Al2O3-supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil

Transesterification of palm kernel oil (PKO) and coconut oil (CCO) with methanol was investigated under a heterogeneous catalysis system. Various Al2O3-supported alkali and alkali earth metal oxides prepared via an impregnation method were applied as solid catalysts. The supported alkali metal catalysts, LiNO3/Al2O3, NaNO3/Al2O3 and KNO3/Al2O3, with active metal oxides formed at calcination temperatures of 450–550 °C, showed very high methyl ester (ME) content (>93%). XRF analysis suggests this is likely to be due to a homogeneous catalysis of dissoluted alkali oxides. On the other hand, Ca(NO3)2/Al2O3 calcined at 450 °C yielded the ME content as high as 94% with only a small loss of active oxides from the catalyst, whereas calcined Mg(NO3)2/Al2O3 catalyst possessed an inactive magnesium-aluminate phase, resulting in very low ME formation. At calcination temperatures of >650 °C, alkali metal- and alkali earth metal-aluminate compounds were formed. Whilst the water-soluble alkali metal aluminates formed over NaNO3/Al2O3 and KNO3/Al2O3 were catalytically active, the aluminate compounds on LiNO3/Al2O3 and Ca(NO3)2/Al2O3 are less soluble, giving very low ME content. The suitable conditions for heterogeneously catalyzed transesterification of PKO and CCO over Ca(NO3)2/Al2O3 are the methanol/oil molar ratio of 65, temperature of 60 °C and reaction time of 3 h, with 10 and 15–20% (w/w) catalyst to oil ratio for PKO and CCO, respectively. Some important physical and fuel properties of the resultant biodiesel products meet the standards of diesel fuel and biodiesel issued by Department of Energy Business, Ministry of Energy, Thailand.