Chemical separation accounts for roughly ten to fifteen percent of the world’s energy consumption, due to conventional methods, like distillation, relying heavily on heat-driven processes. This however may be about to change, with metal-organic framework (MOF) membranes offering a greener alternative.

But for MOF membranes to reach wider commercialisation, with adoption in chemical and pharmaceutical manufacturing, inefficient fabrication methods must first be overcome. Addressing this bottleneck, NUS researchers have detailed a simple two-step strategy for producing scalable MOF membranes. This was published in Science Advances.

Why are membranes greener?

Membranes are selective barriers that block larger molecules but allow smaller ones to pass through. They perform this task under mild conditions and without additional chemicals, making the process energy efficient and environmentally friendly. This seemingly simple technology is key to today’s water purification and desalination processes.

The fact that membranes can function at room temperature makes them an especially good fit for separating temperature-sensitive products, including the intermediates of active pharmaceutical ingredients. However, in chemical industries where large volumes of harsh organic solvents are utilised, membranes made of conventional polymers can swell, even dissolve.

This is where MOF membranes, which are stable in organic solvents, stand out. These materials are highly promising for Organic Solvent Nanofiltration (OSN) applications, where liquid solutions containing organic solvents are filtered via pressure-driven membrane-based processes.

Sieving molecules through MOF membranes

MOF membranes are made up of metal ions and organic linkers in the form of sand-like porous crystals. Lead researcher Professor Zhao Dan from NUS Chemical and Biomolecular Engineering explained that turning these crystals into a continuous film is challenging because crystals will stop growing when they touch each other, leaving cracks and gaps larger than the material’s pores and allowing solutes to escape.

Defect-healing methods have already been developed to combat this problem, but these methods involve repetitive trial-and-error procedures, which impede their implementation in industrial-scale OSN. Thus, Zhao remarked that the “holy grail” in MOF membrane fabrication is to develop a method that can produce scalable yet defect-free MOF membranes.

No more trial-and-error

The research team zeroed in on the production of aluminium MOF (Al-MOF) membranes, which are particularly stable MOFs, and adopted a two-step metastable phase crystallization (MPC) strategy that combines sol-gel coating with vapor-phase reactions and ligand insertion. This strategy homogenises the way the crystals are formed in intermediate phases, accelerating crystallisation and producing structurally uniform membranes, minimising or neutralising the gaps between crystals.

Experiments showed that by attaching different ligands with the MPC strategy, the researchers could regulate the membranes’ retention performances within small increments of 200 g mol⁻¹ in molecular weight cut-off (MWCO), spanning 400 to 830 g mol⁻¹; lower MWCO indicate smaller pores, while higher MWCO indicate larger. This suggests that their fabrication method can produce designable pore sizes that can separate molecules with a precision sufficient for solute-solute separations, where the differences in molecular sizes can be extremely small.

Getting MOF membranes to market

To demonstrate the membranes’ scalability, the team tested their chemical stability under long runtimes typical to industry conditions. First author Dr Shi Dongchen highlighted one peak result: An Al-MOF membrane constructed with naphthalenedicarboxylate linkers stayed stable even after ten days of continuous filtration in harsh industrial organic solvents. After which, it still blocked over 99 percent of methyl blue dye, indicating that no major defects had formed.

The researchers also assembled a 10-tube membrane module with a large effective area of 230 square centimetres and tested it under high operating conditions. They observed over fivefold product enrichment, with over twofold dilution of the reactants.

Riding new pharmaceutical trends

Further, the emerging OSN field is especially relevant today because of the growing interest within the pharma industry towards continuous-flow manufacturing, said Zhao. Continuous manufacturing operates in a continuous flow, unlike the multi-stage unit operations seen in traditional batch-processing – optimising resources and reducing waste. Membrane separation does not require phase changes, making it a natural fit for continuous processes.

Major industry players have been closely following the team’s research. To further bridge the gap between lab-scale success and real-world demands, Zhao and his colleagues are now refining their designs based on industry feedback. With this momentum, they are optimistic about moving scalable MOF membranes out of the lab and into industrial application.