The decomposition process and kinetic analysis of benzene-based liquid crystal in hydrothermal system

•Hydrothermal method can effectively decompose EMOPE into innocuous products.•Reaction kinetic of EMOPE under optimized conditions is studied.•Two possible decomposition processes of EMOPE are proposed.•The mixture of EMOPE and OCBPh can be hydrothermally treated together sufficiently.

Liquid crystal, while being a core component in liquid crystal displays, also has potential harmful effects on ecosystems and human health, especially when it is not treated properly. In this study, a hydrothermal process is proposed to dispose liquid crystal of 1-ethyl-4-[(4-methoxyphenyl)ethynyl]benzene (EMOPE) and mixed liquid crystal of EMOPE and 4-octoxy-4′-cyanobiphenyl (OCBPh). The effect of reaction time, reaction temperature, oxidant addition and solid-to-liquid ratio on decomposition ratio is evaluated. Products from hydrothermal decomposition of EMOPE are characterized, and two possible hydrothermal decomposition pathways are proposed. To get complete decomposition of 4 mg of EMOPE, the conditions required are: 0.3 mL of H2O2 addition, 3.5 mL of water supply, 275 °C and 5 min. The decomposition kinetic analysis of EMOPE indicates that its decomposition process fits the apparent first-order reaction model. For the mixture's decomposition, given conditions of 275 °C, reaction time of 6 min, 3.5 mL of water supply, and 1.5 time of oxidant addition, the decomposition ratio of OCBPh reaches to 99.75%, and EMOPE can be completely decomposed.