Sylwia G ł owniak
a, Barbara Szcze z sniak
a, Jerzy Choma
a, Mietek Jaroniec
b,⇑aInstitute of Chemistry, Military University of Technology, 00-908 Warsaw, Poland
bDepartment of Chemistry and Biochemistry & Advanced Materials and Liquid Crystal Institute, Kent State University. Kent 44-242, OH, USA
Metal–organic frameworks (MOFs) have attracted a special attention due to outstanding porosity, adjustable pore sizes, and huge opportunities in varying organic–inorganic compositions. Enormous studies conducted so far on MOFs indicate their high potential in catalysis, gas adsorption, drug delivery, water treatment, energy storage, among others. However, mass production of MOFs is still limited mainly due to the non-economic, non-green and complex synthesis methods. Mechanochem- istry is an alternative solution for ef fi cient and environmentally friendly syntheses of various MOFs.
Fast and solvent-free or solvent-less mechanosynthesis seems to be a very powerful versatile method for obtaining these advanced porous materials. The mechanochemical concept was used for the preparation of various MOFs including the most popular structures: MOF-5, ZIF-8, HKUST-1, MIL- 101, UiO-66. These MOFs feature high speci fi c surface areas, comparable to those prepared by conventional solvent-based methods. Furthermore, mechanochemistry was successfully used for the synthesis of non-conventional multimetallic MOFs and previously unreported solid phases. This review shows the recent developments, challenges and perspectives of green synthesis of diverse MOF structures using mechanochemistry. Besides describing the mechanochemical synthesis of MOFs, some achievements in green applications are also summarized. Importantly, current trends in research suggests for further development of these fields i.e., harmful gas adsorption, water treatment, and energy storage.
Introduction
Recent research activities in the field of nanomaterials have been directed towards improving their synthesis and perfor- mance in various applications. Mechanochemistry has been shown as a promising strategy for synthesizing the variety of advanced nanomaterials such as: alloys, nanoparticles, organometallic frameworks, bioconjugates, polymers, covalent organic frameworks, zeolites, carbonaceous materials, supramolecular complexes, Perovskites, energy materials, and different composites. Recently, we reviewed mechanochemical syntheses of various highly porous materials including activated carbons, graphene- and C3N4-based materials, metal oxides as
well as metal–organic frameworks (MOFs)[1]. However, the pre- vious work provides only a brief overview of MOFs and is focused on those with large specific surface areas. Readers are also referred to a few reviews devoted to the synthesis of MOFs via mechanochemical methods [2–4]. Herein, we elaborate on the mechanochemically-prepared various MOFs with special emphasis on green synthesis aspects and recent developments in thisfield. In addition, we present the foremost advancements in applications of mechanochemically prepared MOFs with spe- cial emphasis of green applications such as adsorption of harm- ful gases and volatile organic compounds as well as removal of pollutants from contaminated water. Furthermore, we compare the solvent-based synthesis methods with mechanosynthesis including properties of MOFs prepared by both synthetic strategies.
Mechanochemistry: Toward green
synthesis of metal – organic frameworks
⇑ Corresponding author.
E-mail address:Jaroniec, M. ([email protected])
RESEARCH:Review
Overview of metal–organic frameworks Main types of MOFs
Metal–organic frameworks (MOFs) are three dimensional porous crystalline materials consisting of metal ions or clusters and organic linkers connected via coordination bonds. An extensive range of components of both metal nodes and linkers creates almost unlimited opportunities for creating a variety of metal–or- ganic structures. Among the countless number of MOFs, several structures, so-called archetypal MOFs, have attracted special attention because of outstanding properties, such as high specific surface area (SSA determined by Brunauer–Emmett–Teller method), uniform pore structures and tunable porosity. Fig. 1 presents examples of the most studied metal–organic frameworks [5–10].
The most popular archetype MOF, denoted as MOF-5, consists of zinc clusters and terephthalic linkers. It wasfirst synthesized two decades ago by Yaghi’s group, but still numbers of papers are reported, mainly devoted to its adsorption/storage capability [11–15]. MOF-5 is also known as IRMOF-1, since belongs to isoreticular metal–organic frameworks (IRMOFs) and possess high SSA even up to 3800 m2/g[15]. IRMOFs have emerged as a large family of MOFs consisting of many structures with the same zinc-containing clusters [Zn4O]6+, and organic carboxylate linkers differed in length and attached functional groups. Vari- ous MOFs with versatile functionalities and pore sizes can be achieved by varying ligands. Other well-known MOFs are zeolitic imidazolate frameworks (ZIFs) composed of transition metal ions (usually Zn2+or Co2+) tetrahedrally bonded to imidazole deriva- tives. Topology of ZIFs is isomorphic to zeolites and thereby they show combined merits of the both MOFs and zeolites. For instance, ZIFs exhibit high surface areas, high crystallinity and flexibility as well as great thermal and chemical stabilities, as opposed to some other MOFs. The representative ZIF structure is ZIF-8 also known under the commercial name Basolite TMZ1200 [16,17]. ZIF-8 is one of the most stable MOFs com-
posed of zinc-containing clusters and 2-methylimidazole ligands.
Another group of intensively studied MOFs based on transition metals are MIL-type materials. The acronym means Materials Institute Lavoisier, place where these MOFs were synthesized for thefirst time by Ferey’s group. MILs contain trivalent metal ions (V3+, Cr3+, Fe3+, Al3+, Ga3+, In3+) and linkers derived from diverse carboxylic acids, usually terephthalic acid (BDC) and tri- mesic acid (BTC). These MOFs feature high SSA even above 3000 m2/g and large pore sizes (>2,5 nm). Among this type of MOFs, the most extensively studied are MIL-101, MIL-53 and MIL-100 [18]. Another well-studied MOF, denoted as UiO-66, consists of zirconium octahedral clusters and BDC2ligands. It wasfirst synthesized at the University of Oslo (UiO) and is one of the most frequently reported material among Zr-containing MOFs. Most of Zr-containing MOFs feature high stability and have the potential to meet the requirements of practical applica- tions e.g., in gas storage and separation, water purification or catalysis[19–22].
Main synthetic methods
Experimental conditions used for synthesis of MOFs influence their morphology, porosity, and crystallinity. Therefore, the proper choice of synthesis method is very important and deter- mines physicochemical properties of the obtained products.
Moreover, economic and environmental aspects, which are par- ticularly important in large-scale synthesis, should be considered too. Among many diverse synthetic procedures reported so far for the preparation of MOFs, the solvothermal synthesis is still most often used. This method involves a solvent-based reaction of metal ions with organic linkers and crystallization in a closed vessel where high temperature and pressure (above boiling point of a solvent) facilitate the self-assembly and crystal growth. The selected solvent considerably affects both solubility of reactants and reaction temperature. The most widely used are organic sol- vents such as acetone, dimethylformamide or ethanol. A similar route, hydrothermal synthesis is carried out in aqueous solu- tions. Typically, energy required to initiate and stimulate synthe- sis reactions during solvothermal/hydrothermal processes is supplied by conventional electric heating within several dozen hours. Alternative forms of energy include electromagnetic (mi- crowave and ultrasonic waves), electrochemical and mechanochemical energies (the latter is described in Section:
Mechanochemical methods)[23,24].
In microwave-assisted methods, waves at a frequency ranged from 300 MHz to 300 GHz are used to shorten reaction time even to a few hours or minutes without deterioration of the product quality. The frequency used influences interactions between microwaves and electric charges of the irradiated molecules. Heat is generated due to the collisions of rotating polar solvent mole- cules. Ultrasounds, namely waves at a frequency range from 20 kHz to 10 MHz, induce the cavitation effect which relies on the formation of gas bubbles, their growth, and subsequent dis- integration. This process produces high local temperatures (up to 5000 K) and pressures (up to 1000 bar) intensifying and speed- ing up chemical reactions. An electrochemical method is associ- ated with anodic dissolution of a metal cathode. Typically, MOFs are formed when the electric currentflows through the electro- chemical cell consisting of a metal cathode immersed in a solu- FIGURE 1
Scheme of the most known metal–organic frameworks composed of pictorial structures reported in Refs.[5–10].
