Ionic liquids based materials for air purification

Resumen

Exposure to air contaminated with volatile organic compounds (VOCs) is proven to cause acute and chronic health effects in humans, including eye, nose and throat irritation, headaches, fatigue and damage to internal organs. Some VOCs are known to be carcinogenic and endocrine disruptors, posing even more serious long-term risks. VOCs are most damaging when concentrated in indoor environments. What is needed is a small, relatively inexpensive air purifier device that can effectively, efficiently remove even low concentrations of VOCs from indoor environments. Current commercial air purifiers rely on activated carbon layer for removing indoor VOCs, having relatively short lifespan and the impossibility of regeneration, fact that implies the material disposal and replacement after usage. Ionic liquids (ILs) may be the answer to this problem. Taking advantage of their unique properties that characterize them as environmentally benign “green solvents”, a deliberate screening of commercially-available ionic liquids potentially useful for VOC capture was conducted. An extensive literature on ionic liquids (ILs) and metal organic frameworks (MOFs) provides a rich selection to formulate novel composite materials with the desired physicochemical properties for capturing VOCs, outperforming traditionally used activated carbon, charcoals and molecular sieve materials. Key parameters affecting the solubilities of gases in ionic liquids were compiled from an in-depth literature review, and analyzed, producing seven commercially available ionic liquids that were potentially suitable for VOC capture in an indoor device. A screening of these ionic liquids array was conducted, and three ionic liquids were selected as candidates for VOCs capture. Thermodynamics (in terms of Henry’s law constants, partition coefficients, enthalpies and entropies of adsorption) and kinetics (diffusion coefficients) of these three ionic liquids were assessed and modelled in detail, establishing a proper criterion for the selection of the most appropriate ionic liquid. Selected ionic liquid exhibited a 40-60% toluene partition coefficient improvement compared to conventional solvents as PEG 400, oleyl alcohol and n-hexadecane, a saturated absorption capacity of 586 mg/g at 20 °C and diffusion coefficients two times higher than other ionic liquids. It was found that anion selection within ionic liquids played a key role in toluene capture performances. Metal organic frameworks (MOFs) have been selected as suitable materials for supporting ionic liquids due to their high surface areas, various pore geometries, facile functionalization and tunable porosities, with the purpose of developing novel composite materials for more efficient VOCs capture, overcoming common issues related to ionic liquids, namely high viscosities and low partition coefficients. Internal pore properties were optimized by the addition of ionic liquids for favorable VOC capture without damaging the framework structure at optimum ionic liquid loading. The selected IL/MOF composite material showed excellent performances in BTEX (benzene, toluene, ethylbenzene and xylenes) capture compared to traditionally used activated carbons (commercial and novel activated carbon fibers), zeolites and common MOFs such as ZIF-8 and HKUST-1. An improvement of 10-12 times in toluene uptake at low concentrations (0.5-3mbar) compared to parent MOF material was achieved; this represents an overall 100% sorption improvement with respect to commercial activated carbons. Kinetics of adsorption were modelled, studied and compared between materials and were generally boosted by the impregnation of ionic liquids in the supporting material. The selected composite would substitute activated carbon layer’s place in an air purifier device, for a more effective capture of indoor VOCs, with the possibility of regeneration and use in subsequent sorption cycles, expanding the lifespan and reducing the technology costs.