Porous metal-organic framework-based membranes for environmental and energy applications

Conventional separation processes have high-energy consumption and cause carbon emissions and environmental pollution. Advanced environmental treatment technologies are urgently needed to remediate complex environmental problems, including the capture of radioactive elements and compounds and valuable metals from water or wastewater. Membrane separation technology with low carbon and environmental footprints can open new routes for the exploitation of new resources, purify chemicals and extract valuable metals from air and water. Metal-organic frameworks (MOF) have high crystallinity, large porosity, and tuneable molecule-/ion-specific functional groups, making them potential candidates for improving membrane working performance in targeted applications by customizing the nanochannel structure and chemistry of the membranes. Two main problems, the dispersion of MOF and MOF-to-substrate adhesion of MOF-based membranes, restrict their wider application. Through the rational design of their structures and functionalities, MOF-based membranes can offer superior absorption of radioactive pollutants and separation performance of valuable metals. Therefore, this thesis aimed to develop innovative and efficient approaches for the preparation of cutting-edge MOF-based membranes for highly efficient molecule/metal separation to solve energy and environmental problems.

My work resulted in a state-of-the-art review of the latest developments and challenges of high-performance MOF-based membranes for air and water purification. This review emphasized efforts to overcome current problems which are a significant focus of membrane structure and design, in conjunction with the development of controllable and scalable approaches to improve MOF-based membrane performance in target applications. Established techniques to fabricate MOF-based membranes, including electrospinning, freeze-drying, post-synthetic polymerization, in-situ growth, and hot-pressing, which can achieve high MOF loading, good MOF dispersion, and MOF-to-substrate adhesion in MOF-based membranes, were summarized. Available MOF-based membrane architecture and substrate materials were reviewed and discussed. Promising strategies to customize MOF-based membrane morphologies, and control preferential reactions, transport pathways, or interactions with contaminants were suggested. Potential environmental applications and underlying mechanisms to evaluate MOF-based membrane performance to identify their effectiveness in improving pollutant transport properties are also presented. Perspectives on future developments of MOF-based membranes with facile, universal, energy-saving processes are offered to inspire researchers to design next-generation MOF-based membranes for the treatment of environmental problems.

Direct growth of MOF is a facile and scalable route to modify existing membrane materials. Ceramic membranes are potential candidates as substrates because of their stable structure and high-temperature resistance. Precisely tuning the irregular pore system of the ceramic membranes was crucial in this research. I attempted seed-assisted growth of in situ ZIF-L on and in microporous silicon carbide (SiC) membranes under mild conditions. The hybrid membranes contain highly loaded ZIF-L nanoflakes on the surface and pore channels of the SiC membrane. These hierarchical ZIF-L@SiC membranes with relatively large surface area, high ZIF-L loading levels, numerous active sites, and mass transportation channels exhibited a high molecular iodine removal efficiency of near 100 % and 96 % at a high flux of 9.37×10³ L m^-2 h^-1 with iodine/cyclohexane concentrations of 5 mg L-¹ and 50 mg L^-1, respectively. This study should inspire novel MOF-based ceramic membrane fabrication by constructing pore systems and offering a new radioactive waste treatment route.

The ever-growing global demand for lithium requires energy-efficient techniques to separate lithium ions (Li⁺) from natural resources and commercial wastewaters. A well-defined MOF-on-MOF membrane (UiO-66-X membrane, X=(COOH)₂ and NH₂) was prepared via twin zirconium source seed-assisted growth and was used for lithium extraction from synthetic Taijiner salt lake brine. By adjusting the media layer of a bilayer UiO-66-based membrane with different functional groups (-(COOH)₂ and -NH₃), the separation performance of UiO-66-(COOH)₂-on-UiO-66-NH₂ (90.8) for Li⁺ /Mg2+ selectivity was higher than UiO-66-NH₂-on-UiO-66-NH₂ (65.0). The Li⁺ transportation behavior in membrane nanochannels with different surface charge polarity, feed concentrations, and driving forces (e.g. concentration or electrical-driving forces) and underlying mechanisms of direct Li⁺ extraction were systematically investigated and elucidated. The UiO-66-(COOH)₂-on-UiO-66-NH₂ membrane showed excellent Mg²⁺/Li⁺ separation efficiency in synthetic Qinghai Taijiner salt lake brine containing a high Mg²⁺/Li⁺ molar ratio. This research will help inspire the design of MOF-based membranes, optimize ion-selective membrane architecture and functionality, explore universal guidelines for Li⁺ extraction in future industrial production and boost the circular economy.

Finally, developing artificial membranes with adaptive filtering and chemical/molecular species selective transport is essential for improving membrane-working performance. A new class of programmable UiO-66-(COONa)₂ membrane for ion-selective transport was fabricated by post-synthetic modification. This Zr-based MOF membrane, forming a network of ionic channels with carboxylic functional groups that exhibited smart ion permeability and selectivity by regulating the order of exposing ionic environments. The experimental results assess the effects of different ionic activators, including Mg²⁺ and K⁺ ions, respectively, on ionic selective transport in artificial subnanometer channels of UiO-66-(COONa)₂ membranes in single-component or multi-component systems. Pre-activated membranes exhibit excellent Li⁺/Mg²⁺ selectivity, including Li+/Mg2+ of 468.8±36.3 with Mg2+-activated membrane and Li⁺/Mg²⁺ of 331.31±14.04 with K⁺-activated membrane at a high salinity of 0.5 M Mg²⁺ and 0.01 M K⁺, Na⁺, Li⁺. Molecular dynamics simulations further assess ion-selective transport behaviors in different ionic activated channels. These findings will inspire next-generation MOF-based design, preparation, and adaptive functionality for mineral recovery.

My research focused on (i) designing and developing MOF-based membranes with three types of transport channels: MOF coating in/on the microporous SiC ceramic membrane and two MOF-on-MOF membranes; (ii) preparing these porous MOF-based membranes via different synthesis methods (e.g. seed-assisted or twin metal source-assisted in-situ growth and post-synthetic modification); (iii) evaluating their separation performance for radioactive pollutant (e.g. iodine) capture and energy-critical metal (lithium) recovery. My research will enhance understanding of well-designed principles of MOF-based membrane and underlying molecular species/ion-selective transport mechanisms and offer novel insights for the development of next-generation MOF-based membrane and the uptake of MOF-based membrane into the industry.