacity and short life due to the low conductivity and the serious shuttle effect of soluble polysulfides
[145]. Xu et al. introduced a π-conjugate with the function of "electron reservoir" into the triazine-based skeleton as a separator in lithium-sulfur batteries, which promoted the charge transfer between the triazine unit and lithium polysulfide, thus improving the immobilization and electrocatalytic conversion ability of polysulfide
[146].
Supercapacitor is a kind of energy storage and power supply device. The main mechanisms of charge storage are electric double layer capacitor (EDLC) and pseudocapacitance (PC). EDLC operates on the electric double layer through the accumulation of charge at the electrode/electrolyte interface, and its stored energy is proportional to the capacitance of the electrode. PC utilizes the rapid redox reaction of electroactive species. Therefore, electrode materials are the key to the development of supercapacitors. Activated carbon is one of the most studied electrode materials, and its capacitance has been reported to be as high as 270 F·g
-1[147]. The specific capacitance of Aza-CMP is four times that of activated carbon (<270 F·g
-1)
[44]. Aza-CMP is superior to nitrogen-rich carbon (50~330 F·g
-1) and to the latest nanostructured carbon, with a capacitance even greater than the maximum of a supercapacitor with a redox-active ruthenium oxide electrode (720 F·g
-1). The maximum energy and power density of Aza-CMP are 53 Wh·kg
-1 and 2.25 kW·kg
-1, respectively. The energy density is higher than that of nanostructured porous carbon and reaches battery States such as lead-acid, nickel-cadmium, and lithium-ion (10~150 Wh·kg
-1) batteries. The powder density of Aza-CMP is an order of magnitude higher than that of the battery (<0.3 kW·kg
-1). In recent years, conjugated microporous polymers have also been used in potassium ion batteries. The electronic structure of CMP can effectively improve the storage capacity of potassium. The optimized PyBT has a reversible capacity of up to 428 mA·h·g
-1 and can be stably cycled for 500 times
[148]. Feng et al. synthesized lamellar conjugated microporous polymers based on two-dimensional graphene (as shown in Figure 11), and introduced thiophene, thiazole and pyridine into the materials to generate GMC-S, GMC-NS and GMC-N. The series of materials have good energy storage performance, and the specific weight capacitance is increased by 14% -35% compared with similar materials without graphene template
[149]. In recent years, Liao et al. Synthesized a three-dimensional polyaminoanthraquinone network (PAQ) by B-H coupling
[150]. The material has good dispersibility in a polar solvent and can be processed into a flexible electrode, and the specific capacitance of a double-electrode and a three-electrode formed by the material is 168 and 576 F·g
-1 respectively. The coulombic efficiency is more than 95%. These results demonstrate the great potential of CMP in energy storage applications.
However, the shortcoming of CMPs in the field of energy storage is that the volume conductivity of such materials is usually poor, so it often requires the use of more additives, such as carbon. This also results in that the storage capacity of the device as a whole is not ideal even if CMPs with high charge storage energy are used. In contrast, sp2 carbon-based COFs have made great progress in [151~153]. In the future, highly porous, functionalized CMPs with higher electrical conductivity need to be developed.
4.8 Biological Hybrid
Currently, CMPs are also used to fuse with biological organisms to produce functional biological hybrids
[154⇓~156]. For example, solar-powered functional biological hybrids for carbon dioxide fixation
[157,158]. In addition, it can also be used to manufacture new antibacterial agents. Wang et al. Introduced hydantoin groups into the CMPs network in a covalent manner, and prepared the first CMP-based antibacterial agent with a maximum specific surface area of 1411 m
2·g
-1, which could completely inhibit the growth of bacteria within 120 min, and a dispersion concentration of 100μg·ml
-1, showing great potential in the practical application of water cleaning
[159]. This study shows that a series of antimicrobial materials based on CMPs can be designed and manufactured by introducing various antimicrobial substituents into CMPs. In addition, the global pandemic of the new coronavirus in 2020 has further promoted the application of various materials in the field of biological hybridization.
