Polybenzodiazine Aerogels: All-Nitrogen Analogues of Polybenzoxazines─Synthesis, Characterization, and High-Yield Conversion to Nanoporous Carbons

Abstract

Tetrahydroquinazoline (THQ) was designed as an all-nitrogen analogue of main-stream benzoxazine monomers. THQ solutions in DMF gelled at 100 °C via HCl-catalyzed ring-opening polymerization to polybenzodiazine (PBDAZ) wet gels, which were dried in an autoclave with supercritical fluid CO2 to aerogels. These as-prepared PBDAZ-100 aerogels undergo ring-fusion aromatization at 240 °C under O2. This oxidized form is referred to as PBDAZ-240. Chemical identification of PBDAZ-100 and PBDAZ-240 relied on consideration of all nine possible polymerization pathways, in combination with elemental analysis, infrared and solid-state 13C NMR spectroscopy, and 15N NMR spectroscopy of aerogels from the selectively 15N-enriched THQ monomer. Fully oxidized PBDAZ-240 aerogels were carbonized at 800 °C under Ar to carbon aerogels with 61% w/w yield and with retention of the nanomorphology of the parent PBDAZ-100 aerogels. Direct pyrolysis of PBDAZ-100 at 800 °C, i.e., without prior oxidation, resulted in only 40% w/w yield and complete loss of the fine nanostructure. The evolution of PBDAZ-240 aerogels along pyrolysis toward carbonization was monitored using progressively higher pyrolysis temperatures from 300 to 800 °C under Ar. Aerogels received at 600 and 800 °C (referred to as PBDAZ-600 and PBDAZ-800, respectively) had relatively high surface areas (432 and 346 m2 g–1, respectively), a significant portion of which (79% in both materials) was assigned to micropores. The new polymer aerogels, together with polybenzoxazine aerogels, comprise a suitable basis set for comparing N-rich versus O-rich porous carbons as adsorbers.