Hydrogen’s Critical Role in the Chemical Deconstruction of Plastic Waste
Synthetic polymers were first made in the early 20th century. These long-chain macromolecules are composed of many repeating sub-units, called monomers, and have versatile properties and applications, with their most notable use in plastics. As a result, the large-scale production of synthetic polymers has grown exponentially since the 1950s. Today, plastic materials have become ubiquitous, primarily due to their durability and low-cost production. Over 400 million tons of plastic are manufactured each year from fossil fuel derived carbon.1 Of this plastic, almost half is used as packaging, meaning it is produced, used, and discarded on a very short time scale.1 Thus, the question arises, what happens to all this plastic once it is discarded?
Currently, there are three common fates for plastics: landfilling, incineration, and recycling. Landfilling is the prevailing method; up to 60% of plastic produced has accumulated in landfills or the environment.1 Incineration, in which plastic is burned with or without energy recovery, accounts for 12% of plastic waste.1 Lastly, only 9% of all plastic has been recycled, largely due to technological difficulties of collecting and sorting this waste and the reality that the secondary material produced from recycling is of lower quality and lesser value.1 Evidently, the current solutions are lacking.
To tackle this problem, recent research efforts have focused on developing new recycling routes for the reuse of plastic waste. The umbrella term for this approach is called chemical deconstruction of plastic waste, in which the macromolecular long-chain structures of polymers are chemically cut into smaller molecules. One method of chemical deconstruction is depolymerization, in which the plastic is cut into its primary small monomers or oligomers, which can then be polymerized back into the original polymer with identical quality, enabling a true circular loop for plastic waste. Alternatively, chemical conversion can be used, in which polymers are deconstructed into smaller molecules that do not resemble the monomer or oligomer. Instead, the targeted molecules are called platform chemicals, which may have a multitude of uses as feedstocks for other polymers, fuels, or other important chemical commodities. While the chemical products resulting from plastic deconstruction may vary, the common goal between all approaches is to divert plastics from landfills and enable the reutilization of the precious carbon content of these polymers that derive initially from fossil fuels.
Scientists at the Center for Plastics Innovation (CPI) Energy Frontier Research Center (EFRC) have been focused on a specific category of chemical conversion—hydrogenolysis—which utilizes a catalyst and hydrogen to ‘cut’ chains of ordinary polyolefins, such as polyethylene, polypropylene, and polystyrene.2 Catalysts are materials that speed up a chemical reaction without being consumed during the reaction, and they are critical in the manufacturing of many major chemicals in the global economy. Notably, scientists at CPI have developed ruthenium-based heterogeneous (solid) catalysts, which have emerged as a dominant material for the hydrogenolysis of plastic waste due to ruthenium’s high activity for hydrogenolysis.3 The one drawback of ruthenium, however, is its propensity to produce methane, a low-value ‘C1’ product.4 For better economic feasibility of this process, the objective is to increase the catalyst’s probability of producing longer-chain hydrocarbon products that have higher value. To achieve this, CPI scientists have delved into the mechanism of polyolefin hydrogenolysis to design catalysts that shift the hydrogenolysis mechanism away from methane-forming pathways.
The hydrogenolysis of polyolefins on ruthenium catalysts proceeds through a multi-step mechanism (Figure 1). First, the polymer comes in proximity with the ruthenium on the catalyst surface through a process called ‘adsorption’. Then, ruthenium removes two hydrogen atoms from the polymer and adsorbs them onto its surface via ‘dehydrogenation’. Ruthenium then cuts the bond between the two adsorbed carbons, called ‘C-C scission’, resulting in two shorter chains that are adsorbed separately on the ruthenium surface. Following scission, the ruthenium uses nearby adsorbed hydrogen to hydrogenate the adsorbed carbon chains (‘hydrogenation’), and then finally the chains desorb from the catalyst surface and diffuse back into the polymer melt (‘desorption’).
