Microwave-assisted heterogeneous catalysis

Since the invention of the magnetron 80 years ago, microwave heating has found numerous applications across industries such as agriculture and food, ceramics, analytical chemistry, pharmaceutical and cosmetics, vulcanization, and material synthesis. Because of its ability to directly heat the subjected material, microwave heating, also known as dielectric heating, has been extensively studied for various applications in academia and industry. Advantages of microwave heating over conventional heating stem mainly from its ability to interact with the material at the molecular level. Heat losses (conductive and convective) associated with traditional heating methods are negligible with microwave heating. These properties of microwave heating make it an efficient heating method, with a conversion efficiency of electrical energy to heat reaching >60%–65%. Microwave heating is well-established in organic chemistry and homogeneous catalytic reactions, and is gaining momentum in heterogeneous reactions as microwaves can selectively heat the catalyst or active sites on the catalyst.

The three pillars of heterogeneous catalyst are activity, selectivity and stability. All three have been shown to be positively affected during microwave-assisted heterogeneous catalysis. Catalyst activity refers to the nature and strength of the chemical bond between the reactants and reaction intermediates and the catalyst surface. The activity of the catalyst dependent on the structure and composition of these sites and their ability to increase the rate of reaction. Microwave heating has shown to enhance the catalyst activity, where the polar reactant molecules have faster rotation rates and enter the transition state more rapidly than in conventional heating system, resulting in increased reaction rates. Microwave-assisted reactions have also found to increase product selectivity due to rapid heating rates and higher temperatures at certain sites (such as metal sites on catalyst surface). Microwave heating has been associated with generation of microplasmas on the metal sites, which leads to increased product selectivity and rapid activation of reactants such as methane. Catalyst stability could be affected through changes in grain sizes, deactivation due to poisoning and coking. Microwave-assisted reactions in some cases have shown to slow the process of catalyst deactivation and increase catalyst life.

This chapter will focus on the fundamentals of microwave heating applicable to solid catalysts and discuss the advantages and disadvantages, applications, designs, scale up and challenges of microwave-assisted heterogeneous catalysts for gas-phase reactions.

Microwaves are a form of electromagnetic radiation with wavelengths ranging from 1 m to 1 mm, and frequencies ranging between 300 MHz and 300 GHz. The operational frequencies are administered by the Federal Communication Commission (FCC) and all microwave systems are required to operate in the ISM (industrial, scientific and medical) frequency range. The most common frequencies used by industrial microwave systems are 915 MHz and 2450 MHz. Therefore, the heterogeneous catalysts should be designed such that it absorbs microwaves at these frequencies. When a material is exposed to microwave frequencies, it results in electromagnetic relaxation. Electromagnetic relaxation is a lag in dielectric constant of the material caused by the delay in polarization of material when responding to changing electric or magnetic fields. This relaxation results in friction between the molecules and energy loss causing heat generation. While polar molecules react to the change in electromagnetic field, not all of the responses result in dissipation of energy in the form of heat. At lower frequencies for instance, the molecules have ample time to align themselves with the changing electric field, resulting in little or no energy dissipation. At higher frequencies, molecules do not have enough time to respond to the fast oscillating electric field, causing a marked decrease in dielectric permittivity. Thus, selecting the right frequency of operation is crucial for efficient heating applications.

The amount of energy loss because of electromagnetic relaxation depends on the dielectric, magnetic, and thermal properties of the material. Dielectric relaxation induced by electric field is largely governed by material's dielectric properties namely; dielectric constant and dielectric loss. The ability of the material to store electrical charge is defined by the dielectric constant(ε′) of the material whereas the dielectric loss(ε″) of the material is the inherent dissipation of electric energy to heat. The dielectric permittivity is a complex function of dielectric constant and loss factor and is given by eqn(1).¹ 

A quantitative measure of dissipation of electric energy that dictates the amount of heat generated is given by dielectric loss tangent defined as the ratio of loss factor and dielectric constant.

Similarly, dielectric permeability is a complex function of magnetic loss and susceptibility given as

The amount of energy absorbed and dissipated as heat is given as

where, P_abs is volumetric power absorbed (W m⁻³), σ is the conductivity (S m⁻¹) given as 2πfε⁰ε′, f is frequency (Hz), ε⁰ and µ₀ is the dielectric constant and magnetic permeability in the vacuum, ε′ and µ″ is relative dielectric and relative magnetic loss,|E| and|H| are the is electric and magnetic field intensity.² 

Another important equation defining the microwave heating of material is the penetration depth equation. Penetration depth of the material is defined as the depth in the material where the microwave power drops to 37% of the initial value. Penetration depth(D_p) is given by the following equation

where λ is the wavelength of microwave radiation. The penetration depth of the material is dependent on the frequency of operation as well as its dielectric properties. Since dielectric properties could change with change in temperature, the penetration depth of the material changes with temperature.^3,4 

Generally dielectric loss is more dominant than magnetic losses for most materials. Dielectric properties for a given material are frequency and temperature dependent and vary significantly from material to material. Therefore, not all materials are heated equally in microwave.⁵  Based on their interactions with the electric field in microwave frequencies, materials can be classified as insulators, perfect conductors, and absorbers. Insulators are transparent to microwaves such that microwaves pass through them with little or no losses, such as glass. Perfect conductors are materials where incident microwaves are reflected, for instance metals; whereas absorbers are materials where microwaves can penetrate beyond the skin depth.⁶  Other material properties such as viscosity, density, temperature, molecular weight, concentration etc. also affect the response of material to microwave field, specifically those that are temperature dependent.

Dielectric heating is primarily governed by two known mechanisms: polarization and conduction in material.

In conduction, ions or mobile charged particles oscillate in response to changing electric field, generating electric current. The internal resistance to this current caused by collision of the charged particles and other molecules, generates heat. Heating may occur due to conduction of charged ions moving freely in heterogeneous mixture. If these ions are in a space bounded by other molecules and cannot freely couple with the changing electric field, the charge accumulates, and the energy is dissipated as heat.

When a dielectric material is exposed to high frequency electromagnetic field, the electric field component displaces the charge particles within the material from their equilibrium positions, inducing dipoles. These dipoles respond to the applied electric field and rotate with high frequency, generating friction and heat. Polar materials have permanent dipoles that also result in polarization. Polarization can arise through three main mechanisms (Fig. 1) which are discussed below in detail.

Displaced electrons around the nuclei or relative displacement of atomic nuclei could cause displacement polarization. Displacement polarization does not contribute to microwave heating since the relaxation frequency for these mechanisms is higher than the microwave frequency.

