Synthesis of MMoO 4/SiO 2 catalysts (M=Ni or Co) by a sol–gel route via silicon alkoxides : Stabilization of β-NiMoO 4 at room temperature

Abstract

MMoO 4/SiO 2 composites (M=Ni or Co) were prepared by a sol–gel procedure involving Si(OMe) 4, M(NO 3) 2 and (NH 4) 6Mo 7O 24 as starting materials. The dried gels were treated at increasing temperatures until crystalline grains of MMoO 4 highly dispersed in the amorphous silica matrix were formed (675°C). They were evidenced by XRD and TEM measurements. Beside supporting the MMoO 4 mixed oxide, the silica xerogel appears to play the important role of stabilizing the β-phase of nickel molybdate, which otherwise would turn into the α-phase at room temperature. The catalytic activity of the β-NiMoO 4/SiO 2 composite in the oxidative dehydrogenation of isobutane was compared with related systems containing the metastable β-molybdate.