Transition metal substituted polyoxometalates supported on amine-functionalized silica

Abstract

A series of transition metal substituted polyoxometalates (POM) have been anchored to propylamine-functionalized mesoporous silica (silicaNH2). These include V, Co and Mn Keggin-type anions such as [PMo10V2O40]5− and [PMo11VO40]4−, or [SiW11CoII(H2O)O39]6− and [SiW11MnIII(H2O)O39]5−, and sandwich-type anions, [(PW9O34)2 Co 4 II (H2O)2]10− and [(PW9O34)2Mn 4 II (H2O)2]10−. Experiments at different initial pHi of aqueous suspension of silica were performed for the Co and Mn substituted Keggin anions. The novel silicaNH2-POM materials having between 12–17% (w/w) of polyoxometalate were characterized by elemental analysis, solid-state 29Si, 13C and 31P n.m.r., diffuse reflectance spectroscopy, FTIR spectroscopy, thermal analyses, and BET surface area measurements. The elemental analyses and spectroscopic results pointed to the integrity of POM structures after immobilization on the silica support while the latter one showed a slight decrease of BET surface area. Results of the visible diffuse reflectance spectroscopy for the Co-substituted Keggin anion revealed the coordination of cobalt centers in the cluster with the nitrogen atom of amine groups in modified silica at pHi ≥ 5.5 or the electrostatic bonding between polyoxoanion and protonated C3H6NH 3 + group at pHi = 3.5. For the Co-substituted sandwich anion, external Co centres of the polyoxoanion have coordinated with the amine groups of modified silica under the experimental conditions used. 31P MAS n.m.r. showed a shifting of phosphorus atom resonances in H5[PMo10V2O40] and H4[PMo11VO40] to a single resonance at ca. − 4.1 ppm (Δv 1/2 ~ 70 Hz) when these POM were immobilized on the functionalized silica under weak acidic conditions (pH 3), evidencing electrostatic interaction of POM clusters with the C3H6NH 3 + groups.