CTC Highlight: Influence of First and Second Coordination Environment on Structural Fe(II) Sites in MIL-101 for C–H Bond Activation in Methane

3/11/21

Title: Influence of First and Second Coordination Environment on Structural Fe(II) Sites in MIL-101 for C–H Bond Activation in Methane
Date of Publication: December 28, 2020
Journal: ACS Catalysis
Authors: Jenny Vitillo (CTC), Connie C. Lu, Chris Cramer (CTC), Aditya Bhan, and Laura Gagliardi

Natural gas is an important fuel and a largely available resource, and methane is its main component. The handling of gas requires the construction of major infrastructures because gas pipelines are very costly. As a result, the equivalent of about 1/6 of the annual consumption of natural gas in the US is flared—burned off in a controlled manner—each year in the oilfields because it is too costly to transport it. 

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As a versatile chemical feedstock that is biodegradable and an excellent energy vector, methanol is the starting reagent for the production of many commodities, including fuels, pharmaceuticals, and fertilizers. Methanol’s global demand is about 100 million tons per year with market size of 30 billion US dollars, and it is steadily increasing. Methanol is used as an additive in gasoline to improve engine efficiency and decrease car pollution, but it can also be used as fuel on its own. It is not surprising that the “methanol economy” is suggested as a solution for replacing fossil fuels in their applications.

Methanol is obtained primarily from methane through an indirect mechanism that involves complex and energy-intensive industrial processes. This direct conversion has inspired academic and industrial research for decades. The technological gap is the identification of materials—namely catalysts—that enable the methane to methanol conversion at a low cost. The impact of the identification of such catalysts will be global. It will lower the dependence of the United States on petroleum, while also reducing vehicular emissions of hydrocarbons and greenhouse gases.

Even though enzymes can catalyze the methane to methanol reaction efficiently at room temperature, they’re expensive and have short lifetimes in reaction conditions, making their large-scale use impracticable. Most studies have suggested combining the biological and the synthetic approaches to obtain more efficient synthetic catalysts with a lower cost and longer lifetime. However, it has been historically difficult to compare the diverse strategies of different catalysts, presenting the need for a systematic study. This would be nearly impossible to do efficiently in an experimental lab due to the large number of catalysts and behaviors. In this study, researchers employed state-of-the-art theoretical methods to further understand these catalysts.

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Due to their flexibility, enzymes—which are found in biological systems—can change their geometry several times during a reaction, making them great catalysts. The most efficient catalysts adopt a strain-releasing strategy, meaning they start from a tense geometry—like a contracted spring—and then, as the reaction proceeds, this strain is gradually released, lowering the energy. This holds true for the methane to methanol conversion, too. As the strain is gradually released, the energy required for the methane to methanol conversion decreases.

Using computational methods, researchers in this study were able to replicate enzymatic behavior—which is a biological material—in a catalyst, which is a synthetic material. They systematically explored how small modifications in the catalyst composition alter the reaction performances by mimicking the strategies employed by enzymes in their natural evolution. In total, 26 new catalysts were modeled, corresponding to about 1000 structures simulated.

According to Jenny Vitillo, a CTC alum and the first author of this publication, the most interesting finding was that through the comparison of the results obtained among the catalysts candidates, they were able to delineate very general rules that can be used to improve the performances of most of the catalysts already reported in the literature for the methane to methanol reaction, as well as more generally for reactions based on the carbon-hydrogen bond cleavage, which are at the basis of several industrial processes already in operation. This study will reduce the energy costs of these processes and increase their efficiency.

Ultimately, findings from this study provided much-needed information about the conversion from methane to methanol. This information will be used to make better-informed decisions about how to meet global demand for methanol, a feedstock that holds promise for a cleaner environment.