Diffusion mechanisms of normal alkanes in faujasite zeolites

Molecular dynamics simulations of C1 through C14 $n$ -alkanes in a siliceous faujasite zeolite are used to elucidate diffusion mechanisms. Additional simulations of the bulk liquids are conducted to compare the liquid and adsorbed phases. Heats of adsorption, diffusivities, and activation energies are compared with experimental values and agree well. Particularly good agreement is found between predicted self-diffusivities and those measured with pulsed field gradient nuclear magnetic resonance (NMR). Analysis of the chain conformations and motional frequencies indicates that the liquid and adsorbed phases are quite similar, most likely due to the open nature of the faujasite structure. Insights into the influence of local environment on molecular relaxation are also given. Siting results reveal an interesting progression of adsorption sites as the chain length is increased. Shifts of preferred adsorption sites are due to matching between the length scales of sorbate and adsorbent. Alkanes below C6 adsorb preferentially above the 4-rings and 6-rings inside the supercages. For longer chains, the preferred sites shift more toward the center of the supercages. For C8 and above, the molecules spend much of their time in the window regions. These window sites are separated from the supercage sites by unexpected orientational barriers. Consequently, diffusion of longer alkanes in faujasite type structures is not characterized by simple supercage-to-supercage hopping but by hopping between supercage and window sites.