Tunable porosity of nanoporous organic polymers with hierarchical pores for enhanced CO2 capture

Abstract

A series of cost-effective nanoporous organic polymers (NOP-50–NOP-52) with hierarchical pores for efficient CO₂ capture were successfully synthesized via one-step Friedel–Crafts alkylation promoted by anhydrous FeCl₃. Two chloromethyl monomers, i.e. dichloroxylene (DCX) and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMBP) were utilized as crosslinkers to tailor the pore sizes. The porosity of the resultant polymers can be well-tuned by varying the length of crosslinkers and type of building blocks. More specifically, shorter linkers provide the polymers with greater microporosity, whereas longer linkers prove to block the microporosity created by the high-degree crosslinking of organic network. Introducing a series of highly rigid, nitrogen-containing, or metal-decorated building blocks features the obtained amorphous networks abundant microporosity and mesoporosity, and large Brunauer–Emmett–Teller (BET) surface areas up to 1650 m² g⁻¹ as measured by N₂ adsorption at 77 K. Notably, such networks possess hierarchical pores with wide pore size distributions ranging from 0.55 to 4.3 nm. NOP-50A derived from DCX and carbazole exhibits competitive CO₂ uptake up to 18.8 wt% at 273 K and 1 bar, surpassing most known HCPs. Remarkable selectivity ratios for CO₂ adsorption over N₂ (39–72) at 273 K and high CO₂ isoteric heats of adsorption (34.2–46.7 kJ mol⁻¹) were obtained. The high CO₂/N₂ selectivity and CO₂ isosteric heat values could be ascribed to the good binding affinity of abundantly available electron-rich basic heteroatom or metal sites of the networks towards CO₂. These results are significant for the construction of NOPs with hierarchical pores by introducing optimum building block and suitable length linkers for enhanced CO₂ capture.