Revealing the correlation between the performance of silica-based DAC adsorbents and their pore natures

Direct air capture (DAC) technology is a potential carbon negative process, which can effectively reduce the emission of carbon dioxide (CO 2 ) in the atmosphere. Amine-functionalized porous materials are promising because of their high ratio of accessible amino groups and fast adsorption/desorption kinetics. This work reveals the correlation between the specific surface area, pore radius, and pore volume of the silica-based DAC adsorbents and their CO 2 adsorption capacity and adsorption rate. Four typical silica-based DAC adsorbent materials loaded with polyethylenimine (PEI) were studied, including hydrophilic fumed silica (HFS), Santa Barbara Amorphous-15 (SBA-15), Mobil Crystalline Materials-41 (MCM-41), and silica gel (SG). It is observed that when the PEI loading is constant, adsorbent materials with larger specific surface area, larger pore radius, and larger pore volume tend to show the higher adsorption capacity and faster adsorption-desorption rate. Humidity is beneficial for the CO 2 capture capacity, but it slows down the CO 2 uptake rate at the same time. The capacity of 50%PEI@SBA-15 adsorbent can reach to 3.24 mmol/g under 90% relative humidity (RH). Supports with small pore radius tend to have unfavorable performance at dry conditions. After the inlet gas is humidified, the CO 2 capacity of the MCM-41 material is enhanced greatly, from 0.91 mmol/g to 2.92 mmol/g. This work provides guiding principle for synthesizing appropriate adsorbents with higher DAC capacity, higher adsorption and desorption rate at ambient conditions.