ZIF-8 Synthesis Induced By Imidazolium Functionalized Polysulfone And Its Mixed Matrix Membranes

Membrane CO₂ separation has many advantages,such as no phase change,simple and reliable process,no secondary pollution,etc.,but the traditional polymer membrane has the balance effect of selectivity and permeability,so the separation efficiency is difficult to meet the requirements.The mixed matrix films doped with metal organic framework microporous materials,especially the ZIF-8 nanoparticles with appropriate pore size(3.3?=d_CO2<3.4?<d_N2=3.64?),provide a new direction for the development of high-performance CO₂ separation films.However,ZIF-8 is a crystal material with regular surface,which is different from polymer in physical properties.In addition,it is easy to agglomerate when nanoparticles are transferred,resulting in agglomerate defects and interfacial compatibility defects in the mixed matrix films.In this study,ZIF-8 nanoparticles were synthesized in situ in aprotic polar casting solvent to solve the problem of agglomeration caused by particle transfer.Imidazolized polymer was introduced into the casting solution to enhance the interface bonding strength between the particles and the polymer by coordination with Zn?II?,and a CO₂ separation membrane with high selectivity and permeability was developed.Firstly,the synthesis conditions of ZIF-8 in DMF,DMAC and NMP were investigated.The results show that due to the strong coordination and solvation of aprotic polar solvents to metal cations,it is much more difficult to synthesize ZIF-8 nanoparticles by these systems than by the systems of methanol,water and other proton solvents,which require the higher ratio of Zn?II?/Hmim?from 1:8 to 1:4?and the higher reaction temperature?>105°C?.Even so,ZIF-8nanoparticles synthesized in DMF,DMAC and NMP has some problems,such as imperfect crystal form,wide particle size range?55²000 nm?,which is difficult to meet the requirements of preparing high-performance ZIF-8 mixed matrix films.Secondly,in view of the special role of non proton polar solvents,based on the heterogeneous nucleation promotion mechanism,it is proposed to add imidazolized polymer PSF-MimCl in ZIF-8 synthesis system,and form heterogeneous micro interface with relative enrichment of Zn?II?in polymer molecular neighborhood through coordination adsorption,so as to promote the rapid formation of ZIF-8 nucleation starting point and improve the synthesis conditions.The results show that a small amount of PSF mimcl?concentration?1.0 wt%?can promote the synthesis of typical Zn?II?/Hmim?1:8?.For DMF,DMAC and NMP,the lower limit of synthesis temperature of zif-8 was reduced to about 25?.In addition,the addition of PSF-MimCl improved the degree of crystal perfection and the range of particle size distribution.Further comparison shows that polymers such as PEI,PES and PVDF also have ZIF-8 growth promoting effect due to the existence of atoms with lone pair electrons,but the strengthening degree is obviously weaker than PSF-MimCl.The quantitative calculation shows that the charge density of Mulliken of nitrogen atom in PSF-MimCl is higher,and it is easier to capture Zn?II?to form ZIF-8 nucleation starting point when it is fully exposed in solvent,so the induction and promotion effect is the strongest.Finally,based on the above synthesized ZIF-8 nanoparticles,PSF-MimCl was used as the matrix to prepare the mixed matrix membrane.The results of gas permeation and tensile strength test showed that the best synthesis conditions of ZIF-8 were as follows:DMF as solvent with PSF-MimCl content of 1.0 wt%,Zn?II?/Hmim ratio of 1:8,reaction temperature of 45°C,reaction time of 24 hours.The corresponding ZIF-8 particle size range is 30-45 nm;the properties of the mixed matrix membrane prepared by in-situ synthesis method:the selectivity of?CO₂/N₂reaches 48.2,the permeability of P_CO2 reaches 49 barrer,and the tensile strength reaches 58 MPa.Compared with the mixed matrix membrane prepared by particle transfer after methanol solution synthesis,the selectivity is increased by 53.5%,the tensile strength is increased by 28.9%,and the permeability is basically unchanged.