Development of carboxyl-layered double hydrotalcites of enhanced CO₂ capture capacity by K₂CO₃ promotion

Layered double hydroxides have been comprehensively studied as adsorbent precursors for CO₂ capture at 250–500 °C. Layered double hydroxides take advantages of partial reversibility by pressure change of adsorbate and good selectivity to CO₂ contributed by active basic sites. However, they suffer from comparatively lower CO₂ capture capacity, commonly 0.28–0.70 mmol/g in previous studies, compared with typical physical adsorbents like zeolites or chemical sorbents such as calcium oxide. In this paper, K₂CO₃ promoted n-caprylate (CH₃(CH₂)₆COO⁻) LDH and K₂CO₃ promoted stearate (CH₃(CH₂)₁₆COO⁻) LDH were synthesized by combining long carbon-chain carboxylic anion intercalation and K₂CO₃ promotion. K₂CO₃ surface promotion raised CO₂ capacity to 1.36 mmol/g for K-promoted magnesium–aluminum n-caprylate LDH and 1.97 mmol/g for K-promoted magnesium–aluminum stearate LDH respectively. Thermal gravimetric analysis was used to test CO₂ uptake up to 1000 kPa. It was found that adsorption kinetics fitted Elovich equation. X-ray diffraction was deployed to detect the crystal patterns of both precursors and adsorbents. In situ fourier transformation infrared spectroscopy and temperature programmed desorption were used to monitor the bonding between surface basic sites and adsorbate during adsorption and desorption process, in order to illustrate the adsorption mechanism of CO₂ on LDHs only by changing the partial pressure of CO₂ at the same operation temperature. Both novel LDHs show good potential for PSA application.