An efficient green route for hexamethylene-1,6-diisocyanate synthesis by thermal decomposition of hexamethylene-1,6-dicarbamate over Co3O4/ZSM-5 catalyst:An indirect utilization of CO2

The utilization of CO₂ as rawmaterial for chemical synthesis has the potential for substantial economic and green benefits. Thermal decomposition of hexamethylene-1,6-dicarbamate (HDC) is a promising approach for indirect utilization of CO₂ to produce hexamethylene-1,6-diisocyanate (HDI). In this work, a green route was developed for the synthesis of HDI by thermal decomposition of HDC over Co₃O₄/ZSM-5 catalyst, using chlorobenzene as lowboiling point solvent. Different metal oxide supported catalysts were prepared by incipientwetness impregnation (IWI), PEG-additive (PEG) and deposition precipitation with ammonia evaporation (DP) methods. Their catalytic performances for the thermal decomposition of HDC were tested. The catalyst screening results showed that Co₃O₄/ZSM-5₂₅ catalysts prepared by different methods showed different performances in the order of Co₃O₄/ZSM-5_25(PEG) N Co₃O₄/ZSM-5_25(IWI) N Co₃O₄/ZSM-5_25(DP). The physicochemical properties of Co₃O₄/ZSM- 5₂₅ catalyst were characterized by XRD, FTIR, N₂ adsorption-desorption measurements, NH₃-TPD and XPS. The superior catalytic performance of Co₃O₄/ZSM-5_25(PEG) catalyst was attributed to its relative surface content of Co³⁺, surface lattice oxygen content and total acidity. Under the optimized reaction conditions: 6.5% HDC concentration in chlorobenzene, 1 wt% Co₃O₄/ZSM-5_25(PEG) catalyst, 250 ℃ temperature, 2.5 h time, 800 ml·min-1 nitrogen flow rate and 1.0 MPa pressure, the HDC conversion and HDI yield could reach 100% and 92.8% respectively. The Co₃O₄/ZSM-5_25(PEG) catalyst could be facilely separated from the reaction mixture, and reused without degradation in catalytic performance. Furthermore, a possible reaction mechanism was proposed based on the physicochemical properties of the Co₃O₄/ZSM-5₂₅ catalysts.