Multinuclear magnetic resonance study on the occurrence of phosphorus in activated carbons prepared by chemical activation of lignocellulosic residues from the babassu production

Abstract

Activated carbons with well-developed microporosity and containing dispersed nanostructured phosphorus compounds were produced by the chemical activation with H3PO4 of an abundant Brazilian lignocellulosic residue (endocarp of babassu coconut). The chemical nature of the phosphorus-containing species in the produced materials was studied by 1H, 13C and 31P solid-state NMR spectroscopy, including 2D 1H-31P correlation experiments and the use of 1H homonuclear decoupling. With this approach, it was possible to detect the formation of different phosphorus compounds (mostly containing phosphate groups) embedded into the porous carbon network and exhibiting strong dipolar coupling between 31P nuclei and 1H nuclei in hydrogen atoms bonded to aromatic carbons. The chemical activation with H3PO4 was found to be noticeably effective when starting from the raw lignocellulosic precursor, leading to the achievement of activated carbons with large surface area (up to 1900m2/g). Due to the natural occurrence of silicon compounds in the endocarp of babassu coconut, the chemical activation with H3PO4 was found to produce silicon pyrophosphate nanocrystals in addition to phosphate groups in non-crystalline clusters dispersed into the porous structure of the activated carbons, suggesting that these materials can be useful for applications as solid phosphoric acid catalysts.