Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

Abstract

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH₃, CO₂/CH₄ and CO₂/N₂, C₂H₂/C₂H_e and C_eH₆, C₂H₂/CO₂, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H₂ and CO₂ reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

1. Introduction

With the accelerating process of industrialization and urbanization in today’s society, environmental pollution is aggravating and pollution types are becoming more and more complex, thus threatening human health and the natural environment in which we live [1]. There are many air pollutants such as particulate matter and gases and water pollution such as organic dyes, heavy metal ions, pharmaceutical and personal care products (PPCP) [2]. These pollutants involve a wide range, harmful degrees, strong aggression, and difficulty to control, even trace pollutants can cause huge environmental problems and transmit their toxicity to humans. Therefore, environmental problems have become one of the major problems urgently to be solved by human beings.

Porous materials, such as porous organic polymers (POPs) [3], covalent organic frameworks (COFs) [4], metal-organic frameworks (MOFs) [5] and hydrogen-bonded organic frameworks (HOFs) [6], have attracted more and more attention because of their broad application prospects in the environmental remediation fields. MOFs and COFs have been extensively studied due to their high specific surface area, flexible structure, abundant porosity, and good thermal stability [7,8]. However, there are some drawbacks to MOFs and COFs. The stability of MOFs (especially in water) has always been a difficult problem to solve [9,10]. In the framework of MOFs, metal nodes have certain toxicity, which limits their application in biological and drug delivery fields. Compared with MOFs, COFs have no metal nodes, but the synthesis conditions are relatively harsh [11]. Overall, the catalysts required in the synthesis process are expensive, the reaction conditions are strict, and it is difficult to obtain large-scale crystalline products, which hinders their application and development in many fields.

Compared with traditional crystalline porous materials, HOFs have attracted extensive attention due to their unique characteristics. HOFs are pure organic framework materials that use hydrogen bonding as one of the main forces to connect the organic structure primitives with electron-acceptor bodies, and at the same time, they are formed through self-assembly in other weak interactions, such as π-π interaction, van der Waals interaction, and dipole-dipole interaction [12,13,14,15,16]. Actually, the history of hydrogen-bonded organic frameworks goes back even further. Figure 1a presents some crucial time points for the HOFs’ development. As early as 1969, Duchamp et al. reported a two-dimensional network crystal structure formed by hydrogen bonding forces of 1,3,5-benzoic acid as an organic unit [17]. In the decades that followed, however, HOFs research stalled. In the 1990s, Wuest and his colleagues discovered a large number of HOFs, pioneering the development of diaminotriazine (DAT) groups as hydrogen bond interaction sites to construct stable hydrogen bond networks [18,19,20,21]. However, their work only focused on the crystal structure, and did not explore the pore and function of the material. Until 2011, he et al. reported the first microporous HOF material (HOF-1). Its small pore size and strong interaction with acetylene made HOF-1 can selectively separate ethylene/acetylene gas, which caused chemists to pay great attention to the development of functional materials based on HOFs [22]. Accordingly, the number of research documents related to HOFs has drastically increased every year, as shown in Figure 1b.

Because of the inherent properties of hydrogen bonds (flexibility, multi-directivity, reversibility), this means that HOFs have many advantages that differ from MOFs and COFs. First of all, compared with MOFs, HOFs are easier to synthesize and characterize and can be easily regenerated and recycled by self-healing under certain synthetic conditions [23]. In addition, HOFs have low density and low toxicity due to the absence of metal ions. Secondly, compared with COFs, the weak interaction force makes it easier for single crystals to grow, so as to obtain more detailed structural information. Most importantly, we can take advantage of the easy regulation and designability of framework structure and pore size, and design the frame structure with target structure and specific function by controlling the assembly conditions [15,24]. Furthermore, as HOFs are constructed from organic components, they generally exhibit higher chemical and thermal stability. HOFs have great potential applications in gas adsorption and separation [25,26], fluorescence recognition [27], proton conduction [28], catalysis [29], and drug transport [30] due to their large specific surface area, long-range ordered pore structure and modifiability of pore. Furthermore, HOFs have attracted vast interest in recent years because of their potential in the environmental remediation field. The major application fields of HOFs in environmental remediation are shown in Figure 2a [31,32,33,34,35,36].

Metallized HOFs and HOF-derived materials can expand the application and performance of HOFs in various fields [37,38,39]. Based on the redox properties of metal ions and their excellent performance in light, electricity, magnetism, and catalysis, introducing metal centers into porous frameworks has become a new method to build HOFs [40]. Metallized HOFs not only strengthen the stability of building units, but also increase the adjustability of hydrogen-bonded supramolecular frameworks, which can greatly enrich the types of HOFs [41,42]. In addition, the functional diversity of metal complexes undoubtedly provides unlimited possibilities for the creation of new multifunctional HOFs [43]. With the deepening of the research on HOFs, people begin to use HOFs to build composite materials so as to give them more abundant properties and applications. At the molecular level, by mixing HOFs with one or more additional materials and imbuing them with improved properties either physically or chemically, composites can be produced that offer the advantages of multiple materials at the same time [44,45]. In addition, HOFs-derived pyrolysis materials with high porosity can be obtained by carbonizing HOFs with dopant atoms at high temperature, which has great application prospects in energy and environment fields [37].

But in fact, developing HOFs faces many challenges. One of the reasons is that compared with the coordination bond and covalent bond in MOFs and COFs structures, the hydrogen bond force in HOFs is a flexible and weaker interaction force, which makes the pore collapse easily when HOFs remove the free solvent molecules in the pore [46]. Moreover, the weak hydrogen bonding force of HOFs cannot meet the energy requirement of the stable frame, so it is difficult to establish permanent pores. The rigidity and strength of the skeleton structure and the number of hydrogen bonding sites play a crucial role in the stability of HOFs materials. In addition, hydrogen bond interactions between structural units and guest molecules are also very important. Strong interactions between structural units and weak interactions with guest molecules are needed so that HOFs remain firm after the removal of guest molecules [47]. Figure 2b summarizes HOFs synthesis approaches [23]. The selection of different synthesis methods has different effects on the synthesis of HOFs and can create HOFs with different structures. With significant advances in the field, more and more researchers are preparing HOFs with inherent porosity, high chemical and thermal stability, and functional properties.

Due to the continuous development of the functions of HOFs-based materials and their increasing popularity in the field of environmental remediation, it is important to summarize relevant research. This work first outlines the design principles and supramolecular synthons for the construction of stable HOFs. Then, HOF modification (metallization) and HOF-derived materials (composites and HOF-derived pyrolysis materials) are summarized. Furthermore, the recent progress and application of HOFs in gas adsorption and separation, adsorption in aqueous solution, catalysis and sensing are introduced. Importantly, as a promising new environmental remediation material, we proposed selecting different functional groups to pre-design the structure of HOFs to bring additional functions, explaining the mechanism of the interaction between adsorbent and adsorbent pore sites through molecular simulation technology, and applying HOFs to the remediation of contaminated land by adsorption and chemical reduction. It is also meaningful to combine DFT high-throughput computing with machine learning to develop low-cost and efficient HOFs.

2. Design Rules of Stable HOFs

HOFs are mainly composed of monomers connected with each other through hydrogen bonding. Hydrogen bonding is a long-range force with strong non-covalent bonding and clear direction, which has a great influence on the formation of HOFs structure. Because the hydrogen bond that constitutes HOFs is much weaker than the coordination bond and covalent bond, the framework is easy to collapse and loses porosity when the solvent is lost. In addition, HOFs materials are generally crystal structures. Due to the reversibility of hydrogen bonds, the stability of HOFs materials formed is poor, which greatly limits the development and application of HOFs. In order to generate HOFs with high stability, the following methods can be adopted.

