Zeolitic Imidazole Frameworks for Super Selective Separation of Propylene from Propane

Olefin and paraffin gases are important products of the petrochemical refining industry wherein their effective separation could provide high economic gains and competitiveness in the emerging energy markets amidst state-of-the-art technology. Important chemicals such as polyethylene and polypropylene are derived from raw materials in the form of light olefins, including ethylene and propylene, processible via naphtha cracking. However, since this leads to its production among other valuable by-products separation/ refinement is required and this is commonly achieved through technical and energy-intensive cryogenic distillation due to the similarity in thermophysical properties of the process effluents. Thus, cost-effective and energy-efficient processes are required among which membrane-based separation techniques stand out. In that line, zeolitic imidazolium frameworks (ZIFs) have shown a superior potential to provide high selectivity and high permeability to particular species in a mixture they are used to refine. This superior effectiveness is attributed to ZIF's uniform pore sizes that enable sharp molecular sieving, as well as its highly porous structure that enables fast species transport through it, with minimal mass transfer hindrance for the targeted/preferably selected specie. Among ZIFs, there exists one called ZIF-8 which is especially suited to propylene/propane separation and has been widely reported for its sharp molecular sieving performance for this mixture. ZIF-8 is also distinctly stable, and relatively easy to synthesize from cheap and readily available starting materials. Recent advances in the fabrication methods reported for ZIF-8 synthesis are presented in this work, along with a comparison of the separation performance for propylene and propane resulting from different types of ZIF-8 produced by these methods. The potential effect of utilizing this ZIF material in the refining units applied for olefin/paraffin separation is also critically evaluated towards its industrial utilization.