甲烷部分氧化制合成气的研究进展

综述了近年来甲烷部分氧化制合成气的催化剂，反应机理及活性中心的研究进展及反应中存在的主要问题。     

甲烷部分氧化制合成气催化剂的研究进展

甲烷部分氧化（POM）反应制合成气是化学利用甲烷的有效途径之一。研究表明,甲烷部分氧化反应的工艺有能耗低和反应速率较快等优点,而且所得的H2和CO的比例适于合成甲醇等工业化学品。在该工艺过程中,所需反应容器体积小,反应效率高,可大幅度降低制备合成气的成本。开发POM反应的高效催化剂是进一步提高反应效率、实现工业化的关键途径,因此,是当前国际催化领域研究的热门课题之一。本文主要介绍了传统的金属负载型催化剂和金属氧化物催化剂用于POM反应的研究进展。     

天然气甲烷部分氧化制合成气的研究进展

甲烷部分氧化制合成气是高转化率、高选择性、高空速、低H2/CO、温和的放热反应,综述了近几年来甲烷部分氧化制合成气的催化剂、反应机理及活性中心的研究进展及反应中的存在问题。     

甲烷部分氧化制合成气催化剂的研究进展

综述了甲烷部分氧化制合成气的研究意义和现状，从金属活性组分，载体效应，载量选择，助剂添加和制备方法等因素对催化剂活性的影响及研究进行了系统。结合本课题组的研究结果及文献报道，分析了Ni基催化剂的失活特性，并提出使用等离子体技术对Ni基催化剂进行改笥处理，以提高其催化稳定性的技术展望。     

甲烷部分氧化制合成气反应的研究

用粒度为5mm的α－Al2O3、β-Al2O3、γ-Al2O3为载体，用浸渍法制备了10％（质量）Ni基催化剂。在固定床流动反应器中，在反应温度500－850℃，大空速和不同的CH4／O2摩尔比下，测定了该催化剂用于甲烷部分氧化制合成气的活性和CO选择性。500℃用H2对催化剂还2h后，进行活性测试结果,10%Ni／β-Al2O3、Ni／γ-Al2O3对POM反应无活性，只有10％Niα－Al2O3对POM反应有活性。TPR测试结果表明，这是由于10％Ni／β-Al2O3和Ni／γ-Al2O3催化剂在700℃以下未被还原所致。另外，合成气的生成速率和CO选择尾均随反应温度和空速的增大而增大，并在CH4／O2摩尔比2时有最大值。     

甲烷催化部分氧化制合成气的研究进展

文章叙述了甲烷催化部分氧化制合成气研究的进展情况。介绍了两种主要的反应机理：间接反应机理和直接反应机理;对负载型镍系催化剂的研究现状进行了叙述,并简单介绍了非负载型催化剂;另外还对固定床、流化床、膜反应器以及一些新开发的反应器的特点进行了讨论。     

甲烷催化部分氧化制合成气催化剂的研究进展

天然气是一种前景广阔的清洁燃料,甲烷作为天然气的主要成分,其高效利用具有重要的现实意义。在众多甲烷转化途径中,甲烷催化部分氧化（CPOM）具有能耗低、合成气组分适宜、反应迅速等优势。本文简要介绍了CPOM反应机理,即直接氧化机理和燃烧-重整机理;重点综述了过渡金属、贵金属、双金属和钙钛矿这四类CPOM催化剂的研究现状;分析了反应温度、反应气体碳氧比和反应空速对CPOM反应特性的影响;阐述了积炭和烧结这两种催化剂失活的主要原因及应对措施。根据研究结果可知,通过选取合适的催化剂组分、采用优化的制备方法、精确控制催化剂活性组分分布和微观结构等措施,可以保证更多的有效活性位更稳定地暴露在催化剂表面,以此提高催化性能（包括甲烷转化率、合成气选择性、合成气生成率、反应稳定性等）。最后指出了对CPOM催化剂微观结构的合理设计与可控制备以及对CPOM反应机理的深入研究仍将是今后关注的重点。     

微波场中的甲烷部分氧化制合成气

介绍了微波场中甲烷部分氧化制合成气反应的原理及特性,对甲烷部分氧化制合成气反应的催化剂在微波场中的性能进行了简要介绍和分析,表明了甲烷部分氧化的微波反应技术具有较好的工业应用前景。     

微波辐照引发甲烷部分氧化制合成气

考察用微波辐照方式引发反应后,１０％Ｎｉ／Ｌａ２Ｏ３催化氧化甲烷制合成气的反应行为。结果表明,当催化剂用量为０．１０ｇ,原料气空速为１５０～１８０Ｌ·ｇ－１·ｈ－ｌ,原料气中ＣＨ４与Ｏ２配比为２：ｌ,且原料气从催化床层的上端通入时,催化活性最佳     

催化部分氧化甲烷制合成气的平衡热力学研究

The catalytic partial oxidation of methane to syngas (CO H2) has been simulated thermodynamically with the advanced process simulator PRO/Ⅱ. The influences of temperature,pressure,CH4/O2 ratio and steam addition in feed gas on the conversion of CH4 selectively to syngas and heat duty required were investigated, and their effects on carbon formation were also discussed. The simulation results were in good agreement with the literature data taken from a spouted bed reactor.     

