Acidic ionic liquid-decorated covalent organic framework TpPa–SO3H: a reusable solid acid catalyst used for sustainable biodiesel production from low-grade acidic oils

Hierarchical porous solid catalysts, with features such as large surface area, low mass transfer resistance, and high accessibility of active sites, have emerged as ideal catalysts for promoting large biomass molecule-involving reactions. In this study, an acidic ionic liquid [PrSO₃HMIM] [HSO₄] loaded covalent organic framework (COF) solid catalyst (xAIL@TpPa–SO₃H) was developed as an effective and recoverable catalyst for the efficient production of biodiesel. The solid catalysts featuring hierarchical porous structure and high surface acidities effectively improved the mass diffusion rate of oily macromolecules and easy accessibility of active sites. This catalyst demonstrated enhanced activities in the transesterification of triglycerides and esterification of free fatty acids (FFAs) simultaneously, which could achieve one-step production of biodiesel from low-grade acidic oils. Under the optimal conditions of a methanol to soybean oil molar ratio of 30 : 1, 10 wt% of catalyst loading (relative to the used soybean oil), and reaction temperature of 120 °C for 8 h, an oil conversion of 93.9% and full FFA conversion could be concurrently attained by adopting this developed catalyst. Kinetic studies have identified that the apparent activation energy E_a of the oil transesterification was 45.36 kJ mol⁻¹. Additionally, this catalyst exhibited satisfactory acid- and water-resistance even with an FFA level of 40% and water content of 4%, and after four times of reuse, over 80% oil conversion could still be attained, hereby posing its high potential for sustainable and green production of biodiesel particularly with low-grade acidic oils as feedstocks.