Transition metal substituted polyoxometalates supported on amine-functionalized silica

A series of transition metal substituted polyoxometalates (POM) have been anchored to propylamine-functionalized mesoporous silica (silicaNH^sub 2^). These include V, Co and Mn Keggin-type anions such as [PMo^sub 10^V^sub 2^O^sub 40^]^sup 5-^ and [PMo^sub 11^VO^sub 40^]^sup 4-^, or [SiW^sub 11^Co^sup II^(H^sub 2^O)O^sub 39^]^sup 6-^ and [SiW^sub 11^Mn^sup III^(H^sub 2^O)O^sub 39^]^sup 5-^, and sandwich-type anions, [(PW^sub 9^O^sub 34^)^sub 2^ Co ^sub 4^ ^sup II^ (H^sub 2^O)^sub 2^]^sup 10-^ and [(PW^sub 9^O^sub 34^)^sub 2^Mn ^sub 4^ ^sup II^ (H^sub 2^O)^sub 2^]^sup 10-^. Experiments at different initial pH^sub i^ of aqueous suspension of silica were performed for the Co and Mn substituted Keggin anions. The novel silicaNH^sub 2^-POM materials having between 12-17% (w/w) of polyoxometalate were characterized by elemental analysis, solid-state ^sup 29^Si, ^sup 13^C and ^sup 31^P 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 pH^sub i^ ≥ 5.5 or the electrostatic bonding between polyoxoanion and protonated C^sub 3^H^sub 6^NH ^sub 3^ ^sup +^ group at pH^sub i^ = 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. ^sup 31^P MAS n.m.r. showed a shifting of phosphorus atom resonances in H^sub 5^[PMo^sub 10^V^sub 2^O^sub 40^] and H^sub 4^[PMo^sub 11^VO^sub 40^] to a single resonance at ca. - 4.1 ppm (Δv ^sub 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 C^sub 3^H^sub 6^NH ^sub 3^ ^sup +^ groups.[PUBLICATION ABSTRACT]