The adsorption of several classes of volatile organic compounds by materials with a range of pore size distributions and chemistries were assessed gravimetrically in both dry and wet carrier gas conditions. Measurements carried out at room temperature, and a range of relative humidity values (RH) from 0 to 70%, reflected real-world conditions similar to those of indoor air. Dry removal performance appeared to be dependent on the surface area of adsorbents and, for polar compounds, the relative hydrophobicity of the material. Performance of sorbents with hydrophilic surface chemistry, such as silica gel and molecular sieve 13X, decreased drastically with small increases in pre-exposed humidity. Activated charcoal and high-silica faujasite Y retained their capacities for toluene in relative humidities up to 50% and 70% respectively, with their selectivity for non-polar species credited to hydrophobic pore structure and low water vapour uptake. These conclusions help to emphasise the importance of process humidity as a key parameter when designing or selecting adsorbents in realistic process conditions. Additionally, the methods used in this study provide a simple and reproducible way of testing porous materials for applications requiring or involving high levels of relative humidity.
The vapour sorption properties of solid materials are recognized as critical factors in determining their storage, stability, processing and application performance. These properties are routinely determined for many natural and man-made materials and have traditionally been evaluated by storing samples in sealed jars containing saturated salt solutions of established relative humidity and then regularly weighing these samples until equilibrium is reached. However, there are a number of disadvantages associated with these manual jar methods.