Experiments‐Guided Modeling of MCM‐41: Impact of Pore Symmetry on Gas Adsorption

Abstract

by combining sol-gel synthesis with a proper surfactant templating agent. The latter was not a single solvated organic or inorganic species, but a self-assembled surfactant array. Three main mesophases are typically produced, namely, the cubic MCM-48, the lamellar MCM-50, and the hexagonal MCM-41. The latter is characterized by a regular 2D hexagonal array (P6mm space group symmetry) of highly uniform parallel non-interconnected channels, with the pore diameter depending on the specific templating agent. Compared to the other members of the M41S mesoporous family, MCM-41 has a higher thermal stability, and it is easier to prepare, thus attracting greater attention. [1,12] The ordered pore architecture of MCM-41 can be clearly observed through transmission electron microscopy (TEM) [2,13] and assessed through low-angle X-ray diffraction (LA-XRD) techniques or small-angle x-ray scattering (SAXS). [2] Overall, porosity can be as high as 80%, with the pore walls composed of amorphous silica. [1] The analysis of the N 2 adsorption isotherms according to the Brunauer-Emmett-Teller (BET) theory [14] is widely employed to estimate the Specific Surface Area (SSA), which is typically >700 m 2 g -1 . [11] Analysis of the same adsorption isotherm through the Barrett-Joyner-Halenda (BJH) model [15] is the typical choice to determine the pore size distribution (PSD). However, other approaches are available to retrieve these textural details, which are more recent and have a more rigorous physical base, such as the Density Functional Theory (DFT), allowing the determination of both the SSA, the pore volume, and the PSD. [16] The former models and theories have been recently questioned. [17] Computer calculations and simulations are able to unveil the microscopic details and the energetic aspects of the interaction between the guest molecules and the solid surface. Adsorption capacity, selectivity, diffusivity, and so on, can be investigated at the microscopic level under different pressure, temperature and humidity, so to obtain fundamental information to tailor material synthesis and functionalization to meet specific goals. Computational approaches can be used to complement the experiments, but also to possibly predict the system behavior, and to save time and resources on the laboratory bench. To this aim, the preparation of a 3D model that matches the characteristics of the real sample and that is able to reproduce the macroscopic behavior, as seen by the experiments, is crucial. Mesoporous materials cannot be modeled as simple flat surfaces or slits. At Among mesoporous ordered silica-based materials, MCM-41 is widely employed in both experimental and computational investigations of gas adsorption. Specific surface area and pore size distribution are typically obtained by analyzing the N 2 adsorption isotherm. To this aim, different models and theories are available, and which one is more accurate is under debate. On the computational side, the in silico model ought to match the characteristics of real samples and reproduc...

Surface Heterogeneity Affects Adsorption Selectivity for CO2 Over CH4 in Bare Mesostructured Silica with 2D Hexagonal Symmetry and Different Pore Size

“…The 3D structure of three different mesostructured silica was obtained as described in details elsewhere. [35] These three materials differed in the pore diameter, but shared the same 2D Figure 2. Density profile of pure CO 2 (top row) and CH 4 (bottom row) inside the pores of Meso-2.4 (left column), Meso-3.4 (middle column) and Meso-6.4 (right column) materials.…”

“…The remaining surface O atoms were treated through an in-house computer code, [35] to possibly match the SS-NMR experimental f) This model was referred to as MCM-41/BJH in a previous publication; [35] g) This model was referred to as MCM-41/DFT in a previous publication. [35] ratio (the index refers to the number of ─O─Si─O─ linkages to the material bulk). In the case of Meso-2.4, all the surface O atoms were hydrogenated to obtain the surface ─Si─OH groups.…”

“…. [35] In the case of Meso-6.3, per-hydrogenation was similarly applied, due to the lack of a real sample on which SS-NMR analysis could be performed (the model has is based on SBA-15 but has no micropores). In the case of Meso-3.4, a number of surface Si atoms were randomly selected to form new ─Si─O─Si─ bridges.…”

“…In the case of Meso-3.4, a number of surface Si atoms were randomly selected to form new ─Si─O─Si─ bridges. The remaining surface O were hydrogenated, matching the SS-NMR experimental [35] Finally, the materials were subjected to the amorphization stage, i.e., a random maximum displacement of 0.5 Angstrom was applied to each atom. A thousand steps of energy minimization were performed with conjugate-gradients.…”

“…Deviations of the adsorption behavior from the ideality could be explained by focusing on the energetic inequivalence of the different adsorption sites, namely, Q 3 and Q 2 sites on the silica surface. [4,[33][34][35] Such surface heterogeneity appears to promote selectivity for CO 2 over CH 4 at low CO 2 concentration, which is granted at low molar fraction and low (atmospheric) pressure.…”