Orientation polarization occurs in dielectric that contain permanent dipoles due to asymmetric charge distribution within the molecule. Orientation or dipole polarization is more dominant in liquids and semi-solid materials. Dipole polarization in solids are limited to localized sites and on the surface. The electric field polarizes the charges in the material, the polar molecule then tries to align itself in the direction of the electric field at a rate equivalent to the microwave frequency (2.5 billion per second for 2450 MHz). However, the inability of the molecule to trail the fast changing electric field leads to dissipation of energy in the form of heat.⁷  Specifically in catalysts, dipole moments could exits due to crystalline lattice structure, surface oxygen vacancies, deficit sites, or even impurities. While designing a microwave compatible catalyst, these criteria should be taken into account. The support materials in catalyst are generally poor microwave absorbers since they are often made of insulating solids such as silica, alumina etc. For these materials to heat in microwave reactors, they should possess significant vacancies or deficit sites on the surface, enough to trigger dipole polarization. Therefore, topography of surface sites mainly govern the microwave heating of catalysts. The lingering atoms or electrons at these sites respond to the applied field and result in orientation polarization. The source of these atom clusters on a catalyst surface could be from oxygen vacancies, hydroxide groups or metal loadings. Applied electric field causes displacement of charged particles from their equilibrium position, causing induced dipole polarization. The supported metal catalysts have metal particles that are dispersed on a microwave transparent support. When placed in the electric field, the free electrons on the metal surface move, polarizing the metal at each individual metal site. While bulk metal is opaque to microwaves, metal powders or supported metal such as Fe on ZSM-5 with particle size smaller than 5um have lower electric conductivity and can absorb microwaves.⁸ 

Interfacial polarization occurs in solids or powdered material within a heterogeneous system, commonly known as Maxwell–Wagner-Sillar polarization, by accumulation of mobile charges at grain surfaces and is an important loss mechanism during microwave heating of catalysts. In heterogeneous dielectric media, consisting of a fraction of a conducting medium (catalysts) in a pool of a non-conducting medium (gas reactants), charge particles build up at the interfaces when subjected to alternating electric field. The movement of these charged particles is often subjected to resistance, resulting in relaxation and dielectric loss and consequently heating. In certain materials such as carbon, some charged particles such as electrons that are free to move in the delimited region of the material, current is induced which produces similar effect as that of ionic conduction. The charged particles vibrate in the constricted region of the material, dissipating heat energy. Lossy solids heat readily in microwave environment and are electrically conductive (0 < σ < ∞). When electric current flows through these materials, some of the energy is converted to heat. For catalyst materials with metal oxides, magnetic resonance losses contribute to heating. Magnetic resonance losses are caused by electron spin resonance,⁹  as studied in the microwave heating of a zeolite catalyst. They described the heating mechanism for these catalysts as ionic rattling effect within the catalyst structure. This mechanism is evident through XRD analysis of spent zeolites that shows disruption of the lattice structure. The mechanism described is in accord with the interfacial polarization effect.⁹  Another study examined the effect of microwave heating on hydrated zeolites. According to their observation, the trapped moisture in this catalyst responds to microwave irradiation and heats in the initial stage. Once the moisture is desorbed, the dehydrated lossy zeolite heats up to 500 °C. Beyond this temperature, they observed a thermal runaway within the catalyst.¹⁰ 

Electric field losses are considered the dominant mechanism during microwave heating for most materials. However, material (ferromagnetic, magnetic), magnetic field heating offer advantage over electric losses. Magnetic field heating in microwaves happen due to losses from hysteresis, eddy currents, and residual losses. Hysteresis loss due to magnetic field are similar to dipole polarization due to electric field. Hysteresis losses occur when a magnetic material is placed in an oscillating magnetic field that results in irreversible magnetization. The magnetic dipoles cannot often keep up with the altering magnetic poles, which results in friction between neighboring molecules leading to heat generation.¹¹ 

When a conductive material is placed in a magnetic field, the generated electromagnetic field creates circulating currents within the body of the magnetic material. These circulating currents are called eddy currents. Any resistance offered to these currents results in eddy current losses. Any other observed losses, other than hysteresis and eddy current loss are categorized as residual losses. Residual losses are caused by magnetic relaxations and resonances mainly caused by domain wall resonance or rotational resonance. Residual losses are dominating losses in ferrites. In heterogeneous catalysis, magnetic losses could be advantageous in terms of selective heating of certain catalyst zones.¹¹ 

Microwave catalysis for homogeneous catalysis has been explored for over three decades with some processes reaching commercial stages, specifically in pharmaceutical industry. However, by the early 2000s only a limited number of groups were studying microwave irradiation for heterogeneous gas phase catalysis.^12–14  Since advances in temperature measurement techniques, power and temperature controls, generator systems, and advanced system designs, microwave-assisted heterogeneous catalysis has recently garnered increased attention. The conspicuous advantage of microwave heating over existing heating methods is the rapid heating rates and high temperatures, and is often coined as the ‘thermal-effects’.^15,16  The thermal effects of microwave heating are evident through above discussed dielectric heating mechanisms and provide reaction rate enhancement, higher process efficiency, product selectivity and higher yields over conventional thermal processes. Microwave thermal effects are also enhanced through hotspot generation, superheating, and selective species heating – peculiar only to microwave-assisted heating.

Non-uniform distribution of electromagnetic field on the catalyst surface results in hotspot zones. Generation of hotspots in turn facilitates increase in reaction rate as well as movement of products from the active sites in to the bulk since the bulk temperature is often lower than the catalyst temperature. A systematic study¹⁷  on “hot-spot” generation in catalyst bed showed that the electric field intensity is significantly higher at the point of contact between two catalyst particles. The concentrated electric field results in higher temperature regions or hotspots in the catalyst bed. The temperature difference between bulk catalyst and vicinal points depends on the thermal conductivity of the catalysts. Hotspots can also be generated in non-contact regions in case of non-uniform distribution of electromagnetic fields. The size of these hotspots range from 90–1000 um and temperature difference between bulk and hotspots could range from 15–200°C (Fig. 2).

In the case of catalysts with metal loadings, microwave-metal discharge could trigger hot-spot formations, local microplasmas and arcing at metal sites. Depending on the reaction, electric arc discharge could benefit the reaction by creating a temperature gradient between the catalyst and metal site.¹⁸ 

Catalyst particle geometry and morphology has also found to have significant effect on electric field distribution and dielectric properties of material. A comparison of spiked vs cubed catalyst particle study showed that spiked particles have about 20% higher permittivity values that cubed ones. A numerical study validated these effects and showed an increased electric field at the tip of the spiked particles and induced a change in dipole density of the matrix material, increasing the dielectric properties of the catalyst material (Fig. 3). Addition of electrically conductive particles within a composite matrix of high permittivity also generates highly localized electric fields resulting in high localized temperature zones or hotspots.¹⁹ 

Selective heating of catalyst material occurs in microwave assisted heterogeneous reactions, leaving the bulk at a much lower temperature than the catalyst. This temperature gradient leads to migration of product radicals from catalyst surface to bulk gas phase. The selective heating of catalyst as opposed to gas bulk, could result in enhanced selectivity towards desired products.¹³  Another way microwaves can be selective is by heating a certain part of the catalyst particle better than the rest. For instance, low loss materials such as alumina loaded with microwave active metal (Fe, Pt, Mo, Pd). The microwave active material will heat to very high temperatures leading to selective overheating of metallic sites that cause increased reaction rates. Such increased reaction rates have been reported in the literature, Zhang et al.,²⁰  reported overheating at the center of about 200 K when MoS₂/gamma-Al₂O₃ catalyst was heated at constant microwave energy input.

Microwave heating leads to dielectric heating of adsorbed molecules as in the case of Langmuir–Hinshelwood mechanism and Eley–Rideal mechanism, where either one or both reactants adsorb on the catalyst surface. As these adsorbed species have a dipole moment, they are selectively heated under microwave irradiation, leading to accelerated adsorption–desorption cycle and enhanced rate of reaction (Fig. 4). The accelerated adsorption–desorption cycle is also supported by the temperature gradient between gas molecules and catalyst surface. For Mars van Krevelen mechanism, where the catalyst surface itself is an active form of the reaction, forming a thin layer of metal-reactant (metal-oxide, metal-sulfide etc.) on the surface, microwave heating causes selective acceleration of primary reaction step, resulting in increased overall rate of reaction.

