Role of Iron and Cobalt in 4-Component Bi-Mo-Co-Fe-O Catalysts for Selective Isobutene Oxidation Using Complementary Operando Techniques

tructure-activity correlations in the selective oxidation of lower olefins over Bi-Mo-based mixed metal oxides are challenging but essential for a knowledge-based catalyst design and improvement of process efficiency. One important aspect is the extension of the 2-component Bi-Mo oxides to more complex mixed metal oxide (MMO) systems like Bi-Mo-Co-Fe oxides, as their higher activity and selectivity are traced back to synergistic metal oxide phase interactions. Hence, three Bi-Mo-Co-Fe oxide catalysts differing in elemental composition, especially iron and cobalt content, were tested during the selective oxidation of isobutene. The catalyst structure was studied using operando Raman spectroscopy, (synchrotron) X-ray diffraction, and X-ray absorption spectroscopy. The formation of ternary phases beta-Co0.7Fe0.3MoO4 and Bi3FeMo2O12 resulted in an increased selectivity to methacrolein, but the overall performance strongly depended on the variety of metal oxide phases, thus pointing to the role of phase cooperation. The highest selectivity was observed with the simultaneous presence of alpha-Bi2Mo3O12, while the interplay of gamma-Bi2MoO6 and Fe3O4 led to highly active but less selective catalysts. In this context, the reducibility of Fe3+ to Fe2+ was found to be crucial for moderate and controllable catalytic activity. Co3O4 performed unselective by mainly favoring total oxidation product formation. Comparison with previous findings in selective propylene oxidation showed similar structural changes during the catalytic reaction, hence emphasizing similar behavior of Bi-Mo-Co-Fe oxides in lower olefin oxidation. The systematic investigation of 4-component catalysts by advanced characterization techniques is a powerful approach to gain new insights into the roles of individual catalyst phases by considering their complex phase interplay and thus receiving more profound understanding of their working principles during the selective oxidation of olefins.