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tion containing organic linkers. This approach resolves the prob- lem of toxic anions, derived from metal salts, abundantly intro- duced to the reaction systems. A large-scale electrochemical synthesis of MOFs was elaborated by scientists from BASF[23–
29]. Other innovative synthesis approaches include ion- thermal and surfactant-thermal methods or the use of deep eutectic solvents and are worth to be mention because can open up new possibilities for obtaining MOFs with unique properties [30–32].
Most procedures used for the synthesis of MOFs are performed in liquid media using various solvents. The choice of a solvent is usually based on the dissolution efficiency, cost and subse- quently environmental benefits. The principles of green chem- istry suggest reducing the amounts of solvents and use of more environmentally friendly ones. High toxicity of many conven- tional solvents and their excessive use in large-scale syntheses of MOFs result in generating substantial amounts of waste liq- uids. Therefore, to make synthesis protocols more sustainable, the employment of alternative greener solvents is important. It is difficult to point a universal green solvent that can be used for synthesizing organics and inorganics. For instance, supercrit- icalfluids (SCFs) and ionic liquids (ILs) constitute a significant part of the research on alternative solvents[33]. The utilization of ILs and SCFs for the synthesis of MOFs has many unique advantages compared to the conventional solvent-based meth- ods. ILs are generally molten salts with a melting point below 100°C. They provide high solubility of both organic and inor- ganic substances, thus can dissolve organic ligands and inorganic MOF precursors. ILs can also act as structure-directing agents contributing to the preparation of novel MOF structures. A typi- cal supercriticalfluid CO2with critical pressure of 7.38 MPa and temperature of 31.18°C is inexpensive, non-toxic, easily avail- able, and can be recycled after use. Importantly, some properties of MOFs obtained by using SC-CO2can be tuned just by control- ling the pressure of CO2. A crucial aspect of the synthesis is that MOF precursors need to be compatible with CO2. Both approaches using SCFs and ILs have been reviewed elsewhere [34].
Mechanochemical synthesis Mechanochemical methods
Mechanochemical reactions rely on a direct absorption of mechanical energy by reagents, usually solids, during milling or grinding processes. In a typical ball milling process, the energy required to initiate chemical reactions is supplied by friction and impact between balls and reactants. High ball impact is needed to induce a chemical reaction, otherwise only elastic deformations can occur. A high energy grinding process causes structural stress, bonds breakage and formation of reactive radicals. As a result, reactive layers of atoms are exposed, which facilitates chemical reactions at the interface of solid reactants. Initially, mechanochemical reactions were performed by using mortar and pestle. Nowadays, automatic ball mills/grinders are predominantly used. These special instruments provide high grinding energy which facilitates mechanochemical reactions. Such devices enable to conduct mechanosynthesis under well-defined reproducible conditions [35–43].
A typical mechanochemical synthesis involves grinding/
milling the mixture of solid precursors in a ball miller under solvent-free conditions (Neat Grinding, NG). Thus, it gives an opportunity to use insoluble metal sources, which sometimes are difficult to dissolve in the solvents used in conventional syn- theses of MOFs. For example, using insoluble metal oxides as metal precursors instead of salts is safer, more environmentally friendly and opens possibilities for the synthesis of new materi- als. It was noticed that using hydrated reactants, which release water during the mechanochemical reaction, enables to conduct NG process more efficiently. The presence of a solvent improves the mobility of metal ions and organic linkers, which facilitates chemical reactions including the formation of coordination bonds. Following this lead, Liquid-Assisted Grinding (LAG) was introduced, in which small amounts of solvents are added to improve the mechanochemical synthesis process. Comparing NG and LAG, the latter enhances both the reaction rate and crys- tallinity of the resulting products. Moreover, LAG allowed extending the scope of mechanochemically-synthesized materi- als, because it was difficult to initiate and stimulate some reac- tions under solvent-free conditions. Another approach, Ion- and Liquid-Assisted Grinding (ILAG) is a type of LAG concept, in which both liquid and salt are added. These additives promote dissolution of solid reagents providing a homogeneous reaction mixture, which intensifies reactivity of substrates and thus improves efficiency of the milling process. Proper additives can also enhance both the reaction selectivity and product quality [4,37–39,41,42,44]. Fig. 2 schematically illustrates types of mechanochemical synthesis methods.
Mechanosynthesis vs green chemistry
Designing a synthesis of nanoporous materials involves many aspects including a proper selection of precursors, method, sol- vents (if required), and conditions (temperature, pressure). Addi- tionally, all syntheses should comply with Green Chemistry principles. According to IUPAC green chemistry concept
FIGURE 2
Schematic illustration of mechanochemical methods: Neat Grinding (NG), Liquid-Assisted Grinding (LAG), Ion- and Liquid-Assisted Grinding (ILAG).
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includes “The invention, design, and application of chemical products and processes to reduce or to eliminate the use and gen- eration of hazardous substances”[45]. A greener synthesis pro- cess can be achieved by reducing solvent and energy consumptions and using renewable precursors. Mechanosynthe- sis is inherently a green process that provides fast chemical reac- tions between solids under solvent-free conditions (or with a small addition of solvents). Thermal energy released during mechanosynthesis is sufficient to initiate and stimulate chemical reactions without additional external heating. Furthermore, in this process, cheap and widely available insoluble materials can serve as precursors. Mechanosynthesis successfully solves prob- lems with extensively used expensive and hazardous solvents in conventional syntheses, thus contributes to the reduction of liquid wastes[45–47]. A high-speed milling or grinding can effec- tively shorten synthesis time and decrease energy consumption compared to solution-based methods. Importantly, these pro- cesses assure a high reaction yield and are easy to scale-up. There- fore, mechanosynthesis can be established as a green synthetic method. Table 1 summarizes pros and cons of the solvent- based and mechanochemical methods.
Mechanochemical synthesis of MOFs
ZIFsAs shown above, to perform the synthesis of metal–organic frameworks one should put a special emphasis on various factors including ecological andfinancial aspects. Thus, the use of inex- pensive and easily available precursors (i.e., metal oxides) is of particular interest[48–53]. Thefirst dry conversion of ZnO into ZIF-8 was performed in 2013 by Tanaka et al.[51]. Grinding a nanosized ZnO and 2-methylimidazole (HmIm) for 96 hours resulted in porous material with SSA of 1480 m2/g, while typi- cally this type of MOF features SSA around 1200 m2/g. Its I- type adsorption isotherm and X-ray diffraction (XRD) analysis suggest that the mechanochemically obtained ZIF-8 possessed an open-framework with sodalite topology. The as-obtained ZIF-8 was thermally stable in air up to 300°C, which is compara-
ble to other ZIFs reported so far. Additionally, the weight loss ratio permitted calculation of the content ratio in the milled ZIF-8 product, yielding 80 % of ZIF-8 after milling for 96 h. Ima- waka et al.[52]obtained bimetallic CoZn-ZIFs by milling ZnO and 2-methylimidazole with cobalt acetate. The addition of cobalt ions resulted in the enlargement of SSA and CO2uptake of the obtained bimetallic MOFs. The sample with Co/Zn molar ratio of 0.9:0.1 exhibited the highest SSA (1690 m2/g) and CO2
uptake capacity of 0.85 mmol/g measured at 25°C and 1 bar.
