Efficiency of Polyoxometalate-Based Mesoporous Hybrids as Covalently Anchored Catalysts

Abstract

Polyoxometalate (POM) hybrids have been covalently immobilized through the formation of amide bonds on several types of mesoporous silica. This work allows the comparison of three POM-based mesoporous systems, obtained with three different silica supports in which either the organic functions of the support (amine vs carboxylic acid) and/or the structure of the support itself (SBA-15 vs mesocellular foams (MCF)) were varied. The resulting POM-based mesoporous systems have been studied in particular by high resolution transmission electronic microscopy (HR-TEM) in order to characterize the nanostructuration of the POMs inside the pores/cells of the different materials. We thus have shown that the best distribution and loading in POMs have been reached with SBA-15 functionalized with aminopropyl groups. In this case, the formation of amide bonds in the materials has led to the nonaggregation of the POMs inside the channels of the SBA-15. The catalytic activity of the anchored systems has been evaluated through the epoxidation of cyclooctene and cyclohexene with H2O2 in acetonitrile. The reactivity of the different grafted POMs hybrids has been compared to that in solution (homogeneous conditions). Parallels can be drawn between the distribution of the POMs and the activity of the supported systems. Furthermore, recycling tests together with catalyst filtration experiments during the reaction allowed us to preclude the hypothesis of a significant leaching of the supported catalyst.

Selective Formation of Epoxylimonene Catalyzed by Phosphonyl/Arsonyl Derivatives of Trivacant Polyoxotungstates at Low Temperature

“…In a second experiment, a sample of the protonated ( n Bu 4 N) 3 [NaHAsW 9 O 33 {P(O) t Bu} 2 ] ( 1 ) was treated with one equivalent of a methanolic solution (1 mol L –1 ) of n– Bu 4 NOH, leading to the in situ formation of the [NaAsW 9 O 33 {P(O) t Bu} 2 ] 4– anion. Using this sample for the cyclooctene epoxidation led to a dramatic decrease of the catalytic performances, attaining values obtained in our previous publication . It is, thus, possible to assign the previously reported results to the non‐protonated ( n Bu 4 N) 4 [NaAsW 9 O 33 {P(O) t Bu}2] compound (see Table ).…”

“…This catalyst was also covalently grafted onto a {NH 2 }‐functionalized mesoporous SBA‐15 silica support. In these heterogeneous conditions, interesting conversions for both alkenes were maintained and, last but not least, a better epoxide selectivity for the cyclohexene, compared to the homogeneous conditions, was observed …”

“…In this context, we recently synthesized and used a family of heteropolytungstates‐based hybrids for their covalent immobilization onto the surface of various silica‐based supports . The performances of these catalysts were first evaluated in the epoxidation of cyclooctene and cyclohexene with aqueous H 2 O 2 as a model reaction, in homogeneous conditions or after their anchoring onto the support . In the course of this study, B,α‐[NaHAsW 9 O 33 {P(O)R} 2 ] 3– (R = ‐C(CH 3 ) 3 = ‐ t Bu ( 1 ), ‐CH 2 CH 2 CO 2 H ( 2 )) and A,α‐[NaHPW 9 O 34 {P(O) p ‐C 6 H 4 NH 2 } 2 ] 3– ( 3 ) complexes demonstrated increased catalytic performances in homogeneous conditions.…”

“…Protocols with hydrogen‐ or alkyl‐peroxides involving efficient W(VI)‐peroxides catalytic species are generally characterized by negligible oxidant decomposition pathways and good to excellent selectivities . Some of us recently reported the promising catalytic performance of an organophosphonyl derivative of the arsenotungstate B,α‐{AsW 9 O 33 } 9– , namely B,α‐[NaHAsW 9 O 33 {P(O)CH 2 CH 2 CO 2 H} 2 ] 3– (Figure ) . This particular compound has proved to be an efficient catalyst when used with H 2 O 2 at room temperature.…”

Current advances in the utility of functionalized SBA mesoporous silica for developing encapsulated nanocatalysts: state of the art

“…The authors believed that using amine-functionalized SBA-15, the best dispersion and loading of polyoxometalate could be achieved. 148 Moreover, the POM-CO 2 H@SBA-NH 2 was proved to be a truly anchored catalyst and signicant leaching of the catalytic species did no observed.…”