New catalyst enables lower-temperature methane conversion with sustained performance

Oxidative coupling of methane (OCM) is a chemical reaction that directly converts methane and oxygen, the main components of natural gas, into higher-value hydrocarbons such as ethane and ethylene. However, the reaction presents a fundamental selectivity challenge.

The C–H bonds in methane are highly stable and difficult to activate at low temperatures. Furthermore, as ethane and ethylene are more reactive than methane, they readily overoxidize to carbon oxides (COx). Conventional catalysts used for OCM require operating temperatures near 800°C (1,472°F) and can undergo substantial deactivation during prolonged operation, hindering widespread industrial adoption.

While scientists have tested a wide range of catalysts, simultaneously achieving low-temperature activity, high product yields and long-term stability remains a critical hurdle.

Lanthanoid oxides offer a stable alternative

To address these challenges, researchers at Institute of Science Tokyo (Science Tokyo), Japan, have developed lanthanoid-based high-entropy oxide (HEO) catalysts, materials in which five or more metallic elements are incorporated into a common crystal structure.

The research team, led by Professor Keigo Kamata from the Materials and Structures Laboratory, Institute of Integrated Research, Science Tokyo, and Assistant Professor Keiju Wachi from the Department of Applied Chemistry, School of Engineering, The University of Tokyo, Japan, demonstrated that HEOs combining lanthanoid elements exhibit exceptional low-temperature activity and long-term stability during OCM.

Their study was published online in the Journal of the American Chemical Society.

Screening narrows the catalyst field

The researchers screened 55 HEO compositions spanning five crystal structures and 24 elements. "As a result of the screening, lanthanoid-containing systems emerged as promising candidates, with C-type rare-earth HEOs combining relatively high C2 yields with structural stability," says Kamata.

Seven HEOs were synthesized as nanoparticle catalysts using a sol–gel method developed in-house. HEOs featuring five homogeneously integrated elements were synthesized by calcining amorphous precursors prepared from metal acetates and aspartic acid at 750–800°C (1,382–1,472°F).

Further screening led to the selection of HEO-2, composed of lanthanum (La), samarium (Sm), europium (Eu), gadolinium (Gd) and dysprosium (Dy), (LaSmEuGdDy)0.4O3, as a representative catalyst for detailed investigation.

Moderate basicity enables lower-temperature conversion

In catalytic tests, HEO-2 initiated C2 hydrocarbon formation at 525°C (977°F), a temperature significantly lower than those required by previously investigated catalysts. At 600°C (1,112°F), HEO-2 reached a 12.3% C2 yield.

The researchers linked this low-temperature activity to the catalyst's surface basicity. Their analysis showed that moderately basic sites, rather than the strongest basic sites, are most favorable for OCM, as excessively strong basicity can promote overoxidation of C2 products or strong CO2 adsorption.

Importantly, the team discovered that surface basicity could be systematically tuned through the average ionic radius of the constituent lanthanoid elements.

HEO-2, with an average ionic radius of 0.957 Å, exhibited CO2 desorption predominantly in the 300–400°C (572–752°F) range, associated with moderately basic sites effective for C2 production. The high-entropy configuration of the HEO also appears to stabilize these active surface functionalities.

Sustained performance over 240 hours

This stabilization was particularly evident during long-term operation. At 600°C (1,112°F), HEO-2 maintained a C2 yield after 240 hours that was essentially unchanged from its initial performance. Its calculated deactivation rate was 1.0 mmolC2 gcat-1 h-2, more than 25 times lower than those of the corresponding single oxides.

Surface analysis showed that the moderately basic sites were substantially preserved after the reaction, while HEO-2 also retained its C-type polycrystalline framework. In contrast, single oxides either underwent structural transformation or progressively lost their active basic sites.

Together, these findings demonstrate how compositional entropy simultaneously tunes surface chemistry and stabilizes active catalyst structures.

"By establishing the average ionic radius as a descriptor for surface basicity, our study provides a rational framework for designing selective oxidation catalysts that convert methane at lower temperatures with sustained activity," concludes Kamata.