| CO2separation is of great significance in the fields of chemical industry, environment and energy. Membrane technology receives much attention in recent years because it offers potential as an energy-efficient, low-cost and environment-friendly CO2separation option. Ionic liquids (ILs) are nonvolatile, tunable solvents of which many show high CO2selectivity and permeability. The use of ILs in CO2membrane separation processes is widely researched in last years. Supported ionic liquid membranes (SILMs) and polymerized ionic liquid membranes (PILMs) are mainly two kinds of ionic liquid membrane (ILM) ever investigated. However, the negative aspect of SILMs is the leaching of ILs from the support at high pressure gradient; the drawbacks of PILMs are low permeability and rareness of monomers. In this study, ILs-polymer composite membranes with high CO2separation performance and good stability were prepared and optimized.[bmim][PF6]-PVDF/PP and [bmim][Tf2N]-PVDF/PP membranes were prepared by solvent evaporation method, with [bmim][PF6] and [bmim][Tf2N] as ILs phase respectively, PVDF as polymer matrix and PP porous membrane as support. The importance order of ILs content, PVDF concentration and solvent evaporation temperature were firstly decided by orthogonal test. And then, preparation conditions were optimized in the study of their effect on the morphology and performance of composite membranes.The crystallization property of PVDF weakens gradually with the addition of ILs. ILs channels which facilitate CO2permeation will form as ILs content increasing to a certain amount. CO2separation performance is augmented with the increase of ILs. The maximum CO2/N2and CO2/CH4selectivity of [bmim][PF6]-PVDF/PP membranes are40.8and24.1respectively with [bmim][PF6] weight ratio to be60%, the corresponding CO2permeance is42GPU. CO2/N2separation performance begin to surpass Robeson’s upper bound line when [bmim][PF6] content reach to50%, which means the composite membranes prepared in this work outperform common polymer membranes. Composite membrane with [bmim][PF6] content as high as60%shows stable performance when the operating pressure increasing from0.2MPa to0.6MPa, which indicated a better pressure-resistant property compared with SILMs. It also demonstrates that the permeation of gases in composite membranes follows solution-diffusion mechanism. Composite membranes exhibit good stability when operating at0.4MPa for thirty days without any performance degradation. Gas permeance decreases and selectivity increases with the increase of PVDF concentration. An appropriate range of PVDF concentration is determined to be18%to24%. The rise of solvent evaporation temperature will cause declines in both CO2permeance and selectivity. A temperature range feasible to prepare membranes with good performance is30℃to60℃. CO2and CH4permeability of composite membranes was simulated using Maxwell model. The results shows that the values predicted with ILs as continuous phase are closer to experimental data, especially when ILs volume fraction surpasses50%.[bmim][Tf2N]-PVDF/PP membranes exhibit better separation performance with CO2permeance to be93.7GPU, maximum CO2/N2and CO2/CH4selectivity to be36.2and23.8. Both CO2/N2and CO2/CH4separation performance of [bmim][Tf2N]-PVDF/PP membranes can exceed Robeson’s upper bound line when [bmim][Tf2N] content reaches to50%. CO2permeance of [bmim][Tf2N]-PVDF/PP membranes is two or three times the permeance of [bmim][PF6]-PVDF/PP membranes with the same ILs content. The former also shows higher pressure resistance. |
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