ReviewStrategies for conversion between metal–organic frameworks and gels
Introduction
Metal-organic framework (MOF) materials have been popular for over 20 years, during which the MOF family has grown rapidly shown in the continuous increase in kinds and numbers. The outstanding features of this class of crystalline materials that arouse extensive interest include their high surface areas, exceptional porosity and topological and compositional tunability [1], [2]. Numerous advances have been reported in applying MOFs to different fields such as catalysis [3], [4], sensing [5], [6], drug delivery [7], [8], adsorption and separation [9], [10]. Still, efforts to further improve the properties and device performances of MOFs have never stopped. At present, research on MOFs focuses more on efficient synthetic methods of MOFs as well as design and modification of new materials based on MOFs [11], [12], [13], [14]. The former is about the construction of various morphological structures of MOFs, such as the synthetic methods of two-dimensional films and three-dimensional microporous or mesoporous materials [11], [12]. The latter is based on the ideology that MOFs are excellent precursor materials with superior structural diversity and exceptional properties which their derivatives may inherit, and diverse approaches have been developed to obtain various derivatives of MOFs, such as doping method, embedding, gelation, etc. [13], [14].
So far, the investigations on disordered states, including the design of crystal defects and amorphous phase are increasing. This is a new and unstoppable trend which brings focus to the materials with these features, especially for gels [15]. According to IUPAC, a gel refers to colloidal network or polymer network which is formed through covalent or supramolecular bonds of colloidal particles or polymer monomers to make the spatial network structure develop continuously and gradually lose the fluidity [15], [16]. Gels are usually soft with a finite and rather small yield stress. Recently, an excellent review has analyzed the structure–property relationships of polymer networks including gels and pointed out that crystalline framework materials such as MOFs and COFs can have some associations with gels [16]. In fact, metal–organic gels (MOGs) which consist of crystalline nanoparticles that aggregate via non-covalent interactions, mainly van der Waals forces, have been connected to MOFs in many reports. MOGs have been demonstrated to exhibit applicable and outstanding performances in adsorption [17], drug delivery [18] and catalyst [19], as well as sensing including detection of nitro aromatic compounds [20], ammonia [21], and antibiotics [22]. Gels show superior mechanical and chemical stability and they have advantages over MOFs in multiple complex application scenarios. For instance, easier and gentler preparation than MOFs makes MOGs more attractive for synthesizing porous carbon materials (PCMs) [23].
MOFs and MOGs are both multicomponent systems with metal ions and organic ligands, which makes the conversion from MOFs to MOGs and vice versa possible. In fact, to date MOGs are mainly synthesized from MOF nanoparticles instead of crystalline MOF monoliths. MOF nanoparticles are precursors formed during the very initial stage of MOF crystallization, and in this paper we still consider the synthesis of MOGs from MOF nanoparticles as a kind of conversion from MOFs to MOGs. In addition to MOGs, the formation of organic gels can also be realized through crosslinking of MOF monoliths and removal of metal ions. Moreover, conversion of gels into MOFs has also attracted much attention and contributed a lot to the search for more efficient and facile synthesis of MOFs. Since MOFs exhibit crystalline and rigid structures while gels are flexible and amorphous materials, their very different structures and properties can guide the conversions between them to meet various commands on the functions of practical devices.
On one hand, research on conversion of MOFs into gels, including both organic polymer gels and metal–organic gels, has achieved some substantial progress. For synthesis of organic gels, it is usually realized through crosslinking of the organic ligands in MOFs and then hydrolysis for removal of metal ions. As for metal–organic gels, it is essential to have a deep understanding of the crystallization and aggregation of MOF nanoparticles [24]. In general, MOF crystallization involves two processes, namely nucleation and crystal growth, whereas MOGs will come into being when nanoparticle aggregation outcompetes crystal growth. Thereby, modulating the competition between crystal growth and nanoparticle aggregation is the major principle for formation of MOGs. Concluded from the available literature, mismatch growth and optimization of synthetic parameters are general strategies for conversion of MOFs to MOGs.
