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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
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
generation of Nb or mixed Nb/VV phosphate . The latter compound might facilitate the
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
-butane-rich (10% n
-butane, 17% oxygen, remainder He).
spectra of Nb-doped VPP recorded in function of T
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.
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
, 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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