Other notable advantages of using microwave reactor for heterogeneous catalytic reactions include the modular reactor design, rapid start up and shut down, and low carbon footprint. Microwave reactors provide an optimal platform for modular design by intensifying the energy used and directly interacting with the materials during reaction. Small chamber sizes along with decrease in required processing parameters make them ideal for modularization. By lowering processing temperatures and pressures, additional downstream processing steps can be typically eliminated. Microwave reactors also provide near instantaneous response times to varying processing conditions and quickly shut down/start up allowing for the integration of renewable energy and lowering process carbon footprint. The advantage of modular mid-size designs is that the reactor can be placed at the site location. For instance, in case of methane conversion or waste plastics conversion, a modular reactor can be stationed at the natural gas plant site or the recycling unit, eliminating the need for transportation logistics. These advantages great reduce running costs of the process.

Owing to the exciting prospects that microwave catalysis heating offers in terms of energy efficiency, heating and reaction rates, microwave technology is being developed and tested to various heterogeneous catalyst applications. Some of these studies are noted below.

Natural gas is a robust and more environmentally friendly alternative to fossil oil resources for transportable fuels and chemicals production. The main components of natural gas are methane and ethane. Effective conversion of natural gas to higher value-added chemicals is highly desired. High-valued aromatics, hydrocarbons, methanol, and synthesis gas have been synthesized from natural gas through various conversion processes. Due to the high stability of natural gas, especially of methane, it is difficult to achieve methane-activation – a crucial step for conversion to other chemicals, under traditional thermal conditions. Microwave heating has been tested for conversion of methane to overcome this problem and has been widely explored for production of array of chemicals and fuels.

Coal and petroleum have long been the sole source of aromatic hydrocarbons, however, advances in natural gas conversion technologies and its relatively low prices in the United States, direct natural gas conversion to aromatics is as a promising pathway for aromatic production. Direct, single-step conversion of light alkanes into liquid aromatic hydrocarbons in the absence of oxygen, e.g. dehydroaromatization (DHA), is considered an economical and greener approach for natural gas utilization. However, direct natural gas DHA requires high operating temperatures to activate highly stable natural gas molecules to achieve a reasonable single-pass conversion. The downside of high temperature operation is the coking side-reaction that leads to catalytic deactivation. Utilizing microwave-assisted heterogeneous catalysis to activate natural gas at relative low reaction temperature^21,22  has shown significant enhancement in ethane conversion to aromatics at temperatures as low as 400 °C bulk temperature which otherwise would take up to 600–800 °C in a thermally heated fixed-bed reactor. As shown in Fig. 5, the aromatic hydrocarbons (benzene and toluene) yields in microwave reactor is twice of that observed in the thermally-heated fixed-bed reactor under same operating conditions of 400 °C. The increase of aromatics production is attributed to the synergistic effects of microwave heating through activating catalyst and forming activated hydrocarbon intermediates. The formation of microwave-induced dipoles on catalyst surface facilitates the electron shift and sharing between catalyst and reaction intermediates. This catalytic activation process has significantly lowered activation energy, thus permitting the reaction to proceed under lower bulk temperature. The conversion and selectivity of the products have also been compared between microwave irradiation and conventional thermal heating methods.²²  A variable frequency microwave controller (Lambda MC1330-200) was used to supply the microwave irradiation, with frequency set at 6650 MHz. Ethane conversion reaches 80% at 375 °C under ambient pressure, with 4% Mo-0.5% Fe/ZSM-5 as catalyst. In a conventional reactor, the reaction temperature of 660 °C is needed to achieve similar ethane conversion rates. The higher conversion at relatively lower temperatures is due to the selective heating feature of microwave-assisted reactions. It has been experimentally determined that the microscopic “hot spot” temperature of metal particles on the zeolite could reach about 660 °C while the bulk catalyst temperature was only 375 °C. Moreover, the aromatics production rate reached 32.5 µmol min⁻¹ with microwave as compared to 20 µmol min⁻¹ obtained from high temperature thermally-heated fixed-bed reactor. The higher aromatics production is due to the formation of MW-induced dipoles on catalyst surface, which facilitates the formation of active reaction intermediates.

Microwave-assisted methane conversion to C₂ hydrocarbons has found to improve the selectivity and decrease the overall reaction temperature. Using microwaves for methane conversion into ethane and ethylene, can increase the selectivity to 98% with rare-earth oxides or rare-earth alkali metal mixed oxides as catalysts (SmLiO₂). This selectivity is significantly higher than the conventional heating processes which cap at 80%.¹³  On the other hand, C₂ yield increases linearly as the conversion increases, but is much lower than the maximum theoretical yield in a conventional thermal reactor. While in microwave reactor, experimental yield approaches the theoretical value at low temperatures, but it departs from theoretical value as the temperature and conversion increase (Fig. 6).²³  Moreover, even at low reaction temperature (500 °C) microwave processes saw an exceedingly higher selectivity, and at 700 °C the reaction reaching a significantly higher conversion with almost the same value of selectivity. During microwave-assisted methane conversion, the selectivity of ethane, ethylene and acetylene changed depending on the applied microwave power. H subtraction from CH₄ and CH₃* are low-energy reactions and they occur predominantly at low microwave power levels. CH₃* and CH₂* are more desirable products since they lead to formation of ethane and ethylene. CH* recombination reaction leads to acetylene which is also a low-energy reaction. However, acetylene production via ethylene dehydrogenation demands more energy. Through this mechanism, low-power microwave irradiation has seen 70% methane conversion to ethylene and hydrogen, without observable side reactions.²⁴  By substitution of plasma jet with microwave discharge, the energy consumption in the Dupont acetylene-from-methane process reduced significantly with better temperature control.²⁵  In the 100–250 W microwave power range, the acetylene selectivity is above 92%, further increase in power led to the deposition of carbonaceous/coke matter on the catalyst surface as well as the reactor walls.

Oxidative coupling of methane (OCM) process can obtain higher hydrocarbons from methane in a single step. Traditional methods of conversion of methane to higher hydrocarbons involve multiple steps (i.e., methane conversion into syngas and subsequently Fischer–Tropsch synthesis to obtain higher hydrocarbons). OCM reaction is an exothermic reaction that requires high reaction temperatures (750–900 °C). High reaction temperatures pose several operational challenges for OCM process such as shortened catalysts life, high equipment investment cost due to high exothermicity. Microwave heating dramatically reduces the required reaction temperature, with products appearing at lower reaction temperatures. Moreover, the product species and product selectivity significantly changes with microwave heating.