2.1. Stronger Intermolecular Interactions

In order to obtain stronger intermolecular forces from molecular assembly to HOFs formation, multiple hydrogen bond interactions between organic ligands are favorable for the construction of stable HOFs. HOF-2 is a robust 3D porous material in which each organic building block is connected to six adjacent organic building blocks by multiple strong hydrogen bonds in 1,2-diamino-triazine groups [48]. The frame was retained after HOF-2 was soaked in different alcohols for 24 h. TGA indicates that HOF-2 is thermally stable at 350 °C. Therefore, organic molecules with multiple functional groups would be promising candidate ligands. The other approach is to create charge-assisted hydrogen bonds (especially highly charged ionic bonds) between anions and cations. For charged hydrogen bonds, the acid-base of the ligands involved is directly related to the strength of the hydrogen bond, and the combination of a strong acid and a strong base can produce permanent porosity. Nicks reported a highly stable HOF based on strong charge-assisted hydrogen bonding between carboxylic acid groups and amide groups [49]. Monolayer hydrogen-bonded organic nanosheets (HONs) were produced by ultrasonic-assisted liquid stripping. HONs exhibit remarkable stability, maintaining their extended crystallinity and monolayer structure even after three days of suspension in 80 °C water. Notably, the use of charge-assisted hydrogen bonding results in stronger materials.

2.2. Rigid Stereoscopic Framework

HOFs are constructed using rigid organic ligands with stereoscopic scaffolds. Although bonding of hydrogen bond units may be sensitive to desolvation in most porous organic solids with weak hydrogen bonds, some supramolecular scaffolds retain permanent porosity due to insufficient intermolecular stacking due to stereoscopic effects [50,51]. In 2020, Li et al. reported PFC-11 (PFC = Porous materials from FJIRSM, CAS), a new hydrogen-bonded organic framework material obtained by hydrothermal reaction using the tricarboxylic acid ligand H₃TATB as the construction unit [52]. PFC-11 can maintain a stable state for aqueous solutions and common organic solvents in the pH range of 0–10. When solvent was exchanged with ethanol and solvent molecules were removed at 90 °C, the original skeleton did not collapse. Therefore, although most organic frameworks characterized by weak hydrogen bonds are sensitive to solvent removal, there are still some rigid skeletons with inherent pores. To sum up, a rigid stereoscopic framework plays a crucial role in the stability and permanent porosity of HOFs, which is helpful to the construction of HOFs.

2.3. Highly Interpenetrated Networks

Proper introduction of interpenetrating structure into the HOFs skeleton is also an effective means to improve the stability of HOFs. It is well known that interpenetration reduces aperture but enhances skeleton stability because interpenetrating frames have lower energy and are thermodynamically advantageous. By controlling crystallization conditions (e.g., solvent, temperature), interpenetrating frames are easily achieved (e.g., polycarboxylic acids). Hu et al. used interpenetration to construct a diamond topology network of HOF-TCBP (H₄TCBP = 3,3′,5,5′-tetrakis-(4-carboxyphenyl)-1,1′-biphenyl) with permanent three-dimensional porous structure and five-fold interpenetration structure [53]. The activated HOF-TCBP has a high specific surface area of 2066 m²·g⁻¹, which can adsorb and separate light hydrocarbons with high selectivity. More importantly, the structure of HOF-TCBP did not change after soaking in water and some common organic solvents for one day, and the activated HOF-TCBP could be stable to 395 °C in the N₂ atmosphere, with extremely high thermal and chemical stability, which is a rare combination of HOF structures. The interpenetrating frame constructed by modifying the structure of the building unit and adjusting the assembly conditions plays an irreplaceable role in the thermal and chemical stability of HOFs

2.4. Additional Intermolecular Interaction

Although HOFs are formed primarily through self-assembly through intermolecular hydrogen bond interactions, additional intermolecular forces can also stabilize the construction of HOFs. Weak interactions between molecules, such as π-π stacking, are also important driving forces for stabilizing organic porous solids. The solid redox active hydrogen-bonded organic framework CPHATN-1a is formed by using the hexaazatrinaphthalene (HATN) unit [54]. The hydrogen bonds and strong π-π interactions between organic ligands form a stable HOF with the periodic arrangement. CPHATN-1a has good solvent resistance and prevents solvent hydrolysis. Therefore, the existence of π-π packing can further enhance the stability of the framework structure and obtain excellent stability of HOFs.

2.5. Avoiding D/A Structure

Avoid the presence of donors and recipients that form terminal hydrogen bonds. HOFs generally show poor stability in highly polar organic solvents such as dimethyl sulfoxide, while highly polar solvents are generally good hydrogen bond donors/acceptors and can form strong hydrogen bonds with organic ligands, resulting in leaching and etching of organic molecules from HOFs. Therefore, weakly polar solvents such as tetrahydrofuran, dimethyl ether, and isopropyl alcohol should be used in the synthesis of HOFs [52,55]. These HOFs can remove solvent molecules from the pores at room temperature during the activation process to ensure the integrity of the frame. Therefore, avoiding the formation of D/A structures is crucial for the construction of high porosity and stable HOFs.

In conclusion, the construction of stable HOFs still requires the coordination of the above strategies, namely, the combination of directional hydrogen bond construction units and rigid organic framework of co-planar p-conjugated large aromatic molecules. This enables porous HOFs with strong intermolecular interactions, which play a critical role in achieving molecular stability and permanent porosity.

3. Supramolecular Synthons for Designing HOFs

3.1. DAT Synthons

Diaminotriazines (DAT) are commonly used synthons for the construction of porous HOFs materials. The DAT functional group contains two amino and nitrogen atoms, which can be used as hydrogen bond acceptors or donors, and between the two DAT through N—H···N hydrogen bonds can form three kinds of supramolecular synthons with different structures. The remaining two amino groups of each DAT dimer are interlinked with neighboring DAT by hydrogen bonds, thus extending into a two-dimensional or three-dimensional hydrogen bond network (Figure 3a). A series of diaminotriazine HOFs can be constructed by using this property [56,57,58]. Wuest and colleagues prepared the first diaminotriazine HOFs [19]. Later, He et al. combined DAT into the main chain to synthesize 3D micropore HOF, HOF-1 with permanent porosity [22]. The BET of HOF-1 is 359 m²·g⁻¹, and the thermal stability is up to 420 °C. It can selectively separate C₂H₂/C₂H₄ under environmental conditions. At the same time, a series of diaminotriazine HOFs with different crystal structures, pore shapes, sizes, and functions have been reported successively. Some reported synthons for the construction of diaminotriazine HOFs are shown in Figure 3b.

3.2. Carboxylic Acid Synthons

Carboxyl ligands are the simplest functional group to form hydrogen bonds. As shown in Figure 4a, there is only one type of hydrogen bonds: O—H···O [59], it not only has a strong force but also has a strong direction. The construction units are connected by carboxylic acid dimers to form a honeycomb hydrogen bond network or two-dimensional layer with two-dimensional folds. Therefore, carboxy ligands are very suitable for the construction of porous HOFs.

Figure 4b shows the structural formula of carboxylic acid synthons selected for some carboxylic acid HOFs. The carboxylic acid synthons form simple hydrogen bonds and mature mechanisms, which are the most mature synthons in HOFs synthesis. Many recently reported carboxylic acid synthonic structures contain carboxyphenyl instead of carboxylic groups, which can be attributed to three reasons: (1) the introduction of carboxyphenyl improves the solubility of structures in solvents; (2) phenylene as a spacer to produce a void; (3) metal-catalyzed cross-coupling reactions between aryl groups have been commercialized, allowing the easy synthesis of various structures. In 2021, Yu et al. selected N,N,N′,N′-tetra-(4-carboxyphenyl)-1,4-phenylenediamine (H₄TPA) as synthons to assemble microporous HOFs in alcohol solutions of different polarities and named them HOF-30 [60]. The HOF-30 has a 3D decad interpenetrating network structure, with the narrowest channel size of about 3.5 Å along the direction. The solvent-removed HOFs (HOF-30a) also retained the 3D decad interpenetrating network structure, and the channel size was about 4.2 Å along the A-axis.

