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A heterogeneous solid acid catalyst was successfully derived from coir fiber, which is a cheap and abundantly available waste material. The coir fiber was first transformed into carbon aerogel to enhance the surface area and pore volume using a sol-gel method. H₂SO₄ immersion was then used to graft the single bondSO₃H functional group onto the produced carbon aerogel. Surprisingly, the NH₄OH-urea system could maintain the honeycomb-like internal pore structure of the cellulose fiber in the resulting carbon aerogel, while the NaOH-urea system could not. Good physical properties, including a high surface area and large pore volume, could be achieved in the catalyst at a sulfonation temperature of 100 °C. The obtained sulfonated carbon catalyst also had excellent performances, with an ionic capacity of 4.45 meq/g, a conversion of 81–83% in a short time reaction, and good recyclability, with a slight …