Adsorption microcalorimetry as a tool in the characterization of amine-grafted mesoporous silicas for CO2 capture

In this work, amine-grafted mesoporous silica were investigated using a adsorption microcalorimetry and supplementary techniques in order to study the influence of increasing the density of grafted amine on the CO2 binding mechanisms. A Tian–Calvet microcalorimeter coupled to a manometric setup was used to evaluate the energetic heterogeneity of adsorption sites and to calculate the thermokinetic parameter from the differential enthalpy curves. With such device, equilibrium adsorption isotherms of CO2 were simultaneously measured at 25 °C for all samples up to 1 bar. The adsorption microcalorimetric study suggests a change in active sites distribution as the density of grafted amines increases. The maximum thermokinetic parameter of 471 s for the pure silica support at 30.7 kJ mol−1 suggests that physisorption is the dominant binding mechanism. A different behavior occurs with the grafted samples: they have considerably higher enthalpy values corresponding to the formation of reacted species on the surface (chemisorbed CO2), which depend on grafted amine density and available free surface silanols. Kinetics of formation of chemisorption products, together with hindered CO2 diffusion, seriously impair the approach to equilibrium, which leads to a decrease in uptake upon isothermal pressure-swing cycles at low temperatures (25 °C). CO2 could be successfully desorbed at 120 °C from the sample with highest uptake (MSG60), which showed a constant uptake for three adsorption–desorption cycles using only vacuum at 50 °C.