The study of composition of novel high temperature catalysts for oxidative conversion of methane

A direct method of converting natural gas into ethylene is the heterogeneously catalyzed oxidative coupling of methane (OCM), however, only with hydrocarbon yields limited to 30-35% despite enormous efforts to optimize the catalysts. By combining the exothermic OCM with a secondary process, namely steam reforming of methane (SRM), the methane conversion can be increased significantly while improving temperature control and simultaneously producing valuable synthesis gas. In this thesis, two different reactor concepts were developed to integrate the OCM and SRM reactions in an overall autothermal process, so that the OCM process is effectively cooled and the generated reaction energy is efficiently used to produce synthesis gas. The integration is most optimally achieved on the catalyst particle scale, which would eliminate the need for external heat exchange and opens up the possibility to use distributive oxygen dosing with which much higher product yields can be achieved. It is proposed to use a dual function catalyst particle in which the two chemical processes are physically separated by an inert, porous layer, such that additional diffusional resistances are intentionally created to control the reaction rates. This concept was studied with numerical simulations on the scale of a single catalyst particle and on reactor scale. It was found that the SRM and OCM reaction rates could be effectively tuned to achieve autothermal operation at the reactor scale, while the methane conversion was enhanced from 44% to 55%. An alternative integrated process can be achieved by combining OCM and SRM in a heat exchange reactor comprising of two separate reaction chambers which are thermally coupled. The OCM is carried out in packed bed reverse flow membrane reactor tubes submerged into a fluidized bed where the unconverted methane and byproducts from OCM are reformed, thus producing synthesis gas and consuming the reaction heat liberated by OCM. The feasibility of this concept is supported by experiments of OCM on a Mn/Na2WO4/SiO2 catalyst in a packed bed (porous Al2O3) membrane reactor. The results demonstrated that a C2 yield of 25-30 % can be achieved and that distributed feed of oxygen is optimal for the combined OCM/SRM reactor concept.

Analysis of oxidative coupling of methane in dense oxide membrane reactors

Monolithic composites with geometry controlled by polymeric 3D printed templates: Characterization and catalytic performance in OCM

The oxidative coupling of methane (OCM) is an attractive technology for the production of ethane (C2H6) and ethylene (C2H4); and significant performance and efficiency gains as well as reduced carbon dioxide (CO2) emissions are expected when OCM takes place within mixed ionic and electronic conducting (MIEC) ceramic membrane reactors (CMRs). So far, research on OCM in CMRs has been limited to unstable and incompatible materials investigated under short-term measurements that hinder upscaling and commercial application. To this end, this work demonstrates long-term stable OCM performance enabled by a BaFe0.9Zr0.1O3−δ (BFZ91) perovskite utilized as the oxygen-ion MIEC membrane and lanthanum oxide (La2O3) used as the OCM catalyst. Experimental measurements conducted in the temperature (T) range of 750–900 °C and at inlet methane (CH4) mole fractions (XCH4in) of 0–30% revealed a highly stable performance during 23 days of continuous operation, which was further confirmed by material characterization. Under the aforementioned operating conditions, BFZ91 offers a high oxygen (O2) permeation flux (JO2) between 0.5−1.5 (μmol/cm2/s); CH4 conversion (CCH4) reached ∼35% while the selectivities to C2H6 (SC2H6) and C2H4 (SC2H4) were as high as ∼50% and ∼40%, respectively, showing a strong dependency on the operating conditions. Yields of C2H6 (YC2H6) and C2H4 (YC2H4) in the range of 1–5% and 1–7%, respectively, were measured, with more C2H4 being produced at higher T. In the absence of La2O3, CCH4 and C2 (C2H6 and C2H4) yields are lower confirming that BFZ91 does not promote CH4 oxidation, reforming, or coupling on its surface at high rates. The OCM performance of BFZ91 with La2O3 was also found to be stable under partial O2 consumption and pure CH4 conditions. Furthermore, a detailed analysis of the mixture composition allowed the identification of the primary reactions in the OCM chemistry. Our results reveal that within our reactor, CH4 full oxidation to CO2 and steam (H2O) happens simultaneously with CH4 oxidation to C2H6 and H2O (both on the La2O3 catalyst), but the production of the valuable C2H4 is primarily taking place through the C2H6 non-oxidative dehydrogenation in the gas phase; this reaction was not found to proceed on the La2O3 catalyst. Besides the promise of the investigated materials toward commercialization, the methods to study the OCM chemistry and the membrane catalyst coupling presented here are expected to promote further advances in the field of OCM.

