Screening of MgO- and CeO_2-Based Catalysts for Carbon Dioxide Oxidative Coupling of Methane to C_(2+) Hydrocarbons

The catalyst screening tests for carbon dioxide oxidative coupling of methane (CO2-OCM) have been investigated over ternary and binary metal oxide catalysts. The catalysts are prepared by doping MgO- and CeO2-based solids with oxides from alkali (Li2O), alkaline earth (CaO), and transition metal groups (WO3 or MnO). The presence of the peroxide (O2-2) active sites on the Li2O2, revealed by Raman spectroscopy, may be the key factor in the enhanced performance of some of the Li2O/MgO catalysts. The high reducibility of the CeO2 catalyst, an important factor in the CO2-OCM catalyst activity, may be enhanced by the presence of manganese oxide species. The manganese oxide species increases oxygen mobility and oxygen vacancies in the CeO2 catalyst. Raman and Fourier Transform Infra Red (FT-IR) spectroscopies revealed the presence of lattice vibrations of metal-oxygen bondings and active sites in which the peaks corresponding to the bulk crystalline structures of Li2O, CaO, WO3 and MnO are detected. The performance of 5%MnO/15%CaO/CeO2 catalyst is the most potential among the CeO2-based catalysts, although lower than the 2%Li2O/MgO catalyst. The 2%Li2O/MgO catalyst showed the most promising C2+ hydrocarbons selectivity and yield at 98.0% and 5.7%, respectively.

Effect of CO_2 on the structural variation of Na_2WO_4/Mn/SiO_2 catalyst for oxidative coupling of methane to ethylene

In this work,the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/Si O2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation method and characterized by XRD,Raman and XPS techniques. Appropriate amount of CO2 in the reactant gases enhanced the formation of surface tetrahedral Na2WO4 species and promoted the migration of O in MOx,Na,W from the catalyst bulk to surface,which were favorable for oxidative coupling of methane. When the molar ratio of CH4/O2/CO2 was 3/1/2,enriched surface tetrahedral Na2WO4 species and high surface concentration of O in MOx,Na,W were detected,and then high CH4 conversion of 33.1% and high C2H4 selectivity of 56.2% were obtained. With further increase of CO2 in the reagent gases,the content of active surface tetrahedral Na2WO4 species and surface concentration of O in MOx,Na,W decreased,while that of inactive species（Mn WO4 and Mn2O3） increased dramatically,leading to low CH4 conversion and low C2H4 selectivity. It could be speculated that Na2WO4 crystal was transformed into Mn WO4 crystal with excessive CO2 added under the reaction conditions. Pretreatment of Na2WO4/Mn/Si O2 catalyst by moderate amount of CO2 before OCM also promoted the formation of Na2WO4 species.

Rule of 100:An inherent limitation or performance measure in oxidative coupling of methane?

The oxidative coupling of methane over La203/CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions （973〈T〈 1103 K, 55〈 Ptotal 〈220 kPa, and 3.7〈 mcat/VTp 〈50 kg.s/m^3）. A ten-step kinetic model incorporating all main products was used to predict the behavior of the system. Methane conversions and C2 selectivities were calculated by varying methane to oxygen ratios in the feed under different operating conditions which were also compared with the rule of 100. The results show that deviation from this rule depends on the operating conditions. Within the range studied, an increase in pressure, temperature and contact time results in smaller deviation from the rule. This rule is best approximated when the catalyst operates near its optimal performance. For negative deviations, common to the most catalysts, it is found that the optimal performance should occur at methane conversion levels lower than 50%. A plot of selectivity versus conversion for high-yield reported performance data of a large variety of catalysts shows that data points concentrated roughly in 20%-50% methane conversion region, confirming the generality and prediction of modeling.

From fundamentals to chemical engineering on oxidative coupling of methane for ethylene production:A review

A comprehensive overview is presented to summarize the research works since 1982 on oxidative coupling of methane(OCM),a complex reaction network combining heterogeneous and homogeneous reaction steps.Fundamentals on reaction mechanisms and thermodynamics have revealed that the OCM process is highly exothermic and its C2+selectivity and yield is critical in evaluating its commercial viability.Catalytic strategies have been put to enhance C2+selectivity,improve C2+yield and lower reaction temperature.The catalyst MnNa2WO4/SiO2 enables methane activation at a temperature of 800◦C and simultaneously a high C2+selectivity of 70–80%,while the nanowire and La2O3-based catalysts enable to lower the reaction temperature to 200–300◦C and 500◦C,respectively.Reaction engineering aspects have also been dealt in many investigations in order to make the process technically feasible.Particularly,research works on reaction kinetics,reactor selection and reactor operating mode choice have been addressed.Intermediate cooling and distributed oxygen feed have been integrated into a multi-stage adiabatic fixed-bed reactor system to suppress the side oxidation reactions and improve the performance of the catalysts towards CH4 conversion and C2+yield.This review paper proposes employing a circulating reactor system coupled with catalyst fine particles but having little internal diffusion resistance to maximize one-pass C2+selectivity and yield of the OCM reaction and evaluate its industrial application potential.

在ThO_2－SrCO_3／BaSO_4催化剂中甲烷氧化偶联反应的研究

研究了ＴｈＯ２－ＳｒＣＯ３／ＢａＳＯ４催化剂在甲烷氧化偶联反应中的催化作用，发现在ＳｒＣＯ３／ＢａＳＯ４中添加ＴｈＯ２可明显提高催化剂的活性，其Ｃ２选择性及Ｃ２收率可达６１．３１％和１８．１７％，而且经过３００ｈ的连续考察，活性稳定。用ＸＲＤ方法研究了３组份催化剂的物相组成，推测结构与催化性能的关系。用ＴＰＤ方法研究３组份催化剂的酸碱性与活性的关系。