With a Ni catalyst for OCM reaction, C₂ hydrocarbons selectivity ranged from 85–97% and C₃ hydrocarbons selectivity was between 3–15%, under high-power (2 kW) pulsed microwave reactor.²⁶  Compared to conventional heating, product species and product selectivity were much higher with the microwave heating, both with SrCe_0.95Yb_0.05O₃ and BaCe_0.93La_0.07O₃ catalysts. Microwave heating also lead to formation of C₂H₂ in addition to C₂H₄, C₂H_6, CO, CO₂, H₂, and H₂O that are formed in conventional reactor. With (SmLiO₂)_0.8(CaOMgO)_0.2 as catalyst, the product selectivity significant increased with microwave heating compared to conventional thermal heating. Under microwave irradiation, the C₂ selectivity was up to 100% at low methane conversion and decreased as the methane conversion increased. On the contrary, C₂ selectivity is 0% at lower methane conversion in the conventional thermal reactor and increases as the methane conversion increases. With BaBiO₃-x and Li/MgO as catalysts, the OCM have been studied with microwave heating.²⁷  The C₂₊ selectivity improved at low temperature over Li/MgO and smaller BaBiO₃-x catalyst in a microwave-assisted process. Over BaBiO₃-x catalyst, the C₂₊ selectivity improvement is attributed to the decrease of catalytic oxidation rate of the CH₃⁻ carbanion into CO_x. This decrease in oxidation rate is caused by the electromagnetic field on the catalyst, which decreases the oxygen species concentration at the catalytic surface. Over Li/MgO catalyst, the C₂₊ selectivity increases due to the decrease of the oxidation rate of the CH₃* radicals in the gas phase.²⁷ 

Processes involving methane conversion reactions demand very high reaction temperatures and highly active catalysts due to high stability of the methane molecules. The microwave-assisted process demonstrates obvious advantages over conventional thermal process, including the higher methane conversion at a lower temperatures and better product selectivity. The 50–250 °C difference between microwave and thermal conditions that resulted in similar methane conversions is mainly due to generation of hotspots in the catalysts bed under microwave conditions.

Synthesis gas, more commonly known as syngas, primarily consists of CO and H₂, and serves as an important energy source in chemical industry such as ammonia, acetic acid, and oxo-alcohols production. Syngas is produced from methane reforming, such as steam reforming and CO₂ reforming. Under microwave conditions and at H₂O/CH₄ molar ratio over 0.5, H₂ selectivity reaches 92.7%, while methane conversion reaches to 91.6% at H₂O/CH₄ ratio of 1.0.²⁸  Nanocarbon powder, CO₂, C₂H₂, C₂H₄, and HCN were also observed during the microwave-assisted methane steam reforming process,⁵  Ni/Al₂O₃ catalyst quickly deactivated as methane conversion and hydrogen selectivity increased.

CO₂ reforming of methane is a newly developed reforming processes, with theoretical 1 : 1 ratio H₂/CO production. However, due to the high endothermicity of CO₂ reforming of methane (ΔH° = 247 kJ mol⁻¹), very high temperatures are required to obtain high conversions. In conventional reactor, another issue associated with CO₂ reforming of methane is significant amount of carbon deposition on catalyst surface and reactor walls. Microwave heating is considered as a promising technology for CO₂ reforming of methane due to its ability to selectively heat catalyst material. With Pt catalysts, the H₂/CO product ratio obtained was lower than the equilibrium at low temperatures with conventional heating compared to microwave-assisted heating. The H₂/CO ratio advanced towards the thermodynamic equilibrium for in thermal and microwave heating model at high temperatures. The high H₂/CO ratio at low temperatures with microwave was attributed to selective catalyst heating at hotspot generation within the catalyst bed. The enhanced conversion of both methane and CO₂ by microwave due to hot-spots, which is generated by the interaction between the microwave field and the catalyst, has been confirmed by Domìnguez et al.²⁹  Over microwave-assisted methane dry reforming, the catalysts and process parameters are systematically studied. Compared to conventional heating, methane conversion was significant enhanced by applying microwave heating at the same catalyst bed temperature with the same activated carbon (AC) supported catalysts (Ni/AC, NiMgO/AC, and Ni/MgO/AC). The higher conversion rates were attributed to the formation of microplasmas and hotspots on the catalyst active sites, which leads to significantly higher temperatures than the measured bulk temperatures in the reactor.³⁰  National Energy Technology Laboratory (NETL) research indicated that the doped M-LSC (La_0.8Sr_0.2Co_0.9M_0.1O₃, M = Mn, Fe, Ni, Cu) is promising catalyst for microwave-assisted dry reforming of methane. With 90 watts microwave power, the CO₂ conversion nearly reaches to 100% over Mn-LSC.

Ammonia is one of the most consumed chemicals, it is the essential raw material for many important chemicals, including fertilizers, polymers, and explosives. In addition, it also being considered as a future fuel alternative and hydrogen storage molecule. Ammonia production at commercial scale using the Haber–Bosch (HB) process is a highly energy intensive requiring 400–520 °C and 15–25 MPa. It is carried out in the presence of highly effective iron catalyst (iron [Fe] promoted with potassium oxide and aluminum oxide and other metal oxides):³¹ 

The high operating pressure and temperature requires massive capital investments, thus an alternative approach that can lead to efficient conversion of N₂ and H₂ to ammonia with low-energy consumption is desired. Microwave-catalyst interaction could selectively stimulate the active catalyst sites on the surface of heterogeneous catalysts, which in turn can affect reaction rates. In the presence of promoters, microwaves can facilitate electron transfer between catalyst and reaction intermediates.³²  Microwaves have also been associated with the decrease in the overall activation energy of the reaction. A recent study shows an efficient integration of microwave reaction chemistry with heterogeneous catalysis for ammonia production. They demonstrated that microwaves can synergistically decouple dinitrogen activation from requiring high-temperature and high-pressure reaction conditions, which lead to alteration in the reaction pathways and ultimately increasing the rate of ammonia formation as shown in Fig. 7.³³  The catalyst used for ammonia synthesis under microwave irradiation was supported Ru catalysts with promoters. Ammonia synthesis was carried out under ambient pressure conditions and relatively low temperature conditions of 260–300 °C and noted an ammonia yield of ∼1 mmol ammonia g_cat⁻¹ h⁻¹ at 280 °C. The study tested various reaction parameters and process variables that affect ammonia synthesis in a microwave catalyzed reactor.

Microwave heating is also tested in petroleum refinery processes, including catalytic hydrotreating, catalytic polymerization, catalytic reforming, hydro-dealkylation, catalytic cracking, catalytic hydrocracking etc. Petroleum oil often contains compounds containing sulfur, nitrogen, and oxygen. Sulfur compounds in crude oil are in the form of hydrogen sulfide, sulfides, benzothiophenes, sulfur dioxide, thiophenes, mercaptans, and dibenzothiophenes. Sulfur compounds could cause corrosion and other issues in the refinery and causes pollution and environmental issues. Hydrodesulfurization (HDS) is an effective catalytic hydrogenation processes for sulfur removal from crude oil. HDS is usually performed at high hydrogen pressures (>5 MPa) and temperature (>300 °C). High hydrogen requirements and energy consumption limits the HDS application. Due to the selective heating, microwave application is considered as a promising alternative technology for HDS reaction. High dielectric constants of sulfur compounds make them susceptible to microwave irradiation. HDS reaction rates have shown to significantly improve by microwave-assisted heating, as selective catalyst heating avoids the requirement of high temperature for the whole reaction system. This involves reducing reaction volumes, temperature and pressure, resulting in a substantial savings in operating cost.

Several catalysts including iron, MoS₂/Al₂O₃, Ni-Co-Mo/Al₂O₃ etc. are used for microwave-assisted HDS. In MoS₂/Al₂O₃, active species MoS₂ shows much higher dielectric constants compared to Al₂O₃ support, in a wide range of temperatures (200–800 °C).^34,35  The different dielectric constants of active species and support cause remarkable temperature gradients or hot-spots inside the catalyst beds under microwave-assisted heating. The pulse mode microwave with variable frequency was able to resolve the temperature gradients problem; the desulfurization rate reaches 98%. The high microwave sensitive catalysts are desired for HDS to achieve the energy saving and high activity. Iron and copper-based catalysts show high sulfur removal activity from bitumen pitch (70%), under high power pulsed microwave condition. The pulsed microwave irradiation mode shows better control of temperature and product selectivity, because heat can be conducted to the microwave transparent region during the off-mode of pulsed microwave setting.

