Carbon dioxide adsorption over amine modified silica: Effect of amine basicity and entropy factor on isosteric heats of adsorption

SBA-15 mesoporous silica was modified with 3-aminopropyl (AP), N-propyl ethylenediamine (DA) and N-propyl diethylenetriamine (TA) ligands. The prepared materials were characterized by thermogravimetry (TGA), small angle X-ray scattering (SAXS), nitrogen adsorption/desorption and transmission electron microscopy (HRTEM). The carbon dioxide adsorption/desorption experiments were performed at 273 K, 293 K, 313 K and 333 K. The adsorption capacity of CO2 at pressures below 2 kPa well correlates with number of amine centers in the respective ligand. Based on the adsorption isotherms, measured at different temperatures, the isosteric heats of adsorption (Qst) were calculated. The Qst values correlate with the basicity of the nitrogen atoms of respective amine ligands and their steric availability. The AP and DA ligands with the higher basicity of amine nitrogen showed the higher isosteric heats of adsorption at zero coverage (Qst = 72 kJ/mol), while TA ligand, with the lower basicity and steric availability of nitrogen atoms exhibited at zero coverage the lowest value of isosteric heat of adsorption (Qst = 40 kJ/mol). This low value shows, that in addition to amine basicity, also the entropic factor may influence the differences in isosteric heats of adsorption. While the mono- and diamine ligands loaded in the pores of SBA-15 are flexible enough to react with the carbon dioxide, the lower isosteric heats of adsorption of carbon dioxide over the TA ligands may be due to unfavorable entropic factor associated with filling of the pores of SBA-15 by bulkier TA ligands. Steric hindrance among TA ligands may result in lower flexibility of TA ligands and unfavor the formation of carbamates through intramolecular reaction in the pores of SBA-15.