Characterization of nanoporous materials by gas adsorption and density-functional theory

Nanoporous materials with pores ranging from several molecular diameters to ca. 10 nm currently find numerous applications in modern separation and catalytic technologies. Adsorption is one of the most informative experimental techniques for structural characterization of nanoporous materials. Practical problems of characterization and prediction of adsorption properties of active carbons, zeolites, pillared clays, mesoporous molecular sieves, carbon nanotubes, and many other traditional and newly synthesized adsorbents gave rise to a number of theoretical models capable of constructing adsorption isotherms in model pores. Modern methods of statistical thermodynamics such as Monte Carlo (MC) simulations, molecular dynamics (MD) and density functional theory (DFT) provide molecular level understanding of adsorption in pores, and can be used for characterization of nanoporous materials and predicting their adsorption properties.; The main focus of the present work is experimental and theoretical studies of adsorption in nanoporous materials. A nonlocal density functional theory (NLDFT) model has been developed for predicting adsorption/desorption isotherms in nanopores of different geometries over a wide range of pore sizes (0.3–100 nm), and for calculating pore size distributions from the experimental adsorption isotherms based on given intermolecular fluid-fluid and fluid-solid potentials. The NLDFT model has been applied to studies of N₂ and Ar adsorption and hysteresis phenomena in mesoporous molecular sieves of MCM-41-type, N ₂ and CO₂ adsorption on activated carbons.; An important issue of comparison of the theoretical and experimental excess adsorption isotherms have been studied in details. A method of “virtual helium calibration” has been introduced, which makes the theoretical and experimental isotherms entirely consistent. The method is applicable to any molecular model of adsorption.; Several new methods for calculating pore size distributions from experimental adsorption isotherms have been developed. Using regularization techniques NLDFT-based methods make possible the calculation of pore size distributions of (1) mesoporous molecular sieves of M41S type from low temperature N₂ and Ar isotherms, (2) microporous carbons from low temperature N₂, Ar isotherms, and from CO₂ isotherms at ambient temperature.; Theoretical results have been validated by extensive comparisons with the experimental data, and results of Monte Carlo simulations. Pore size distributions of mesoporous molecular sieves calculated from the NLDFT method are consistent with other independent methods for characterization of pore structure, such as X-ray diffraction and Transmission Electron Microscopy. The NLDFT method is a rigorous approach which is recommended for characterization of nanoporous materials.