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This book is an expert multi-author endeavour which has covered diverse topics, namely, basics of microwave heating, the role of microwaves in heterogeneous catalysis, microwave-assisted synthesis of nanostructured oxides and porous materials, the application of microwave-assisted heterogeneous catalysis in various fields of bulk and specialty chemicals synthesis, energy conversion and environmental remediation, and microwave augmented reactor technology. We have tried our best to bring together a host of outstanding contributors, with diverse expertise, in a fairly consistent way. The arena of microwave chemistry, which incorporates the application of microwaves into chemical reactions, has witnessed an extensive solicitation ever since the turn of the 21st century. It is no longer uncommon to find microwave chemical reactors installed in various research laboratories all over the globe. In general, the microwave heating mechanism is different from the conventional heating method as it is a volumetric process in which the heat is generated within the material itself. Therefore, it is very rapid and highly selective. In this process, microwave-susceptible materials absorb the energy embodied in the microwaves. Therefore, the application of microwave heating techniques to a chemical process is expected to lead to both a reduction in the processing time and an increase in the production rate, which is achieved by improving the chemical reactions and results in energy saving.

For the convenience of presentation, the book is structured into four parts, consisting of a total of 17 chapters. Part 1 of the book deals with the basics of microwave heating and the role of microwaves in heterogeneous catalysis providing a greater in-depth understanding of the subject with recent advances. Chapter 1 contributed by Meloni et al., from Italy, presents several creative ideas on basics of microwave heating and associated advantages in various chemical processes. The elaborative discussion supported by a literature survey reveals that better performance of microwave-assisted catalytic reactions could be due to both thermal and non-thermal effects. Some recent exclusive investigations have focused on understanding the mechanisms responsible for heating when a material is exposed to microwaves. The phenomena frequently referred to as “hotspots” originating from microwaves have been explored and accepted for a long time as the foremost mechanism responsible for microwave heating. However, different studies in the recent past have demonstrated the existence of a catalytic mechanism of the microwaves that decreases the activation energy of reactions, especially in the gas-phase. It is clear now that in heterogeneous catalysis, an appropriate selection of the material acting as the active species and/or support is mandatory in order to harness its heating effect and enable a successful microwave-assisted catalytic reaction.

The second chapter in this part is authored by Horikoshi and Serpone from Japan. This chapter elegantly deliberates on solid-state catalytic reactions employing microwave heterogeneous microscopic thermal energy (MHMTE) and its positive effects on chemical reactions. The distribution of heat in solid catalysts during the generation of MHMTE is also explained using a combined analysis of electromagnetic field and heat transfer in computer simulations. These authors emphasize that microwaves are electromagnetic waves of high energy and their application is wasteful if used as a heat source for simply heating the entire bulk in a chemical reactor. Microwaves should be considered whenever one wishes to use their inherent potential, because it is possible to perform special heating that cannot otherwise be conducted with existing conventional heating methods. Lessening of fossil fuel energy utilization and reduction of industrial energy consumption have been cited as prime issues that must be solved urgently on a global scale. Through designing creative chemistry using microwaves as an energy source, one should be able to build innovative industries to respond to societal issues of the current times.

Part II of the book focuses on the microwave-assisted synthesis of various catalytic materials with great industrial relevance. In Chapter 3, Perala et al. have focused their attention on the synthesis of nanostructured oxides having numerous catalytic applications. In the last two decades, the use of microwave irradiation, either alone or in conjunction with other synthesis techniques, has grown in popularity among scientists and researchers for the production of nanostructured materials. This chapter provides a detailed insight into microwave-assisted synthesis of different nanostructured materials including pure metal oxides, mixed metal oxides, spinels, perovskites and composite oxides, since these materials have attracted significant research interest in various fields. Nanostructured metal oxides show great potential in exhibiting superior catalytic performance due to the enhanced structural, morphological, and surface properties compared to conventionally synthesised materials. Metal oxides with nanostructures are widely used in diverse fields, namely, materials science, catalysis, and biotechnology. This is primarily because they are more reactive compared to their micro-sized counterparts. Nanoscale materials, in general, are highly reactive because they have a large surface area-to-volume ratio and provide unsaturated surface atoms.

