Increasing carbon utilization in Fischer-Tropsch synthesis using H2-deficient or CO2-rich syngas feeds

Fischer-Tropsch technology has become a topical issue in the energy industry in recent times. The synthesis of linear hydrocarbon that has high cetane number diesel fuel through the Fischer-Tropsch reaction requires syngas with high H₂/CO ratio. Nevertheless, the production of syngas from biomass and coal, which have low H₂/CO ratios or are CO₂ rich may be desirable for environmental and socio-political reasons. Efficient carbon utilization in such H₂-deficient and CO₂-rich syngas feeds has not been given the required attention. It is desirable to improve carbon utilization using such syngas feeds in the Fischer-Tropsch synthesis not only for process economy but also for sustainable development. Previous catalyst and process development efforts were directed toward maximising C₅₊ selectivity; they are not for achieving high carbon utilization with H₂-deficient and CO₂-rich syngas feeds. However, current trends in FTS catalyst design hold the potential of achieving high carbon utilization with wide option of selectivities. Highlights of the current trends in FTS catalyst design are presented and their prospect for achieving high carbon utilization in FTS using H₂-deficient and CO₂-rich syngas feeds is discussed.     

Fischer-Tropsch synthesis and the generation of DME in situ

Ternary physical mixtures comprised a Fischer-Tropsch catalyst, a methanol synthesis catalyst and a zeolite employed in the hydrocarbon synthesis from syngas. Two Fe-based catalysts (i.e., one promoted by K and the other by Ru), two HY zeolites with different acidities, a commercial HZSM-5 and Cu/ZnO/Al₂O₃ (methanol synthesis catalyst) were used in these systems. The main products obtained were dimethyl ether, methanol and hydrocarbons. First of all, it was observed that by adding Cu/ZnO/Al₂O₃ catalyst to a binary physical mixture comprised of a Fischer-Tropsch catalyst and HZSM-5, the CO conversion increases more than 20 times. Second, during the reaction transient period the dimethyl ether selectivity decreases as the conversion increases. Third, the hydrocarbons synthesized followed the ASF distribution in the C₁-C₁₂ range and finally, it was also verified that the Y zeolites and the Fischer-Tropsch synthesis catalyst promoted by Ru generated the most active physical mixtures. The results showed that the role of zeolites in the ternary physical mixture is only associated with the dimethyl ether synthesis. The following reaction pathway was suggested: first, methanol is synthesized from syngas using Cu/ZnO/Al₂O₃ catalyst; after that, this alcohol is dehydrated by an acid catalyst generating DME; and lastly, DME initiates Fischer-Tropsch synthesis, which is then propagated by CO.     

Enhancement of gasoline selectivity in combined reactor system consisting of steam reforming of methane and Fischer-Tropsch synthesis

A two-stage, one-dimensional configuration model including the steam reforming of methane (SRM) and Fischer-Tropsch (FT) synthesis has been developed for the production of hydrocarbons. This configuration is used to investigate hydrocarbon product distribution, such as gasoline. The first SRM reactor is fed by methane and steam, and the products are converted to hydrocarbons by the second FT reactor. The model was solved numerically by applying the finite difference approximation, and the set of first-order ODEs was solved in the axial direction. The results show that complete conversion of hydrogen in the second reactor can be achieved although a small amount of carbon monoxide remains. Furthermore, at higher H₂O/CH₄ ratio (and low CO in feed), lower C₂-C₅ yield and selectivity is obtained.     

ZSM-5 supported iron catalysts for Fischer-Tropsch production of light olefin

Fischer-Tropsch Synthesis (FTS) for olefin production from syngas was studied on Fe-Cu-K catalysts supported on ZSM-5 with three different Si/Al ratios. The catalysts were prepared by slurry-impregnation method of metallic components, and were characterized by BET surface area, XRD, hydrogen TPR and ammonia TPD. Fe-Cu-K/ZSM-5 catalyst with a low Si/Al ratio (25) is found to be superior to the other catalysts in terms of better C₂-C₄ selectivity in the FTS products and higher olefin/(olefin + paraffin) ratio in C₂-C₄ because of the facile formation of iron carbide during FTS reaction and also due to a larger number of weak acidic sites that are present in these catalysts.     

