Natalia Rey-Raap

This chapter aims to provide a concise overview of the synthesis of carbon materials with spherical morphology, obtained through different sol–gel methods, highlighting the key parameters that influence the formation of their structure and physicochemical properties. The sol–gel methodology is a highly versatile process that enables precise control over composition, porosity, particle size, and surface chemistry through relatively simple adjustments to synthesis conditions. Particular attention is given to the role of the nature and concentration of precursors and solvents, pH, and drying methods in tailoring the final morphology of carbon sol–gel materials. The use of alternative precursors and the possibility of designing hybrid systems is also discussed as an interesting strategy to move toward more sustainable processes, while maintaining the ability to design materials with advanced functionalities. Special emphasis is placed on the advantages of spherical morphology compared to irregular particles or monolithic structures, which result in lower pressure drop in fixed-bed systems, better electrochemical performance, and more efficient catalytic and adsorption processes. The relationship between synthesis parameters and the structural, chemical, and textural characteristics of the resulting material is highlighted, along with the influence of these factors on its applicability, with a discussion of its relevance in advanced technological applications, including energy storage, catalysis, and adsorption.

5-Hydroxymethylfurfural (HMF) is a versatile platform molecule with the potential to replace many fossil fuel derivatives. It can be obtained through the dehydration of carbohydrates. In this study, we present a simple and cost-effective microwave-assisted method for producing HMF. This method involves the use of readily available sucrose as a substrate and glucose-derived bifunctional hydrochars as carbocatalysts. These catalysts were produced via hydrothermal carbonisation using thiourea and urea as nitrogen and sulphur sources, respectively, to introduce Brønsted acidic and basic sites into the materials. Using a microwave reactor, we found that the S, N-doped hydrochars were active in sucrose dehydration in water. Catalytic results showed that HMF yield depended on the balance between acidic and basic sites as well as the types of S and N species present on the surfaces of these hydrochars. The best-performing catalyst achieved an encouraging HMF yield of 37%. The potential of N, S-co-doped biochar as a green solid catalyst for various biorefinery processes was demonstrated. A simple kinetic model was developed to elucidate the kinetics of the main reaction pathways of this cascade process, showing a very good fit with the experimental results. The calculated rate constants revealed that reactions with a 5% sucrose loading exhibited significantly higher fructose dehydration rates and produced fewer side products than reactions using a more diluted substrate. No isomerisation of glucose into fructose was observed in an air atmosphere. On the contrary, a limited rate of isomerisation of glucose into fructose was recorded in an oxygen atmosphere. Therefore, efforts should focus on achieving a high glucose-to-fructose isomerisation rate (an intermediate reaction step) to improve HMF selectivity by reducing humin formation.

There is currently an effort to scale up sol–gel nanomaterials without compromising quality, and microwave heating can pave the way for this due to its heating efficiency, resulting in a fast and homogeneous process. In this work, the sol–gel synthesis of transition metal aerogels, specifically iron-based aerogels, is studied using a microwave-assisted sol–gel methodology in an open-system multimode device as a potential route to scale-up production. Different approaches were tested to evaluate the best way to increase yield per batch, with different vessel shapes and volumes. It is shown that the shape and size of the vessel can be determinant in the interaction with microwaves and, thus, in the heating process, influencing the sol–gel reactions and the characteristics and homogeneity of the obtained nanomaterials. It has been found that a wide vessel is preferable to a tall and narrow one since the heating process is more homogeneous in the former and the sol–gel and cross-linking reactions take place earlier, which improves the mechanical properties of the final nanomaterial. For mass production of nanomaterials, the interaction of the reagents with the microwave field must be considered, and this depends not only on their nature but also on their volume, shape, and arrangement inside the cavity.

The chapter presents a general overview of the fundamentals of electrochemical sensors, describing the materials used and their critical characteristics, such as porosity and electrical conductivity. In this sense, the sol-gel methodology is presented as an effective solution to design a porous structure and other physicochemical properties to meet the needs of electrochemical sensors. This approach allows us to obtain a versatile and customized design of the properties of the materials, improving the sensitivity and durability of these devices.