Abstract
A new mixed-ligand H-bonded coordination polymer {[Cu2(Or)2(Bimb)3]·4H2O}n (KA@CP-S) has been prepared hydrothermally using basic copper carbonate with 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (Bimb) and potassium orotate (OrK) ligands. According to topological studies, KA@CP-S has a new topology with a three-connected uninodal net with point symbol (PS) {82·12}2{8}3. The KA@CP-S was employed as a catalyst for screening of a series of harmful cationic, anionic, and neutral organic dyes in contaminated water. The photocatalytic degradation study shows that it exhibits good catalytic efficiency for cationic dyes like Crystal Violet (CV, 75.8%), Methyl Violet (MV, 76.8%), and Rhodamine 6G (Rh6G, 86.5%) and Rose Bengal (RB, 76.1%), which is an anionic dye, while for a neutral dye, its catalytic efficiency is only 72% (Neutral Red) at ambient temperature. The effect of pH on photocatalytic degradation was also analyzed. The degradation experiment reveals that the detection limits of KA@CP-S for mostly catalyzed colorant concentrations in contaminated water are 0.60 ppm (CV), 0.20 ppm (RB), 0.33 ppm (MV), and 0.20 ppm (Rh6G) at pH 12, 4, and 10. The degradation of dyes follows pseudo-first-order kinetics. The excellent catalytic property and regeneration ability of KA@CP-S make it a potential and efficient future remedial material for the detection and separation of toxic dyes from wastewater contaminated by industrial effluents.
Introduction
Coordination polymers (CPs) have rapidly emerged due to their varied network topologies and extensive applications, such as in gas adsorption,1 separation,2 magnetism,3 photocatalysis,4 luminescence,5 drug delivery,6 high thermal and mechanical stability,7 and so on. The key parameter used to identify the coordination polymers is the number of organic linkers and metal nodes used. CPs comprising of mixed-ligand polycarboxylate (rigid) and N-donor linkers (with flexible arms) are useful building blocks in the assembly of CPs among the functional organic ligands. The design and synthesis of suitable CPs and MOFs for well-organized dye removal from wastewater/industrial effluent is both thought-provoking and desirable. Although numerous methods using natural and synthetic ingredients have been employed for the segregation of these contaminants from water, much attention has been paid to MOFs/CPs as a remedial solid.
The loss of water resources as a result of contamination from numerous pollutants, including organic and inorganic compounds, is one of the most serious issues.8 Synthetic organic dyes are one of the most common types of water contaminants as they are frequently utilized in textile, paper, printing, leather, rubber, plastic, carpet, food processing, cosmetic, and pharmaceutical industries and are regarded as the most serious threat to humans and environments.9 The textile sector is extremely important to the economy of Asian and other countries. The unwarranted proclamation of these dyes as frequent environmental contaminants raises significant global concern. Owing to rapid urbanization and industrial development, a growing number of harmful pollutants are entering into the freshwater supplies. In terms of noteworthy impact, the World Economic Forum recognized water crisis as the most serious threat.8 Dye hues can be seen with concentrations as low as 1 ppm in water (however, textile production wastewater often contains dye concentrations of 10–200 ppm), causing esthetic issues, disrupting photosynthesis in aquatic plants, and having hazardous and carcinogenic effects on mammals.
To remove dyes from wastewater, various biological treatments, physicochemical, ion-exchange, chemical oxidation, membrane separation, and photocatalytic processes, and solid-phase adsorption methods are used.10 The emergence of new photocatalysts for the efficient degradation of aromatic dye pollutants prevalent in wastewater discharge has become one of the most crucial among them.11 The presence of many catalytic units in these polymers with a suitable coordination environment for metal ions is vital for improving catalytic activity.12 The side effects of severe exposure to such dyes are increased heart rate, shock, cyanosis, vomiting, tissue necrosis, and jaundice in humans.13
In this work, we have designed a coordination polymer using Cu(II) ions with the flexible 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (Bimb) ligand and rigid N/O-donor ligand potassium orotate (OrK) to build a new mixed-ligand H-bonded coordination polymer, {[Cu2(Or)2(Bimb)3]·4H2O}n (KA@CP-S), hydrothermally. The physicochemical analysis of KA@CP-S is corroborated by VT-FTIR, VT-PXRD, and VT-luminescence. Single-crystal X-ray diffraction shows that KA@CP-S forms a 1D chain with a coordination bond and a 2D layer supramolecular network, which is stabilized by strong O–H···O and N–H···O, H-bonding and the formed 2D chain. The physical properties of the as-synthesized KA@CP-S and KA@CP-S after heating shows luminescence within the region of 459–542 nm (2.70–2.28 eV) with a red emission.
The screening of 13 toxic dyes like Methyl Orange (MO), Methylene Blue (MB), Eriochrome Black T (EBT), Bromophenol Blue (BPB), Rhodamine B (RhB), Congo Red (CR), Safranin O (SO), Neutral Red (NR), Crystal Violet (CV), Rose Bengal (RB), Methyl Violet (MV), Rhodamine 6G (Rh6G), and Fluorescein (FS) has been performed over KA@CP-S. It was found to exhibit good efficiency for cationic dyes between 98 and 94% (Rhodamine 6G > Methyl Violet > Crystal Violet), whereas Rose Bengal showed the highest uptake percentage (98.0%) among the anionic dyes screened. KA@CP-S showed the maximum degradation efficiency compared with rice husk (RH) and saw dust (SD) for most of the dyes among all 13 screened colorants. The detection limit of KA@CP-S with respect to dye concentrations as detected by photocatalytic performance are as follows: CV, 0.60 ppm; RB, 0.20 ppm; MV, 0.33 ppm; and Rh6G, 0.20 ppm.
