Preparation And Characterization Of One Crown-Shaped Selenium-Containing Polytungstophosphate And A Cobalt-Substituted Tungstosilicate/Aniline/Graphene Composite

Polyoxometalates?POMs?are a class of oxo-metal clusters built up by early transition metal?e.g.V?V?,Nb?V?,Ta?V?,Mo?VI?,W?VI??in their highest oxidation states.POMs can be applied in diverse fields such as green catalysis,magnetism and biomass treatment.These species are usually formed through the self-assembly of building units in acidic environment.An increasing basicity of the synthetic media will facilitate the tendency of forming lacunary POMs,which features the removal of one or more WO₆ octahedra from the intact polyanions,such as the well-known Keggin or Wells-Dawson types.These resultant lacunary POMs can be used as precursors for the incorporation of electrophiles,such as transition metal,main-group metal and lanthanide ions,to form heterometal-substituted POMs with an enhanced structural and compositional variety.K₁₂H₂P₂W₁₂O₄₈·24H₂O?KLi-P8W48,P8W48?represents a promising lacunary polyanion precursor with good solubility and stability.The terminal oxygen of WO₆ octahedral in this structural could incorporate various electrophilic to form high neclear complex structures.However,the previous reported work mainly focused on transition metal ions,lanthanides and organic ligands,while the studies about non-metal elements were little.Preparation of polyoxometalate/carbon nanocomposite material is another hot research areas in POM chemistry for their multiple functions that might be applied in renewable energy storage and conversion technologies.As an important Keggin type polyacid,K-CoSiW11 can be stably existed at pH=4.8-7 and is easy to synthesize.It has been widely used in cycloaddition and CO₂ reduction in homogeneous reactions.The selectivity and conversion is as high as 90%.Owing to the high solubility of most polyanions,the above-mentioned property studies are commonly performed in homogeneous conditions,which are hardly applicable in industrial practice that involves facile separation process for the recycling of the POM catalysts.However,it is urgent to develop heterogeneous material preparation methods.The work of this thesis mainly focused on two acpects:?1?Synthesis,structure and characterizations of the first selenite-incorporated derivative of P₈W₄₈;?2?Develop an effective method to immobilize POMs on rGO nanosheets via ANI bridges.Details are as follows:1.Synthesis of the tetraselenite-incorporated 48-tungsto-8-phosphate[?SeO?₄P₈W₄₈O₁₈₄]^32–?Se4P8W48?by treating SeO₂ with the superlacunary,macrocyclic polyanion precursor[P₈W₄₈O₁₈₄]^40–(P₈W₄₈)in aqueous solution.The resultant KLi-Se₄P₈W₄₈ bulk material was characterized by X-ray diffraction,FTIR,UV-vis,and NMR spectroscopy,as well as thermogravimetric and elemental analysis.Single-crystal X-ray diffraction analysis was carried out on K₂₆Li₆[?SeO?₄P₈W₄₈O₁₈₄]·98H₂O[KLi-Se4P8W48,monoclinic space group C_2/m,a=35.3618?9??;b=21.7869?6??;c=24.6187?7??;?=132.5822?15?°;V=13965.4?7??³;Z=2].The polyanion Se4P8W48 contains four SeO₃^2–groups,adopting a trigonal-pyramidal coordination geometry,grafted on the rim of the central cavity of P8W48.These four Se atoms are arranged in a coplanar fashion that is perpendicular to the main plane of P8W48,resulting in an overall D_2h point group symmetry for Se4P8W48.The FTIR spectrum of the KLi-Se₄P₈W₄₈is almost identical as that of the KLi-P8W48 precursor,suggesting their close structural relationship.At the same time,there are some new peaks appeared,which mean new chemical bonds exist.³¹P NMR spectroscopy reveals the increase of the Se₄P₈W₄₈ concentration as a function of the reaction time in the first four hours.A longer reaction time did not increase the relative intensity of these two peaks any more,indicating the establishment of a chemical equilibrium in solution.2.Preparation of the novel CoSiW11/ANI/rGO nanocomposite by fixing monocobalt-substituted tungstosilicate K6-[?-SiW11O39Co?OH?2]·15H2O?K-CoSiW11?onto the reduced graphene oxide?rGO?involving the use of aniline?ANI?species as linkers.The ANI species attached on rGO through?-?interactions can be protonated to generate strong electrostatic attraction towards the negative POM clusters.The FTIR,TEM and EDX mapping analysis indicate the uniform dispersion of the nanosized CoSiW11 clusters?¹ nm?on the surface of rGO.The XPS data demonstrates that graphene oxide?GO?undergoes a one-step reduction to rGO during the preparation and that there are electron transfers from the POMs to the rGO involved in the formation of the nanocomposite.Additionally,the effects of reagents,temperature,acidity and reaction time had been systematically investigated and discussed for the purpose of obtaining optimal assembly product,which has been characterized by SEM,TEM,XPS,XRD,FTIR,TG,elemental analysis and EDX.Thus the preparation of the CoSiW11/ANI/rGO material not only represents a success on the synthetic methodology for POM-based nanocomposite,but also supply a platform for further functionalization by applying various POM guests.