Induction Heating: An Enabling Technology for the Heat Management

Perspective Cite This: ACS Catal. 2019, 9, 7921−7935

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Induction Heating: An Enabling Technology for the Heat Management in Catalytic Processes Wei Wang,†,# Giulia Tuci,‡,# Cuong Duong-Viet,† Yuefeng Liu,§ Andrea Rossin,‡ Lapo Luconi,‡ Jean-Mario Nhut,† Lam Nguyen-Dinh,∥ Cuong Pham-Huu,*,† and Giuliano Giambastiani*,†,‡,⊥

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Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES), UMR 7515 CNRS- University of Strasbourg (UdS), 25, rue Becquerel, 67087 Strasbourg Cedex 02, France ‡ Institute of Chemistry of OrganoMetallic Compounds, ICCOM-CNR and Consorzio INSTM, Via Madonna del Piano, 10, 50019 Sesto F.no, Florence, Italy § Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, 116023 Dalian, People’s Republic of China ∥ The University of Da-Nang, University of Science and Technology, 54, Nguyen Luong Bang, 550000 Da-Nang, Vietnam ⊥ Kazan Federal University, 420008 Kazan, Russian Federation ABSTRACT: This perspective illustrates the electromagnetic induction heating technology for a rational heat control in catalytic heterogeneous processes. It mainly focuses on the remarkable advantages of this approach in terms of process intensification, energy efficiency, reactor setup simplification, and safety issues coming from the use of radio frequency heated susceptors/catalysts in fixed-bed reactors under flow operational conditions. It is a real enabling technology that allows a catalytic process to go beyond reactor bounds, reducing inefficient energy transfer issues and heat dissipation phenomena while improving reactor hydrodynamics. Hence, it allows pushing catalytic processes to the limits of their kinetics. Undoubtedly, inductive heating represents a twist in performing catalysis. Indeed, it offers unique solutions to overcome heat transfer limitations (i.e. slow heating/cooling rates, nonuniform heating environments, low energy efficiency) to those endo- and exothermic catalytic transformations that make use of conventional heating methodologies. KEYWORDS: induction heating, heterogeneous catalysis, endothermic processes, exothermic processes, flow catalytic processes

1. INTRODUCTION Advances in catalysis, process design, and reactor engineering have the task of demonstrating how energy losses, scale-up issues, byproduct formation, and costly equipment can be minimized while saving the process intensification principles are retained.1 Making innovations in catalysis does not mean squeezing a few percent units from a consolidated protocol but rather guaranteeing a quantum leap in the process efficiency in terms of time, energy costs, employment of raw materials, easy process scale-up, and environmental impact. The electromagnetic induction heating (IH) or radio frequency (rf) heating of magnetic nanoparticles (NPs) or electrically conductive susceptors has been exploited for a wide range of applications, spanning from metallurgic manufacturing of metals and alloys2 to biomedical technologies for drug release3,4 and disease treatment by magnetic hyperthermia.5 Whatever the application, induction heating provides unique features in comparison to the more classical heating systems based on heat convection, conduction, and/or radiation (i.e. flame and resistance heating or traditional furnaces). Thus, IH is © XXXX American Chemical Society

a powerful tool that can be exploited to achieve specific and highly challenging tasks not easily addressed otherwise. IH takes advantage of the electromagnetic properties of a magnetically susceptible medium (susceptor) exposed to a varying magnetic field (H) produced by an alternating current (ac) generator. The capacity of IH to target heat directly where it is needed through the electromagnetic energy adsorption/conversion on dedicated materials (noncontact heating technology) is not just an alternative heating approach but rather a powerful tool that allows overcoming the heat transfer limits encountered in classical “contact” heating reactors. With IH, high temperatures can be reached more quickly on the target sample (catalyst) without the need of heating the catalyst support (if any), liquid/ gaseous carriers and reagents, or even the entire reactor. The heat can be originated by induction directly on the catalyst without the need to cross the whole reactor, from its outer walls Received: June 13, 2019 Revised: July 19, 2019 Published: July 24, 2019 7921

DOI: 10.1021/acscatal.9b02471 ACS Catal. 2019, 9, 7921−7935

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outlook on the direction this technology is taking in catalysis and on which additional advantages its wider catalytic exploitation will offer in the upcoming future.

up to the catalyst core. Accordingly, IH occurs almost instantaneously on the target sample without appreciable thermal inertia and heating efficiency considerably higher than those provided by conduction/convection/radiation schemes. Furthermore, heat loss (dissipation phenomena) and “heat waste” mainly due to the prolonged exposure of reagents and products to large temperature gradients into the reactor6,7 can be deeply mitigated. As a matter of fact, catalyst fouling problems for a wide variety of inductively heated processes are significantly reduced in comparison to protocols using conventional heating systems and catalysts show markedly increased lifetimes.8,9 Such a contactless heating technology makes any transformation safer, cleaner, and more reproducible. Overall, a careful reactor design (coil) and an appropriate susceptor choice can optimize the inductor power required for running a catalytic process to the bounds of its inherent kinetics10−14 while ensuring an optimal energy balance. Many of these key features have been summarized in the “cold catalysis” concept15 that illustrates how challenging catalytic transformations can be conveniently accomplished using IHbased energy-saving protocols. This concept looks even beyond the high benefits coming from the implementation of IH in catalysis. Indeed, it aims at the development of protocols where the energy (heat) required for a given process to occur can be provided directly where it is needed and where the target susceptor simultaneously holds the role of catalyzing the process. The pioneering work by Kirschning and co-workers on induction-heated catalysis for continuous flow organic synthesis16 has certainly given a decisive boost to research in this field. Since the seminal contributions by the German team, IHassisted catalysis has stimulated the interest of various research groups who have contributed to unveil the unique potentiality of this heating technology in a series of endo- and exothermic transformations that we are going to systematically convey in this perspective. It should be noted that certain numbers of literature contributions focusing on the use of IH deal with chemical transformations that hardly fall in the context of “catalytic” processes in a strict sense. They rather focus on highlighting the key advantages of IH over conventional heating methods coming from the targeted and rapid control of the heating rate of solid feedstock7,17−20 or volatile21,22 reagents on dedicated susceptors. Despite their general relevance from a chemical viewpoint, rf-heated noncatalytic transformations do not fit with the purposes of this perspective article and thus they will not be further detailed. In addition, the use of IH for the enhancement of catalytic performance in biohybrid catalysts is out of the energy- and process-intensification issues of continuous-flow catalytic transformations that represent the aim of this perspective; hence, they will be not handled in the contribution. We will start this perspective with a brief theory introduction on the magnetic material properties as to provide the readership with the fundamental basis of induction heating technology. Afterward, we will provide a comprehensive overview of the pioneering contributions to inductively heated catalytic processes mainly devoted to fine chemicals synthesis. We will focus on the potentiality of the IH methodology applied to continuous flow catalytic transformations in terms of process intensification, energy-saving issues, simplified reactor setup, and improved catalyst performance. A special emphasis will be given to comment on inductively heated high-temperature exoand endothermic catalytic processes before concluding with an

2. BRIEF DESCRIPTION OF INDUCTION HEATING THEORY This section aims to provide the readership with a brief description of the cornerstones of materials magnetic properties and the key elements of rf heating theory, mandatory to discuss inductively heated catalysis. When a susceptor is immersed in an alternating magnetic field, rf energy is directly targeted to the material and locally transformed into heat with only minor energy losses due to convection, conduction, or thermal radiation. There are three main electromagnetic dissipation phenomena responsible for rf energy conversion into heat, whose occurrence depends on the susceptor nature: (1) the hysteresis heating in the case of ferromagnetic (FM) materials, cycling through their magnetic hysteresis loops,23,24 (2) the magnetic field heating mechanism by relaxation losses (Néel relaxation mechanism) taking place in superparamagnetic (SPM) nanoparticles,25,26 and (3) the Joule heating due to eddy currents27,28 (or Foucault currents) occurring on electrically conductive samples. The magnetic properties of metal nanoparticles (NPs) rely on a complex set of structural, chemicophysical, and morphological features whose thorough discussion goes beyond the scope of this perspective article, as most of them have already been extensively reviewed elsewhere.29,30 Therefore, we will focus on the most relevant features of radio frequency (rf) heated ferromagnetic (FM), superparamagnetic (SPM), or simply electrically conductive samples to be applied in catalysis. FM particles possesses distinctive magnetization (M/H) profiles that allow the conversion of rf into heat by means of hysteresis losses. These samples are characterized by (1) the remnant magnetization (Mr, Figure 1a), namely the magnet-

Figure 1. Schematic illustration of (a) a typical hysteresis loop of an array of FM NPs and (b) a typical curve for a SPM sample. Reproduced from ref 32. Copyright 2009 Royal Society of Chemistry.

ization retained by the material in its magnetically saturated state (Ms) after removal of the external magnetic field (H = 0),and (2) the coercive field (Hc, Figure 1a), which is the magnetic field required to demagnetize the sample. Mr and Hc define the area subtended by the FM material hysteresis (Figure 1a). which is proportional to the amount of heat dissipated during the sample magnetization reversal (Appendix A). The greater the hysteresis area, the higher the energy absorbed per mass unit and subsequently the heat produced. For superparamagnetic (SPM) particles, they lack the hysteresis contribution to the heat production (Figure 1b) and dissipate the electromagnetic energy through the Néel relaxation mechanism (Appendix A).25 The latter describes the rotation of the individual magnetic moments in single-domain NPs (single giant magnetic moment) 7922


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Figure 2. Preparation of MagSilica-Au(0) (a) and MagSilica-Pd(0) (b) composites and their use as heterogeneous, rf-heatable catalysts in a flow reactor for alcohol oxidation (a) and cross-coupling reactions (b). Freely adapted from refs 16 and 40. Copyright 2008 and 2014 John Wiley and Sons.

