Chemical Recycling of Polystyrene to Valuable Chemicals via Selective Acid-Catalyzed Aerobic Oxidation under Visible Light

Abstract

Chemical recycling is one of the most promising technologies that could contribute to circular economy targets by providing solutions to plastic waste; however, it is still at an early stage of development. In this work, we describe the first light-driven, acid-catalyzed protocol for chemical recycling of polystyrene waste to valuable chemicals under 1 bar of O 2 . Requiring no photosensitizers and only mild reaction conditions, the protocol is operationally simple and has also been demonstrated in a flow system. Electron paramagnetic resonance (EPR) investigations and density functional theory (DFT) calculations indicate that singlet oxygen is involved as the reactive oxygen species in this degradation process, which abstracts a hydrogen atom from a tertiary C–H bond, leading to hydroperoxidation and subsequent C–C bond cracking events via a radical process. Notably, our study indicates that an adduct of polystyrene and an acid catalyst might be formed in situ, which could act as a photosensitizer to initiate the formation of singlet oxygen. In addition, the oxidized polystyrene polymer may play a role in the production of singlet oxygen under light.

Bridging Plastic Recycling and Organic Catalysis: Photocatalytic Deconstruction of Polystyrene via a C–H Oxidation Pathway

“…Recent reports have demonstrated the feasibility of driving photocatalytic deconstruction of PS with FeCl 3 or acid. 32 − 34 In our report, the photocatalysis is driven by a blue LED under mild temperature and pressure (50 °C, 1 bar, aerobic O 2 ) with an aromatic ketone as the HAT photocatalyst. The HAT catalytic cycle and the formation of the peroxyl radical intermediate are comparable to small-molecule −CH 2 – oxidation, but the absence of the second hydrogen induces the cleavage of the C–C bonds in the polymer backbone to give the degradation products.…”

Catalytic oxidation of polystyrene to aromatic oxygenates over a graphitic carbon nitride catalyst

“…Previous works have exhibited that polystyrene can be selectively converted into the monomer styrene 24 or other arenes such as benzene 25 by heterogeneous catalysts, while the upgradation into functionalized aromatics with higher value was mainly achieved by homogeneous catalysts 26 . Very recently, oxidative deconstruction of polystyrene was reported, which depolymerized and transformed the polymer into high-value products with oxygen-containing groups, such as benzoic acid [27][28][29][30][31] . The photocatalytic benzylic C-H bond activation was involved in most of these catalytic processes.…”

Accelerated Degradation of Microplastics at the Liquid Interface of Ice Crystals in Frozen Aqueous Solutions

“…Some novel advanced oxidation technologies for rapid degradation of MPs have been widely studied, such as photodegradation, photocatalytic oxidation, and electrochemical oxidation, with promising achievements to date. [2] However, these methods either involve catalysis or they are externally stimulated by UV and thermal activation. Notably, the widespread distribution of MPs in the natural environment has been disregarded in the past decade, although some slow degradation induced by light and microorganism have been discovered.…”

“…Microplastics (MPs) are an emerging, ubiquitous pollutant that are resistant to degradation, [1] and consequently, they have attracted global attention in recent years. Some novel advanced oxidation technologies for rapid degradation of MPs have been widely studied, such as photodegradation, photocatalytic oxidation, and electrochemical oxidation, with promising achievements to date [2] . However, these methods either involve catalysis or they are externally stimulated by UV and thermal activation.…”

“…Notably, such combined effects improved selectivity to 1 O 2 production, rather than ⋅OH in traditional aqueous systems, and enriched the as‐formed 1 O 2 in the confined space [22] . Due to the low energy barrier of 1 O 2 ‐mediated PS oxidation, [2] highly selective generation of 1 O 2 could boost the degradation of PS easily through radical cross‐linking reactions (Figure S12) [23] in a frozen system. Without foreign energy input, this process is completely different form a traditional PS degradation process, which has no concentration effects (Figure 3b; Figure S10) and thus needs external stimulation of UV radiation, thermal activation, and so on [13, 19] .…”