Chapter 4 discusses the important subject of microwave-assisted synthesis of porous materials of catalytic significance. Sounak Roy et al. from India have authored this chapter. For both industrial and household needs, the modern society is extensively relying on different types of porous solid materials. The historical start of porous materials began with porous carbon and clay-type minerals, and has come a long way to zeolites and presently metal organic frameworks (MOFs). Microwave-assisted synthesis of porous materials has recently gained attention as an alternative for conventional heating methods owing to its several advantages such as less energy consumption, shorter synthesis time, increased phase purity and phase selectivity, narrow particle size distribution, and versatile composition of the resulting products. Microwave heating is believed to facilitate rapid energy transfer into the reaction system, thereby accelerating the heating rate and consequent crystallization as well as the nucleation rate. This chapter encompasses the different aspects of microwave-assisted synthesis of two popular porous materials; zeolites and MOFs. The microwave-assisted synthesis offers a significant advantage of being time- and energy-saving compared to the classical hydro(solvo)thermal method. Microwave synthesis can also be considered as environmental friendly as it is reasonably cleaner and more environmentally pleasant than conventional heating methods.

Part III of the book covers 11 chapters with focus on application of microwave-assisted heterogeneous catalysis in a wide spectrum of chemical reactions. These chapters are highly relevant to bulk, fine, speciality, and knowledge intensive segments of the chemical industry, which collectively possess a current global production value exceeding several billion USD. A large tool box of chemical reactions is available for their synthesis developed through traditional R&D approaches which are more developmental than efficiency oriented. Their environmental burden is rather high compared to other segments of the chemical industry. Significant investments are anticipated in the coming years in intensifying and greening the microwave-assisted technologies. From this perspective, various chapters covered in part III of this book assume an extraordinary prominence.

Chapter 5 authored by David Daggett and Béla Török from USA covers microwave-assisted reactions with solid acid and base catalysts. This chapter includes three main parts, namely, (i) a brief survey of solid-acid and solid-base catalysts and the application of microwave irradiation in environmentally benign processes, and synthetic applications of (ii) solid-acid and (iii) solid-base catalysts. Acid–base catalysis has undergone significant changes since its inception over a century ago. These reactions remain an essential tool in organic syntheses and transformations. The dawn of green chemistry and the growing environmental consciousness brought about the development and application of solid acids and bases to replace traditional, often dangerous and corrosive alternatives. In this chapter the recent advances in the application of solid-acid and solid-base catalysts in microwave-assisted environmentally benign synthetic processes are described. Based on a survey of recent literature, it is suggested that further development of green catalytic processes will remain an important driving force in the expansion of sustainable syntheses and with this, the combined application of solid acids/bases and microwave activation will grow both in scope and importance.

Chapter 6 by Lili Li et al. from USA deals with the activation of stable molecules through microwave catalytic processing. This chapter introduces the application of microwave energy combined with heterogeneous catalysis for the conversion of stable molecules, namely, CH4, N2 and CO2 to value added chemicals. Microwave irradiation exhibits a strong influence on the activation of stable molecules such as CH4, ethane, N2 and CO2. Their own experimental results suggest that methane and N2 can be simultaneously activated in a single-stage microwave-heated reactor to form ammonia, ethylene and acetylene at atmospheric pressure. Microwave irradiation can also induce oxidative dehydrogenation of methane and ethane with CO2 as the soft oxidant. This chapter highlights that microwave catalysis as a transformative technology has the potential to increase energy efficiency across industries and decarbonize high greenhouse gas emitting industrial subsectors including ammonia, ethylene, and BTX (benzene, toluene and xylene).

