Each year, more than $20 trillion worth of products depend on the chemical reactions sparked by catalysts.
So what makes catalysts so important? They help molecules do something that they might not do on their own. Like little matchmakers, catalysts pair up molecules in exactly the right position to make things like the dye for your jeans or the fuel for your car.
But sometimes the catalyst gets tangled up with the chemicals it helps to pair. For example, if both the catalyst and the chemicals are in liquid form, they may run together just like coffee and cream. These are called homogeneous catalysts because they are in the same physical state as the product. In other words, if both the catalyst and product are liquids, like milk mixed with coffee, it can be laborious and costly to separate them.
Yet separation and recovery is essential for precious metal catalysts, which are worth as much as $26,000 per pound. These pricey catalysts must be recyclable in order to be commercially viable.
Scientists and engineers at the Center for Environmentally Beneficial Catalysis, or CEBC, are designing ingenious techniques for incarcerating catalysts inside the reactor so they can be reused.
There are a variety of ways to confine homogeneous catalysts inside reactors. But most current incarceration systems suffer from drawbacks. Some use exotic or costly chemicals. Others have poor reproducibility or take a long time to make. And in some cases the metal escapes its shackles and leaches out under reaction conditions.
“We need an efficient yet simple method for immobilizing catalysts,” says R.V. Chaudhari, Distinguished Professor of Chemical Engineering at the University of Kansas.
To find a solution, Chaudhari and his postdoctoral researcher, Bibhas Sarkar, looked to nature for inspiration. They noticed that bones, coral reefs and shells are all made by a similar process.
Take bone cells, for example. They excrete a liquid form of calcium that mixes with oppositely-charged ions in the surrounding fluid to form clumps that slowly deposit on the bone matrix. Similarly, calcium excreted from sea creatures mixes with carbonate in the water to form shells and coral reefs.
“We wondered if homogeneous metal catalysts could be immobilized in the same way,” said Chaudhari.
Sure enough, Chaudhari’s lab has patented a novel technique for doing just that. He coined the name ‘ossification’ for this method because of its similarities to bone growth.
In this simple and adaptable technique, negatively charged sulfonate groups are first linked to the metal catalyst. This complex is soluble in water, but clumps together and falls out of solution when positively-charged calcium or barium salts are added.
Best of all, Chaudhari’s method does not destroy the catalyst. “We’ve found that our ossified catalysts show high activity and selectivity for a number of industrially important reactions such as palladium catalyzed carbonylation and rhodium catalyzed hydroformylation,” said Chaudhari.
Chaudhari and Sarkar are currently advancing this technology at CEBC. Ultimately, their catalysts could be shackled to a solid support inside a chemical reactor. Instead of getting tangled up with the product, these costly catalysts would be sentenced to life without parole.
--Story by Claudia Bode
Sarkar, B.R. and Chaudhari, R.V. "'Ossification' - A Novel Approach for Immobilisation of Platinum Group Metal Complex Catalysts." Platinum Metals Rev., 2010, 54(2):31-37.