In order to investigate the details of the process of pore condensation and hysteresis mechanisms in three-dimensional (3-D) pore networks, we performed a systematic study of the adsorption and pore condensation behavior of N2 (77.4 K) and Ar (77.4 and 87.3 K) in a 3-D ordered pore system, i.e., cubic Ia3̅d mesoporous KIT-6 silica materials with mode pore diameters ranging from ca. 5 nm up to 11 nm. KIT-6 silica is a porous material composed of two intertwined mesoporous subnetworks similar as in MCM-48, but this material can be prepared with much larger mean pore diameters. Accurate pore size analysis was performed by X-ray diffraction modeling and by state-of the art application of nonlocal density functional theory (NLDFT) on N2 (77.4 K) and Ar (87.3 K) sorption data. Furthermore, our data suggest that the width of the adsorption/desorption hysteresis loop observed for 3-D KIT-6 silica can be narrower as compared to that of pseudo-one-dimensional SBA-15 silica of the same pore size (i.e., in the pore diameter range from 6 to 8 nm). This specific behavior correlates well with the existence of the highly interconnected 3-D pore network of the KIT-6 material. Moreover, the results of our investigations are also consistent with previous observations that the SBA-15 pore system becomes more and more interconnected with increasing aging temperatures, i.e., SBA-15 changes from being a material with a pseudo-one-dimensional mesopore system to a material exhibiting a three-dimensional pore system resembling KIT-6 silica. These results provide new insights into the effects of pore interconnectivity on pore condensation and hysteresis behavior in both KIT-6 and SBA-15 silica materials and enable a more thorough understanding of the pore structure and textural properties of these materials.

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Regarding the design of SBA-15 silica, substantial efforts were deployed in the past decade in order to understand the mechanism of formation and the effects of the different synthesis conditions on the structure and porosity of the resulting materials. However, better insights into both the tailoring and the characterization of the pore structure of such mesoporous materials are still needed in order to enable the accurate control of adsorption and pore condensation properties in SBA-15. For this, the influence of the synthesis parameters on the properties of SBA-15 silica must be rationalized in terms of their implications for pore architecture, i.e., pore structure and network interconnectivity. Herein, it is demonstrated that pore condensation and hysteresis behavior of inert gases in subcritical conditions confined in ordered mesoporous SBA-15 silica can precisely be modulated as a function of the synthesis parameters. Synthesis conditions were found for generating SBA-15 samples that can be described entirely as a pseudo one-dimensional (1-D) pore system (i.e., pore condensation and hysteresis behavior are an intrinsic property of the liquid–vapor transition in a finite volume, in agreement with the independent open pore model). However, the data also revealed that distinct synthesis conditions allow for the preparation of SBA-15 with pore condensation properties affected by cooperative pore network effects, mimicking the behavior observed for materials with a pristine three-dimensional (3-D) interconnected pore network topology, such as KIT-6 silica. Therefore, this comprehensive study shows that SBA-15 should be best regarded as a family of solids with easily adjustable porosity, ranging from corrugated and/or distorted pore systems to highly interconnected networks of channels. The effect of each different synthesis parameter on the final pore size of SBA-15 was carefully monitored, and a threshold acid concentration range for optimal pore size variation was found. In addition to substantial progress in the SBA-15 synthesis, such in-depth characterization of a “model” ordered mesoporous material coupled with advanced application of state-of-the-art NLDFT methods is of prime importance both for the development of fundamental research on the topic and for the applications requiring tailored high surface area materials with selectively tuned pore structure.

A new synthesis procedure that is based on the grafting of titanium alkoxide species chemically modified by acetylacetone (acac) on the surface of a P123/SBA-15 composite material is proposed to prepare Ti-SBA-15 catalysts and studied by a combination of elemental analysis, Fourier transform infrared spectroscopy (FTIR), solid-state 13 C (CP) NMR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), diffuse reflectance UV-vis (DR-UV-vis), and N 2 physisorption at -196 °C. In the absence of the acac chelates, the observed formation of anatase TiO 2 onto the surface of the material demonstrates that the coordinating ligand acts as an inhibitor for the crystallization of anatase. Furthermore, FTIR and 29 Si NMR results show that the chelated titanium alkoxide precursor interacts strongly with the silanol groups, which, in turn, greatly enhances the dispersion of the titanium species in the mesoporous silica matrix. Moreover, a decrease of the temperature applied for the postgrafting and an increase of both the acac/Ti ratio and pH are shown to favor the retention of titanium on the materials surface without affecting the titanium dispersion. According to the X-ray photoelectron spectroscopy (XPS) results, a maximal titanium content of 13.8% can be well-dispersed on the surface of the mesopores without formation of an excess on the external surface of the solids. However, the results of the DR-UV-vis analyses and the catalytic epoxidation of cyclohexene reveal that the maximal concentration of titanium species in tetrahedral coordination is obtained for materials with a Ti/Si ratio of 5.6%. Even if materials with higher titanium content do not show higher conversion of cyclohexene, they do exhibit remarkably low catalytic deactivation during the recycling tests. A higher hydrothermal stability is suggested as an explanation for the lower deactivation of Ti-SBA-15 at high titanium content.