CO2 and CH4 adsorption on periodic mesoporous organosilica: A DFT study

Abstract Periodic mesoporous organosilicas (PMO) were proposed as potential adsorbents for CO2/CH4 adsorption-separation due to their strong interactions with CO2 and low affinity for CH4. Herewith, we present a comprehensive density functional theory (DFT) study about the binding properties of CO2 and CH4 with the pore walls of different PMO materials. The calculations considered the M06-2X DFT exchange-correlation functionals, and cluster models of the walls of the PMO materials with organic bridges based on phenylene (Ph-), biphenylene (Bph-), pyridine (Py-) and bipyridine (Bpy-) fragments or on mixtures of Ph/Py- and Bph/Bpy- moieties. All materials showed stronger interactions with CO2 than with CH4, which suggest they are potentially selective for CO2 over CH4. From the calculated data it is demonstrated that the PMO materials with phenylene moieties establish more favorable contacts with CO2 than those found in their counterparts with bridges based on the homologous heteroaromatic nitrogen compounds, viz. pyridine. Independently of the size of the organic bridge, both phenylene- and biphenylene- PMO materials showed similar CO2 adsorption behavior. The presence of bipyridine-bridges improved the CO2 adsorption behavior only when mixed with biphenylene moieties. The adsorption behavior of CO2 gas can be directly related to the SiOH···OCO distance, i.e. the adsorption strength increases with the decrease of this distance. In the case of the CH4 adsorption, the lowest adsorption energies were calculated for PMO materials with pyridine and phenylene/pyridine bridges.