With increased availability and reduced cost of bio-ethanol, conversion of this particular bio-based feedstock to highly valuable fuels and chemicals has been an especially important research goal. Currently, research on bio-ethanol conversion to value-added chemicals focuses mainly on ethanol dehydration to ethylene, or ethanol dehydrogenation to acetaldehyde and then to acetone via Aldol-condensations pathways...

Research on direct bio-ethanol transformations to other types of highly valuable fuels and chemicals has not been carried out. In large part, this is due to the fact that such a process requires catalysts with multiple functions in order to yield more valuable chemicals such as isobutene. Isobutene is of special interest because it is widely used as an intermediate for the production of a variety of industrially important products.

For example, the trimerization of isobutene produces tri-isobutenes, which can be used as a premium (odorless, no aromatics) solvent and as an additive for jet fuel. Isobutene dimerization and hydrogenation to produce isooctane is used to increase the octane number of gasoline, and butyl rubber is produced from isobutene polymerization. Isobutene also reacts with alcohols such as ethanol to form ethyl tert-butyl ether (ETBE), a gasoline additive.

Currently, isobutene is obtained from catalytic or steam cracking of fossil feedstocks. With the depletion of fossil resources and increased demand for the isobutene market, it is desirable to explore alternative routes to synthesize isobutene from renewables.

—Sun et al.

The PNNL and WSU researchers had been trying to make hydrogen from ethanol. To improve on a conventional catalyst, they had taken zinc oxide and zirconium oxide and combined both into a mixed oxide—the zinc and the zirconium atoms woven through a crystal of oxygen atoms. Testing the new material, PNNL postdoctoral researcher Junming Sun found not only hydrogen, but unexpectedly quite a bit of isobutene.

Investigating the catalyst in greater depth, the researchers found that a catalyst made from just zinc oxide converted the ethanol mostly to acetone; if the catalyst only contained zirconium oxide, it converted ethanol mostly to ethylene. Isobutene only arose in useful amounts when the catalyst contained both zinc and zirconium.

Zirconium oxide is capable of converting acetone into isobutene; for the isobutene yield found, however, something would have to prevent zirconium oxide from turning ethanol into 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% isobutene, indicating the high isobutene selectivity of their 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 transmission electron microscopes, the team saw that the mixed oxide catalyst was made up of nanometer-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.

The unique combination of Zn- and Zr-oxides provides a balance of the surface acid-base chemistry in the mixed oxides. We find that, with an appropriate Zn/Zr ratio, most of the stronger Lewis acidic sites of ZrO₂ are selectively passivated and Brönsted acidic sites are weakened by the addition of ZnO. Consequently, undesirable bio-ethanol dehydration reactions are largely suppressed, while the surface basic site-catalyzed ethanol dehydrogenation and aldol-condensation reactions followed by Brönsted acid site-catalyzed acetone to isobutene conversion dominate on the Zn_xZr_yO_z mixed oxides. Passivation of the stronger Lewis acidic sites by ZnO on the Zn_xZr_yO_z mixed oxides also mitigates the polymerization/coking of acetone.

In this way, a highly selective (as high as 83% yield) process for direct conversion of bio-ethanol to isobutene on the nanosized Zn_xZr_yO_z catalyst has been achieved.

—Sun et al.

Experimental 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 their lifespan.

Future 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.

This work was supported by the US Department of Energy Offices of Science and of Energy Efficiency and Renewable Energy.

Resources

• Junming Sun, Kake Zhu, Feng Gao, Chongmin Wang, Jun Liu, Charles H.F. Peden, Yong Wang (2011) Direct Conversion of Bio-ethanol to Isobutene on Nanosized Zn_xZr_yO_z Mixed Oxides with Balanced Acid-Base Sites, J. Am. Chem. Soc., doi: 10.1021/ja204235v