Design of nanomaterials through mechanochemical grinding processes for catalytic and electrochemical applications

Abstract

The present doctoral thesis project deals with the design of nanomaterials with potential applications in two important fields: 1) the design of energy storage devices and 2) heterogeneous catalysis, in both cases for the development of more sustainable processes that contribute to ameliorate climate change. Today, most of the challenges humanity faces are related to energy and environment, including the scarcity of water and resources, energy requirements and the depletion of fossil fuel sources. In particular, energy demands, whether in the transport sector or for stationary applications, are priorities in all the scientific programs and agendas of the world. Hence, many investigations are currently aimed to find new materials with better electrochemical results for the development of a new generation of sustainable energy storage devices. In addition, the development of a more sustainable chemical industry requires high efficient processes and therefore the preparation of active and selective catalytic systems. Mechanochemical methods have been employed in this doctoral thesis for the design of nanomaterials, including various nanobioconjugates based on proteins and magnetic nanoparticles, as well as metal oxide nanoparticles supported on mesoporous supports and biomass-templated metal oxide nanomaterials. Mechanochemistry offers several advantages regarding its high reproducibility, versatility, simplicity and, specially, to its green character related to the possibility to avoid the use of solvents and additional reagents. Studies on mechanochemistry have greatly increased in the past two decades, nonetheless as an open research line, a lot of efforts still need to be devoted in order to take full advantages of its great potentialities. Biomass valorization has been one of the main issues covered in this thesis, towards both chemicals and materials. Biomass constitutes, together with CO2, one of the most abundant renewable carbon sources. Therefore, the use of biomass-derived platform molecules for the preparation of added-value chemicals, replacing the petro-based chemical industry, is a highly attractive option. In particular, along this thesis, biomass derived platform molecules such as levulinic acid and isoeugenol have been employed for the synthesis of Nheterocycles and vanillin, respectively. It is important to highlight that, through the valorisation of biomass, added value materials can be also obtained, representing an environmentally friendly methodology. In this regard,, several biomass residues, including spent coffee grounds, egg-white form expired eggs and orange peel have been treated by mechanochemical protocols for the preparation of nanostructured materials with controlled morphology and textural properties. In particular, through this work a mechanochemical synthetic strategy have been designed for the preparation of bioconjugates, minimizing reaction times and costs associated with the use of solvents and other reactants. As an alternative to inorganic materials, organic electroactive products, such as proteins have opened new opportunities for the design of innovative energy storage devices with a greater theoretical capacity, safety, sustainability and low cost. Rechargeable batteries and electrochemical supercapacitors (ECs) are among the most representative examples of energy storage devices. The construction of ECs with high energy density and high power has become a priority issue for the development of future devices. Research in this field has focused on the synthesis of active porous nanomaterials such as metal oxides, hydroxides, or carbonbased materials. These materials provide a high capacity, but have several disadvantages including high costs, manufacturing stages, difficult scalability, besides not being respectful with the environment. In order to overcome the inherent drawbacks of conventional inorganic materials, metalloproteins containing heme groups have recently been employed. However, the development of hybrid systems of hemoproteins and nanoparticles (NPs) for the design of sustainable supercapacitors has not yet been deeply investigated, being a highly innovative and relevant idea for the design of materials that could become a potentially applicable product. Therefore, this doctoral thesis also aimed to synthesize hybrid nanostructures of metalloproteins and nanoparticles through a mechanochemical methodology, as a cheap, sustainable, and versatile strategy, for the design of a new generation of electrochemical supercapacitors. Another potential application of these hybrid protein-nanoparticle systems is their use in biomimetic catalysis processes. Inspired by the in vivo synthesis of natural polymers, where enzymes play an important role as catalysts, in vitro enzymatic polymerization has been widely developed to design a wide range of advanced materials. In particular in this project, bioconjugates based on hemoproteins have been used for the preparation of fluorescent carbon-based nanoparticles following a bottom-up strategy. Similarly, carbon quantum dots with fluorescent behavior have been prepared by a top-down metholody from biomass residues and using an iron oxide containing material as catalyst. In summary, mechanochemistry, as a powerful tool, has been successfully employed to produce a wide range of nanomaterials, from bioconjugates to metal oxides, with applications in catalysis and energy storage. Taking into account the above mentioned premises, the objectives of the proposed work are very relevant for the development of a Sustainable Chemistry and to integrate the strategies of Green Chemistry and Engineering.