Catalysts screening, optimization and mechanism studies of dimethylhexane-1,6-dicarbamate synthesis from 1,6-hexanediamine and dimethyl carbonate over Mn(OAc)(2) catalyst

The catalytic synthesis of dimethylhexane-1,6-dicarbamate (HDC) from 1,6-hexanediamine (HDA) and dimethyl carbonate (DMC) over a series of homogeneous catalysts (e.g., metallic nitrates and acetates) were investigated. Mn(OAc)(2) was found to be the most effective catalyst for this reaction. Under the optimum reaction conditions (reaction temperature of 363 K, DMC-to-HDA molar ratio of 4:1, catalyst concentration of 7 wt% (based on HDA), and reaction time of 5 h), HDC yield can reach as high as 98.0% with HDA totally converted. Online in-situ Fourier Transform Infrared (FTIR) and Density Functional Theory (DFT) calculations were used together to investigate the interactions between Mn(OAc)(2) and substrates. The results indicated that both of HDA and DMC had interactions with Mn(OAc)(2), but the energy barriers of the transition states (TS) of DMC (DMC-TS) was higher than that of HDA (HDA-TS), which meant that HDA was easier to interact with Mn(OAc)(2) to form new complex than DMC. The whole process can be proposed that, the new species II, which was formed by the interaction of HDA and Mn(OAc)(2), attacked carbonyl carbon of DMC to generate intermediate HMC and finally to HDC. The experimental data and calculation results are in good agreement with each other. (C) 2016 Elsevier B.V. All rights reserved.