The effect of synthesis gas composition on the Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 catalysts

The Fischer–Tropsch synthesis over Co/γ-Al₂O₃ and Co–Re/γ-Al₂O₃ was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H₂/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H₂/CO ratios in the feed, the CO conversion and the CH₄ selectivity decreased, while the C₅₊ selectivity and olefin/paraffin ratio for C₂–C₄ increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H₂/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H₂/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H₂/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C₅₊, lower selectivity to CH₄, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted.     

Effect of Ru addition to Co/SiO2/HZSM-5 catalysts on Fischer–Tropsch synthesis of gasoline-range hydrocarbons

Microporous HZSM-5 zeolite and mesoporous SiO₂ supported Ru–Co catalysts of various Ru adding amounts were prepared and evaluated for Fischer–Tropsch synthesis (FTS) of gasoline-range hydrocarbons (C₅–C₁₂). The tailor-made Ru–Co/SiO₂/HZSM-5 catalysts possessed both micro- and mesopores, which accelerated hydrocracking/hydroisomerization of long-chain products and provided quick mass transfer channels respectively during FTS. In the same time, Ru increased Co reduction degree by hydrogen spillover, thus CO conversion of 62.8% and gasoline-range hydrocarbon selectivity of 47%, including more than 14% isoparaffins, were achieved simultaneously when Ru content was optimized at 1 wt% in Ru–Co/SiO₂/HZSM-5 catalyst.     

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al₂O₃ and SiO₂ supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co₃O₄ was high in the region of 0–1.64 wt.% of Ru in Co/SiO₂ catalysts. Co/γ-Al₂O₃ displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al₂O₃ catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO₂ catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.     

Modeling a slurry CSTR with Co/P–Al2O3 catalyst for Fischer–Tropsch synthesis

A kinetic model of cobalt-based Fischer–Tropsch synthesis was developed based on detailed reaction mechanisms. A mathematical model of a slurry CSTR was also constructed. Kinetic parameters and physical properties were estimated by fitting experimental data under a variety of conditions. Simulated values were in good agreement with experimental data, and the suitability of the estimated parameters was discussed by comparison with reported values. The developed model was used to evaluate the effects of operating conditions on CO conversion and the distribution of hydrocarbon products. Temperature and H₂/CO ratio were found to be important variables, affecting the entire product distribution.     

Raising Co/Al2O3 catalyst lifetime in Fischer–Tropsch synthesis by using a novel dual-bed reactor

Accelerated deactivation of 15 wt.% Co/Al₂O₃ catalyst in Fischer–Tropsch synthesis (FTS) in a single-bed and a dual-bed reactor is reported. Water was found to have a remarkable effect on the deactivation of Co/Al₂O₃ catalyst during FTS. Synthesis at higher temperatures and lower space velocities resulted in higher values of P_H2O/(P_CO + P_H2) and P_H2O/P_CO and higher catalyst deactivation rates. Water-induced back-oxidation of cobalt, cobalt–alumina interactions, irreducible cobalt aluminates formation and refractory coke formation are the main sources of deactivation. When the water to carbon monoxide plus hydrogen ratio P_H2O/(P_CO + P_H2) is greater than about 0.55 or water to carbon monoxide ratio P_H2O/P_CO is greater than about 1.5, it is not uncommon to find rapid catalyst deactivation. Separation of water and heavy hydrocarbons between the two catalytic beds of the dual-bed reactor, resulted in 62% lower catalyst deactivation rate than that of the single-bed reactor. The amount of refractory coke formation on the catalysts of the dual-bed reactor is 34% lower than that of the single-bed reactor. It was revealed that activity recovery of the used catalysts of the dual-bed is higher than that of the single-bed reactor.     

Fischer–Tropsch synthesis over Re-promoted Co supported on Al2O3, SiO2 and TiO2: Effect of water

The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al₂O₃, TiO₂ and SiO₂ have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al₂O₃ supported catalyst, while the activity of the TiO₂ and SiO₂ supported catalysts increased. However, at high concentrations of water both the SiO₂ and TiO₂ supported catalyst deactivated. Common for all catalysts was an increase in C₅₊ selectivity and a decrease in the CH₄ selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H₂ chemisorption and X-ray diffraction (XRD).     

