INTRODUCTION:
The study of MOFs (Metal Organic Frameworks) is one of the fields of chemistry that is rapidly developing because of their functional and structural tunability1. MOFs' enormous variation has drawn a lot of attention within the last ten years.
In theory, the characteristics of a known MOF can be systematically altered by switching out the functional group of an organic linker or a different type of metal2. MOFs, also known as porous coordination polymers (PCPs), are a promising family of materials with high porosity, variable composition, and tuneable pore topologies. They are made up of inorganic nodes connected by organic linkers3,4. These properties have sparked substantial research interest in a wide range of domains, including catalysis5, biomedicine6, sensing7, gas adsorption and separation8,9. To increase the catalytic, electrical and luminous properties of MOFs, it has been suggested to insert second metal ions into framework nodes to create bimetallic MOFs. The bimetallic system demonstrates synergistic effects because of the partial substitution of second metal ions in the secondary-building units (SBUs) or inorganic nodes of the framework. The metal proportions in bimetallic MOFs may be altered or even regulated, allowing us to modify the physicochemical features of bimetallic MOFs10,11. Bimetallic MOFs with tunable compositions and structures outperform their monometallic counterparts in a variety of applications, including catalysis, gas adsorption, luminescence sensing, energy conversion and storage12-14. Bimetallic MOFs may be manufactured using a variety of methods, including direct synthesis, one-pot synthesis, post-synthetic modification, template synthesis, seed-induced growth and post-synthetic exchange.
Water is an indispensable element on Earth, playing a vital role in the survival of all living things. Water covers around 70% of the surface of the Earth and can be found in many different forms, including lakes, rivers, glaciers, and seas15. Nitroaromatic compounds (NACs) are important anthropogenic pollutants, thus their breakdown has received a lot of attention. NACs are carcinogenic, tumorigenic, toxic, genotoxic or reproductive-toxic; hence, their widespread presence in all domains of the environment and numerous waste sources, as well as their persistence, constitute a serious danger to human and animal life. As a result, significant efforts have been made to successfully repair NACs16. Catalytic reduction under mild circumstances is especially essential in this regard because NAC catalytic reductions produce aromatic amines (AAs), which are excellent chemical products and crucial building blocks for large-scale pharmaceutical production. Therefore, there has been a lot of interest in creating novel catalysts that improve the efficiency of NACs reduction. 2-nitroaniline (2-NA) is a pollutant that is released into the environment as a consequence of human activities such as the manufacturing of colours, explosives and medications. It is occasionally discovered in the waste effluents of textile and military sectors, as well as being utilized as a precursor for the production of latex, pesticides and medications. Owing to its extremely poisonous, carcinogenic and mutagenic properties, 2-NA is detrimental to aquatic life as well as human life, even at low concentrations in water17. Natural abundance builds up in food chains, upsetting the natural order. 2-NA has also been designated by the US Environmental Protection Agency classifies it as dangerous waste and a priority toxic pollutant. Consequently, more work is needed to create efficient methods for removing 2-NA from additional sources including industrial wastes. Adsorption, thermal breakdown, photo catalysis, biological degradation, electrochemical degradation and catalytic reduction are among the methods used to remove or degrade 2-NA. However, catalytic reduction is superior to other ways since the catalytic reduction product, 2-aminoaniline (2-AA) or o-phenylenediamine (O-PDA), has numerous applications in various sectors. O-phenylenediamine (O-PDA) is utilized as a precursor in the production of a wide range of goods, including medicines, colours, poultry products, antioxidant and antibacterial agents, surfactants, certain polymers, and fine chemicals18. It has been discovered that 4-nitroaniline (4-NA), an aromatic amine that is widely utilized as an intermediary in the synthesis of numerous industrial and high-volume compounds, including insecticides, is a pollutant in agricultural biosolid that is produced from sludge from industrial effluent. 