NON-ASF PRODUCT DISTRIBUTIONS DUE TO SECONDARY REACTIONS DURING FISCHER-TROPSCH SYNTHESIS

Since Fischer-Tropsch (FT) synthesis is a chain growth reaction, its t otal product yield decreases exponentially with chain length forming a so-called Anderson-Schulz-Flory (ASF) distribution. Such a distributi on is unselective toward middle distillates for all possible chain gro wth probabilities. Chain-length-dependent secondary reactions, however , cause deviations from the ASF-type distribution and can thus be used to improve the selectivity to the desired product range. To investiga te secondary reactions we set out to study FT synthesis on flat model catalysts, a cobalt foil and cobalt particles on a SiO2 wafer, allowin g a much better definition of the experiments than porous ones. On a C o foil olefin hydrogenation is the main chain-length-dependent seconda ry reaction, causing an exponential increase in the paraffin-to-olefin ratio with carbon number, but not resulting in a deviation from the A SF distribution. On Co/SiO2 model catalysts chain-length-dependent rei nsertion of alpha-olefins into the chain growth process is the main se condary reaction, causing an increase of the growth probability with c hain length. To a lesser extent, hydrogenolysis also plays a role, sho rtening long hydrocarbons by successive demethylation. On Co/SiO2 the interplay of chain-length-dependent reinsertion and hydrogenolysis res ults in sigmoid product distributions with a high selectivity to middl e distillates. These product distributions can be fitted with a simple model in which a chain growth reaction is combined with chain-length- dependent secondary hydrogenation, reinsertion, and hydrogenolysis. (C ) 1996 Academic Press, Inc.