Low-temperature feedstock: XRD and oxidation

Hormonally yours

X-ray diffraction has been used to study the low-temperature oxidation of the important feedstock chemical ethylene over platinum nanoparticles supported on mesoporous silica, according to a study published in Angewandte Chemie.

Intriguingly though, this work is not about chemical modification for the sake of manufacturing and organic synthesis as one might expect, but about keep fruit fresh longer. Ripening fruit, vegetables, and flowers all naturally generate a ripening hormone, the small organic molecule ethylene, C2H4, which causes fruit to ripen and flowers to wilt even at 0 Celsius. Injecting ethylene into a batch and bringing fruit, such as bananas or tomatoes, up to temperature can be used to quickly ripen them for sale or use. However, it is not always possible to halt the natural process and this can often be a problem for fruit packers and shops, leading to premature spoilage of their stock. Writing in the journal Angewandte Chemie, a team in Japan has developed a new catalytic system that can quickly and completely degrade ethylene and so could be used in warehouses to filter the air surrounding fresh produced and help to prevent stock from perishing and flowers wilting so quickly.

Mesoporous support

Atsushi Fukuoka, Director and Professor of the Catalysis Research Center, at Hokkaido University, in Sapporo, Japan, usually specialises in heterogeneous catalysis for the conversion of cellulosic biomass and processing of mesoporous materials. He and his colleagues, Chuanxia Jiang and Kenji Hara, turned their expertise to an unsolved problem in the food industry. Previous biotechnological methods for removing ethylene from warehouses have been expensive, complex, or just ineffective. Other chemists have not been successful in finding a catalyst material that might rapidly oxidise ethylene, fundamentally because it must work at low temperatures otherwise one might break down the gaseous hormone but one would end up with unripe fruit stew or flowers that wilt because of the heat instead.

Fukuoka and his colleagues tested a range of different metals in combination with a variety of support materials to help them to find an effective catalyst for this problem. They prepared their catalyst by stirring the support material with an aqueous solution of a platinum salt for 18 hours. They then dried the support and heated it first under oxygen and then under hydrogen gas. This led to inclusion of platinum nanoparticles within the large pores of the silicon dioxide.

Storage solution

Ultimately, they were successful with platinum nanoparticles on a support made of special mesoporous silicon dioxide (MCM-41). This system was highly active from 0 Celsius up to room temperature, which is perfectly suitable for food and flower storage. Their experiments demonstrated a 99.8 percent conversion rate of ethylene to hormonally inactive products even at a concentration of 50 parts per million (ppm) at 0 Celsius. This catalyst was able to maintain a reduced level of ethylene over long periods and even with multiple usage. It was pore and particle size that seemed to work together to endow the catalyst with its high level of activity even at this low temperature.

The researchers suggest that the ethylene gas reacts with oxygen rapidly on the catalyst to form formaldehyde as an intermediate, which is then adsorbed on to the platinum nanoparticles and degraded to carbon monoxide and hydrogen-containing species. These then degrade further generating carbon dioxide and water, as one might anticipate in such a process. The team points out that a small amount of formic acid is also formed as a by-product of the reaction. They are currently investigating the precise details of the reaction mechanism with a view to optimising the catalyst still further or finding ways to exploit it chemically in reactions beyond fruit and vegetable storage.