The fischer—tropsch synthesis over a ruthenium catalyst under composition cycling

The influence of feed composition cycling on the activity and selectivity of the Fischer-Tropsch synthesis over an alumina-supported ruthenium catalyst was investigated using a fixed-bed reactor at 211°C and 446 kPa total pressure. Forced composition cycling suppressed the overall synthesis rate but increased the rates of formation of the C₁ to C₄ paraffins and shifted the paraffin/olefin ratio.     

Synthesis and catalytic properties of eggshell cobalt catalysts for the Fischer-Tropsch synthesis

CO diffusional restrictions decrease C₅₊ synthesis rates and selectivity within large (1–3 mm) catalyst pellets often required in Fischer-Tropsch (FT) synthesis reactors. Eggshell catalysts, where Co is located preferentially near outer pellet surfaces, reduce the severity of these transport restrictions and lead to higher synthesis rates and C₅₊ selectivity. Maximum C₅₊ selectivities occur on catalysts with intermediate shell thickness, within which transport restrictions limit the removal of reactive olefins but not the arrival of reactants at catalytic sites. A new synthetic technique leads to sharp distributions of active sites near outer pellet surfaces by controlling the rate of imbibition of cobalt nitrate melts. Also, slow reduction of the impregnated salt leads to moderate Co dispersions (0.05–0.10) even at high local Co loadings present within shell regions.     

Selective synthesis of C2-C4 hydrocarbons from CO + H2 on composite catalysts

A copper-zinc-aluminum methanol synthesis catalyst has been prepared using a precipitated hydrotalcite-type precursor that decomposes to a mixture of the corresponding amorphous oxides at a low temperature. TPR studies show that such a mixture is easy to reduce giving a highly dispersed catalyst. When this is mixed with a zeolite, the resulting hybrid catalyst gives C₂-C₄ hydrocarbons with very high selectivity. This may be useful in obtaining LPG from synthesis gas.     

Support effect of Co/Al2O3 catalysts for Fischer-Tropsch synthesis

Cobalt supported on different γ-alumina carries prepared by incipient wetness impregnation are used to investigate the effect of support on the performance of cobalt catalysts for Fischer-Tropsch synthesis (FTS). It is found that the acidity of support has a great influence on the interaction between metallic cobalt and support and then the reducibility of cobalt. The support with low acidity leads to the higher active FTS catalysts. Furthermore, the high reducibility and more bridged type CO which is favored by γ-alumina with low acidity appears to be responsible for high C₅⁺ hydrocarbon selectivity and low methane selectivity.     

The skeletal isomerization of C4-C7 1-olefins over ferrierite and ZSM-5 zeolite catalysts

The skeletal isomerization of C₄-C₇ 1-olefins was studied on ferrierite (FER) and ZSM-5 (MFI) zeolites to elucidate the effect of the molecular distribution in zeolite pores on the selectivity foriso-olefin formation. Regardless of the difference in molecular length of 1-olefins, the FER zeolite showed high selectivity foriso-olefins, while the selectivity became slightly low at the skeletal isomerization of long olefin molecules. The drastic decrease in the selectivity of MFI zeolites by increasing the conversion is concurrently observed in the skeletal isomerization of C₄-C₇ 1-olefins. The high selectivity of FER zeolites is explained by their sparse distributions of olefin molecules in pores, which induces a high preference for monomolecular skeletal isomerization.     

Selective synthesis of light olefins from syngas over potassium-promoted molybdenum carbide catalysts

In the hydrogenation of CO at atmospheric pressure, unsupported molybdenum carbide catalyst produced mostly C₁-C₅ paraffins. Promotion of the catalyst with K₂CO₃ yielded C₂-C₅ hydrocarbons consisting of 80–100% olefins and reduced the methane selectivity. The selectivity of C₂-C₅ olefins among all hydrocarbon products was 50–70 wt% at CO conversions up to 70%.This work has been supported by Korean Science and Engineering Foundation through a contract 88-03-1302.     

Fischer-Tropsch synthesis: Relations between structure of cobalt catalysts and their catalytic performance

Fischer-Tropsch synthesis has been experiencing a strong revival in recent years, due to the resource utilization considerations and environmental concerns. Cobalt supported catalysts represent the optimal choice for the synthesis of long-chained hydrocarbons from syngas with high H₂/CO ratio. This paper reviews the state of the art related to the influence of cobalt particle size and cobalt phase composition, catalyst support and support texture, and promotion with noble metals on Fischer-Tropsch reaction rates, hydrocarbon selectivity and catalyst stability. Possible mechanisms of catalyst deactivation and modification of cobalt active sites during the reaction are also discussed. Several requirements to the design of cobalt Fischer-Tropsch catalysts have been specified.     

