Study On The Surface Reaction Behaviors Of Propylene Partial Oxidation To Propylene Oxide

Propylene oxide is an important organic-chemicals intermediate. In present work,the catalytic reaction behaviors of propylene partial oxidation with water vapor andoxygen to propylene oxide were studied systemically on complex phosphates.Three kind of phosphates Fe_1/2Al_1/2PO₄, FePO₄ and AlPO₄ were prepared fromthe sol-gel method. The results characterized by the techniques of IR, XRD, BET,TEM and TPR showed that these materials were composed of phosphonium-oxygentetrahedrons with Lewis acid sites Fe³⁺ or Al³⁺ and Lewis base sites P=O or Fe(Al)–O–P presented on the surfaces, that the Fe_1/2Al_1/2PO₄ catalyst was an uniformcompound composed with 10nm particles of FePO₄ and AlPO₄, and that the activityof the lattice oxygen was in the order of Fe_1/2Al_1/2PO₄ > FePO₄ >> AlPO₄.The chemisorption characters measured from chemisorbed-IR andchemisorbed-TPD showed that the C=C bond and the allylic H in propylenemolecular can be chemisorbed on the Lewis acid sites(Fe³⁺ or Al³⁺) and the Lewisbase sites(P=O) respectively, the O and H in H₂O molecular can be chemisorbed onthe Lewis acid site Fe³⁺ and the Lewis base site P=O, forming to a mutil-siteadsorption state, which desorbed at a higher temperature with a result of inducingsurface Fe–O-sites. bond in propylene oxide can be chemisorbed on the Lewisacid site Fe³⁺ of the solid surface to form a molecular adsorption state.The results obtained from micro-reactor experiments showed that propylenepartial oxidation reaction was occurred remarkably on Fe_1/2Al_1/2PO₄ and FePO₄catalysts at normal pressure and a temperature higher than of 100℃, the main productof C₃H₆ reacting with O₂ was acrolein and the main product of C₃H₆ reacting with O₂and H₂O was propylene oxide. The water vapor adding into the reaction system notonly increased the conversion of propylene, but also promoted the formation ofpropylene oxide. Under the conditions of 0.1Mpa, SV 1200h-1 and 200℃, thepropylene conversion of 5.8% and the propylene oxide selectivity of 77.6% can beobtained on the Fe_1/2Al_1/2PO₄ catalyst.A reaction mechanism of C₃H₆ reacting with O₂ and H₂O to form propyleneoxide is proposed as follows: the surface Fe–O-species on the Fe_1/2Al_1/2PO₄ catalyst isformed with adsorption-desorption of H₂O molecular at first, then the C=C bond inC3H6 molecular chemisorbs on the surface Fe–O-site and reacts with the O-species toform the propylene oxide, at last molecular O₂ chemisorbs on the Lewis acid site Fe²⁺and splits into O^2- to compensate the lattice oxygen which consumed in the reaction.