utilization as “magnetic catalysts” to be easily recovered under the action of an external magnetic field.37 Although the ability of MNPs to dissipate heat on exposure to a remote radio frequency is historically known, the exploitation of this feature in catalysis is relatively new. It received an important boost as an innovative and contactless heating technology since the terrific achievements in another enabling technology for synthesis and catalysis: the flow chemistry in micro- and mesofluidic devices.38,39 The extremely rapid and locally targeted heating of magnetic/ conductive materials upon their radio frequency exposure is ideally suited for flow reactions where the reagent residence time on the catalyst is generally short. Furthermore, the homogeneous heating of a target susceptor/catalyst under a continuous flow of reagents in a carrier (whether its nature is inert or reactive, liquid or gas) is managed much more easily and effectively by IH than by other conventional heating schemes. Pioneering outcomes on the use of IH in catalysis are due to the group of Kirschning, who first reported on the use of SPM iron oxide-silica core−shell nanocarriers (Fe2O3/Fe3O4@SiO2; MagSilica), impregnated with Pd16 or Au40 NPs as catalytically active sites for heterogeneous transformations (cross-coupling or alcohol oxidations) under flow conditions. The German team focused on the ability of rf-powered SPM MagSilica NPs to act as an effective heating system for a broad scope of liquid-phase stoichiometric reactions under flow conditions (heterocyclic condensations,13 reductive cyclizations,13 oxidation,41 and multicomponent reactions42) before exploring their successful use in catalysis. It is fair to note that the idea of employing rfheatable nanocarriers for the heat transfer to catalytically active metal NPs at their outer surface is the forerunner concept that has opened the way to a great number of recent achievements in the field of inductively heated catalytic transformations under flow conditions. To accomplish their goal, the authors have exploited the stability and chemical versatility of silica shells in their SPM NPs. Indeed, in addition to preserving the iron oxide core from undesired NPs clustering and chemical alterations that might compromise its SPM properties,13 the silica shell in MagSilica increases the NPs surface area and creates a versatile platform to its surface engineering via exohedral chemical

toward an applied external magnetic field. Such a rotation moves the magnetic moment away from the crystal “easy axis” with subsequent heat release into the system.31 In addition to hysteresis loss and Néel relaxation phenomena, eddy currents induced by an ac magnetic field, that flow through the resistance of an electrically conductive susceptor, dissipate energy by the Joule effect (Appendix A).33 At odds with the other heat contributions, eddy currents generated by an ac magnetic field do not contribute to the homogeneous warmup of the conductor but they concentrate heat to the surface of the susceptor mainly (skin-depth effect; Appendix A).34,35 All of these heat contributions outline the hyperthermic efficiency or specific absorption rate (SAR) of a given sample: i.e., its own capacity to act as heat mediator36 once it is immersed in an ac magnetic field. As mentioned above, the preparation of nanostructures joining high magnetic heating efficiency (SAR) with excellent catalytic performance for a given process currently represents the most challenging task in the field of inductively heated catalytic transformations. Two further distinctive features of magnetic susceptor/ catalysts applied to inductively heated catalytic processes are the Curie temperature (TC) and the blocking temperature (TB). These T values mark the material phase transition from FM to paramagnetic (PM) and from FM to superparamagnetic (SPM), respectively. Overall, they indicate the temperature limits beyond which a given FM sample loses its permanent magnetism and thus the related hysteresis loss contribution responsible for the material heating.

3. INDUCTIVELY HEATED CATALYTIC PROCESSES FOR FINE CHEMICAL SYNTHESIS AND FLOW PROCESS INTENSIFICATION Magnetic nanoparticles (MNPs) have long been engaged in a wide collection of catalytic transformations. They are considered the bridge between homogeneous and heterogeneous catalysis, as they can be easily handled, recovered, and recycled. Indeed, their engineering by surface decoration with catalytically active metal sites or by heterogenization of homogeneous single-site complexes (quasi-homogeneous catalysts) has widened their 7923


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Figure 3. Preparation of a physical mixture of MagSilica and Pd (10 wt %) on charcoal and its use as a catalytic fixed-bed material in a mesofluidic reactor for hydrogen transfer reactions. Freely adapted from data of ref 13. Copyright 2011 John Wiley and Sons.

Figure 4. Core−double-shell nickel ferrite nanoparticles (a) NiFe2O4@SiO2@ZSM-5 and (b) NiFe2O4@SiO2@TiO2 for inductively heated isomerization (a) and amidation (b) catalysis under liquid flow conditions. Freely adapted from data of refs 44 and 52. Copyright 2017 Royal Society of Chemistry and 2017 Elsevier.

MagSilica-Au(0) catalyst (Figure 2a) gave almost complete and chemoselective substrate conversions regardless of the nature of the starting primary or secondary alcohol and without generation of overoxidation byproducts.40 With MagSilicaPd(0) (Figure 2b), the authors have finally shown the ability of their radio frequency heatable Pd composite to catalyze the Heck (1/3 aryl iodide:styrene, 3 equiv of nBu3N, 2.8 mol % of Pd(0), DMF, 1 h, flow rate 2 mL/min, IH (25 kHz), 120 °C)43 and Suzuki−Miyaura (1.5/1 phenylboronic acid/aryl bromide, 2.4 equiv of CsF, 2.8 mol % of Pd(0), DMF/H2O, 1 h, flow rate 2 mL/min, IH (25 kHz), 100 °C)43 cross-coupling reactions under flow conditions with fairly good substrate conversions.16 In a different reactor scheme,13 plain MagSilica NPs (as an inductively heatable susceptor) were physically mixed with Pd (10 wt %) on charcoal (as the catalytically active phase) to set up a packed-bed flow reactor for ethanol hydrogen transfer on different substrates: benzyl ethers, nitroarenes, alkenes, and alkynes (Figure 3) (conditions: MagSilica/Pd (10 wt % on C), 1/1 cyclohexene/ethanol, flow rate 0.2−0.5 mL/min, IH (25 kHz), 70 °C).43

functionalization and/or metal NPs decoration. Accordingly, the German team prepared amino and ammonium (Merrifieldtype ion-exchange groups) decorated MagSilica NPs to be employed as core−shell systems for the controlled growth of Au0 (Figure 2a)40 and Pd0 NPs (Figure 2b),13,16 respectively. While the former composite (MagSilica-Au0) was successfully scrutinized for the catalytic O2-promoted oxidation of allylic and benzylic alcohols, MagSilica-Pd0 was tested in a fixed-bed reactor as a catalyst for the Suzuki−Miyaura and Heck crosscoupling reactions. Both of these engineered nanoarchitectures were demonstrated to undergo rapid heating upon radio frequency excitation, thus allowing heat to be directly transferred from the SPM iron oxide core to the neighboring transition metal and catalytically active sites. When they are used in mesofluidic reactors, both catalyst composites gave good to almost complete substrate conversion in a single pass as a function of the applied reactor temperature and flow rate. Under optimized conditions (0.5 g of MagSilica-Au(0), 6.0 mg of Au, and 0.27 mmol of substrate in benzene (3 mL) at a flow rate of 0.1 mL/min, p(O2) = 7 bar, IH (25 kHz), 150 °C)43 the 7924


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the heat is directly and homogeneously transferred to the active sites without the need of heating the reactor walls, hence allowing very high local temperatures to be reached at the catalyst surface. This reactor configuration is well emphasized by these authors with the figurative concept of “hot spots” in a “cold environment”. Indeed, the heating targeted to the catalytic bed (hence, where it is needed for the process to occur) can reach temperatures well above those of the solvent boiling point. This can be explained on the basis of the fact that heat transfer from the susceptor to the solvent is slowed down by the formation of a “vapor shell” around the catalyst particles.55 Overall, the proposed IH configuration allows the creation of local “hot spots” at the catalyst bed surrounded by a vapor phase where higher temperatures and gas pressures (ideal for running the HDO process) may be reached. The power to hydrogen concept, i.e. hydrogen production via water electrolysis promoted by electricity coming from low carbon footprint sources, is currently at the forefront of many of our future energy technologies. Indeed, it can comply with the steadily increasing hydrogen demand (as fuel and chemical) for a large portfolio of applications. In this contest, Chatenet and Carrey have reported on inductively heated Ni-coated iron carbide (FeC-Ni) NPs as exceptionally performing electrocatalysts for water electrolysis under alkaline conditions.12 They have shown how radio frequency heated FeC-Ni core−shell NPs significantly decrease the overpotential value at which the two half electrochemical reactions start (OER, oxygen evolution reaction; HER, hydrogen evolution reaction). Although IH applied to the intensification of electrochemical reactions in a liquid electrolyte cell in principle might have detrimental interferences with electric charges transported and exchanged throughout the process, the localized rf heating in the immediate proximity of the metal active phase has shown only largely beneficial effects. Indeed, when MNPs underwent a highfrequency alternating magnetic field in an electrochemical cell designed to operate under IH (Figure 5), the electrocatalyst overpotential value (at 20 mA cm−2) was markedly reduced by ∼200 and ∼ 100 mV for the OER and HER, respectively. In particular, these authors claimed that the observed kinetic enhancement in the OER (i.e., the kinetically sluggish process in

Remarkably, both catalytic systems have shown negligible leaching of active metallic phases either throughout long-term catalytic runs40 or upon catalyst recycling.16 Zeolite-encapsulated magnetic nickel ferrite nanoparticles (NiFe2O4@TiO2@ZSM-5) have also been prepared as an acid catalyst for the inductively heated citronellal isomerization to isopulegol (Figure 4a).44 The combination of acidity and adsorption capacity of the zeolite double-shell system45 with the radio frequency heatable magnetic core has allowed BerenguerMurcia et al.44 to demonstrate the effectiveness of their nanoreactors in a classically acid-promoted organic transformation.46 Indeed, IH afforded an exceptional control on the reactor conditions while providing a catalytic system with outstanding performance and long-term stability on runs (conditions: citronellal (40−80 mM) in 1,4-dioxane, flow rate 0.1 mL/min, He flow 3.0 mL/min (STP), IH (300 kHz), 80 °C, cat. 82.2 mg). Similarly, Š těpánek and co-workers have reported on the acidcatalyzed vapor-phase dehydration of ethanol to ethane using rfheated catalyst composites consisting of iron microparticles (mean particle diameter 25 μm) embedded in porous alumina.47 The employment of iron microparticles allows the controlled and local catalyst heating via eddy currents and hysteresis losses with positive effects on the process intensification. Finally, the rapid catalyst heating/cooling rate ensured by IH has been used by the authors to demonstrate the easy and fast on/off switching of the endothermic process (typically occurring at 300 and 500 °C),48 introducing the concept of catalyst remote control for “on-demand” product formation. Magnetic core−shell NiFe2O4@TiO2 NPs have also been employed by Rebrov49 and co-workers as an acid catalyst for a continuous flow and inductively heated direct amide synthesis (Figure 4b).50 Despite the effectiveness of their IH protocol for one of the most important reactions in the pharmaceutical industry,51 the authors have recently demonstrated a significant increase of the process rate by the use of double-shelled magnetic core NPs. In particular, the use of an intermediate silica layer between nickel ferrite particles and the external titanium oxide shell (NiFe2O4@SiO2@TiO2) was found to increase the specific surface area of the catalytic layer while preventing undesired and detrimental interactions between TiO2 and the magnetic core.52 An additional and beneficial effect on the catalyst performance finally comes from the optimization of the catalyst acidity by TiO2-shell sulfation (Figure 4b). The sulfated core−double shell catalyst showed markedly lower deactivation rates under long-term runs in comparison to the nonsulfated counterpart or plain NiFe2O4@ TiO2 core−single shell nanoreactors.52 Very recently, Asensio and Chaudret have proposed an elegant and highly efficient protocol for the inductively heated hydrodeoxygenation (HDO) of acetophenone derivatives and of biomass-derived molecules under remarkably mild operative conditions.53 They prepared hybrid functional materials based on the combination of magnetic susceptors with excellent heating power (Fe2.2C NPs)54 and Ru NPs featuring high catalytic activity in HDO (Fe2.2C@Ru NPs). This ideal combination has allowed the authors to run a high-temperature catalysis in solution, ensuring excellent catalyst activity and high product selectivity already in the presence of low catalyst loadings and low hydrogen pressures. The use of magnetic nanoparticles (MNPs) as a local transducer for the radio frequency conversion into heat (through hysteresis loss) was found to improve the energy efficiency of the process. Indeed,

