Mid-temperature CO2 Adsorption over Different Alkaline Sorbents Dispersed over Mesoporous Al2O3

Abstract

CO₂ adsorbents comprising various alkaline sorption active phases supported on mesoporous Al₂O₃ were prepared. The materials were tested regarding their CO₂ adsorption behavior in the mid-temperature range, i.e., around 300 °C, as well as characterized via XRD, N₂ physisorption, CO₂-TPD and TEM. It was found that the Na₂O sorption active phase supported on Al₂O₃ (originated following NaNO₃ impregnation) led to the highest CO₂ adsorption capacity due to the presence of CO₂-philic interfacial Al-O^--Na⁺ sites, and the optimum active phase load was shown to be 12 wt % (0.22 Na/Al molar ratio). Additional adsorbents were prepared by dispersing Na₂O over different metal oxide supports (ZrO₂, TiO₂, CeO₂ and SiO₂), showing an inferior performance than that of Na₂O/Al₂O₃. The kinetics and thermodynamics of CO₂ adsorption were also investigated at various temperatures, showing that CO₂ adsorption over the best-performing Na₂O/Al₂O₃ material is exothermic and follows the Avrami model, while tests under varying CO₂ partial pressures revealed that the Langmuir isotherm best fits the adsorption data. Lastly, Na₂O/Al₂O₃ was tested under multiple CO₂ adsorption-desorption cycles at 300 and 500 °C, respectively. The material was found to maintain its CO₂ adsorption capacity with no detrimental effects on its nanostructure, porosity and surface basic sites, thereby rendering it suitable as a reversible CO₂ chemisorbent or as a support for the preparation of dual-function materials.

Figure 1

CO₂ breakthrough curves for different alkaline compounds dispersed over Al₂O₃. Adsorption conditions: 0.5 g adsorbent, 50 mL/min of 1 vol % CO₂/Ar flow, T = 300 °C.

Figure 2

(a) N₂ adsorption–desorption isotherms with pore size distribution (inset) for the Al₂O₃ support and NaNAl sorbent. (b) XRD patterns for the different alkaline compounds dispersed over Al₂O₃. (c,d) CO₂-TPD profiles with (d) focus on the temperature region below 500 °C.

Figure 3

CO₂ breakthrough curves for Na₂O/Al₂O₃ with different Na₂O loads. Adsorption conditions: 0.5 g adsorbent, 50 mL/min of 1 vol % CO₂/Ar flow, T = 300 °C.

Figure 4

(a) N₂ adsorption–desorption isotherms and pore size distribution (inset) for Na₂O/Al₂O₃ with different Na₂O loads. (b) XRD patterns. (c,d) CO₂-TPD profiles with (d) focus on the temperature region below 500 °C.

Figure 5

TEM images of the (a) Al₂O₃ support and the (b) Na6Al and (c,d) Na12Al sorbents.

Figure 6

CO₂ breakthrough curves for NaAl at different temperatures. Adsorption conditions: 0.5 g adsorbent, 50 mL/min of 1 vol % CO₂/Ar flow.

Figure 7

Comparison of the observed CO₂ uptake values and the fitted ones via the PSO and Avrami equations for NaAl at 1 atm, 1 vol % CO₂ concentration and at different adsorption temperatures.

Figure 8

Effect of adsorption temperature on the Avrami’s kinetic constant for NaAl using the modified Arrhenius equation as a regression function.

Figure 9

CO₂ breakthrough curves for NaAl at different CO₂ feed concentrations. Adsorption conditions: 0.5 g adsorbent, T = 300 °C, 50 mL/min of CO₂/Ar flow at different concentrations.

Figure 10

CO₂ adsorption isotherm for NaAl obtained from the experimental CO₂ adsorption values under different CO₂ initial feed concentrations at 300 °C, along with the theoretically obtained values from the fitted Langmuir, Freundlich and Sips equations.

Figure 11

(a) CO₂ breakthrough curves and (b) CO₂ adsorption capacity values in mg/g for NaAl after multiple adsorption–desorption cycles (0.5 g adsorbent). Adsorption conditions: 50 mL/min of 10 vol % CO₂/Ar flow at 300 °C for 15 min. Desorption conditions: 50 mL/min of Ar flow at 500 °C for 15 min.

Figure 12

(a) N₂ adsorption–desorption isotherms with pore size distribution (inset) for the fresh NaAl adsorbent, as well as after the first and tenth cycle. (b) XRD patterns for the NaAl adsorbent undergoing various treatments. (c,d) CO₂-TPD profiles with (d) focus on the temperature region below 500 °C. (e,f) TEM images of the NaAl adsorbent following (e) the 1st and (f) the 10th adsorption–desorption cycles.