Unsupported molybdena catalysts: precipitation, characterization and catalytic activity

Abstract

Three different MoO₃ samples have been prepared by spontaneous precipitation from (NH₄)₆Mo₇O₂₄ and Na₂MoO₄ aqueous solutions at pH 2.30 and 1.65. The supersaturated solutions were stable over the entire concentration range examined for pH values over 4.0 and below 1.50. The spontaneous precipitation process showed a high apparent order (p= 3) typical for a surface-controlled mechanism. The solution pH and the nature of the counter-ions influenced the size, texture and symmetry of Mo in the precipitated oxide.

Catalytic tests over MoO₃ samples prepared by spontaneous precipitation followed by sulfidation, were performed using the hydrosulfurization of thiophene as a probe reaction. The tests showed that the activity increased with the ratio Mo (tetrahedral): Mo (octahedral). The maximum value of this ratio was obtained for the sample prepared by ammonium heptamolybdate at pH 2.30.