Synthesis and Applications in Chemistry and Materials

The following sections are included:

• Preface
• About the Editors
• Contents

The utilization of biomass from different sources for obtaining valuable products in the form of chemical compounds or energy has led to the development of a new field. Its aim is to promote the technological development and industrial application of new refinery processes and the development of specific valorization routes for biomass research. In Portugal, several biomasses have started to be studied for valorization, such as forest cuttings and agricultural waste, with the added value of developing a biomass-based energy system. Also, the use of by-products (e.g., insect exuviate) has started with promising results. Therefore, in this chapter, we propose a review of the status of those complementary approaches in Portugal and an important comparison with the most developed Mediterranean country, Italy. One approach is the revision of the literature concerning the valorization of selected biomasses which have been investigated or are under investigation in the country.

In particular, spent coffee grounds, spent tea, eucalyptus leaves and insects exuviae are taken into consideration.

As for coffee, tea and eucalyptus biomasses, their valorization involves:

• – extraction with solvents (in particular, ethanol) to identify organic compounds of industrial interest;
• – thermal treatment of solid residues to prepare biochar for applications in agriculture or activated carbon depending on the reaction conditions: an overview of the analytical techniques used to characterize these carbonaceous materials is presented.

The second approach is the utilization of green metrics to deepen our understanding of greener treatment methods and how sustainable they really are. Concepts such as the E-factor (one of the most adopted metrics by the chemical industry worldwide) or the Eco-Footprint, EATOS and Eco-Scale are still not available. The use of sustainable metrics, such as value-added, employment, the impact of biomass supply chains, and environmental impacts are parameters already used by EU countries, such as Italy, to identify key points in biomass research.

In this chapter, we aim to summarize the current state of knowledge and its application to the field of the developmental implementation of biomass-related industries in Portugal.

Vegetable biomass (VB) is a renewable source of organic carbon used for producing alternative chemicals that may contribute to the shift from a strongly fossil-fuel-based economy to a bio-based one. However, the development of sustainable VB conversion processes is important. The field of heterogeneous catalysis has greatly contributed to make up chemical processes more sustainable. Zeolite catalysts, which have served well the petrochemical industry, continue to receive considerable attention from several research groups across the globe for developing efficient VB conversion processes. This versatile family of materials has been modified in many different ways to meet superior catalytic performances. This chapter focuses on zeolites/zeotypes for the selective chemocatalytic conversion of carbohydrates (which are the main components of VB) and related compounds, highlighting the contributions from scientists in Portugal.

Plastics have become indispensable in our everyday lives for a large number of reasons, including health and safety issues, as became apparent during the COVID-19 pandemic. However, their use has brought about a very significant disturbance in the environment, in particular concerning the marine ecosystems, and adequate plastic waste management is crucial to the continued development of our industrialized society. In this chapter, we aim to show that the proper use of catalysis can help improve the way plastic waste is handled and provide tools to implement a circular economy approach to plastics.

The effect of climate change and the need for sustainable products and production processes are crucial issues, which are of enormous concern to the global community. Great efforts have been made to develop new efficient methodologies for converting biomass resources and plastic waste into value-added chemicals and raw materials for industry. This chapter highlights selected examples of catalytic methodologies for the valorization of biomass resources and plastic waste.

Plastics are one of the world’s greatest industrial innovations. Nevertheless, the sheer scale of their production and the poor disposal practices have led to a dramatic increase in plastic waste (PW) and, consequently, to serious environmental sustainability problems. Chemical recycling (CR) is an emerging theme in PW management, which has the potential to contribute to a low-carbon, resource-efficient and sustainable economy. Among CR processes, hydrocracking (HDC) is one of the most promising technologies for the conversion of PW into high-quality liquid fuels and other value-added chemical products. Contrary to other thermal and catalytic routes, it allows, in a single stage, the breakdown of heavy polymer molecules into much lighter molecules in the gaseous and liquid ranges and the reduction, by hydrogenation reactions, of the olefin and aromatic contents in the products. In addition, the HDC process includes the excellent ability for handling troublesome heteroatoms and the reduction of the coke precursors responsible for catalyst deactivation. This chapter presents the current state of the art of the application of HDC technology to convert PW into high-added-value products. First, the most common PW management strategies, especially the CR ones, are considered for comparative purposes. Then, an overview of the typical HDC mechanism, the type of catalytic systems and process operating conditions (temperature, H₂ pressure, time and reactor type) are discussed.

