(7g) Gravimetric and Spectrometric Assessment of Porous Materials for Removing Harmful Volatiles from Humidified Environments

Volatile organic compounds, or VOCs, are a broad class of chemicals commonly emitted from a range of sources, with elevated levels being associated with acute conditions described by ‘sick building syndrome’, encompassing a range of symptoms including headaches, fatigue, throat irritation, and nausea (Nakaoka et al., 2014). Finding ways to minimize exposure to these chemicals using reactive or adsorptive techniques could provide significant quality-of-life improvements to at-risk individuals. Typically the adsorption of VOCs is carried out using porous materials such as molecular sieves, zeolites, and activated charcoals (Zhang, Gao, Creamer, Cao, & Li, 2017), with emerging technologies like metal-organic frameworks (Vellingiri et al., 2016) and carbon nanotubes (Shih & Li, 2008) being applied to the field more recently. Indoor air-cleaning provides a unique process variable compared to most industrial VOC adsorption: high levels of humidity, exceeding the concentration of volatiles by several orders of magnitude. This discrepancy leads to significant levels of competition for adsorption sites, necessitating the design or use of adsorbents with high selectivity for volatiles over water, while still possessing sufficient free space to accommodate large quantities of the former.

Initial adsorption measurements combined with material characterisation identified high surface area and high microporosity as preferential properties for VOC adsorption. Materials with these properties, such as activated carbon and zeolite adsorbents, were highly sensitive to low concentrations of volatiles. After exposure to partial pressures of 0.5 % P/P₀, activated charcoal was found to have adsorption capacities of 264.1, 199.2, and 167.2 mg/g for toluene, n-hexane, and 2-butanone respectively. Adsorbents most commonly displayed type I and II isotherms following exposure to a variety of species, best modelled by the Dubinin-Astakhov volume-filling model, with the exception of an uncommon type V isotherm following dealuminated zeolite Y’s interaction with light alcohols. Two-component measurements found the impact of humidity on VOC removal performance depended strongly on the surface hydrophobicity of each material. Molecular sieve and silica gel adsorbents, due to their lack of selectivity, had negligible uptake of volatiles in the presence of water. Despite its extensive use as an adsorbent, the removal efficiency of activated charcoal was found to drop by a factor of five in the presence of 70% relative humidity. Only dealuminated zeolite Y was found to retain its capacity for toluene at the highest humidity studied, which has been credited to the hydrophobic nature of its pore apertures.

More in-depth studies into the impact of humidity on removal performance were carried out in breakthrough experiments through the use of humidity-resistant photoionisation detectors (PIDs). The sensitivity PIDs for VOCs, and their ability to analyse high-humidity gas streams , provides a unique opportunity for measuring multicomponent adsorption at concentration ranges approaching those in the real world. The adsorption of aromatic, aliphatic, and alcohol species in humid environments was carried out using activated charcoal, zeolite Y, and ZIF-8, due to their high surface area and hydrophobicity. This data, collected at concentrations approaching 5 part-per-million and in excess humidity, provide a link between industrial measurement techniques and the operating conditions typical in air-cleaning applications.

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