While investigating this mechanism, CPI researchers found that the hydrogenolysis of polyolefins is ‘hydrogenation-limited’, meaning that the hydrogenation step of the adsorbed carbon chains is hindered due to a lack of available hydrogen. As a result, the carbon chains remain adsorbed on the catalyst, allowing for cascade C-C scission to occur, thus explaining the production of excess methane on these ruthenium catalysts (Figure 1A). In an ideal case, the hydrogenation and desorption of the carbon chains would take place after a single C-C scission event, resulting in long-chain carbon products (Figure 1B).
Based on these findings, CPI scientists determined that hydrogen plays a key role in deciding reaction pathways and the resulting products in plastic deconstruction. A higher hydrogen concentration on the catalyst surface amplifies the likelihood of single-event chain scission, optimizing the yield of longer-chain, higher-value hydrocarbon compounds.5
While boosting hydrogen pressure in the reactor can be effective, this method has limitations due to cost and safety concerns. Thus, researchers have turned to catalyst design. Recent investigations have looked into the effect of doping other elements onto the ruthenium catalyst surface—including Ti, Nb, Ce, W, V, and Mo—to see their impact on C-C scission.5 While Ti, Nb, and Ce did not change the likelihood of cascade C-C scissions, it was found that highly reducible oxides such as W, V, and Mo greatly increase the likelihood of single-event C-C scission, leading to decreased methane yields and increased C4-C30 yields. Ruthenium on its own has limited hydrogen storage capacity, meaning that its surface quickly becomes saturated with hydrogen atoms. However, the doped metal oxides work synergistically with the ruthenium to effectively ‘store’ hydrogen on the catalyst surface through a phenomenon called hydrogen spillover, increasing the total hydrogen storage of the catalyst surface.5 In this way, these dopant metal oxides increase the available hydrogen around the ruthenium on the catalyst surface. This enables the hydrogenation of the carbon chains and the subsequent desorption from the catalyst surface after single-event C-C scission, mitigating the occurrence of cascade C-C scission to produce methane.5
The work highlighted above, conducted by scientists at the CPI, has uncovered fundamental insight into the hydrogenolysis mechanism for polyolefins. By designing improved catalysts for more selective plastic deconstruction, CPI researchers have paved a path forward for the effective chemical deconstruction of plastic waste into valuable fuels, lubricants, and platform chemicals.
(1) Geyer, R.; Jambeck, J.; Law, K. Production, use, and fate of all plastics ever made. Sci. Adv. 2017, 3 (7). DOI: 10.1126/sciadv.1700782
(2) Kots, P.A.; Vance, B.C.; et al. Polyolefin plastic waste hydroconversion to fuels, lubricants, and waxes: A comparative study. Reaction Chemistry and Engineering. 2022, 7, 41- 54. DOI: 10.1039/d1re00447f
The University of Delaware Center for Plastics Innovation is supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award Number DE-SC0021166.
(3) Kots, P. A.; Liu, S.; et al. Polypropylene Plastic Waste Conversion to Lubricants over Ru/TiO2 Catalysts. ACS Catalysis. 2021, 11, 8104-8115. DOI: 10.1021/acscatal.1c00874
This work was ﬁnancially supported by the Center for Plastics Innovation (CPI), an Energy Frontier Research Center funded by the U.S. Department of Energy, Oﬃce of Science, Oﬃce of Basic Energy Sciences, award number DE-SC0021166.
(4) Wang, C.; Xie, T.; et al. Polyethylene Hydrogenolysis at Mild Conditions over Ruthenium on Tungstated Zirconia. JACS Au. 2021, 1, 1422-1434. DOI: 10.1021/jacsau.1c00200
This work was supported as part of the Center for Plastics Innovation, an Energy Frontier Research Center, funded by the U.S. Dept. of Energy, Oﬃce of Science, Oﬃce of Basic Energy Sciences, under Award Number DE-SC0021166.
(5) Wang, C.; Yu, K.; et al. A general strategy and a consolidated mechanism for low-methane hydrogenolysis of polyethylene over ruthenium. Appl. Cat. B: Environ. 2022, 319, 121899. DOI: 10.1016/j.apcatb.2022.121899
This work was supported as part of the Center for Plastics Innovation, an Energy Frontier Research Center funded by the US Dept. of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0021166.