Catalytic synthesis of biodiesel with silica gel-based Brønsted–Lewis acidic ionic liquid as catalysts

Sol–gel was used to successfully construct a series of silica gel supported Brønsted-Lewis acid ionic liquid (BLAIL/SG) catalysts. The structures of the catalysts were characterized by FT-IR, TGA-DTG, BET, and ³¹P MAS NMR. Meanwhile, the catalytic performance of these series catalysts in the preparation of ethyl laurate was investigated. The results showed that 20%[DMBPSH]⁺(½Cu²⁺)SO₄²⁻/SG catalyst exhibited good catalytic activity in the preparation of ethyl laurate owing to strong Brønsted acidity and the acidic synergy between the ionic liquid (Brønsted), metal ions, and the support (Lewis). A maximum ethyl laurate yield of 94.6% was reached for the 20%[DMBPSH]⁺(½Cu²⁺)SO₄²⁻/SG catalyst under the optimal conditions: ethanol (EtOH)/lauric acid (LA) molar ratio of 5, catalyst amount of 4wt%, time 3 h of reaction at 383 K. After being reused 6 times, the yield of ethyl laurate was still as high as 90.5%, indicating that 20%[DMBPSH]⁺(½Cu²⁺)SO₄²⁻/SG catalyst had excellent reusability. Further kinetic studies showed that the reaction order was 1.15 with activation energy of 25.69 kJ/mol.