Alternative Oxidants for the Catalytic Oxidative Coupling of Methane.

The catalytic oxidative coupling of methane (OCM) to C2 hydrocarbons with oxygen (O2‐OCM) has garnered renewed worldwide interest in the past decade due to the emergence of enormous new shale gas resources. However, the C2 selectivity of typical OCM processes is significantly challenged by overoxidation to COx products. Other gaseous reagents such as N2O, CO2, and S2 have been investigated to a far lesser extent as alternative, milder oxidants to replace O2. Although several authoritative review articles have summarized OCM research progress in depth, recent oxidative coupling developments using alternative oxidants (X‐OCM) have not been overviewed in detail. In this perspective, we review and analyze OCM research results reporting the implementation of N2O, CO2, S2, and other non‐O2 oxidants, highlighting the unique chemistries of these systems and their advantages/challenges compared to O2‐OCM. Current outlook and potential areas for future study are also discussed.

Products of catalytic oxidative coupling of methane to improve thermal efficiency in natural gas engines

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.

The catalytic oxidative coupling of methane (OCM), which can be used to obtain ethylene, is a major challenge in heterogeneous catalysis. This chapter mainly discusses the active sites of OCM catalysts, their reaction mechanisms, and their catalytic performance under various oxidative reaction conditions, including the OCM reaction network. In the OCM reaction, CH4 is oxidatively converted to C2H6 and then C2H4. After activation of CH4 on catalysts such as metal oxides, the formation of C2H6 proceeds in a homogenous gas phase via a free-radical mechanism. Thus, C2H6 is produced mainly by the coupling of the surface-generated •CH3 radical (methyl radical) in the gas phase. The C2H4/C2H6 yields are limited by the secondary reaction of •CH3 radicals with the catalyst and reactor surfaces and the further oxidation of C2H4 on the catalyst surface and in the gas phase. The nature of the active sites and the reaction mechanism have been investigated. Reactive oxygen ions, such as O− or O22−, are required for the activation of methane on catalysts. However, no feasible processes have resulted, despite a reasonable understanding of the elementary reactions in the OCM reaction. The non-oxidative coupling of methane (dehydrogenative coupling of methane) gives C2H4 and aromatic hydrocarbons at ~1000 K. Although the dehydrogenative coupling of methane is thermodynamically disadvantageous due to the large positive change in free energy, over-oxidation does not occur, and CO and CO2 are not formed. The catalytic performance of supported Fe catalysts, such as SiO2-supported Fe, are discussed, along with their catalytic properties.

On the design, development and operation of an energy efficient CO2 removal for the oxidative coupling of methane in a miniplant scale

Performance Analysis of Bubbling Fluidised-bed Reactors for the Catalytic Oxidative Coupling of Methane

In this study, mathematical modeling of Oxidative Coupling of Methane (OCM) to C2 hydrocarbons (C2H6 and C2H4) over La2O3/CaO catalyst in a fixed-bed reactor operated under isothermal and non-isothermal conditions was investigated. The kinetic model proposed for OCM process consisted of 10 elementary reactions and 8 chemical species. In this process, methane and acetylene were the inputted feed and ethane, ethylene, propylene, propane, i- butane and n-butane were the output products. The amount of methane conversion obtained was 12.7% for the former feed, however, if pure methane was inputted this conversion rose to 13.8%. Furthermore, the plasma process would enhance the conversion, selectivity towards desired product and yield. In the present study, when methane and acetylene were fed at a molar ratio of CH4/C2H2=10 to the reactor, the selectivity of C2, C3 and C4 hydrocarbons was determined to be 30, 24 and 44% respectively. Concurrently, a higher yield was obtained for n-butane at about 3.9% and the minimum yield achieved for propylene was approximately 0.7%. A comparison between the thermal and the plasma process showed that the methane conversion and yield production in the plasma were higher than in the thermal process under the same operating conditions. On the other hand, product selectivity in the plasma process was determined to be lower than that of the thermal process. Finally, the results of the catalytic OCM and methane conversion processes in the plasma phase were compared with one another.

