N2 physisorption is the experimental method to determine the surface area in porous and non-porous materials, and pore size distribution (PSD) in micro (pore size less than 2 nm) and mesoporous materials (pores from 2 to 50 nm). In mesoporous materials, Barrett-Joyner-Halenda (BJH) method and, more recently, Density Functional Theory (DFT) model are used to estimate PSD from N2 adsorption-desorption isotherms. BJH is the most employed method, because it can be easily applied to a great variety of materials and no information about the pore shape is required. Mesostructured siliceous materials, offers the possibility to compare the two models, being the pores ordered and the structure well know. Among the highly ordered mesostructured silicas, we selected MCM-41 due to the 2D-pore structure P6mm (mesochannels). Transmission Electron Microscopy (TEM), Small Angle X-Ray Diffraction (SA_PXRD) and 29Si Solid-State NMR were combined to N2-phisisorption measurements to extract information about pore size, pore structure, pore wall thickness and Q2/Q3/Q4 sites. Starting from the two pore size values obtained through the application of either BJH or DFT models to the same experimental N2-isotherms of MCM-41, we propose two computer models at the atomic level to reconstruct these isotherms by combining GCMC and MD simulations. Although pore size difference between the two models is about 1 nm, large difference is observed in the simulated gas adsorption isotherms. Saturation of the pore surface and the progressive formation of additional gas layers, as well as the eventual condensation, occur at a relative pressure that strongly depends on the pore size. The simulations of other gases and their mixtures bolster such dependence, with potential impact on selectivity.

Pore size from N2 adsorption isotherm in mesostructured siliceous materials. BJH or DFT model? GCMC/MD simulations answer the question

ACS Fall 2021

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