On the Influence and Role of Alkali Metals on Supported and Unsupported Activated Hydrotalcites for CO2 Sorption

To increase the CO2 capture capacity of hydrotalcites, the influence of alkali (K, Na) metal carbonate loading of activated supported and unsupported hydrotalcites (HTact) on their CO2 capture properties was investigated. The alkali-loaded supported hydrotalcites adsorb at 523 K, depending on the alkali metal (Na or K) and the preparation method, 1.7-2.2 mmol CO2 g(HT)(-1) which exceeds the capacity of unloaded supported HT (1.3 mmol CO2 g(HT)(-1)) and K-loaded unsupported HT (similar to 0.3 mmol.g(-1)). The key for the increase in capacity in alkali-loaded HT is a close contact at least at a mesoscopic level between HT and alkali metal carbonate. The alkali metal carbonate could be successfully introduced either by impregnation of a K2CO3 solution on as-synthesized HT or by leaving residual K (or Na), from the synthesis, in the final material. The latter method is advocated since it omits a washing step after precipitation. The increase in capture capacity for alkali loaded HTs points to a higher concentration of defects (low-coordination oxygen sites) on the surface of the activated alkali loaded HTs compared to the unloaded HT. We propose that the higher concentration of adsorption sites is caused by the presence of Na+/K+ on the surface of Mg(Al)O-x.