A novel single-step hydrogenation of 2-imidazolecarboxaldehyde to 2-methylimidazole over Pd-impregnated Al–Ti mixed oxide and kinetics

Abstract

2-Methyl imidazole (2MIMZ) is a very essential chemical feedstock and intermediate used in the manufacturing of pharmaceutical drugs, metal organic frameworks (MOFs), agricultural chemicals, pigments and dyes, and coating additives. The currently used processes for the synthesis of 2MIMZ include dehalogenation and dehydrogenation routes, which use homogeneous catalysts and hazardous chemicals such as CH₃CN and aryl-halides leading to wastes, and ultimately increase the cost of production. Therefore, a green route was developed in this work using Al–Ti mixed oxide (ATMO) based catalysts, which were developed by co-precipitation and hydrothermal methods. ATMO was further doped with various metals (M = Ni, Cu, Fe and Pd) to give respective M/ATMO catalysts. These prepared M/ATMO catalysts were screened for selective hydrogenation of 2-imidazolecarboxaldehyde (IMZC) to give 2-methyl imidazole (2MIMZ). The order of activity was 2% Pd/ATMO_CP > 5% Ni/ATMO_CP > 2% Pd/Al₂O₃ > 5% Cu/ATMO_CP > 5% Fe/ATMO_CP > 2% Pd/TiO₂. At 140 °C and 10 atm H₂ pressure, there was >99% conversion of IMZC and 95% selectivity to 2MIMZ in 2 h. It is a consecutive reaction with 1H-imidazole-2-methanol (IMZ2MOL) as an intermediate, which on further hydrogenation gave 2MIMZ. The catalysts were characterized by sophisticated techniques such as FESEM, EDS, XRD, NH₃-TPD, FTIR, BET analysis, HRTEM, XPS, TPR, and TGA. The Langmuir–Hinshelwood–Hougen–Watson (LHHW) dual-site reaction mechanism was found to be consistent with the experimental findings. The activation energies for the hydrogenation steps were 11.4 and 13.6 kcal mol⁻¹, respectively. The catalysts were found to be robust, active and selective. Here, we unveil for the very first time a new route for the selective single-step hydrogenation of IMZC to 2MIMZ.