Figure 5. Schematic representation of a water electrolyzer powered by rf heating of FeC-Ni core−shell MNPs under a high-frequency alternating magnetic field. Reproduced with permission from ref 12. Copyright 2018 Springer Nature. 7925


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ACS Catalysis the water splitting) would refer to a cell temperature rise of ∼200 °C, whereas an average increase of only 5 °C was measured in practice. Overall, this seminal contribution has provided unique hints to the use of rf heating aimed at intensifying key electrochemical processes at the heart of renewable energy technology. As a further example of catalytic process intensification resulting from the transition from a contact to contactless heating technology, Lee and co-workers have investigated for the first time IH in the catalytic degradation of organic pollutants from wastewater.56 The authors have developed a multiplemetal-based catalyst supported on carbon-coated iron sponge particles (MnNiLa/C@Fe) to be employed for the inductively heated CWPO (catalytic wet peroxide oxidation) protocol. Their reaction scheme has shown how energy saving and process intensification can be achieved by transferring electromagnetic energy generated by an external alternating field directly to the magnetic core of their catalytic system and in turn to the catalyst particles decorating its C coating. By this way, heat can be targeted directly to the catalytic bed without additional energy transfer pathways involving the liquid phase. Accordingly, the catalyst temperature can be rapidly increased up to 503 K and kept much higher than that of the surrounding liquid phase. The high temperature generated by IH at the catalyst/ water interface has a significant influence on the removal of dyeing wastewater. This translates into improved catalyst performance along with a remarkable process energy saving. Three model azo dye aqueous solutions (i.e. direct blue (D3GL), direct violet (D-BL), and direct scarlet (D-GLN)) underwent H2O2 treatment using alternately either IH or a traditional heating system. The authors have shown that IH remarkably improves the process efficiency with an almost complete dye removal in comparison to a modest 20% of pollutant abatement reached under identical reaction conditions when a traditional heating setup was at work (conduction/ convection/radiation). A conceptually different IH catalytic setup has finally been used to perform one of the most studied metal-mediated reactions in synthetic organic chemistry: the 1,3-dipolar cycloaddition of an azide with a terminal alkyne (also known as the Huisgen reaction or “click” reaction)57 to give the 1,2,3triazole core. Kirshning’s team has shown for the first time the effectiveness of electrically conductive (not magnetic) Cu(0) wires (or turnings) in an IH flow microreactor for the azide− alkyne cycloaddition.58,59 Copper wires exposed to an external ac magnetic field have a double role: (1) they provide the source of catalytically active Cu(0) sites60 to the process and (2) they offer a local transducer for the radio frequency conversion into heat (through eddy currents). The fast and localized heating of the copper susceptor inside the flow microreactor has given excellent outcomes with a full substrate conversion in a single pass upon optimized reaction parameters (concentration of reactants, flow rates, IH parameters).58 All of these seminal findings have had and currently have a significant effect on fixed-bed flow chemistry in general and in catalysis in particular. It can be inferred that rf heating of magnetic or simply electrically conductive susceptors represents something more than a further “direct” heating method.61,62 As an enabling technology, IH has allowed overcoming many of the major limitations encountered with the more classical heating schemes based on convection/conduction or radiation, and it has deeply contributed to boost research on other and more challenging inductively heated catalytic flow processes.

4. INDUCTION HEATING IN HIGH-TEMPERATURE ENDOTHERMIC CATALYTIC PROCESSES 4.1. Introduction. Literature reports on high-temperature catalytic processes further illustrate how energy losses, scale-up issues, undesired byproduct formation, and costly equipment can be minimized by adapting a given catalytic transformation to a reactor operating under IH technology. The very rapid up/ down temperature ramps and the high temperatures reachable along with a well localized heating management are just some of the strengths of IH. Heat transfer in reactors operating highly endothermic transformations is claimed as a rate-limiting and highly energy consuming step when heat is provided by convection, conduction ,and radiation. Indeed, heat transfer from the outer reactor surface to the catalyst bed (where the process takes place) often results in slow start-up times due to the high heat capacity of the system and high energy losses. This aspect is of particular relevance for many industrial processes, where the catalyst performance is often limited by physical constraints related to the effective heat transfer from the reactor walls to the catalyst bed rather than to the intrinsic catalyst activity. Rostrup-Nielsen and co-workers have calculated in 200163 that only about 50% of the external heating provided to reformer reactors is used for running the catalytic process, the remaining part being dissipated by heat transfer restrictions encountered in contact heating modes from the heat source to the catalyst. In this scenario, the noncontact rf-heating technology has given a unique contribution in terms of energetics, product quality, process safety, and accurate power control. 4.2. 1D and 2D Carbon-Nanomaterial Synthesis by Radio Frequency Heated Catalytic Chemical Vapor Deposition (rf-cCVD). The reduced energy footprint of IH makes it an attractive technology for highly energy demanding catalytic transformations. This feature matches well with a hightemperature and energy-consuming process such as the catalytic chemical vapor deposition applied to C-nanomaterials production. In this regard, rf heating has been successfully applied to the high-quality production of single-walled (SW),64 double-walled (DW),65 and multiwalled carbon nanotubes (MWCNTs)66−68 by lowering the energy consumption and shortening the overall reaction times in comparison to classical oven-heated reactors. Preliminary data have shown the use of various mono- or binarytransition-metal mixtures as such (Ni,66 Pd,66 Co,66 Co/Ni, Pd/ Rh, Pt/Rh, Fe/Mo,65,68 Fe/Co66,67) or as NPs on metal oxides (TiO2, Al2O3,66 MoO2,65,68 CaCO367) to be housed into the reactor on molybdenum or copper foils or graphite vessels as susceptors for the rf heating. Heat is originated by the electromagnetically induced eddy currents flowing through the electrical resistance of the susceptor (Mo, Cu, or graphite foils, Fe or Ti rods), and it is transferred by conduction/radiation to the metal active sites. The external reactor walls are kept cold (“cold reactor”), and the thermal decomposition of gaseous reagents is significantly reduced, whatever the nature of the carbon source used.66,68 All of these contributions outline the deeply beneficial effects of rf-heated catalysis in comparison to the traditional conduction-based (outer reactor furnaces) protocols. rf-cCVD significantly reduces the energy consumption and shortens the reaction time. In addition, the much faster and more accurate control of the heating/cooling rates within the catalytic runs provides better-quality materials in terms of carbon purity66 and control of their ultimate morphology.64,65,67 7926


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ACS Catalysis Ruoff and co-workers have proposed an original “no-flow” rfcCVD protocol for the graphene synthesis from a methane/Ar mixture,69 using an inductively heated copper foil as catalyst for the material growth, yielding graphene of quality comparable to or even higher than that obtained by a more conventional hotwall setup. The rapid heating/cooling rate of the copper susceptor (typically 30 °C/s) along with its annealing under a pure hydrogen (first) and argon (later) atmosphere were critical steps for the high-quality monolayer graphene (>99%) full coverage of the metal susceptor. In addition, the simple use of a Cu foil as a heating source and catalyst for the process represents an important step forward in terms of catalytic protocol simplification for the scaling up of graphene synthesis at an industrial level. More recently, Li and Guo have demonstrated the versatility of the rf-cCVD for the controlled formation of single crystals of graphene with varying morphologies70 again using Cu foil as a catalyst (Figure 6). With the IH approach, they

atomic phenomena responsible for the single-crystal graphene growth. 4.3. Dry and Steam Methane Reforming. Dry and steam methane reforming (eqs 1 and 2, respectively) are two highly endothermic transformations for efficient hydrogen production. While the former is considered the most cost-effective way for the large-scale biogas conversion into syngas,71,72 the latter certainly represents one of the most industrially relevant catalytic processes used to date.73,74 CH4 + CO2 ⇆ 2CO + 2H 2

ΔH °298 = 247 kJ mol−1 (1)

CH4 + H 2O ⇆ CO + 3H 2

ΔH °298 = 206 kJ mol−1 (2)

These endothermic reactions are thermodynamically favored at temperatures higher than 700 °C (under ambient-pressure conditions) or even higher temperatures (up to 950 °C) in industrial plants where higher operational pressures are used.75 The first application of an rf-heated reactor for hydrogen production via biogas reforming was presented in 2015 by Rooney and collaborators.76 The authors have shown how an inductively heated stainless-steel reactor filled with a perovskitetype mixed metal oxide catalyst (Na0.5La0.5Ni0.3Al0.7O0.25), underwent much faster heating/cooling cycles that translated into lower coke formation rates in comparison to the classical thermally heated system. Although their IH protocol lies in the classical concept of a “hot reactor” where heat is still transferred from the hot reactor walls to the catalyst via conduction/ convection, the more rapid up/down temperature control within the system was found to minimize the occurrence of sidecracking processes typically responsible for the catalyst stability under long-term operations. In addition, the use of a ferromagnetic reactor has introduced a key concept related to the Curie temperature, above which a given material becomes paramagnetic and loses its hysteresis contribution to the heating. Such a temperature for stainless steel is around 750 °C,2 but it varies with the alloy composition.77,78 This idea has been developed and strengthened later by other research groups for

