Effect of H₂S in Syngas on the Fischer–Tropsch Synthesis Performance of a 0.5%Pt–25%Co–Al₂O₃ Catalyst

Abstract

The effect of 1.0 ppm H₂S in the syngas feed on initial activity and selectivity of a 0.5%Pt–25%Co/Al₂O₃ catalyst was studied by comparing the catalyst performance under H₂S and sulfur free conditions. The reaction tests were performed using a 1-L slurry phase reactor for 141–212 h under constant reaction conditions: 220 °C, 2.0 MPa, H₂/CO = 2.0 and 6.0 Nl/g-cat/h. In the H₂S poisoning test, an H₂S in N₂ gas mixture was added to the syngas feed (1.0 ppm) after running the Fischer–Tropsch synthesis (FTS) reaction for ca. 6.0 h; as such, the impact of H₂S on the initial deactivation of the cobalt catalyst (unsteady state) was successfully assessed. The used catalysts were characterized by XANES to investigate if Co–S surface species were formed during the deactivation. The initial deactivation under 1.0 ppm H₂S condition was significantly higher (by 2.0–2.4 times) than that under clean conditions. CH₄ selectivity increased substantially and C₅₊ selectivity decreased greatly with time regardless of whether H₂S was added or not; however, the addition of H₂S accelerated the changes in the hydrocarbon selectivities, which were likely caused by the sharp deactivation of the catalyst in the presence of H₂S. After co-feeding the sulfur for 141 h, a comparison was made at similar conversions by adjusting space velocity; the sulfur pretreated cobalt catalysts favored heavier hydrocarbons (C₅₊) formation and suppressed lower hydrocarbon formation. The addition of H₂S to the feed increased CO₂ selectivity and the secondary reaction of 1-olefins. The XANES results revealed that surface species involving Co–S bonding formed on the cobalt catalyst after exposure to H₂S during FTS. This was likely the primary reason for the abnormal selectivity trends observed during and after the deactivation of the catalyst by sulfur. This study points out a possible approach to increase the selectivity to heavy hydrocarbons by properly sulfiding the cobalt catalyst prior to the FTS reaction.

Graphical Abstract