Selective Oxidation of n-Butane over Vanadium Phosphate Based Catalysts: Reaction Network and Kinetic Analysis

The reaction network of n-butane selective oxidation was comparatively analyzed over a novel alpha(11)-V0.8W0.2OPO4 orthophosphate and a reference vanadyl pyrophosphate (VPP) based catalyst via parameter field studies (T = 360-420 degrees C, x(n-butane) = 0.5-2.0%, x(O-2) = 10-20%) as well as cofeed and pulse experiments with presumed reaction intermediates. For VPP the selectivity to the target product maleic anhydride (MAN) is particularly sensitive to the reaction temperature, the n-butane concentration, and the amount of cofed H2O, whereas the selectivity to MAN over the W-containing catalyst is almost independent of all reaction parameters. The roles of 1-butene, acetylene, furan, acetaldehyde, water, and 2,5-dihydrofuran are discussed. Pulsing of possible C4 intermediates indicates that their desorption from the catalyst surface is detrimental to MAN selectivity. Ethylene and acetylene may be formed from MAN. The consecutive reaction of acetylene with H2O can lead to acetic acid, whereas all other byproducts are predominantly formed directly from n-butane. The pronounced stability of both samples was confirmed by repeated catalyst performance test under reference conditions, XRD, and SEM. Mass and heat transport limitations were experimentally excluded. A formal kinetic model including n-butane, MAN, CO, CO2, acetic acid, and acrylic acid was developed, in which the acids were found to be less relevant on the V0.8W0.20PO4 catalyst. However, the main reaction pathways were found to be similar over both catalysts, which differ mainly in product selectivities.