4-NA is a common environmental pollutant that can reach concentrations of up to 100 mg L-1, particularly in agricultural soil where it is applied as a fertilizer component and naturally occurs as a result of pesticide transformation. Its chemical stability, persistence and toxicological impacts on human health and living things, even at low concentrations, have made it one among the main priority pollutants that needs to be treated19. Since the product 4-Aminoaniline (4-AA) or p-phenylenediamine (P-PDA) is a component of engineering polymers and composites like Kevlar, catalytic reduction is recommended. It is also occasionally used in place of henna as a component in hair colours. Para-nitrophenol (4-NP), also known as 4-hydroxy nitrobenzene, is regarded as a dangerous contaminant. When 4-NP is dissolved in water, a somewhat acidic aqueous solution is produced. Therefore, 4-NP is an essential chemical component of many industrial processes, such as those in the pulp and paper mill, oil refineries, dye and paint, pesticide, petrochemical, and pharmaceutical industries. As a result, 4-NP is present in the effluents produced by these businesses. The 4-NP-containing effluents will thereafter be dumped into drinking water, rainwater, and soil. In humans, 4-NP enters the body through the gastrointestinal tract, skin, or lungs by light droplets. Because of its presence in the blood, haemoglobin is converted to methaemoglobin, which results in anaemia, liver damage, palpitations, and other associated symptoms. Furthermore, it has been claimed that 4-NP irritates the eyes and causes ataxia, headaches, nausea, and tiredness when consumed or inhaled. The US Environmental Protection Agency recently revealed that 4-NP is one of the most toxic, bio accumulative and indestructible chemical compounds, having negative effects on both humans and animals even at very low doses. Consequently, it is crucial to focus more on studies that deal with 4-NP reduction or removal in aqueous media20. Since 4-Aminophenol (4-AP) is more biocompatible than 4-NP, catalytic reduction is the preferred way of elimination among the available techniques. For the production of medications such analgesics and antipyretics, the product 4-AP is a crucial intermediary21.
Numerous Cu-Ni Bimetallic MOFs, such as Nix Cu(3-x) (HITP)2 MOFs22, Ni-Cu-BTC23, Ni-Cu-BDC24 and so on, have been synthesized. Ni-Cu bimetallic MOF was produced with EDTA acting as the ligand. To my knowledge, this is the first literature on the Cu-Ni-EDTA Bimetallic MOF (BMOF) that has been published to date. Fourier Transform Infrared Spectroscopy (FT-IR), Powder X-ray Diffraction (PXRD), Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (FESEM-EDS), Brunauer-Emmett-Teller (BET), X-ray Photoelectron Spectroscopy (XPS) and Thermogravimetric Analysis (TGA) are used to further characterize the synthesized Cu-Ni bimetallic MOF.
MATERIALS AND METHODS:
Materials:
All chemicals were acquired from well-known businesses such as Coastal Chemicals and Sigma-Aldrich Chemicals and utilized without additional purification. The ethanol used to wash the Cu-Ni-EDTA Bimetallic MOF is A.R. grade. The solutions were produced with water that had been doubly distilled. To eliminate the potential of contamination, each piece of glassware was thoroughly cleaned and rinsed with double-distilled water before being dried in an oven before use.
Synthesis of Cu-Ni-EDTA Bimetallic MOF (BMOF):
To begin with, precisely weigh 2mM of copper nitrate trihydrate, 2mM of nickel chloride hexahydrate and 2mM of disodium salt of ethylenediamine tetra acetic acid using a digital weighing balance. Then, transfer the weighted materials into a 100ml reagent bottle. The contents of the reagent bottle should be dissolved by adding 5ml of double-distilled water. Now, add 35 millilitres of dimethyl formamide (DMF) to the mixture. Take a pH paper and note and measure the pH of the solution. The solution's pH needs to be between 6 to 8. If not, add 0.1N NaOH or 0.1N CH3COOH to adjust the pH. Using a hotplate and magnetic stirrer, mix the contents of the reagent bottle for 120 minutes to obtain a transparent solution. The solution is green at first, but eventually turns blue. After filling the reagent bottle, place it in the hot air oven set at 150°C for ten hours. The 100ml reagent bottle should be removed from the hot air oven after 10 hours. Centrifuge the precipitate after removing the 100 ml reagent vial from the hot air oven. To get rid of any contaminants that may have dissolved in water or an organic solvent, wash the residue four to five times with double-distilled water and then four to five times with ethanol. After letting the residue dry overnight at 70°C in a hot air oven, crush it into a fine powder.
