The plastic problem – accumulation before alternatives

The plastic problem – accumulation before alternatives

By Karlee Bamford, Treasurer for the GCI

Plastics undoubtedly play a central role in our daily lives and played a pivotal role in the development of consumer societies across the globe for over a century. Concurrent with newfound materials and newfound possibilities, unprecedented environmental problems have emerged as a result of our reliance on plastics. The accumulation of plastics in allocated disposal sites (e.g. landfills) and in otherwise uninhabited spaces (e.g. beaches, open ocean) present threats to human health, water security, and food supply. These challenges now impact communities globally, irrespective of their actual contribution to the generation of plastic waste, and affect individuals of all economic backgrounds.

Figure 1. Examples of waste plastic accumulation in landfills and the environment. Images source: Pixabay.

Given the scale and significance of these challenges, is there anything that chemists can do to resolve this panhuman problem? A recent blog post from the Green Chemistry Initiative (https://greenchemuoft.wordpress.com/category/author/molly-sung/) highlighted the advances that have been made in synthetic and materials chemistry towards plant-derived and biodegradable plastics as alternatives to traditional petroleum-derived plastics. While this is undoubtedly a crucial area of research as humanity has become permanently dependent on plastics, the design of next generation plastics that are inherently sustainable will not mitigate the overwhelming impacts of existing plastic waste. Arguably, attenuating the problem of plastic waste is more important than finding alternatives to traditional plastics. Indeed, the decomposition time for products made from the top four families of commodity plastics (PP, PE, PVC, PET), produced on a 224.6 million tonne-scale alone in 2017,1 is estimated at 1 to 600 years in marine environments2 and considerably longer in landfills due to lack of moisture.4

Figure 2. Examples of the top five most-produced commodity polymers and their production scale in 2017.1,3

Traditional plastic-recycling methods are not equipped to resolve the issue of waste plastic accumulation either. Recycling can be broken down into three distinct varieties: primary, secondary, and tertiary.5 Primary recycling, which is equivalent to repurposing or reusing, is used limitedly for products such as plastic bottles, typically made of PET, which be directly reused following the necessary sterilization. Secondary recycling involves mechanical processing of plastics into new materials and frequently results in reduction of the plastics overall quality or durability due to the thermal or chemical processes involved. Primary and secondary recycling account for the majority of recycling efforts, however, as a consequence of poor consumer compliance (e.g. <10 % in the US and 30-40 % in the EU)6 and the deteriorating value of plastics with repeated secondary recycling, all plastics eventually become waste. The last and most underutilized form of recycling is tertiary recycling, the degradation or depolymerization of plastics into useful chemicals or materials. In the last year alone, numerous high profile editorial and review articles have appeared in Science7,8,9 and Nature6,10 emphasizing the incredible potential of chemical (tertiary) recycling as means of reducing plastic waste and as a new, sustainable chemical feedstock for the polymer (plastics) industry.

The challenge of chemical recycling is immediately evident: plastics have been expertly designed to be highly durable and chemically resistant, and thus, plastics cannot be easily transformed chemically. Ideally, polymers used in plastics could be depolymerized to monomer for subsequent repolymerization. For condensation polymers, such as polyethylene terephthalate (PET), the reverse of the polymerization reaction is the addition of a small molecule to the polymer to reform monomer. While completely reversible on paper or in theory, such depolymerization strategies have had limited success for PET.

Reacting the polymeric PET material with protic reagents (e.g. amines, alcohols) followed by hydrolysis to give monomers that can be repolymerized, if of sufficient purity (Figure 3), requires high temperature (250-300 °C) and high pressure (0.1-4 MPa) conditions unless additives, such as strong acids and bases or metal salts, are used.11 The action of many additives is not well understood, thus precluding rational improvement of the system. Hydrolysis of PET itself, especially at neutral pH, is the most challenging approach to PET chemical recycling as water is a relatively poor nucleophile. Hence stronger nucleophiles, such as ethylene glycol, are preferred.

Figure 3. Depolymerization of PET by glycolysis.

One practical problem in the chemical recycling of any plastic is its insolubility. Phase transfer catalysts –  species capable of transferring from one phase to another – have been used to address the insolubility of PET12 and have permitted the direct hydrolysis of PET at operating temperatures as low as 80 °C, as in the work of Karayannidis and coworkers (Figure 4). The phases in these systems are the insoluble PET polymer (the organic phase) and the basic solution (the aqueous phase) surrounding it.13

Figure 4. Phase transfer catalyzed hydrolysis of PET (catalyst shown in blue).