Figure 6. Schematic representation of the rf-cCVD setup for the singlecrystal graphene growth. Reproduced from ref 70. Copyright 2015 Springer Nature (work licensed under a Creative Commons Attribution 4.0 International License).

have developed an original methodology to the synthesis of circular, hexagonal, and dendritic graphene domains through a rapid heating and temperature quenching (from more than 1000 to 700 °C in about 5 s) at the catalyst bed. Furthermore, they have contributed to shed light on the nature of competing

Figure 7. CH4 conversion and equilibrium temperature (insets) for steam reforming reactions at ambient pressure and variable gas flows, using IH as the unique heat source. (A) Curves refer to the ferromagnetic susceptor/catalyst NiCo/MgAl2O4 (Ni, 12.6 wt %; Co, 9.0 wt %). Reproduced with permission from ref 79. Copyright 2017 American Chemical Society. (B) Curves refer to the susceptor/catalyst CoNi (Co0.5Ni0.5/Al2O3) and Cu⊂CoNi (Cu0.01⊂Ni0.45Co0.45/Al2O3). Reproduced from ref 11. Copyright 2018 John Wiley and Sons. 7927


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Figure 8. Heating profiles (A, B) of macroscopic alloys (A, FeCr; B, AlNiCo) vs time, powered by an ac magnetic field (55.7 kHz) at increasing magnetic field strength between 0.01 and 0.08 T (A′ and B′). Spikes over 800 °C in the temperature profile of Figure 8B have to be seen as measurement errors. Images courtesy of J. S. Engbæk and S. B. Vendelbo from the Danish Technological Institute (DK) and P. Mortensen from Haldor Topsøe A/S (DK).80

observables have confirmed the ability of the rf-heated composites to catalyze the transformation efficiently by removing many of the heat transfer limitations typically encountered in the classical “hot-wall” reactors and thus pushing the process to the bounds of its intrinsic kinetics.11 Interestingly, the Cu⊂CoNi system showed significantly improved catalytic performance in comparison to its copper-free counterpart (CoNi) despite the very small content of (nonmagnetic) copper. Indeed, at higher flow rates (152 NL/h) the Cu-doped catalyst reached 95% of CH4 conversion at a magnetic field (Bapplied) 15% lower than that required to get similar outcomes with the plain CoNi (Figure 7B).11 In addition, none of the reactors outlined above have shown significant catalyst degradation or appreciable coke formation on the time scale of each catalytic run or on dedicated long-term tests. The latter in particular were claimed by the authors to be a strong proof of evidence of the key advantages arising from the exploitation of a noncontact heating technology while running potentially “coke-suffering” catalytic transformations. The absence of temperature gradients within the IH reactor can preserve the system from the generation of “hot spots”, thus hampering carbonaceous deposit growth while ensuring high catalyst stability. Haldor Topsøe has also patented the invention of macroscopic ferromagnetic alloys used as rf susceptors for catalytically active metal NPs to be used in a series of highly endothermic transformations (i.e., steam reforming, tar reforming, and reverse water-gas shift).80 The applicants have demonstrated that low-frequency (55 kHz) magnetic fields (between 0.01 and 0.08 T) can be used to rapidly heat macroscopically shaped ferromagnetic alloys by eddy currents and hysteresis loss to maximum temperature values substantially close to the inherent Curie point of each composite. Indeed, above the respective TC values, samples become paramagnetic and any further temperature increase induced by stronger (low-frequency) applied magnetic fields (due to residual eddy currents only) is negligible (Figure 8). The authors have prepared self-regulating ferromagnetic susceptors suitable for operating catalytic runs at the upper temperature limit of the given endothermic process. This finding ensures a heat transfer from a rf-heated (oxide

the hysteresis heating of either magnetic nanoparticles or macroscopic ferromagnetic supports as “self-regulating” systems that operate at their Curie point.11,79−81 Indeed, a first and decisive step forward in the field of syngas production by IH methane steam reforming was given by the Danish team at Haldor Topsøe A/S in 2017.79 Mortensen and co-workers described a conceptually new flow reactor system where the hysteresis heat supplied by magnetic Ni-Co NP alloys on a magnesium aluminate (MgAl2O4) spinel support was used to carry out the reaction. Through a simple and sequential metal impregnation on an inert support these authors have combined the catalytic activity of Ni in methane reforming (Ni;82 TC = 354 °C) with the higher Curie point of Co (TC = 1115 °C) to increase the ferromagnetic alloy temperature to the values required for the process. With 12.6 wt % of Ni and 9.0 wt % of Co they prepared a catalytic alloy with a Curie point higher than 800 °C, with a dual functionality: it allows rf conversion by hysteresis heat to the target steam reforming temperature, and it catalyzes the endothermic transformation efficiently because of the presence of nickel. Methane conversion obtained with the NiCo/MgAl2O4 composite (Figure 7A) at variable flow rates (from 20 to 102 NL/h) increases with the applied power at the induction heater, and it is nearly complete for the lowest flow rates. Remarkably, the way that curves behave for higher flow charges shows that the system is limited by the process kinetics rather than by heat transfer phenomena as typically occurs with classical “hot-wall” reformers. More recently, Vinum, Bendix, and Mortensen have proposed a different approach to catalyst manufacturing by replacing the trivial metal impregnation approach with a more accurate bottom-up NPs engineering. Accordingly, they synthesized a series of spinel-type composites of general formula MyCo1−x−1/2y)Nix−1/2yAl2O4 that can accommodate different metal ions (M) at variable ratios.11,81 They focused on two composites: mainly, CoNi (Co0.5Ni0.5/ Al2O3; TCCoNi = 892 °C) and Cu⊂CoNi (Cu0.01⊂Ni0.45Co0.45/ Al2O3; TCCu⊂CoNi = 875 °C) as susceptors/catalysts for the inductively heated methane steam reforming (Figure 7B). As reported in their first paper,79 the authors have shown similar trends in methane conversion as a function of the flow rate and applied magnetic field strength (Figure 7B). These 7928


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Figure 9. (a) Schematic representation of a magnetically induced Sabatier reaction in a continuous-flow reactor using ICNPs-Ru/SiRAlox catalyst. (b) TEM of ICNPs and Ru NPs on a SiRAlox as carrier. (c) Magnification of the catalyst surface containing 1 wt % of Ru. (d) Model representation of the catalytic system. Reproduced with permission from ref 54. Copyright 2016 John Wiley and Sons.

controlled and tuned in almost real-time, any sudden temperature runaway due to the exothermicity of the reaction can be limited and properly redirected. Thus, a typical drawback (hot spot generation) for the catalyst stability and efficiency can be turned into an extra (self-induced) energy resource to be exploited for running the process. To date, carbon oxides (CO and CO2) catalytic hydrogenations are the most relevant exothermic processes investigated using IH technology. The following paragraphs highlight the advantages of the IH mode to operate highly exothermic catalytic processes and provide an overview of the different catalytic setup (susceptor/catalyst) reported so far. 5.2. Fischer−Tropsch and Methanation Reactions. Chaudret, Respaud, and co-workers have first reported on the successful “cold catalysis” applied to one of the most challenging synthetic processes for the hydrocarbons production from CO/ H2 mixtures:15 the Fischer−Tropsch process.85 The French group started from the tailored bottom-up synthesis of magnetic core−shell NPs combining a ferromagnetic core (Fe) featuring a high hyperthermic efficiency (SAR) with a highly catalytically active shell (FeCo or Ru). Hence, monodispersed bimetallic core−shell systems (Fe@FeCo and Fe@Ru; ∼12.3 ± 1 nm) were prepared and exploited as heterogeneous catalysts for inductively heated CO hydrogenation.15 Under experimental conditions relatively far from those normally operating in real industrial plants (Fischer−Porter bottle), the authors have demonstrated how their magnetic susceptors/catalysts were able to catalyze the reaction efficiently, after being loaded in a CO/ H2 statically pressurized reactor and heated by an external alternating magnetic field to the target reaction temperature (>200 °C). They have also shown how a proper catalyst engineering, aimed at designing NPs featuring high heating power and tailored catalytic surface properties (i.e., Fe@Ru vs Fe@FeCo), could largely prevent undesired catalyst deactivation phenomena.15 Most recently the same team has extended the concept of “cold catalysis” to another exothermic and even more challenging catalytic process in the so-called “power to gas” (PtG) technology:86,87 the methanation reaction or Sabatier process.88 Such a catalytic reaction represents a powerful tool to the production of synthetic natural gas (SNG) as fuel (CH4),89 and it copes with timely issues such as (1) the chemical storage of extra and intermittent renewable energy sources and (2) the dramatic environmental and climatic impact arising from the steady increase of CO2 concentration in the atmosphere. However, the development of a continuous-flow catalytic process using IH of magnetic NPs requires the development of nanoreactors featuring exceptionally high SAR values and catalytic performance. In this regard, the French team has shown how a prolonged (>140 h) carbidization treatment of Fe(0) NPs

coated) ferromagnetic susceptor to catalytically active metal NPs that decorate its outer surface up to a target temperature that substantially does not exceed the susceptor Curie point (TC), thus preventing undesired and risky temperature runaway effects. With an approach conceptually similar to that described above, Varsano and co-workers have recently reported the use of micrometric powders of Ni60Co40 in the form of compacted disks for CH4 dry reforming powered by magnetic induction.83 The Italian team has described the highly beneficial effects arising from the use of IH in terms of energy efficiency, temperature control at the catalytic sites, and coke poisoning phenomena for long-term catalytic runs. An additional and common feature of all these reforming protocols comes from the fact that heat can be directly targeted to the catalyst and immediately used at the rate of the chemical reaction for a given endothermic transformation. As a matter of fact, this heating methodology solves the general problems encountered in steam reforming industrial plants, where start-up times under conventional radiation, convection, and conduction heating schemes typically require several days to bring the plant under the regime conditions safely. In addition to offering fast heating responses, large process intensification, and reduced energy costs, IH opens up important simplification and size reduction of steam reforming units (i.e., removal of expensive waste-heat sections commonly employed in the methane reforming setup, thus reducing the complexity of the overall process design). This paves the way for the development of small-scale reactors for hydrogen production on demand in the scenario of the future hydrogen economy.11,79,84

