Anthropogenic CO₂ emissions are primarily responsible for the greenhouse effect and their prompt limitation is essential in containing global warming to safer levels. Among the various mitigation techniques, post-combustion capture setups may be installed onto pre-existing industrial or power plants with minimal disruption to operation, showing obvious potential for CO₂ emission reduction from stationary sources. However, the high temperatures and low CO₂ concentrations of flue gases mandate the choice of sorbents with good selectivity and thermal stability, in addition to suitable performance at elevated temperatures which excludes common solid adsorbents such as zeolites and activated carbons. In contrast, layered double oxides (LDOs) obtained through the thermal activation of hydrotalcite and other layered double hydroxides (LDHs) were shown to readily adsorb CO₂ at temperatures as high as 500°C making them promising candidates for post-combustion carbon capture processes.
In this study, porous geopolymer-hydrotalcite composite scaffolds with 40wt% active phase were produced by Direct Ink Writing and evaluated for CO₂ adsorption at 300°C. Geopolymers are attractive matrix candidates for composite adsorbents due to their low cost and facile synthesis route, good mechanical properties and significant porosity which improves access to the active phase. Furthermore, they can act as adsorbents themselves due to their mesoporous nature and moderate Specific Surface Area (SSA). Na-geopolymers (NaGPs) and K-geopolymers (KGPs) were characterised individually in terms of their textural properties and CO₂ capacity in order to select suitable candidates for the matrix of the composite adsorbents. KGPs displayed superior SSA and performance overall, however during optimisation of the printing slurry they were found to require the use of organic additives to obtain a stable dispersion of the hydrotalcite filler.
The composites were tested for CO₂ adsorption at 300°C after thermal activation at 400°C to induce the LDH-LDO transformation in the hydrotalcite active phase. Both NaGP- and KGP-based composites showed higher CO₂ capacity compared to the expected value for 40wt% hydrotalcite, indicating that the geopolymer matrices assume simultaneous structural and functional roles in the composites. The organic fraction in KGP-hydrotalcite composites could not be completely removed by heat treatment and resulted in a significant performance loss compared to the entirely inorganic NaGP-hydrotalcite formulation; moreover, the latter displayed substantially better mechanical properties. Scaffolds printed with finer struts were found to expose a larger geometric contact area resulting in higher performance, while simultaneously allowing for a better load distribution and mechanical strength. Regeneration strategies were evaluated in order to obtain a stable performance throughout several CO₂ adsorption cycles.