Polyoxometalates Intercalated Layered Double Hydroxides:From Synthetic Approaches To Functional Material Applications

Polyoxometalates （abbreviated as POMs） are complex anions formed via condensation of anionic metal oxides polyhedra mainly from groups 5 and 6. They possess special and useful physical-chemical properties such as strong Br（?）nsted acidity, strong redox agents, and their synthesis result into a variety of molecules. Consequently, they have been applied in industry as efficient adsorbents, pollutant degraders, green catalysts in fine chemical production as well as photo-luminescence, especially the lanthanide containing POMs. Nevertheless, POMs are highly soluble and easily form crystals in solution. Furthermore, anion-cation interaction between POMs crystals is very weak resulting into low lattice energies. In addition, some of the lanthanide containing POMs exhibit reduced photoluminescence emissions, hence affecting their applications. These aforementioned demerits largely restrict POMs application in reality. For instance, research in POMs catalysis is mostly directed towards heterogenizing POMs anions so as to achieve high catalyst recoverability, high rate of catalyst re-use, effective selectivity and thermally stable catalysts.Immobilization and solidification of POMs on a number of substrates resulting into various multifunctional POMs heteroginized materials are two widely applied techniques in research. These techniques generate new POM based multifunctional materials of wide application encompassing POMs advantages while overcoming its limitations. Particularly important is the immobilization of POMs by intercalating them within the gallery space of layered double hydroxide materials （LDHs）.Several techniques are reported for the synthesis of POM/LDHs including ion exchange, reconstitution, co-precipitation, electrochemical reduction, ultrasound, and delamination. In this work, we employ both the delamination and reconstitution technologies to engineer some new POM/LDHs materials and use them to solve various environmental perturbing problems.Firstly, unilamellar nanosheets are synthesized from a europium containing LDHs material. And for the first time, the nanosheets are successfully isolated at nanoscale level and characterized as positively charged layers of formula [Eu₈（OH）₂₀·nH₂O]Cl₄ （LEuH）. The isolated nanosheets are successfully used in sorption of environmentally poisonous fluoride anions from aqueous medium. And by calcination of the fluoride adsorbed material at 480℃, the nanosheets are recycled in the fluoride adsorption process.Secondly, condensation of WO₄^2- POM sub-units on the LEuH nanosheets at pH= 5 yields a versatile new POM/LDHs material, Eu₂（OH）₅[H₂W₁₂O₄₀]0.17-7H₂0 (LEuH-H₂W₁₂O₄₀) with well-ordered hierarchical super channels. The resultant porous material exhibits high surface area and enhanced liquid/molecular/ion transport. Heavy metal adsorption by this material is reported to be 1.8,2.4 and 4.1 mmol/g for Cd²⁺, Pb²⁺, and Cr⁶⁺within contact times of 18,25 and 20 minutes respectively. The sorption mechanism is pseudo-second order which obeys Langmuir sorption model.The delaminated positively charged unilamellar LEuH nanosheets provide not only a very unique support material for POMs anions, but also a particular important pathway in the development of novel functional materials. In a very innovative approach, POMs anions of [PW₁₀O₃₆]7-(PW₁₀) are electrostatically immobilized on the surface of a magnetite core-shell structure of Fe₃O₄@SiO₂ through the [Ln₈（OH）₂₀·nH₂O]⁴⁺ nanosheets （LEuH） to get a new nanocomposite material of Fe304@SiO₂@LEuH@PW₁₀. And it is demonstrated to be able to catalytically brominate phenol red to bromophenol blue. The bromination reaction is 99% selective at the rate of 5.5 x 10-3 mmol g^-1s^-1,298 K, and 1 atm. In addition, the nanocomposite material is recycled for a minimum of ten times with similar catalytic activity.Furthermore, the newly synthesized nanocomposite material, Fe₃O₄@SiO@L2EuH@PW₁₀, is demonstrated to effectively adsorb chromate anions from aqueous solutions. The adsorption isotherms fit Langmuir model. Its adsorption capacity is 23 mmol g^-1 after 42 minutes at 25℃. The reaction is spontaneous at room temperature with 44.22 kJ mol^-1 of activation energy required. In addition, heating the chromate adsorbed nanocomposite material at 40 ℃, results in dissociation of the chromate anions from the nanocomposite material. As such, the recycled adsorbent Fe304@Si02@LEuH@PW₁₀ is re-used in chromate removal from aqueous medium for a minimum of ten cycles successfully. This spontaneous reversible chemisorption mechanism for chromate adsorption provides a new pathway for separation and cleaning of industrial wastewater contaminated with chromate ions.Finally, an extensive study is done on various LDHs intercalated with [EuW₁₀O₃₆]^9- POM anion through the reconstitution method. Lanthanide containing polyoxometalates （Ln-POMs）, especially europium, have attracted great applications in industry including photo-luminescence and catalysis due to their line-like emissions, long decay times and redox properties. However, Eu-POM exhibits reduced red light luminescence hence limiting it is use in applications requiring the red light luminescence such as agriculture and horticulture. The reduced red light luminescence of [EuW₁₀O₃₆]^9- ion is converted into an intense red light luminescence by intercalation of the anion within the restricted regions of various LDHs. The positive nanosheets in LDHs provide a conducive environment for strong transitions of ⁵D₀â†'⁷F₂ to occur that are responsible for the red light emissions. The ratio I（⁵D₀â†'⁷F₂）/I（⁵D₀â†'⁷F₁） for the observed intensities （I） vary from 0.44 for [EuW₁₀O₃₆]^9- ion to 14.08,6.20,1.75 and 1.59 in MgAl-EuW₁₀O₃₆, LYbH-EuW,0O36, ZnAl-EuW,0O36 and LEuH-EuW₁₀O₃₆ POM/LDHs materials respectively. In addition, the [EuW₁₀O₃₆]^9- ion does not lose its redox property after heterogenization within the restricted gallery spaces of the LDHs as is demonstrated by catalytic extractive desulfurization experiments using a model diesel oil. The efficiency of removing sulfur from the oil is 90%,94% and 99% for BT,4,6-dMdBT and dBT in 120,125 and 25 minutes respectively using MgAl-EuW₁₀ as a catalyst.