Renewable ethanol for fuel additives, rubber and solvents
3 Aug 2011
Richland, Washington – US researchers claim to have developed catalysts that could replace chemicals currently derived from petroleum and be the basis for products including octane-boosting gas and fuel additives, bio-based rubber for tyres and solvents for the chemicals industry.
Currently, so-called bioethanol’s main values are as a non-polluting replacement for octane-boosting fuel additives to prevent engine knocking and as a renewable replacement for a certain percentage of gasoline.
In a bid to turn bioethanol into other useful products, researchers at the US department of energy’s Pacific Northwest National Laboratory and at Washington State University have developed a catalyst material that will convert it into isobutene in a cost-effective, single production step.
The new catalyst also requires the presence of water, allowing producers to use dilute and cheaper bio-ethanol rather than having to purify it first, potentially keeping costs lower and production times faster.
The PNNL and WSU researchers were trying to make hydrogen fuel from ethanol. To improve on a conventional catalyst, they had taken zinc oxide and zirconium oxide and combined both into a new material called a mixed oxide – the zinc and the zirconium atoms woven through a crystal of oxygen atoms. Testing the mixed oxide out, PNNL postdoctoral researcher Junming Sun saw not only hydrogen, but – unexpectedly – quite a bit of isobutene.
The research showed that a catalyst made from just zinc oxide converted the ethanol mostly to acetone, an ingredient in nail polish remover. If the catalyst only contained zirconium oxide, it converted ethanol mostly to ethylene, a chemical made by plants that ripens fruit.
Isobutene, however, only arose in useful amounts when the catalyst contained both zinc and zirconium. With a 1:10 ratio of zinc to zirconium, the mixed oxide catalyst could turn more than 83% of the ethanol into isobutene.
“We consistently got 83% yield with improved catalyst life,” said chemical engineer Yong Wang, who has a joint appointment at PNNL in Richland, and at WSU in Pullman, and leads research efforts at both institutions.. “We were happy to see that very high yield.”
In single metal oxides experiments, the zinc oxide created acetone while the zirconium oxide created ethylene. The easiest way to get to isobutene from there, theoretically speaking, is to convert acetone into isobutene, which zirconium oxide is normally capable of.
And the route from ethanol to isobutene could only be sufficiently productive if zirconium oxide did not get side-tracked turning ethanol into ethylene along the way, the researchers found.
Something about the mixed oxide, then, prevented zirconium oxide from turning ethanol into the undesired ethylene. The team reasoned the isobutene probably arose from zinc oxide turning ethanol into acetone, then zirconium oxide – influenced by the nearby zinc oxide – turning acetone into isobutene.
At the same time, the zinc oxide’s influence prevented the ethanol-to-ethylene conversion by zirconium oxide. Although that’s two reaction steps for the catalyst, it’s only one for the chemists, since they only had to put the catalyst in with ethanol and water once.
To get an idea of how close the reactions had to happen to each other for isobutene to show up, the team combined powdered zinc oxide and powdered zirconium oxide. This differed from the mixed oxide in that the zinc and zirconium atoms were not incorporated into the same catalyst particles.
These mixed powders turned ethanol primarily into acetone and ethylene, with some amounts of other molecules and less than 3 percent isobutene, indicating the magic of the catalyst came from the microstructure of the mixed oxide material.
So, the researchers explored the microstructure using instruments and expertise at EMSL, DOE’s Environmental Molecular Sciences Laboratory on the PNNL campus. Using high-powered tools called transmission electron microscopes, the team saw that the mixed oxide catalyst was made up of nanometre-sized crystalline particles.
A closer look at the best-performing catalysts revealed zinc oxide distributed evenly over regions of zirconium oxide. The worst performing catalyst – with a 1:1 zinc to zirconium ratio – revealed regions of zinc oxide and regions of zirconium oxide. This suggested to the team that the two metals had to be close to each other to quickly flip the acetone into isobutene.
Experimental results from other analytical methods indicated that the team could optimise the type of chemical reactions that lead to isobutene and also prevent the catalyst from deactivating at the same time. The elegant balance of acidic and basic sites on the mixed oxides significantly reduced carbon buildup on the catalysts, which cuts their lifespan.
Future work will look into optimisations to further improve the yield and catalyst life. Wang and colleagues would also like to see if they can combine this isobutene catalyst with other catalysts to produce different chemicals in one-pot reactions.
