LAUSR

Welive in the “plastics age” where plastics are the most commonly engineered material and found in almost everything we purchase, ranging from food packaging and clothing to electronic and medical devices. Polyolefins are the most common type of plastics, and there is tremendous interest from consumers, governments, and industry to have more efficient technologies for their recycling. In this month’s issue of JACS Au, Julie E. Rorrer and Yuriy Romań-Leshkov at Massachusetts Institute of Technology and Gregg T. Beckham at National Renewable Energy Laboratory have demonstrated that polyolefins can be converted into alkanes over a ruthenium on a carbon support (Ru/C) catalyst under mild conditions of 200 °C and hydrogen pressure of 20 bar. Globally, more than 92.2 million tons of polyethylene (PE) and 52.6 million tons of polypropylene (PP) are produced each year. However, most plastics are not efficiently recycled due to the lack of viable recycling technologies. Plastic recycling has several challenges including the low-density of plastics, contamination, and challenges with collecting and sorting the waste plastics. It has been estimated that only 14% of plastics that are produced are recycled (with only 2% being recycled in closed-loop processes), 40% are being landfilled, and 32% are leaking into the environment. Plastic recycling today primarily involves mechanical recycling where pure plastic materials are cleaned and then converted into pellets which can have slightly degraded properties compared to the virgin plastics. Rorrer et al.’s work fits into the area of chemical recycling of plastics where catalysts, heat, and solvents are used in the recycling process. The plastics industry is making numerous efforts to use chemistry to recycle waste plastics. For example, an industrial consortium of more than 80 companies, called the Alliance to End Waste Plastics, has committed over $1.0 billion dollars to develop improved technologies for plastic recycling. However, understanding the chemistry of these chemical processing reactions and designing stable and robust catalysts needs to occur before economical processes can be used at the industrial scale. The alkanes produced by Rorrer et al. can be used to produce diesel fuel and lubricants or as a feed to produce more olefins in a steam cracker. The Ru/C catalyst is a heterogeneous or solid catalyst, which is the most commonly used industrial catalyst. Heterogeneous catalysts have the advantage of being recyclable and are easily separable from the reactants and products. Thus, heterogeneous catalysts can last for years in a chemical reactor before they need to be replaced. As shown by Rorrer et al., during the hydrogenolysis reaction, C−C bonds along the polyolefin chain can be cleaved by hydrogen to produce lighter alkanes. Rorrer et al. first tested a number of noble metal catalysts using n-octadecane as a PE model compound. Model compounds are often used to simplify the product analysis and reaction chemistry, compared to reactions with the actual feedstock. Complete conversion of n-octadecane was observed with a 5 wt % Ru/C catalyst at 250 °C and a hydrogen pressure of 50 bar. This catalyst was selected for further study with PE feeds to produce lighter alkanes at similar reaction conditions used for n-octadecane conversion. The hydrogenolysis proceeds via both terminal and internal C−C bond cleavage. The catalyst was stable in a continuous flow reactor with a n-dodecane feed, but more work will be needed to see if this catalyst is stable with actual plastic material. Following the results with n-octadecane, three different PE substrates were tested for the hydrogenolysis: (1) a model PE (MW of 4000), (2) a low-density PE (LDPE MI25), and (3) a postconsumer LDPE bottle. The gaseous alkanes produced ranged between C1 and C7 with methane being the primary product (Figure 1b). For the LDPE bottle,