Partial Oxidation Reaction Pathways Over Metal and Metal-Oxide Catalysts

Abstract

Catalytic partial oxidation of organic molecules is critically enabling for production of commodity and specialty chemicals. Metal and metal-oxide surfaces can facilitate specified selective oxidation routes, but concurrent unselective reactions also occur. Minimizing the profusion of deleterious pathways through increasing selectivities of target oxidation products offers economic and environmental benefits. Although unselective oxidation of some molecules, such as ethylene, produces almost exclusively CO2, multiple organic byproducts of varying size (C1-C7+) are formed in appreciable quantities during oxidations of other molecules such as acrolein. Along with decreasing overall process yield, these byproducts lead in many cases to difficulties in downstream separation and processing of target oxidation products. This dissertation focuses on the identification of deleterious oxidation pathways responsible for the formation of undesired products during partial oxidation reactions through utilization of product stability analysis, co-feed reactions, isotopic labeling studies, and probe molecule co-feed reactions conducted in gas-phase batch and flow reactors. We present studies of oxidation pathways in two catalytic systems: acrolein oxidation to acrylic acid over a mixed-metal oxide (promoted MoVOx) catalyst and ethylene epoxidation over a supported metal (promoted Ag/α-Al2O3) catalyst. Results of acrolein oxidation reaction studies performed in a gradientless, recirculating batch reactor are rationalized by a mechanistic reaction network and a kinetic model rationalizing unselective C-C bond scission and formation pathways and show that reactions of both acrolein and acrylic acid generate byproducts. Batch and flow reactor studies of degradation of ethylene oxide show that catalyst metal and support surfaces contribute to its consumption.