Title : Computational studty of pillararenes as selective sorbents for greenhouse gases
Abstract:
Pillar[n]arenes (P[n]A) are emerging as highly tunable supramolecular materials for CO2 capture and industrial gas separation, yet the molecular factors governing their adsorption performance remain incompletely understood. Unlike traditional rigid frameworks, the unique "pillar" architecture of these macrocycles provides a symmetrical, electron-rich cavity that can be precisely engineered for specific guest molecules. In this work, we combine high-level Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) calculations with rigorous experimental characterization to systematically investigate how cavity size, guest polarity, and functionalization influence adsorption in P[n]A-based systems. Our computational results reveal that CO2 binds most strongly at "cavity-in" sites of P[4]A and P[5]A through a delicate balance of multiple non-covalent interactions. Specifically, the binding energy is dominated by C-H ××× O hydrogen bonding and p-p interactions between the aromatic host rings and the guest molecule. In contrast, common flue gas components such as CH4, N2, and CO exhibit significantly weaker physisorption, as they lack the significant quadrupole moment and polarizability required to stabilize within the confined macrocyclic space. Furthermore, toxic gases including NO2 and NH3 show enhanced adsorption profiles, suggesting that these materials could serve a dual purpose in both carbon mitigation and environmental remediation. To further refine these properties, we investigated functionalized derivatives, including P[5]A-OCH3, P[5]A-OCOH, and the oxidized P[5]Q (Pillar[5]quinone). These modifications tune the binding strength by modulating electronic redistribution across the phenolic units and inducing subtle shifts in the cavity geometry. Experimentally, the optimized synthesis and recrystallization of P[5]-OH and P[5]Q confirm these host-guest interactions via Infrared (IR) spectroscopy and ab initio analysis, showing clear vibrational shifts that correlate with predicted binding affinities. Upon activation, P[5]A-OH forms stable, crystalline porous frameworks that achieve high CO2/N2 selectivity. Crucially, these materials maintain their structural integrity and performance under both dry and humid conditions, a vital requirement for real-world flue gas processing. By bridging the gap between quantum mechanical simulations and macroscopic gas-uptake measurements, this study provides a unified structure-property framework. These results offer a predictive roadmap for the rational design of next-generation P[n]A sorbents tailored for high-efficiency carbon capture and industrial gas purification.
