Fischer–Tropsch Synthesis over Ordered Mesoporous Carbon Supported Cobalt Catalysts: The Role of Amount of Carbon Precursor in Catalytic Performance

Abstract  

Ordered mesoporous carbon supported cobalt-based catalysts (Co/MC) were synthesized via incipient wetness impregnation with different amounts of furfuryl alcohol (FA) as carbon precursor. The characterizations of obtained Co/MC were subjected to N₂ adsorption, XRD, XPS, TEM, H₂-TPR, H₂-TPD and H₂-TPSR. The results indicate that the reducibility and dispersion of Co active species vary significantly due to the difference of FA amount. By simply tuning the FA content from 25 to 100 wt%, the reduction temperature of deriving metallic Co shifts gradually to lower. The catalytic performance of Co/MC was evaluated for Fischer–Tropsch (FT) synthesis. The observed FT activity exhibits a volcano-type curve with the amount of FA due to the effect of both reducibility and dispersion of active species. As the precursor concentration overweighs 50 wt%, the ability of CO to dissociate over the active surface and the selectivity to the C₅₊ products level off after experiencing an initial increase. Substantially, the catalysts with higher concentration of FA render the larger crystallites having an average size of more than 6 nm, which facilitates the CO hydrogenation by way of carbon chain propagation. It seems that the sample with FA content of 50 wt% is optimum in terms of FT activity and C₅₊ selectivity.     

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.     

Potassium and nickel doped β-Mo2C catalysts for mixed alcohols synthesis via syngas

Potassium and nickel doped β-Mo₂C catalysts were prepared and their performances of CO hydrogenation were investigated. The main products over β-Mo₂C catalyst were hydrocarbons, and only few alcohols were obtained. The potassium promoter resulted in remarkable selectivity shift from hydrocarbons to alcohols over β-Mo₂C. Moreover, it was found that the potassium promoter enhanced the ability of chain propagation of β-Mo₂C catalyst and resulted in a higher selectivity to C₂₊OH. When doped by potassium and nickel, β-Mo₂C catalyst showed high activity and selectivity for mixed alcohols synthesis, the Ni promoter further enhanced the whole chain propagation to produce alcohols especially for the step of C₁OH–C₂OH. From the XPS analysis, it had been proved that the formation of higher alcohols might be attributable to the presence of Mo^IV species, whereas the formation of hydrocarbons was closely associated with the presence of Mo^II species on the surface of the catalysts.     

Vapour phase hydrogenation of phenol over Pd/C catalysts: A relationship between dispersion,metal area and hydrogenation activity

A series of palladium supported on activated carbon catalysts, with Pd varying from 0.5 to 6.0 wt%, were prepared via wet impregnation method using PdCl₂ · xH₂O as a precursor salt. The dried samples were further reduced at 573 K in hydrogen and characterized by CO adsorption at room temperature in order to determine the dispersion, metal area and particle size. The catalysts were tested for vapour phase phenol hydrogenation in a fixed-bed all glass micro-reactor at a reaction temperature of 453 K under normal atmospheric pressure. The decrease in metal surface area as well as dispersion with corresponding increase in turn-over frequency (TOF) against palladium loadings suggest the unusual inverse relationship that exist between Pd dispersion and phenol hydrogenation activity over Pd/carbon catalysts. The stability of TOF at larger crystallite size indicates that phenol hydrogenation is less sensitive reaction especially beyond 3 wt% of Pd content. It is evident from the results that structural properties of the catalysts strongly influence the availability of Pd atoms on the surface for CO chemisorption and hence for phenol hydrogenation. A comparison between selectivity and product yield of the reaction against overall phenol conversion indicates that changes in reaction selectivity for cyclohexanone or cyclohexanol is independent of phenol conversion level and either of the product is not formed at the cost of another. The stability of the catalysts with reaction time suggests that coke formation on the surface of the catalyst is less significant and the formation of cyclohexanone remains almost total even at higher reaction temperatures.     

Cobalt Particle Size Effects in the Fischer–Tropsch Synthesis and in the Hydrogenation of CO2 Studied with Nanoparticle Model Catalysts on Silica

We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H₂) and have compared it to data obtained for carbon dioxide hydrogenation (CO₂/H₂) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO₂ hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO₂ produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.     

Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts

The hydrogenation of CO, CO + CO₂, and CO₂ over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm³ min^− 1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 8000 cm³ g_cat^− 1 h^− 1), with a H₂:(CO + CO₂) ratio of 1:1. A comparative study of CO and CO₂ hydrogenation shows that while Rh and Rh–Fe/TiO₂ catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO₂ hydrogenation. The Fe/TiO₂ sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm.     

Fischer–Tropsch synthesis: Effect of CO2 containing syngas over Pt promoted Co/γ-Al2O3 and K-promoted Fe catalysts

The effect of CO₂ was studied for cobalt and iron Fischer–Tropsch (FT) synthesis. CO₂ behaves differently in the presence of CO over cobalt and iron catalysts in terms of hydrogenation. A systematic increase of the CO₂ mole fraction of carbon in the feed gas mixture alters the product distribution dramatically for cobalt FT synthesis with CO₂ behaving like an inert gas at higher partial pressure of CO. With cobalt, CO appears to compete with CO₂ for adsorption. Using an iron FT catalyst, hydrogenation of CO₂ was effected due to the presence of the reverse water–gas shift activity of the catalyst which converts CO₂ to hydrocarbons through the formation of CO. Unlike the cobalt catalyst, the product distribution was only slightly altered with increasing CO₂ content in the feed gas mixture to the iron catalyst. This difference in behavior of CO₂ over cobalt and iron could be attributed to the absence of reverse water–gas shift activity on cobalt and hydrogenation of CO₂ to hydrocarbons—other than methane—will be derived through the formation of CO.     

Co and Fe Catalysed Fischer–Tropsch Synthesis in Biofuel Production

Synthesis of liquid biofuels from synthesis gas is considered. A series of Co, Co/Ru and Fe catalysts supported by three Al₂O₃ based supports were prepared and tested for the Fischer–Tropsch (FT) reaction. The effects of supports and precursor salts on the activity of the catalysts were studied in the hydrogenation of CO using H₂/CO in a ratio of 2:1. The most active catalysts were tested with gas mixture having a composition close to synthesis gas derived by gasification of biomass. The combination of precursor salt and support is significant in order to get an active catalyst. Cobalt-based catalysts with traces of ruthenium on a small particle support proved to be the most active in the production of hydrocarbons with FT reaction.     

Promotional SMSI Effect on Supported Palladium Catalysts for Methanol Synthesis

High-temperature reduction (HTR) of palladium catalysts supported on some reducible oxides, such as Pd/CeO₂, and Pd/TiO₂ catalysts, led to a strong metal-support interaction (SMSI), which was found to be the main reason for their high and stable activity for methanol synthesis from hydrogenation of carbon dioxide. But low-temperature-reduced (LTR) catalysts exhibited high methane selectivity and were oxidized to PdO quickly in the same reaction. Besides palladium, platinum exhibited similar behavior for this reaction when supported on these reducible oxides. Mechanistic studies of the Pd/CeO₂ catalyst clarified the promotional role of the SMSI effect, and the spillover effect on the HTR Pd/CeO₂ catalyst. Carbon dioxide was decomposed on Ce₂O₃, which was attached to Pd, to form CO and surface oxygen species. The carbon monoxide formed was hydrogenated to methanol successively on the palladium surface while the surface oxygen species was hydrogenated to water by spillover hydrogen from the gas phase. A reaction model for the hydrogenation of carbon dioxide was suggested for both HTR and LTR Pd/CeO₂ catalysts. Methanol synthesis from syngas on the LTR or HTR Pd/CeO₂ catalysts was also conducted. Both alcohol and hydrocarbons were formed significantly on the HTR catalyst from syngas while methanol formed predominantly on the LTR catalyst. Characterization of these two catalysts elucidated the reaction performances.     