Gasoline-range hydrocarbon synthesis over Co/SiO2/HZSM-5 catalyst with CO2-containing syngas

Selective synthesis of gasoline-range hydrocarbons (C₅-C₁₂) was investigated in a fixed-bed micro reactor using two series of CO₂-containing syngas with various mole CO₂/(CO + CO₂) and H₂/(CO + CO₂) ratios, where Fischer-Tropsch synthesis(FTS) and in situ hydrocracking/hydroisomerization were performed over bifunctional Co/SiO₂/HZSM-5 catalyst. CO₂ was converted at 0.15-0.55 of CO₂/(CO + CO₂) ratio under H₂-rich condition (H₂/(CO + CO₂) = 2.0), highest conversion of 20.3% at 0.42. Further increasing CO₂ content decreased CO₂ conversion and quite amount of CO₂ acted as diluting component. For the syngas with low H₂ content or H₂/(CO + CO₂) ratio(< 1.85, H₂/CO = 2.0), the competitive adsorption of CO, H₂ and CO₂ resulted in low CO, CO₂ and total carbon conversion, which was 57.9%, 12.7% and 31.4% respectively at 0.74 of H₂/(CO + CO₂) ratio(H₂/CO/CO₂/N₂ = 40.8/20.4/34.8/4). FTS results indicated that high H₂ content and proper H₂/(CO + CO₂) ratio were favorable for the conversion of CO₂-containing syngas. More than 45% selectivity to gasoline-range hydrocarbons including isoparaffins was obtained under the two series of syngas. It was also tested that the catalytic activity of Co/SiO₂/HZSM-5 kept stable under CO₂-containing syngas(< 7.5%). And the quick catalytic deactivation under high CO₂ containing syngas(H₂/CO/CO₂/N₂ = 45.3/23.2/27.1/3.06) was due to carbon deposition and pore blockage by heavy hydrocarbon, tested by thermal gravimetry, N₂ physisorption and scanning electron microscopy(SEM).     

Characterization of Pd catalysts supported on USY zeolites with different SiO2/Al2O3 ratios for the hydrogenation of naphthalene in the presence of benzothiophene

A series of ultra-stable Y-type (USY) zeolites with different SiO₂/Al₂O₃ ratios in the range of 10–80 were used as supports for preparing Pd/USY at 2 wt% Pd loading. The FT-IR of hydroxyl groups of USY zeolites, the n-butylamine chemisorption and the temperature-programmed desorption were used in combination to characterize the zeolite acidity. TPR, H₂-TPD and chemisorption using H₂ were used to characterize the Pd reduction and dispersion. The hydrogenation of naphthalene was conducted at 200 °C in the presence of benzothiophene at different sulfur/metal ratios. The hydrogenation activity, selectivity, and the sulfur tolerance strongly depended on the SiO₂/Al₂O₃ ratio (thus the acidity) of the zeolites. The activity decreased with increasing SiO₂/Al₂O₃ in this range. The IR and n-butylamine TPD showed that both the amount and strength of Brönsted acidity decreased with the increase of the SiO₂/Al₂O₃ ratio. The good relationship between the acidity modification and catalytic performance suggests that the sulfur tolerance of Pd/USY zeolite might be due to the desired metal-support interaction, which resulted in larger amount of electron-deficient Pd. However, as shown in TGA and TPO-IR studies, the higher hydrogenation performance on more acidic zeolite also caused higher amount of carbonaceous species on the catalyst.     

