Chemists Group of the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences and the University of California has developed a method for the conversion of polyethylene into liquid hydrocarbons and waxes. To do this, the researchers used a combination of two catalysts, working “in tandem”. The study is published in the journal Science Advances.

Polyethylene molecules are long and often branched hydrocarbon chain – (CH2-CH2) n-. In one molecule of commercial plastics which consist of bottles and bags may be from hundreds to tens of thousands of carbon atoms. Gasoline, diesel oil and fuel oil also is a mixture of hydrocarbons whose molecules are, in turn, are composed of 7-21 carbon atoms. Therefore, creating a way to effectively “shorten” the polyethylene, chemists will be able to process recycled into fuel.

The authors propose a new operation method based on a combination of three conventional reactions of hydrocarbons – dehydrogenation, hydrogenation and metathesis. Dehydrogenation – the process of separation of hydrogen from hydrocarbon skeleton molecule in this process -CH2-CH2- single bonds are converted to double -CH = CH-. Hydrogenation – the inverse process, converting multiple bonds by hydrogen in a single connection. The polyethylene of multiple bonds is almost no.

Driving “shortening” of polyethylene
Xiangqing Jia et al. / Science Advances, 2016

The key reaction, which reduces the number of atoms in the carbon chain, a metathesis reaction. It only take molecules with multiple bonds. The essence of the metathesis is as follows: to a special substance-catalyst join two molecules containing double bonds can be imagined in the form of chains, blue and red. Catalyst redistributes double bonds initially “gluing” together chains (formed by the square of the carbon atoms), and then tearing the sides of the square. As a result, we get two new red and blue chains.

If we assume that red was the long chain – 1000 carbon atoms, and blue short – only 10, then 505 for two chains of atoms are formed by metathesis. Adding excessive amounts of short chains (by orders of magnitude greater than the length), successive metathesis reaction will lead to a gradual decrease in the average length of the chains.

Each of these reactions requires its catalyst. Thus, hydrogenation and dehydrogenation goes iridium complex catalysts used in industrial metathesis rhenium oxide supported on alumina. In the experiments set forth in the new work, chemists were mixed in a laboratory reactor with two catalyst polyethylene, and n-octane hydrocarbon consisting of 8 18 carbon atoms and hydrogen atoms.

In the first step of the process, and polyethylene subjected to dehydrogenation octane – formed double bond. Then there is a metathesis reaction and hydrogenation of its products.

The mixture was heated to 175 ° C and allowed to stand for three days. In the case of low molecular weight polyethylene (about 120 carbon atoms in the chain), the authors failed to reduce chain length of 20-40 atoms in the chain or less. Importantly, the metathesis can occur between molecules and octane. Similarly chemists conducted several experiments, in particular, by reacting an ordinary polyethylene industrial and petroleum ether.

A – used types of plastics and yields of liquid and waxy product. B – process photos. C – mass spectrum processed products
Xiangqing Jia et al. / Science Advances, 2016
Scientists were able to achieve complete recycling of plastic into liquid hydrocarbons and paraffin waxes per day reaction. As chemists said conversion conditions so soft polyethylene has not previously been described. In addition, the catalysts used do not lose its activity even when present in polyethylene stabilizing additives – for example, zinc stearate or polyphenols.

Many industrial polymers has a very high chemical stability in environmental conditions, for example, polyethylene practically decomposed. Recently, however, Japanese biologists have found a strain of bacteria that is capable of processing PET – plastic, is actively used for the manufacture of bottles.