Adsorption And Diffusion In Porous Materials

Porous materials science is still expanding, and many kinds of porous materials such as activated carbons and zeolites are increasingly important in the modern chemical industry. For a long time, scientists and engineers have been very interested in the research field of adsorption and diffusion in porous materials. With the development of theory and experimental techniques as well as the improvement of molecular simulation, studies on adsorption and diffusion become more prosperous.Indeed adsorption and diffusion are very important for many applications such as the characterization of porous materials, adsorption-based separation processes, catalysis, especially catalytic kinetics. This MSc research has been focused on adsorption and diffusion in porous materials, and the main results are summarized as follows:Combined with carbon dioxide (CO₂) adsorption and a multiple-process adsorption equilibrium (MPAE) model, activated carbons have been characterized in detail. Single component adsorption equilibria of CO₂ on three commercial activated carbons were systematically investigated by means of a volumetric method at pressures up to 1 bar and at temperatures ranging from 195 K to 323 K. The MPAE model describes the present adsorption isotherm data very well over the whole range of the investigated conditions. The inconsistency of the isotherms correlated by the Tóth model with those predicted by Monte-Carlo simulation for an adsorption system in the literature was interpreted, i.e. it is probably caused by the non-equilibrium isotherm data measured and the correction behavior of the Tóth model at low pressures. The estimated saturation capacity by the MPAE model is reasonably consistent with the total pore volume determined from N₂ adsorption at 77 K, indicating that CO₂ has a high degree of occupancy in the activated carbons investigated. The current work also confirms that CO₂ molecular is a good candidate for the charactrization of porous carbons, especially microporous carbons. The thermodynamic properties such as adsorption equilibrium constant and enthalpy associated with adsorption are derived. The adsorption affinity decreases gradually along with the adsorption processes occurring. The approach using the MPAE model provides an excellent interpretation of the adsorption of CO₂ on activated carbons in terms of pore filling processes and yet gives much detailed information on the textural properties of activated carbons. The consistency between experimental results and model predictions emphasizes the strength of the proposed approach.The harm of greenhouse gases such as CO₂ and nitrous oxide (N₂O) has been aroused great attention. Adsorption is one of the effective strategies to control and decrease the concentration of CO₂ and N₂O in the atmosphere. The adsorption equilibria of CO₂ and N₂O on different activated carbons were systematically investigated by means of a volumetric method. The MPAE model appropriately describes the equilibrium data over the whole range of the applied conditions. To recover the low concentrations of CO₂ and N₂O, activated carbons with more available micropores could be suitable as adsorbents. On the other hand, to adsorb the high concentrations of CO₂ and N₂O, activated carbons with high pore volumes would be more applicable.The separation of chloropentafluoroethane (CFC-115) and pentafluoroethane (HFC-125) is an important industrial process. The technology used nowadays is based on cryogenic extractive distillation. However, the distillation-based technology makes it an energy-intensive separation scheme. A process configuration based on an adsorption-based separation could be an attractive commercial arrangement. In order to achieve this objective, it is of utmost importance to find an effective adsorbent. Thus two kinds of adsorbent, i.e. activated carbon and zeolite, were used to find out the possibility for the separation of CFC-115 and HFC-125. The adsorption isotherms of CFC-115 and HFC-125 on VAC series activated carbon and silicalite-1 have been accurately measured. The Tóth and Dual-site Langmuir (DSL) models were used to describe the adsorption isotherm data on activated carbons and silicalite-1, respectively, and both models give a good description. The thermodynamic properties such as adsorption equilibrium constant and enthalpy associated with adsorption are discussed in detail. Based upon the model of the single component adsorption isotherms, the ideal adsorbed solution (IAS) theory was employed to predict the mixture adsorption and selectivity. The predicted results show that both of the VAC series activated carbons and silicalite-1 exhibit a high separation performance for CFC-115 and HFC-125 mixtures, and these adsorbents could be industrially applicable in the separation process.Zeolite molecular sieves have been extensively used in the chemical processes such as fluidized catalytic cracking and alkane isomerization. For a long time, diffusion in zeolites has been a pop subject in terms of scientific research. Many experimental techniques have been developed for investigating diffusion in zeolites. One of the widely used techniques is the so-called Zero Length Column (ZLC) method, which is introduced in this thesis in detail. A home-made ZLC setup was used to show its capability to measure diffusivities in microporous materials via two systems n-butane in silicalite-1 and ethylene in Kureha activated carbon.