First transition series metal–organic frameworks: synthesis, properties and applications

Sandeep Kaushal *, Gurmeet Kaur , Jasmeen Kaur and Prit Pal Singh * Department of Chemistry, Sri Guru Granth Sahib World University, Fatehgarh Sahib, Punjab, India. E-mail: [email protected]; [email protected]

First published on 28th September 2021

Metal organic frameworks (MOFs) have captured immense attention in the last decade, owing to their better adsorption properties as compared to those of organic as well as inorganic materials, like enormous surface area, highly porous nature, tunable pore size, and high stability. Herein, the regular advancements in MOFs of metals of the first transition series, their morphology, properties, synthesis methods, and numerous applications like gas storage, separation, ion-exchange and catalysis have been discussed. The doping of MOFs and their composites has been classified and discussed, in addition to fabrication of MOFs with various types of organic linkers. MOFs have been discussed for their sustainable and convenient use in future. Various characterization techniques confirmed the rigid, porous and fine structure of MOFs.

Herein, the synthesis, structure and applications of MOFs based on first transition series elements have been discussed. Commonly used organic linkers in the frameworks are fumaric acid, oxalic acid, terephthalic acid, benzene-dicarboxylate, adipic acid, etc. In addition to organic linkers, some ionic liquids are also used for the fabrication of MOF composites. The commonly used ionic liquids for fabrication of composites are N-alkyl-methyl-imidazolium, N-alkyl-pyridinium, tetraalkyl-ammonium, tetraalkyl-phosphonium, etc. The reactions of most of the metal ions with numerous organic linkers are feasible and given in Fig. 1 and 2.

Here, we are considering metals of first transition series for fabrication of MOFs. These elements have a partially filled d-subshell or can result in cations with an incomplete subshell with general electronic configuration (n − 1)d1–10ns0–2. Later in this paper, MOFs of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn have been discussed. The discussion includes the capability of MOFs for the environmental systems that may include sterilization, gas storage and separation, and gas fuel stockpiling. Thus, MOFs show their advantageous role in energy generation and storage. In this way, MOFs are leading to an ecologically sustainable and convenient future.44,45

Similarly, 3D porous bi-metallic frameworks of scandium metal ions along with lithium or sodium metal ions, coordinated to pyrimidine dicarboxylate were synthesized to investigate its capability to arrest CO2 at room temperature. A heat assisted solvent-free process was established to be most effective, yielding materials with nearly the same adsorption capacities as anticipated from Grand Canonical Monte Carlo (GCMC) calculations. The synthesized materials possessed high specific surface area, and systematic distribution of alkaline ions in the crystal structure. The ionic conductivity was also enhanced (10−4 S cm−1) by partial substitution of scandium ions with Li+ and Na+ ions.59 Pillai et al. synthesized and explored a series of functionalized scandium terephthalate MOFs for selective CO2 adsorption. The computational studies revealed that Sc2(BDC-NO2)3 MOFs exhibited exceptional adsorption for CO2 over N2 and CH4, beating the vast majority of MOFs as well as other reported porous materials.

The sample exhibited very good water stability and can be easily regenerated at room temperature under vacuum.60 Khan et al. introduced scandium-triflate (Sc(OTf)3) MOFs and carried out adsorptive desulfurization and denitrogenation of fuels. In comparison to sole MOFs, Sc(OTf)3 MOFs exhibited much better adsorption for benzothiophene (BT) from liquid fuel. Basic quinolone (QUI) was also adsorbed preferentially on acidic Sc(OTf)3 MOFs (Fig. 3). The improved adsorption might be attributed to interactions between acidic Sc(OTf)3 and basic adsorbates. The efficiency of the adsorbent was enhanced by solvent washing and thermal treatment.61

Graham et al. fabricated a scandium terephthalate MOF (Sc2(BDC)3, BDC = 1,4-benzenedicarboxylate) and its derivative (Sc2(NO2-BDC)3). The as-synthesized MOFs were employed for methanol uptake at high pressure. Diffraction experiments revealed that molecules of methanol occupied two sites, and one site was preferentially occupied in comparison to the other. The likely orientation of methanol molecules was controlled by H-bonding.62 Furthermore, Cabello and co-workers utilized variable-temperature infrared (VTIR) spectroscopy to report interactions between carbon dioxide and coordinatively unsaturated MIL-100(Sc) MOFs. The ΔH° value was found to be −48 kJ mol−1 which is the highest reported for adsorption of CO2 on MOFs comprising open metal sites. The corresponding ΔS° value was found to be −178 J mol−1 K−1 which displayed a positive correlation between ΔH° and ΔS°.63 Similarly, Mitchell et al. also reported porous MOFs with scandium and different linkers. The synthesized MOFs were used as catalysts in numerous reactions catalyzed by Lewis acid catalysts which are very significant in organic synthesis but have not been studied using MOF catalysts.64 Arean et al. reported the thermodynamics of hydrogen interaction with MIL-100(Sc) MOF with VTIR spectroscopy. The corresponding values of ΔH° and ΔS° were observed to be −11.2 (±1) kJ mol−1 and −120 (±10) J mol−1 K−1, respectively, in the temperature range of 80–125 K. The enthalpy for adsorption of hydrogen was observed to be the highest on the MOFs reported so far.65

