Clean, green way to make ammonia fertilisers

A team of Australian scientists have developed a new, efficient way to make the critical agricultural product ammonia. The research could be used to make fertilisers directly on farms, using renewable energy, by the end of the decade.

Ammonia (NH₃) is a key ingredient in fertilisers, with 150 million metric tonnes made in 2019 alone. The cheapest production method requires natural gas (methane) and is extremely energy and carbon-intensive.

Ammonia production is responsible for around 1.8% of global greenhouse gas emissions, and a tonne of commercially made ammonia produces around 1.9 tonnes of carbon dioxide.

But a paper published in Science, from Monash University researchers, shows a clean and more energy-efficient way to make it.

As it’s made from nitrogen and hydrogen, ammonia can theoretically be created from air and water with the right catalysts and energy.

Chemists have known since the 1990s that this can be done with a lithium catalyst, but it was “not producing the kinds of productivity that were remotely commercial,” according to Douglas MacFarlane, a professor of chemistry at Monash University and senior author on the paper.

This is partially because it requires a source of protons (or hydrogen, H⁺, ions) to work, and chemists haven’t yet been able to find a proton source that generates high yields.

“In our study, we have found that a phosphonium salt can be used as a ‘proton shuttle’ to resolve this limitation,” says Bryan Suryanto, lead author on the paper.

This allows them to “produce ammonia at room temperature at high, practical rates and efficiency”, according to Suryanto.

“It’s really a big jump forward in the productivity,” says MacFarlane. He adds that this process could now compete with the traditional, methane method.

The phosphonium salts aren’t consumed in the process, and they’re readily available in commercial amounts. “They’re actually made in quite large quantities by several big chemical companies around the world,” says MacFarlane.

The reaction they describe could also have applications in other areas of organic chemistry – for instance Wittig reactions, which are a very powerful method of synthesis.

“Our discoveries have been licensed to a new Monash spin-out called Jupiter Ionics P/L, who will be scaling up the process to demonstrate operation in commercial applications,” he adds.

Within a couple of years, MacFarlane hopes the company will be selling small, ammonia-generating cells that can be run off renewable power, for use in greenhouses and hydroponic operations.

The aim is then to scale up, eventually to “a shipping container with all the gear inside it, which could literally be put on the back of a truck and taken to a workstation”.

“In principle, such a device could generate a tonne of ammonia a day, which is the sort of numbers that is very interesting for agricultural applications.”

A farm could generate its own ammonia, and thus fertilisers, using renewable energy also produced on the farm.

MacFarlane says this will dramatically change the fertiliser industry.

He hopes generators like this will be available by 2025.

A video explaining the research can be found here.

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