Co-pyrolysis of polyethylene with products from thermal decomposition of brominated flame retardants

Highlights

• Hydro-dehalogenation of aromatic compounds constitutes a key step in refining pyrolysis oil of BFRs.

• We describe the interaction of a bromphenoxy radical with a PE-Cu(Br) configuration.

• Such a scenario arises in primitive recycling of e-waste and their open burning.

• Computed parameters are useful for understanding the combustion chemistry of BFRs-polymers blends.

Abstract

Co-pyrolysis of brominated flame retardants (BFRs) with polymeric materials prevails in scenarios pertinent to thermal recycling of bromine-laden objects; most notably the non-metallic fraction in e-waste. Hydro-dehalogenation of aromatic compounds in a hydrogen-donating medium constitutes a key step in refining pyrolysis oil of BFRs. Chemical reactions underpinning this process are poorly understood. Herein, we utilize accurate density functional theory (DFT) calculations to report thermo-kinetic parameters for the reaction of solid polyethylene, PE, (as a surrogate model for aliphatic polymers) with prime products sourced from thermal decomposition of BFRs, namely, HBr, bromophenols; benzene, and phenyl radical. Facile abstraction of an ethylenic H by Br atoms is expected to contribute to the formation of abundant HBr concentrations in practical systems. Likewise, a relatively low energy barrier for aromatic Br atom abstraction from a 2-bromophenol molecule by an alkyl radical site, concurs with the reported noticeable hydro-debromination capacity of PE. Pathways entailing a PE-induced bromination of a phenoxy radical should be hindered in view of high energy barrier for a Br transfer into the para position of the phenoxy radical. Adsorption of a phenoxy radical onto a Cu(Br) site substituted at the PE chain affords the commonly discussed PBDD/Fs precursor of a surface-bounded bromophenolate adduct. Such scenario arises due to the heterogeneous integration of metals into the bromine-rich carbon matrix in primitive recycling of e-waste and their open burning.

Introduction

To comply with safety codes, brominated flame retardants (BFRs) are added to electronics, furniture, building materials, and a wide range of consumer products (Covaci et al., 2011). “Waste-to-energy” approach has emerged as a main stream strategy in treating plastic-based objects laden with BFRs at the end of their lifetimes (thermal recycling) (Evangelopoulos et al., 2019). Likewise, a great deal of applied and fundamental research has focused on the extraction of non-brominated fractions from bromine-treated plastics (chemical recycling) (Zhang and Zhang, 2012). Effective elimination of bromine constitutes a major obstacle in chemical recycling of BFRs-containing polymers (Pivnenko et al., 2017). Unit operations of BFRs-polymer systems require a detail understanding of interaction of brominated species with the polymeric structural entities.

Thermal decomposition of BFRs yields a very broad spectra of brominated species spanning HBr, aliphatic, and aromatic compounds, in addition to PAHs (Altarawneh et al., 2019). Of a notable concern is the formation of the notorious polybrominated dibenzo-p-dioxin and dibenzofurans; PBDD/Fs (Wang et al., 2018). The latter predominantly forms via the condensation of structurally related precursors or through the gradual build-up of short carbon cuts; i.e., de novo synthesis (Yu et al., 2011). Pyrolysis of polymers treated with BFRs dictate reactions of brominated entities with the parent skeleton of the polymer (Luda et al., 2010; Yu et al., 2011; Koch et al., 2019). It follows that, it is necessary to describe on a precise atomic basis, chemical events that mark the synergistic interaction of BFRs’ pyrolysates with the structural entities of polymers. Luda and Balabanovich (2011) investigated the hydro-dehalogenation capacity of a series of polymeric materials (polystyrene, polyamide, polybutadiene, polypropylene (PP), and polyethylene (PE)) toward the conversion reaction bromophenols → phenol + HBr. It was found that PE and PP entail a significantly higher dehydrohalogenation reactivity than other polymers. This prompted authors (Luda and Balabanovich, 2011) to restrict the hydro-debromination reactivity to the aliphatic moieties in the macromolecule chains.

Formation of lower brominated congeners of aromatic compounds in case of BFRs-polymers systems concurs with the role of polymers as hydrogen donors (Luda et al., 2010; Luda and Balabanovich, 2011). Pyrolysis of synthetic hydrocarbon polymers proceeds in a highly reducing environment. Hydrogen atoms preferentially abstract aromatic bromine leading to lower brominated isomers. However, the role of polymeric chains on the chemical transformation of brominated compounds is far more complex than merely serving as a hydrogen reservoir. Balabanovich et al. (2005) co-pyrolyzed PP with polybrominated diphenyl ethers (PBDEs) reporting rapid decomposition of PBDEs; in reference to neat feed of PBDEs. Fission of C–C linkages in PP affords primary, secondary, and diradical sites. Abstraction of a bromine atom by these potent active sites triggers a lower-temperature decomposition pathway of PBDEs. In a previous study, we addressed (via DFT calculations) interaction of a bromophenol molecule with a cyclohexane moiety; as a model compound of the carbon matrix (Altarawneh and Dlugogorski, 2014b). A facile abstraction of phenolic’s O–H and aromatic C–Br bonds by radical sites in the cyclohexane adduct was reported.

In a recent review article, we provide a comprehensive account on pathways that operate in the decomposition of neat BFRs with a prime focus on reaction routes leading to major primary products and PBDD/Fs (Altarawneh et al., 2019). However, interaction of decomposition products of BFRs with the polymeric chain remains highly speculative. For instance; it is generally viewed that HBr (the major Br carrier in pyrolysis of BFRs) (Altarawneh et al., 2019), mediates fission of O–CH₂ bond in brominated epoxy resins. Such pathways are yet to be confirmed via quantum chemical calculations.

Overall, chemical reactions prevailing in the chemical recycling of e-waste in general and those related to emission of notorious brominated compounds in specific are rather poorly understood. To this end, this contribution attempts to shed a mechanistic insight into chemical events encountered during the co-pyrolysis of BFRs with polymeric materials. A periodic-boundary solid configuration of PE is deployed herein to simulate polymeric entities predominantly formed of CH₂ groups deploying accurate density functional theory (DFT) calculations. Intriguing questions to be answered are pertinent to a plausible function of polymeric Br site as a shuttle to transfer organic Br into phenyl-type radicals and a likely role of Cu-decorated PE chain in the bromination reaction sequence. The latter scenario prevails during primitive e-waste recycling operations, in which Cu species dominate the metallic content in the ensuing uncontrolled and highly random combustion medium (Fujimori et al., 2016). Formation of HBr molecules via free Br abstraction reactions of an H atom from the polymeric chain is investigated as a likely source for the abundant hydrogen bromide. Reactions pathways are mapped out for the interaction of a bromophenol model compound with the PE. Computed thermo-kinetic parameters shall be useful to form a robust understanding of the combustion chemistry germane to BFRs-polymers blends.