Beyond Solution-Based Protocols: MOF Membrane Synthesis in Supercritical Environments for an Elegant Sustainability Performance Balance

Abstract

Supercritical fluids (SCFs) represent materials at temperatures and pressures above their critical points where distinct liquid and gas phases do not exist. SCFs possess several unique properties rendering them excellently adapted for the reaction medium of MOF membranes (like ZIF-8) which, although offering an energyefficient and eco-friendly option for large-scale industrial separation, are still experiencing a dilemma between process sustainability and performance superiority. In this study we addressed this issue through a facile SCF synthetic protocol employing supercritical CO 2 (scCO 2 ) as the reaction medium. Relying on the near-zero surface tension, low viscosity, and high diffusivity of scCO 2 , nanoscale intercrystalline defects could be effectively patched, resulting in the formation of well-intergrown ZIF-8 membranes with commercially attractive olefin/paraffin (C 3 H 6 / C 3 H 8 ) separation performance. Moreover, after the SCF processing, both discharged CO 2 and unreacted ligands could be conveniently recovered and reutilized, resulting in zero pollutant discharge due to the controllable phase transition, variable solubility, and chemical inertness of scCO 2 . This work sheds lights on the promising prospects of SCF processing for sustainable fabrication of high-performing MOF membranes.

Role of supercritical carbon dioxide (scCO2) in fabrication of inorganic-based materials: a green and unique route

“…Then, sprayed CO 2 into the chamber was spontaneously transformed to scCO 2, resulting in supercritical growth. Finally, the recovery of both discharged CO 2 and unreacted 2-mIm ligands was conveniently performed ( Figure 9 ) [ 122 ]. Notably, the intercrystalline defects could be productively reinforced through this process, leading to the formation of complete ZIF-8 membranes with paraffin/olefin (C 3 H 8 /C 3 H 6 ) departure efficiency.…”

“… (a) Schematic illustrating the proposed scCO 2 processing of ZIF-8 membrane, (b) Schematic illustrating the in situ growth of ZIF-8 membrane in scCO 2 . Reproduced with permission from Ref [ 122 ]. Copyright 2020, American Chemical Society

Figure 10.

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Sustainable Fabrication of the Zeolitic Imidazolate Framework-67 Membrane via Supercritical Fluid Processing of the Co-Based Gel Layer

“…Very recently, we further prepared propylene-permselective ZIF-8 membranes relying on a supercritical CO 2 (scCO 2 ) fluid technique. 27 Compared with other protocols, with this technique, both used CO 2 and residual 2-methylimidazole (2-mIm) ligands were able to be sufficiently recovered, giving rise to no pollutant discharge and therefore a satisfactory balance between process sustainability and performance superiority. In contrast, there remained few studies that focused on sustainable fabrication of other kinds of ZIF membranes.…”

“…In particular, several unique physicochemical properties of SCFs (e.g., supercritical CO 2 , abbreviated as scCO 2 ), such as variable solubility toward organic compounds, zero surface tension, low viscosity, high diffusivities, and chemical inertness, have rendered them quite promising for the sustainable production of high-quality polycrystalline MOF membranes. Based on our previous expertise in SCF processing, 27 herein, we aim to prepare a continuous ZIF-67 membrane on a porous α-Al 2 O 3 substrate via in situ supercritical growth. It is expected that through the introduction of an appropriate Co-based gel layer, the highperformance ZIF-67 membrane could be facilely prepared under mild conditions in a sustainable manner.…”

“…Subsequently, ground powders were placed at the bottom of a homemade chamber. 27 Then, the Co-based gel layer-modified substrate was placed on top of the chamber with the gel layer side facing down. Finally, the sealed semi-permeable cylindrical cell was placed inside the SCF vessel.…”