Precipitated Iron-based Catalysts Research Articles

Effect of alkaline-earth metals (Mg, Ca, Sr, and Ba) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis of light olefins

For the high-temperature Fischer-Tropsch synthesis (HTFT) of light olefins (C2=-C4=), alkaline-earth metals (Mg, Ca, Sr, and Ba) modified FeMn catalysts were prepared using precipitation and impregnation methods. It was demonstrated that the electron-giving interaction of Ca, Sr, and Ba decreased the adsorption of H species while improving the dissociative adsorption of CO. This interaction resulted in the production of χ-Fe5C2, which increased CO conversion and C2=-C4= selectivity. Compared with Ca and Ba, the FeMnSr catalyst had the strongest CO dissociative adsorption capacity and the highest χ-Fe5C2 content with the best FTS performance. When Sr and Na were introduced simultaneously, the synergistic effect of Sr and Na inhibited the formation of θ-Fe3C and the aggregation of catalyst particles, and also increased the χ-Fe5C2 content. This synergistic effect resulted in the highest surface basicity of FeMnSrNa catalyst, the highest electron cloud density on the surface of Fe atoms, the weakest H species adsorption, the strongest CO dissociative adsorption, and the highest total amount of active iron carbide, which improved the FTS performance. The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kgCat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kgCat)) and FeMnNa catalyst (80.6 g/(h·kgCat)).

Unravelling acidity–selectivity relationship in the bifunctional process of Fischer-Tropsch synthesis and catalytic cracking

Fischer-Tropsch synthesis (FTS) was carried out over bifunctional model catalysts prepared by physically mixing precipitated iron-based FTS catalysts (P-Fe) and H-ZSM-5. We adjusted the acidity of model catalysts by varying the ratio of P-Fe/H-ZSM-5 and the ratio of Si/Al in the H-ZSM-5. The physical mixing of H-ZSM-5 was adequate to supply the Brønsted acid sites (BAS), which are nonexistent in the P-Fe, to the bifunctional model catalysts without deteriorating the catalytic activity of P-Fe. As a result, we found a linear correlation between the cracking rate and the BAS concentration in the bifunctional process of FTS and cracking for the first time. Furthermore, the model catalysts showed high CO conversion (73–78 %), comparable to that of P-Fe (73 %), and improved C5–C20 selectivity with increased BAS concentration. The highest C5–C20 selectivity obtained in this study (72 wt%) was twice higher than that obtained in the P-Fe (36 wt%).

Effect of alkalis (Li, Na, and K) on precipitated iron-based catalysts for high-temperature Fischer-Tropsch synthesis

Alkalis (Li, Na, and K) decorated FeMnZr catalysts prepared by coprecipitation and impregnation methods were used to investigate the effects of three alkali metal promoters and promoter loading (for Na and K) on the iron-based catalyst for high-temperature Fischer-Tropsch synthesis (HTFT). The results showed that alkalis donated electrons to iron species, which promoted CO dissociative adsorption while suppressing hydrogen adsorption, then boosted the selectivity to light olefins and the improvement was enhanced with the increase of the atomic number of alkali metals and the content of alkali metals. Moreover, compared to alkalis-free catalysts, alkalis-promoted catalysts facilitated the formation of χ-Fe5C2 and ɛ’-Fe2.2C. In alkalis decorated catalysts, the χ-Fe5C2 content decreased and the ɛ’-Fe2.2C content increased with increasing basicity of the catalyst surface. In addition, compared with χ-Fe5C2, ɛ’-Fe2.2C has a stronger CC coupling ability and promotes chain growth more. Compared to Li and Na promoters, K was easier to facilitate the formation of ɛ’-Fe2.2C due to its strongest basicity and CO dissociative adsorption ability. The lowest CH4 and highest C5+ selectivity were accompanied by the highest content of ɛ’-Fe2.2C on K-promoted catalysts, which suggested that ɛ’-Fe2.2C had outstanding CC coupling ability.

Effect of the Zr promoter on precipitated iron-based catalysts for high-temperature Fischer–Tropsch synthesis of light olefins

FeMnxZr and FeMnxZr2Na catalysts prepared by coprecipitation and impregnation methods were applied to investigate the promoting effects of Zr on iron-based catalysts for high-temperature Fischer–Tropsch synthesis (HTFT).