5 Conclusion and prospect
As a unique class of polymers, CMPs effectively combine π-conjugation with porous structure, and have become a hot spot in the field of material chemistry.
From the point of view of structural design, monomers are closely related to the structure, characteristics and functions of materials. Although CMPs are disordered amorphous materials, the flexibility of molecular design and structural variability make CMPs still have the characteristics of "pre-design", and the development of CMPs with customized functions is a major development trend. The model development and application of amorphous materials is still a major difficulty. For amorphous porous organic materials such as CMPs, the structural design and analysis still need further research, and the relevant calculation and simulation software should start from the mechanism of material synthesis and application to explore the potential mechanism. Taking PAF-1 as an example, the CRN model can explain part of the structure of the material, but more research is needed to fully understand the material structure. IAS model can be regarded as a typical example to analyze the topology of CMPs, but it still needs to be explored and enriched. At present, a series of CMP networks have been constructed, and the controllable adjustment of carbon dioxide adsorption enthalpy has been successfully realized through theoretical calculation.The theoretical design ideas of this study have certain reference value for the structure design of such materials in the future.
From the point of view of chemical synthesis, a large number of coupling reactions and a variety of synthesis conditions have created a rich variety of CMPs. Although some progress has been made in the study of CMPs film formation, it is still a challenge to make CMPs into films because a large number of CMPs are insoluble powders and it is still difficult to prepare films by conventional methods. At the same time, great progress has been made in making microporous polymers into monolithic materials, which provides more ideas for the synthesis of CMPs monolithic materials. Although the synthesis cost of CMPs has been significantly reduced compared with other materials, most CMPs still need to add precious metals as catalysts in the synthesis process, and the production cost is still high, which is not suitable for commercial promotion. Therefore, reducing the cost of material synthesis is also a major research focus. In addition, most CMPs need to use carcinogens such as tetrahydrofuran in the synthesis process, which poses a greater threat to the health of researchers, does not conform to the concept of green synthesis, and limits their wider application, so it is also a major trend to use safe reagents to synthesize high-performance materials.
From an application perspective, many applications of CMPs are based on conjugation. According to the literature, CMPs are the earliest conjugated materials, and now, conjugated COFs also belong to this kind of materials
[13,160]. Earlier, it was suggested that the high specific surface area of CMPs offered the potential to fill a second conjugated polymer
[26]. Now, the formation of donor-acceptor heterojunction materials for photocatalysis still embodies this theory
[161,162]. Crystalline conjugated COFs, as opposed to amorphous conjugated microporous polymers, have their own advantages and disadvantages in the field of functional materials. For crystalline conjugated COFs, most of them have a moderate or higher level of long-range order. Cooper et al. Studied the photochemical hydrogen uptake activity of crystalline COFs and their amorphous or semi-crystalline analogues, and showed that the hydrogen uptake activity of crystalline COFs was much higher
[163]. Beyond this specific example, studies comparing the effects of crystalline and amorphous systems on function are still too few to be conclusive. Crystalline systems have advantages over amorphous systems in that atoms can be precisely positioned at specific locations, disadvantages in that properties such as the electronic gap are expensive to calculate, subtle changes in crystal packing can cause changes in electronic properties, and the sequential synthesis of specific structures remains challenging. In contrast, amorphous CMPs can explore the optimal functional products by adjusting the size of the linking units, while conjugated COFs can not be achieved, because crystalline solids need to use node units and linking units of the same size to ensure the isotope-network structure.
All in all, CMPs are an interesting class of materials with great potential for application. Past research on design, synthesis and function exploration has shown that CMPs are a universal platform for structural and functional design, which will continue to drive further development in the field of materials. CMPs are expected to have a bright future and will continue to play a strong role as a platform for addressing challenging environmental and energy issues.
National Natural Science Foundation of China(51978569)
PDF
