This NSF award by the Chemical and Biological Separations Program supports work to advance the use of metal-organic framework materials as the active components in membranes for large-scale chemical separations. Membranes can play an enormous role in energy-efficient chemical separations, but traditional polymeric membranes suffer from a trade-off between selectivity and throughput. The premise of this proposal is that metal-organic frameworks (MOFs) have enormous promise as membranes for large-scale separations because of their potential for rational design of micropore size, shape, and functionality. Almost nothing is currently known about the performance of MOFs as membranes. Equally importantly, almost nothing is known regarding which of the thousands of MOF materials would be productive to pursue for membrane fabrication. Preliminary modeling work by us and experiments by others have demonstrated, however, that MOFs can be fabricated into membranes and that the performance of these membranes can be predicted under practical operating conditions. This field is poised for rapid growth, and our collaborative team is well positioned to lead this area. The proposed work will use a combination of computational modeling and experimental studies to develop new membranes based on MOFs.
This project will create many noteworthy opportunities for education and training. The research efforts will be linked with an innovative high school curriculum being developed in Gwinnett County, GA. The highly interdisciplinary approach will create learning opportunities for a variety of students: K-12, undergraduate, and PhD-level at Georgia Tech, including external undergraduates from underrepresented groups recruited through the SURE program.
Crystalline nanoporous materials such as zeolites are critical in multiple large-scale industrial applications, including key steps in petroleum refining and air separation. In recent years, great progress has been made in developing metal-organic framework (MOF) materials, which mirror the crystalline structure of zeolites but have greater ease of synthesis. The structured nanoconfinement defined by the pores of these materials gives rise to multiple properties that cannot be achieved with bulk or mesoporous materials. During this project, funds were primarily used to support a PhD student, Mr. Jaeyub Chung. Funds were also used to partially support a second PhD student, Mr. Jason Gee, and a postdoctoral fellow, Dr. Ji Zang. Work was also performed by Mr. Lasse Vilhelmsen, a PhD student from Denmark who visited Georgia Tech for six months. The PIs have also had 7 undergraduates perform research on MOF materials. Our work included computational modeling of several novel nanoporous materials. Molecular Dynamics (MD) simulations of CF4 diffusion in aluminosilicate nanotubes were combined with PFG-NMR experiments performed by Sergey Vasenkov and co-workers (U. Florida) to give the first assessment of molecular diffusion in these materials. DFT calculations and subsequent GCMC simulations with classical potentials were used to refine the structure and predict adsorption in new functionalized variants of the water stable MOF UiO-66, in collaboration with experiments at Georgia Tech by Krista Walton’s group. DFT calculations were also used as a basis for developing a force field that correctly accounts for the range of bonding environments available to water adsorbed in MOFs with open Cu sites such as CuBTC. Standard classical potentials cannot even qualitatively capture the experimentally observed uptake of water in these materials because they do not include the possibility of coordinative bonds forming with open metal centers. In a rather different application of DFT to MOFs, we combined DFT calculations with a genetic algorithm to study the stability and structure of small metal clusters in MOF-74. A number of high profile studies have introduced the possibility of creating small metal clusters in or on MOFs and using these clusters as catalysts. Our work was the first modeling study of these materials and we anticipate it will be influential in the future development of this part of the field. A key aspect of our work was to perform initial experiments and simulations to understand the adsorption and transport of hydrogen bonding species in MOFs. Our work focuses on a subset of MOFs called zeolitic imidazolate frameworks (ZIFs), which have very good thermal and chemical stability. The structure of ZIFs can be understood by analogy with zeolites by replacing the T atoms in zeolites with metal centers (typically Zn, although other metals are also possible) and O atoms that connect T atoms in zeolites with imidazole groups. This description highlights two importance characteristics of ZIFs. First, an enormous number of distinct ZIF crystal structures can be formed, just as there are many polymorphs of SiO2 zeolites. Second, ZIFs can be prepared containing a wide range of internal functionality by using substituted imidazolate linkers. As a result, ZIFs are a useful class of materials for systematically probing the effect of pore structure and functionality on adsorbed molecules. Our work used ZIF-8 and ZIF-90, two well-known ZIFs. The imidazole linkers in ZIF-8 (-90) are functionalized with –CH3 (-CHO), so only ZIF-90 has potential to form hydrogen bonds with adsorbates. The pore characteristics of the two materials are similar, although ZIF-8 has a slightly smaller pore limiting diameter. We have used Pulsed Field Gradient (PFG) NMR to measure self diffusion of MeOH and EtOH in each ZIF. To make reliable PFG-NMR measurements it is critical that large crystals are used. To this end, we adapted literature synthesis methods for each ZIF to grow crystals of diameter > 100 mm. Crystallinity was assessed using X-Ray Diffraction (XRD), and great care was taken to activate these large crystals to remove residual solvent from the synthesis procedure. The diffusivities measured in ZIF-8 and ZIF-90 with PFG-NMR showed that the activation energy for both alcohols is ~14 (17) kJ/mol in ZIF-8 (ZIF-90). The slightly larger activation energy in ZIF-90 is likely to be associated with the formation of hydrogen bonds between adsorbed alcohol molecules and the –CHO groups in the pores of this material. Although the activation energies for each example are similar, the net diffusion coefficients vary significantly. Most importantly, the diffusion selectivity for the two alcohols, DMeOH/DEtOH, is > 200 in ZIF-90, while it is < 10 in ZIF-8. This indicates that ZIF-90 membranes could have very high selectivities for MeOH/EtOH mixtures. This observation underlines the possibility of developing MOF-based membranes with exceptional properties.
