(667b) Effects of Acid Gases in a Chemically Stable Metal-Organic Framework

Metal-organic frameworks (MOFs) are promising materials for a variety of applications, including adsorption separation processes. Some of these potential gas separation applications involve exposure to acid gases (e.g. sulfur dioxide and nitrogen oxides), which may cause framework degradation or poisoning of adsorption sites, limiting the applicability of MOFs in adsorption processes or making their repeated use more difficult. In this work, MIL-101(Cr)—a chemically stable MOF predicted in literature to experience poisoning by acid gases and derivatives—is exposed to SO₂ and NO₂ at varying concentrations under both dry and humid conditions. Post-exposure nitrogen physisorption measurements demonstrate a loss of adsorption capacity—as measured by BET surface area—caused by exposure to SO₂ and NO₂, but without structural degradation (as indicated by powder x-ray diffraction). Repeated single-component breakthrough experiments (with sample regeneration conducted in situ) done with both SO₂ and NO₂ on MIL-101(Cr) show that this loss in adsorption capacity applies to multiple adsorbates and cannot be completely reversed by normal activation conditions. X-ray photoelectron and infrared spectroscopy conducted post-exposure and after regeneration conditions show retention of acid gas species and their derivatives in the MIL-101(Cr) samples, suggesting that the cause of the reduced post-exposure capacity includes acid gas species strongly bound to the framework structure. These results experimentally confirm a difficulty that acid gases may present in MOF adsorbents in addition to well-known issues of chemical stability. Hence, the results draw attention to a detrimental effect of acid gases to be accounted for in future work on chemically stable MOFs.