Highly selective CO2 uptake in Calix[4]arene compounds immobilized on silica gel

Abstract

Macrocyclic calix[4]arene molecules were immobilized onto silica surface and shown to be highly selective hybrid adsorbents for CO2 capture application. These hybrid adsorbents were prepared by immobilization of various calix[4]arene molecules, with differing upper rim functional groups: tert-butyl, amine, and calix[4]arene, onto surface of silica gel through 2,4-toluene diisocyanate (TDI) linker. Synthesized hybrid materials were characterized by Fourier transform infrared spectroscopy, thermal gravimetric analysis, nitrogen adsorption–desorption measurements, scanning electron microscopy and elemental analysis. Single component adsorption of pure gas CO2, CH4 and N2 were measured experimentally at three different temperature and pressure up to 30 bar using the volumetric method. Among all the materials, amino functionalized calix[4]arene, with calixarene loading of (~0.21 mmol g−1), exhibited improved CO2 uptake as well as exceptionally high ideal adsorption selectivity (CO2:N2 = 443) and (CO2:CH4 = 148) at 25 °C and 1 bar, mainly due to the presence of amine functional groups. Moreover, for investigating the affinity between CO2 and adsorption sites, isosteric heat of adsorption were calculated by Clausius–Clapeyron equation. Results of systematic experimental investigation in this report may pave the way toward later studies on these promising highly potential hybrid adsorbents in CO2 adsorption field area, according to their unique properties and designable structure.