Reactions of Olefins Added during Fischer-Tropsch Synthesis over Supported Cobalt Catalyst

The control on product distributions in Fischer-Tropsch synthesis by feeding olefins in the n-paraffin solvent over a 20 wt.% Co/SiO2 catalyst under the typical reaction conditions of 503K, 2.1MPa, H2/CO = 2, and co-fed olefins 16 mol% in CO base in a trickle bed reactor has been studied. The selected co-fed olefins include 1-octene, 2-octene and 1-dodecene. Results showed that the addition of 1-octene or 1-dodecene could greatly enhance the selectivity of heavy wax products with a marked increase in CO conversion while suppressing the formation of lighter hydrocarbon product by incorporation of co-fed 1-olefins into chain growth compared to the conventional FT reaction. In the case of co-fed 1-dodecene, disproportionate reaction may occur, which resulted in a symmetrical product distributions and gave a high selectivity in middle distillates in 1-dodecene-added FT reaction. The disproportionate reaction occurred due to the successive demethylation of 1-dodecene and higher hydrocarbons formed by 1-dodecene incorporated with CH2 species from demethylation of 1-dodecene. The disproportionate reaction of co-fed olefins was competitive with incorporation into chain growth and were controllable by tuning process conditions. The disproportionate reaction was increased with chain length n. When the 2-octene was used as co-fed olefin, the incorporation into chain growth was much less and hydrogenation reaction was also relatively more difficult to occur, which was ascribed to the sterical hindrance.