We investigated the effect of sodium cation modification on the acid-base and adsorption properties of alumina synthesized via centrifugal thermal activation of gibbsite. Two methods of modification were compared: post-synthesis impregnation of finished granules and in situ incorporation of sodium cations during the hydration stage of the precursor. The resulting adsorbents were characterized using X-ray diffraction (XRD) and low-temperature nitrogen adsorption. Their acid-base properties were assessed using a pH-metric method and X-ray photoelectron spectroscopy (XPS). All of the samples obtained consisted of low-temperature modifications, had a specific surface area above 200 m2/g, and were characterized by the presence of small mesopores (2–15 nm). The surface basicity of the alumina samples was found to increase directly with the concentration of introduced sodium, as quantified by the isoionic point pH (pHiip). A strong correlation was observed between the XPS data and the pH-metric method results, confirming the applicability of XPS for evaluating the acid-base characteristics of modified adsorbents. The experimental data on the effect of introducing sodium into alumina on water vapor adsorption were compared with previous results obtained by researchers using various modification methods. The results demonstrated that dynamic water vapor adsorption capacity is influenced by the characteristics of Al2O3 and the method of sodium introduction. The most effective method for modifying aluminum oxide is introducing it at the hydration stage during synthesis. The results demonstrate the possibility of controlling the adsorption properties of alumina by varying the concentration of sodium cations and the synthesis conditions.
Meshcheryakov et al. (Sun,) studied this question.