Heterogenized homogenous nanocatalysts

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2013

A new gold catalyst combines the performance benefits of heterogeneous and homogeneous catalysts.

KEY CONCEPTS
• While heterogeneous and homogeneous catalysts are widely used in industry, both have limitations.
• A new catalyst that contains gold clusters encapsulated in a polyamidoamine-based dendrimer polymer combines the performance benefits of heterogeneous and homogeneous catalysts.
• In a specific organic chemistry reaction, the new catalyst displayed good conversion and better stereoselectivity than the homogeneous catalyst, gold trichloride and was easily recycled in a similar fashion to heterogeneous catalysts.

THE CONTINUING RESEARCH IN NANOTECHNOLOGY is leading to the development of improved catalysts for use in a wide variety of processes. In lubrication, catalysts perform important functions ranging from preparation of synthetic lubricant basestocks to processing base oils.

In a previous TLT article, research revealed how zeolite catalysts can be used on the nanoscale (1). Difficulties have been encountered in preparing nanocatalysts from zeolite due to either destruction of the crystal structure initially formed or aggregation of the layers. To overcome these issues, a solid-state exfoliation process using a polymer was utilized to prepare stable zeolite nanocatalysts that have typical dimensions of 0.5 micron x 0.5 micron x 3 nanometers. The resulting nanocatalysts were incorporated into a filtration membrane that effectively separated two xylene isomers.

The two main industrial catalyst types are heterogeneous and homogeneous. Each has its advantages and disadvantages. Gabor Somorjai, senior scientist with Berkeley Lab’s Material Sciences Division and professor of chemistry at the University of California Berkeley in Berkeley, Calif., says, “Heterogeneous catalysts are very effective at preparing small molecules and can be readily recycled. They are very suitable for use in fixed-bed flow reactors. During a specific reaction process, intermediates can be readily separated to better understand the mechanism. But heterogeneous catalysts cannot be used in many organic reactions and also cannot be easily modified to improve their performance.”

Somorjai continues, “Homogeneous catalysts are typically prepared by surrounding transition metal ions with organic ligands. They exhibit high product selectivity and are very effective in preparing complex organic molecules such as those used in the pharmaceutical industry. But homogeneous catalysts are frequently used in organic solution, which makes it very difficult to separate them from the product generated during the reaction.”

It is desirable to combine the beneficial properties of heterogeneous and homogeneous catalysts to produce a material with even better performance. Somorjai says, “We have developed transition metal catalysts based on palladium, platinum and rhodium clusters that are anchored to a polymer support. These heterogeneous materials were able to catalyze reactions previously done only through the use of homogeneous catalysts.”

One transition metal that has exhibited strong catalytic properties is gold. A new type of catalyst based on gold has now been developed that exhibits high reactivity and selectivity.

DENDRIMER-ENCAPSULATED NANOCATALYSTS
Somorjai and his fellow researchers have developed a new gold catalyst that combines the performance benefits of heterogeneous and homogeneous catalysts. He says, “We prepared gold clusters that are encapsulated in a polyamidoamine-based dendrimer polymer that is placed on a mesoporous silica support.”

The dendrimer acts in a similar fashion to a polymer-like tree with branches. Somorjai considers a dendrimer to be very similar to branched peptide or protein compounds. Metal clusters are allowed to grow within the branches up to a size of one nanometer in diameter. This represents a cluster of 30 to 40 atoms.

The encapsulated metal cluster is anchored to the mesoporous silica support through the hydrogen bonding interaction of hydroxyl terminated groups residing on the dendrimer with the silica support.

To evaluate the catalyst, the researchers selected a reaction that leads to the formation of cyclopropane derivative from a substituted alkyne (propargyl pivalate) and styrene (see Figure 3). The resulting products are cis and transdiastereomers of the cyclopropane. Somorjai says, “This is a reaction that would be typically catalyzed by a homogeneous catalyst. It involves carbon-carbon bond formation with a pie-bonded molecule.”


Figure 3. A new catalyst prepared by encapsulating gold clusters in a polyamidoamine-based dendrimer combines the performance benefits of heterogeneous and homogeneous catalysts in the formation of a cyclopropane derivative from a substituted alkyne and styrene. (Courtesy of Lawrence Berkeley National Laboratory and University of California-Berkeley)

The study was conducted by placing the raw materials in a Schlenk tube that was sealed under a nitrogen atmosphere. Reactions were run at temperatures ranging from ambient up to 70 C. Iodobenzene dichloride was used to activate the catalyst through oxidation of the gold nanoparticles to gold (III).

The encapsulated metal cluster catalyst demonstrated good conversion and much higher stereoselectivity than the homogeneous catalyst, gold trichloride. In a similar fashion to other heterogeneous catalysts, the encapsulated metal cluster can be easily recycled through filtration. Somorjai says, “High stability of the hydrophilic catalyst, without any leaching of the metal ions to the solution phase, was obtained while employing a hydrophobic solvent such as toluene.”

Transmission electron microscopy images of the encapsulated metal catalyst were taken before and after the cyclopropane forming reaction. No difference in the cluster size of the catalyst was observed, showing that it is durable. Once the encapsulated metal cluster catalyst was recycled, its reactivity and selectivity were similar to those of the fresh catalyst.

This step may lead to the development of new catalysts that will provide better performance in the production of lubricant raw materials. Somorjai is hopeful that this approach will lead to more effective catalysts that exhibit even better selectivity. He adds, “My dream is to eventually develop a catalyst that can handle complex reactions in a similar fashion to an enzyme.”

Further information can be found in a recent article (2) or by contacting Somorjai at somorjai@berkeley.edu.

REFERENCES
1. Canter, N. (2012), “More Effective Zeolite Nanocatalysts and Membranes Based on Layers,” TLT, 68 (4), pp. 10-11.
2. Gross, E., Chang, J., Toste, F. and Somorjai, G. (2012), “Control of Selectivity in Heterogeneous Catalysis by Tuning Nanoparticle Properties and Reactor Residence Time,” Nature Chemistry, 4 (11), pp. 947-952.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.