A molecular dynamics study of the diffusion of a hydrocarbon in AlPO4-5 molecular sieve

In this paper a computational study of the ethane diffusion in AlPO4-5 molecular sieve by classical molecular dynamics techniques is presented. Calculations were performed at different temperatures and loading of ethane molecules per AlPO4-5 unit cell. Static and dynamics AlPO4-5 framework conditions in the calculations were analyzed. According to the results of the simulations the molecular diffusion of ethane in a defect-free AlPO4-5 crystal structure occurs via a normal regime and not as single-file diffusion in agreement with the quasi-elastic neutron scattering (QENS) experiments. The simulated activation energy for a particular loading of the ethane molecules is calculated. Moreover, the calculations performed in dynamical conditions show the influence of the framework vibrations on the molecular diffusion. Particulary, the framework vibrations can be a factor to take into account when the molecular dimensions are close to the channel diameter. However, according to our calculations this fact does not produce a qualitative change in the diffusion regime of the ethane molecules (with a molecular diameter ~3.8 Å) in the 12-member channel of the AlPO4-5 unit cell.