X-ray Structural Modeling and Gas Adsorption Analysis of Cagelike SBA-16 Silica Mesophases Prepared in a F127/Butanol/H₂O System

Abstract

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.

Improvement in the Pore Size Distribution for Ordered Mesoporous Materials with Cylindrical and Spherical Pores Using the Kelvin Equation

“…As a result, a pore sizes analysis in the entire range of micro and mesopores can be performed by the NLDFT model [11]. The pore sizes obtained by this model agree with the results derived from experimental techniques based on X-ray diffraction (XRD) or transmission electronic microscopy (TEM) for OMM type MCM-41 [48,49], SBA-15 [50] and SBA-16 [51,52].…”

“…The shape of this hysteresis is related to the percolation phenomenon occurring throughout the desorption process at a P/P 0 of approximately 0.42 [53]. For all the samples, the capillary condensation on the primary mesopores takes places in one step, suggesting a defined pore size and a material with a high degree of arrangement [52].…”

Physical Adsorption Characterization of Nanoporous Materials

“…An improvement for pore size analysis can be obtained by calibrating the Kelvin equation using a series of MCM-silicas of known pore diameter (obtained from XRD interplanar spacing and the mesopore volume). In this manner, a relation between capillary condensation pressures and pore size can be established and used to obtain an empirically corrected Kelvin equation valid over the calibrated range (∼ 2 -10 nm) [118,119].…”

High-pressure synthesis of mesoporous stishovite: potential applications in mineral physics

“…The isotherm obtained may be interpreted as a type IV isotherm with an extremely broad capillary condensation step and an H3-type hysteresis loop (IUPAC classification; Kaneko 1994). The pore size distribution (PSD) calculated by NLDFT method (adsorption branch model for spherical mesopores) is broad, indicating a range from 0.5 to 30 nm with two maxima centered at ~5.3 nm (corresponding to the pore entrance size) and ~9.1 nm, respectively (Kleitz et al 2006), with the latter likely representative of the main spherical mesopores. This result is in contrast with that obtained for mesostructured stishovite quenched at 14 GPa (Stagno et al 2014) after plasma treatment that showed a type II isotherm with no hysteresis.…”