Preparation of manganese-based metal organic framework (MOF) and its characterization properties

Metal-organic frameworks (MOFs) are produced by the reaction of metal ions and organic linkers with extremely crystalline and porous coordination networks. The applications of MOF cover from gas purification, gas separation, catalysis and super-capacitors. This work reports on the synthesization of metal-organic framework (MOF), using mangan (II) nitrate tetrahydrate as source of metal ions, 2-methylimidazole as organic ligand and ethanol as solvent. The material was prepared using precipitation method, at room temperature for 48 hours. The characterization of this material were carried out including X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The SEM analysis shows an irregular structure with a few petals. XRD shows several peaks, indicating crystallinity of the material, and amorphous state. To study the electrochemical property of the material, Cyclic Voltammetry (CV) was conducted. The cyclic voltammetry result shows peak at 0.23 V, with current output of 0.14 μA, with no changes in peak position as the scanning rate increases from 10 to 100 mV/s.


Introduction
Metal-organic frameworks (MOFs), a class of crystalline porous organic-inorganic materials with distinctive physical and chemical properties, composed of organic ligands with coordinated selfassembled bonds and metal ions.It has been widely applied for various applications in many industries such as catalysis [1], gas storage, gas separation [2], lithium-ion batteries [3], supercapacitors [4], and electrochemical sensors [5].Due to its easily modifiable structure, good chemical stability, high surface area, and low density, MOFs has received the majority of attention to date.
In addition, MOF has been acknowledged as a novel class of battery-grade materials for energy storage applications [6,7].In addition, zinc-based MOF-20 and MOF-177, collected 6.7 and 7.5 wt.% of ‫2ܪ‬ at 77 K, respectively.Some metal-organic frameworks that are manganese-based also exhibit a comparable capacity for hydrogen uptake; where, Dinca and Long [8] achieved an uptake of 6.9 wt%.Due to this qualities, including as its magnetic characteristics, the synthesis of new manganese-based MOFs have grown in interests in research field.In a publication by Jiang et al. [9], and the results showed that the manganese atoms next to each other exhibited antiferromagnetic behavior.Both Qi et al. [10] and Bai et al [11] concludes the same result after measuring the magnetic susceptibility of MOFs containing manganese.Many transition metal combinations have been studied in detail, although manganese is still rarely used.XRD study has indicated that the addition of manganese to silver results in a face-centered cubic crystal structure.Greater manganese concentrations result in larger crystals.
Because of its intricate crystal structure, manganese is the subject of research on particle size and surface area.
This study shows the studies of chemical, physical and electrochemical properties of MOF synthesized by introducing manganese nodes.Other than that, this study adds further information for usage of future research in the field of hydrogen storage applications.

Methodology 2.1. Synthesis of Manganese-based Metal Organic Frameworks (Mn-MOF)
The material was successfully synthesized via precipitation approach.4 mol of mangan (II) nitrate tetrahydrate was mixed with 1 mol of 2-methylimidazole.0.66 g of 2-methylimidazole were dissolved in 30 mL of ethanol and 0.50 g of mangan (II) nitrate tetrahydrate were also dissolved in 30 mL of ethanol, stirred separately for 30 minutes continuously.Then, both solution were mixed and further stirred for 1 hour.The bottle used to store the mixture was wrapped with aluminium foil, before leaving it to incubate for 48 hours in room temperature.After that, the mixture was washed multiple times using methanol, and dried at 60℃ for 24 hours, resulting in harvestable precipitate.

Morphology and electrochemical characterization
Rigaku Minifex and Foto Thermoscienrific : Quanta 650 Scanning Electron Microscope were used to characterize the crystallinity and morphology of the synthesized material.Cyclic voltammetry measurements were used to do electrochemical characterisation, which involved dropping the material onto a screen-printed carbon electrode that had working, counter, and reference electrodes.The substance was added to the working electrode in a 10 μL drop and left for 24 hours to set.The electrolyte ‫6)ܰܥ(݁ܨ3ܭ‬ (5 mM) and KCl (0.1 M) in a 0.01 M PBS (pH = 7.4) solution were then used for cyclic voltammetry experiments, which were performed at a scan rate from 10 to 100 mV/s.A computer device and PalmSens4 were connected to the setup.XRD characterizations were conducted in order to determine the crystalline phase of Mn-MOF.Figure 2. depicts the XRD patterns of the Mn-MOF.The obtained patterns indicate that material possess a crystalline nature due to their sharp and strong intensity peaks.Mn-MOF exhibits high intensity peak suggesting a high degree of crystallinity.The diffraction peaks of Mn-MOF are at 9.5°, 27.5°, 32.8°, 36.1°, and 38.0°.The diffraction peak of the synthesized Mn-MOF shows the same peak compared with ZIF-67 at 9.5° [12].There is broad diffraction around 25°, showing possibility of presence of amorphous state.Shown in Figure 3, the cyclic voltammetry test results for the synthesized material.As the scanning rate increases from 10 -100 mV/s, the current increases.This is because the scanning rate determines how quickly the applied voltage is scanned.Higher currents are found as a result of a reduction in the size of the diffusion layer caused by faster scan speeds [13].At scanning rate 50 mV/s, the current peaks can be observed at potential 0.23 V and 0.14 V for forward (cathodic peak) and reversed (anodic peak) scan respectively.The peak is caused by high mass transfer rates in the non-steady state and progressive reactant depletion in the diffusion layer.In cyclic voltammetry, the oxidation and reduction processes mirror one another, causing the anodic and cathodic peak currents to have distinct signs but identical magnitudes.

Conclusion
In this work, the synthesize of Mn-MOF was successful.The 3D morphology observed through SEM analysis shows irregular with petals structure, of non-uniform diameter of nanostructure and thickness of petals.XRD results shows several peaks and an amorphous state can be observed.Through cyclic voltammetry testing, the synthesized Mn-MOF shows good electrochemical property by producing a peak in both cathodic and anodic trace at 0.23 V and 0.14 V respectively.

Acknowledgement
The research conducted was supported by Universiti Teknologi PETRONAS (Malaysia), and Institut Teknologi Bandung (Indonesia).