5. INDUCTION HEATING IN HIGH-TEMPERATURE EXOTHERMIC CATALYTIC PROCESSES 5.1. Introduction. At odds with the relatively high number of endothermic catalytic processes, much less attention has been devoted to the use of IH in exothermic transformations. This may be partially due to the inherent nature of exothermic reactions, where any sudden temperature increase at the catalytic sites (induced by the process itself) can be difficult to control. However, the almost instantaneous “heat on/off” switching at the nanometer scale provided by IH technology holds key potentialities that have been only partially exploited in catalysis up to now. The “cold catalysis” concept recently introduced by Chaudret and co-workers15 mirrors the reduced energy losses and the higher reactor safety (cold walls) encountered in rf-heated systems. In the case of exothermic catalytic transformations, any endogenous heating generated in the catalyst bed by the process exothermicity (self-induced hot spots) can be regarded as an “extra energy source” to carry out the process. Indeed, where the local heating can be finely 7929


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ACS Catalysis afforded iron carbide (Fe2.2C) NPs (ICNPs; ∼15.1 ± 0.9 nm) with excellent hyperthermic efficiency already upon exposure to an ac magnetic field with amplitudes as low as 20−25 mT.54 They proposed two alternative ways to the preparation of active and selective methanation catalysts by combining iron carbide susceptors with classically active metals for the process: i.e. Ni and Ru. Hence, ICNPs coated with a nickel shell (ICNPs@Ni; 29 wt % of Ni) or a binary mixture of plain ICNPs and Ru NPs (1 wt %) have been supported on a silica−alumina carrier (SiRAlox) and employed as methanation catalysts in a continuous-flow reactor powered by a 300 MHz ac magnetic field. In contrast with a more intimate susceptor/catalyst action foreseen in the case of the core−shell system (ICNPs@Ni), the idea behind the use of binary NP mixtures (as in the case of the Ru-based catalyst) was to allow a full exploitation of the hyperthermal efficiency of plain ICNPs for the (hysteresis loss) heat transfer to the neighboring Ru NPs for catalysis (Figure 9). Indeed, SAR of ICNPs@Ni was found to be appreciably lower in comparison to that of plain ICNPs and CH4 production with the ICNPs-Ru/SiRAlox catalyst was significantly improved in comparison to ICNPs@Ni under identical conditions. Furthermore, the Ru-based system gave excellent catalytic performance (CH4 yields up to 93% and complete selectivity) on longterm hydrogenation runs even for relatively high H2/CO2 flow rates (up to 125 mL/min) at a magnetic field strength of 28 mT. Despite outstanding catalytic outcomes, the methanation setup described above foresees a “static” electromagnetic environment of the catalytic system (f = 300 MHz and fixed magnetic field strengths between 20 and 64 mT) without any temperature control in “real time” in the catalytic bed. This aspect is even more relevant in the case of highly exothermic processes, where a discontinuous reagent supply can be responsible for sudden temperature deviations at the active sites with the occurrence of severe and potentially irreversible catalyst deactivation phenomena. To cope with that, our research group has recently proposed an alternative setup to the catalytic methanation reaction in a rf-heated flow reactor. An electrically conductive susceptor based on an oxidized carbonfelt matrix (OCF) decorated with homogeneously sized Ni NPs (∼4 ± 1 nm) was employed as susceptor/catalyst for the methanation process.14 Notably, the active metal phase was synthesized using commercial nickel salts under straightforward impregnation conditions, a simplified synthetic scheme in comparison to the more complex core−shell systems that allows an easy and cheap catalyst scale-up production. The susceptor/catalyst was located in a transparent tubular quartz reactor, housed inside the inductor heating coils, and the average catalyst temperature was monitored by a laser pyrometer shot up on the catalytic bed, coupled to the inductor heater via a PID controller (proportional integral derivative controller). The latter ensured a continuous loop feedback between the laser and the induction heater (Figure 10a). Thus, a real-time measurement of the average catalyst temperature was used to finely tune the intensity of the current supplied to the inductor coils and to modulate dynamically the amplitude of the applied magnetic field responsible for the susceptor heating. Heat transfer on the rf-heated Ni/OCF system occurs through conduction/convection between OCF and Ni NPs, the former having eddy current losses as the unique heating source. The combination of IH with a susceptor (OCF) featuring excellent thermal conductivity allows for very rapid heating/cooling rates, preventing the local generation of undesired “hot spots” detrimental for the process performance

Figure 10. (a) Representation of an inductively heated reactor setup made of the Ni/OCF catalyst located inside a quartz tubular reactor housed within the coils of the inductive heater and PID-coupled with a laser pyrometer shot up on the catalytic bed. (b) Ni/OCF catalytic performance in the inductively heated methanation reaction operated under dynamic conditions (i.e., successive shutdown periods, variable GHSVs, and target catalyst temperatures). Reproduced with permission from ref 14. Copyright 2019 Elsevier.

and catalyst stability with time. This feature is of great importance for methanation systems operating under harsh dynamic conditions (i.e. under variable gas hourly space velocities (GHSV), variable reactor temperatures, and pure reactant flows) such as those of real industrial plants, where problems related to the intermittent supply of renewable reagents (e.g. H2) can occur more frequently. With their IH setup, the authors have demonstrated an excellent and long-term stability of the catalytic system when harsh and dynamic operational conditions are applied (Figure 10b). The forced dynamic methanation conditions stem from the application of successive start/stop cycles, with a very rapid temperature decrease and sudden reagent supply shutdown. No appreciable catalyst deviations from its pristine performance (XCO2, CO2 conversion; SCH4, methane selectivity) or deactivation/poisoning phenomena were observed under these conditions. As already mentioned above, such a high stability can be explained by the fast and accurate catalyst temperature control that prevents the generation of local hot spots or sudden temperature runaway at the catalytic bed, thus reducing the occurrence of undesired catalyst degradations (sintering or fouling). Worthy of note, these conditions are not technically feasible with classical “hot-wall” reactors because of markedly longer thermal induction times and uncontrollable hot spot generation at the catalyst sites caused by sudden variations of reagent concentrations that are not easy to be promptly compensated when conventional heating schemes are employed. Whether the rapid heating/cooling rate of inductively heated susceptors/catalysts holds the positive feature of reducing the detrimental generation of local hotspots, their “controlled 7930


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this heating technology has answered urgent catalysis issues such as those related to energy saving and process intensification, reactor and equipment simplification, and (more generally) reduction of production chain costs. The capacity of IH to drive heat directly where it is needed through the selective radio frequency absorption onto dedicated materials (magnetic or electrically conductive) turned out to be an enabling technology capable of pushing several catalytic transformations beyond the reactor heat transfer limits, to the bounds of the process kinetics. Moreover, the removal of several heat transfer restrictions typically encountered in classical contact heating schemes has largely contributed to the improvement of process energetics as well as the reduction of side products classically indicated as “heat wastes”. Overall, the advantages of a transition from a contact to contactless heat management in catalysis based on IH technology are manifold: more favorable energy balance, process intensification, reactor setup simplification, reduced safety issues, minor operational costs, and increased process productivity. The catalyst engineering aimed at joining materials featured by high hyperthermic efficiency (SAR) with excellent catalytic performance and the development of ferromagnetic “selfregulating” susceptors working at the temperature limit set by their Curie point have radically changed the way of managing heat within a catalytic reactor, with important implications on the catalyst efficiency, selectivity, and stability on run. In light of the results listed above, different perspectives can be drawn for the future development of inductively heated catalytic schemes. First, the combination of magnetic susceptors featuring high hyperthermic efficiency (SAR) with metal NPs or metal coatings with catalytic properties is a research area in its infancy that still has a great deal of room for further and more challenging fundamental developments. Second, the “contactless heating technology” is well suited for the industrial scale-up of a number of exo- and endothermic transformations. Indeed, IH strongly reduces the amount of energy required to catalyze a given process, it is much safer with respect to traditional heating schemes, and the reactor setup can be deeply simplified. Electrically conductive (nonmagnetic) metallic or nonmetallic macroporous foam structures used as such as susceptors/catalysts or exploited for their outer surface decoration with another catalytically active phase represent a new item in inductively heated heterogeneous catalysis. Indeed, they can be used to operate catalytic reactions with extremely low pressure drops throughout the catalytic bed, reduce costs and synthetic efforts typically encountered for the preparation of MNPs or magnetic core−shell systems, and open new horizons for a rapid laboratory- to industrial-scale transition. The use of ferromagnetic “self-regulating” susceptors working at the temperature limit of their Curie point represents a further breakthrough that holds key advantages not yet fully exploited for running endothermic catalytic transformations. Finally, the unique control exerted by IH on the almost real time “heat on/ off” switching at the catalytic bed holds potentialities that have been marginally exploited in catalysis up to now. This tool offers a valuable solution to the problem of hot spot generation (sudden and uncontrollable temperature increase) at the catalyst bed typically occurring in case of exothermic processes: a problem even more relevant for those exothermic transformations where reagents are potentially supplied discontinuously (such as those derived from renewable sources). The redirection of the energy produced by an exothermic catalytic run to accomplish the process itself by controlling the “hot spot”

ignition” deserves to be studied further. Indeed, part of the heat required by an exothermic reaction to start can be provided by the reaction itself with a net and positive energy balance to carry out the process. This is certainly a research area still in its infancy that holds huge potential for future developments in heterogeneous catalysis. 5.3. Catalytic Combustion of Volatile Organic Compounds (VOCs). The removal of volatile organic pollutants through their complete catalytic oxidation to harmless products such as H2O and CO2 is currently a key research area. Its effective exploitation has important consequences from a societal and political viewpoint, as it is strictly related to environmental protection issues. In this regard, Gaillard and coworkers have investigated for the first time the potentialities of IH in the catalytic degradation/combustion of volatile organic compounds (VOCs).90 They have demonstrated the high efficiency of a catalytic system made of a SnO2 thin film deposited on stainless steel as a susceptor (SnO2/SS) and applied to the total oxidation of isopropyl alcohol (IPA) as a model VOC. The easy and fast on/off heat switching at the catalytic bed along with the ability of IH to ensure very rapid heating/cooling rates of the catalyst/susceptor turned out to be fundamental tools for an efficient and fast VOCs oxidation treatment on demand. This feature can be of great practical interest when fast abatement of accidental VOC contaminations is required. SnO2/SS under IH has given almost complete IPA conversions to CO2 and H2O without any appreciable decrease of its performance even when the catalytic system underwent fast heating rates (up to 800 °C min−1). As a proof of concept, large amounts of IPA (1 vol %) have been completely converted to CO2 and H2O in less than 30 s with a catalyst heating rate of 800 °C/min. This result provides clear-cut evidence of the great potentiality of inductively heated catalytic systems for practical applications in the treatment of air for organic volatile pollutants. The same team has reported on the use of IH for the fast toluene vapor conversion into CO2 and H2O using a model reactor system joining in series a toluene adsorbent (e.g., Al2O3) and an efficient catalytic system for complete low-temperature VOC oxidation (1% Pt on ceramic yttria-stabilized zirconia, Pt/ YSZ).91 Catalyst and toluene adsorbent were finally arranged on a SS susceptor for the rf heating. In their reactor setup, IH was used to rapidly heat both the catalytic system and VOC adsorbent at the respective optimal working temperatures. Hence, the catalyst was located within the IH coils while toluene adsorbent was intentionally housed outside the coils. Thus, two different temperatures were optimally and rapidly managed with a unique IH source: a lower temperature for the optimal toluene desorption from the Al2O3 adsorbent and a higher temperature for its complete oxidation. Such a scheme proposes a valuable example of a continuum VOC adsorption/decomposition system where the contaminant desorption rate and its fast catalytic oxidation are controlled by IH.

