Impact of ZIF flexibility for aromatic vapor capture

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.