Enabling Kinetic Light Hydrocarbon Separation via Crystal Size Engineering of ZIF-8

Recent increases in shale gas production provide an excellent opportunity for advancements in the energy-efficient separation of natural gas liquids because these are increasingly used as fuel sources and chemical feedstocks. Current fractionation schemes generally involve cryogenic distillation of C-1-C-4 hydrocarbons and are extremely energy intensive. Here, we describe the first steps toward a lower energy, kinetic pressure-swing-adsorption cycle for the separation of ethane, propane, propylene, and butane using ZIF-8 as a diffusionally selective adsorbent. Crystal engineering techniques were employed to control the diffusive time scale of the separation, allowing for multiple separations using the same adsorbent within reasonable process times. Equimolar separation of ethane/propane mixtures at 293 K exhibited separation factors of 2.7 in the gas phase under nonoptimized conditions, which enhances the concentration of the feed mixture to 7.5 mol % propane. The separation performance was shown to improve to 3.8 at lower temperatures (81 mol % propane), which is attributed to differences in the activation energy of permeation of the two components. Propane/butane mixtures demonstrated a lower diffusive selectivity and almost negligible enhancement, while propylene/propane showed enhancement beyond ethane/propane due to a strong diffusive selectivity and sorption selectivities closer to unity. Single-component adsorption and diffusion results were incorporated into a computational model of the system and shown to be in relatively good agreement with the experimental values. The model was used to predict the separation system performance and recovery at various temperatures.