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

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Abstract
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 (Mn) 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 Mn (3030) and lowest hydroxyl value (43.5).

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