(381g) Structural Characterization of Sba-15 Silica by 29Si-NMR and in-Situ Synchrotron Small-Angle Diffraction Studies of Physisorbed Films

A combination of ²⁹Si NMR spectroscopy and small-angle x-ray diffraction (SAXD) studies has been used to characterize the pore structure of periodic mesoporous silica materials of the MCM-41 and SBA-15 type. The two methods are complementary, as ²⁹Si NMR yields structural information at the atomic length scale, viz. the proportions of Si atoms with four, three, and two nearest neighbors (Q⁴ : Q³ : Q²), while SAXD provides information about the mesoscopic scale (pore diameter and wall thickness, width and porosity of the corona) and about the long-range order of the silica matrix. The integrated intensities of the diffraction peaks arising from the periodic pore structure are analyzed in terms of a model electron density profile of the unit cell of the matrix structure, and appropriate values the model parameters are derived by data fitting. Further information about the corona structure and the pore filling process is obtained by admitting adsorbable gases and studying the changes in the integrated intensities of the diffraction peaks induced by gas adsorption. A quantitative analysis of the in-situ SAXD intensities indicates that the filling of the porous corona and the growth of a liquid-like film at the pore walls occur simultaneously over a rather wide range of relative pressures. Such a behavior is indeed expected for highly corrugated pore walls, due to a concomitant smoothing of the rough surface and the growth of the liquid-like film. Hence the present results support the picture that the corona of SBA-15 silica represents a highly corrugated pore wall.