On the other hand, MOGs can be designed with the same components in nature as MOFs, and they can be turned into MOFs by appropriate inducing factors [25]. Progress in conversion of MOGs into MOFs has been made in the past years. Gel degradation strategy utilizes the interaction of anions or oxidants with MOGs to disrupt the gel systems, making them crystallize into MOFs. And dry-gel conversion, which involves the formation of intermediate gel phase, has become a significant method for fabrication of MOFs. Furthermore, growth of MOF films, MOF membranes and polyMOFs provides another possibility for conversion of gels into MOFs, which is promising for integration of MOFs with practical devices.
While some recent reviews have focused on the gels and their connections with MOFs [15], [16], none of them has been oriented at the concrete strategies for the conversion between MOFs and gels. Herein, strategies for conversion between MOFs and gels, together with detailed synthetic process and corresponding mechanisms of conversion, are concluded, as shown in Scheme 1. In the meantime, some advantages and potential applications of each strategy are also introduced. It is believed that with further in-depth investigations into conversion between MOFs and gels, more and more functional materials with exceptional properties will be fabricated and greatly meet the commands of speeding-up development of modern industry.
Section snippets
The conversion from MOFs to gels
As the number of MOFs climbs up, many suitable candidates among MOFs have been designed to turn into organic gels or MOGs to carry on better properties in catalysis and sorption [26], [27]. There are several advantages to turn MOFs into gel materials. For example, turning MOFs into MOGs can increase the chemical homogeneity of this metal–organic multicomponent system. And in general, the resulting MOGs usually exhibit higher porosity than the corresponding MOFs. In addition, owing to the high
The conversion from gels to MOFs
Synthesis of MOFs from gels have attracted much attention since the flexible structures of gels can help to finely control the morphologies and porosities of MOFs. However, conversion of gels into MOFs is quite challenging as the crystalline and rigid frameworks of MOFs are somehow in conflict with the flexible and amorphous characteristics of gels. And the formation of MOFs is limited by some specific features of gels; for example, hydrophilic gels are not suitable for the conversion to MOFs
Conclusions
As we demonstrate in this review, novel mutual transformation strategies between MOFs and gels have emerged in recent years. For conversion of MOFs into gels, the obtained gels usually combine both the porosity of MOF crystals and the flexible and amorphous features of gels. Crystal crosslinking method is one of the strategies for conversion of crystalline MOFs into gels. In general, this strategy refers to gelation reaction between MOF crystals and crosslinking agents and subsequent hydrolysis
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51603052) and the Fundamental Research Funds for the Central Universities (Grant No. 18lgpy02).
Zeyu Zhuang was born in 2000, Guangdong Province, China. He is studying at School of Materials Science and Engineering, Sun Yat-Sen University in Guangzhou, China. He is now doing his research under the supervision of Dr. Prof. Dingxin Liu.
References (115)
- et al.
J. Solid State Chem.
(2019) - et al.
Energy Storage Materials
(2019) - et al.
Chem. Sci.
(2020) - et al.
Chem. Mater.
(2009) - et al.
Tetrahedron Lett.
(2016) - et al.
Coord. Chem. Rev.
(2013) - et al.
Cem. Concr. Res.
(2018) - et al.
J. Mech. Phys. Solids
(2012) - et al.
Colloids Surf., A
(2011) - et al.
Carbon
(2000)
Int. J. Biol. Macromol.
J. Mol. Catal. A: Chem.
Chem. Sci.
Thin Solid Films
Mater. Lett.
Microporous Mesoporous Mater.
Chem. Rev.
Angew. Chem. Int. Ed.
Mater. Interfaces
Mater. Interfaces
Adv. Mater.
Dalton Trans.
J. Am. Chem. Soc.
Adv. Mater.
J. Mater. Chem. A
RSC Adv.
Chem. Commun.
Appl. Organometal. Chem.
Angew. Chem. Int. Ed. Engl.
Nat. Mater.
J. Am. Chem. Soc.
Chem. Commun.
Eur. J. Inorg. Chem.
Chem. Commun.
Adv. Mater.
Mater. Interfaces
Chem. Soc. Rev.
Chem. Sci.
Adv. Mater.
Sci. Rep.
