Design and environmental applications of polyoxometalate-ionic liquid (POM-IL)-based molecular and composite materials

Polyoxometalate-based molecular and composite materials were investigated for eco-friendly sustainability concerns like acid corrosion protection and water purification. A comprehensive literature investigation on the classical and non-classical roles of counter-cations in polyoxometalate (POM) chemistry was also achieved. One principal project deals with the investigation of POM based ionic liquids (POM-ILs) as transparent and highly viscous coatings to prevent the corrosion of different typical mineral stones with different elemental composition and porosity. Corrosion caused by acid rain and biodeterioration from bacterial films are worldwide problems for building construction or preservation of cultural heritage objects such as statues and sculptures. The project demonstrates how typical building stones can be protected from weathering and biofilm formation by application of thin films of monolacunary Keggin POM {SiW11} -based water-repellent ionic liquids (POM-ILs). Exposure of the stones samples to simulated acid rain showed negligible corrosion compared to the major deterioration of the uncoated samples and the biocidal properties of the POM-ILs inhibited the formation of microbial films on coated stone slabs. This class of bifunctional modular molecular materials could also be developed for usage in building construction, cultural heritage conservation and environmental protection. The acid-stable surface coating was observed to be mechanically stable and is not removed even under accelerated acid rain simulation (surface adherence). The rheological, physical and chemical properties of the POM-ILs can be tuned at the molecular level and the results elucidate that the modification of the cation affects both the anti-corrosive properties and the antibacterial activity. The literature review study encompasses the state-of-the-art and emerging perspectives of POM-cation interplay. This covers the classical (POM dissolution, precipitation and purification) and non-classical roles of counter-cations in the domain of POM-chemistry, with an overview of fundamental POM-cation interactions in solutions leading to the formation of solid-state molecules, and also the exclusive chemical properties resulting from such interactions. The combination of anionic POMs with a huge library of organic and inorganic cations is crucial to control the self-assembly, stabilization, solubility, and functional diversity of polyoxometalates. Apart from considerably explored simple alkali metal or ammonium cations, a cation-pool of dendrimers, metal complexes, amphiphiles, alkaloids etc. offer a vast domain of targeted applications possibly achieved via molecular level tuning, i.e. cation modification. The advanced definition of counter-cations coined in the review article includes surfactants, polycations, biomolecules, and even positively charged surfaces and matrices. The non-classical roles of POM counter-cations include framework construction, templating and stabilization of POMs supramolecular assembly, and regulating the reactivity and speciation. A deeper understanding of the roles of the counter-cations is emerging and expanding along with cutting-edge instrumentation and modern computational capacity, which would definitely enrich the scope of fundamental and applied POM research. The other major project presents an alternative water- purification approach to address the global scarcity of pure drinking water, reporting the first example of magnetic polyoxometalate supported ionic liquid phases (magPOM-SILPs) and their use in water purification. The magPOM-SILP composite was employed to remove organic, inorganic, bacterial and microplastic pollutants, some of the typical surface contaminants, from water. The monolacunary Keggin ion {SiW11}-based water-insoluble polyoxometalate ionic liquid (POM-IL) having long-chain alkylammonium cations ((n-C7H15)4N+) with established antimicrobial properties was adsorbed onto magnetic microporous core-shell Fe2O3/SiO2 nanoparticles, giving rise to a magnetic POM supported ionic liquid phase (magPOM-SILP). These lacunary Keggin polyoxometalate anions ([α-SiW11O39]8-) feature pre-designed binding sites for uptake of toxic heavy metals such as lead, nickel, chromium etc. The pollutant removal efficiencies reported were quite high compared to the reference composite without the POM-ILs. Initial insights into a new mode of often quantitative microplastics removal by surface binding of magnetic particles were also attained. The lipophilic POM-IL facilitated the adsorption of organic contaminants such as textile dyes, resulting in a multifunctional water purification system, with facile removal of the purifier particles using a permanent magnet. Efficient optimization of the individual components by molecular-level tuning can improve the overall capacity of the systems and their coupling to electromagnetic recovery systems can be investigated for use under realistic operating conditions and task-specific applications such as centralized and decentralized water purification systems. So, both the major projects have sustainability as the core driving concept and the targeted applications are achieved through the conceptual understanding and modular design approach of POM-ILs and POM-SILPs.