Editorial Catalysts: Special Issue on “Microwave-Assisted Catalysis”

The concept of circular economy is based on several principles, such as the use of renewable energy resources, including those obtained from the sun, wind, or water; the use of natural raw materials; the manufacturing of products avoiding the generation of wastes and pollution; keeping products and materials in use for a longer time; or giving new applications to waste. Within this concept, microwaves appear as an alternative to thermal-based processes, since this technology increases energy efficiency and, therefore, can improve the overall economy of industrial processes.

The use of microwaves applied to catalysis has received considerable attention from both the industry and academics in the last few decades as an alternative to conventional heating [1,2]. The benefits of microwave heating for catalysis mainly lie in the fact that they accelerate the reaction rates, can be used at milder reaction conditions than conventional heating (lower temperature and time) with a subsequent energy saving, and can lead to higher chemical yields. Additionally, considering that molecules or solid surfaces have different abilities to transform electromagnetic energy into heat, a different reaction selectivity could be obtained by controlling the catalyst properties. On the other hand, the main drawbacks reported until now are related to the scale-up of microwave-based processes; the non-uniform microwave fields generated in most microwave ovens, which can involve the formation of superheating spots; the impacts on reaction kinetics; or the arcing phenomena basically linked to the use of large metal particles.

This Special Issue on “Microwave-Assisted Catalysis” collects original research papers focused on the recent research on this topic in order to highlight its importance. Hence, four representative works have been published about the use of microwaves in catalysis, paying special attention to the development of catalytic processes for producing energy and chemicals of interest from renewable resources as an alternative to the traditional use of fossil feedstocks. Another important point to remark on is that, in the field of catalysis, microwaves can be used not only in the reaction but also in the preparation of catalysts. Thus, the properties of the catalysts prepared under the microwave action, when compared to those synthesized by conventional heating, can have a significant influence on the final catalytic activity.

The first paper, entitled “Overcoming Stability Problems in Microwave-Assisted Heterogeneous Catalytic Processes Affected by Catalyst Coking” by Julian et al. [3], is remarkably interesting since the authors tried to find solutions to problems associated with the use of microwaves in catalytic reactions that work without oxygen. In this work, different catalyst arrangements and reactor characteristics were explored in order to minimize the effect due to coke deposition in microwave-assisted heterogeneous catalytic processes. The carbon deposited not only supposes a fast deactivation of the catalyst but also involves significant stability problems. Carbon species may promote the formation of hot spots and temperature gradients on the catalyst, which results in the loss of controllability and reproducibility in the process. Other negative effects of coke could be disturbing the electromagnetic field, leading to cavity uncoupling and causing a decrease in the temperature. The aim of this work was to study the role of coke deposits and work out how to minimize their negative effect on the non-oxidative methane coupling reaction (MNOC) process. This was evaluated following the thermal evolution of the dielectric properties of the catalyst and the catalytic support used in the reaction. Additionally, a study about the effect of coke on the scaling process working at two different microwave radiation frequencies and cavity sizes was performed. The results of this paper showed that the negative effect of coking can be controlled using a suitable catalyst and specific reactor arrangement. The use of an Mo/ZSM-5 catalyst coated on SiC monoliths enabled working for several hours under reacting atmospheres even in the presence of coke deposits. With respect to the scalability of microwave-assisted heterogeneous catalytic processes, it was found that microwaves’ energy input and sample size were not linearly correlated, since the increase in the microwaves power by six times allowed processing methanol flow 150 times.

The second and third papers were focused on the study of catalytic reactions related to the valorisation of lignocellulosic biomass as an alternative to petroleum-derived products [4]. To improve biomass transformation into useful chemical is essential to improve sustainability in industrial processes. Lignocellulose, the most abundant renewable biomass, is considered the main raw material on a biorefinery concept, since its non-edible nature does not compete with food crops and is less expensive than conventional agricultural feedstocks. Lignocellulose biomass consists of three types of polymers—cellulose (40–50%), hemicellulose (25–35%), and lignin (15–20%). The conversion of biomass into functionalized, targeted platform molecules, which includes sugars (glucose, xylose), polyols (sorbitol, xylitol), furans (furfural, 5-hydroxymethylfurfural), and acids (levulinic, lactic), is unique to hydrolysis-based methods and allows for the production of a wide range of fuels and chemicals [5]. The use of microwaves as a source of heating is usual for biomass treatment, since it decreases the reaction times and also can enhance the selectivity of the process to the intermediates of interest. In contrast to molecules coming from fossil feedstocks, which are essentially unfunctionalized alkanes, platform molecules from biomass are already functionalized compounds; this allows transforming them into more valuable chemicals through a lower number of steps than are required when starting from alkanes. Levulinic acid, obtained from cellulose, and xylose, obtained from hemicellulose, were the platform molecules employed in these two papers, respectively, to obtain chemical products of interest.

