Characterization of regular and plugged SBA-15 silicas by using adsorption and inverse carbon replication and explanation of the plug formation mechanism

A series of SBA-15 silicas was synthesized using triblock copolymer templates with the same length of poly(propylene oxide) block (POm; m = 70) and different average lengths of poly(ethylene oxide) blocks (EOn, n = 17-28). The average EOn length was varied by mixing EO20PO70EO20 copolymer with either EO5PO70EO5 or EO106PO70EO106 copolymer, whereas the tetraethyl orthosilicate (TEOS)/EO-unit molar ratio was kept constant. In addition, samples with the higher ratios were synthesized for the EO20PO70EO20 template. In all cases, 2-D hexagonally ordered SBA-15 silicas with interconnected primary pore structure (as inferred from carbon inverse replication) were obtained, but higher TEOS/EO-unit molar ratios resulted in the formation of samples with plugged pore structures, which were recently reported by others and referred to as plugged hexagonal templated silicas (PHTSs). Nitrogen adsorption data showed that n equal to about 19 was optimal from the point of view of formation of high-pore-volume, large-pore SBA-15 structure. All of the replicas exhibited structures with high specific surface area and pore volume and tended to have a similar pore diameter but more optimized SBA-15 structures tended to afford better ordered inverse carbon replicas. Regular and PHTS SBA-15 samples were additionally studied using argon adsorption at 77 K, which provided an important insight into the plugged structure. In particular, a PHTS sample that on the basis of nitrogen adsorption at 77 K appeared to be fully plugged, that is, with all primary mesopore channels closed by finely porous plugs, was found to be only partially plugged based on argon adsorption at 77 K, although there was evidence for the presence of some constrictions in all of the channels. A mechanism of the plug formation at high TEOS/copolymer molar ratios is proposed on the basis of similarity of the PHTSs with as-synthesized SBA-15 subjected to postsynthesis modification with TEOS. It is proposed herein that when the TEOS/EO-unit molar ratio is excessively high, only a part of TEOS present initially interacts with the EOn blocks of the copolymer template and thus hydrolyzes and condenses faster to form the SBA-15 structure. Subsequently or concurrently, the remaining TEOS, which hydrolyzes and condenses more slowly, solubilizes in the copolymer template in the SBA-15 pores (perhaps in the poly(propylene oxide) core of the micelles), or displaces the template, and then condenses to form the plugs.