Surface dynamics of a vanadyl pyrophosphate catalyst for n-butane oxidation to maleic anhydride: an in situ raman and reactivity study of the effect of the P/V atomic ratio.

This work focused on investigating the effect of the P/V atomic ratio in vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride, on the nature of the catalytically active phase. Structural transformations occurring on the catalyst surface were investigated by means of in situ Raman spectroscopy in a non-reactive atmosphere, as well as by means of steady-state and non-steady-state reactivity tests, in response to changes in the reaction temperature. It was found that the nature of the catalyst surface is affected by the P/V atomic ratio even in the case of small changes in this parameter. With the catalyst having P/V equal to the stoichiometric value, a surface layer made of alpha(Iota)-VOPO(4) developed in the temperature interval 340-400 degrees C in the presence of air; this catalyst gave a very low selectivity to maleic anhydride in the intermediate T range (340-400 degrees C). However, at 400-440 degrees C delta-VOPO(4) overlayers formed; at these conditions, the catalyst was moderately active but selective to maleic anhydride. With the catalyst containing a slight excess of P, the ratio offering the optimal catalytic performance, delta-VOPO(4) was the prevailing species over the entire temperature range investigated (340-440 degrees C). Analogies and differences between the two samples were also confirmed by reactivity tests carried out after in situ removal and reintegration of P. These facts explain why the industrial catalyst for n-butane oxidation holds a slight excess of P; they also explain discrepancies registered in the literature about the nature of the active layer in vanadyl pyrophosphate.

A systematic review of diagnostic procedures to detect midgut neuroendocrine tumors.

The aim of this work was to assess the diagnostic yield of some clinical diagnostic procedures utilized to establish a diagnosis of midgut neuroendocrine tumors (NETs). Medical databases published between 1982 and 2007 were analyzed. Seventeen observational studies, including 629 patients, met the inclusion criteria. Urinary 5-HIAA is the first test to prescribe in patients with flushing or persistent, unexplained diarrhea. Abdominal CT scan and OctreoScan should be prescribed whenever NET is suspected.