Amine-Grafted Silica Gels for CO₂ Capture Including Direct Air Capture

Abstract

A series of commercially available (and low-cost) silica gels were amine-grafted under dry conditions using N1-(3-trimethoxysilylpropyl)diethylenetriamine (i.e., “triamine”). CO2 capture performance, pore properties, and amine loading were investigated, and the results showed that the silica gel with the largest pores (150A) was the most suitable for further enhancement. Amine loading needed to be increased in order to further enhance the CO2 adsorption capacity. The addition of water during the grafting process was implemented on the 150A silica gel. The wet-grafted 150A silica gel exhibited a 2.3 mmol/g adsorption capacity at 75 °C and 1 bar, which is comparable to the best literature reported values that used similar synthesis methods. The wet-grafted 150A exhibited good cycle stability as well as fast CO2 uptake rates. The fixed-bed breakthrough capacity for air capture (ambient air with 415 ppm of CO2 at 25 °C) under dry (0.773 mmol/g) and wet (1.098 mmol/g) conditions are among the highest CO2 capacities for air capture. The capacity of the low-surface area silica gel (150A, S.A. = 309 m2/g) rivaled those of the best of the SBA and MCM type materials, which was unexpected. These results indicate that wet-grafted silica gel 150A is a promising and low-cost sorbent for direct capture of CO2 from ambient air and flue gas applications.