Tandem Heterogeneous Catalysis for Polyethylene Depolymerization via an Olefin-Intermediate Process

Abstract

The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al2O3 and Re2O7/γ-Al2O3. This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C20 alkane, n-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an n-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.

Recent Advancements in Plastic Packaging Recycling: A Mini-Review

“…Another SnPt/γ-Al 2 O 3 and Re 2 O 7 /γ-Al 2 O 3 heterogeneous catalyst was used to produce linear alkanes from HDPE. This type of catalyst promotes a tandem reaction via which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly active olefin catalyst [160].…”

Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia

“… 9 , 11 , 13 − 16 Olefin-mediated metathesis 17 has recently been applied to polyethylene decomposition using a large volume of pentane and long residence times. 6 Bifunctional hydrocracking has been proven feasible at moderate temperatures and H 2 pressures. 5 , 12 , 18 − 22 High branching imparts desirable properties to fuels and lubricants.…”

Waste to Wealth: Chemical Recycling and Chemical Upcycling of Waste Plastics for a Great Future

“…[39b] Alkane metathesis is also a promising strategy for breaking CÀ C bonds via a sequence in which both PE and light alkanes were catalytically dehydrogenated, the intermediate unsaturated PE chain and light alkene underwent metathesis to produce two new olefins, which were subsequently hydrogenated back to produce saturated hydrocarbons. [40] The ultralong PE chain will be cut into short hydrocarbons suitable for transporting oils after several cross alkane metathesis (CAM) cycles if it provides a large excess of light alkanes, although no ethylene monomer was obtained in the final products (Figure 7e). [40a] This catalytic system is highly selective to produce linear alkanes without the generation of alkenes and aromatics.…”