Adding an oxygen atom to a molecule – called oxidation – in a controlled fashion is a long sought but challenging goal of chemistry. The procedure is important because most molecules used in industrial and consumer applications have unique properties due to the exact position on the molecule of specific groups of atoms – their chemical functional groups. Many of these functional groups are oxygen based.
For example, introducing one oxygen atom would convert methane – the main component of natural gas and an abundant resource – into methanol, which could be used as a liquid fuel. While simple on paper, the actual transformation is challenging. It often leads to more than one oxygen attaching and ultimately, to the complete burning of methane into carbon dioxide.
More generally, the chemical industry heavily relies on molecules that contain oxygen at key positions. They are used as bulding blocks to make other chemicals. Among them, benzyl alcohol, benzaldehyde, benzoic acid and benzylbenzoate have important economical impacts. They constitute starting points in the manufacture of important products, such as pharmaceuticals, dyes, solvents, perfumes, plasticizers, preservatives and flame retardants.
But, harmful chemicals are needed to make this family of molecules. The process starts with a simple, carbon-based molecule called toluene. Toluene is a component of crude oil. Its transformation currently requires harsh processes involving chlorinated chemicals, acidic solvents or toxic metals. The process has very poor yields (around 15 percent) and generates copious quantities of waste.
Alternatives starting with biomass are desirable and intensely researched, but to date, no alternative process exists to create this important family of compounds.
Current research on oxidation reactions focuses on three key points. First, any required additive needs to be solid to ease separation. Second, the oxygen atom introduced should preferably come from oxygen gas present in air, as opposed to more wasteful or toxic sources. Third, the process must be very efficient – that is, yield a lot of material– and selective – create only one molecular product and thus limit separating the wheat – what’s wanted – from the chaff – what’s not – after the reaction.
The research team from the United Kingdom and the United States exposed toluene to oxygen at 160 degrees Celcius and 10 bars of oxygen – equivalent to 10 times normal atmospheric pressure – for two days. Under these conditions, a very small fraction – 2.9 percent – of toluene was converted into a mixture of benzaldehyde, benzoic acid and a small quantity of the desired benzylbenzoate.
Then, they tested several materials for their ability to improve the reaction. One was gold nanoparticles deposited onto titania, because it actively promotes oxidation of alcohols and carbon monoxide.
Gold nanoparticles deposited onto carbon – similar to porous charcoal – was also tested.
In another approach, they deposited tiny particles of gold and palladium inside the carbon pores. In this setting, gold and palladium form an alloy that has unique properties, different from both pure gold and pure palladium.
They also varied the proportion of gold and palladium in the system and measured how it was impacting the chemical process.
Each system was analyzed using two criteria: 1) conversion – the number of toluene molecules transformed – and 2) selectivity – Did one toluene transform into one or several kinds of molecules?
The gold and palladium nanoparticles on the porous carbon offered the best materials to spur the difficult reaction that transforms toluene to benzylbenzoate.
About half of the toluene was converted with this media. The process was very selective – 94.3 percent of the product was benzylbenzoate. This process required 6,500 times less metal than toluene.
The authors then tried to improve toluene conversion by altering the amount of metal. By adding more metal – 1,650 times less metal than toluene – they reached a conversion of 94.4 percent.
Researchers found that the other materials were not as good. Gold nanoparticles on titania did not perform well despite its other successes. Gold alone on carbon was not very active, either.
The mixture of the gold with palladium inside each nanoparticle proved essential to activity. The optimum proportion was 1 gold atom for 1.85 palladium atoms. Size mattered too, and a small size particle was critical to obtaining high conversion.
In this work, researchers developed a novel material that takes oxygen from the air to transform toluene into the valuable benzyl benzoate. The material is composed of carbon, gold and palladium at specific ratios for optimum performance.
This research is very important because it brings chemists closer to defining the reaction that could convert methane from natural gas to methanol. Such a process could allow two important developments.
First, methane is a flammable gas and is thus impractical and dangerous to use as a fuel for vehicles. Methanol, on the other side, is a liquid similar in properties to ethanol. Ethanol currently makes up a small fraction of automobile fuel sold in the United States.
Second, methanol has other important applications in the chemical industry. It is used as a solvent, as an antifreeze and as a building block for a host of commodity chemicals.
The new reaction is also remarkable because of its high yield, chemical selectivity, low waste and use of available resources. The use of a very simply reagent – oxygen gas from air – generated no waste.
The material was very convenient to use. After the reaction, the ending product separated completely from the starting material and was simply filtered off. This allows for two good things: 1) no metal or carbon ended up in the product and 2) the material can be reused in subsequent reactions. The materials was reused four times without measuring any alteration of reactivity.
The porous carbon support played an important role in the success of the reaction. The same material based on porous titania was less active. The researchers suggest the improved performance originates from the difference in surface structure between the carbon and titania based nanoparticles. They believe carbon causes the metal particles to be rougher and thus more reactive with the oxygen.
In all cases, the chemists did not detect any carbon dioxide waste. This means that the oxidation reaction – that is, adding an oxygen atom to the starting carbon, gold and palladium materials – proceeds well. Only one atom of oxygen is added, not several, which happens during complete combustion. This process affords a valuable chemical from toluene without burning toluene itself.
More development and studies are needed to turn this discovery into an industrial process.