Addition polymers, such as polypropylene (PP) or polyethylene (PE), cannot be depolymerized to monomer form using the above strategies as their polymerization does not involve the loss of small molecules. Until very recently, the best end-of-life purpose for the majority of plastics has been energy recovery through incineration. The work of Huang and coworkers on the chemical degradation of PE plastics is a break-through for the field of plastic recycling. While previous studies have reported that thermolysis of PE yields poorly defined mixtures of hydrocarbons, these authors have found a remarkable, highly targeted method for converting PE to a narrow distribution of fuels (3 to 30 carbons in length) using a dehydrogenative metathesis strategy (Figure 5).14 The homogeneous iridium catalysts employed were previously reported in the literature for alkane dehydrogenation (step 1) and hydrogenation (step 3), but no such polymer substrates had apparently been attempted for main-chain dehydrogenation. Similarly, the authors used a previously-established rhenium oxide/aluminium oxide catalyst for olefin metathesis (step 2).

Figure 5. The transition-metal catalyzed degradation of PE to liquid fuels reported by Huang and Guan (catalysts shown in blue).14

The chemical recycling of PET by phase transfer catalysis and of PE by dehydrogenative-metathesis have very little in common with one another on a technical level. What unites these two strategies is the desire to transform the problematic, highly abundant and inexpensive resource that is waste plastic into useful commodities. Perhaps more importantly, these two examples both take revolutionary approaches to old problems through inspiration from fundamental research and parallels found in small molecule catalysis. Rethinking the plastic problem into a challenge for catalysis, rather than solely a call for clever materials design, is critical if we wish to reduce the threats that waste plastics pose to our health and our environment.

References:

  1. Tavazzi, L., et al., The Excellence of the Plastics Supply Chain in Relaunching Manufacturing in Italy and Europe, The European House, Ambrosetti, 2013 (as cited in Bühler‐Vidal, J. O. The Business of Polyethylene. In Handbook of Industrial Polyethylene and Technology; Spalding, M. A.; Chatterjee, A. M., Eds.; John Wiley & Sons: Hoboken, NJ, 2017; p. 1305).
  2. Mote Marine Laboratory Biodegradation Timeline; 1993. Available from: https://www.mass.gov/files/documents/2016/08/pq/pocket-guide-2003.pdf ; accessed July 10, 2018.
  3. Image sources: Image sources: (Plastic recycling symbols) http://naturalsociety.com/recycling-symbols-numbers-plastic-bottles-meaning/ ; (PP) https://www.screwfix.com/p/stranded-polypropylene-rope-blue-6mm-x-30m/98570 ; (LLDPE) https://www.polymersolutions.com/blog/differences-between-ldpe-and-hdpe/ ; (HDPE) https://chemglass.com/bottles-high-density-polyethylene-hdpe-wide-mouths ; (PVC) https://omnexus.specialchem.com/selection-guide/polyvinyl-chloride-pvc-plastic ; (PET) https://ecosumo.wordpress.com/2009/06/04/what-does-the-recycle-symbol-mean-part-2/
  4. Andrady, A. L. Journal of Macromolecular Science, Part C: Polymer Reviews, 1994, 34(1), 25-76.
  5. Hopewell, J.; Dvorak, R.; Kosior, E. Trans. R. Soc. B, 2009, 364, 2115–2126.
  6. Rahimi, A.; García, J. M. Nature Reviews Chemistry, 2017, 1, 0046.
  7. MacArthur, E. Science, 2017, 358 (6365), 843.
  8. García, J. M.; Robertson, M. L. Science, 2017, 358(6365), 870-872.
  9. Sardon, H.; Dove, A. P. Science, 2018, 360(6387), 380-381.
  10. The Future of Plastic. Nature Communications, 2018, 9, 2157.
  11. Venkatachalam, S.; Nayak, S. G.; Labde, J. V.; Gharal, P. R.; Rao, K.; Kelkar, A. K. Degradation and Recyclability of Poly (Ethylene Terephthalate). In Polyester; Saleh, H. E. M., Ed.; InTech: London, 2004; p. 78.
  12. Glatzer, H. J.; Doraiswamy, L. K. Eng. Sci. 2000, 55(21), 5149-5160.
  13. Kosmidis, V. A.; Achilias, D. S.; Karayannidis, G. P. Mater. Eng. 2001, 286(10), 640-647.
  14. Jia, X.; Qin, C.; Friedberger, T.; Guan, Z.; Huang, Z. Science Advances 2016, 2(6), e1501591.

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Ecocatalysis: Harnessing Phytoextraction for Chemical Transformations

Ecocatalysis: Harnessing Phytoextraction for Chemical Transformations

By Karlee Bamford, Treasurer for the GCI

What is ecocatalysis? I had never heard this term before until reading a recent publication from Grison and coworkers in the RSC journal Green Chemistry entitled “Ecocatalyzed Suzuki cross coupling of heteroaryl compounds”.1 In this work, the authors perform the familiar Suzuki cross-coupling of arylboronic acids (Figure 1) with heteroaryl halides. However, they use a thoroughly unfamiliar palladium catalyst: the common water hyacinth (Figure 2).

Karlee_blog1

Figure 1. The general reaction for Suzuki cross-coupling  (Ar = substituted phenyl, thiophene, or indole groups).