Oxidative conversion of methane to syngas over NiO/MgO solid solution supported on low surface area catalyst carrier

Influence of time-on-stream (0.5–15 h), CH₄/O₂ ratio in feed (1.8–8.0), space velocity (6000–510,000 cm³ g⁻¹ h⁻¹), catalyst particle size (22–70 mesh), and catalyst dilution by inert solid particles (diluent/catalyst weight ratio=4) on the performance at different temperatures (600–900°C) of the NiO/MgO solid solution deposited on SA-5205 [which is a low surface area macroporous silica-alumina catalyst carrier] in the oxidative conversion of methane to syngas (a mixture of CO and H₂) has been investigated. The dependence of conversion and selectivity on the space velocity is strongly influenced by the temperature. Both the conversion and selectivity for H₂ and CO are decreased markedly by increasing the CH₄/O₂ ratio in the feed. The catalyst dilution resulted in a small but significant decrease in both the conversion and selectivity for H₂ and CO. The increase in the catalyst particle size had also a small but significant effect on both the conversion and selectivity in the oxidative conversion process. Both the heat and mass transfer processes seem to play significant roles in the oxidative conversion of methane to syngas at a very low contact time or very high space velocity (5.1×10⁵ cm³ g⁻¹ h⁻¹).     

Partial oxidation of methane to syngas over BaTi1 − xNixO3 catalysts

Performances of BaTi_1 − xNi_xO₃ perovskites, prepared using sol–gel method, as catalysts for partial oxidation of methane to syngas have been studied. The catalysts were characterized by XRD, BET and TEM. The experimental studies showed the calcination temperature and Ni content exhibited a significant influence on catalytic activity. Among catalysts tested, the catalyst BaTi_0.8Ni_0.2O₃ exhibited the best activity and excellent stability.     

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Mixed LaNi_xFe_(1−x)O₃ perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi_0.3Fe_0.7O₃.     

Optimum washcoat thickness of a monolith reactor for syngas production by partial oxidation of methane

In this paper, syngas (hydrogen and carbon monoxide) production was investigated by a numerical model of an adiabatic monolithic reformer (e.g. for a micro fuel cell system). The study includes the thermal and diffusive properties of a washcoat of finite thickness that is modeled as a porous layer composed of a ceramic support containing catalytic active rhodium sites. It was combined with a two-dimensional radially symmetric model of a single tubular mini-channel, considering both the thermal and the diffusive transport phenomena in all domains. It was found that both the methane conversion and the hydrogen yield depended markedly on the washcoat thickness. An interesting result was obtained by implementing the common experimental conditions into the numerical model: if the inlet volume flow and the amount of catalyst per washcoat volume are constant, an optimum washcoat thickness of was found, where the hydrogen yield is maximal. For a thinner washcoat, the smaller amount of catalyst is limiting, leading to a low methane conversion. For a thicker washcoat, the limiting effect is the reduced residence time, which stems inherently from the constraint of constant volume flow, rather than the increased diffusive resistance.The demonstration of the existence of an optimum washcoat thickness is important, because it can have economic effects due to saving of the precious catalyst rhodium.     

Promotion of palladium-based catalysts on metal monolith for partial oxidation of methane to syngas

Four different modifications of alumina were prepared for use as the support for a Pd catalyst used for the partial oxidation of methane to syngas. The catalysts were washcoated on a metallic monolith in order to determine their activities at high gas flow rates. Compared with the Pd/Al₂O₃ catalyst, enhanced partial oxidation activities were observed with the Pd/CeO₂/Al₂O₃, Pd/CeO₂/BaO/Al₂O₃ and Pd/CeO₂/BaO/SrO/Al₂O₃ catalysts. The palladium particles were better dispersed in the presence of CeO₂ and SrO. Adding BaO, CeO₂ and BaO–CeO₂ to γ-Al₂O₃ prevented the transformation of the alumina phase during the 3-day aging process at 1000 °C, providing the support with some level of thermal stability. The addition of small amounts of SrO to the CeO₂/BaO/Al₂O₃ support enhanced the thermal stability of the Pd particles and minimized their sintering. The triply promoted Pd catalyst studied in this work was effective in carrying out partial oxidation at high temperatures, with BaO and CeO₂ promoting the thermal stability of the support, CeO₂ and SrO dispersing the Pd particles and SrO anchoring the Pd particles strongly to the support. The composition of the catalyst which gave both the highest partial oxidation activity and the best thermal stability was Pd(2)/CeO₂(23)/BaO(11)/SrO(0.8)/Al₂O₃.     

Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Effect of composition

Nickel-calcium phosphate/hydroxyapatite catalysts have recently been reported to exhibit high activity and selectivity in partial oxidation of methane (POM). In this work the optimum composition was determined. The optimum mole ratio of Ca/PO₄ was around 10/6 and that of Ni/PO₄ was in a range from 1.0/6 to 3.0/6 with the optimum Ca/PO₄, and the activity could be related with the amount of metallic nickel. In a temperature range from ca. 400 to 700 K, an apparent autothermal reaction was observed to occur in some cases. This is due to the fact that the actual catalyst temperature is higher than the measured temperature, which comes from the exothermic nature of the reaction. The mixing sequence of the precursors during the catalyst preparation does little affect the catalyst activity and characteristics. Deactivation of the catalyst occurred slowly, but the catalyst could easily be regenerated. Moreover, the nickel-calcium phosphate/hydroxyapatite catalyst showed higher activity than the nickel-strontium phosphate catalyst. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Overall chemical kinetics model for partial oxidation of methane in inert porous media

An overall three-step six-component chemical kinetics model which includes CH₄ + O₂ → CO + H₂O + H₂ and reversible 2CO + O₂ ←→ 2CO₂ and CH₄ + H₂O ←→ CO + 3H₂ reactions is elaborated for the simulation of partial oxidation of methane in inert porous media. Procedure of the model adjusting to the experimental data is described. Kinetic parameters of the model are derived on the basis of temperature–flow rate, H₂ and CO output concentration–flow rate and temperature–pressure experimental correlations. It is found that extremely slow solid body temperature growth with flow rate T_s,max(G) reported in the works on partial oxidation of methane (and other hydrocarbons) in inert porous media may be reproduced by the model. The model is designed for optimization, scale up and design assistance of the reactors of partial oxidation of methane.It is demonstrated that the overall chemical kinetics model can be combined with detailed gas-phase kinetics model for the investigation of detailed composition of syngas and intermediary components.     

Characterization of a nickel-strontium phosphate catalyst for partial oxidation of methane

A nickel-strontium phosphate catalyst was recently reported to exhibit high activity and selectivity in partial oxidation of methane (POM). Further characterizations have been performed in the present work in order to better understand the characteristics of this catalyst. TEM showed that very fine nickel particles of a few nanometers of the size were present in the needle shape in the sample after the reaction. TPR showed the presence of three nickel species, confirming that the nickel in the strontium hydroxyapatite and phosphate structures comes out as fine particles and is reduced during the reaction. As the calcination time during the preparation increased, the total re-oxidation temperature became higher. This is considered due to larger crystallite size owing to the longer calcination, which results in a smaller amount of nickel coming out of the matrix to the surface.     

Catalyst instabilities during the liquid phase partial oxidation of methane

A promising catalytic system for the low temperature oxidation of methane to a methanol derivative has been investigated under both batch and semi-continuous operation in two different reactor types. The system comprises of a bimetallic palladium and copper(II) chloride catalyst contained in a trifluoroacetic acid (TFA) and an aqueous phase. Methane, oxygen and a co-reductant carbon monoxide constitute the gas phase. Typical operating conditions were a temperature of 85 °C and a pressure of 83 bar.

The yields of the methyl trifluoroacetate product observed in this present work were less than those obtained in other batch autoclave works, which employed only 4 ml of liquid phase, compared with 50 ml in this study. Furthermore, an encouraging initial product formation rate of ca. 40 mol/m³ h, quickly decreased after the first hour, and came to an apparent end after only 2 h. This observation had not been reported previously.

Work performed in a semi-continuous porous tube reactor (300 ml of re-circulating liquid phase) also showed the same reaction characteristics as in the batch reactor. Thus, the deteriorating product formation rate cannot be attributed to gaseous reactant depletion (batch operation). The results suggest problems associated with catalyst instabilities, e.g. with the previously elucidated Wacker chemistry.     

Development of a dual functional structured catalyst for partial oxidation of methane to syngas

Rh-LaCoO₃ structured catalysts for the oxidative production of syngas from methane were developed by deposition of the active components on La-γ-Al₂O₃ washcoated honeycomb monoliths. SEM/EDS analysis showed a good adhesion of the washcoat layer and a uniform distribution of La and Co, while Rh was favourably located on the outer shell. Catalytic partial oxidation of methane was tested under both isothermal and pseudo-adiabatic conditions showing that the process can be conducted with high yield and selectivity and stable performance at short contact times over the novel catalysts, characterised by a limited content of noble metal and no need for pre-reduction. Further experiments of CO₂ autothermal reforming indicated the possibility to enhance CO production and to reduce the H₂/CO ratio through secondary endothermic reactions consuming CO₂, which are autothermally self-sustained in a single catalytic reactor operated at short contact time by the heat generated through partial oxidation reactions.