Characterizations:
The Bruker D8 Advance X-Ray Diffractometer was employed for the Powder-XRD measurements, Bruker alfa-II was used for FT-IR, Quanta FEG 250 was used for FESEM-EDS, Hitachi STA 7300 model was employed for TGA, AXIS SUPRA X-Ray Photoelectron Spectrometer was used for XPS, Quantachrome NOVA touch 4 LX instrument was used for BET analysis using N2 isotherms at 77 K and the Cary 60 UV-Visible Spectrophotometer was used to observe the reduction of Nitroaromatic Compounds (NACs).
Catalytic Performance:
To create a deep yellow solution, first add 25 ml of an aqueous 1 mM nitroaromatic compounds (NACs) solution to a beaker. Next, mix with 100 mM of a recently made 25 ml aqueous NaBH4 solution. The yellow solution mentioned above was then mixed with 2 mg of the catalyst, and the reaction was continued until the solution lose its entire colour. By monitoring the reaction mixture's UV-vis absorption spectra, the reaction's progress was tracked. Equation shown below was used to compute the percentage reduction of NACs:
Percentage degradation/ Reduction = 
where Ci and Cf represent the NACs' initial and final concentrations.
RESULTS AND DISCUSSIONS:
Fourier Transform Infrared Spectroscopy (FT-IR):
The FT-IR spectra of the BMOF was obtained in the 4000-400 cm-1 region, as shown in Figure 1. The Cu–O stretching and bending modes are represented by the peak at 481.9 cm-1. The peaks at 1605 and 1654 cm-1 indicate the asymmetric stretching vibrations of carboxylate, whereas the significant absorption bands at 1396.9 and 1459.3 cm-1 indicate the symmetric vibrations25. At 678.68 cm-1, the absorption peak was one of the characteristic vibration peaks of Ni-O. The absorption band at 1070–1100 cm-1 in EDTA chelates has been determined to be the C–N bond26.
Figure 1. FT-IR Spectra of the BMOF
Thermogravimetric Analysis (TGA):
BMOF underwent a TGA study from 30 to 1000℃. Figure 2’s BMOF thermogram displayed a three-step weight decrease. The first weight loss, which was approximately 6.1 percent, was brought on by ethanol and water loss between 31 and 75 degrees Celsius. The second weight loss, which was estimated to be 5.19 percent, was a result of the DMF solvent evaporating at a temperature between 86 to 159℃. The collapse of the organic linker in MOF and the temperature range of 184 to 407℃ were the causes of the third loss, which was approximately 69.9%. In the end, a black powder including copper oxide and nickel oxide was the residue that was generated.
Figure 2. Thermogram of the BMOF
Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (FESEM-EDS)
The FESEM images displayed in Figure 3(a-d) demonstrate distinct morphology, indicating the presence of open pores on the structure's surface. The EDS Spectra (Figure 4) shows the purity of the BMOF by showing the presence of the Copper and Nickel metals with 20 % and 8 % respectively. The Spectra also shows the presence Carbon, Nitrogen and Oxygen elements with 41 %, 8 % and 22 % respectively. From the SEM images the particle size ranges from 100 nm to 0.900 µm.
