Sulfonated N‑Doped Carbon Nanotubes as Magnetic Solid Acid Catalysts for the Synthesis of Diphenolic Acid

This manuscript introduces an effective method for preparing a magnetic solid acid and its application in the synthesis of diphenoic acid. The prepared catalyst (Fe@NC-SO₃H, Co@NC-SO₃H, or Ni@NC-SO₃H) was a sulfonated N-doped carbon nanotube encapsulating Fe, Co, or Ni nanoparticles, respectively. Remarkably, the magnetic core was formed in situ during the calcination of the catalyst. The metal nanoparticles gave the catalyst excellent magnetic separation performance. The −SO₃H group could be capable of efficiently catalyzing the condensation reaction of levulinic acid with phenol to diphenolic acid with a 95% yield. Significantly, this encapsulated structure of the catalyst can effectively improve the stability of the catalyst, and this catalyst still obtained good stability in acidic reaction conditions, which was proved by recycled experiments and a series of catalyst characterizations.