3.3. Pyridine Synthons

Small organic molecules containing pyridine modules have also been used in the construction of HOFs because the N atom in pyridine has a good electron-donating ability and can form strong hydrogen bonds with the H atom. As with the carboxyl group, two pyrazole functional groups are connected by N—H···N hydrogen bonds interconnect to form stable HOFs (Figure 5a) [23]. Some reported synthons for the construction of pyridine HOFs are shown in Figure 5b. It is worth noting that in the reported pyrazole cycle-based HOFs, the pattern of hydrogen bonds between the pyrazole cyclids at the end of the trunk is not the same as that of paired hydrogen bonds between carboxyl groups. Luo et al. synthesized a HOF (HOF-8) using tris (4-pyridyl)-1,3,5-triamidobenzene (TPBTC), which has good selectivity for benzene adsorption [32]. Moreover, the activated HOF-8 still maintains its structural integrity even at 350 °C. Even in MOFs and COFs, a material with such high thermal stability is very limited.

3.4. Sulfonate-Guanidium Sheet Synthons

In the process of supramolecular self-assembly, the guanidinium cations (G = C(NH₃)³⁺) with triple axial symmetry and organic sulfonic root ions (S = R-SO₃⁻) can be connected by a hydrogen bond to form a honeycomb sulfonic acid-guanidinium (GS) hydrogen bond network. Sulfonate-guanidine is one of the most common two-component supramolecular synthons, and the relationship between G and S is not only through multiple N—H···O—S hydrogen bonds are connected to each other (Figure 6a), there is electrostatic action, and GS grid has the good folding ability, so using different organic sulfonate ions and guanidine cations self-assembly can become very rich structure [61]. The structural formulas of some sulfonic acid synthons that can form sulfonic acid HOFs are summarized in Figure 6b. In 2019, Kang et al. dissolved tetra-(4-sulfonyl)methane and guanidine hydrochloride in a mixed solvent of methanol and acetone, and then diffused slowly into the mixed solvent through methylene chloride vapor, and finally formed a sulfonic acid HOFs with ammonia adsorption properties in the solution, named Korea University Framework-1 (KUF-1) [62]. KUF-1 has a thermal stability of up to 300 °C and is completely regenerated at room temperature and vacuum.

3.5. Carboxyl-Pyridine Dimer Synthons

The carboxyl-pyridine dimer is also a common two-component supramolecular synthons, and the dimer has the double hydrogen bonds effect of O—H···N and O···H—C (Figure 7) [63,64]. Lü’s research group reported the synthesis of a hydrogen-bonded 3D supramolecular organic framework (SOF-7) using 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene and 5,5′-bis (acetylamino)diisobenzoic acid as binary building blocks [65]. The material starts with an O—H···N hydrogen bond and has a quadruple interpenetrating structure, good thermal stability at 350 °C, excellent structural durability against water and organic solvents, and permanent porosity.

Although the structural formulas of synthons are varied, there are still some problems, such as the synthesis process being relatively complex, the hydrogen bond formation type being relatively complex, and the hydrogen bond formation mechanism being immature. These problems will affect the preparation of other synthons and the synthesis of other HOFs. The comparisons of different supramolecular synthons for designing HOFs are summarized in

Table 1. Comparison of different supramolecular synthons for designing HOFs.

Supramolecular Synthons	Construction Component	Hydrogen Bonding Mode	Numbers of Hydrogen Bond Type	Chemical Stability
DAT	Single-component	N—H···N	3	Good
Carboxylic acid	Single-component	O—H···O	1	Good
Pyridine	Single-component	N—H···N	1	Good
Sulfonate-guanidium sheet	Two-component	N—H···O—S	2	Sulfonic acid is unstable
Carboxyl-pyridine dimer	Two-component	O—H···N and O···H—C	1	Good

.

4. HOFs Modification/HOF-Derived Materials

The inherent characteristics of HOFs, such as large specific surface area, long-range ordered pore structure, and modifiability of pore, provide favorable conditions for the application in environmental remediation. However, the framework of HOFs is easy to collapse and the poor solubility and dispersion in most solvents limit the catalytic activity and adsorption performance. Therefore, it is necessary to provide additional physical and chemical properties for HOFs to meet the needs of practical applications. Various HOFs modification (metallization) and HOF-derived materials (composites and HOF-derived pyrolysis materials) have received wide attention and development. The comparisons of HOFs modification are summarized in

Table 2. Comparison of HOFs modification.

Materials		Guest Species/Other Materials	Porosity	Obtaining Property
Metallized HOFs	Metal-organic cations	[M(H2O)6]n+ (n = 1, 2, 3···); [M(NH3)6]n+; [M(en)3]n+; [M(tame)2]n+; [M(lmdz)6]n+; Metal-organic cage	Microporous	Oxidation-reduction; Electronics; Optics; Magnetism
	Metal-organic anions	[M(CN)n]m−) (n = 2, 3, 4···; m = 1, 2, 3···); [M(C2O4)n]m−
	Neutral metal-organic complexes	Metal salt hydrate
	Metal-porphyrin complexes	Metal centers of porphyrins and their derivatives
HOF composites		Nanoparticles; Metal oxides; Polymers; Carbon materials	Micro/mesoporous	Catalysis; Adsorption; Photothermal
HOF-derived pyrolytic materials		Heteroatoms	Micro/mesoporous	Oxidation-reduction; Adsorption

.

4.1. Metallized HOFs

Metal ions with different valence states (neutral, positively charged, or negatively charged) have been widely used as effective catalysts [66,67,68]. Based on the redox characteristic of metal ions and their excellent performance in optical, electric, magnetic, and catalytic aspects, metal centers are introduced into porous frames as a new method to construct porous materials. The modular nature of HOFs allows for integration through various molecular building blocks and for these molecular building blocks (i.e., organic ligands) to be evenly distributed throughout the HOFs. It is worth mentioning that some metallized HOFs are achieved by using metallized monomers such as metallized porphyrin monomers [69,70].

4.1.1. Metallized HOFs Containing Metal-Organic Cations

Cationic metal complexes can be combined with anionic sulfonates or phosphates because they can replace water or ammonia molecules that coordinate with metals to form structurally stable HOF mixtures [71,72]. In addition, various amines such as “en” (ethylenediamine) and “tame” (1,1,1-tri(aminomethyl)ethane), “lmdz” (imidazole), etc., are also used in the construction of metallized HOFs containing organic cations [73,74,75,76]. Assembling metal-organic cages (MOCs) into highly ordered, functional hybrid HOFs is also a viable strategy [77]. Luo et al. reported a series of mesoporous supramolecular frameworks based on Co₈L₆(H₂O)₆(Y)₆], Y = ClO₄⁻, BF₄⁻ and PF₆⁻, H₂L = 1,3-bis[(5-methyl-1h-imidazole-4-yl) methylene aminomethyl] benzene and various anions, respectively [78]. Here, the Co₈ molecular cage serves as a template for the construction of a three-dimensional mesoporous framework, and the anion Y is formed by the hydrogen bonding mode of C−H···X (X = O, F and π). It was found that the series of Co-HOFs had a cavity of a certain size, and the active site was exposed in the cavity. Moreover, the introduction of hydrogen bonds makes the structural units of the series of Co-HOF very close to the structural units of diamond-like protein crystals. Therefore, it is worth studying to construct metal-organic cations with appropriate spatial configuration to overcome the dilemma that directly affects the mixed HOFs structure.

4.1.2. Metallized HOFs Containing Metal-Organic Anions

Organometallic anions containing N or O atoms are large hydrogen bond acceptors, the most typical of which are the metalcyanide anion ([M(CN)_n]^m−) and the oxalate anion ([M(C₂O₄)_n]^m−) [79,80]. The metal-cyanide anion can be assembled with the cationic N hydrogen bond donor diamidine to form a strong hydrogen bond network. Another typical hydrogen bond acceptor metal oxalate anion can be assembled with hydrogen bond donors to form hybrid HOFs. Most of these hybrid HOFs are compact solids with low porosity or 3D frames. In addition, rigid molecules can also coordinate with metal ions to form square planar metal-organic complexes [81]. They can combine with hydrogen bond donors to form layered or pore-closed structures with overlapping accumulation. Beach et al. reacted DL-α-methylbenzylammonium with cupric pyridine dicarboxylate ions to form [α-methylbenzylammonium]₂Cu(PDCA)₂(H₂O) (H₂PDCA = pyridine-2,4-dicarboxylic acid) through acid-base reaction [82]. Methyl benzylammonium ion is oriented from edge to surface due to adjacent π···π interactions between the layers, which results in the formation of a closed stacked bilayer framework. In order to achieve the functionalization of hybrid HOF, robust and high-dimensional hydrogen-bonding entities with sufficient extension in non-planar directions need to be selected to construct stable porous HOFs.

