Porous carbon as support for iron and ruthenium catalysts

Iron and ruthenium catalysts have been supported on a porous carbon prepared by pyrolysis and activation of the copolymer Saran. For comparison, a graphitized carbon black (V3G) has also been used as support for both metals. The catalysts have been characterized by chemisorption of H₂ and CO₂ at 298 K (373 K in some cases) and by X-ray line broadening. The hydrogen chemisorption on iron catalysts was very low and increased with adsorption temperature, whereas the CO chemisorption results indicate the formation of subcarbonyl species. However, H₂ and CO uptakes led to similar dispersion values for the ruthenium catalysts. The X-ray results were in good agreement with the chemisorption results except in the case of highly dispersed Fe catalysts. The results obtained in the hydrogenation of CO indicate that in the case of Fe catalysts the highest selectivity toward hydrocarbons was given by the catalyst supported on V3G, with large metal particle size which, at the same time, exhibited a lower decrease in activity with reaction time than the other Fe catalysts with smaller average particle size. The olefin/paraffin ratio is very large for the catalyst prepared from Fe(CO)₅.The Ru catalysts are essentially of the methanation type.     

Effect of Palladium on Iron Fischer–Tropsch Synthesis Catalysts

Studies were conducted to investigate the effect of Pd on the Fischer–Tropsch Synthesis (FTS) selectivity, activity and kinetics as well as on the water–gas shift activity of an iron catalyst. Two palladium promoted catalysts (Pd0.002/Fe100 and Pd0.005/Fe100) were prepared from a base Fe100/Si5.1 (atomic ratio) catalyst. Results of FTS over the two palladium promoted catalysts were compared to those obtained from the K/Fe/Si base catalyst and a Cu/K/Fe/Si catalyst. The results indicate that Pd enhanced the FT activity while the selectivity for CO₂ and CH₄ changed little compared to the results for the base catalyst and the Cu promoted catalyst. Palladium promotion had a negative effect on the C₂—C₄ olefin to paraffin ratio. Pd promotion led to a higher WGS rate than the other two catalysts at high syngas conversions. A higher WGS rate compared to the FTS rate was obtained only for the Pd promoted catalysts. The FTS rate constant for the Pd promoted catalyst is higher than the base catalyst but lower than for the Cu promoted catalyst.     

Effect of Structural Promoters on Fe-Based Fischer–Tropsch Synthesis of Biomass Derived Syngas

Biomass gasification and subsequent conversion of this syngas to liquid hydrocarbons using Fischer–Tropsch (F–T) synthesis is a promising source of hydrocarbon fuels. However, biomass-derived syngas is different from syngas obtained from other sources such as steam reforming of methane. Specifically the H₂/CO ratio is less than 1/1 and the CO₂ concentrations are somewhat higher. Here, we report the use of Fe-based F–T catalysts for the conversion of syngas produced by the air-blown, atmospheric pressure gasification of southern pine wood chips. The syngas from the gasification step is compressed and cleaned in a series of sorbents to produce the following feed to the F–T step: 2.78 % CH₄, 11 % CO₂, 15.4 % H₂, 21.3 % CO, and balance N₂. The relatively high level of CO₂ suggests the need to use catalysts that are active for CO₂ hydrogenation as well is resistant to oxidation in presence of high levels of CO₂. The work reported here focuses on the effect of these different structural promoters on iron-based F–T catalysts with the general formulas 100Fe/5Cu/4K/15Si, 100Fe/5Cu/4K/15Al and 100Fe/5Cu/4K/15Zn. Although the effect of Si, Al or Zn on iron-based F–T catalysts has been examined previously for CO+CO₂ hydrogenation, we have found no direct comparison of these three structural promoters, nor any studies of these promoters for a syngas produced from biomass. Results show that catalysts promoted with Zn and Al have a higher extent of reduction and carburization in CO and higher amount of carbides and CO adsorption as compared to Fe/Cu/K/Si. This resulted in higher activity and selectivity to C₅+ hydrocarbons than the catalyst promoted with silica.     

