FISCHER-TROPSCH SYNTHESIS: EFFECT OF CO CONVERSION ON PRODUCT SELECTIVITIES DURING DEACTIVATION BY OXIDATION OR BY CHANGING SPACE VELOCITY AT STABLE CONDITIONS OVER UNPROMOTED AND RU PROMOTED 25%Co/Al_2O_3 CATALYSTS

摘要

The effects of CO conversion on hydrocarbon and CO_2rnselectivities were studied under both deactivation and periods ofrnstability but with changes in contact time. Essentially identical trendsrnin changes of selectivities to CH4 (negative), C5+ (positive), 1-olefinrn(negative) and 2-olefin (positive) with CO conversion were observedrnregardless of whether a decrease in CO conversion was caused byrncatalyst deactivation or by an increase in gas space vel℃ity (stablernconditions). However, selectivities to CO_2, total paraffins and totalrnolefins in the C_2-C_4 and C_5-C_(10) ranges did not change significantlyrnduring the marked deactivation from 320 h (XCO: 40-20%) for a cobaltrncatalyst, whereas different trends were observed under stablernconditions with changing contact time, (i.e., CO_2 selectivity and totalrnparaffin content increased almost linearly and total olefin contentrndecreased almost linearly with increasing CO conversion).rnDifferent mechanisms were responsible for the observedrnrelationships between CO conversion and various selectivities underrndeactivating and stable conditions. The formation of cobalt oxidernspecies during catalyst deactivation enhanced methanation and WGSrnreaction pathways, as well as the hydrogenation reaction of olefins orrnC_nH_(2n+1)-S species to paraffins. It is proposed that cobalt oxide speciesrnplay a role in enhancing methanation, perhaps by enhancing the l℃alrncoverage of hydrogen on the catalyst surface by promoting the WGSrnreaction. Therefore, changes in the product selectivities with COrnconversion on the cobalt catalyst under deactivating conditions was arncombination of the effect of contact time and changes in cobaltrnstructure, but the latter factor became dominant in the later portion ofrndeactivation in the long kinetic test run. In contrast, the intrinsicrnrelationship between CO conversion and various selectivities of the Corncatalyst obtained under stable conditions with changing contact timernmainly resulted from the changes in partial pressures of reactantsrnand/or products with changes in contact time.rnUnder natural deactivation conditions, isomerization of 1-olefinsrnwas not observed to be enhanced on cobalt oxidized species; thus thernisomerization of 1-olefins to internal olefins was mainly governed byrnpartial pressure of CO.