Adv. Mater.
Eur. J. Inorg. Chem.
Angew. Chem. Int. Ed.
Bull. Korean Chem. Soc.
Chem. Commun.
Angew. Chem. Int. Ed.
Polym. J.
Bull. Chem. Soc. Jpn.
Nat. Commun.
Sci. Rep.
Cited by (33)
ZIF-8 Gel/PIM-1 mixed matrix membranes for enhanced H<inf>2</inf>/CH<inf>4</inf> separations
2024, Chemical Engineering JournalImproving the proton conductivity of MOF materials by regulating the pore space
2024, Microporous and Mesoporous MaterialsRational design of recyclable metal-organic frameworks-based materials for water purification: An opportunity for practical application
2023, Science of the Total EnvironmentRoom temperature synthesis of monolithic MIL-100(Fe) in aqueous solution for energy-efficient removal and recovery of aromatic volatile organic compounds
2023, Journal of Hazardous MaterialsCitation Excerpt :MOGs are constructed by mismatched growth of MOF nanoparticles, which are aggregated into an interconnected three-dimensional network (Bennett et al., 2017). Synthesis temperature and time, reactant concentration and solvent, drying temperature and modulator will affect the gelation process of MOGs as well as their mechanical stability and shapeability (Horcajada et al., 2009; Zhuang et al., 2020). However, such synthesis procedures must satisfy three requirements for practical application: (1) thermal and water satability, (2) low cost and easy scaling up, (3) green synthesis route.
Hierarchical-pore UiO-66-NH<inf>2</inf> xerogel with turned mesopore size for highly efficient organic pollutants removal
2022, Journal of Colloid and Interface ScienceCitation Excerpt :Therefore, it is highly desired to develop a simple and feasible routine to prepare H-MOFs for persistent organic pollutant removal. In recent years, the metal–organic gel is gradually developing into a new routine for H-MOFs preparation [30–34]. The metal–organic gel is formed by aggregating the tiny MOF particles into three-dimensional frameworks through π-π stacking, hydrogen bonding interactions, metal–ligand coordination interactions, and van der Waals interactions [33], and then removing the solvent in air or under supercritical CO2 will form monolithic or granular MOF material.
FeSe and Fe<inf>3</inf>Se<inf>4</inf> encapsulated in mesoporous carbon for flexible solid-state supercapacitor
2022, Chemical Engineering JournalCitation Excerpt :Herein, we propose a novel strategy to synthesize mesoporous carbon-encapsulated iron selenides by employing metal–organic framework gel (MOG) as a template. Specifically, MOG is an aggregation of MOFs driven by metal–ligand interactions, possessing both the excellent porous features of MOFs and the uniform dispersion properties of gels [35,36]. The strategy was realized based on a fast-formed MOG of MIL-100-Fe (illustrated in Fig. 1), whose gelation process can be controlled into a few seconds by changing the concentration of reactants.
Zeyu Zhuang was born in 2000, Guangdong Province, China. He is studying at School of Materials Science and Engineering, Sun Yat-Sen University in Guangzhou, China. He is now doing his research under the supervision of Dr. Prof. Dingxin Liu.
Zehan Mai was born in 1998, Guangdong Province, China. He is studying at School of Materials Science and Engineering, Sun Yat-Sen University in Guangzhou, China. He is now doing his research under the supervision of Dr. Prof. Dingxin Liu.
Tianyi Wang was born in 1998, Jiangxi Province, China. He is studying at School of Materials Science and Engineering, Sun Yat-Sen University in Guangzhou, China. He is now doing his research under the supervision of Dr. Prof. Dingxin Liu.
Prof. Dr. Dingxin Liu received his PhD from Shanghai Jiao Tong University. After graduation he work as an assistant Professor in National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing, China. In 2019 he began his career as an associate Professor at School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou, China. His current research focuses on Metal-Organic Frameworks (MOFs), Porous Coordination Polymers (PCPs), Morph-Genetic MOFs, Biomimetic functional composite, Nanomaterials, Materials for Energy and Environment, Photoelectric Functional Materials and so on.
- 1
These authors contributed equally to this work.