The concentration and Brønsted acidity of surface silanol groups on mesoporous silica (SBA-15) has been studied by following the adsorption of benzylamine, BA, from water as a function of pH. The adsorbed amount of BA from water was compared to the maximum amount of BA that could be adsorbed from cyclohexane. Furthermore, the surface concentration and acidity of carboxylic acid functions on surface-functionalized SBA-15 was also studied, which allowed the relative surface concentration of remaining silanols to be obtained. Two types of silanols can be identified, where about 1/5 of the silanols have a pKa = 2 and the remaining 4/5 of the silanols have a pKa of about 8.2. According to the literature, these two types of silanols can be identified as Q3 and Q2 silanols, respectively, of which the Q3 silanols are more acidic. For the surface-functionalized materials, pKa values close to their respective intrinsic values are found for the carboxylic acid functions. However, irrespective of the method of surface functionalization, 50% or more of the accessible surface groups are silanols. The results thus suggest that the effective charge density is largely controlled by the (de)protonation of the silanol groups even for surface-functionalized mesoporous silica, which render the surface chemistry of the surface-functionalized silicas to be radically different from what simple schematic representations would suggest. The results are suggested to be of importance for reaching a predictive level of understanding for the behavior of mesoporous silica in a range of applications, as many of the foreseen applications for mesoporous silica involves water as the medium.
A detailed characterization of large-pore cagelike mesoporous SBA-16 silica materials with tailored pore dimensions is reported. The materials were synthesized in a EO106PO70EO106 (F127)−butanol−H2O system under mildly acidic conditions, and the pore diameters were tailored by varying the hydrothermal treatment temperature. Structural information was acquired by full-profile analysis of powder X-ray diffraction (XRD) patterns. High-resolution diffraction data were obtained for all the materials using synchrotron radiation as the X-ray source, enabling a comprehensive XRD modeling supplemented with the generation of electron density distribution maps. The structural parameters derived from the XRD modeling were compared with data obtained from nitrogen and argon physisorption experiments performed at −196 °C. An excellent agreement was found between the XRD modeling results and those obtained by a new nonlocal density functional theory (NLDFT) kernel developed for pore size analysis based on gas adsorption in spherical pores, while NLDFT analysis based on a cylindrical pore model was shown to systematically underestimate the pore dimensions by about 30% which exceeds previous expectations. Furthermore, the Barrett−Joyner−Halenda model was shown to give errors up to about 45% in the pore size range above 4 nm. The structure of the surfactant−silica hybrid materials was also analyzed by XRD, which shed more light on the structural changes accompanying the thermal surfactant removal process. The present study is expected to provide a reference source for the accurate characterization of large cagelike mesoporous silica materials, on the basis of a direct comparison of suitable data collected independently by gas physisorption and comprehensive XRD modeling.
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