Stabilization of n-aminopropyl silica gel against hydrolysis by blocking silanol groups with TiO2 or ZrO2

Silanol groups of n-aminopropyl silica gel (APSG) were blocked with TiO2 or ZrO2 to produce Ti-APSG and Zr-APSG, respectively. The silica materials were characterized by infrared, Raman spectra, thermogravimetric, elemental analyses, magic angle spinning C-13-nuclear magnetic resonance, specific surface area measurements, pH-metric titration and inductively coupled plasma-optical emission spectrometry-monitored silica hydrolysis. The stability of APSG against hydrolysis was found to be mainly affected by the specific surface area and the basicity of the n-aminopropyl groups which acquire additional strength from their intramolecular interaction with the silanol groups. The hydrolysis of silica in Ti-APSG and Zr-APSG remarkably decreased in the range of pH 1.0-9.1 due to the interruption of that intramolecular interaction. The hydrolyzed silica of Ti-APSG and Zr-APSG was decreased to 22.7 and 29.9%, respectively, of that of APSG at pH 4.5. Capacity and stability of Ti-APSG and Zr-APSG were improved in comparison with APSG upon application in the extraction of Cu2+ for 20 cycles of extraction and regeneration.