Gas-solid contactors and catalytic reactors with direct microwave heating: current status and perspectives

Microwave heating (MWH) transforms energy from an electromagnetic wave to heat. In contrast to conventional heating (CH) mechanisms that use slower heat transfer processes via conduction, convection or radiation, microwaves (MW) directly interact with MW susceptor materials and induce a rapid conversion of the electromagnetic energy into heat. This rapid heating provides MWH with distinct features that can be leveraged to increase conversion, selectivity and/or energy efficiency of chemical processes. Here we discuss recent significant advances reported in MWH processes involving gas-solid interactions. Special attention is devoted to key aspects such as the methodologies to accurately determine local temperatures under the influence of electromagnetic (EM). Other relevant aspects such as the consideration of the solid catalyst dielectric properties or the design of novel gas-solid contactor configurations will be discussed. Emerging aspects such as the potential of MWH to minimize secondary by-products in high temperature reactions or to efficiently perform in transient processes, e.g. adsorption-desorption cycles, are highlighted. Finally, current challenges and perspectives towards a wide application of MWH in gas solid contactors will be critically discussed

Ethylene epoxidation in microwave heated structured reactors

In the present work we show the microwave-induced heating of monolithic reactors containing a thin-layered catalyst that exhibits a strong and selective heating susceptibility under microwave irradiation. The combination of microwave radiation and structured reactors has been successfully applied for the intensification of the selective oxidation of ethylene to ethylene oxide (epoxidation) while operating at lower power consumptions and with higher energy efficiencies than in conventional heating conditions. The microwave radiation selectively heats the catalyst and the monolith walls while maintaining a relatively colder gas stream thereby creating a gas/solid temperature gradient of up to ~70 °C at a reaction temperature of 225 °C. Moreover, the influence of different parameters such as the distribution of the catalyst onto the structured monoliths or the temperature measurement techniques employed to determine the heating profiles (Optic Fibers and/or IR thermography) have been also thoroughly evaluated to justify the obtained catalytic results

Non-oxidative methane conversion in microwave-assisted structured reactors

The main problem to be addressed in the valorization of methane under non-oxidative conditions (MNOC) is to reduce or even avoid coke formation. In this work we report the use of microwave-assisted heating for MNOC. We have developed a system able to heat-up a Mo-ZSM5 catalyst coated on silicon carbide monolith that could operate stable for at least 19¿h at reaction conditions, 700°C. We demonstrate that under MW-heating the selectivity shifts to C2s and benzene. In contrast, the operation under conventional heating (CH) produces more coke and polyaromatics. The selective microwave heating has two effects in this reaction: i) during the activation of the catalyst the formation of the active catalytic species of Mo2C inside the microporous support is different affecting the selectivity and product distribution; ii) a gas-solid temperature gradient is established that prevents the formation of coke from PAHs in the gas phase. The MNOC process under controlled MW heating at high space velocity (3000¿mL/gcat·h) gives a hydrocarbon yield of around 6% with a very low deactivation rate. These results open up new possibilities for process intensification using alternative sources of energy, as is the case of microwaves, for heating structured catalytic reactors

Laser-Assisted Production of Carbon-Encapsulated Pt-Co Alloy Nanoparticles for Preferential Oxidation of Carbon Monoxide

C-encapsulated highly pure PtxCoy alloy nanoparticles have been synthesized by an innovative one-step in-situ laser pyrolysis. The obtained X-ray diffraction pattern and transmission electron microscopy images correspond to PtxCoy alloy nanoparticles with average diameters of 2.4 nm and well-established crystalline structure. The synthesized PtxCoy/C catalyst containing 1.5 wt% of PtxCoy nanoparticles can achieve complete CO conversion in the temperature range 125–175°C working at weight hourly space velocities (WHSV) of 30 L h−1g−1. This study shows the first example of bimetallic nanoalloys synthesized by laser pyrolysis and paves the way for a wide variety of potential applications and metal combinations

A Nanoarchitecture Based on Silver and Copper Oxide with an Exceptional Response in the Chlorine-Promoted Epoxidation of Ethylene