Pyrolysis is a thermochemical process that dependents on operational parameters such as reaction temperature, heating rates and reaction times, for the selectivity of desired products. Coal, biomass, municipal solid waste, and waste from the agriculture, forest, and food processing industry serve as the primary feedstock for pyrolysis process.^36,37  Microwave heating offers a number of advantages over conventional heating methods for pyrolysis, such as, rapid heating rates, volumetric heating, high efficiency, and lower energy losses.

During pyrolysis process, microwave energy absorption is largely affected by the dielectric properties of catalyst such as, carbon-based materials, metal-based materials and zeolites. Carbon-based materials draws a lot of attention due the high microwave adsorbing capacity. With activated carbon, CaO, and SiC as catalyst, the pyrolysis of microalgae was studied under microwave condition.³⁸  The activated carbon showed the best catalytic activity among all the catalysts, and the maximum bio-oil yield obtain with 1500 W microwave energy. Activated carbon was also used as a catalyst in the lignin microwave-assisted pyrolysis, the main reaction product was bio-oils composed of phenols, guaiacols, hydrocarbons, and esters.³⁹  Bio-oil yield was observed to increase with the increase in temperature, and reaches the highest value at 450 °C, with phenol and phenolic selectivity reaching the highest at 550 °C. On the contrary, biochar yield decreases to the minimum at the same temperature. The char and char-supported catalysts largely improved the microwave adsorption capability, and the products component during microwave pyrolysis of rice husks. With the addition of catalysts, the yield of simple organic compounds increased, while the aromatics and sugar yields decreased. Palm kernel shell pyrolysis has been compared between thermal heating and microwave heating with activated carbon and lignite char as catalysts.⁴⁰  The results showed a significant increase in phenol selectivity with microwave catalytic pyrolysis. Highest concentration of phenols in bio-oil were obtained at 500 °C with activated carbon as catalyst, mainly because activated carbon promoted the demethylation, decarboxylation and dehydration reactions, increasing phenol yields.

Zeolite is also a promising catalyst for the microwave pyrolysis, with HZSM-5 and MgO mixture as catalyst, Fan et al.³³  investigated co-pyrolysis of lignin and low-density polyethylene (LDPE) using microwave-assisted catalytic reactor. They studied the effect of the lignin to LDPE ratio, MgO to HZSM-5 ratio, reaction temperature, and feedstock: catalyst ratio, on products yields and chemical composition. The proportion of aromatics increased with the increasing HZSM-5 to MgO ratio, while the alkylated phenols proportion decreased. They observed that with transition metal modified ZSM-5 (cobalt, nickel, and zinc) the bio-oil yields increased during MW-assisted rice straw pyrolysis, compare to parent ZSM-5 catalyst, more than 50% of the oil was composed of phenols, ketones and aldehydes.⁴¹  Thermal behavior and reaction kinetics analysis showed that the addition of transition metal modified zeolites was able to improve the reaction rates. Higher char yields, aromatics selectivity and syngas yields with ex-situ pyrolysis were observed compared to insitu catalytic upgrading lignin pyrolytic vapors with HZSM-5 in a microwave reactor, see Fig. 8. In ex-situ process, with the catalyst/lignin ratio and temperature increase, the bio-oil and gas yields increased while the coke formation decreased. The catalyst/lignin ratio increase largely improved the aromatics selectivity and reduced the selectivity towards methoxy-phenols, the highest selectivity to alkyl phenols was obtained at 0.2 catalyst/lignin ratio.

Array of catalysts have been tested for microwave pyrolysis of variety of feedstocks including metal salts and acids (KAc, K₂Cr₂O₇, ZnCl₂, H₃BO₃, MgCl₂, Al₂O₃, CoCl₂, Na₂HPO₄, AlCl₃),⁴²  metal oxides (NiO, CaO, MgO, and CuO),⁴³  metal oxide and salt (CaO, CaCO₃, NiO, Ni2O₃, Al₂O₃, and TiO₂)⁴⁴  metal oxides and chlorides (MgO, Fe₂O₃, MnO₂, CuCl₂, and NaCl).⁴⁵  These catalysts have shown to have significant improvement in performance due to microwave assisted heating methods.

Due to selective and volumetric heating advantage, microwave heating has been widely applied to environmental catalysis for air and water pollution control. These applications include microwave-assisted catalytic conversion of nitrogen oxides (NOx), sulfur oxides (SOx), carbon dioxide (CO₂), and volatile organic compounds (VOCs), which are emitted by chemical industries, power plants, and transportation.

NOx (NO, NO₂) is one of the main greenhouse gases which originate from transportation (55%) and power plants and chemical industries (45%).⁴⁶  The selective catalytic reduction (SCR) by ammonia is one of the most used technologies for power plants NOx removal,⁴⁷  selective NOx recirculation (SNR) is the commonly used method for NOx thermally decomposed in vehicles.⁴⁸  Using ammonium bicarbonate (NH₄HCO₃) as a reducing agent with Ga-A zeolites as a catalyst, NOx removal efficiency reaches to 95.45% with 259–280 W microwave power.⁴⁶  An innovative de-NOx technique developed for NOx removal with activated carbon catalyst in a circulating fluidized-bed with microwave system.⁴⁹  This novel system reduces the reductant cost and demonstrated continuous operation. The adsorption and destruction efficiency largely improved by microwave while energy consumption was largely reduced. In the NOx abatement process, the NO and NO₂ adsorbs on the activated carbon (AC) surface. Microwave heating can be efficiently used to regenerate the this activated carbon since AC readily absorbs microwaves. The results of the study suggested that the MW process is more effective than the conventional thermal method because microwave heating leads in an increase in specific surface areas of activated carbon, leading to an increase in NOx adsorption capacity.

Sulfur dioxide (SO₂), is mainly emitted from fuel combustion, is also one of the main air polluters. SO₂ is simultaneously abated with NOx. During electron beam flue gas treating process, with 2.45 GHz frequency microwave accelerated electron beams, 80% of NOx and 95% of SO₂ was removed.⁵⁰  Microwaves increase the NOx removal efficiency. Simultaneous SO₂ and NOx removal from flue gas over potassium permanganate on zeolite was achieved with microwave heating at higher efficiency. The microwave reactor could be used for oxidation of SO₂ to sulfate with the desulfurization efficiency of 96.8% and oxidize NOx to nitrates with NOx removal efficiency of 98.4%. Microwave heightens catalytic oxidation treatment, and increases the SO₂ and NOx removal efficiency by 7.2% and 12.2% respectively. The addition of zeolite to microwave potassium permanganate increases SO₂ removal efficiency from 16.5% to 43.5%, and the NOx removal efficiency from 85.6% to 98.2%.⁵¹ 

The volatile organic compounds (VOC) has high vapor pressure at room temperature, for example isoprenoids (isoprene and monoterpenes), alcohols, alkenes, esters, ethers, alkanes, carbonyls, and acids.⁵²  VOC primarily originates from biological, vehicular, industrial sources as well as tobacco smoke and is a serious threat to air quality.

