Trivacant heteropolytungstates as building blocks for new polyoxometalates and catalytic materials.

摘要

Polyoxometalates (POMs) are a class of compounds having a rare versatility that makes them amenable to both fundamental studies and practical applications. In particular, the catalytic properties of these complexes are of great interest. Two efforts toward extending the current capabilities in this area are the synthesis of new transition metal substituted POMs and the development of new polyoxometalate-containing materials for heterogeneous catalysis.; A new family of transition metal substituted POMs has been synthesized by the controlled decomposition of the sandwich-type polyoxometalates K 12[(M(OH2)2)3(A-alpha-PW9O 34)2] (where M = Mn(II) or Co(II)) in 0.5 M NaCl. Crystallographic studies show that the new polyanions consist of a hexavacant B-type Keggin unit linked to two B-alpha-(MOH2)M2PW9O 343- units (where M = Mn(II) or Co(II)). Additionally, these complexes have been characterized by UV-visible and FT-IR spectroscopies and differential scanning calorimetry (DSC). The methyltricaprylammonium salt of [((MnIIOH2)MnII2PW 9O34)2(PW6O26)]17- is a selective catalyst for the H2O2-based epoxidation of alkenes.; Electrostatic immobilization of polyoxometalates on the surface of cationic silica nanoparticles results in the formation of a new heterogeneous catalyst that has been characterized by several physicochemical methods. Streaming potential measurements combined with elemental analysis and dynamic light scattering data have allowed the polyoxometalate-to-nanoparticle stoichiometry to be determined. Electron microscopy techniques confirm polyoxometalate binding and reveal that the natural silica gelation process is inhibited by the presence of POMs on the cationic silica nanoparticles. EPR data shows a change in the local Fe(III) environment upon polyoxometalate binding which has been further investigated by catalytic oxidation experiments.; A second new material has been prepared by modification of the surface of cotton with diethylaminoethyl (DEAE) groups followed by the electrostatic binding of polyoxometalate nanoclusters to the cellulose fibers. The immobilized POMs belong to the family of sandwich-type POMs having the general formula [(M(OH2)2)3(A-a-PW 9O34)2]n- where M = Fe(III) or Co(II). SEM-EDS, TEM, and FT-IR spectroscopy have been used to confirm the presence of the POMs on the cellulose fibers and the thermal stability of the POM-cotton material has been assessed by TGA and DSC. The polyoxometalate-decorated cottons have been found to be heterogeneous catalysts for the H2O 2-based epoxidation of alkenes and the aerobic autoxidation of the indoor air pollutant acetaldehyde to acetic acid.