Functionalization of Two-Dimensional Coordination Polymer in Small Organic Matter Removal from Organic Wastewater

Multicobalt(II) two-dimensional layer coordination polymer {[Co(bitbu-OMe)2(NCS)2]·2MeOH}n (1) was synthesized using a bis-imidazole ligand having a steric hindrance, tert-butyl, and one methoxy group expressed by bitbu-OMe (bitbu-OMe = 1,1′-[(5-tert-butyl-2-methoxybenzene-1,3-diyl)dimethanediyl]bis(1H-imidazole). Since there are two methanol molecules trapped inside each void within 1, the investigation of small organic matter removal from organic wastewater was conducted in order to reveal its ability in the purification process. The research findings indicate that 1 has the ability to capture methanol, acetone, acetonitrile, and tetrahydrofuran in organic wastewater with removal ratios of 29.17%, 63.22%, 42.77%, and 21.24%, respectively.


Introduction
Possessing clean water has long been a significant topic [1], and access to clean water is a key component of Sustainable Development Goal 6 [2]. Water pollution is caused by a variety of waste releases [3] from a variety of sources [4][5][6][7]. The creation of organic wastewater is one of the challenges that arise in chemical laboratory research [8][9][10][11]. In general, a chemical laboratory's organic wastewater consists of used water and waste organic solvent mixtures [12]. Various efforts to recycle organic wastewater to generate clean water have been established, such as using zeolite as a purification agent to absorb organic matter dissolved in organic wastewater by leveraging its microporous [13][14][15][16][17][18][19].
Because of the metal ions and ligands used, these molecules have piqued curiosity, and they should have no negative impacts on the human body or the environment. As a result, the creation of two-dimensional coordination polymers in the absorption process of small molecules [25,26] as a purification agent has proven to be advantageous in the purification of organic wastewater. We reported on the synthesis and characterization of {[Co(bitbu-OMe) 2 (NCS) 2 ]·2MeOH} n , a two-dimensional layered coordination polymer made from Co(SCN) 2 and a bis-imidazole ligand with a steric hindrance tert-butyl and a methoxy group called bitbu-OMe, in which methanol molecules are encapsulated within its voids [27] ( Fig. 1).

Materials and Methods
All of the solvents and reagents were commercially available and were used without further purification. On a Rigaku VariMax with a Saturn diffractometer, single-crystal X-ray structural diffraction data for 1 were acquired. On a Rigaku Smart Lab, powder X-ray Diffraction (PXRD) spectrum for 1 was gathered. On the Euro Vector EA3000, elemental analyses (CHN) for bitbu-OMe and 1 were performed. IR spectra for bitbu-OMe and 1 were collected in the 400-4000 cm −1 range using a PerkinElmer FT-IR Spectrometer Frontier. In the range of 300-800 nm, UV-Vis spectra for 1 were collected with a JASCO V-570 UV/Vis/NIR Spectrometer. On a Rigaku TG 8121, thermogravimetry (TG) for 1 was conducted. JEOL ECA-600 Spectrometer was used to generate nuclear magnetic resonance (NMR) spectra for bitbu-OMe and small organic matter such as methanol, acetone, acetonitrile, and tetrahydrofuran before and after their removal treatment.

Crystal Structure Characterization of 1
VESTA software [35] was used to characterize the crystal structure for 1. Table 1 contains the crystallographic data for 1.

