Despite the great success of Microwave Assisted Organic Synthesis (MAOS) there is still a lack of knowledge about the interaction of the electromagnetic radiation with matter. In consequence, it has been very difficult to rationalize the effect of microwave irradiation in chemistry, to determine the existence of microwave effects (thermal and non-thermal) and to develop predictive models on the characteristics required for a reaction to be improved under microwaves. This has been a handicap to develop new chemistry under microwave irradiation and the origin of many controversies. This personal account collects some new findings in this field and our work on the use of computational chemistry to develop predictive models and to determine parameters related to thermal and non-thermal effects, with clear advantages over experimental methods where separation of these effect is almost impossible.

A Critical Overview on the Effect of Microwave Irradiation in Organic Synthesis

The aim of this review is to discuss the applications of microwave heating to separation and purification processes in chemical engineering from the perspective of its three unique features, namely rapid heating, selective heating and specific microscopic effects. These features render microwaves an alternative clean energy source that can replace or complement the conventional heating in various separation processes. The rapid heating and evaporation of water under the influence of microwaves renders the technology valuable in materials drying and organic extraction. The direct and selective heating of materials with high dielectric loss in a mixture enables energy savings in the treatment of petrochemical products. In adsorption processes, microwave heating can selectively heat solid adsorbent material leading to the formation of micro “hot spots” (localized zones of high temperature), increasing the efficiency in adsorbent regeneration. Finally, a deeper understanding of the impact of microwaves on microscopic structure has led to novel separation methods, such as microwave-assisted pervaporation and microwave-assisted reactive distillation. An overview of the key mechanisms and developments of microwave heating in all these applications during the recent five years is provided and is followed by identification of the associated challenges in scale up. Fundamental research is still required for guiding further industrial application, while numerical simulations considering both heat transfer and distribution of the electromagnetic field in various reactor geometries can be helpful in the design of microwave reactors for large scale applications.

Fundamentals and applications of microwave heating to chemicals separation processes

Covalent organic frameworks (COFs) are relatively recent materials. They have received great attention due to their interesting properties. However, the application of microwaves in their synthesis, despite its advantages such as faster and more reproducible processes, is a minority. Herein, a comprehensive compilation of the research results published in the microwave-assisted synthesis (MAS) of COFs is presented. This review includes articles of 2D and 3D COFs prepared using microwaves as source of energy. The articles have been classified depending on the functional groups including boronate ester, imines, enamines, azines, and triazines, among others. It compiles the main parameters of synthesis and characteristics of the materials together with some general issues related with COFs and microwaves. Additionally, current and future perspectives of the topic have been discussed.

Microwave-Assisted Synthesis of Covalent Organic Frameworks: A Review

Microwave absorption and catalytic activity have been synergistically integrated into a core/shell structured CNT/polyaniline (PANI) hybrid catalyst that enables localized heating on the catalyst surface and uplifts energy efficiency in the dehydration reaction of saccharides to 5-hydroxymethylfurfural (HMF). Results reveal that the integration of CNT and polyaniline significantly improves the overall saccharide conversion and HMF yield. The PANI shell thickness and morphology are controlled by both reactant feeding ratio and polymerization reaction rate. A thinner and rougher PANI shell is favourable to enhance the overall microwave absorption, improve heat transfer efficiency to catalyst surface and thus reaction efficiency. The highest energy efficiency of 7.6 mmol kJ−1 L−1 is reached in 10 min with CNT/PANI catalyst (30 wt% CNTs) and 19 W input power. Moreover, the packing of polyaniline shell on CNT core facilitates its separation from reaction mixture and effectively prevents catalyst loss. These catalysts show excellent recyclability that the catalyst activity can be completely recovered by a simple re-doping process in 1.0 M sulfuric acid. This work opens up a new window in microwave responsive catalyst design that enhances energy efficiency in saccharide-HMF conversion reactions and promotes the economic feasibility of such reactions in practice.

Structurally tuning microwave absorption of core/shell structured CNT/polyaniline catalysts for energy efficient saccharide-HMF conversion

Using renewable ethanol and isopropanol for lipid transesterification in wet microalgae cells to produce biodiesel with low crystallization temperature

The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.

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D. Raúl Martín

Papers

Influence of Polarity and Activation Energy in Microwave-Assisted Organic Synthesis (MAOS).