The 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
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.