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The cyclic process of CO2 adsorption and CH4 hydrogenation on dual function materials (DFMs) has shown increasing interest in recent years. This form of operation eliminates the costly CO2 purification step required for methanation with continuous CO2 and H2 feed. The DFMs present a CO2 adsorbent compound and a metal that assists its methanation. The objective of the work is to modulate the basicity of a DFM through the joint presence of Ca and Na to extend the temperature window with high CH4 production.

Five DFMs identified as Ru-xNa/yCa (x%/y%=0/16, 4/12, 8/8, 12/4, 16/0) were prepared on g‑Al2O3 as support. The synthesis was performed by sequential wet impregnation, first the adsorbent material. The physico-chemical properties of the DFMs were determined by N2 adsorption-desorption, H2 chemisorption, XRD, H2-TPR, CO2 TPD and TEM. All temperature programmed experiments were monitored with a Hiden Analytical HPR-20 EGA mass spectrometer. The catalytic activity was evaluated in a vertical tubular reactor, with downward flow. Prior to analysis, the sample was reduced in a stream composed of 10% H2/Ar at 400 °C for 60 min. CO2, CH4, CO and H2O concentrations were continuously monitored by on-line FTIR, during the CO2 storage/capture period (10% CO2/Ar, 1 min) followed by its hydrogenation/methanation (10% H2/ Ar, 2 min), with an intermediate Ar purge. The experiments were carried out with a flow rate of 1200 ml min-1 (45000 h‑1), in the temperature range 280-400 °C.

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The incorporation of Ca, Na and Ru reduces the specific surface of the DFMs compared to g‑Al2O3 due to the decrease in the amount of mesoporous solid (g-Al2O3) and, to a lesser extent, by the partial blocking of the pores of smaller size. Values ​​between 104-111 m2 g-1 were obtained. In the XRD spectra of the calcined DFMs, a seen with peaks corresponding to RuO2 and nitrogenous compounds remaining from the precursor salts. However, for the already reduced DFMs, only peaks belonging to metallic Ru were observed. This confirms that the reduction treatment at 400 °C, prior to the activity tests, is sufficient to obtain the DFMs with the desired phases, as can also be deduced from the H2-TPR results.

The joint presence of Na and Ca in the DFM favours the Ru dispersion. The distribution of basic sites was studied by CO2-TPD. Weak basicity predominates in the DFM containing only Na and strong basicity in the DFM with only Ca. In the DFMs with both adsorbents, medium basicity predominates, presenting the highest DFM Ru-8Na/8Ca. Therefore, by adding a given Na/Ca distribution, the basicity strength of DFMs can be modulated.

The catalytic activity in cycles of CO2 adsorption and hydrogenation to CH4 presents different trends with the operating temperature according to the Na/Ca ratio. The Na presence favours the CH4 production at intermediate temperatures (310 °C), while the Ca presence favours production at high temperatures (400 °C), which correlates with the basicity results. Importantly, the presence of both adsorbents, in general, boosts the CH4 production in the entire temperature range studied (280-400 °C). The DFM Ru-8Na/8Ca, achieved higher activity and selectivity. The improved ruthenium dispersion and the presence of a greater number of basic sites with medium strength boost the CO2 adsorption and hydrogenation to CH4, and enhances the contact between the carbonates and the metal sites, which favours a fast kinetics of CH4 production. 

Project summary by: Bermejo-López, B. Pereda-Ayo, J.A. Onrubia-Calvo, J.A. González-Marcos, J.R. González-Velasco Department of Chemical Engineering, Faculty of Science and Technology, Universidad del País Vasco UPV/EHU, Barrio Sarriena, s/n, Leioa, Bizkaia 48940, Spain

Paper Reference: “Tuning basicity of dual function materials widens operation temperature window for efficient CO2 adsorption and hydrogenation to CH4Journal of CO2 Utilization 58 (2022) 101922. https://doi.org/10.1016/j.jcou.2022.101922

Hiden Product: HPR-20 EGA

Reference: AP-HPR-20-202289

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