A facile and scalable approach was used to fabricate porous anatase TiO2 and Li4Ti5O12 nano-tablets consisting of nanoparticles, grown from a Ti-MOF template. Both the nano-tablets exhibited outstanding electrochemical properties, and were employed as anode materials for lithium-ion batteries. TiO2 and Li4Ti5O12 delivered capacity up to 228 mA h g−1 and 158 mA h g−1, respectively, at 0.1 A g−1. The outstanding electrochemical properties were attributed to the typical porous structure which augments active sites to lodge lithium ions and enhance ionic diffusion.70 Lian et al. constructed a distinct core–shell titanium-based MOF (Fe3O4@Cys@MIL125-NH2) and applied it as a magnetic adsorbent for fluoroquinolones from aqueous samples. The developed magnetic solid phase extraction (MSPE) method showed large linear concentration range and low LOD (0.05–0.2 μg L−1).71 In another work, Ti-based MOFs were synthesized by addition of tetraethyl orthosilicate. The size of synthesized Ti-based MOFs was less by 42.78% as compared to that with traditional methods. Ibuprofen (IBU) was used to investigate the controlled release property of MOFs. Approximately 95% of IBU was released from MOFs after exposure to a phosphate buffered saline system for 24 h.72 In another study, titanium-based MOF, MIL-125 was sulfated to increase its acidic property. The catalytic performance of raw and sulfated MIL-125 was investigated in esterification of acetic acid. It was observed that approximately 88.2% of acetic acid was converted to ester in 7 h.73 Wang and co-workers synthesized titanium(IV) ion-modified COFs with outstanding performance for detection and identification of trace phospho-peptides in the complex biological samples of milk and HeLa cells.74 It was revealed in another investigation that Ti-oxo clusters are highly selective and efficient towards photocatalysis, CO2 reduction and pollutant degradation due to their high optical response and photo-redox properties.75 In order to enhance the photocatalytic and degradation results, titanium–carboxylate molecular clusters and functional group tolerances of clusters were investigated by adding organic functionalities. During CO2 cycloaddition reaction, 80% of polypropylene oxide gets converted into cyclic carbonates. The degradation studies were performed on methylene blue (MB) dye and complete degradation was achieved in 4 h.76 George et al. reported microwave synthesis of titanium-based MOFs (Ti-BDC). The MOFs were observed to be photo-catalytically active (band gap energy = 3.14 eV), and effectively degraded 96.77% of MB dye in 6 h.77 Furthermore, a hydrothermal method was used to fabricate titanium-based MOFs (porous NH2-MIL-125) having high surface area (1540 m2 g−1). This MOF was successfully employed as a prospective photosensitizer and pH-responsive nano-carrier for cancer therapy, bio-imaging, drug delivery and ROS therapy (Fig. 5). Excellent results were reported during examination of breast cancer (MCF-7) cell line.78

Fe3O4@SiO2@MOF/TiO2 nano-composite was prepared and utilized as an adsorbent for MSPE of tri-azole fungicides in water. The limit of detection was observed to be 0.19–1.20 ng L−1 for the selected fungicides. This method was employed to estimate the concentration of fungicides in real life water samples.79 Water splitting reaction and CO2 reduction was attained with Ti doped MOFs, followed by conversion of light energy into chemical energy.80 Guo and co-workers reported amine-functionalized MOFs (NH2-MIL-125-Ti) which were used to prepare polysulfone-based mixed matrix membranes. The amalgamation of NH2-MIL-125-Ti in the membrane enhances CO2 permeability in comparison to that of the pure polymer membrane.81

