Promoted cobalt metal catalysts suitable for the production of lower olefins from natural gas

Abstract

Due to the surge of natural gas production, feedstocks for chemicals shift towards lighter hydrocarbons, particularly methane. The success of a Gas-to-Chemicals process via synthesis gas (CO and H₂) depends on the ability of catalysts to suppress methane and carbon dioxide formation. We designed a Co/Mn/Na/S catalyst, which gives rise to negligible Water-Gas-Shift activity and a hydrocarbon product spectrum deviating from the Anderson-Schulz-Flory distribution. At 240 °C and 1 bar, it shows a C₂-C₄ olefins selectivity of 54%. At 10 bar, it displays 30% and 59% selectivities towards lower olefins and fuels, respectively. The spent catalyst consists of 10 nm Co nanoparticles with hcp Co metal phase. We propose a synergistic effect of Na plus S, which act as electronic promoters on the Co surface, thus improving selectivities towards lower olefins and fuels while largely reducing methane and carbon dioxide formation.

Fig. 1

Catalytic performance of Co₁Mn₃–Na₂S at different reaction temperatures or pressures. a Activity and selectivity at 240–280 °C, 10 bar, and H₂/CO = 2, and b activity and selectivity at 240 °C, 3–10 bar, and H₂/CO = 2. Activity is shown here as % CO conversion and product selectivity is shown in terms of methane, CH₄ (blue solid diamonds), C₂–C₄ olefins (red solid squares), C₂–C₄ paraffins (light red open squares), and C₅₊ (grey solid triangles) which include all other products except CO₂ and C₁–C₄ hydrocarbons

Fig. 2

Catalytic performance over 70 h time-on-stream. Reaction conditions: 240 °C, 3 bar, and H₂/CO = 2. a Activity in terms of %CO conversion (black solid circles) and selectivity towards methane, CH₄ (blue solids diamonds), C₂–C₄ olefins (red solid squares), and C₅₊ (grey solid triangles) of Co₁Mn₃–Na₂S over time, and b activity in terms of cobalt-time-yield, CTY, of various Co-based catalysts, namely Co₁Mn₃ (black open circles), Co₁Mn₃–Na₂O (grey solid circles), Co₁Mn₃–Na₂S₂O₃ (grey solid with black outline circles), Co₁Mn₃–Na₂S (black solids circles), Co₃Mn₁ (blue open diamonds), Co₃Mn₁–Na₂S (blue solid diamonds), Co₃Mn₁–Na₂O (light blue solid diamonds), and bulk Co (red squares) over time

Fig. 3

Distribution of 1-olefins and n-paraffins of C₁–C₈ hydrocarbon products. Reaction conditions: 240 °C, 10 bar, H₂/CO = 2, 18–30% CO conversion. a Co₁Mn₃–Na₂S, b Co₁Mn₃–Na₂O, c Co₁Mn₃ and d Co. Red bar corresponds to olefin product flow and light red bar corresponds to paraffin product flow

Fig. 4

XRD analysis of spent Co₁Mn₃, Co₁Mn₃–Na₂S, Co₃Mn₁–Na₂O. Reaction conditions: 240–280 °C, 10 bar, and H₂/CO = 2. a Background corrected XRD patterns and b rietveld QPA-based crystalline phase compositions, which shows the Mn_xCo_yO₄ phase (blue), Mn_0.95O (cyan), MnCO₃ (green), hcp Co (violet) and Co₂C (brown)

Fig. 5

Electron microscopy images of spent Co₁Mn₃–Na₂S. Reaction conditions: 240–280 °C, 10 bar, and H₂/CO = 2. a bright-field TEM image with a scale bar corresponding to 100 nm, and blue arrows to point out the presence of wax, b dark-field TEM images with a scale bar corresponding to 50 nm, c particle size distribution of Co nanoparticles supported on MnO, and d–f STEM-EDX maps of Co and Mn, and the scale bars correspond to 200 nm

Fig. 6

DFT-calculated binding geometries of Na₂S and Na₂O on the Co (0001) surface. a Na₂O and b Na₂S bind in a very similar fashion, although the O atom ends up above a subsurface cobalt atom and the S atom above an empty site. Atoms outside the calculation unit cell are depicted as smaller spheres; blue is Co, orange is Na, yellow is S and red is O

Fig. 7

Schematic drawing of the structure of the active catalysts based on TEM and XRD analysis. a Co₁Mn₃–Na₂S consists of metallic Co nanoparticles of ~10 nm dispersed on MnO support and b Co₃Mn₁–Na₂O consists of Co₂C nanoprisms of ~10–50 nm from Zhong et al. Whereas the former catalyst restricts WGS and thus CO₂ formation, the latter leads to large amounts of CO₂