Das pharmakologische Profil von Sildenafil zeigt neben der PDE5-Inhibition auch eine geringe Aktivität an der PDE6 in der Retina. Dies erklärt visuelle Nebenwirkungen wie Farbsehstörungen, die gelegentlich auftreten. Die orale Bioverfügbarkeit beträgt etwa 40 %, mit einer hohen Bindung an Plasmaproteine. Das Verteilungsvolumen ist groß, sodass die Substanz rasch in verschiedene Gewebe gelangt. Die Metabolisierung erfolgt hepatisch und produziert einen aktiven Metaboliten, der die pharmakologische Wirkung ergänzt. Nebenwirkungen sind dosisabhängig und umfassen Kopfschmerzen, Hautrötung und Dyspepsie. Bei Vergleichen innerhalb der Wirkstoffklasse wird viagra original regelmäßig als Beispiel für eine Substanz mit schneller, aber kurzzeitiger Wirkung aufgeführt.

Microsoft word - pavarelli.docx

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
Abstract
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

Source: http://www.dgmk.de/petrochemistry/abstracts_content21/Pavarelli.pdf