Abstract
At 550{degree}C, the 1 wt % SO{sub 4}{sup 2}{minus}/l wt % Sr/La{sub 2}O{sub 3} catalyst, one of the most active catalyst for the selective conversion of methane at these moderate temperatures, showed a very good stability with a reactant mixture of CH{sub 4}/air = 1/1 with GHSV = 70,040 {ell}/kg catal/hr. During a 25 hr catalytic test, the conversion of CH{sub 4} and the C{sub 2}{sup +} selectivity did not change, indicating good stability of the catalyst. At the same time, the CO{sub 2}/CO ratio remained steady at about 2.2, but the C{sup 2}{sup =}/C{sub 2} ratio shifted slightly with time from 0.74 to 0.68. The oxidative coupling of CH{sub 4} to C{sub 2}-hydrocarbons was very sensitive at 550{degree}C to the alteration of the CH{sub 4}/air reactant ratio at GHSV = 70,040 {ell}/kg catal/hr. It was observed that the conversion of CH{sub 4}, the C{sub 2}{sup +} selectivity, and the %yield of C{sub 2}{sup +} hydrocarbon products decreased very rapidly with increasing CH{sub 4}/air ratio from 1 to 3-4. At the largest CH{sub 4}/air ratio of 40.77 that was used utilized, the CH{sub 4} conversion was less than 0-5 C-mol%. The reverse process of decreasing the CH{sub 4}/air ratio from {approx}40 to 1 showed nearly reversible catalytic performance, but some deactivation was apparent at the lowest reactant ratios. The 1 wt % SO{sub 4}{sup 2}{minus}/1 wt % Sr/La{sub 2}O{sub 3} catalyst used in the experiments in which the CH{sub 4}/air ratio was varied subsequently revealed a good stability in the CH{sub 4} conversion level during testing at 550{degree}C for 15 hr (GHSV = 70,040 {ell}/kg catal/hr and CH{sub 4}/air = 1/1). Indeed, the C{sub 2}{sup +} selectivity even increased by 3 to 4 C-mol%. Increasing the temperature from 550 to 600{degree}C resulted in a further recovery of the activity of the partially deactivated catalyst.
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