Non-Oxidative Coupling of Methane over Heterogeneous Catalysts: Understanding Surface and Gas-Phase Processes

October 4, 2023

Hao Zhang defended his PhD thesis at the department of Chemical Engineering and Chemistry on October 4th.

iDPC-STEM image of Mo/ZSM-5 catalyst. (b)-(d) Enlarged areas of 1 (empty channel), 2 (channels containing a MoO3H cluster bonded at T8 site), and 3 (MoO3H sites bonded at T1 sites). Each panel includes the STEM image (top), calculated structural model (middle), and the simulated projected electrostatic potential (bottom); Si blue, Al green, H white, Mo pink, and O red. (e) Line scanning profiles of the intensities in image (b)-(d). (f) Statistical analysis of Al occupancy at different T sites. Reproduced with permission from Ref. 41. Copyright 2020 Wiley-VCH.

Methane is the main component of the abundant natural gas. In addition to using natural gas for heating or coking in our daily life, it is also attractive to use it as a chemical feedstock for the production of value-added chemicals. The structure of methane molecule is simple, it only contains one carbon atom and four hydrogen atoms. However, the activation of methane into more valuable products is very challenging because of the strong connection between carbon and hydrogen atoms (the C-H bond in methane). Among the techniques developed for methane utilization, employing heterogeneous catalysts is effective to activate methane.

There is already significant progress for methane valorization to produce high-value hydrocarbon products. Through multi-step routes, methane is first converted to synthesis gas (syngas, a mixture of carbon monoxide and hydrogen) by steam reforming or gasification processes, followed by the catalytic conversion of syngas through Fischer-Tropsch synthesis or methanol synthesis processes. Although these multi-step routes have already been commercialized, the nature of indirect methane conversion can be only carried out at large scale in a cost-effective manner. Therefore, it would be beneficial to also develop medium- or small-scale methane valorization routes in which methane can be converted to value-added products in a single step. This technique will be attractive for natural gas utilization in remote areas that methane cannot be transported in an economically favorable way to end use markets.

This thesis focuses on the understanding methane non-oxidative reaction over heterogeneous catalysts, which is one of the methane direct conversion routes. By using the methane non-oxidative coupling technique, the low-value methane can be converted to hydrocarbon products like ethylene, the key building block of plastics, in a single step. Nevertheless, commercialization of this technique is hampered the lack of suitable catalysts that exhibit high selectivity and stability. And sufficient understanding on this promising reaction can aid the rational design of more effective catalysts. To achieve reasonable product yield, this process requires high reaction temperature, for example, above 600 °C, and a major challenge of this reaction is that coke (low value carbon materials) is thermodynamically preferred over all the other products. Another feature of this reaction is the involvement of gas-phases processes such as radical reactions in the reactor besides the surface catalytic reactions. The harsh reaction conditions under high temperature in combination with the coke formation and gas-phase reactions make the understanding and development of this process challenging.

In this thesis, three classes of heterogeneous catalysts with different properties, for example, different kinds of active sites or porosity, were prepared and tested in methane non-oxidative coupling reaction at temperatures higher than 750 °C. In addition to analyzing the activity and stability of these catalysts, multiple advanced characterization tools were used to monitor the processes occurred on the catalyst surface and in the gas-phase regions of the reactor. This study tries to reveal the relation between catalytic performance, the gas-phase reactions, and the structure of active sites. The new insights obtained in this thesis may help in the design of more efficient heterogeneous catalysts for methane non-oxidative coupling reaction.

Hao Zhang defended his thesis "Non-Oxidative Coupling of Methane over Heterogeneous Catalysts: Understanding Surface and Gas-Phase Processes" on Wednesday 4th of October 2023. He was supervised by Emiel Hensen and Dr. Nikolay Kosinov.


Bianca Moonen-Tossaint
(Departmental Communication Advisor)