Mechanistic Insights into the Competitive Adsorption-Desorption of Volatile Organic Compounds in Mesoporous Matrices

Volatile organic compound (VOC) adsorption in porous materials is primarily governed by surface interactions and capillary forces and becomes further complicated when multiple compounds compete for the same adsorption sites. Since VOC emissions are multicomponent, it is imperative to understand the competitive nature of multiple VOCs adsorbing and desorbing in a porous adsorbent. This study explores real-time adsorption-desorption of polar ethanol and nonpolar heptane in 40 μm thick mesoporous silicon (p-Si) and silica (p-SiO₂) membranes, using in-situ transmission Fourier transform infrared spectroscopy (FTIR). The surface areas for the p-Si and p-SiO₂ membranes are 211 and 154 m² g^-1, respectively. All single-component and multicomponent sorption studies were conducted under dynamic flow conditions. Single-component sorption kinetics were fitted to pseudo-first- and pseudo-second-order models and equilibrium adsorption capacities to the Freundlich isotherm. Overlapping IR bands (CH region, 2800-3000 cm^-1) were deconvoluted to independently quantify each VOC in binary mixtures. In these experiments, competitive uptake between the two VOCs was strongly asymmetric. On p-Si, heptane displaced already adsorbed ethanol molecules at around 40% heptane concentration in the mixture, whereas, to replace already adsorbed heptane from the p-Si pores, approximately 65% ethanol concentration was needed. However, on p-SiO₂, the ethanol molecules resisted displacement until almost 70% heptane vapor concentration was reached, due to the hydrogen bonding between the polar ethanol and silanol bonds present in the p-SiO₂ membrane. These results demonstrate that dynamic selectivity in multicomponent adsorption depends on both surface chemistry and adsorption kinetics and cannot be inferred from equilibrium desorption studies alone. This in-situ spectroscopic study enables us to understand competitive adsorption-desorption kinetics without having to reach adsorption equilibrium. Such an approach can facilitate the design of porous material-based sensing and separation strategies to achieve rapid VOC uptake and recovery.