Characterisation of porous materials for the adsorption of volatile organic compounds

Abstract

Volatile organic compounds (VOCs) are a broad class of chemicals which, at elevated levels, can contribute negatively to the health and quality-of-life of affected individuals. Adsorption using porous materials a common method of reducing the levels of VOCs in indoor spaces. This work investigates the VOC adsorption performance of industrial and synthetic porous materials in real-world conditions via a comprehensive experimental study. Adsorbent hydrophobicity is a key parameter when considering application in environments with high relative humidity. To illustrate the humidity-dependant nature of a material’s adsorption hydrophobicity, single- and two-component gravimetric adsorption experiments were carried out. A new method of determining hydrophobicity indexes as a function of humidity was developed to illustrate the importance of the adsorption environment on selectivity, which correlated well with toluene adsorption following pre-exposure of humidity. To better understand the mechanism of competition occurring between water and toluene, a breakthrough analyser was designed and constructed. Fixed-bed measurements largely confirmed findings from gravimetric experiments, while revealing that displacement of toluene could occur in microporous adsorbents providing the critical pore filling pressure for water vapour was reached. A single-step, modulator-based synthesis procedure was devised for ZIF-8/67 and MIL-100, producing millimetre-centimetre scale monoliths with enhanced density while still possessing the base MOF’s physicochemical and morphological characteristics. ZIF monoliths demonstrated gravimetric VOC capture performances up to 217 and 232% higher than their powder counterparts at 0% and 80% RH respectively. These monoliths may be a promising technology for applications where volume is a critical consideration, such as packed-bed columns and air filtration. A comprehensive evaluation of VOC adsorption behaviour as a function of relative humidity for a range of adsorbents is reported. These results aim to highlight the interplay between pore size, aperture chemistry, and selectivity in determining the effectiveness of adsorbents in real world VOC capture.