Studies On MPV Reduction Of Ketones Over MgMAlO Oxides Derived From Hydrotalcite Precursor (M=Zr, Ti, Sn, Zn)

The reduction of carbonyl group to alcohol is an important reaction in organic chemistry, fine chemicals and medical industry. Compared with other reaction approaches, the heterogeneous MPV reduction shows some advantages of high chemical selectivity for C=O,mild conditions for reaction process and simple treatment after reaction.A kind of mixed metal oxide catalysts were derived from the calcination of hydrotalcite precursors MgAl-LDHs and MgMAl-LDHs (M=Zr, Ti, Sn, Zn). MgZrAl-LDHs were prepared via the direct method (Coprecipitation) and indirect method (Structure Memory). The other hydrotalcite precursors were obtained via the direct method. The catalytic activities of catalysts were investigated in the MPV reaction of cyclohexanone or acetophenone with the reductant of 2-propanol. The structural, thermal, textural and basicity of mixed oxides were examined by using XRD, BET, FT-IR, SEM, TG-DTA, ICP, XPS and CO2 adsorption-in situ-FT-IR.The results manifest that the pairs of strong Lewis basic sites and weak Lewis acid sites (O2--Mn+) are critical for MPV reduction over various mixed oxides. The composition of catalyst significantly affected its activity. The catalytic activities of MgMAlO (M=Zr, Ti) were higher than that of MgAlO, but the catalytic activities of MgMAlO (M=Sn, Zn) were lower than that of MgAlO.Calcination temperature of catalyst precursor had a significant effect on the activity of MgZrAlO obtained by the direct and indirect methods. The activity of MgZrAlO obtained by the direct method was higher than that of MgZrAlO obtained by the indirect method calcined at 250-400℃. The similar activity existed in the two types of MgZrAlO catalysts calcined at 400-600℃. When calcined above 600℃, the activity of MgZrAlO obtained by indirect method is higher compared to MgZrAlO obtained by direct method.The content of Zr had also effect on the activity of MgZrAlO. The activity of MgZrAlO-500 improved firstly and then decreased with the content of Zr. The catalysts show the better activity at the Zr content of 5wt%. The reusability and regeneration of MgZrAlO mixed oxides were better than that of MgAlO. All the mixed oxide catalysts had poor water resistance.The kinetics of the MPV reactions of cyclohexanone with 2-propanol over various mixed oxides suggest that the MPV reactions were all first-order. The apparent activation energies of the MPV reactions decreased in the order of Mg2Sn0.5Al1O-500 > Mg2Zn0.5Al1O-500 > Mg2Al1O-500 > Mg2Zr0.06Al1O-500 > Mg2Ti0.15Al1O-500. But the real activation energies of catalysts were almost same. It indicated that the mechanism of the MPV reaction did not change with the addition of amphoteric elements in mixed oxides of MgAlO. It changes the number of the pairs of Lewis basic site and acid site in catalysts. The number of the pairs increased with the addition of Zr or Ti, while the number of the pairs decreased with the addition of Sn or Zn.