Experimental Section
General Procedure
All chemicals were of reagent grade and were purchased from Sigma-Aldrich and used without further purification. Potassium orotate, 1,4-bis[(1H-imidazol-1-yl)methyl]benzene, methanol, and NaOH were commercially available. Single-crystal X-ray diffraction was performed using an XtaLAB Synergy, Dualflex, HyPix3000 diffractometer fitted with a graphite monochromator and Mo Kα radiation source (λ = 0.71073 Å). The elemental study (CHN) was conducted on a PerkinElmer 2400 Series II element analyzer. Powder X-ray diffraction (PXRD) was performed on a PANalytical 3 kW X’pert multifunctional powder X-ray diffractometer. The Brunauer–Emmett–Teller (BET) surface area of KA@CP-S was analyzed using N2 adsorption isotherms (Quantachrome Instruments). FTIR spectra were collected in the range of 4000–400 cm–1 by FTIR spectroscopy (Bruker Optics, GmBH, Germany). To evaluate thermal properties, a thermal analyzer TG–DTA 7200 (Hitachi, Japan) was used. A field-emission scanning electron microscope (FESEM) and a field-emission scanning electron microanalyzer (ZEISS GeminiSEM 500) were used to examine the shape and size of as-synthesized KA@CP-S and prepared CuO nanoparticles. An energy-dispersive X-ray spectrometer (EDAX) equipped with FESEM was used for elemental analysis. Luminescence measurements were obtained with an RF-5301PC spectrofluorophotometer (Shimadzu) in the solid state. Photocatalytic spectroscopic measurements were obtained using an LT-2900 double-beam UV–vis spectrophotometer with 190–1100 nm wavelength range, AC: 220 V + 10%, 50 Hz, photometric range of −0.3 to 3.5 A, and 0–220% T with cuvettes having a 1 cm path length.
Synthesis of {[Cu2(Or)2(Bimb)3]·4H2O}n (KA@CP-S)
KA@CP-S was synthesized hydrothermally from a mixture of basic copper carbonate (40 mg), potassium orotate (20 mg), 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (20 mg), deionized water (4 mL), methanol (1 mL), and NaOH (1 M, 0.2 mL), which was sealed in a 25 mL Teflon-lined steel autoclave and heated at 120 °C for 72 h, and then slowly cooled to room temperature at a rate of 10 °C h–1. After 3 weeks of keeping the solution as such, phase-pure single crystals of KA@CP-S suitable for single-crystal X-ray diffraction were collected (yield 58%, based on Cu). Calcd. (%) for C52H54Cu2N16O12: C, 51.10; H, 4.45; N, 18.33. Found (%): C, 51.08; H, 4.41; N, 18.30. FTIR (cm–1, KBr): 3437 (wb), 3119 (w), 2995 (w), 1659 (vs), 1634 (vs), 1520 (m), 1463 (s), 1445 (m), 1404 (w), 1376 (vs), 1238 (m), 1094 (s), 1023 (m), 943 (w), 793 (w), 779 (w), 728 (w), 717 (w), 656 (w), 615 (w). The synthesis of KA@CP-S is depicted schematically in Scheme 1.
Scheme 1. Outline of the Synthesis of KA@CP-S.
Photocatalytic Experiments
KA@CP-S (0.025 mmol) was added to 40 mL of aqueous solutions of dye (10 mg/L) under magnetic stirring. The clear solution was then transferred to a quartz cell, and then the characteristic electronic absorption bands of dyes were recorded using a UV–vis spectrophotometer under sunny day morning ambient laboratory tube light illumination (at 25 °C) every 15 min. The experiment was deprived of any additional oxidants or cocatalysts. Additionally, a control experiment was conducted under similar reaction conditions but without the addition of catalyst, KA@CP-S. The recyclability experiments were conducted over five cycles under similar reaction conditions. The photocatalytic activity of dyes improved with time from 0 to 150 min. The same method is applied for RH and SH by taking 30 mg of each.
Determination of X-ray Crystal Structure
A single crystal of the C52H54Cu2N16O12 (KA@CP-S) was placed in a nylon loop and measured on an XtaLAB Synergy, Dualflex, HyPix3000 diffractometer at 293(2) K temperature with graphite monochromated Mo Kα radiation (λ = 0.071073 nm). The crystal was kept at 293(2) K throughout the data collection process. Using Olex2,14 the structure was solved with the help of olex2.solve15 structure solution program with Charge Flipping and refined with the olex2.refine15 refinement package using Gauss–Newton minimization. Detailed data collection and refinement for the KA@CP-S are given in Table 1, and the selected bond lengths and angles are listed in Table S1. The hydrogen bond parameter is listed in Table S2, and solvent mask information for KA@CP-S is given in Table S3.
Table 1. Crystal Structure Refinement Parameters for KA@CP-S.
