A series of Co–Fe bimetallic
catalysts was prepared, characterized,
and studied for the hydrogenation of carbon dioxide. The catalyst
precursors were prepared via an oxalate coprecipitation method. Monometallic
(Co or Fe) and bimetallic (Co–Fe) oxalate precursors were decomposed
under a N2 flow at 400 °C and further pretreated under
a CO flow at 250 °C. The catalysts (before decomposition of the
oxalates or after activation) were characterized by BET, TGA-MS, X-ray
diffraction, CO-TPR, SEM, HR-TEM, and Mössbauer spectroscopy
techniques. The hydrogenation reaction of CO2 was performed
using Co–Fe bimetallic catalysts pretreated in situ in a fixed-bed
catalytic microreactor operating in the temperature range of 200–270
°C and a pressure of 0.92 MPa. With increasing Fe fraction, the
selectivity to C2–C4 for Co–Fe
catalyst increased under all operating conditions. The alcohol selectivity
was found to increase with increasing iron content of the Co–Fe
catalyst up to 50%, but then it dropped with further addition of iron.
Among the three different activation conditions, the CO pretreated
Co–Fe (50Co50Fe) catalyst exhibited a much lower selectivity
for methane. Addition of 1 wt % Na or 1.7 wt % K to 50Co50Fe catalyst
increases its olefinic (C2–C4) and oxygenate
selectivities.
The bulk of the products that were synthesized from Fischer–Tropsch synthesis (FTS) is a wide range (C1–C70+) of hydrocarbons, primarily straight-chained paraffins. Additional hydrocarbon products, which can also be a majority, are linear olefins, specifically: 1-olefin, trans-2-olefin, and cis-2-olefin. Minor hydrocarbon products can include isomerized hydrocarbons, predominantly methyl-branched paraffin, cyclic hydrocarbons mainly derived from high-temperature FTS and internal olefins. Combined, these products provide 80–95% of the total products (excluding CO2) generated from syngas. A vast number of different oxygenated species, such as aldehydes, ketones, acids, and alcohols, are also embedded in this product range. These materials can be used to probe the FTS mechanism or to produce alternative chemicals. The purpose of this article is to compare the product selectivity over several FTS catalysts. Discussions center on typical product selectivity of commonly used catalysts, as well as some uncommon formulations that display selectivity anomalies. Reaction tests were conducted while using an isothermal continuously stirred tank reactor. Carbon mole percentages of CO that are converted to specific materials for Co, Fe, and Ru catalysts vary, but they depend on support type (especially with cobalt and ruthenium) and promoters (especially with iron). All three active metals produced linear alcohols as the major oxygenated product. In addition, only iron produced significant selectivities to acids, aldehydes, and ketones. Iron catalysts consistently produced the most isomerized products of the catalysts that were tested. Not only does product selectivity provide a fingerprint of the catalyst formulation, but it also points to a viable proposed mechanistic route.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.