Activity and selectivity control in iron catalyzed Fischer-Tropsch synthesis

We present results of a catalyst structure-function study that supports a CO hydrogenation model with -olefins formed as the principal primary products and n-paraffins formed during secondary hydrogenation reactions. The interplay of catalyst composition and reaction environment controls the extent of secondary reactions. Catalysts that contain mainly oxidic phases or carbides with large concentrations of excess matrix carbon favor secondary reactions. The relative concentrations of oxide and carbide phases depends on the ease of reduction of the catalyst, which can be changed by cation substitutions. For example, cobalt substitution into Fe₃O₄ lowers the reduction temperature by 20 ° C. Excess matrix carbon has been intentionally introduced (by treatment in high temperature H₂/CO) into model iron carbide catalysts produced by laser synthesis. Increased paraffin selectivity as matrix carbon is introduced suggests the influence of the diffusion constraints on product selectivity. Alkali promotion will affect secondary hydrogenation pathways. We illustrate how catalysts with low levels of alkali become increasingly more selective to paraffins at high conversions (and high effective H₂/CO ratios).Reaction environment also controls catalyst composition and selectivity. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts are sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, catalysts partition into carbide phases in the front of the bed but show increasing amounts of oxide near the exit, whereas the catalysts in the stirred tank reactor remain all carbides. Product selectivities reflect the phase differences.Other examples illustrating secondary hydrogenation phenomena will be presented.     

CO hydrogenation over potassium-promoted Fe/carbon catalysts

The effects of potassium on the catalytic behavior in CO hydrogenation over K-promoted Fe/carbon catalysts having low K/Fe ratios were investigated. Even though the doses of potassium were low the promotional effects were pronounced, especially on the olefin-to-paraffin ratio, and theC ₃ toC ₄ olefin selectivities of the K-promoted catalysts were as high as 51 to 66 mol%. Over the catalysts having no or low potassium content the olefin-to-paraffin ratio and the ratio of the CO₂ formation rate to the rate of CO conversion to hydrocarbons remained roughly the same regardless of temperature, while over the K-promoted catalysts having higher potassium content they increased with temperature. Formation of significant amounts of filamentous carbon was observed in the K-promoted catalysts; however, the carbon deposition did not appear to affect the inherent activity and selectivity of the K-promoted catalysts.     

Effect of Vanadium Promotion on Activated Carbon-Supported Cobalt Catalysts in Fischer–Tropsch Synthesis

The effect of vanadium promotion on activated carbon (AC)-supported cobalt catalysts in Fischer–Tropsch synthesis has been studied by means of XRD, TPR, CO-TPD, H₂-TPSR of chemisorbed CO and F-T reaction. It was found that the CO conversion could be significantly increased from 38.9 to 87.4% when 4 wt.% V was added into Co/AC catalyst. Small amount of vanadium promoter could improve the selectivity toward C₁₀–C₂₀ fraction and suppress the formation of light hydrocarbon. The results of CO-TPD and H₂-TPSR of adsorbed CO showed that the addition of vanadium increased the concentration of surface-active carbon species by enhancing CO dissociation and further improved the selectivity of long chain hydrocarbons. However, excess of vanadium increased methane selectivity and decreased C₅⁺ selectivity.     

Direct synthesis of dimethyl ether from carbon-monoxide-rich synthesis gas: Influence of dehydration catalysts and operating conditions

Various dehydration catalysts were studied in the synthesis of dimethyl ether (DME) directly from carbon-monoxide-rich synthesis gas under a series of different reaction conditions. The investigated catalyst systems consisted of combinations of a methanol catalyst (CuO/ZnO system) with catalysts for methanol dehydration based on γ-Al₂O₃ or zeolites and γ-Al₂O₃ was identified as the most favorable dehydration catalyst. Various reaction parameters such as temperature, H₂/CO ratio and space velocity were studied. The impact of water on Cu/ZnO/Al₂O₃-γ-Al₂O₃ catalysts was investigated and no deactivation could be observed at water contents below 10% during running times of several hours. A running time of several days and a water content of 10% led to a significant increase of CO conversion but the water gas shift reaction became dominating and CO₂ was the main product. After termination of water feeding significant deactivation of the catalyst system was observed but the system returned to high DME selectivity. Catalyst stability and the influence of CO₂ in the gas feed were studied in experiments lasting for about three weeks. The presence of 8% of CO₂ caused an approximately 10% lower CO conversion and an about 5% lower DME selectivity compared to the reaction system without CO₂.     

