Electrosynthesis of Metal-Organic Frameworks (MOFs)Based on Nickel(II) and Benzene 1,3,5-Tri Carboxylic Acid (H3BTC): An Optimization Reaction Condition

Electrosynthesis of metal-organic frameworks based on nickel(II) and benzen 1,3,5-tricarboxylic acid (H3BTC) to form [Ni3(BTC)2] has been conducted. This study aims to determine the optimum electro-synthetic conditions of [Ni3(BTC)2] by varying the solvents, electrolytes, as well as the voltages. The optimum condition was determined based on the percent yield of the product which upon washing and drying at room temperature showed pale green precipitate. The materialhas high crystallinityaccording to XRD analysis with the main peak observed at 2θ 19° and 28° and appropriate with [Ni3(BTC)2] pattern (CCDC No. 1274034). The refinement results using Le Bail methods revealed the Rp and Rwp values are 3.29% and 3.47%, respectively. The coordination between nickel(II) and carboxylate moeties of the linker has been characterized using FTIR and showed significant shift from 1723 cm-1 to 1608 cm-1. The compound has thermal stability up to 400 °C according to TG/DTA analysis. The SEM analysis confirmed that compound has morphology nanoplates shape with a thickness of 75 ± 0.023 nm. Another interesting feature of the obtained material is the occupancy of the reversible frameworks, which proved after methanol absorption. The optimum condition of the electro-synthesis of [Ni3(BTC)2] achieved in the methanol with TBATFB (0.1 M) as electrolyte, and the voltage of 15 V at room temperature with a yield of 99.99%.


Introduction
Metal-organic frameworks (MOFs) or called as porous coordination polymers are crystalline hybrid porous materials constructed from metal ions or metal oxide clusters and bi-or poly dentate organic linkers based on carboxylate, pyridil, phophonat, sulfonat end group through strong covalent coordinative bonding to form infinite two or three-dimensional structures [1,2]. The tunability in designing MOFs structure by varying the metal cations and functionalized organic linkers resulted very rich variety in MOFs topology and function [1]. The occupancy of unsaturated metal center, large surface area and high porosity found in MOFs make them very potential as functionalized host materials and can be applied in gas storage and separation [

Materials and methods
High purity (99.99%) of nickel plate with dimension (16x1x0.01 cm 3 ) was used as working electrodes. Benzene 1,3,5-tricarboxylic acid (H 3 BTC, 95%), tetrabutylammonium tetrafluoroborate (TBATFB, 99%), tributylmethylammonium methyl sulfate (MTBS, 99%) were purchased from sigma Aldrich and used without any further purification. Solvents such as ethanol, methanol and dimethyl formamide (DMF) was obtained from Merck with high grade and used as purchased. The nickel plates were initially activated by immersing in HNO 3 (1 M) then scrubbed with sand paper till shiny before used in electro-synthesis. yellow precipitate after activation till 200 °C but the color remain unchanged under activation up to 150 °C. To determine the optimum conditions of the synthesis, the applied voltage were varied from 13 to 17 V, while TBATFB concentration varied from 0.02 to 0.1 M. Other than TBATFB, MTBS (0.1 M) was also applied as supporting electrolyte. To test the reversibility of the frameworks, the activated [Ni 3 (BTC) 3 ] were immersed in methanol for 24 hours then filtered and dried at room temperature.

Characterization
The electrosynthesized products were characterized using powder X-Ray diffraction (JEOL JDX-3530 diffractometer) using Cu-Kα radiation (λ = 1.5406 Å), with the voltage and current were held at 40 kV and 30 mA (2θ = 5-50°) at a step size of 0.02°/s. Thermal stability of compound was analyzed using STA Linseis PT-1600 under nitrogen atmosphere with heating rate of 10 °C/minute. Scanning Electron Microscopy FEI type Inspect S50 were used to determine the morphology of the material. FTIR spectra were recorded on Shimadzu IR Prestige-21 PC using KBr pellet in the range of 400-4000 cm -1 . BET surface area determined based on BET adsorption isotherms of nitrogen at 77 K using a static volumetric apparatus Micrometrics, NOVA 1200e after the sample activated at 150 °C.

