The Preparation Of Alkaline Lithium Phosphates And Their Catalytic Properties For Propylene Oxide Isomerization

Allyl alcohol is a very useful product in petrochemistry. In order to improve the conversion of propylene oxide and selectivity of allyl alcohol, this paper was focused on the preparation of lithium phosphates. These catalysts were analyzed by different ways of characterization to study their different morphologies and structures. The properties of these catalysts were examined by the isomerization of propylene oxide. A mechanism of the isomerization was also proposed.Stoichiometric Li3PO4and a series of basic lithium phosphates with different pH values and ripening times were employed to the isomerization of propylene oxide. The result showed that stoichiometric Li3PO4had the lowest catalytic property among these catalysts. With the pH value decreasing, basic lithium phosphate had a higher catalytic property. The ripening time of basic lithium phosphate also affected the crystalline size and the catalytic properties, and the optimum ripening time was3h.Alkaline lithium phosphates were prepared at different calcination temperatures and their catalytic properties were evaluated.These alkaline lithium phosphates were also characterized by TG-DTA, XRD, IR and BET. The results showed that there were two forms of lithium phosphates:γ-form and β-form, and the transition temperature was in the range of620-720℃. With the increase of calcination temperature, BET surface area and catalytic property of Li3PO4decreased, and their crystalline sizes increased. The β-Li3PO4partially transformed into y-Li3PO4in the reaction process.With different addition sequences, three types of lithium phosphates were prepared:NLi3PO4, LLi3PO4, SLi3PO4. These samples were characterized by XRD, TEM, SEM and BET. The three types of lithium phosphates all displayed peaks characteristic of hollow β-Li3PO4. The LLi3PO4sample showed the maximal pore diameter and more advantageous crystallinity which were probably conducive to the best catalytic property.The fresh and used lithium phosphates were characterized by IR, XPS, CO2-TPD and7Li NMR. There were some moderate alkaline sites and weak acid sites existing on the surface of the fresh catalyst. After the isomerization, there were peaks characteristic of alkyl and olefinic groups on the surface of the used catalysts. The Li ions in the crystal lattice were located on two crystallographically different sites based on the XPS and NMR patterns, and the motion and shift of Li ions could be taken place after the isomerization.