High catalytic activity over novel Mg-Fe/Ti layered double hydroxides (LDHs) for polycarbonate diols (PCDLs): synthesis, mechanism and application

A series of novel activated Mg-Fe/Ti layered double hydroxides (LDHs) with high crystallinity, which were used for the transesterification between dimethyl carbonate (DMC) and aliphatic diols as acid-base bifunctional catalysts were successfully synthesized by co-precipitation. The structures of the LDHs were characterized by both X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), indicating that the formation of LDHs is strongly affected by the initial pH of the suspension. A general precipitation-recombination-crystallization mechanism was proposed to explain the LDHs formation and growth. The results of the CO2 and NH3 temperature-programmed desorption (CO2/NH3-TPD) tests showed that with increasing the amount of Ti4+ cations, the basicity of LDHs decreased, while the acidity of LDHs increased. An acid-base cooperative manner to catalyze the transesterification reaction was found to improve the catalytic activity of LDHs. The polycarbonate diols (PCDLs) with a high number-average molecular weight (M-n) and low hydroxyl value were obtained via a two-step transesterification method, which can avoid the DMC/methanol azeotrope during the reaction. With LDH-10 catalyst, the yield of methanol is up to 89.12%, and the PCDL shows the highest M-n (3030) and lowest hydroxyl value (43.5).