The dynamic competition in adsorption between gaseous benzene and moisture on metal-organic frameworks across their varying concentration levels

Abstract

Metal-organic frameworks (MOFs) have opened a new path for the construction of air/gas purification systems through a number of technological routes (e.g., adsorption and/or catalysis). To develop MOFs with upgraded adsorption performance, the breakthrough (BT) behavior of two MOF sorbents (e.g., MOF-199 and UiO-66-NH2) was investigated using gaseous benzene under the interactive conditions between the two key variables: (i) the inlet benzene vapor concentration (10 to 200 ppm) and (ii) relative humidity (RH = 0 to 100%). According to the comparison of adsorption performance derived using gaseous benzene, MOF-199 outperformed UiO-66-NH2 by 2 to 16-fold when assessed at 10% BT and zero RH. This observation thus reflects the high reactivity of Cu-metal sites for interaction with benzene ring compared with Zr sites. The dynamic adsorption data for the two MOFs at various RH conditions also indicated the competitive inhibition effect of water vapor on benzene adsorption mechanism, especially at low benzene concentrations (< 100 ppm) and high RH level of > 50%. Adsorption kinetics of benzene onto the two MOFs showed good fitting with both pseudo-first and -second-order kinetics at low RH (0–20%) conditions, whereas the results at high RH (> 50%) failed to fit with both kinetics (e.g., R2 ≤ 0.87) because of the high roll-up effect. The water vapor acted as a competitive component, reducing the performance of the MOF adsorbents in purifying humidified gaseous streams. That effect became more prominent as the benzene concentration decreased (≤ 50 ppm), reflecting the dynamic interaction between multiple adsorbates (water and benzene) and the adsorbent. In addition, the exothermic adsorption property was evident for both MOFs under the dry conditions, while the patterns were reversed under humid conditions (e.g., 100% RH). Overall, the adsorption properties of VOCs onto MOFs and their uptake mechanisms are affected sensitively by an interplay between the key variables (e.g., concentration levels of competing adsorbates) along with the structural stabilities of sorbents against water vapor