A microwave-heated adsorbent-reactor system was studied for the continuous cleaning of air streams containing n-hexane at low concentrations.⁵³  A single catalytic bed (PtY zeolite) and a double (adsorptive DAY zeolite + catalytic PtY zeolite) fixed-bed reactor configurations were studied under dry and humid conditions. The zeolites were selectively heated by pulsed microwaves, which caused the desorption of n-hexane and its subsequent catalytic combustion. The double bed configuration was more efficient with half the catalyst amount than single bed configuration. Continuous gas cleaning was demonstrated with short (3 min, 30 W) microwave heating pulses every 5 min. A Cu-Mn-Ce/cordierite honeycomb catalyst was demonstrated for the catalytic oxidation of gaseous toluene with microwave heating. 98% of toluene was removed at 200 °C with 0.12 m³ h⁻¹ gas flowrate, even after four consecutive cycles, the toluene removal efficiency was still higher than 90%. The selective heating and dipole polarization of microwave can form high temperature hotspots and reduce activation energy of toluene catalytic oxidation. The active sites, such as, Cu, Mn, Ce monometallic oxides, CuMnO₂ and Ce(Mn,Cu)₆, improved toluene oxidation effectively.⁵⁴ 

Oxidative dehydrogenation of ethylbenzene reaction, with multi-walled carbon nanotubes (MWCNTs) supported iron oxide as catalyst showed higher conversion and styrene selectivity under microwave conditions. The catalyst was affected by the heating method, under microwave heating condition, no carbon deposition on the catalyst surface was observed for the microwave-assisted reaction as opposed to conventional reaction which saw a large amount of carbon deposition with no distinguishable pattern.⁵⁵ 

The dehydrogenation of ethylbenzene to styrene at atmospheric pressure using O₂ and CO₂ as oxidants under conventional and microwave heating was compared by Tarasov et al.⁵⁶  Replacing oxygen with CO₂ lead to a substantial reduction in ethylbenzene conversion with a simultaneous increase in the selectivity. Microwave heating with O₂ used as an oxidizer provided increased process efficiency for moderate operating (up to 420 °C) temperatures. The phase composition of the mixed oxide catalyst changed at higher temperatures under microwave, mostly due to the hot spot generation at the active reaction sites.

The influence of heating method on oxidative dehydrogenation of ethane (ODE) over BaO–CeO₂–ZrO₂ and BaCl₂–TiO₂–SnO₂ catalysts showed⁵⁷  that with BaO–CeO₂–ZrO₂ catalyst, the MW heating decreases the carbon dioxide selectivity and increases carbon monoxide selectivity. While, conventional heating led to complete oxidation of CO to CO₂. With BaCl₂–TiO₂–SnO₂ catalyst, microwave heating suppressed consecutive deep oxidation of ethylene and ethylene selectivity increased remarkably. The high efficiency of microwave heating for ethane oxidative dehydrogenation can be accounted for the effective “quenching” of the target ODE product via inhibiting undesirable homogeneous reactions in the gas phase.⁵⁷ 

Microwave heating demonstrated high selectivity to propylene in the oxidative dehydrogenation of propane.⁵⁸  The preferential microwave heating of the solid monolith (SiC, a good microwave susceptor), allowed working with a lower gas phase temperature. With MW-heated straight channel monolithic reactors coated with a VMgO catalyst, the propane conversion reached 21% and selectivity to propylene up to 70%. This was attributed to the cooler gas phase preventing homogeneous formation of ethylene and methane that takes place intensely at gas phase temperatures above 550 °C. Consequently, higher selectivity to propylene was achieved in comparison with conventional heating. Compared to conventional heating, the propylene selectivity increased around 5 percentage with the lower gas phase temperature by microwave heating.⁵⁸ 

Laboratory scale microwave-assisted heterogeneous reactions are generally tested in a fixed bed catalyst setup and continuous gas flow. The design and setup varies with the reaction and catalyst type. In general, microwave reactors can be classified based on applicator design, waveguide, flow, and power source type.

Microwave energy can be delivered to the material from the source through propagating between a transmitter and a receiver or it can also be guided through an enclosed channel. These channels can be in the form of enclosed metal casing, known as waveguides, of different shapes. The microwave energy guided through these waveguides end up in an applicator (also known as a cavity) where the dielectric material to be heated is placed. An applicator is a hollow, enclosed metal structure that confines the field and the microwaves bounce back and forth between the walls of the applicator. The applicator is responsible for generation of standing waves of certain frequencies, known as the resonant frequencies of the cavity. Applicators can be designed according to the process requirements, to limit the resonant frequencies, allowing a small range of microwave frequencies to pass while blocking the rest. Common microwave applicators include multimode applicators, single mode applicators and travelling wave applicators.

Multimode microwave applicators are the most commonly used for both domestic and laboratory applications. Common household microwave ovens operate in multimode settings. A typical multimode applicator consists of a closed metal box with a port for microwave power coupling. The metallic box dimensions are such that it hosts large number of resonant modes for the operational frequency. To achieve this, the box is several wavelengths long in all directions. When power is applied, the microwaves bounce back and forth, reflecting from the metallic reactor walls. At resonant frequency, they cause standing waves within the cavity. Multimode applicators are simple in design, robust and can accept array of different dielectric loads. Multimode applicators can be used for processing large loads and can accommodate several reactor vessels; hence are suitable for industrial batch applications. A striking disadvantage of a multimode microwave applicator is non-uniform energy fields and hence, non-uniform temperature distribution within the dielectric load. Mode stirrers are used to mitigate the non-uniformity of fields. Mode stirrers are simple metal blades bent at an angle that rotates inside the cavity and continuously perturb the field distribution (Fig. 9). For a high loss material, mode stirrers deflect the incident energy from the generator. While, for low loss dielectric load, the mode stirrers are tuned to resonance and act as an antenna to maximize the incident energy at the dielectric load.

Multimode applicators have been reported in literature for applications in heterogeneous catalysis. Will et al.⁵⁹  reported oxidation of propane on heterogeneous (perovskite) catalyst in a multimode microwave field. The researchers modified a commercial multimode reactor to create appropriate field homogeneity. They noted comparable and reproducible results for propane conversion with a multimode microwave reactor.

Singe mode applicators are most widely studied for heterogeneous gas phase reactions because of its ability to reach high temperatures at low powers. Single-mode (mono-mode) applicator delivers highly focused energy field, resulting in faster heating rates. In single mode applicators, a guided wave passes through a waveguide into the circular or elliptical microwave cavity. A reactor tube is placed directly within the applicator. The dimensions are such that only one mode of operation is present. The electromagnetic radiations bounce back from all directions to a single focus area on the reactor tube, creating standing wave (Fig. 10). The focused intense energy fields leads to homogeneous temperatures and can achieve high temperatures at lower microwave powers. High temperature uniformity is achieved with single mode applicator. Shape and size of applicator and reactor tube, penetration depth of the material, field homogeneity, frequency and mode of operation all affect the temperature uniformity of the catalyst bed. The biggest disadvantage of single-mode applicator design is limitations on the size of the reactor, which poses issues during scale-up. This could be overcome with novel microwave reactor designs such as multiple applicators in series or in parallel or using a multimode applicator in place of single mode design.