4.1.3. Metallized HOFs Containing Neutral Metal-Organic Complexes

Hybrid HOFs’ self-assembly of neutral metal-organic complexes generally requires organic ligands with metal coordination sites (N, O atoms) and hydrogen bonding sites (carboxyl, amino, imidazole, pyridine, etc.). For example, although HMAC (4-imidazolarboxylic acid) and BIPYMO (4,4′-bipyridine derivative) have two sites, the low-dimensional metal structures they form result in dense stacked structures [83]. The teeming molecular building blocks of the bulk have large spatial sites, forming a compact hydrogen-bonded supramolecular imidazole salt framework (HIF) with low porosity [84]. Moreover, even though the molecules have two sites, there are not many additional functional sites when hybridizing HOFs, which has great limitations in practical application.

4.1.4. Hybrid HOFs Containing Metal-Porphyrin Complexes

Metal-attached ligands can be bound by monomers that have the function of pre-binding ligands. Porphyrins are the most widely studied macrocyclic compounds. They play an important role in nature and have special coordination, spectral and redox characteristics. Meanwhile, the four N atoms in the center of the porphyrin ring have good coordination activity and can be modified by various metal ions to form different metal porphyrin complexes, which can be used to construct functional HOFs containing specific metal porphyrins [85,86]. Yin et al. produced a series of stable metallic porphyrins HOFs with large specific surface areas (named PFC-72 and PFC-73 series) by introducing transition metals into the center of the tetrohydroxy-carboxylate porphyrins ligand [29]. This series of HOFs can effectively catalyze the conversion of CO₂ to CO, and cobalt porphyrin HOF shows the best performance. This work extends their use in photocatalytic reduction.

4.2. HOF-Derived Materials

4.2.1. HOF Composites

Combining HOFs with one or more additional materials and giving improved properties in physical or chemical aspects, HOF composites with the advantages of multiple materials can be obtained. Various composites have been successfully prepared, such as nanoparticles/HOF composites, metal oxides/HOF composites, polymers/HOF composites, and carbon/HOF composites [38,87,88,89]. Nanoparticles (NPs) have been widely used in various practical applications due to their unique physical and chemical properties [90,91]. However, their high surface energy tends to cause serious aggregation, which inevitably increases their difficulties in storage, processing and further use. HOFs are excellent framework materials that can be chemically combined with NPs as guest molecules. By introducing the guest unit into HOFs, we can obtain the HOFs that have the advantages of both the subject frame and the guest unit. Song et al. successfully prepared HOF composed of discrete organic molecular cage modules with cyanogen chloride, phloroglucinol and ammonium hydroxide, and then soaked HOF in RuCl₃ solution to prepare Ru³⁺@HOF [45]. The Ru precursor was reduced by NaBH₄, and the Ru NPs with an average diameter of 0.47 nm were encapsulated in the HOF pore. In a similar manner, materials containing Ag NPs were prepared from AgNO₃ solution by HOF-101 [92]. Using HOF as Membrane filler, HOF-based Mixed Matrix Membrane (MMM) was synthesized by preparing HOF and polymer mixture solution. Lin et al. prepared HOF-PyTTA (PyTTA = 4,4′,4″,4‴ -(pyrene-1,3,6,8-tetrayl) tetraaniline)/glass fiber membrane by soaking it in HOF-PyTTA solution and drying it at 50 °C [44]. Li et al. formed a uniform dispersion by dispersing HOF-30 and PolyImide (PI) in CHCl₃, poured the solution into a glass plate, and stripped it for vacuum drying [89]. In addition, Liu et al. prepared HOF-based core-shell composites by ligand grafting step-by-step method [88]. Shi et al. synthesized Z-scheme hydrogen-bonded organic frameworks (HOFs)/g-C₃N₄ nanosheets (CNNS) heterojunction photocatalyst by in-situ electrostatic method [38]. The detailed synthesis methods of composites and exciting examples show that HOF composites can now be prepared by combining multiple synthesis strategies with multiple functional compounds to achieve better HOFs application efficiency.

4.2.2. HOF-Derived Pyrolytic Materials

In the last decade, a large number of MOFs [93] and COFs [94] have been used as precursors to prepare carbon materials for use in the field of environmental remediation. Similar to MOFs and COFs, HOFs consist of ordered skeletons with well-organized pores and uniformly distributed heteroatoms. In addition, since HOF is a pure organic framework that does not contain any metallic elements, it is easier to obtain clean carbon materials. In this context, the availability of HOF as a precursor for the preparation of porous carbon materials is worth further investigation. Among many heteroatomic doped carbon materials, nitrogen-doped carbon materials generally have high activity and good stability, which is the most widely studied [95]. Liu et al. prepared nitrogen and iodine-doped carbon material C_KI through direct pyrolysis by using an abnormally heat-stable HOF-8 constructed from nitrogen-rich ligand N,N′,N′′-tri(4-pyridinyl)-1,3,5-benzenetricarboxamide (TPBTC) as a precursor and pretreated with potassium iodide (KI) [96]. At high temperatures, the presence of iodine vapor during pyrolysis expands the pore of HOF-8, resulting in a carbon material with a larger surface area, wider pore size distribution, higher pyridine N content, and a higher degree of defect sites. The results of the electrocatalytic performance of C_KI on the oxygen reduction reaction (ORR) show that the catalytic performance of C_KI is comparable to that of commercial 20% Pt/C, and can even be compared with that of MOF and COF-derived carbon, such as Co nanoparticle-embedded carbon@Co₉S₈ double-shelled nanocages (Co-C@Co₉S₈ DSNCs) and highly graphitic nitrogen-doped porous carbon electrocatalyst (GC@COF-NC), etc. [97,98]. Therefore, this work also provides new guidance for the efficient preparation of carbon-based ORR electrocatalysts. Although HOF-derived pyrolytic materials have not yet been used in adsorption, separation, and many other environmental applications, HOF-derived pyrolytic materials, such as MOF and COF-derived pyrolytic materials, show great promise in environmental remediation.

5. Applications of HOFs in Environmental Remediation

5.1. Gas Adsorption and Separation

HOFs are a kind of crystalline porous materials with adjustable pore shape and size and modified pore surface, so they are very suitable to be used as adsorbents for gas adsorption and separation. Since most HOFs are constructed from pure organic building units, that is, HOFs usually contain only elements with small atomic weights such as C, H, N, O and S, and have a high specific surface area, HOFs as gas adsorbents can usually obtain larger mass adsorption capacity. It is possible to achieve good gas adsorption and separation performance by reasonably designing HOFs with specific pore shapes and sizes or by introducing unique adsorption sites into the HOFs framework.

Table 3. Gas adsorption and separation functionalities of HOFs and their derivatives.

