Pore Structure and Fluid Sorption in Ordered Mesoporous Silica. I. Experimental Study by in situ Small-Angle X-ray Scattering

Abstract

Ordered and disordered pores in SBA-15 silica and the gradual filling of these pores by an adsorbed fluid (dibromomethane, CH2Br2) are investigated by in situ small-angle X-ray scattering. Adsorption into the microporous corona and film formation at the corrugated surface of the ordered cylindrical pores is described by two different geometrical models. The analytical form factor resulting from these models is used to fit the integrated intensities of up to 10 Bragg diffraction peaks from the mesopore lattice. Model fits for the evacuated sample yield the porosity caused by the ordered pores. From these results and the total porosity obtained by nitrogen sorption, we determine the contribution of the disordered porosity, which is nearly 20% for the present sample. The model fits also provide new insight into the adsorbate structure in the ordered pores at different stages of pore filling, while the analysis of diffuse scattering provides information about fluid adsorption in the disordered pores in the walls. It is shown that the filling of the wall porosity affects the evaluation of the adsorbed amount in the ordered pores and leads to a distinction between relative and absolute adsorbed amounts. Using absolute adsorbed amounts, the filling isotherm of the ordered pores and the overall pore filling isotherm can be derived and compared with direct adsorption measurements. On the basis of the results of the present study, a quantitative modeling of the pore morphology and fluid sorption in the ordered and disordered pore regions of SBA-15 is presented in a subsequent paper (part II, DOI 10.1021/jp810040k).

Relationship Between Pore Structure and Sorption-Induced Deformation in Hierarchical Silica-Based Monoliths

“…From the perspective of SAXS data, however, it is possible to assume that by far not all porosity present within the struts is "ordered" in terms of contributing to the Bragg reflections. A similar situation was found during studies of SBA-15 silica powders by Jähnert et al [39]. In that work, the "corona-model" originally proposed by Imperor-Clerk et al [47] and later refined by Zickler et al [48] was adopted to describe surface roughness and related microporosity within the mesopore walls of SBA-15 silica.…”

“…2 (see text). The corresponding fraction of the disordered pores dis , is calculated including Gurvich Summarizing the results from structural characterization with SAXS and nitrogen sorption, we conclude that the mesopores in the present sample contain a corona with a rough surface and micropores within the pore walls, similar to SBA-15 silica [39]. The detailed SAXS analysis shows that a considerable fraction dis (about 1/3) of the pore volume in the struts is not related to the ordered pores seen with SAXS.…”

“…It should be noted that the solid phase density of 1.75 g/cm 3 is considerably lower than the density of non-porous silica in similar sol-gel derived materials (around 2.1-2.2 g/cm 3 , see e.g. [39]). This low solid density may be related to the fact that the sample was not calcined, and therefore still contained a considerable amount of organics.…”

Capillary condensation, freezing, and melting in silica nanopores: A sorption isotherm and scanning calorimetry study on nitrogen in mesoporous SBA-15

“…Our results with regard to the bimodal, hierarchical pore radius distribution in SBA-15 are in good agreement with small-angle X-ray 24,29,[29][30][31][32][33][34] and neutron 34 studies as well as atomistic, lattice Monte Carlo simulations 35 addressing the intrawall porosity of SBA-15. The controversially debated question, whether this pore wall porosity is corona-like, confined to a small region around the main pores, or whether it affects the entire pore walls, can not be answered by our analysis.…”

Neutron and X-Ray Porosimetry

“…Such behavior is expected for pore walls with highly corrugated surface due to the simultaneous smoothening of the surface and the growth of the thickness of the adsorbed film with pressure. The step-density model used in [40] for the data analysis can be extended to account for the possible finite density gradients in the adsorbed films [41][42][43].…”