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

• Published in: Adsorption : journal of the International Adsorption Society Vol. 26; no. 2; pp. 165 - 175
• Main Authors: Sánchez-Zambrano, K. S., Vilarrasa-García, E., Maia, D. A. S., Bastos-Neto, M., Rodríguez-Castellon, E., Azevedo, D. C. S.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01-02-2020; Springer Nature B.V
• Subjects: Adsorption; Amines; Binding; Carbon dioxide; Carbon sequestration; Chemisorption; Chemistry; Chemistry and Materials Science; Density; Engineering Thermodynamics; Enthalpy; Free surfaces; Grafting; Heat and Mass Transfer; Industrial Chemistry/Chemical Engineering; Low temperature; Organic chemistry; Parameters; Reaction kinetics; Silicon dioxide; Surface chemistry; Surfaces and Interfaces; Thin Films; APTES-grafted silica; Thermokinetic parameter; adsorption; Microcalorimetry; CO
• ISSN: 0929-5607; 1572-8757
• DOI: 10.1007/s10450-019-00064-y

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 CO
2
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 CO
2
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 CO
2
), which depend on grafted amine density and available free surface silanols. Kinetics of formation of chemisorption products, together with hindered CO
2
diffusion, seriously impair the approach to equilibrium, which leads to a decrease in uptake upon isothermal pressure-swing cycles at low temperatures (25 °C). CO
2
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.