Molecular Simulation Study On Prediction Of Adsorption And Diffusion Properties Of Guest Molecules In Porous Materials

A wide variety of porous materials such as silica nano-pores, zeolites andmetal-organic frameworks are used in adsorptive separations and catalysisprocesses. In such applications, the adsorption and diffusion behaviors of theguest molecules represent the thermodynamic and dynamic properties of thesystem, respectively. For a proper design of both the materials and relatedprocesses, a basic understanding and proper description of both thethermodynamic and dynamic properties are inevitably necessary. To this end,with a particular focus on the separation and catalysis processes, in the presentthesis, the prediction methods of mixture adsorption equilibria and purecomponent self-diffusivities are investigated respectively by adoptingmolecular simulation methods.Firstly, for adsorption of CH₄and CO₂in silica pore and rho-ZMOFsystems, IAST was shown to give a good description of the adsorption ofbinary mixtures at the low-pressure range which is not the case at thehigh-pressure range. The real adsorbed solution theory (RAST) is applicableto the mixture adsorption of CO₂and CH₄in rho-MOFs with various extra-framework cations in all pressure range. For the competition adsorptionof benzene and propene in MCM-22zeolite, it is found the zeolite account forthe structural and chemical heterogeneity for the adsorbates and can bedivided into three adsorption sites according to a simulated annealing method.It is noted that the competition adsorption in the three adsorption sites for thetwo adsorbates can fall into three successive steps and the adsorption order ofpropene in mixture in these three sites is S3→S1→S2. A new model ispresented to predict the benzene and propene adsorption equilibrium inMCM-22. This approach yields better multicomponent equilibrium predictionsthan IAST.Secondly, combining with theoretical method, molecular simulationswere carried out to study the diffusion of the host-guest systems. We extendthe use of the excess entropy scaling laws originated from bulk fluids andserve as a first step for evaluating single component diffusion coefficients inporous media other than the ones in bulk Lennard-Jones (LJ) fluids.For CH₄fluid, the relationship between self-diffusivity and excessentropy formulated by the two laws was found to hold qualitatively for thepresent case of confined fluid in porous materials, i.e, slit pore, silica pore,zeolites (AFI, MFI, MCM-22, and FAU) and MOF (IRMOF-1andNa-rho-ZMOF) materials.For CH₄molecules confined in slit pore with different frameworks, it isfound the roughness of pore surface make the pre-exponential parameter A smaller than that of bulk fluid. Studying the CH₄fluid confined in silicanano-pores of different pore sizes from1to5.8nm, we found thetwo-dimensional confinement can arouse the scale-dependent of thepre-exponential scaling parameters. For zeolites and MOF materials, A isroughly in direct proportion to V_free.For CO₂confined in several porous materials, i.e, slit pores (unchangedand charged),2nm silica pore, MCM-22and MFI zeolites, we find that theelectrostatic interactions between CO₂and the host frameworks induce thetemperature dependence of scaling results, i.e, a lower temperature results inan extremely non-uniform microscopic fluid distribution, which leads to thebreakdown of the excess entropy scaling. The parameters A and B in equationof scaling laws depend extremely on the species investigated. Quantitatively,A is in direct proportion to V_free.For the diffusion of propene molecules confined in several porousmaterials, i.e, slit pores (unchanged), silica pore, MCM-22and MFI zeolite,the scaling results are found to have temperature and concentrationdependence. The flexible model of propene arouses the scale dependence ofexponential parameter B in the equation of scaling law which can becharacterized by the most probable distribution of the intramolecular angle θof propene.