Cobalt Fischer Tropsch Synthesis Research Articles

PROSPECTS FOR USING A COBALT CATALYST FOR FISHER-TROPSCH SYNTHESIS BASED ON SYNTHETIC SILICA ALUMINA

The possibility of the use of commercially available silica-alumina hydrate Siral-40, containing 40 wt.% of SiO2, in the composition of a composite cobalt Fischer–Tropsch synthesis (FTS) catalyst was studied. The use of silica aluminas may become a new promising direction in the development of catalysts for a shortened technological chain based on the use of bifunctional catalysts that make it possible to eliminate the hydroprocessing stage. In addition, such catalysts can be useful for expanding the range of products obtained, including those unconventional for single-reactor Fischer–Tropsch synthesis. The initial silica alumina powder was pretreated in air flow at different temperatures to adjust the concentration of acid cites on its surface. The initial and calcined powders were studied using X-ray diffraction analysis, IR spectroscopy and sorption methods. The catalysts were investigated by diffraction and sorption methods. It has been shown that an increase in the calcination temperature of the initial Siral-40 powder leads to a decrease in the specific surface area and pore volume of catalysts based on it, with temperatures above 900 °C having the greatest effect. The synthesized catalysts were active in FTS, and the composition of the obtained C5+ hydrocarbons depended more on the properties of the Siral-40 powder than the catalytic performance. The composition of C5+ hydrocarbons, obtained in the presence of all investigated catalysts contained at least 60% of diesel fraction and more than 30% of wide base oil fraction. Thus, synthetic amorphous silica aluminas are a promising component for new Fischer–Tropsch synthesis catalysts, which make it possible to obtain a wide range of products without the use of a deep hydroprocessing stage in the technological chain. For citation: Sineva L.V., Asalieva E.Yu., Gryaznov K.O., Mordkovich V.Z. Prospects for using a cobalt catalyst for Fisher-Tropsch synthesis based on synthetic silica alumina. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 10. P. 88-98. DOI: 10.6060/ivkkt.20246710.7y.

Effect of different alkaline earth metals on the adsorption and catalytic behavior of cobalt Fischer–Tropsch synthesis

Alkali earth metals are effective promoters in cobalt-based Fischer–Tropsch synthesis. To compare the similarity and difference of the effect of the same main group of alkali earth metals on Fischer–Tropsch reaction, a series of alkali earth metals-modified alumina was prepared and applied as the supports for Co catalysts. Promotion of Co/Al2O3 catalyst by alkali earth metals achieved a low CH4 selectivity,high C5+ selectivity and C3 o/p due to the lower H*/CO* ratio and enhanced CO dissociation on the surface of catalyst than the unmodified one.

The water assisted vinylene mechanism for cobalt Fischer-Tropsch synthesis assessed by multi-catalyst modelling of kinetics and deactivation

The paper describes development of a mechanism and a consistent rate expression for Fischer-Tropsch (FT) synthesis over cobalt-based catalysts. The developed mechanism relies on a two-step hydrogen assisted activation of CO. The carbon atom of CO is first hydrogenated by surface hydrogen to formyl; followed by the rate-limiting step whereby the oxygen atom is hydrogenated by adsorbed water. The produced CH* monomer is incorporated into the growing chain giving vinylene intermediate. The vinylene intermediate is either terminated to an olefin by adding hydrogen to the α-carbon atom or propagates by adding hydrogen to the β-carbon position. The resulting expression for CO consumption, the Fischer-Tropsch rate, can respond positively or negatively to the partial pressure of water, in agreement with experimental observations. A special feature is that the chain propagation probability does not depend on the partial pressure of hydrogen. The resulting kinetic model is tested on several cobalt catalysts supported on alumina; spanning from γ-alumina with average pore sizes ranging from 6.1 to 18.3 nm to α-alumina with a wide pore structure; and with cobalt particle sizes from 8 to 19 nm. Water was added sequentially to the syngas feed, causing enhanced deactivation, for testing the water response on activity and selectivity. A deactivation model comprising sintering and cobalt oxidation, and the FT-kinetics, describe the observed CO conversions with great precision for all catalysts. Selectivities are also well described, but with slight deviations at least partly due the effect of deactivation. Trends in some of the kinetic parameters are rationalized in terms of cobalt crystallite and pore sizes.