Catalytic wall reactor: Catalytic coatings of stainless steel by VOx/TiO2 and Co/SiO2 catalysts

Catalytic wall (structured) reactors and structured supports are suitable to study the catalytic properties of nanosized materials. The coating of metallic (aluminum and stainless steel) plates by thin layers of active phase is presented in two cases, VO_x/TiO₂ and Co/SiO₂, catalysts used in the oxidative dehydrogenation (ODH) of propane and in Fischer–Tropsch synthesis (FTS) of clean fuels, respectively. The preparation of coated plates and their characterisation by various methods of physicochemical analysis are described. Both chemical and physical methods were used for coating. VO_x/TiO₂ layers were obtained by grafting of Ti (on Al or stainless-steel plates) and V (on TiO₂) alkoxides and use of sol–gel media or suspension. A silica primer was deposited (on stainless-steel plate) by plasma-assisted chemical vapour deposition (PACVD) onto which Co oxide and silica were coprecipitated from sol–gel. The catalytic experiments in the respective reactions were carried out in special plate reactors and compared with those of catalytic powders. The study shows that the coating of a metallic substrate by a catalyst is not straightforward and requires specific studies dealing with both chemistry (chemical affinity between substrate and catalytic layers) and catalytic engineering (catalytic performance in taylor-made reactors).     

Fischer-Tropsch synthesis: Support and cobalt cluster size effects on kinetics over Co/Al2O3 and Co/SiO2 catalysts

The influence of support type and cobalt cluster size (i.e., with average diameters falling within the range of 8-40 nm) on the kinetics of Fischer-Tropsch synthesis (FT) were investigated by kinetic tests employing a CSTR and two Co/γ-Al₂O₃ catalysts having different average pore sizes, and two Co/SiO₂ catalysts prepared on the same support but having different loadings. A kinetic model that contains a water effect constant “m” was used to fit the experimental data obtained with all four catalysts. Kinetic parameters suggest that both support type and average Co particle size impact FT behavior. Cobalt cluster size influenced kinetic parameters such as reaction order, rate constant, and the water effect parameter. In the cluster size range studied, decreasing the average Co cluster diameter by about 30% led to an increase in the intrinsic reaction rate constant k, defined on a per g of catalyst basis, by 62-102% for the γ-Al₂O₃ and SiO₂-supported cobalt catalysts. This increase was due to the higher active Co⁰ surface site density as measured by hydrogen chemisorption. Moreover, less inhibition by adsorbed CO and greater H₂ dissociation on catalysts having smaller Co particles was suggested by the higher a and lower b values obtained for the measured reaction orders. Interestingly, irrespective of support type, the catalysts having smaller average Co particles were more sensitive to water. Comparing the catalysts having strong interactions between cobalt and support (Co/Al₂O₃) to the ones with weak interactions (Co/SiO₂), the water effect parameters were found to be positive (indicating a negative influence on CO conversion) and negative (denoting a positive effect on CO conversion), respectively. No clear trend was observed for b values among the different supports, but greater a and a/b values were observed for both Al₂O₃-supported Co catalysts, implying greater inhibition of the FT rate by strongly adsorbed CO on Co/Al₂O₃ relative to Co/SiO₂. For both supports, the order on P_CO was always found to be negative (i.e., suggesting an inhibiting effect) and positive for P_H2 for all four catalysts. The order of the reaction on P_H2 was close to 0.5, suggesting that dissociated H₂ is likely involved in the catalytic cycle. Finally, in the limited range of average pore diameters studied (13.5 and 18.2 nm), the average pore size of the Al₂O₃-supported Co catalysts displayed no observable impact on the reaction rate or water effect, suggesting either that the reaction is kinetically controlled, or that the pore size difference was not significant enough to elicit a measurable response.     

Fischer–Tropsch synthesis over Co–SiO2 catalysts prepared by the sol–gel method

Fischer–Tropsch synthesis was carried out in slurry phase over uniformly dispersed Co–SiO₂ catalysts prepared by the sol–gel method. When 0.01–1 wt.% of noble metals were added to the Co–SiO₂ catalysts, a high and stable catalytic activity was obtained over 60 h of the reaction at 503 K and 1 MPa. The addition of noble metals increased the reducibility of surface Co on the catalysts, without changing the particle size of Co metal significantly. High dispersion of metallic Co species stabilized on SiO₂ was responsible for stable activity. The uniform pore size of the catalysts was enlarged by varying the preparation conditions and by adding organic compounds such as N,N-dimethylformamide and formamide. Increased pore size resulted in decrease in CO conversion and selectivity for CO₂, a byproduct, and an increase in the olefin/paraffin ratio of the products. By modifying the surface of wide pore silica with Co–SiO₂ prepared by the sol–gel method, a bimodal pore structured catalyst was prepared. The bimodal catalyst showed high catalytic performance with reducing the amount of the expensive sol–gel Co–SiO₂.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Influence of H4SiW12O40 loading on hydrocracking activity of non-sulfide Ni-H4SiW12O40/SiO2 catalysts

