Macroporous Monoliths with Modified Compositions and Structures
Prof. Kazuki Nakanishi
Division of Materials Research, Institute of Materials and Systems for Sustainability, Nagoya University; Institute for Integrated Cell-Material Sciences, Kyoto University Institute for Advanced Study
https://www.icems.kyoto-u.ac.jp/en/people/1421/
Utilizing the sol-gel process accompanied by phase separation, monolithic materials with controlled macropores have been synthesized in a variety of chemical compositions. In most cases, oxides of corresponding metals in the precursor is the final phase comprising the solid component of resultant porous materials. There are, however, various possibilities of modifying chemical compositions and structures using externally added and/or initially embedded components via appropriate treatments enhancing dissolution-reprecipitation, reduction-oxidation, and crystallization. Examples are shown below adopting typical mother compositions of macroporous gels.
Macroporous pure silica gels in the presence of solvent exhibit appreciable reactivity for reorganization of micropores into mesopores through partial dissolution and reprecipitation under weakly basic conditions. Hierarachically meso/macroporous silica monoliths have widely been used as separation media of HPLC as well as bioreactors and DNA purification devices. Macroporous hydridosilica with abundant Si-H groups on pore surfaces can accommodate noble-metal nanoparticles via on-site reduction of individual metal ions and their aggregation into fine nanoparticles. Macroporous wet pure titania can be converted to various titanates with perovskite structure by incorporating alkaline earth oxides. 2D structured titanium phosphates are grown on the titania particles with similar compositions to those for macroporous monolith by hydrothermal reactions with phosphoric acid. Fully crystallized titania are subjected to forced reduction by heating in a evacuated closed vessel with metallic zirconium as an oxygen getter to give various Magneli phases with regular lattice defects and appreciable electric conductivity. By incorporating second/third components to mother alumina composition, various aluminate phases, e.g. spinel, garnet, mullite, cordierite and LDHs are crystallized by calcination without damaging the macropore structures. External addition of silica source together with appropriate template molecule precipitates zeolite crystals onto the macropore surface.
Physical aggregation of colloidal particles, gelation of water glass on neutralization, as well as purely organic polymerization, have been known to be applied to the macropore formation process by phase separation with well-defined structures. Recent developments include reactions between coordinating ligands and metal ions. Aggregation and gelation by multi-valent carboxylic acids, often used as structural components of metal-organic frameworks (MOFs), can also combined with phase separation to give well-defined macroporous monoliths of oxides containing divalent metals that are otherwise hard to be fabricated into monolith. Well-defined trimodal pore distribution is obtained by combining macro/mesopores and highly ordered microporous arrays of crystalline MOFs. HKUST-1 precipitated on macropore surfaces of copper hydroxide is additional example of external modification with coordinating molecules.