Effect of the Support and Its Surface Modifications in Cobalt-Based Fischer–Tropsch Synthesis

Abstract

Support effects in heterogeneous catalysis are evolving as an important field of investigation to optimize catalyst properties. The cobalt-based Fischer−Tropsch (FT) catalysts usually consist of metallic cobalt nanocrystallites dispersed on a support material. The present review surveys the progress that has been made over the last couple of decades in the area of the effect of the support and its surface modifications in cobalt-based FT synthesis. Different catalyst supports such as alumina, silica, titania, niobia, zirconia, zeolite, ceria, carbon-based materials, silicon carbide, aluminum phosphate, hydrotalcite, metal−organic frameworks, and metal foams are discussed and compared with classical supports like alumina or silica wherever permitted. Properties such as metal−support interactions, the support-induced size and morphology of the cobalt nanocrystallites (textural properties of the support), changes in the electronic properties of the cobalt clusters, and the acid/base nature of the support are examined, and wherever possible the activity and/or selectivity in FT synthesis is discussed. This review also summarizes findings on new and promising supports for FT catalysts. Guidance for support modifications and choice of the support as functions of the product selectivity in the FT process is also proposed. Figure 1. Effect of Zr loading and pore size on (a) CO rate, (b) CH 4 selectivity, and (c) C 5+ selectivity. Reaction conditions: 220°C, 22 bar, H 2 / CO = 2.1, X CO = 49−51%. Adapted with permission from ref 49.

Phosphorus‐Modified Mesoporous Inorganic Materials for Production of Hydrocarbon Fuels and Value‐Added Chemicals

“…[24] On the other hand, the Cobased catalysts are attractive for commercial interest due to its relatively long life time and high deactivation resistance toward an oxidation by water formed during FTS besides higher activity and selectivity to long chain C 5 + hydrocarbons. [23,[25][26][27] The particle sizes and crystallographic structures of Co nanoparticles have been considered as the key parameters for achieving a good FTS activity. For example, Bezemer et al revealed that both FTS activity and C 5 + selectivity were found to be strongly dependent on the Co particle sizes in the range of 6-8 nm.…”

“…[30] Therefore, the selection of catalyst support is particularly noteworthy for achieving a high FTS activity with C 5 + hydrocarbon selectivity. [25] Besides the above factors, the suitable interactions between the Co species and supports, and optimal amount of Co metals on the supports are another important feature to obtain the catalytically active species. Very strong interaction would lead to the low degree of Co reduction, whereas a weak interaction would cause a poor dispersion of active Co metal.…”

“…Hence, variety of irreducible metal oxide supports such as alumina (Al 2 O 3 ), silica (SiO 2 ), zirconia (ZrO 2 ) and titania (TiO 2 ) have been largely used for FTS reaction. [25,27,31,33] Especially for Co-based FTS catalysts, the strength of the interactions between Co metal and supports decreases in the order of Al 2 O 3 > TiO 2 > SiO 2 . [34] Due to the strong interactions of Al 2 O 3 with cobalt oxides, relatively smaller Co crystallites are formed.…”

“…is the most commonly utilized support for FTS reaction. [25,41] The lower thermal conductivity of such support renders it more sensitive to hot-spot formation on the catalyst surfaces, which induces secondary cracking reactions leading to the formation of light hydrocarbons by reducing the value-added C 5 + hydrocarbons. [42] However, fluidized-bed or slurry-phase reactors relatively diminish the hot-spot formation than the fixedbed reactor.…”

“…[43] Besides, the incorporation of saccharide (sucrose) into Co/PÀ Al 2 O 3 , designated as P(x)S(y), where the notations of P and S are the phosphorus and sucrose species respectively, efficiently increased the dispersion and regulated crystallite sizes and shapes of the Co nanoparticles, which overcome the lowered dispersion caused by phosphorus treatment, and thus an enhanced CO conversion ( Table 2). [38] Although FTS activity on the Co/PÀ Al 2 O 3 catalyst was superior, the steady decreases of FTS activity with time on stream (for instance, Figure 3(A)) was generally observed over phosphorus-modified catalysts, which may be largely attributed to the heavy wax depositions on the active sites or possible formations of volatile phosphorus-containing species [17,25,44] under H 2 -rich FTS reaction conditions. In addition, the applications of the ordered mesoporous metal oxides such as Co 3 O 4 and Fe 2 O 3 for FTS reaction were limited due to their intrinsically unstable metal oxide phases by transforming them into corresponding metallic states, which attributed to the severe disintegrations of the mesoporous structures with rapid catalytic deactivation.…”