On the performance of a Co-based catalyst supported on modified γ-Al₂O₃ during Fischer–Tropsch synthesis in the presence of co-fed water

Abstract

The effect of water on the Fischer-Tropsch performances of a supported cobalt catalyst has been studied in a fixed bed reactor by running co-feeding experiments for more than 1000 h at industrially relevant process conditions. The adopted γ-Al 2 O 3 support (D p =300 µm) has been modified with superficial cobalt-aluminates so to prevent the formation of these species during the reaction as a consequence of the strong interaction between the active phase and the support. In spite of this precaution, the addition of water results in a slow deactivation of the catalyst, possibly caused by the oxidation of the reduced cobalt metal centers. More complex is the effect of water on the product distribution: in this case water has both irreversible effects, which become stronger upon increasing the concentration of co-fed water, and reversible effects, which are clearly visible even with small concentrations of co-fed water. The two effects bring, in the presence of co-fed water, to decreased CH 4 and alcohols selectivities, and increased C 25+ hydrocarbons, olefins and CO 2 selectivities. These results can be explained by assuming that water suppresses hydrogenation reactions.

Effect of Co-fed Water on a Co–Pt–Si/γ-Al2O3 Fischer–Tropsch Catalyst Modified with an Atomic Layer Deposited or Molecular Layer Deposition Overcoating

“…Water has been shown to increase the chain propagation α-value, resulting in enhanced C 5+ selectivity. 4,5 The results shown in Figure 8 support this observation as all the catalysts showed increased C 5+ selectivity during moderate water addition (P H 2 O /P H 2 = 0.42). However, upon increased water partial pressure at the reactor inlet (P H 2 O /P H 2 = 0.71), 10c and 15c MLD catalysts showed a decreasing C 5+ trend compared with other catalysts.…”

“…Our results support previous studies on added water partial pressure regimes, where moderate water addition (P H 2 O /P H 2 = 0.4) had a negative effect on the FT rate with γ-Al 2 O 3supported catalysts. 4,11 However, the addition of MLD overcoating resulted in the opposite as the activity was increased during moderate added water conditions. Furthermore, in addition to moderate water partial pressure regime, the previous literature findings 4,6 were confirmed for waterinduced irreversible deactivation under a high partial pressure regime (P H 2 O /P H 2 > 0.6) for all the studied catalysts.…”

“…4,11 However, the addition of MLD overcoating resulted in the opposite as the activity was increased during moderate added water conditions. Furthermore, in addition to moderate water partial pressure regime, the previous literature findings 4,6 were confirmed for waterinduced irreversible deactivation under a high partial pressure regime (P H 2 O /P H 2 > 0.6) for all the studied catalysts. Especially, the thickest MLD overcoated catalyst (20 cycles MLD) showed indications of overcoat degradation during added water periods.…”

“…Several studies discuss the effect of added water on the catalyst performance. 1−5 Often reported effects include increased C 5+ and lowered CH 4 selectivity, 4,5 while catalyst activity and deactivation are dependent on several factors including support composition, cobalt loading, cobalt crystallite size, promoter selection, reactor type, and reaction conditions. 4,6,7 Bertole et al 3,8 have shown that water enhances catalyst activity by lowering the CO activation energy barrier and increasing the carbon species (CH x ) surface coverage via an assisted CO dissociation rate on the catalyst surface.…”

“…In addition to activity effects, an added water amount has been shown to have reversible and irreversible effects on the catalyst performance. Fratalocchi et al 4 studied co-fed water on the Co/γ-Al 2 O 3 catalyst in a fixed-bed tubular reactor and reported both reversible and irreversible effects on the FT reaction rate. They showed that operating at a low partial pressure regime (P H 2 O /P H 2 < 0.32) had no remarkable effect on the FT reaction rate, although slow deactivation induced by co-fed water was observed.…”