Insight into the Influence of the Graphite Layer and Cobalt Crystalline on a ZIF-67-Derived Catalyst for Fischer–Tropsch Synthesis

Due to the special framework structure, ZIF-67 is a promising material as the precursor to prepare the Co@C catalysts with high cobalt loading and superior cobalt dispersion. Unfortunately, these Co@C-X catalysts exhibit not only unsatisfied activity but also high CH₄ selectivity. This limited its further application due to the lack of in-depth analysis of the reasons behind it. In this work, the Co@C-X catalysts were prepared by pyrolyzing the ZIF-67 precursor at different temperatures. A series of characterizations were conducted to explore the behavior of the graphite carbon coated on cobalt species, realizing that the role of active Co sites on these Co@C catalysts was restricted by the graphite carbon layer since it suppressed the adsorption and activation of syngas on Co sites. TEOS was introduced to suppress the aggregation of cobalt species and more active sites were exposed after the graphite carbon layer was eliminated. As a result, the FTS performance was greatly improved by a factor of 5. The effect of O₂ concentration on the microcrystalline size of Co and the reconfinement effect of SiO₂ were investigated. The model catalyst was prepared and the key factors determining CH₄ selectivity of the ZIF-67-derived Co@C catalyst were revealed. This provides a good basis for rational designing ZIF-67-derived Co-based FTS catalysts.