Oxidative Dehydrogenation of Propane over Ni–Al Mixed Oxides: Effect of the Preparation Methods on the Activity of Surface Ni(II) Species

Abstract

4Ni–Al layered double hydroxide (LDH) precursors were prepared by the following three methods: constant pH coprecipitation, variable pH coprecipitation and urea hydrolysis. The 4Ni–Al mixed metal oxide (MMO) catalysts were obtained by the calcination of LDH precursors at 600 °C, and their catalytic performances in ODHP from 350 to 550 °C were tested. The 4Ni–Al MMO from the urea hydrolysis method showed a higher and more stable propylene selectivity of ca. 40% with propylene yield of ca.11% at 500 °C, and that from the constant pH coprecipitation method was followed, while oxidative cracking of propane occurred on the 4Ni–Al MMO from the variable pH coprecipitation method. It is considered to be closely related to the dispersion and stability of the surface Ni(II) species through comprehensive analysis of XRD, N2-adsorption-desorption, TEM, XPS, H2-TPR and In-situ electrical conductivity. The urea hydrolysis method with a low salt concentration, leading to an excellent stability of the surface Ni(II) species at high reaction temperature, should be selected to prepare Ni–Al MMO catalysts for ODHP instead of the traditional variable pH coprecipitation method with a high salt concentration.