Effects of Sodium on the Catalytic Performance of CoMn Catalysts for Fischer–Tropsch to Olefin Reactions

Abstract

The effects of a sodium (Na) promoter on the catalytic performance of cobalt-manganese (CoMn) catalysts for Fischer–Tropsch to olefin (FTO) reactions were investigated. For the sample without Na, Co0 was found to be the active phase for the traditional Co-based Fischer–Tropsch reaction with low CO2 selectivity. The olefin/paraffin (O/P) ratio was found to be low with a C2–4 = selectivity of only 15.4 C%. However, with the addition of Na, cobalt carbide (Co2C) quadrangular nanoprisms with the (101) and (020) facets exposed were formed. The Co2C nanoprisms displayed a high C2–4 = selectivity (54.2 C%) as well as a low methane selectivity (5.9 C%) under mild reaction conditions. The O/P ratio for C2–4 reached 23.9, and the product distribution deviated greatly from the classical Anderson–Schulz–Flory (ASF) distribution. Co2C nanoprisms were considered to be an effective FTO active phase with strong facet effects. The Na promoter played a key role in the evolution of the FTO catalysts. The addition of Na, which acted as an electronic donor to cobalt, resulted in stronger CO adsorption and enhanced CO dissociation, which also benefited the formation of the Co2C phase, leading to highly stable and active catalysts. The effects of other alkali promoters were also studied, and only the K promoter had an effect similar to that of Na on the CoMn catalysts for promoting the FTO reaction.

Tuning the Facet Proportion of Co2C Nanoprisms for Fischer‐Tropsch Synthesis to Olefins

“…The molar ratio of Mn/Co was close to the nominal composition. For all of the studied catalysts, a small quantity of Na existed in various CoMn catalysts, which could accelerate the formation of Co 2 C …”

“…Based on theory study, HCP-Co was belonged to the D 3h point group and it exhibited a dihedral-like shape with two closed packed (0001) facets, while FCC-Co had an octahedron-like shape composed of (100), (110), (310) and (111) facets. In addition, the reaction rates for (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21), (10)(11), (10)(11)(12), and (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) facets of HCP-Co were higher than that for the exposed facets of FCC-Co. [5] During the FT reaction process, many products including paraffins, olefins and oxygenates are generated simultaneously. For the traditional FT catalysts, paraffins are the main products with ASF distribution.…”

“…[13] Although the formation of Co 2 C was often responsible for the deactivation of cobalt-based FTS reaction, [14] we found that Co 2 C nanoprisms with exposed facets of (020) and (101) were very promising for FTO reaction. Furthermore, we found that Na promoter could improve the formation rate of Co 2 C. [15] In addition, it was found that Mn promoter was important in modifying the active metal properties, and the addition of Mn could increase the reaction rate and enhance the olefins selectivity and olefin/paraffin (O/P) ratio. [16] We also investigated the effect of the Mn promoter and found that Mn could combine with Co element to form Co x Mn 1-x O spinel, which was the essential precursor for the formation of Co 2 C nanoprisms under FTO reaction conditions.…”

“…The molar ratio of Mn/Co was close to the nominal composition. For all of the studied catalysts, a small quantity of Na existed in various CoMn catalysts, which could accelerate the formation of Co 2 C. [15] The BET surface area of the calcined and reduced samples was measured by N 2 physisorption as shown in Table 1. For the calcined catalysts, the specific surface area increased with increasing of Mn content as Mn could improve the dispersion of Co. For the reduced catalysts, once the Mn promoter was added, the BET surface area was raised from 18.7 m 2 /g (Co 3 O 4 ) to 98.3 m 2 /g (Co3Mn1), then decreased to 43.4 m 2 /g with further increasing of Mn content.…”

Carbon–Silica Composite as an Effective Support for Iron Fischer–Tropsch Synthesis Catalysts

“…The performance of iron catalysts can be greatly improved by adding “electronic” or chemical promoters to the catalyst formula . Among such promoters, potassium, sodium, copper, and manganese have been proposed. Recently, the TPR‐EXAFS technique confirmed that doping FTS catalysts with copper and alkali metals remarkably promotes the carburization rate relative to the undoped catalyst.…”