A new and very promising application of auto-thermal reactors is the coupling of endothermic and exothermic reactions where the product of the endothermic reaction is the desired one. Therefore, in this work, a reactor in which oxidative coupling of methane (OCM) and steam re-forming of methane (SRM) reactions take place simultaneously was modeled. The results were obtained in a wide range of different conditions such as inlet feed, inlet temperature, portions of OCM and SRM catalysts, and inlet velocity. In selection of the catalysts, more attention was drawn to prevent re-forming of OCM products. The main parameters of each reaction under different conditions such as conversion of the feed components, products selectivity and yield, temperature in the length of reactor, and component's concentration in the reactor were considered in course of this study. The results revealed that simultaneous OCM and SRM reactions in one reactor will tend to be auto-thermal, and the waste of energy will be reduced. The results also show that complete conversion of water and majority of methane and oxygen will decrease the amount of unwanted products at the reactor's discharge-a constraint that exists in single reactors of each reaction specially OCM.

The enhancement effects of BaX2 (X = F, Cl, Br) on SnO2-based catalysts for the oxidative coupling of methane (OCM)

The effects of the introduction of tetrachloromethane into the feedstream for the partial oxidation and oxidative coupling of methane

The oxidative coupling of methane (OCM) is being actively studied for the production of higher hydrocarbons from natural gas. The present study concentrated on the oxidative conversion of methane in an atmospheric pressure, nonthermal plasma formed by ac or dc corona discharges. Methyl radicals are formed by reaction with negatively-charged oxygen species created in the corona discharge. The selectivity to products ethane and ethylene is affected by electrode polarity, frequency, and oxygen partial pressure in the feed. Higher C2 yields were obtained with the ac corona. All the ac corona discharges are initiated at room temperature (i.e., no oven or other heat source is used), and the temperature increases to 300−500 °C due to the exothermic reactions and the discharge itself. The largest C2 yield is 21% with 43.3% methane conversion and 48.3% C2 selectivity at a flowrate of 100 cm3/min when the ac corona is at 30 Hz, 5 kV (rms) input power was used. The methane conversion may be improved to more than 50%...

Investigations of the selective partial oxidation of methanol and the oxidative coupling of methane over copper catalysts

Oxidative coupling of methane has been studied over (Mn+A+W)/SiO2 catalysts in a continuous–flow micro reactor, where A represents an alkali ion of Li, Na, K or Cs having different weight percents. The main aim of this study is to find the role of alkali ions in interaction between Mn and W species with SiO2 to make a proper structure for catalyzing oxidative coupling of methane (OCM) reaction. The catalysts were characterized by XRD, SEM, FTIR, TPR and also the electrical conductivity was measured in air and under OCM reaction. It was found that for the formation of crystallized catalyst, the amount of alkali ion should be such that the catalyst containing tungsten transforms into A2WO4. Using a smaller amount of alkali ions does not result in crystalline catalyst by calcination under the same condition of temperature and atmosphere. However, under the OCM reaction condition the catalyst gradually turns into a crystalline structure and its catalytic performance, i.e. conversion and selectivity, for the OCM reaction is almost similar to the (Mn+A2WO4)/SiO2 catalyst. The transformation of the catalyst containing alkali ions from amorphous to crystalline one indicates a kind of structural flexibility of the catalyst under OCM atmosphere. The structural flexibility of the catalyst under the OCM reaction is considered to be the main chemical role of the alkali ions.

For more than three decades, Oxidative Coupling of Methane (OCM) process has been comprehensively investigated as an attractive alternative for the commercially available ethylene production technologies such as ethane and naphtha cracking. Developing a suitable catalyst and proper reactor feeding policy, reviewing and deploying the efficient methods in the separation and purification of the undesired and desired products, possible energy saving and process intensification in different sections of the OCM process, each has been the subject of many researches in the past. In this paper, the interconnections of these aspects will be addressed by reviewing the performance of different alternative structures and unit operations in different sections of the OCM process. As a systematic approach in this analysis, a concurrent engineering approach supported by the experimental data which was extracted from an OCM miniplant scale facility was applied. Using an efficient porous packed bed membrane reactor and a proper set of operating conditions, up to 25% C2-yield, 20% C2H4-yield, and 52% C2H4-selectivity and the highest observed fluidized bed reactor C2-yield was achieved in this OCM miniplant. This experimental analysis was performed in the chair of process dynamics and operation at Berlin Institute of Technology under the main framework of Unifying Concepts in Catalysis (UniCat) project. The economic analysis of the industrial-scale operation showed promising potentials and also advantages of the final proposed OCM process-structure in this research.

Oxidative coupling of methane on fluorite-structured samarium–yttrium–bismuth oxide

Doped samarium oxide xerogels for oxidative coupling of methane—Effects of high-valence dopants at very low concentrations

Dissecting La2Ce2O7 catalyst to unravel the origin of the surface active sites devoting to its performance for oxidative coupling of methane (OCM)

Novel microdesign of oxidation catalysts. Part 2. The influence of fluorination on the catalytic properties of glass crystal microspheres