Microcalorimetric study of functionalized mesoporous silicas for CO2 capture under dry conditions

Abstract

CO2 adsorption on functionalized mesoporous silica was studied by microcalorimetry in order to investigate the influence of increasing the density of grafted amine and the type of functionalization on the CO2 capture mechanism. A Tian-Calvet microcalorimeter coupled to a manometric setup was used to evaluate the energy distribution of adsorption sites and calculate the thermokinetic parameters from the differential adsorption enthalpy curves. CO2 equilibrium adsorption isotherms were also measured at 25 °C for all samples up to 1 bar in order to know which grafted material has the highest CO2 adsorption capacity at 25°C contrasting it with a sample double functionalized in post-combustion scenario. Besides that, adsorption-desorption cycles were performed at 25 and 50°C and binary CO2/N2 isotherms at 50 and 75°C up to 10 bar. The adsorption calorimetric study suggests a change in active sites distribution as the density of grafted amines increases. The maximum thermokinetic parameter was calculated for the samples; 471 seconds for the pure silica at 30.7 kJmol-1 suggests that physisorption is the dominant binding mechanism. A different behavior occurs with the grafted samples, which have considerably higher enthalpy values corresponding to the formation of intermediate products (chemisorbed CO2), which depend on grafted amine density and available free surface silanols. The MSG60, chosen among the grafted samples, could be successfully regenerated at 120 °C, maintaining a constant adsorption capacity for 3 adsorption–desorption cycles using molecular vacuum at 50°C. The sample double functionalized presented these same good characteristics and also presented the API parameter for post-combustion process higher than MSG60; therefore, this type of functionalization probably would be more appropriate for this process under dry condition.