Chinese new catalysts

Scientists from the Dalian Institute of Chemical Physics in China have helped to develop two new catalysts that offer novel ways for deriving fuels from plant biomass. The first catalyst is able to convert biomass into both hydrogen and green diesel, while the second catalyst is able to convert cellulose directly into ethanol in a one-pot process.

When hydrogen is being considered as a clean fuel, it is often generated by the photocatalytic splitting of water, with the energy provided by sunlight. The resultant hydrogen can then be used to generate electricity in a fuel cell, all without releasing any carbon dioxide.

But hydrogen can also be generated by catalytically breaking down plant biomass. While this process does release carbon dioxide, it can still be carbon neutral, because the released carbon dioxide should be taken up by the growth of new plants. In other ways, generating hydrogen from plant biomass is actually more efficient than generating it by splitting water.

The process involves light releasing electrons from the photocatalyst, which combine with protons in the biomass to produce hydrogen. However, this leaves behind ‘holes’, or positively charged gaps where electrons used to be, in the photocatalyst. For the process to work efficiently, these holes need to be filled with external electrons, otherwise they simply recombine with the released electrons. One potential source of external electrons is the biomass, but the resulting oxidation reactions tend to produce a load of unusable waste products, which need to be dealt with, ruining the economics of the whole process.

Far better would be if the oxidation reactions, as well as providing electrons, also converted the biomass into useful fuel-like compounds, and this is what an international team of scientists, led by Feng Wang at the Dalian Institute of Chemical Physics, has now managed to achieve. The scientists have done this by developing a catalyst made from zinc, indium and sulfur that is doped with ruthenium.

As they report in a paper in Nature Energy, this catalyst was able to convert methylfurans, which can easily be derived from plant biomass, into both hydrogen and carbon compounds that are precursors to the kind of alkanes found in diesel fuel. The only problem with these precursors is that they contained too much oxygen, and this oxygen could easily be removed with a standard hydrodeoxygenation reaction, using some of the hydrogen generated by the catalyst, to produce just the kind of alkanes found in diesel.

Catalysts aren't generally employed for turning plant biomass into ethanol. At the moment, this is usually an almost entirely biological process, using enzymes to break down the cellulose into simple sugars and yeast or other microbes to ferment those sugars into ethanol. But now another team from the Dalian Institute of Chemical Physics, led by Aiqin Wang, has come up with a catalytic process for doing this, via a slightly different route.

They have developed a multifunctional catalyst containing molybdenum, platinum and tungsten oxide that first converts cellulose into ethylene glycol and then converts this ethylene glycol into ethanol. This all happens in a single process, with the catalyst converting cellulose into ethanol with a yield of around 43%. Aiqin Wang and her colleagues report this work in a paper in Joule.

For industrial-scale applications, the efficiencies of both these catalytic processes would need to be enhanced substantially, while problems such as how to separate the diesel precursors from the reaction media also remain to be solved. But these two studies amply demonstrate that novel catalytic routes from biomass to fuels are there to be found.

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.