Considerable amounts of dyes in wastewater causes harmful to the environment system owing to their resistance to be biodegradable due to their chemical stability [37, 38]. So, elimination of such hazardous materials is an important environmentally target. Here, methylene blue (MB) as one of the wide used dye was used as a selective pollutant for mineralization experiments to estimate the catalytic efficiency of 1 utilizing the eco-friendly H2O2. MB is usually utilized for dyeing wood, cotton and silk which suffers hard decomposition under light irradiation [39]. To address the catalytic activity of SCP1 for degradation of MB, the absorption spectra of MB at 664 nm were measured at regular time intervals. The spectra of the [H2O2/ MB] solution, as blank experiment, exhibit noteworthy change in the absorbance of MB however, up on addition of the heterogeneous catalyst 1 to [H2O2/ MB] solution, the reaction propagates very fast causing bleaching of MB where 94.6% was decolorized within 90 min to the mineral components, Fig.5. Irradiation of the catalytic solution by UV-light enhances the reaction rate giving 95% degradation efficiency within 40 min, Fig.6. It is worth mentioning that the absorbance bands at 250, 296 and 664 nm of MB dye disappear supporting mineralization of MB dye via destruction of the phenyl ring and the hetero- poly-aromatic system. The observed rate constant, Kobs., was calculated from the slope of the first-order plot which verifies:
ln At= ln A0- kobs. t or ln A0/At= kobs.t (2)
Ao and At are the absorbance at time t= 0 and time= t, respectively. The plot exhibits pseudo first-order rate with respect to MB dye concentration, Kobs = 0.0189 min-1 (R = 0.994) for normal degradation and Kobs = 0.0252 min-1 (R = 0.980) for degradation under UV-light radiation. On the other hand, the catalyst 1 reserved the catalytic activity for at least five cycles, Fig. 7. The catalyst SCP1 was washed several times with double distilled water and was dried after each catalytic cycle. After that the catalyst was utilized in starting a new experiment. The degradation efficiencies of the first three cycles are the same while after that they suffer very slight decrease indicating that the SCP1 catalyst maintained its efficient catalytic activity, Fig. 7. The IR spectra of the catalyst 1 had been carried out after each cycle which displays the same bands of the fresh prepared SCP indicating that the catalyst was not oxidized by H2O2, the case which was further supported by elemental analyses indicating that the structure of the catalyst 1 retains its identity.
6.1 Hydroxyl radical determination
Applying the disodium salt of terephthalic acid (NaTA) technique the •OH radicals can be detected since NaTA reacts selectively with •OH radicals giving a bright stable fluorescence [40]. This reaction represents selective probe for detecting •OH radicals since it is not reactive to H2O2, HO2• and O2•- species [41]. The fluorescence intensity of the [SCP1/H2O2/ NaTA] system increases dramatically to 920 within 5 min supporting that •OH radicals produced in solution, Fig. 8a, while on adding NB dye to the [SCP1/H2O2/ NaTA] system the fluorescence intensity significantly decreased reaching to 375 within 35 min, Fig. 8b. Thus, most of the •OH radicals reacted rapidly with MB, as the discoloration was also observed. So, a mechanism of oxidation reaction can be formulated involving the combination of H2O2 and the [SCPCuI] catalyst producing •OH radicals via electron transfer from the catalyst [SCPCuI] to H2O2 as well as the effect of UV-radiation according to the following equations:
[SCPCuI] + H2O2 → [SCPCuI-H2O2]
[SCPCuI-H2O2] → [SCPCuII] + •OH + OH-
[SCPCuII] + OH- → [SCPCuI] + •OH
MB + •OH → Intermediates → oxidation products + CO2
Under UV-radiation, the electrons (e−) in the [SCPCuI] catalyst moved from (HOMO) or (CB) to (LUMO) or (VB) leaving positive holes (h+) in (CB). The electrons in (VB) react with O2 producing •O2− which may affect degradation reaction. The holes can directly oxide the dye. Meanwhile, the HOMO acquires electrons to have the stable state, therefore, one electron can be captured from H2O producing •OH radicals which able to further destroy MB dye efficiently.
[SCPCuI] + hν → h+VB + e−CB
h+VB + H2O → •OH + H+
[SCPCuI] + h+ → [SCPCuII]
[SCPCuII] + OH− → [SCPCuI] + •OH
MB + •OH/ h+ → degradation products
Also, H2O2 under UV-radiation can accelerate the degradation process according to the following equations.
