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 CO_2 as raw material 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 CO_2 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 Co_3O_4/ZSM-5 catalyst, using chlorobenzene as low boiling point solvent. Different metal oxide supported catalysts were prepared by incipient wetness 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 Co_3O_4/ZSM-5_(25) catalysts prepared by different methods showed different performances in the order of Co_3O_4/ZSM-5_(25)(PEG)N Co_3O_4/ZSM-5_(25)(IWI)N Co_3O_4/ZSM-5_(25)(DP). The physicochemical properties of Co_3O_4/ZSM-5_(25) catalyst were characterized by XRD, FTIR, N2 adsorption-desorption measurements, NH3-TPD and XPS.The superior catalytic performance of Co_3O_4/ZSM-5_(25)(PEG)catalyst was attributed to its relative surface content of Co^(3+), surface lattice oxygen content and total acidity. Under the optimized reaction conditions: 6.5% HDC concentration in chlorobenzene, 1 wt% Co_3O_4/ZSM-5_(25)(PEG)catalyst, 250 °C 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 Co_3O_4/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 Co_3O_4/ZSM-5_(25) catalysts.

Non-phosgene synthesis of hexamethylene-1,6-diisocyanate from thermal decomposition of hexamethylene-1,6-dicarbamate over Zn–Co bimetallic supported ZSM-5 catalyst

A non-phosgene route for the synthesis of hexamethylene-1,6-diisocyanate(HDI) was developed via catalytic decomposition of hexamethylene-1,6-dicarbamate(HDC) over Zn–Co bi-metallic supported ZSM-5 catalyst.The catalyst was characterized by FTIR and XRD analyses. Three solvents dioctyl sebacate(DOS), dibutyl sebacate(DBS) and 1-butyl-3-methylimidazolium tetrafluoroborate(BMIMBF_4) were investigated and compared; DOS gave better performance. The catalytic performances for thermal decomposition of HDC to HDI using DOS as solvent were then investigated, and the results showed that, under the optimized reaction conditions, i.e.,10 wt%concentration of HDC in DOS, 250 °C temperature, 60 min reaction time, 83.8% yield of HDI had been achieved over Zn–Co/ZSM-5. Decomposition of the intermediate hexamethylene-1-carbamate-6-isocyanate(HMI) over Zn–Co/ZSM-5 in DOS solvent was further studied and the results indicated that yield of HDI from HMI reached to 69.6%(98.6% HDI selectively) at 270 °C, which further increased the yield of the total HDI(HDI_(tol)) to as high as 95.0%. Recycling of catalyst showed that HDI and HMI yield slightly decreased, and by-product yield increased after the catalyst was reused for 4 times. At last possible reaction mechanism was proposed.