The Water Assisted Vinylene Mechanism for Cobalt Fischer-Tropsch Synthesis Assessed by Multi-Catalyst Modelling of Kinetics and Deactivation

The Water Assisted Vinylene Mechanism for Cobalt Fischer-Tropsch Synthesis Assessed by Multi-Catalyst Modelling of Kinetics and Deactivation

Fischer–Tropsch Synthesis: Study of Different Carbon Materials as Cobalt Catalyst Support

In this work, cobalt Fischer–Tropsch synthesis (FTS) catalyst supported on various carbon materials, i.e., carbon nanotube (CNT), activated carbon (AC), graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanofiber (CNF), were prepared via impregnation method. Based on TGA, nitrogen physisorption, XRD, Raman spectroscopy, H2-TPR, NH3-TPD, ICP, SEM, and TEM characterization, it is confirmed that Co3O4 particles are dispersed uniformly on the supports of carbon nanotube, activated carbon and carbon nanofiber. Furthermore, the FT catalyst performance for as-prepared catalysts was evaluated in a fixed-bed reactor under the condition of H2:CO = 2:1, 5 SL·h−1·g−1, 2.5 MPa, and 210 °C. Interestingly, the defined three types of carbon materials exhibit superior performance and dispersion compared with graphene oxide and reduced graphene oxide. The thermal stability and pore structure of the five carbon materials vary markedly, and H2-TPR result shows that the metal–support interaction is in the order of Co/GO > Co/CNT > Co/AC > Co/CNF > Co/rGO. In brief, the carbon nanofiber-supported cobalt catalyst showed the best dispersion, the highest CO conversion, and the lowest gas product but the highest heavy hydrocarbons (C5+) selectivity, which can be attributed to the intrinsic property of CNF material that can affect the catalytic performance in a complicated way. This work will open up a new gateway for cobalt support catalysts on various carbon-based materials for Fischer–Tropsch Synthesis.

Zeolites as a tool for intensification of mass transfer on the surface of a cobalt Fischer–Tropsch synthesis catalyst

Zeolite-containing Co catalysts for Fischer–Tropsch synthesis (FTS) were investigated in catalytic transformations of liquid hydrocarbons thus emulating secondary transformations of FTS products. Three different zeolites were employed, namely HBeta, HZSM-5 and HY. The conditions for catalytic testing were similar to those observed during FTS, i.e. typical temperatures in 170–260 °C range and the presence of water as a main byproduct of FTS. The results of hydrocarbon transformation investigation were analyzed vs performance of those same catalysts in FTS. It was established that all three investigated zeolites caused lower average molecular weight of the products of both FTS and liquid hydrocarbon transformations thus clearing the pore system of the catalyst from heavier hydrocarbons. It is remarkable that, despite the average molecular weight drop, the introduction of zeolites does not lead to increase in methane selectivity at Co catalysts in FTS. It was shown, in particular, that the composition of FTS-generated liquid hydrocarbons C5+ depends primarily on the properties of a particular zeolite emplyed in the Co catalyst, more precisely on the number and availability of Brønsted acid centers since these centers determine the activity of a catalyst in cracking and isomerization. Cracking was revealed to be decisive in elimination of wax-like heavy hydrocarbons while isomerization and oigomerization balanced both molecular weight distribution and group composition. Thus, zeolites can be instrumental in controlling composition of the products of Fischer–Tropsch synthesis produced over hybrid catalysts due to intensification of mass transfer.

Exploring the reasons for Zr-improved performance of alumina supported cobalt fischer-tropsch synthesis

In order to investigate the enhancement of Zr in FTS performance, a series of Zr-modified 20 wt% Co/Al2O3 catalysts has been prepared by regulating the Zr and Co impregnation order and Zr amount and then the catalytic performance were evaluated in a fixed reactor. The Zr addition leads to a significant increase in activity, olefin/paraffin ratio for C3 and C4 at a similar CO conversion, and selectivity to C5+ compared with the Co/Al2O3 catalysts. The co-impregnation of Zr and Co shows the most attractive catalytic performance. This can be attributed to the higher reducibility of the catalyst as well as the formation of Co–Zr interface. The introduction of Zr can enhance the interaction between C∗ and cobalt active sites and weaken the CO bond, making CO easier to be dissociated and the selectivity of C5+ is improved.

