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New Technologies and Alternative Feedstocks in Petrochemistry and Refining
DGMK Conference October 9 – 11, 2013, Dresden, Germany
Improved Performance of Nb-doped Vanadyl Pyrophosphate, Catalyst for n-
butane Oxidation to Maleic Anhydride
G. Pavarelli*, A. Caldarelli*, F. Cavani*, C. Cortelli**, S. Luciani**
*Università di Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Bologna, Italy
Polynt SpA, Via E. Fermi, Scanzorosciate (BG), Italy
Vanadyl pyrophosphate is the main component of the industrial catalyst (V/P/O) for the
selective oxidation of n-butane to maleic anhydride. It has been established that the key
point to obtain an active and selective catalyst is the in-situ generation, under reaction
conditions, of discrete amounts of -VOPO4 on the vanadyl pyrophosphate surface, and that
the formation of this compound is favoured in the presence of a slight excess of P [1,2]. In
order to further improve the catalytic performance of the industrial catalyst, we doped the
vanadyl pyrophosphate with controlled amounts of a Nb5+ compound [3]. Here, we report
about an investigation of catalytic performance of V/P/O and Nb-doped V/P/O, carried out by
means of both reactivity experiments and in-situ Raman spectroscopy, in order to
understand the role of Nb on the generation of the selective phase.
Catalytic experiments were carried out with samples having different V/Nb atomic ratios.
The effect of Nb on selectivity to maleic anhydride was a function of temperature; in fact, the
catalyst having the greater amount of Nb (V/Nb=46) showed the worst performance at all
the temperatures tested; however, the sample containing the smaller Nb content
(V/Nb=150) showed the highest MA selectivity at low temperature. Finally, at high
temperature the catalyst showing the better selectivity was that one with the intermediate Nb
content (V/Nb=80). These results showed that the optimum Nb amount in Nb-doped V/P/O
is a function of the reaction conditions used.
The effects observed were attributed to an in-situ modification of the characteristics of
vanadyl pyrophosphate, i.e., of the ratio between V5+ and V4+ species on catalyst surface,
the latter being in turn affected by both the gas-phase composition and the Nb content in
samples. We hypothesized that Nb5+ affects the in-situ generation of the selective phase, -
VOPO4; an excessive amount of the oxidized phase led to a non-selective catalyst, but the
generation of a discrete amount of it, in the catalyst containing an intermediate amount of
Nb, led to the more selective behavior. In-situ raman experiments confirmed the
hypothesized role of the Nb dopant.
1 F. Cavani, S. Luciani, E. Degli Esposti, C. Cortelli, R. Leanza, Chem. Eur. J. 16 (2010)
[2] F. Cavani, D. De Santi, S. Luciani, A. Lofberg, E. Bordes-Richard, C. Cortelli, R. Leanza, Appl. Catal. A 376 (2010) 66.
[3] F. Cavani, F. Pierelli, F. Ghelfi, G. Mazzoni, C. Fumagalli, EP 1514598 (2003), assigned


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