Chapter 7 by Vishal Tuli et al. from USA focuses on microwave-assisted depolymerisation of polymeric materials. Over the last few decades, plastics have played a key role in enhancing the quality of life for human civilization. The application of plastics has been massive resulting in major innovations in different sectors such as construction, healthcare, consumer electronics, automotive segment, packaging, and so on. With the rapid growth of world population, the demand for plastics has been growing continuously. Most of the plastics (≈99%) are sourced from fossil fuel-based chemicals, e.g., ethylene and BTX aromatics. It is projected that, by 2050, the plastic industry could account for 20% of the world’s total fossil fuel consumption. However, around 79% of plastics end up in landfills or oceans, where their embodied energy and value are lost, causing enormous environmental pollution. Therefore, recycling or upcycling of plastics is essential to prevent their release into the environment, reduce fossil-fuel consumption and achieve sustainable production. Over the decades, many recycling and upcycling approaches have been explored for depolymerisation. However, the majority of them are environmentally unfriendly and economically unsustainable. As highlighted by the authors, microwave-assisted depolymerisation is one of the potential approaches that can address both the issues. Due to site-specific heating, microwave pyrolysis has an inherent advantage over other conventional thermal technologies. Additionally, the usage of a metal catalyst in the depolymerisation process not only ensures complete degradation but also leads to the production of high-value products selectively. With the application of specific catalysts, high-yield and highly selective products can be achieved.

Chapter 8 by Leilei Dai et al. from USA provides an authoritative outlook on microwave-assisted pyrolysis of municipal solid wastes (MSW) for the production of energy, fuels and chemicals. This chapter begins with a brief overview of MSW composition and management followed by a description of the microwave-assisted pyrolysis process, mechanisms involved in microwave heating, and advantages of microwave heating over the conventional heating. Currently, the rapid urbanization and ever-increasing population in the world has led to fast-growing consumption of finite resources, which is accompanied by the massive generation of MSW each year. Effective recycling and recovery technologies for MSW collection, storage and treatment can prevent it from threatening our environment and health. Unfortunately, about half of the MSW generated is thrown away in many developing countries, thereby resulting in serious environmental issues, such as contamination of air and water, and the potential spread of diseases. It is imperative to develop advanced MSW recovery processes to achieve environmentally friendly and sustainable MSW treatment. Pyrolysis of MSW is a promising technology for commercial production of low carbon energy, fuels and chemicals. Pyrolysis of MSW, employing microwave as a heating method, has many advantages over various options using conventional heating. Some of these advantages include selective, rapid, reduced equipment size, and high energy utilization efficiency. When considering examples such as plastic and biomass conversion, microwave-assisted pyrolysis revealed a great potential for energy and fuel recovery, with high energy conversion efficiency.

Chapter 9 by Fattah and co-authors from Australia discusses the significance of microwave-assisted catalytic biodiesel production. Because of the global concern for sustainable energy, the conversion of bio-based feedstocks into biodiesel has become an important study subject. Various technologies have been explored for biodiesel production, one of which as pointed by the authors, the application of microwaves, has been shown to hold a lot of promise. Microwave-enhanced biodiesel synthesis is a favoured approach due to various advantages such as decreased energy usage, a significant reduction in the reaction time, no solvent use, higher selectivity and improved conversion with less by-product generation. Because of the increasing greenhouse gas emissions worldwide and diminishing supply of fossil fuels, alternative fuels have received an unprecedented attention recently. In many industrialised nations, there is a growing interest in the usage of new information and applying a range of biofuels to transform bio-energy into a more cost-effective form than fossil fuels. The higher production cost of biodiesel in comparison to fossil-based diesel fuel has stimulated more research to optimize biodiesel synthesis economically. Microwave-assisted biodiesel production is one of the most promising synthesis approaches to overcome this disadvantage. The capacity of microwave systems to directly transfer heat to the reactants makes the microwave-assisted biodiesel production more advantageous compared to the conventional heating methods.