Comparative Study on CO2 Hydrogenation to Higher Hydrocarbons over Fe-Based Bimetallic Catalysts

This paper reports on notable promotion of C₂ ⁺ hydrocarbons formation from CO₂ hydrogenation induced by combining Fe and a small amount of selected transition metals. Al₂O₃-supported bimetallic Fe–M (M = Co, Ni, Cu, Pd) catalysts as well as the corresponding monometallic catalysts were prepared, and examined for CO₂ hydrogenation at 573 K and 1.1 MPa. Among the monometallic catalysts, C₂ ⁺ hydrocarbons were obtained only with Fe catalyst, while Co and Ni catalysts yielded higher CH₄ selectively than other catalysts. The combination of Fe and Cu or Pd led to significant bimetallic promotion of C₂ ⁺ hydrocarbons formation from CO₂ hydrogenation, in addition to Fe–Co formulation discovered in our previous work. CO₂ conversion on Ni catalyst nearly reached equilibrium for CO₂ methanation which makes this catalyst suitable for making synthetic natural gas. Fe–Ni bimetallic catalyst was also capable of catalyzing CO₂ hydrogenation to C₂ ⁺ hydrocarbons, but with much lower Ni/(Ni+Fe) atomic ratio compared to other bimetallic catalysts. The addition of a small amount of K to these bimetallic catalysts further enhanced CO₂ hydrogenation activity to C₂ ⁺ hydrocarbons. K-promoted Fe–Co and Fe–Cu catalysts showed better performance for synthesizing C₂ ⁺ hydrocarbons than Fe/K/Al₂O₃ catalyst which has been known as a promising catalyst so far.     

Study of an iron-manganese Fischer–Tropsch synthesis catalyst promoted with copper

The metal–silica interaction and catalytic behavior of Cu-promoted Fe–Mn–K/SiO₂ catalysts were investigated by temperature-programmed reduction/desorption (TPR/TPD), differential thermogravimetric analysis, in situ diffuse reflectance infrared Fourier transform analysis, and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO₂ catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions. In addition to the well-known effect of Cu promoter on easing the reduction of iron-based FTS catalysts, it is found that Cu promoter can increase the rate of carburization, but does not vary the extent of carburization during the steady-state FTS reaction. The basicity of the Cu and K co-promoted catalyst is greatly enhanced, as demonstrated by CO₂-TPD results. In the FTS reaction, Cu improves the rate of catalyst activation and shortens the induction period, whereas the addition of Cu has no apparent influence on the steady-state activity of the catalyst. Promotion of Cu strongly affects hydrocarbon selectivity. The product distribution shifts to heavy hydrocarbons, and the olefin/paraffin ratio is enhanced on the catalyst due to the indirect enhancement of surface basicity by the copper promotion effect.     

Investigating the effect of metal oxide additives on the properties of Cu/ZnO/Al2O3 catalysts in methanol synthesis from syngas using factorial experimental design

The Cu/ZnO/Al₂O₃ catalysts, prepared by co-precipitation method, have been modified by adding small amount of Mn, Mg, Zr, Cr, Ba, W and Ce oxides using design of experiments (1/16 full factorial design). The structure and morphology of catalysts were studied by X-ray diffraction (XRD) and BET. Performance of the prepared catalysts for CO/CO₂ hydrogenation to methanol was evaluated by using a stainless steel fixed-bed reactor at 5 MPa and 513 K. The oxide additives were found to influence the catalytic activity, dispersion of Cu, Cu crystallite size, surface composition of catalyst and stability of catalysts during their operations. The results showed that the Mn and Zr promoted catalysts have high performance for methanol synthesis from syngas.     

Study on Fe–Al2O3 interaction over precipitated iron catalyst for Fischer–Tropsch synthesis

Two model spherical iron catalysts (100Fe/0Al₂O₃ and 100Fe/15Al₂O₃) with free Cu and K promoters were prepared by the combination of co-precipitation and spray drying method for the application of slurry Fischer–Tropsch synthesis (FTS). The effect of Fe–Al₂O₃ interaction on the reduction/carburization behavior in H₂/CO/syngas, surface basicity and the change of phase structure were comparatively studied by means of H₂ or CO temperature-programmed reduction (TPR), CO₂ temperature-programmed desorption (TPD) and Mössbauer effect spectroscopy (MES). The results showed that the catalyst incorporated with Al₂O₃ exhibits a strong Fe–Al₂O₃ interaction, which obviously weakens the surface basicity, stabilizes the FeO phase and inhibits the reduction of iron catalyst in H₂ or syngas. Furthermore, Fe–Al₂O₃ interaction also restrains the carburization of iron catalyst in CO or syngas. In slurry FTS process, it was found that the strong Fe–Al₂O₃ interaction decreases the FTS activity and suppresses the water gas shift (WGS) reaction, but can stabilize the active sites of iron catalyst and improve its run stability. Due to the strong Fe–Al₂O₃ interaction, the weak surface basicity on the catalyst incorporated with Al₂O₃ greatly decreases the selectivity of heavy hydrocarbon products.     

