Transesterification of triglycerides with methanol over thermally treated Zn5(OH)8(NO3)2×2H2O salt

Methanolysis of natural oil, i.e. castor oil and triacetin, a model compound for the transesterification of triglycerides in biodiesel production was studied under atmospheric pressure at temperature of 50-60 °C. As-received zinc hydroxy nitrate Zn₅(OH)₈(NO₃)₂×2H₂O (Zn-5) and samples obtained by thermal treatment of Zn-5 for 2 h in the temperature range of 105-300 °C were used as the catalysts. The catalysts were characterized by thermogravimetric (TG) analysis, X-ray powder diffraction (XRD), infrared spectroscopy (FTIR), nitrogen sorption (BET) and scanning electron microscopy (SEM). The effect of thermal treatment of Zn-5 salt on the activity and reusability in methanolysis of triglycerides was studied. During thermal treatment of as-received Zn-5 salt a gradual decomposition via various hydroxy nitrates intermediates such as Zn₅(OH)₈(NO₃)₂ and Zn₃(OH)₄(NO₃)₂ to ZnO occurred. This was accompanied by significant morphological and textural changes. Plate-like particles of Zn-5 salt reorganized into spherically shaped particles of ZnO. Moreover, decrease in specific surface area and porosity occurred. In methanolysis of both triglycerides, the activity of Zn-catalysts gradually decreased as the temperature of thermal treatment increased and the activity of ZnO, a final product of thermal decomposition was very low. The most active was as-received Zn-5 salt and its morphological/chemical properties did not change during methanolysis reaction performed at temperature of 50-60 °C. Moreover, the activity of original Zn-5 salt was fully restored after methanol/THF washing of spent catalysts. The activity of as-received Zn-5 catalyst was preserved under successive use in catalytic tests. The activity of thermally treated Zn-5 salt (at 140 °C) did not restore after methanol/THF washing and during subsequent use of Zn-5-140 catalyst its activity successively decreased.