HOFs	Pore Size/Å	BET Surface Areas (m2·g−1)	Functionality	Ref.
SOF-1a	7.4	474	CO2/C2H2 adsorption	[99]
Trispyrazole-1	16.5	1159	fluorocarbon and hydrocarbon adsorption	[51]
PFC-2	19.7, 10.7	1014	C2H2/C2H4 separation	[100]
HOF-14	31.2 × 24.1	2573	hydrocarbon separation	[101]
HOF-102	34.0 × 35.58	2500	mustard gas detoxification	[102]
benzotrisimidazole-CF3	3.8	131	O2/Ar, O2/N2 separatio	[103]
HOF-16a	6.7	279	C3H6/C3H8 separation	[26]
IISERP-HOF1	9.4 × 9.1	1025	CO2/N2 separation	[104]
HOF-9a	6.9 × 8.8	286	CO2/N2 separation	[105]
HOF-8d	6.8 × 4.5	-	CO2 capture	[32]
CP-PP-1a	18.1 × 30.3	114	CO2/N2 separation	[106]
HOF-5a	3.9 × 5.5	1101	CO2 capture	[36]
PFC-5	5.2 × 4.0	256	hydrocarbon separation	[107]
HOF-76a	7.0	1121	C2H6/C2H4 separation	[108]
HOF-6a	~6.4, ~7.5	130	CO2/N2 separation	[109]
HOF-7a	3.4 × 4.7, 4.2 × 6.7	124	CO2/N2 separation	[110]
HOF-11a	6.2 × 6.8	687	CO2 capture	[55]
HOF-FJU-1a	3.4 × 5.3	385	C2H6/C2H4 separation	[111]
Tcpb/HOF-BTB	18.5	1095	CO2/CH4,CO2/N2 separation	[112]
PFC-11	-	751.3	CO2, benzene and toluene capture	[52]
PFC-12	-	653.6	CO2, benzene and toluene capture	[52]
HOF-12a	8.6 × 10.8	320	CO2 capture	[113]
ZJU-HOF-1	4.6	1465	C2H6/C2H4 separation	[114]
HOF-40	4.15 × 3.85	234	Xe/Kr separation	[115]
HOF-30a	~4.2	361	propyne/propylene separation	[60]
HOF-TCBP	17.8 × 26.3	2066	light hydrocarbons separation	[53]
ZJU-HOF-10	14.07 × 16.73	1169	C2H6/C2H4 separation	[116]
HOF-4a	3.8 × 8.1	312	C2H6/C2H4 separation	[117]
UPC-HOF-6	~2.8	237	H2/N2 separation	[118]
SOF-7a	13.5 × 14.0	900	CO2/CH4 separation	[65]
CPOS-5	5.3	760	CO2 capture	[119]

concludes the adsorption and gas separation functionalities of HOFs and their derivatives.

5.1.1. CO₂/CH₄ and CO₂/N₂ Separation

The main component of natural gas is methane (CH₄). Because it is a green, clean, and efficient energy, it is one of the most promising energy sources in replacing fossil fuels [120]. However, in addition to methane, natural gas also contains other unwanted impurities, such as carbon dioxide (CO₂) and nitrogen (N₂) [121,122]. These impurities will reduce the calorific value of natural gas and corrode equipment and pipelines during operation, so it is very necessary to remove these impurities. On the other hand, the burning of fossil fuels and other human activities leads to the increasing concentration of greenhouse gases (mainly CO₂) in the atmosphere, resulting in global warming [123]. Therefore, it is of great significance to capture carbon dioxide from the flue gas after combustion. In 2022, Ding et al. proposed a binary solvent synthesis strategy for the construction of C₂-symmetric molecular-based hydrogen bond organic frameworks, namely BTBA-1, PTBA-1 and TCBP-1 (BTBA = 4,4′,4′′,4′′′-(benzene-1,2,4,5-tetrayl)tetrabenzoic acid, PTBA = 4,4′,4′′,4′′′-(pyrazine-2,3,5,6-tetrayl) tetrabenzoic acid, TCBP = 3,3′,5,5′-tetrakis-(4-carboxyphenyl)-1,1′-biphenyl) [25]. BTBA-1a and PTBA-1a give significant IAST selectivity for CO₂/N₂ (20:80) mixtures at 298 K and 100 kPa, up to 2051 and 2340 (Figure 8), which are significantly higher than previously reported HOFs and comparabled to zeolite, and activated carbon selectivity, due to matching channel and CO₂ sizes [124,125,126,127]. It also shows excellent separation potential of CO₂/CH₄ mixtures.

5.1.2. C₂H₂/C₂H₄ and C₂H₄/C₂H₆ Separation

Ethylene and acetylene are important basic raw materials in the petrochemical industry. The production level of ethylene is an important index to measure the development level of a country’s petrochemical industry. These low-carbon hydrocarbons are mainly derived from naphtha cracking, and cracking gas in addition to ethylene, acetylene and byproduct ethane. In polymerization grade ethylene, the acetylene content has a very strict limit, acetylene concentration of more than 40 ppm, which will produce serious harm to the production of polyethylene, such as catalyst poisoning [128]. In addition, acetylene may neutralize metal in the pipeline and form metal acetylene compounds, which will lead to pipeline blockage and explosion. Therefore, the separation of acetylene and ethane from ethylene is of great significance [129,130]. In 2011, He’s research group reported for the first time the separation of C₂H₂/C₂H₄ from HOFs (HOF-1) [22]. The molar ratio separation selectivity of HOF-1a for C₂H₂/C₂H₄ at 273K was 7.6, significantly higher than the previously reported mixed-metal-organic frameworks (M’MOFs) (<2.5) [131]. This may be attributed to the interaction between the relatively narrow pore size of HOF-1 and the H atom in C₂H₂ molecule and the amino functional group in the structure of HOF-1 material, resulting in the strong adsorption force of HOF-1 on C₂H₂. Therefore, in 2021, Zhang et al. prepared a new HOF named rod-packing desolvated framework (ZJU-HOF-1) [114]. The activated ZJU-HOF-1 has a specific surface area of 1465 m²·g⁻¹ and cavity size of 4.6 Å, achieving efficient and selective separation of C₂H₄/C₂H₆. At 0.5 atmosphere and 298 K, ZJU-HOF-1 showed high C₂H₆ absorption (88 cm³·g⁻¹) and C₂H₆/C₂H₄ selectivity (2.25), superior to most reported MOFs (

Table 4. Comparisons of C₂H₆/C₂H₄ mixture uptake and IAST selectivity between ZJU-HOF-1 and MOFs.

Absorbent	T (K)	P (Bar)	C2H6 UptakeMixed (mmol·g−1)	C2H4 UptakeMixed (mmol·g−1)	IAST Selectivity (C2H6/C2H4)	Ref.
ZJU-HOF-1	298	1	3.2	1.42	2.25	[114]
MFU-15	293	1	3.13	1.6	1.96	[130]
PCN-250	298	1	2.96	1.6	1.85	[132]
Ni(bcd)(ted)0.5	298	1	2.48	1.38	1.8	[133]
IRMOF-8	298	1	2.16	1.25	1.7	[134]
MAF-49	316	1	1.21	0.44	2.7	[134]
Fe2(O)2(dobdc)	298	1	2.53	0.57	4.4	[135]
MIL-142-A	298	1	2.1	1.39	1.51	[136]
PCN-245	298	1	1.8	1	1.8	[137]
Cu(Qc)2	298	1	1.65	0.48	3.45	[138]
ZIF-8	275	1	1.26	0.7	1.8	[139]
ZIF-7	298	1	1.2	0.8	1.5	[140]

). The breakthrough experiment on the wet C₂H₆/C₂H₄ mixture with a humidity of 60% confirmed that the presence of water vapor had no effect on the separation ability of ZJU-HOF-1, further making it an ideal adsorbent for C₂H₄ purification under real industrial conditions.

5.1.3. C₂H₂/CO₂ Separation

Acetylene (C₂H₂) is an important fossil fuel, but also in the industry to manufacture acetaldehyde, acetic acid, benzene, synthetic rubber, synthetic fiber, and other basic raw materials. The purification of C₂H₂ from a C₂H₂/CO₂ mixture is not only valuable but also scientifically challenging due to their very similar molecular sizes and physical properties. Wang’s research group reported a case of ECUT-HOF-30, a bicomponent for the separation of CO₂/C₂H₂ [141]. It is composed of [NH₂(CH₃)₂]⁺ ion and N,N′-bis(p-benzoyl-2-yl)naphthalimide ion by self-assembly of ionic hydrogen bonds. It belongs to the microporous HOFs and has a one-dimensional rectangular channel with a pore size of 4.0 × 4.1 Ų.The BET of ECUT-HOF-30 was calculated as 402 m²·g⁻¹. It has a C₂H₂ absorption of 53.8 cm³·g⁻¹ at 1 bar and 273 K, a C₂H₂ absorption of 43.7 cm³·g⁻¹ at 298 K, and a CO₂ absorption of 17 cm³·g⁻¹ at 273 K and 9 cm³·g⁻¹ at 298 K. It has a C₂H₂/CO₂ selectivity of up to 4.8, due to the fact that C₂H₂ can form strong hydrogen bonds with carbonyl groups on the framework, and the size matches the pore size. The complete separation of C₂H₂/CO₂ also strongly supports its superior application in the separation of C₂H₂/CO₂.

