Series Of Molecular Imprinted TiO₂ Photocatalysts And Their Application In Selectively Rapid Mineralization Of Highly Toxic Organic Pollutants

Nowadays, TiO₂ photocatalysis has been well studied in concerns of environmental protection, and it has been found to show a good advantage in degradation of Highly Toxic Organic Pollutants (HTOPs). However, it is difficult to realize selective removal of low-level HTOPs from complicated wastewaters via photocatalytic treatment, because TiO₂ has very poor selectivity and cannot differentiate between these pollutants. Thus, we develop a new method to obtain high selectivity by coating imprinted polymers on the photocatalyst's surface. Using a photocatalytic treatment with the resulted simulated enzyme photocatalysts, we can selectively and rapidly mineralize the target HTOPs in the presence of high-level less toxic pollutants.Firstly, considering the molecularly imprinted polymers should be stale in the oxidizing environment, we prepared three types of organic conductive MIPs/TiO₂ composite photocatalysts. During the preparation of the catalysts, we use o-phenylenediamine as a functional monomer, and selected Real Substrate, (RS), Substrate Analogue (SA), and Transition State Analogue (TSA) as the template molecules for different HTOPs. Based on the photocatalytic experiments, these types of MIPs/TiO₂ photocatalysts are compared: 1) By using 2-NP and 4-NP as templates (the target pollutants), RS-MIPs/TiO₂ photocatalysts were successfully prepared via surface molecular imprinting, and were observed to have high selectivity toward the photodegradation of the target pollutants. The promoted selective removal of the target pollutant over the RS-MIPs/TiO₂ is mainly attributed mainly to the special recognition of the MIPs to the target. 2) SA-MIPs/TiO₂ can control the transformation of the target moleculeduring the photocatalysis. To confirm this, SA-MIPs/TiO₂ were prepared by using structural analogues (DNP) of PCP as pseudo templates. The accelerated photodegradation of the target PCP and depressed the generation of toxic intermediates is partly due to the affinity between target to SA-MIPs/TiO₂ is as high as the affinity between template to SA-MIPs/TiO₂. The other reason is that the amino groups at the footprint cavities provided a well-defined micro reaction environment, which made the benzene ring of the adsorbed PCP be better exposed to photo-generated reactive·OH radicals, leading to easier cleavage of the benzene ring. 3) By using transition state analogue (2-NP or 4-NP) as template, TSA-MIPs/TiO₂ photocatalysts is produced. In comparison with neat TiO₂, the obtained TSA-MIPs/TiO₂ photocatalysts not only increase the photodecomposition of the target pollutant (NB), but also inhibit the accumulation of unwanted byproducts, such as NPs. This finding indicates the TSA-MIPs/TiO₂ prepared with proper template possesses special molecular recognition to the reaction transition state, which would accelerate the generation and the degradation of the reaction transition state.Secondly, it is observed that the selectivity of the above MIPs/TiO₂ gradually worsened during a long period of UV-light illumination if there was only a low level of the target pollutant in solution. To overcome the shortcoming of the organic conductive MIPs/TiO₂ composite photocatalysts, inorganic molecular imprinted polymer (IMIP) coated photocatalyst for photodegradation of diethyl phthalate (DEP) was synthesized by coating a layer of molecular imprinted silica/alumina on the surface of TiO₂ nanoparticles with DEP as the template. The 27Al MAS NMR measurements revealed that the IMIP layer consisted of framework tetrahedrally coordinated aluminum and non-framework octa-coordinated aluminum species, both of which function as the hot spots for the adsorption of target molecules on the catalyst during photocatalysis. It was found that the IMIP layer provided the photocatalyst with special molecular recognition ability, leading to selective adsorption and rapid mineralization of the target pollutant from complicated wastewaters. Unlike the neat TiO₂ photocatalyst, the use of IMIP-coated TiO₂ photocatalyst almost eliminated the generation of toxic aromatic byproducts.Thirdly, due to nano powder imprinted photocatalysts possess some defects, such as difficult to reuse, we developed a novel approach for the preparation of photocatalytic thin film with high selectivity. Among different approaches of removing the template in the preparation of MIFs, a calcination treatment was the best to produce more 3D"footprint"cavities of the template, which promoted further the photocatalytic activity of the MIF in comparison with the films pre-treated by extraction or photodegradation. Compared with the non-imprinted TiO₂ film (NIF), the MIF enhanced the photodegradation of the target pollutants by increasing the adsorption of the target pollutants on the surface of the MIF. From the Langmuir-Hinshelwood model, the value of the apparent reaction rate constant on the MIF was obtained, which was much larger than that on the NIF. The equilibrium adsorption constant on the MIF was more than 7 times that on the NIF. Because of this high affinity, the MIF exhibited special molecular recognition ability, leading to selective adsorption and photodegradation of the target pollutant. Moreover, the MIF was confirmed to have good stability during long-time photocatalysis.In short, this paper firstly prepared a series of imprinted TiO₂ photocatalysts or film via different methods. The synthesis of all the catalysts was carried out in water phase, leading these hybrid TiO₂ photocatalyts to selectively remove low-level HTOPs. Furthermore, by tailoring the template molecule, the prepared catalysts can effectively choose the type of intermediate products with inhibit the accumulation of highly toxic intermediates. Thus, these molecular imprinted photocatalyst (film) have promising applications in selectively removal of low-level target pollutant in the presence of other high-level pollutants, with good market prospects and economic benefits.