Microwave Heating-Assisted Catalytic Dry Reforming of Methane to Syngas

Natural gas is a robust and environmentally friendlier alternative to oil resources for energy and chemicals production. However, gas is distributed globally within shales and hydrates, which are generally remote and difficult reserves to produce. The accessibility, transportation, and distribution, therefore, bring major capital costs. With today’s low and foreseen low price of natural gas, conversion of natural gas to higher value-added chemicals is highly sought by industry. Dry reforming of methane (DRM) is a technology pathway to convert two critical greenhouse gas components, CH4 and CO2, to syngas, a commodity chemical feedstock. To date, the challenges of carbon deposition on the catalyst and evolution of secondary gas-phase products have prevented the commercial application of the DRM process. The recent exponential growth of renewable electricity resources, wind and solar power, provides a major opportunity to activate reactions by harnessing low-cost carbon-free energy via microwave-heating. This study takes advantage of differences in dielectric properties of materials to enable selective heating by microwave to create a large thermal gradient between a catalyst surface and the gas phase. Consequently, the reaction kinetics at the higher temperature catalyst surface are promoted while the reactions of lower temperature secondary gas-phase are reduced.


Methods
Development of the C-SiO2 receptors. The receptor particles were developed by induction heating-assisted chemical vapor (CVD) deposition of methane (99.92%) over silica sand substrates ( " = 2.6 g/cm 3 , " = 212-250 µm) 51 Table 1). The accelerator and sample temperature were adjusted at 1800 o C and 1400 o C, respectively. Detailed analyses on the morphologies of the samples prepared at various operating conditions were conducted by field emission scanning electron microscopy using SEM-FEG; model JSM-7600 TFE, JEOL, Japan (Extended Data Fig.   1). The SEM was performed in LEI imaging mod, operating distance of 15 mm and 5 kV.
Furthermore, focused ionized beam (FIB) milling (FIB; model 2000-A, Hitachi, Japan) was deployed to measure the carbon coating layer, whereas the samples were initially cavum coated with tungsten and later milled through in a rectangular area of 30x10 µm 2 at 20 kV. The milled samples were further investigated with the SEM-FEG, and the thickness of the coating layers was determined using the open source Image-J software (Extended Data Fig. 2). The composition of the coating layer was investigated by energy-dispersive X-ray spectroscopy (EDX; Oxford Instruments), and the results were investigated by the corresponding software (Extended Data Fig.   3 and Supplementary Table 2). Finally, the surface characterization of the coating layer was accomplished by X-ray photoelectron spectroscopic (XPS) analysis carried out on a VG scientific ESCALAB 3 MK II X-ray photoelectron spectrometer using a MG Kα source (15 kV, 20 mA).
Each scan was performed at pass energy of 100 eV and energy step size of 1.0 eV at 10-nanometer penetration depth (Supplementary Table 3).
MW heating performance. The microwave heating performance of the developed C-SiO2 receptor particles was executed in a single-mode microwave heating-assisted lab-scale fluidized bed reactor. The results were further compared with 50% and 90% graphite/sand samples (Fig. 1).
A water-cooled 2.5 kW, 2.45 GHz Genesys Systems microwave generator was employed to transmit the microwave radiation. The microwave radiation was directed to a 2.5 cm OD and 8 cm length tubular quartz reactor (Extended Data Figure 9). For each experiment, 30 gr of the sample was loaded to the quartz reactor, and the temperature was monitored with a K type thermocouple.
The receptor bed was fluidized with nitrogen in a bubbling regime and maintained by adjusting the superficial gas velocity through the experiments. The samples were heated up to 500 o C to determine the corresponding heating rate.