| empirical formula | C52H54Cu2N16O12 |
| formula weight | 1222.20 |
| temperature, K | 293(2) |
| crystal system | triclinic |
| space group |  |
| a, Å | 9.41790(10) |
| b, Å | 9.88970(10) |
| c, Å | 16.3110(2) |
| α, deg | 102.2980(10) |
| β, deg | 106.2470(10) |
| γ, deg | 94.0070(10) |
| volume, Å3 | 1411.43(3) |
| Z | 1 |
| ρcalc, g/cm3 | 1.4378 |
| μ, mm–1 | 0.829 |
| F(000) | 632.9 |
| crystal size, mm3 | 0.38 × 0.23 × 0.17 |
| radiation | Mo Kα (λ = 0.71073) |
| 2Θ range for data collection, deg | 4.26–54.7 |
| index ranges | –11 ≤ h ≤ 12, –12 ≤ k ≤ 12, –21 ≤ l ≤ 21 |
| reflections collected | 41 177 |
| independent reflections | 6089 [Rint = 0.0388, Rσ = 0.0279] |
| data/restraints/parameters | 6089/0/377 |
| goodness-of-fit on F2 | 1.020 |
| final R indexes [I ≥ 2σ(I)] | R1 = 0.0429, wR2 = 0.1112 |
| final R indexes [all data] | R1 = 0.0537, wR2 = 0.1177 |
| largest diff. peak/hole, e Å–3 | 0.76/–0.52 |
| CCDC no. | 2178636 |
Results and Discussion
Structure Description of {[Cu2(Or)2(Bimb)3]·4H2O}n (KA@CP-S)
The complex {[Cu2(Or)2(Bimb)3]·4H2O}n(KA@CP-S) crystallizes in the triclinic space group Pi̅. The asymmetric unit of KA@CP-S contains one Cu(II) ion, one potassium orotate ligand, 1.5 molecules of Bimb, and two lattice water molecules. As shown in Figure 1, the environment around the Cu1 ion has a somewhat distorted square pyramidal geometry. The central copper ion is coordinated by four nitrogen (N1, N3, N6, and N7) atoms from three distinct Bimb ligands and one orotate ligand with bond lengths (Cu–N) that fall within the range of 1.9971(19) to 2.0209(19) Å and one oxygen (O1) atom from one potassium orotate ligand with a bond distance (Cu–O) of 2.2096(19) Å. All Bimb ligands coordinated through nitrogen atoms of imidazolyl rings in μ2:η1:η1 bidentate bridging mode between two Cu(II) ions and orotate ligands coordinated through nitrogen and carboxylate oxygen atoms in η2 bidentate mode and formed five-membered chelate rings with a metal ion. A 1D polymeric chain of KA@CP-S formed resulting from Bimb bridging Bimb ligands (Figure S1). The adjacent Cu1 ions are connected by a bidentate bridge from the Bimb ligands with the Cu1–Cu1 bond distances within the linear chain and in between strands are 14.1252(5) and 14.4317(4) Å, respectively.
Figure 1.
Diagrammatic representation of coordination environments of KA@CP-S (hydrogen atoms are omitted for clarity).
KA@CP-S also forms 1D and 2D hydrogen-bonded polymeric structures via classical strong N2–H2···O3 interactions, whereas adjacent 1D coordination polymeric ladders are interconnected through N2–H2···O3 hydrogen bonds and form 2D hydrogen-bonded networks. The 2D networks further expand and form 3D supramolecular networks via weak nonclassical C23–H23b···O2 hydrogen bonds in between the hydrogen bond donor active methylene carbons of Bimb and the hydrogen bond acceptor carboxylate oxygen atoms of orotate ligands (Figure 2). The water molecules situated in the lattice points are also stabilized by strong O5–H5a···O3 and O7–H7b···O1 hydrogen bonding interactions where an oxygen atom from the lattice water molecule acts as a hydrogen bond donor and acceptor (O7–H7b···O1 and C7–H7···O7, respectively), showing a [1 + 1] type of hydrogen bonding.16 All hydrogen bond parameters for KA@CP-S are given in Table S2.
Figure 2.
Hydrogen bonding-driven 3D supramolecular structure along the a-axis (hydrogen atoms are omitted for clarity).
Topological Analysis
The structure of KA@CP-S (Figure S2) is a mononuclear coordination polymer, where copper atoms are bonded with the ligand (Bimb) and coligand. KA@CP-S is a 3D periodic structure.
The rod-net representation shows how the rods are connected. The standard representation of the coordination polymer shows a new topology type with a 3-connected uninodal net with a point symbol {42·6} (Figure 3a). Using ToposPro,17 having applied the cluster representation, the adjacent matrix was simplified to include the 0,1,2 nodal net, and the topology of the underlying net obtained a new topology with a point symbol (PS) {82·12}2{8}3 (Figure 3b,c).
Figure 3.
(a) 3-c uninodal net, (b) rod-net representation of the structure KA@CP-S, (c) new topological type after structural fragments in the (010) direction.
FTIR Analysis of KA@CP-S
The FTIR analyses for as-synthesized KA@CP-S and KA@CP-S after heating are shown in Figure 4. The FTIR peaks of KA@CP-S at 3437 cm–1 are attributed to the υ(O–H) stretching vibration of the aqua ligand, and those from 2995 to 3119 cm–1 are attributed to the υ(C–H) stretching vibrations. The vibrations at 1659 cm–1 are attributed to the υ(CO) stretching vibrations of coordinated orotate ligands. The bands appearing from 1094 to 1634 cm–1 are attributed to the υ(C–N) and υ(C–C) stretching vibrations of ligands. The bands from 779 to 943 cm–1 could possibly be attributed to the characteristic vibrations of υ(Cu–N).18 With further heating of the as-synthesized complex at 373 K (II), 473 K (III), and 573 K (IV), the intensity of some vibrational peaks in the region below 1600 cm–1 reduced or vanished (Figure 4). It could be due to the structural changes in the complex.
Figure 4.
FTIR spectra for KA@CP-S.
The FTIR analysis of CuO nanoparticles after heating at 723 K (V) revealed a peak at 3436 cm–1 attributed to the υ(O–H) stretching vibration of absorbed water molecules. The bands at 2854 and 2925 cm–1 are attributed to the υ(C–H) stretching vibrations. The bands appearing at 1385 and 1638 cm–1 are attributed to the υ(CO) stretching vibrations. The bands from 534 to 584 cm–1 could be attributed to the characteristic vibrations of υ(Cu–O), confirming the formation of CuO nanoparticles.19
Thermogravimetric and Thermal Decomposition Analysis
The TG/DTA/DTG experiment was conducted in a N2 environment on the crystalline sample of KA@CP-S. The temperature was increased at 10 °C/min from 40 to 900 °C. The TG–DTA–DTG curves are shown in Figure 5. For KA@CP-S, the destruction of the complex occurring in the range of 60–115 °C is attributed to a weight loss of ∼5.7% of four lattice water molecules. The resultant anhydrous complex is thermally stable up to 240 °C, following which the complex exhibits two weight loss steps that occur between 240 and 600 °C corresponding to the continuous disintegration of coordinated Bimb (∼57.2%) and orotate (∼24.1%) ligands. The leftover mass is equivalent to the formation of ∼12.9% of CuO nanoparticles. The DTA curve of KA@CP-S shows two peculiar endothermic peaks at 94 and 270 °C, which could be attributed to the release of lattice aqua molecules and both ligands, respectively. The DTG curve of KA@CP-S shows three distinct peaks at 91, 270, and 326 °C, which can be attributed to the release of water molecules, Bimb, and orotate ligands, respectively. The thermal behavior of KA@CP-S is shown in Scheme 2.