6. CONCLUSIONS AND PERSPECTIVES This perspective highlights the most exciting applications and current trends in the field of rf-heated catalytic transformations and in particular within a series of highly energy demanding and industrially relevant processes. Despite the widespread longterm application of rf heating in several key technological sectors (spanning from metallurgy to biotechnologies and magnetic hyperthermia treatment of important diseases), its exploitation in catalysis is relatively recent. However, the interest of the scientific community has rapidly grown in recent years because 7931


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where σ is the material electrical conductivity (S/m), μ is the magnetic permeability (H m−1), f is the frequency (Hz) of the applied AC magnetic field.

APPENDIX A Appendix A describes the cornerstones of the IH theory with a particular attention to the magnetic and chemicophysical properties of a given sample with respect to its ability and effectiveness in rf conversion into heat, on the basis of the three dissipation phenomena outlined above: hysteresis losses, Néel relaxation phenomena, and Joule heating by eddy currents. Accordingly, the expression of the hyperthermic efficiency (SAR) of a given sample exposed to an ac magnetic field can be outlined as follows. Hysteresis Losses.10

ORCID

■

*E-mail for C.P.-H.: [email protected]. *E-mail for G.G.: [email protected].

■

SAR (W/g) = fμ0

Giulia Tuci: 0000-0002-3411-989X Yuefeng Liu: 0000-0001-9823-3811 Andrea Rossin: 0000-0002-1283-2803 Giuliano Giambastiani: 0000-0002-0315-3286 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions #

W.W. and G.T. contributed equally.

∮ MdH

Notes

(A1)

The authors declare no competing financial interest.

where f is the frequency (Hz) of the applied ac magnetic field, μ0 is the magnetic permeability in vacuum (H m−1), M is the magnetization (A m2 kg−1), and H is the magnetic field amplitude (kA m−1). Néel Relaxation.82,92 SAR (W/g) = P /ρ

■

ACKNOWLEDGMENTS G.G. and C.P.-H. thank the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the “Make our Planet Great Again” program (ref. ANR-17-MPGA-0017) for support. The Italian team also thanks the Italian MIUR through the PRIN 2015 Project SMARTNESS (2015K7FZLH) “Solar driven chemistry: new materials for photo- and electrocatalysis” for financial support to this work. W.W. thanks the China Scholarship Council (CSC) for financial support to his study at ICPEES.

(A2) −3

where P is the volumetric power dissipation (kW m ) and ρ is the material density (kg m−3). The volumetric power dissipation, P, can be expressed as P = πfμ0 χ0 H0 2

2πfτ 1 + (2πfτ )2

■

(A3)

ABBREVIATIONS ac, alternating current; CWPO, catalytic wet peroxide oxidation; DW, double-walled; FM, ferromagnetic; GHSV, gas hourly space velocity; HDO, hydrodeoxygenation; HER, hydrogen evolution reaction; ICNPs, iron carbide nanoparticles; IH, induction heating; IPA, isopropyl alcohol; MNPs, magnetic nanoparticles; MWCNTs, multi-walled carbon nanotubes; NPs, nsnoparticles; OCF, oxidized carbon felt; OER, oxygen evolution reaction; PID, proportional integral derivative; PM, paramagnetic; PtG, power to gas; rf, radio frequency; rf-cCVD, radio frequency-heated catalytic chemical vapor deposition; SAR, specific absorption rate; SNG, synthetic natural gas; SPM, superparamagnetic; SS, stainless steel; STP, standard temperature and pressure; SW, single-walled; TEM, transmission electron microscopy; VOCs, volatile organic compounds

where f is the frequency (Hz) of the applied ac magnetic field, μ0 is the magnetic permeability in vacuum (H m−1), χ0 is the magnetic susceptibility, H0 is the magnetic field amplitude (kA m−1), and τ is the relaxation time (s). The Néel relaxation time, τ, can be expressed as τ = τ0e KaV / kT

(A4)

where τ0 is the attempt time (s), Ka is the magnetic anisotropy constant (J m−3), V is the average particle volume (m3), k is the Boltzmann constant (J K−1), and T is the temperature (K). Eddy Current Dissipation.35 For spherical particles SAR (W/g) =

σ(πμrfH )^ 2 5ρ

(A5)

■

where σ is the material electrical conductivity (S/m), μ is the magnetic permeability in vacuum (H m−1), r is the average particle radius (m), f is the frequency (Hz) of the applied ac magnetic field, H is the magnetic field amplitude (kA m−1), and ρ is the material density (kg m−3). The skin depth, δ, defined as the effective depth of penetration of an electromagnetic wave in a conductive material, can be evaluated as34 δ=

1 σπfμ

AUTHOR INFORMATION

Corresponding Authors

REFERENCES

(1) Stankiewicz, A. I.; Moulijn, J. A. Process Intensification. Ind. Eng. Chem. Res. 2002, 41, 1920−1924. (2) Lozinskii, M. G. Industrial Applications of Induction Heating; Pergamon: New York, NY, USA, 1969; p 690. (3) Timko, B. P.; Whitehead, K.; Gao, W.; Kohane, D. S.; Farokhzad, O.; Anderson, D.; Langer, R. Advances in Drug Delivery. Annu. Rev. Mater. Res. 2011, 41, 1−20. (4) Norris, M. D.; Seidel, K.; Kirschning, A. Externally Induced Drug Release Systems with Magnetic Nanoparticle Carriers: an Emerging Field in Nanomedicine. Adv. Therap. 2019, 2, 1800092.

(A6) 7932


Perspective

ACS Catalysis (5) Hedayatnasab, Z.; Abnisa, F.; Wan Daud, W. M. A. Review on Magnetic Nanoparticles for Magnetic Nanofluid Hyperthermia Application. Mater. Des. 2017, 123, 174−196. (6) Ovenston, A.; Walls, J. R. Generation of Heat in a Single Catalyst Pellet Placed in an Electromagnetic Field for Endothermic Reforming of Hydrocarbons. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1073−1084. (7) Muley, P. D.; Henkel, C.; Abdollahi, K. K.; Boldor, D. Pyrolysis and Catalytic Upgrading of Pinewood Sawdust Using an Induction Heating Reactor. Energy Fuels 2015, 29, 7375−7385. (8) Bursavich, J.; Abu-Laban, M.; Muley, P. D.; Boldor, D.; Hayes, J. D. Thermal Performance and Surface Analysis of Steel-Supported Platinum Nanoparticles Designed for Bio-Oil Catalytic Upconversion During Radio Frequency-Based Inductive Heating. Energy Convers. Manage. 2019, 183, 689−697. (9) Abu-Laban, M.; Muley, P. D.; Hayes, D. J.; Boldor, D. Ex-Situ UpConversion of Biomass Pyrolysis Bio-Oil Vapors Using Pt/Al2O3 Nanostructured Catalyst Synergistically Heated with Steelballs via Induction. Catal. Today 2017, 291, 3−12. (10) Houlding, T. K.; Rebrov, E. V. Application of Alternative Energy Forms in Catalytic Reactor Engineering. Green Process. Synth. 2012, 1, 19−31. (11) Vinum, M. G.; Almind, M. R.; Engbæk, J. S.; Vendelbo, S. B.; Hansen, M. F.; Frandsen, C.; Bendix, J.; Mortensen, P. M. DualFunction Cobalt−Nickel Nanoparticles Tailored for High-Temperature Induction-Heated Steam Methane Reforming. Angew. Chem., Int. Ed. 2018, 57, 10569−10573. (12) Niether, C.; Faure, S.; Bordet, A.; Deseure, J.; Chatenet, M.; Carrey, J.; Chaudret, B.; Rouet, A. Improved Water Electrolysis Using Magnetic Heating of FeC−Ni Core−Shell Nanoparticles. Nat. Energy 2018, 3, 476−483. (13) Ceylan, S.; Coutable, L.; Wegner, J.; Kirschning, A. Inductive Heating with Magnetic Materials Inside Flow Reactors. Chem. - Eur. J. 2011, 17, 1884−1893. (14) Wang, W.; Duong-Viet, C.; Xu, Z.; Ba, H.; Tuci, G.; Giambastiani, G.; Liu, Y.; Truong-Huu, T.; Nhut, J.-M.; Pham-Huu, C. CO2 Methanation Under Dynamic Operational Mode Using Nickel Nanoparticles Decorated Carbon Felt (Ni/OCF) Combined with Inductive Heating. Catal. Today 2019, DOI: 10.1016/j.cattod.2019.02.050. (15) Meffre, A.; Mehdaoui, B.; Connord, V.; Carrey, J.; Fazzini, P. F.; Lachaize, S.; Respaud, M.; Chaudret, B. Complex Nano-objects Displaying both Magnetic and Catalytic Properties: a Proof of Concept for Magnetically Induced Heterogeneous Catalysis. Nano Lett. 2015, 15, 3241−3248. (16) Ceylan, S.; Friese, C.; Lammel, C.; Mazac, K.; Kirschning, A. Inductive Heating for Organic Synthesis by Using Functionalized Magnetic Nanoparticles Inside Microreactors. Angew. Chem., Int. Ed. 2008, 47, 8950−8953. (17) Lee, M.-K.; Tsai, W.-T.; Tsai, Y.-L.; Lin, S.-H. Pyrolysis of Napier Grass in an Induction-Heating Reactor. J. Anal. Appl. Pyrolysis 2010, 88, 110−116. (18) Muley, P. D.; Henkel, C.; Abdollahi, K. K.; Marculescu, C.; Boldor, D. A Critical Comparison of Pyrolysis of Cellulose, Lignin, and Pine Sawdust Using an Induction Heating Reactor. Energy Convers. Manage. 2016, 117, 273−280. (19) Henkel, C.; Muley, P. D.; Abdollahi, K. K.; Marculescu, C.; Boldor, D. Pyrolysis of Energy Cane Bagasse and Invasive Chinese Tallow Tree (Triadica Sebifera L.) Biomass in an Inductively Heated Reactor. Energy Convers. Manage. 2016, 109, 175−183. (20) Daniel, D. J.; Ellison, C. R.; Bursavich, J.; Benbow, M.; Favrot, C.; Blazier, M. A.; Marculescu, C.; Nokes, S. E.; Boldor, D. An Evaluative Comparison of Lignocellulosic Pyrolysis Products Derived from Various Parts of Populus Deltoides Trees and Panicum Virgatum Grass in an Inductively Heated Reactor. Energy Convers. Manage. 2018, 171, 710−720. (21) Li, C.; Burke, N.; Gerdes, K.; Patel, J. The Undiluted, nonCatalytic Partial Oxidation of Methane in a Flow Tube Reactor - an Experimental Study Using Indirect Induction Heating. Fuel 2013, 109, 409−416.

