Oxidative coupling of methane

XRD, XPS/XAES, TPR analysis and catalytic testing have been applied to Na/W/Mn/SiO 2 methane coupling catalysts and to corresponding formulations without one or more of Na, Mn, and W. We find a clear correlation between catalyst performance and support structure in the final calcined material. Amorphous silica yields active but very unselective catalysts. Crystalline SiO 2 (α-cristobalite) generates active and highly selective catalysts-especially with respect to the formation of ethylene. We demonstrate that the presence of Na is essential for the anomalous low temperature silica → cristobalite support phase transition to occur. The structural, catalytic, and spectroscopic results indicate that Na plays a dual role as both structural and chemical promoter.

In this study a mathematical model of a small scale single pellet for the oxidative coupling of methane (OCM) over titanite pervoskite is developed. The method is based on a computational fluid dynamics (CFD) code which known as Fluent may be adopted to model the reactions that take place inside the porous catalyst pellet. The steady state single pellet model is coupled with a kinetic model and the intra-pellet concentration profiles of species are provided. Subsequent to achieving this goal, a nonlinear reaction network consisting of nine catalytic reactions and one gas phase reaction as an external program is successfully implemented to CFD-code as a reaction term in solving the equations. This study is based on the experimental design which is conducted in a differential reactor with a Sn/BaTiO 3 catalyst (7−8 mesh) at atmospheric pressure, GHSV of 12000 h −1 , ratio of methane to oxygen of 2, and three different temperatures of 1023,1048 and 1073 K. The modeling results such as selectivity and conversion at the pellet exit are in good agreement with the experimental data. Therefore, it is suggested that to achieve high yield in OCM process the modeling of the single pellet should be considered as the heart of catalytic fixed bed reactor.

The effects of ceria loading (0.1-6.0 wt%) on the properties of Li/MgO catalysts and on their activities for oxidative coupling of methane (OCM) were studied. The catalysts were characterised by surface area measurement, XRD, XPS, SEM and CO 2 temperature programmed desorption. The catalyst activity study suggested that the addition of 0.5-1.0 wt% ceria improved the methane conversion of Li/MgO catalyst significantly at reaction temperatures below 800 • C with little change in C 2 selectivity compared to un-promoted Li/MgO. Further increases in ceria loading resulted in lower methane conversion and C 2 selectivity. The CH 4 conversion was related to the surface distribution of O − and O 2− on the catalysts. An increase in reaction temperature beyond 800 • C or a decrease in CH 4 :O 2 ratio from 7:1 to 3:1 imparted negative effects on the methane conversion and C 2 selectivity of the ceria (0.5 wt%) doped Li/MgO catalyst. All ceria doped catalysts did not show any methane coupling activity in the absence of gaseous oxygen, suggesting that they were ineffective to work with alternate feeds of CH 4 and O 2 (redox) mode. Based on the OCM activity and characterisation results, a new pathway that describes the formation of active sites through the electron transfer between Ce 4+ /Ce 3+ and Li/MgO was proposed to help explain the improved OCM activity by using ceria doped Li/MgO catalysts.

The paper addresses the multiobjective optimization of a simulated moving-bed chromatographic reactor (SCMCR). The selection of the operating parameters such as the length and number of columns, switching time, and liquid flow rates in different sections is not straightforward in an SCMCR. In most cases, conflicting requirements and constraints govern the optimal choice of the decision (operating or design) variables. A mathematical model that predicts single-column experimental results well was modified for MTBE synthesis in an SCMCR. Thereafter, a few simple multiobjective optimization problems were solved that included both existing and designstage SCMCRs using a nondominated sorting genetic algorithm (NSGA). Optimal design and operating conditions for three cases of practical relevance are studied in this work, namely, (a) simultaneous maximization of the purity and yield of MTBE; (b) simultaneous maximization of the purity and yield of MTBE, together with minimization of the total amount of adsorbent/ catalyst required; and (c) maximization of the purity of MTBE with simultaneous minimization of the eluent consumption, to illustrate the procedures and interpret the results obtained. Paretooptimal solutions were obtained in all cases, and moreover, it was found that the performance of the SCMCR could be improved significantly under optimal operating conditions.

The effects of ceria loading (0.1-6.0 wt%) on the properties of Li/MgO catalysts and on their activities for oxidative coupling of methane (OCM) were studied. The catalysts were characterised by surface area measurement, XRD, XPS, SEM and CO 2 temperature programmed desorption. The catalyst activity study suggested that the addition of 0.5-1.0 wt% ceria improved the methane conversion of Li/MgO catalyst significantly at reaction temperatures below 800 • C with little change in C 2 selectivity compared to un-promoted Li/MgO. Further increases in ceria loading resulted in lower methane conversion and C 2 selectivity. The CH 4 conversion was related to the surface distribution of O − and O 2− on the catalysts. An increase in reaction temperature beyond 800 • C or a decrease in CH 4 :O 2 ratio from 7:1 to 3:1 imparted negative effects on the methane conversion and C 2 selectivity of the ceria (0.5 wt%) doped Li/MgO catalyst. All ceria doped catalysts did not show any methane coupling activity in the absence of gaseous oxygen, suggesting that they were ineffective to work with alternate feeds of CH 4 and O 2 (redox) mode. Based on the OCM activity and characterisation results, a new pathway that describes the formation of active sites through the electron transfer between Ce 4+ /Ce 3+ and Li/MgO was proposed to help explain the improved OCM activity by using ceria doped Li/MgO catalysts.