The CoZn-ZIF obtained by the solvent-based method exhibited a similar CO2 adsorption capacity of 0.90 mmol/g. The thermal stability of CoZn-ZIFs decreases with increasing Co/Zn ratio, which shows better thermal stability of ZIF-8 (Zn-ZIF) than ZIF- 67 (Co-ZIF). Analysis of the weight loss data for all prepared CoZn-ZIFs reveals that the addition of cobalt acetate improves the conversion of ZnO. Extensive studies of these MOFs by Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and nitrogen adsorption suggest that the mechanochemically prepared CoZn-ZIFs have hierarchical superstructures with interparticle porosity. Recently, Taheri et al.[53] reported a one-step mechanochemical conversion of ZnO to ZIF-8 (Fig. 3). Adding small amounts of a liquid triggered dispersion of ZnO particles and then the deagglomerated ZnO particles easily reacted with 2-methylimidazole ligands leading to a full conversion into ZIF-8. These mechanochemically syn- thesized materials had SSA up to 1885 m2/g, which is close to the theoretical value for ZIF-8 (1947 m2/g). The rate of ZnO con- version into ZIF-8 depends on the milling time, which was con- firmed by the XRD analysis. The intensity of diffraction peaks originating from ZIF-8 increases with increasing milling time in contrast to the peaks of ZnO. However, after 12 h of grinding, an amorphous sample was obtained. The optimal milling time was adjusted to 8 hours. The average crystallite size determined by TEM and XRD was about 61 nm. According to thermogravi- metric analysis (TGA), the prepared ZIF-8 was thermally stable up to 400°C. Moreover, smaller weight losses reported for the samples prepared by prolonged milling show that the reaction between ZnO and HmIm requires a few hours of milling, which is consistent with XRD data. The mechanochemically synthe- sized ZIF-8 samples showed an enhanced adsorption of dyes Rhodamine B (RhB) and Methylene Blue (MB) compared to TABLE 1
Comparison of typical solvent-based and mechanochemical methods.
Solvent-based method Mechanochemical method High temperature Room temperature or slightly
elevated Multiple steps (time-
consuming)
One pot synthesis (timesaving)
Large amounts of solvents Solvent-free or small amounts of solvents
Expensive Cheap and easy (cost-effective) Large amounts of liquid
wastes
Minimum wastes
Highly crystalline products NG may lead to amorphization Basic equipment Special grinders/mills or mortar and
pestle
Pure products Possible impurities from milling reactors
Precise control of synthesis
Precise control of synthesis is difficult
FIGURE 3
Schematic diagram of the mechanochemical reactions of ZnO nano powder and HmIm[53]. Adapted with permission from Ref.[53]. CopyrightÓ2020 American Chemical Society.
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ZIF-8 obtained by the solvent-based precipitation. After 30 min of the exposure in the dark, 29.1 % of MB and 93.9 % of RhB were removed by the mechanochemically synthesized ZIF-8 and only 3.3 % of MB and 41.6 % of RhB by the conventionally synthesized counterpart. The difference in the dye adsorption capacities was explained by the differences in their surface chem- istry. Depending on the synthesis conditions, different func- tional groups are attached to the external surface of ZIFs, which results in different surface charges and consequently, in altering in interactions between ZIF-8 and dye molecules.
Recently, Zhong’s group[54]reported a solvent-free synthesis of ZIF-8 with SSA of 1377 m2/g from Zn(OH)2 used as a zinc source. Furthermore, the authors demonstrated a convenient synthesis route for synthesizing bimetallic ZIF-8-based catalysts.
For instance, the addition of cobalt ions and an ionic liquid dur- ing mechanosynthesis of ZIF-8 gave a good IL@ZIF-8(Zn/Co) cat- alyst after 60 minutes of simple manual grinding. The presence of the catalytically active components was verified by High- Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) and Energy Dispersive Spectroscopy (EDS). According to TEM imaging data, the crystal size of the cat- alyst is slightly larger (80–100 nm) compared to ZIF-8. A compar- ison of the catalytic activity of IL@ZIF-8(Zn/Co) in cycloaddition of CO2and epoxide with the catalytic performance of ZIF-8(Zn), ZIF-8(Zn/Co), and IL@ZIF-8(Zn) in the same reaction showed an improved catalytic performance of the former. The reported high activity was explained by the synergistic effect of multiple active sites existing in the framework of IL@ZIF-8(Zn/Co). The pre- sented strategy is a simple way to design multifunctional MOF catalysts. Samal et al. [55] adopted a kitchen grinder to carry
out an analogous mechanosynthesis of ZIF-8 at multi-gram- scale (20 g). Briefly, grinding Zn(OH)2in the presence of a small amount of ethanol for 60 minutes gave product with SSA of 1024 m2/g. This tool was also employed to synthesize Cu-BTC and MIL-100(Fe) MOFs. The reported TGA data show that ZIF- 8, Cu-BTC, and MIL-100(Fe) are thermally stable up to 600°C, 250°C, and 350°C, respectively, which is consistent with other literature data. The pore size distributions calculated using the non-local density functional theory (NLDFT) method for all three MOFs as well as their SSA differ from those of the analogous materials reported elsewhere. The authors suggested that it is due to their moderate crystallinity and insoluble oligomeric frag- ments present in the pores because of grinding. SEM and TEM images clearly reveal crystals with rhombic dodecahedron shape.
The average size of ZIF-8, Cu-BTC, and MIL-100(Fe) crystals is about 100–200 nm, 40 –50lm, and less than 50 nm, respec- tively. MIL-100 and ZIF-8 were examined for the removal of organic dyes from aqueous solutions. The as-obtained MIL-100 (Fe) was shown to be an efficient adsorbent for methylene blue removal (ca. 50 % within 5 min and 98 % within 3 h) mainly due to the favorable pore size (0.89 nm) within its framework.
The inherent microporous structure of most MOFs including ZIFs can have a negative effect on the mass transport and acces- sibility for molecules. Therefore, an effort has been made to obtain hierarchical MOF structures, i.e., having at least two levels of pore systems, micropores and mesopores/macropores. How- ever, the development of dual porosity in MOFs is still challeng- ing. Tanaka’s group[56]presented synthesis of hierarchical ZIF-8 via a salt-assisted grinding (Fig. 4). Zinc oxide with various parti- cle sizes and HmIm linker were milled along with zinc acetate
FIGURE 4
N2adsorption isotherms and Field Emission Scanning Electron Microscopy (FESEM) images of ZIF-8 prepared by using (a) a solvent-based method and a mechanochemical method with (b) and without (c) adding zinc acetate[56]. Adapted with permission from Ref.[56]. CopyrightÓ2017 American Chemical Society.
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additive. Using nano-sized particles of ZnO, ZIFs with hierarchi- cal porosity and SSA up to 1720 m2/g were successfully synthe- sized. The XRD analysis revealed benefits associated with the addition of zinc acetate. The presence of the salt accelerated the mechanosynthesis process regardless of ZnO powder size.
FESEM analysis revealed that the addition of zinc salt led to a change in the shape of agglomerated ZIF-8 particles formed dur- ing mechanical milling into dendritic porous architectures. Due to the presence of zinc acetate, a competitive dissolution process occurred in the gradually produced acetic acid, which caused an enhancement of microporosity and contributed to the formation of crystal defects in the resulting MOFs. Both FESEM and TEM images reveal that the ZIF-8 synthesized by the salt-templating method exhibit hierarchical superstructure with interconnected porosity throughout micro-, meso-, and macroporous regions.
Due to the presence of pores greater than 100 nm, the porosity and pore size distribution were examined by using the Mercury Intrusion Porosimetry (MIP) technique, which confirmed differ- ent pore sizes in the framework centered around 3.6 nm, 60 nm, and 30lm. The ZIF-8 material with nanometer-scale hier- archical superstructure obtained by the mechanosynthesis can be used for adsorption of molecules with a kinetic diameter larger than its window size. For instance, it showed even 10 times faster uptake of n-butanol and n-hexane and 3 times higher adsorption capacity of rhodamine B as compared with data for monocrys- talline ZIF-8 prepared in solution.