A single mode cavity is most commonly used applicator design for laboratory scale studies, mainly due to its uniform and accurate temperature control. An ambient pressure and low temperature ammonia synthesis was demonstrated using a single mode microwave cavity catalytic reactor operating at 2450 MHz.³²  Similarly, significant number of studies (some noted above) have demonstrated the use of single mode microwave catalytic reactor for carbon conversion and hydrocarbon transformations.⁶⁰ 

In a traveling wave microwave applicator, microwaves travel only in one direction. This ensures that no waves reflect back causing non-uniform interference in to the waveguide and avoids standing wave formation. A reactor tube is placed directly inside the waveguide either vertically or horizontally and does not require a specially designed chamber (Fig. 11). The waveguide, if carefully designed does not require continuous tuning to deal with change in frequency during operation, leading to low equipment costs since automatic tuning elements can add significant cost to the process. They have a potential for uniform microwave heating by avoiding resonant conditions.

A traveling-wave applicator designed specifically for catalyst heating can demonstrate a highly uniform heating throughout the catalyst bed.^61,62  Coaxial design for microwave excitation is simple due to the coaxial nature of the microwave generators. The dimensions have to be carefully designed to avoid impedance mismatch and interference that can cause standing wave patterns. The catalyst bed dimensions positon also needs to be determined before designing and operating a traveling-wave applicator to achieve maximum temperature uniformity. This poses a limitation on variety of catalysts that can be heated. If the dielectric properties are significantly different from the original catalyst for which the reactor is designed, the reactor may not be used for these materials. The studies^61,62  claim high scale up potential, however one has not been demonstrated to date.

Type of waveguide used is important because each waveguide type can support only modes of wave propagation. For instance, rectangular waveguide supports TE10 mode but circular waveguide supports a TE11 mode as the most common mode. Each mode of propagation has a cutoff frequency above which the wave does not travel through the waveguide. Thus, selection of waveguide is a crucial design decision. Rectangular waveguides are most commonly used because of its compatibility with desired operational modes and ease of manufacturing. Other waveguide types include circular waveguides and coaxial waveguides. Circular waveguides are primarily used because of its ease of installation. Circular waveguides are also used extensively for microwave plasma generation. Coaxial waveguides have recently been used for microwave reactors primarily because coaxial configuration does not have a cut-off frequency in Transverse electric-magnetic (TEM) mode of propagation making a broad range of frequency available for operation^61,63–66  (Fig. 12). Variable frequency generators could be used to sweep most suitable frequency of operations using coaxial waveguides and obtain longitudinal uniform heating.⁶⁷ 

Microwave reactors are commonly powered by magnetron tubes. Magnetrons have high efficiency (typically >85%) for converting electric power to microwave energy. A magnetron is essentially a high-powered vacuum tube that generates microwaves through interaction of electrons with a magnetic field. Magnetrons operate at only one frequency and has a short lifetime extending from 2000–10 000 working hours. An emerging technology of solid-state amplifiers is now used in a number of laboratory and commercial microwave reactors. The solid-state generators can vary frequency during operation, giving it a significant advantage over magnetron tube. The varying frequency can help mitigate hotspots and random wave interactions. Solid-state generators can last up to 30 years and are enabled by wide band gap semiconductors such as gallium nitride. The disadvantage of these new generation solid-state power amplifiers is the cost, which is about 50 times more than traditional magnetrons. The cost is expected to lower as the technology matures in the coming years.

A microwave system can be tailored and optimized for the flow reaction flow conditions. A multimode cavity with mode stirrers is more suitable for batch reactions, however, heterogeneous catalysis is seldom performed under batch conditions. Here reacting gases flow over a packed catalyst bed for a certain period. The reaction is either stopped or paused for regeneration of catalyst.

A monomode or a traveling wave microwave system is commonly used for continuous reactors. Some multimode designs as shown elsewhere in this chapter have been demonstrated, however, multimode cavities are not widely used due to its poor temperature uniformity and unpredictability.

A fluidized bed reactor has certain advantages over a fixed bed catalytic reactor. Fluidized bed provides extended exposure of catalyst particles to the reacting gases, increasing product yield. Fluidized bed also offers higher heat transfer efficiency, and uniform temperature distribution due to flowing gases. High velocity of gases avoids pressure drop issues commonly observed in fixed bed reactors with smaller catalyst particle size. Catalysts can be used in powder form or smaller sized particles can be used with ease in a fluidized bed reactor. Fluidized bed also has some limitations with particle entrainments and particle agglomeration. In conventional fluidized bed reactors, the wall temperature is significantly higher than the bulk temperature that leads to a temperature gradient. Certain catalyst types agglomerate at higher temperatures and deposit on the surface. Some of these issues can be resolved with the use of microwave technology for heating the catalyst particles directly and reducing the temperature gradient.

Fluidized bed reactors demonstrated at laboratory scale has shown to increase productivity and selectivity of certain products (Fig. 13). Microwave heating also showed higher conversion compared to conventional fluidized bed reactor.⁶⁸ 

In continuous flow reactors, catalyst bed either could be a packed bed or fluidized bed system. The gas flows over the catalyst bed continuously and the catalyst is regenerated simultaneously without pausing the process. Microwave heating has been demonstrated for regeneration of catalyst as an independent process. However, commercial implementation or lab scale demonstration of continuous heterogeneous gas phase reactions heated by microwave power are currently non-existent and primarily limited by complexities of the microwave-material interaction, design complexity of a continuous system, and microwave system limitations.

A typical lab-scale unit for heterogeneous gas phase catalysis includes a microwave power generator, waveguides, cavity that holds the quartz reactor tube with catalyst bed, an autotuner, sliding short, and temperature and power control devices, gas supply units and condensing lines. A microwave system scale up is not as aligned as a conventional process scale up typically is. In a conventional reactor, scale up poses challenges in terms of decreased efficiency and performance with an increase in size and volume of the reactor. In microwave-assisted systems, lab-scale experiments are focused on microwave-catalyst interactions and temperature uniformity. Scale up of microwave systems pose unique set of challenges, for instance, increasing reactor size is possible via change in operational frequency from 2450 MHz to 915 MHz. as the penetration depth of catalyst changes with change in frequency, rigorous studies should be performed before designing a scaled up reactor. Multimode cavities offer simpler scale up solutions and are commonly applied for liquid phase reactions. However, for heterogeneous catalysis, multimode cavities can lead to non-uniform temperatures if not carefully designed. Some interesting scale up designs have been proposed over past decades as microwave-assisted catalysis has gained momentum.

A novel modular scale of microwave reactor was proposed in⁶⁹  where multiple microwave waveguide and cavities were arranged in series (Fig. 14). The design was tested for packed bed reactors and achieved over 99% conversion for esterification of acetic acid and ethanol catalyzed by ion-exchange resin in 18 cavities placed in series. For continuous flow systems, the production capacity was limited to smaller reactor sizes due to limited microwave penetration depths. The proposed scale-up was deemed successful and modular.⁶⁹ 

A large scale multimode reactor for agri-food processing was proposed⁷⁰  which can be used for heterogeneous catalysis. The reactor consists of a 2450 MHz vertical microwave system of 100 cm diameter and 25 cm height. Mode stirrers were placed uniformly throughout the inside of the reactor to attain uniform electric field distribution and chokes were designed to minimize microwave leakage. The author's noted a consistent electric field and minimum reflections, as well as uniform temperature distribution within the sample.

A commonly used lab scale microwave operates at 2450 MHz, for scale up however, 915 MHz makes a more feasible alternative. Continuous liquid flow studies have explored this option because of the possibility of an increase in the size of reactor tube as most materials have greater penetration depths at 915 MHz frequency. A scaled up rectangular cavity for methane non-oxidative coupling reaction showed that a 915 MHz microwave can be effectively used for a greater throughput and an increase of 6 fold in power increased methane processing flow by 150 times⁷¹  (Fig. 15). Similar systems could have more scope in heterogeneous catalysis.

