Simple One-Step Synthesis of Aromatic-Rich Materials with High Concentrations of Hydrothermally Stable Catalytic Sites, Validated by NMR

We report a facile one-step synthesis of aromatic-rich materials that contain high concentrations of Brønsted acidic or basic groups attached via alkyl linkers, which previous work has shown to be particularly hydrothermally stable. The method is based on the Maillard reaction and low-temperature (250 °C) pyrolysis of glucose with primary amines linked to acidic sulfonic or phosphonic functionalities or basic piperidine or pyridine groups. The resulting black, carbon-rich materials were characterized using one- and two-dimensional solid-state 13C and 15N NMR, supplemented by elemental analysis. Synthesis with 13C-enriched glucose enabled a selective NMR characterization of the aromatic scaffold, which is composed mostly of interlinked pyrrole, indole, and pyridine rings. The fraction of sp2-hybridized C in the matrix is 72%; the aromaticity is ∼60% for the sulfonic-acid functionalized material made from glucose and taurine in a 1:1 molar ratio, where sulfur exceeds 11 wt %. The alkyl linkers remained intact in the synthesis at 250 °C, as proved by distinctive NMR signals of CH2 groups bonded to heteroatoms. The incorporation of the amine-derived nitrogen into the aromatic matrix was characterized by 15N, 15N–13C, and 13C­{15N} NMR of a material made from 15N-taurine with 13C-enriched glucose. 15N NMR shows that no significant unreacted alkyl amine groups remain in the material. Hydrothermal stability as well as catalytic activity of the materials for an esterification reaction was verified.