The selective oxidation of ethylene to ethylene epoxide is highly challenging as a result of competing reaction pathways leading to the deep oxidation of both ethylene and ethylene oxide. Herein we present a novel catalyst based on silver and copper oxide with an excellent response in the selective oxidation pathway towards ethylene epoxide. The catalyst is composed of different silver nanostructures dispersed on a tubular copper oxide matrix. This type of hybrid nanoarchitecture seems to facilitate the accommodation of chlorine promoters, leading to high yields at low reaction temperatures. The stability after the addition of chlorine promoters implies a substantial improvement over the industrial practice: a single pretreatment step at ambient pressure suffices in contrast with the common practice of continuously feeding organochlorinated precursors during the reaction

Block copolymer based novel magnetic mixed matrix membranes-magnetic modulation of water permeation by irreversible structural changes

This contribution focuses on understanding the effect of magnetic field intensity on the performance of novel hydrophilic and hydrophobic mixed matrix membranes (MMMs). The hydrophilic MMMs were made up of polymeric nanoparticles (PNPs) that were synthesized through polymerization-induced self-assembly (PISA) and iron oxide nanoparticles prepared in presence of poly (methacrylic acid)-b-poly quaternized (2-dimethylamino)ethyl methacrylate. The hydrophobic MMMs were prepared by the addition of iron oxide nanoparticles with different surface properties to a linear poly (methacrylic acid)-b-poly (methylmethacrylate) diblock copolymer dissolved in tetrahydrofuran (THF). Three different types of hydrophilic membranes were prepared with polymeric nanoparticles of different morphologies (spherical, vermicular and vesicular). In case of the hydrophobic membranes, six different membranes containing different iron oxide core coated with different stabilizers such as poly (methacrylic acid), quaternized poly(2-dimethylamino)ethyl methacrylate and meso-2, 3-dimercaptosuccinic acid were prepared. An external magnetic field with intensity values up to 1.15 T was used for the permeation studies and the results were compared with those obtained in the absence of magnetic field. The collected data indicate an increase in the water flux of up to 16% and 29% under the magnetic field for hydrophobic and hydrophilic membranes, respectively. The STEM analyses suggest that the magnetic nanoparticles move within the membrane structure during the application of the magnetic field. This displacement/rearrangement causes constant changes in the membrane structure (structure of the active layer) and consequently on the membrane permeability. These results suggest that the application of the magnetic field could be used as a pretreatment step to obtain high flux membranes

Supercritical solvothermal synthesis under reducing conditions to increase stability and durability of Mo/ZSM-5 catalysts in methane dehydroaromatization

Natural gas is currently envisioned as a potential energy and hydrocarbon feedstock in the forthcoming years. To overcome the detrimental flaring of this natural gas and the partial release of its major component, methane, novel and more effective strategies are required. These include the development of new, efficient and seemingly stable catalysts able to rapidly convert methane into valuable feedstocks. We show a novel synthesis method of Mo/ZSM-5 based on a solvothermal synthesis under supercritical conditions and reducing atmosphere (SC-STSE) to improve metal dispersion and enhance catalyst stability and durability during the methane dehydroaromatization (MDA) reaction. In contrast to the conventional impregnation method, SC-STS-E provides a superhigh atom-like metal dispersion at the zeolite pores resulting in the most stable Mo/ZSM-5 catalyst for MDA with the highest long-term hydrocarbon yield (xCH4=11.6% and yC2+ = 8.9%, after 15 h on stream) among the catalysts reported in literature for this reaction

Polyoxometalates as alternative Mo precursors for methane dehydroaromatization on Mo/zsm-5 and Mo/mcm-22 catalysts

The conversion of methane into higher molecular weight hydrocarbons of greater added value has emerged as one of the grand challenges of the 21st century. The non-oxidative methane dehydroaromatization (hereafter MDA) reaction is a promising methane valorisation reaction since it transforms methane into added-value aromatics and olefins, namely benzene, naphthalene and ethylene. Molybdenum-promoted ZSM-5 zeolite has proven to be one of the most effective catalysts for MDA providing a shape-selective environment for the conversion of methane into benzene. However, one of the principle disadvantages of using aluminosilicates in the presence of methane is that the catalyst suffers from rapid deactivation induced by coke formation, which ultimately leads to a decrease in activity and aromatics selectivity, making the process unsuitable for large-scale industrial applications. Better control of the metal dispersion on the surface of the aluminosilicate supports represents a crucial factor to partially suppress catalyst coking and improve stability. Here we show how different molecular polyoxomolybdate (POM) anions can be used as alternative Mo precursors to conventional Mo salts for the preparation of catalysts for the MDA reaction. Molecular dynamics simulations and experimental testing were conducted to characterize the interphase interactions between polyoxomolybdates and zeolite surfaces at the atomistic level and to evaluate the MDA performance of different POM-based catalysts supported on ZSM-5 and MCM-22, respectively. The catalysts prepared using hexamolybdate anions, [Mo6O19]2-, were found to be more active and selective towards benzene than those employing the commercial heptamolybdate, [Mo7O24]6-. The Mo loading and dispersion of MoOx species were found to be the key factors leading to enhanced catalytic stability on ZSM-5 and MCM-22-based supports for MDA where the 5% Mo6/MCM-22 catalyst provided a constant aromatics yield above 7% for more than 18 hours time-on stream operating at 700 °C with a diluted methane flow under atmospheric pressure. The zeolitic catalysts prepared with the Mo6 precursor were found to be amongst the most promising MDA catalysts in the literature and the results of this study pave the way for the selection and use of different POMs as innovative metal precursors to formulate new catalysts and further improve the MDA reaction process

