TAP vacuum pulse-response and normal-pressure studies of propane oxidation over MoVTeNb oxide catalysts

The economic advantages of switching to a propane feedstock in the production of acrylic acid have been a motivating force for extensive research studies focused on developing alkane partial oxidation catalysts. While catalyst systems containing MoVTeNb oxides have received considerable attention, the activity of these catalysts has not been systematically characterized over a wide domain of working conditions (different gas compositions, temperatures, etc.) and a detailed kinetic model of the propane-to-acrylic-acid reaction has not been developed yet. Extensive steady-state studies (conversion/product yields vs temperature at different feed compositions and contact times) and non-steady-state studies (pulse-response temporal analysis of products (TAP) experiments) of the propane-to-acrylic-acid reaction were performed using a TAP-2 reactor system under normal and vacuum conditions. TAP pump-probe experiments indicate that there is an optimum time for the introduction of water and that water reacts with an adsorbed intermediate to produce acrylic acid. Under steady-state conditions, the temperature was slowly cycled and the reaction progress was monitored with a temperature hysteresis clearly distinguished. The upper branch of the hysteresis corresponds to a cooling regime and is characterized by complete oxygen conversion. No distinctive hysteresis behavior was observed for propane, while the product yields exhibit clear hysteresis loops, clockwise for acrylic acid and counterclockwise for CO2/CO and acrolein. In this paper, the hysteresis-type dependence is interpreted in terms of a possible detailed mechanism of selective oxidation of propane into acrylic acid.