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

Abstract

Structure−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 β-Co 0.7 Fe 0.3 MoO 4 and Bi 3 FeMo 2 O 12 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 α-Bi 2 Mo 3 O 12 , while the interplay of γ-Bi 2 MoO 6 and Fe 3 O 4 led to highly active but less selective catalysts. In this context, the reducibility of Fe 3+ to Fe 2+ was found to be crucial for moderate and controllable catalytic activity. Co 3 O 4 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.

Exploring structure, temperature and activity correlations in the selective oxidation of lower olefins over Bi–Mo–Co–Fe–O catalysts by spatial reactor profile measurements

“…Hence, these results support the selective role of Bi 3 FeMo 2 O 12 and the unselective role of Co 3 O 4 found in the context of selective isobutene oxidation. 28 Moreover, Bi 3 FeMo 2 O 12 might moderate the oxygen mobility through the catalyst bulk, while Co 3 O 4 enhances the oxygen mobility drastically. This supports the findings of Liotta et al , who attributed an increased activity of Co 3 O 4 to the higher mobility of lattice oxygen forming mainly CO 2 in total oxidation experiments.…”

“…Two Bi–Mo–Co–Fe–O catalysts (denoted as FSP-Co and FSP-U) were prepared by flame spray pyrolysis (FSP) as described in ref. 28. The catalysts differed in their elemental composition (Table 1), with FSP-Co containing the highest amount of cobalt (40 mol%) and FSP-U containing the highest amount of molybdenum (50 mol%), referring to an elemental composition firstly reported and recommended by Moro-Oka and Ueda.…”

“…24–26 Parallels between the selective oxidation of propylene to acrolein and isobutene to methacrolein were also found. 27,28…”

“…In this study, we report on spatial profiling experiments during both the selective oxidation of propylene and isobutene over Bi–Mo–Co–Fe-oxides in a CPR setup. Two flame-spray prepared catalysts differing strongly in selectivity 28 were investigated with respect to temperature and gas phase concentration gradients along the catalyst bed. Flame spray pyrolysis is advantageous for the systematic study of such complex mixed metal oxides in terms of easy adjustment of elemental composition including high reproducibility and accessibility to certain metastable phases in a single step.…”

Comparison of Structure and Reactivity of Hydrothermally Prepared Bi−Mo−Co−Fe−O Catalysts in Selective Propylene and Isobutene Oxidation

“…However, because the catalyst structure is highly dependent on the reaction conditions, it is crucial to monitor structural changes of BiÀ MoÀ CoÀ Fe-oxides by in situ/operando techniques. [36][37] Moreover, their complex composition requires a set of complementary characterization methods to fully deconvolute the various crystalline and amorphous metal oxide phases. [38] In this work, we systematically investigate both the influence of the synthesis conditions during HS and compare propylene and isobutene as hydrocarbon reactants with respect to their catalytic performance (activity, selectivity) in lower olefin oxidation.…”