Confined Ru Nanoclusters for the Low-Temperature Cyclizationof Diamines to Piperidines/Pyridines

Piperidines and pyridines are valuable cyclic amines with versatile applications in pharmaceutical and chemical industries. Although various production methods exist, achieving a high catalytic efficiency and selectivity under mild reaction conditions remains a significant challenge. Herein, ultrasmall Ru nanoclusters (NCs, ∼1.26 nm) are homogeneously confined within SBA-15 via the slow evaporation (SE) method using triruthenium dodecacarbonyl (Ru₃(CO)₁₂) as a precursor. The pore confinement and partial decarbonylation stabilize the Ru NCs against sintering, limiting growth to 1.41 nm even after reduction at 600 °C. This approach was extended to other mesoporous materials, such as KIT-6 and MCM-41, with similar results in terms of Ru NC dispersion. The resulting Ru NCs in SBA-15 exhibit high catalytic performance for the cyclization of 2-methylpentanediamine to 3-methylpiperidine, initiating at 50 °C without byproducts. Importantly, deactivation of catalysts by ammonia (NH₃) was observed and conquered by transforming the reaction from a batch model to a continuous-flow model. Under optimal conditions (130 °C and 0.5 MPa), the reaction achieves >99% selectivity with minimal change in catalyst activity over 200 h in a fixed-bed reactor. Control experiments, in situ Fourier transform infrared (FT-IR) spectroscopy, and density functional theory (DFT) calculations identified the optimal reaction channel. The confined Ru NCs developed in this study significantly enhance the stability of NCs at high temperatures and are expected to be more widely used in similar amine cyclization reactions.