Studies On The Reaction Mechanism Of The Selective Oxidation Of Propane To Acrolein Over MoPO/SiO₂ Catalyst

The selective oxidation of propane to acrolein is an important reaction for the utilization of light alkanes. The main challenge for this process is that propane is less reactive than the desired products, and activation of the reactant needs operating condition (higher reaction temperature for example) which is detrimental to the stability of the reaction intermediates and products. A better understanding of the reaction mechanisms and the involved intermediates as well as their conversion dynamics on the catalyst surface is the key for the design of the active and selective catalysts.In the present work, the selective oxidation of propane (SOP) to acrolein over MoPO/SiO2 catalysts was studied. The effects of P loading and support on the structure, acid-base and redox properties as well as the catalytic performance of the supported Mo catalysts were investigated. To elucidate possible reaction pathway for propane selective oxidation to acrolein over MoPO/Si02 catalyst, the oxidative conversions of propane, possible intermediates or their probe molecules as well as the reaction products of SOP to acrolein on the catalyst were studied. The surface species formed by adsorption of these molecules on the catalyst and the transformation of these adspecies at elevated temperature were characterized by in situ IR spectroscopy. The main results are summarized as follows:1) The addition of certain amount of phosphorus to MoO/SiO2 increases the activity and selectivity of the catalyst for propane selective oxidation to acrolein. By forming Mo-O-P bond with the Mo species, phosphorus component in the catalyst improves the dispersion of Mo oxide species that will be favorable to the formation of isolate active sites on the catalyst. The effects of supports on the catalytic behavior of the supported MoPO catalysts for SOP to acrolein were also investigated.2) The results of in situ IR studies showed that the IR bands of the adsorbed acetone and acrolein species were observed over the MoPO/SiO2 catalyst after interacting with a gas mixture of C3Hg/02 at 200 癈. Similar IR spectrum arising from the adsorption of isopropanol over the catalyst at the same temperature was observed. The adsorption of isopropanol and 2-Br-propane at 100 癈 result in isopropoxy species, which can either convert to propene or to acetone at higher temperature. Propene can then convert to acrolein through allylic process. While the adsorptions of l-propanol and 1-Br-propane at 100 癈 give rise to 1-propoxy species, which mainly converted to propanal at higher temperature. IR spectrum arising from the interaction of propene with the catalyst at 100 癈 shows the IR bands of allyl alcoholate and isopropoxy species. As the temperature was increased to 150 癈, the bands ascribable to the adsorbed acetone and acrolein were observed. Based on these results, it is suggested that isopropoxy species is one of the intermediates for SOP to acrolein over MoPO/SiO2 catalyst. The isopropoxy species can either convert to propene by p-hydrogen elimination or to acetone by dehydrogenation and the former can further convert to acrolein through allylic process.