Post-Synthetic Thioesterification of Carboxyl-Functionalized HCP for Enhanced CO₂ Capture and Selectivity

Post-synthetic modification (PSM) is an effective route to improve the CO₂ capture and CO₂/N₂ selectivity of hyper-cross-linked polymers (HCPs) by introducing tailored adsorption sites. A key challenge, however, is achieving these improvements without substantially compromising porosity. Here, we report a simple PSM strategy that incorporates multiple CO₂-philic functionalities through a condensation reaction between a carboxyl-functionalized HCP (HCP-TRIP) and thiourea-formaldehyde resin, yielding a thioester-functionalized material (HCP-TRIP-P). The modification largely preserves the porous framework, with only a minor reduction in Brunauer-Emmett-Teller (BET) surface area from 843 to 794 m² g^-1. In parallel, CO₂ uptake increases by 43%, from 15.4 wt.% to 22.0 wt.% at 273 K and 1 bar. The CO₂/N₂ selectivity is also markedly enhanced, rising from 31 to 82 as estimated by the Henry initial slope method, and from 25 to 53 based on ideal adsorbed solution theory (IAST) predictions for a CO₂/N₂ (15/85 v/v) gas mixture at 298 K and 1 bar. Dynamic breakthrough experiments confirm effective CO₂/N₂ separation under dry and 50% RH conditions; HCP-TRIP-P retains > 99% adsorption capacity over five cycles at 273 K. This work demonstrates a facile PSM route to simultaneously enhance CO₂ adsorption and CO₂/N₂ selectivity while maintaining the intrinsic porosity of the HCPs.