The conversion of syngas on Fe/MnO (2): The Roie of K~+ in Modified Fe/MnO Catalysts

摘要

We have sudied Fe/Mn oxide for selective production of short chain olefines in FT-synthesistl]. In our earlier work, the addition of K~+ to this system could increase the ratio of olefines/paraffines in C_2-C_4 effluent, but it has been found that the conversion of CO was a little inhibited by K~+. In this paper, Cu, Co, Pd and Rh were loaded on K-Fe/MnO catalyst with the intention of studying the catalytic behavior of the multi-metal loaded Fe/MnO catalysts in CO hydrogenation and their effects on the production of C2-C4 olefms. The results are discussed in combination with XPS surface studies and determination of coke amount.rnThe Fe/Mn oxide (Fe/Mn=1/4) was prepared by continuous co-precipitation and the 3%KNO_3(mol%) was added to Fe/Mn-solution and other promotorions were loaded by impregnating the calcined K-Fe/Mn oxide precursor. The exact composition of the catalysts was determined by AAS and XPS. The catalytic reactions were carried out in a fixed bed tubular microreactor, at 543K, 11bar. The GHSV was held constant at 215/h. The gaseous products were analysed on-line by a Carle AGC 111 gaschromatograph with 4 columns. The liquid effluent was separated and then analysed by a Perkin-Elmer Sigma 2B capillary-gaschromatograph. The syngas was composed of 60% H_2, 30% CO and 10% Ar as a standard The results are showed in Table 1.rnThe activity over K-containing catalysts varied with time on stream. At the beginning of the reaction, the conversion rose with time. After 10 hr it reached a maximum. Then the conversion declined slowly to a stable level (about after 20hr).The addition of Cu, Co, Pd, Rh could enhance the time when the higher activity stayed. The loading of 1%Cu and 1%Co played a positive role in converting CO+H_2 to hydrocarbons, while l%Pd and l%Rh as well as 0. l%Co did not benefited the conversion in K-containing system.rnThe 3%K~+ led to a decrease in CO conversion comparing with unloaded Fe/MnO (from 41 to 36%)[1]. The product distribution was also changed. The catalysts with potassium hadrnlower C_1-C_4 selectivity than K-free catalysts. A similar selectivity to olefines was found withrnall catalysts, which may be attributed to the effect of the 3%K The ratio of C_2-C_4 olefines tornC_2-C_4 paraffins over multi-metal-catalysts are higher thaa K-free samples. The O/P ratio variedrnfrom 2.4 to 4.5. The larger the amount of K, the higher was the O/P-ratio(3.96 over 1%K and 4.5 over 3%K). Comparing with our earlier work [1], it can de concluded that the K + acts as a main promoter in multi-metal system because no great differences in the product distribution over all 3%K-containing catalysts.rnThe XPS-studies were performed for all catalysts after calcination and reaction. The K+-content from AAS and from XPS is about the same value. After reactions, the Fe-amount at the surface decreased to a great content. In contrast, the K~+ enriched from 3% to 18-34%. From the coke deposit it was concluded that K~+ has a very strong influence on the carbon deposition during synthesis. All K containing catalysts snow surface carbon contents of 66 to 70%, while only about 20-25% were observed for the other catalysts. This deposited C can cover the active sites of the Fe/MnO. That is why the K-containing catalysts had lower activity for CO-hydrogenation.