Phosgene-free synthesis of hexamethylene-1,6-diisocyanate by the catalytic decomposition of dimethylhexane-1,6-dicarbamate over zinc-incorporated berlinite (ZnAlPO4)

Highlights: • Synthesis of HDI via HDU decomposition over ZnAlPO4 heterogeneous catalyst. • Employment of self-designed reliable fixed bed reactor for HDU decomposition to HDI. • As high as 89.4% yield of HDI over ZnAlPO4 catalyst. • High stability and large ability for repeating usage of ZnAlPO4 catalyst. -- Abstract: The phosgene-free synthesis of hexamethylene-1,6-diisocyanate (HDI) by the decomposition of dimethylhexane-1,6-dicarbamate (HDU) was carried out on a self-designed fixed-bed catalytic reactor, using zinc-incorporated berlinite (ZnAlPO4) as catalyst, dioctyl phthalate (DOP) as solvent and N2 as carrier gas. Factors influencing the yield of HDI, including the Zn/Al molar ratio, HDU concentration and liquid space velocity (LHSV), were investigated. Under the optimized reaction conditions, i.e., 4.8 wt.% concentration of HDU in DOP, 100 ml/min N2 flow rate, 0.09 MPa vacuum, 623 K reaction temperature, 1.2 h−1 LHSV and catalyst usage 2.0 g, a 89.4% yield of HDI had been achieved over the ZnAlPO4 (molar ratio Zn/Al = 0.04) catalyst. The ZnAlPO4 catalyst was found to exhibit a considerable large on-stream stability and could be repeatedly used in the decomposition of HDU to HDI, after its regeneration