Highly selective and robust single-atom catalyst Ru1/NC for reductive amination of aldehydes/ketones

Single-atom catalysts (SACs) have emerged as a frontier in heterogeneous catalysis due to the well-defined active site structure and the maximized metal atom utilization. Nevertheless, the robustness of SACs remains a critical concern for practical applications. Herein, we report a highly active, selective and robust Ru SAC which was synthesized by pyrolysis of ruthenium acetylacetonate and N/C precursors at 900 °C in N2 followed by treatment at 800 °C in NH3. The resultant Ru1-N3 structure exhibits moderate capability for hydrogen activation even in excess NH3, which enables the effective modulation between transimination and hydrogenation activity in the reductive amination of aldehydes/ketones towards primary amines. As a consequence, it shows superior amine productivity, unrivalled resistance against CO and sulfur, and unexpectedly high stability under harsh hydrotreating conditions compared to most SACs and nanocatalysts. This SAC strategy will open an avenue towards the rational design of highly selective and robust catalysts for other demanding transformations.

5wt% Ru/HZSM-5 catalyst was prepared according to the reported procedure. 5 1.0 g HZSM-5 (molar ratio of SiO 2 /Al 2 O 3 = 46) was mixed with 1.50 g solution of RuCl 3 (0.32 mg· mL -1 Ru). Then the mixture was slowly evaporated at 60 o C under stirring and dried at 80 °C for 12 h. The obtained sample was calcined at 400 °C for 4 h and reduced at 300 °C for 2 h under H 2 atmosphere. After being cooled to room temperature, the catalyst was passivated with 1% (v/v) O 2 /N 2 for 8 h.
Catalytic tests. The reductive amination reaction was conducted in a 50 mL stainless-steel autoclave (Parr Instrument Company, America). Typically, the substrate (2 mmol), Ru 1 /NC catalyst (molar ratio of Ru: substrate was 1:400), dodecane (internal standard) and 3 mL methanol (solvent) were added to a teflon lining. Then the autoclave was sealed and purged by N 2 for three times, followed by charging with 0.5 MPa NH 3 and 2 MPa H 2 . The reaction mixture was stirred at 100 °C for 10 h. The products were identified using a Varian G450/320 GC/MS system, and were quantitatively analyzed using an Agilent 7890A GC system equipped with a HP-5 capillary column and a FID detector.
Purification experiment. Selected primary amine products were purified by column chromatography (silica; ethyl acetate-methanol mixture). The purified amines were converted to their corresponding hydrochloride salt by reacting with 2.0 M HCl followed by water removal through rotary evaporator for characterization by NMR and FT-ICR-MS spectral analysis.

Characterization methods.
In the sample characterization section, the actual Ru loadings were determined by inductively coupled plasma spectroscopy (ICP-OES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation).
X-ray diffraction (XRD) patterns were recorded on a PANalytical X' pert diffractometer with a Cu-Kα radiation source (40 kV and 40 mA). A continuous mode was used to record data in the 2θ range from 10° to 80°. X-ray photoelectron spectroscopy (XPS) spectra were obtained on a Thermo ESCALAB 250 X-ray photoelectron spectrometer equipped with Al Kα excitation source and with C as internal standard (C 1s = 284.6 eV).
The X-ray absorption spectra (XAS) including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at Ru K-edge of the samples were measured at the beamline 14W of Shanghai Synchrotron Radiation Facility (SSRF) in China. The output beam was selected by Si(311) monochromator, and the energy was calibrated by Ru foil. The data were collected at room temperature under transmission mode. Athena software package was employed to process the XAS data.
Microcalorimetric measurement was performed by a BT2.15 heat-flux calorimeter, which was connected to a gas handling and a volumetric system employing MKS Baratron Capacitance Manometers for precision pressure measurement. The ultimate dynamic vacuum of the microcalorimetric system was 10 -7 Torr by calculation. First, the fresh sample was treated in a special tube in H 2 at 5 / 37 100 o C for 1h and then high pure He at 200 o C for 1h to eliminate the adsorption.
Then, the tube was transferred into the the high vacuum system and stabilized for (6-8 h). After thermal equilibrium was reached, the H 2 -microcalorimetric data was collected by sequentially introducing small doses (10 -6 mol) of H 2 (CO 2 or NH 3 ) into the system until it became saturated (5-6 Torr). Simultaneously, the differential heat versus adsorbate coverage plots and adsorption isothermals can be obtained after a typical microcalorimetric experiment.
NMR spectra of selected products were recorded at room temperature in C 2 D 6 OS on 400 MHz Bruker DRX-400 NMR and 700 MHz Bruker DRX-700 NMR ( Table 3