Brief Review of Precipitated Iron-Based Catalysts for Low-Temperature Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis (FTS) is a promising way to produce clean liquid fuels and high value-added chemicals from low-value carbon-containing resources such as coal, natural/shale gas, waste wood/plastics, and CO2 via syngas (CO + H2). Precipitated iron-based catalysts are one group of commercially viable FTS catalysts that are important in academic circles as well as in industry. Here, we briefly review the promoters for precipitated iron-based FTS catalysts, which critically influence the catalyst performance. Precipitated iron-based catalysts typically contain three types of promoters: reduction promoters, alkali promoters, and structural promoters. The major effects of reduction promoters on catalytic performance include shortening of the induction period and increasing the CO conversion. Alkali promoters significantly influence the catalyst activation and product selectivity. They facilitate carburization of iron and increase selectivity for the target hydrocarbons C5+ and 1-olefin. The heavier the alkali metal, the stronger the effect. The structural promoters are effective for increasing the resistance of precipitated iron-based catalysts to thermal sintering and physical attrition. However, the impact of structural promoters on chemical properties and resultant catalytic performance is still considered a matter of debate. In the case of alkali promoters and structural promoters, the optimal amount of loading is very important for obtaining high catalytic performance, and this deteriorates severely if loading exceeds the optimal range.

Eco-friendly prepared iron-ore-based catalysts for Fischer-Tropsch synthesis

The iron-ore-based catalysts (IO-CAT) with catalytic properties similar to those of conventional precipitated iron-based catalysts (PFe-CAT) were successfully prepared through a combination of a wet-milling process and a wet impregnation method. This approach was much more economical and eco-friendly than a conventional precipitation technique in terms of the amounts of water and chemicals used and discharged. The IO-CAT exhibited higher surface area, a larger pore volume, and a smaller crystallite size than the unmodified iron ore samples (IO-U). In particular, the pore volume of IO-CAT was as large as that of PFe-CAT. Furthermore, the reducibility and surface basicity of IO-CAT were much higher than those of IO-U and comparable to those of PFe-CAT, which implies that a reduction promoter, Cu, and an alkali promoter, K, are used successfully to impregnate the well-developed pore structure of IO-CAT. As a result, the IO-CAT showed catalytic performance favorable for low-temperature Fischer-Tropsch synthesis (LT-FTS) using hydrogen-deficient syngas (H2/CO = 1) in all aspects of CO conversion (75%), CO2 selectivity (43 C-mol%), and C5+ selectivity in hydrocarbons (71 wt%). The overall catalytic performance of IO-CAT was much greater than that of IO-U and comparable to that of PFe-CAT. This strongly demonstrates new potential for economical and eco-friendly preparation of iron-based catalysts for LT-FTS.

Increased CO2 hydrogenation to liquid products using promoted iron catalysts

The effect of alkali promoter (K, Rb and Cs) on the performance of precipitated iron-based catalysts was investigated for carbon dioxide (CO2) hydrogenation. Characterization by temperature-programmed reduction with CO, Mössbauer spectroscopy, and transmission electron microscopy were used to study the effect of alkali promoter interactions on the carburization and phase transformation behavior of the catalysts. Under similar reaction conditions, cesium (Cs) and rubidium (Rb) promoted catalysts exhibited the highest initial CO2 conversions to higher hydrocarbons. CO2 conversions then decreased to reach steady state conversions around 170 h on stream. At steady state conversion, all three catalysts exhibited similar CO2 conversions and selectivities. For comparison, a lower loaded Cs (1.5 Cs) promoted iron-based catalyst was prepared. It exhibited slightly lower initial conversion than the higher loaded Cs catalyst, but remained very stable. Among all the catalysts at steady state conversion, the 1.5 Cs promoted catalyst exhibited the highest stability. Results indicate a synergistic effect brought on by these promoters that, if balanced, could potentially yield superior CO2 hydrogenation catalysts.

