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.     

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: C labeled 1-alkene conversion using supercritical conditions with Co/A12O3

Under a pressure tuned supercritical fluid mixture of pentane/hexane as the media and using Co/A1₂O₃ as the catalyst, the Fischer-Tropsch synthesis (FTS) reaction was conducted in a fixed bed reactor. ¹⁴C-labeled decene, tetradecene, and nonadecene were chosen as probe compounds to study the chemical behavior of the added mid-carbon number olefin under FTS supercritical reaction conditions. In the case of the run with decene, 21.8% of the added decene was reduced to decane, 73.5% remained unchanged, and only 4.7% of the added decene was incorporated into higher carbon number compounds. For the tetradecene and nonadecene runs, the percentages of reduction of the added olefin are 23.4 and 2.5%, while the percentages of incorporation are 2.6 and 1.5%, respectively. The molar radioactivity of the incorporated compounds decreases with increasing molecular size, indicating the presence of some accumulated products under these reaction conditions. The α value calculated based on the relative radioactivity of the incorporated compounds is 0.62, which is smaller than the value obtained from normal FTS under the same reaction conditions. These results indicate that the added mid-carbon number olefin cannot significantly alter the product distribution in favoring the heavier products.     

Experimental studies and modeling of a fixed bed reactor for Fischer-Tropsch synthesis using biosyngas

The aim of this work was to study the Fischer-Tropsch (FT) synthesis of a model biosyngas (33% H₂, 17% CO and 50% N₂) in a single tube fixed-bed FT reactor. The FT reactor consisted of a shell and tube with high-pressure boiling water circulating throughout the shell. A spherical unpromoted cobalt catalyst was used with the following reaction conditions: a wall temperature of 473 K, a pressure of 20 bars and a gas hour space velocity (GHSV) of 37 to 180 NmL.g_cat^− 1.h^− 1. The performance of the FT reactor was also validated by developing a 2D pseudo-homogeneous model that includes transport equations and reaction rate equations. Good agreement between the model predictions and experimental results were obtained. This developed model was extended to predict and quantify the influence of the FT kinetics as well as determine the influence of the tube diameter and the wall temperature. The predicted behaviors for CO and H₂ conversion, productivity of hydrocarbons (mainly CH₄ and C₅₊) and fluid temperature along the axis of the reactor have been analyzed.     

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 with Co/SiO2–Al2O3 catalyst and steady-state modeling using artificial neural networks

This paper reports the results of an experimental study involving Fischer–Tropsch synthesis on Co/SiO₂–Al₂O₃ catalyst. The objective of the study was to find the reaction conditions for achieving an optimal selectivity with respect to -liquid hydrocarbons. The experimental data on reaction conversion and steady-state concentrations of different product species has been used to develop artificial neural-network-based models which are generic and can be used for predicting the reaction behaviour under different operating conditions.     

Fischer-Tropsch diesel production over calcium-promoted Co/alumina catalyst: Effect of reaction conditions

The effects of reaction conditions on the Fischer-Tropsch activity and product distribution of an alkali-earth metal promoted cobalt based catalyst were studied. The influence of the promoter on the reducibility and cobalt particle size was studied by different techniques, including N₂ adsorption, X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption and acid-base titrations. Experiments were carried out on a bench-scale fixed bed reactor and catalysts were prepared by incipient wetness impregnation. It was observed that addition of a small amount of calcium oxide as a promoter (0.6 wt.%) improved the cobalt oxide reducibility and reduced the formation of cobalt-aluminate species. A positive correlation between basicity and particle size was observed. In terms of FTS results, CO conversion and selectivity were found to be enhanced by the addition of this promoter. It was important to note that the addition of calcium shifted the distribution to mainly C₁₆-C₁₈ hydrocarbons fraction, which could be greatly considered for a diesel formulation. Furthermore, the variation of the reaction conditions seemed to influence product distribution in a lesser extent than unpromoted catalyst. Also, a displacement of hydrocarbon distribution to higher molecular weight with decreasing space velocity and temperature was observed. Moreover, the addition of calcium to the cobalt based catalyst was found to greatly maintain selectivity to for a wide range of H₂/CO molar ratios.     

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.     

