Oxidized pectin modified by sulfonic acid groups as a bio-derived solid acid for the synthesis of 1-amidoalkyl-2-naphthols

The widespread use of homogeneous acids faces enormous environmental problems due to their corrosive nature and recovery issues. On the other hand, developing heterogeneous catalysts with high acid content and benign nature is challenging. Thus, pectin was selected as a cost-effective and eco-friendly support material for preparing a solid acid with high Bronsted acidity. Therefore, pectin was first oxidized by NaIO4 to produce oxidized pectin (OPec) with aldehyde groups as additional sites to effectively anchor melamine (Mel) and chlorosulfonic acid (SA). After oxidation, the dialdehyde content (Ac) was found to be 53.2%. Then, the as-produced OPec-Mel-SA was compared with the catalyst based on non-oxidized pectin (Pec-Mel-SA) to ensure the positive effect of oxidation on the catalytic activity and morphology. Field emission scanning electron microscopy (FESEM) images revealed that OPec-Mel-SA is more ordered with fewer aggregated particles relative to Pec-Mel-SA. The results of energy dispersive X-Ray analysis (EDX) indicated a higher sulfur content for OPec-Mel-SA than Pec-Mel-SA. Moreover, BET (Brunauer-Emmet-Teller) analysis showed a higher SBET and pore volume for OPec-Mel-SA. The higher acidity of OPec-Mel-SA over Pec-Mel-SA nanoparticles was confirmed by titration and Hammet acidity function ( H 0 ) studies using H2SO4 as a benchmark. Accordingly, the OPec-Mel-SA nanoparticles presented a higher catalytic activity compared to Pec-Mel-SA for the three-component synthesis of 1-amidoalkyl-2-naphthols from aldehyde, 2-naphthol, and benzamide, under solvent-free conditions. As a bio-inspired and metal-free heterogeneous solid acid, the OPec-Mel-SA nanoparticles were reusable and highly stable for at least five runs. (c) 2023 Published by Elsevier B.V.