Highly performant and biodegradable polyesters are currently in high-demand as polymeric materials for sustainable applications. Interestingly, such polymers could be prepared through ring-opening polymerization (ROP) using biorenewable monomers. Two aspects have been critically considered, practice catalyst and monomer availability, in which the efficiency and properties of polymeric production would rely on the catalyst used. This review illustrates the recent progress in using metal complexes as catalysts for the ROP of cyclic esters, including commercial tin compound and the transition (Fe, Zn, Ti, Sc, Y) and main group (Li, Na, K Ca, Mg, Al, Ge) elements. The auxiliary ligands used significantly affect the catalytic efficiency and fine controllability of the molecular weights of the resultant polymers as well as their stereoselectivity. More specifically, various multidentate ligands, including bidentate, tridentate and tetradentate, bearing nitrogen, oxygen, sulfur, selenium or phosphorus donors, have been employed to coordinate with various metals in achieving high performance. Besides the huge literature that has emerged in recent years, the new aspects of the clear trends in precatalysts for biodegradable polymers are summarized.

The use of polymers in functional nanostructured materials covers a wide range of applications and requires different synthesis techniques to respond to the diverse requisites of the final materials and their applications. Polymer chains of precise size and architecture provide extremely well-controlled vehicles of only a few nanometers, with applications in the precision templating of nanomaterials and as high-brightness sensors, for example. On the other hand, for application as carriers, polymer nanoparticles offer extreme simplicity of production and good flexibility of composition and surface properties. Stimuli-responsive polymers, in particular, have huge potential for applications in controlled release and retrieval of cargo and in adhesion modulation. By combining silica with polymers into hybrid nanomaterials, the range of applications widens even more. These nanostructures can cover length scales from tens to hundreds of nanometers, with high chemical and mechanical stability and huge functionalization versatility. They offer the possibility to easily incorporate different functionalities in the same vehicle, to transport and protect cargo, fine-tune colloidal stability and interaction with the environment, and enhance biocompatibility, biological targeting and stealth from the immune system.

Lighting, including that used for displays, consumes over 20% of world-wide energy consumption, with the majority coming from traditional light sources and displays, which leads to a pressing demand for materials with minimized heat losses, low costs and high efficiencies. Tetracoordinate organoboron fluorophores are examples of low-cost luminescent materials that can be employed in the fabrication of highly efficient devices. Among the various ligands utilized to make luminescent organoboron complexes, the 2-iminopyrrolyl scaffold turns out to be highly versatile. With ease in preparation and modifications of steric and electronic nature of their core, the 2-iminopyrrolyl-derived organoboron complexes exhibit different colors of emission, ranging from violet to orange-red. This chapter describes the main achievements in luminescent tetracoordinate boron compounds comprising 2-iminopyrrolyl ligands and their applications to OLEDs.

This chapter summarizes the research work that has been developed in the past two decades in the area of polyoxometalates (POMs) by our research group, which is currently developing the investigation at the Associated Laboratory for Green Chemistry LAQV-REQUIMTE. The work herein reported has been largely dedicated to the structural modification, modulation and functionalization of the discrete units of POM structures. Various structures, integrating different transition metals and lanthanides, have been studied and modified according to the application planned. During these nearly two decades of research, POM structures have been associated with various organic units, integrated into different porous solid supports and encapsulated in distinct inorganic materials. A straight bridge between inorganic chemistry and materials science was built to activate and modulate the application of POMs, with an aim to increase the sustainability and cost-efficiency of their systems.

