(247f) Transient Kinetic Characterization of Selective Propene Oxidation on Industrial Bismuth Molybdate Catalysts

Selective oxidation of propene to acrolein, the chemical foundation of numerous plastics and commodity chemicals (i.e., acrylamide, adiponitrile, acrylonitrile-butadiene-styrene (ABS), etc.), remains challenged by CO_x and acetaldehyde byproduct formation. Though the binary bismuth molybdate (Bi₂Mo₃O₁₂) catalyst continues as the central ingredient, industrial catalysts include several promoter metals, each with different function. Such catalysts are at the foundation of site isolation concept whereby the cooperation of distinct active sites for propene and oxygen activation are connected by oxygen diffusion pathways. While the steady-state flow reactor is useful in the assessment of performance under different operating conditions, transient experiments can provide more detailed information about selective and nonselective pathways as well as the role of gas phase, lattice oxygen and surface diffusion. In particular, the Temporal Analysis of Products (TAP) pulse response methodology provides such distinction in absence of any disguising effects of gas phase reactions, transport limitations or thermal gradients. TAP transient experiments are presented that deconvolve selective and nonselective routes under varied catalyst redox state, propene/oxygen feed ratio and temperature. The influence of oxygen content on changes in reactant and product rate constants (including quantitative analysis of numbers of active sites, frequency factor and activation energy) are determined from time-dependent analysis of rate, gas and surface concentration measurements that are unique to the TAP method. The precise kinetic distinction of competing reaction pathways is helpful for both benchmarking complex industrial catalyst compositions as well as informing operating strategies at higher pressures.