Karlee_blog2

Figure 2. The common water hyacinth (Eichhornia crassipes). Credit: Richard A. Howard, provided by Smithsonian Institution, Richard A. Howard Photograph Collection (Montréal, Canada). [2]

In broad terms, ecocatalysis is the use of plant-derived, metal-based catalysts in chemical reactions. If it were not for the author’s graphical abstract illustrating the plant body performing catalysis, I would have assumed that this was a more standard bioinorganic paper featuring a plant extract as catalyst. While the propensity of certain plants and microbiota to uptake (“phytoextract”) particular contaminants has long been used in waste water purification, for example in the uptake of inorganic phosphates (e.g. [PO4]3-),3 ecocatalysis represents a clever progression from plants being used in chemical sequestration to chemical transformation.

Plants currently used in phytoremediation, that is, the removal of contaminants such as heavy metals from anthropogenically polluted environments, could clearly be used in the production of ecocatalysts.4 One current use for such metal-laden plants is in phytomining as so-called bio-ore.5 This metal extraction process ultimately results in the majority of the plant bio-mass being wasted through the energy-consumptive process of incineration, whereas an ecocatalyst such as EcoPd requires that same bio-mass as a kind of ligand support.

Grison and colleagues report reaction times, conditions, and yields (typically >90 %) for their “EcoPd” catalyst that are competitive with typical Suzuki cross coupling experiments and catalysts, both homogenous and heterogeneous. Remarkably, the primarily root-based EcoPd catalyst can be reclaimed and reused without loss of activity, as the authors demonstrated in a control study that involved recycling the catalyst four times over. Finally, the palladium content of the used catalyst can be quantitatively recovered by rhizofiltration, that is, by returning the elemental palladium obtained in post-synthesis work-up to a new plant specimen for metal uptake. In practical terms, this involves filtering the post-synthesis solution, dissolving the isolated solids with aqua regia, and diluting the resultant palladium-containing solution with water before reintroducing it to the roots of E. crassipes.

Ecocatalysis is an entirely new and emerging field of chemistry (circa 2013) being pioneered by Grison and coworkers at The Laboratory of Bio-inspired Chemistry and Ecological Innovations (University of Monpellier) in France. Their research has furnished several other noteworthy ecocatalysts (EcoM’s) featuring nickel (EcoNi),6 zinc (EcoZn),7 manganese (EcoMn),8 copper (EcoCu),9 which have proven effective in Biginelli, Diels-Alder, reductive amination, and Ullmann reactions, respectively.

This new approach to catalysis is not only charmingly novel – at least to a non-bioinorganically-minded chemist such as myself – but it also offers a real solution to the problematic dependence of catalysis on pure precious metals. The plants themselves provide a means for both harvesting and using low-abundance metals in a format that does not require complicated ligand design and is consistent with homogenous catalysis. Clearly, EcoPd and other such EcoM may not be suitable replacements in every metal-catalyzed transformation, but they nonetheless provide a new avenue for recycling precious metals and realising catalyst sustainability.

The range of possible ecocatalysts is, in my mind, astounding. Plant species that are known to preferentially accumulate heavy metals, known as accumulators and hyperaccumulators, are greater than 500 in number and sequester metals from across the p- and d-block of the periodic table, each to varying extents.10 As the tolerance and preference for certain transition metals is in part gene-regulated,11 it is conceivable that genetic modification and controlled environmental conditions could in the future yield heavy metal-specific plant species for sequestration and, perhaps, subsequent ecocatalysis.

 

References:

  1. G. Clavé, F. Pelissier, S. Campidelli and C. Grison, Green Chemistry, 2017, DOI: 10.1039/c7gc01672g.
  2. Used with permission from Larry Allain, hosted by the USDA-NRCS PLANTS Database.
  3. J. Lv, J. Feng, Q. Liu and S. Xie, Int. J. Mol. Sci., 2017, 18.
  4. C. Grison, Environmental Science and Pollution Research, 2015, 22, 5589-5591.
  5. R. R. Brooks, M. F. Chambers, L. J. Nicks and B. H. Robinson, Trends in Plant Science, 1998, 3, 359-362.
  6. C. Grison, V. Escande, E. Petit, L. Garoux, C. Boulanger and C. Grison, RSC Adv., 2013, 3, 22340.
  7. V. Escande, T. K. Olszewski and C. Grison, Comptes Rendus Chimie, 2014, 17, 731-737.
  8. V. Escande, A. Velati, C. Garel, B.-L. Renard, E. Petit and C. Grison, Green Chemistry, 2015, 17, 2188-2199.
  9. G. Clavé, C. Garel, C. Poullain, B.-L. Renard, T. K. Olszewski, B. Lange, M. Shutcha, M.-P. Faucon and C. Grison, RSC Adv., 2016, 6, 59550-59564.
  10. H. Sarma, Journal of Environmental Science and Technology, 2011, 4 118-138.
  11. S. Jan and J. A. Parray, Approaches to Heavy Metal Tolerance in Plants, Springer Singapore, Singapore, 2016.