(22) Li, C.; Kuan, B.; Lee, W. J.; Burke, N.; Patel, J. The non-Catalytic Partial Oxidation of Methane in a Flow Tube Reactor Using Indirect Induction Heating - an Experimental and Kinetic Modelling Study. Chem. Eng. Sci. 2018, 187, 189−199. (23) Ruta, S.; Chantrell, R.; Hovorka, O. Unified Model of Hyperthermia via Hysteresis Heating in Systems of Interacting Magnetic Nanoparticles. Sci. Rep. 2015, 5, 9090. (24) Hergt, R.; Dutz, S.; Röder, M. Effects of Size Distribution on Hysteresis Losses of Magnetic Nanoparticles for Hyperthermia. J. Phys.: Condens. Matter 2008, 20, 385214. (25) Zhang, X.; Chen, S.; Wang, H.-M.; Hsieh, S.-L.; Wu, C.-H.; Chou, H.-H.; Hsieh, S. Role of Néel and Brownian Relaxation Mechanisms for Water-Based Fe3O4 Nanoparticle Ferrofluids in Hyperthermia. Biomed. Eng. 2010, 22, 393−399. (26) Pearce, J.; Giustini, A.; Stigliano, R.; Hoopes, P. J. Magnetic Heating of Nanoparticles: The Importance of Particle Clustering to Achieve Therapeutic Temperatures. J. Nanotechnol. Eng. Med. 2013, 4, 011005. (27) Moses, A. J., Eddy Current Losses in Soft Magnetic Materials. Wiley Encyclopedia of Electrical and Electronics Engineering; Wiley: 2016; pp 1−22. (28) Appino, C.; De La Barrière, O.; Fiorillo, F.; Lobue, M.; Mazaleyrat, F.; Ragusa, C. Classical Eddy Current Losses in Soft Magnetic Composites. J. Appl. Phys. 2013, 113, 17A322. (29) Coey, J. M. D. Magnetism and Magnetic Materials; Cambrige University press: New York, 2010; p 633. (30) Cullity, B. D.; Graham, C. D. Introducion to Magnetic Materials; Wiley: Hoboken, NJ, 2009; p 568. (31) Lisjak, D.; Mertelj, A. Anisotropic Magnetic Nanoparticles: A Review of their Properties, Syntheses and Potential Applications. Prog. Mater. Sci. 2018, 95, 286−328. (32) Frey, N. A.; Peng, S.; Cheng, K.; Su, S. Magnetic Nanoparticles: Synthesis, Functionalization, and Applications in Bioimaging and Magnetic Energy Storage. Chem. Soc. Rev. 2009, 38, 2532−2542. (33) Ruffini, R. S.; Madeira, R. J. Production and Concentration of Magnetic Flux for Eddy Current Heating Applications. Mater. Sci. Forum 1992, 102−104, 383−392. (34) Corcoran, J.; Nagy, P. B. Compensation of the Skin Effect in Low-Frequency Potential Drop Measurements. J. Nondestruct. Eval. 2016, 35, 58. (35) Skumiel, A.; Kaczmarek-Klinowska, M.; Timko, M.; Molcan, M.; Rajnak, M. Evaluation of Power Heat Losses in Multidomain Iron Particles Under the Influence of AC Magnetic Field in rf Range. Int. J. Thermophys. 2013, 34, 655−666. (36) Carrey, J.; Mehdaoui, B.; Respaud, M. Simple Models for Dynamic Hysteresis Loop Calculations of Magnetic Single-Domain Nanoparticles: Application to Magnetic Hyperthermia Optimization. J. Appl. Phys. 2011, 109, No. 083921. (37) Hudson, H. Coupling the Magnetic and Heat Dissipative Properties of Fe3O4 Particles to Enable Applications in Catalysis, Drug Delivery, Tissue Destruction and Remote Biological Interfacing. RSC Adv. 2016, 6, 4262−4270. (38) Wegner, J.; Ceylan, S.; Kirschning, A. Ten Key Issues in Modern Flow Chemistry. Chem. Commun. 2011, 47, 4583−4592. (39) Kirschning, A.; Kupracz, L.; Hartwig, J. New Synthetic Opportunities in Miniaturized Flow Reactors with Inductive Heating. Chem. Lett. 2012, 41, 562−570. (40) Chaudhuri, S. R.; Hartwig, J.; Kupracz, L.; Kodanek, T.; Wegner, J.; Kirschning, A. Oxidations of Allylic and Benzylic Alcohols under Inductively-Heated Flow Conditions with Gold-Doped Superparamagnetic Nanostructured Particles as Catalyst and Oxygen as Oxidant. Adv. Synth. Catal. 2014, 356, 3530−3538. (41) Wegner, J.; Ceylan, S.; Friese, C.; Kirschning, A. Inductively Heated Oxides Inside Microreactors − Facile Oxidations under Flow Conditions. Eur. J. Org. Chem. 2010, 4372−4375. (42) Hartwig, J.; Ceylan, S.; Kupracz, L.; Coutable, L.; Kirschning, A. Heating under High-Frequency Inductive Conditions: Application to the Continuous Synthesis of the Neurolepticum Olanzapine (Zyprexa). Angew. Chem., Int. Ed. 2013, 52, 9813−9817. 7933


Perspective

ACS Catalysis

Regioselective “Ligation” of Azides and Terminal Alkynes. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (61) With “direct heating” we intend a method where heat is generated within the material (e.g., microwave, induction, or controlled exothermic reaction), whereas “indirect heating” methods transfer energy from a heat source to the target material by conduction, convection, radiation, or a combination of these functions. (62) Thekdi, A.; Atkinson, M.; Barry, J.; Ciampini, M.; Greene, J.; Marker, A. Improving Process Heating System Performance: A Sourcebook for Industry; 2nd ed.; United States Department of Energy, Lawrence Berkeley National Laboratory, and Resource Dynamics Corporation: 2004. (63) Aasberg-Petersen, K.; Bak Hansen, J.-H.; Christensen, T. S.; Dybkjaer, I.; Seier Christensen, P.; Stub Nielsen, C.; Winter Madsen, S. E. L.; Rostrup-Nielsen, J. R. Technologies for Large-Scale Gas Conversion. Appl. Catal., A 2001, 221, 379−387. (64) Gennett, T.; Dillon, A. C.; Alleman, J. L.; Jones, K. M.; Parilla, P. A.; Heben, M. J. Laser Synthesis of Single-wall Carbon Nanotubes Utilizing High Temperature Induction Heating. Mater. Res. Soc. Symp. Proc. 2000, 633, A.2.3.1−A.2.3.6. (65) Biris, A. R.; Lupu, D.; Gruneis, A.; Ayala, P.; Rummeli, M. H.; Pichler, T.; Li, Z.; Xu, Y.; Misan, I.; Dervishi, E.; Biris, A. S. HighQuality Double-Walled Carbon Nanotubes Grown by a Cold-Walled Radio Frequency Chemical Vapor Deposition Process. Chem. Mater. 2008, 20, 3466−3472. (66) Lupu, D.; Biris, A. R.; Jianu, A.; Bunescu, C.; Burkel, E.; Indrea, E.; Mihailescu, G.; Pruneanu, S.; Olenic, L.; Misan, I. Carbon Nanostructures Produced by cCVD with Induction Heating. Carbon 2004, 42, 503−507. (67) Biris, A. R.; Biris, A. S.; Lupu, D.; Trigwell, S.; Dervishi, E.; Rahman, Z.; Marginean, P. Catalyst Excitation by Radio Frequency for Improved Carbon Nanotubes Synthesis. Chem. Phys. Lett. 2006, 429, 204−208. (68) Ramakrishnan, S.; Jelmy, E. J.; Dhakshnamoorthy, M.; Rangarajan, M.; Kothurkar, N. Synthesis of Carbon Nanotubes from Ethanol Using rf-CCVD and Fe−Mo Catalyst. Synth. React. Inorg., Met.Org., Nano-Met. Chem. 2014, 44, 873−876. (69) Piner, R.; Li, H.; Kong, X.; Tao, L.; Kholmanov, I. N.; Ji, H.; Lee, W. H.; Suk, J. W.; Ye, J.; Hao, Y.; Chen, S.; Magnuson, C. W.; Ismach, A. F.; Akinwande, D.; Ruoff, R. S. Graphene Synthesis via Magnetic Inductive Heating of Copper Substrates. ACS Nano 2013, 7, 7495− 7499. (70) Wu, C.; Li, F.; Chen, W.; Veeramalai, C. P.; Ooi, P. C.; Guo, T. Electromagnetic Induction Heating for Single Crystal Graphene Growth: Morphology Control by Rapid Heating and Quenching. Sci. Rep. 2015, 5, 9034. (71) Asencios, Y. J. O.; Rodella, C. B.; Assaf, E. M. Oxidative Reforming of Model Biogas Over NiO−Y2O3−ZrO2 Catalysts. Appl. Catal., B 2013, 132−133, 1−12. (72) Jang, W.-J.; Shim, J.-O.; Kim, H.-M.; Yoo, S.-Y.; Roh, H.-S. A Review on Dry Reforming of Methane in Aspect of Catalytic Properties. Catal. Today 2019, 324, 15−26. (73) Iulianelli, A.; Liguori, S.; Wilcox, J.; Basile, A. Advances on Methane Steam Reforming to Produce Hydrogen Through Membrane Reactors Technology: A Review. Catal. Rev.: Sci. Eng. 2016, 58, 1−35. (74) LeValley, T. L.; Richard, A. R.; Fan, M. The Progress in Water Gas Shift and Steam Reforming Hydrogen Production Technologies − A Review. Int. J. Hydrogen Energy 2014, 39, 16983−17000. (75) Rostrup-Nielsen, J.-R.; Sehested, J.; Nørskov, J. K. Hydrogen and Synthesis Gas by Steam- and CO2 Reforming. Adv. Catal. 2002, 47, 65− 139. (76) Pérez-Camacho, M. N.; Abu-Dahrieh, J.; Rooney, D.; Sun, K. Biogas Reforming Using Renewable Wind Energy and Induction Heating. Catal. Today 2015, 242, 129−138. (77) Sechovský, V., In Encyclopedia of Materials: Science and Technology; Buschow, K. H. J., Cahn, R. W., Flemings, M. C., Ilschner, B., Kramer, E. J., Mahajan, S., Veyssière, P., Eds.; Elsevier: Amsterdam, 2001; p 5021.

