(235f) Catalytic CO2 Hydrogenation to C2+ Hydrocarbons
AIChE Annual Meeting
Monday, October 29, 2018 - 4:57pm to 5:15pm
In our most recent investigation, Fe-Cu bimetallic catalysts supported on Î³-Al2O3 with a wide range of Cu/(Cu + Fe) atomic ratios were prepared. Hydrocarbon synthesis activity was tested in a fixed-bed flow reactor at 573 K under 1.1 MPa using 24% CO2/72% H2/4% Ar as feed gas after H2 reduction at 673 K. Results show that upon combining Cu and Fe together, the CO2 conversion increases with the increasing Cu content with the maximum at Cu/(Cu + Fe) atomic ratio = 0.17, but CO2 conversion decreases with a further increase in Cu content beyond this ratio. The STY of C2âC7 reveals an identical trend as that for CO2 conversion and maximizes at Cu/(Cu + Fe) atomic ratio of 0.17; however, CH4 is significantly suppressed by the increase of Cu loading, while CO is promoted in the meantime. Considering Cu is a reverse water gas shift catalyst (RWGS) to generate CO but cannot produce hydrocarbons, FeâCu bimetallic catalysts show strong synergetic promotion in the higher hydrocarbon synthesis. The AndersonâSchulzâFlory (ASF) distribution was also examined for the chain growth. At the optimal composition (i.e., Cu/(Cu + Fe) = 0.17), the alpha value also maximizes, indicating the promoting effect of FeâCu combination on carbon-carbon chain growth. These results reveal that the Fe-Cu bimetallic catalyst with a small amount of Cu demonstrated a strong bimetallic promotion on CO2 conversion and C2+ hydrocarbons selectivity . The addition of Cu to Fe successfully suppresses the undesired CH4 formation while enhancing CO and C2+ hydrocarbon formation.
The effect of K addition on catalytic performance of FeâCu bimetallic catalysts was also studied. The results show that K addition to FeâCu gives higher CO2 conversion and C2âC7 STY at all the Cu/(Cu + Fe) atomic ratios than the corresponding unpromoted FeâCu catalysts, while at the same time, CH4 is suppressed after adding K. At lower loading level of K, although paraffins still dominate, light olefins are observed evidently. C2âC4 light olefins are selectively produced and become dominant among light hydrocarbons when the K/Fe atomic ratio was increased to 0.5 and 1.0. Such K loading-dependent behavior becomes more distinct when plotting the STY of light olefins (C2âC4) and corresponding olefin/paraffin ratio (O/P) as a function of K/Fe atomic ratio. Clearly, the addition of K into FeâCu bimetallic catalysts significantly enhances the formation of light olefins as the STY increases with the increase of K/Fe atomic ratio. In addition, the corresponding alpha values of these K promoted FeâCu catalysts display a similar K loading dependent behavior as both C2âC4 olefin STY and O/P ratio. Hence, the addition of K into Fe-Cu dramatically enhances the production of olefin-rich C2-C4 hydrocarbons over Fe-Cu bimetallic catalysts. The Fe-Cu/K catalysts exhibit superior selectivity towards C2+ hydrocarbons synthesis than Fe-Co/K catalysts under the same reaction conditions.
XRD results of the reduced catalysts suggest the Fe-Cu alloy formation in FeâCu(0.17)/Al2O3 catalyst, which is in accordance with the DFT calculations by Nie et al. [7,8], that adding Cu to Fe can create alloy-like species, and thus leads to a completely different pathway without going through CO as the intermediate product, which has been considered the dominant pathway for Fe monometallic catalyst. We also find the shifts of H2 peaks in Fe-Cu bimetallic catalysts compared with Fe and Cu monometallic ones from H2-TPR tests, which demonstrate the surface species transformation after adding Cu to Fe. Furthermore, the reduction degrees of Fe-Cu bimetallic catalysts differ from those monometallic ones physically combined in proportion, which, again reveals the new adsorption site formation on the Fe-Cu bimetallic catalysts, compared with Fe monometallic one. The CO2 adsorption property is also proved to be affected by adding Cu into Fe, by testing CO2-TPD. Larger peaks appear around 500K in Fe-Cu bimetallic catalyst, and there exist stable adsorbed CO2 from 600 to 700K only in Fe-Cu bimetallic catalysts, which corresponds to both CO2 conversion and C2+ hydrocarbon promotions. More characterization tests are ongoing to explore the relationship between physico-chemical properties of the Fe-Cu bimetallic catalysts with the synergetic effects in CO2 hydrogenation to hydrocarbons.
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