Isomorphic titanium-substituted mesoporous SBA-16 as support for cobalt Fischer–Tropsch synthesis catalysts: balance between dispersion and reduction

The delicate balance between dispersion and reduction of the Co-based Fischer–Tropsch synthesis catalyst is the golden key to enhancing catalytic performance, which highly depends on an optimized metal–support interaction.

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).

Fischer–Tropsch Synthesis over a Cobalt Catalyst Supported on Titania-Doped Silicon Carbide

A cobalt Fischer–Tropsch synthesis catalyst supported on titania-doped silicon carbide is synthesized and comprehensively studied by a number of physicochemical means. The dynamics of the behavior and deactivation of the catalyst was studied after its stable operation for 1500 h during continuous experimental resource testing with a periodic change in the syngas flow rate and pressure and the process temperature. This catalyst has two types of active sites, on which products with different Schulz–Flory distribution parameters form. Depending on the process regime, the effective formation of soft waxes or more high-molecular-weight products is possible.

Fischer-Tropsch synthesis: Effect of nitric acid pretreatment on graphene-supported cobalt catalyst

As a support for cobalt Fischer-Tropsch synthesis catalyst, graphene needs to be pretreated to modify its surface properties for desired catalytic performance. Graphene was pretreated by thermal reflux using 20–60 % nitric acid. The pretreated graphene and catalysts were characterized with TPR, TEM, SEM, XRD, XPS and Raman spectroscopy to study the effect of treatment on microstructures of the materials. This work showed that cobalt metal uniformly dispersed on graphene, and with increasing pretreating acid concentration, surface defects increased and the cobalt particle size decreased, suggesting alleviated agglomeration of cobalt particles on graphene. Pretreating improved the reduction of the catalyst due to the increased defect sites, improved support-metal interaction and cobalt dispersion. Pretreated catalyst yielded better activity, higher C5+, but lower methane selectivity than unpretreated catalyst. Therefore, acid pretreatment of graphene can be an effective approach to modify the microstructure of the material, and can thus enhance the performance of the prepared catalyst.

Amino-Ended Hyperbranched Polyamide Modified SBA-15 as Support for Highly Efficient Cobalt Fischer-Tropsch Synthesis Catalyst

Among the Fischer-Tropsch synthesis (FTS) catalysts, Cobalt catalysts are currently attracting a lot of research interests due to their high activity and high selectivity. But the dispersion and reducibility of cobalt catalysts with moderate interaction between cobalt and support are still a challenge. A novel amino-ended hyperbranched polyamide (AEHPA) was used to modify SBA-15 and then was applied as support for obtaining cobalt catalyst (15Co/SBA-15-N). The catalysts were characterized by XRD, TEM, XPS and H2-TPR techniques. The results showed that AEHPA doped mesoporous SBA-15 caused the generation of N species in the SBA-15 pore channels. The N-Co bonds resulted in the formation of highly-dispersed cobalt nanoparticles with uniform sizes inside the ordered mesopores of support. AEHPA doping was an effective way to modify the surface properties of the SBA-15 for immobilizing cobalt nanoparticles. Compared with the conventional 15Co/SBA-15 catalyst without doping AEHPA, the AEHPA doped 15Co/SBA-15-N catalyst showed improved cobalt dispersion and stabilized cobalt location, which led to much better reaction stability as well as C5+ selectivity.