The effect of H₄SiW₁₂O₄₀ loading on the catalytic performance of the reduced Ni-H₄SiW₁₂O₄₀/SiO₂ catalysts for hydrocracking of n-decane with or without the presence of thiophene and pyridine is studied. The catalysts were characterized by BET, XRD, Raman, XPS, H₂-TPR, H₂-TPD, NH₃-TPD and FT-IR of pyridine adsorption. It was found that addition of H₄SiW₁₂O₄₀ to the system increases the catalytic activity and the promoting effect is a function of the H₄SiW₁₂O₄₀ loading. The best result was obtained on 5%Ni-50%H₄SiW₁₂O₄₀/SiO₂ catalyst which shows the highest activity for hydrocracking of n-decane and excellent tolerance to the sulfur and nitrogen compounds in the feedstock. The results showed that a suitable amount of H₄SiW₁₂O₄₀ loading on the 5%Ni/SiO₂ catalyst increases the amount of both hydrogen adsorbed and Brønsted acid and Lewis acid sites on the catalyst. The high catalytic performance of the catalyst can be related to the nature of H₄SiW₁₂O₄₀ and the proper balance between metal and acid functions.     

Syngas production via combined oxy-CO2 reforming of methane over Gd2O3-modified Ni/SiO2 catalysts in a fluidized-bed reactor

In this research, Ni/SiO₂ catalyst was modified with different amount of Gd₂O₃ and characterized with temperature-programmed desorption of CO₂ (CO₂-TPD) and NH₃ (NH₃-TPD), temperature-programmed reduction with H₂ (H₂-TPR) and X-ray diffraction (XRD). It was found that Gd₂O₃-modified Ni/SiO₂ catalysts possessed higher CO₂ adsorption and activation ability due to the formation of surface carbonate species. H₂-TPR and XRD characterizations found that the strong interaction among nickel, Gd₂O₃ and SiO₂ took place, which improved the dispersion of Ni. Gd₂O₃-modified Ni/SiO₂ catalysts exhibited higher activity and stability for the combined oxy-CO₂ reforming of methane in fluidized-bed reactor. The H₂/CO ratio in produced syngas could be controlled via controlling reaction temperature and CO₂/O₂ ratio in feed.     

Intrinsic kinetics of Fischer–Tropsch reactions over an industrial Co–Ru/γ-Al2O3 catalyst in slurry phase reactor

The rate of Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al₂O₃ catalyst was studied in a laboratory well mixed, continuous flow, slurry reactor under the conditions relevant to industrial operations as follows: temperature of 200–240 °C, pressure of 20–35 bar, H₂/CO feed ratio of 1.0–2.5, gas hourly space velocity of 500–1500 N cm³ g_cat^− 1 h^− 1 and conversions of 10–84% of carbon monoxide and 13–89% of hydrogen. The ranges of partial pressures of CO and H₂ have been chosen as 5–15 and 10–25 bar respectively. Five kinetic models are considered: one empirical power law model and four variations of the Langmuir–Hinshelwood–Hougen–Watson representation. All models considered incorporate a strong inhibition due to CO adsorption. The data of this study are fitted fairly well by a simple LHHW form − R_H2 + CO = ap_H2^0.988p_CO^0.508 / (1 + bp_CO^0.508)² in comparison to fits of the same data by several other representative LHHW rate forms proposed in other works. The apparent activation energy was 94–103 kJ/mol. Kinetic parameters are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Also, the performance of the catalyst for Fischer–Tropsch synthesis and the hydrocarbon product distributions were investigated under different reaction conditions.     