Figure 5.
TG–DTA–DTG curve of KA@CP-S.
Scheme 2. Thermal Behavior of KA@CP-S.
Copper oxide (CuO) nanoparticles were synthesized by thermal decomposition of as-synthesized KA@CP-S at 723 K in air with a decomposition time of 90 min. This was characterized by FESEM, EDAX, FTIR, and PXRD analyses.
The morphology and size of KA@CP-S and prepared CuO nanoparticles were also investigated by FESEM micrographs. According to the findings, the roofing sheetlike crystals belong to KA@CP-S (Figure 6a–d). The average diameter of CuO nanospheres was about 18.2 nm with good homogeneity and uniform distribution. No obvious aggregation of CuO nanospheres was observed (Figure 7a,b). The particle size predicted from the FESEM analysis is in good agreement with PXRD data. Energy-dispersive X-ray spectroscopy (EDAX) was used for elemental detection (Figure S4).
Figure 6.
FESEM images of KA@CP-S at different magnifications: (a) 30k×, (b) 202×, (c) 5k×, and (d) 10k×.
Figure 7.
FESEM images of formed CuO nanoparticles from thermal decomposition of KA@CP-S under different magnifications: (a) 10k× and (b) 25k×.
VT-PXRD Analysis
The analysis of the VT-PXRD spectrum of KA@CP-S indicates that the experimental patterns of the as-synthesized form adequately match the simulated patterns at main positions. The study suggests that KA@CP-S has good phase purities. The disparate peak intensities in II, III, IV, and V after heating of KA@CP-S at 373, 473, 573, and 723 K, respectively, may result from the favored orientation in KA@CP-S (Figure 8).
Figure 8.
VT-PXRD patterns for as-synthesized KA@CP-S and KA@CP-S after heating.
The CuO nanoparticles synthesized from the thermal decomposition of KA@CP-S were reported by the characteristic peaks observed in the PXRD patterns (Figure S5). In this present work, peak positions were observed at 2θ values of 32.61, 35.52, 38.77, 48.72, 53.7, 58.4, 61.69, 6627, 68.11, 72.46, and 75.15°, confirming the formation of a crystalline monoclinic structure. Well-defined and acute CuO reflections in the detected PXRD patterns confirm the crystalline nature of CuO nanoparticles.20
BET Analysis
The BET method was used to determine the surface area of KA@CP-S through N2 adsorption isotherms at 77 K using the Quantachrome Instruments (St 1 on NOVA touch 4LX). The BET surface area of KA@CP-S from the N2 adsorption measurement is 1.84 m2 g–1.
VT-Photoluminescence Analysis
Coordination polymers based on metal ions, particularly those of Cu(I), are appropriate since they may show rich structural variety combined with bright luminescence emission from blue to red light, but variable temperature luminescence emission for both Cu(I)- and Cu(II)-based coordination polymer/MOFs is not studied yet. In the solid state at room temperature, photoluminescence studies of as-synthesized KA@CP-S, KA@CP-S after heating at 373 K (II), 473 K (III), 573 K (IV), and 723 K (V), and ligands were conducted with a spectrofluorophotometer. The photoluminescence spectra for OrK and Bimb ligands exhibited photoluminescence emission at 362 and 438 nm upon excitation at 290 and 360 nm, respectively (Figure 9a), and the photoluminescence emission spectra of as-synthesized KA@CP-S and KA@CP-S after heating and images of physical facets after heating are summarized in Figure 9b,c. It can be seen that the as-synthesized complex shows two maximum emissions at 542 and 467 nm, while other heated forms of the complex show maximum emissions at 463 nm (II), 464 nm (III), 463 nm (IV), and 459 nm (V) upon excitation at a wavelength of 380 nm. Intensities of emission bands decrease with increasing temperature, which means that at a higher temperature, quenching of the luminescence intensity was observed as well as shifting of emission bands toward a lower wavelength (blue shift) was also pragmatic. After heating the sample at various temperatures, significant changes were seen in the region of higher wavelength at 542 nm, which could be attributed to the release of Bimb ligands. The physical properties of as-synthesized KA@CP-S and KA@CP-S after heating show strong luminescence in the range of 459–542 nm (2.70–2.28 eV) with a red emission in the solid state at room temperature. Generally, the electronic transitions responsible for the luminescence properties of polymeric KA@CP-S can be assigned to intraligand (IL), metal-centered (MC), ligand-to-metal (LMCT), or metal-to-ligand charge transfer (MLCT).21 Temperature (K) vs wavelength (nm)/PL intensity (au) plot for KA@CP-S is shown in Figure S3.
Figure 9.
(a) Luminescence emission spectra of Bimb and OrK ligands; (b) luminescence emission spectra of KA@CP-S (as-synthesized and heated form of KA@CP-S) at 373, 473, 573, and 723 K temperature and (c) picture obtained after heating.
Photocatalytic Activities
Photocatalytic Activity of {[Cu2(Or)2(Bimb)3]·4H2O}n (KA@CP-S)
The photocatalytic activity of as-synthesized KA@CP-S as well as RH and SD were assessed by examining the degradation of organic dyes such as Methyl Orange (MO), Methylene Blue (MB), Eriochrome Black T (EBT), Bromophenol Blue (BPB), Rhodamine B (RhB), Congo Red (CR), Safranin O (SO), Neutral Red (NR), Crystal Violet (CV), Rose Bengal (RB), Methyl Violet (MV), Rhodamine 6G (Rh6G), and Fluorescein (FS) in an aqueous solution under room illumination. Thus, only the dyes CV, RB, MV, and Rh6G that exhibit effective photocatalytic performance with KA@CP-S are covered in depth here (Table 2).
