Cheap catalysts are cool

Catalysts, hydrogen and heat are all required for generating useful fuels and chemicals from lignin, but there can be a bit of flexibility in exactly how they are used together. They can all be used at the same time, with the lignin heated in the presence of a catalyst and hydrogen to break it down into a bio-oil containing useful hydrocarbons.

Alternatively, they can be used a bit more separately, in which case the lignin is first heated in the absence of air, known as pyrolysis, to produce a bio-oil, which is then upgraded by treating it with a catalyst in the presence of hydrogen. This upgrading is required to drive off excess oxygen and replace it with hydrogen, a process known as hydrodeoxygenation, to produce a bio-oil with a higher concentration of useful hydrocarbons.

Scientists are actively investigating both routes as an efficient way to generate useful fuels and chemicals from lignin. They have, for example, developed a whole range of catalysts for depolymerizing lignin. Many of these catalysts are based on precious metals such as platinum and palladium, which have a high natural catalytic ability but are also expensive. 

So, in recent years, scientists have been trying to develop catalysts based on cheaper metals. This has led them to develop bimetal catalysts containing two different metals, in the hope that the combination will have a higher catalytic activity than either metal on its own. In a new paper in ChemistrySelect, Hui Pan and his colleagues at Nanjing Forestry University, China, describe one such bimetal catalyst that combines copper and nickel loaded onto activated carbon. 

They developed several versions of this catalyst with different amounts of nickel and copper, finding that a version containing loadings of 5% nickel and 5% copper worked best. This was able to convert lignin from poplar into a bio-oil containing various phenolic monomers with a yield of 40%.

Although the alternative route uses heat to convert the lignin to a bio-oil and then a catalyst to upgrade this bio-oil, the upgrading process does still require heat, needing to take place at temperatures of 200–500°C. Furthermore, the hydrogen needs to be delivered at high pressures, up to 200 bar, to drive forward the hydrodeoxygenation reaction. This all requires energy, which makes this route quite expensive as well.

Now, though, Yulin Deng and his colleagues at Georgia Institute of Technology, US, have developed a version that can take place at low temperatures (below 100°C) and ambient pressures. They have managed this by combining a conventional platinum-on-carbon catalyst with a far-from-conventional superacid known as polyoxometallate acid (SiW₁₂). As they report in a paper in Nature Energy, this involves dissolving SiW₁₂ in water, adding particles of the catalyst to this solution and then bubbling hydrogen through it. 

When they added phenol and guaiacol as model lignin bio-oil compounds to this solution, the two compounds were upgraded into useful hydrocarbons such as cyclohexane and benzene with a yield of 90%. The SiW₁₂ appeared to perform three functions in this process. First, it transfers the hydrogen to the surface of the catalyst particles, concentrating it there. Then, it helps lower the energy required for removing the oxygen from the bio-oil. Finally, it promotes the reaction between the hydrogen and the bio-oil.

"The super-acid can reduce the activation energy required for removing the oxygen, and at the same time, you have more active hydrogen in the solution, which reacts on the molecules of oil," explained Deng. This removes the need for high temperatures and pressures.

The one downside is that the catalyst contains expensive platinum, but Deng and his team are now looking for cheaper catalysts that are also more selective for specific hydrocarbons. Perhaps bimetal catalysts could help here as well.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.