Timofeev et al. reported isostructural metal-carboxylates MIL-100(M) and MIL-53(M) (M: V, Al, Fe and Cr) which were used in catalysis of condensation reaction of glycerol with acetone. It was observed that the materials with higher vanadium content exhibited enhanced activity and reusability at room temperature.85 Wang et al. fabricated microporous MIL-47 frameworks VO (1,4-benzenedicarboxylate) with CS2, tetrahydrothiophene, thiomorpholine and thioxane. The channel opening of framework was significantly expanded by intercalation with CS2 by breathing deformation mode, in contrast to numerous other molecules that typically lead to contraction of channel opening. With incorporation of CS2, maximum spherical void diameter increases to 8.0 Å from 7.6 Å of VO (1,4-benzenedicarboxylate) with empty channels.86 The development of more stable novel materials is required as extensive leaching of active V species is observed in vanadium-based solid catalysts in liquid phase oxidation reactions. Consequently, Kim et al. synthesized V-based MOFs by dispersion on a carbon support, of vanadium oxide and carbide species by pyrolysis of MIL-47(V) (Fig. 6). A number of carbon catalysts containing different surface and bulk phases of V were synthesized by affecting the phase transitions of V by altering the pyrolysis temperature. Vanadium carbide catalysts displayed better performance and increased stability as confirmed by lesser amounts of V leached (65%, and after 10 catalytic cycles, the activity was found to be 60% over 6 h. This synthesis of highly dispersed compound is an economically convenient, surfactant free and green synthesis.151 Anbia and co-workers synthesized iron terephthalate, MOF-235 hydrothermally that was used for gas adsorption. Among CH4, H2 and CO2 gases, maximum adsorption capacity and selectivity was shown for CH4. The presence of more open metal sites and large pore volume were responsible for better adsorption of CH4 on MOF-235. The higher selectivity of MOF for CH4 as compared to other two gases, advocated that MOF-235 was a good adsorbent for separation of CH4 from gaseous mixtures.152 The removal of methyl orange, methylene blue or other harmful dyes from contaminated water was carried out via adsorption on iron terephthalate (MOF-235). It was observed that adsorption of dyes was a spontaneous and endothermic process. Furthermore, with the adsorption of MB and MO dyes, entropy also increased.153

Furthermore, intensification of catalytic activity of cobalt-based MOF, Co-MOF-74@NDHPI doped with NDHPI under mild conditions was detected. The modification resulted in high selectivity of the catalyst for oxidation of toluene where molecular oxygen acted as a primary oxidant. Experimental studies showed the successful synthesis of benzoic alcohol, benzaldehyde and benzoic acid with 18%, 48% and 34% selectivity, respectively and conversion of toluene was found to be 16%.155 Various electrochemical techniques were employed for insertion of titanium carbide in cobalt-based MOFs on nickel foam. The advantages of titanium carbide-based Co-MOF as the binder free supercapacitor electrode, increasing active sites and promoting fast ion transport, provide a new way for further research on such composites as it has high gravimetric capacitance.156 A distinct MOF with 2-D structure was prepared by coordination of cobalt ions with bis-carboxyphenyl-triazole and bis-imidazolebenzene. On interaction with graphene, the MOF gave well organized electro-catalytic hydrogen evolution reaction (HER) with overpotential of 125 mV.157 An investigation on the convenient ultrasonic technique for synthesis of cobalt terephthalate-based MOF nano-sheets having oxygen vacancies was carried out. The synthesized novel MOFs exhibited higher anodic performance that enhanced their application in sodium ion batteries. The improved working of batteries was attributed to increased ion dispersion rate and reversible sodium ion storage by the induction of electric field through oxygen vacancies.158 Jiang et al. fabricated a single phase Co/Mn-MOF-74 composite by a hydrothermal method. The catalytic activity at low temperature was demonstrated by using it as a control catalyst for industrial NH3-SCR NOx degradation with 96% efficiency. Studies showed that morphology of Co/Mn-MOF-74 was affected by the incorporation of Co ions.159 The MOFs are also used as outstanding electrode materials in supercapacitors. Co/Ni-MOF and Ni-MOF was fabricated hydrothermally by altering the proportion of nickel. The MOF with a dandelion like hollow structure displayed splendid specific capacitance with 75% activity retention after 5000 cycles. Experimental studies showed enhancement in electrochemical properties and improved morphology of the MOFs.160 High porosity and surface area make the MOF a fascinating electrode material for energy storage and conversion. Zhu et al. reported a layered Co-based MOF on a nickel foam support (Co-MOF/NF) which was used as a high-performing electrode in supercapacitors. Furthermore, Co-MOF/NF demonstrated a very high specific capacitance (13.6 F cm−2) at 2 mA cm−2 in 2 M KOH which was higher in comparison to other MOFs.161

