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conditions, i.e., with 10% n-butane and 17% oxygen in feed. As shown in Figure 1 (left), there Doping vanadyl pyrophosphate with NbV: improved efficiency of
was only a minor effect on n-butane conversion, but the yield to maleic anhydride was higher with the Nb-doped system, especially at temperatures higher than 400°C. The higher selectivity the catalyst for n-butane oxidation to maleic anhydride
did correspond to the decrease of selectivity to both COx and phthalic anhydride. It is worth noting that the formation of phthalic anhydride, a minor compound under hydrocarbon-lean Aurora Caldarelli1, Francesca Pierelli1, FabrizioCavani1*, Carlotta Cortelli2, and Federico conditions, but an important one at hydrocarbon-rich conditions, derives from the Diels-Alder type reaction between maleic anhydride and butadiene, the latter being an intermediate 1Diaprtimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, ALMA MATER STUDIORUM Università di Bologna, 40136 Bologna, Italy In order to explain the better efficiency of the Nb-doped catalyst in maleic anhydride formation, 2Polynt SpA, Via E. Fermi 51, 20097 Scanzorosciate (BG), Italy we carried out in-situ Raman experiments on the V/P/O catalysts. We found that the Nb-doped VPP developed the desired δ-VOPO4 during heating in air at 380°C (Figure 1, right); on the opposite, the undoped VPP developed the desired oxidized phase only after treatment in an Introduction
air/steam flow, an environment that favoured the transformation of the VPP to δ-VOPO4 via Vanadyl pyrophosphate is the main component of the industrial catalyst for the selective hydrolysis to VOPO4.2H2O. We concluded that the enhanced formation of the desired δ- oxidation of n-butane to maleic anhydride. Various structural and morphological characteristics VOPO4 was the reason for the better selectivity shown by the Nb-doped catalyst; in fact, the affect the catalytic behavior of the V/P/O system, but the most important one is the presence of transformation of the intermediately formed butadiene into maleic anhydride turned out to be a slight excess P with respect to the stoichiometric amount required for the vanadyl more favoured, because of the higher concentration of the selective δ-VOPO4 compound. pyrophosphate formation. In this regard, we recently reported that the key point to obtain a moderately active but highly selective catalyst is the in-situ generation, under reaction conditions, of discrete amounts of δ-VOPO 4 on the vanadyl pyrophosphate surface, and that the generation of this VV phosphate is favoured in the presence of the forementioned P excess [1,2]. In order to further improve the catalytic performance of the industrial catalyst, we doped the Sel MA, Nb-doped VPP
vanadyl pyrophosphate with controlled amounts of a NbV compound, precursor for the o, Mnnio 40
generation of Nb or mixed Nb/VV phosphate [3]. The latter compound might facilitate the 1013
Nb-doped VPP
C4 conv, VPP
The V/P/O catalyst was prepared according to the procedure described in refs [1,2]; the Nb- Sel MA, VPP
doped V/P/O were prepared by addying the desired amount of NbCl 2O5, H3PO4 and isobutanol, raw materials for the the synthesis of the vanadyl pyrophosphate precursor. Catalysts were prepared with atomic V/Nb ratio equal to 44. Catalysts were characterized by means of X-ray Diffraction, Raman Spectroscopy and UV-Vis Diffuse Figure 1. Left: n-butane conversion and selectivity Reflectance. Catalytic experiments were carried out in a continuous flow reactor; different inlet to MA of Nb-doped VPP and undoped VPP under feed compositions were used, either n-butane lean (1% n-butane, 17% oxygen, remainder He), hydrocarbon reach conditions. Right: in-situ Raman 1 or n-butane-rich (10% n-butane, 17% oxygen, remainder He). spectra of Nb-doped VPP recorded in function of T Results/Discussion
Doping the V/P/O catalyst with Nb had important implications on the catalytic behavior. Under F. Cavani, S. Luciani, E. Degli Esposti, C. Cortelli, and R. Leanza, Chem. Eur. J., 16,
hydrocarbon-lean conditions, e.g., with 1.7% n-butane in feed, the improvement was that of an increase of n-butane conversion, with respect to the undoped system. Since the selectivity to F. Cavani, D. De Santi, S. Luciani, A. Lofberg, E. Bordes-Richard, C. Cortelli, and R. maleic anhydride did not decrease, the activity increase led to an higher yield to the desired Leanza, Appl. Catal. A, 376, 66 (2010).
compound. Even more remarkable was the effect of Nb doping under hydrocarbon-rich F. Cavani, F. Pierelli, F. Ghelfi, G. Mazzoni, and C. Fumagalli, EP 1514598 (2003).


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