Supercritical CO₂ Assisted Preparation Of Zeolitic Imidazolate Framework Membranes For Gas Separation Application

Metal-organic framework（MOF）membranes have been widely utilized in diverse industrially-relevant gas separation processes.In recent decades,diverse synthetic protocols have been developed for the preparation of various MOF membranes with superior gas separation performance.Nevertheless,it remained a great challenge for balancing process sustainability and performance superiority.Recently,supercritical fluid（SCF）processing has emerged as a sustainable and efficient technique and as an alternative to conventional solution-based protocols.In particular,several unique physicochemical properties of SCFs,such as variable solubility toward organic compounds,zero surface tension,and high diffusivities,rendering them excellently adapted for the reaction medium of MOF membranes.Herein,a novel SCF processing was employed for sustainable preparation of various polycrystalline MOF membranes for efficient gas separation,with an elegant sustainability-performance balance.（1）As a demonstration of this concept,we aim to prepare polycrystalline ZIF-8membranes with commercially attractive C₃H₆/C₃H₈ separation performance via novel SCF processing.In this work,we pioneered the sustainable fabrication of well-intergrown ZIF-8membranes through in situ supercritical growth on ZnO buffer layer-modified porousα-Al₂O₃substrate with supercritical CO₂（scCO₂）as the reaction medium.Relying on the unique physicochemical properties of scCO₂,after SCF processing,both used scCO₂ solvents and residual 2-methylimidazole（2-m Im）ligands were able to be sufficiently recovered.Meanwhile,gas permeation results indicated that separation factor of equimolar C₃H₆/C₃H₈ gas mixture through prepared ZIF-8 membrane reached 47.1 with a C₃H₆ permeance of 29.2×10^-10mol·m^-2·s^-1·Pa^-1,which easily met the proposed performance criteria for commercial applications.All the above results clearly demonstrated that SCF processing held great promise for sustainable preparation of high-performance ZIF-8 membranes.（2）To verify the generality of this protocol,we further to prepared versatile MOF membranes via SCF processing.Taking ZIF-based membranes as an example,herein,well-intergrown ZIF-67 membrane showing attractive H₂/CO₂ selectivity was prepared on the Co-based gel layer-modified porousα-Al₂O₃ substrate through in situ supercritical growth in scCO₂environments.It was found that in comparison to solution-based protocol,only under scCO₂conditions could the Co-based gel layer be converted to continuous ZIF-67 membrane.Specifically,elaborate tailoring the drying temperature of Co-based gel layer and the SCF processing parameters led to the generation of continuous ZIF-67 membrane with satisfying gas separation performance.Prepared ZIF-67 membrane displayed H₂ permeance of 2.6×10^-7mol·m^-2·s^-1·Pa^-1and ideal H₂/CO₂,H₂/N₂and H₂/CH₄selectivity of 11.4,8.1 and 7.1.In addition,we could also prepare well-intergrown pure MAF-6（[Zn（e Im）₂]）membrane via SCF processing of Zn-based gel layer,thereby demonstrating that this protocol in general holds great promise for sustainable preparation of divrese MOF membranes.（3）A seed-assisted SCF processing was proposed for the preparation of highly C₃H₆-selective ZIF-8 membranes with improved microstructural morphology.In this work,we report a seed-assisted SCF synthetic strategy to prepare highly propylene-selective ZIF-8 membrane through deposition of the ZIF-8 seed layer followed by supercritical growth in scCO₂environments.Our research indicated that not only surface deposition of uniform and closely packed seed layer but also maintenance of scCO₂ in the gas-like region were indispensable for the preparation of qualified ZIF-8 membranes with superior gas separation performance.Prepared ZIF-8 membrane displayed C₃H₆ permeance of 149.6×10^-10 mol·m^-2·s^-1·Pa^-1and C₃H₆/C₃H₈ selectivity of 51.8,which was superior to that of above-mentioned ZIF-8 membrane prepared via in situ supercritical growth,thus demonstrating the significance of the introduction of ZIF-8 seed layer in improvement of C₃H₆/C₃H₈permselectivity of prepared ZIF-8 membrane under scCO₂environments.（4）We pioneered the all-solvent-free fabrication of well-intergrown ZIF-8 membranes via coupled atomic layer deposition（ALD）processing and SCF processing,which is expected to improve the process sustainability.Relying on the unique advantages of coupled ALD-SCF processing,pre-deposited ZnO buffer layer with high chemical activity could be transformed to continuous ZIF-8 membrane in scCO₂ environments.Prepared ZIF-8 membrane exhibited attractive ideal C₃H₆/C₃H₈ selectivity of 37.5 as well as considerable C₃H₆ permeance of58.1×10^-10 mol·m^-2·s^-1·Pa^-1.Moreover,after all-solvent-free ALD-SCF processing,Zn-based metal sources（ZnO phase）,2-m Im ligands and scCO₂ solvents all could be fully recovered so that zero pollutant discharge could be realized.This work convincingly demonstrated that coupled ALD and SCF processing holds great potential for highly sustainable fabrication of high-quality ZIF-8 membranes.