Everyone knows that plastics are destroying the world. It’s really no secret. Yet since the 1950s, when mass produced plastics started appearing everywhere from Tupperware to telephones, the amount of non-biodegradable, barely destructible plastics manufactured around the world has increased every year. In time, they won’t only full landfills but also our streets. Microplastics already plague our oceans. And all because nobody has figured a decent way to break the bonds that make plastic do durable.
Until now, perhaps.
Saito Susumu from Nagoya University in Japan has developed a catalyst capable of destroying the pesky double bonds in plastic. If his method can scale up sufficiently, the world might stand a chance. SCINQ spoke with him to learn more about his plastic-reducing catalyst.
SCIENTIFIC INQUIRER: First some background. Why are plastics so notoriously difficult to break down?
SAITO SUSUMU: Because many plastics such as Kevlar and nylons have extremely strong amide bonds. The C–N bond of amides (in plastics and peptides) has a double bond character, which is stronger than a single bond, and such an “essential double bond” is the most thermodynamically stable carboxylic acid derivatives (acid chloride, acid anhydride, ester, carboxylic acid, and amide). The amide bond has remained the last impregnable fortress.
SI: What are some approaches to breaking them down and why are they inefficient?
SS: The breakdown of amides by hydrogenation has been examined extensively, usually using either molecular catalysts (metal complex catalysts) or heterogeneous catalysts. The former show low reactivity and decompose easily under strenuous reaction conditions. The latter only works under harsh reaction conditions (high temperature and high hydrogen pressure), and thus undesirable side reactions accompany with the desirable amide bond breaking.
For promoting the hydrogenation of amide bonds using metal complexes, milder reaction conditions are required to avoid decomposition of the catalyst, accordingly. However, under such conditions, metal complexes can only hydrogenate a range of strongly or moderately “activated” amides (amides specifically manipulated as they could be more easily hydrogenated), including N-aryl-, N-acyl-, N-(di)methyl-, and -alkoxy amides and morpholino ketones, as well as relatively simple and small amides (e.g., formamides, acetamides, trifluoroacetamides). The carbonyl carbons of these amides have a more electrophilic character or the C–N bonds have a more single bond character. Unfortunately, however, those “activated” amides are rarely found in plastics or in nature. Plastics and natural peptides/proteins have much “unactivated” amide bonds, which are far more inert than “activated” amide bonds.
Our method might pave a new avenue for recycling monomer derivatives, as an alternative to the depolymerization of polyamides (e.g., 6-nylon) that proceeded at 300 °C (reported previously in Org. Lett.), much higher temperature than that we used in our system.
SI: Can you describe your work with catalysts prior to this current experiment?
SS: Our first attempt to use a Ru catalyst and successful results for amide hydrogenation, the narrative of our primary findings was published in Tetrahedron Lett. in 2013. During the course of the previous research, we used a (PN)2Ru complex as precatalyst, in which two PN-type bidentate (phosphinyl-CH2-pyridine) ligands composed of a pyridine-phosphine framework is coordinated to the Ru center. This (PN)2Ru complex was able to hydrogenate and cleave C–N bonds of relatively small “unactivated” amides under rather strenuous conditions. However, due to an inherent drawback in bidentate, ligands easily undergo detachment from the metal center especially at a high temperature, low catalytic activity was a serious problem with which plastics cannot be hydrogenated. Detachment of the ligand causes significant damage in the catalyst that is detrimental to maintaining catalytic activity for a long period of time. How is it possible to avoid the ready detachment of the ligand from the Ru center? To solve this issue, we modified the ligand from “bidentate” to “tetradentate”.
SI: How were you able to improve the catalysts so that catalysis occurred under more favorable conditions?
SS: We had three points in our original precatalyst design.
The first was “structural robustness” bolstered by the tetradentate “PNNP” (6,6’-bis(phosphinyl-CH2)-2,2’-bipyridine) ligand bearing two strongly electron-donating dialkylphosphines. The “tetradentate” makes the Ru center saturated and prevents the ligand detachment from the Ru center – through a “dissociative ligand exchange mechanism” – is effectively avoided even at a high reaction temperature.
