Selecting microporous materials for direct CO₂ capture from air and conversion into methane

A CO2 Direct Air Capture and Conversion (DACC) system has been designed and implemented at laboratory bench scale using up to 100 g of adsorbent material at atmospheric pressure trying to approach industrially relevant levels. The general objective is to process indoor air from buildings and other spaces having CO2-rich atmospheres compared to ambient air and subsequently couple the regeneration of the adsorbent with the conversion of the captured CO2 into methane via the Sabatier reaction. Several microporous adsorbent materials, including zeolites, activated carbon and metal-organic-frameworks (MOFs) were evaluated both at small scale in powder form and in a prototype system, focusing on their CO2 adsorption and desorption (regeneration) performance. Pelletized zeolite 5A proved to be the most effective sorbent investigated, achieving a breakthrough volume of 792 mL, a dynamic sorption capacity of 6.30 mgCO₂·gZeolite−1 and a saturation sorption capacity of 7.79 mgCO₂·gZeolite−1 for indoor air containing ca. 460 ppmV CO2. To valorize the captured CO2, desorption was performed using 10 mL·min−1 H2 stream at 200 ºC. The resulting CO2/H2 mixture was subsequently fed to a downstream fixed-bed catalytic reactor containing a Ru-based catalyst, where complete CO2 conversion to methane was achieved at 350 ºC. This prototype demonstrates a combined adsorption-catalytic conversion strategy for Power-to-Gas (PtG) applications employing microporous adsorbents and ambient CO2 as carbon source.