Mesostructured Sr and Ti mixed oxides as heterogeneous base catalysts for transesterification of palm kernel oil with methanol

Abstract

In this work, two mesoporous Sr–Ti mixed oxides (MST) were synthesised via the evaporation-induced self-assembly (EISA) method and sol–gel combustion (SGC) method using Pluronic P123 as a structure-directing agent and citric acid as a complexing agent (for SGC). The physicochemical properties of MST were characterised by various techniques. The mixed oxide attained by SGC (MST–SGC) possessed greater crystallite size and crystallinity of cubic perovskite SrTiO3 than that synthesised by EISA (MST–EISA). An aggregation of mixed oxide nanoparticles generated mesopores with bimodal size distribution. Due to the higher mesostructured order of MST–SGC, it exhibited a relatively high specific surface area, large pore volume and narrow pore size distribution when compared to MST–EISA. The MST catalysts were applied to the transesterification of palm kernel oil (PKO) with methanol using an autoclave reactor. Although the reaction over MST–SGC required the temperature of 170°C, which was higher than that over MST–EISA with higher total basicity (150°C), to achieve the fatty acid methyl esters (FAME) yield of >99.9wt%, the MST–SGC showed superior chemical stability compared to the MST–EISA in a methanol washing and the transesterification. The study on the effect of adding free fatty acid (FFA) on the FAME yield indicated that both MST catalysts possessed an esterification activity. The FFA and water added in the feedstock affected the transesterification over MST–SGC much less than in the case of MST–EISA. The mesostructure of highly crystalline perovskite SrTiO3 should be a key for good catalytic performance of MST–SGC in the transesterification of PKO with methanol.