Abstract The performance of the Mn-Na2WO4/SiO2 catalyst for oxidative coupling of methane (OCM) has been ascribed to crystalline phases that are not present at reaction temperatures (>700 °C). The evolution of W and Mn sites structure was monitored via in situ TPO-XRD, -Raman, and -XANES spectroscopies. TPO-XRD shows that the crystalline phases identified on the Mn-Na2WO4/SiO2, Na2WO4/SiO2, and WO3/SiO2 catalysts at room temperature do not exist at relevant OCM temperatures. The γ → β → α-WO3, α → β-cristobalite, and cubic → orthorhombic → molten-Na2WO4 phase transitions occur upon heating. TPO-Raman spectra show that the bond order of W sites with octahedral (Oh) and tetrahedral (Td) symmetry changes during the δ → γ → β → α-WO3 and cubic → orthorhombic → molten-Na2WO4 transitions, respectively. TPO-XANES spectra show that bond order changes are due to distortion degree variations because all samples preserve essentially W6+ valence and Oh-Mn3+ sites are always present on Mn-Na2WO4/SiO2 catalyst. Steady-state OCM tests show that Oh-W6+ sites are inactive and Td-W6+ sites are less distorted and more active towards methane activation in the presence of Oh-Mn3+ sites.

. Laser Raman spectroscopy LRS is one of the most powerful tools for the in situ study of catalytic materials and surfaces under working conditions. Raman characterizations can be carried out at temperatures as high as 1000 K and in controlled atmospheres. Modern high light-throughput spectrometers permit the recording of the whole spectral range from 100 to 4000 cm y1 at once and time resolutions in the subsecond regime for materials with high Raman cross-sections. Transient temperature or pressure response studies, e.g. pulse experiments with isotope labels, are thus possible, and kinetic and spectroscopic characteristics can be related. Modern quartz fiber optics render possible easy spectroscopic access to catalytic reactors of defined and well characterized operation conditions. Quantitative relation of real catalytic steady state operation, e.g. catalytic activity and selectivity, to changes in the catalyst structure is thus made possible.

... Vasant R. Choudhary,; Shafeek AR Mulla,; Balu S. Uphade. Article first published online: 4 MAY 1999. ... Publication History. Issue published online: 4 MAY 1999; Article first published online: 4 MAY 1999; Manuscript Accepted: 13 JAN 1998; Manuscript Revised: 26 SEP 1997; ...

... Vasant R. Choudhary,; Shafeek AR Mulla,; Balu S. Uphade. Article first published online: 4 MAY 1999. ... Publication History. Issue published online: 4 MAY 1999; Article first published online: 4 MAY 1999; Manuscript Accepted: 13 JAN 1998; Manuscript Revised: 26 SEP 1997; ...

ABSTRACT Properties of Ag–La2O3/SiO2 catalysts in oxidative coupling of methane (OCM) carried out in redox cyclic mode were studied. Methane and air were injected as separate and alternating pulses, and the oxidative conversion of methane proceeded due to participation of mobile lattice oxygen of the catalysts. It was found that joint action of Ag and La2O3 in OCM could give considerable synergetic effect. Such catalysts as 3%Ag–10%La2O3/SiO2 and 6%Ag–10%La2O3/SiO2 exhibited high efficiency in the OCM reaction performed in the redox cyclic mode. In the case of 3%Ag–10%La2O3/SiO2, the yield of C2 hydrocarbons reached about 30% at C2 selectivity close to 60%. Active sites which were responsible for OCM reaction were generated during reduction of silver by the injected methane. OCM selectivity increased remarkably if a small amount of hydrogen was injected over the catalyst before the methane pulse.

In this study a mathematical model of a small scale single pellet for the oxidative coupling of methane (OCM) over titanite pervoskite is developed. The method is based on a computational fluid dynamics (CFD) code which known as Fluent may be adopted to model the reactions that take place inside the porous catalyst pellet. The steady state single pellet model is coupled with a kinetic model and the intra-pellet concentration profiles of species are provided. Subsequent to achieving this goal, a nonlinear reaction network consisting of nine catalytic reactions and one gas phase reaction as an external program is successfully implemented to CFD-code as a reaction term in solving the equations. This study is based on the experimental design which is conducted in a differential reactor with a Sn/BaTiO 3 catalyst (7−8 mesh) at atmospheric pressure, GHSV of 12000 h −1 , ratio of methane to oxygen of 2, and three different temperatures of 1023,1048 and 1073 K. The modeling results such as selectivity and conversion at the pellet exit are in good agreement with the experimental data. Therefore, it is suggested that to achieve high yield in OCM process the modeling of the single pellet should be considered as the heart of catalytic fixed bed reactor.

The simulated countercurrent moving-bed chromatographic reactor (SCMCR) has been reported to significantly enhance methane conversion and C 2 product yield for oxidative coupling of methane (OCM) reaction, which is otherwise a low per pass conversion reaction. A mathematical model of an unconventional three-section SCMCR for OCM was first developed and solved using numerically tuned kinetic and adsorption parameters. The model predictions showed good agreement with available experimental results of SCMCR for OCM. Effects of several process parameters on the performance of SCMCR were investigated. A multi-objective optimisation problem was solved at the operating stage using state-of-the-art AI-based non-dominated sorting genetic algorithm with jumping genes adaptations (NSGA-II-JG), which resulted in Pareto Optimal solutions. It was found that the performance of the SCMCR could be significantly improved under optimal operating conditions.