Earth-abundant transition metal-based catalysts for sustainable reactions

Noble metal nanoparticles have long been playing a central role as heterogeneous catalysts but due to their high cost, their toxicity and scarce availability, they have always been less attractive for industrial application. Moreover, the immediacy of global climate change and the need for a greener chemistry have intensified the efforts in the research of sustainable alternatives. One of the most promising ones, is represented by the use of earth-abundant transition metals that are much more available and offer appealing properties able to overcome the disadvantages of the precious metals. In this context, the objective of this research thesis has been to develop and optimize innovative procedures for the preparation of new polymer supported transition metal nanoparticles to employ as catalysts for selected model reactions. In particular, the first part of the work has been focused on the synthesis of cobalt catalysts obtained by copolymerizing the metal-containing monomer Co(AAEMA)2 with suitable comonomers. The obtained catalyst was tested in the reduction of nitroarenes to the corresponding anilines, showing good capabilities and avoiding the formation of side products. The reactions were performed under mild conditions and the catalyst was easily removed from the reaction products and reused. The second part of this work deals with the preparation of supported nickel nanoparticles following different procedures, differing from each other for the calcination conditions or for the matrix employed. All materials were characterized by advanced techniques such as transmission of electron microscopy (TEM) demonstrating that the variation of experimental conditions leads to the formation of nanoparticles with different morphology. This feature resulted in different catalytic behavior that permitted to develop a highly selective reduction system for a variety of structurally different nitroarenes towards azoxyarenes. Under optimized reaction conditions, more than twenty azoxyarenes were obtained in good or excellent yields with wide functional group compatibility. The recovery and reuse of the catalytic system were also achieved for up to five runs without appreciable Ni leaching, loss of activity and selectivity as well as nickel nanoparticles agglomeration.