Loading and promoter effects on the performance of nitrogen functionalized graphene nanosheets supported cobalt Fischer-Tropsch synthesis catalysts

The effects of nitrogen functional groups on graphene surface and also the effects of cobalt loading and ruthenium promoter on the performance of nitrogen functionalized graphene nanosheets (N-GNS) supported cobalt catalysts in Fischer–Tropsch synthesis (FTS) are investigated. A 15 wt% Co/PGNS catalyst, a series of Co/N-GNS (15–30 wt% loading) and ruthenium promoted catalysts (25 wt% Co,0.5 wt% Ru/N-GNS and 25 wt% Co,0.5 wt% Ru/PGNS) were prepared by impregnation method. The purified GNS and functionalized GNS and all catalysts were characterized by Raman spectroscopy, FTIR, SEM, EDS, ICP, BET, TEM, XRD and TPR. The catalysts assessed in FTS in a fixed bed micro-reactor at 220 °C, 1.8 Mpa and H2/CO ratio of 2. Functionalization of GNS shifted the TPR reduction peaks to lower temperature, increased the dispersion of cobalt particles and increased the percentage CO conversion from 70.6% to 74.5%. Increasing the cobalt loading resulted in increasing the average cobalt cluster size, improvements in the reducibility of Co3O4. The maximum FTS activity for N-GNS supported catalyst is achieved at 30 wt % cobalt loading. The C5+ selectivity for the 30 wt % cobalt catalyst was higher than that of the 15 wt % Cobalt catalyst. Addition of 0.5 wt%Ru increased the FTS rate (gCH/(gcat·h)) from 0.377 to 0.412 and the liquid products selectivity from 86.5% to 91.2%.

Effect of ammonia on cobalt Fischer–Tropsch synthesis catalysts: a surface science approach

Undercoordinated (defect) sites on cobalt surfaces favor NH3 decomposition and dehydrogenated products adsorb strongly around these defect sites.

Direct conversion of syngas to isoparaffins over hierarchical beta zeolite supported cobalt catalyst for Fischer-Tropsch synthesis

A hierarchical micro-meso-macroporous Beta zeolite (HB) was developed by a steam-assisted crystallization method and was employed for cobalt Fischer-Tropsch synthesis. Meso-macroporous silica (HS), alumina modified meso-macroporous silica (HAS) and conventional Beta zeolite (CB) were used for comparison. Structural characterization indicated that the as-synthesized HB exhibited nanosized Beta zeolite crystallites and a hierarchical micro-meso-macroporous structure. The HB supported catalyst showed very high isoparaffins selectivity because of the isomerization reaction at the strong acidic sites of the Beta zeolite, while due to its good macroporous structure, which is in favor of reducing the internal diffusion limitations, the methane selectivity was kept as low as HAS supported catalysts. In this paper, we use large particle catalysts for evaluation, which are very close to industrial condition. And the conclusions obtained may be very important for the design of new Fischer-Tropsch catalysts.

Effects of Ag promotion and preparation method on cobalt Fischer-Tropsch catalysts supported on silica-modified alumina

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100 °C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ∼0.050 s−1 (equivalent to a CO consumption rate of ∼65 mmol gcat−1 h−1), which are 20–30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with essentially the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.

Consorted Vinylene Mechanism for Cobalt Fischer–Tropsch Synthesis Encompassing Water or Hydroxyl Assisted CO-Activation

A mechanistic model for Fischer–Tropsch synthesis is proposed. The model contains four key steps. The first step is hydrogen assisted CO activation to formyl. This is followed by hydrogen transfer from either water or hydroxyl to a hydroxycarbene intermediate to generate CH* surface species on cobalt as chain building blocks. Further, chain propagation is by adding methylidyne (CH*) to an alkylidene (CHCH2R) chain to create a vinylene (CHCHR) chain end unit. Last, hydrogenation finalizes the chain-building step or terminates the chain depending on whether hydrogen is added to the β- or α-carbon atom, respectively. The result is that the main product is α-olefins. Chain growth probability is independent of hydrogen partial pressure and depends solely on the surface coverage of CH* monomers. Short-chain (Me) branching and variation in chain growth with chain length are also rationalized. Derived kinetic equations depend on details of the mechanism, but generally encompass water in the expressions and can account for both positive and negative water responses. The expression for the Anderson–Schulz–Flory (ASF) α is dependent on CO, and can contains the water vapor pressure as well, in a way consistent with higher α when more CO and water are present for production of CH* chain growth monomers.

