Cleaner, cooler, and cheaper: Green chemistry gets a low-temperature oxidation breakthrough

What if chemical manufacturers could cut their energy costs while eliminating toxic heavy metals from their processes? Researchers at Nagoya University have developed a catalyst system that does exactly that by converting alcohols to valuable chemical products at lower temperature using safer iodine compounds instead of dangerous heavy metals, expensive precious metals, and reagents. 

In 2009, a team led by Professor Kazuaki Ishihara from the Graduate School of Engineering successfully replaced toxic heavy metals and expensive precious metals used in traditional oxidation reactions with safer, more abundant iodine. Their new iodine-oxone catalyst system allowed them to sustainably convert alcohols into aldehydes or carboxylic acids and ketones, compounds used to manufacture diverse consumer products. However, one problem remained: the process required temperatures of 70°C to work effectively.  

Now, the team has decreased the oxidation reaction temperature from 70°C to 30°C by using their catalyst in its pre-activated form and adding a helper chemical to improve mixing. This allowed them to remove the slow startup steps that required high heat. Combined with their earlier replacement of toxic metals such as chromium and manganese with iodine-based catalysts, their method produces cleaner chemical reactions at lower temperatures, cutting costs and energy consumption significantly. The research was published in Green Chemistry. 
 
Identifying the problem 

Oxidation of alcohols to aldehydes and ketones is fundamental to chemical manufacturing. These molecules are essential ingredients for countless consumer products, including medicines, fragrances, and plastics. Any improvements in efficiency or environmental impact have important effects on multiple industries.  

To find out why the oxidation process was taking a long time and needed high temperatures to work, the researchers used a technique called nuclear magnetic resonance spectroscopy to observe what happened during the reaction. They assumed that the main reaction that transformed an alcohol to an aldehyde was the slow part of the process. However, they found that their catalyst, 2-iodoxybenzenesulfonic acid (IBS), was not activating properly at the start of the reaction. 

Before it could work, IBS had to be converted from its inactive form pre-IBS to its active form IBS(III). This conversion process was very slow at low temperatures. Making matters worse, oxone, the oxidizing agent that drives the conversion of alcohol to aldehydes and ketones, is a powder that does not dissolve well in organic solvents. Therefore, it could not effectively activate the catalyst. This meant pre-IBS took a long time to become active at 30°C, forcing researchers to use high heat (70°C) to speed up the activation process. 

Smart solutions, green benefits 

“A major limitation in green chemistry is that high temperatures often prevent the synthesis of heat-sensitive compounds used in specialty chemicals and medicines,” Professor Ishihara said. 

“To overcome these obstacles, we used a pre-activated catalyst by preparing IBS in its ready-to-work form ahead of time. We also added a helper chemical, tetrabutylammonium hydrogen sulfate, that acts like soap to allow oxone to dissolve and mix properly.” 

The improved system has several advantages: it can perform multiple chemical reactions in one container, called “one-pot synthesis,” where the product of the first reaction immediately becomes the starting material for the next reaction. This removes costly and time-consuming purification steps between reactions. Moreover, the low-temperature conditions allow oxidation of many heat-sensitive alcohols that are difficult to process. 

Japan, the world's second largest iodine producer, could particularly benefit from the new iodine-oxone-based catalyst system, which could make its chemical industry more efficient and sustainable.  

Future research will focus on replacing the remaining chemicals with more environmentally friendly options and finding ways to recycle the catalyst so it can be used repeatedly, making the process even cleaner and more cost-effective.