Comparative Study of Olefin Production from CO and CO2 Using Na- and K-Promoted Zinc Ferrite

Abstract

Using a zinc ferrite catalyst system, we investigated the effect of sodium and potassium promoters on the concurrent conversion of CO and CO2 to olefins, focusing on the productivity and product distribution. We found that the use of promoters alters the balance between iron oxides and iron carbides in the catalyst, which affects the CO and CO2 conversion. The Na- and K-promoted catalysts facilitated the production of C-2-C-32 olefins, and a parametric study with 12 feedstock compositions (CO/CO2 = 0.2-5 and H-2/(CO + CO2) = 1-3) revealed that the Na/Fe-Zn catalyst exhibited a 6.1-times higher apparent CO consumption rate and 2.7-times higher apparent CO2 consumption rate than the K/Fe-Zn catalyst at 340 degrees C and 2.0 MPa. At a CO/CO2 ratio of 0.2 and H-2/(CO + CO2) ratio of 2, the Na/Fe-Zn catalyst achieved the maximum linear alpha-olefin yield (17.9%) at 70.3% apparent CO conversion and 26.0% apparent CO2 conversion (58.4% higher than those of the K/Fe-Zn catalyst) over 200 h. The Na/Fe-Zn catalyst activity for apparent CO conversion was more than twice that of the K/Fe-Zn catalyst, and it also exhibited better reactivity in terms of chain growth probability and secondary reactions, such as isomerization and hydrogenation. Characterization experiments revealed that the spent Na/Fe-Zn catalyst contained 43.2% iron carbides (mainly Fe5C2), and these were distributed within 19 nm of the catalyst particle surface. In contrast, the spent K/Fe-Zn catalyst was mostly composed of core-shell-type iron carbides (74.3% Fe5C2 and 21.2% Fe7C3) surrounded by carbonate/carbonyl carbon species. H2O isotherms of the spent catalysts were studied to understand factors affecting CO adsorption and CO2 reactivity, and theoretical calculations were used to probe CO hydrogenation productivity. The reactivity of Na/Fe-Zn toward CO and CO2 was analyzed with respect to the temperature, pressure, weight hourly space velocities, and optimal olefin productivity.