(520f) Characterization of LTA Zeolites Based on Pore Typification

Despite the great industrial importance of zeolites, the characterization of these materials based on adsorption is still very limited. Adsorption has traditionally been used to determine the pore size of zeolites. However, the available models are semi-empirical, based on an average surface potential to calculate the gas-solid interactions. Unlike carbonaceous materials, which intensively apply molecular simulation and density functional theory methods in their characterization, zeolites did not have an equivalent development. In addition, several zeolites are unsuitable for characterization with N₂ or Ar due to diffusional limitations of these gases. In a recent study, Dantas and Neymark (2020)* have introduced a methodology – the pore type distribution (PTD) – for pore structure characterization based on matching a theoretical isotherm generated by the combination of isotherms of adsorption in individual pore types to an experimental isotherm. Here we propose to extend this methodology to zeolites. We selected Na-LTA (4A) zeolite as an application example. Based on the 8 local isotherms, one from each supercage of the LTA unit cell, we developed a kernel that estimates the most likely energy distribution levels among supercages from the best fits of experimental isotherms. The method was applied in the characterization of LTA commercial and laboratory synthesized samples. The pore type distribution of the samples, showed that at least 4 supercages with strong interaction potential are a probable signature of the LTA zeolites studied. The new simulated isotherm based on this pore type distribution considerably improved the agreement between simulated and experimental isotherms. The heat of adsorption variation in supercages is related to the relative positioning of cations between the S3 and S2 sites. The theoretical average volume obtained was used to assess the level of defects of samples obtained from unconventional precursors raw material or those that underwent hydrothermal degradation. The new methodology enables the rational use of adsorptive methods in the characterization of small pore zeolites, which can be extended to other classes of zeolites, allowing new information to be obtained and expanding the use of adsorption as a characterization tool.

(*) Dantas, S.; Neimark, A. V. Coupling Structural and Adsorption Properties of Metal-Organic Frameworks: From Pore Size Distribution to Pore Type Distribution. ACS Appl. Mater. Interfaces 2020, 12, 15595.