5.1.4. NH₃ Capture

Ammonia is a highly toxic gas, but it is also an irreplaceable chemical. It is mainly used as raw material for fertilizer, medicine, and hydrogen source [142]. So far, many porous material adsorbents have been used to capture NH₃ efficiently, such as MOFs, COFs, and POPs [143,144,145,146,147]. However, they usually exhibit a type I isotherm, that is, high selectivity but low adsorption capacity, and harsh regeneration conditions [148]. In order to overcome these deficiencies, materials with S-shaped adsorption types (types IV and V) have attracted great attention due to the presence of STEP pressure (P_STEP) in the isothermal curve. In 2019, kang et al. reported a case of HOFs (KUF−1) for ammonia adsorption [62]. At 298 K, its NH₃ isotherm exhibits an unprecedented type IV adsorption behavior, which is the first of its kind among porous materials. It is worth noting that at 298 K and 1 bar, the adsorption of NH₃ is 6.67 mmol·g⁻¹, about seven times that at 283 K, that is, the adsorption capacity increases with the increase of temperature, which is contrary to the adsorption trend of type I. Because it belongs to the surface adsorption at 283 K, not involved in the restructuring of the hydrogen bonds. At 298 K, hydrogen bonds participate in recombination, so the adsorption capacity is greatly increased (Figure 9). This adsorption mechanism is completely different from the open-door effect of MOFs or the mechanism of gas embedding. Of course, this particular mechanism and the characteristics of the hydrogen bonds themselves give them the characteristics of mild and lower temperature regeneration. The regeneration of KUF−1a after NH₃ adsorption is only required at room temperature and vacuum, which provides a new idea for the design of the NH₃ trapping agent.

5.1.5. Xe/Kr Separation

The efficient separation of xenon (Xe)/krypton (Kr) is very important for nuclear environmental detection and fuel processing. The structure and polarizability of Xe and Kr are very similar, and the traditional low-temperature distillation process consumes a lot of energy to separate Xe and Kr, so porous material adsorption separation becomes an ideal alternative separation technology. In 2019, Lee et al. synthesized highly microporous HOF (HOF-BTB) based on 1,3,5-tri(4-carboxyphenyl)benzene and first used it for Xe/Kr separation [112]. The adsorption capacity of HOF-BTB for Xe at 273 K and 295 K was 3.37 mmol·g⁻¹ and 2.01 mmol·g⁻¹, respectively, while the adsorption capacity for Kr was 0.672 mmol·g⁻¹ and 0.416 mmol·g⁻¹, which was much lower than Xe. Moreover, the adsorption capacity of HOF-BTB on Xe was similar to that of carbon-based porous materials, such as CC3 (2.4 mmol·g⁻¹, 295 K) and IISERP-POF8 (1.72 mmol·g⁻¹, 298 K) and MOFs without open-metal sites, such as Co₃(HCCO)₆ (2 mmol·g⁻¹), IRMOF-1 (2 mmol·g⁻¹) and CROFOUR-1-Ni (1.8 mmol·g⁻¹) [149,150,151,152,153,154]. The pore structure is known to be one of the key factors for Xe/Kr selectivity. The lower the largest cavity diameter (LCD)/pore limiting diameter (PLD) ratio (between 1 and 2), the higher the Xe selectivity, and the calculated LCD/PLD ratio of HOF-BTB (1.65) falls within this range. Due to its solvent stability, this recirculable water adsorption property of HOF-BTB plays an important role in industrial applications.

5.2. Adsorption in Aqueous Solution

The regular porous structure and large specific surface area of HOFs make them very suitable for the interception of macromolecules in solution. In recent years, HOFs adsorbents have made great progress and have been applied to the removal of pollutants from aqueous solutions. The adsorption of HOFs in metal ions and organic pollutants will be mainly introduced below.

5.2.1. Metal Ion Adsorption

Metal ions are difficult to biodegrade. Instead, they accumulate in organisms through the food chain and finally enter the human body. Metal ions can not only inactivate proteins in the body, but can also accumulate chronic diseases in the body. Therefore, it has always been a hot topic to alleviate metal ion pollution and protect water resources [155]. It is reported that HOFs exhibit superior adsorption performance and good adsorption effect on metal ions in water. This is because HOFs can promote the selective adsorption of natural groups and metal ions. Moreover, the abundant ordered pores of HOFs can provide active sites for the adsorption of metal ions and accelerate mass transfer. In 2022, Kaushik et al. reported a single-component HOF (^CSMCRIHOF-1), which is constructed with a bis(phenoxy-imine)-based organic module (PMAP, (2-((E)-(6-((E)-(2-hydroxyphenylimino)methyl)pyridin-2-yl)methylene-amino)phenol)), to extract uranium (U) efficiently [156]. The design of porous ^CSMCRIHOF-1 conjugated by pyridine and phenoximine provides a suitable binding site for U. In addition, ^CSMCRIHOF-1 has excellent stability in water, acid, alkali, salt, and high ionic strength media, and still have excellent thermal stability at 340 °C, so it is strong and durable. The synthesized HOF has a 2D porous hydrogen bond supramolecular structure, a flow channel with a pore size of 3.6~3.8 Å, and a BET surface area of 328 m²·g⁻¹. The researchers also demonstrated for the first time that porous ^CSMCRIHOF-1 can be treated with the solution into large-area crystalline self-supporting films with the adjustable thickness (TFCH). The 40 nm TFCH shows excellent adsorption performance (17.80 mg U/g TFCH), which is higher than all existing adsorbents reported on uranium extraction from seawater (UES) to date (

Table 5. Comparison of the maximum U-adsorption capacity between ^CSMCRIHOF-1 with other materials.

Materials	Adsorbent	pH	Qm (mg·g−1)	Ref.
HOFs	CSMCRIHOF-1	7	1185	[156]
MOFs	Cu-BTC	3	617	[157]
	MIL-101-His	6	345	[158]
	UIO-66-AO	5.5	106	[159]
	ZIF-67	4	397.6	[160]
	ZIF-90-OM	5	482.5	[161]
COFs	COF-PDAN-AO	4	256	[162]
	COF-TpPa-1	6	120	[163]
	DP-COF	4.5	317	[164]
	ACOF	4.5	169	[165]
	TPB-BPTA-COF-AO	7	90.6	[166]
Inorganic	CMLH	6	99	[167]
	Fe3O4@SiO2 composites	6	52.36	[168]
	PFG MSs	8	207.6	[169]
Organic	PAMAMG3-SDB	6	99.05	[170]
	CTPP	5	140	[171]
	HCP-2	8	33.4	[172]
Biology	Ca-pretreated cystoseira indica alga	4	318.15	[173]
	Bacillus cereus 12-2	5	94.14	[174]
	Biosynthesized melanin	5	49	[175]

). Moreover, it has the advantages of a U/V selectivity ratio greater than 1.2 times, and long service life of five sorption-desorption cycles. This work highly reveals the great potential of HOFs in the field of metal adsorption.

5.2.2. Adsorption of Organic Contaminants

In environmental wastewater, we mainly focus on persistent organic pollutants (POPs). POPs refer to artificial and natural organic pollutants that can persist in the natural environment for a long time and migrate through various environmental media to produce adverse effects on the ecological environment and human health [176]. POPs are generally persistent, bioenriched, long-distance migration, and ecotoxic. POPs have strong chemical stability and cannot be decomposed by organisms. Once inside the human body, pops will accumulate in organs and cause damage. This can cause significant damage to aquatic organisms, including weight loss, fecundity failure, and developmental and immune system disorders [177,178]. The high surface area and controllable pore structure of HOFs make them very suitable for the removal of various pollutants from the environment as a new solid adsorbent. Such materials can give higher adsorption capacity to pollutants by introducing high-affinity active sites. In terms of the adsorption mechanism, the adsorption process mainly depends on hydrogen bonding, electrostatic interaction, π-π interaction, and Lewis acid-base interaction [179,180].