ZIF-8 is undoubtedly one of the most studied zeolitic imida- zole frameworks, because of its high stability provided by strong bonds between Zn ions and imidazole linkers. Taheri et al.[57]
utilized ball milling to obtain ZIF-8, cobalt-containing ZIF-8 and ZIF-67 materials. In this synthesis, metal acetate (zinc and/
or cobalt) and 2-methylimidazole were milled for 2 hours. ZIF- 67 was formed by replacing all Zn2+ions of ZIF-8 by Co2+ions (Fig. 5). Based on SEM images and XRD patterns, it was found that all as-obtained ZIF powders featured particles with similar sizes (200–400 nm) and sodalite topology regardless of the cobalt content. The authors investigated the influence of cobalt-doping on their stability in water. The results showed that increasing cobalt content leads to lower stability in water. Therefore, it was evidenced that ZIF-8 is more suitable for water-involved
applications than ZIF-67. Additionally, ZIF-8 possesses higher SSA (1881 m2/g) than the cobalt-containing MOFs (1525 m2/g for ZIF-67). This study was directed towards selection of appropri- ate MOFs for catalytic and photocatalytic water treatment or heavy-metal scavenging in water.
Although, mechanosynthesis of MOFs can be conducted under solvent-free conditions, their subsequent purification is usually needed. The post-synthesis solvent washing is an emerg- ing problem that may hinder scale-up fabrication of these mate- rials. Recently, Brekalo et al.[48] demonstrated a fully solvent- free mechanosynthesis of ZIF-8 employing NG of zinc carbonate ([ZnCO3]2[Zn(OH)2]3) and imidazole linker used in excess to pre- vent the formation of byproducts. Subsequently, the ligand excess was removed by vacuum sublimation at 200°C and reused. SSA of the as-prepared ZIF-8 (1785 m2/g) was comparable to that of commercially available ZIF-8 (1758 m2/g).
The coexistence of diverse metal ions in MOF lattices can expand their usability. The synergetic effect of different active metal centers has emerged as a particularly desirable feature in catalytic applications. Thus, many efforts have been made to develop an efficient strategy to introduce additional metal ions into MOF structures. For instance, Wei et al.[58]prepared series of guest-ZIF complexes using 18 functional guest molecules, which differ in terms of size, shape, and properties (Fig. 6). Var- ious guest-sod-ZIF and guest-rho-ZIF complexes were obtained from zinc oxide, 2-methylimidazole and the corresponding guest molecules via a mechanochemical method. The reactants were grinded in the presence of liquid/salt additives and zirconium balls for 60 min. In as obtained complexes, guest molecules were encapsulated in the ZIF structure, but their release led to the destruction of the host framework. Guest-ZIF complexes showed high crystallinity and the lack of diffraction peaks of guest mole- cules, which indicate that the encapsulation of guest molecules caused no damage of the integrity within ZIF frameworks. The guest-ZIF materials showed potential application in magnetic res- onance imaging and heterogonous catalysis.
Designing high-entropy MOFs is a quite new concept. Such materials can contain even five metal species randomly dis- tributed in a single MOF lattice and may possess unique proper- ties that extend their applications[59]. For instance, ball milling
FIGURE 5
Schematic illustration of the synthesis of cobalt-containing-ZIF-8 via ball milling[57]. Adapted with permission from Ref.[57]. CopyrightÓ2019 Elsevier Ltd.
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facilitates chemical synthesis by local short-range heating and has emerged as an optimal solution for the synthesis of high- entropy MOFs. Xu et al.[59]assembledfive types of metal ions into a zeolitic framework yielding a high-entropy ZIF (Fig. 7).
In brief, metal precursors (ZnO, CuO, CdO, Ni(OAc)2 and Co (OAc)2) were ball milled with an excess of 2-methylimidazole for 2 h. Then, the product was washed with methanol and dried under vacuum. It was shown that the characteristic intensive diffraction peak of HE-ZIF-BM is slightly shifted compared to the ZIF-8 simulated pattern due to the introduced metal ions, which led to a lattice expansion. Furthermore, TGA, Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), and X-ray Photoelectron Spectroscopy (XPS) confirmed the successful introduction offive metal ions into the ZIF structure. Elemental maps obtained using EDS show randomly dispersed metals in the ZIF lattices (Fig. 7). The obtained high-entropy ZIF had SSA of 1147 m2/g and exhibited superior catalytic properties towards conversion of CO2 to carbonate compared to single metal- containing ZIF materials. It is worth noting, that the material obtained by ball milling contained a few times higher amount of Cd, Cu and Ni components than the analogues sample obtained by a solvothermal method.
IRMOFs
Thefirst mechanosynthesis of IRMOF-1 was performed by a so- called “SMART” strategy based on the acid-base reaction of a proper ligand with a pre-assembled oxo-zinc cluster followed by a simple ligand exchange process. For example, 30 min NG of terephthalic acid and [Zn4(l4-O)(HNOCPh)6] clusters gave material with SSA of 2345 m2/g[60]. Activation with anhydrous tetrahydrofuran (THF) was required to obtain IRMOF-1 whose XRD pattern is fully consistent with the previously reported ones. The activated samples werefinally guest-free as confirmed by1H NMR (Hydrogen Nuclear Magnetic Resonance) and TGA.
This method has been extended to the synthesis of other isoretic- ular MOFs such as IRMOF-3 and IRMOF-10 (Fig. 8). These mate- rials were obtained using the same oxo-zinc amidate cluster, but different linkers derived from 2-aminoterephthalic acid and 4,40- biphenyldicarboxylic acid, respectively. After mechanochemical synthesis (NG or LAG), the materials were activated in THF solu- tion. Activated IRMOF-3 samples exhibited a microcrystalline structure with crystal sizes in the range of 400–500 nm and ther- mal stability up to 400°C. IRMOF-3 showed high crystallinity and SSA of 1272 m2/g. However, in the case of IRMOF-10 consist- ing of longer linkers and consequently larger pores, the activa- FIGURE 6
Schematic illustration of the one-pot mechanochemical synthesis of guest-ZIF complexes[58]. Adapted with permission from Ref.[58]. CopyrightÓ2018 American Chemical Society.
FIGURE 7
(a) Schematic illustration of mechanochemical synthesis of a high entropy-ZIF; (b) TEM image and EDS elemental maps of high-entropy ZIF (b)[59]. Adapted with permission from Ref.[59]. CopyrightÓ2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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tion procedure contributed to the amorphization of its structure and needed to be modified, which was confirmed by XRD analy- sis[61].
Recently, isoreticular metal–organic frameworks were also syn- thesized from Zn(OAc)22H2O used as a metal source. Abedi et al.
[62]synthesized IRMOF-1 and halogen-containing IRMOF-2 car- rying out 30-minute LAG of zinc cluster and terephthalic acid or terephthalic acid halogen derivatives in the presence of a small amount of DMF using mortar and pestle (Fig. 9). XRD analysis confirmed the formation of these MOF structures during mechan- ical grinding. SEM images showed cubic topology of these frame- works with slight differences in their particle size. Luminescent properties of the as-obtained halogen-containing products were tested in sensors for nitrobenzene. A similar synthesis of highly porous MOF-5 (IRMOF-5) was reported by Lv et al.[63]. An appro- priate adjustment of synthesis parameters (e.g., metal/ligand molar ratio, grinding time and speed) together with an efficient post-synthetic activation process afforded highly porous MOF-5 with SSA of 3466 m2/g and excellent adsorption capacity for lin- ear alkanes. Such high SSA was achieved for optimized conditions by using Zn2+/BDC ratio of 3:1, 60 min of grinding time, and 1100 rpm of grinding speed. XRD and TGA analysis revealed that the as-prepared MOF-5 shows well-defined crystalline structure. The adsorption capacity of MOF-5 towards hydrocarbons nC4–nC7 increases with decreasing adsorbate chain length. However, at low adsorption pressures, higher adsorption capacities for longer alkanes are obtained due to the stronger adsorbate–adsorbent interactions, which increase with increasing molecular length of the adsorbates.