Chapter 10 deals with microwave-augmented carbon capture (CC) by Ramanarayanan et al., from India. This chapter focuses solely on microwave assistance as well as the future challenges and prospects it presents in this field. Over the last few decades, global warming has become a significant concern due to its adverse effects on the environment and living systems. A major reason for global warming has been the ever-increasing emissions of CO2. Most CO2 adsorbents are expensive to synthesize; however, microwave assistance was found to reduce the cost involved and the production time. Similarly, the energy consumed during the regeneration of CO2 absorbents and the efficiency of this process were improved when microwave heating was used. In general, microwave assistance improved CO2 selectivity and capacity as it positively affected the morphology of the sorption materials. Thus microwave energy has been explored to assist in CC in several ways. The CC methods largely belong to either adsorption or absorption with a few being used in membrane separation and oxyfuel combustion. Among the various options available for CC, the most common methods are absorption and adsorption. Of course, both these methods have their own advantages and disadvantages. As contemplated by these authors that microwave augmentation in CC applications provides a multitude of advantages over conventional heating. As such there are fewer studies on microwave amplified CC; however, there is a large scope for research to make microwave assistance viable on a large scale.

Chapter 11 by Komal Sharma et al. addresses the microwave-assisted catalytic transformation of biomass to platform chemicals. Due to the scarcity of fossil fuel-derived sources, alternative resources of chemicals and fuels have become necessary. The perception of utilizing renewable feedstock such as biomass and its conversion to platform chemicals and fuels is increasing in demand day by day. In this chapter, these authors highlight the catalytic upgradation of biomass and biomass-derived molecules to platform chemicals, including furans, levulinic acid, levulinates, xylitol and sorbitol by employing metal nanoparticles, single metal atoms, metal oxides and graphene oxide as catalysts. It was also pointed out that there are significant challenges and drawbacks of MW-mediated reactions, such as poor interaction of substrates having low-dielectric properties and the scope of large-scale applications, compared to the pyrolysis and hydrothermal processes. As a growing technology, microwaves reduce energy consumption, having a fast conversion process with the improved quality and yield of the products. It is also emphasized that waste biomass serves as the benchmark raw material for establishing a controlled circular bioeconomic route.

The subject of microwave-assisted extraction of lignin from biomass has been dealt in Chapter 12 by Ahmad et al. Microwave-assisted extraction (MAE) of lignin from biomass is considered as an efficient and environmentally friendly method due to its reduced amount of energy and chemical requirements. Furthermore, it can be performed at a lower operating temperature and pressure compared to conventional methods, which helps to preserve the structural integrity of lignin. Therefore, microwave-assisted lignin extraction from various feedstocks such as wood, straw, and agricultural waste has been highlighted in this chapter. Furthermore, the effects of biomass, nature of solvent, microwave power and frequency, extraction time and temperature, solid-to-solvent ratio, and the presence of catalysts in lignin extraction are enlightened. On these grounds, biomass has been projected as a potential replacement for fossil fuels in the production of fuels and value-added chemicals because of its renewable source of organic carbon compounds and its ability to form a closed carbon cycle. In particular, lignocellulosic biomass (LCB) is available in abundance at an affordable cost which makes it a potential candidate for alternative renewable energy to substitute fossil fuels. The lignocellulosic biomass could be used to produce an array of value-added chemicals and fuels such as biofuel, biodiesel, biochemicals, bioethanol and hydrocarbons. The extracted lignin has numerous potential applications, including environmental remediation, biofuel production, preparation of bio-composite materials, production of carbon fibers and chemicals. Despite many advantages of MAE, the selective and effective extraction of near-native lignin remains a major challenge.

Chapter 13 deals with microwave catalysis in energy and environmental applications and is authored by Jicheng Zhou and Wentao Xu from China. In this chapter, in particular, a highly efficient conversion of NO and decomposition of H2S through microwave catalysis have been addressed. The reaction temperature was found to decrease by several hundred degrees centigrade in microwave catalysis. Most importantly, the apparent activation energy (Ea) of the investigated reactions was decreased substantially under microwave irradiation. This study also proposes a model of interactions between microwave electromagnetic waves and the reactant molecules to elucidate the intrinsic reason for the reduction in Ea under microwave irradiation, and a formula for the quantitative estimation of the decrease in Ea was also determined. The authors also describe their developed microwave catalysis model for application in energy and environmental catalytic reactions. They have also dealt with microwave catalytic oxidation reaction technology for degradation of organics in wastewaters.