Influence of CO2 co-feeding on Fischer–Tropsch fuels production over carbon nanofibers supported cobalt catalyst

Nowadays, the syngas which is obtained from the reforming of coal, biomass or natural gas contain significantly amounts of CO₂ that cannot be separated and consequently, it can take part into the Fischer–Tropsch (FTS) catalytic activity. Therefore, the presence of CO₂ in the syngas flow should be taken into account. In the present study, the FTS CO hydrogenation process was compared to that of CO₂ on a carbon nanofibers supported Co catalyst. The influence of CO₂ content in the feed stream (H₂/CO/CO₂ ratio) on the reaction performance in terms of conversion and selectivity to the different products was described. Both the support and the prepared catalyst were characterized by nitrogen adsorption–desorption, temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Results showed that CO hydrogenation was controlled by a Fischer–Tropsch regime, whereas CO₂ hydrogenation was controlled by a methanation process. When feed was composed of CO and CO₂ mixtures, the catalytic activity decreased with respect to that obtained with a CO₂-free feed stream. Moreover, the presence of CO₂ in feed stream favored the formation of lighter hydrocarbons and could block the production of further CO₂ via Water-Gas-Shift (WGS) reaction.     

Hydrogenation of CO2 Over Skeletal Copper Surfaces Containing Precipitated ZnO

Catalysts have been prepared by precipitating different amounts of ZnO on the surface of skeletal copper particles. The precursor copper catalysts were prepared by fully leaching CuAl₂ particles with concentrated NaOH solutions in the presence and absence of sodium chromate. The resulting catalysts contained surface zinc oxide concentrations in the range 0-1.36 mg per m² of catalyst surface.The influence of different concentrations of ZnO on the surface of skeletal copper was examined by the hydrogenation of CO₂ using a mixture of 24% CO₂/76% H₂ at 4 MPa, 493-533K and space velocities up to 410 700 h^-1. The catalysts were found to be highly active and selective for methanol synthesis. Methanol synthesis activity increased linearly with ZnO loadings with a maximum being reached at a loading of around 1.1 mg m^-2 for higher surface area skeletal copper prepared by leaching in the presence of chromate. ZnO loadings were also shown to improve the selectivity of CO₂ hydrogenation over copper by reducing the formation of CO by the reverse water-gas shift reaction.     

原位红外光谱法研究Pd-Ag/Al2O3和Pd/ Al2O3乙炔加氢催化剂

以一氧化碳和乙炔为探针分子,采用原位红外光谱技术研究了Pd-Ag/ Al₂O₃和Pd/ Al₂O₃催化剂上乙炔加氢反应以及催化剂本身的表面形态,动态考察了乙炔加氢的气相反应行为、CO吸附以及催化剂表面吸附物种的变化。结果表明,在Pd-Ag/ Al₂O₃催化体系中,由于Ag的加入而受到几何效应和电子效应的共同影响,引起了催化剂表面形态的改变从而改变了催化剂的性能。另外,乙炔加氢反应会导致钯催化剂表面形成由长分子链的饱和烃组成的碳氢化合物层,该碳氢化合物层有可能是加氢反应形成的绿油。     

Mixed alcohols synthesis from carbon monoxide hydrogenation over potassium promoted β-Mo2C catalysts

Potassium-promoted β-Mo₂C catalysts were prepared and their performances in CO hydrogenation were investigated. The main products over β-Mo₂C catalyst were C₁-C₄ hydrocarbons, only ∼4 C-atom% alcohols were obtained. The products of hydrocarbons and alcohols obeyed traditional linear Anderson-Schultz-Flory (A-S-F) distribution. However, modification with K₂CO₃ resulted in a remarkable selectivity shift from hydrocarbons to alcohols. Moreover, it was found that potassium promoter enhanced the ability of chain propagation of β-Mo₂C catalysts and resulted in a higher selectivity to C₂₊OH. For K/β-Mo₂C catalysts, the hydrocarbon products also obeyed traditional linear A-S-F plots, whereas alcohols gave a unique linear A-S-F distribution with remarkable deviation of methanol compared with that on β-Mo₂C catalyst. It could be concluded that potassium promoter might exert a prominent function on the whole chain propagation to produce alcohols. A surface phase on the K/β-Mo₂C catalysts such as the “K-Mo-C” explained the higher value for C₂₊OH, especially could promote the step of C₁OH to C₂OH, or could have a role in producing directly C₂OH, but again this would be speculative. At the same time, the influence of the loadings of K₂CO₃ on the performances of β-Mo₂C catalyst was investigated and the results revealed that the maximum yield of alcohol was obtained at K/Mo molar ratio of 0.2.     