Temperature profile distribution.
A single-mode microwave heating-assisted lab-scale fluidized bed reactor was developed.
Microwave radiation was generated by a 2.5 KW and 2.45 GHz frequency water-cooled Genesys system magnetron and transferred to a 20-cm height and 2.24-cm ID quartz tube reactor (Extended Data Fig. 10). For each experiment, 30 g of the receptor samples were loaded to the reactor. The experiments were performed in 3.4 cm/s, 6.7 cm/s and 10 cm/s and 500, 600 and 700 o C superficial gas velocities and particle surface temperatures, respectively (Extended Data Figs. 5 and 6). The temperature measurement of the dielectric solid particles (C-SiO2) and bed bulk was performed using radiometry and thermometry methods, respectively, whereas a thermopile was calibrated to convert the voltage generated from the light capturing of the particles radiation exposed to microwave heating to a corresponding temperature. Due to the complex measurement of the gaseous components exclusively, the gas phase temperature was estimated with energy balance equation and empirical correlations developed by the attained experimental data (Fig. 2) 25 .
MW-assisted DRM. The DRM reactions were attained in a lab-scale single-mode microwaveheated fluidized bed reactor. The microwave radiation was generated by a 2.5 kW and 2.45 GHz frequency Genesys system magnetron with an internal water cooling mechanism and transferred to a 2.54-cm ID and 20-cm long fused quartz tube reactor (Extended Data Fig. 11). For each experiment, a mixture of 12 gr of the HiFUEL R110 (15-10% Ni, alumina supported) and 28 gr of the C-SiO2 receptor/promoter were loaded to the quartz reactor. The bed was fluidized by nitrogen with a superficial gas velocity of 3.3 cm/s to 10 cm/s depending on the system temperature to maintain a bubbling fluidization regime. The temperature of the solid particles was monitored with a thermopile. Due to the insufficient dielectric properties of the system components and the bed constituents, the C-SiO2 receptors mitigated for the heat generation inside the reactor, exclusively.
The gas flow was switched to a CO2/CH4=1:1 following the accomplishment of the operating temperature. The exhaust gas was continuously monitored by a Varian CP-4900 micro gas chromatographer (GC) to detect the volumetric fraction of each component. The reactions were terminated upon the complete deactivation of the catalyst active sites due to the extended carbon deposition (Extended Data Fig. 12).    The carbon, silicon and oxygen composition of the carbon coating layer for multiple receptor grades were investigated by EDX (Extended Data Fig. 3) to determine the uniformity of the coating layer accordingly.     The possible DRM reactions pathways according to the thermodynamic studies. Reaction (1) is the anticipated root to produce syngas, while reactions (2) to (17) deteriorate the quality of the final product by evolution of the secondary by-products or deactivation of the catalyst active sites through carbon deposition. The dielectric properties are a function of operating temperature frequency, whereas the frequency value is stationary at 2.45 GHz for microwave heating purposes. The dielectric constant, the ability of the dielectric material to absorb microwave radiation, does not project significant fluctuation while enhancing the operating temperature. However, the loss factor, the ability to dissipate the absorbed microwave radiation to heat, is enormously manipulated by the operating temperature, whereas the loss factor was enhanced by 100% in a temperature range of 25 o C to 1000 o C. It implies that the efficiency of the receptors could be doubled during the reaction period, hence enhancing the effect of the selective microwave heating mechanism.

Extended Data Figure 5 | Effect of the Operating Temperature on the Solids and Bulk
Temperature in the C-SiO2 Receptor Bed at = 6.7 cm/s. The particle surface temperature affects the solid phase and the bed bulk temperature gradient. Increasing the solid surface temperature causes the gradient value to expand, while the effect is similar to the superficial gas velocity (See Extended Data Fig 6.). Thus, operating at higher temperatures demonstrates a more prominent opportunity to advocate the effect of the microwave selective heating mechanism due to the evident larger temperature gradient. Superficial gas velocity dominantly influences the temperature gradient between the gas and solid phase. Due to the inability of the gases to adapt to the reactor temperature at lower residence times, the gradient expands by increasing the superficial gas velocity. Consequently, the application of higher superficial gas velocities demonstrated the effect of microwave selective heating mechanism on the productivity of the reactions extensively. While the gas enters the reactor at ambient temperature, due to insignificant dielectric properties it would not project microwave interaction. Concurrently, the catalyst active sites retain extremely high temperature due to the exceptional dielectric properties of the C-SiO2 receptors. While the reaction proceeds on the active site, the temperature of the products and the reactants drastically drops while leaving the catalyst surface. Consequently, the gas does not acquire sufficient energy to evolve secondary gas-phase reactions. Consequently, microwave selective heating mechanism promotes catalytic reactions while restricting the production of secondary undesired by-products, simultaneously.

Extended Data Figure 8 -Induction Heating-Assisted Fluidized Bed CVD Experimental
Setup. The induction heating mechanism, while generated by renewable electricity resources, provides an esteemed opportunity to develop environmental friendly and economically feasible chemical reactions. The CVD reactions were successfully performed in the present induction heating-assisted fluidized bed reactor or develop multiple grades of C-SiO2 microwave receptors. The induction heating setup facilitated a comprehensive evaluation of the effect of the CVD operating conditions on the quality and performance of the developed particles.