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More scalable method for membrane separations

By Mary Page Bailey |

Membranes present a lower-energy alternative to distillation for the separation of hydrocarbons, but these processes are often limited by prohibitively expensive materials or intricate fabrication steps. Now, a team of researchers from Texas A&M University (TAMU; College Station, Tex.; www.tamu.edu) have developed a membrane-based separation technology for light olefins that is not only highly selective between similarly sized molecules, but also uses more scalable materials and methods.

The project centers around the separation of propylene and propane using a membrane composed of a zeolite-imidazolate framework (ZIF-8). ZIF-8 has been proven to effectively separate propylene and propane, due to its very small pore size. However, since polycrystalline ZIF-8 membranes are quite expensive, the team sought ways to decrease the cost of ZIF-8 membranes by drastically increasing propylene flux without adversely impacting its separation capabilities. The team integrated ZIF-90 into the membrane using a method called post-synthetic linker exchange (PSLE). Since ZIF-90’s pore size is much larger than that of ZIF-8, strategic layering of ZIF-90 into ZIF-8 resulted in a membrane that reduced by four times the effective thickness of the ZIF-8 layer, thereby increasing propylene flux by four times, without compromising the selectivity of the membrane. This type of PSLE can be scaled up to commercial levels, says Hae-Kwon Jeong, associate professor of chemical engineering at TAMU.

Currently, the membranes have been demonstrated on 2.5-cm alumina discs, but the team is now working to prepare the membranes on pre-formed modules made of polymer hollow fibers, making the membrane fabrication step much more readily scalable, explains Jeong. The next steps of the research will focus on applying the membranes to the polymer modules and further reducing the thickness of ZIF-8 layers, while still maintaining optimal separation factors. According to Jeong, chemical companies in the U.S. and Japan have shown interest in further developing the technology.

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