Table 2. Chemical Structures of Neutral, Cationic, and Anionic Dyes Included in This Work.
As a result, KA@CP-S is thought to have a competitive edge for the photocatalytic degradation of organic dyes. The degradation of organic dyes under room illumination without any catalysts was also carried out for comparison. Furthermore, the peaks nearly vanished after 150 min, revealing the equilibrium point at which KA@CP-S had degraded the maximum dye molecules. At equilibrium, cationic dyes like Rh6G (86.5%), MV (76.8%), and CV (76.1%) had a higher exclusive percentage removal by KA@CP-S than anionic (RB, 75.8%) and neutral dyes (NR, 72.0%).
The wavelength maxima and photocatalytic degradation of KA@CP-S, RH, and SD for all dyes were examined in this work (Table S5). The decrease in the concentration of dyes over degradation time, which can be shown in the UV–vis absorption spectra, implies that the degradation efficiency of the dye contaminant from aqueous solution resulted in a significant change in color: for the dye Rh6G, from dark pink to light; for MV, from violet to clear; for RB, from dark purple to light; and for CV, violet to clear solution; this can be attributed to the higher photocatalytic degradation of these dyes. All kinetic parameters for the degradation of dyes by KA@CP-S, RH, and SD were calculated (Tables 3 and S4). The UV–vis absorption spectra of NR, SO, CR, RhB, BPB, EBT, MB, and MO dyes by KA@CP-S are illustrated in Figures S9a–d and S10a–d. The photocatalytic degradation kinetic graph of 12 dyes on KA@CP-S and their degradation efficiencies are shown in Figure 12a–d. The degradation efficiency of KA@CP-S for 12 dyes with time intervals is shown in Figure S11. The UV–vis absorption spectra of dyes without catalyst are shown in Figures S13a–f and S14a–f. RH shows the uptake of Rh6G (75.8%), SO (73.6%), MB (72.5%), and CV (71.6%), whereas MB (84.0%), SO (75.4%), Rh6G (75.3%), and MV (73.0%) were moderately degraded by SD. The UV–vis absorption spectra and photocatalytic degradation kinetic graph of 12 dyes by RH (Figures S15–S18) and SD (Figures S19–S22) are illustrated.
Table 3. Relevant Kinetic Parameters of Photocatalytic Degradation of Rh6G, MV, RB, and CV by KA@CP-S, RH, and SD at 150 min.
| pseudo-first-order |
||||
|---|---|---|---|---|
| dyes | catalyst | R2 | k (min–1) | t1/2 (min) |
| Rh6G | KA@CP-S | 0.654 | 0.0134 | 51.90 |
| RH | 0.905 | 0.0095 | 73.19 | |
| SD | 0.966 | 0.0093 | 74.31 | |
| MV | KA@CP-S | 0.529 | 0.0097 | 70.98 |
| RH | 0.727 | 0.0078 | 88.45 | |
| SD | 0.613 | 0.0087 | 79.76 | |
| RB | KA@CP-S | 0.95 | 0.0095 | 72.61 |
| RH | 0.984 | 0.0021 | 335.69 | |
| SD | 0.787 | 0.0006 | 1156.54 | |
| CV | KA@CP-S | 0.365 | 0.0095 | 72.97 |
| RH | 0.788 | 0.0084 | 82.46 | |
| SD | 0.731 | 0.0038 | 181.61 | |
Figure 12.
(a) UV–vis absorption spectra; (b) photocatalytic dye degradation rate; (c) kinetic plot between ln(C0/Ct) vs time interval; and (d) histogram of the photodegradation percentage of 12 organic dyes recorded after 150 min with KA@CP-S under room illumination.
Influence of Metal Ions on the Photocatalytic Process
A potential chain of events takes place on the surface of a photocatalyst. The e– and h+ transfer to the surface of the catalyst. When light with sufficient energy (photons) is absorbed by a photocatalyst, molecular excitation occurs on the surface of the photocatalyst. As a result, electrons (e–) are stimulated from the valence band (VB) to the conduction band (CB) via suitable band gap energy (Eg), and holes (h+) are generated on the valence band of the associated photocatalyst. Through a sequence of chemical interactions, the activation effect of coordination polymers on the photocatalytic degradation reactions would eventually result in the production of HO• (hydroxyl radical) using H2O molecules. This one is the most potent oxidizer. The electrons in the conduction band simultaneously interact with the dissolved O2 to produce an O2•– (superoxide radical) anion, which may then combine with H2O to convert HO• radicals. The most crucial step in the degradation of organic dyes is the reactivity of the HO• generated with organic pollutants/dyes.22 The aforementioned mechanism is universally applicable and can be applied to our system (Scheme 3).
Scheme 3. Possible Schematic Representation of the Photocatalytic Degradation of Dyes Using KA@CP-S.
The absorption intensities of MO, MB, EBT, BPB, RhB, CR, SO, NR, CV, RB, MV, and Rh6G in water reduced markedly as the reaction time increased with as-synthesized KA@CP-S, RH, and SD. The absorbance at 483.5, 463.8, 663.5, 530.6, 592, 554, 498.6, 519.6, 531.4, 590, 549.5, 584, and 526 nm for FS, MO, MB, EBT, BPB, RhB, CR, SO, NR, CV, RB, MV, and Rh6G, respectively, was used to evaluate the concentration of organic dyes.
