Modeling Diffusion of Linear Hydrocarbons in Silica Zeolite LTA Using Transition Path Sampling

The diffusivities of linear hydrocarbons (CH4, C2H6, C2H4, C3H8, C3H6, and C4H10) in pure silica zeolite LTA (ITQ-29) are computed at 300 K and infinite dilution. To overcome the time scale problem arising from the slow diffusion process at room temperature, we used transition path sampling (TPS). The influence of framework flexibility on diffusion is investigated by combining TPS simulations with fully flexible molecular dynamics performed in the NpT ensemble. The ensemble of the collected reactive trajectories was used to characterize sets of transition states, and the corresponding configurations were analyzed to construct window size distributions during the molecular hopping events. The diffusion process is affected by framework flexibility, and the influence of framework flexibility on diffusion of propane and butane is much larger than for methane and ethane.