Separating mixed substances can demand considerable amounts of energy—currently estimated to be approximately 15% of the total energy consumption—part of which is wasted due to process inefficiencies. In days of abundant and inexpensive fuel, this was not a major consideration when designing industrial separation processes such as distillation for purifying gasoline and polymer precursors. But as energy prices rise and policies promote efficiency, the need for more energy-efficient alternatives has grown.

One promising option for more energy-efficient separations is high-resolution molecular separation with membranes. They are based on preferential adsorption and/or sieving of molecules with minute size and shape differences. Among the candidates for selective separation membranes, zeolite materials (crystals with molecular-sized pores) show particular promise.

High-aspect-ratio zeolite single crystals with thickness in the nanometer range (zeolite nanosheets) are desirable for applications including building blocks for heterogeneous catalysts and the fabrication of thin molecular sieve films and nanocomposites for energy-efficient separations. They could also be of fundamental importance in probing the mechanical, electronic, transport, and catalytic properties of microporous networks at the nanoscale. Despite steady advances in the preparation and characterization of layered materials containing microporous layers and of their pillared and swollen analogs, the synthesis of suspensions containing discrete, intact, nonaggregated zeolite nanosheets has proven elusive because of structural deterioration and/or aggregation of the lamellae upon exfoliation.

Here, we report the isolation and structure determination of highly crystalline zeolite nanosheets of the MWW and MFI structure types, and we demonstrated the use of their suspensions in the fabrication of zeolite membranes.

—Varoon et al.

Images of the MFI nanosheet coating on porous supports. (A) SEM image (top view) of the coating of MFI nanosheets on an Anopore disk. The top half of the image shows the bare Anopore support; the bottom half shows a uniform coating of nanosheet on the 200-nm pores of the support. (B) SEM image (top view) of the coating of an MFI nanosheet on a homemade porous α-alumina support. (C) FIB image of the cross section of the coating in (B). The nanosheet coating is sandwiched between the FIB-deposited platinum (to protect the coating from milling) and the alumina support. (D) TEM image of the cross section of the coating in (B). The dark layer on top of the coating is FIB-deposited platinum. (E) HRTEM image of the coating cross section. Scale bars in (A) to (D), 200 nm; in (E), 20 nm. Varoon et al. Click to enlarge.

While zeolites have been used as adsorbents and catalysts for several decades, there have been substantial challenges in processing zeolitic materials into extended sheets that remain intact. To enable energy-savings technology, scientists needed to develop cost-effective, reliable and scalable deposition methods for thin film zeolite formation.

The University of Minnesota team used sound waves in a specialized centrifuge process to develop “carpets” of flaky crystal-type nanosheets that are not only flat, but have just the right amount of thickness. The resulting product can be used to separate molecules as a sieve or as a membrane barrier in both research and industrial applications.

Members of the research team include Ph.D. candidates Kumar Varoon and Xueyi Zhang; postdoctoral fellows Bahman Elyassi and Cgun-Yi Sung; former students and Ph.D. graduates Damien Brewer, Sandeep Kumar, J. Alex Lee and Sudeep Maheshwari, graduate student Anudha Mittal; former undergraduate student Melissa Gettel; and faculty members Matteo Cococcioni, Lorraine Francis, Alon McCormick, K. Andre Mkhoyan and Michael Tsapatsis.

This research is being funded by the United States Department of Energy (including the Carbon Sequestration Program and the Catalysis Center for Energy Innovation – An Energy Frontier Center), the National Science Foundation and a variety of University of Minnesota partners.

Resources

• Kumar Varoon, Xueyi Zhang, Bahman Elyassi, Damien D. Brewer, Melissa Gettel, Sandeep Kumar, J. Alex Lee, Sudeep Maheshwari, Anudha Mittal, Chun-Yi Sung, Matteo Cococcioni, Lorraine F. Francis, Alon V. McCormick, K. Andre Mkhoyan, and Michael Tsapatsis (2011) Dispersible Exfoliated Zeolite Nanosheets and Their Application as a Selective Membrane. Science 334 (6052), 72-75. DOI: 10.1126/science.1208891