TPR study of the mechanism of rhenium promotion of alumina-supported cobalt Fischer-Tropsch catalysts

The effect of small amounts of Re on the reduction properties of -alumina supported cobalt catalysts has been studied by temperature-programmed reduction (TPR). An intimate mixture of Co/Al₂O₃ and Re/Al₂O₃ catalysts showed a promoting effect of Re similar to that for coimpregnated Co-Re/Al₂O₃. A loose mixture of Co/Al₂O₃ + Re/Al₂O₃ did not show any effect of Re on the reduction of cobalt. However, when the loose mixture of Co/Al₂O₃ + Re/Al₂O₃ was pretreated with Ar saturated with water before the TPR, a promoting effect of Re on the reduction of Co was observed. It is suggested that Re promotes the reduction of cobalt oxide by hydrogen spillover, and that no direct contact between Re and Co seems to be necessary in order to obtain the promoting effect as observed by TPR. It is also shown that the presence of a high temperature TPR peak at 1200 K assigned to cobalt aluminate is mainly a result of Co-ion diffusion during the TPR and not during calcination.     

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.     

Fischer-Tropsch合成钴基催化剂研究进展

综述了近年来钴基催化剂在费托合成反应中的研究进展.重点评述了钴源、负载量、载体、助剂、制备方法、焙烧和还原条件等因素在负载型钴基催化剂制备过程中对催化剂活性、选择性控制等方面的影响,概述了钴基催化剂在费托合成反应中的催化机理,并对今后催化剂的研究提出了一点建议.     

Fischer-Tropsch synthesis on supported cobalt catalysts promoted by platinum and rhenium

An investigation of the CO hydrogenation of Pt- or Re-promoted 8.7 wt% Co/Al₂O₃ (1.0 wt% Pt or 1.0 wt% Re) has been carried out at two different conditions: 473 K, 5 bar, H₂/CO = 2 and 493 K, 1 bar, H₂/CO = 7.3. The addition of Pt or Re significantly increases the CO hydrogenation rate (based on weight of Co), but the selectivity was not changed by the presence of Pt or Re. The results show that the observed increases in the reaction rates are caused by increased reducibility and increased number of surface exposed Co-atoms. Steadystate isotopic transient kinetic analysis (SSITKA) with carbon tracing was used to decouple the effects of the concentration of active surface intermediates and the average site reactivity of intermediates during steady-state CO hydrogenation. The SSITKA results show that the concentration of active surface intermediates leading to CH₄ increased as a result of the addition of a noble metal promoter. However, the average site activity was not significantly affected upon Re or Pt addition.     

费托合成钴催化剂的失活与再生研究进展

为降低费托合成钴催化剂成本,提高费托合成钴催化剂的稳定性和寿命,分析了费托合成钴催化剂的失活与再生机理,论述了费托合成钴催化剂的再生方法。结果表明,中毒、烧结长大、积碳、氧化及固相反应是造成费托合成钴催化剂失活的主要原因。通过净化原料气、调节钴与载体相互作用、调整催化剂配方及工艺条件、控制钴颗粒尺寸及体系中水和H2的分压比等措施可以提高催化剂稳定性和寿命。通过脱蜡、氧化、还原可实现催化剂再生,氧化是再生的关键步骤,要注意氧化过程中升温速率、热量移除等问题;多次的氧化和还原过程可增加再生催化剂中金属钴与还原助剂的均匀性,提高再生催化剂的活性。     

费托合成催化剂失活动力学模型的研究进展

费托合成催化剂由于自身的化学性质在反应中不可避免会发生失活，为了保证生产的连续稳定，需要建立费托合成催化剂失活的动力学模型来预测催化剂的活性变化，并及时对失活的催化剂进行置换和再生。本文论述了费托合成失活动力学模型的研究进展，讨论了通用型失活动力学模型，根据机理建立的失活动力学模型的特点。通用型失活动力学模型与催化剂种类和失活原因没有直接关联，包括线性模型、简单幂律模型、通用幂律模型、韦伯分布模型、S型分布模型。根据机理建立的失活动力学模型则与催化剂种类和失活发生的机理相关，包括硫中毒失活模型、烧结失活模型、表面氧化失活模型等。通用型失活动力学模型准确性好、容易建立，但比较粗略，适用于费托合成的生产管理、过程模拟。根据机理建立的失活动力学模型建立过程复杂，只适用于特定催化剂，但能够从中研究催化剂的失活机理。费托合成失活动力学模型未来的发展趋势是融合两类模型，从失活机理的角度理解通用型失活动力学模型里参数的含义。     

Preparation and properties of supported cobalt catalysts for Fischer-Tropsch synthesis

Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.     

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.     

费托合成Fe基催化剂失活研究进展

Fe基催化剂是具有工业化应用潜力的一种费托合成催化剂,但在反应中易因积炭、相变和烧结等原因失活,从而影响Fe基催化剂的实际应用。主要综述围绕上述3个引起铁基费托合成催化剂失活的主要原因所取得的研究进展。在此基础上,进一步探讨催化剂组成和预处理条件对Fe基催化剂的影响以及提高Fe基催化剂性能领域所取得的研究进展。     

载体焙烧温度对Co/Al2O3催化剂F-T合成反应催化性能的影响

采用等体积浸渍法,以不同温度焙烧的Al2O3为载体制备Co/Al₂O₃催化剂,通过N₂物理吸附、X射线衍射和程序升温还原等方法对催化剂进行表征,并在固定床反应器中评价催化剂的F-T合成反应性能。结果表明,低温焙烧可获得较大比表面积的Al₂O₃载体,有利于提高活性组分的分散度,但增强了钴与载体之间的相互作用,降低催化剂活性和选择性。高温焙烧的Al2O3载体有利于提高C⁺₅选择性,尤其是柴油组分的选择性。     

The importance of passivation in the study of iron Fischer-Tropsch catalysts

In the study of iron catalysts, careful passivation is necessary for study of microstructure by ex situ analytical techniques. The passivation procedure used in our study consists of heating the sample in He at the reaction temperature, cooling to room temperature and introducing small amounts of O₂ (<1%) in a flowing He stream. A properly passivated sample shows no more than a few nm of surface Fe₃O₄ on-Fe, when examined in a high resolution TEM. Proper passivation is also characterized by an exotherm of no more than 2–3 K. We show that a Fischer-Tropsch catalyst carbided in CO will show substantial amounts of magnetite, if exposed to air without proper passivation. Such surface oxidation may cause errors in determining the relative amounts of the magnetite and carbide phases in Fischer-Tropsch catalysts, which are important for proper identification of the catalytically active phase.     

Fischer-Tropsch synthesis over iron catalysts

Because of the decreased profitability of making synthetic fuels, Sasol intends expanding its production of the higher valued chemicals, in particular waxes and olefins. The advantages and disadvantages of using Fe, Co and Ru catalysts are discussed from the point of view of costs, availability, product selectivity, activity and sensitivity to poisons.The loss of activity and selectivity of iron based catalysts in both fixed and fluidized bed reactors is discussed. The main contributing factors are sulfur poisoning, oxidation and coke fouling. In fixed bed reactors sulfur poisoning and coke laydown deactivates the front end of the bed while hydrothermal sintering/oxidation deactivates the back end. In fluidized beds the deposition of large amounts of Boudouard carbon doesnot markedly lower the activity. The smaller catalyst particles end up consisting of small iron carbide entities embedded in a matrix of carbon. The larger catalyst particles consist of cores of inert magnetite surrounded by the carbide/carbon matrix.FT reactor development at Sasol is briefly reviewed.     

Reduction of supported cobalt catalysts by hydrogen

The reduction of cobalt catalysts supported on Al₂O₃, SiO₂, and TiO₂ was investigated using a closed system filled with hydrogen gas. Effects of support and metal loading on the rate of reduction were also discussed. The activation energy of reduction increased in the following order: Co/TiO₂2O₃2. For different metal loadings, it was found that the catalyst with the higher loading was more readily reducible than that with the lower metal loading. This was confirmed using the results from measurements of particle size, amount of CO adsorbed and activity.     

合成气费托合成制重质烃Ru-Co/SiC催化剂的制备及性能

费托合成是以合成气生产清洁燃料和其他化学品的重要途径。传统费托合成产物遵循A-S-F分布,只有甲烷和重质烃的选择性没有极限值。因此,费托合成研究以最大程度地合成重质烃,提高合成产物中重质烃的选择性为目标。基于此,首先详细探究了Al₂O₃、SiO₂和SiC载体对费托反应性能的影响。结果表明 Co/SiC催化剂具有最高的CO转化率（83.5%）和C₅₊选择性（80.3%）。与浸渍法相比,原位还原法更为有效地引入Ru到Co/SiC催化剂,将C₅₊选择性提高至90.1%。Ru助剂能在保持较高催化活性不变的前提下,有效提高Co/SiC催化剂C₅₊选择性。催化剂表征（XRD、H₂-TPR、XPS、H₂-化学吸附和TEM）结果表明,Ru能与Co发生相互作用,提高了催化剂的可还原性和活性组分的分散性,进而改善了Co/SiC催化剂重质烃的选择性。     

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.     

Conventional and wet proofed CuO/Al2O3 catalysts for phenol oxidation: deactivation studies in a trickle bed reactor

A large variety of catalytic systems have been studied for the catalytic wet air oxidation of phenolic solutions. Most of them show good activity, but serious stability problems. In this contribution, stability studies were performed over CuO/Al₂O₃ conventional (CNT) and polytetrafluorethylene coated (C3T) catalysts used for the oxidation of 5 g L^?1 phenol solutions in a trickle bed reactor (140 °C and 7 atm of oxygen pressure). For the hydrophilic catalyst, phenol conversion decreased with usage due to the formation of Cu₂O and copper oxalate phases. For the wet proofed catalyst, the hydrophobic layer prevented the appearence of those phases, and conversion levels remained practically constant with reaction time. After usage, both catalysts were oxidized at 400 °C and tested for reaction: in the case of the C3T catalyst, the phenol conversion was increased over its initial level; for CNT catalyst, the phenol conversion was also increased, but initial levels were not completely restored. The deactivation mechanism of the CNT catalyst is associated with the formation of the Cu₂O and copper oxalate phases during reaction. For catalyst C3T, practically no deactivation was observed. Copyright © 2007 Society of Chemical Industry     

Ni基重整催化剂失活机理研究进展

H₂是一种清洁、绿色的燃料和能源载体。目前工业上应用较为成熟的生产工艺是重整反应制氢。其中,Ni基重整催化剂由于其高储量、高活性和低成本的优点而受到研究人员的广泛关注,但在反应过程中存在易因烧结、积炭和中毒等原因而失活的问题。因此,如何提高Ni基重整催化剂的反应稳定性是一个急需解决的问题。本文介绍了上述三种引起Ni基重整催化剂失活的主要原因,并从调控金属Ni粒子粒径、增强金属-载体相互作用、形成晶格氧或表面氧物种以及Ni粒子纳米结构调控四个方面阐述了近年来在抑制失活并提高Ni基重整催化剂反应性能和稳定性领域所取得的研究进展,并且提出优化反应条件、调变化学组成和调控Ni粒子纳米结构将是提高Ni基催化剂在重整反应过程中的稳定性的有效方法。     

费-托合成钴基催化剂研究进展

尽管Co基费-托（Fischer-Tropsch,FT）合成催化剂相对较为成熟,但高活性、高稳定性以及高α-烯烃等特定产物选择性Co基催化剂的研发,依然是FT合成过程更大规模工业化应用面临的重大挑战。本文总结分析了Co基FT合成催化剂的结构敏感性、分散度与还原度矛盾、催化剂失活以及产物选择性调控等方面的最新进展和动向。根据Co的尺寸、晶相结构及Co与载体间相互作用影响催化剂活性的规律,认为除通过调变金属载体间相互作用以提高Co的分散度和还原度外,设计制备具有更高本征活性的hcp相Co是提高其质量比活性的有效策略;而进一步提高工业Co基催化剂寿命的关键是抑制Co的烧结和积炭。最后,总结了合成气一步高选择性合成液体燃料的新进展,认为提高双功能催化剂的稳定性以及解决工程化制备问题是实现该过程工业化应用的关键。     

Propylene metathesis over Re2O7/Al2O3-B2O3 catalysts

A series of alumina aluminum borate (AAB) with various Al/B molar ratios were prepared by the coprecipitation method. The supported rhenium oxide catalysts with various contents of Re₂O₇ were also prepared by the impregnation method with perrhenic acid. The catalytic activity and stability of Re₂O₇/AAB catalysts for the reaction of propylene metathesis were tested in a fixed-bed microreactor. It was found that Re₂O₇/AAB is more active, stable and regenerable than Re₂O₇/Al₂O₃ for propylene metathesis. The optimum Al/B molar ratio was found to be in the range of 4–10.     

费托合成钴催化剂载体改性研究进展

载体和钴物种之间的相互作用对费托钴催化剂分散度和还原度具有重要影响,载体改性可以调节载体与钴物种之间的相互作用。简要介绍了费托合成钴催化剂载体改性技术及其对载体与钴物种相互作用的影响,结果表明:改性载体和钴物种之间相互作用增加时,分散度升高,还原度降低;改性载体和钴物种之间互相作用降低时,则分散度降低,还原度升高;通过多载体掺杂改性、无机助剂改性及载体表面有机改性可有效调节载体和钴物种之间的相互作用,使得催化剂具有适中的还原度和分散度,从而提高催化剂的活性。