Design and Development of an Ionic Liquid–Halloysite Nanohybrid Palladium Nanoparticle Catalyst: Synthesis and Catalytic Applications in Mizoroki–Heck, Suzuki–Miyaura, and Copper Free Sonogashira Reactions

Recent progress in catalyst design has emphasized the development of innovative support systems to improve catalytic performance and support more sustainable synthetic strategies. Among these, hybrid inorganic–organic support materials, particularly those derived from aluminosilicate clays such as halloysite, have gained considerable attention. Halloysite's unique nano scrolled structure and chemically modifiable surfaces make it an attractive platform for heterogeneous catalysis. In this study, selective functionalization of the halloysite lumen with an imidazolium-based ionic liquid enabled the encapsulation of palladium nanoparticles, resulting in a fine-tuned nanohybrid catalyst that demonstrated both high reactivity and stability. A reliable and environmentally friendly method was developed to synthesize ionic liquid-modified, halloysite-supported palladium nanoparticles (Pd@Hal-IL). This process involved grafting an imidazolium ionic liquid into the halloysite lumen, followed by in situ reduction of palladium acetate using ascorbic acid under mild aqueous conditions. The resulting nanocomposite was then evaluated across a series of palladium-catalyzed cross-coupling reactions. The Pd@Hal-IL catalyst efficiently facilitated the Mizoroki–Heck olefination reaction in water, producing a broad range of cinnamic acid derivatives in excellent yields over reaction times ranging from six to twenty-four hours. The catalyst displayed excellent water stability and could be recovered and reused without significant loss of activity. In Suzuki–Miyaura cross-coupling reactions between benzoyl chlorides and various arylboronic acids, Pd@Hal-IL enabled the synthesis of benzophenone derivatives in moderate to high yields. The reactions proceeded with high selectivity and clean product formation, underscoring the system’s efficiency and environmentally friendly characteristics. The nanohybrid catalyst also proved effective in promoting copper-free Sonogashira couplings, enabling the synthesis of diphenylacetylenes and ynones through the reaction of terminal alkynes with aryl and acyl halides. The transformations of α,β-acetylenic ketones occurred efficiently under ambient conditions, demonstrating the mild and versatile nature of the Pd@Hal-IL system in C–C bond-forming reactions, even in the absence of a copper co-catalyst. Overall, Pd@Hal-IL represents a robust and green catalytic platform with broad applicability in fundamental organic transformations.