Theoretical Study On Thermal Degradation Mechanism Of Tetrabromobisphenol A(TBBPA)

Tetrabromobisphenol A(TBBPA)is mainly used to increase the fire resistance of substances.TBBPA is widely used in plastics for its excellent performance and affordable pricing,and its output represents nearly 60% of the total BFR output.TBBPA-containing waste plastics are difficult to spontaneously degrade in the environment of nature.Currently,pyrolysis and co-pyrolysis technology are considered a promising and valuable decomposition strategy for recovering pyrolyzed oil from waste plastics,and have become the main-stream treatment technology for materials laden with BFRs in general.A deeper understanding of the pyrolysis mechanisms of TBBPA could greatly provide the theoretical basis for thermal degradation techniques on TBBPA-containing waste plastics.Therefore,this work presented an investigation on pyrolysis mechanism of TBBPA by employing density functional theory(DFT)B3P86/6-311+G(d,p)method.For the formation mechanisms of TBBPA pyrolysis products,polybrominated dibenzo-p-dioxins and dibenzofurans(PBDD/Fs),and mechanisms of catalytic pyrolysis,this paper presented various possible reaction pathways and acquired their thermodynamic parameters and analyzed the effects of H and Br radicals on the formation mechanisms of pyrolysis products.Some significant conclusions are drawn as follows:(1)The initial pyrolysis of TBBPA begins with demethylation reaction(268.8k J/mol).Abstraction of H atom from phenolic hydroxy groups by Br radical is barrierless and is also the optimal path to the generation of HBr.The keto-enol tautomerism reactions are optimal unimolecular debromination channels,but the formation of low-brominated bisphenol A has lower activation energies than that of other paths in the initial reaction involving H radicals.TBBPA can be transformed into low-brominated bisphenol A through consecutive hydrodebromination reactions with trivial activation energies of 8.7～9.5 k J/mol.The demethylation reaction and subsequent H transfer contribute to BPA and brominated BPA producing phenolic compounds.During the pyrolysis of TBBPA,the energy barriers of rate-determining steps of the optimal reaction paths for the formation of 2,4,6-tribromophenol,2,6-dibromophenol,2,4-dibromophenol,2-bromophenol,4-bromophenol and phenol are 108.8,7.6,8.7,8.1,9.5,and 8.7 k J/mol,respectivel.(2)The 2-bromophenoxy radical is a major decomposition product in initial pyrolysis of 2,6-dibromophenol.Dimerization of bromophenoxy radicals occurs mainly through O-C coupling.The formation of PBDDs proceeds mainly via dehydrogenation or debromination and cyclization reaction.The involvement of H and Br radical can obviously lower the energy barrier of the formation of PBDDs,and1-MBDD is the major pyrolysis product and 1,6-DBDD and DD are the major competitive pyrolysis products.The formation of PBDD/Fs proceeds mainly via H-abstraction or H-transfer,cyclization and debromination or dehydroxylation.The formations of brominated dibenzo-p-dioxins are superior to the formations of brominated dibenzofurans.(3)The possible reaction channels were studied and the effect of calcium ions on charge distribution was discussed during catalytic pyrolysis of TBBPA.The results show that the initial reaction of catalytic pyrolysis is calcium oxide capturing hydrogen protons of phenolic hydroxyl group of TBBPA.The elimination of hydrogen bromide may the reaction of capturing protons from hydrogen bromide via calcium oxide.Compared with pure pyrolysis,when calcium ions participate in the reaction,the negative charge of oxygen atoms significantly increases,leading to lower the reaction energy barrier of TBBPA keto-enol tautomerism.