Exploring the catalytic activity of –NHSO₃H functionalized natural asphalt: a sustainable and efficient catalyst for condensation reactions in water

Herein, we report the synthesis of natural asphalt sulfamic acid (NA-NHSO₃H) <i>via</i> functionalization of natural asphalt (NA). The -SO₃H groups on the surface of natural asphalt function as acid catalytic sites. This configuration not only enhances the acidity but also increases the surface area available for catalytic activity, making NA-NHSO₃H a promising candidate for various acid-catalyzed reactions. The synthesized catalyst was characterized using Fourier Transform Infrared (FT-IR) spectroscopy, Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX) and X-ray mapping analysis. The catalyst in question has been previously explored for its role in synthesizing various heterocyclic compounds such as polyhydroquinolines, tetrahydrobenzo[<i>b</i>]pyran, and 3,4-dihydropyrimidine-2(1<i>H</i>)-one/thiones. The NA-NHSO₃H demonstrated good to excellent yields in the Knoevenagel and Claisen-Schmidt condensations in water at room temperature, indicating its efficiency in these reactions. Conducting these reactions at room temperature and in water can be particularly advantageous, as it enhances safety and sustainability by minimizing the use of hazardous solvents and heat. Conducting these reactions in water at room temperature in the presence of a reusable catalyst aligns with green chemistry principles, which favor the use of benign solvents and milder conditions. This enhances the appeal of NA-NHSO₃H for industry and academia by reducing environmental impact and energy usage. Furthermore, the catalyst's heterogeneity was evidenced by its excellent reusability and results from hot-filtration tests.