(395al) CO2 Adsorption On Amine-Tethered MCM-41: Equilibrium Studies and Isotherm Modeling

Recently, amine-tethered mesoporous silica materials have attracted considerable attention as a promising chemical sorbent for post combustion CO₂ capture applications. In the present work, we explored the influence of pore size distribution for amine-tethering application in order to enhance the CO₂ equilibrium uptake. MCM-41 samples with two different pore sizes (30 nm and 3.5 nm) were synthesized by sol-gel method. Then, a mono-amino silane compound named as “3-aminopropyl triethoxy silane” was allowed to react with silanol (Si-OH) groups of the MCM-41 samples through silane coupling chemistry. The adsorption capacity measurements of CO₂ on to mono-amine tethered MCM-41 samples were measured at four different temperatures (30, 45, 60, 75 ⁰C) and pressures ranging from 0-5 bar using Rubotherm magnetic suspension balance. Interestingly, it was observed that MCM-41 sample with pore diameter of 30 nm functionalized with monoamine silane exhibited highest CO₂ uptake of 1.2 mmol/g at 1 bar pressure and 30 ⁰C, when compared to that of other sample (0.7 mmol/g). From the outcome of  CO₂ adsorption experiments, it can be seen that pore-expanded MCM-41 served as a promising support material for amine grafting applications. Ten adsorption-desorption cycles were conducted to check the stability of the adsorbent. No change in the performance of the adsorbent was noticed, indicative of reusable nature of the adsorbent and its applicability for multi-cycle operations. In addition to this, modeling for CO₂ uptake in amine-tethered MCM-41 was done using Freundlich and Toth isotherm models. The Freundlich model parameter n and Toth model parameters t and n significantly deviated from unity, indicative of heterogeneous nature of the surface. Surface heterogeneity is due to the presence of two active sites, which in turn indicates that CO₂ interacts with siloxane bridges through physisorption and amine groups through chemisorption. Isosteric heat of adsorption (~ΔH= 50 kJ/mol) calculated by Clasius-Clapeyrone equation revealed the dominance of chemisorption process in the current study.