Multi-component ppm level adsorption of VOCs on the ZIF-8 and UiO-66 MOFs: Breakthrough analysis with selected ion flow tube mass spectrometry

Abstract

In the recent years, Metal-Organic Frameworks (MOFs) emerged at a high and increasing pace and have been widely studied in the frame of applications such as gas storage, mixtures separation and heterogeneous catalysis. Nevertheless, the study of Volatile Organic Compounds (VOCs) adsorption on Metal-Organic Frameworks (MOFs) is still lagging behind compared to zeolites and other conventional adsorbent materials. The selection of an optimal adsorbent for the trapping of environmental VOCs, only present at ppmv levels, strongly relies on its low-coverage adsorption properties, which are often challenging to evaluate through conventional experimental methods. In this work, adsorption of VOCs on Metal-Organic Frameworks (MOFs) was investigated using a novel experimental methodology to measure breakthrough adsorption at ultra-low concentration amid 4–12 ppmv. Multicomponent breakthrough curves of a mixture of ethanol, ethyl acetate, 2-butanone and 2,3-butanedione were measured in anhydrous condition at 25 °C and 6 °C on two highly stable MOFs, namely ZIF-8 and UiO-66, using Selected Ions Flow Tube Mass Spectrometry (SIFT-MS) for monitoring. Furthermore, adsorption experiments were carried out at 6 °C and 90% RH to evaluate the effect of humidity on the VOCs uptake onto the selected MOFs. While ZIF-8 adsorption capacities were ranging between 0.3 and 1.8 mg g−1, UiO-66 exhibited significantly higher uptake towards the target compounds, with a concentration comprised between 1.8 and 51.9 mg g−1. Despite this relevant discrepancy, both MOFs shared a pronounced preferentiality towards the most hydrophobic molecules. Very large enrichment factors (between 100 and 5000) showed that these materials can increase many-fold the adsorbed phase over gas phase concentration, even when present at ppmv levels, which is crucial for the good operation of sensors based on affinity layers. Furthermore, VOCs adsorption on UiO-66 unveils intricate competitive mechanisms, leading to multiple steps of ethanol displacement while simultaneous adsorption of ethyl acetate, 2-butanon and 2,3-butanedione occurs.