Enhanced CO2 capture in nitrogen-enriched microporous carbons derived from Polybenzoxazines containing azobenzene and carboxylic acid units

Abstract

In this study, we prepared nitrogen-doped microporous carbons (NMCs) from two different benzoxazine monomers (AMBZ, AEBZ), each containing both azobenzene and carboxylic groups, through a simple and environmentally friendly process of ring-opening polymerization (ROP), calcination, and KOH activation. We synthesized the AMBZ and AEBZ monomers through Mannich condensations of 4,4′-diaminodiphenylmethane (M) and 4,4′-diaminodiphenyl ether (E), respectively, with paraformaldehyde and 4-(4-hydroxphenylazo)benzoic acid (Azo-COOH). Differential scanning calorimetry (DSC) revealed that the thermal curing temperatures of AMBZ and AEBZ (both ca. 230 °C) were lower than that of a typical Pa-type benzoxazine monomer (263 °C), suggesting that the azobenzene and COOH units acted as promoters and catalysts for the ROP of the benzoxazine units. In addition, after ROP of the benzoxazine units of AMBZ and AEBZ, the polymers PAMBZ and PAEBZ, respectively, displayed high glass transition temperatures (Tg) and high thermal stability, as evidenced using DSC and thermogravimetric analysis (TGA), due to their greater cross-linking densities. We used Brunauer–Emmett–Teller analysis, TGA, wide-angle X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy to examine the specific surface areas, porous structures, chemical compositions, and thermal stabilities of the resulting KOH-activated NMCs PAMBZ-A and PAEBZ-A. The CO2 capture abilities and thermal properties of these two highly-nitrogen-doped microporous carbons, synthesized from polybenzoxazine (PBZ) resins containing azobenzene and COOH groups, were excellent when compared with those of other N-doped porous carbons derived from other PBZ matrices.