Selectivity control of Cu promoted iron-based Fischer-Tropsch catalyst by tuning the oxidation state of Cu to mimic K

The effect of activation gas on the Fischer-Tropsch synthesis (FTS) performance of copper and potassium promoted precipitated iron-based catalysts was investigated in an earlier study [Catal. Lett. 144 (2014) 1624-1635]. In that study, CO and syngas (H2/CO=0.7) activation methods were tested after varying the copper promoter loading (0, 2, and 5%, atomic ratios relative to iron). At similar CO conversion level, the selectivities for the CO-activated catalysts were identical for the various copper loaded catalysts, whereas for the syngas activated catalysts, light hydrocarbon (methane and C2–C4) selectivities decreased and higher hydrocarbon (C5+) selectivities increased with increasing copper loading. To understand the reason for that behavior, the catalyst samples were characterized by XANES/EXAFS spectroscopy following each activation procedure before and after exposure to FTS conditions. Cu K-edge normalized XANES results indicate that for the syngas activated catalyst copper is present primarily in the Cu1+ state, whereas for CO activated catalyst the copper is more reduced (greater fraction in the Cu0 state). The Cu1+ in the syngas activated catalyst behaved like K1+; therefore, the C5+ selectivity was enhanced and light hydrocarbon selectivity was suppressed.

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The effect of activation gas on the Fischer-Tropsch synthesis (FTS) performance of copper and potassium promoted precipitated iron-based catalysts was investigated in an earlier study [Catal. Lett. 144 (2014) 1624-1635]. In that study, CO and syngas (H2/CO = 0.7) activation methods were tested after varying the copper promoter loading (0, 2, and 5%, atomic ratios relative to iron). At similar CO conversion level, the selectivities for the CO-activated catalysts were identical for the various copper loaded catalysts, whereas for the syngas activated catalysts, light hydrocarbon (methane and C2–C4) selectivities decreased and higher hydrocarbon (C5+) selectivities increased with increasing copper loading. To understand the reason for that behavior, the catalyst samples were characterized by XANES/EXAFS spectroscopy following each activation procedure before and after exposure to FTS conditions. Cu K-edge normalized XANES results indicate that for the syngas activated catalyst copper is present primarily in the Cu1+ state, whereas for CO activated catalyst the copper is more reduced (greater fraction in the Cu0 state). The Cu1+ in the syngas activated catalyst behaved like K1+; therefore, the C5+ selectivity was enhanced and light hydrocarbon selectivity was suppressed.

Fischer–Tropsch Synthesis: Effect of Activation Gas After Varying Cu Promoter Loading Over K-Promoted Fe-Based Catalyst

The effects of activation gas and copper promoter loading on the Fischer–Tropsch synthesis performance of potassium promoted precipitated iron-based catalysts were investigated using a continuously stirred tank reactor. In this study, CO and syngas (H2/CO = 0.7) activated catalysts were tested after varying the copper promoter loading (0, 2 and 5 %, atomic ratios relative to iron). After attaining a steady-state conversion for the CO-activated catalysts, similar or slightly higher CO conversions were exhibited with increasing copper loading, and the induction period was reduced with increasing copper loading. Partial pressure of hydrogen in the activation gas influenced the resulting activity of the catalysts. For syngas activated catalysts, CO conversion was found to increase with increasing copper loading up to 2 %, and slightly decrease with further increases in copper (5 %) loading. For similar CO conversion levels, the selectivities were similar for the CO activated catalysts, whereas for the syngas activated catalysts, the selectivity varied with copper loading. With increasing copper loading, lower hydrocarbon (methane and C2–C4) selectivities decreased and the corresponding higher hydrocarbon (C5+) selectivity increased.

Effects of Zr and Ni promoters on the activation and deactivation of a precipitated iron-based catalyst for Fischer–Tropsch synthesis

Iron-based catalysts with or without Zr and Ni promoters were prepared by the co-precipitation method. The specific surface area, pore volume and average pore size of the fresh catalysts was analyzed by N2-physisorption, and the effects of Zr and Ni promoters on the structure, activation and deactivation were investigated by H2-TPR, XRD and Mossbauer spectroscopy techniques. The Fischer–Tropsch reaction performance of catalysts was also studied in a fixed bed reactor. The results indicated that Zr promoter inhibited the reduction and carburization of catalysts due to the strong interaction between Fe and Zr and led to a lower activity and C5+ selectivity. The addition of Ni promoter improved the activity of iron-based catalyst, but caused a lower C5+ selectivity and higher CO2 selectivity. Meanwhile, Ni was preferential for the reduction of iron phases but against the carburization of catalysts. To some extent, the addition of Zr, Ni promoters improved the stability of iron-based catalysts.

Mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis over the precipitated iron-based catalysts

In-situ DRIFTS and chemical trapping techniques were employed to investigate the adsorbed species over the surface of precipitated iron-based catalysts and the mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis. The results showed that both linear and bridged CO molecules are present on the catalyst surface, which leads the formation of numerous oxygenated precursors. Some crucial surface intermediates are detected by the in-situ DRIFTS, such as acetate, acetyl and methoxide. The surface of precipitated iron-based catalysts is characterized by following facts: alcohols are able to react with free surface hydroxyls to form alkoxy species; surface adsorbed molecules exhibit certain oxidizing ability; basic sites such as OH− and lattice oxygen may react with CH3OH or CH3CHO molecules. By chemical trapping of the CH3OH + CO and CH3I + CO + H2 reactions, it was found that acetyl is an important intermediate for oxygenates and the hydrogenation of acetyl is a crucial step for the formation of oxygenates. On the basis of these observations, the mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis over the precipitated iron-based catalysts was then proposed.

Effect of incorporation manner of Zr promoter on precipitated iron-based catalysts for Fischer-Tropsch synthesis

The promotional effects of Zr on the structure, reduction, carburization and catalytic behavior of precipitated iron-based Fischer-Tropsch synthesis (FTS) catalysts were investigated. The catalysts were characterized by N2 physisorption, temperature-programmed reduction (TPR), and Mossbauer effect spectroscopy (MES) techniques. As revealed by N2 physisorption, Zr decreased the BET surface area and pore volume of the catalyst. The results of TPR and MES show that Zr suppresses the reduction and carburization of Fe catalysts because of the interaction between Fe and Zr. The FTS reaction results indicate that Zr decreases the FTS activity of Fe catalysts but improves the catalysts’ stability. In addition, Zr promoter restraines the formation of light hydrocarbons (methane and C2∼C4) and shifts the production distribution to the heavy hydrocarbons.

Study of phase transformation and catalytic performance on precipitated iron-based catalyst for Fischer–Tropsch synthesis

Detailed phase transformation in syngas (H 2/CO = 1.2) on a precipitated iron-based catalyst was studied by N 2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (LRS). Fischer–Tropsch synthesis (FTS) performance of the catalyst was investigated in a slurry-phase continuously stirred tank reactor (STSR). The hematite in the fresh catalyst was reduced initially to magnetite, and then the magnetite in the bulk reached steady state slowly with increasing reduction time. Simultaneously, the Fe 3O 4 on the surface layers converted gradually to iron carbides, accompanied with the continual increase in the amounts of surface carbonaceous species. In the FTS reaction, the catalytic activity presented an increased trend with gradual carburization of the catalyst by keeping the stability in the bulk Fe 3O 4, suggesting that the conversion of magnetite to iron carbides in the near-surface regions provides probably the active sites for FTS. In addition, the chain growth reaction was restrained and the hydrogenation reaction was enhanced with increasing reduction duration.

Mechanistic pathway for methane formation over an iron-based catalyst

The methanation reaction mechanism under Fischer–Tropsch conditions is investigated with the Steady State Isotopic Transient Kinetic Analysis (SSITKA) technique over a precipitated iron-based catalyst. The 13CH 4 transients resulting from a CO 12 → CO 13 switch (330 °C, 1.2 bar, and H 2 /CO = 15 ) provided kinetic information for the methanation reaction. Six methanation models were screened and only three of these could describe the methane transient. These models were subsequently extended to account for the Fischer–Tropsch higher hydrocarbon products by considering C C coupling reactions and the kinetic rate parameters were estimated. The result was two indistinguishable mechanisms which could describe the methane transient as well as the experimental steady-state concentrations. Both mechanisms have two active pools of carbon ( C α and C β ) on the catalyst surface with both leading towards the formation of methane. The C β pool is 25 to 50 times less active than the C α pool for methanation and occupies 92% of the total CH x coverage (0.25 ML). The C C coupling reaction was shown to involve both the C α and C β pools. The concentration of molecularly adsorbed CO on the Fe-based catalyst is shown to be extremely low, with an estimated surface coverage of 9 × 10 −4 ML .

