Depolymerization of Waste Plastics to Monomers and Chemicals Using a Hydrosilylation Strategy Facilitated by Brookhart’s Iridium(III) Catalyst

Abstract

Plastic waste management is a major concern. While the societal demand for sustainability is growing, landfilling and incineration of waste plastics remain the norm and methods able to efficiently recycle these materials are desirable. Herein, we report the depolymerization, under mild conditions, of oxygenated plastics in the presence of hydrosilanes with the cationic pincer complex [Ir(PCP)H(THF)][B(C6F5)4] (PCP = 1,3-(tBu2P)2C6H3)) as catalyst. The iridium catalyst, with a low loading (0.3-1 mol%), proves selective toward the formation of silyl ethers or the corresponding alkanes depending only on the reaction temperature. It is noteworthy that the depolymerization of real household waste plastics such as PET (from plastic bottles) and polylactic acid (PLA) from 3D printer filaments is not altered by the presence of dye or other plastic's additives.

Zinc Complexes for PLA Formation and Chemical Recycling: Towards a Circular Economy

“…Compared to other waste management strategies such as incineration and biodegradation, chemical recycling offers the greatest value with potential to reduce the cost of PLA and increase market uptake . Previous examples of PLA recycling processes include hydrolysis, alcoholysis, hydrogenation and hydrosilylation …”

Mono- and dimeric zinc(ii) complexes for PLA production and degradation into methyl lactate – a chemical recycling method

“…[27][28][29] A number of different PLA transesterification methods have been reported in the literature. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] DuPont possess a patent for PLA degradation in the presence of H 2 SO 4 , achieving high conversion (69-87%) to various lactate esters (R = Me, Et and n Bu) within 2 h between 150-190°C. 30 Coszach et al 17 demonstrated the hydrolysis of PLA to lactic acid, observing enhanced PLA dissolution and polymer separation with lactate esters as the solvent of choice.…”

“…Catalysts based on rare metals, including iridium and ruthenium, have also been reported for hydrogenation and hydrosilylation processes, but are unattractive options to industry due to a high metal cost. [36][37][38] Both Fliedel et al 40 and McKeown et al 41 have reported systems based on Zn(II)-complexes, with the latter achieving up to 100% Me-LA conversion within 1 h at 90°C. Interestingly, McKeown et al 42 recently demonstrated shifting to an analogous propylenediamine system afforded superior activity, achieving 81% conversion within 30 minutes at 50°C, highlighting the influence of metal-ligand relationships.…”

Current Technologies in Depolymerization Process and the Road Ahead

“…Cantat [ 66 ] and his group have depolymerized polycarbonates, polyesters, and polyethers using hydrosilanes as reductants and metal-free catalysts to generate functional chemicals such as alcohols and phenols at room temperature. They also reported the depolymerization of polyesters (such as polylactic acid, or PLA) in the presence of hydrosilanes with the cationic pincer complex as catalyst toward the formation of silyl ethers or the corresponding alkanes under mild conditions [ 67 ]. Liu et al [ 68 ] reported a series of imidazole-anion-derived ionic liquid were facilely synthesized and used for efficient catalyzing alcoholysis of polyester wastes, such as PLA and polyhydroxybutyrate (PHB).…”