Thermal Stability of Ionic Compounds on Nanostructured Metal Catalysts: Conversion of Quaternary Ammonium to Amines on Gold Nanoparticles

Quaternary ammonium halides are an important class of compounds because they are not only used as structural templates ororganic surface modifiers for inorganic minerals and other aluminosilicates such as zeolites, but are also well-known and widespread as phase-transfer catalysts and stabilizing agents for colloidal metal nanoparticle synthesis. Moreover, quaternary ammonium ions are frequently exploited as weakly coordinating cations in various ionic liquids. Thus, it is important to monitor, understand, and utilize thermal effects and limits of application. Herein, we report the thermal conversion of didodecyl-dimethylammonium bromide (DDAB) into organic amines on the surface of gold nanoparticles. In contrast to the pure compound, in which this decomposition appears as an endothermic process, the on-particle reaction is exothermic. This in situ modification of the ligand shell not only involves the essential step of halide removal through the gas phase but also turns out to be a valuable tool to produce uncovered metal sites and thus impacts the fabrication of nanostructured metal catalysts for ligand-directed chemistry.