High temperature CO2 sorption using mixed oxides with different Mg/Al molar ratios and synthesis pH

Abstract

Scheme of post- and pre-combustion processes for CO 2 capture at high temperature using hydrotalcite-derived mixed oxides as CO 2 sorbents. • Several hydrotalcite-derived mixed oxides were prepared to be used as CO 2 sorbents. • Different Mg/Al molar ratios and synthesis pH were used to prepare the materials. • The CO 2 sorption isotherms were determined at 300 °C for screening purposes. • 8.5HT7 showed a high CO 2 sorption capacity: 1.62 mmol/g @ p CO 2 = 1 bar and 300 °C. • The effect of steam over successive CO 2 sorption–desorption cycles was assessed. Several hydrotalcites were prepared by the co-precipitation method with different Mg/Al molar ratios (from 2 to 20) and synthesis pH (8.5 and 10); upon calcination, the hydrotalcite-derived mixed oxides (HDMOs) were obtained, to be used as CO 2 sorbents at high temperature, for instance in post- or pre-combustion CO 2 capture processes. The sorption equilibrium isotherms were determined for screening purposes of all materials prepared using a magnetic suspension microbalance (static experiments) at 300 °C, along with a detailed physicochemical characterization (TG-DTG, ICP-OES, XRD, FTIR, physical sorption–desorption of N 2 at −196 °C and TPD-CO 2 ). In addition, the most promising HDMOs, in terms of CO 2 sorption capacity, were submitted to successive CO 2 sorption–desorption cycles (dynamic experiments) in a packed column at 300 °C, with and without water in the feed, to assess their potential in a sorption–desorption cyclic operation. The results indicate that a synthesis pH of 8.5 provides samples with better CO 2 sorption capacities and that there is an optimum Mg/Al molar ratio (between 4 and 10). The sample with a synthesis pH of 8.5 and a Mg/Al molar ratio of 7 (called 8.5HT7) provided an outstanding CO 2 sorption capacity (1.62 mmol·g −1 at 1 bar of CO 2 and 300 °C) when compared with the other samples herein prepared, but also when compared with commercial (MG63 and MG70) and with other similar materials found in the literature. Finally, an average working capacity of 1.04 mmol·g −1 and 2.11 mmol·g −1 was obtained after repeated sorption–desorption cycles at 0.4 bar of CO 2 and 300 °C for the 8.5HT7 HDMO under dry and wet conditions (45 vol.% H 2 O), respectively.