The right balance of zinc and zirconium oxides in this catalyst (purple block) converts ethanol to isobutene with low amounts of unwanted byproducts such as acetone and ethylene. Credit: PNNL
at the Pacific Northwest National Laboratory (PNNL) have developed a new
catalyst material that could replace chemicals currently derived from
petroleum and be the basis for more environmentally friendly products
including octane-boosting gas and fuel additives, bio-based rubber for
tires and a safer solvent for the chemicals industry.
make sustainable biofuels, producers want to ferment ethanol from
nonfood plant matter such as cornstalks and weeds. Currently, so-called
bio-ethanol’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. To turn
bio-ethanol into other useful products, researchers at the PNNL and at
Washington State University have developed a new catalyst material that
will convert it into a chemical called isobutene. And it can do so in
one production step, which can reduce costs.
by researchers in the Institute for Integrated Catalysis at PNNL and in
the Gene and Linda Voiland School of Chemical Engineering and
Bioengineering at WSU, the findings appeared July 21 in the Journal of
the American Chemical Society.
is a versatile chemical that could expand the applications for
sustainably produced bio-ethanol,” said chemical engineer Yong Wang, who
has a joint appointment at PNNL in Richland, Wash. and at WSU in
Pullman, Wash., and leads research efforts at both institutions.
addition, this catalyst 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
No Z-Z-Z for the weary
important key to unlocking renewables to replace fossil fuel products
is the catalyst. A catalyst is a substance that promotes chemical
reactions of interest. The catalytic converter in a car, for example,
speeds up chemical reactions that break down polluting gases, cleaning
up a vehicle’s exhaust.
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
is great, but isobutene is better. Chemists can make tire rubber from
it or a safer solvent that can replace toxic ones for cleaning or
industrial uses. Isobutene can also be readily turned into jet fuel and
gasoline additives that up the octane?that value listed on gas pumps
that prevents an engine from knocking?such as ETBE.
one had ever seen a catalyst create isobutene from ethanol in a
one-step chemical reaction before, so the researchers realized such a
catalyst could be important in reducing the cost of biofuels and
the catalyst in greater depth, the researchers examined what happened
when they used different amounts of zinc and zirconium. They 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.
the isobutene? That only arose in useful amounts when the catalyst
contained both zinc and zirconium. And “useful amounts” means “a lot.”
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 Wang. “We were happy to see that very high yield.”
researchers analyzed the chemistry to figure out what was happening. In
the 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 road
from ethanol to isobutene could only be as productive as Sun found if
zirconium oxide didn’t get side-tracked turning ethanol into ethylene
along the way.
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.
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.
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
nanometer-sized crystalline particles.
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
results from other analytical methods indicated that the team could
optimize 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 from building up and gunking up the catalysts, which cuts
work will look into optimizations 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.
Direct Conversion of Bio-ethanol to Isobutene on Nanosized ZnxZryOz Mixed Oxides with Balanced Acid-Base Sites