Catalytic conversion of C4 hydrocarbons on vanadium-phosphorus oxides: factors influencing the selectivity in 1-butene oxidation

Abstract

The influence of the valence state of vanadium and of the P:V ratio on the activity and selectivity in maleic anhydride in the oxidation of 1-butene on vanadium-phosphorus oxides was investigated. Chemical and spectroscopic analyses indicated that two redox couples are mainly involved in the mechanism of C4 fraction oxidation to maleic anhydride: (1) Viv reduction to V111 for the dehydrogenation of 1-butene to butadiene and (2) Vv reduction for the formation of maleic anhydride from adsorbed butadiene. A slight excess of phosphorus strongly stabilizes vanadium(IV), avoiding excessive formation of surface Vv ions, which influence the ratio between rates of maleic anhydride formation and decomposition; on the other hand, a greater excess of phosphorus has a negative effect on the overall rate of butene oxidation, Inhibiting the reduction of Viv to V111. It is suggested that the nonstoichiometry of the catalyst structure (B phase), which is able to maintain high contents (up to approximately 20%) of both Vv and V111 as well as an excess of phosphorus without the formation of new crystalline phases, plays an important role in the catalyst activity.

Structure and reactivity of vanadium-phosphorus oxides

“…However, an excess of P principally affects the rate of hydrocarbon activation. Similar results are obtained in the oxidation of 1-butene [5,13,16].…”

“…Analysis of the valence distribution of vanadium in the above-mentioned zones shows the presence of i) V(III) and V(IV), ii)V(IV) and a low content of both V(V) and V(III), and iii) V(IV) and V(V), respectively [13].…”

“…During the oxidation of 1-butene, it is possible to observe that the reaction temperature has a considerable effect on the distribution of products; more specifically, three different zones can be distinguished, and the main products which are obtained in these zones as the temperature increases are i) butadiene, ii) maleic anhydride and iii) carbon oxides [13].…”

“…This fact suggests that the redox couple V(IV)-V(III) is involved in the formation of butadiene from 1-butene, while the couple V(IV)-V(V) is involved in the formation of maleic anhydride [13]. However, when the surface is excessively oxidized, the maleic anhydride formed is overoxidized to carbon oxides.…”

“…Therefore, thermal methods of analysis of the solid-state reaction are particularly useful, because they are very sensitive to modifications in the transformation mechanism. Another peculiarity of these oxides is the possibility of varying the P : V atomic ratio within the range 0.9-1.3 without apparent modification of the (VO)2P207 structure, even though there is a drastic change of the catalytic activity [13,16]. Moreover, the content of phosphorus in the catalyst also affects the redox John Wiley & Sons, Limited, Chichester Akad~miai Kiad6, Budapest …”