The second paper of this issue, entitled, “Rapid Microwave-Assisted Polyol Synthesis of TiO₂-Supported Ruthenium Catalysts for Levulinic Acid Hydrogenation” by Edwards et al. [6], is focused on the use of microwaves to prepare catalysts with specific characteristics to improve their activity in catalysis. In the field of nanomaterial synthesis, microwave heating has been shown to decrease the nanoparticle preparation time from hours, in the case of conventional heating, to minutes with microwaves. Additionally, microwaves enabled the access of the reagents to highly faceted and crystalline nanomaterials. In the case of lignocellulosic biomass, the major component is cellulose, which is readily hydrolysed under acidic conditions to Levulinic acid (LA), which has been considered as a bio-derived platform chemical. One important transformation of LA is its hydrogenation to γ-Valerolactone (GVL), a valuable intermediate for producing fuels or fuel additives. The supported catalysts of metal nanoparticles of Ir, Rh, Pt, Re, Ni, and Ru have shown a high activity for this reaction. A good dispersion of the metal phase is determinant in achieving a high conversion and high selectivity to GVL for this reaction. Nanoparticles can be prepared by several methods. The authors of this paper employed the microwave-assisted solvothermal method in previous works, showing that is an efficient and fast procedure. The aim of this paper was to check the effect of microwave treatment time and temperature, and the influence of the metal precursor on the activity of the Ru nanoparticles supported on TiO₂ prepared by microwave-assisted one pot polyol synthesis, for the hydrogenation of levulinic acid. The catalyst prepared using RuCl₃ as a metal precursor under microwaves at 150 °C for 5 min was the most active catalyst, with a maximum LA conversion of 67%. An important characteristic of this catalyst was the high dispersion of the Ru nanoparticles with a size lower than 3 nm. The general results showed that precursor, temperature, and preparation time are critical for producing catalysts active for levulinic acid hydrogenation.

The third paper of this issue, entitled “Microwave-Assisted Furfural Production Using Hectorites and Fluorohectorites as Catalysts” by Cesteros et al. [7], can be considered strategic for developing processes directed to furfural production from renewable lignocellulosic biomass using microwaves during reactions. Furfural is an important building block in the chemical industry, since it can be applied for the synthesis of an important number of chemical products of interest. The aim of this work was to study the catalytic behaviour of several hectorites with different acidities to produce furfural from xylose aqueous solutions using microwave heating and toluene as a co-solvent. The commercial xylose and xylose aqueous solutions obtained from almond shells were compared. The use of microwave heating together with the presence of co-solvents were previously reported as a way to improve the furfural production from xylose solutions by acid catalysis. Solid acid catalysts can be a good alternative to mineral acids, since they can prevent corrosion and can be reused. In this work, delaminated hectorites (Na⁺ and H⁺) and fluorohectorites (Li⁺ and H⁺) were tested as catalysts for this reaction and compared with one H-beta zeolite used as a reference catalyst. The optimization of the synthesis methodology to obtain fluorohectorites was another important objective of this paper. The results concluded that the crystalline fluorohectorite, which had a high acidity strength, was the most efficient catalyst at 1 h of reaction for the transformation of xylose to furfural, independently of the xylose source. The furfural yield was 20% for commercial xylose and 60% for xylose produced from almond shells.

Another clean technological approach within a sustainable zero emission concept is the use of hydrogen as a fuel due to its high energy density per mass and taking into account that its combustion only generates water vapour. Among the numerous methods investigated for the production of hydrogen, the use of H-containing liquid organic compounds as a source of hydrogen is becoming a topic of interest [8].

The fourth paper published in this special issue, entitled “Microwave-Mediated Continuous Hydrogen Abstraction Reaction from 2-PrOH Catalyzed by Platinum on Carbon Bead” by Sajiki et al. [9], constitutes a challenging approach in the field of clean energy to mitigate the climate crisis. Microwaves were applied in this paper for producing hydrogen from 2-propanol (2-PrOH). The combustion of hydrogen, the combustion of which does not emit a greenhouse gas, has been proposed in the last few decades as an environmentally friendly energy vector. The direct production of hydrogen from liquid organic hydrides has been demonstrated to have a high potentiality. 2-Propanol is cheap, readily available, reusable via the reduction of acetone, and has a low toxicity, so it could be an excellent hydrogen raw material. The continuous hydrogen production and the use of microwaves as an efficient heating method were considered key points in a new process. The use of MoS₂/Al₂O₃ as a catalyst in a flow reactor allowed the continuous production of hydrogen gas, but with similar results using microwave or conventional heating [10]. Other catalysts should be studied to improve the microwave action. Considering the high electron permittivity of carbon, adequately heated by microwave irradiation, carbon could be a component in a good catalyst for an effective hydrogen production from 2-PrOH in a continuous flow. The aim of this paper was to study the catalytic behaviour of a catalyst with 5% platinum supported on spherical carbon beads, using microwaves of low power for this reaction. The results showed the production of high-purity hydrogen from 2-PrOH at high conversion. This method requires only 10 W of microwave irradiation and 80 mg of catalyst to achieve a yield of 324 mol/kgcat/h. This can be considered efficient in comparison to other catalysts described in the literature. This system was stable, since any loss of activity was observed during 13 h of continuous operation.

Regarding the previous literature related to microwaves applied to catalysis, this Special Issue showed how the application of microwaves to catalysis has gone from an initial state of seeing what possible applications it had to being currently applied for solving the most important problems that society has today, such as the obtention of chemicals from biomass instead of fossil fuels or the formation of hydrogen from chemicals to be used for producing clean energy.