Synthesis of isoalkanes over Fe–Zn–Zr/HY composite catalyst through carbon dioxide hydrogenation

The reaction path of isoalkanes formation via CO₂ hydrogenation was studied over the Fe–Zn–Zr/HY composite catalyst, which gives high selectivity to isoalkanes. The results indicate that the reverse water–gas shift reaction is not the indispensable step for the synthesis of hydrocarbons. And i-C₄ (iso-butane) is formed from propylene and methanol through MTG (methanol to gasoline) reaction and i-C₅ (iso-pentane) obtained from the reaction of C₂ and C₃ through the additive dimerization. A part of C₁, C₄ is formed on the sole Fe–Zn–Zr catalyst from methanol for the CO₂ hydrogenation over Fe–Zn–Zr/HY composite catalyst.     

Synthesis of Light Hydrocarbons from Biogas and Hydrogen: Investigation of a Fe-Mn-K/MgO Catalyst

Fischer-Tropsch synthesis of the CO₂ in biogas aims at producing light hydrocarbons and increasing its calorific value for feeding into the grid. Fe catalysts with Mn and K as promoters are supposed to yield high amounts of light hydrocarbons. Using a Fe-Mn-K/MgO catalyst, a parameter screening and long-term experiments were carried out. The catalyst shows, within the examined range, the highest selectivity to C₂–C₄ hydrocarbons at 450 °C, 8 bar(a), and a gas hourly space velocity of 350 h⁻¹. Calcination of the catalyst resulted in a significant drop of activity and an almost complete loss of selectivity to hydrocarbons. Admixture of steam to the reactant gas lowers the tendency to carbon deposition but also promotes the water-gas shift reaction and results in lower yields of hydrocarbons.     

Bell-boudoir and water gas shift reactions under conditions of the Fischer-Tropsch synthesis

Underground coal gasification gases contain CO and H₂; therefore, they can be used as raw materials in the Fischer-Tropsch synthesis of higher hydrocarbons. The profitability can be increased by decreasing the selectivity of this process for undesirable products. The water gas shift and Bell-Boudoir reactions are considered; CO₂ is formed as a result of these reactions. Promoted and unpromoted cobalt catalysts on the basis of silica gel and aluminum oxide were tested. The processes occurring on these samples were evaluated quantitatively under the conditions that simulate the Fischer-Tropsch synthesis.     

Hydrogenation of CO on supported cobalt γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in fixed bed and slurry bubble column reactors

Fischer-Tropsch synthesis for the production of C₅+ hydrocarbons from syngas was carried out in a tubular fixed bed reactor (TFBR) and in a slurry bubble column reactor (SBCR). The Co-based catalysts for FTS were prepared by the conventional wet-impregnation of γ-Al₂O₃. Effects of operating conditions such as GHSV (1,000–4,000 ml/g·hr), reaction temperature (220–250°C) and pressure (0.5–3.0MPa) on the CO conversion and product selectivity of Co/γ-Al₂O₃ catalyst were examined in the TFBR and SBCR. The C₅+ selectivity and olefin selectivity in an SBCR were found to be higher than that in a TFBR, whereas C₂–C₄ selectivity showed a reverse trend. The CO conversion and product distribution in an SBCR were less sensitive than that in a TFBR with variations of reaction conditions.     

Slurry-phase CO2 hydrogenation to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst: Investigation of reaction conditions

The hydrogenation of CO₂ to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst was studied in a slurry reactor for the first time. Reducibility of the catalyst and effect of reaction variables (temperature, pressure and H₂/CO₂ ratio of the feed gas) on the catalytic reaction performance were investigated. The reaction results indicated that the Fe-Cu-Al/K catalyst showed a good CO₂ hydrogenation performance at a relatively low temperature (533 K). With the increase of reaction temperature CO₂ conversion and olefin to paraffin (O/P) ratio in C₂-C₄ hydrocarbons as well as the selectivity to C₂-C₄ fraction increased, while CO and CH₄ selectivity showed a reverse trend. With the increase in reaction pressure, CO₂ conversion and the selectivity to hydrocarbons increased, while the CO selectivity and O/P ratio of C₂-C₄ hydrocarbons decreased. The investigation of H2/CO₂ ratio revealed that CO₂ conversion and CH₄ selectivity increased while CO selectivity and O/P ratio of C₂-C₄ decreased with increasing H₂/CO₂ ratio.     