Metal oxides are an important class of materials with unique properties that are attractive for various applications, ranging from electronics to photocatalysis. Thus, the sustainable production of metal oxides is highly desirable. Solution-based synthesis methods provide a versatile and facile route to obtaining metal oxide nanomaterials with properties tailored to meet the demands of specific applications. This chapter provides an overview of wet-chemical synthesis routes to produce metal oxide nanomaterials, focusing on solution combustion synthesis and hydrothermal synthesis methods. Solution combustion synthesis is widely explored for the preparation of semiconducting (In₂O₃, ITO, IZO, ZTO and IGZO), insulating (Al₂O₃, HfO₂ and ZrO₂) and conductive (doped-In₂O₃) oxide thin films for application in printed electronic devices. Hydrothermal synthesis is preferred to prepare nanostructured powders or films (TiO₂, ZnO, ZTO and WO₃) with a high surface area for photocatalysis of pollutant dyes (such as methylene blue and rhodamine B) under ultraviolet (UV) radiation and/or sunlight. For each method/application, emphasis is given to processing technologies that are aligned with the Sustainable Development Goals.

Photocatalytic synthesis has shown an enormous potential for development in critical areas for society’s progress, impacting sustainability and the circular economy. Owing to the increased performance of artificial light sources, allied with the possibility of solar activation and the strong resilience of the photocatalytic systems, synthetic applications are boosting in almost all the quadrants of the chemical industry. The out-standing activity, selectivity and stability of the optical semiconductors used as photocatalysts reflect their unique electronic structures, which can be tailored and characterized in ever-increasing detail. The techniques used for characterization are addressed in traditional and contemporary approaches to establish the fundamentals of the reported structure–performance relationships. The potential applications are demonstrated through a selection of examples in three groundbreaking domains: oxidation of organic alcohols, oxidative coupling of amines and reduction of benzophenone. An outlook toward a new world of possibilities addressing photocatalytic process intensification is also included. References are made to the options opened up by combining photocatalytic reactors with traditional separation techniques in over-coming the photocatalytic processes’ drawbacks, especially the selectivity of organic synthesis.

The presence of antibiotics, such as oxytetracycline, in the aquatic environment is of significant concern, as it poses a risk to global health. The aim of this work was to study the application of Ti-doped SrFeO₃ perovskites as heterogeneous Fenton-like catalysts in the degradation of the antibiotic oxytetracycline. The perovskites were successfully synthesized via a solid-state reaction and characterized by XRD, SEM and XPS. The XRD data showed the existence of a predominant well-crystallized phase belonging to the perovskite cubic system. All samples exhibited catalytic activity on the degradation of oxytetracycline (50 mg/L) in the presence of 10 mM of H₂O₂ and at a pH of 3.0, with drug degradations above 70% after 3 h being the best result obtained by the undoped perovskite. Both •OH and •HO₂/O₂•⁻ were assessed as being the main species involved in the oxytetracycline degradation mechanism, with their formation being catalyzed by the presence of Fe²⁺/Fe³⁺ at the perovskite surface. The SrFeO₃ catalyst proved to be reusable up to three times without any loss of catalytic activity and maintained structural and morphological stability, which is a promising characteristic for practical applications.

Supercapacitors and fuel cells are essential energy devices for the implementation of a real renewable energy economy. The development of highly efficient and low-cost electrode materials is one of the major challenges to improve these devices. Biomass-derived carbon materials are postulated as a very interesting alternative, as they can be obtained from inexpensive precursors and abundant resources obtained directly from nature. However, they commonly exhibit an underdeveloped porous structure and a lack of controlled surface functionalities that limit their real application. Therefore, it is essential to apply different approaches to modify and design their properties to fit the requirements of energy storage and conversion devices. This chapter attempts to provide a broad overview of the most promising strategies that can be followed to design biomass-derived carbon materials with highly efficient performance in energy applications, such as supercapacitors and fuel cells.

