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May 2013

Catalytic Sieves: A New Route to Selectivity

Nano-sieve templates can make simple and inexpensive bulk metal oxide catalysts selective

Brandon O’Neill

The new sieving layers are synthesized by attaching large bulky groups (at right) to the TiO2 surface (green). After that, atomic layer deposition is used to fill in the spaces around the bulky groups with Al2O3 (blue). Once the alumina is deposited, the bulky groups are burned off, leaving behind small cavities that act like a sieve on top of the TiO2 catalyst.

It was a bit of a Goldilocks dilemma. Not too big. Not too small. Only just right would work. Scientists at the Institute for Atom-Efficient Chemical Transformations, IACT, found a way to create pores of just the right size by modifying materials that could be used to selectively convert biomass molecules into renewable fuels and chemicals based on size. They drew inspiration from materials such as zeolites, a type of shape- and size-selective catalyst that has been important for the production of petroleum-based fuels and commodities for years.

“Most biomass molecules and many precursors to commodity chemicals are too large to fit in zeolite pores,” notes Justin Notestein, one of the authors of a paper describing the work that appeared in Nature Chemistry.

To address this problem, IACT scientists started with titanium dioxide, TiO2, a catalyst that reacts fairly indiscriminately with a specific arrangement of atoms known as a functional group. This is useful for some applications like wastewater treatment, but less useful for making fuels or chemicals. The goal of the IACT scientists was to make TiO2 differentiate among molecules that have the same functional group, but different sizes.

Using a series of steps, they attached “bulky” template molecules, about 1 nanometer across, more than 10,000 times thinner than a human hair, to the TiO2. Next, they filled the space between the template with aluminum oxide, Al2O3, in a process known as atomic layer deposition and then burned off the template, exposing the TiO2 underneath. This process left behind a coating of Al2O3 less than 1 nanometer thick that was full of 1-nanometer cavities that worked just like a sieve.

When TiO2 was used for the reaction of two alcohols that differed only slightly in size, the reaction rates were essentially the same. However, when they used the new sieve-like material, they selectively converted the smaller alcohol at a rate nearly 10 times higher than the larger one.

This type of reaction selectivity could be a breakthrough for large industrial applications because it would allow a reaction to be performed as a mixture. This means it would not be necessary to first separate the mixture into its pure components. Skipping separations would save energy and reduce costs. Notestein says, “The types of reactions possible with oxide catalysts, and specifically with the nanocavity catalysts, are potentially much greater than for zeolites.”

The collaborative nature of the Energy Frontier Research Centers, EFRCs, was critical in fostering this breakthrough. Lead author Christian Canlas, a post-doctoral researcher on the project, said, “Demonstrating the existence of 1- to 2-nanometer cavities is challenging, but fortunately, IACT has the resources.”

These resources include a variety of synthesis, characterization, and testing techniques. “This is the type of project that could only be carried out within the structure of an EFRC,” said Notestein.

Both scientists foresee future development. “There is still so much to be learned with this discovery,” says Canlas. This discovery could pave the way towards energy savings, reduced waste, and a sustainable future via new opportunities for catalyst design and optimization.

More Information

Canlas CP, J Lu, NA Ray, NA Grosso-Giordano, S Lee, JW Elam, RE Winans, RP Van Duyne, P Stair, and JM Notestein. 2012. “Shape-selective Sieving Layers on an Oxide Catalyst Surface.” Nature Chemistry 4:1030-1036. DOI: 10.1038/nchem.1477

Acknowledgments

This work was funded by the Institute for Atom-Efficient Chemical Transformations, an Energy Frontier Research Center funded by the Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. JM Notestein acknowledges a 3M Non-Tenured Faculty Grant, a DuPont Young Professor Grant, a Camille and Henry Dreyfus New Faculty Award and a DOE award.

The Advanced Photon Source, Argonne National Laboratory, was used. The transmission electron microscope at the Electron Probe Instrumentation Center of NUANCE, Northwestern University, was used.

About the author(s):

  • Brandon O’Neill is a graduate research assistant at the University of Wisconsin-Madison and a member of the Institute for Atom-Efficient Chemical Transformations based at Argonne National Laboratory. His research interests center around the development of new catalytic materials and elucidation of reaction kinetics and mechanisms focused on the production of sustainable fuels and chemicals.

Building a Better Way to Sift Through Biomass Molecules

Material selects molecules by size to react with catalyst

The new sieving layers are synthesized by attaching large bulky groups to the titanium dioxide surface (green). After that, atomic layer deposition is used to fill in the spaces around the bulky groups with aluminum oxide (blue). Once the alumina is deposited, the bulky groups are burned off, leaving behind small cavities that act like a sieve on top of the catalyst.

Sifting through a mixture so that only select molecules react is challenging, especially when the mixture contains molecules destined for biofuels. Most catalyst sieves, as such materials are called, are designed with pores that are simply too small. Scientists devised an elegant way to create a material with pores that are right size for bio-molecules. They began with a layer of a catalyst, which drives the reaction. They dotted the catalyst’s surface with bulky molecules, about 1 nanometer wide. Next, they deposited aluminum oxide, layering the material around the bulky molecules. When they removed the sacrificial molecules, they had perfectly sized cavities with a layer of catalyst underneath. The cavities kept the wrong molecules away from the catalyst, boosting the reaction’s selectivity for smaller molecules. This grating can be used to obtain high selectivity in reactions. The Institute for Atom-Efficient Chemical Transformations, led by Argonne National Laboratory, conducted this work.

More Information

Canlas CP, J Lu, NA Ray, NA Grosso-Giordano, S Lee, JW Elam, RE Winans, RP Van Duyne, P Stair, and JM Notestein. 2012. “Shape-selective Sieving Layers on an Oxide Catalyst Surface.” Nature Chemistry 4:1030-1036. DOI: 10.1038/nchem.1477

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.