In the chemical industry, polymer-grade propylene, which exceeds 99.5% purity, is a key raw material used in the production of plastics and other polymers. However, its production is not without challenges. As propylene is produced, propane is generated as an unavoidable byproduct. The separation of these two molecules is crucial for obtaining polymer-grade propylene, but it is also an energy-intensive process due to the striking similarity in their physical and chemical properties. The conventional methods to separate propylene from propane, typically involving complex distillation processes, consume large amounts of energy and are costly.
Molecular sieve membranes have emerged as an energy-efficient alternative for this separation, but the journey to optimize these materials for high-efficiency separation has been challenging. Metal-organic frameworks (MOFs), in particular, have shown promise as the materials of choice for molecular sieving membranes due to their tunable pore sizes, structural diversity, and chemical stability. Despite their advantages, however, MOF membranes face significant hurdles that limit their performance in large-scale applications, especially when it comes to separating molecules of similar sizes, such as propylene and propane.
One of the key challenges with MOF membranes lies in their “gate-opening” effect, where flexible cage windows within the material may inadvertently open or close depending on various external conditions. Additionally, non-selective intercrystalline defects in MOF membranes—tiny spaces or gaps between the individual crystals—also contribute to reduced selectivity and molecular sieving capability. Furthermore, while MOFs are incredibly efficient in separation, they are notoriously brittle. Collisions or abrasions during use can cause structural damage, degrading the performance of MOF membranes in industrial settings.
A research team led by Prof. Yang Weishen and Assoc. Prof. Peng Yuan from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences recently published a study in Nature Communications offering a solution to these issues by developing an innovative wear-resistant MOF membrane that can achieve highly efficient propylene–propane separation, even in the face of practical stresses.
The Wear-Resistant ZIF-67 MOF Membrane: Bioinspired Design
The key to this breakthrough lies in the membrane’s unique structure, inspired by the wear-resistant features found in nature. Throughout evolution, animals and plants have developed intricate surface textures designed to protect them from environmental wear and tear. For instance, certain species of fish and insects possess scales that resist abrasion, while others have evolved armor-like textures capable of withstanding collisions. The researchers at DICP took a cue from these natural structures, employing bioinspired design principles to enhance the stability and performance of their ZIF-67 MOF membrane.
The team’s approach centered on creating a precursor layer with an interwoven structure that was later converted into a ZIF-67 membrane. ZIF-67 (a type of zeolitic imidazolate framework) is widely known for its tunable pore size and stability. Upon further refinement, the structure mimicked the scale-like interlacing patterns observed in natural armor, providing the membrane with both efficient molecular sieving capabilities and impressive physical durability.
The final MOF membrane contained two distinct structural sections:
1. Tangential Section (T): This part of the membrane played a crucial role in propylene–propane separation. The tangential section was specifically engineered to prevent ligand flipping motions of the ZIF-67 cage windows. In doing so, it minimized intergranular defects—spaces between individual ZIF-67 crystals that could otherwise allow unwanted permeation of propane. This structural precision ensured that the membrane could selectively separate propylene from propane based on size exclusion and molecular interaction.
2. Normal Section (N): The bulging normal section functioned as a protective armor. This region of the membrane was designed to withstand abrasion and wear, especially under conditions that might otherwise cause mechanical stress. The scale-like, interlacing structure offered a durable barrier that protected the membrane’s sieve functions from external damage.
Outstanding Separation Performance and Durability
The novel bioinspired ZIF-67 membrane demonstrated remarkable performance in separating propylene from propane, with an exceptional separation factor of over 220—an indicator of its ability to distinguish between molecules of nearly identical size. This level of separation factor is significantly better than traditional separation technologies such as distillation, underscoring the promise of this MOF membrane for future industrial applications.
Furthermore, the membrane’s ability to maintain its separation efficiency over time was a significant breakthrough. Even after being subjected to a 1.5-year storage period in ambient conditions, the ZIF-67 membrane demonstrated excellent long-term stability. Crucially, the structure’s paralyzed framework did not degrade, and the separation capabilities were maintained for up to nearly 1,000 hours of operation. This is a critical achievement because longevity is a key requirement for industrial membrane materials.
The team’s bioinspired design also proved to be incredibly resistant to wear and tear. The N section, which served as the “armor” of the membrane, was subjected to three rounds of severe sanding, simulating physical stress in a manufacturing or operational setting. Remarkably, even after these abrasive tests, the membrane retained its effective separation performance. This demonstrated that the design was not only effective for molecular sieving but also durable under harsh physical conditions, making it far more reliable than traditional MOF membranes that often crack or degrade when exposed to mechanical wear.
New Manufacturing Opportunities
Another exciting aspect of this innovation is the potential to scale the production of the bioinspired membrane at a low cost. The researchers suggested that the advanced architecture could be transferred onto high-curvature, small-diameter capillary ceramic substrates. These substrates, with outer diameters as small as 4 mm, provide a platform for integrating the new ZIF-67 membrane into high-efficiency filtration systems, making it possible to mass-produce large-area membranes that are both energy-efficient and durable.
By adhering to bioinspired design principles and developing a more stable and durable MOF membrane, the DICP team has created a product that offers significant advantages for industrial applications, such as propylene–propane separation, and beyond.
Potential for Industrial Application
The development of bioinspired MOF membranes presents not only a powerful technological advancement but also a practical pathway to more cost-effective, energy-efficient separation technologies. Traditionally, the propene separation process is associated with high energy demands and operational costs, especially because of the near-identical chemical and physical properties of propylene and propane. The wear-resistant ZIF-67 membrane could allow for a more energy-efficient and cost-effective approach to this crucial chemical separation process.
The team’s solution could also be applied to other challenging molecular separation tasks in various industries, such as gas separation, water filtration, and more. The improved mechanical resilience and molecular selectivity of the new membrane open up avenues for applications where existing technologies might fail or be inefficient.
This research aligns with a growing trend of bioinspired materials science, which borrows strategies from nature to solve complex engineering challenges. The team, led by Prof. Yang Weishen and Assoc. Prof. Peng Yuan, expressed their confidence that their bioinspired MOF membrane addresses crucial challenges in the membrane field, including fragility and selectivity, and that it will continue to influence future research into complex, demanding separation processes.
Conclusion
This study represents an exciting step forward in membrane technology, combining bioinspiration, material innovation, and sustainability to solve real-world problems. The bioinspired ZIF-67 membrane developed by the DICP team has addressed critical challenges facing MOF membranes: durability, selectivity, and wear resistance. The team’s pioneering work in combining these features promises to revolutionize the way industries approach molecular separation, paving the way for more efficient, cost-effective, and sustainable practices in industries ranging from petrochemicals to water treatment.
By advancing the understanding and application of MOF membrane technology, this research sets the stage for future industrial-scale applications, where durable, energy-efficient, and environmentally friendly solutions will be paramount.
Reference: Lun Shu et al, Metal-organic framework membranes with scale-like structure for efficient propylene/propane separation, Nature Communications (2024). DOI: 10.1038/s41467-024-54898-4