How the Calcination Procedure Affects the Morphology and the Catalytic Activity of Polymer-Supported Nickel Nanoparticles

A nickel containing monomer, Ni(AAEMA)(2) (AAEMA(-) = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate) is co-polymerized with ethyl methacrylate (co-monomer) and ethylene glycol dimethacrylate (cross-linker). The obtained polymer is a green methacrylic resin containing Ni(II) centers homogeneously dispersed in the catalyst, which results insoluble in all common organic solvents and in water. The specific features of this material classify it as an amphiphilic resin, air and moisture stable, with the peculiarity to swell in halogenated solvents, acetone and water and to shrink in diethyl ether and petroleum ether. The polymer is calcined under reductive conditions (dihydrogen with initial pressure of 5 bar) following two procedures, differing from each other for the cooling conditions. In the first procedure, the calcined material is cooled under dihydrogen gas, while in the second one the cooling step occurs under air. After calcination, the green Ni(II)-based co-polymer turns into black resins, Ni-res1 (obtained by cooling under hydrogen) and Ni-res2 ((obtained by cooling under air). FESEM analyses show that both Ni-res1 and Ni-res2 support Ni nanoparticles with different morphologies, being the metal nanoparticles onto Ni-res1 smaller than the ones dispersed in Ni-res2, that have an urchin-like shape. Both Ni-based co-polymers are tested as catalysts in the reduction of nitrobenzene with NaBH4. Ni-res1 results more active and selective towards aniline with respect to Ni-res2.