Methanol synthesis over Cu/SiO2 catalysts

The rates of CO and CO/CO₂ hydrogenation at 4.2 MPa and 523 K are reported for a series of Cu/SiO₂ catalysts containing 2 to 88 wt.% Cu. These catalysts were prepared on a variety of silica sources using several different Cu deposition techniques. In CO/CO₂ hydrogenation, the rate of methanol formation is proportional to the exposed Cu surface area of the reduced catalyst precursor, as determined by N₂O frontal chromatography. The observed rate, 4.2×10^–3 mole CH₃OH/Cu site-sec, is within a factor of three of the rates reported by others over Cu/ZnO and Cu/ZnO/Al₂O₃ catalysts under comparable conditions. These results suggest that the ZnO component is only a moderate promoter in methanol synthesis. Hydrogenation of CO over these catalysts also gives methanol with high selectivity, but the synthesis rate is not proportional to the Cu surface area. This implies that another type of site, either alone or in cooperation with Cu, is involved in the synthesis of methanol from CO.     

Role of promoters on Rh/SiO2 in CO hydrogenation: A comparison using DRIFTS

La, V, Zn, Cu, Fe, Li and Ag promoted Rh/SiO₂ catalysts were investigated for the synthesis of ethanol during CO hydrogenation at 230 °C and 1.8 atm. As is well known, the activity and selectivity depend heavily on the choice of promoter. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to probe the effects of La, V, Zn and Cu on CO adsorption and hydrogenation. From the IR study, it was found that the behavior of CO adsorbed on the differently promoted catalysts was very different. While La enhanced total CO adsorption, the addition of V, Zn and Cu suppressed CO adsorption to different extents. The doubly promoted Rh-La/V/SiO₂ showed only moderate CO adsorption. Results from DRIFTS suggest that the higher catalytic activity (compared to the non-promoted catalyst) observed for the La singly promoted Rh/SiO₂ catalyst may primarily be caused by an increase in the concentration of the adsorbed CO species in the presence of H₂, possibly due to the formation of new active sites at the LaO_x-Rh interface. The higher catalytic activity of the V singly promoted Rh/SiO₂ catalyst could be ascribed to an increased desorption rate/reactivity of the adsorbed CO species. The La and V doubly promoted catalyst showed both new adsorbed CO species and increased desorption rate/reactivity of the adsorbed species during CO hydrogenation due to a synergistic promoting effect of La and V. The addition of Zn or Cu promoters significantly reduced the desorption rate/reactivity of the adsorbed CO species on Rh/SiO₂, leading apparently to the much reduced activities for CO hydrogenation observed.     

The difference in the active sites for CO2 and CO hydrogenations on Cu/ZnO-based methanol synthesis catalysts

The effect of Zn in copper catalysts on the activities for both CO₂ and CO hydrogenations has been examined using a physical mixture of Cu/SiO₂+ZnO/SiO₂ and a Zn-containing Cu/SiO₂ catalyst or (Zn)Cu/SiO₂. Reduction of the physical mixture with H₂ at 573–723 K results in an increase in the yield of methanol produced by the CO₂ hydrogenation, while no such a promotion was observed for the CO hydrogenation, indicating that the active site is different for the CO₂ and CO hydrogenations. However, the methanol yield by CO hydrogenation is significantly increased by the oxidation treatment of the (Zn)Cu/SiO₂ catalyst. Thus it is concluded that the Cu–Zn site is active for the CO₂ hydrogenation as previously reported, while the Cu–O–Zn site is active for the CO hydrogenation.     

Hydrogenation of CO2 to dimethyl ether on La-, Ce-modified Cu-Fe/HZSM-5 catalysts

Cu–Fe–La/HZSM-5 and Cu–Fe–Ce/HZSM-5 bifunctional catalysts were prepared and applied for the direct synthesis of dimethyl ether (DME) from CO₂ and H₂. The catalysts were characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, H₂-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density of Cu and improved the reduction ability of the Cu–Fe catalyst in comparison to the Cu–Fe–Zr catalyst, which may increase the selectivity for DME. The Cu–Fe–Ce catalyst had a greater specific surface area than the Cu–Fe–La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the DME selectivity and the CO₂ conversion. The catalysts were stable for 15 h.     

