Biodiesel production by soybean oil methanolysis over SrO/MgO catalysts: The relevance of the catalyst granulometry

The methanolysis of soybean oil was studied over magnesia supported SrO heterogeneous catalysts. The samples (0.05–0.35Sr/Mg atomic ratio) were prepared by contacting commercial MgO with aqueous solutions of strontium nitrate. Both Mg and Sr oxides were identified by XRD even for the lowest Sr content. The calcination temperature is a critical issue for the catalytic performances because carbonated and hydroxilated surface species are only removed for calcination temperatures higher than 575 °C. Carbonated species seems to be more soluble in methanol than oxide species thus increasing the rate of Sr leaching. In tested conditions (67 °C, methanol/Oil=9 and WCₐₜ/WOᵢₗ=5%) prepared catalysts presented interesting initial methanolysis activities. Catalysts with atomic ratios Sr/Mg higher than 0.10, showed analogous catalytic behaviors giving FAME (biodiesel) contents higher than 94%.The catalysts deactivation can be ascribable to strontium loss detected by XRD and SEM–EDS. The residual strontium remains trapped in MgO pores and thus inaccessible to the reactants. This result emphasizes the relevance of the external surface area of the catalysts due to the size of the triglycerides molecules and of the oil micelles, which cannot diffuse into catalysts pores. No soap formation was observed in the tested conditions.