Three-dimensional molecular basket sorbents for CO₂ capture: Effects of pore structure of supports and loading level of polyethylenimine

Three-dimensional (3-D) mesoporous materials including mesocellular siliceous foam (MCF), MSU-J and hexagonal mesoporous silica (HMS) were examined as supports of "molecular basket" sorbents (3-D MBS) by loading CO₂-philic polyethylenimine (PEI). The CO₂ sorption performance of the 3-D MBS was evaluated in comparison with the MBS by using MCM-41, SBA-15 and carbon black (CB) as the supports. The effect of PEI loading on the sorption capacity is associated with the sorption temperature and pore structure of the support. At 30 wt% PEI loading, the increase in temperature from 30 to 75 °C has a slight and even negative effect on the sorption capacity; while at 65 wt% PEI loading, it has a significant, positive effect. Superior CO₂ sorption capacity and sorption rate of 3-D MBS over 2-D and 1-D MBS were observed. MCF-based MBS with 65 wt% PEI loading (PEI(65)/MCF) gave the highest CO₂ sorption capacity of 201 mg-CO ₂/g-sorb. The maximum PEI loading for MCF was up to 80 wt%, which is the largest among the support materials studied in this work, and is related to its largest pore volume. The highest sorption capacity and sorption rate of PEI(65)/MCF are ascribed to its largest pore size and unique 3-D pore structure, which facilitate the CO₂ diffusion, promote mass transfer and offer more accessible sorption sites. The present work demonstrates that the 3-D mesoporous solid amine sorbents are more effective for CO₂ capture in comparison with 1-D and 2-D materials in terms of higher sorption capacity and faster sorption rate. The pore structure (pore dimension, pore size, pore volume) of the support, PEI loading and temperature are the three key factors that determine the sorption capacity and sorption rate.