Manufacture of highly loaded silica-supported cobalt Fischer-Tropsch catalysts from a metal organic framework

Abstract

The development of synthetic protocols for the preparation of highly loaded metal nanoparticle-supported catalysts has received a great deal of attention over the last few decades. Independently controlling metal loading, nanoparticle size, distribution, and accessibility has proven challenging because of the clear interdependence between these crucial performance parameters. Here we present a stepwise methodology that, making use of a cobalt-containing metal organic framework as hard template (ZIF-67), allows addressing this long-standing challenge. Condensation of silica in the Co-metal organic framework pore space followed by pyrolysis and subsequent calcination of these composites renders highly loaded cobalt nanocomposites (~ 50 wt.% Co), with cobalt oxide reducibility in the order of 80% and a good particle dispersion, that exhibit high activity, C5 + selectivity and stability in Fischer-Tropsch synthesis.

Fig. 1

Schematic illustration of the synthesis of the Co@SiO₂ catalysts. (1) Impregnation and hydrolysis of TMOS molecules in the porosity of ZIF-67. (2) Pyrolysis of the mixture of ZIF-67@SiO₂ in N₂ to decompose ZIF-67 and form Co@C-SiO₂. (3) Calcination of the Co@C-SiO₂ in air leads to carbon removal and oxidation of Co. (4) Reduction of the Co@SiO₂ in H₂ leads to the formation of metallic Co for Fischer–Tropsch synthesis. The resulting composite is an excellent catalyst for the low temperature Fischer–Tropsch synthesis

Fig. 2

Electron microscopy images and corresponding nanoparticle size distributions of cobalt based samples. a High-angle annular dark-field scanning electron (HAADF-STEM) micrograph of ZIF-67@SiO₂ (scale bar 200 nm). Elemental mapping of b Si, c Co, and d C in ZIF-67@SiO₂ sample (scale bars 200 nm). TEM micrograph of e Co@SiO₂-cal, f Co@SiO₂-773 with an inset of the observable needle-like structure, g Co@SiO₂-873 and h Co@SiO₂-973 (scale bars from (e) – (h) 50 nm). Particle size histograms obtained from TEM analysis for i Co@SiO₂-cal, j Co@SiO₂-773, k Co@SiO₂-873, and l Co@SiO2-973. Electron tomography results for m, n, o Co@SiO₂-cal (scale bar 50, 50, and 100 nm, respectively), and p, q, r Co@SiO₂-873 (scale bar 50, 50, and 100 nm, respectively)

Fig. 3

TPR(H₂) profiles of Co@SiO₂ catalysts. a Co@SiO₂-773, b Co@SiO₂-873, c Co@SiO₂-973, and d Co@SiO2-cal. The TPR(H₂) experiments were performed from 303 to 1223 K at a ramp of 5 K min⁻¹ in 10 vol.% H₂/Ar

Fig. 4

Catalytic performance. a Time-on-stream evolution of CO conversion for the Co@SiO₂ catalysts. b Molar fraction distribution of FTS products from Co@SiO₂-873 after 201 h on stream. Chain growth probability (α = 0.94) obtained from the ASF plot in the C15-C100 hydrocarbon range. Reaction conditions: 483 K, 20 bar, and H₂/CO = 1, and syngas flow of 40 ml min⁻¹

Fig. 5

Catalytic performance. a Time-on-stream evolution of CO conversion for the Co@SiO₂-873 and Co/SiO₂ catalysts prepared using conventional methods. M’ refers to melt infiltration. IWI refers to incipient wetness impregnation. A refers to Aerosil-200 support and F refers to CARiACT Q-10 support. Reaction conditions: 483 K, 20 bar, H₂/CO = 1, and syngas flow of 40 ml min⁻¹. b Time-on-stream evolution of CO conversion for the Co@SiO₂-873 and Co/SiO₂ -F-TIWI catalysts prepared using two-step incipient wetness impregnation method (TIWI). Reaction conditions: 483 K, 26 bar, H₂/CO = 2, and syngas flow of 40 ml min⁻¹