Catalytic Hydrogenation of Polyurethanes to Base Chemicals: From Model Systems to Commercial and End-of-Life Polyurethane Materials

Abstract

Polyurethane (PU) is a highly valued polymer prepared from diisocyanates and polyols, and it is used in everyday products, such as shoe soles, mattresses, and insulation materials, but also for the construction of sophisticated parts of medical devices, wind turbine blades, aircrafts, and spacecrafts, to name a few. As PU is most commonly used as a thermoset polymer composed of cross-linked structures, its recycling is complicated and inefficient, leading to increasing PU waste accumulating every year. Catalytic hydrogenation represents an atom-efficient means for the deconstruction of polyurethanes, but so far the identification of an efficient catalyst for the disassembly of real-life and end-of-life PU samples has not been demonstrated. In this work, we reveal that a commercially available catalyst, Ir- i Pr MACHO, under 30 bar H 2 and 150–180 °C, is a general catalyst for the effective hydrogenation of the four cornerstones of PU: flexible solid, flexible foamed, rigid solid, and rigid foamed, leading to the isolation of aromatic amines and a polyol fraction. For the first time, a variety of commercial PU materials, including examples of foams, inline skating wheels, shoe soles, and insulation materials, has been deconstructed into the two fractions. Most desirable, our reaction conditions include the use of isopropyl alcohol as a representative of a green solvent. It is speculated that a partial glycolysis at the surface of the PU particles is taking place in this solvent and reaction temperatures in the presence of catalytic amounts of base. As such a more efficient hydrogenation of the solubilized PU fragments in isopropyl alcohol becomes possible. As the isolated anilines are precursors to the original isocyanate building blocks, and methods for their conversion are well-known, the work reported in this paper provides a realistic indication of a potential circular plastic economy solution for PU. Preliminary experiments were also undertaken applying Mn- i Pr MACHO for the deconstruction of a commercial flexible PU foam. Although successful, more forcing conditions were required than those when applying Ir- i Pr MACHO.

Evaluation of Manganese Catalysts for the Hydrogenative Deconstruction of Commercial and End‐of‐Life Polyurethane Samples

“…Although, a recent study by Liu and co-workers would suggest PNN-based Mn-6 and Mn-7 to outperform PNP-based Scheme 1. Catalytic hydrogenation of PU samples using ruthenium (Milstein and Schaub), [25,26] iridium (Skrydstrup), [27] and manganese (this work).…”

“…[25,26] Subsequently, we demonstrated the catalytic hydrogenation of a wide variety of PU materials covering all four PU cornerstones (rigid solid, rigid foamed, flexible solid and flexible foamed), employing the commercially available Ir-iPr MACHO catalyst (Scheme 1a). [27] In our efforts to identify more sustainable catalysts based on earth abundant metals, which demonstrate high activity for the hydrogenative deconstruction of end-of-life PU commercial products, we turned our attention towards manganese. [28] Representing the third most abundant transition metal in the earth's crust (after iron and titanium), the biocompatibility of manganese with low toxicity and as part of metabolic processes has made this particular metal an interesting substitute for noble metals.…”

“…These results build on initial tests with Mn-iPr MACHO for deconstruction of flexible foam reported by our group. [27] This procedure offers a base metal alternative to ruthenium or iridium for catalytic hydrogenative depolymerization of PU, while a high efficiency and turnover is maintained. Most importantly, the catalyst was demonstrated to effectively deconstruct four commercial PU products, representing different subclasses of this polymer.…”

Hydrogenative Depolymerization of Polyurethanes Catalyzed by a Manganese Pincer Complex

“…Recently, Skrydstrup and co-workers reported the catalytic hydrogenation of polyurethanes utilizing Ir pincer complex. [10] The procedure features the application of a green solvent isopropanol and can be performed at relatively low temperature (150-180 °C) and pressure (30 bar). A range of different PU samples were tested and depolymerized.…”

Recent Advancements in Photocatalytic Valorization of Plastic Waste to Chemicals and Fuels

“…Plastic as one of the most influential inventions of 20th century has been widely applied in our daily life (Andrady and Neal, 2009;Bai et al, 2019;Jie et al, 2020;Gausas et al, 2021;Zhang F et al, 2021). Currently, about 360 million tonnes (Mt) of plastic are produced each year (Martín et al, 2021), and the number is expected to double over the next 2 decades (MacArthur 2017).…”