Methane utilization by oxidative coupling: I. A study of reactions in the gas phase during the cofeeding of methane and oxygen

Gas-phase methyl radicals have been long proposed as the key intermediate in catalytic oxidative coupling of methane, but the direct experimental evidence still lacks. Here, employing synchrotron VUV photoionization mass spectroscopy, we have directly observed the formation of gas-phase methyl radicals during oxidative coupling of methane catalyzed by Li/MgO catalysts. The concentration of gas-phase methyl radicals correlates well with the yield of ethylene and ethane products. These results lead to an enhanced fundamental understanding of oxidative coupling of methane that will facilitate the exploration of new catalysts with improved performance.

Feasibility of Ethylene Synthesis via Oxidative Coupling of Methane

The major goal of this research is to make progress towards isolating and identifying the most active and selective phase(s) in the catalytic oxidative coupling of methane to C{sub 2} hydrocarbons by one of the most active and selective REO'' catalysts, La{sub 2}O{sub 3}.'' As discussed in the foregoing literature review, the catalyst preparation methods and the reaction conditions employed can result in the catalyst consisting of several coexisting rare earth compounds. Furthermore, it is conceivable, if not likely, that either the pure REO phase or one of these coexisting compounds is much more catalytically active towards the desired reaction than all others. It is therefore proposed that by employing strict catalyst preparation methods and multi-technique characterization, pure oxide, hydroxide, carbonate, hydroxycarbonate and oxycarbonate phases of lanthanum may be prepared and subsequently tested for catalytic performance such that the extent to which each of these phases are involved in the catalytic oxidative coupling of methane may be determined. 157 refs., 60 figs., 6 tabs.

The homogeneous and catalytic oxidative coupling of methane (OCM) for converting methane directly into higher hydrocarbons has been the subject of a large body of research. The present study on conversion of methane in dc corona discharge packed bed reactors may significantly improve the process economics. Experimental investigations have been conducted in which all the reactive gases pass through a catalyst bed which is situated within the corona-induced plasma zone. In this study, a typical OCM catalyst, Sr/La2O3, was used to investigate experimentally the corona discharge OCM reactions. Experiments were conducted over a wide range of temperatures (823−1023 K) and input powers (0−6 W) with both positive and negative corona processes. Compared to the catalytic process in the absence of corona discharge, the corona discharge results in higher methane conversion and larger yield of C2 products even at temperatures at which there is no C2 activity for the catalyst alone . The methane conversion and C2 yield increase with O2 partial pressure during the corona-enhanced catalytic reactions, while the selectivity decreases slightly with increasing O2 partial pressure. Compared to results obtained in the absence of corona discharges, methane conversion in the presence of the dc corona was nearly five times larger and the selectivity for C2 over eight times higher at 853 K. A great enhancement in catalytic activity has also been achieved at a temperature at which the catalyst alone shows no C2 activity. The conversion at higher temperature (more than 953 K) is limited by the poor corona performance and the availability of active oxygen species.

Direct and selective catalytic oxidative coupling of methane (OCM) into high‐carbon products is a great challenge in C1 chemistry. Herein, the successful fabrication of a series of Au clusters loaded on different MgO facets of (111), (110), and (100) is reported. This work demonstrates the Au‐loaded MgO(111) (denoted as Au/MgO(111)) shows the highest C2H6 yield of 12733.4 µmol g−1 h−1 and selectivity of 90.4%, which is 2.46 times higher than that of Au/MgO(110) (5171.4 µmol g−1 h−1) and 25.14 times higher than that of Au/MgO(100) (506.4 µmol g−1 h−1). Moreover, the high activity of Au/MgO(111) can be well maintained over 100 h. Detailed in situ experiments and theoretical calculation reveal such great performance can be attributed to 1) the strong electronic affinity of the surface oxygen species on polar MgO(111) and CH4with the lowest adsorption energy of −0.82 eV; 2) the Au/MgO(111) shows the lowest ΔERDS of 0.53 eV in the rate‐deterime step of OCM that comes in activating the second CH4 molecule; 3) the Au can act as a hole acceptor under light irradiation and adsorb *CH3 with a strong d‐σ hybridization, resulting in an increased C2H6 selectivity.

The role of initiation in oxidative coupling of methane

The oxidative coupling of methane over tin promoted lithium magnesium oxide: a TAP investigation