MILs
A successful employment of mechanochemistry in the synthesis of various MOFs afforded MILs under solvent-less or HF-free con-
ditions[53,54]. The synthesis procedures involved two stages: (i) grinding and (ii) subsequent heating in an autoclave for several hours. For instance, Leng et al. [64], synthesized chromium- containing MIL-101 via 30 min ball milling of Cr(NO3)39H2O and BDC and subsequent heating of the obtained product in an autoclave at 220°C for 4 h (MIL-101-BM). Although, the syn- thesis reaction was conducted under solvent-free conditions, a post-synthetic washing with hot ethanol was needed. The as- obtained MIL-101(Cr) showed high SSA of 2764 m2/g. The authors also prepared analogous MOFs under hydrothermal con- ditions with the addition of hydrofluoric acid (MIL-101-HF) and/
or other solvents (MIL-101-solvent) for comparison. Such wet conditions promoted development of porosity in these MOFs leading to higher SSA up to 3517 m2/g. The authors tried to explain the reasons for these differences in the SSA values of the as-prepared MILs. Elemental analysis shows that MIL-101- BM contained less C and H elements and had even three times higher N content compared to MIL-101-solvent (5.2 wt % N and 1.6 wt % N, respectively). Fourier Transform Infrared Spec- troscopy (FTIR) spectra suggest the presence of C-NO2 group because of a strong absorption band at 1538 cm1 and a weak absorption band at 1350 cm1, which can be assigned to the asymmetric and symmetric stretching vibrations in this group.
1H NMR confirmed the existence of the NO2group attached to benzene ring. These results suggest a nitration of the BDC ligands during the mechanochemical synthesis, which led to a lower SSA of the as-obtained MOF compared to those prepared under solvothermal conditions. According to the Barrett-Joyner- Halenda (BJH) method, all the prepared MOFs possess pore sizes centered around 1.5 and 2.0 nm. XRD data confirm the crys- talline structure of MIL-101(Cr). However, the width of the diffraction peaks of the mechanochemically-synthesized MIL are larger indicating its smaller crystal size than those of the FIGURE 8
Schematic illustration of the mechanochemical“SMART”strategy for synthesizing Zn-based MOFs[61]. Adapted with permission from Ref.[61]. CopyrightÓ 2018 American Chemical Society.
FIGURE 9
Schematic illustration of mechanochemical synthesis of IRMOF-1 and IRMOF-2-X (X = Cl, Br, I) series and representation of their structure[62]. Adapted with permission from Ref.[62]. CopyrightÓ2015 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
RESEARCH:Review
MOF samples prepared under solvothermal conditions. The dif- ferences in the crystal size and morphology are confirmed by SEM imaging. Mechanosynthesis gave crystals with the size of 40–200 nm and irregular granular shape, while the conventional methods produced MIL crystals with sizes of 300–500 nm and regular octahedral shape. TG analysis showed decomposition of the MIL-101-BM framework around 400°C indicating its rela- tively good thermal stability. Despite lower SSA value, MIL-101- BM exhibited superior catalytic activity for oxidation of cyclo- hexene compared to that of MIL-101-solvent. This can be explained by smaller particle size and more exposed Cr active sites in MIL-101-BM. Elsewhere, milling CrCl36H2O and H3BTC for 40 minutes followed by heating in an autoclave at 220°C for 15 h yielded MIL-100(Cr) with SSA of 1848 m2/g, while the same MOF synthesized by a traditional wet chemistry method possessed SSA of 1741 m2/g [65]. The results from XRD, FTIR and thermal analysis are in good agreement with those obtained for the MIL sample prepared by the solvent- based method. SEM images indicate that MIL-100(Cr) consists of smaller crystalline particles than the product of the solvent- based method (50–200 nm and 200–500 nm, respectively).
ICP-AES and elemental analysis data show comparable elemental compositions of both samples. The mechanochemically obtained MIL exhibited an enhanced catalytic performance in the oxidation reaction of benzyl alcohol (conversion of 53 % within 3 h) compared to the MIL obtained by using solvents as well as some other catalysts such as noble metal catalyst (conver- sion of 52 % within 6 h), graphene oxide (conversion of 46 % within 24 h) and ionic liquid (conversion of 23 % within 4 h).
Importantly, heterogeneous catalysts can be reused, which is favorable for potential applications[65].
Fe-containing MOFs attract a lot of interest because of their distinctive catalytic performance, semiconductor properties, and highly flexible frameworks. In addition, iron is an earth- abundant metal, thus iron-based-raw materials are low-cost and easily available. Sun’s group [66]reported a solvent-free proce- dure to synthesize MIL-100(Fe) with a yield of 93 %. This MOF characterized by SSA of 1940 m2/g and a micropore volume of 0.56 cm3/g was prepared by grinding iron nitrate and trimesic acid and subsequent heating at 160°C for 4 h. It consisted of stick-like crystals with a size of 1–4mm. For comparison, the anal- ogous MIL prepared under hydrothermal conditions was com- posed of flaky-like crystals with a size of 100–300 nm.
However, both samples featured similar elemental compositions, surface functional groups and thermal stabilities (up to 400°C).
The mechanochemically prepared MIL-100 has a potential for effective acetalization of benzaldehyde with methanol due to its higher TOF value (Turn Over Frequency; moles of converted benzaldehyde per mole of metal site per hour) among other MOF catalysts such as UiO-66, Fe-BTC, Cu3(BTC)2. Moreover, its catalytic activity did not change much even afterfive runs. Pil- loni et al.[67]prepared MIL-100(Fe) by conducting 1 h grinding of H3BTC and Fe(NO3)39H2O in the presence of liquid additive– an aqueous solution of tetramethyl ammonium hydroxide (TMAOH). After centrifugation at 2500 rpm and air-drying at room temperature, the obtained MIL possessed SSA of 1033 m2/ g. This value was lower than that of an electrochemically synthe- sized commercially available counterpart. The authors pointed
out a significant difference in the SSA values of commercial MILs-100(Fe) samples (1300–1600 m2/g) and those reported in their paper (398 m2/g). Nevertheless, the mechanochemically synthesized material exhibited higher adsorption capacity toward 4,6-dimethyldibenzothiophene (4,6-DMDBT) from a sim- ulated diesel fuel at ambient temperature than the commercially available MIL-100. The ball-milled sample exhibited higher crys- tallinity than the commercial MIL. Moreover, the determined FTIR spectra indicate the presence of unreacted 1,3,5-benzene tri- carboxylic acid in the commercial sample. There are also differ- ences in the thermal stability of these samples, i.e., the mechanochemically obtained sample is more thermally stable.
Jeong et al. [68]showed a facile, fast, and eco-friendly method to prepare Fe-containing MIL-88A. They grinded FeCl36H2O and sodium fumarate using mortar and pestle. Fe-MIL-88A was obtained after 10 minute NG and exhibited five times higher SSA (108 m2/g) than the corresponding materials prepared by a solution-based method (21 m2/g). As noted in the previously cited examples, the method of synthesis influences the morphol- ogy and crystal growth of MOFs. In this case, the mechanochemically-prepared MIL-88A features a 3D hierarchical superstructure with macroscopic pores and relatively broad XRD peaks, which is associated with small crystal size due to the fast nucleation under grinding/milling conditions. Hou et al. [69]
carried out a two-step synthesis of MIL-88B(Fe). At first, ball milling of reactants H2BDC and Fe(NO3)39H2O was proceeded for 1 or 2 h, and then the obtained mixture was stirred in differ- ent volumes of ethanol (5, 10, and 15 mL) for 1, 3, 6, and 12 h at ambient temperature. This strategy gave MIL-88B(Fe) with a rel- atively high SSA of 261 m2/g and high crystallinity. SEM and TEM images show irregular needle-type crystals with an average size of 200–300 nm. TG analysis reveals the collapse of the struc- ture between 300°C and 500°C. The as-obtained product exhib- ited excellent adsorption performance for the treatment of arsenate-contaminated water. According to the Langmuir model, the maximum adsorption capacity of MIL-88B(Fe) towards arsen- ate is 156.7 mg/g reaching the arsenic threshold for drinking water.
