Rhenium catalysts reducibility

Valence State of Rhenium in Reduced Bimetallic Catalysts With and Widiout Alkali Metals... 

Platinum-rhenium catalysts have been reduced in one atmosphere of flowing hydrogen and then examined, without exposure to the atmosphere, by ESCA. The spectra indicate that the Group VIII metal is present in a metallic state in the reduced catalyst and that the majority of the rhenium is present in a valence state higher than Re(0). 

For the composite Pt-Re/Al203-ZSM-5 catalyst, the increased activity and aromatic selectivity can be ascribed to the suppression of hydrogenolysis by sulfur adsorption on the rhenium surface reducing the ensemble size of platinum [3]. As a result, the... 

The catalysts were prepared by contacting alumina with aqueous solutions of chloroplatinic acid and ammonium perrhenate. The consumption of hydrogen during reduction corresponded to complete reduction of platinum from the +4 oxidation state to the metal and of rhenium from the +7 to the +4 state. The X-ray diffraction data on the metal residue from the leached catalysts showed no evidence for the presence of rhenium metal or a platinum-rhenium alloy. Most of the rhenium was found in the leaching solution. Finally, the authors stated that data from an electron spin resonance experiment on one of the reduced platinum-rhenium catalysts were consistent with their conclusion that the rhenium was present in the +4 state. 

Therefore, in the catalytic experiment prior to activity tests the alumina supported Re-Pt catalysts were calcined in oxygen at 400 °C and reduced at the same temperature. After such a treatment the catalyst was not completely reduced since further hydrogen consumption was observed above 400 °C in TPR. ° As already shown in Table 22 this is an indication on the presence of ionic rhenium species in the catalysts reduced at 400 °C for 2 hours. 

Reduction of Carboxylic Acids to Alcohols. In addition to the nonsupported catalysts mentioned for the hydrogenation of amides to amines, mthenium and rhenium on alumina can be used to reduce carboxyHc acids to alcohols. The conditions for this reduction are somewhat more severe than for most other hydrogenation reactions and require higher temperatures, >150° C, and pressures, >5 MPa (725 psi) (55). Various solvents can be used including water. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

T-4)-J or Raney nickel and hydrogen.8 Alcohols of the benzylic type have also been reduced directly with hydrogen under pressure in the presence of various catalysts,9 and benzoic acids have been reduced to toluenes with rhenium-type catalysts and hydrogen at high temperatures and pressures.10... 

Pt-Re-alumina catalysts were prepared, using alumina containing potassium to eliminate the support acidity, in order to carry out alkane dehydrocyclization studies that paralleled earlier work with nonacidic Pt-alumina catalysts. The potassium containing Pt-Re catalyst was much less active than a similar Pt catalyst. It was speculated that the alkali metal formed salts of rhenic acid to produce a catalyst that was more difficult to reduce. However, the present ESCA results indicate that the poisoning effect of alkali in Pt-Re catalysts is not primarily due to an alteration in the rhenium reduction characteristics. 

Platinum loadings, reducing, 19 628 Platinum metals plating, 9 822-823 Platinum oxides, volatilized, 17.T80 Platinum-palladium thermocouple, 24 461 Platinum reforming catalysts, rhenium and, 21 695-696... 

In the preparation of faujasite zeolite-supported Pt-Re catalysts, bimetallic PtRe clusters have been reported to be predominantly formed when a carbonyl rhenium precursor (Re2(CO)io) is contacted with zeolite in which platinum has been previously introduced and reduced. The preexisting Pt clusters may act as nucleation sites. After reduction, these Pt-Re systems show a high selectivity to CH4 in the hydrogenolysis of n-heptane [58]. 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

Usually, the aquametal(III) is reducible to aquametal(II) and oxidizable to the corresponding oxometal(IV), hydrox-ometal(IV) and oxometal(V) derivatives depending on the character of the incorporated metals. Further oxidations to oxometal(VI) and-(VII) are possible in the cases of metals like rhenium [87]. The interest of hydroxometal and ox-ometal species as oxidation catalysts in synthetic applications must be stressed. 

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