Reactions of 1-Olefins Added during the Fischer-Tropsch Synthesis over Supported Cobalt Catalyst
Xiaohao Liu, Xiaohong Li, and Kaoru Fujimoto. Department of Chemical Processes and Enviroments, Faculty of Enviromental Engineering, The University of Kitakyushu, 1-1, Hibikino, Wakamatsu, Kitakyushu, Japan
Abstract: Since Fischer-Tropsch (FT) synthesis is a polymerization process, its total product yield decreases exponentially with chain length resulting in a so-called Anderson-Schulz-Flory (ASF) distribution. Such a distribution places a significant limitation towards more valuable products such as diesel and heavy wax. Chain-length-dependent secondary reactions for primary 1-olefin products, however, cause deviations from the ASF-type distribution. Thus, the 1-olefin adsorption reactions can be used to control the product selectivity. In this study, our major aim is to obtain more insight into the role of secondary reactions and to improve the selectivity to the desired product range. For this purpose, we have done some co-fed 1-olefin studies over supported cobalt catalyst in a trickle bed reactor. Results show that addition of 1-octene or 1-dodecene in the n-paraffin solvent could greatly enhance the selectivity of heavy wax and increase middle distillates to a certain extent directly from syngas, with a marked increase in CO conversion while suppressing the formation of lighter hydrocarbon products compared to the conventional FT synthesis. In the case of co-fed 1-dodecene, the disproportionate reaction was obviously observable for the first time, which resulted in a symmetrical product distribution and gave a much higher selectivity in middle distillate. The disproportionate reaction has experimentally been evidenced by the present authors that added 1-dodecene undergoes significant chain growth and chain degradation on Co/SiO2 catalyst in the prosence H2 (not in syngas). Based on the experiments, a modified secondary reaction network for the reactive 1-olefins is presented and,expecially, the reaction process for the disproportionate reaction of 1-olefins is discussed in details.