Microwaves and Heterogeneous Catalysis: A Review on Selected Catalytic Processes

Abstract

Since the late 1980s, the scientific community has been attracted to microwave energy as an alternative method of heating, due to the advantages that this technology offers over conventional heating technologies. In fact, differently from these, the microwave heating mechanism is a volumetric process in which heat is generated within the material itself, and, consequently, it can be very rapid and selective. In this way, the microwave-susceptible material can absorb the energy embodied in the microwaves. Application of the microwave heating technique to a chemical process can lead to both a reduction in processing time as well as an increase in the production rate, which is obtained by enhancing the chemical reactions and results in energy saving. The synthesis and sintering of materials by means of microwave radiation has been used for more than 20 years, while, future challenges will be, among others, the development of processes that achieve lower greenhouse gas (e.g., CO 2 ) emissions and discover novel energy-saving catalyzed reactions. A natural choice in such efforts would be the combination of catalysis and microwave radiation. The main aim of this review is to give an overview of microwave applications in the heterogeneous catalysis, including the preparation of catalysts, as well as explore some selected microwave assisted catalytic reactions. The review is divided into three principal topics: (i) introduction to microwave chemistry and microwave materials processing; (ii) description of the loss mechanisms and microwave-specific effects in heterogeneous catalysis; and (iii) applications of microwaves in some selected chemical processes, including the preparation of heterogeneous catalysts.A chemical process is conventionally energized by means of conductive heating with a steam boiler as a typical heat source. Nevertheless, a large variety of other forms of energy can be applied for PI, including ultrasounds (for reactions or crystal nucleation), light (in photocatalytic processes), electric fields (in extraction or for orientation of molecules), or microwaves. The microwave (dielectric) heating of materials has been known for a long time, and microwave ovens have been developed from more than 60 years. The studies by Gedye et al. in 1986 and 1988 [3,4] opened a period of very intensive investigation of the microwave effects on chemical reactions in homogeneous systems. Since then, hundreds of research papers have been published, and research has also expanded toward heterogeneous catalysis and its related chemical processes. This review gives an overview of the application of microwave technology to heterogeneous catalysis, including various chemical processes, as well as to the preparation of catalysts.

Microwaves in Chemistry

“…Our recent results have demonstrated the use of microwaves for effective monitoring of catalyst deactivation [1], production of highly pure hydrogen from fossil fuels and the destruction of plastic waste [42]- [44]. Palma et al [45] have given a recent and extensive review of many fields of application, including preparation of catalyst materials and applications in oil and gas processing such as hydrocarbon conversion processes, partial oxidation reactions, production of syngas and water-gas shift reactions, desulfurization, and dehydrogenation of hydrocarbons.…”

“…Though the conditions do not always lead to enhanced reaction rates or increased yields of desired products, basic microwave technology is accessible and convenient to use, and has become a standard tool in chemical laboratories for rapid experimentation. Microwave reactions are carried out routinely in synthetic chemistry and are emerging in microwave catalysis [32], [45].…”

Interactions of Zinc Oxide Nanostructures with Mammalian Cells: Cytotoxicity and Photocatalytic Toxicity

“…Increasing solution temperature accelerates the reactions between zinc salt precursor, solvent and capping agent as expected [ 81 , 96 , 97 , 98 , 99 , 100 ]. In comparison to conventional heating in hydrothermal and solvothermal processes, microwave heating offers several advantages including rapid heating, high reaction rate, and increased production yield [ 101 , 102 , 103 , 104 , 105 , 106 ]. These result from the ability of microwaves to react with polar molecules directly [ 101 ].…”

A Review about the Recent Advances in Selected NonThermal Plasma Assisted Solid–Gas Phase Chemical Processes

“…Since the most increasingly problems connected to the diesel-soot emissions, several technologies have been proposed in the last years to allow the respect of the European Union regulations in this matter and, among them, the diesel particulate filters (DPFs), are nowadays, the most common after-treatment systems employed in the particulate matter (PM) trapping from the exhaust gases [98]. These systems usually operate in two steps, accumulation and regeneration, as described elsewhere [99], and the main challenges encountered in their on-board adoption regard the regeneration step. Indeed, during the DPF working the carbon deposition gradually blocks the filter, thus generating an increase of the exhaust backpressure and a decrease of the engine power.…”