Yu et al. fabricated a Co-based MOF that was carbonized and then acid etched to get mesoporous carbon, to use it as a lithium–sulfur (Li–S) battery cathode scaffold. After etching, mesoporous carbon displayed an enhancement in porosity and/or in sulfur-loading amount. In comparison to the composite-1 cathode, composite-2 (1C = 1675 mA g−1) delivered a high discharge capacity of 925.1 mA h g−1 in second cycle and maintained a specific value of 781.1 mA h g−1 in 140th cycle.162 Enzymes, the catalysts of nature, are highly selective and efficient. In recent times, MOFs have attained great consideration as an interesting porous support for immobilizing enzymes. However, their handling and separation is still challenging, owing to less density and higher dispersion of enzyme-MOF composites. The magnetic-MOF can be a probable contender due to its advanced structural design, coupled with magnetic characteristics. Definitely fabricated magnetic-MOFs have unique properties like adjustable composition, huge surface area and speedy collection, making them a prospective candidate for immobilizing enzymes.163 In recent years, MOFs have been considered advantageous materials for electrocatalysis. However, electrocatalytic performance and stability of synthesized MOFs was diminished by their low conductance. Murthy and co-workers prepared a cobalt hydroxide functionalized multi-walled carbon nanotube (MWCNT) composite with improved electro activity using Co-MOF, ZIF-67 templates. The composite was used in the fabrication of a MWCNTs-Co(OH)2 modified glassy carbon electrode (GCE) for estimation of hydrazine and hydrogen peroxide. The MWCNTs-Co(OH)2/GCE based sensor demonstrated linear range, detection limit and sensitivity, comparable with that reported earlier for sensing of hydrazine and hydrogen peroxide.164 The enhanced activity, selectivity, stability and synergic effect of dissimilar components than single component earned extensive attention. Yu et al. fabricated mesoporous hybrid dual-metal MOFs (Co/Fe-MOF, Fe-MOF, and Co-MOF) and studied their electrochemical properties in an ionic liquid (IL)/supercritical CO2 (SC)/surfactant emulsion system. It was revealed that due to coexistence of two metals, Co/Fe-MOF displayed 4 times higher specific capacitance (319.5 F g−1 at 1 A g−1) as well as superior cycling stability compared to that of single metal Co-MOF and Fe-MOF.165 In alkaline medium, electrocatalysis of glucose was performed with Co-MOF by using cobalt ions and terephthalic acid on nickel foam. Selective and sensitive examination of glucose through the production and development of non-noble metal nano-array arrangement was found to exhibit 10886 μA mM−1 cm−2 sensitivity and 1.3 nM detection limit. The results revealed that a Co-MOF/NF compound has long-range stability and powerful reproducibility.166 Owing to the ill use of antibiotics in creature farming and horticulture, it is profoundly important to estimate anti-microbials in biological systems by a straightforward, easy, and rapid strategy. Liu et al. fabricated a novel nano-construction, Co-MOF@TPN-COF using Co-MOF and terephthalonitrile-(TPN-COF) which was utilized as an aptasensor for sensing commonly used β-lactam antibiotic, ampicillin (AMP). The aptasensor exhibited a very low detection limit of 0.217 fg mL−1 in concentration range 1.0 fg mL−1–2.0 ng mL−1.167 The electrode property of cobalt MOF was enhanced by doping it with heteroatoms which can be further used in lithium sulfur (Li–S) batteries. By arranging the activation time and addition of a small number of clews of polymer nano belts, physiochemical properties of the doped MOFs were altered. The areal mass loading was enhanced by co-doping of the MOF defined carbon arrangement which contains cobalt and oxygen containing groups in matrix medium. This hybrid material showed transformation in activity efficiency from 1300.69 to 1020.6 mA h g−1 for 0.1 and 0.5 current density after several catalytic revolutions.168 The novel microporous and mesoporous cobalt-based MOF was synthesized in reduced reaction time with a simple method at low temperature. The as-synthesized lithium-ion batteries electrode displayed a high discharge capacity and respectable cyclability with a high coulombic efficiency. The micropores and mesopores of particles in materials provide sufficient space for loading and diffusion of electrolyte which is responsible for enhanced electrochemical performance.169 A slow evaporation technique was used for the preparation of cobalt pyridine-dicarboxylate MOF. L-Cysteine was detected with the help of cobalt pyridine-dicarboxylate. High resistance was used for observation of signals at pH 4. The slow evaporation technique was used for preparation of {(C5H5NH)[Co(C7H4NO4)(C7H3NO4)]·(C7H5NO4)·(H2O)}n composite bonded through covalent and non-covalent bonds.170 Mehak et al. investigated electrochemical oxidation of methanol in an alkaline medium employing Co-MOF and its composites with GO (Co-MOF-71/GO). The composite presented exceptional peak current density (29.1 mA cm−2) at lower potential (0.1 V). The boosted activity and stability was accredited to the synergy between the MOF and GO substrate.171 In another study, Co-MOF designed by coordination of cobalt ions with benzene-tricarboxylic acid in the presence of tri-ethylamine and nonanoic acid Co@NPC-900 displayed many advantageous properties like large surface area, proper pore sizes, highly ordered arrangement and diverse constitutions with various organic linkers, and was utilized for catalytic reaction of oxygen reduction and oxygen evolution.172 Two types of nano-crystals of Co3O4 were designed by indirect solid state thermolysis of cobalt terephthalate MOF (MOF-71). The MOF possessed an energy band gap of 3.7 eV and displayed outstanding stability by retaining 85% capacitance after 2000 revolutions, excellent energy density and electrochemical properties.173 DFT calculations were performed for investigation of M-MOF-74 (M = Mg, Co or Mn). Experimental data indicated structural design, band gap energies and many other properties of MOFs. The results showed that primarily, ionic bonds and covalent bonds were observed between the metal cations and oxygen atoms. Cobalt and magnesium MOFs also possessed hydrogen bonds.174 Choi et al. reported isostructural 3-D, Co(II) and Cd(II) MOFs using rigid 1,4-naphthalenedicarboxylate (1,4-NDC) as linkers. The 3-dimensional framework was generated by coupling of 1-dimensional chains which were formed by the bridging of metal ions with ligand. The performance of synthesized MOF as a catalyst in cyanosilylation reaction of aromatic aldehydes was investigated, and approximately 50% improved results were obtained compared to previously reported data.175 The attraction for MOFs has increased owing to their probable use in high efficiency energy storage. Liu and co-workers synthesized cobalt based MOFs using tetrafluoroterephthalic acid as a linker. The MOF, with large specific capacitance and splendid cycling stability, was used as an electrode in supercapacitors (Fig. 16). The properties of Co-MOF like enough space for storage, suitable pore sizes and dispersion of electrolytes lead to its outstanding electrochemical behavior. The electrode had a specific capacitance of 2474 F g−1 at 1 A g−1 current density, and about 94.3% capacity was retained after 2000 cycles.176