Reaction modeling on the phosphorous-treated Ru/Co/Zr/SiO2 Fischer-Tropsch catalyst with the estimation of kinetic parameters and hydrocarbon distribution

The catalytic performances and the kinetics model of the cobalt-based Fischer-Tropsch (FT) catalyst were investigated on Ru/Co/Zr/SiO₂ with different amounts of phosphorous. The preliminary analysis of the CO conversion, the hydrocarbon selectivity and the activation energy revealed that the 0.5 wt.% phosphorous-treated Ru/Co/Zr/SiO₂ catalyst was found to have the highest catalytic activity because of the good dispersion of the cobalt species and the suppressed aggregation during the FT reaction. Kinetics model was concomitantly developed on the selected catalyst. The model that was reported in the literature was modified in order to explain the characteristics that were observed in the experimental studies, and the kinetic parameters were estimated to fit the experimental data under various reaction conditions. The simulation results along with the estimated kinetic parameters satisfactorily predicted the effects of the operating conditions on both the CO conversion and the entire hydrocarbon distribution.     

费托合成铁基催化剂浆态床反应性能的研究

为了获得工艺参数对铁基催化剂费托合成产品分布的影响规律,在浆态床反应器中考察了反应温度、反应压力、氢碳比、空速对铁基催化剂费托合成反应性能的影响。结果表明,温度升高时,催化剂活性、CO_2和CH_4选择性均升高,产物向轻组分分布;压力增大时,催化剂活性和CO_2选择性升高,CH_4选择性下降,产物向重组分分布;随氢碳比的增加,催化剂活性和CH_4选择性升高,CO_2选择性下降,C_(5+)呈下降趋势;随空速增加,催化剂的活性和CO_2选择性下降,CH_4选择性上升,C_(5+)向轻质烃分布。选择合适的工艺条件,可有效改善铁基催化剂的费托合成反应性能,控制碳链长度和产物的分布,提高费托合成反应的经济性。     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

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).     

干燥温度对Co/SiO2费托合成催化剂结构和性能的影响

采用浸渍法制备了钴负载质量分数15％的Co/SiO2催化剂,考察干燥温度对催化剂性能的影响.随着干燥温度升高,催化剂活性先增大后减小,但对产物选择性没有明显影响.BET、SEM、XRD和H2 - TPR测定结果表明,改变干燥温度可以影响钴在催化剂中的分布状态,从而使催化剂结构、晶粒度和还原性能发生相应变化,这些效应的叠...     

Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/Al2O3 in slurry reactor

The Fe₂O₃/Al₂O₃ catalyst was studied to selectively synthesize mixed alcohols from syngas in a continuously stirred slurry reactor with the oxygenated solvent Polyethylene Glycol-400 (PEG-400). The selectivity of mixed alcohols in the products reached as high as 95 wt.% and the C₂₊ alcohols (mainly ethanol) was more than 40 wt.% in the total alcohol products at the reaction conditions of 250 °C, 3.0 MPa, H₂/CO = 2 and space velocity = 360 ml/g_cat h. The hydrogen temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) measurements of the catalyst confirmed that the FeO phase was responsible for the high selectivity to mixed alcohols in the process. And the oxygenated solvent PEG-400 was also necessary for the selective synthesis of mixed alcohols in the reaction system.     

Catalytic oxidation of naphthalene using a Pt/Al2O3 catalyst

Polycylic aromatic hydrocarbons (PAHs) are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs in the environment stem from the atmospheric deposition and diesel emission. Consequently, the elimination of PAHs in the off-gases is one of the priority and emerging challenges. Catalytic oxidation has been widely used in the destruction of organic compounds due to its high efficiency (or conversion of reactants), its economic benefits and good applicability.

This study investigates the application of the catalytic oxidation using Pt/γ-Al₂O₃ catalysts to decompose PAHs and taking naphthalene (the simplest and least toxic PAH) as a target compound. It studies the relationships between conversion, operating parameters and relevant factors such as treatment temperatures, catalyst sizes and space velocities. Also, a related reaction kinetic expression is proposed to provide a simplified expression of the relevant kinetic parameters.

The results indicate that the Pt/γ-Al₂O₃ catalyst used accelerates the reaction rate of the decomposition of naphthalene and decreases the reaction temperature. A high conversion (over 95%) can be achieved at a moderate reaction temperature of 480 K and space velocity below 35,000 h⁻¹. Non-catalytic (thermal) oxidation achieves the same conversion at a temperature beyond 1000 K. The results also indicate that Rideal–Eley mechanism and Arrhenius equation can be reasonably applied to describe the data by using the pseudo-first-order reaction kinetic equation with activation energy of 149.97 kJ/mol and frequency factor equal to 3.26 × 10¹⁷ s⁻¹.     