Application of microwave irradiation (MIR) in Fischer–Tropsch synthesis and fuel cells has been deliberated in Chapter 14 by Christel Olivier and Linda Jewell from South Africa. This chapter mainly focusses on different ways and different benefits that can be achieved when MIR is applied in Fischer–Tropsch synthesis (FTS) and fuel cells. The FTS is an industrial process that consists of valorizing abundant natural resources such as coal, methane, biomass and municipal waste by converting them into a mixture of syngas (CO and H2). The syngas is then subjected to conversion over a catalyst at a certain temperature and pressure, leading to the production of valuable products such as liquid fuels and chemicals. The utilization of MIR as both a preparation and post-synthesis method for catalysts; or as a heating source for chemical processes, offers more advantages compared to catalytic systems using conventional heating. For the FTS, the application of MIR can enable the preparation of Co and Fe catalysts with better metal dispersion and distribution than those prepared by the conventional method. The MIR also prevents the agglomeration of Co and Fe. It was also demonstrated that favourable metal–support interactions can be achieved by using MIR in the case of inert supports such as β-SiC. As a result of improvement of the catalyst’s bulk properties with MIR, an increase in CO conversion and lower methane selectivity during FTS operation have also been reported. In H2 and CH3OH fuel cells, factors such as size of the active particles, metal dispersion and distribution play a crucial role in determining their performance. It has been demonstrated that MIR could be used to obtain small active particles that are very active due to the increase in the number of active sites with better dispersion and distribution. The catalysts prepared using MIR displayed higher electrochemical surface areas than those prepared by the conventional method. Pertaining to the catalyst strength, the MIR has also been reported to improve the durability of the fuel cells. There are some promising alternatives to Pt that can be exploited to make the fuel cells more affordable.

Chapter 15 deals with microwave-assisted glycerol conversion into valuable chemicals, contributed by Rafael Estevez et al. from Italy. This chapter describes the effectiveness of microwave irradiation as a new synthetic method for the conversion of glycerol into more valuable chemicals. In particular, microwave irradiation quickly activates the reactants, facilitating their interaction with the catalysts and leading to the conversion of glycerol towards the formation of several important chemicals. It is mainly used in the manufacture of various foods and beverages, cosmetics, pharmaceuticals, polyether, polyols, alkyd resins, etc. In the last fifteen years, the huge production of biodiesel has resulted in a notable increase in the production of so called “crude glycerol”. The high production of biodiesel can be attributed to its inherent advantages to substitute fossil fuels, such as its low toxicity, as well as its biodegradable, renewable and biocompatible character and reduced emission characteristics. Furthermore, biodiesel can be easily integrated into the logistics of the global transportation system and fits into existing engines with little or no modifications needed. In fact, biodiesel dominated the glycerol market with a 61% share in 2019. With the use of appropriate catalytic materials, the described research studies enabled the accomplishment of very fast and efficient reactions. These features make the processes sustainable in terms of energy, time and economy savings. Moreover, the use of waste as secondary raw material not only promotes the sustainability of the process but also reduces environmental pollutants. In particular, the use of glycerol as a significant by-product of biodiesel production holds great potential for the sustainable synthesis of value added products.