Numerical analysis of microwave heating cavity: Combining electromagnetic energy, heat transfer and fluid dynamics for a NaY zeolite fixed-bed

[EN] Three-dimensional mathematical model was developed for a rectangular TE10n microwave heating cavity system, working at 2.45 GHz. Energy/heat, momentum equations were solved together with Maxwell's electromagnetic field equations using comm. MULTIPHYSICS (R) simulation environment. The dielectric properties, epsilon' and epsilon '', of NaY zeolite (Si/Al = 2.5) were evaluated as a function of temperature. Considering these values, the microwave heating of a porous fixed-bed made of dry NaY zeolite was simulated. Electric field distribution, axial and radial temperature profiles and temperature evolution with time were obtained. The zeolite fixed bed was heated up to 180 degrees C in 5 min, with 30 W power. The fixed-bed temperature evolution under non-steady state conditions showed the same trend as the one observed experimentally with only an average deviation of 10.3%. The model was used to predict microwave heating of other materials improving energy efficiency of the microwave cavity. Furthermore, the developed model was able to predict thermal runaway for zeolites.Financial support from the European Research Council ERC-Advanced Grant HECTOR-267626 is gratefully acknowledged. Hakan Nigar acknowledges financial support from the Spanish Ministry of Education for the FPU grant (Formacion del Profesorado Universitario - FPU12/06864), and also for the academic short stay grant (Estancia Breve - FPU2016) at the Delft University of Technology, Delft, The Netherlands.Nigar, H.; Sturm, GSJ.; García-Baños, B.; Penaranda-Foix, FL.; Catalá Civera, JM.; Mallada, R.; Stankiewicz, A.... (2019). Numerical analysis of microwave heating cavity: Combining electromagnetic energy, heat transfer and fluid dynamics for a NaY zeolite fixed-bed. Applied Thermal Engineering. 155:226-238. https://doi.org/10.1016/j.applthermaleng.2019.03.117S22623815

Microfluidic devices as gas – Ionic liquid membrane contactors for CO2 removal from anaesthesia gases

This work proposes a microfluidic gas – ionic liquid contactor for CO2 removal from anaesthesia gas, containing Xe. The working principle involves the transport of CO2 through a polymer flat membrane followed by its capture and enzymatic bioconversion in the ionic liquid solvent. Microfluidic devices enable a rapid and inexpensive screening of potential CO2 absorbers. The alveolar – type design of the ionic liquid chamber was adopted to reduce mass transfer limitations of CO2 through the liquid phase. Polydimethylsiloxane (PDMS) was the chosen polymer for dense membrane, as well as for the microfluidic device fabrication, mainly due to the high permeability of gases, O2 and CO2, and low cost. The selected ionic liquid was cholinium propionate (CP) with a water activity of 0.753, due to its high affinity towards CO2 and biocompatibility with the enzyme used for CO2 conversion to bicarbonate, carbonic anhydrase (CA). The CO2 and Xe permeability and CO2/Xe selectivity were determined in the microfluidic devices developed and compared to those exhibited by free standing PDMS membranes mounted on a standard permeation cell. The performance of the microfluidic devices as gas – ionic liquid contactors was evaluated for a given solvent flow rate with pure gas streams of CO2 and Xe. The obtained results show that cholinium propionate with or without the enzyme has no effect on the Xe transport, but remarkably enhances the affinity towards carbon dioxide leading to enhancement factor up to 1.9 in the presence of 0.1 mg CA/gIL