Adsorption of CO2 on amine-functionalised MCM-41: experimental and theoretical studies

Abstract

Adsorption of CO₂ on MCM-41 functionalised with [3-(2-aminoethylamino)propyl]trimethoxysilane (MCM-41-N2), N¹-(3-trimethoxysilylpropyl)diethylenetriamine (MCM-41-N3), 4-aminopyridine (MCM-41-aminopyridine), 4-(methylamino)pyridine (MCM-41-methylaminopyridine) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (MCM-41-guanidine) was investigated. The amine-functionalised materials were characterised by ²⁹Si and ¹³C solid-state nuclear magnetic resonance, N₂ adsorption/desorption isotherms, X-ray diffraction and transmission electron microscopy. CO₂ adsorption at 1.0 bar and 30 °C showed that the amount of CO₂ (n_ads/mmol g⁻¹) adsorbed on MCM-41-N2 and MCM-41-N3 is approximately twice the amount adsorbed on MCM-41. For MCM-41-aminopyridine, MCM-41-methylaminopyridine and MCM-41-guanidine, the CO₂ adsorption capacity was smaller than that of MCM-41 at the same conditions. The proton affinity (computed with wB97x-D/6-311++G(d,p)) of the secondary amino groups is higher than that of the primary amino groups; however, the relative stabilities of the primary and secondary carbamates are similar. The differential heat of adsorption decreases as the number of secondary amino groups increases.