III International Conference

PP-II-151linewidth 500 Oe. This state is probably attribute to the fine-dispersed X-ray amorphous phaseof chromium (III) or copper/magnesium chromite. Third, an isolated Cr 5+ ions (g 0 = 1.97) areobserved. The ratio between Cr 3+ associates and isolated Cr 5+ ions depends on chromiumloading and calcination temperature. The concentration of Cr 3+ associates is raised and theconcentration of isolated Cr 5+ ions is diminished when chromium loading and calcinationtemperature are increased. The concentration of isolated Cr 3+ ions is practically constant.Although, pre-sulfiding is well known and commonly used, there is still debate regardingthe optimal conditions of active sites formation. In order to explain the purpose of presulfidingin thiophene synthesis we studied a chemical composition of the Cu-Cr-Al and Mg-Cr-Al oxide catalysts pre-sulfided in H 2 S or DMDS. A formation of weakly and stronglyfixed sulfur was observed after pre-sulfiding by both DMDS and H 2 S. When DMDS was usedfor the catalysts pre-sulfiding, a formation of carbon-containing fragments was also observed.These pre-sulfided catalysts were characterized by the ESR signal with g o -2.0 and a linewidth5 Oe and the weight loss followed by the exothermal effect at 425-450 o C observed bythermogravimetric analysis. These signals in ESR and TGA were also observed for thesamples after thiophene synthesis from butane and dimethyldisulfide, and were absent forsample pre-sulfided by H 2 S. Formation of the carbon-containing fragments can explain thelarger catalytic activity of the samples pre-sulfided by DMDS in comparison with the samplespre-sulfided by H 2 S.It has been shown that the calcination of a spent catalyst in air at 580 o C results in aregeneration of the initial electronic state of Cr n+ ions and removal of coke formed duringreaction. Strongly bonded sulfur is removed at temperature above 800 o C.Thus, the Cu-Cr-Al and Mg-Cr-Al oxide catalysts have three electronic states of Cr n+cations. Further studies are needed to evaluate their role in the catalytic activity in thiophenesynthesis and reduction of sulfur dioxide by methane or synthesis-gas.AcknowledgementsFinancial support by the Russian Foundation for Basic Research (Grants 06-08-01450) isacknowledged with gratitude.References1. Ismagilov Z.R., Kerzhentsev M.A., Khairulin S.R., Kuznetsov V.V. Chem. for SustainableDevelopmet, 7 (4) (1999) 375.2. US 4147763.3. Mellor J.R., Copperthwaite R.G., Coville N.J. Appl.Cat. 164 (1997) 69.4. Ryashentseva M.A.. Uspechi chimii 63 (5) (1994) 456.5. Southward B.W.L., Fuller L.S., Hutchings G.J., Joyner R.W., Stewart R.A. Catal. Lett. 55 (1998)207.444