272481 Carbon Dioxide Adsorption On Mesoporous Amorphous Silica Surfaces with Tethered Propylamine Groups
Using a combination of classical and ab initio modeling methods, we study adsorption of carbon dioxide and nitrogen on amorphous silica grafted with propylamine chains at moist conditions that may be found in post-combustion gases. We model physisorption and chemisorption aspect separately, using classical and ab initio tools, respectively, and consider different chemisorption mechanisms leading to formation either carbamates or bicarbonates. The carbamate mechanism requires two amine groups per each CO2 molecule chemisorbed, while bicarbonate formation requires a single amine. Thus, bicarbonate formation is desired from the practical point of view, but favorable only when reactants are surrounded by water molecules. Earlier experimental results indicated bicarbonate mechanisms; however, a few recent studies insist on the carbamate mechanism of CO2 chemisorption.
Ab initio modeling shows that activation barriers are rather low for both chemisorption mechanisms. Therefore, in these systems, physisorption mechanisms strongly influence chemisorption mechanisms, because physisorption precedes chemisorption. Since published experimental results on water sorption at amine-grafted silica vary dramatically, and we modeled CO2 physisorption at various levels of hydration using classical molecular dynamics and grand canonical Monte Carlo methods. Generally, we conclude that the physisorption scenarios are unfavorable to bicarbonate formation at all but highest hydration levels, which are not typically observed in experiments. In particular, water resides closer to the silica surface than CO2. CO2 molecule in the process of chemisorption reaction act is typically surrounded by alkylamines that are known to favor carbamate mechanism.