Nature‐Inspired Depolymerization of Soda Lignin: Light‐Induced Free Radical Promoted Cleavage of β‐O‐4 Bonds

Abstract

The development of sustainable valorization methods for lignin is a challenging task because the vast majority of the reported studies involve either harsh conditions or expensive transition metal catalysts. Inspired by the sunlight degradation of lignin compounds, known as lignin yellowing, the use of a commercially available cheap organic photoinitiator, namely, phenacyl bromide (PAB) is reported here, for the efficient cleavage of lignin model compound 2‐phenoxyacetophenone (2‐PAP) and for the depolymerization of soda pulped lignin (SL) under UV‐A irradiation and ambient conditions. Real‐time NMR investigations of the photoreaction between 2‐PAP and PAB shed light on the possible reaction mechanisms involving different radical species, HBr, and molecular oxygen. Interestingly, combined spectral, chromatographic, powder X‐ray diffraction, and thermal studies of the photoreaction between PAB and SL indicate the formation of guaiacyl alcohol as the main product. The unprecedented performance of PAB is attributed to the excess generation of phenacyl radicals, the generation of photolabile brominated species, and HBr playing key roles in the cleavage of β‐O‐4 linkages. This work represents a new edge for sustainable lignin valorization under mild reaction conditions and offers the opportunity for large‐scale production of valuable aromatics using technical lignins as feedstock.

Efficient Photolytic Breakdown of Waste Polystyrene Foam Using an “All-in-One” Photo-HAT Reagent at Ambient Conditions

“…In another study, Reisner and co-workers successfully obtained BA by reacting a commercial PS foam with organic HAT catalysts, such as fluorenone and other aryl ketones, in conjunction with an acid catalyst (H 2 SO 4 ) in an O 2 atmosphere and visible light (Scheme 1C). 35 Inspired by these recent works and our long-term interest in utilizing ambient conditions for the photochemical valorization of waste polymer, 25 we investigated the efficacy of phenacyl bromide (PAB) as a metal-free photo-HAT reagent/catalyst in the degradation of waste PS foam under ambient conditions. We used ethyl acetate (EtAc) as an environmentally friendly solvent (Scheme 1E) and hypothesized that the photodecomposition of PAB under near UV light (365 nm) leads to the in situ generation of bromine and phenacyl radicals.…”

“…Light-induced methods to degrade waste polymers are sustainable alternatives to thermal methods. , Photochemical treatment of synthetic and natural waste polymers has recently become the main focus of scientists because of the lower energy requirements and greener nature, which meet the criteria of sustainability. , Although hydrogen atom transfer (HAT) reactions have always been a versatile tool for C–H bond chemistry, exploitation of light-induced HAT reactions for the degradation/valorization of waste polymers through oxidative C–C bond breakages has recently proved to be an effective and sustainable strategy. − Moreover, recent photochemical studies have also demonstrated that halogen radicals can act as the HAT reagent and selectively abstract benzylic hydrogens under mild reaction conditions. , …”

“…Inspired by these recent works and our long-term interest in utilizing ambient conditions for the photochemical valorization of waste polymer, we investigated the efficacy of phenacyl bromide (PAB) as a metal-free photo-HAT reagent/catalyst in the degradation of waste PS foam under ambient conditions. We used ethyl acetate (EtAc) as an environmentally friendly solvent (Scheme E) and hypothesized that the photodecomposition of PAB under near UV light (365 nm) leads to the in situ generation of bromine and phenacyl radicals.…”