Studies on the reaction pathways for the selective oxidation of propane to acrolein over MoPO/SiO2 catalyst by IR spectroscopy

To understand possible reaction pathway for propane selective oxidation to acrolein over the MoPO/SiO2 catalyst, the surface species formed by adsorption of the reactant (propane), possible intermediates or their probe molecules (propene, isopropanol, 1-propanol, 2-Br-propane, allyl alcohol) as well as the reaction products (acrolein, acetone and propanal) on the catalyst and the transformation of these species at elevated temperature under vacuum or under O2/He atmosphere were studied by IR spectroscopy. When a gas mixture of propane/oxygen was introduced to the catalyst at 473 K for 4 h, the band of adsorbed acetone with a shoulder assigned to adsorbed acrolein was observed. Using 2-Br-propane or isopropanol as the probe molecule, characteristic IR bands associated with isopropoxy species were detected over MoPO/SiO2 catalyst at 323 K. Upon heating to 423 K, the band of adsorbed acetone and adsorbed acrolein was also observed. The overall spectrum actually seems closely similar to that described above, arising from the adsorption of propane at the same temperature. On the other hand, if 1-Br-propane or n-propanol was used as the probe molecule, the formation of propanal was detected via n-propoxy. It was found that when propene was adsorbed on the catalyst at 373 K, besides the σ-allyl species, isopropoxy was also detected by the IR spectra. Upon heating to 423 K, two new bands ascribable to the acrolein and adsorbed acetone were observed. Based on these results and density functional calculations, the reaction network for selective oxidation of propane to acrolein over this catalyst was proposed. The propane activation is initiated by the H abstraction from the methylene group, leading to the formation of isopropyl species, which rebounds to the surface to form the isopropoxy species. The isopropoxy species can either undergo α-H elimination to form byproduct acetone or undergo β-H elimination to form the propylene. Propylene can undergo α-H abstraction to form a π-allyl, which is then followed by O insertion to form a σ-allyl, and is eventually dehydrogenated to acrolein.

The reaction networks for propane selective oxidation to acrolein were proposed by using the IR spectroscopy studies the adsorbed species and gaseous species formed by adsorption of the reactant, possible intermediates or their probe molecules as well as the reaction products on the MoPO/SiO2 catalyst and the transformation of these species at elevated temperature under vacuum and under O2/He atmosphere. Figure optionsDownload as PowerPoint slide