Palladium doped magnetic nanocatalysts for sustainable 5-hydroxymethylfurfural oxidation

Diminishing reserves of the easily available petroleum resources and environmental impact of fossil fuels are the driving forces to the search of alternative energy and new carbon sources. Undoubtedly, carbon-containing molecules found in renewable biomass represent a sustainable feedstock for the chemical industry. Specifically furan compounds, such as 5-hydroxymethylfurfural (HMF) and its oxidation derivatives, have high potential for the production of chemicals and fuels. For instance, 5-hydroxymethyl-2-furancarboxylic acid (HFCA) derived from the selective oxidation of the aldehydic moiety of HMF, serves not only as a monomer in the synthesis of various polyesters, but also shows antitumor activity. Much more research interest has been devoted to the synthesis of 2,5-furandicarboxylic acid (FDCA) considered a top selected value-added chemical from biomass and a possible replacement monomer for terephthalic acid used to produce polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). The aim of this study is the environmentally friendly synthesis of HFCA and FDCA through heterogeneously catalyzed reactions in water at mild conditions so as to guarantee a sustainable development. Pd doped Fe3O4 is proposed as easily recoverable and versatile catalyst being able to reach up to 86% yields of HFCA when subjected to 10 atmospheres of O2 and 60°C in the as synthesized form. On the contrary, this catalyst can reach up to 70% FDCA yields when working at 40°C and atmospheric O2 pressure in a mildly reduced form. In order to design a truly sustainable process also from the economic point of view, biomass derived HMF raw liquors must be employed. Therefore, catalysts here proposed were also tested in a cascade process of biomass valorization using raw hydrolysates derived from cellulose. These aqueous liquors obtained by hydrolysis and dehydration with a solid acid contained mostly HMF and furfural as well as small amounts of unreacted sugars. Quite good results were obtained from the cascade process using Pd doped Fe3O4 leading to 81% yield of HFCA, 4% FDCA and 80% yield of furoic acid (main furfural oxidation product).