The selective oxidation of lower olefins over bismuth molybdate based multicomponent catalysts are key reactions in chemical industry for the functionalization of hydrocarbons. To understand the catalysts’ working principles, three Bi−Mo−Co−Fe-oxides were synthesized, tested in the selective oxidation of propylene and isobutene and characterized by complementary Raman spectroscopy, X-ray absorption spectroscopy and synchrotron X-ray diffraction. Comparison of their catalytic behavior in both reactions provided fundamental insights into the corresponding structure-activity correlations.

Abstract

The elucidation of structure-activity correlations in selective oxidation of propylene and isobutene over mixed metal oxides (MMO) is attractive for knowledge-based catalyst design and process optimisation. Particularly, 4-component Bi−Mo−Co−Fe−O catalysts need to be studied since their complex metal oxide phase mixture leads to higher activity and selectivity than 2-component Bi−Mo−O. Hence, three Bi−Mo−Co−Fe-oxides with different metal ratios were prepared by hydrothermal synthesis and compared during selective oxidation tests with propylene and isobutene. The active phases after several days on stream were investigated by synchrotron X-ray diffraction (XRD) and Raman spectroscopy, while the structural evolution under reaction conditions was followed by operando Raman spectroscopy, synchrotron XRD, and multi-edge X-ray absorption spectroscopy. Similar structural transformations were observed during selective oxidation of propylene and isobutene, with similar influence on catalytic performance. A phase mixture of β-CoMoO₄/β-Co_0.7Fe_0.3MoO₄, γ-Bi₂MoO₆, Fe₃O₄ and Bi₃FeMo₂O₁₂ was observed, whereby high amounts of β-CoMoO₄/β-Co_0.7Fe_0.3MoO₄ increased selectivity to acrolein/methacrolein. In contrast, high amounts of γ-Bi₂MoO₆ and Fe₃O₄ favoured total oxidation to CO and CO₂. The simultaneous presence of β-CoMoO₄/β-Co_0.7Fe_0.3MoO₄, Bi₃FeMo₂O₁₂ and Fe₂O₃ increased selectivity to methacrolein in isobutene oxidation, whereas no comparative increase in acrolein selectivity was observed in propylene oxidation. This suggests different key active phases in both reactions.