Effect of the method and impregnation time on the surface acidity of zirconia modified with boron

Abstract: In this paper, the synthesis and characterization of boron-modified zirconia is reported. The zirconium hydroxide obtained by sol-gel was modified with boric acid, in order to increase the surface acidity of the zirconia. For this purpose, two impregnation methods were used: conventional and ultrasonic, and the impregnation time was set at 1 or 4 h. The synthesized materials were characterized by thermal analyses, X-ray diffraction, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The boron incorporation delayed the crystallization of solids, which were mainly amorphous but with tendency toward the formation of the tetragonal phase. The modified materials were micro-mesoporous and their specific area was improved, while the pure zirconia showed a lower nitrogen adsorption. Units BO₃ and BO₄ were identified, confirming the boron presence. In general, the solids showed spherical particles; however, the ultrasonic treatment originated elongated particles. In addition, the acidic properties were evaluated by potentiometric titration and ethanol decomposition. The maximum acid strength of the zirconia increased from −28 mV to over 200 mV; consequently, all the boron-modified materials showed strong and very strong acid sites. The influence on the catalytic activity, due to the method and impregnation time, was clearly observed in the ethanol decomposition. The most active catalysts were those obtained by the ultrasonic method and the best was that impregnated during 1 h. Graphical Abstract: [InlineMediaObject not available: see fulltext.]