Photocatalytic Degradation Kinetics of Catalysts for Dyes
The pseudo-first-order kinetic is used to determine the photocatalytic degradation behavior of the dye catalyst. The statistical histogram of ultimate dye degradation efficacy is shown in Figure 12d. The rate of degradation of organic dyes catalyzed by as-synthesized KA@CP-S was calculated using eq 1
 |
1 |
where C0 and Ct are the initial dye concentration and the dye concentration at time t, respectively, and k is the rate constant of the reaction.23 In most of the cases, the photocatalytic degradation of organic dyes by photocatalysis governs the pseudo-first-order kinetics. For each dye included in this investigation, the estimated rate constant (k) and the R2 value are listed (Tables 3 and S4).
The absorption intensities of Rh6G (Figure 10) and MV (Figure S6) dyes dropped dramatically with the increase of the illumination time in the presence of as-synthesized KA@CP-S under room illumination under different pH conditions.
Figure 10.
UV–vis absorption spectra of Rh6G at (a) pH ∼7.5, (b) pH 4, (c) pH 10, and (d) pH 12 in aqueous solutions under room illumination using KA@CP-S.
The absorption intensities of RB (Figure S7) and CV (Figure S8) dyes dropped dramatically with the increase of illumination time in the presence of as-synthesized KA@CP-S under room illumination under different pH conditions (Figure 11).
Figure 11.
(a–d) Plot of the Ct/C0 concentration ratios of Rh6G, MV, RB, and CV vs time interval, (e–h) kinetic plot between ln(C0/Ct) vs time interval for Rh6G, MV, RB, and CV, respectively, under different pH conditions (4, 10, and 12) under room illumination using KA@CP-S.
The degradation efficiency was calculated using eq 2
 |
2 |
where C0 and Ct are the initial dye concentration and the dye concentration at time t, respectively.24 The structure of all dyes is presented in Table 2. After 150 min of exposure to as-synthesized KA@CP-S, the photocatalytic degradation efficiencies of MO, MB, EBT, BPB, RhB, CR, SO, NR, CV, RB, MV, and Rh6G were 9.5, 17.8, 21.2, 23.7, 29.6, 40.2, 59.0, 72.0, 75.8, 76.1, 76.8, and 86.5%, respectively (Figure 12). The overall degradation efficiencies for the dyes used in this work with the catalysts KA@CP-S, RH, and SD and without the catalyst are shown in Figure S23. Also, a comparison of the photocatalytic degradation of dyes for KA@CP-S and other known photocatalysts is presented (Table 6).
Table 6. Comparison of the Photocatalytic Degradation of Dyes for KA@CP-S and Other Known Photocatalysts.
| catalyst | dye | condition | time | degradation efficiency (%) | ref |
|---|---|---|---|---|---|
| [Zn(bpe)(fdc)]·2DMF | CV | UV-Hg, 500 ppm | 120 min | 92.5 | (27) |
| {[Co3(BTC)2(Bimb)2.5]·2H2O}n | MV | UV, 10 ppm | 135 min | 68.7 | (4a) |
| CV | 53.9 | ||||
| [Cu2(L1)·5DMF], H4L1: 3,5-di(3,5-dicarboxyphenyl)nitrobenzene | MV | UV, 500 ppm | 45 min | 70 | (28) |
| {[Cu2(L)(H2O)2]·H2O·3DMA·(CH3)2NH2}n, H2L: 2,5bis(3′,5′-dicarboxylphenyl)benzoic acid | MV | Hg, 10 ppm | 100 min | 100 | (29) |
| {[Zn(1,4-ndc)(tpcb)0.5]}n | MB | UV, 500 ppm | 10 h | 50 | (30) |
| {[Pb(Tab)2(bpe)]2(PF6)4}n | MO | UV, 100 ppm | 300 min | 95 | (31) |
| {[Cd2(H2O)2(tpeb)2(1,2-CHDC)2]·H2O}n | CR | visible light, 200 ppm | 90 min | 90 | (32) |
| Zn-U3 | RB | UV, 5 ppm | 4 h | 93 | (33) |
| {[Zn(PMBD)(DPB)]·DPB}n | Rh6G | Hg, 10 ppm | 75.8 | (34) | |
| KA@CP-S | Rh6G | room illumination, 10 ppm | 150 min | 86.5 | this work |
| MV | 76.8 | ||||
| RB | 76.1 | ||||
| CV | 75.8 | ||||
| CR | 40.2 | ||||
| MB | 17.8 |
We investigated the band gap energies in KA@CP-S using a reflectance UV–vis spectrophotometer at room temperature prior to the catalysis experiments. The degradation rate decreased over time as the band gap energy (Eg) increased. To calculate the band gap energy (Eg) for the as-synthesized KA@CP-S, the Kubelka–Munk (K–M) method and Tauc plot ((αhν)2 vs energy) (Figure 13b) and UV–vis absorption spectra were used (Figure 13a),25 and it was determined to be 3.55 eV (349.3 nm) for as-synthesized KA@CP-S. This band gap energy implied that KA@CP-S might be photocatalytically active and might react to room illumination.
Figure 13.
(a) UV–vis absorption spectra of KA@CP-S and (b) Tauc plot ((αhν)2 vs energy) for band gap analysis of KA@CP-S.
Capture Study
According to the UV–vis data, the KA@CP-S swiftly removed dyes, namely, CV (2.40 ppm), RB (2.38 ppm), MV (2.31 ppm), and Rh6G (1.35 ppm) from the polluted water samples at about pH ∼7.5 (Figure 14a–d). Removal efficiencies of more than 75% of the dyes indicated were achieved in 150 min from 1.35 to 2.40 ppm concentration of detection limit. However, when pH levels of 12, 4, and 10 were maintained for the dyes CV, RB, MV, and Rh6G, detection limits of 0.60, 0.20, 0.33, and 0.20 ppm, respectively, were reached within 75 min (Figure 14e–h). Having said that, all of the dyes chosen are observed as one of the most hazardous and alarming water toxins owing to their adverse effects on human health as well as the global environment.
