Synthesize 2-methylpyrazine using aqueous glycerol and ethylenediamine over zinc oxide–zinc chromite catalysts: Structure–activity relationship

Abstract ZnO–ZnCr 2 O 4 mixed oxides (Zn–Cr–O) were obtained by decomposition of Zn–Cr hydrotalcite precursors synthesized by co-precipitation method at pH ∼ 7 and 9, with a constant Zn/Cr ratio of 2. Based on the product distribution, mechanisms were proposed. The differences in dehydrocyclization activity of Zn–Cr–O mixed oxides were established based on adsorption and spectroscopic data that emphasized changes in the structural properties of Zn–Cr–O obtained at different pHs. X-ray photoelectron (XPS) lines indicated Cr 6+ and these species were reduced to Cr 3+ during H 2 -temperature programmed reduction (H 2 -TPR) analysis. The higher NH 3 and O 2 uptakes of Zn–Cr–O (pH ∼ 7) than Zn–Cr–O (pH ∼ 9) are attributed to the presence of Cr 6+ ions on the catalyst surface. Disorder in the cubic ZnCr 2 O 4 sub-lattice may result in multiple Raman bands appeared in the Zn–Cr–O samples. The characteristic bands of Lewis and Bronsted acid sites at 1450 and 1540 cm −1 assigned by FT-IR spectra of pyridine adsorbed on Zn–Cr–O samples, and their relative peak intensities corresponding to Lewis and Bronsted suggested higher ratios for Zn–Cr–O (pH ∼ 7) than for Zn–Cr–O (pH ∼ 9). It is demonstrated that the dehydrocyclization reaction proceeds on ZnO and/or ZnCr 2 O 4 resulting in 2MP synthesis, and catalysts that possess mainly acid sites show intermolecular cyclization of EDA leading to high selectivity of pyrazine.