Small Organic Matters Removal Process
The removal of small organic matter in organic wastewater with 1 commenced with thermogravimetric analysis (TG analysis) to determine the optimum temperature for the desolvation process of voids in 1. UV/Vis spectra were recorded before and after the desolvation process to compare 1 powder with and without methanol molecules in the voids. In addition, the UV/Vis spectra for 1 were taken before and after the small organic matter removal treatment to distinguish the differences.
2,2-Dimethylpropan-1-ol (neopentyl alcohol, NPA) (0.018 g, 0.200 mmol) was added to 20 mL of D 2 O to create the organic wastewater representative. Four vials of NPA solution were created (5 mL of solution for each vial). Each vial contained 0.400 mmol of each small organic matter (methanol, acetone, acetonitrile, and tetrahydrofuran). For 1 H NMR analysis, 0.500 mL of each small organic matter solution was introduced to the NMR tube (Fig. 2).
The desolvated 1 powder (0.040 g, 0.05 mmol) was mixed with four small organic matter solutions to capture small organic matter. The desolvated 1 powder and small organic matter solutions were mixed at room temperature for one hour. A filter attached to the syringe was used to filter the mixes. For 1 H NMR analysis, 0.500 mL of each solution was put to the NMR tube.
The equation used for the small organic matter removal ratio was derived from the relative quantitative NMR (qNMR) [36], Eq. (1), where n x /n y , I x , I y , N x , and N y are molar ratio of two compounds, the integrated signal area of compound x, the integrated signal area of compound y, the number nuclei of compound x, and the number nuclei of compound y, respectively. Equation (1) was used to calculate the molarity of organic molecules after removal treatment (n a ) from  molarity of organic molecules before removal treatment (n b ), and by assuming the molarity of standard compound, the number nuclei of standard compound, the number nuclei of organic molecules before and after treatment to remain the same. The molar ratio of organic molecules before and after removal treatment was calculated using the Eq. (2).
The percentage of organic molecules removal was calculated using the Eq. (3) where I b and I a are the integrated signal area of organic molecules before and after removal treatment, respectively.

Synthesis and Characterization of 1
The 1 is generated by mixing Co(SCN) 2 salt with bitbu-OMe bridging ligand in a 1:2 ratio, according to the empirical formula of 1. Elemental analysis, IR spectra, UV-Vis spectra, powder X-ray diffraction, thermogravimetric analysis, and single-crystal X-ray diffraction were used to characterize the synthesized complex. Because the variation of the results for C, H, and N is higher than 0.4%, the CHN elemental analysis results for 1 do not match the calculated ones. However, since an element-agnostic deviation of 0.4% is not a realistic journal criterion for synthetic samples [37], elemental analysis results are not the only technique to validate the purity of a synthesized chemical. As a result, we could carry out more characterizations.
Percentage of organic molecules removal = The FT-IR spectrum of bitbu-OMe exhibits extremely distinct bands in the region of 2800 and 3000 cm −1 , which are attributed to asymmetric and symmetric stretching vibrations of the tert-butyl group of the bitbu-OMe. Strong bands at 1209 cm −1 and medium bands at 1485 cm −1 in the FT-IR spectrum of bitbu-OMe represent scissoring vibrations of H atoms on the benzene ring and scissoring vibrations of H atoms along with the imidazole rings and tert-butyl group, respectively (see Fig. S1). In comparison to the FT-IR spectrum of bitbu-OMe, the FT-IR spectrum of 1 has a relatively consistent band. It indicates that bitbu-OMe is indeed used to make 1 (see Fig. S2). The 1 powder's powder X-ray diffraction (PXRD) pattern is highly consistent with 1 crystal's simulated PXRD pattern (see Fig. S3a, b). It signifies that the compound in the 1 powder is the same as in the produced crystals.
The percentage by weight of methanol contained in 1 is calculated to be 7.23% using the empirical formula of 1. The thermogravimetric study shows a rapid mass drop of 1 up to 7.21% at 42.8 °C and a gradual mass decrease of 1 below 250 °C (see Fig. S4). It implies that below 45 °C, all methanol molecules are entirely eliminated, and the structure of 1 remains stable. The UV-Vis spectrum of 1 powder (initial/ before being heated below 45 °C) shows that 1 possesses the highest absorbance (approximately 0.3) at 300 nm, whereas the UV-Vis spectrum of 1 powder after being heated below 45 °C reveals that 1 has the highest absorbance (above 0.3) at 300 nm (see Fig. S5), and the 1 powder after being heated below 45 °C has a different color than that of the initial 1 powder (Fig. 3).

