Ruthenium hydrodesulfurization

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

Ruthenium carbonyl-derived catalytic systems have also been studied in hydrodesulfuration [118, 119], Highly active catalysts for the hydrodesulfuration of diben-zothiophene have been obtained by supporting on alumina MHRu3(CO)n (M = group 1 metal), which was the product of the reaction between Ru3(CO)i2 and MOH. The activity increased from Li to Cs [119]. 

Ruthenium clusters were evaluated for catalytic activity for hydrodesulfurization (HDS) of benzo[b]thiophene. At 423 43 K and 100 atm, hydrogen benzo[b]thiophene was converted to 2,3-dihydrobenzo[b]thiophene and 2-ethylthiophenol. The best catalyst precursor was H4Ru4(CO)g(PPh3)4, which gave 81% conversion to 2,3-dihydro-benzo[b]thiophene after 384 h at 443 K about 5% ethylbenzene was also formed, showing some HDS activity. The clusters were more active than the monometallic precursor Ru(CO)3(PPh3)2. However, the reaction conditions are rather severe and the involvement of decomposition products is very possible. ... 

The hydrogenolysis to thiols can be carried out effectively in a biphasic system, with the catalyst exclusively soluble in the polar phase, thus enabling easy catalyst recycling. However, to introduce this biphasic technique to industrial hydrodesulfurization, much research has to be carried out to design catalysts that are suitable for biphasic catalysis and that contain inexpensive metals (cobalt, ruthenium). Furthermore the catalysts have to tolerate the great thermal and chemical stress of the reaction conditions. 

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