Effect of calcination atmosphere and temperature on γ-Al2O3 supported cobalt Fischer-Tropsch catalysts

The effect of calcination temperature and atmosphere on the properties of γ-Al₂O₃ supported cobalt Fischer-Tropsch catalysts has been investigated. One common precursor for all the catalysts was prepared by incipient wetness impregnation of γ-Al₂O₃ with an aqueous solution of cobalt nitrate hexahydrate. It was subjected to four different calcination atmospheres (air/50% steam: 30 mL/min, air: 30 mL/min, air: 50 mL/min, N₂: 30 mL/min) and eight different calcination temperatures (range: 473–723 K), making the total number of samples 32. Both the post calcination nitrogen content and the cobalt dispersion were measured. The results demonstrated that in order to maximise the cobalt dispersion, it is necessary to use low calcination temperatures and remove the precursor decomposition products (NO, NO₂, H₂O) efficiently. The Fischer-Tropsch synthesis performance of two catalysts calcined at the same temperature, but at different air flow rates was evaluated. No significant effect of the air flow rate was found on the turnover frequency or C₅₊ selectivity, but a high flow rate resulted in 30% higher activity per gram catalyst.     

Nickel-iron catalysts prepared by homogeneous deposition-precipitation of cyanide complexes on a titania support

Nickel-iron catalysts have been prepared by homogeneous deposition-precipitation of complex nickel-iron cyanides on a titania support. The nickel-iron alloys obtained after calcination and reduction of the cyanide precursors were characterized by Mössbauer spectroscopy, high-temperature X-ray diffraction, and magnetic measurements. Fischer-Tropsch experiments show remarkable results. Catalysts prepared from K₃Fe(CN)₆ and Na₂Fe(CN)₅NO cyanide precursors exhibit a high activity and selectivity, whereas catalysts prepared from K₄Fe(CN)₆ do not show any activity. This lack of activity is caused by the presence of potassium in catalysts prepared from K₄Fe(CN)₆. Potassium or iron titanate inhibits the adsorption of CO on the nickel-iron surface. Deactivation of active nickel-iron catalysts was caused by the deposition of inactive carbon during Fischer-Tropsch synthesis.     

Steric effects of alkyl dibenzothiophenes: The root cause of frustrating efficacy of heterogeneous desulfurization for real diesel

High-carbon alkyl dibenzothiophenes (DBTs) in real diesel are difficult to remove. Their steric effect has been studied and identified as the controlling factor in heterogeneous catalytic oxidation desulfurization (ODS) process. Mixed alkyl DBTs (C₂-DBT, C₂-C₂-DBT, C₃-DBT, and C₃-C₃-DBT) were synthesized via DBT's Friedel–Crafts alkylation, and their reactivity is DBT > C₂-DBT > C₂-C₂-DBT > C₃-DBT > C₃-C₃-DBT. C₃-DBT, C₂-C₂-DBT, and C₃-C₃-DBT can be hardly removed by any of three efficient heterogeneous ODS processes, but are removed efficiently in a homogeneous ODS process of HAc-HPW-H₂O₂, which contrasts the great importance of their steric effects in heterogeneous catalytic process. Thus, the frustrating efficacy of various ODS processes for real diesel is likely originated from the unidentified high-carbon alkyl DBTs and their least reactivity in heterogeneous catalytic systems. And, the steric hindrance is diminished greatly in a homogeneous catalytic process. The results are instructive for the development of new catalysts and catalytic ODS processes for real diesel.     

Catalytic hydrogenation of carbon monoxide over silica-supported Ir-Mo-Rh catalyst

We present here some results on Ir-Mo-Rh metallic catalysts for the synthesis of C₂-C₄ alcohols from syngas. It was found that Ir-Mo-Rh supported on silica containing small amounts of Rh exhibited much higher activity for CO hydrogenation than Ir-Mo bimetallic catalyst. The selectivity to various alcohols did not change very much upon the addition of Rh. The activity was greatly affected by the impregnation procedure of the metals in the catalyst preparation.