Entrance size analysis of silica materials with cagelike poer structure by thermoporometry

Abstract

Ordered mesoporous materials offer many potential applications in catalysis, chromatography, and drug delivery. If the pores form cages rather than straight channels, the entrance size to these cages is a crucial parameter but notoriously difficult to asses. For example, classical physisorption techniques are limited by forced closure of the hysteresis loop. Here we apply thermoporometry by differential scanning calorimetry (DSC) of confined water to quantify the entrance sizes in a series of mesoporous silica materials with cagelike pore structure, i.e. SBA-16 and FDU-12. With DSC, entrance sizes of materials with cages of up to 15 nm were determined which could not be assessed by nitrogen physisorption and were validated by argon physisorption at 77 K. For cage sizes exceeding 15 nm, entrance sizes were quantified by thermoporometry that were inaccessible by either of the physisorption techniques. In addition, the size distribution of the widest-entrance path toward the cages was determined by applying a step-equilibration method with DSC, providing essential structural information beyond the limits of physisorption with N2 or Ar at 77 K. We show that thermoporometry extends the range of classical physisorption techniques and is a powerful tool for the determination of entrance sizes in mesoporous materials.