Hannah Rhoda with with the resonance Raman spectroscopy equipment

Scientists turn methane into methanol at room temperature

Image credit: Courtesy of Hannah Rhoda/Stanford University

An international team of scientists have developed a novel process that could be an important step toward a methanol fuel economy with abundant methane as the feedstock, an advance that could change how the world uses natural gas.

Methanol is used to make various products, such as paints and plastics, and as an additive to gasoline. According to experts, methanol, which is rich in hydrogen, can drive new-age fuel cells that could yield significant environmental benefits.

If natural gas, of which methane is the primary component, could be converted economically into methanol, experts believe the resulting liquid fuel would be much more easily stored and transported than natural gas and pure hydrogen. That also would greatly reduce the emissions of methane from natural gas processing plants and pipelines.

Today, escaped methane, a greenhouse gas many times more potent than carbon dioxide, nearly negates the environmental advantages of natural gas over oil and coal. To use this knowledge, a team of researchers from Stanford University and the University of Leuven in Belgium have created a low-energy way to produce methanol from methane.

“This process uses common crystals known as iron zeolites that convert natural gas to methanol at room temperature,” explained Benjamin Snyder, who earned his doctorate at Stanford studying catalysts to address key facets of this challenge. “But, this is extremely challenging chemistry to achieve on a practical level, as methane is stubbornly chemically inert.”

When methane is infused into porous iron zeolites, methanol is rapidly produced at room temperature with no additional heat or energy required. By comparison, the conventional industrial process for making methanol from methane requires temperatures of 1,000°C and extreme high pressure, according to experts.

“That’s an economically tantalising process, but it’s not that easy,” said Edward Solomon, Stanford professor of chemistry and of photon science at SLAC National Accelerator Laboratory at Stanford. “Significant barriers prevent scaling up this process to industrial levels.”

However, the scientists said most iron zeolites deactivate quickly – the process peters out as the zeolites cannot process more methane. So scientists have been keen to study ways to improve iron zeolite performance.

The new study, co-authored by Hannah Rhoda, a Stanford doctoral candidate in inorganic chemistry, uses advanced spectroscopy to explore the physical structure of the most promising zeolites for methane-to-methanol production. “A key question here is how to get the methanol out without destroying the catalyst,” Rhoda said.

Choosing two attractive iron zeolites, the team studied the physical structure of the lattices around the iron. The researchers discovered that the reactivity varies dramatically according to the size of the pores in the surrounding crystal structure. The team refers to it as the 'cage effect', as encapsulating lattice resembles a cage.

Illustration of cage effect

An illustration of the cage structures of two iron-based zeolites used in the study. The red and gold spheres (representing oxygen and iron, respectively) make up the active site. The cage structure, in grey, is formed of silicon, aluminium and oxygen. The blue sphere quantifies the size of the largest molecule that can diffuse freely in and out of the active site cage (the diameter of methane is ~4.2 Å).

Image credit: Benjamin Snyder

If the pores in the cages are too big, the active site deactivates after just one reaction cycle and never reactivates again. When the pore apertures are smaller, however, they coordinate a precise molecular dance between the reactants and the iron active sites – one that directly produces methanol and regenerates the active site.

Leveraging this so-called ‘cage effect’, the team said they could reactivate 40 per cent of the deactivated sites repeatedly – a significant conceptual advance toward an industrial-scale catalytic process. “Catalytic cycling – the continual reactivation of regenerated sites – could someday lead to continual, economical methanol production from natural gas,” Snyder said.

Synder and his team next aim to tackle the process not only at room temperature but using ambient air rather than some other source of oxygen, such as the nitrous oxide used in these experiments. Dealing with a powerful oxidising agent like oxygen, which is notoriously hard to control in chemical reactions, will be another considerable hurdle along this path, they said.

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