Mechanical and Microstructural Properties of Two-Step Acid-Base Catalyzed Silica Gels

Abstract

ABSTRACTThe mechanical and microstructural properties of two-step acid-base catalyzed silica gels were examined as functions of aging time, catalyst concentration, and hydrolysis time. Cylindrical gels were prepared using Si(OC2H5)4, C2H5OH, and H2O, with HCl followed by NH3 as catalysts. Mechanical properties were obtained from three-point bend tests, and the microstructures of dried gels were analyzed using nitrogen adsorption/desorption techniques. Gel strength initially increased with aging time at 70 °C, then leveled off after about one week. When the sol was hydrolyzed for less than two hours, there were significant differences in the properties of gels catalyzed with relative molar amounts of 0.0001 and 0.0002 HCl. However, as the hydrolysis time was increased, the gels all had similar properties, independent of the amount of HC1. The amount of NH3 influenced gelation time and to a lesser extent, the strength, but had no observable effect on pore size. The two-step catalysis procedure produced gels with strength and pore size combinations intermediate to those of either single acid or base-catalyzed gels.