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A modeling and simulation study of a hydroxy sodalite (H-SOD) membrane reactor for in situ steam removal during CO2 methanation reaction is presented. A nonisothermal, steady-state, pseudohomogeneous 1D mathematical model with axial dispersion was proposed to describe and compare the performance obtained with traditional fixed-bed and membrane reactors. The model was validated against experimental data taken from the literature, first concerning CO2 conversion in a fixed-bed reactor and then regarding mixture separation factors through the membrane. The reactor’s performance was assessed considering the CO2 conversion and CH4 outlet purity under different operating conditions. The membrane selectivity toward H2 is more significant than that for the other species (i.e., CH4 and CO2). Hence, if hydrogen is used as sweep gas it can be co-delivered from the permeate side, which was found to significantly enhance the membrane reactor performance. The permeate-to-retentate pressure ratio (P ratio) increases the CO2 conversion but only up to an optimum value (P ratio*), which depends on the reaction conditions and feed composition. This holds true particularly when processing raw biogas: increasing the P ratio leads to a trade-off situation, where CO2 conversion increases but CH4 purity decreases. High water selectivity of the membrane enables a significant improvement compared to the traditional reactor performance, particularly at high temperature, pressure, and contact time, even if some reactants are lost to the permeate. Notably, the membrane reactor excels over the traditional fixed-bed reactor even at milder temperatures and pressures, which is particularly relevant for improving the overall energy efficiency of power-to-methane processes.

In this work, in-house synthesized NiMgAl, Ru/NiMgAl, and Ru/SiO 2 catalysts and a commercial ruthenium-containing material (Ru/Al 2 O 3 com. ) were tested for CO 2 methanation at 250, 300, and 350 • C (weight hourly space velocity, WHSV, of 2400 mL N,CO2 ·g −1 ·h −1 ). Materials were compared in terms of CO 2 conversion and CH 4 selectivity. Still, their performances were assessed in a short stability test (24 h) performed at 350 • C. All catalysts were characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), N 2 physisorption at −196 • C, inductively coupled plasma optical emission spectrometry (ICP-OES), and H 2 /CO chemisorption. The catalysts with the best performance (i.e., the hydrotalcite-derived NiMgAl and Ru/NiMgAl) seem to be quite promising, even when compared with other methanation catalysts reported in the literature.Extended stability experiments (240 h of time-on-stream) were performed only over NiMgAl, which was selected based on catalytic performance and estimated price criteria. This catalyst showed some deactivation under conditions that favor CO formation (high temperature and high WHSV, i.e., 350 • C and 24,000 mL N,CO2 ·g −1 ·h −1 , respectively), but at 300 • C and low WHSV, excellent activity (ca. 90% of CO 2 conversion) and stability, with nearly complete selectivity towards methane, were obtained. This is particularly relevant whenever the destination of methane is the injection into gas grid infrastructures, where the content of species like CO should be in accordance with natural gas specifications (typically a content up to 0.5 mol % can be tolerated (e.g., [8])). Hence, highly active and methane-selective catalysts for CO 2 methanation are required. In addition, catalyst stability under dynamic operation, i.e., with the capacity to withstand temperature variations, is also quite important and particularly relevant for application in PtM processes, where the reactor is operated intermittently and whenever surplus renewable power for H 2 production is available [6].Many metals have been tested for CO 2 methanation, for instance, Ni, Ru, Rh, Pd, and Co. Among these, ruthenium and nickel catalysts supported over various materials (e.g., Al 2 O 3 , SiO 2 , TiO 2 , CeO 2 , or ZrO 2 ) stand out [9,10]. Ruthenium-based catalysts have been reported in the literature, as well as in the catalogs of some catalyst suppliers (e.g., [11,12]), to be more suited for operation at low temperatures (T<200 • C), where CO formation is inhibited due to both restricted kinetics and the endothermic nature of the parallel RWGS reaction. On the other hand, nickel-based are the most widely investigated and commercialized catalysts for CO 2 methanation due to their high activity, availability, and low cost [4]. Improvement in their catalytic performance has been reported with hydrotalcite-derived Ni catalysts [13][14][15], as well as when combining nickel with ruthenium in the same bimetallic catalyst [16]. The use of hydrotalcite-derived Ni materials has also another important feature,...

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