Catalytic Hydrogenolysis of Polyolefins into Alkanes.

We live 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 Román-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.[1]

Globally, more than 92.2 million tons of polyethylene (PE) and 52.6 million tons of polypropylene (PP) are produced each year.[2,3] 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.[4] 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.[5] 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.[6]

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 b). For the LDPE bottle, longer reaction times at 225 °C produced methane with near 100% selectivity, which at larger scales could be an option for natural gas production. The liquid alkanes produced from the reactions with the different PE feeds were from C8 to C45+ as shown in Figure c–e. This demonstrated that postconsumer LDPE waste can be depolymerized without pretreatment via hydrogenolysis over a 5 wt % Ru/C catalyst under mild conditions of 225 °C and a hydrogen pressure of 22 bar in the absence of a solvent to produce gaseous and liquid alkanes.

Figure 1

Results for the hydrogenolysis of different PE substrates over a 5 wt % Ru/C catalyst. (a) Product distributions for PE 4K, LDPE MI25, and LDPE bottle. (b) Distribution of gaseous products from the different PE substrates. (c) C8 to C45+ distribution for PE 4K. (d) C8 to C45+ distribution for LDPE MI25. (e) C8 to C45+ distribution for LDPE bottle. (Reprinted from ref (1). Copyright © 2020 The Authors. Published by American Chemical Society.)

The hydrogenolysis of polyolefins into alkanes as described by Rorrer et al. is one approach for utilization of waste polyolefins using chemical technology. This approach will ultimately need to be compared with other approaches for upcycling of waste plastics such as pyrolysis and liquefaction.[7] Pyrolysis of PE at temperatures ranging from 500 to 600 °C produces a liquid oil that contains primarily C5–C40+ olefins, alkadienes, and cycloparaffins and a gaseous product that contains C1–C5 olefins, paraffins, and dienes.[8] This plastic oil can be used as a feed to produce olefins through steam cracking.[7] Another technology is the Plast-TCat approach, being developed by Anellotech, which uses a zeolite catalyst in a fluidized bed reactor to produce olefins and aromatics in a single catalytic step.[9] To properly evaluate these technologies, a rigorous process model will be needed to estimate the economics and environmental impacts of each technology. This model would be based on laboratory data and then on larger integrated pilot plants before the technologies are used at the commercial levels. These technologies will have to deal with impurities that are in mixed plastic waste streams. For example, polyvinyl chloride (PVC) is a contaminant that is ubiquitous in plastic waste streams. The chloride can form hydrochloric acid in the reactor, which causes catalyst deactivation and corrosion in steel. Past work in plastic recycling has discussed some of the challenges and technology options in dealing with the PVC.[7] The pretreatment of the postconsumer waste plastic streams is one of the most challenging aspects of plastic recycling that will need to be understood in more detail.

As discussed by Rorrer et al., it is important to consider how this reaction could be applied to other common polymers found in plastic waste. The authors are looking into the design of improved heterogeneous catalysts for the scission of specific bonds pertaining to different polymers. Studies like the work by Rorrer et al. are critical to provide a fundamental understanding of the chemistry that occurs during catalytic upgrading of waste plastics. Society needs to reduce the amount of plastic that leaks into the environment and goes into landfills. Clearly, chemistry and catalysis will be critical components of realistic technologies for plastic recycling as has been demonstrated by Rorrer et al.