Chlorination of para-substituted phenols: Formation of α, β-unsaturated C4-dialdehydes and C4-dicarboxylic acids

Effective boosting of halogenated α, β-unsaturated C4-dicarbonyl electrocatalytic hydrodehalogenation by 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) heterojunctions: A theoretical study

Halogenated α, β-unsaturated C4-dicarbonyl (X-BDA), a novel family of high-toxicity ring cleavage products, is produced during the disinfection of phenolic compounds. The technique of electrocatalytic hydrodehalogenation (ECH) is efficient in rupturing carbon-halogen bonds and generating useful chemicals. This study used first principles to examine the ECH reaction mechanism of X-BDA and the subsequent hydrogenation reaction of the toxic derivative BDA over the 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) catalysts. The catalytic activity of Ti3C2T2 (T = O, OH, F) catalysts decreases gradually with -OH, -F, -O functional group. The loading of 1 T'-MoS2 onto the Ti3C2T2 surface improves the stability and selectivity of Ti3C2T2. In particular, 1 T'-MoS2/Ti3C2(OH)2 is most conducive to the ECH reaction of X-BDA via a direct-indirect continuous reduction process. It exhibits excellent removal capability towards Cl-BDA, with decreasing reactivity in the order of the Cl-, Br-, and I-BDA. The material offers a solution to the challenging dechlorination issue. The dehalogenated product BDA can be hydrogenated to produce 1,4-butanedial, 1,4-butanediol, and 1,4-butenediol. Three valuable chemicals can be obtained by exerting an applied potential of - 0.65 V. This work suggests that the formation of heterojunction catalyst may lead to new strategies to improve ECH for environmental remediation applications.

Ipso Substitution of Aromatic Bromine in Chlorinated Waters: Impacts on Trihalomethane Formation

Parabens and salicylates were examined as disinfection byproduct (DBP) precursors to explore the possible influence of ipso substitution (i.e., halogen exchange) on the yield and speciation of trihalomethanes (THMs) formed during water chlorination. Substoichiometric conversion of C-Br bonds into C-Cl bonds was confirmed for several parabens and salicylates. The co-occurrence of (mono)brominated and nonhalogenated precursors in the presence of free chlorine (but in the absence of added Br-) generated polybrominated THMs, implicating ipso substitution. The THM molar yield, bromine incorporation, and bromine recovery from brominated and nonhalogenated precursor mixtures were commensurate with those observed from equimolar additions of NaBr, indicating efficient displacement of aromatic bromine by free chlorine followed by reincorporation of liberated HOBr into DBP precursors. The THM molar yield from brominated precursors was enhanced by a factor of ≤20 relative to that from nonhalogenated precursors. Trends in THM molar yields and bromine incorporation differed between brominated parabens and brominated salicylates, suggesting that the influence of ipso substitution on THM formation varies with the structure of the organic precursor. Collectively, these results provide new evidence of the often-overlooked role ipso substitution can play in promoting halogen exchange and bromine enrichment among DBPs in chlorinated waters.

Chlorination of bisphenols in water: Understanding the kinetics and formation mechanism of 2-butene-1,4-dial and analogues

While it is widely accepted that 2-butene-1,4-dial (BDA) is a toxic metabolite with genotoxic and carcinogenic properties, little is known about BDA and its analogues (BDAs) formation during water disinfection. In this study, the effects of different chlorination conditions on the formation of BDAs from bisphenol and its analogues (BPs analogues) were evaluated. A transformation pathway for the formation of BDAs upon chlorination of BPs analogues is proposed. The time profile of the transformation of BPs analogues into BDAs reveals that the generation of dichlorohydroquinone, dichloro-hydroxybenzenesulfonic acid and 2,4,6-trichlorophenol, are significantly associated with the formation of BDAs in the disinfected water. Owing to the different bridging groups contributing to the electrophilicity of BPs analogues in varying degrees, the stronger the electrophilicity of BPs analogues the more BDAs are formed. In addition, the type of BDAs produced is also affected. Four types of BDAs were detected in this study, one of which was newly identified. This study confirms that BPs analogues are an important source of BDAs and provides more insights into the formation of BDAs during chlorination. Greater attention should be given to the formation of BDAs in chlorinated water and their potential threat to humans and the ecosystem.