Application of metal organic frameworks ZIF-8-based solid catalyst with hierarchical porous structure and Brønsted-Lewis dual acid sites ionic liquids for sustainable biodiesel production from acidic soybean oil

Abstract

In order to alleviate the mass transfer limitation of triglyceride macromolecules on the solid catalyst surface during the catalytic oil transformation process, the utilization of hierarchical porous solid catalyst is a feasible method. For reaching this goal, the hierarchical porous support of H-ZIF-8 was initially prepared by incorporating the three-dimensional ordered macro and mesoporous into the ZIF-8 using polystyrene spheres (PS) as a hard template. Subsequently, the phosphotungstate anion PW₁₂O₄₀³⁻ (PW) decorated sulfonated 4,4′-dipyridinium ionic liquid with Keggin structure, namely [DPySO₃H]_1.5PW, was encapsulated in the H-ZIF-8 support, with the formation of the novel [DPySO₃H]_1.5PW@H-ZIF-8 catalyst. The so-synthesized solid catalysts featured with ordered hierarchical porous structure and dual Lewis and Brønsted acidic properties, with large BET surface area of 267.38 m²/g. This catalyst had high catalytic performances for the concurrent transformation of triglycerides and free fatty acids to biodiesel in a heterogeneous manner. Under the optimal reaction parameters (methanol/oil molar ratio: 30:1; catalyst dosage: 4 wt%; reaction time: 8 h; reaction temperature: 130 °C), the oil conversion level of 91.2 % and entire conversion of free fatty acids could be attained by applying this [DPySO₃H]_1.5PW@H-ZIF-8 catalyst. The high catalytic activity of this catalyst was due primarily to the increased mass transfer efficiency and synergism of Lewis and Brønsted acid sites. Moreover, the good FFA and moisture-resistance capacity was also shown for this catalyst even in the case of moisture content of 5 % and FFA content of 40 % in the oil feedstock. This solid catalyst could be reused by simple filtration and displayed good reusability, still attaining over 80 % oil conversion at the fourth reuse cycles thanks to the robust interactions between the acidic active centers and the hierarchical porous support. The kinetic analysis indicated that the activation energy Ea and Arrhenius constant A for this solid acid-catalyzed transesterification process were 56.0 kJ/mol and 7.8 × 10⁴ min⁻¹, respectively. This research would provide a new approach for the development of hierarchical porous solid catalysts, enabling the one-step transformation of low-grade acidic oils into biodiesel in a more sustainable and environmentally friendly way.