Formation of 2-alkenes as secondary products during Fischer–Tropsch synthesis

•There is an inverse isotope effect during Fe catalyzed FT reactions.•The KIE during FTS over iron catalysts (αH/αD) is 0.88 at 270 °C.•1-alkenes and 2-alkenes are formed through two different pathways during FTS.•There is a normal isotope effect in the formation of 2-alkenes during FTS at 270 °C.•Internal alkenes produced during FTS are products of secondary reactions.

The H2/D2 switching experiments showed that there is an inverse isotope effect during iron catalyzed Fischer–Tropsch (FT) reactions. The rate of hydrocarbon production (r) increased when syngas CO/H2 was switched to CO/D2 with the ratio of rH/rD being 0.55–0.58. The inverse isotope effect in every propagation step was calculated to be 0.88, which is a reasonable value for an inverse isotope effect at 270 °C that originates from the C–H (D) bond hybridization change from sp2 to sp3. These results are similar to cobalt catalyzed FT reactions that were explained by the modified alkylidene mechanism by which 1-alkenes and n-alkanes are the primary products of the FT reaction and the others are products of secondary. The detailed analysis of products produced during iron catalyzed FT reactions with CO/H2 or CO/D2 as the syngas feeds showed that while the ratios of [1-alkenes]H/[1-alkenes]D are less than 1, the ratios of [2-alkenes]H/[2-alkenes]D are more than 1; this indicates that 2-alkenes are produced through a pathway that is different from 1-alkenes. These results also suggest that there is a normal isotope effect during the formation of 2-alkenes through the corresponding 1-alkenes, which will result in the ratio of [1-alkenes]H/[1-alkenes]D being less than 1. The formation of 2-alkenes is explained by the alkylidene mechanism.

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