Figure 14.
(a–d) Dye degradation efficiency/concentration of KA@CP-S with dyes (Rh6G, MV, RB, and CV) at pH ∼7.5 and (e–h) with pH as a function of contact time.
Recyclability and Stability of KA@CP-S
For commercial or industrial applications, convenient regeneration of CPs after dye degradation and retention of their degradation capacity are very important. After catalytic degradation of Rh6G, MV, RB, and CV by KA@CP-S, it was washed several times with ethanol until all dye molecules were liberated, dried, and again employed for the photocatalytic study. In terms of dye degradation from aqua systems, the feasibility of regeneration and reusability of KA@CP-S has been investigated. The acquired data reveal that even after five cycles, the KA@CP-S maintains its initial catalytic activity, with just a minor drop in the CV, RB, MV, and Rh6G degradation efficiency. Degradation efficiency for Rh6G decreases from 86.5% (1st cycle) to 72.0% (5th cycle), while it decreases from 76.8% (1st cycle) to 60.3% (5th cycle) for MV, from 76.1% (1st cycle) to 58.0% (5th cycle) for RB, and from 75.8% (1st cycle) to 56.4% (5th cycle) for CV (Figure 15). The PXRD data of the recovered catalyst indicates that the structure of KA@CP-S remained substantially unchanged after five photocatalytic cycles, further supporting the chemical stability of KA@CP-S during the photocatalytic test (Figure S12). Accordingly, the recycling tests indicated that the KA@CP-S was stable and could be employed as photocatalysts for the degradation of CV, RB, MV, and Rh6G for at least five cycles.
Figure 15.
Catalytic plot of KA@CP-S for CV, RB, MV, and Rh6G over five cycles.
The color of the dye aqueous solution changes from light to dark yellow for FS dye. The patterns of absorption spectra of the Fluorescein (FS) dye with KA@CP-1, RH, and SD are unique from those of other dyes, implying that there is no photocatalytic enhancement and no electronic interaction between the Fluorescein (FS) dye and KA@CP-1, RH, and SD (Figure S24). None of them exhibit extra peaks emerging at longer wavelengths, which would suggest aggregation of the particles, nor do they show any photocatalytic amplification.26
Effect of pH: Analysis of Initial and Equilibrium Concentration toward Dye Degradation
The remarkable degradation efficiency of KA@CP-S was demonstrated by the rapid removal of CV, RB, MV, and Rh6G from the aqueous medium. The pH of the solution is an important parameter in the dye degradation process because it can influence the surface charge of the catalysts, the degradation tendency of the catalyst, owing to its effect on the functional groups, the degree of ionization, and the structure of the dye molecules.24
To further investigate the degradation behavior for CV, RB, MV, and Rh6G dyes of KA@CP-S, several experimental setups were applied at various pH values ranging from 3 to 12 by regulating the solution pH using 0.1 M HCl and NaOH solutions at ambient temperature. There are two different setups used: (i) maintaining the pH of the solution between 3 and 12 after reaching the equilibrium at 150 min. The effect of pH on the degradation rate of the dyes CV, RB, MV, and Rh6G by KA@CP-S was examined for this purpose. The entire degradation experiment was conducted at pH ∼7.5, and at this pH, KA@CP-S showed the maximum degradation rate for CV (75.8%), RB (76.1%), MV (76.8%), and Rh6G (86.5%). When the pH was increased from 3 to 12, the degradation rate of CV, RB, MV, and Rh6G increased from 75.8 to 82.5% (pH 12), 76.1 to 81.3% (pH 4), 76.8 to 83.8% (pH 12), and 86.5 to 87% (pH 10), respectively (Figure 16). The equilibrium degradation efficiencies for cationic dyes CV, MV, and Rh6G were found to be reduced in acidic conditions, with a considerable drop in degradation behavior, whereas for anionic dye RB, a higher degradation rate was observed at pH 4. The maximum degradation was noticed at pH 10–12, which is regarded as the ideal pH for photocatalytic degradation at room temperature. In another setup for the pH effect, (ii) the pH of the solution is initially fixed at pH 4, 10, and 12 separately before the degradation experiment. Figure S25 shows the PXRD pattern of KA@CP-S under different pH conditions before the photocatalytic degradation of CV, RB, MV, and Rh6G. In these experiments, KA@CP-S showed excellent catalytic degradation rates for CV (94.0%) at pH 12, RB (98.0%) at pH 4, and MV (97.0%) and Rh6G (98.0%) at pH 10 (Figure 17). Kinetic studies of photocatalytic degradation of dyes at different pH values are depicted in Figure 11, and kinetic parameters are listed in Table 4. Photocatalytic degradation of Rh6G, MV, RB, and CV by KA@CP-S at different pH values with different contact times is shown in Table 5. Under severe pH conditions, the degradation efficiency may decrease due to structural changes in the complex or interference by ionic species in the surrounding environment. For example, under highly acidic conditions, nitrogen atoms of ligands might be protonated, resulting in a positively charged surface of the KA@CP-S. Electrostatic repulsion between the cationic components of the dye and KA@CP-S might then occur, resulting in a considerable reduction in the photocatalytic degradation process (Table 6).
Figure 16.
Effect of pH on dye (CV, RB, MV, and Rh6G) removal efficiencies for KA@CP-S.
Figure 17.
Degradation plot of KA@CP-S for CV, RB, MV, and Rh6G over pH 4, 10, and 12 at 75 min.
