CO 2 /N 2 /CH 4 adsorption on pseudo-boehmite: Effective synthesis, adsorption experiments, and DFT simulation

In this study, a simple, cost-effective, and environmentally friendly synthesis method of high-purity, bayerite-free pseudo-boehmite (PB) using sodium aluminate and nitric acid was developed for application in CH4, N2, and CO2 adsorption. The effects of pH and temperature on PB’s structure and pore characteristics were examined using FTIR, ESEM, XRD, and BET analyses. Optimal conditions were found at pH 8 and 85°C, resulting in the specific surface area of 336.56 m2/g and the pore volume of 0.65 cm3/g. Gas adsorption measurements were conducted at 0–8 bar and 30–50°C. The results revealed CO2 uptakes of 0.63 mmol g−1 (30°C, 8 bar) and ideal selectivities of 4.5 (CO2/N2) and 5.9 (CO2/CH4). Among several isotherm models evaluated, the Freundlich isotherm was identified as the most appropriate model, providing both good statistical agreement and physically meaningful parameters. The Freundlich constants indicate favorable adsorption (n > 1) and a stronger affinity of PB toward CO2 compared to CH4 and N2, consistent with surface heterogeneity and multilayer adsorption behavior. The isosteric heat of adsorption, supported by DFT simulations (≈ −34 kJ mol−1), confirms that the adsorption process is physical in nature. Other thermodynamic parameters, ΔG, ΔS, and ΔH, were also calculated. Overall, this work demonstrates that the controlled precipitation of NaAlO2 produces a reproducible, bayerite-free PB with enhanced CO2 affinity and competitive separation selectivity for CO2, N2, and CH4.