(Nanowerk Spotlight) The petrochemical industry relies on separating chemicals that differ by just fractions of a nanometer in size. Methanol must be purified from similarly-sized molecules in the production of plastics, pharmaceuticals, and fuels. Currently, this separation requires heating massive quantities of liquid mixtures until they boil at slightly different temperatures – an energy-intensive process that consumes as much electricity as the entire continent of Africa.
Previous attempts to replace distillation with membranes that filter molecules by size have failed because even tiny defects in the membrane structure create holes large enough for similarly-sized molecules to pass through together. A molecular sieve needs pores with dimensions controlled to within one-tenth of a nanometer – like building a fishing net where every hole must be exactly the same size down to the width of an atom.
Metal-organic frameworks (MOFs) – materials built from metal atoms linked by organic molecules – can theoretically achieve this precision. Their crystal structure naturally forms pores of consistent size. But manufacturing defect-free MOF membranes has proved impossible. The materials develop flaws at multiple scales: visible cracks between crystals, missing metal clusters that create large holes, and absent molecular links that distort the pore structure.
Scientists from Nanjing Tech University have now developed a method that eliminates these defects at every scale. Their two-step process, reported in Advanced Materials (“Angstrom-Scale Defect-Free Crystalline Membrane for Sieving Small Organic Molecules”), first tackles the missing metal clusters by slowing down the crystal formation. They replaced the standard zirconium-containing starting material with one that reacts more slowly, giving the metal clusters time to arrange perfectly.
The team then addressed the missing molecular links by exposing the membrane to additional linking molecules that fill in gaps in the structure. Using electron microscopes capable of imaging individual atoms, they confirmed the elimination of both types of defects. The resulting membrane achieves unprecedented control over pore size.
Schematic of construction of angstrom-scale lattice defect-free MOF membrane. Changing the metal source from ZrOCl2 to ZrCl4 was used to achieve the complete connection of metal clusters into a framework owing to the reduced reaction rate, thus realizing the elimination of clustermissing defects. The resultant MOF membrane (labeled as MOF-ZrCl4) was then immersed into ligands solution to heal the linker-missing defect, thus assembling the intrinsic angstrom-sized lattice pore in the membrane, labeled as MOF-ZrCl4(H). To illustrate the defects, the red ball and red stick stand for the missed cluster and ligand, respectively. (Image: Reprinted with permission by Wiley-VCH Verlag)
Testing demonstrated the membrane’s precision. It separated mixtures of methanol and methyl acetate – molecules that differ in width by just 0.9 angstroms (less than the diameter of a single atom). The membrane processed 3,700 grams of mixture per square meter per hour while capturing 99.6% of the methanol. Current commercial membranes cannot separate molecules with size differences this small.
The membrane maintained this performance over 300 hours of continuous operation – a crucial requirement for industrial use. It also worked effectively across different temperatures and concentrations of starting materials. The manufacturing process requires temperatures of only 80 degrees Celsius, compared to the 120-180 degrees needed for existing MOF membrane production.
The researchers validated their technique works with different types of MOF materials, suggesting broad applicability. They demonstrated similar results separating methanol from other industrial solvents like dimethyl carbonate and methyl tert-butyl ether.
This development marks a key step toward replacing energy-intensive distillation with membrane-based separation. The combination of precise molecular filtering, high processing speed, and operational stability addresses the core technical barriers that have prevented membrane adoption in industry. The manufacturing approach also provides insights into controlling crystal formation at the atomic scale – knowledge that may advance the development of other materials requiring molecular-level precision.
The advancement demonstrates that atomic-scale control over material structure can be achieved in practical applications. As industries face pressure to reduce energy consumption, this technology offers a concrete path to eliminating one of manufacturing’s most energy-intensive processes.
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