Oxidative coupling of methane (OCM) is a promising technique for converting methane to higher hydrocarbons in a single reactor. Catalytic OCM is known to proceed via both gas-phase and surface chemical reactions. It is essential to first implement an accurate gas-phase model and then to further develop comprehensive homogeneous–heterogeneous OCM reaction networks. In this work, OCM gas-phase kinetics using a jet-stirred reactor are studied in the absence of a catalyst and simulated using a 0-D reactor model. Experiments were conducted in OCM-relevant operating conditions under various temperatures, residence times, and inlet CH 4 /O 2 ratios. Simulations of different gas-phase models related to methane oxidation were implemented and compared against the experimental data. Quantities of interest (QoI) and rate of production analyses on hydrocarbon products were also performed to evaluate the models. The gas-phase models taken from catalytic reaction networks could not adequately describe the experimental gas-phase performances. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics; it is recommended for further use as the gas-phase model for constructing homogeneous–heterogeneous reaction networks.

Oxidative coupling of methane (OCM) is a promising process for converting natural gas into high-value chemicals such as ethane and ethylene. The process, however, requires important improvements for commercialization. The foremost is increasing the process selectivity to C 2 (C 2 H 4 + C 2 H 6 ) at moderate to high levels of methane conversion. These developments are often addressed at the catalyst level. However, optimization of process conditions can lead to very important improvements. In this study, a high-throughput screening (HTS) instrument was utilized for La 2 O 3 /CeO 2 (3.3 mol % Ce) to generate a parametric data set within the temperature range of 600−800 °C, CH 4 /O 2 ratio between 3 and 13, pressure between 1 and 10 bar, and catalyst loading between 5 and 20 mg leading to space-time between 40 and 172 s. Statistical design of experiments (DoE) was applied to gain insights into the effect of operating parameters and to determine the optimal operating conditions for maximum production of ethane and ethylene. Rate-of-production analysis was used to shed light on the elementary reactions involved in different operating conditions. The data obtained from HTS experiments established quadratic equations relating the studied process variables and output responses. The quadratic equations can be used to predict and optimize the OCM process. The results demonstrated that the CH 4 /O 2 ratio and operating temperatures are key for controlling the process performance. Operating at higher temperatures with high CH 4 /O 2 ratios increased the selectivity to C 2 and minimized CO x (CO + CO 2 ) at moderate conversion levels. In addition to process optimization, DoE results also allowed the flexibility of manipulating the performance of OCM reaction products. A C 2 selectivity of 61% and a methane conversion of 18% were found to be optimum at 800 °C, a CH 4 /O 2 ratio of 7, and a pressure of 1 bar.

Summary A reliable data set covering a parametric space of process conditions is essential for realizing catalyst informatics. A high-throughput screening (HTS) instrument was used to obtain a parametric data set to develop a detailed reaction microkinetic model for the oxidative coupling of methane (OCM) over a La2O3/CeO2 catalyst. The model was combined with well-validated gas-phase kinetics to describe the interactions between homogeneous and heterogeneous reaction chemistry. Methane and oxygen conversions and selectivities of ethylene, ethane, carbon monoxide, and carbon dioxide were measured experimentally in the temperature range of 500 to 800°C, CH4/O2 ratio between 3 and 13, and pressure between 1 bar and 10 bar. The proposed reaction network consists of 52 irreversible elementary steps describing catalytic reactions between 11 surface species and 123 reversible steps describing the contribution of gas phase between 25 species. A packed-bed reactor model was developed based on the dimensions of the experimental setup and catalyst characterization results to account for homogeneous-heterogeneous interactions. The proposed mechanism was tested and validated over a wide range of operating conditions and showed a reasonable fit with an average difference of less than 5% compared to experimentally measured methane conversion and selectivities of ethylene and ethane. Rate of production (ROP) and species sensitivity analyses were performed to identify the main reaction pathways and highlight the important reactions in the OCM.

The reliance on methanol as an alternative and sustainable source to produce monoaromatics is challenged by many side reactions. Although the strategy of intentionally incorporating a number of active functionalities (metals and acid sites) in individual catalyst particles can enhance product selectivity, it promotes undesired hydrogen-transfer reactions leading to paraffins and the formation of polyaromatic species causing catalyst deactivation. Here, we report a catalysis relay strategy to control the chemistry by activating methanol differently. Methanol is selectively converted to formaldehyde and light olefins, which follow Prins and Diels-Alder reactions to yield monoaromatics. High selectivity is achieved by the precise location of active sites of different nature in a reactor. The catalyst, which is a combination of inexpensive and commercial ZnO, HZSM-5 and a Zn-ion exchanged HZSM-5, achieves aromatic selectivities of 71.4% initially and above 40% after a 400-h test with full methanol conversion, showing potential for industrial application. In addition, monocyclic aromatics make up nearly all (99%) the liquid products, and BTX (benzene, toluene, and xylenes) account for 91% of monoaromatics, which is advantageous for product separation. This work provides a sound approach to balance product selectivity and catalyst stability via catalysis relay for complex heterogeneous catalytic processes.