Some MOFs have the ability of changing cell parameters with- out permanent destruction of their frameworks under various stimuli, i.e., temperature, pressure, the addition of solvents. This phenomenon occurs especially forflexible MOFs including Cr- MIL-88B and Cr-MIL-53. The Cr-based MOFs were obtained anal- ogously as described above using a two-step synthesis strategy of milling and heating (Fig. 10) [70]. Terephthalic acid and chro- mium salts [Cr(NO3)39H2O or CrCl36H2O] were milled in a ball-miller at 30 Hz for 30 min and then heated in an autoclave at 190°C for 3 h. XPS spectra determined for the both MOFs con- firm the presence of Cr and exclude the presence of Fe that could be introduced from the grinding jar and balls. TG curves and FTIR spectra obtained for MOFs are comparable to those reported in the literature for analogous MILs [70]. The SSA value of the mechanochemically obtained Cr-MIL-53 is about 1000 m2/g, while Cr-MIL-88B is rather nonporous. Apparently, the crys- talline MIL-88B features a close-pore conformation when desol- vated, resulting in a very low N2 uptake and specific surface area. However, agglomeration of its small crystals and impurities trapped in its pores, which hinder the pore-closing, may result in
RESEARCH:Review
a higher porosity determined for this material by gas adsorption.
Overall, the SSA values of flexible MOFs underestimate their porosity in solvents (becauseflexible frameworks can open up), which does not reflect their performance of adsorption-related applications in solvents.
Lanthanide-containing MIL-78 has been also synthesized by a mechanochemical method. For example, Singh et al. [71]
prepared MIL-78 using only a mechanical force as energy source (without heating step). Trimesic acid and yttrium hydride were milled for 7 h under an argon atmosphere.
SEM images reveal its non-uniform crystals of different shapes that could be assigned to the grinding effect. The XRD pattern of the MIL-78 and its FTIR spectrum indicate the presence of characteristic carbon–oxygen–yttrium bonds. The TG analysis shows that MIL-78 is thermally stable up to 450°C similarly as other MIL-78 samples reported in the literature. A series of MIL-78 structures with rare-earth elements were synthesized by using the same ligand and different metal nodes. In gen- eral, the mechanochemical synthesis of this type of MILs relied on grinding a mixture of H3BTC and rare-earth carbon- ates or carbonate hydrates[72–74]. Yuan et al.[72]proposed a 20-minute LAG synthesis of rare-earth(III)-containing MOFs (i.e., Y, Sm, Gd, Tb, Dy, Er, and Yb). These synthesis reactions were performed in the presence of a small amount of DMF and the as-obtained materials were additionally thermally acti- vated. The highest SSA of 655 m2/g had the sample activated at 300°C (Dy-BTC). The use of H2O as a liquid additive instead of DMF led to some structural changes in the resulting MOFs. XRD analysis indicated new phases that are isostruc- tural to the 1-D ribbon-like coordination polymer. Liu et al.
[75] presented a similar synthesis strategy for the preparation of diverse Ln-containing MOFs, which involved 5 min NG of Ln(NO3)3nH2O (Ln = Eu, Er, Dy, Y, Tm) and H3BTC and sub- sequent solvent-free thermal treatment at 160°C for 24 h (Fig. 11). The neat grinding process afforded both amorphous metal complexes and crystal seeds of MIL-78, which grew dur- ing thermal heating thanks to strong chelating interactions.
The prepared samples are highly crystalline and show a special
morphology of accordion-like microrods with different sizes;
namely, numerous ultrathin nanosheets are arranged vertically on the surface of Ln-MOFs crystals forming a hierarchical architecture. The obtained Ln-MOFs show superior photolumi- nescence properties at room temperature, thus can be used as ratiometric luminescent thermometer. For instance, Eu-Dy-Y- MOF with characteristic Eu3+and Dy3+ dual red/blue emission exhibited maximum relative sensitivity (0.640 % °C1 at 103°C) comparable to those of the previously reported Ln- MOF-based thermometers.
UiOs
Mechanosynthesis was also effectively used to obtain the Zr- containing UiO-66 MOFs [76–78]. The first mechanochemical synthesis of UiO-66 and their ammonium derivatives (UiO- 66-NH2) was performed via LAG of pre-assembled Zr-based clus- ters {[Zr6O4(OH)4(C6H5CO2)12] or [Zr6O4(OH)4(C2H3CO2)12]}
and terephthalic acid in the presence of a small amount of methanol or DMF as liquid additives (Fig. 12). Ball milling pro- cess conducted for 75 min afforded MOF material with SSA of 1020 m2/g. Identification of the products while optimizing the synthesis conditions was performed by using XRD. The as-prepared MOF showed similar catalytic activity for the degra- dation of the nerve agent simulant, dimethyl 4-nitrophenyl phosphate (DMNP) as that for the solvothermally obtained materials [76]. Karadeniz et al. [77] synthesized UiO-66 and UiO-66-NH2 via water-assisted grinding for 90 min using the pre-assembled dodecanuclear zirconium acetate cluster [Zr12O8(OH)8(CH3COO)24] and terephthalic acid linker, which led to MOFs with SSA up to 1145 m2/g. Highly crystalline mate- rials with face-centered cubic topology and particle size below 100 nm were obtained. Thermal decomposition of samples occurs between 350 and 500°C. In this paper, the catalytic activity of as-prepared MOFs towards degradation of DMNP was examined. Similarly, as in the case of solvothermally obtained materials, the mechanochemically prepared UiO-66- NH2 showed a greater activity in the catalytical decomposition of DMNP than the bare UiO-66. Interestingly, the UiO-66-NH2
sample prepared by Twin Screw Extrusion (TSE) in larger quan- tity (about 100 g) showed an enhanced catalytic activity proba- bly due to better dispersibility in water. Further enhancement of their porosity can be achieved by using extended ligands as linkers. As a consequence of this approach, larger pores are formed in the resulting MOFs [79,80]. For instance, Fidelli et al.[79] demonstrated mechanosynthesis of UiO-67 with an outstanding surface area of 2250 m2/g by utilizing a dodecanuclear zirconium source [Zr12O8(OH)8(CH3COO)24] and 4,40-H2BPA as a linker. The synthesis required small amounts of organic solvent DMF. In situ monitoring revealed that the mechanosynthesis of UiO-67 from the typically used 4,40-H2BPA precursor leads to an intermediate Zr-cluster bearing DMF ligands while the use of a dodecanuclear precursor enables the direct synthesis of Zr-MOFs, without any intermedi- ate. The UiO-67 sample prepared from a dodecanuclear zirconium acetate cluster showed an excellent activity as a catalyst in the degradation of DMNP, even twice higher than that for the catalysts obtained in solution (an initial half-life of 2.5 min and 5 min, respectively).
FIGURE 10
Schematic illustration of solvent- and HF-free syntheses of Cr-based MIL-53 and MIL-88B[70]. Adapted with permission from Ref.[70]. CopyrightÓ2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
RESEARCH:Review
In recent years, a special emphasis has been given toward preparation of functionalized MOFs to enhance their adsorption capacities for various applications, e.g., selective adsorption of hazardous gases. A common modification of MOFs is introduc- tion of nitrogen species. Typically, the preparation of N- containing MOFs relies on the attachment of N-rich functional groups to linkers (most often ammonium groups). N- containing MOFs can be also obtained by milling their precur- sors with a nitrogen source[81,82]. For instance, Hu et al.[81]
obtained N-coordinated UiO-66(Zr) by grinding a mixture of ZrCl4, terephthalic acid and dopamine (N-source, DA) for 30 min at 50 Hz in the presence of liquid additives (Fig. 13).
