The chemical and pharmaceutical industries spend vast amounts of money each year on catalysts that are needed to process drugs and other high-value chemicals.
‘There are some industrial reactions where drug makers have no choice but to use platinum and palladium catalysts, but the majority of these are homogenous, which means they mix readily with reactants and are very difficult to remove,’ said lead researcher Eugene Zubarev, associate professor in chemistry at Rice. ‘Because these heavy metals are toxic, they must be completely removed from the drug after its synthesis is completed.
‘However, the removal of homogeneous catalysts is very time consuming and expensive, which creates a big problem for pharmaceutical companies.’
Among catalysts, platinum and palladium are well known for reactions involving hydrogen because atoms of hydrogen typically join together in pairs. Platinum and palladium are suited to cleaving these pairs and leaving the individual hydrogen atoms available for reactions with other molecules.
Zubarev and Rice graduate student Bishnu Khanal set out to make a heterogeneous platinum catalyst that was soluble enough for industrial use, but that could also be easily removed.
Previous studies had shown that combining platinum with gold in tiny nanoparticles could enhance the platinum’s catalytic effect. Zubarev and Khanal started with tiny rods of pure gold and coated them with a layer of platinum so thin that it left the gold exposed in some places.
After confirming the structure of the gold-platinum nanorods, Zubarev and Khanal then had to make them soluble in organic solvents that are favoured by industry.
Building on Zubarev’s previous work in making soluble gold nanorods, the pair found a way to attach molecules of polystyrene to the surface of the gold-platinum rods.
Zubarev and Khanal found that the coated particles were easy to remove from solution with a conventional centrifuge. In addition, the polystyrene shells made them soluble in organic solvents and enhanced their catalytic selectivity.
‘The selectivity of the coated gold and platinum nanorods will be very attractive to industry,’ Zubarev said. ‘For example, we found they had nearly 100 per cent catalytic selectivity for the hydrogenation of terminal olefins.’
Zubarev’s group is using similar methods to produce gold-palladium catalysts in a follow-up study. Palladium is another high-demand catalyst.