(674b) Towards a Generalized Understanding of the Acid Gas Stability of Zeolitic Imidazolate Frameworks (ZIFs)

The use of nanoporous materials as separation agents can provide attractive alternatives to thermally-driven, energy-intensive separations that account for 10-15 % of the global energy consumption. Acid gases are present in many environments in which the use of nanoporous materials is envisaged. The overall focus of this work is to evaluate, understand, and control acid gas (SO₂, CO₂, NO₂) interactions in an important class of nanoporous materials, namely Zeolitic Imidazolate Frameworks (ZIFs). ZIFs are a subset of metal-organic frameworks (MOFs), and have been extensively explored as membranes or adsorbents for different separation processes. However, there exists little systematic knowledge of the effects of acid gas exposure on the structure of ZIFs, in particular the mechanistic aspects of ZIF degradation by acid gases as well as the effects of crystal topology and linker composition.

In this talk, we will discuss our significant recent progress towards a generalized characterization of the kinetic and thermodynamic acid gas stability of a diverse range (>15) of ZIF materials under different environments – humid air, liquid water, and acid gases CO₂, and SO₂ (dry, humid, and aqueous). Two main findings that will be discussed in detailed are (1) the quantification of “stability” under humid SO₂ exposure through the measurement of degradation rate constants for ZIFs that can be used to compare materials in a meaningful way, as well as (2) the development of quantitative correlations of ZIF stability as a function of different structural and chemical descriptors. We have also compiled our characterization results in the form of an easy-to-use stability chart to aid researchers in selection of stable ZIF materials. In addition, we expand our investigation of ZIF stability to the acid gas NO₂ using selected ZIF materials from the stability chart under varying relative humidity conditions. NO₂ is observed to be more potent in destabilizing ZIFs compared to CO₂ or SO₂. Finally, we show the important role of humidity in a quantitative manner by measuring ZIF stability as a function of relative humidity of the acid gas. Overall, this work provides a comprehensive guide to understanding the acid gas stability of ZIFs, and its main features could be extended to other classes of porous materials and reactive species.