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

ZnO–ZnCr2O4 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 Cr6+ and these species were reduced to Cr3+ during H2-temperature programmed reduction (H2-TPR) analysis. The higher NH3 and O2 uptakes of Zn–Cr–O (pH ∼ 7) than Zn–Cr–O (pH ∼ 9) are attributed to the presence of Cr6+ ions on the catalyst surface. Disorder in the cubic ZnCr2O4 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 ZnCr2O4 resulting in 2MP synthesis, and catalysts that possess mainly acid sites show intermolecular cyclization of EDA leading to high selectivity of pyrazine.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (92 K)Download as PowerPoint slideHighlights► Dehydrocyclization of EDA and glycerol produce 2-methylpyrazine over ZnO–ZnCr2O4. ► Nature of ZnO–ZnCr2O4 plays an important role on selectivity of 2MP and pyrazine. ► Intermolecular cyclization of EDA is predominant over crystalline Cr2O3.