Effect of Pt Impregnation on a Precipitated Iron-based Fischer–Tropsch Synthesis Catalyst

Effect of Pt impregnation on the textural properties, surface element distributions and catalytic behavior of a precipitated iron-based catalyst for Fischer–Tropsch synthesis (FTS) was investigated by N2 physical adsorption, temperature-programmed reduction (TPR), Mossbauer effect spectrometer (MES), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). Low levels of Pt addition lead to an increase in BET surface area. The result of XPS indicates that Pt enriches on the catalyst surface after being calcined. HRTEM shows that Pt crystallites with diameter about 2 nm are well dispersed on the surface of the catalyst (100Fe/1Pt/4 K/16SiO2). The results of TPR and MES clearly indicate that Pt facilitates the reduction and carburization of Fe2O3 to some extend. The reaction tests in a slurry reactor give the result that the Pt impregnation remarkably increases the FTS activity, and suppresses the selectivities of the light hydrocarbons and the olefins.

Promotional effects of Cu and K on precipitated iron-based catalysts for Fischer–Tropsch synthesis

The effects of Cu and K promoters on precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated by using N 2 physical adsorption, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer effect spectroscopy (MES). The FTS performances of the catalysts were tested in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that Cu promoter facilitates the high dispersion of Fe 2O 3, significantly promotes the reduction and H 2 adsorption, but severely suppresses CO adsorption and the carburization. However, K promoter severely retards the reduction and suppresses the H 2 adsorption, facilitates the CO adsorption and promotes the carburization. In the FTS reaction, it was found that Cu promoter decreases the FTS initial activity and water gas shift (WGS) reaction activity, promotes the oxidation of iron carbides to Fe 3O 4 and accelerates the deactivation of iron-based catalyst. However, K promoter improves the FTS activity and WGS reaction activity, suppresses the oxidation of iron carbide to Fe 3O 4 and significantly improves the stability of iron-based catalyst. As compared with individual promotion of Cu or K, the double promotions of Cu and K significantly improve the FTS and WGS activities and keep excellent stability. Due to weaker CO adsorption and stronger H 2 adsorption than the catalysts without Cu, Cu promoted catalysts have higher selectivity to light hydrocarbons and methane and lower selectivity to heavy hydrocarbons. However, the opposite result is obtained on the catalyst incorporated with K promoter.

Role of rare-earth oxides and thoria as promoters in precipitated iron-based catalysts for Fischer-Tropsch synthesis

The catalytic results in Fischer-Tropsch synthesis indicate that the addition of La, Ce, Nd, Eu and Th oxides to a precipitated iron-based catalyst increases the syngas conversion to hydrocarbon products and shifts the selectivity to the gasoline and middle fractions with significant suppression of both methane and wax formation. Rare-earth (RE) oxides are more effective than thoria, of which Eu 2O 3 shows a unique promoting effect. The results of catalyst characterization obtained from the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR),carbon monoxide temperature-programmed desorption (TPD), carbon dioxide TPD and temperature-programmed surface reaction (TPSR) measurements demonstrate that the presence of these promoters greatly changes both the bulk structure and surface states of the iron catalyst, especially when promoted with europium. It is found that they increase the dispersion of the iron particles, inhibiting their growth and further reduction due to the formation of a mixed Fe-RE(Th) oxide interface, encourage the adsorption and dissociation of carbon monoxide and decrease the reactivity of the surface carbon species towards hydrogen due to the electronic donation properties brought about by their basicities. These results explain the observed variation in the catalytic performances. Eu 2O 3 exhibits a unique promoting effect on the catalytic activity and selectivity due to its superior structural and chemical effects.