The selective oxidation of propylene to acrolein is the first step in the production of high value acrylate-based chemicals. The formation of C2 and C3 aldehydic and acidic organic byproducts, even in small quantities (less than 10% yield), poses significant challenges to the down-stream processing of acrolein and its derivatives. We present in this talk a reaction network detailing the formation and consumption of all C3 and smaller products during propylene oxidation to acrolein on α-bismuth molybdate (Bi2Mo3O12). The kinetics and mechanisms of sequential and parallel oxidation reactions were investigated in a recirculating gas-phase batch reactor in absence of concentration and temperature gradients. The temporal evolution of all compounds formed in 1% or higher carbon selectivity was measured up to 25% conversion of propylene to determine their ranks in the reaction network. We identified (i) acrolein, acetaldehyde, acetone, and acetic acid as primary products of propylene oxidation, (ii) acrylic acid and ethylene as secondary products, and (iii) CO and CO2 as both primary and secondary products. Experiments in which acetaldehyde, acetic acid, acetone, and acrylic acid were co-processed, separately, with propylene, oxygen, and water revealed (i) the oxidation pathways of acrolein to acrylic acid and ethylene, (ii) the promotional effects of water on the synthesis rates of acetaldehyde, acetone, and acetic acid, and (iii) the inhibitory effects of water on the decomposition of acetic acid and acrylic acid. We describe a power law kinetic model that includes rate expressions with first-order dependency on the partial pressure of organic compounds in conjunction with terms describing the promotional and inhibitory effects of water on various reaction rates and show the accuracy of the model in quantitatively describing the kinetic behavior of all products as a function of propylene conversion. We also discuss a postulated mechanistic network that includes relevant surface intermediates to describe all molecular transformations associated with both selective and unselective pathways in the partial oxidation of propylene.

Mechanistic origins of unselective oxidation products in the conversion of propylene to acrolein

253rd National Meeting (2017)

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