Methanation is a potential large-scale option for CO₂ utilization, and it is one of the solutions for decreasing carbon emission and production of synthetic green fuels. However, the CO₂ conversion is limited by thermodynamics in conventional reaction conditions. However, around 100 % conversion can be obtained using sorption enhanced CO₂ methanation according to Le Chatelier's principle, where water is removed during the reaction using zeolite as a sorbent. In this work 5%Ni5A, 5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic and water adsorption properties were tested in a fixed bed reactor. The overall performance of the bifunctional materials decreased on going from 5%Ni2.5%Ce13X, 5%Ni13X, 5%Ni5A, to 5%NiL. The CO₂ conversion and CH₄ selectivity were approaching 100 % during prolonged stability testing in a 100 reactive adsorption – desorption cycles test for 5%Ni2.5%Ce13X, and only a slight decrease of the water uptake capacity was observed.

Zeolite 13X and 5A supported Ni catalysts were synthesized for CO₂ methanation using the evaporation impregnation method. The influence of using different Ni precursors (nitrate, citrate, and acetate) as well as calcination temperatures on the catalyst properties and performance were investigated. XRD, SEM-EDX, TEM, STEM-EDX, N₂ physisorption, H_2-TPR, TPD-NH₃ and TG/DTA were used for detailed characterization of the catalysts. The parent structure of the zeolites did not change during catalyst synthesis. Using nickel citrate and acetate resulted in smaller NiO particle size compared to nitrate. STEM-EDX results showed that all the Ni-precursor complexes entered more efficiently the 13X zeolite structure, which is mainly due to steric hindrance resulting from the smaller pore size of 5A. Methanation experiments revealed that the 13X catalysts synthesized using nickel citrate (5% Ni) displayed clearly higher activity, compared to the catalysts synthesized using nickel nitrate or nickel acetate. A 79% conversion at 320 °C was obtained with 100% selectivity towards CH₄ and the catalyst showed excellent stability during 200 h testing. Overall, it can be concluded that the Ni precursor significantly influences the physico-chemical characteristics and catalytic properties of Ni 13X and Ni 5A zeolite catalysts in CO₂ methanation: complex size and pore size matter.

Zeolites 13X and 5A were modified with nickel and/or ruthenium for CO₂ methanation. The catalysts were prepared by evaporation impregnation and XRD, SEM-EDX, TEM, STEM-EDX, nitrogen physisorption, H₂-TPR and NH₃-TPD were used to characterize the physico-chemical properties of the catalysts. The physico-chemical characterization results show that the zeolites structure did not change after the Ni, Ru modification, however. Ni was able to enter the pores of 13X, in the other case, 5A, an egg shell type structure was formed. Methanation experiments were performed in a lab scale fixed bed reactor system, the results showed that the mono-metallic catalysts out-performed the bi-metallic ones with Ni being the more active. One of the factors influencing the performance of the bi-metallic catalysts was the difficulty to obtain good dispersion when both metals were used. Also the morphology of the catalyst significantly influenced the selectivity. The catalysts with lower weak acidity benefit for getting a higher activity. The single metal catalysts 2.5 %Ru13X and 5%Ni13X showed good catalytic stability with around 97 % CH₄ selectivity at 360 °C, with no catalyst deactivation during the 200 h catalyst stability test.