Greening the Path to Useful Chemicals: Development of a New Wacker-Type Route to Aldehydes
Winter 2015 - Written by Resnick Postdoctoral Scholar Christopher Prier
Chemicals are fundamental to all aspects of modern society as the components of our plastics, textiles, electronics, and medicines. The field of green chemistry aims to deliver the chemicals that society demands in a manner that is as sustainable, energy-efficient, and resource-efficient as possible. Primary goals of green chemistry are thus the development of chemical processes that reduce the generation of waste, use less hazardous inputs, and overall consume less energy compared to conventional approaches.
In a recent advance in this area, Caltech Professor Bob Grubbs and co-workers Zachary Wickens, Bill Morandi, and Kacper Skakuj have developed a novel approach to the synthesis of aldehydes. Aldehydes are broadly useful chemicals used in plastics, detergents, fragrances, and as precursors to complex molecules such as pharmaceuticals. The approach developed at Caltech uses a combination of three catalysts and provides an alternative route to currently employed energy-intensive processes for the synthesis of the same materials.
Currently, aldehydes are often produced industrially from alkenes – readily available feedstock chemicals derived from petroleum – via a process known as hydroformylation. In this process, alkenes are reacted with carbon monoxide and hydrogen in the presence of a metal catalyst. The reaction typically requires high temperatures (up to 200 ºC) and pressures (10 to 100 atmospheres), features that render it energy intensive. Despite these energy demands, hydroformylation is currently employed to produce more than 20 billion pounds of aldehydes per year.
Taking an alternative approach, the Grubbs team has found that a combination of three catalysts – a palladium catalyst, a copper catalyst, and a nitrite catalyst – enables the efficient and selective conversion of alkenes to aldehydes at ambient temperature and pressure (23 ºC, one atmosphere). Their findings have recently been published in Angewandte Chemie International Edition and the Journal of the American Chemical Society.
The new process bears similarities to the Wacker oxidation – a process employing palladium and copper used on large industrial scale for the production of acetaldehyde. Critically, however, traditional Wacker conditions convert alkenes to a different class of products known as ketones. The inclusion of nitrite in the Grubbs three-catalyst system dramatically alters the outcome of the process, such that it no longer produces ketones and instead produces aldehydes. The reaction has been demonstrated to be efficient for the conversion of a broad range of different alkene inputs to aldehydes.
While the process may represent a green alternative to hydroformylation, certain drawbacks such as the high amount of metal required provide a stimulus for further reaction development. Wickens suggests that improving the process to make it greener, more efficient, and more selective may be possible via a deeper understanding of the reaction mechanism, which remains poorly understood. In particular, he hopes that further research will identify the nature of the actual active catalyst – likely a particular formulation of palladium, copper, and other reaction components. Understanding the nature of the mechanism and catalyst will likely drive the development of a more input- and energy-efficient process.
A further potential application of this science is the conversion of renewable, bio-derived materials to commodity chemicals. The Grubbs laboratory has recently disclosed a separate strategy for the synthesis of linear α-olefins (LAOs) from biomass; this process could be coupled to the new palladium/copper/nitrite process to achieve an overall conversion of biomass to valuable aldehyde products.