Low-temperature high-pressure cryogelation of nanooxides

High-pressure cryogelation (HPC) of individual (silica, alumina, their mechanical blends) and complex (silica/titania, alumina/silica/titania, AST) nanooxides were studied to reveal the influence of nanooxide composition and gelation conditions on changes in the textural characteristics of the materials. The temperature–pressure behavior of different phases (silica, alumina, and titania) under HPC can result in destroy of complex nanoparticles, especially large (100–200 nm in size) core–shell secondary nanoparticles composed of a number of small primary nanoparticles (~10 nm in size) covered by a shell destroyed under HPC. The main changes in the texture of HPC or gelled nanooxides are due to rearrangement (compaction) of secondary structures such as aggregates of primary nanoparticles and agglomerates of aggregates. This compaction enhances the pore volume but much weakly affect the specific surface area (with one exception of AST) because small primary nanoparticles (10–20 nm in size) are relatively stable during HPC. The HPC materials are characterized by enhanced mesoporosity shifted to macroporosity with decreasing specific surface area and increasing size of primary nanoparticles. Diminution of the freezing temperature from 208 to 77.4 K during HPC results in an increase in pressure (up to 1,050 atm in stainless steel freezing bombs) and enhanced compaction of aggregates and agglomerates but this does not practically affect the primary nanoparticles. The degree of decomposition of secondary core–shell nanoparticles of AST does not practically increase with this decreasing freezing temperature. High-pressure cryogelation can destroy core–shell nanoparticles.