Fabrication of Magnetic NH₂-MIL-88B (Fe) Confined Brønsted Ionic Liquid as an Efficient Catalyst in Biodiesel Synthesis

A magnetically recyclable and efficient catalyst for biodiesel synthesis was fabricated by confining the Brønsted ionic liquid 1,4-butanediyl-3,3′-bis(3-sulfopropyl) imidazolium dihydrogensulfate in an amino-functionalized magnetic metal-organic-framework composite (Fe₃O₄@NH₂-MIL-88B(Fe)) through a simple and efficient strategy via the amino groups. The catalyst was systematically characterized, proving ionic liquid was grafted in the amino-functionalized carrier, and the catalyst was spindle-shaped with superparamagnetism (7.2 emu/g). The prepared catalyst with high acidity (1.76 mmol H⁺ g^-1) exhibited good catalytic activity in biodiesel synthesis from oleic acid and ethanol with a yield of 93.2%, optimized by Box-Behnken response surface methodology. The fine catalytic performance of the catalyst was mainly due to the combination of the characteristics of the ionic liquid and the structure of Fe₃O₄@NH₂-MIL-88B(Fe). Moreover, the catalyst was recycled without significant loss of activity after being used for six runs and could be magnetically separated in magnetic force without obvious mass loss.