Co-MOF was used as a catalyst for oxidative desulfurization reaction under mild conditions (Fig. 17). Cobalt MOFs were fabricated solvothermally by the coordination of cobalt metal ions with dicarboxylate ligand. Model oil to be desulfurized, was generated by mixing dibenzothiophene in n-hexane. Production of the corresponding sulfone from oxidation was confirmed by FT-IR and mass analysis techniques, and 500 ppm of sulfur was removed with 75.2% efficiency in 8 h.177

Trends in structural, catalytic and magnetic properties of microporous cobalt MOF, [Co3(lac)2(pybz)2]·3DMF and its derivatives prepared by ligand exchange or solvent exchange were investigated. The effect of water, DMF and other solvents on the crystal structure of MOF was studied by crystallographic technique. Upon interaction with water molecules, adsorption efficiency of Co-MOF for CH4, CH3OH, C6H6 and I2 gets reduced whereas post synthetic changes in structure showed transformation from antiferromagnetic to ferromagnetic.178 Response of MOFs towards adsorption of gases, separation and even storage of gases in both humid and aqueous medium was studied. Almost all the MOFs are unstable in an aqueous medium whereas few such MOFs are known which show water stability. XRD, BET and N2 adsorption studies were carried out to determine the stability of MOFs, and a useful collaboration of data was reported for advancements in MOFs.179 The supercapacitor properties of cobalt MOF, synthesized from cobalt metal ion and benzene-dicarboxylate in DMF solvent, were investigated. Porous cobalt hydroxide was produced by the surface hydrolysis of Co-MOF in a basic medium. The newly formed porous cobalt hydroxide is active, and has excellent pseudo capacitance and electro-catalytic activity. Large nano-scale porosity was introduced into the products because of conformal transformation.180 In current energy gadgets, efficient hydrogen evolution by electrocatalytic water splitting offers a lot of potential. Wang et al. reported that localized surface plasmon resonance (LSPR) excitation of Au nanorods efficiently boosted the hydrogen evolution property of CoFe-MOF nanosheets. Studies performed suggest that improved hydrogen evolution is mostly due to the injection of hot electrons from Au nanorods into CoFe-MOF nanosheets which raises the Fermi level of CoFe-MOF nanosheets, allowing for the reduction of H2O and a lower activation energy for HER.181

