Modification of zeolites with organic ligands for improved separations

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The separation of mixtures to yield high purity components accounts for a large fraction of the world's energy consumption. Separations of alkanes from alkenes, for example propane from propylene, are among the most difficult and energy intensive. Improving these separations processes is vital to decreasing the cost and environmental impact of commodity chemicals and plastics production. Microporous materials such as zeolites enable separations based on differences in molecular size. Efforts to further enhance zeolite separation efficiencies require new methods for controlling interactions between the gases and sorbents. This project will develop a new approach to selectivity control that uses deposition of tailored organic ligands on the zeolite surface and pores. By controlling the properties of the ligands, it is possible to regulate their location on the microporous material as well as the effective size and chemical functionality of the pores. Achieving this enhanced level of control can yield dramatic improvements in separations of molecules of similar size, including propylene and propane. Both experimental and computational methods will be used to understand how the organic ligands bind with zeolites and how the organic-modified material affects sorption and diffusion of alkanes, alkenes, and other key gases. The overall research effort is directed toward enabling the design of improved materials for gas separations. This research is also closely linked with discovery-based learning for undergraduate and high school students and with development and dissemination of online learning tools for science and engineering students at different education levels. This project aims to develop a new approach to selectivity control in separations. The approach employs the controlled deposition of organophosphonic acid ligands used in organic self-assembly processes to tailor the zeolite external surface and pores. Changes in the chemical structure of the ligands can be used to control their organization within the zeolites and can yield dramatic improvements in the selective adsorption of molecules of similar size, including propylene and propane. A combination of experimental and computational techniques will be used to develop structure-property relationships for ligand-modified zeolites. By varying both the zeolite structure and the size of the ligands, these studies will examine cases in which deposition is confined to the external surface of the zeolites, as well as situations where the modifiers can partially or fully enter the zeolite pores. The project will also focus on determining how the nature of the organic ligand can be tuned to alter diffusion of gases into the zeolite based on changes in ligand-gas affinities. Finally, quantum and classical molecular modeling tools will be employed to design improved combinations of ligands and zeolites tuned for selected separations such as propane-propylene and CO2-methane. The project will develop understanding of and the ability to control separations processes that account for major energy expenditures in the chemical industry. The proposed research will be carried out in conjunction with graduate, undergraduate, and high school students; this team will collaboratively develop and implement level-appropriate research, mentoring, and outreach goals. The team will also develop student-accessible active learning tools related to sorption- and membranes-based separations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.