MnOx promotional effects on olefins synthesis directly from syngas over bimetallic Fe‐MnOx/SiO2 catalysts

The direct synthesis of lower olefins via the Fischer‐Tropsch reaction (FTO) has been performed over a series of Fe‐MnO_x/SiO₂ catalysts. The addition of MnO_x could improve the dispersion of iron species, and promote the reduction of iron oxide during the activation and subsequent carburization. Moreover, the results of characterization demonstrated that MnO_x could enhance the surface basicity of the catalysts due to electronic effects and promote the formation of iron carbides. For the first time, the intrinsic power‐law kinetics for FTO was obtained for both Fe₂₀/SiO₂ and Fe₂₀‐Mn₁/SiO₂ catalysts. Kinetic parameters and structure characterizations indicated that MnO_x could facilitate the CO dissociation on the catalyst surface, thus enhancing the adsorption strength and capacity of surface carbonaceous intermediates. The weak hydrogenation of carbonaceous species would boost the selectivities toward lower olefins. Finally, a plausible mechanism for FTO, involving the promotional effects of MnO_x on Fe, has been proposed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4451–4464, 2017     

Hydrogenation of carbon dioxide over alumina supported Fe-K catalysts

The hydrogenation of CO₂ has been studied over Fe/alumina and Fe-K/alumina catalysts. The addition of potassium increases the chemisorption ability of CO₂ but decreases that of H₂. The catalytic activity test at high pressure (20 atm) reveals that remarkably high activity and selectivity toward light olefins and C₂₊ hydrocarbons can be achieved with Fe-K/alumina catalysts containing high concentration of K (K/Fe molar ratio = 0.5, 1.0). In the reaction at atmospheric pressure, the highly K-promoted catalysts give much higher CO formation rate than the unpromoted catalyst. It is deduced that the remarkable catalytic properties in the presence of K are attributable to the increase in the ability of CO₂ chemisorption and the enhanced activity for CO formation, which is the preceding step of C₂₊ hydrocarbon formation.     

Fischer–Tropsch Synthesis: Characterization Rb Promoted Iron Catalyst

Rubidium promoted iron Fischer–Tropsch synthesis (FTS) catalysts were prepared with two Rb/Fe atomic ratios (1.44/100 and 5/100) using rubidium nitrate and rubidium carbonate as rubidium precursors. Results of catalytic activity and deactivation studies in a CSTR revealed that rubidium promoted catalysts result in a steady conversion with a lower deactivation rate than that of the corresponding unpromoted catalyst although the initial activity of the promoted catalyst was almost half that of the unpromoted catalyst. Rubidium promotion results in lower methane production, and higher CO₂, alkene and 1-alkene fraction in FTS products. Mössbauer spectroscopic measurements of CO activated and working catalyst samples indicated that the composition of the iron carbide phase formed after carbidization was χ-Fe₅ C₂ for both promoted and unpromoted catalysts. However, in the case of the rubidium promoted catalyst, ?′-Fe_2.2C became the predominant carbidic phase as FTS continued and the overall catalyst composition remained carbidic in nature. In contrast, the carbide content of the unpromoted catalyst was found to decline very quickly as a function of synthesis time. Results of XANES and EXAFS measurements suggested that rubidium was present in the oxidized state and that the compound most prevalent in the active catalyst samples closely resembled that of rubidium carbonate.     

Hydrogenation of carbon dioxide by hybrid catalysts,direct synthesis of aromatics from carbon dioxide and hydrogen

Direct synthesis of aromatics from carbon dioxide hydrogenation was investigated in a single stage reactor using hybrid catalysts composed of iron catalysts and HZSM-5 zeolite. Carbon dioxide was first converted to CO by the reverse water gas shift reaction, followed by the hydrogenation of CO to hydrocarbons on iron catalyst, and finally the hydrocarbons were converted to aromatics in HZSM-5. Under the operating conditions of 350°C, 2100 kPa, and CO₂/H₅ = 1/2, the maximum aromatic selectivity obtained was 22% with a CO₂ conversion of 38% using fused iron catalyst combined with the zeolite. Together with the kinetic studies, thermodynamic analysis of the CO₂ hydrogenation was also conducted. It was found that unlike Fischer Tropsch synthesis, the formation of hydrocarbons from CO₂ may not be thermodynamically favored at higher temperatures.     

Fischer–Tropsch Synthesis: Deuterium Kinetic Isotope Study for Hydrogenation of Carbon Oxides Over Cobalt and Iron Catalysts

Abstract  

The hydrogenation of CO₂ using Pt promoted Co/γ-Al₂O₃ and doubly (Cu, K) promoted iron catalysts exhibits an inverse isotope effect (r ^H/r ^D < 1). The observed inverse isotope effect for hydrogenation of CO₂ shows that hydrogen addition to CO₂ should be involved in the kinetically relevant step. The systematic increase of inverse isotope effect with carbon number of products obtained during H₂–D₂–H₂ switching experiments suggests the possible existence of a common intermediate (CH _x O) for hydrogenation of CO₂ over both cobalt and iron FT catalysts. The magnitude of the inverse isotope effect is lower for CO₂ compared to CO hydrogenation under similar reaction conditions. The deuterium isotope effect does not provide a definite conclusion regarding the mechanism which CO₂ hydrogenation follows (alkyl, enol, or alkylidine mechanisms).     

