Accessibility and strength of H-acceptor hydroxyls of ordered mesoporous silicas probed by pyridine donor

Ordered mesoporous silica (OMS) is an important and useful material for a variety of applications, including catalysis, adsorption, sensing and controlled drug delivery. The surface chemistry and the silanol groups on OMS pores are key properties for the potential modification and application of this material. This research aimed to synthesize (using standard protocols) and differentiate the accessibility and strength of the H-acceptor Si-OH from FDU-12, SBA-16, MCM-41 and SBA-15 by pyridine (Py) donor, where the first two have cubic pore structures and the last two have hexagonal pore structures. Donor-acceptor properties were assessed by calculation of the surface Si-OH densities by thermogravimetry (TG), H2O-TPD/MS, and(29)Si MAS and CP/MAS NMR. The nature of the Si-OH groups on these materials was determined to be hydrogen-bonding sites using FT-IR spectroscopy of Py adsorption. The reactivity of these silanol groups was probed by Py-TG and slurry microcalorimetry of Py adsorption in cyclohexane. Differences in accessibility and reactivity were discussed considering the total potential sites on the surface (n(OH)) versus the actual sites that can react with the Py molecule (n(Py)). By using microcalorimetry, it was possible to quantitatively distinguish the strength of the sites: The acidity order was approximately the same as the relative amount of silanol groups (Si-OH) and Py on the surface of the OMS materials (alpha(Py)): FDU-12 > MCM-41 >= SBA-16 > SBA-15.

Automated summary

This research compared the accessibility and strength of silanol groups on ordered mesoporous silica materials with different pore structures. Various analytical methods were used to evaluate the donor-acceptor properties, including thermogravimetry and NMR. Differences in reactivity were discussed, highlighting the impact of surface silanol groups on the adsorption and reaction properties of the materials.