Low-cost preparation of carbon-supported cobalt catalysts from MOFs and their performance in CO hydrogenation

With terephthalic acid (H₂BDC) as ligand and cobalt acetate as Co source, metal-organic frameworks (Co-BDC MOFs) were synthesized in water by co-precipitation; after that, core-shell Co@C catalysts were prepared by chemical vapor deposition (CVD) of Co-BDC MOFs in acetylene and Ar atmosphere. The structure, composition and properties of Co@C catalysts were characterized by XRD, nitrogen physisorption, SEM, TEM, XPS, TGA and Raman spectroscopy and their catalytic performance in Fischer-Tropsch synthesis (FTS) were investigated in a fixed-bed tubular reactor. The results demonstrated that the carbonization atmosphere has an important influence on the graphitization degree of carbon shell, whereas has little effect on the phase and size of Co core. The pore of graphite shell is significantly improved by CVD in acetylene, which can enhance the selectivity to heavier hydrocarbons (C₅₊) for CO hydrogenation; in particular, the Co@C-C₂H₂ catalyst shows a high selectivity of 82.66% to the C₅₊ hydrocarbons. As the carbon shell can effectively inhibit the cobalt nanoparticles from migration and agglomeration during the FTS reaction, the Co species were well distributed in both the fresh and spent catalysts and no significant sintering and deactivation are observed for the Co@C catalysts upon the FTS tests. During the FTS reaction, the active phase changes from metallic Co to a mixture of metallic Co and Co₂C, whilst the catalytic activity of Co@C-C₂H₂ keeps almost unchanged, suggesting that Co₂C may also be an active phase for the Fischer-Tropsch synthesis.