The as-obtained functionalized MOF was next activated at 200°C under vacuum. This work suggests that N-rich groups are directly coordinated with Zr-O clusters. However, coordina- tion of the amino group with Zr4+is very weak considering the Pearson acid-base concept. Moreover, the coordinated amino
groups would be easily replaced by O-ligands (such as methanol) in the synthesis. Apparently, the dihydroxyphenyl group from N-rich dopamine can coordinate with Zr-O clusters. Neverthe- less, an additional coordination with Zr-O cluster generated defects in UiO-66 leading to higher porosity of the resulting MOFs. For instance, MOF denoted as M-UiO-66(Zr-N3.0), with the mole content of added DA of 3 %, possesses higher SSA of 1550 m2/g and pore volume of 0.50 cm3/g, than the parent material (1220 m2/g and 0.40 cm3/g, respectively). The con- ducted characterization including XRD analysis of the obtained modified MOFs reveals that the DA content influences the parti- cle size and crystallinity of the resulting frameworks. For instance, the M-UiO-66(Zr-N3.0) sample featured particle sizes up to 400 nm and high crystallinity. Importantly, the prepared dopamine-doped MOF particles showed a more regular cubic- like shape compared to the parent material. The successful dopa- mine incorporation was confirmed by FTIR spectroscopy by the FIGURE 11
Schematic illustration of the crystalline phase formation of accordion-like Y-MOF[75]. Adapted with permission from Ref.[75]. CopyrightÓ2019 American Chemical Society.
FIGURE 12
Structures of (a) Zr6O4(OH)412+cluster, (b) terephthalic and 2-aminoterephthalic acids, and (c) schematic illustration of UiO-type MOFs synthesis[76]. Adapted with permission from Ref.[76]. CopyrightÓ2016 The Royal Society of Chemistry.
RESEARCH:Review
appearance of new peaks at 3420 and 1030 cm1on the spec- trum, which correspond to the acrylamide group. Additionally, the content of N and N/Zr molar ratio increase with the increas- ing amount of DA used. N-coordinated UiO-66(Zr) was shown to have higher thermal stability as evidenced by a shift in the start- ing temperature of decomposition from 430 to 459°C. The TG curves recorded for all samples studied show only one weight loss step. Moreover, the N-containing UiO-66 possessed superior chlorobenzene and acetaldehyde uptake capacities of 4.94 mmol/g and 9.42 mmol/g, respectively. Using dopamine as N-source had a positive effect on the volatile organic com- pounds (VOCs) adsorption in contrast to linker modifications.
N-containing UiO-66 was also prepared using other nitrogen sources such as pyrrole (NP-UiO-66), 2-methylimidazole (NI- UiO-66), dopamine (ND-UiO-66)[82]. SSA of the obtained mate- rials ranged from1080 to1480 m2/g, depending on the nitro- gen precursor used. ND-UiO-66 and NP-UiO-66 samples showed the well-arranged architectures and larger particle sizes (346 nm and 362 nm, respectively) compared to those of NI-UiO-66 (257 nm). The FTIR spectra of these MOFs feature typical bands associated with asymmetric and symmetric stretching vibrations of O–C–O in terephthalic linkers (1410 and 1573 cm1) and N–H symmetric stretching (3420 cm1). Furthermore, a sharp peak at 1019 cm1observed on the spectrum of NP-UiO-66 suggests the presence of pyrrolic N groups. Similarly, as in the case of the dopamine-doped UiO-66, the addition of N-rich reagents results in more thermally stable MOF structures. Decomposition tem- peratures for UiO-66, NI-UiO-66, ND-UiO-66 and NP-UiO-66 are 428, 439, 449, and 494°C, respectively. Material with the high- est SSA (NP-UiO-66) exhibited high adsorption capacity for Rho- damine B (384.1 mg/g) and outstanding selective adsorption for a mixture of Rhodamine B and Safranine T dyes, even 223 times higher than parent UiO-66[82].
Other MOFs and MOF-based composites
Mechanochemistry was successfully implemented to synthesize enormous number of other MOFs composed of different metals and ligands[83–99]. For example, an ultrafast mechanochemical synthesis of Ni-MOF was proposed by Wang’s group[89]. One
minute grinding of Ni(OAc)24H2O and H3BTC without any additives was sufficient to prepare MOF crystals. The product showed a good crystallinity and a rod-like morphology. After prolonged milling time, no significant differences in porosity and crystallinity of the obtained products were observed. How- ever, the length and width of the rods depends on the milling time (varied from 10mm and 500 nm to 500 nm and 100 nm, respectively). According to the pore size analysis, the size of micropores in the MOF structure is around 1.7 nm regardless of milling time, while pore sizes in the mesopore range are depen- dent on the milling duration. The Ni-MOF prepared within one minute exhibited a good electrochemical performance with the specific capacitance of 640 F/g at the current density of 1 A/g.
The result is comparable to the performance of the hydrothermally-synthesized Ni-MOF. Friscic’s’ group [87]
reported a mechanosynthesis procedure to obtain MOF-74 from zinc oxide and 2,5-dihydroxyterephthalic acid. Neat grinding afforded MOFs with SSA up to 960 m2/g, whereas using small addition of liquid gave frameworks with higher SSA up to 1145 m2/g. Room-temperature liquid-assisted grinding of Cu (OAc)2H2O and biphenyl-3,30,5,50-tetracarboxylic acid (H4bptc) led to MOF-505 with high SSA of980 m2/g and CO2 uptake capacity of 2.01 mmol/g at 25°C and 1 bar. Additionally, its CO2/CH4 and CO2/N2 selectivities determined by using ideal adsorbed solution theory (IAST) were as high as 5.5 and 26.6, respectively [88]. The XRD analysis indicates that the mechanochemically synthesized MOF-505 shows lower degree of crystallinity compared with its solvothermally obtained coun- terpart. However, the crystallinity of the milling product can be improved by the addition of solvents in the synthesis. The SEM images demonstrate that the MOF-505 synthesized by the mechanochemical method exhibits a semiregular cubic mor- phology and less defined and nonuniform faces and edges in comparison with MOF-505 synthesized by a solvothermal method. TGA measurements show that the MOF framework col- lapses at about 250°C[88].
One of the most extensively explored MOFs is HKUST-1 (HKUST stands for Hong Kong University of Science and Tech- nology) also known as CuBTC. HKUST-1 is composed of copper FIGURE 13
(a) Schematic illustration of N-coordinated UiO-66(Zr) material prepared via mechanochemical method; (b) vapor adsorption isotherms of chlorobenzene [81]. Adapted with permission from Ref.[81]. CopyrightÓ2017 Elsevier B.V.
RESEARCH:Review
nodes and 1,3,5-benzenetricarboxylic linkers. This MOF can be obtained by ball milling of Cu(OAc)2H2O and H3BTC under solvent-free conditions, but adding some liquids to the milling bowl can enhance porosity of thefinal product[100–102].
For instance, Schlesinger et al.[101]obtained CuBTC with SSA of 1119 m2/g via a 20-min NG at 30 Hz. The addition of 1 ml of DMF to the synthesis system resulted in the product with higher SSA of 1421 m2/g after a 30-min NG at 20 Hz. This paper shows that mechanochemical synthesis of MOFs is facilitated by the presence of small amounts of organic liquids. In situ and exsitu monitoring of mechanosynthesis of CuBTC indicated the promi- nent role of liquid additives. The added liquid plays multiple roles, briefly: (i) enters pores stabilizing the resulting framework, (ii) dissociates intermediates, thus promotes activation of MOF, and (iii) accelerates products formation[103–105]. Polar and pro- tic liquids (e.g., alcohols) emerged as the most efficient solvents for a fast formation of CuBTC with a high reaction yield. For example, using methanol afforded HKUST-1 just after 5 min of milling, whereas the addition of other alcohols required a pro- longed milling up to 20 min to obtain the desired product [105]. Interestingly, mechanochemical concepts are utilized not only to synthesize MOFs from substrates (metal sources and organic linkers) but also to e.g., reconstruct structures of degraded MOFs. Sun et al. [103] reported mechanochemical reconstruction of moisture-degraded HKUST-1 (Fig. 14). The degraded material was ball milled in the presence of EtOH/H2O mixture for 30 min, which was sufficient to reconstruct its struc- ture yielding HKUST-1 with similar SSA and benzene capacity as the parent material (95 % of the surface area and 92 % of the ben- zene capacity of the fresh HKUST-1). The reconstructed material showed the same physicochemical characteristics (evidenced by XRD patterns, SEM images, FTIR spectra, and thermal analysis) as the fresh HKUST-1 and better than that of the sample recov- ered by liquid immersion.