Fischer–Tropsch Synthesis Bifunctional Catalysts: Cobalt Supported on 3D Mesoporous Cellular Silica Foams Assembled by Using ZSM‐5 Seeds

AbstractMesoporous cellular silica foams (MCF) have 3D pore networks in which large spherical cell pores interconnect with small window pores. This novel characteristic provides an open pore structure that favors the accessibility of reactants and mass transfer in reactions. In this work, MCF was functionalized by assembling ZSM‐5 seeds to improve the acidity and the ZSM seed was found to disperse in the MCF structure homogeneously (MCF‐Z). The obtained MCF‐Z materials showed high surface acidity and were then used as supports for cobalt Fischer–Tropsch synthesis (FTS) bifunctional catalysts (20 wt % Co). Compared with Co/MCF catalyst, the Co/MCF‐Z catalysts showed higher CO conversion and improved durability in FTS at 260 °C and 1.0 MPa. Moreover, on increasing the amount of ZSM‐5 seed, the activity of Co/MCF‐Z catalysts gradually increased and then decreased. Co/MCF‐Z catalyst assembled with the highest amount of ZSM‐5 seed (Al/Si=1.5 %, molar ratio) exhibited the lowest deactivation rate, which was owing to the superior stability of the MCF‐Z support and an increased interaction between cobalt and MCF‐Z. The Co/MCF‐Z bifunctional catalyst showed higher iso‐paraffin/n‐paraffin ratios, higher olefin/paraffin ratios as well as lower C2–C4 alcohol selectivity in comparison to Co/MCF catalyst. For the Co/MCF‐Z catalyst, the decrease in space velocity led to hydrocarbon selectivity shifting toward middle distillate products.

Perspectives on the Effect of Water in Cobalt Fischer–Tropsch Synthesis

Water is an inherent component in Fischer–Tropsch synthesis on Co catalysts, and the effect of water is discussed with an emphasis on alumina and aluminates as support materials. Water may affect the selectivity, activity, deactivation, and state of the catalyst, and without exception, water is known to enhance the C5+ selectivity by increasing the chain propagation α value. The effect of water depends on the catalyst. Small-pore γ-Al2O3 is less efficient at high water content than large-pore γ-Al2O3. The effect of water on selectivity is independent of its origin: i.e., adding water to the feed has the same effect as water produced by the reaction. Arguments are provided for the effect of water being partially mechanistic in nature and occasionally due to pore condensation. In particular, we introduce water-assisted CO activation to methylidyne as an option for generation of polymerization monomers. An additional factor that needs to be considered is that high water partial pressure is concurrent with re...

Enhancement of performance and stability of Graphene nano sheets supported cobalt catalyst in Fischer–Tropsch synthesis using Graphene functionalization

Graphene nano sheets (GNS) were functionalized and used as cobalt Fischer–Tropsch synthesis (FTS) catalyst support. The effects of nitrogen functional groups on graphene surface on the activity, selectivity and stability of cobalt catalyst in FTS was investigated using a fixed bed micro-reactor. 15wt.% of cobalt was loaded on the supports by impregnation method. Physico-chemical properties of pure and functionalized graphene, calcined fresh and used catalysts were studied by Raman spectroscopy, FTIR, BET, XRD, TEM, TPR and H2chemisorption techniques. According to the TEM and H2chemisorption tests, 480hrs continuous FT synthesis increased the average cobalt particle size from about 7.8 to 8.8nm for Co/GNS catalyst and from about 6.8 to 7.2nm for Co/N-GNS catalyst. The proposed cobalt catalyst supported on functionalized GNS increased the initial %CO conversion from 70.6 to 74.5. The CO conversion over the Co/N-GNS after 480hrs on stream decreased by 4.3% and that for Co/GNS decreased by 6.2%. For the Co/N-GNS catalyst 0.6% of total activity loss and for the Co/GNS catalyst 3.1% of total activity loss cannot be recovered after regeneration of the catalyst at the same conditions of the first regeneration step.Catalysts supported on functionalized GNS showed better stability.