Hydrodesulfurization of model diesel using Pt/Al2O3 catalysts prepared by supercritical deposition

Pt/Al₂O₃ catalysts with Pt loadings ranging from 0.5 to 11 wt.% were synthesized by supercritical carbon dioxide (scCO₂) deposition method. Transmission electron microscopy (TEM) images showed that the synthesized catalysts contained small Pt nanoparticles (1–4 nm in diameter) with a narrow size distribution, no observable agglomeration, and uniformly dispersed on the alumina support. The catalysts were found to be active for hydrodesulfurization of dibenzothiophene (DBT) dissolved in n-hexadecane (n-HD) without sulfiding the metal phase. The reaction proceeded only via the direct hydrogenolysis route in the temperature range 310–400 °C and at atmospheric pressure. The activity increased with increasing the metal loading. Increasing [H₂]₀/[DBT]₀ by either increasing [H₂]₀ or decreasing [DBT]₀, increased the DBT conversion. At a fixed weight hourly space velocity and feed concentration, conversion did not increase with increasing temperature beyond 330 °C. The presence of toluene inhibited the catalyst activity presumably due to competitive adsorption between DBT and toluene. Under the operating conditions, the reaction was far from equilibrium.     

The modification of SiO2 by various organic groups and its influence on the properties of cobalt-based catalysts for Fischer-Tropsch synthesis

SiO₂ was modified by various organic groups before the impregnation of cobalt precursor. These modified supports and the corresponding catalysts were characterized by BET, ²⁹Si CP MAS NMR, XRD, Raman, XPS and H₂-TPR. These characterizations clearly show the changes of morphology as well as reducibility of the catalysts. The organic modification of SiO₂ remarkably influences the reducibility and catalytic properties of Co catalysts. Co catalyst supported on (CH₃)₃-modified SiO₂ exhibits high activity and C₅⁺ hydrocarbon selectivity. However, COOH-, NH₂-, and NH₂(CH₂)₂NH-modified SiO₂ distinctly suppress the catalytic activity of Co catalysts.     

Preparation and structural characterization of Co/Al2O3 catalysts for the ozonation of pyruvic acid

A series of Co/Al₂O₃ catalysts were prepared by the incipient wetness impregnation method using γ-Al₂O₃ support and (CH₃COO)₂Co·4H₂O solutions, followed by calcination at 500–800 °C. Characterization of catalysts was accomplished by several techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), physisorption of nitrogen, mercury and helium-based pycnometries, Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and pH of zero charge (PZC). Impregnation of support produced a moderate decrease of its surface area and pore volume and also led to minor changes of its PZC. Depending on preparation conditions (i.e., calcination atmosphere and temperature and metal loading), one or more of the following Co-containing compounds were identified: CoO, Co₃O₄ and CoAl₂O₄. The support and prepared Co/Al₂O₃ catalysts were tested to catalyze the ozonation of aqueous pyruvic acid at pH 2.5. Pyruvic acid was shown refractory towards single ozonation but the use of γ-Al₂O₃ and Co/Al₂O₃ catalysts resulted in 56–96% pyruvic acid conversion and 41–78% decrease in DOC after 2 h of ozonation of phosphate-buffered solutions. In the absence of the buffer, conversion rate was enhanced likely as a result of pH increase during the course of the process thus giving rise to the indirect way of ozonation through hydroxyl radicals. Acetic acid was found as the main by-product of pyruvic acid ozonation. Depending on the catalyst used, yield of acetic acid varied from 32 to 49%, values noticeably lower that that obtained from the control non-catalytic ozonation experiment (73%). Differences in catalytic activity amongst the various Co/Al₂O₃ catalysts investigated were attributed to the different Co active phases deposited on the γ-Al₂O₃ surface. The following sequence of increasing activity can be inferred from experimental results: CoO, CoAl₂O₄ and Co₃O₄. All the Co/Al₂O₃ catalysts prepared showed good stability as the percentage of cobalt leached out was rather low.     

Preparation of V2O3 nanopowders by supercritical fluid reduction

V₂O₃ nanoparticles, with diameters of 30-60 nm, have been synthesized by supercritical ethanol fluid reduction of VOC₂O₄. It was found that the reaction time has little influence on the purity and morphology of as-prepared products. And carbon oxides decomposed from C₂O₄²⁻ were conducive to the formation of the uniform sphere-like nanoparticles. The reaction mechanism of synthesizing uniform V₂O₃ nanoparticles is believed to be the redox reaction between VOC₂O₄ and ethanol in the supercritical condition. The crystal growth mechanism was proposed on the basis of the discussion about the formation process of the uniform sphere-like nanoparticles.