Figure 3. a, b, c and d are the FESEM images of the BMOF
Figure 4. EDS Spectra of the BMOF
X-ray Photoelectron Spectroscopy (XPS):
The XPS spectra presented in Figure 5 further demonstrates the elements Cu, Ni, O, N and C present in the BMOF. Copper is present in both Cu 2p3 and Cu 2p1 at 932 and 952 eV respectively. Cu 2p and Cu Auger is also present at 941 and 568 eV respectively. Ni 2p1 can be seen at 872 eV, Ni 2p3 peak is observed at 854 eV and Ni Auger was shown at 648 eV. Sharp O 1s peak can be seen at 529, N 1s is shown at 397 eV and sharp C 1s peak is shown at 283 eV.
Figure 5. XPS Spectra of the BMOF
Powder X-ray Diffraction (PXRD):
According to my knowledge, the PXRD pattern shown in
the Figure 6 has been seen for the first time, as the BMOF has been synthesized
for the first time. The peaks at 16.3º and 32.4º with the miller indices (001)
and (0
3) indicates the presence of the complex
Cu-EDTA27. The peaks at 39.8º with the hkl plane of (111) suggest
that the Ni is not in metallic (because metallic nickel peak should be at
44.5º) nor it is nickel oxide (since NiO peak should be at 37.2º) form, which
means that the Ni can be bonded with the EDTA28. Further peaks from
50º to 70º indicate Cu-Ni's bimetallic composition29. The strong
peak at 16.3º, 32.4º, and 39.8º demonstrates the crystallinity of the
synthesized bimetallic MOF. The average crystallite size was calculated from
Debye Scherer’s Equation:
0.9 λ
D= ------------------
FWHM Cos θ
where FWHM stands for angle full width at half maximum Bragg peak, and θ is reflection angle and λ for X-ray wavelength. The Average crystallite size estimated by Scherer’s equation was 4.9136 nm.
Figure 6. PXRD Pattern of the BMOF
Brunauer-Emmett-Teller (BET):
The produced BMOF sample's specific surface area and pore size distribution were determined using the N2 gas adsorption/desorption isotherms that came from the Brunauer-Emmett-Teller (BET) gas-sorption experiments. According to the IUPAC classification, the N2 adsorption-desorption curve of BMOF exhibits a type IV isotherm (Figure 7), which is associated with mesoporous materials30. The average pore diameter of the BMOF was determined to be 3.84119 nm and its BET specific surface area was discovered to be 293.14 m2/g.
Figure 7. N2 Adsorption/Desorption isotherms of the Synthesized BMOF
Catalytic Reduction of Nitroaromatic Compounds (NACs):
Three NACs i.e., 4-nitrophenol (4-NP), 2-nitroaniline (2-NA) and 4-nitroaniline (4-NA), were investigated for their ability to be reduced by the BMOF catalytic activity. The absorbance of 4-NP is 400 nm, 4-NA is 380 nm and 2-NA has two peaks at 283 nm and 413 nm, respectively, representing their maximal absorbance peaks (λmax). These absorbance changes indicate the reduction reactions. When NACs are reduced entirely to a non-toxic condition using only NaBH4, it takes one to two days in the absence of an adequate catalyst31. Indications of a quicker reduction of NACs include a sharp decline in peak intensity with adding 2 mg of BMOF catalyst in combination with NaBH4.
As illustrated in Figure 8a, in the instance of 4-NP, the intensity of the 400 nm peak progressively decreases over 180 seconds, and a new peak at 300 nm corresponding to 4-aminophenol (4-AP) appears. The peak at 413 nm becomes weaker as the 2-NA catalytic reduction process moves on (Figure 8b). The peak shifted from 283 to 290 nm as 2-aminoaniline (2-AA) was formed. Figure 8c illustrates how 4-aminoaniline (4-AA) is created in contrast to 4-NA, which shows a decline in intensity at 380 nm, the formation of a new high peak at 238 nm and a transient peak at 305 nm of 4-AA. The catalytic decline of 4-NP, 2-NA and 4-NA peaks disappears after 180, 120 and 120 seconds respectively, showing a quick NACs reduction.