Preparation, characterization and application of a novel solid BrÖnsted acid catalyst SO₄ ²⁻/La³⁺/C for biodiesel production via esterification of oleic acid and methanol

Waste camellia seed shell was placed in the fluidized bed tubular reactor and carbonized at 700 °C for the preparation of a carbon material. The obtained-carbon material was simultaneously modified by La³⁺ and concentrated sulfuric acid for the synthesis of a novel solid Brönsted acid catalyst SO₄ ²⁻/La³⁺/C by performing impregnation method. Physical and chemical properties and structural characteristics of the SO₄ ²⁻/La³⁺/C catalyst were characterized and analyzed by several methods. The catalytic activity and stability of SO₄ ²⁻/La³⁺/C were evaluated from the catalytic synthesis of biodiesel via esterification of oleic acid and methanol. The highest conversion of oleic acid was 98.37 wt% when the mass ratio of catalyst/reactant, mass ratio of methanol to oleic acid, reaction temperature and reaction time were fixed at 0.75 wt%, 9:1, 62 °C and 5 h, respectively. Additionally in terms of reusability, the conversion of oleic acid was still 81.9 wt% after SO₄ ²⁻/La³⁺/C catalyst had been recycled for ten times. The high catalytic activity and stability can be ascribed to the formation of a six-coordination bond from the interactions of SO₄ ²⁻/La³⁺, C–OH oxygen of the carboxyl group and H₂O. Hence, SO₄ ²⁻/La³⁺/C will exhibit strong Brönsted acidity and keep high stability in a reaction medium with large amount of H₂O.