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Critical catalytic reaction understood at molecular level

Researchers from Lehigh and the Stevens Institute of Technology have announced a major advance in the fundamental understanding of a catalytic reaction that directly converts natural gas into valuable liquid fuels and feedstocks for the chemical industry.

In an article published in Science magazine on April 9, the researchers reported a molecular-level understanding of the critical reaction, which employs catalysts with molybdenum nanostructures supported on shape-selective zeolites. They identified the initial molybdenum nanostructures and their anchoring sites on the zeolite supports by combining state-of-the-art spectroscopy techniques, reaction rate measurements and quantum-chemical calculations.

In addition, the researchers demonstrated that the initial molybdenum nanostructures can be regenerated after catalyst deactivation, fully restoring catalytic activity. They also showed that the distribution of the molybdenum nanostructures can be controlled, which can lead to enhancements in catalytic activity. The three-year project was funded by the National Science Foundation.

Their achievement, the researchers wrote, “opens new opportunities for the rational design of improved catalyst formulations and for optimizing the reaction conditions” for the conversion of natural gas.

The Science article, titled “Identification and Regeneration of Molybdenum Oxide Nanostructures on Zeolites for Catalytic Conversion of Natural Gas to Liquids,” was coauthored by Jie Gao, Yiteng Zheng and Simon Podkolzin of Stevens Institute and Jih-Mirn Jehng, Yadan Tang and Israel E. Wachs of Lehigh.

Gao and Zheng are graduate students in Prof. Podkolzin’s group in the department of chemical engineering and materials science at Stevens. Tang completed her Ph.D. in chemistry at Lehigh in 2014 under Wachs’ supervision and is now an engineer at Cummins Inc. in Indiana. Jehng is a professor in the department of chemical engineering at National Chung Hsing University in Taiwan. He worked on the project in Wachs’ group as a visiting scientist at Lehigh.

Wachs is the head of the Operando Molecular Spectroscopy and Catalysis Laboratory and the G. Whitney Snyder Professor of Chemical and Biomolecular Engineering at Lehigh.

(Note: To view the article, visit this page of the Operando Lab website, scroll down to “Publications” and click on “2015”)

An often stranded resource

Natural gas is abundant and inexpensive but underutilized. More than half the world’s known reserves are classified as stranded for lack of efficient technologies to process natural gas at remote locations. It is usually not economical to transport natural gas in its gas phase over long distances, and the cost of converting it to a shippable liquid or building a pipeline can be prohibitive.

Natural gas is often produced when an oil well is drilled, but at remote sites, this gas is typically flared—burned off—or vented into the atmosphere. Worldwide, more than 140 billion cubic meters of natural gas are flared or vented every year. This is equvalent to about 20 per cent of all the natural gas consumed annually in the United States.

This venting and flaring, say the Stevens and Lehigh researchers, contribute significantly to global warming. Methane, the main component of natural gas, traps about 86 times more heat over a 20-year period than carbon dioxide. Therefore, new technologies for natural gas conversion are urgently needed.

One of the technologies under development, which can address this technical challenge, is the direct conversion of natural gas into liquid aromatic hydrocarbons in a single step without oxidizing reagents. This dehydroaromatization of methane is achieved using catalysts with molybdenum nanostructures supported on shape-selective zeolites. This technology, say the researchers, offers unique advantages over other methane activation chemistries because it does not require the transportation of reagents to remote locations.

In their NSF project, the researchers studied molybdenum nanostructures supported on zeolites, which are crystalline porous silica-alumina materials. They combined steady-state and transient reaction rate measurements with multiple spectroscopic techniques, including in situ infrared spectroscopy, operando Raman spectroscopy and in situ ultraviolet-visible diffuse reflectance spectroscopy. The experimental results were interpreted with the help of quantum chemical and molecular mechanical modeling calculations.

“The close integration of experimental and computational techniques was essential for the success of our collaborative project,” said Wachs.

“Our jointly developed synergistic methodology of catalyst testing, characterization and modeling can be readily extended to studies of other materials and reactions,” said Podkolzin.

Regenerating catalytic activity

The researchers said their published results could help overcome one of the biggest obstacles to the commercialization of natural gas over catalytic molybdenum nanostructures—the rapid deactivation of the catalyst. Progress in developing improved catalyst formulations and in optimizing reaction conditions has been hindered by the lack of molecular-level understanding of the reaction.

“The catalyst deactivates rapidly over time,” said Wachs. “We showed that it’s possible to reverse this process and fully restore the initial catalytic activity with regeneration in air.”

By combining experimental and computational studies, the researchers characterized the catalyst under reaction conditions. This made it possible, they wrote, “to determine the identity and anchoring sites of initial molybdenum oxide nanostructures and establish a relationship between the identity of molybdenum nanostructures and catalytic performance.”

In this manner, the researchers demonstrated how to reverse the deactivation of the catalyst and even enhance its performance. The key, they said, is to reverse the formation of carbonaceous deposits and growth of molybdenum nanostructures under reaction conditions. It is also important to control the anchoring sites of the molybdenum nanostructures.

“Most molybdenum nanostructures anchor on one aluminum atom or on two aluminum atoms in the zeolite framework,” said Wachs. “However, some nanostructures also anchor on silicon atom sites on the external surface of the zeolite.

“Our results demonstrate that the distribution of different types of molybdenum nanostructures and their anchoring sites can be controlled. Higher catalytic activity is observed when molybdenum nanostructures are restricted to anchoring sites with framework aluminum atoms.”

Discovering the identity of catalytic molybdenum nanostructures and the means of their regeneration will be helpful in the development of new natural gas processing technologies, the researchers said. The new technologies will address not only the economic issue of natural gas conversion into liquid fuels and chemical feedstocks but also a significant environmental issue. If the current venting and flaring of natural gas can be eliminated, such a reduction in greenhouse gas emissions will by itself more than meet the requirements of the Kyoto Protocol in the United Nations Framework Convention on Climate Change for all the participating countries combined.

Story by Kurt Pfitzer