Microwave technology for heterogeneous gas phase catalysis is still in exploration stage, and shows a promising future.

A classical thermocouple sensor cannot be used in a microwave reactor for temperature measurements as the metal in the thermocouple couples with the electromagnetic fields, causing arcing and inaccurate readings. In low temperature reactions, a fiber optic thermo-sensor is commonly used. The advantage of this kind of thermal sensor is accuracy and the ability to be installed within the material to measure local temperatures. Fiber optic thermal sensors can only measure temperatures up to 250 °C as the sensor material cannot withstand higher temperatures. For high temperature reactions, IR sensors are typically used. These sensors can measure temperatures up to 1500°C and are installed outside the catalyst bed. The drawback of IR sensor is low accuracy of readings. Largely because of direct microwaves-material interactions, significant temperature gradient exists between the sample and surrounding as well as the reactor walls. This temperature gradient increases with increasing temperatures. Another difficulty with using infrared sensors to determine temperature is that quartz is transparent at wavelengths shorter than 2.5 um but not at longer wavelengths, hence, to cover a larger temperature range two sensors are required. IR sensors, even when transparent to quartz, only measure surface temperature of the sample section at which they are pointed, making hotspots detection within the bed impossible. Thus, temperature measurement at one spot by IR sensor cannot be representative of the total bulk temperature (Fig. 16). Moreover, microwave heating causes a unique phenomenon of inverted temperature gradient, where the surrounding air temperature is cooler than the sample temperature. This causes a decrease in sample surface temperature, which should be picked up instantly by the sensor, but a delay in IR sensor's reading could result in inaccuracy.

Some recent studies show newer approaches to solve these limitations. While conventional thermocouples cannot be directly used inside the microwave, some studies show that optimal placement of the thermocouples is possible and can avoid electromagnetic interferences.⁷²  Based on this, a methodology was developed to measure temperature within the catalyst bed as well as on the surface, employing a combination of thermocouple and a thermal camera. Another study investigated use of chemical probe⁷³  for temperature measurements of droplets in a millifluidic device within a temperature range of 50–90 °C. These studies are promising steps towards developing novel technology for temperature sensing within microwave reactors.

Apart from the thermal effects peculiar to microwave heating, microwave non-thermal effects have also been proposed. However, the existence of non-thermal effects of microwaves have long been debated in the scientific community.^74–76  The enhanced reaction rates and product yields observed specifically in microwave environment has been subjected to various hypothesis explaining the underlying mechanisms including non-thermal effects of microwaves. These effects include molecular interaction with the microwave electric or magnetic fields such as changes in bond energy or length with electric field, or shifting reaction equilibrium. Some notable hypothesis include a study by Xu et al.⁷⁷  investigated NO decomposition over BaMn_0.8B_0.2O₃ (B = Cu, Co, Ni) and BaCoO₃ catalysts in the microwave and conventional reaction systems. They compared the reactions by setting the microwave hotspot temperatures in conventional reactor and observed low NO conversions than in microwave reactor. The authors reported significant decrease in activation energies in microwave-assisted NO decomposition over these catalysts, noting this to be the reason for high NO decomposition rates at lower temperatures in microwave reactor. Non-thermal microwave effects are also explained and experimentally verified in non-catalytic processes.⁷⁸  Microwave heating can decrystallize certain oxides, alloys and doped silicon. Rise in temperature would normally result in crystallization and grain growth. The hypothesis on non-thermal effect causing decrystallization of certain materials states that the mobility of charged particles on the crystal grain boundaries increases due to the gradient of the high frequency electric field.⁷⁹  The argument against the existence of non-thermal effects highlight that due to limitations in accurate temperature measurement techniques at multiple points within the microwave cavity, the observed rate enhancements and selectivity could be the result of very high local temperature zones inside the reactor. More studies that rigorously focus on this aspect need to be conducted to rest the debate around thermal and non-thermal microwave effects.

Numerical modeling is a widely applied technique to understand the behavior of complex systems through mathematical means. For microwave-assisted systems, often, multiphysics modeling is used to describe and predict the model behavior. Electromagnetic heating uses Maxwell's’ differential equations to describe electric and magnetic fields and how they are affected in time. These are coupled with Fourier's heat transfer equations to solve for dielectric heating in solids and fluids as a result of microwaves. In case of heterogeneous catalysis, electromagnetics is often also coupled with fluid flow and reaction chemistry domain, to understand the effect of reacting gas flow within the catalyst bed. A numerical modeling approach can help understand the heat distribution within the catalyst bed and location of hotspots and low temperature regions. The studies mapping the hotspots in the catalyst bed have modeled this phenomenon for few different catalysts. Two main hypothesis exists; the hotspots form at the point of contact of the catalysts, and hotspots form at the metal loading site on the catalysts. Both hypotheses have been tested numerically. Zhang et al.³⁴  showed that the metal loading site on the catalyst has a clear preferential heating over the bulk catalyst support of Al₂O₃. However, a nanoscale study showed that the difference in temperature between the metal particle loading and alumina support is very low. Experimental results showed that the overall temperature of the catalyst bed increased as the amount of metal loading increased.^34,80  The generation of hotspots at the point of contact was also shown numerically by Haneishi et al.¹⁷ 

Numerical modeling is also an effective tool in designing and optimization of microwave reactor.⁸¹  optimized a microwave assisted catalytic reactor for biofuel upgrading and explored the effect of size and shape as well as the position of the catalyst bed within the microwave reactor (Fig. 17). A clear difference in temperature profile of indie out heating was observed with numerical model comparison of conventional bed reactor and microwave catalytic reactor. Numerical modeling can also help spot effects that are rather challenging to observe experimentally, especially in microwave reactors due to temperature measurement limitations. A model developed for understand the thermal profile inside NaY zeolite packed bed reactor predicted a thermal runway behavior of zeolites at certain conditions⁸²  (Fig. 18).

Numerical modeling in most certainly an effective tool in understanding the heating mechanism with microwave irradiation as well as to optimize and design microwave heterogeneous catalytic reactor for specific applications.

This chapter provides an insight into application of microwave heating to heterogeneous catalysis. The discussion covers the fundamentals of microwave heating, numerous applications of microwave-assisted heterogeneous catalytic reactions, technological advances, and challenges of applying microwave technology to heterogeneous catalysis. The advantages that microwave heating offers over conventional heating mechanism include enhanced reaction rates, faster heating, modularity, rapid shutdown and start up, high product selectivity, increased catalytic activity and opportunity to be coupled with renewable energy sources. Microwave-assisted heterogeneous catalysis shows promising potential as the next generation of catalytic reactors.

This work was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with Leidos Research Support Team (LRST). Neither the United States Government nor any agency thereof, nor any of their employees, nor LRST, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favouring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

NETL: This work was performed in support of the US Department of Energy's Fossil Energy Crosscutting Technology Research Program. The Research was executed through the NETL Research and Innovation Center's [Advanced Reaction Systems.] Research performed by Leidos Research Support Team staff was conducted under the RSS contract 89243318CFE000003.

WVU: The authors acknowledge financial support from AIChE RAPID under the contract #DEEE0007888-6.7. The authors also acknowledge financial support from West Virginia Higher Education Policy Commission under the grant number HEPC.dsr.18.7.