Synthesis and characterization of La_1–xSr_xMn_1-yZn_yO₃ perovskites as an efficient and recoverable catalyst for the Hantzsch reaction

Five perovskite materials of the general formula La_1–xSr_xMn_1-yZn_yO₃ (x = 0, 1, 2, 3, and y = 0, 2) were successfully synthesized by the citric acid sol-gel route and characterized by scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunner-Emmet-Teller (BET) technique and X-ray photoelectron spectroscopy (XPS). The materials were employed as heterogeneous catalysts in the Hantzsch reaction, producing 1,4-dihydropyridine derivatives. Incorporating Zn into the parent perovskite LaMnO₃ improved the catalytic activity, which was greatly enhanced by the simultaneous introduction of Sr. In addition to the positive effect of Zn and Sr incorporation, the results also showed that the catalytic activity was related to the surface Mn⁴⁺ content and acidic character. The La_0.9Sr_0.1Mn_0.8Zn_0.2O₃ catalyst was recycled and reused four times after demonstrating significant catalytic activity in a polar protic solvent (ethanol). After four cycles of use, the catalyst did not exhibit a significant reduction in catalytic activity, despite the formation of some secondary phases revealed by XRD and XPS analyses. Graphical abstract: Zn and Sr doubly partial substitution in lanthanum manganite perovskite resulted in a high catalytic activity of the Hantzsch reaction. The results showed that the catalytic activity is related to the creation of oxygen vacancies and the defect associations, in addition to surface acidity and Mn⁴⁺ content [Figure not available: see fulltext.]