The development of high-performance heterogeneous catalysts is critical to meet the industry decarbonization goals due to their easy separation from the reaction media and reusability. In heterogeneous catalysis, the reaction takes place at the surface level. Thus, the active area of the catalyst is critical to the extent of determining its final activity. For that reason, the preparation of porous materials, in particular 3D architectures with highly accessible surface area and reaction centers, is a cornerstone of heterogeneous catalysis. Among the methods employed to prepare large-surface 3D materials, synthesis based on electrochemical methods stand out due to their low cost and minimal environmental impact, their safety and their adaptability. Electrochemical techniques can be used to obtain large active-surface materials with controlled surface properties (composition, morphologies, etc.). Therefore, electro-chemical synthesis and engineering are becoming valuable tools for the mass production of porous catalysts. This chapter overviews the efforts reported in the literature to prepare porous catalysts by electrochemical techniques. These are also described in the context of a broad range of applications, from renewable energy to the removal of pollutants. A brief explanation of the fundamental principles of electrochemical synthesis introduces an overview of the different electrochemical synthesis used to prepare high-surface materials. To conclude, the main applications of these materials are discussed.

During the last two decades, supercritical CO₂ (scCO₂) emerged as an environmentally benign medium for sustainable chemical synthesis and catalysis. This chapter provides an overview of the applications of scCO₂ as an alternative for harmful organic solvents in chemical reactions. Special emphasis will be given to the importance of phase behaviour and on how scCO₂ unique properties can be successfully explored in the implementation of process intensification strategies.

Catalysis is a key subject when it comes to sustainability in chemistry. Additionally, the catalysts to be used and the processes to produce them must also be sustainable. Nowadays, there are several methods that can be used to obtain these materials; however, not all of them are environmentally friendly. In the past two decades, the supercritical processes and preparation of micro- and nanocatalysts have grown, namely by using supercritical CO₂. The remarkable properties of the supercritical fluids, which depend on their operational conditions, allow the tune-ability of several characteristics of the final material, such as morphology, size and size distribution or surface area. This chapter offers an overview of recent advances in catalysts’ production using supercritical techniques, and the trends and necessities for the use of supercritical fluid technology are also discussed.

Recently, there has been an increase in public awareness on the health, environmental and safety hazards associated with the use of toxic organic solvents in food processing. The high cost of organic solvents, the increasingly stringent environmental regulations and the new requirements from the medical and food industries for ultrapure and high-added-value products have pointed out the need for the development of new and clean technologies for the processing of food products. On the other hand, the 12 principles of green chemistry emphasize the importance of sustainable processes, which minimize waste generation and energy consumption. The compliance of these concepts will lead to a cleaner environment and a more cost-effective use of resources. Thus, the solution passes by the development of more sustainable technologies that significantly increase the efficiency of chemical processes and reduce the operation time of such technologies. Conventional extraction techniques for solid vegetable matrices include Soxhlet, hydrodistillation, shake-flash methods, maceration and pressing. These methods present many disadvantages since they use toxic solvents, with the associated risks for human health and the environment, are time consuming, labor-intensive, present low yield and selectivity, and have high energy consumption. Therefore, the design of green, efficient and sustainable extraction techniques is mandatory. Thus, the green extraction processes should consider the minimization of the use of toxic solvents, reduce waste production, avoid the presence of toxic solvents in the final product and lower the energy consumption of the processes. Among the several techniques complying with these requirements, supercritical fluid extraction (SFE) is considered one of the most suitable for the extraction of compounds from vegetable matrices. Supercritical CO₂ extraction has provided an excellent alternative to the extraction and isolation of valuable compounds from natural products. In this section, SFE studies of compounds from microalgae and aromatic plants are described in more detail. Some of the most interesting experimental results obtained at the Experimental Thermodynamic Laboratory at the Technical University of Lisbon are presented and discussed.

The following section is included:

• Index