H2O2 + hν → 2 •OH
eCB− + H2O2 → •OH + OH−
•O2− + H2O2 → •OH + OH− + O2
General Discussion
One of the most important characteristics of supramolecular coordination polymers (SCP), of course, is the structural variability which can be affected by many factors as the combination of the type and number of ligands and metal ions, coordination geometry and the presence of ionic charge. Also, structural flexibility is usually accompanied with structural variability when approaching supramolecular coordination polymer solids. Self-assembly of R3Sn+cation (R=Ph or Me), [Cu(CN)4]3- anion and aliphatic diamine ligands [L=urea 2, ethylenediamine (en) 3, propane-1,3-diamine (tn) 4, 5, 1,4-diaminobutane (DAB) 6, 1,5-diaminopentane (DAP) 7, 1,6-diaminohexane (DAHX) 8, and 1,7-diaminoheptane (DAHP) 1] afford the 3D-SCPs, 1-8 which represent self- recognition process of supramolecular chemistry paradigms. Eight SCP had been synthesized namely: Ph3SnOH 2, 3∞{[CuII(en)2H2O][CuI2(CN)4]}, 3,3∞{[H2tn][(Cu2(CN)3)2].H2O}, 4,3∞{[CuII(tn)2][Cu2I(CN)3]2}, 5, {[H2DAB][Cu4(CN)6].2H2O}, 6, {[H2DAP][H3O][Cu4(CN)7].2H2O}, 7, ∞ 3[(Cu3(CN)3)2.(DAHX)], 8 and ∞ 3[(Cu3(CN)3)2.(DAHP)], 1. Surprisingly, using Ph3SnCl with different diamines and K3[Cu(CN)4] gave Ph3SnOH, 2 which is CuCN free. The hydroxy anion acts as µ2-ligand bridging the (Ph3Sn)+ fragment forming 1D-corrugated chains which extends to 2D-network via H-bonds [30]. In spite of SCP 1 and 3-8 had been created by similar reactants, they have different structures exhibiting diverse building blocks; [Cu2(CN)3]-, [(Cu2(CN)3]2-2, [(Cu2(CN)4]2-, [Cu4(CN)6]2-, [Cu4(CN)7]3- and [Cu3(CN)3]2. It is a matter of fact that the structures of SCPs 1-8 do not contain the starting reactant anion, [Cu(CN)4]3-. The structure of 3 contains tetrahedral CuI creating 3D-network via formation of interpenetrating fused six-membered rings [Cu6(CN)6] adopting wide cavities capable to accommodate the voluminous [CuII(en)2H2O] complex. The anionic [Cu2(CN)3]2- building blocks of the colorless 4 exhibit CuI distorted trigonal planar sites creating 2D- layers containing 3D-fused honeycomb rings forming wide voids. These layers alternatively accommodate the guest (H2tn)2+ cations and H2O molecules. On the other hand, red violet crystals of 5 had been formed utilizing the same reactants which created the colorless compound 4. In this case, the liquid tn ligand was directly used without solving in H2O. The structure of 5 contains the mixed valence CuI/CuII systems which are so far rare where it consists of the anionic [Cu2(CN)3]2- fragments and the cationic [CuII(tn)2]2+ unit. The puckered CuCN chains interpenetrate creating 6-membered rings forming box-like structures encapsulating the guest [CuII(tn)2]2 cation [30]. The aliphatic diamines DAB and DAP affect the topology of 6 and 7 creating colorless [Cu2(CN)3]22- and [Cu4(CN)7]3- networks, respectively. 6 adopts interpenetrating framework with box-like structure while 7 starts to form the rhomboidal [Cu2(μ3-CN)2] motifs which construct wide cavities enough for the longer H2DAP guest and the water molecules to locate as guests into the network structure of 7. The guest H2O molecules are present in the structures of 4, 6 and 7 to fill the space since nature tends to avoid empty space [29]. As the chain of the diamine extends to 6 (DAHX) and 7 (DAHP) CH2 fragments, the structures consist of 3D-(Cu3(CN)3)2 with tetrahedral CuI geometry. The 3D-networks are constructed of the unique bifold [Cu2(μ3-CN)2] units creating complex structure to construct wide cavities capable to encapsulate the long chain (DAHX) and (DAHP). It is observed that ethylenediamine (en) and propane-1,3-diamine (tn) can oxidize part of CuI to CuII that forms guest CuII complexes which is not the case for the longer diamines. So, the size and shape of the aliphatic diamine and the nature of solvent represent an important role for constructing such diverse structures. The packing of the structures is highly influenced by the capability of the aliphatic diamines to form H-bonds and π–π stacking formed via the CH and H2O. On the other hand, the most interesting phenomenon observed regarding these structures is the formation of interpenetrating framework on going from short to long diamine ligand accompanied with the formation of box-like structures containing large cavities (pores) enable to encapsulate the bulky ligand. The box-like structures are also created by the formation of the rhomboidal [Cu2(m3-CN)2] motifs which afford wider cavities in the case of longer diamines than five CH2 groups. These SCPs are efficient heterogeneous catalysts for elimination of hazardous dyes in wastewater with H2O2.