Performance and characterization of Ru/Al2O3 and Ru/SiO2 catalysts modified with Mn for Fischer–Tropsch synthesis

Mn effect and characterization on γ-Al₂O₃-, -Al₂O₃- and SiO₂-supported Ru catalysts were investigated for Fischer–Tropsch synthesis under pressurized conditions. In the slurry phase Fischer–Tropsch reaction, γ-Al₂O₃ catalysts showed higher performance on CO conversion and C₅⁺ selectivity than -Al₂O₃ and SiO₂ catalysts. Moreover, Ru/Mn/γ-Al₂O₃ exhibited high resistance to catalyst deactivation and other catalysts were deactivated during the reaction. From characterization results on XRD, TPR, TEM, XPS and pore distribution, Ru particles were clearly observed over the catalysts, and γ-Al₂O₃ catalysts showed a moderate pore and particle size such as 8 nm, where -Al₂O₃ and SiO₂ showed highly dispersed ruthenium particles. The addition of Mn to γ-Al₂O₃ enhanced the removal of chloride from RuCl₃, which can lead to the formation of metallic Ru with moderate particle size, which would be an active site for Fischer–Tropsch reaction. Concomitantly, manganese chloride is formed. These schemes can be assigned to the stable nature of Ru/Mn/γ-Al₂O₃ catalyst.     

Reaction performance and characterization of Co/Al2O3 Fischer–Tropsch catalysts promoted with Pt, Pd and Ru

A series of noble metal (Pt, Ru or Pd) promoted Co/Al₂O₃ catalysts were prepared by sequential impregnation method. The catalysts were characterized by XRD, TPR, H₂-TPD and TPSR techniques, and their catalytic performance in Fischer–Tropsch synthesis was investigated in a fixed-bed reactor. The results of activity measurements show that the addition of small amounts of noble metal greatly improved the activity of the Co/Al₂O₃ catalyst. TPR experimental results demonstrate that hydrogen spillover from the noble metal to cobalt oxide clusters facilitated the reduction of cobalt oxide and, thus significantly increased the reducibility of Co/Al₂O₃ catalyst. The presence of noble metal increased the amount of chemisorbed hydrogen and weakened the bond strength of Co–H. TPSR results indicate that CO was adsorbed in a more reactive state on the promoted catalysts.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Evidence of strong metal–oxide interactions in promoted Rh/SiO2 on CO hydrogenation: Analysis at the site level using SSITKA

It has been suggested that the behavior of Group VIII metal catalysts supported on transition metal oxides can be significantly affected by pretreatment conditions due to strong metal–oxide interactions (SMOI). However, the origins for the SMOI effect are still in debate. In this research, SMOI of Rh and vanadium oxide (as a promoter) supported on SiO₂ were studied at the site level for the first time, which provides an insight into the modification of surface properties after high temperature reduction. H₂ chemisorption, Fischer–Tropsch synthesis (FTS), and SSITKA (steady-state isotopic transient kinetic analysis) were used to probe the SMOI effects. The catalytic properties of the catalysts for CO hydrogenation were investigated using a differential fixed bed reactor at 230 °C and 1.8 atm, while for SSITKA, a reaction temperature of 280 °C and an excess of H₂ was used to maximize methane production. The addition of V to Rh/SiO₂ suppresses H₂ chemisorption, and high reduction temperature further decreases H₂ chemisorption on Rh/V/SiO₂ but has little effect on Rh/SiO₂. As reduction temperature increases, the activity for CO hydrogenation on Rh/SiO₂ remains essentially unchanged, but the activity of Rh/V/SiO₂ decreases significantly. SSITKA shows that the concentration of surface reaction intermediates decreases on Rh/V/SiO₂ as the reduction temperature increases, but the activities of the reaction sites increase. The results suggest that Rh being covered by VO_x species is probably the main reason for the decreased overall activity induced by high reduction temperature, but more active sites appear to be formed probably at the Rh–VO_x interface.