It has been widely reported that organic dyes are adsorbed by HOF-based materials. In 2020, Yang et al. reported the formation of 1,3,6,8-tetri (p-benzoate) pyrene through O—H···O PFC-1 assembled by hydrogen bonds and π-π interactions [181]. The synthesized HOF has a 2D porous hydrogen bond supramolecular structure, with a large channel size of 18 × 23 Ų and a surface area of more than 2000 m²·g⁻¹. Clearly, the aperture of PFC-1 is large enough to accommodate rhodamine B (RhB) and methyl orange (MO) molecules. PFC-1 showed excellent removal capacity of RhB and MO, and the maximum adsorption capacity of 317 mg·g⁻¹ and 252 mg·g⁻¹ at 298.15 K, which was better than some commercially available activated carbons (58~87 mg·g⁻¹ for RhB and 79.7 mg·g⁻¹ for MO) [182,183]. In addition, the adsorption capacity of PFC-1 is comparable to that of some representative MOFs and COFs (

Table 6. Comparison of RhB and MO adsorption capacity between PFC-1 with MOFs and COFs.

Materials	Absorbent	Dye	Qm (mg·g−1)	Ref.
HOFs	PFC-1	RhB	317	[181]
	PFC-1	MO	252	[181]
MOFs	NMIL-100(Fe)	RhB	76.69	[184]
	Ni@MOF-74(Ni)	RhB	177.8	[185]
	UiO-66-NH2	MO	148.4	[186]
	ZIF-67	MO	16.3	[187]
	MIL-68(Al)/GO	MO	400	[188]
COFs	TFP-PPDA	RhB	704.3	[189]
	CX4-BD-2	RhB	40	[190]
	CuP-DMNDA-COF/Fe	RhB	378	[191]
	Benzodiimidazole-COF	MO	256	[192]
	P-TH COF	MO	25.9	[193]

). The electrostatic attraction, hydrogen bond, and π-π interaction are the main reasons for the adsorption of RhB on PFC-1, and the van der Waals force contributes greatly to the adsorption of MO. In addition, PFC-1 showed high stability and structural integrity during adsorption. In conclusion, HOFs have great potential as efficient green adsorbents for organic dyes.

In recent studies, pesticides have also been successfully adsorbed using HOFs. Lv et al. synthesized negatively charged MP-HOF using 1,3,6,8-tetra (p-benzoic acid) pyrene (H₄TBAPy) as the ligand [179]. The obtained MP HOF has richer oxygen-containing groups (carboxyl), π-π conjugated structure, large specific surface area, and high stability, which is conducive to their adsorption and desorption/ionization efficiency for paraquat (PQ) and chlormequat (CQ). In addition, the possible ionization mechanism was discussed, and the proposed method was successfully applied to the detection of PQ and CQ in tap water, river water, and soil, confirming its potential for practical application.

5.3. Catalysis

HOF-based photocatalytic and electrocatalytic materials are considered to be among the best candidates in the field of catalysis in terms of performance, preparation, and application, driving clean energy conversion (e.g., H₂ production and CO₂ reduction) and pollutant degradation applications [194]. HOFs based on hydrogen bond assembly often lack catalytic active centers, so it is necessary to use catalytic molecules as catalytic centers to complete the catalytic reaction. In the case of metallized HOFs, due to some special properties of metal ions (such as oxidation-reduction, and luminescence), metal complexes show excellent performance in catalysis, light, electricity, and other aspects [40]. In addition, π-π stacking, C···H···π interaction interactions, and van der Waals forces can further enhance the stability of the skeleton structure. Therefore, HOFs catalysts can be coupled with other semiconductor supporting materials, such as graphite carbonitrides and metal oxides, to form heterostructural composites [38,87]. This can complement the strengths of both and overcome their weaknesses. In addition, the carbonization of HOFs by pyrolysis is also considered a potential catalyst.

Table 7. Summary of HOFs-based materials catalytic performance.

Materials	Reaction	Performance	Ref.
PFC-42	H2O Photocatalysis	H2 production rate (2265 μmol·g−1·h−1 )	[195]
TBAP-α	H2O Photocatalysis	H2 production rate (3108 mmol·g−1·h−1)	[196]
PFC-72-Co	CO2 Photocatalysis	CO production rate (14.7 μmol·g−1·h−1 )	[29]
PFC-73-Ni	CO2 Photocatalysis	CO production rate (9.8 μmol·g−1·h−1 )	[29]
PFC-73-Cu	CO2 Photocatalysis	CO production rate (4.4 μmol·g−1·h−1 )	[29]
PFC-58	CO2 Photocatalysis	CO production rate (3.2 μmol·g−1·h−1 )	[86]
PFC-58-61	CO2 Photocatalysis	CO production rate (4.6 μmol·g−1·h−1 )	[86]
PFC-58-30	CO2 Photocatalysis	CO production rate (7.3 μmol·g−1·h−1 )	[86]
HOF-25-Re	CO2 Photocatalysis	CO production rate (1448 μmol·g−1·h−1 )	[197]
Pt@nano-HOF	H2O Photocatalysis	H2 production rate (1480 μmol·g−1·h−1 )	[31]
PFC-1/CNNS	H2O Photocatalysis	H2 production rate (4450 μmol·g−1·h−1)	[38]
PFC-45/Cu2O@CP	CO2 Photocatalysis	CO production rate (11.81 μmol·g−1·h−1 )	[87]

concludes the catalytic performance of HOFs-based materials.

5.3.1. Energy Conversion

H₂ production and CO₂ reduction catalyzed by HOF-based materials is an important environmental catalytic application of HOFs [31,197]. Recently, it has been reported that HOF-based materials can be used as photocatalytic and electrocatalytic catalysts for water decomposition production of H₂ and CO₂ reduction to CO. In the design of HOFs, the use of π-electron-rich elements such as porphyrins can lead to semiconductor materials with excellent performance in photon absorption and electron transport. With the urgent demand for global energy and climate change, the conversion of CO₂ into high-value-added products and the reduction of CO have been a wide concern. In a study, porphyrin materials containing different metals (named PFC-72-Co and PFC-73-Ni/Cu/Zn) were constructed by simply introducing transition metals into the porphyrin structure, showing good photocatalytic activity for CO₂ reduction [29]. Remarkably, although the introduction of porphyrin core metal did not change the topological structure of the material, it caused significant changes in the non-covalent interaction between the adjacent porphyrin rings in the middle layer of the structure, orbital overlap area/area, and molecular structure. Thus, the thermal and chemical stability of the material is significantly improved (the structural integrity of metalloporphyrin HOFs can be maintained after heating at 27 °C or soaking in concentrated hydrochloric acid/boiling water for 1 day). In addition to robustness, the metallization of porphyrin centers gives the framework for different catalytic activities for CO₂ photoreduction. PFC-72-Co showed the best performance (14.7 μmol·g⁻¹·h⁻¹), 1.5 times higher than PFC-73-Ni (9.8 μmol·g⁻¹·h⁻¹), and 3 times higher than PFC-73-Cu (4.4 μmol·g⁻¹·h⁻¹) (Figure 10). These works enrich the library of stable functional HOFs and expand their applications in photocatalytic CO₂ reduction.

Hydrogen, as a sustainable renewable energy source, is widely regarded as the best alternative to fossil fuels. In recent years, more and more attention has been paid to hydrogen production from water cracking using HOFs as catalysts. In 2020, Aitchison’s research group applied the HOF material TBAP-α to photocatalytic hydrogen production [196]. The pore size of TBAP-α is 1.8 × 2.3 nm and has good stability. TBAP-α has excellent visible light response due to the presence of pyrene units. When the amount of catalyst TBAP-α was 1 mg·mL⁻¹, Pt and 0.1 mol·mL⁻¹ ascorbic acid were used as co-catalyst and sacrificant reagent, respectively. Under visible light irradiation for 5 h, the hydrogen generation rate reached 3108 μmol·g⁻¹·h⁻¹, which was 20 times that of amorphous 1,3,6,8-tetrapyrene (4′-carboxyphenyl) (TBAP) under the same conditions. Using monochromatic light from LED sources, TBAP-α has an external quantum efficiency (EQE) of about 4.1 % at 420 nm, this is higher than many conjugated polymer catalysts such as 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene co-bipyridine polymer PCP4e (EQE_{350 nm} = 0.34%) and olefin-bridged porous polymer OB-POP-3 (EQE_{420 nm} = 2.0%) [198,199]. The crystallinity of TBAP-α decreases slowly with the progress of the reaction, and the hydrogen production rate also decreases, indicating that the photocatalytic performance of HOFs is closely related to its structure and crystallinity.

