Performance evaluation of sulfated-TiO2 as a modifier to Co-Ni-ZrO2 catalyst in a dual-bed reactor for the selective production of C-4 hydrocarbons from syngas

Modified Fischer-Tropsch (MFT) synthesis was performed at atmospheric pressure in a fixed-bed microreactor with a dual-bed configuration using Co-Ni-ZrO2 and sulfated-TiO2 solid acid catalysts. The sulfate loading in the sulfated-TiO2 catalyst was in the range 5-15 wt % whereas sulfated-TiO2 to Co-Ni-ZrO2 catalyst mass ratio was 1-3. The catalyst system was activated by reduction at temperatures ranging from 573 to 723 K for 16 h whereas sulfated-TiO2 catalyst was calcined at temperatures in the range 723-923 K for 4 h. MFT reactions were performed at weight hourly space velocities (WHSV) in the range 5-20 h(-1) and reaction temperatures ranging from 513 to 533 K. Though these parameters did not have substantial effects on the hydrocarbon product composition, the optimum selectivities for C-5(+) hydrocarbons, i-C-4 hydrocarbons, and C-2-C-4 olefins from syngas using Co-Ni-ZrO2 due to the presence of the modifier sulfated-TiO2 were increased from 26 to 31 wt %, 2 to 4 wt % and 34 to 44 wt %, respectively. However, the presence of sulfated-TiO2 catalyst had no substantial effect on the selectivity for total C-4 hydrocarbons. Though the sulfated-TiO2 catalysts possessed large amounts of acid sites, these sites were weak, and as such, could only catalyze reactions such as isomerization, oligomerization, dehydrogenation and alkylation. The more demanding cracking reactions could not be promoted on sulfated-TiO2 catalysts. The weak acidity exhibited by sulfated-TiO2 catalysts is an asset in that it was able to maintain activity stability for a long period of time in contrast to the performance of sulfated-ZrO2 catalysts, which deactivated rapidly because of its strong acid sites.