Low methane selectivity using Co/MnO catalysts for the Fischer-Tropsch reaction: Effect of increasing pressure and Co-feeding ethene

CO hydrogenation using cobalt/ manganese oxide catalysts is described and discussed. These catalysts are known to give low methane selectivity with high selectivity to C₃ hydrocarbons at moderate reaction conditions (GHSV < 500 h^–1, < 600 kPa). In this study the effect of reaction conditions more appropriate to industrial operation are investigated. CO hydrogenation at 1–2 MPa using catalyst formulations with Co/Mn = 0.5 and 1.0 gives selectivities to methane that are comparable to those observed at lower pressures. At the higher pressure the catalyst rapidly deactivates, a feature that is not observed at lower pressures. However, prior to deactivation rates of CO + CO₂ conversion > 8 mol/1-catalyst h can be observed. Co-feeding ethene during CO hydrogenation is investigated by the reaction of¹³C0-¹²C₂H₄-H₂ mixtures and a significant decrease in methane selectivity is observed but the hydrogenation of ethene is also a dominant reaction. The results show that the co-fed ethene can be molecularly incorporated but in addition it can generate a C, species that can react further to form methane and higher hydrocarbons.     

CO2 hydrogenation reactivity and structure of Rh/SiO2 catalysts prepared from acetate,chloride and nitrate precursors

Silica-supported Rh catalysts (Rh/SiO₂) were prepared from acetate, chloride and nitrate precursors by an impregnation method and were applied to CO₂ hydrogenation reaction. CO₂ conversion over the catalyst prepared from chloride precursor was lower than that over acetate or nitrate one, because of fewer active sites on catalysts, as estimated by H₂ chemisorption. The main product was CO over the catalysts prepared from acetate and nitrate, but it was CH₄ over the catalyst prepared from chloride precursor. Characterization of catalysts by TEM, FT-IR and XPS was carried out in order to elucidate the effect of metal precursor on the CO₂ hydrogenation reactivity. The results of XPS showed that the O atomic ratio to Rh on surface hydroxyl groups increased in the order: chloride<nitrate<acetate precursor. The ratio of hydroxyl groups to Rh particles on SiO₂ surface was expected to have a significant influence on the reactivity.     

Preparation and characterization of Co/SiO2, Co-Mg/SiO2 and Mg-Co/SiO2 catalysts and their activity in CO hydrogenation

Co/SiO₂, Mg-Co/SiO₂ and Co-Mg/SiO2 catalysts were prepared from acetate, nitrate or carbonyl precursors. The catalysts were characterized by XRD, XPS, SIMS and TGA. The steady-state activity and product distribution of the catalysts were evaluated in synthesis gas reactions at 0.5 MPa and 235-290°C using 3 : 1 : 3 molar ratio of Ar : CO : H₂. The activity in CO hydrogenation decreased in the precursor order Co₂(CO)₈>Co(NO₃)₂> Co(CH₃COO)₂, and the probability of chain growth decreased in the precursor order Co(NO₃)₂>Co₂(CO)₈>Co(CH₃COO)₂. Alcohol yields were highest with Co₂(CO)₈, and lowest with Co(NO₃)₂, Magnesium promotion influenced the catalyst activity and decreased the CO₂ formation, but the promotion effects were less profound than those of the precursor. Surface studies on partially magnesium covered cobalt foil model catalysts suggested that magnesium promotes CO dissociation and chain growth, neither of which were, however, observed in the supported catalysts.