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

Abstract

Improving process efficiency in selective oxidation of lower olefins over mixed metal oxide catalysts requires profound knowledge of the dynamic behaviour of exothermic reactions along the reactor. For this purpose, structure–activity correlations of two Bi–Mo–Co–Fe–O model catalysts were investigated by means of structure, temperature and activity profiling in selective propylene and isobutene oxidation. Both catalysts showed pronounced differences in selectivity, which strongly affected the temperature and gas phase concentration gradients along the reactor, and thus the reaction network of each olefin oxidation process. Complementary structure profiling by synchrotron XRD identified the evolution of crystalline metal oxide phases after testing in propylene oxidation. Molybdate-based structures (e.g., α-Bi₂Mo₃O₁₂, Bi₃FeMo₂O₁₂) were found to moderate oxygen mobility during catalytic reaction and increase selectivity towards acrolein/methacrolein, while particularly single metal oxides (i.e., Co₃O₄, Fe₃O₄) enhanced oxygen mobility drastically and favoured total oxidation. Comparison of selective propylene and isobutene oxidation revealed the metal oxide phase ensembles within each catalyst had comparable effects on both reaction networks. Hence, the spatially-resolved testing and characterization allowed a systematic study of the catalytic processes along the reactor, showing great promise for knowledge-based optimization of selective oxidation processes.