Towards Photothermal Acid Catalysts Using Eco-Sustainable Sulfonated Carbon Nanoparticles—Part II: Thermal and Photothermal Catalysis of Biodiesel Synthesis

The main goal of this work is to evaluate the ability of sulfonated carbon nanoparticles (SCNs) to induce photothermal catalysis of the biodiesel synthesis reaction (transesterification of natural triglycerides (TGs) with alcohols). Carbon nanoparticles (CNs) are produced by the carbonization of cross-linked resin nanoparticles (RNs). The RNs are produced by condensation of a phenol (resorcinol or natural tannin) with formaldehyde under ammonia catalysis (Stober method). The method produces nanoparticles, which are carbonized into carbon nanoparticles (CNs). The illumination of CNs increases the temperature proportionally (linear) to the nanoparticle concentration and exposure time (with saturation). Solid acid catalysts are made by heating in concentrated sulfuric acid (SEAr sulfonation). The application of either light or a catalyst (SCNs) (at 25 °C) induced low conversions (<10%) for the esterification reaction of acetic acid with bioethanol. In contrast, the illumination of the reaction medium containing SCNs induced high conversions (>75%). In the case of biodiesel synthesis (transesterification of sunflower oil with bioethanol), conversions greater than 40% were observed only when light and the catalyst (SCNs) were applied simultaneously. Therefore, it is possible to use sulfonated carbon nanoparticles as photothermally activated catalysts for Fischer esterification and triglyceride transesterification (biodiesel synthesis).