Zeolitic Imidizolate Frameworks (ZIFs) are nanoporous materials that have considerable potential in CO2/CH4 gas separation applications due to high CO2 adsorbance and selectivity. An extremely large number of possible ZIF structures can be constructed by varying the functionalization of the bridging imidazole, as well as the overall network topology. Combining molecular modeling and experimental studies we seek, in this work, to devise design principles for the optimization y of ZIF adsorption and selectivity for CO2/CH4 separation with respeect to both ZIF functionalization and topology. To examine the role of functionality on the uptake of CO2 and CH4 we study a series of ZIFs with identical topology, but varying functionalization (ZIF-25, -71, -93, -96, -97). By calculating both the adsorption and kinetic selectivities, we determine the membrane selectivities for these five ZIFs and find this quantity to be highest in ZIF-93, with a value of 26.7. First-principles calculations are used to study the barriers to transport for CO2 in ZIF-93 and ZIF-97. The high membrane selectivity of ZIF-93 is due primarily to electrostatic interactions, that result in smaller energy barriers for CO2 transport than in ZIF-97. Thus, for channels small enough to act as molecular sieves, the electrostatic field in the channel is found to be an important factor in the design of ZIFs for the separation of gas molecules possessing non-negligible electrostatic interactions with the framework. In addition to the work to functionality we also examine the role of topology by studying two sets of ZIFs (ZIF 7 & 11, 93 & 94) in which ZIFs with identical imidizolate functionalizations give rise to multiple network structures. These studies show that topologies with smaller pores have larger adsorptions than their large-pore counterparts at pressures below 1 bar; however, the opposite is true at higher pressures where the larger pore topologies have higher adsorption.

Functionality and topology dependence of CO2/CH4 adsorption and selectiviity in zeolitic imidizolate frameworks (ZIFs)

249th National Meeting (2015)

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