The second point is the “steric bulkiness” of the ligand. This prevents incoming substrates other than H2 from making an attack to the Ru center. Many Ru complexes easily undergo deactivation and structural decomposition by the attack of external organic substrates or solvents, which promotes detachment of ligands from a metal center of a catalyst through an “associative ligand exchange mechanism”. Attack of amides to the Ru center should also be circumvented effectively at both low and high temperatures.
The third point is that only the smallest molecule, H2, could favorably attack and coordinate with the sterically encumbered bipyridine-Ru framework, corresponding with the “molecularly well-designed site to confine adsorbed H2 in”. The adsorbed H–H bond is activated and cleaved, leading to active H+ and H– species that subsequently are transferred to amides.
SI: What specific plastics do the catalysts work on?
SS: We tested several plastic nylons including AQ nylon P-70, AQ nylon A-90, AQ nylon T-70, AQ nylon P-95. All are commercially available from Toray Co. and degraded into their monomer units using our hydrogenation method. These polymers cannot dissolve or mix homogeneously at ambient temperature in toluene, which we used as a common solvent for many hydrogenation experiments, but to some extent it dissolved at a higher temperature. Kevlar is the tough guy, since this cannot dissolve effectively in any organic solvents. The molecular structures of the monomer units we obtained after hydrogenation cannot be disclosed to the public due to a confidentiality agreement with Toray.
SI: What would be necessary in order to scale the process up for commercial industrial use?
SS: Higher turnover number (TON: the quantity of product generated using one catalyst, calculated as product (mol)/Ru complex (mol)), and catalyst should work under milder reaction conditions to save money and energy input to the reaction. As a general trend observed in the present results, we still need hydrogen pressure greater than 10 atm. We also need to have a discussion with researchers in chemical companies about how to use and boost our system, and also, to what extent and level we should improve the current performance of the catalyst.
SI: Why brought you to a life in the sciences? Have you always wanted to be a scientist?
SS: When I was a junior high school kid, I was so impressed by many historic and milestone achievements made by Albert Einstein. I admired him so much as an enthusiastic and sensitive student, who seriously thought, “How I could be like him?” However, my father admonished me for wanting physics. He told me, “Chemistry left so many issues to be addressed. You have more chances to find a new thing and is more suitable for your future career.” Although I changed my main interest and discipline from physics to my second favorite, chemistry, when I enter the Nagoya University, I still love to think about natural phenomenon deeply, widely, rationally, and thus scientifically.
SI: How does this current study fit into your past work? How will it guide future lines of inquiry?
SS: It was one of the toughest challenges in chemistry/catalysis I ever had. As I said, hydrogenation of amide was the last impregnable fortress when I started the project. It took for more than eight years to publish the current results since I considered tackling this project. The current achievement encouraged and strongly pushed me to think, “What would be the next important and high-value-added issue to cope with?” I realized suddenly that hydrogenation of amide used to be, but no longer is the last impregnable fortress. The transformation of phosphine oxides into phosphines may be one of the important goals of my long journey of hydrogenation. As you may know, phosphorus is now disappearing from our life and living spaces to somewhere unknown as wastes. Human beings need to secure the source of ATP, NADH, DNA and RNA. We should recycle and reuse missing phosphorus atoms.
SI: What has been your most fulfilling moment as a researcher so far?
SS: When I realized that only I, no one else, accomplished something tough. When I achieved one of the most difficult chemical transformations – breaking an amide bond – . Nobody else’s efforts were able to achieve this.
When I grasped that my discovery would be a breakthrough, and that it might push science forward by leaps and bounds while having the potential to contribute to society (potentially contributing to a sustainable society).
When I achieved the hydrogenation of unactivated amides, an unforgettable, priceless, and irreplaceable moment showed up in front of me.
For more information on Saito Susumu. [Info]
For the original Nature article. [Article]
For more background on the damage plastics do to our environment: