Study On Nickel-based Catalysts For Partial Oxidation Of Methane To Synthesis Gas

Compared with methane reforming with steam or with carbon dioxide, the catalytic partial oxidation of methane (POM) to synthesis gas is a better approach for methane utilization, since it has many advantages such as higher space velocity, lower power energy consumption, smaller equipment investment and proper H2/CO molar ratio in the products about 2/1, which is favorable to the synthesis of methanol or other higher alcohols. Therefore, the POM becomes one of the most attractive and challenging research tasks in the field of chemical conversion of nature gas.In this work, the supported nickel-based catalyst was selected as the research object, and the effects of some promoters (including MgO, CeO₂ and CaO), catalyst particle size, nickel component distribution and calcination temperature on the physico-chemical properties and the POM reactivity of Ni/γ-Al₂O₃ catalyst were systematically investigated by means of BET, XRD, H2-TPR, TEM, EDS, IR, CH₄-TPSR, TGA, temperature-programmed decomposition and catalytic activity evaluation.Ni/γ-Al₂O₃ catalyst had some shortcomings, such as the poor dispersity of nickel species and easy formation of NiAl2O4 spinel, all of which were not beneficial to its POM reactivity. Using the proper amount of MgO promoter, the homogeneity of nickel species in Ni/γ-Al₂O₃ catalyst could be improved and the formation of NiAl2O4 spinel was inhibited, and thus the performance of Ni/γ-Al₂O₃ catalyst modified MgO for POM were enhanced. However, the excessive MgO promoter might be unfavorable to the dispersity of nickel species. The suitable content of MgO promoter in Ni/γ-Al₂O₃ catalyst was about 7wt.%.When CeO₂ and CaO were used as composite promoters to modify Ni/(7wt.%)MgO-γ-Al₂O₃ catalyst, they could form the solid solution, and their remarkable synergetic effect was detected. It was deduced that the formation of CeO₂-CaO solid solution could not only result in higher dispersity of active nickel species, easier conversion of Ce4+ to Ce3+ species, and smaller size of nickel crystallites, but also lead to the better oxygen adsorption ability, more oxygen-vacant sites and easier migration of lattice oxygen anions, all of which benefited the improvement of the catalytic POM performances. When Ni/MgO(7wt.%)-γ-Al₂O₃ catalyst was promoted with CeO₂ or CaO alone, no obvious improvement of the catalyst reactivity was observed. The inner diffusion was eliminated over the catalyst with small particle size to a large extent. For the aim of application, the spherical catalysts with ca. 1.5 mm diameter were applied and investigated for POM reaction process, where the intra-particle mass transport became the rate-limiting step. Compared with the homogeneous catalysts, the eggshell-typed catalysts had higher effectiveness factors because the active nickel component was distributed in the outer surface layer, and thus owned better catalytic POM performance, especially on the methane conversion and hydrogen selectivity. Besides nickel component distribution, catalyst particle size, nickel loadings, reaction temperature, and space velocity also affected the POM reactivity to some extent. Since oxygen molecule encountered higher diffusion resistance than methane molecule due to its twice higher molecular weight, the inner diffusion of oxygen became the rate determine step in POM process. The CH₄/O₂ molar ratio could be increased along the axial length of pores, which might promote the direct partial oxidation and prevent the deep oxidation of methane.MgO promoter could also improve the catalytic performance of eggshell-typed Ni/γ-Al₂O₃ catalyst with larger particle size (in millimeter grade) to some extent. As for eggshell-typed Ni/MgO-γ-Al₂O₃ catalysts, CH₄ conversion increased with the increase of nickel loadings. There existed an obvious turning point of nickel loading at 2wt.%. If Ni loadings were lower than 2wt.%, the catalysts exhibited rather poor catalytic performance at the temperature range between 773 K and 1073 K, and the methane conversion was nearly zero because POM could not be ignited. However, after Ni loading reached 2wt.%, the catalysts showed good ignition property and catalytic performance in POM reaction, and the reaction could be ignited at the temperature of about 773 K. With the increase of nickel loadings, the POM activity increased, and it changed very slightly at the nickel loading of higher than 12 wt.%.It was found that by means of CeO₂ and CaO composite promoters, the ignition and reactivity of 1wt.%Ni /MgO-γ-Al₂O₃ catalyst could be improved, which was closely related to the concentration and oxidizability of surface oxygen as well as lattice oxygen. Furthermore, suitable amount of CeO₂ and CaO composite promoters could also improve the catalytic POM reactivity of millimeter grade eggshell-typed 10wt.%Ni/ MgO-γ-Al₂O₃ spherical catalyst furtherly. The addition of CeO₂ and CaO composite promoters would weaken the oxidizability of surface NiO species, which favored the direct partial oxidation of methane and avoided some side reactions, such as CH₄ deep oxidation (complete combustion). However, excessive promoters caused the formation of CeAlO3, which was unfavorable to POM reaction. It was found the suitable content of CeO₂ and CaO composite promoters was about 1wt.%.The properties of Ni/CeO₂-CaO-MgO-γ-Al₂O₃ catalyst were greatly affected by the calcination conditions. Although the calcination ofγ-Al₂O₃ support at 1273 K would cause the serious decrease of specific surface area and the sintering of nickel crystallites, which might result in negative effect to the catalytic performance, however, it could also result in the increase of average pore radius, which was beneficial to the inner diffusion of CH₄ and O₂ molecules, thus the effectiveness factor of the catalysts was enhanced. For the catalysts with larger particle size, the latter effect was more important, so the catalytic performance was improved after the calcination ofγ-Al₂O₃ support at 1273 K. It was found that the catalyst impregnated with Ni(NO3)2 aqueous solution without calcination showed good catalytic reactivity. Based on theγ-Al₂O₃ supports calcined at 1273 K, the catalyst Ni/CeO₂-CaO-MgO-γ-Al₂O₃, which was prepared by the impregnation with Ni(NO3)2 aqueous solution but without calcination, showed the best catalytic POM performance. At the temperature of 1073 K, CH₄ conversion, CO selectivity and H2 selectivity could reach 97.5%, 94.3% and 94.3%, respectively. In the stability investigation, it showed no deactivation during first 20 h, thereafter, its reactivity decreased slightly. In comparison to the initial activity, the CH₄ conversion, CO selectivity and H2 selectivity after 100 h test decreased by 3%, 1.3% and 1.2%, respectively. The characterization results of the deactivated catalyst sample suggested that NiO-MgO solid solution and/or NiAl2O4 spinel formed during reaction process caused the catalyst deactivation to a great extent due to their reduction difficulty. Additionally, the phase transformation ofγ-Al₂O₃ and the aggregation of Ni species could also lead to the catalyst deactivation. Coke deposition might not be the main reason for the catalyst deactivation.In this work, two different methods were used to prepare the eggshell-typed nickel-based catalysts. In the first one,γ-Al₂O₃ was impregnated with acetone solution of nickel nitrate, which directly led to the nickel eggshell-typed catalysts; In the second one,γ-Al₂O₃ support was firstly modified with MgO promoter, and then impregnated with the water solution of nickel nitrate, and the eggshell catalyst was also prepared. These two approaches were based on different principles to lower the impregnation rate and thus to control the impregnation depth effectively, i.e. the thickness of eggshell layer. There were abundant O-H groups on the surface of γ-Al₂O₃ support, which was beneficial to the impregnation of aqueous solution, thus the impregnation rate of aqueous solution of nickel nitrate onγ-Al₂O₃ support was over 0.097 mm?s-1/2, so the prepared catalyst showed the homogeneous nickel distribution. Owing to the weak polarity of acetone, the impregnation rate of acetone solution of nickel nitrate onγ-Al₂O₃ was only about 0.0058 mm?s-1/2, and the impregnation depth was about 0.20 mm at the impregnation time of 30 minutes. When acetone was used as impregnation solvent, the formation of eggshell-typed distribution of nickel component was attributed to the slower impregnation rate. The composite support with proper average pore radius and less surface O-H groups could be obtained by impregnatingγ-Al₂O₃ with aqueous solution of magnesium nitrate and then calcining at 723 K. On this composite support, the impregnation rate of aqueous solution of nickel nitrate was much slower than that onγ-Al₂O₃ support, and so the eggshell-typed supported nickel-based catalyst could be prepared. By the adjustment of impregnation time, drying temperature and system pressure, the eggshell thickness could be controlled effectively.