While the catalytic capabilities of tungsten oxide have long been known, it takes nanotechnology to maximize their potential, said Rice Professor Michael Wong.

After the initial separation of crude oil into its basic components—including gasoline, kerosene, heating oil, lubricants and other products—refineries crack (by heating) heavier byproducts into molecules with fewer carbon atoms that can also be made into gasoline. Catalysis further refines these hydrocarbons.

We have a way to make a better catalyst that will improve the fuels they make right now. At the same time, a lot of existing chemical processes are wasteful in terms of solvents, precursors and energy. Improving a catalyst can also make the chemical process more environmentally friendly. Knock those things out, and they gain efficiencies and save money.

—Michael Wong

Wong and his team have worked for several years to find the proper mix of active tungsten oxide nanoparticles and inert zirconia. The key is to disperse nanoparticles on the zirconia support structure at the right surface coverage.

The pentane isomerization activity of WO_x/ZrO₂ is strongly affected by the nature of the support, calcination temperature, and tungsten oxide surface density. WZrOH samples demonstrated a volcano-shape dependence on tungsten surface density with maximum activity at 5.2 W·nm^-2, above ML coverage and at the onset of the WO₃ crystallization, in contrast to model WZrO₂ that were inactive. The calcination temperature of 973 K, not 773 K, favored the formation of sub-nm Zr-WOx clusters and in the overall activity of WZrOH, without promoting their surface acidic properties. The induction period during catalysis is critical for the activation of the clusters, which results in the increased isomerization activity and selectivity seen most pronouncedly at intermediate ρ_surf. A bimolecular isomerization mechanism, which plays a significant role and requires further investigation, appears to be promoted by these in situ activated Zr-WO_x sites.

—Soultanidis et al.

Now that the catalyst formula is known, making the catalyst should be straightforward for industry.

Because we’re not developing a whole new process—just a component of it—refineries should be able to plug this into their systems without much disruption. There’s a lot of talk about biofuels as a significant contributor in the future, but we need a bridge to get there. Our discovery could help by stretching current fuel-production capabilities.

—Michael Wong

Co-authors of the paper are Nikolaos Soultanidis, a Rice chemical engineering graduate student in Wong’s lab; Israel Wachs, Wu Zhou and Christopher Kiely of Lehigh University; Antonis Psarras and Eleni Iliopoulou of the Centre for Research and Technology Hellas; and Alejandro Gonzalez of the DCG Partnership, Pearland, Texas.

The National Science Foundation’s Nanoscale Interdisciplinary Research Team Program supported the project, with additional support from SABIC Americas and 3M.

Resources

• Nikolaos Soultanidis, Wu Zhou, Antonis C. Psarras, Alejandro J. Gonzalez, Eleni F. Iliopoulou, Christopher J. Kiely, Israel E. Wachs, and Michael S. Wong (2010) Relating n-Pentane Isomerization Activity to the Tungsten Surface Density of WO_x/ZrO₂. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja105519y