Facile synthesis of CuMAl (M = Cr, Mn, Zn, and Co) with highly dispersed Cu and tailorable surface acidity for efficient 2-methylpyrazine synthesis

Abstract

Synthesis of 2-methylpyrazine (2-MP) from 1,2-propylene glycol (PG) and ethylenediamine (ED) was investigated in the presence of multifunctional catalytic systems (CuMAl) possessing acidic and metallic functional sites. Catalytic systems were prepared from mixed CuMAl-layered double hydroxides (CuMAl-LDHs, M = Cr, Mn, Zn, and Co) via their thermal decomposition. CuMAl-LDH were prepared from Cu(NO₃)₂, M(NO₃)_x and Al(NO₃)₃ and NaOH/Na₂CO₃ as a precipitating agent. X-ray diffraction (XRD), N₂ adsorption–desorption, temperature-programmed desorption with ammonia (NH₃-TPD), N₂O chemisorption, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were used to characterize the physical and chemical properties of the catalysts. The results showed that the nature of the secondary metal M inserted into the LDH structure significantly affected the crystalline structure, the dispersion of copper nanoparticles, and the density of surface acidic sites of the catalysts. The as-prepared CuMAl catalysts displayed promising catalytic performances towards the synthesis of 2-MP. Among them, CuCoAl showed the highest PG conversion (97%) and 2-MP selectivity (55%). These high catalytic activities were found to be associated with the ultra-small Cu nanoparticles (∼2 nm) and high surface acidity (2433 μmol g⁻¹).