Introduction
1,3-Butadiene (BD) is a very versatile organic raw material, a petrochemical-based VOC, used as a monomer for the manufacture of synthetic rubber1. Due to the presence of conjugated double bonds in its molecule, addition, cyclization, substitution, and polymerization reactions can take place, which makes this VOC a widely used compound in synthetic rubber and organic synthesis2. Due to its high consumption for the synthetic rubber industries, it became so essential to control it from the tail gases of synthetic rubber plants. In addition, the international agency for research on cancer has designated 1,3-Butadiene as a possible carcinogenic material, while environmental protection agency (EPA) has listed it as the 10th most carcinogenic material3. Butadiene mainly mixed with the nitrogen during its post-treatment, i.e., drying process, where butadiene evaporates from the synthesized rubber and mixed with the tail gas of the plant. At present, the catalytic oxidation method used to remove the BD from tail gas before emitted to air, which completely converted the VOCs into CO2 and H2O. Although catalytic oxidation removed BD from tail gas of the synthetic rubber plant, it is a destructive method to control the VOC emission so that there must be some constructive method required, which can effectively recover the BD from tail gas and can be reused as well.
Membrane-based technology is one of the best possible solutions for complete VOC recovery. The membrane separation process developed over the last decades and considered to be very attractive due to its ease of operation, small footprint, low energy usage, easily scale up production, and more significantly, it offers the ability to meet future stricter environmental limits4. In the recent works, MMMs showed good results for many applications such as water separation, flue gas purification, petrochemical separation, and natural gas separation5–9. The synthesis of mixed matrix membranes (MMMs) is the most promising approach for using both organic and inorganic material properties at the same time, which also improves gas transport properties and may surpass the Robeson upper bound limit4,10. It involves the fabrication of composite membranes between polymeric materials as a base, and inorganic material as filler particles. Theoretically, by use of MMMs, benefits of both polymer and inorganic content can be used at the same time. Moore et al. identified the different non-ideal structures in MMMs such as rigidified polymer layer around the inorganic fillers, interface voids or sieve in-a-cage, and particle pore blockage11. In order to surpass the Robeson upper bound, the MMMs structure must be defect-free at the polymer/filler interface. For overcoming the poor adhesion, it requires careful selection of the filler and polymer, which must have good interaction with each other. However, the selection of fillers and polymers is limited to form a defect-free interface on the basis of these factors12. Polydimethylsiloxane (PDMS)13–15, polyether block amide (PEBA)5,16, matrimid17, polymers of intrinsic micro porosity  (PIM)18, polyethylene oxide (PEO)19, polyvinyl alcohol (PVA)20, and several polyimides21,22 are polymers that have been commonly used in the fabrication of MMMs, while carbon molecular sieves23, zeolites24, carbon nanotubes13, graphene oxide (GO)25,26, and metal-organic frameworks (MOFs)27,28, are the most extensively used nano porous materials as fillers in MMMs.
There is no data available for the separation of BD from its nitrogen mixture through the membrane separation process in the literature. Baker et al. stated that rubbery membranes are beneficial for the 99% recovery of hydrocarbons with a suitable process design29. Polydimethylsiloxane (PDMS) is a rubbery polymer and well-known membrane material due to its excellent chain flexibility makes it very permeable even at high operating temperature and low trans-membrane pressure. By the end of the last century, PDMS predominated and widely studied polymer for hydrocarbon separation. Past results have shown that the PDMS membrane has excellent potential for recovering hydrocarbons from nitrogen mixed streams30–32. Additionally, it is worth noticing that the 1,3-butadiene permeation through rubbery polymers such as PDMS is superior to its permeation through any glassy polymer since there exist two to three orders of magnitude difference in permeation33.
Zeolite imidazolate framework (ZIF) is a subfamily of MOFs, which is coordinated between the transition metals such as zinc and cobalt and imidazole linkers34. In recent years, ZIFs have engrossed deep interest as a versatile crystalline porous material for gas storage and gas separation applications. The highly crystalline and nano-porous structures of ZIFs, make them an ideal candidate for the separation of small kinetic diameters gas particles. The imidazolate linkers in ZIFs frameworks made them more hydrophobic as compared with any other filler types, which deliberates the excellent interfacial compatibility between polymers and ZIFs35. Moreover, the imidazolate linker property of structural flexibility makes its role more essential and vital as it involves in gate opening effect of ZIFs for large molecules than the pores of ZIFs36. ZIF-8 is the most studied ZIF material among all of its types in MMMs for the gas separation34,37,38. It is worth noticing that; besides the extensively researched linker-substituted effect, metal-substitution also influences the molecular sieving efficiency of the associated ZIF crystals. By introducing a second metal ion in the MOF cluster, the stability and affinity towards the target gases can be largely improved39–41. Many recent works have reported in which researchers have used mixed metal MOFs by adding another metal in the ZIF-8 crystal for properties enhancement41. The results were improved by the addition of another metal in the ZIF-8 cluster. To the best of our knowledge, the separation of low boiling point hydrocarbons (gases at room temperature) from permanent gases, using ZIF-based membranes, is very limited. Just one study reported by Fang et al. in which they used ZIF-8/PDMS/PVDF MMMs to separate propane from nitrogen mixture42. The C3H8/N2 selectivity was 38% improved by using 10% ZIF-8-MMMs relative to the pure PDMS membrane. Therefore, the ZIF-8 based MMMs should be further examined for the hydrocarbon separation from permanent gases.
This work, for the first time, employed Zn/Ni-ZIF-8 as a filler for the preparation of MMMs with PDMS polymer to improve the gas-separation properties of the membranes for the separation of BD/N2. Ni-ZIF-8 has the same zeolite nets-sodalite (SOD) topological structure with ZIF-8, while Ni just partially substitutes the ZnN4in the backbone of the ZIF structure, and tetrahedrally connected with the nitrogen, which makes it a very stable structure of four coordinated with Ni centers. Mixed metal ZIF was used to enhance BD affinity, which resulted in a sufficient adsorption capacity due to the presence of two metals in its cluster. The difference of gas permeation results was tested for single and mixed metal ZIF-8 in their MMMs. The effects of Ni-ZIF-8 loading on the microstructure of the membranes were investigated. The influence of Ni-ZIF-8 nanoparticle loadings feed temperature, and feed pressure on gas-permeation performance were also observed. Importantly, the gas transport properties across these MMMs for 1,3-butadiene and N2 were investigated on a preliminary basis by exploring the effects of different MMMs. This study will introduce the permeation and solution-diffusion properties of Ni-ZIF-8/PDMS MMMs for the removal of BD from nitrogen mixture, which will also provide a scientific contribution in the literature.