NOVELTY - The process comprises depositing zeolitic imidazole framework (ZIF) seed particles having a pore size of 0.2-0.5 nm on a support forming a ZIF seed layer, exposing the support comprising the ZIF seed layer to an ZIF synthesis solution comprising metal ions or metal clusters and imidazolate under conditions suitable for ZIF crystal growth or exposing a support material to an ZIF synthesis solution comprising metal ions/metal clusters and imidazolate under conditions suitable for ZIF crystal formation, and forming a ZIF membrane having a thickness of 1-100 micron and a pore size of 0.2-0.5 nm.    USE - The process is useful for making zeolitic imidazole framework (ZIF) membranes, which are useful for separating C2- and C3+ components, where the C2- component includes hydrogen, nitrogen, oxygen, methane, ethane or ethanol and the C3+ component includes propane, propylene, butane, butylenes, isobutene, propanol, isopropanol or butanol (all claimed).    ADVANTAGE - The method is capable of easily making the zeolitic imidazole framework membranes with high strength and improved permeability and performance thus ensuring energy-efficient and economical separation of multi-component gas mixtures with high selectivity.    DETAILED DESCRIPTION - The process comprises depositing zeolitic imidazole framework (ZIF) seed particles having a pore size of 0.2-0.5 nm on a support forming a ZIF seed layer, exposing the support comprising the ZIF seed layer to an ZIF synthesis solution comprising metal ions or metal clusters and imidazolate under conditions suitable for ZIF crystal growth or exposing a support material to an ZIF synthesis solution comprising metal ions or metal clusters and imidazolate under conditions suitable for ZIF crystal formation, and forming a ZIF membrane having a thickness of 1-100 micron and a pore size of 0.2-0.5 nm. The membrane material is a ZIF material such as a ZIF-8 material or ZIF-67 material having a pore size of 0.3-0-4 nm. The membrane is synthesized in aqueous solution at a temperature of 10-100 degrees C. The support material is flat-sheet, tube, hollow fiber material or monolith material. INDEPENDENT CLAIMS are included for:    (1) a method to separate mixtures of C2- and C3+ components; and    (2) a method to separate mixtures of propylene and propane components.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a membrane separation process.    Membrane separation unit (2)    Non-permeating stream (3)    Permeate stream. (4)