Degradation Of Dyes By Bioinspired Catalytic Fibers Based On High-valent Cobalt-oxo Species

Water pollution has become one of the major environmental issues in the world. The totalamount of dyeing wastewater discharge has been growing with the increasing consumer demandfor textiles, posing a long-term threat to aquatic life and human health. With the characteristics ofhigh temperature, high solids, high pH and high backgrounds of complicated constituents, dyeeffluents are difficult to be effectively eliminated by the conventional advanced oxidationprocesses (AOPs), because hydroxyl radical (OH) inevitably react with the high-concentrationand easily handled constituents, leading to the inefficient oxidation of low-concentration targetpollutants, even the oxidative destruction of the catalysts themselves. Thus, a desirable oxidationsystem for removing recalcitrant pollutants must be shielded from the interference of externalcomplicated constituents or employ these constituents to promote the elimination of the targetcontaminants. Notably, enzymes such as cytochrome P450and peroxidase play important rolesin the oxidation of living organisms, and exhibit high activity by generating the powerfulhigh-valent metal-based oxidants, which have been identified as reactive intermediates inbiology and are highly selective to the target substrate. Inspired by the enzymatic reactions,effective strategies for designing catalytic systems should minimize the formation of unselective OH, and the heterolytic cleavage of the O-O bond is more desirable. Here, we developed thebioinspired catalytic systems based on cobalt phthalocyanine (CoPc) and fibers to eliminate dyeswith H2O2as the oxidant. The heterolytic cleavage of the O-O bond that can be achieved bychanging the coordination sphere of the central Co ion, generating the high-valent cobalt-basedoxidant (Co-oxo) intermediates. Three series of work were carried out as follows:1. A bioinspired catalytic system based on cellulosic fiber–boned cobalt phthalocyanine(CoPc-F) has been developed for capturing and oxidizing dyes by H2O2activation in thepresence of high additive concentrations. In this system, the cellulosic fibers provided theamorphous regions, functioning as cavities in the same manner as the protein backbones ofenzymes for the selective accessibility of dyes; CoPc was employed as the catalytic entity;and linear alkylbenzene sulfonate (C12-LAS, one of the most widespread surfactants inindustrial and domestic wastewater) was introduced as the fifth ligand to help generatehigh-valent cobalt-oxo intermediates by the heterolytic cleavage of the peroxide O–O bond.The high-valent cobalt-oxo (PcCoIV=O) intermediates were identified as the major active species evidenced by the in situ EPR experiment at room temperature, together with theresult of cyclic experiments, indicating that such high oxidation state intermediates can beanchored in the cellulose matrix without autooxidation. According to detailed densityfunctional theory (DFT) calculations, the spin populations are predominantly located aroundthe cobalt-oxo center, achieving an electrophilic attack on the electron-rich azo bond and thearomatic ring of the target model dyes. This work provides new insight into the eliminationof recalcitrant dyes, as well as that of low-concentration contaminants in high backgroundsof complex constituents.2. We developed the bioinspired catalystic systems, CoPc with an axial ligands of pyridine (py)or thiophenol (tph) anchored on the py-/tph-modified multiwalled carbon nanotubes(MWCNTs-py, MWCNTs-tph) and carbon fibers (CF-py, CF-tph), forming the bioinspiredcatalysts of CoPc-py-MWCNTs, CoPc-tph-MWCNTs, CoPc-py-CF and CoPc-tph-CF basedon the coordination interaction between the central Co ions and py/tph groups. The catalyticsystems based on CoPc-py-MWCNTs and CoPc-py-CF present the higher activity for H2O2activation than that of CoPc-tph-MWCNTs and CoPc-tph-CF systems respectively. It wasfound that H2O2can be effectively activated by CoPc-py-MWCNTs or CoPc-py-CF without OH generation at pH10and the catalysts have exhibited long-term stability during thecontinuous cycle oxidations of the dye substrate. On the contrary, both theCoPc-py-MWCNTs and CoPc-py-CF catalytic systems give the OH-dominated oxidationprocesses at pH3, as well as the disappointing stability. According to the results of EPRexperiments and DFT calculations, the activation of H2O2occurs with the heterolyticcleavage of the peroxide O–O bond, which might lead to the high-valent Co-oxointermediates production to oxidize the substrates. Furthermore, the high-concentrationadditives could improve the dye oxidation in the CoPc-py-CF/H2O2system at pH10,holding great promise for wide-ranging applications to waste water treatment.3. For transition metal-based catalysts, the coordination spheres of the central metal ions areessential to their catalytic performance by influencing the electron configuration. Here, wepresent the bioinspired catalysts by high temperature oxidation of CoPc on MWCNTs andCF in the air atmosphere, introducing the Co-O bond for changing the coordination sphereof CoPc, and the catalysts of oxidized-CoPc/MWCNTs (o-CoPc/MWCNTs) and oxidized-CoPc/CF (o-CoPc/CF) have been obtained at different temperatures (290,370,410,450and530°C). It can be found that the Co ions of o-CoPc/MWCNTs and o-CoPc/CFobtained at290°C have the similar inner coordination spheres with CoPc, the oxidation onlyoccured on the outer substituents and partial benzene ring, and the lower catalytic activityhas been presented in systems with o-CoPc/MWCNTs (290°C) or o-CoPc/CF (290°C).With the increase of oxidation temperature, Co-O bonds have been introduced and graduallyincreased, which are different from the Co-O bonds of Co3O4. But, partial Co-O bonds ofo-CoPc/MWCNTs (450°C) and nearly all Co-O bonds of o-CoPc/MWCNTs (530°C) havebeen changed into the Co-O bonds of Co3O4, presenting the decreased catalytic activity. Andthe o-CoPc/MWCNTs obtained at410°C, with Co-O bonds and Co-N bonds, performed thebest catalytic activity. According to the results from X-ray diffraction, X-ray photoelectronspectroscopy and differential thermal analysis-thermogravimetry, the combination of Co-Oand Co-N bonds is the key to exhibiting high activity for o-CoPc/MWCNTs (410°C) ando-CoPc/CF (410°C) by changing the coordination sphere of Co. Further, the high oxidationactivity and selectivity for the dye substrate have been achieved in the presence ofhigh-concentration additives in systems of o-CoPc/CF (410°C), and the excellent stabilityhas been presented during continuous cycle experiments. The activation of H2O2has beencatalyzed by o-CoPc/MWCNTs (410°C) without OH production, and the reactive Co-oxointermediates might generate with the heterolytic cleavage of peroxide O–O bond, indicatingthe reaction channel of H2O2-activation has been influenced by the coordination sphere ofCo resulting from the interaction of O and N ligands.