Studies of carbonaceous species in alkali promoted iron catalysts during Fischer–Tropsch synthesis

The effects of La, Mg and Ca promoters on carbonaceous surface and bulk iron carbide species formed in the alkali promoted iron catalysts are studied under realistic Fischer–Tropsch synthesis (FTS) conditions. Compositions of bulk iron phase and phase transformations of carbonaceous species during pretreatment and FTS reaction were characterized using the temperature-programmed surface reaction with hydrogen (TPSR-H₂) and XRD techniques. Many carbonaceous species on surface and bulk were qualitatively and quantitatively identified by combined TPSR-H₂ and XRD spectra of the alkali promoted iron catalyst. These species, sorted by the their reactivity with H₂ from high to low, were recognized as (a) adsorbed, atomic carbon; (b) amorphous, lightly polymerized hydrocarbon or carbon surface species; (c) bulk carbides and (d) disordered and moderately ordered graphitic surface carbons. The results revealed that while the surface basicity of the iron catalyst increased the CO dissociation proceeds faster than carbon hydrogenation. This phenomenon leads to excessive carbon deposition and formation of inactive iron carbide phases and graphitic type carbonaceous surface species, and consequently leads to catalyst deactivation.     

Performance evaluation under supercritical conditions of Fe–Mn–Cu–K composite catalysts synthesized by supercritical methods

A series of nanosized Fe–Mn–Cu–K composite catalysts was prepared by a supercritical combined technology. The nanosized catalysts were characterized by means of XRD, TEM and BET techniques, and tested for catalytic performance under Fischer–Tropsch synthesis (FTS) reaction conditions. The catalyst synthesized by the supercritical combined technology has some excellent properties. Additionally, the drying and crystallization of nanosize catalyst could be achieved simultaneously by this supercritical combined technology. The addition of Mn, Cu and K promoters can improve the catalytic performance properties of the catalyst, including lower CH₄ and CO₂ formation rates, and higher production rates of desired light-olefin. The optimal performance with a 95.7% CO conversion and a 46.5% light-olefin yield was obtained by using a catalyst component of Fe/Mn/Cu/K = 60:25:10:8.5. In summary, optimal catalytic performance was obtained by using the nanosized catalyst under supercritical reaction conditions, resulting in higher CO conversion, less byproduct CO₂ formation, and a higher yield of light-olefin.     

Carbon dioxide hydrogenation over nickel-, ruthenium-, and copper-based catalysts: Review of kinetics and mechanism

This study critically reviews the mechanism of CO₂ hydrogenation over Ni, Ru, and Cu, and the effect of catalyst properties and operating conditions on reaction kinetics. Most studies have reported the presence of CO and formate species on Ni-, Ru-, and Cu-based catalysts, where subsequent conversion of these species depends on the type of catalyst and the physicochemical properties of the catalyst support. Methane is the major product that forms during CO₂ hydrogenation over Ni and Ru catalysts, while methanol and CO are mainly produced on Cu catalysts. A different approach for catalyst formulations and/or process development is required where long chain hydrocarbons are desired.     

Olefin selective carbon-supported K-Fe-Mn CO hydrogenation catalysts: A kinetic,calorimetric, chemisorption,infrared and Mössbauer spectroscopic investigation

Carbon-supported Fe, Fe-Mn, and K-Fe-Mn catalysts derived from stoichiometric mixed-metal carbonyl clusters were pretreated at either 473 K or 673 K in hydrogen. Chemisorption and kinetic measurements were conducted following these pretreatments. The iron remained highly dispersed at all times except after high temperature reductions when potassium was present. The single promotion by either Mn or K increases the olefin/paraffin ratio, while the doubly promoted catalyst gave very high selectivities for light olefins. Isothermal, integral heats of adsorption of CO were determined at 300 K, and they increased from 15 kcal/mole for Fe₃/C to nearly 17 kcal/mole for both the singly promoted Fe₂Mn/C and KFe₃/C catalysts to 21 kcal/mole for the doubly promoted KFe₂Mn/C sample. A model of the decomposition of these carbonyl clusters is proposed based on calorimetric, Mössbauer effect spectroscopic and diffuse reflectance Fourier transform infrared spectroscopic studies. The state of the MnO_x and K phases on the iron surface, as well as the Fe crystallite size, appears to play a dominant role in determining the catalytic behavior.