Polymer-grade propylene (>99.5%) is an important raw material in the chemical industry. The primary methods for producing propylene are steam cracking and propane dehydrogenation, which inevitably generate propane as a byproduct in the product steam. Achieving polymer-grade propylene requires the critical step of separating propylene from propane. However, this process is highly energy-consuming due to the extremely similar physical and chemical properties of the two molecules.
Molecular sieve membranes provide an energy-saving and effective solution for the separation process. Metal-organic frameworks (MOFs), known for their diverse structures and adjustable pore environments, are ideal candidates for these membranes. However, one challenge is that the "gate-opening" effect of flexible cage windows and non-selective intercrystalline defects, caused by weak grain intergrowth, limit the MOF membranes' molecular sieving capability. Additionally, MOF membranes are brittle, and collisions or abrasions during practical use can lead to a significant reduction in separation performance.
High wear resistance from replicating the biological armor texture (Image by PENG Yuan)
Recently, 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 (CAS) has developed a wear-resistant MOF membrane, ZIF-67, featuring a tangential-normal interweaving configuration, that enables accurate separation of propylene from propane. This was achieved by utilizing the paralyzed cage windows and addressing intergranular defects. The study was published in Nature Communications.
To survive in harsh natural environments, many plants and animals have evolved concave and convex surface textures that offer exceptional wear and collision resistance, such as crocodile skin, pangolin scales, and the skeletons of desert scorpions. Inspired by these natural, wear-resistant armor textures, the researchers first grew a precursor layer with an interlacing structure on a porous substrate. This layer was then elegantly transformed into a ZIF-67 membrane with a scale-like, interlacing architecture. The researchers demonstrated that this bioinspired ZIF-67 membrane could effectively separate propylene from propane with high performance.
The bioinspired membrane structure consisted of two distinct sections:the tangential (T) section, responsible for accurate separation, and the bulgy normal (N) section, which served as a wear-resistant armor. In the T section, the residual precursor, which was linked to the ZIF-67 grains inhibited the ligand flipping motions of the six-membered ring sieving windows in the sod cage, effectively eliminating intergranular defects.
The separation performance results revealed that the defect-free membrane achieved excellent separation of propylene and propane, with a separation factor exceeding 220. After 1.5 years of storage in ambient conditions, the paralyzed ZIF-67 framework remained stable, and its molecular sieving capability was well maintained, leading to a long-term stability for nearly 1,000 hours. Furthermore, after subjecting the membrane surface to severe sanding three times, the N armor section retained its propylene sieving performance, demonstrating a remarkable wear resistance.
Additionally, the researchers proposed that this innovative membrane architecture could be "transplanted" onto high curvature capillary ceramic substrates with an outer diameter of just 4 mm. This provided a practical guideline for the low-cost, large-area production of wear-resistant, high-performance MOF membranes, presenting promising potential for industrial application.
"MOF membranes are often regarded as an ideal solution for separation challenges. This bioinspired MOF membrane, developed by our team, addressed key issues in the MOF membrane field and demonstrated the versatility of MOF membranes for highly complex and demanding separation processes," said Prof. YANG.