Optimization reaction condition in electrosynthesis of [Ni 3 (BTC) 2 ]
The formation of [Ni 3 (BTC) 2 ] through electrochemical method occurred based on redox reaction. The nickel(II) cations formed from the oxidation of Ni metal plate as anode, while the H 3 BTC ligand deprotonated and released protons as H + and BTC 3indicated by the decrease of pH value from 4.09 to 2.89. The presence of H + in the system will be further reduced to be hydrogen and released as a side product indicated by the bubble formation in the electrochemical cell. The nickel(II) coordinated further with BTC 3to form main product appeared as pale green precipitate found in cathode. Explanation about the redox process is clearly shown in scheme 1.

Anode
:  1 M) as the electrolyte at ambient temperature. The addition of electrolyte is an important factor not only acting as a media for ions migration but also increasing the charge mobility required in a solution [17]. The increase in electrolyte concentration is directly proportional to the speed of ion transport in the reaction system. The use of methanol as solvent produced the highest yield in the reaction (reaching 99.99%). In this case, methanol and ethanol are the types protic polar solvent that has the ability in dissolving metal cations and H 3 BTC ligand better than DMF which is an aprotic polar solvent [18]. In addition, the solvent also affects the deprotonated ligand, in which the use of suitable solvents will make it easier to deprotonate the ligand [12]. The higher voltage applied in the reaction system gave the higher energy supply, thus increase the reaction yield. However, electrosynthesis of [Ni 3 (BTC) 2 higher than 15 V tend to decrease the reaction product.
The reason could be that if the voltage was too high lead to the deformation of the frameworks [13].

Material Characterization
The obtained [Ni 3 (BTC) 2 ] in several voltage and electrolyte variation were characterized by XRD analysis to confirm phase purity and crsytallinity of the product compared to the standart pattern (CCDC No. 1274034). Two main peaks are observed at 2θ 19 dan 28° according to figure 3 and indicates that [Ni 3 (BTC) 2 ] have been synthesized successfully.  To know the water molecules content either as coordinated water or as guest molecule in the surface or pore of H 3 BTC thermogravimetric analysis had been conducted. The TG/DTA profile (figure 5) showed 3 step of mass loss with the first step (22.34 %) occurred from RT to 175 °C indicating the removal of ten water molecules as guest molecules occupied in the pore and surface of [Ni 3 (BTC) 2 ]. Secondly, (5.02%) in region 175 to 260 °C corresponds to the loss of two coordinated water molecules from the nickel(II) based MOFs affect the change of coordination number from 6 to 4 indicating by the change of the color from pale green to yellow. The third weight loss (36.98 %) observed at a temperature range from 260 to 405 °C assigned as decomposition process of the organic linker. This result is in line with the publication reported by Reimer et al [19]. In general, [Ni 3 (BTC) 2 ] is thermally stable to 400 °C and afterward, the compound totally decomposed (73.62%) into NiO [20]. The scanning electron microscopy confirmed that the synthesized [Ni 3 (BTC) 2 ] has good crystallinity and nanoplate shape with particle size distribution of 40 -130 nm and average particle thickness of 75±0.023 nm (figure 6).

Reversibility test of [Ni 3 (BTC) 2 ] frameworks
Color change of [Ni 3 (BTC) 2 ] from pale green into yellow after activation until 200 °C for 2 hours indicates the change of coordination number in [Ni 3 (BTC) 2 ] from six to four, consequently the geometry of the compound changed from octahedral into square planar [21] (figure 7 b). Moreover, the frameworks structure tend to collapse from 3dimensional structure into sheet structure (2dimesion) as shown from X-Ray diffractogram. The change of sharp peak intensity into broad peak confirmed that the crystallinity of the compound totally changed into amorf. However, after immersed in methanol for 24 h, the color turned back into the pale green which stipulates the reversibility of the frameworks after contacting with the solvent as observed by XRD analysis (figure 7 c). Thus, the stability of the frameworks is highly dependent on the availability of solvents in the structure. In addition, BET surface area of [Ni 3 (BTC) 2 ] obtained from nitrogen sorption isotherm measurement was 4.365 m 2 /g.