SiO2-Amine Materials from Geothermal Silica Waste with Potential Application in Direct CO2 Air Capture

Abstract Direct CO2 air capture (DAC) has become a key strategy for tackling climate change, as it can help reduce the concentration of this greenhouse gas (GHG) in the atmosphere, which, as of July 2026, has risen to 429.12 ppm. Most DAC techniques are based on adsorption processes, in which sorbents must have high adsorption capacity, good selectivity, easy regeneration, low energy penalty, suitable adsorption–desorption kinetics, excellent chemical stability, and low cost. Amine functionalization of porous materials has been proposed to improve CO2 adsorption capacity and selectivity, and using waste to obtain economically viable adsorbents has attracted interest. Thus, this research aims to develop an efficient SiO2-amine adsorbent for direct CO2 capture from air using geothermal silica waste. The silica waste comes from two Mexican geothermal power plants. Silica waste was characterized by XRF, XRD, and N2 adsorption–desorption, then functionalized with TEPA, PEHA, and PEI by wet impregnation. The resulting SiO2-amine materials were characterized by FT-IR and TG. In addition, CO2 capture tests were carried out at 25, 30, and 35 °C with 400 ppm of CO2. CO2 uptake ranged from 0.17 to 0.65 mmol CO2/g, with the CT-TEPA material achieving the highest value at 30 °C (0.65 mmol CO2/g). The kinetic analysis showed that the nonlinear PSO model adequately described the temporal evolution of CO2 capture. Stability evaluation over 10 consecutive adsorption–desorption cycles shows a 3.09% mass loss, mainly due to amine decomposition during desorption. This study systematically evaluates the influence of waste origin, surface treatment, and amine type on adsorbent performance under representative DAC conditions, offering an alternative strategy for developing CO2 capture materials from locally available waste resources.