Part IV of the book covers 2 chapters with focus on reactors and technological implementation of the microwave energy in practical applications. Chapter 16 by Chaouki et al. from Canada presents an interesting account on the pros and cons of microwave heating with the chapter title ‘Non-uniform microwave heating of heterogeneous systems: How to turn problems into opportunities’. Energy and the environment are currently the main apprehensions prompting researchers and industries to use novel approaches to replace the energy-intensive chemical processes with green and energy efficient alternatives. Microwave heating with unique characteristics, such as selective, rapid, and volumetric heating, is a promising approach that addresses various concerns by electrifying low energy efficiency processes, leading to reduction in energy consumption and CO2 emission by up to 3–5 times. Turning problematic, non-uniform microwave heating into an opportunity is a promising approach to enhance energy efficiency and reduce environmental impact due to the chemical processing of heterogeneous systems. Selective microwave heating of materials provides an adequate temperature for an efficient chemical reaction at the desired sites while the surrounding media remain unheated, diminishing heat loss and enhancing energy efficiency. Most importantly, the side reactions at non-desired sites, including reactor walls and the surrounding media, are prevented owing to lower bulk temperature compared to that of the target material, which improves the quality of the product. Application of this approach in various microwave-assisted catalytic and non-catalytic processes, including pyrolysis, cracking and hydrogen production, has confirmed an extraordinary improvement in the product quality and reduction in energy consumption. Some of these underlining advantages are expected to increase the demand for scaling up microwave-assisted processes. Selective microwave heating provides a non-uniform temperature distribution within the heterogeneous system. The target material is heated via microwaves to reach the desired temperature while the temperature of the reactor walls, surrounding media, and other components is lower, generating a significant temperature difference. Therefore, the overall heat loss during microwave processing is less than that in conventional heating where all components are heated to the desired temperature. Accordingly, microwave processing requires less energy to produce a product similar to conventional heating. Energy costs and environmental considerations certainly prompt the industries to apply high energy efficiency technologies as substitutes for energy-intensive processes.

Chapter 17 deals with scaling up of microwave-assisted heterogeneous catalytic processes and is authored by Ignacio A. Julian and Alejandro Fresneda-Cruz from Spain. This chapter presents an overview of reported heterogeneously catalyzed thermo-chemical transformations assisted by microwave irradiation, highlighting their technological readiness level, industrialization degree and scale-up strategies, advantages, drawbacks and the challenges of each application. As exemplified by the authors, microwave-assisted systems (MASs) represent a very promising technology to overcome the limitations of traditionally heated reactors in the implementation of various heterogeneous catalytic processes of interest in the chemical industry. In particular, MASs are envisioned to have a key role in the development of thermo-chemical process industry intensification and decarbonization strategies, thanks to their inherent energy-efficiency and easy integration with power supply from renewable energy sources. When applied to chemical reaction processes that require mild-to-high temperatures, MAS provides numerous advantages such as very fast start-up and shutdown processing times, selective heating of target absorbing materials, controlled bulk sample temperature, possibility to establish a very significant temperature gradient between the absorbing material and the surrounding atmosphere to enhance the selectivity of certain catalytic processes while inhibiting detrimental gas-phase interactions, lower reaction temperature for a given conversion rate, significant reduction of the required downstream cooling energy, and low thermal inertia upon appropriate sample temperature monitoring, which is very relevant from the safety point of view. This chapter is a highlight of the book with detailed discussion on large scale exploitation of microwave energy and more useful illustrations from various sources.

To sum up, a systematic effort has been made in this book to illustrate the prospects and recent advances made in developing novel microwave-assisted heterogeneous catalytic approaches to make chemical processes energy efficient, greener and environmentally sustainable. The importance of microwave irradiation in catalyst preparation, specialty and bulk chemicals synthesis, biomass utilization, CO2 capture, depolymerisation of polymeric materials, etc., and the use of well-designed reactors for enhanced process performance has also been highlighted in this book. It has unequivocally stressed the need for chemists and engineers to adopt innovative microwave augmented strategies to reduce the production cost and environmental burden in various sectors.

We would like to express our sincere gratitude to all the authors for their enthusiastic contributions and invaluable support. We thank the reviewers for their helpful comments and suggestions, and for respecting our publication schedules. We appreciate the support received from the Royal Society of Chemistry and its editorial staff for their unconditional assistance in publishing this book.

Jianli Hu

Department of Chemical & Biomedical Engineering, West Virginia University, USA

Benjaram M. Reddy

Department of Chemistry, Birla Institute of Technology & Science (BITS) Pilani, Hyderabad Campus, India

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