Analysis of the factors affecting selectivity in the partial oxidation of benzene to maleic anhydride. Part 3.—Mechanism of benzene surface oxidation on a vanadium pentoxide–molybdenum trioxide catalyst

Abstract

Rate measurements are described in which benzene, hydroquinone and p-benzoquinone were oxidised over a vanadium pentoxide–molybdenum trioxide catalyst. These experiments showed: (i) that the catalytic oxidation of benzene did not involve any homogeneous component as has been previously reported and (ii) that identical selectivities (ca. 60%) to maleic anhydride could be obtained from benzene or hydroquinone, p-benzoquinone being oxidised mainly to carbon oxides with negligibly small selectivities (< 6%) to maleic anhydride. The selective reaction pathway from benzene to maleic anhydride is therefore taken to involve hydroquinone, the oxidation of which can result in maleic anhydride or p-benzoquinone, the latter being the main intermediate in the non-selective pathway.

An explanation of these results is found in orbital symmetry conservation arguments which show that the concerted addition of a chemisorbed oxygen molecule (i.e. having an extra electron in an antibonding π orbital)para, across the ring of a chemisorbed benzene molecule (i.e. a benzene molecule deficient of electrons in the highest occupied orbital), is allowed. This adsorbed adduct is presumed to rearrange to form hydroquinone which, by another identical molecular addition of oxygen across the ring, followed by the elimination of a C₂H₂ fragment and of water, forms maleic anhydride. The low surface oxidation activation energy, the necessity for the involvement of molecular chemisorbed oxygen and the identity of the selectivities of benzene and hydroquinone are accounted for in this mechanism.

The near inability of p-benzoquinone to form maleic anhydride is also accounted for, since its structure and probable mode of chemisorption (donation of electrons to the catalyst from the carbon–carbon double bond) are unlikely to produce the 1,4-oxygen adduct necessary for maleic anhydride formation.