The coordination of nickel metal ion with terephthalic acid was carried out by solvothermal synthesis. The synthesized composite had a 3-D flower like structure, and its chemical stability and electrochemical activity were enhanced by ultrasonic method. A non-enzymatic electrochemical glucose sensor, with a detection limit of 4.6 μM over a large concentration range (20 μM–4.4 mM), was developed by the modification of Ni-MOF with a glassy carbon electrode.185 The synthesis of a stable crystalline 3-D mesoporous network by coordination of a metal ion with poly-dentate organic linkers was performed through a solvothermal pathway. Nickel metal ions were coordinated with terephthalic acid using DMF solvent. X-ray studies were carried out to evaluate the crystalline nature and crystallite size. Catalytic adsorption of NO2 with different molar ratios was studied to analyze the composition of designed MOFs. The band gap was observed to be approximately 3.94 eV from UV-vis studies.186 Oxidation of tetramethylbenzidine in the presence of H2O2 was examined with Ni-MOF. Nickel MOF is also useful for food environment analysis and in clinical medicine. In addition, a Ni-MOF nanosheet was used to design a H2O2 colorimetric sensor having a detection limit of 8 nM over a large concentration range (0.04–160 μM).187 Hierarchical porous nickel MOF, with a perfect pore size and specific surface area was fabricated by the coordination of nickel ions with trimesic acid. By varying the concentration of trimesic acid and nickel ions, different morphologies of nickel MOFs were obtained. The studies revealed that Ni-MOF has powerful specific capacitance of 649 F g−1, electrochemical properties, 63.4% rate capability and 70% retention of activity.188 Microwave accelerated hydrothermal method was employed for the polymerization of coordinated 3-D MOFs of nickel metal (Ni-MOF). The experimental investigations indicated excellent surface area (5131 m3 g−1), presence of drilling open pores with radius 4.2–10.1 Å and stabilization of a 3-D structure by various hydrogen bonds (inter and intra-molecular).189 The low biodegradability of industrial dye effluents makes them profoundly harmful and carcinogenic for both human and aquatic lives, and thus, they are considered unfavorable for the biodiversity of environment. Ezugwu et al. reported the capability of cationic Ni-MOFs for adsorbing charged and neutral dye molecules. Nickel metal ions and bis-carboxyphenyl-imidazolium chloride were used for the synthesis of cationic nickel MOF which was used for preferential elimination of methyl orange (81.08%) and congo red dyes (98.65%) by adsorption.190 An investigation of the synthesis of porous Ni2P/C composite from microwave accelerated hexagonal rods of nickel MOF was carried out. The newly prepared composite has long-term stability, highly porous arrangement, powerful electrocatalytic activity and strong electrical conductivity. The hydrogen evolution reaction (HER) was catalyzed by the electrocatalytically active Ni2P/C composite at a potential of −64 mV and possessed capacity for long term stability.191 He et al. reported hexagonal micro-rods of MOF-74-Ni prepared with a fast and facile microwave-assisted method and applied the material as a predecessor for the generation of a porous Ni2P/C composite. The as-synthesized Ni2P/C displayed exceptional electrocatalytic activity for the HER. The structure of generated NiO can be optimized by varying some features of nickel MOF such as pore size, thermal behavior, specific surface area and intrinsic structural properties.192

Synthesis of MOFs with nickel, cobalt and zinc ions, using benzene-tricarboxylic acid as a ligand, was carried out for the determination of the morphological effect on electrochemical applications and electrochemistry. XRD, FT-IR, SEM and electrochemical techniques were employed for determination of structure. Studies revealed that three MOFs have some properties similar to each other. Investigation on ponceau-4R gave the detection limit of 80 pM in 0.5–150 nM concentration range.193 The gravimetric-type gas sensing application of Ni-MOF-74 was explored with resonant cantilever sensors in the ultrasensitive examination of carbon monoxide. Ni-MOF-74 with nanoporous structure and huge surface area holds great adsorption capacity for CO gas and it might be due to definite interaction among Ni2+ in Ni-MOF-74 crystal and CO molecules (Fig. 19).