(43) The temperature of the inductively heated SPM NPs under steady-state conditions was approximately assumed as that of the external reactor surface, and it was measured using an IR pyrometer shot up on three different points (top, middle, bottom) of the reactor outer surface. The pyrometer provided the average reactor temperature, but it failed in measuring the real NP temperature. However, the use of a thermocouple positioned in contact with the catalytic bed was forbidden, as it was incompatible with the IH technology. (44) Garcia-Aguilar, J.; Fernandez-Garcia, J.; Rebrov, E. V.; Lees, M. R.; Gao, P.; Cazorla-Amorosa, D.; Berenguer-Murcia, A. Magnetic Zeolites: Novel Nanoreactors Through Radiofrequency Heating. Chem. Commun. 2017, 53, 4262−4265. (45) It should be noted that a single TiO2 shell around the NiFe2O4 core is critical for the successful preparation of the double-shell nanoreactors. Indeed, TiO2 is claimed by the authors as an essential linker to obtain NiFe2O4@TiO2@ZSM-5 for the present study. (46) Fuentes, M.; Magraner, J.; De Las Pozas, C.; Roque-Malherbe, R.; Pariente, J. P.; Corma, A. Cyclization of Citronellal to Isopulegol by Zeolite Catalysis. Appl. Catal. 1989, 47, 367−374. (47) Zadražil, A.; Š těpánek, F. Remote Control of Reaction Rate by Radiofrequency Heating of Composite Catalyst Pellets. Chem. Eng. Sci. 2015, 134, 721−726. (48) Sundaram, K. M.; Shreehan, M. M.; Olszewski, E. F. Kirk-Othmer Encyclopedia of Chemical Technology: Ethylene; Wiley: New York, 2010. (49) Houlding, T. K.; Gao, P.; Degirmenci, V.; Tchabanenko, K.; Rebrov, E. V. Mechanochemical Synthesis of TiO2/NiFe2O4 Magnetic Catalysts for Operation Under Rf Field. Mater. Sci. Eng., B 2015, 193, 175−180. (50) Liu, Y.; Gao, P.; Cherkasov, N.; Rebrov, E. V. Direct Amide Synthesis Over Core−Shell TiO2@NiFe2O4 Catalysts in a Continuous Flow Radiofrequency-Heated Reactor. RSC Adv. 2016, 6, 100997− 101007. (51) Montalbetti, C. A. G. N.; Falque, V. Amide Bond Formation and Peptide Coupling. Tetrahedron 2005, 61, 10827−10852. (52) Liu, Y.; Cherkasov, N.; Gao, P.; Fernández, J.; Lees, M. R.; Rebrov, E. V. The Enhancement of Direct Amide Synthesis Reaction Rate Over TiO2@SiO2@NiFe2O4 Magnetic Catalysts in the Continuous Flow Under Radiofrequency Heating. J. Catal. 2017, 355, 120− 130. (53) Asensio, J. M.; Miguel, A. B.; Fazzini, P.-F.; van Leeuwen, P. W. N. M.; Chaudret, B. Hydrodeoxygenation Using Magnetic Induction: High-Temperature Heterogeneous Catalysis in Solution. Angew. Chem., Int. Ed. 2019, DOI: 10.1002/anie.201904366. (54) Bordet, A.; Lacroix, L.-M.; Fazzini, P.-F.; Carrey, J.; Soulantica, K.; Chaudret, B. Magnetically Induced Continuous CO2 Hydrogenation Using Composite Iron Carbide Nanoparticles of Exceptionally High Heating Power. Angew. Chem., Int. Ed. 2016, 55, 15894− 15898. (55) Riedinger, A.; Guardia, P.; Curcio, A.; Garcia, M. A.; Cingolani, R.; Manna, L.; Pellegrino, T. Subnanometer Local Temperature Probing and Remotely Controlled Drug Release Based on AzoFunctionalized Iron Oxide Nanoparticles. Nano Lett. 2013, 13, 2399− 2406. (56) Chen, H.; Lee, J.; Zheng, Y.; Duan, Q. A Non-Traditional Energy Transfer Process in CWPO Heterogeneous Reaction for Wastewater Treatment. Chem. Eng. Res. Des. 2016, 114, 142−147. (57) Liang, L.; Astruc, D. The Copper(I)-Catalyzed Alkyne-Azide Cycloaddition (CuAAC) “Click” Reaction and its Applications. An Overview. Coord. Chem. Rev. 2011, 255, 2933−2945. (58) Ceylan, S.; Klande, T.; Vogt, C.; Friese, C.; Kirschning, A. Chemical Synthesis with Inductively Heated Copper Flow Reactors. Synlett 2010, 2010, 2009−2013. (59) Kupracz, L.; Hartwig, J.; Wegner, J.; Ceylan, S.; Kirschning, A. Multistep Flow Synthesis of Vinyl Azides and their Use in the CopperCatalyzed Huisgen-Type Cycloaddition Under Inductive-Heating Conditions. Beilstein J. Org. Chem. 2011, 7, 1441−1448. (60) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, B. A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed 7934


Perspective

ACS Catalysis (78) Frederikse, H. P. R. Handbook of Chemistry and Physics; Properties of magnetic materials; CRC Press: Boca Raton, FL, 2009. (79) Mortensen, P. M.; Engbæk, J. S.; Vendelbo, S. B.; Hansen, M. F.; Østberg, M. Direct Hysteresis Heating of Catalytically Active Ni−Co Nanoparticles as Steam Reforming Catalyst. Ind. Eng. Chem. Res. 2017, 56, 14006−14013. (80) Mortensen, P. M.; Ostberg, M.; Nielsen, P. E. Induction Heating of Endothermic Reactions. WO2017/036794 A1, 2017. (81) Vinum, M.; Mortensen, P. M. Ferromagnetic Materials for Induction Heated Catalysis. WO 2017/186608 Al, 2017. (82) Lartigue, L.; Innocenti, C.; Kalaivani, T.; Awwad, A.; del Mar Sanchez Duque, M.; Guari, Y.; Larionova, J.; Guérin, C.; Georges Montero, J.-L.; Barragan-Montero, V.; Arosio, P.; Lascialfari, A.; Gatteschi, D.; Sangregorio, C. Water-Dispersible Sugar-Coated Iron Oxide Nanoparticles. An Evaluation of their Relaxometric and Magnetic Hyperthermia Properties. J. Am. Chem. Soc. 2011, 133, 10459−10472. (83) Varsano, F.; Bellusci, M.; La Barbera, A.; Petrecca, M.; Albino, M.; Sangregorio, C. Dry Reforming of Methane Powered by Magnetic Induction. Int. J. Hydrogen Energy 2019, DOI: 10.1016/j.ijhydene.2019.02.055. (84) Almind, M. R.; Vendelbo, S. B.; Hansen, M. F.; Vinum, M. G.; Frandsen, C.; Mortensen, P. M.; Engbæk, J. S. Improving Performance of Induction-Heated Steam Methane Reforming. Catal. Today 2019, DOI: 10.1016/j.cattod.2019.05.005. (85) Khodakov, A. Y.; Chu, W.; Fongarland, P. Advances in the Development of Novel Cobalt Fischer−Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels. Chem. Rev. 2007, 107, 1692−1744. (86) Gahleitner, G. Hydrogen from Renewable Electricity: an International Review of Power-to-Gas Pilot Plants for Stationary Applications. Int. J. Hydrogen Energy 2013, 38, 2039−2061. (87) Götz, M.; Lefebvre, J.; Mörs, F.; McDaniel Koch, A.; Graf, F.; Bajohr, S.; Reimert, R.; Kolb, T. Renewable Power-to-Gas: a Technological and Economic Review. Renewable Energy 2016, 85, 1371−1390. (88) Centi, G.; Perathoner, S. CO2-Based Energy Vectors for the Storage of Solar Energy. Greenhouse Gases: Sci. Technol. 2011, 1, 21−35. (89) Li, W.; Wang, H.; Jiang, X.; Zhu, J.; Liu, Z.; Guo, X.; Song, C. A Short Review of Recent Advances in CO2 Hydrogenation to Hydrocarbons over Heterogeneous Catalysts. RSC Adv. 2018, 8, 7651−7669. (90) Leclercq, J.; Giraud, F.; Bianchi, D.; Fiaty, K.; Gaillard, F. Novel Inductively-Heated Catalytic System for Fast VOCs Abatement, Application to IPA in Air. Appl. Catal., B 2014, 146, 131−137. (91) Leclercq, J. Dépollution de l’Air Intérieur par Catalyse É conome en É nergie sur Catalyseurs en Film Mince Chauffés par leur Support Métallique; Université Claude Bernard, Lyon, France, 2013. (92) Rosensweig, R. E. Heating Magnetic Fluid with Alternating Magnetic Field. J. Magn. Magn. Mater. 2002, 252, 370−374.

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