Crystal Structure Characterization of 1
The single-crystal X-ray diffraction analysis reveals that 1 consists of two-dimensional layered coordination polymers with rhombic grid-style [20] crystallized in the monoclinic Fig. 2 The illustration of small organic matter removal in organic wastewater using the desolvated 1 powder system with the space group P2 1 /c, as previously reported [27] (Fig. 4).
For the size of the void in 1, the single-crystal X-ray diffraction analysis reveals that 1 possesses voids where each void possesses an area of approximately 5.370 Å × 4.424 Å whose length and width are calculated based on the distance between C1 and C1* minus the van der Waals diameter of the carbon atom and the distance between N3 and N3* minus the van der Waals diameter of the nitrogen atom by considering the van der Waals modeling with 100% radius relativity, as previously reported [27] (Fig. 5).

Small Organic Matter Removal in Organic
Wastewater Using 1 The study of encapsulating small organic matter in organic wastewater using 1 was conducted by calculating the ratio of the changes in the concentration of small organic matter and was interpreted from the changes in the integrated signal area of small organic matter obtained from the 1 H NMR measurement. Neopentyl alcohol was used as a standard to calculate the removal ratio of small organic matter in organic wastewater. Neopentyl alcohol was used due to its size, which is relatively larger than the size of the void within 1.
The van der Waals volume of neopentyl alcohol is approximately 87.01 Å 3 , according to the van der Waals molecular volume calculation proposed by Bondi [38]. Assuming neopentyl alcohol is deemed to possess a spherical shape, the diameter of neopentyl alcohol is approximately 5.5 Å, calculated with the formula for the volume of a sphere. Therefore, the neopentyl alcohol was not captured by 1. The van der Waals molecular volume and diameter of small organic matter such as methanol, acetone, acetonitrile, and tetrahydrofuran were calculated with the same formula as the van der Waals molecular volume and diameter of neopentyl alcohol described above. The van der Waals molecular volume of methanol, acetone, acetonitrile, and tetrahydrofuran was approximately 36.1 Å 3 , 64.85 Å 3 , 47.13 Å 3 , and 76.61 Å 3 , respectively. Hence, the van der Waals diameter of these molecules was approximately 4.1 Å, 4.99 Å, 4.48 Å, and 5.27 Å, respectively. Meanwhile, the 1 H NMR results reveal that the desolvated 1 powder possesses a capturing ability of methanol, acetone, acetonitrile, and tetrahydrofuran with the removal ratios of 29.17%, 63.22%, 42.77%, and 21.24%, respectively (see Fig. S6a-d). Technically, the smaller the van der Waals volume of the small organic matter is, the higher the capture capacity of 1 powder becomes. Possibly the voids are easier to capture tinier organic matter. In contrast, 1 possesses a capturing ability of acetone better than other organic matter despite being bigger than methanol. It possibly occurs because methanol possesses a strong interaction with water due to the presence of hydrogen bonding between hydroxy groups in methanol molecules and water molecules, so methanol is more likely to remain in water rather than being trapped by 1. The low removal ratio of tetrahydrofuran seemed plausible due to its relatively large van der Waals volume and diameter. Interestingly, the color of 1 powder after small organic matter removal returned to the color of the initial 1 powder (Fig. 6), supported by the UV-Vis spectra of 1 powder after small organic matter removal where these spectra exhibit the highest absorbance (approximately 0.3) at 300 nm (see Fig. S7a-d).

Conclusions
Further to the synthesis of 1, there are two methanol molecules encapsulated by each void within 1 and thus it became interesting to study the organic molecules removal in organic wastewater using 1. Based on the experimental results, 1 has an ability to capture methanol, acetone, acetonitrile, and tetrahydrofuran with the removal ratios of 29.17%, 63.22%, 42.77%, and 21.24%, respectively. The findings of this research work can help towards the purification of organic wastewater.

Acknowledgements
The first author thanks M. K. of Shizuoka University for providing the opportunity to conduct the research and providing Fig. 6 a The 1 powder before small organic matter removal treatment. b The 1 powder after small organic matter removal treatment for (i) methanol, (ii) acetone, (iii) acetonitrile, (iv) tetrahydrofuran