It has been observed that the gravimetric sensor developed by doping of Ni-MOF-74 on the resonant microcantilever, exhibited highly sensitive detection of CO at a trace level with less than 10 ppb (parts per billion in volume) detection limit. In addition, the sensor also exhibited outstanding reproducibility and long lasting stability.194 In addition to copper or iron-based MOFs, nickel MOF has high thermal stability for the catalytic activity of NH3-SCR. Heat treatment in N2 atmosphere was carried out to enhance its catalytic performance and 92% of catalytic NO conversion was achieved in temperature range of 275–440 °C.195 Wang et al. synthesized a flower-shaped MWCNTs/nickel-trimesic acid composite (MWCNTs@Ni/TA) by a solvothermal method. It was revealed that the existence of carboxyl functionalized MWCNTs helped to transform the shape of solid spherical Ni/TA. Electrochemical studies have shown that in comparison to spherical Ni(TA), flower-shaped composite achieved higher specific capacity as well as improved rate capability.196 In another study, a flexible carbon cloth substrate was employed to grow Co3O4/Ni-based MOFs by a two-step hydrothermal method. The synthesized MOFs were employed as electrode materials and exhibited a brilliant specific capacity and good cycling stability.197 The effectively peeled nanosheets from a conductive 2D-MOF Ni3(HITP)2 were found to be a proficient co-catalyst for CO2 decrease in a cross breed photocatalytic framework under visible light brightening, exhibiting 97% selectivity and 3.45 × 104 μmol g−1 h−1 yield of carbon monoxide. Active sites for redox reaction and powerful conductivity for charge transfer are advantageous. This work gave fundamental insights into future structure and advancement of more powerful MOFs for CO2 reduction.198 Nickel-based Ni-MOF-74 was blended by a solvothermal strategy. Ni-MOF-74 showed excellent catalytic properties for immediate arylation of azoles by means of C–H initiation while other Ni-based MOFs, nickel-based heterogeneous frameworks, and homogeneous partners showed lower catalysis.199 Tran et al. synthesized a Ni-MOF and MWCNTs based Ni-MOF/MWCNTs catalyst for non-enzymatic urea detection. The electrode with Ni-MOF/MWCNTs presented a tremendous sensitivity (685 μA mM−1 cm−2, LOD 3 μM) with 10 s response time.200 Qu et al. reported nickel-based MOF, [Ni(L)(DABCO)0.5] where L represents a functionalized BDC (1,4-benzenedicarboxylic acid) linker and DABCO represents 1,4-diazabicyclo [2.2.2]-octane. MOF capacitors exhibited brilliant performance as electrode materials. The Ni-DMOF-ADC electrode had a specific capacitance of 552 at 1 A g−1 current density and 438 F g−1 at 20 A g−1 current density.201 Xu et al. established a superficial and effective one-step solvothermal method for the synthesis of a magnetic and porous Ni@MOF-74(Ni) [Ni2(DOBDC)] [(DOBDC = 2,5-dihydroxyterephthalate)] composite. The as-prepared Ni@MOF-74(Ni) composite showed magnetic characteristics as well as high porosity, making the composite a competent contender for dye removal and targeted drug delivery systems. The composite displayed an adsorption capacity of 177.8 mg g−1 and 4.1 mg g−1 for rhodamine B and ibuprofen, respectively.202 A nickel(II) pillared paddle wheel MOF was prepared via amalgamation of solvent-assisted linker exchange and transmetallation. The MOF showed boosted N2 sorption performance in comparison to the isostructural Zn(II) counterpart.203 Because of their ultrahigh activity and selectivity, single-atom catalysts (SACs) are of tremendous interest. As it is challenging to make model SACs using a generic synthetic technique, distinguishing between the activity of different single-atom catalysts is difficult. Beginning from multivariate MOFs, a comprehensive technique for the production of single-atom metals implanted in N-doped carbon (M1–N–C; M = Fe, Co, Ni, and Cu) has been established. When used in electrocatalytic CO2 reduction, Ni1-N–C demonstrated a very high CO faradaic efficiency (FE) of up to 96.8% which considerably outperformed other materials.204