Table 4. Relevant Kinetic Parameters of the Photocatalytic Degradation of Rh6G, MV, RB, and CV by KA@CP-S at Different pH values at 75 min.
| pseudo-first-order |
|||||
|---|---|---|---|---|---|
| dyes | pH | dye degradation (%) | R2 | k (min–1) | t1/2 (min) |
| Rh6G | 4 | 93.8 | 0.867 | 0.0372 | 18.61 |
| 10 | 98.0 | 0.837 | 0.0520 | 13.31 | |
| 12 | 94.0 | 0.874 | 0.0376 | 18.43 | |
| MV | 4 | 96.0 | 0.732 | 0.0428 | 16.20 |
| 10 | 97.0 | 0.826 | 0.0455 | 15.22 | |
| 12 | 91.0 | 0.355 | 0.0322 | 21.54 | |
| RB | 4 | 98.0 | 0.630 | 0.0519 | 13.35 |
| 10 | 23.0 | 0.930 | 0.0035 | 196.03 | |
| 12 | 21.3 | 0.814 | 0.0031 | 226.18 | |
| CV | 4 | 89.5 | 0.872 | 0.0301 | 23.01 |
| 10 | 83.0 | 0.635 | 0.0236 | 29.36 | |
| 12 | 94.0 | 0.709 | 0.0374 | 18.52 | |
Table 5. Photocatalytic Degradation of Rh6G, MV, RB, and CV by KA@CP-S at Different pH values with Different Contact Times.
| dye degradation
(%) |
||||
|---|---|---|---|---|
| KA@CP-S/dye | pH value | 15 min | 75 min | 150 min |
| KA@CP-S/Rh6G | ∼7.5 | 64.12 | 81.48 | 86.50 |
| 4 | 62.96 | 93.8 | ||
| 10 | 72.51 | 98.0 | ||
| 12 | 62.79 | 94.0 | ||
| KA@CP-S/MV | ∼7.5 | 60.69 | 73.98 | 76.80 |
| 4 | 87.28 | 96.0 | ||
| 10 | 80.92 | 97.0 | ||
| 12 | 89.59 | 91.0 | ||
| KA@CP-S/RB | ∼7.5 | 16.11 | 58.94 | 76.10 |
| 4 | 93.26 | 98.0 | ||
| 10 | 8.94 | 23.0 | ||
| 12 | 9.49 | 21.3 | ||
| KA@CP-S/CV | ∼7.5 | 68.13 | 72.52 | 75.80 |
| 4 | 58.51 | 89.5 | ||
| 10 | 65.99 | 83.0 | ||
| 12 | 72.52 | 94.0 | ||
Conclusions
Finally, we developed a new mixed-ligand H-bonded copper-based coordination polymer. According to variable temperature luminescence analysis, as-synthesized KA@CP-S and KA@CP-S after heating exhibits luminescence in the region of 459–542 nm (2.70–2.28 eV) with a red emission. They seem to be promising candidates for new photosensitive materials in the future. KA@CP-S shows excellent catalytic degradation rate for CV (94.0%) at pH 12, RB (98.0%) at pH 4, and MV (97.0%) and Rh6G (98.0%) at pH 10. KA@CP-S swiftly removed these dyes, namely, CV (0.60 ppm), RB (0.20 ppm), MV (0.33 ppm), and Rh6G (0.20 ppm), from the polluted water samples. The excellent photocatalytic degradation and regeneration properties of KA@CP-S make it a potential and efficient future remedial material for the detection and separation of toxic dyes from wastewater contaminated by industrial effluents that affect the Algal growth, which plays many important and beneficial roles in maintaining the freshwater environment.
Acknowledgments
One of the authors, Somnath, conveys his heartfelt appreciation to the National Institute of Technology Raipur for awarding him with an Institutional research fellowship. The authors are also grateful for the research facilities provided by the Department of Chemistry. K.A.S. expresses his gratitude to the Director, National Institute of Technology Raipur, India, for his cooperation with the “Crystal Engineering” research.
Supporting Information Available
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c04669.
Selected bond lengths and bond angles of KA@CP-S; hydrogen bond parameter and solvent mask information for KA@CP-S; relevant kinetic parameters of photocatalytic degradation of NR, SO, CR, RhB, BPB, EBT, MB, and MO by KA@CP-S, RH, and SD; wavelength maxima (λmax) and degradation (%) of dyes with and without catalysts; temperature (K) vs wavelength (nm)/PL intensity (au) plot for KA@CP-S; 1D chain of KA@CP-S resulting from Bimb bridging; topological structural fragment of KA@CP-S coordination polymer: fragment of metal–ligand connectivity via hydrogen bonding formed a 3D periodic structure along b-axis; EDAX diagram and powder X-ray diffraction spectra for CuO nanoparticles; UV–vis absorption spectra of MV, RB, and CV at pH ∼7.5, pH 4, pH 10, and pH 12 in aqueous solutions under room illumination using KA@CP-S; UV–vis absorption spectra of NR, SO, CR, RhB, BPB, EBT, MB, and MO in aqueous solutions using KA@CP-S under room illumination; degradation of dyes using KA@CP-S with time intervals; PXRD pattern of KA@CP-S before and after photocatalytic degradation of CV, RB, MV, and Rh6G; UV–vis absorption spectra of dyes in aqueous solutions in the absence of catalysts under room illumination; UV–vis absorption spectra and kinetic plot, photodegradation percentage of dyes in aqueous solutions using RH and SD under room illumination; dye degradation (%) in the absence and presence of catalysts (RH, SD, and KA@CP-S); UV–vis absorption spectra for the FS without catalyst, KA@CP-S, RH, and SD in aqueous solutions under room illumination; PXRD pattern of KA@CP-S in different pH conditions before photocatalytic degradation of CV, RB, MV, and Rh6G (PDF)
Accession Codes
Single-crystal data of KA@CP-S with CCDC 2178636. This data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; or Email: deposit@ccdc.cam.ac.uk.
Author Contributions
Somnath: synthesis of coordination polymer, prestructure property interpretation, batch experiment for dye adsorption, preanalysis of data, prewriting of the manuscript. M.A.: crystal data collection and structure solution. K.A.S.: concept, final structure property interpretation, final data analysis, manuscript writing and editing, and supervision.
The authors declare no competing financial interest.
Supplementary Material
References
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