Sulfonated carbons from agro-industrial residues: simple and efficient catalysts for the Biginelli reaction

Abstract

The Biginelli reaction is a one-pot acid-catalyzed cyclocondensation of an aromatic aldehyde, urea, and ethyl acetoacetate to synthesize dihydropyrimidinones (DHPMs). DHPMs are extremely important due to their wide-ranging pharmacological activities, and, for this reason, the Biginelli reaction has been at the forefront of investigations by various research groups. Here, we report the production of sulfonated carbons from agro-industrial wastes (rice husk and tomato bagasse) applied as an inexpensive and efficient heterogeneous catalyst for the synthesis of DHPMs. Catalysts were easily prepared by carbonization using sulfuric acid at 200 °C for 6 hours. The coal surface was characterized by N₂ adsorption–desorption isotherms at −196 °C, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and textural analysis. Infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and Raman analysis proved the sulphonation of catalysts. In addition, other complementary analyses were also carried out, such as elemental analysis of the contents of carbon, nitrogen, oxygen, and sulphur. Then, exploratory catalytic tests were carried out to define the optimal reaction conditions. For this purpose, the effects of different solvents, reaction temperatures, molar ratios of the reactants, and catalyst loads were investigated. The best results were achieved using the catalyst prepared from rice husk (CRH). A yield of 92% of DHPMs was achieved in a short reaction time under solvent-free conditions. Moreover, the catalyst was recovered and reused without a significant decrease in the yield. These findings show that the agro-industrial wastes can be explored to find novel and potential catalysts.