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Burning natural gas at oil and gas wells, also called flaring, is a major waste of fossil fuels and contributes to climate change. But to date, capturing the flared natural gas, which the International Energy Agency estimates at around 140 billion cubic meters per year, has not been economically feasible.
At the University of California, Berkeley, chemists have now devised a simple and green way to convert these gases – mainly methane and ethane – into economically valuable liquids, mostly alcohols such as methanol and ethanol. The liquids are also easier to store.
The alcohols can be used as a feedstock for the production of many other petrochemical products, providing an additional source of revenue for oil and gas companies, but also reducing carbon dioxide emissions from flaring. Flaring is used to mitigate the more harmful effects of releasing natural gas (methane is 34 times more powerful as a greenhouse gas than carbon dioxide) directly into the atmosphere.
Details of the trial were published Nov. 3 in the journal Science.
The new process for oxygenating hydrocarbons into alcohols mimics the way plants and animals add oxygen to carbon-hydrogen bonds to produce energy from carbohydrates, fats and proteins. Carbon-hydrogen bonds are equally abundant in the hydrocarbon molecules that make up fossil fuels.
The natural oxidation processes involve an enzyme centered on a reactive metal – in most cases iron – that catalyzes the insertion of an oxygen atom between a carbon and hydrogen bond to produce COH, an alcohol group.
Much research has gone into finding variants of these natural enzymes that would convert gaseous hydrocarbons from fossil fuels into liquid alcohols without the energy input and massive infrastructure required in today’s chemical industry. But most of these processes involve artificial enzymes in a liquid solution.
UC Berkeley’s innovation integrates these reactive iron sites into a rigid and porous crystalline structure – a metal-organic framework, or MOF – that stabilizes the iron and allows easy entry of gas and easy exit of liquid alcohols.
“We have a lot of natural gas resources here that are too small to build a large-scale facility around them to convert ethane to ethanol or methane to methanol,” said Jonas Börgel, a postdoctoral researcher at UC Berkeley and first author of the paper. , along with former graduate student Kaipeng Hou, now a postdoctoral researcher at UCSF. Both worked in the laboratory of Jeffrey Long, a UC Berkeley professor of chemistry and chemical and biomolecular engineering and a faculty scientist at Lawrence Berkeley National Laboratory (Berkeley Lab).
“This system is nice because it is the first purely synthetic and non-enzymatic process that can use oxygen at near ambient temperature to carry out these reactions reminiscent of the reactivity of metalloenzymes,” Börgel said. “That’s where it really shows its potential for converting natural gas components into more easily storable energy sources such as alcohols. The big advantage of this is converting natural gas into something that has more value than the gas itself and that has the potential to actually have more value. Economically achieveable.”
While the process is still being perfected, Börgel said that if it proves efficient at producing alcohols with less energy input than current processes, it could also be useful in large-scale facilities, many of which produce megatons of alcohols per year from natural gas.
“The standard way to oxygenate hydrocarbons is a multi-step process using high-temperature heterogeneous catalysts that only function at those temperatures, and that is very energy intensive,” he said, “while this process has the potential to operate at temperatures much closer to ambient temperatures – 25 to 50 degrees Celsius.”
Hou and Börgel first noted that an iron-containing variant of the MOF called MFU-4l – made mainly of zinc chloride and organic bis-triazole ligands – could be chemically modified to obtain an iron(II) active site that resembles at the active site in a natural enzyme called taurine dioxygenase (TauD), which supplies the amino acid taurine with oxygen.
“We were inspired by the active site of the enzyme in TauD – the geometry around the iron center is very similar to the coordination geometry around the peripheral metals in the nodes in this metal-organic framework,” he said. “We thought that if we modified the MOF in the same way and placed another ligand on it that resembled the cofactor in the enzyme, we might be able to mimic the CH oxygenation reactivity of the enzyme. That turned out to be true. . “
“This is a good example of how the protected boundaries within the pores of a MOF can be used to create the types of highly reactive metal sites found in enzymes,” Long said.
The researchers performed a detailed spectroscopic analysis of the modified MOF and confirmed that the iron(II) active site behaved similarly to the iron(II) site in TauD, converting hydrocarbons to alcohols.
“We’re now trying to move from our Science paper, which covered a lot of fundamental chemistry, to working on making this a more amenable process,” Börgel said.
The research was supported by the US Department of Energy Office of Basic Energy Sciences (DE-SC0019992). Some of the spectroscopic measurements were performed at Berkeley Lab’s Advanced Light Source.
The UC Berkeley team collaborated with colleagues from the Max Planck Institute in Germany, Argonne National Laboratory and Northwestern University in Illinois, the University of Milan in Italy, the University of Missouri at Rolla, Berkeley Lab and UC Davis.
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