The nature of active components in Fe 2O 3MoO 3 System : IV. Bi-doped Fe(III)-molybdate: Characterization and activity

Abstract

An investigation on the FeMoBi oxides system in the Fe-rich region was carried out by adding various amounts of Bi to Fe(III)-molybdate. The principal catalytic modifications which already occurred at very low Bi levels (0.5% Bi 2O 3 by wt), were: (i) strong decreasing in isomerizing power at low temperature; (ii) strong increase in the selectivity to and yield of butadiene; iii) decrease in 1-butene conversion; (iv) strong decrease in CO formation. The characterization of these catalysts with Mössbauer, ir, ATR, diffuse reflectance, X-rays seems to indicate that no new phase is present at these concentrations of Bi. Only at much higher levels (3.5% Bi 2O 3 by wt) where the catalytic activity is already constant, does a new phase form in increasing amounts. This new phase is Bi-molybdate 2:1 modified probably by Fe 3+. It is proposed that the active and selective phase of these ternary systems is Fe(III)-molybdate with Bi as dopant in interstitial or substitutional sites, which might give rise to Bi(V)Mo(VI) continuous groups. ▪ Other particular features of these catalysts seem to be: 1. i. A higher activity than that of the typical values of Bi-molybdates. 2. ii. An activity constant also in very reducing conditions where Bi-molybdates become inactive. 3. iii. A strong influence of oxygen partial pressure: at very low oxygen level the selectivity increases considerably.