Diffusion mechanism of hydrocarbons in zeolites—I. Theory

The properties of single-component diffusion of hydrocarbons in zeolites, especially its dependence on temperature and concentration, are studied theoretically by modeling the molecule-zeolite interactions and the molecule-molecule interactions. The molecule-zeolite interactions determine the orders of magnitude of the diffusivity, whereas the molecule-molecule interactions could alter the concentration dependence of a diffusivity. A unified diffusion theory is developed to describe gaseous diffusion, liquid diffusion, Knudsen diffusion, solid diffusion and configurational diffusion. Based upon the zeolite structure and molecular properties, an estimate of the order of magnitude of the diffusivity can be made for small organic molecules with no fitting parameters. Repulsion between molecules in the zeolites can lower the activation energy for diffusion, which results in a rising trend of apparent diffusivity and a decreasing trend of the Langmuir parameter at high concentration. Diffusion in zeolites takes place in the Knudsen diffusion regime or in the configurational diffusion regime. Transition from Knudsen diffusion to configurational diffusion depends on the properties of molecules and those of zeolites, such as the ratio of molecular diameter to channel diameter, molecular length, zeolite structures, and temperature. For ZSM-5, transition from Knudsen regime to configurational regime may occur for roughly spherical molecules when the ratio of molecular diameter to channel diameter, λ, is greater than approximately 0.6–0.8.