Abstract
Technological innovations allowing the capture of volatile organic compounds (VOCs), in particular aromatic compounds (BTEX), continue to be sought. Activated carbon (AC) is a widely-used adsorbent, but upon moderate temperature increases, typically 50 °C, it tends to release accumulated VOCs and thereby contribute to air pollution peaks. We evaluate herein the adsorption performances of ZIF-7 synthesized from a new and greener protocol as well as ZIF-8. These two MOFs exhibit flexibility triggered by guest–host interactions, for the adsorption of benzene, toluene and p-xylene (BTX). The desorption capacity of different materials at various temperatures is examined. Adsorption/desorption performances are evaluated using both the static method (volumetric analysis) and the dynamic method. The latter allows for the co-adsorption of the pollutant in the presence of water. Unlike traditional AC, ZIFs demonstrate unaffected adsorption performance in the presence of water. Both AC and ZIF-7 undergo significant desorption at low temperatures (55 and 93% of adsorbed toluene, respectively), potentially releasing high concentrations of VOCs when exposed to solar radiation, which eliminates the possibility of its application for indoor air treatment. Remarkably, ZIF-8 does not show significant BTX desorption for temperatures below 50 °C (14%), while maintaining excellent regeneration characteristics at higher temperatures. We propose that this specific property can be depicted by a local dynamic-motion mechanism. Toluene is strongly diffusion-controlled, requiring the overcoming of a high energy barrier to cross the narrow six-membered ring aperture, while the size of the aperture is reduced by a ratio of the linker induced by the toluene packing in the sodalite (SOD) cavity.