Preparation of Single M1 Phase MoVTe(Sb) NbO Catalyst: Study of the Effect of M2 Phase Dissolution on the Structure and Catalytic Properties
Résumé
The preparation of the M1 active phase of the MoVTe(Sb)NbO catalysts by chemical dissolution in an hydrogen peroxide aqueous solution, of the M2 phase concomitantly formed has been studied. The effects of the reaction temperature and of the H2O2 content of the aqueous solution used for the chemical dissolution have been investigated. The washed Te containing catalysts were shown in all cases to be single M1 phase from XRD diffraction but difference in chemical composition were observed depending upon the washing conditions. These differences were explained by the presence of undissolved M2 phase or amorphous phase covering the M1 phase and leading to much less active catalysts. The solids tend to loose tellurium under the most severe washing conditions, which has a direct effect on the catalytic properties of the phase with a decrease of the selectivity to acrylic acid. The M1(Sb) phase could only be obtained pure at 60 degrees C independently of the H2O2 content. No change in the metal content was observed in the washed M1(Sb) phase but a modification of the structure was detected with a distortion of the orthorhombic unit cell to a monoclinic cell with cell parameters a = 2.1143 nm, b = 2.6632 nm, c = 0.80815 nm, alpha = 90.83 degrees. The pure M1(Sb) phase appears active and selective although less efficient that the M1(Te) phase. A decrease of the selectivity to acrylic with the pre-treatment temperature of the M1(Sb) phase catalysts has been observed and related to a lower Sb3+ content of the phase after catalytic test. Such lower Sb3+ content may be explained by the absence of electron transfers between antimony and the other cations and of oxygens between the hexagonal channels and the octahedral network.