5.3.2. Degradation of Pollutants

The increasing pollutant wastewater has brought serious pollution to our environment, which is an urgent problem. Using photocatalysis to completely degrade organic pollutants into non-toxic molecules is considered to be an environmentally friendly and effective environmental protection strategy. HOFs, as emerging advanced crystalline porous materials, have recently been used to degrade these pollutants [38,200,201]. Shi et al. synthesized a novel Z-type PFC-1/CNNS heterojunction photocatalyst by in-situ electrostatic method to study the photocatalytic effect of methyl orange [38]. This Z-scheme heterojunction reduces the recombination probability of photogenerated holes and photogenerated electrons and accumulates more charge carriers to produce highly active substances (•O₂⁻, •OH, and h⁺). In addition, the strong surface hydrophilicity of PFC-1/CNNS can weaken the interaction with water molecules and methyl orange (MO), and significantly improve the visible light catalytic performance. PFC-1/CNNS degraded 80% of MO after 30 min of light exposure, and completely degraded MO after 60 min. This study provides new implications for the photocatalytic degradation of pollutants by heterojunction materials.

The photocatalytic activity of 9,10-diphenylanthracene (DPA) degradation by meso-tetrakis(carboxylphenyl)porphyrin (TCPP)-based HOFs was studied [200]. TCPP-2 (DMF), TCPP-4 (DMF), and TCPP-6 (DMF) were obtained by blocking the “reverse strategy” of the strong hydrogen bond building unit on the main chain of TCPP to control the synthesis of HOF based on TCPP. The results showed that TCPP-6 (DMF) degraded 99% of DPA after 100 min of light exposure, while TCPP-2 (DMF) and TCPP-4 (DMF) required 120 and 150 min, respectively. More importantly, TCPP-6 (DMF) has excellent reusability in photocatalysis. These results indicate that the more DMF molecules bind, the greater the van der Waals forces around TCPP molecules, and the higher the photostability and photocatalytic activity of TCPP-based HOF. Therefore, a positive correlation was established between the number of DMF molecules and the photocatalytic activity of porphyrinyl HOF, which would significantly promote the development of porphyrinyl photocatalytic materials.

5.4. Sensor Applications

In the presence of specific analytes, the luminescence properties of some luminous HOFs change (enhanced, quenched, or shifted), and the interaction between the material and the target analyte is analyzed by the luminescence changes. Effective sensing or detection of contaminants is essential before removal, even if the operation does not directly remove contaminants. HOFs are ideal sensing candidate materials because of their permanent porosity and high guest molecular load, which can improve the sensitivity of detection. HOFs are currently used to detect a variety of contaminants, including metal ions, anilines, salts, antibiotics, explosive vapors, and several other organic compounds [202,203,204,205,206].

Timely monitoring of trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and other sensitive explosive vapors such as trinitrophenol (PA), trinitroresorcinol (TNR) is a top priority in heavy industry and scientific research institutions. The internal voids of the developed three-dimensional topological aggregates provide shelter for explosive vapors and exhibit extraordinary transmission with the help of coordinated complexation and electrostatic attraction. Therefore, as long as electron-rich aggregation-induced emission (AIE) hydrogen bond microporous skeleton pores adsorb electron-deficient nitroaromatics, the charge transfer between each other will quench the fluorescence AIE-HOF emission. The reaction of three tetraphenylethylene-based fluorescent single crystals TPE-AA-1, 2, 3 (TPE = tetraphenylethylene) based on tetraphenylvinyl to nitroaromatics was studied [206]. For TPE-AA-3, the quenching efficiency reaches 90.9%. This is because TPE-AA-3 has a greater BET than the other two, and tends to adhere to nitro aromatic groups, so it shows obvious fluorescence quenching.

Most sensing processes for environmentally relevant species take place in water, where HOF sensors are a challenging feature. Tang and his colleagues synthesized POSS-T₈A (POSS = polyhedral oligomeric silsesquioxane), by a condensation reaction based on an AIE precursor and a siloxane derivative [33]. It was found that it showed high sensitivity to Cu²⁺ ions and fluorescence quenching selectivity. The selectivity is attributed to the ability of the amide bond in HOF to bind to the Cu²⁺ ion, which causes the conformational change of the luminogen tetraphenylethene (TPE) unit in POSS-T₈A, leading to fluorescence quenching. The fluorescence amplification of HOF-20 for aniline in water was studied [207]. The detection limit was 2.24 μM, and the influence of other aromatic interference on its detection performance was negligible. Aniline molecules in HOF-20 will form multiple intermolecular interactions with the skeleton and may restrict the torsional rotation of aromatic rings in the connectome. This limitation can reduce the non-radiative attenuation path after photoexcitation, resulting in the fluorescence attenuation of HOF-20. Zhu et al. prepared Eu³⁺ functionalized HOF(Eu@HOF-GS-10) by ion exchange between Eu³⁺ ions and guanidine cations [205]. Eu@HOF-GS-10 shows a distinct red fluorescence, which can quickly respond to eight quinolones using machine learning (ML) to analyze and process the data.

6. Conclusions

HOFs as a new crystalline porous polymer have attracted extensive attention. Due to their remarkable properties of metal-free, high stability, high surface area, easily adjustable pore size, and easy regeneration and recovery, HOFs show great potential in the field of environmental remediation, including gas adsorption and separation (NH₃, CO₂/CH₄, and CO₂/N₂, C₂H₂/C₂H₄ and C₂H₄/C₂H₆, C₂H₂/CO₂, Xe/Kr, etc.), heavy metals and radioactive metals adsorption, organic dyes and pesticides adsorption, energy conversion (production of H₂ and CO₂ reduction to CO), degradation of organic dyes, and contaminants sensing (metal ions, anilines, antibiotics, explosive vapors, etc.). However, due to their slow development compared with other porous materials and complex chemistry, the application of HOFs in environmental remediation is still in its infancy, and there are still many problems to be solved.

• Functionalization brings additional functionality to the HOFs’ holes. Due to the periodic network chemical structure of HOFs, different functional groups, such as aromatic and aliphatic groups, esters, carboxylic acids, and alcohols, can be selected to pre-design the structure of HOFs, so as to achieve the purpose of precise regulation of its structure and properties. Various functional groups allow HOFs to be applied in specific areas, such as increasing the interaction of HOFs with pollutants to remove them from gases or solutions;
• With the development of computer technology, molecular simulation technology has been widely used in the mechanism study of some materials, which can not only clarify the interaction between adsorbent and adsorption sites on the inner surface of adsorbent channels qualitatively, but also obtain quantitative results. Compared with traditional experimental methods, molecular simulation has the advantages of convenience and speed, and can obtain microscopic information which is difficult to obtain in conventional experiments. Therefore, molecular simulation technology can be hoped to screen whether HOFs have adsorption properties for specific pollutants, and explain the adsorption mechanism at the molecular level;
• HOFs need to be extended beyond traditional water and air pollution to cover new frontiers, such as the remediation of contaminated land (soil). Remediation of contaminated soil is an urgent environmental problem. If not dealt with effectively, it will seriously destroy the whole ecosystem and endanger human health through the biological chain. HOFs can be used to separate pollutants from the soil by adsorption, or reduce pollutants to dissoluble states by chemical reducing agents, thus reducing the migration and bioavailability of pollutants in the soil environment;
• Through DFT high-throughput computing and machine learning, we hoped to build predictive single objective/multi-objective nature-oriented HOFs intelligent platforms to develop low-cost and efficient HOFs to remove pollutants from the environment and facilitate their large-scale application.

With the rapid development of HOFs, the application of HOFs in environmental remediation has shown great prospects. Although there are still some challenges, we believe that more interesting and attractive multifunctional material platforms can be built in the near future to realize their full potential.