Block-copolymer-templated ordered mesoporous silica: Array of uniform mesopores or mesopore-micropore network?

Microporosity and connectivity of ordered mesopores of SEA-15 silica were s tudied using nitrogen adsorption and novel methods based on selective pore blocking via organosilane modification, and on the imaging of inverse plati num replica of ordered mesoporous structure. It was found that SEA-15 exhib its a relation between the pore size, pore volume and specific surface area which is significantly different from that for cylindrical or hexagonal po res, which suggests that the SEA-15 structure is more complex than an array of hexagonally ordered channels, even if they are corrugated. Nitrogen and argon adsorption measurements provided evidence that large mesopores are a ccompanied by a certain amount of significantly smaller pores (of the size below about 3.4 nm) with a broad distribution primarily in the micropore/sm all-mesopore range. The modification of SEA-15 via chemical bonding of smal l trimethylsilyl ligands partially blocked the complementary pores, and the bonding of larger octyldimethylsilyl groups made them essentially: fully i naccessible to nitrogen molecules, which manifested itself in dramatic chan ges in the relation between the pore size, pore volume, and specific surfac e area. After dissolution of the SEA-15 framework, platinum wires grown ins ide the porous structure formed bundles, as seen from transmission electron microscopy. These results provided strong and unambiguous evidence that la rge ordered mesopores of SEA-15 are accompanied by much smaller disordered pores and that an appreciable fraction of the latter is located in the pore walls, providing connectivity between the ordered large-pore channels. The complementary pores are suggested to form as'. result of penetration of po ly(ethylene oxide) chains of the triblock copolymer template within the sil ica framework of as-synthesized SBA-15. We also studied thermal stability o f SEA-15 structure and its complementary porosity. As inferred from nitroge n adsorption data, the complementary porosity was retained to a significant extent even after calcination at 1173 K, but most likely completely disapp eared at 1273 K. The heat treatment was accompanied not only by a significa nt decrease in the specific surface area and pore volume but also by narrow ing the pore size distribution at temperatures up to 1173 K. Thus, we were able to demonstrate for the first time that the SBA-15 sample with nitrogen adsorption properties similar to those of MCM-41 can be obtained via calci nation at 1273 K, although the pore volume and specific surface area of suc h SEA-15 material is relatively low.