Recently, mechanochemical concepts have been imple- mented for preparation of pillar-layered MOFs. Pillaring is an effective strategy to attain three dimensional porous structures from two dimensional layers. Pillared MOFs gain importance
due to their tailored topology and pore sizes as well as enhanced adsorption properties towards guest molecules. For instance, zinc-based pillared MOFs were successfully obtained via a mechanochemical method[106,107]. Chen et al.[107]prepared zinc-based pillar-layered MOF [Zn2(5-aip)2(bpy)] from zinc acet- ate dihydrate, aminoisophthalic acid (5-aip) and 4,40-bipyridine (bpy) using the 2:1:1 molar ratio. The all precursors were milled for 1, 2, 3, 4, and 5 min, then washed with DMF/H2O mixture and dried at 150°C under vacuum for 8 h. Mechanical milling provided MOF particles with high purity and lamellate morphol- ogy showing better crystallinity as evidenced by the XRD analy- sis. Moreover, the obtained MOF exhibited CH4 adsorption uptake of 1.10 mmol/g at 5.0 bar and 25°C and high CH4/N2
selectivity of 7.0 according to IAST (Fig. 15).
Mechanochemistry gives huge opportunities in synthesizing MOF-based nanocomposites just by milling of their components.
Such composites attract an increasing attention because of a syn- ergistic effect of composite components leading to enhanced/
new properties compared to pure MOFs. For instance, graphene-MOF composites are of particular interest due to their high potential in many applications including gas adsorption, catalysis, and energy storage/conversion[108]. Li et al.[109]pre- pared composite of CuBTC and graphite oxide within 30 min of ball milling without adding any solvent. The resulting composite exhibited high specific surface area of 1360 m2/g and toluene uptake of 9.1 mmol/g at 25°C. The high adsorption capacity was even 47 % higher in comparison to a bare CuBTC which makes it a better candidate for applications in thefield of VOCs adsorption. Szczezsniak et al.[110]used mechanosynthesis to pre- pare CuBTC-based three-component composites containing ordered mesoporous carbon and graphene oxide (OMC/GO/
CuBTC). The procedure involved a solvent-free ball milling of tannin and graphene oxide used as carbon precursors and tri- block copolymer Pluronic F127 used as a mesopore-directing agent. The as-prepared composite exhibited SSA of 980 m2/g and CO2 uptake of 5.39 mmol/g. Creating MOF-based compos- ites with graphitic carbon nitride (g-C3N4/MOF) can led to an increased photocatalytic properties compared to single compo-
FIGURE 14
(a) Schematic illustration of solvent-assisted mechanochemical reconstruction of the degraded HKUST-1; (b) isotherms of benzene on fresh HKUST-1, degraded HKUST-1 and reconstructed samples after mechanochemical treatment for 30 min[103]. Adapted with permission from Ref.[103]. CopyrightÓ 2015 The Royal Society of Chemistry.
RESEARCH:Review
nents. Most MOFs show photocatalytic activities only under the UV light range, which limit their potential applications. In case of g-C3N4, the photoinduced electron and hole charges easily recombine affecting its photocatalytic activity. Combining the both photocatalysts can overcome their disadvantages and merge their merits such as high SSA of MOFs and good chemical stability of g-C3N4 materials [111–115]. For instance, Du et al.
[115] reported MIL-100(Fe)/g-C3N4 composite synthesized through a two-step process based on ball milling and annealing.
This composite showed great photocatalytic activity toward reduction of Cr(VI) to Cr(III) and degradation of diclofenac sodium under simulated sunlight. Mechanochemical concept was also employed to prepare host–guest MOF-based systems by encapsulation of various materials, such as enzymes [116,117], drugs [118], Perovskite quantum dots (PQDs) [119,120], Si nanoparticles [121] or polyoxometalate (POM) [122].
It is difficult to precisely control the synthesis of MOFs via mechanochemical methods due to the fast nucleation and limit- ted control over crystals growth[50]. In the case of MOF-based composites, avoiding the heterophase separation originating from high interfacial energy barriers of the components is often an additional challenge [123]. Nevertheless, there are already some works reporting the successful encapsulation/doping of catalytically active species into MOFs during milling or grinding as discussed above.
Application and prospects
Metal–organic frameworks are of special interest due to their high porosity, adjustable pore sizes, variable structures, and hybrid organic–inorganic nature. Through varying inorganic metal nodes and organic linkers a huge number of diverse MOFs can be fabricated. The easy modification of organic linkers gives almost endless opportunities to design MOFs with desired fea- tures for specific applications. Furthermore, a variety of synthetic methods are already known, and some of them seem to comply with principles of Green Chemistry. For instance, mechanosyn- thesis offers new more environmentally friendly avenues for the preparation of MOFs gaining advantages over conventional
solution-based chemical syntheses. Mechanochemistry was suc- cessfully implemented for synthesizing multimetallic MOFs, evenfive metal containing ZIFs via ball milling process. In such structures, metal ions acted as Lewis acids providing higher cat- alytic activity towards CO2conversion into carbonates compared to single-metal-containing MOFs [59]. Nevertheless, it is still challenging to control the distribution of metal centers in these MOF structures. Theoretical studies devoted to ZIFs indicate the possibility of creating a wide spectrum of isocompositional ZIF- 8 (true polymorphs), i.e., materials with the same chemical com- position but different structures[124]. However, conventional solution-based methods usually give only the targeted open sodalite topology (sod) of ZIF-8. Mechanochemistry emerged as a successful strategy to form so far unreported true polymorphs of ZIF-8 with a novel topology named katsenite (kat)[124]. In situ monitoring of the ball milling reaction revealed transforma- tions toward more stable phases leading to different ZIF-8 topolo- gies. In the case of the reaction of ZnO with 2-methylimidazole, ZIF-8 adopts the sod, kat and diamondoid (dia) topologies sequentially[124,125]. This phenomenon is consistent with Ost- wald’s rule of stage. Furthermore, the liquid and/or salt additives can be used to stabilize the topology of the formed intermediate.
Lifespan of intermediates was related to the additives’amount, while their release led to further transformations of the frame- work[125].
A scalable and sustainable synthesis of MOFs is still challeng- ing, and the major known synthetic methods involve the use of large amounts of hazardous organic solvents that contributes to high production costs and environmental pollution. Some mechanochemical methods (LAG and ILAG) also require liquid additives, but only in catalytic amounts. It should be noted that mechanochemical syntheses often involve subsequent washing step, which may require bulk solvents. Nevertheless, it is still a significant reduction of both solvent utilization and accumula- tion of liquid wastes. DeSantis et al.[126]estimated the produc- tion costs of Mg-MOF-75 by using solvent-based methods (solvothermal and aqueous synthesis) and LAG. Utilizing LAG, a high total saving up to 74 % of the material cost and 84 % of the manufacturing cost was achieved compared to solvothermal FIGURE 15
(a) Illustration of mechanochemical method for the synthesis of a pillar-layered metal–organic framework; (b) CH4and N2adsorption isotherms for the MOF obtained during 4 min grinding determined at 15, 25, 35°C[107]. Adapted with permission from Ref.[107]. CopyrightÓ2019 Elsevier B.V.
RESEARCH:Review
