Hydrogen produced by the electrolysis of water using sustainable electricity will play an increasingly important role as an energy and a feedstock vector. Shifting from fossil to renewable resources means that new industrial platforms have to be set up to provide carbon-based fuels and bulk base chemicals to replace the current fossil resources based routes. The global demand cannot be met by indirect use of carbon dioxide via biomass necessitating the use from point sources or direct air capture, which changes the value of CO2 from waste to commodity chemicals. The production of chemicals by hydrogenation of CO2 is typically hampered by the thermodynamic conversion being far from 100% under currently viable reaction conditions. The equilibrium can, however, be shifted to increase conversion by removing one of the reaction products, namely water, from the reaction mixture with sorbents like zeolites. Prerequisite to conversion enhancement and process intensification is the close proximity of sorption and catalytic sites. This review presents the state of the art in synthesis and application of these, in fact, bifunctional materials.

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Chapter 1 is the introduction, which presents thestate of the art in synthesis and application of these, in fact, bi-functionalmaterials for sorption enhanced CO2 methanation.    In Chapter 2, zeolite 13X and 5A supported Ni catalysts wereutilized, which synthesized using the evaporation impregnation method. Theinfluence of using different Ni precursors (nitrate, citrate, and acetate) aswell as calcination temperatures on the catalyst properties and performancewere investigated. Using nickel citrate and acetate resulted in smaller NiOparticle sizes compared to nitrate. Methanation experiments revealed that the13X catalysts synthesized using nickel citrate displayed clearly higheractivity, compared to the catalysts synthesized using nickel nitrate or nickelacetate.    Chapter 3 describes zeolites 13X and 5A that weremodified with nickel and/or ruthenium for CO2 methanation. The results showed that Ni wasable to enter the pores of 13X, in the other cases an egg shell type structurewas formed. Methanation experimental results showed that the mono-metalliccatalysts outperformed the bi-metallic ones with Ni being the more active. Oneof the factors influencing the performance of the bi-metallic catalysts wasthat it was difficult to obtain good dispersion when both metals were present. Thecatalysts with lower weak acidity displayed higher activity. The catalyst2.5%Ru13X and 5%Ni13X showed good catalytic stability with around 97% CH4 selectivity at 360 °C, with no catalystdeactivation during a 200 h catalyst stability test.    Chapter 4 deals with sub-nanometer zeolite13X-supported Ni-ceria catalysts for CO2 methanation. Ce loading affected thecatalysts’ metal dispersion, reducibility, basicity and acidity, and hencetheir activity and selectivity. STEM-EDX elemental mappings showed that Ce andNi were predominantly highly dispersed. Ce had a positive effect on thereduction of NiO and lead to a relatively high number of medium basic siteswith a low Ce loading. Highly stable 5%Ni2.5%Ce13X displayed high activity andnearly 100% CH4 selectivity in CO2 methanation at 360 °C, which was mainlyattributed to the high dispersion of metals and relatively high amount ofmedium basic sites.In Chapter 5, a long-term experimental study employing 5%Ni5A,5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic andwater adsorption properties was performed in a fixed bed reactor. The overallperformance of the bifunctional materials decreased going from 5%Ni2.5%Ce13X,5%Ni13X, 5%Ni5A, to 5%NiL. The highest obtained CO2 conversion and CH4 selectivity were close to 100 % duringprolonged stability testing in 100 reactive adsorption – desorption cyclesamounting to 203 hours in total with 5%Ni2.5%Ce13X. Chapter 6 focuses on determining the kinetics of anickel on zeolite 13X catalyst in comparison with a nickel catalyst on ameso-porous γ-Al2O3 support. In this chapter, the validity ofthe obtained rate equation is discussed. The results showed that 13X zeolitesupported nickel catalyst was more active compared to the one supported on γ-Al2O3. This is mainly due to a better dispersion ofnickel on the 13X zeolite catalyst.    Finally, Chapter 7 provides the overall conclusions of thestudies reported in this thesis. Recommendations for further research are alsoprovided.@en

Sub-nanometer zeolite 13X-supported Ni-ceria catalysts were synthesized for CO₂ methanation. XRD and SEM results show the structure and morphology of 13X zeolite after impregnation and calcination. Ce loading affected the catalysts’ metal dispersion, reducibility, basicity and acidity, and thence their activity and selectivity. STEM-EDX elemental mappings showed that Ce and Ni are predominantly highly dispersed. Ce has a positive effect on the reduction of NiO and leads to a relatively high number of medium basic sites with a low Ce loading. Highly stable 5%Ni2.5%Ce13X had high activity and nearly 100% CH₄ selectivity in CO₂ methanation at 360 °C, which is mainly due to the high dispersion of metals and relatively high amount of medium basic sites. It can be inferred that this catalyst synthesis strategy has great potential for good catalyst dispersion, since metal uptake by the zeolite is selective for the metal citrate complexes in solution.

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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.

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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.

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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.

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