Exploring the role of surface and porosity in CO2 capture by CaO-based adsorbents through response surface methodology (RSM) and artificial neural networks (ANN)

In this study, synthetic CaCO₃ materials were utilized as precursors for CaO-based CO₂ sorbents. The investigation examined how various operating parameters—such as synthesis temperature (ST), stirring rate (SR), and surfactant percentage (SP)—impact the properties of the adsorbents. Samples were firstly characterized by X-ray diffraction and Scanning Electron Microscopy (SEM), which revealed that the prevalence of calcite or aragonite crystal phases in the synthetic CaCO₃ precursors can be tuned by adequately choosing the dose of surfactant (Triton-X100®), so that it can be used as a crystal habit and growth modifier. The calcination process applied to the CaCO₃ precursors leads to the formation of partially sinterized cubic crystals of CaO, accompanied by minor quantities (< 5 %) of additional compounds like Ca(OH)₂ or CaSO₄. Specific surface area (S_BET) and porosity were determined by measuring the N₂ adsorption isotherms. A CaCO₃ sample with an unprecedented value of S_BET as large as 116 m²/g was prepared operating under optimal conditions. S_BET and pore volumes were successfully correlated with the CO₂ uptake capacity of the samples. S_BET is more influential for experiments carried out under diluted CO₂ atmosphere. When pure CO₂ is used, the influence of meso- and micropore volumes (V_me and V_mi) is clearly predominant, which suggests that in this latter case diffusion through the porous texture of the samples plays a more remarkable role. A double-way approach through Response Surface Methodology (RSM) and the use of Artificial Neural Networks (ANNs) has been used to analyze the CO₂ uptake capacity of the samples. Within the operational interval, excellent results were obtained for pure and diluted CO₂ flow, and RSM and ANNs have demonstrated to be a very efficient tool to correlate the behavior of the CaO-based materials as CO₂ sorbents with the surface area and pore volumes of the samples. Valuable information on (i) the importance of the different factors under study; (ii) their influence on the surface and porosity of the CaO-derived sorbents; and (iii) the subsequent CO₂ capture performance of the sorbents has been obtained. The results suggest that four parameters have a statistically significant influence on CO₂ uptake. These parameters are SR, the square of SR, its interaction with SP, and the square of SP. Additionally, the study assessed the stability of the CaO-based sorbents over 11 consecutive calcination-carbonation cycles. By adequately choosing the synthesis strategy and conditions, an almost negligible shrinkage effect can be achieved, resulting in a more sustained uptake capacity throughout the cycles.