Mono- and bimetallic nanoparticles stabilized by functionalized ionic liquids: synthesis and catalytic applications

Metal nanoparticles (NPs) are recognized attractive materials for application in the field of catalysis due to their remarkable high surface-to-volume ratio that induces a high number of potential active sites. It is well known that stabilizers play an important role in synthesis and catalytic performance of metal NPs. They protect the NPs structure, thereby preventing agglomeration or aggregation that may occur due their high surface energy. Additionally, stabilizers can influence the electronic properties of the NPs, optimizing their reactivity and selectivity in catalytic reactions. Thus, appropriate selection and utilization of stabilizers are crucial factors for modifying and enhancing the performance of NPs across a diverse range of catalytic reactions including hydrogenation of various substrates. Among all possible stabilizers for metal NPs, ionic liquids (ILs) can be employed for the synthesis of metal NPs. ILs possesses intriguing physical and chemical properties and presents the advantage to act both as stabilizer and solvent in the reaction, resulting in reduced utilization of chemicals and limit waste, which limits the environmental footprint. Careful tailoring of imidazolium based ILs with functional groups has proven a relevant way to improve catalytic performance of metal NPs. In this context, a series of Ru NPs and Ni NPs were synthesized by the organometallic approach in functionalized imidazolium ionic liquids (FILs) with methoxy- and cyano-groups (abbreviated as MEM, MME and CN). Well-dispersed and narrow-sized Ru NPs ranging from 1.3 to 2.2 nm were obtained as well as Ni NPs ranging between 2.8 and 6.9 nm depending on the IL functionalization. Thermal gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) allowed to study the interaction between the NPs and the ILs. Ru NPs stabilized by the methoxy-containing FILs (MEM and MME) displayed a good balance between catalytic activity and stability when applied in the hydrogenation of styrene at mild reaction conditions. Moreover, the Ru/FILs showed complete selectivity towards ethylbenzene at milder reaction conditions (5 bar, 30 °C) than reported in literature for other Ru NP catalysts. All the Ni/ILs systems were found to be efficient catalysts for the hydrogenation of 2-cyclohexe-1-none under the applied reaction conditions (substrate/Ni ratio of 100/1, 130 °C, 10 bar H₂), providing full substrate conversion and complete selectivity towards hydrogenation of the olefinic bond in short reaction time (1 h). Bimetallic RuRe NPs were also synthesized with an average size of 1.6 and 3.3 nm in unfunctionalized (H) and MEM ILs, respectively, but neither of the systems catalyzed the hydrogenation of amides, which is reputed as a challenging reaction due to the high stability of amides. In summary, the PhD work led to the synthesis of novel Ru, Ni and RuRe bimetallic NPs in MEM and MME ILs, and insights into structure-performance relations in hydrogenation catalysis. The work provides perspectives for future development within both the fields of nanochemistry and selective catalysis.