The effect of water adsorption on the structure of the carboxylate containing metal–organic frameworks Cu-BTC, Mg-MOF-74, and UiO-66

Abstract

Metal–organic frameworks (MOFs) with metal–carboxylate bonds, including Cu-BTC (HKUST-1), Mg-MOF-74 (Mg/DOBDC), and UiO-66, have been shown to have varying degrees of water stability. The three MOFs in this study are three of the most highly studied MOFs in the literature. We investigate here how each MOF degrades at several temperature and humidity conditions over the course of 28 days. At conditions of 90% relative humidity (RH) and 25 °C, water uptake for Cu-BTC is shown to be higher than at 90% RH and 40 °C, causing the degradation of the inner structure of Cu-BTC to occur more readily at the lower temperature. However the external surfaces of Cu-BTC degrade more readily, as shown through SEM images, at conditions of 90% RH and 40 °C. Mg-MOF-74 has a nearly complete loss of surface area after just one day of exposure to each of the conditions studied, however the PXRD patterns show only a change in the [100] peak. We offer here a novel mechanism for the degradation of Mg-MOF-74, involving a 6-coordinate Mg intermediate, which leaves the 1-dimensional channels of Mg-MOF-74 intact. Furthermore, we conclude that UiO-66 is stable to each of the aging conditions for the full 28 days of this study.