Lincke et al. prepared and explored the crystal structure of a copper-based MOF. PXRD technique was used to confirm phase purity of the material whereas TD-PXRD and coupled DTA–TG–MS analysis confirmed its stability up to 230 °C.207 The catalytic property of copper-based MOF (Cu-MOF-74) was also examined. An acid catalyzed reaction of anisole was carried out for its Friedel–Crafts acylation. Effects of various parameters like temperature, acylating agent and solvent were studied on Cu-MOF. The MOF exhibited an extraordinary catalytic performance for anisole alteration and p-MAP yield. Furthermore, the structural MOF-74 phase was conserved when dilute acetyl chloride was used as an acylating agent.208 In another work, a graphene doped copper-based MOF composite was fabricated and employed for adsorption of ammonia at room temperature, under dry and moist conditions. In a dry environment, composites with lesser quantity of GO were adjudged improved adsorbents of ammonia in comparison to those having more quantity of GO. In moist environments, adsorbed ammonia caused the breakdown of MOF structure and release of active groups.209 The sonochemical synthesis of guanine-Cu-MOF was reported by using copper metal ions and biphenyldicarboxylic organic ligand, followed by doping of guanine. Their effect on oprD gene expression was determined by a broth micro-dilution method. Minimum inhibitory concentration (MIC) was observed to be 400 μg mL−1, and minimum bactericidal concentration was 400 μg mL−1 of P. aeruginosa strains.210 Some other transition metal-based MOFs reported for various catalytic activities are listed in Table 1.

In a study, synthesis of zinc-based MOFs through green and facile synthetic methods was discussed. The nano MOF containing biocompatible metal ions, i.e. zinc(II) and benzene 1,3,5-tricarboxylic acid (H3BTC) as a linker to form [Zn-3(BTC)(2)] using electro- and sonochemical methods was compared. The electrochemical method generated a bigger particle size (ca. 18.43 ± 8.10 μm) than the sonochemical method (ca. 87.63 ± 22.86 and 112.23 ± 28.87 nm, for 30 and 60 min, respectively). Slow-release of ibuprofen was conducted in a phosphate buffered saline system at 37 °C and pH 7.4.229 Yadav et al. reported zinc-containing MOF-5(Zn) and MOF-5W(Zn) systems, followed by incorporation of silver which were employed in electrochemical oxidation of accumulated nitrophenols. The electrochemical and structural characterization revealed that silver is present in Ag@MOF-5(Zn) in metallic form. The incorporation of silver changed the electrochemical oxidation performance towards nitrophenols in MOF-5(Zn) from “inactive” to “active”.230 Wang et al. synthesized monodisperse MOF-5 crystals having tunable pore sizes and shapes. The synthesis comprised base-tuned nucleation and surfactant-modulated facet development of MOF-5. Under augmented reaction conditions, monodisperse MOF-5 cubes, truncated cubes, truncated octahedra, and octahedra were obtained with high reproducibility.231 Mohammad et al. investigated adsorption of aqueous tetracycline (TC) by MOF-5, synthesized at room temperature. The MOF has cubic arrangement, 84.3% crystallinity, thermal stability up to 450°C and about 2510 m2 g−1 BET surface area with both the micro- and meso-cavities. The as-synthesized material was employed for adsorptive removal of TC antibiotics, and nearly 96% of TC was removed successfully (adsorption capacity = 233 mg g−1).232 Mixed matrix membranes (MMMs) were fabricated by the introduction of MOFs, to investigate their separation properties for H2, CO2 and CH4 gases. There was no direct connection between the gas adsorption capacities of MOFs and degree of permeation of these gases over the MMMs. The most adsorbed gas (H2) infused gradually over MMMs which could be used for separation of H2 from CO2.233,234 In summary, MOFs have shown various applications such as energy storage, catalytic degradation, electrical conductivity and charge storage. The green and facile synthesis method was used for the preparation of zinc-p-phenylenediamine which was used for energy storage.235 The liberation of U(VI) in the atmosphere with extensive application of radionucleotides has been established. The MOF of zinc was formed by the solvothermal method, and used for the sorption of U(VI) which is spontaneous and endothermic, contingent upon pH and free from ionic strength. The researchers showed the sorption capacity (237) at pH 5 and room temperature. FTIR and XPS showed attribution to oxygen containing functional groups.236 The regulation of selectivity towards aldehydes in aromatic alcohol oxidation utilizing molecular oxygen under gentle conditions remains a major concern. Chen et al.237 fabricated Pt/PCN-224(M) (M = Zn, Ni, Co, Mn and 2H) composites by incorporating Pt nanocrystals and porphyrinic MOFs. Due to the strongest diamagnetism with d10 configuration of Zn2+ among different metals existing in a porphyrin center in the MOF, PCN-224(Zn) demonstrates the greatest ability for O2 activation and 1O2 production among all the PCN-224(M) MOFs investigated.