Palladium-ion-exchanged geopolymer catalyst derived from natural kaolin: an ecofriendly and sustainable catalyst for solventless synthesis of quinoline

The current study addresses the usage of the natural mineral kaolin as a precursor for synthesizing a geopolymer (GNK)-based new catalytic material. The catalyst was synthesized by introducing palladium ions to the geopolymer matrix via an ion-exchange process. It was found that the obtained palladium-ion-exchanged geopolymer (Pd-GNK) was highly efficient for the synthesis of quinoline using cinnamyl alcohol and aniline as reactants under solventless conditions. Both the geopolymer and catalyst have been thoroughly characterized using various techniques, including CP-MAS NMR, FT-IR, XRD, HR-TEM, FE-SEM, EDX, XPS, ICP-OES, and XRF and BET analysis. Remarkably, under optimal reaction at 150 °C in the presence of a Pd-GNK catalyst, a one-pot quinoline synthesis resulted in a good isolated yield without the use of any solvent. Additionally, the synthesized catalyst demonstrated that it exhibits stability, as evidenced by its sustained catalytic activity across four consecutive cycles. This study highlights the development of a novel catalyst derived from low-cost material and having no carbon footprint contribution during the synthesis. This material has been reported for the first time for quinoline synthesis. The important features of the process are easy workup, simple catalyst recovery, and reusability. This approach is sustainable, ensuring economic feasibility and minimizing environmental impact. These materials present novel opportunities for advancement in the field of catalysis.